MIG Welding Aluminum is a fast, adaptable process that is used with direct current reverse polarity and an inert gas to weld heavier thicknesses of aluminum alloys, in any position, from 1/16 in. (1.6 mm) to several inches thick.
Shielding gas for MIG Welding Aluminum. Precautions should be taken to ensure the gas shield is extremely efficient. Welding grade argon, helium, or a mixture of these gases is used for MIG welding aluminum. Argon produces a smoother and more stable arc than helium. At a specific current and arc length, helium provides deeper penetration and a hotter arc than argon. Arc voltage is higher with helium, and a given change in arc length results in a greater change in arc voltage. The bead profile and penetration pattern of aluminum welds made with argon and helium differ. With argon, the bead profile is narrower and more convex than helium. The penetration pattern shows a deep central section. Helium results in a flatter, wider bead, and has a broader under-bead penetration pattern. A mixture of approximately 75 percent helium and 25 percent argon provides the advantages of both shielding gases with none of the undesirable characteristics of either. Penetration pattern and bead contour show the characteristics of both gases. Arc stability is comparable to argon. The angle of the gun or torch is more critical when MIG welding aluminum with inert shielding gas. A 30° leading travel angle is recommended. The electrode wire tip should be oversize for aluminum. Table 7-21 provides welding procedure schedules for the MIG welding of aluminum.
MIG Welding Aluminum Technique
The electrode wire must be clean. The arc is struck with the electrode wire protruding about 1/2 in. (12.7 mm) from the cup. A frequently used technique is to strike the arc approximately 1.0 in. (25.4 mm) ahead of the beginning of the weld and then quickly bring the arc to the weld starting point, reverse the direction of travel, and proceed with normal welding. Alternatively, the arc may be struck outside the weld groove on a starting tab.
When finishing or terminating the weld, a similar practice may be followed by reversing the direction of welding, and simultaneously increasing the speed of welding to taper the width of the molten pool prior to breaking the arc. This helps to avert craters and crater cracking. Runoff tabs are commonly used. Having established the arc, the welder moves the electrode along the joint while maintaining a 70 to 85 degree forehand angle relative to the work. A string bead technique is normally preferred. Care should be taken that the forehand angle is not changed or increased as the end of the weld is approached.
Arc travel speed controls the bead size.
When MIG welding aluminum, it is important that high travel speeds be maintained. When welding uniform thicknesses, the electrode to work angle should be equal on both sides of the weld.
When welding in the horizontal position, best results are obtained by pointing the gun slightly upward.
When welding thick plates to thin plates, it is helpful to direct the arc toward the heavier section. A slight backhand angle is sometimes helpful when welding thin sections to thick sections.
The root pass of a joint usually requires a short arc to provide the desired penetration. Slightly longer arcs and higher arc voltages may be used on subsequent passes.
The wire feeding equipment for aluminum welding must be in good adjustment for efficient wire feeding. Use nylon type liners in cable assemblies. Proper drive rolls must be selected for the aluminum wire and for the size of the electrode wire. It is more difficult to push extremely small diameter aluminum wires through long gun cable assemblies than steel wires. For this reason, the spool gun or the newly developed guns which contain a linear feed motor are used for the small diameter electrode wires.
Water-cooled guns are required except for low-current welding. Both the constant current (CC) power source with matching voltage sensing wire feeder and the constant voltage (CV) power source with constant speed wire feeder are used for welding aluminum. In addition, the constant speed wire feeder is sometimes used with the constant current power source.
In general, the CV system is preferred when welding on thin material and using all diameter electrode wire. It provides better arc starting and regulation. The CC system is preferred when welding thick material using larger electrode wires. The weld quality seems better with this system. The constant current power source with a moderate drop of 15 to 20 volts per 100 amperes and a constant speed wire feeder provide the most stable power input to the weld and the highest weld quality.
MIG Welding Aluminum, Joint Design
Edges may be prepared for welding by sawing, machining, rotary planing, routing or arc cutting. Acceptable joint designs are shown in figure 7-12.