What is the effect of wire diameter in SA (Submerged Arc) welding?
The preferred wire diameter is governed by the welding current required for a particular application. The commonly used SA wire diameters lie in the range 2.0 to 6.0mm. Current ranges for solid wire are shown in the figure. Overlap in these ranges enables exploitation of wire diameter effects. For example, for a given welding current, small diameter wires give increased current density resulting in narrower and more deeply penetrating weld beads and increased metal deposition rates compared to a larger diameter wire. Arc starting and stability may also be improved with smaller wire diameters.
What is the effect of arc voltage in SA (Submerged Arc) welding?
Arc voltage has an important effect on the weld bead shape and the depth of penetration; the precise effect being dependent on the joint preparation. Bead on plate welds and square edge close butt welds have increased bead width and dilution as the arc voltage increases, although the depth of penetration is relatively unaffected. In a prepared V-butt joint, increasing the arc voltage may lead to lack of fusion in the root as the wide arc will not reach the bottom of the root. Reducing the voltage, in this case, will increase the depth of penetration as the narrow arc column is more easily able to reach the bottom of the preparation.
Increasing arc voltage lengthens the arc so that weld bead width, reinforcement and flux consumption are increased as is the risk of arc blow. When alloying the weld metal from the flux, arc length and hence arc voltage must be carefully controlled as at high arc voltages more flux is melted allowing more alloying elements to enter the weld metal thereby affecting weld metal composition.
What is the difference between MIG and MAG?
MIG stands for Metal Inert Gas.
Only inert gases or gas mixtures are used for the shielding gas when MIG welding. Typical inert gases used for MIG welding are argon and helium. These gases are usually used for MIG welding of aluminium and other non-ferrous metals.
MAG stands for Metal Active Gas.
Active gas mixtures have been developed primarily for welding steels. Typical shielding gases are mixtures of argon, carbon dioxide and oxygen e.g. CO2 , Ar + 2 to 5% O2 , Ar + 5 to 25% CO2 and Ar + 10% CO2 + 5% O 2 .
The composition of the shielding gas has a substantial effect on the stability of the arc, metal transfer and the amount of spatter. The shielding gas also affects the behaviour of the weld pool, particularly its penetration and the mechanical properties of the welded joint.
How important is the Contact Tip in MIG welding?
Very important. Make sure the gun tip isn’t worn out or that weld spatter is not on the tip near the exit hole. The contact tip in the gun should be perfectly round and just a few thousandths larger than the wire itself. Worn tips are typically oval and can cause an erratic arc from the random electrical connection and physical movement of the wire inside the worn tip. Genuine Lincoln contact tips are precisely made from a wear-resistant copper alloy for superior welding performance. If the contact tip enters the molten weld pool, it should be immediately replaced. For most casual welders, a good rule of thumb to assure high quality welding is to change the tip after ever 100 lbs. of wire. Another point to remember about contact tips is that they should always be threaded completely into the gas diffuser and tightened prior to welding to give a smooth flow of welding current.
How important is a good electrical ground in MIG welding?
In arc welding, an arc is established from the electrode to the workpiece. To do this properly, the arc requires a smooth flow of electricity through the complete electrical circuit, with minimum resistance. If you crimp a garden hose while watering the lawn, the flow at the sprinkler head is much reduced. Beginning welders often make the mistake of attaching the work clamp (or electrical ground) to a painted panel or a rusty surface. Both of these surfaces are electrical insulators and do not allow the welding current to flow properly. The resulting welding arc will be difficult to establish and not very stable. Other telltale signs of an improper electrical connection are a work clamp that is hot to the touch or cables that generate heat. Another key point to consider when attaching the welding ground is to place the welding ground on the piece being welded. Welding current will seek the path of least resistance so if care is not taken to place the welding ground close to the arc, the welding current may find a path unknown to the operator and destroy components unintended to be in the welding circuit.
Are there any other tips you can provide for higher quality MIG welding?
are .035" and .045", a smaller diameter wire usually will make it easier to create a good weld. Try an .025" wire diameter, which is especially useful on thin materials of 1/8" or less. The reason? Most welders tend to make a weld that is too big - leading to potential burnthrough problems. A smaller diameter wire welds more stable at a lower current which gives less arc force and less tendency to burn through. If you keep your weld current lower, you will have a greater chance of success on thinner materials. This is a good recommendation for thinner materials; but be careful using this approach on thicker materials (>3/16”) because there may be a risk of lack of fusion. Whenever a change like this is made, always verify the quality of the weld meets its intended application.
Does shielding gas affect the quality of the finished weld?
For most mild steel applications, CO2 will provide adequate shielding, but when you must have a flatter bead profile, less spatter or better wetting action, you may want to consider adding 75 to 90% argon to your CO2 shielding gas mix.
Why? Argon is essentially inert to the molten weld metal and therefore will not react with the molten weld metal. When CO2 is mixed with Argon, the reactivity of the gas is reduced and the arc becomes more stable. But, Argon is more expensive. In production welding, selecting the perfect shielding gas can be a science of its own. Attributes such as material thickness, welding position, electrode diameter, surface condition, welding procedures and others can affect results.
Common gas mixes:
· 100% CO2 -Lowest price, generally greatest penetration, and higher levels of spatter. Limited to short circuit and globular transfer.
· 75% Argon - 25% CO2 -Higher price, most commonly used by home hobbyist and light fabricator, lower levels of spatter and flatter weld bead than 100% CO2. Limited to short circuit and globular transfer
· 85% Argon - 15% CO2-Higher price, most commonly used by fabricators, with a good combination of lower spatter levels and excellent penetration for heavier plate applications and with steels that have more mill scale. Can be used in short circuit, globular, pulse and spray transfer
· 90% Argon - 10% CO2- Higher price, most commonly used by fabricators, with a good combination of lower spatter levels and good penetration for a wide variety of steel plate applications. Can be used in short circuit, globular, pulse and spray transfer
Why is hydrogen a concern in welding?
Hydrogen contributes to delayed weld and/or heat affected zone cracking. Hydrogen combined with high residual stresses and crack-sensitive steel may result in cracking hours or days after the welding has been completed. High strength steels, thick sections, and heavily restrained parts are more susceptible to hydrogen cracking. On these materials, we recommend using a low hydrogen process and consumable, and following proper preheat, interpass, and postheat procedures. Also, it is important to keep the weld joint free of oil, rust, paint, and moisture as they are sources of hydrogen.
I need to order some welding cable for our shop, but am not sure the correct size to get.
Welding cable is the electrical conductor for the welding current. It consists of a series of fine copper strands wrapped inside a non-conductive, durable jacket (typically some type of synthetic or natural rubber of various colors). The fine copper strands give welding cable more flexibility than other types of electrical conductors and the insulating jacket is designed to hold up to repeated movement over rough surfaces. As the current level increases (measured in amperage or amps), the diameter of the welding cable and resulting cross sectional area of the copper stranding needs to increase. The concept is similar to the flow of water through a hose. A larger diameter hose is needed in order to carry a greater volume of water. You use a smaller hose to water your garden, while the fire department uses a much larger hose to fight fires.
Welding cable “ampacity”, also known as current capacity or amperage rating, refers to the maximum amount of electrical current that a cable can safely conduct. Besides the cross sectional area, other factors that impact the ampacity of welding cable are its length, ohm rating (i.e., resistance rating), temperature ratings of the insulation material and the ambient temperature. Shorter cables can carry more current than longer cables of the same diameter. Welding cable is often rated with a conductor temperature of 75°C (167° F), 90°C (194⁰F) or 105°C (221°F). While the copper wire itself can handle the high temperatures generated by higher amperages before sustaining damage, the insulation protecting them would melt. Welding cables are also often rated for an ambient temperature of 30°C (86°F). Higher ambient temperatures can reduce their ability to dissipate heat into the surrounding environment, and thus reduce their ampacity. In addition, several cables packed tightly together can also have a reduction in their ability to dissipate heat. Multiple cables should be slightly spread apart.
Note that while copper is an excellent conductor of electricity, it still has a degree of resistance to the flow of electrons through it. Therefore, some amount of resistance heating will occur in the cable. It is normal for a properly sized welding cable to feel warm to the touch after prolonged welding. However, if the diameter of cable is too small for the level of current flowing through it, then the cable will overheat. This can result in a potential fire hazard, as well as damage to the cable itself (and ultimately to cable breakage and failure). A breakdown of the insulation jacket can also be an electrical shock hazard. Conversely, cable that is oversized for a given amperage level does not conduct current any more effectively than properly sized cable. However, larger diameter cable typically costs more per foot or per meter than smaller diameter cable, because of the increased amount of copper strands. Therefore, oversized cables may not be cost effective.
Now when selecting the proper cable size for your welding equipment, it is best to choose cable that can handle the maximum output of the welder. To do this, you need to determine three factors. These include:
• Total length of the welding circuit
• Rated output of welding power source
• Duty cycle of the welding power source
Why is preheat sometimes required before welding?
Preheating the steel to be welded slows the cooling rate in the weld area. This may be necessary to avoid cracking of the weld metal or heat affected zone. The need for preheat increases with steel thickness, weld restraint, the carbon/alloy content of the steel, and the diffusible hydrogen of the weld metal.
Poor Gas Coverage Leads to Contamination?
Contamination caused by lack of shielding gas can happen when the shielding gas is not turned on, there is either too little or too much gas shielding, or the gas shielding is blown away.
To troubleshoot gas contamination issues, first check the gas cylinder label to be sure you’re using the right type of gas for TIG welding, generally 100 percent argon (or perhaps an argon/helium blend for thick aluminum). Attempting to weld with an AR/CO2 mix (used for MIG welding) will cause immediate contamination.
Next, set the proper gas flow rate, which should be 15 to 20 cubic feet per hour (cfh). Welders commonly—and incorrectly—assume that a higher gas flow/pressure provides greater protection. In fact, excessive gas flow creates turbulence and swirling currents that pull in unwanted airborne contaminants (and it can cause arc wandering). Generally, err on the lower side of recommended shielding gas rates to ensure proper shielding coverage without turbulence.
Third, check all the fittings and hoses for leaks. Any breach may pull air into the shielding gas stream, which can cause the weld to be contaminated (and you’ll waste money if gas escapes). Rub soapy water over the hose and all fittings. If bubbles form, you have a leak and need to replace the defective components.
Finally, assuming you have a full cylinder, the right type of gas and no leaks, consider that you may have a tank contaminated with moisture. Shielding gas cylinder contamination does not happen frequently, but it is possible. Check with your gas supplier to resolve this issue.
How do I combat "dirty power" (the voltage fluctuations that hamper my arc stability and weld quality)?
Whether it is other workers running tools and equipment off of the same primary power line, brownouts, power spikes, or generators that don't regulate auxiliary power voltage, voltage fluctuations can cause havoc with welding parameters.
New technologies are ensuring that operators never experience a fluctuation in the welding arc. Line voltage compensation devices have been implemented on units to help curtail such fluctuations. Manufacturers are also creating new technology that makes sure the primary power remains within certain parameters. One of the newest multiprocess units available promises no arc fluctuation or wandering as long as the primary power remains within a 185 to 635-V range. That covers a "low line" 208-V primary all the way through a "high line" 575-V primary. This system takes primary power and converts it to a buss voltage, then using that buss voltage to drive the control part of the inverter mechanism.
This technology is ideal for job sites where many workers run tools off of the same power and where line transients cause voltage fluctuations.
What is Stove Pipe Welding?
Stove pipe welding is one of the chief methods used in the field welding of pipelines for oil, gas, water etc., where the speed of joining pipes is critical in the speed of pipeline construction (ditching, hauling, stringing, etc.). It is a variant of the manual/shielded metal arc welding (MMA/SMAW) technique used for positional welding, enabling steel pipelines to be laid at high production rates.
In pipe joining, the rate of progress is limited by the root pass and the hot (second) pass. In order to speed up the deposition of these two passes without compromising the quality of the weld, the welding is carried out downward from 12 o’clock to 6 o’clock, since the process is faster than the upward direction, especially for pipes of wall thickness below 25 mm. Furthermore, this allows the use of two pairs of welders working simultaneously on both sides of the pipes, as opposed to only one pair in the upward position.
The root pass is the most critical, and requires skilled welders. Cellulosic or cellulosic-iron powder coated electrodes are used. These do not require drying and are coated with cellulose, an organic compound with high levels of hydrogen which gives a high burn-off rate, forceful arc and a light, fast-freezing slag - all very suitable for the vertical-downward technique. The coating also provides a gas shield which is less affected by wind than other electrodes (though weather protection may still be required).
The weld preparation typically consists of a 60-70° bevel (inclusive angle), with a 1-2mm root face and 2-3mm root gap. Stringer beads are deposited into the root at high speeds (250-300 mm/min). This is immediately followed by a hot pass which refines the root pass and reduces the risk of hydrogen cracking associated with these consumables. A minimum level of preheat may be required for the same reason. Filler runs, stripper runs and capping run(s) complete the welding.
The welding of pipelines is usually performed by a team of welders; the larger the diameter of pipe, the greater the number of welders. In most cases each welder performs the same weld run(s) on each successive joint.
Why do we use Alternating Current to weld Aluminum?
我Alternating current alternates between electrode positive and electrode negative. When the tungsten electrode is positive the aluminum oxide on the surface is being cleaned and most of the heat in the arc is concentrated on the tungsten. When the electrode is negative the arc penetrates but very little heat is concentrated on the tungsten. We could use DC electrode positive to weld aluminum but a very large tungsten electrode would be required. By alternating (AC current) between positive and negative we can clean the aluminum surface without using too large a tungsten electrode.
Can I weld AL-6XN to 316L stainless steel?
AL-6XN alloy and 316L stainless steel can be joined easily using standard welding practices for austenitic stainless steel. However, from a corrosion resistance viewpoint, this joining is not recommended if avoidable. Whenever welding two different materials together, it is important to consider the galvanic potential between the two materials.
Can I use an Oxy/Acetylene torch to weld stainless steel?
Yes, stainless steel can be welded with an oxy/acetylene torch. A good welding flux which protects the welding zone from oxygen should be used. Also, the torch should be carefully adjusted minimizing excess oxygen could cause the oxides mentioned above.
What is common welding process for structural steel?
For structural steel applications, bridge construction and heavy equipment repair, self-shielded flux-cored welding (FCAW-SS) has become a standard and reliable process due to its ability to provide high-deposition rates and good weld quality. It also provides the chemical and mechanical properties necessary to withstand low temperatures and is equally well-suited for ship and barge construction applications.
Is It Better to Use Fixed Automation or a Robot?
Each type of automation has its own best applications. Fixed automation is an efficient and cost-effective way to weld simple repetitive straight welds or round welds, where the part is rotated. It is good for high-volume applications of a single part. Fixturing for fixed automation can be expensive, however, so companies will need to factor that cost into the initial investment and determine whether this type of automation is still cost-effective for the long-term. They also need to determine if future jobs will require retooling, as that will add further to costs.
For companies wishing to have the flexibility to weld on multiple applications, a robotic welding system is a better choice. Because a robot can be programmed for multiple jobs, it can often handle the task of many fixed-automation systems.
What Are the Best Applications for a Robotic Welding System?
High-volume, low-variety applications are well-suited to robotic welding; however, lower-volume, higher-variety applications may also work if implemented with the proper tooling. Companies will need to consider the additional cost for tooling to determine if the robotic welding system can still provide a solid return on the initial investment.
In either case, it is critical that the application have simple, consistent parts so that the robot can repeatedly execute the weld in the same location. Having a blueprint or electronic computer-aided design drawing is helpful. Robotic integrators can review the blueprint or create a software simulation that can assess the suitability of the part for welding automation. These assessments not only help to visualize the quality of the part to be welded, but they can also identify ways to fine-tune tooling to optimize the process.
Workflow is also important. Companies should be certain to have a high enough flow of parts to the robotic welding cell for the application so that it can operate consistently. Delays in upstream parts fabrication can cause bottlenecks that result in costly downtime.
How do I weld thick to thin aluminum?
Concentrate the heat on the thick part, as the heat will transfer easily to the thinner aluminum.
What is the most dependable method for welding aluminum with a GMAW machine?
Spray transfer is the desired mode of metal transfer for welding aluminum. Spray transfer offers a smooth transfer of molten metal droplets from the end of the electrode to the molten pool. The droplets crossing the arc are smaller in diameter than the electrode. There is no short circuiting. The deposition rate and efficiency are relatively high and the arc is smooth, stable, and stiff. The weld bead has a nice appearance and a good wash into the sides. In spray transfer mode, a large amount of heat is involved, which creates a large weld pool with good penetration that can be difficult to control and cannot be used on materials thinner than 14 gauge. This transfer will produce a hissing sound and no spatter.
have a dependable GMAW unit and need to weld aluminum. Can I do that or should I look into other options?
The aluminum material thicknesses that can be welded with the GMAW process are 14 gauge and heavier. How heavy depends on the output capacity of the welding machine being used. To GMA weld aluminum thinner than 14 gauge (0.074 in.), either specialized pulsed gas metal arc or AC gas tungsten arc welding equipment may be necessary.
Why is my aluminum welded connection so much weaker than the base material?
In steel weldments, a welded connection can be made as strong as the base material, but this is typically not the case with aluminum. In almost all instances, the welded connection will be weaker than the base material.
To understand why this occurs, consider the two classifications of aluminum alloys: heat treatable and nonheat treatable. The latter category is hardened only by cold working, which causes physical changes in the metal. The more the alloy is cold worked, the stronger it gets. When you weld an alloy that has been cold worked, you locally anneal the material around the weld so it goes back to its zero-tempered (or annealed) condition and it becomes "soft." Therefore, the only time you can make a weld as strong as the base material with a nonheat-treatable alloy is when you start with zero-tempered material.
With heat-treatable aluminum alloys, the last heat treatment step heats the metal to approximately 400°F (200°C). When welding, the material around the weld (the heat-affected zone) becomes much hotter than 400°F so the material tends to lose some of its strength. Unless a postweld heat treatment is applied, the area around the weld will become significantly weaker than the rest of the aluminum - by as much as 30 to 40%. Post-weld heat treatment can restore this loss in strength if a heat-treatable aluminum is used.
Table 1 is a guide as to which series of aluminum alloys are heat treatable and which are not.
Table 1 - Guide to Heat-Treatable Aluminum Alloys
May I overlay a stick weld (6010) with a bead of MIG (gas shielded) for appearance? I am welding 1/2 mild steel.
I don't see any reason why you cannot do this. As always you should make sure that you are welding at high enough setting to obtain sufficient penetration. If you are welding to any codes or other requirements then you need to make sure that this procedure is allowed under those codes or specifications.
Should I ball a pure tungsten electrode for welding thin material?
No. Instead use a 3/32in. tungsten with 2% cerium (2% thorium is second choice), grind it to a point and put a small land on the end. Compared to a balled tungsten, a pointed electrode provides greater arc control and lets you direct the amperage precisely at the joint, minimizing distortion.
When welding aluminum items such as heads or intakes, which is the preferred method? TIG or MIG?
The TIG method is used to weld these items because of its complete fusion and accuracy of the repaired area. MIG being more of a production process would deposit too much material in the affected area causing excessive post repair machining and clean up.
Can I use my MIG welder to weld Aluminum?
Yes you can but you require an inert gas such as Argon. The drive rolls used should be "U" grooved drive rolls and the gun liners and feed guides made of Teflon or nylon. The gun length should be as short as possible and be kept as straight as possible. 5356 aluminum wire feeds better than the 4043 type because it is stiffer. If much Aluminum is going to be welded you should look at whether a spool gun or a push-pull aluminum gun can be adapted to the MIG unit.
How about some tips on arc welding inside corners?
1) If the pieces are 1/8th inch or smaller, I would just butt them together & run a good 1/8" bead, or smaller.
2) For bigger than 1/8", I would bevel one inside edge & then fill it with a bead (or beads), to equal the diameter of the metal.
3) If you CAN get to the outside corner & WANT it welded, I'd go ahead and run a bead there too.
4) If you DON'T want the outside corner welded due to looks or whatever, make sure the inside welding is sufficient!
How is Steel welded to Aluminum?
proper formation of successful fusion welding of such dissimilar metals. These are: widely different melting temperatures, no mutual solubility in molten state, discrepancy in thermal conductivity and in thermal expansion that cause stresses and cracks.
Furthermore during fusion welding, but also during heating to some low temperature like 200 0C (400 0F) melting phases and several brittle intermetallic phases are generated that compromise the integrity of the weld.
If confronted with a similar problem, short of selecting a different material for one of the components, so that the combination be more favorable, one should explore which alternative process is suitable for the application.
Solid state processes like Explosion-, Friction-, Magnetic Pulse-, Ultrasonic-welding, Roll bonding and High Temperature Diffusion joining avoid fusion by definition. Obviously not all of them can be suitable for a given application, because of the specific limitations of each one of them.
Other than those, High Energy like Electron- and Laser-beam welding could sometimes be applied as they are able to concentrate their energy in a very tiny spot limiting their influence in heat, location and time duration.
Finally, if the joint configuration can be adapted to process requirements, brazing, soldering or adhesive bonding might provide a suitable solution.
How is Aluminum welded to Stainless Steel?
It must be realized that fusion welding is generally not suitable for welding together dissimilar materials like aluminum and stainless steels. That is because of widely different melting temperatures, no mutual solubility in molten state, and because of differences in thermal conductivity and in thermal expansion that cause stresses and cracks.
During welding, low temperature melting phases and several brittle intermetallic phases are generated that compromise the integrity of the weld. Also not every aluminum type and not every stainless steel type can be considered for being joined together.
However a highly localized fusion welding process of elevated power density like Electron Beam Welding in vacuum may be sometimes used, provided that a third transition metal, compatible with both base metals, is used in between. In the specific case Silver might be used as a transition element, or to bridge the gap.
Solid state welding is applicable in certain combinations, providing acceptable joints can be realized that meet requirements. One of the most used of these processes is friction welding. Cleaning of the surfaces is of the utmost importance because contaminants entrapped in the joint risk to undermine its properties.
For joining large parts a suitable transition hybrid element (part of which is aluminum, the other part being stainless) can be prepared, welded by friction. The ends of the transition element can then be welded to the main structure parts by more conventional procedures between similar base metals.
Besides that, if alternative solution can be considered, brazing or adhesive bonding, if appropriate, are applicable.
A thin tube has to be welded to a thick plate or bar: why is it so difficult to do so?
The thick element absorbs a large quantity of heat before reaching melting temperature. On the contrary the thin tube melts almost immediately. Therefore to weld properly one has to change the configuration of the joint so that the difference in thickness be kept to a minimum.
The bar or plate has to be machined so that at the joint location the thickness be comparable to that of the tube, or an intermediate transition element of proper shape and size must be welded between the two elements. Alternatively, if the joint shape permits it, one should consider brazing or friction welding.
What shall I use to weld stainless steel?
This question, which I actually received from a reader, is unfortunately not sufficiently defined: therefore it is not possible to give a meaningful answer. One must first describe a few parameters to qualify the solution requested.
Material: as explained in another page on Stainless Steel Welding (opens a new page), there are many different types of steels loosely responding to this category, but they can be grouped in four or five families having important characteristics in common.
Each family/type has to be addressed separately as they behave differently during welding and need specific instructions.
Therefore, before tackling a job, one has to know positively or to have analyzed qualitatively the stainless steel type involved, for identifying at least its family.
Joint: type and dimensions of the needed joint must be stated. One should pay attention to the deformations that may develop as a consequence of welding and take proper precautions.
Process: If we think of a small shop and of an occasional job coming up once in a while, we will try to adapt whatever process is available that will give an acceptable solution. If we have a larger shop with plenty of equipment to choose from, and with experienced workforce with the needed skills, we will be able to select in complete freedom. If we are planning for mass production we will be able to purchase the equipment capable of the most cost effective welding.
Consumables need to be suitable both for base material and for process selected.
Of the common processes that can be used to weld stainless steels we will consider only three:
· the Shielded Metal Arc Welding (SMAW) or Manual Metal Arc (MMA) with covered electrodes, see Shielded Metal Arc Welding Tips (Opens a new page). This manual process is the first to think of, if the material is not extremely thin. By using multiple passes one can weld substantial thickness.
· the Gas Tungsten Arc Welding (GTAW or Tig) with nonconsumable tungsten electrode, see Tig Welding Tips (Opens a new page). This manual or mechanized process can produce very clean welds, as needed for food or pharmaceutical industries. It is not used, generally, for thick materials except for the first pass.
·the Gas Metal Arc Welding (GMAW or Mig) with consumable electrode, see Mig Welding Tips (Opens a new page). This process provides higher deposition rate than the two above, and is best used for industrial applications on substantial thickness or over a root pass made by GTAW or GMAW. Can be used for Robotic Arc Welding (Opens a new page).
Why is preheat used when arc welding steel, and how is it applied?
Preheating is the process applied to raise the temperature of the parent steel before welding. It is used for the following main reasons:
· To slow the cooling rate of the weld and the base material, resulting in softer weld metal and heat affected zone microstructures with a greater resistance to fabrication hydrogen cracking.
· The slower cooling rate encourages hydrogen diffusion from the weld area by extending the time period over which it is at elevated temperature (particularly the time at temperatures above approximately 100°C) at which temperatures hydrogen diffusion rates are significantly higher than at ambient temperature. The reduction in hydrogen reduces the risk of cracking.
Preheat can be applied through various means. The choice of method of applying preheat will depend on the material thickness, weldment size and the heating equipment available at the time of welding. The methods can include furnace heating for small production assemblies or, for large structural components, arrays of torches, electrical strip heaters, induction heaters or radiation heaters.
It is important to apply preheat correctly, with appropriate monitors and controls, and also to monitor the interpass temperature (the temperature of the workpiece between welding the first and subsequent passes), to ensure that it does not fall below the preheat temperature.
Common techniques for monitoring preheat are temperature indicating crayons and thermocouples or contact thermometers. Preheat should be monitored at a distance of 4t (where t is the thickness of the material to be joined) away from the longitudinal edge of the groove for t<50mm or at a minimum distance of 75mm from the joint preparation for t>50mm and on the reverse side of the plate to the heat source.
Can I weld mild steel to high yield steel?
Yes, provided that the welding procedure, particularly any preheat, is designed for the high yield steel. This may mean using basic low hydrogen MMA (SMA) consumables. It is also a convention that the weld metal matches the strength of the weaker of the two components although this is not invariably the case. However, it must be recognised that the strength of such a joint will always be limited by the magnitude of the mechanical properties of the mild steel.
What are the factors involved in the choice of consumables for MMA (SMAW) welding of cast iron?
Several factors must be considered when choosing consumable filler materials for welding cast iron. The following factors need to be looked at although it may not be possible to take the full requirements of each one into account.
2. Matching strength
3. Tolerance of dilution (absorption of base material into weld metal).
5. Tolerance of high cooling rates
6. Weldability at low heat inputs
7. Colour matching
8. Sufficient ductility to absorb welding strains
Common MMA (SMAW) filler materials include:- nodular iron, low carbon steel, nickel based alloys and copper-based alloys.
MIG/MAG and flux-cored arc welding of cast iron
Metal Inert Gas (MIG) and Metal Active Gas (MAG) welding (known collectively in the USA as GMA welding) offer distinct advantages for welding of cast irons, provided that the special requirements of cast iron welding are borne in mind.
Three modes of metal transfer from the electrode tip in MIG and MAG welding are possible, depending on the heat input. These are spray transfer, globular transfer and short-circuit 'dip transfer', in order of decreasing heat input. Since spray transfer has the highest penetration, it is the least desirable condition for cast iron welding, despite its high deposition rate. Provided that care is taken to avoid incomplete fusion, dip transfer is best suited for welding cast irons since it produces the narrowest HAZ, with the minimum of base metal melting. MIG/MAG welding is used successfully with steel, nickel-based and copper-based consumables, but the choice of consumable depends on the joint performance and appearance requirements.
Flux cored arc welding (FCAW) includes many of the best features of MIG or MAG welding in that it uses a continuously fed wire, and can therefore be easily mechanised, as well as using a flux, which can be used to adjust the weld metal composition and solidification rate. Some cored wire consumables can be used in dip transfer mode, although this will create a lot of spatter and may lead to fusion defects.
The range of consumable types available for FCAW of cast irons is confined to the high nickel, nickel-iron, and nickel-iron-manganese types. The choice of consumables depends on the same factors that govern consumable choice for MMA and MIG/MAG welding.
Manual metal arc (MMA) welding of cast iron?
Manual metal arc (MMA) welding (also known as SMAW) is a commonly used industrial method for joining cast iron. The good weld penetration typical of the process is actually a disadvantage, as it increases the tendency for dilution of the filler metal by the parent material. For most filler metals this is not required, and so many commercial electrodes have specially designed coatings to give the softest possible arc characteristics.
For nickel-based fillers, which are the most common choice for MMA cast iron welding, the penetration of the arc is reduced by a special graphite coating, which serves to introduce graphite into the weld pool. (Nickel based consumables designed for welding nickel alloys are therefore unsuitable for cast iron welding).
Very soft arc characteristics are also required for use of mild steel electrodes, which again is achieved by consumables designed specially for use on cast iron.
Heat input in MMA welding should be kept to a minimum, since supplying larger amounts of heat naturally leads to more extensive melting of the parent material. Use of the smallest practical electrode diameter and minimum current is therefore recommended. Some degree of control over the cooling rate, and hence the HAZ hardness can be achieved by choice of an appropriate preheat temperature. The use of post-weld heat treatment may also be beneficial.
How do I weld 7075?
Most aluminum alloys are weldable, but there are a fair number of them that are not, including 7075 aluminum. The reason 7075 is singled out in this example is that it is one of the highest strength aluminum alloys. When designers and welders look for an aluminum alloy to use, many will start by reviewing a table that lists all of the aluminum alloys and their strengths. But what those newcomers don't realize is that few of the higher strength aluminum alloys are weldable - especially those in the 7000 and 2000 series - and they should not be used.
The one exception to the rule of never using 7075 for welding is in the injection molding industry. This industry will repair dies by welding 7075 - but it should never be used for structural work.
Here are some simple guidelines to follow when choosing aluminum alloys:
Main Alloying Elements
Aluminum and copper. (High strength aluminum used in the aerospace industry )
Aluminum and manganese. (Low- to medium-strength alloys, examples of products using these alloys are beverage cans and refrigeration tubing)
Aluminum and silicon. (Most alloys in this series are either welding or brazing filler materials)
Aluminum and magnesium. (These alloys are used primarily for structural applications in sheet or plate metals - all 5000 series alloys are weldable )
Aluminum, magnesium and silicon. (These alloys are heat treatable and commonly used for extrusions, sheet and plate - all are weldable, but can be crack sensitive. Never try to weld these alloys without using filler metal)
Aluminum and zinc. (These are high strength aerospace alloys that may have other alloying elements added)
AOTAI suggests that if you have a need to design something of high strength aluminum, look to a 5000 series high magnesium alloy instead of a 2000 or 7000 series. The 5000 series alloys are weldable and will produce the best results.
How do I TIG weld two dissimilar thicknesses of aluminum?
When an operator has two dissimilar thicknesses, he or she must set the parameters so that they are high enough to TIG weld the thickest piece. When welding, favor the joint and put more of the heat on the thicker piece.
How can I tell different aluminum alloys apart?
There are quite a few different aluminum alloys and for proper and safe welding, you should know what alloy your welding. If you don't, you can follow these general guidelines:
Extrusions are generally 6000 series alloys Castings most often are a combination of aluminum/silicon cast -- some are weldable, others are not. Pieces of sheet, plate or bar are probably 5000 to 6000 series alloys
If you want to be precise, purchase an alloy tester kit that will help you determine the exact makeup of your alloy.
What is the proper stress relieving practice for aluminum welds?
When welding, the operator sets up residual stresses around the vicinity of the weld because the molten material shrinks as it solidifies. Further, when the operator takes this welded structure and begins to remove material by machining, it tends to distort and create dimensional instability. To avoid this in aluminum, operators perform stress relieving by heating the material hot enough to allow the aluminum atoms to move around.
For steel, the stress relieving temperature is approximately 1050° F to 1100° F, but for aluminum, the proper stress relieving temperature is 650° F. This means that in order for post weld stress relief on aluminum to be effective, the material will have to be heated to a temperature where mechanical properties will be lost. For this reason, post weld stress relief is not recommended for aluminum.
How much preheat should I use when welding aluminum?
While a little preheat is good, too much preheat can degrade the mechanical properties of the aluminum.
As was discussed earlier, the last heat treatment for heat treatable alloys is 400° F so if the operator preheats the aluminum to 350° F and holds the temperature in that range while welding, the aluminum's mechanical properties are changed.
For the non-heat treatable alloys such as the 5000 series, if the operator holds the temperature even in the 200° F range - he or she can sensitize the material to stress corrosion cracking. In most cases, some preheat is acceptable to dry the moisture away from the piece, but preheat should be limited.
Many inexperienced aluminum welders use preheat as a crutch. Since equipment for welding aluminum needs to operate at higher capacities, many feel that preheat helps eliminate equipment limitations, but this is not the case. Aluminum has a low melting point -- 1200° F compared to 2600° F to 2700° F for steel. Because of this low melting point, many operators think they only need light duty equipment to weld the aluminum. But, the thermal conductivity of aluminum is five times that of steel, which means that the heat dissipates very quickly. Therefore, welding currents and voltages for welding aluminum are higher than they are for steel so operators actually need heavier duty equipment for aluminum.
For TIG Welding, what type of electrode is best for aluminum?
For most materials, including steel, a two percent thoriated tungsten electrode is recommended, but since aluminum is welded with AC rather than DC, the electrical characteristics are different and the amount of energy put into the tungsten electrode is higher when AC welding. For these reasons, pure tungsten or zirconiated tungsten are recommended for aluminum welding.
In addition, the electrode diameter for AC welding has to be significantly larger than when using DC. It is recommended to start with an electrode that is 1/8" and adjust as needed. Zirconiated tungsten can carry more current than pure tungsten electrodes. Another helpful hint for AC welding is to use a blunt tip - the arc tends to wander around a pointed tip.
What type of shielding gas should I use for aluminum welding?
For both TIG Welding (Gas Tungsten Arc Welding or GTAW) and MIG Welding (Gas Metal Arc Welding or GMAW) use pure argon for aluminum materials up to ½" in thickness. Above ½" in thickness, operators may add anywhere between 25 to 75 percent helium to make the arc hotter and increase weld penetration. Argon is best because it provides more cleaning action for the arc than helium does and it is also less expensive than helium.
Never use any shielding gas that contains oxygen or carbon dioxide, as this will oxidize the aluminum.
Why is my aluminum weld much weaker than the parent material?
In steels a weld can be made as strong as the parent material, but this is not the case with aluminum. In almost all instances, the weld will be weaker than the parent material.
To further understand why this occurs, let's look at the two classifications of aluminum alloys: heat treatable and non-heat treatable. The latter category is hardened only by cold working which causes physical changes in the metal. The more the alloy is cold worked the stronger it gets.
But, when you weld an alloy that has been cold worked, you locally anneal the material around the weld so that it goes back to its 0 tempered (or annealed) condition and it becomes "soft". Therefore, the only time in the non-heat treatable alloys that you can make a weld as strong as the parent material is when you start with 0 tempered material.
With heat treatable aluminum alloys, the last heat treatment step heats the metal to approximately 400° F. But when welding, the material around the weld becomes much hotter than 400° F so the material tends to lose some of its mechanical properties. Therefore, if the operator doesn't perform post-weld heat treatments after welding, the area around the weld will become significantly weaker than the rest of the aluminum - by as much as 30 to 40 percent. If the operator does perform post weld heat treatments, the proprieties of a heat treatable aluminum alloy can be improved.
What welding wire should be used to weld cast iron?
Cast irons are alloys which typically have over 2% carbon plus 1-3% silicon and are difficult to weld. Electrodes with a high percentage of nickel are commonly used to repair cast iron. Nickel is very ductile, making it a good choice to weld on cast iron, which is very brittle.
What precautions should I take when welding T-1 steels?
T-1 is a quenched and tempered steel. Welding quenched and tempered steels may be difficult due its high strength and hardenability. The base steel around the weld is rapidly being heated and cooled during welding, resulting in a heat affected zone (HAZ) with high hardness. Hydrogen in the weld metal may diffuse into HAZ and cause hydrogen embrittlement, resulting in delayed underbead or toe cracking outside of the weld. To minimize heat affected zone cracking:
Faulty Wire Delivery in MIG Welding?
If the wire is not feeding smoothly or if the operator is experiencing a chattering sound within the gun cable, there may be a problem with the wire delivery system. Most of the problems related to wire delivery are attributed to equipment set-up and maintenance.
Faulty Wire Delivery Problem #1: Contact Tip
There is a tendency among operators to use oversized tips, which can lead to contact problems, inconsistencies in the arc, porosity and poor bead shape.
Make sure that the contact tip in the gun is in working order and sized appropriately to the wire being used. Visually inspect the tip and if it is wearing out (becoming egg-shaped), it will need to be replaced.
Faulty Wire Delivery Problem #2: Gun Liner
A gun liner, like the contact tip, must be sized to the wire being fed through it. It also needs to be cleaned or replaced when wire is not being fed smoothly.
To clean the liner, blow it out with low-pressure compressed air from the contact tip end, or replace the liner.
Faulty Wire Delivery Problem #3: Worn Out Gun
Inside the gun are very fine strands of copper wire that will eventually break and wear out with time.
If the gun becomes extremely hot during use in one particular area, that is an indication that there is internal damage and it will need to be replaced. In addition, be certain that the gun is large enough for the application. Operators like to use small guns since they are easy on the hand, but if the gun is too small for the application, it will overheat.
Faulty Wire Delivery Problem #4: Drive Roll
Drive rolls on the wire feeder periodically wear out and need to be replaced.
There are usually visual indications of wear on the grooves of the rolls if replacement is necessary. Also, make sure that the drive roll tension is set properly. To check tension, disconnect the welding input cable from the feeder or switch to the cold feed option. Feed the wire and pinch it as it exits the gun with the thumb and forefinger. If the wire can be stopped by pinching, more drive roll tension is needed. The optimum tension will be indicated by feeding that is not stopped while pinching the wire. If the drive roll tension is too high, it may deform the wire leading to birdnesting (tangling) and a burn back (when the arc climbs the wire and fuses the wire to the contact tip).
Make sure that the drive rolls and the guide tube are as close together as possible. Next, check the path from where the wire leaves the reel to where it enters the drive rolls. The wire must line up with the incoming guide tubes so there is no scrapping of the wire as it goes through the tube. On some wire feeders, the wire spool position is adjustable -- align it so that it makes a straight path into the tube.
Faulty Wire Delivery Problem #5: Wire Coming Off Reel and Tangling
Some wire feeding problems occur because the inertia from the wire reel causes it to coast after the gun trigger is released.
If the reel continues to coast, the wire on the reel will loosen and the wire may come off or become tangled. Most wire feeding systems have an adjustable brake on the wire reel. The brake tension should be set so that the reel does not coast.
How to avoid Lack of Fusion in MIG welding?
If the consumable has improperly adhered to the base metal, a lack of fusion may occur. Improper fusion creates a weak, low quality weld and may ultimately lead to structural problems in the finished product.
In short arc transfer, the wire directly touches the weld pool and a short circuit in the system causes the end of the wire to melt and detach a droplet. This shorting happens 40 to 200 times per second. Fusion problems may occur when the metal in the weld pool is melted, but there is not enough energy left to fuse it to the base plate. In these cases, the weld will have a good appearance, but none of the metal has actually been joined together. Since lack of fusion is difficult to detect visually, it must be checked by dye-penetrant, ultrasonic or bend testing.
To guarantee correct fusion, ensure that voltage and amperage are set correctly. If the operator is still having problems after making those adjustments, it may require a change in the welding technique. For example, changing to a flux-cored wire or using the spray arc transfer method instead. In spray arc transfer, the arc never goes out so cold lapping and lack of fusion are not issues. Spray arc welding takes place at amperages high enough to melt the end of the wire and propel the droplet across the arc into the weld puddle.
Welding Aluminum in the Wrong Polarity/Adjusting Balance?
If TIG weld polarity set on direct current electrode negative (DCEN), the weld did not break through the aluminum oxide layer. This created a weld where the filler metal mixed in with the partially melted oxide and created the contaminated bead seen here. To defeat this, always TIG weld aluminum with the polarity set to alternating current (AC).
TIG welding in AC allows the electrode positive (EP) portion of the cycle to blast away the aluminum oxide while the electrode negative (EN) portion melts the base metal. A feature called AC balance control allows operators to tailor the EP to EN ratio. If you notice a brownish oxidation and or flakes that look like black pepper in your weld puddle, increase the cleaning action. However, note that too much EP causes the tungsten to ball excessively and provides too much etching. Lastly, when TIG welding aluminum, do not start welding until the puddle has the appearance of a shiny dot. This indicates that the oxide has been removed and it is safe to add filler and move forward. Adding filler to the weld zone before the oxide layer is adequately removed will result in contamination.
What are the Most Common MIG Weld Defects on Aluminum and Steel and what cause them?
Some of the most common weld defects are porosity, lack of fusion and burn through, with aluminum presenting a few more welding challenges than steel. Aluminum conducts heat about six times faster than steel, plus it has excellent thermal conductivity coupled with a low melting point, making it extremely susceptible to warping and burn-through. Additionally, aluminum wire has less tensile strength, which can pose wire feeding issues and lead to weld defects if the correct equipment is not used.
Shielding gas protects the molten weld pool from the surrounding atmosphere, which would otherwise contaminate the weld. It shows how the lack of shielding gas on steel can cause porosity (pinholes) in the weld bead are formed in the face and weld interior in the absence of shielding gas. Lack of shielding gas can be caused by improper setting on the equipment, a hole in the gun liner or wind blowing the shielding gas away.
Lack of Fusion
Lack of fusion can occur when the voltage or wire feed speed is set too low, or when the operator's travel speed is too fast. Because aluminum conducts heat much faster than steel, it is prone to lack of fusion at the start of a weld until enough energy is put into the weld. Some welding equipment addresses this by automatically ramping up the current at the start of a weld and then decreasing it to avoid too much heat build up.
With aluminum, craters can form at the end of a weld. If they are not filled in, they create a stress point, which can lead to cracking. This requires the user to quickly trigger the gun again to fill in the crater, although some welding machines offer a crater timer that will fill in the crater when the gun trigger is released.
Too much heat input can be caused by setting voltage or wire feed speed too high or by too slow of a travel speed. This can lead to warping or burn through especially on the thinner materials found in the sign industry, aluminum being more prone to the effects than steel. Generally aluminum requires a faster travel speed than steel to avoid heat build up.
Do I Have To Remove Rust Or Oil Before Stick Welding?
Stick welding is more forgiving on unclean conditions, but it never hurts to clean parts with a wire brush or grind off excess rust. If you prepare well and have average welding ability, you can make a sound weld. However, even great welding skill cannot overcome poor preparation, as it can lead to cracking, lack of fusion and slag inclusions.
What causes porosity during welding?
In any welding process, porosity can be caused by the presence of contaminants or moisture in the welding zone, which includes the base metal, filler metal, shielding gas, and the surrounding atmosphere. Contaminants can include oil, dirt, grease, or cutting fluids. Concurrently, moisture can collect in the flux, shielding gas, or on the base metal, or come from the atmosphere.
Porosity occurring in a welding process that utilizes an external shielding gas can occur from using too much or too little gas flow, poor gas quality, or a defective welding torch, gun, or hose.
Operator technique can also cause porosity. Electrode, torch, or gun angle can lead to porosity, as can excessive arc length, electrode extension, or travel speeds.
I'm getting a lot of melt- through. What am I doing wrong?
There are a number of remedies for this, one of which might be switching to the gas tungsten arc welding (GTAW) process.
You're getting the melt-through due to excessive heating of the base material. This can be dealt with by increasing the travel speed and making shorter welds. Moving the arc around on the part and spreading the heat will also help. Eliminating and reducing any gaps will also prove effective, but you may have to consider switching to a thicker material or going with an AC GTAW or pulsed GMAW machine.
Why do I care about circuit boards in my engine drive?
Because they can be a giant hassle when not protected. The reality of all work areas is that they're dirty. Between dirt, dust, humidity, and other intrusive elements of construction work, there are many things that can cause circuit board failure. While you'll find some constant current (CC) machines that completely eliminate circuit boards, you can't escape them entirely.
Constant voltage (CV) engine drives require at least one circuit board to control the arc and are necessary for many flux cored and gas metal arc welding (GMAW) jobs. Many new engine-drive models are implementing a single circuit board and encasing it in a "vault" of sorts. Since this practice has been implemented, circuit boards with that added protection have had a 99.71% durability rating, which is important because repairing circuit boards can cost upward of $1,000. Tip one would be to look for a machine with the fewest amount of circuit boards that still offers the welding processes you require. Since you can't entirely get rid of them, it's equally as important to find an engine drive that protects its circuit boards.
Why am I being told my current engine drive may not be sufficient for flux cored welding on structural steel?
In the past, engine-driven welding generators with a constant current (CC) output have dominated the rental and construction markets. Many contractors outfit these engine drives with a voltage-sensing wire feeder to enable flux cored welding. If you're looking to buy a new engine drive, save the future headache and go straight for an engine drive that also features constant voltage (CV).
Many engineering firms, construction companies, and building codes no longer allow flux cored welding with a CC power source. It does not provide adequate assurance the weld is being made with the proper voltage. For this reason, CV power sources are being required, especially for nickel-alloy flux cored wires used for structural welds on buildings and bridges. Some of the self-shielded wires are particularly voltage sensitive. A wide variety of multiprocess machines are available that feature both CC and CV capabilities.
I keep making holes when I TIG weld thin material. What can I do?
Try a set-up that give you finer control over amperage adjustments. If your foot pedal and front panel amperage control have a leader/follower relationship, limit output on the machine (e.g. if you need 20 amps, set the machine at 40). Now the entire range of foot pedal motion only controls a fraction of the welder's output. In other words, 1in. of travel might change the heat by 5 amps, not 50.
Why is my wire having a hard time coming out of the gun?
If the wire is having a hard time coming out of the gun, you are having feeding problems. The inlet guides, liners and contact tubes all have a tolerance. Make sure that you have the correct size of guides, liner and tip. With time the liners and tips can become clogged with dirt. A tip can become plugged. Take the tip out of the gun and try to feed the wire through the gun by squeezing the gun trigger with the gun liner as straight as possible. If the wire feeds through without the tip, you know that the problem was the tip. Check for tip size and plugging. Loosen the drive rolls totally and try to pull the wire through the gun. If you can't pull it through the gun, chances are that the liner is plugged. If you can pull it though the gun, put the correct size contact tube in the gun and slowly increase the drive roll pressure while depressing the gun trigger or wire jog button on the machine. Use just enough pressure to feed the wire as too much pressure will distort the wire and cause feeding problems.
Why can't I keep the weld going?
* If you started out "stick" welding, you've been used to manually feeding the rod inward as it is consumed. When it's been awhile since I've "fed" wire, I need to REMEMBER to let the welder do the feeding, & not get closer & closer to the puddle.
* I start out with the heat higher than it needs be, to just get USED to the feel again. Then I PRACTICE before doing a job. Then I make settings at where I recall from previous jobs.
Why is the wire pushing against the metal(Flux Cored Welding)?
* Turn the heat up or the wire speed down, or both.
* Adjust the length of how much the wire is sticking out beyond the gun's tip by ever so slightly advancing or backing off. I use slightly more than an inch "stick-out."
Do arc welding electrodes cause problems if they are damp?
1) They certainly do. If they're wet enough, you may not be able to use them at all.
2) Dampness can cause the coating to fall off in chunks or unevenly & cause "finger-nailing", where part of the coating sticks out beyond the rod end, during welding.
3) Missing chunks of coating will make the rod stick a lot, plus your results may not be satisfactory.
4) Keep your rods in the house if possible or in a heated storage outside.
5) You can also get away with keeping your rods in a SEALED container to keep out moisture.
My arc welder will turn on but it won't weld. Why?
* My first thought would be connections. There must be a complete electrical path for the "current" to flow completely from one terminal of your welder, to the other.
* Paint, or corrosion can keep an arc from starting.
* I would then try a quick touch of the rod (or wire), directly to the ground connector to see if there is a spark. (if not, then there is likely an INTERNAL problem with your welder).
* If you're "electrically inclined", then open it up & look for blackened wiring, broken wires or connections, or even a mechanical breakage of some sort.
After welding, metal won't stay together, why?
*The rod: same material that you're welding? Is it dry & not flaking off?
* Welder setting: Hot enough amperage?
* Material that you're welding: Compatible with the rod you're using? (steel on steel?).
* Your methods: Welding speed OK? Steady & staying in one place long enough for good adhesion? Welding too close?
Why does the welding rod stick to the metal sometimes?
You may have the amperage a bit too low. You may have let the rod get too close to the parent metal (arc length too short). It's as simple as "if the 2 metals (the rod & the workpiece) are in contact & it's hot enough to have molten metal between them, they WILL STICK!
How can spatter be reduced in Flux Cored Arc Welding?
FCAW is often perceived as a low cost process, even for Hobby and Home work, in that shielding gas is not required (flux cored wire is self shielded), thus reducing equipment cost and simplifying procurement of consumables.
For industrial applications shielding gas (for steel mostly Argon with 8-25 %CO2) is almost always employed, with remarkable influence of the gas mix on the arc and on the resulting welds.
It is also claimed to be easier to master than Gas Metal Arc Welding (GMAW), in that only basic skills are required to obtain acceptable welds in all positions. Penetration and deposition rate are higher than for Shielded Metal Arc Welding.
The often cited additional advantage is that flux cored filler material, by virtue of special ingredients in the flux can be more tolerant to the presence of rust or mill scale on the steel.
The production of thicker smoke and fumes is considered an advantage when welding outdoors because an occasional light breeze would not remove the shielding effect around the weld. It can be a nuisance and a health risk if welding indoors, unless fume extraction is in place to protect the welder.
Slag has to be removed in any case after welding and before any additional weld is done on top of the deposited weld beads.
When using traditional constant voltage power supplies the polarity selected is mostly DCEP (Direct Current Electrode Positive) that gives a stable arc, low spatter (at the correct voltage), a good weld bead profile and optimum penetration
It is important to know which metal transfer mode is used. At lower currents the short circuit transfer mode is operating, usually when welding steel less than 3 mm (1/8") thick.
Spatter is best controlled by using voltage adjustment to obtain a crisp, consistent crackle sound. One should learn from practice to recognize the correct sound associated with short circuit welding.
As an indication, the starting voltage for short circuit applications with flux cored wire of size 0.8 - 1.0 - 1.2 mm(0.030 - 0.035 - 0.045") is 16 to 18 V.
The corresponding wire feed speed could be 1.8 to 10.7 m/min (70 to 420 inch per minute), that would provide 50 to 170 Amps, 65 to 200 Amps, and 130 to 220 Amps for the three wire sizes.
If the crackle of the weld consists in a soft plop sound with some spatter, reduce voltage one volt at a time until the correct sound is generated and spatter is eliminated.
If on the contrary the sound is harsh and explosive with no soft sounds, then increase one volt at a time until spatter is substantially reduced.
With higher current levels the metal transfer becomes the spray mode. Here the arc length should be kept minimal and again one should strive to obtain the consistent crackling sound already described.
Voltage for spray mode would preferably be between 24 and 34 V, a good starting point would be 30V.
For 1.0 mm (0.035") wire size the wire feed speed could be between 10.7 and 14.2 m/min (420 and 560 ipm) that would provide 215 to 300 Amps for a normal stickout (electrode extension) between 13 to 16 mm (1/2 to 3/4").
For 1.2 mm (0.045") wire size, the wire feed speed could be between 8.9 and 16 m/min (350 and 630 ipm) that would provide between 250 to 360 Amps. Voltage adjustment in spray mode goes in opposite direction relative to short circuit mode.
Decreasing voltage (one volt at a time) shortens the arc, but too low a value will bring the electrode to plunge in the weld pool with consequent spatter. Then the voltage should be increased again until the optimum is reached and spatter is substantially reduced.
I keep getting porosity when welding aluminium. Any advice?
Porosity in aluminium welds is caused by gas that becomes trapped in the weld pool when the metal freezes before all of the gas in the weld pool has a chance to escape.
The main cause of porosity is entrapment of gases such as air and shielding gases. Gases can be entrapped when turbulence occurs in the weld pool. When welding aluminium by the MAG process, turbulence can occur if too low a welding current is used because large droplets are transferred across the arc. However, excessive currents deposit metal over a gas bubble before it escapes, giving irregular shaped porosity. Hence, the welding current should be sufficiently high to stabilise the droplet transfer, whilst avoiding excessive currents. Erratic wire feeding can also cause turbulence. Erratic wire feeding may be caused by drive-roll slip, excessive bending of the guide liner, using the wrong size liner, kinks in the wire or poorly wound spools.
When using the TIG process porosity is most likely to be caused by contamination or loss of gas shielding.
The main cause for porosity in aluminium is hydrogen, which has very high solubility in molten aluminium but very low solubility in solid as illustrated in Figure 1. This shows a decrease of solubility in the order of 20 times as solidification takes place. Hydrogen gas is therefore evolved as the weld pool solidifies. If the cooling rate is too high, the gas remains in the metal in the form of porosity. Thus, any compound containing hydrogen and contaminating the filler wire or work surface can cause porosity.
Fig.1. Hydrogen Solubility in Aluminium
Oil, moisture or other contaminants may be present on the filler wire. In addition, the oxide layer of aluminium tends to get hydrated and improper cleaning of the oxide layer immediately preceding welding could be a cause for porosity. Ensuring that the plate is clean before welding and switching to clean, high quality electrodes will reduce the likelihood of forming porosity.
The amount of porosity depends on how fast the weld pool solidifies. Increasing the welding current and/or decreasing the travel speed will increase the heat input, and help retard the cooling rate allowing gases to escape from the weld pool and thereby reducing the risk of porosity.
Filler wires should ideally be kept in their packaging until needed; wire that is left out ion open workshop conditions will absorb moisture into its oxide layer. It is advisable when TIG welding aluminium to wipe each wire prior to use with a clean rag dipped in acetone.
Which welding techniques can be used to minimise distortion?
· keep the weld (fillet) to the minimum specified size
· use balanced welding about the neutral axis
· keep the time between runs to a minimum
In the absence of restraint, angular distortion in both fillet and butt joints will be a function of the joint geometry, weld size and the number of runs for a given cross section. Angular distortion (measured in degrees) as a function of the number of runs for a 10mm leg length fillet weld is shown in Fig.1.
Fig.1. Angular distortion, α, as a function of number of runs, N.
If possible, balanced welding around the neutral axis should be done, for example on double sided fillet joints, by two people welding simultaneously. In butt joints, the run order may be crucial in that balanced welding can be used to correct angular distortion as it develops.
What are the main types of distortion?
· Longitudinal shrinkage
· Transverse shrinkage
· Angular distortion
· Bowing and dishing
Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage.
Non-uniform contraction (through thickness) produces angular distortion as well as longitudinal and transverse shrinking.
For example, in a single V butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. The second run causes the plates to rotate using the first weld deposit as a fulcrum. Therefore balanced welding in a double side V butt joint can be used to produce uniform contraction and prevent angular distortion.
Similarly, in a single sided fillet weld, non-uniform contraction will produce angular distortion of the upstanding leg. Double sided fillet welds can therefore be used to control distortion in the upstanding fillet but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.
Longitudinal bowing in welded plates happens when the weld centre is not coincident with the neutral axis of the section so that longitudinal shrinkage in the welds bends the section into a curved shape. Clad plate tends to bow in two directions due to longitudinal and transverse shrinkage of the cladding. This produces a dished shape.
Dishing is also produced in stiffened plating. Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds.
In plating, long range compressive stresses can cause elastic buckling in thin plates, resulting in dishing, bowing or rippling.
Distortion due to elastic buckling is unstable; if you attempt to flatten a buckled plate, it will probably snap through and dish out in the opposite direction.
Twisting in a box section is caused by shear deformation at the corner joints. This is caused by unequal longitudinal thermal expansion of the abutting edges. Increasing the number of tack welds to prevent shear deformation often reduces the amount of twisting.
Increasing the leg length of fillet welds, in particular, increases the shrinkage.
Why do electronics devices fail?
Electronics devices, components and boards are required to fulfil a desired performance for a defined period of time. This enables manufacturers to give a predicted lifetime on their products, and suppliers to provide warranties without fear of too many early failures.
Within the electronics field there are three recognised regimes of failure: early failure (infant mortality), random failure and wearout.
Early failures occur as a result of flaws introduced within the manufacturing process, due to intermittent malfunction of equipment, problems with material supply, etc. These failures need early detection though burn-in or environmental stress screening so that the components do not make their way to customers.
Random failures are proportional to the component population and can only truly be found by retrieval from the field.
Wearout is the natural end-of-life of a component, board or system related to physical phenomena as a result of materials interaction with the environment. This regime of failure is of particular concern in denoting the lifetime of the product. It is possible to describe wearout mechanisms mathematically allowing the concept of reliability and, hence, lifetime prediction.
How can weld porosity be detected and how can it be remedied?
For sub surface imperfections, detection is by radiography or ultrasonic inspection. Radiography is normally more effective in detecting and characterising porosity imperfections. However, detection of small pores is difficult especially in thick sections, and the effectiveness of the inspection technique will be affected by the material of construction.
Remedial action normally needs removal by localised gouging or grinding but if the porosity is widespread, the entire weld should be removed. Further inspection may be necessary to ensure that all traces of the porosity have been eliminated. The joint should be re-prepared and re-welded as specified in the agreed procedure.
You can find more information on non-destructive testing in other parts of the corporate site.
How can I minimise the risk of solidification cracking in Submerged Arc (SAW) welds?
Because of the large weld pools and high welding speeds often associated with submerged arc welds, solidification or 'hot cracking' may be encountered and is usually found along the centreline of the weld.
Solidification cracking is controlled by the composition of the weld, its solidification pattern and the strain on the solidifying weld metal. The problem is aggravated by the presence of phosphorus, sulphur and carbon and if these elements are known to be present in the parent material in higher amounts than usual, a change should be made to a wire with a higher manganese content and steps taken to minimise dilution and ensure good weld bead profiles. The most dangerous element is carbon which, if other considerations allow, can be kept low in the weld by use of high silica fluxes, i.e. manganese and calcium silicate types. Alternatively, if the carbon level is not too high, a basic flux would be more preferable as this can help to reduce weld metal sulphur levels. Sometimes useful improvement to the weld metal composition can be obtained by selecting a wire that is particularly low in carbon, sulphur and phosphorus, so as to reduce the risk of cracking.
The weld bead shape also has a critical effect. Deep narrow welds, with high depth to width ratios, are prone to centreline cracking, Fig.1. Mushroom-shaped beads as shown in Fig.2 should also be avoided.
Fig.1. Form factor for SA weld beads:
a) W > d giving tendency for surface cracks;
b) W < d giving tendency for centreline cracking;
c) W/d ≈ 3/2 giving sound weld
Fig.2. Mushroom shaped weld penetration resulting from high voltage combined with low speed
A formula has been developed to predict the cracking tendency of SAW weld metal composition. ( Ref. 1 ). The crack susceptibility, in arbitrary units known as units of crack susceptibility (UCS) has been related to the composition of the weld metal (in weight%) as follows:
230C + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1
This formula is valid for weld metal containing the following:
C 0.03 to 0.23 (NOTE: Contents of less than 0.08% to be taken as equal to 0.08%)
S 0.010 to 0.050
P 0.010 to 0.045
Si 0.15 to 0.65
Mn 0.45 to 1.6
Nb 0 to 0.07
Alloying elements and impurities in the weld metal up to the following limits do not exert a marked effect on values of UCS:
In the above formula, values of less than 10 UCS indicate a high resistance to cracking and above 30 a low resistance. Within these approximate limits the risk of cracking is higher in weld runs with a high depth/width ratio, made at high welding speeds or where fit-up is near the maximum allowable.
For fillet weld runs having a depth/width ratio of about 1.0, UCS values of 20 and above indicate a risk of cracking whilst for butt welds the values of about 25 UCS are critical. Decreasing the depth/width ratio from 1.0 to 0.8 in fillet welds may increase the allowable UCS by about 9. However, very low depth/width ratios, such as are obtained when penetration into the root is not achieved, also promote cracking.
Cracking is normally only a problem in root runs, as in Fig 3a, where dilution of parent plate into the weld is high giving excessive carbon contents. Long and deep weld pools as in Fig.3b or welds made at high welding speeds or with high restraint and large gaps as in Fig.3c, accentuate the problem. Conversely, a combination of high arc voltage and slow welding speed can produce a mushroom shaped weld bead with solidification cracks at the weld bead sides.
Fig.3. Solidification cracking: a) in the root beads of a multi-run weld
b) caused by high speed giving a long deep weld pool in first pass
c) caused by high restraint and root gap
Occasionally a groove may be found on the surface running along the centre of the weld. This may be caused by shrinkage and although it is sometimes mistaken for incipient solidification cracking it is actually only superficial.
What are the common causes of porosity in SA (Submerged Arc) welds?
Porosity is a fairly common defect which can be influenced by many factors. Sometimes it is clearly visible as pinholes in the weld surface, at other times it is below the surface and is revealed only by X-ray examination or ultrasonic testing. Unless it is gross or preferentially aligned, porosity is unlikely to be harmful.
Common causes of porosity are:
The surface of a weld may sometimes contain small depressions known as surface pocking or gas flats. These are harmless and while the exact cause is not fully understood it is linked to conditions which cause generation of gas or make it difficult for gas to escape; for example, moisture or lack of deoxidants and too many fines in the flux to allow gas to pass readily.
What causes distortion?
Initially, compressive stresses are created in the surrounding cold parent metal when the weld pool is formed due to the thermal expansion of the hot metal (heat affected zone) adjacent to the weld pool. However, tensile stresses occur on cooling when the contraction of the weld metal and immediate heat affected zone is resisted by the bulk of the cold parent metal.
The magnitude of thermal stresses induced into the material can be seen by the volume change in the weld area on solidification and subsequent cooling to room temperature. For example, when welding C-Mn steel, the molten weld metal volume will be reduced by approximately 3% on solidification and the volume of the solidified weld metal/heat affected zone will be reduced by a further 7% as its temperature falls from the melting point of steel to room temperature.
I'm welding with an Innershield FCAW-SS wire and occasionally get porosity. How can I eliminate this?
First, make sure the steel is clean. Vaporization of contaminants on the base metal such as moisture, rust, oil, and paint may cause porosity.
Second, this can be commonly caused by excessive voltage or too short a stickout (the length of wire from the end of the contact tip to the workpiece). Make sure these are within our recommended parameters.
Also, reducing the travel speed also helps minimize porosity.
I am welding with a flux-cored, gas-shielded wire. I notice that occasionally there will be curved lines or scratches on the surface of the weld. What are these and what causes them?
Gas marks are small grooves that sometimes appear on the surface of a weld made with the Flux-Cored Arc Welding process (FCAW), whether it is gas-shielded, flux-cored (FCAW-G) or self-shielded, flux-cored (FCAW-S) welding. Excessive levels of dissolved gases in the weld metal are the cause of gas marks. While these dissolved gases migrate out of or escape from the molten weld metal before it solidifies, some of the gases do not completely migrate through the molten slag before it solidifies. Thus, these remaining pockets of gas become trapped underneath the solid slag and leave indentations or gas marks on the weld surface. “Worm tracks” or “chicken tracks” are other common non-standard terms for gas marks. Gas marks are considered a cosmetic imperfection, rather than a weld defect.
Gas marks are more likely to occur with electrodes that produce a smaller weld puddle and have a faster freezing slag system, versus a large weld puddle with a slower freezing slag. Therefore, smaller diameter electrodes designed for all position welding (i.e. flat, horizontal, vertical and overhead) and run at lower current levels will be more susceptible to gas marks than larger diameter, flat and horizontal only position electrodes run at higher currents.
One potential cause of gas marks with the FCAW-S process is:
• Excessive arc voltage: As voltage increases, the arc length gets longer. If arc length is too long, excessive levels of nitrogen from the air are introduced into the arc (more than can be controlled or locked up with denitriders in the core elements). This extra nitrogen ends up in the molten weld metal, and then must escape.
Other potential causes of gas marks, with either gas-shielded or self-shielded, flux-cored welding are:
• Excessive moisture: If excessive moisture levels are in the arc, then that moisture can also end up in the molten metal as dissolved hydrogen, with some of it not escaping back through the slag before it solidifies, causing gas marks. The potential sources of moisture in the arc are:
a. From the air (high humidity).
b. From the plate surface (condensation and/or excessive hydrocarbon contaminants, such as rust, oil, primer, etc.).
c. From the wire (if wire packaging is damaged and/or wire is exposed to air for long periods of time, allowing condensation on the inner core elements to occur).
d. For gas-shielded electrodes, from the shielding gas (condensation in the cylinder. The dew point of shielding gases should be below -40°F (-40°C)).
• Contact tip to work distance (CTWD) too short: If the CTWD or electrical stickout (ESO) is too short for the particular electrode, moisture in the arc may not have enough time to completely burn off, resulting in excessive levels of dissolved hydrogen in the weld puddle. In addition, as CTWD gets shorter, arc length get longer, causing the same problem with FCAW-S electrodes as explained earlier.
• Welding procedures and/or techniques that result in an increased freezing rate of the slag. A more fluid weld puddle may stay fluid just long enough to allow all dissolved gases to escape before the slag freezes.
Note that if any of these issues became too excessive, the problem would go beyond gas marks (a cosmetic blemish only) and would cause porosity in the weld (a weld discontinuity and possible weld defect).
For gas-shielded electrodes, there is also a higher susceptibility to gas marks with argon (Ar) / carbon dioxide (CO2) blends (i.e. 75%Ar / 25% CO2) than with a 100% CO2 shielding gas. There is more of a spray arc type metal transfer with argon in the shielding gas, which results in a smaller metal droplet size, but also a greater number of droplets. The net effect is an increase in the total surface area of all molten droplets, resulting in a higher level of dissolved gases in the weld metal. This potential increases even more with higher percentage of argon in the gas mix than 75%. Note however, that most FCAW-G electrodes made today have very low diffusible hydrogen levels (i.e. H8 ratings or a maximum of 8 milliliters diffusible hydrogen per 100 grams weld metal). Some electrodes even have H4 ratings. Therefore, the likelihood of getting gas marks with either type of shielding gas has decreased in recent years.