Aluminum is used for many things in today’s automobiles, from cylinder heads, engine blocks, intake manifolds and transmission housings to radiators, body panels, subframes, suspension arms and bumper reinforcements. Aluminum is used for many of these applications because it is lighter than steel and does not rust. It also conducts heat well which makes it a good choice for engine cooling.
But aluminum has some drawbacks. It is much more expensive than steel, and not as strong. Aluminum castings and wheels can sometimes have porosity problems which require a resin or epoxy seal coating to prevent coolant, oil or air leaks. And aluminum is vulnerable to corrosion pitting, especially from salt, which means surfaces exposed to environmental attack must be protected by anodizing or clearcoat paint.
Problems Associated with Welding Aluminum
Aluminum is not as easy to weld as steel. When bare aluminum is exposed to air, a thin layer of oxide forms on the surface just as it does on other metals. The oxide layer that forms on aluminum creates a hard barrier that protects the metal against further corrosion. But the oxide barrier also interferes with welding by contaminating the weld and preventing proper metal fusion from taking place. Unless the oxide layer is removed from the surface of the metal and prevented from reforming, it is very difficult to weld aluminum using an oxyacetylene torch or an arc welder. The weld must be shielded from the atmosphere so oxide cannot form on the surface. This requires using a shielding gas such as Argon to keep oxygen away from the weld while the metal is being welded.
Aluminum melts at around 1200 degrees F. compared to 2500 degrees F. for steel. Although it takes less heat to melt aluminum, the heat must be more concentrated in the weld area because the metal conducts heat away so quickly. Aluminum can be arc welded using special aluminum rods such as those made by Zena, but the best results are usually obtained using a Metal Inert Gas (MIG) welder or a Tungsten Inert Gas (TIG) welder.
There are various types of MIG and TIG welders from which to choose.
MIG Welding Aluminum
Metal Inert Gas (MIG) welding is a popular technique for welding thin gauge and high strength steels. The same basic MIG welding techniques can also be used to successfully repair aluminum components up to about 1/4 inch in thickness. But a few things must be done differently.
For one, pure argon gas is recommended as the shielding gas. Argon is about ten times denser than helium so it tends to hug the weld area better, providing better protection than helium. Argon also takes less voltage than helium to maintain the arc so the arc tends to be more stable and concentrates heat better in the weld area. It is also difficult to establish a good arc with helium below 150 amps.
The only time helium or a helium/argon mixture would be used is when making deep penetration welds or where high travel speeds are desired. This also requires a gas flow rate two to three times higher than normal for equivalent shielding.
You also have to change the wire spool when MIG welding aluminum. Aluminum wire is required. But not just any aluminum wire will work. You should use a wire alloy that is compatible with the base metal. Choosing the right wire for a given weld is not that simple because of the many different types of aluminum alloys that are used. Unfortunately, there is no universal wire that works in every case. But for most hard aluminums (such as high strength forgings), 5356 wire works well. For softer alloys (most castings), 4043 wire is the best choice.
Aluminum alloys are identified by a four digit number. The number can sometimes be found in a service manual. If the alloy is not known, try some trial welds to find the wire that works best.
Using aluminum wire may also require some equipment modifications such as changing the gun liner and/or gas nozzle. The nozzle for welding aluminum is straight rather than tapered to give the proper gas shielding.
Aluminum wire is more sensitive to drive tension adjustment than steel wire. If the tension is not adjusted properly, the aluminum wire can bird nest on the spool and jam. Some MIG welding equipment manufacturers recommend replacing the wire feed drive roller with a soft rubber or urethane roller when using aluminum wire.
For metal up to 1/8 inch think, use wire diameter .030 in. (0.8 mm). For 1/8 inch thickness and up, use .035 in. (0.9 mm) or 3/64 in.
For best results, don't leave you spool of aluminum wire sitting around for long periods of time. The wire tends to oxidize much faster than steel wire and the oxidation is much heavier. The early stages of oxidation are virtually invisible, but as time passes a white powder forms on the wire that can cause extreme arc flutter, wire drive problems, contamination build-up in the MIG liner, wire burn-back into the contact tip, and a poor weld. For best results, use up your aluminum wire as quickly as possible, preferably within three months. When not in use, remove the wire spool and seal it in an airtight plastic bag to keep it from oxidizing.
The power and current settings on the MIG equipment will depend on the application, but generally speaking use Direct Current with Reverse Polarity (DCRP) when welding aluminum.
When welding thin aluminum, it usually works best to hold the gun about 35 degrees to the surface, and move it forward. When welding thicker metal or a casting, pulling the gun backwards toward you produces deeper penetration.
Prepping Aluminum for Welding
Surface preparation is especially important when welding aluminum. Dirt and surface oxidation must be removed prior to welding with a stainless steel wire brush. The brush should not be used for anything else. Use it only to clean aluminum. The reverse polarity of the arc produces a cleaning action at the surface of the metal, but by itself it may not be enough to scour loose all the oxide.
Aluminum is often anodized to protect it against corrosion. An anodized surface will not conduct electricity. So before you can weld anodized aluminum you first have to sand or grind off the anodized coating. The layer is usually less than a few thousandths of an inch thick, so it does not take much sanding to get down to bare metal. You also have to grind off the anodized coating where the ground clamp will be connected.
Though most anodized finishes are black or colored, some are clear. To determine whether or not the metal has an anodized coating, use a 12-volt continuity tester or an ohmmeter to check the metal's conductivity. Touch both probes of the tester or ohmmeter to the surface of the metal about an inch apart. If you get continuity, there is no anodizing on the metal. No continuity means there is an anodized coating that will have to be removed prior to welding.
TIG Welding Aluminum
The other technique for welding aluminum is TIG welding. It is essentially arc welding with a shielding gas and a nonconsumable tungsten electrode. The TIG process creates extremely high temperatures in a concentrated region while the shielding gas protects the weld from contamination. No flux is used, so there is no slag to cause problems. Nor does the process itself produce smoke or toxic fumes, making it a clean welding process. TIG welding is typically used to repair heavy castings such as aluminum cylinder heads, engine blocks and other large aluminum castings.
There is no transfer of metal across the arc in TIG welding so there are no globules of spatter to contend with. There are also no sparks if the metal is free from contaminants. This can be an advantage in situations where spatter might create problems around the weld area or on adjoining parts.
If filler metal is needed, it can be added manually using an aluminum alloy filler rod. The technique is the same as when using a filler rod and oxyacetylene torch. The alloy of the filler rod must be compatible with the base metal as previously described in the section on MIG welding. ER4043 filler rod is one of the most commonly used rods for TIG welding aluminum-silicon alloy castings. For high magnesium alloy castings (which can be identified by chemical tests), a ER5356 filler rod is recommended.
TIG Welding Equipment Setup for Aluminum
TIG welding equipment consists of an arc welder power unit with a tungsten electrode gun and shielding gas supply. High amperage guns are often water-cooled but low amperage guns can be air-cooled.
TIG welding can be done using direct current of either straight or reverse polarity, or alternating current. When alternating current is applied to the surface of the metal, it heats the metal during one half the voltage cycle (electrode negative) and cooks off the oxide during the reverse portion of the cycle (electrode positive). This back-and-forth heating/cooking action keeps the weld free from contamination and makes for a strong weld. Using direct current with straight polarity (DCSP - electrode negative) can produce more heat at the work surface but it does not do as good a job of cleaning the metal. Using direct current with reverse polarity (DCRP - electrode positive) does a fine job of cleaning the surface but it does not produce as much heat. High frequency Alternating Current (AC) works best when TIG welding aluminum.
There are a variety of different electrodes that can be used with a TIG welder. Most experts say tungsten thorium (color coded green) electrodes work best with aluminum. Zirconium tungsten electrodes perform even better but cost five times as much and are hard to find.
Do NOT touch the metal with the tungsten electrode when starting an arc or welding aluminum since doing so can contaminate the electrode. The electrode should be held about an eighth of an inch above the working surface. The arc will start itself as soon as the electrode is brought close enough to the surface.
Surface Preparation for TIG Welding Aluminum
When welding a cracked aluminum casting, it is very important to determine the full extent of the damage so the crack can be completely ground out. Extend the grinding a short distance beyond the visible ends of the crack to make sure you've eliminated all damage. The area can then be cleaned up by bead blasting or brushing prior to welding.
Aluminum is nonmagnetic so magnetic crack detection equipment is no help in finding cracks. You have to use penetrating dye instead. To find cracks with dye, all dirt and oil must first be cleaned from the surface. The dye is then sprayed on and allowed to dry. Wipe the excess dye dust off then spray on the developer. Any cracks will then appear as dark lines on the metal.
If you are welding a casting (such as a cylinder head, manifold or other part that will experience thermal stress in normal use, preheat the casting to 200 to 300 degrees F. with a propane torch or by placing the part in an oven. Use a temperature crayon or very accurate thermometer to prevent overheating because aluminum can soften if it gets much hotter than 450 degrees F. The heat helps cook out oil and grease that might contaminate the weld. It also reduces the chance of the casting cracking after it has been welded and cools down.
After welding, the part should be allowed to cool very slowly. This can be done by placing it back in the oven or wrapping it with an insulating blanket. Once it has cooled back down to room temperature, the part can be ground, machined, polished or painted before it is returned to service.