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When MIG welding aluminum, obtaining good quality and productivity depends on using the right welding process and system. This may seem like common sense, but many fabricators get hung up on purchase price vs. life cycle costs, or they try to weld aluminum with the same processes and systems used for steel.

As Fig. 1 shows, aluminum's properties differ greatly from those of steel, and those properties create welding challenges. Some of the common issues include:

• Excess heat input leading to burn-through or warping on thin materials
• Poor bead appearance
• Wire feeding issues, including bird's nests, burnback and erratic feeding
• Difficulty meeting production goals/customer deadlines
• Difficulty training operators/finding good welders
• Excess spatter, grinding or post-weld rework
• Poor fusion or over-welding
• Bleed-through on the back side of the weld
• Can only weld in the flat or horizontal position (e.g., unproductive time while repositioning weldments)

Fortunately, the newest generation of pulsed MIG welding systems addresses these issues. Even better, they generally provide a quick return on investment through increased productivity, reduced downtime and better weld quality.

The Case for Pulsed MIG

Because aluminum quickly transfers heat away from the weld area, establishing the weld puddle takes more energy than with steel. However, because aluminum has a relatively low melting point, thin sections of aluminum (which have less mass to transfer heat) are especially prone to burn-through and warping. In short, there's a fine line between providing sufficient energy to ensure good fusion, yet controlling the heat input to prevent problems.

In many aluminum applications, such as trailer, truck body and components (e.g., cargo boxes), boat and sign fabrication, material thicknesses tend to be 3/8-in. and thinner. For thin components, the pulsed MIG welding process solves heat control issues.

For heavy sections of aluminum, pulsed MIG provides the ability to weld out-of-position. This eliminates the need to reposition the weldment, saving time (the spray transfer process is only suitable for welding in the flat and horizontal positions because the weld puddle is so fluid that it would otherwise roll out of the joint). Pulsed MIG also provides better control over the weld puddle, enabling new operators to easily learn the process and experienced operators to control bead appearance.

Pulsed MIG works well with both thin and thick sections because it is a modified spray transfer process. The welder switches between a high peak current and a low background current 120 to 200 times per second (see Fig. 2). The peak current pinches off a spray transfer droplet and propels it toward the weldment. The background current maintains the arc, but the heat input is too low for metal transfer to occur.

The peak current pulse ensures good fusion, overcoming concerns related to cold lap, a common issue with short circuit MIG on aluminum. It also provides faster travel speeds, which improves productivity. The background current lowers overall heat input (see Fig. 3), addressing the burn-through and warping issues commonly associated with spray transfer MIG.

A cooler weld puddle also permits all-position welding (the puddle is less likely to sag or look excessively convex), and it helps bridge gaps when fit-up is less than optimal. Pulsed MIG also provides the operator with excellent directional control over the molten weld puddle, which improves bead appearance (see Fig. 4).

As an economic benefit, pulsed MIG welding allows using larger filler wire diameters, such as.047 in. instead of .030 in., to weld thin gauge material. An .047 wire improves feeding performance because larger wires are stiffer and feed better, increases the deposition rate and saves money. A 20-lb. spool of .030-in aluminum filler wire costs $92, where a spool of .047-in. wire costs $74.00, a difference of $0.90 per pound. Add in the decreased time and materials for rejected parts, and pulsed MIG may be the best welding process for your particular application.