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A Guide to Aluminum Welding
Equipment Selection, Material Prep, Welding Technique…
Follow the rules of thumb offered here for selecting welding equipment, preparing base materials, applying proper technique, and visually inspecting weldments to ensure high-quality gas-metal-and gas tungsten-arc welds on aluminum alloys.
Even for those experienced in welding steels, welding aluminum alloys can present quite a challenge. Higher thermal conductivity and low melting point of aluminum alloys can easily lead to burnthrough unless welders follow prescribed procedures. Also, feeding aluminum welding wire during gas-metal-arc-welding (GMAW) presents a challenge because the wire is softer than steel, has a lower column strength, and tends to tangle at the drive roll.
To overcome these challenges, operators need to follow the rules of thumb and equipment-selection guidelines offered here…
To weld aluminum, operators must take care to clean the base material and remove any aluminum oxide and hydrocarbon contamination from oils or cutting solvents. Aluminum oxide on the surface of the material melts at 3,700 F while the base-material aluminum underneath will melt at 1,200 F. Therefore, leaving any oxide on the surface of the base material will inhibit penetration of the filler metal into the workpiece.
To remove aluminum oxides, use a stainless-steel bristle wire brush or solvents and etching solutions. When using a stainless-steel brush, brush only in one direction. Take care to not brush too roughly: rough brushing can further imbed the oxides in the work piece. Also, use the brush only on aluminum work-don’t clean aluminum with a brush that’s been used on stainless or carbon steel. When using chemical etching solutions, make sure to remove them from the work before welding.
To minimize the risk of hydrocarbons from oils or cutting solvents entering the weld, remove them with a degreaser. Check that the degreaser does not contain any hydrocarbons.
Preheating the aluminum workpiece can help avoid weld cracking. Preheating temperature should not exceed 230 F-use a temperature indicator to prevent overheating. In addition, placing tack welds at the beginning and end of the area to be welded will aid in the preheating effort. Welders should also preheat a thick piece of aluminum when welding it to a thin piece; if cold lapping occurs, try using run-on and run-off tabs.
The push technique:
With aluminum, pushing the gun away from the weld puddle rather than pulling it will result in better cleaning action, reduced weld contamination, and improved shielding-gas coverage.
Aluminum welding needs to be performed “hot and fast.” Unlike steel, the high thermal conductivity of aluminum dictates use of hotter amperage and voltage settings and higher weld-travel speeds. If travel speed is too slow, the welder risks excessive burnthrough, particularly on thin-gage aluminum sheet.
Argon, due to its good cleaning action and penetration profile, is the most common shielding gas used when welding aluminum. Welding 5XXX-series aluminum alloys, a shielding-gas mixture combining argon with helium – 75 percent helium maximum – will minimize the formation of magnesium oxide.
In aluminum welding, crater cracking causes most failures. Cracking results from the high rate of thermal expansion of aluminum and the considerable contractions that occur as welds cool. The risk of cracking is greatest with concave craters, since the surface of the crater contracts and tears as it cools. Therefore, welders should build-up craters to form a convex or mound shape. As the weld cools, the convex shape of the crater will compensate for contraction forces.
When selecting a power source for GMAW of aluminum, first consider the method of transfer -spray-arc or pulse.
Constant-current (cc) and constant-voltage (cv) machines can be used for spray-arc welding. Spray-arc takes a tiny stream of molten metal and sprays it across the arc from the electrode wire to the base material. For thick aluminum that requires welding current in excess of 350 A, cc produces optimum results.
Pulse transfer is usually performed with an inverter power supply. Newer power supplies contain built-in pulsing procedures based on and filler-wire type and diameter. During pulsed GMAW, a droplet of filler metal transfers from the electrode to the workpiece during each pulse of current. This process produces positive droplet transfer and results in less spatter and faster follow speeds than does spray-transfer welding. Using the pulsed GMAW process on aluminum also better-controls heat input, easing out-of-position welding and allowing the operator to weld on thin-gage material at low wire-feed speeds and currents.
The preferred method for feeding soft aluminum wire long distances is the push-pull method, which employs an enclosed wire-feed cabinet to protect the wire from the environment. A constant-torque variable-speed motor in the wire-feed cabinet helps push and guide the wire through the gun at a constant force and speed. A high-torque motor in the welding gun pulls the wire through and keeps wire-feed speed and arc length consistent.
In some shops, welders use the same wire feeders to deliver steel and aluminum wire. In this case, the use of plastic or Teflon liners will help ensure smooth, consistent aluminum-wire feeding. For guide tubes, use chisel-type outgoing and plastic incoming tubes to support the wire as close to the drive rolls as possible to prevent the wire from tangling. When welding, keep the gun cable as straight as possible to minimize wire-feed resistance. Check for proper alignment between drive rolls and guide tubes to prevent aluminum shaving.
Use drive rolls designed for aluminum. Set drive-roll tension to deliver an even wire-feed rate. Excessive tension will deform the wire and cause rough and erratic feeding; too-little tension results in uneven feeding. Both conditions can lead to an unstable arc and weld porosity.
Use a separate gun liner for welding aluminum. To prevent wire chaffing, try to restrain both ends of the liner to eliminate gaps between the liner and the gas diffuser on the gun.
Change liners often to minimize the potential for the abrasive aluminum oxide to cause wire-feeding problems.
Use a contact tip approximately 0.015 inch larger than the diameter of the filler metal being used – as the tip heats, it will expand into an oval shape and possibly restrict wire feeding. Generally, when a welding current exceeds 200 A use a water-cooled gun to minimize heat buildup and reduce wire-feeding difficulties.
Crater cracks happen for two reasons:
- High thermal rate of conductivity
- The concave shape of the crater
Aluminum cools so fast that it doesn’t provide adequate time for the weld bead to flatten or the crater to fill.
The deep depression of the crater quickly freezes in a concave shape, exerting high tensile stresses on the surrounding metal. It is in this area that a crack will propagate through the weld metal.
As the weld cools, crater cracking is common if proper steps are not taken to minimize the problem.
The easiest most common way to prevent cracking in aluminum
Instead of releasing the trigger at the end of the weld, continue to feed wire and reverse the direction of travel back into the already-welded material.
Weld far enough back to re-weld the entire crater (One inch should be sufficient), and this will increase the deposit in the crater area, changing its shape from concave to convex.
The convex-shaped crater rapidly cools and reduces the stress on the weld metal in the crater, which causes “crater cracking.”
4043 is designed for welding 6xxx series aluminum alloys. It may also be used to weld 3xxx series alloys or 2xxx alloys. 4043 has a lower melting point and more fluidity than the 5xxx series filler alloys, and is preferred by most welders because it “wets and flows better” and it’s less sensitive to weld cracking with the 6xxx series base alloys. 4043 can also be used for welding castings. 4043 also makes brighter looking MIG welds with less smut because it doesn’t contain magnesium. 4043 gives more weld penetration than 5356, but produces welds with less ductility than those made using 5356. However, 4043 is not well suited for welding Al-Mg alloys and should not be used with high Mg content alloys such as 5083, 5086 or 5456 because excessive magnesium-silicide (Mg2Si) can develop in the weld structure to decrease ductility and increase crack sensitivity. (One exception to this rule is 5052, which has a low magnesium content.)
5356 wire has become the most commonly used of all aluminum filler alloys because of its good strength and its good feed-ability when used as a MIG electrode wire. It is designed to weld 5xxx series structural alloys and 6xxx series extrusions, basically anything other than castings, because castings are high in silicon. Its one limitation is that 5356 is not suitable for service temperatures exceeding 150 degrees Fahrenheit (65 degrees Celsius). The formation of Al2Mg at elevated temperatures at the grain boundaries makes the alloys prone to stress corrosion. For components that will be anodized after welding, 5356 is recommended over 4043, which turns jet black when anodized.
308L (including ER308LSi) is predominately used on austenitic stainless steels, such as types 301, 302, 304, 305 and cast alloys CF-8 and CF-3. For high temperature applications such as in the electrical power industry, the high carbon 308H electrode provides better creep resistance than does 308L.
Use 309L (including ER309LSi) when joining mild steel or low alloy steel to stainless steels, for joining dissimilar stainless steels such as 409 to itself or to 304L stainless, as well as for joining 309 base metal. CG-12 is the cast equivalent of 309. Some 308L applications may be substituted with 309L filler metal, but 316L or 316 applications generally require molybdenum and 309L contains no molybdenum.
Type 347 stainless steel filler metal is ideal for 347 and 321 base materials because it matches these stabilized grades. CF-8C is the cast equivalent of 347. Type 347 filler metal is also suitable most 308L filler metal applications.
Choosing electrodes for welding cast iron typically comes down to three things: cost, machine-ability, and whether the weld is single or multiple pass.
Softweld 99 Ni (AWS class ENi-CI) is a nominally 99% Nickel electrode. Nickel is expensive, and so, therefore, is this premium electrode. The electrode will deposit welds that are machine-able, an important consideration when the casting is to be machined after welding. Repairs made with Softweld 99 Ni are often single pass welds with high admixture. Even with high admixture, the weld deposit will remain machine-able. It works best on castings with low or medium phosphorous contents.
Softweld 55 Ni (AWS class ENiFe-CI)is a nominally 55% Nickel electrode. The lower Nickel content makes this electrode more economical than Softweld 99 Ni. Weld deposits are usually machine-able, but under conditions of high admixture, the welds can become hard and difficult to machine. It is often used for repairing castings with heavy or thick sections. As compared to Softweld 99 Ni, welds made with 55 Ni are stronger and more ductile, and more tolerant of phosphorous in the casting. It also has a lower coefficient of expansion than 99 Ni, resulting in fewer fusion line cracks.
Ferroweld (AWS class ESt) is a lower cost, steel electrode. The weld deposits are hard, and are not machine-able, but can be finished by grinding. This is the lowest cost electrode for welding cast iron, and the electrode has a very user-friendly arc. It can tolerate welding on castings that cannot be completely cleaned before welding. Ferroweld deposits will rust, just like cast iron. This may be important when repairing cast iron parts such as exhaust manifolds on antique cars.
“Nothing,” according to Lincoln Technical Rep. Dan Mahony.
When welding two pieces of steel together, a crack in the weld is a bad thing. However, when you’re applying a hardfacing material to reduce frictional wear on a piece of steel, a crack in the face of the weld is a good thing. Selecting a welding electrode for joining is typically done by identifying the mechanical properties (tensile, yield, ductility and impact strength) of the steel you are welding, and selecting a filler material that duplicates those properties.
Hardfacing electrodes, in contrast, are very highly alloyed electrodes. This is what makes them so hard. They are designed to resist impact or abrasion, or both. Hardfacing electrodes are not designed for joining steels together. Hardfacing deposits can be so hard they surrender any, or all, ductility.
Immediately after welding the deposited hardfacing material begins to contract, or change shape, as the heat dissipates. Since most hardfacing deposits have nearly no ductility, the welds will crack. This alarms many welders. Most often these cracks are transverse (cross) cracks, which are very beneficial to relieve the residual stress in the weld bead. This type of cracking will not affect the wear resistance.
You’re doing nothing wrong.
It does appear (if you do the math) that you should be able to use 1/8 in. and smaller diameter electrodes with an AC/DC 225/125 powered by a 5000 or 6000 watt generator. Unfortunately, the design of the transformer on the AC/DC 225/125 is not efficient enough to be powered by a small generator. If you try and weld with this combination you will most likely experience the electrode being hard-to-strike and also the electrode frequently sticking to the work.
If you are fortunate to establish an arc, the arc will tend to pop out frequently. Also, there will not be adequate heat input to the work, resulting in low weld quality (poor fusion), and poor bead appearance.
To successfully run your AC/DC 225/125 welder you would need a minimum of a 15,000 watt AC generator.