WELDING ALUMINUM' IMPROVE YOUR PRODUCTIVITY WITH PULSED MIG

It goes without saying that the last two years have been a struggle for most fabrication companies. But if there is one lesson to be learned from previous economic downturns, it is this: when the economy does recover, those companies that prepared well and invested in ways to increase their productivity and product quality while reducing input costs are the ones that will thrive in the years ahead.

One way that aluminum fabricators can look toward future growth is through the adoption of pulsed MIG technology. Aluminum has a reputation for being difficult to weld. Depending on temperature, aluminum conducts heat about four times faster than carbon steel and has a relatively low melting point (1,220 deg F/660 deg C), which means that heat input needs to be precisely controlled in order to avoid burn through and warping.

For this reason and others, AC TIG (gas tungsten arc welding ? GTAW) has been the process of choice for most aluminum welding, especially for material thicknesses below 3/16 in. AC TIG excels at producing nice looking welds and controlling heat input, and advanced AC TIG machines can increase productivity over conventional TIG machines. However, TIG is still a slower welding process that requires a high level of operator skill.

By contrast, short circuit MIG (gas metal arc welding ? GMAW) welding offers faster travel speeds and requires less operator skill, but it is prone to poor fusion, excessive spatter and porosity. Short circuit MIG welding with aluminum is, in general, only used to weld thin aluminum sheet metal less than 0.100 in thick.
Spray transfer MIG welding, on the other hand, provides excellent fusion, low spatter, and high travel speed rates, yet because it requires high amperages for the filler metal to achieve a spray deposition, it is often susceptible to burn through and warping on thin materials.

Considering all this, wouldn?t it be nice if there were an ideal process that provided high travel speed rates, good heat control on thin material and visually appealing weld beads? Well, as luck ? or clever engineering ? would have it, pulsed MIG is just such a process.

Technically categorized as a modified spray transfer process, pulsed MIG (GMAW-P) uses an inverter to switch between a high peak current and a low background current ? to the tune of up to 10,000 pulses per second. During the pulsed MIG process, the peak current heats the filler metal enough to create a spray transfer, and then the power source switches to a background current that is too low to generate metal transfer, but high enough to maintain the arc.

By alternating between the high peak and low background currents, pulsed MIG lowers the overall heat input enough to weld aluminum down to 22 ga. As an example, a 1/8 in piece of aluminum with a 3/64 in filler metal calls for an average of 140 amps. Using a 350 amp peak and a 90 amp background current pulsed program, an operator can weld without the risk of burn through or warping associated with short circuit transfer, all while achieving the good penetration and wet-out characteristic of spray transfer. Plus, there will be little to no spatter because the electrode (filler wire) used for spray transfer does not touch the weld pool. And because the weld pool cools slightly during the background current period, the resulting weld often has a TIG-like appearance.

Another benefit of pulsed MIG is that it permits the operator to use larger diameter filler metal and/or wire feed speeds, which in turn increases travel speeds and deposition rates compared to short circuit transfer. In one real-life example, after switching to pulsed MIG, a fabrication company increased its travel speeds by 37 percent ? from 105 ipm to 144 ipm ? on thin gauge (0.060 in to 0.090 in) aluminum lap joints.

Another way for companies to further increase productivity and reduce downtime with the pulsed MIG process is by using a push-pull wire feed system instead of a spool gun. A large boat fabricator that switched from standard spray transfer MIG with a spool gun to pulsed MIG with a push-pull gun (which allowed them to use a 16 lb spool of 3/64 in wire instead of a 1 lb spool of 1/8 in wire) was able to realize $49,695 in annual savings and achieve an equipment payback period of 4.5 months on a purchase of three power source/gun units.

Although there are some applications in which TIG will continue to be the preferred process (including material 22 ga and thinner), pulsed MIG can provide fabricators and manufacturers the productivity and weld quality boost they need to get ahead as the economy begins its fragile recovery.

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Nick Peterson is a welding engineer for Miller Electric Mfg. Co., 1635 West Spencer Street, P.O. Box 1079, Appleton, WI 54912-1079, 920-734-9821, npeter@millerwelds.com, www.millerwelds.com. He is also a contributing editor to Melting Point, a quarterly digital welding publication brought to you by Fabricating & Metalworking.