Pulsed MIG Improves Aluminum Welding Performance

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.

New Pulsed MIG Equipment

Readers with a perception of pulsed MIG equipment that comes from any system older than three years should forget everything they know. Several recently introduced systems (see Fig. 5 [images of 350P and 350 MPa]) completely overcome the two biggest barriers to pulsed MIG welding: system complexity and operator frustration. Two such systems from Miller Electric Mfg. Co. are the Millermatic® 350P and Invision® 350 MPa.

The Millermatic 350 is all-in-one welder with the power source and wire drive mechanism contained within one unit (for welding aluminum, pair this welder with Miller’s new XR?-Aluma-Pro? push-pull gun; more on guns later). This system has an output of 25 to 400 amps (60 percent duty at 300 amps), welds aluminum up to 3/8-in. thick in a single pass and has a list price (with gun) of $6,305. It offers the flexibility of working on single- or three-phase power, and it can be easily wheeled around large weldments.

The Invision 350 MPa has an output of 5 to 425 amps (60 percent duty at 350 amps) and features Miller’s Auto-Line? power management technology (which enables the unit to use single- or three-phase input power, as well as weld flawlessly as long as primary power stays within 190 to 630 VAC). This system works with a remote wire feeding system, such as the XR-Control with XR-Aluma-Pro gun, which many fabricators use to create a large work envelope. Total system list price starts at $8,941.

New Pulsing Technology

New pulsed MIG systems feature built-in pulsing programs for 4000 and 5000 series aluminum wires in .035- and .047-in. (3/64th) diameter (as well as mild steel, stainless steel and other wire types). Most importantly, the systems are operator-friendly. In many cases, all operators do set wire feed speed to match the application: use faster speeds for thicker material and slower speeds for thinner material. The systems adjust all other voltage and pulsing variables automatically.

In this respect, new pulsed MIG systems­ – where operators only need to adjust wire feed speed – are actually easier to use than conventional MIG systems, which require operators to fine tune both wire feed speed and voltage.

New pulsing technology enables operators to easily adapt to different joint configurations by adjusting arc cone width, which in turn controls bead width. A wider bead can tie-in both sides of a joint or help with an outside corner; a narrow bead helps provide good fusion at the root of a joint (see Fig. 6).

One of biggest differences with new pulsing technology compared to old pulsing technology is that it does not force operators to hold a longer arc. It adapts to individual preferences so they don’t have to change their welding style. Using an arc length control function on the front panel, operators can set the arc length the want. Even better, the machine holds arc length constant regardless of electrode stick-out, which helps when welding in deep corners.

With just three variables to adjust ­- wire feed speed, arc length and arc cone width – experienced MIG welders can learn to use new pulsed MIG systems with just a few hours of training. They often become more productive in just one day. For companies that struggle to find and train new welders, pulsed MIG can reduce the learning curve. Some companies have successfully taken fork truck drivers with good work ethics and turned them into productive welders with just two or three weeks of training.

Feeders for Aluminum

As has been shown, many (if not most) aluminum fabricators would benefit from upgrading to welders with pulsed MIG capabilities. The same holds true for their wire feed system. Many fabricators first enter the aluminum marketing by purchasing a spool gun. A spool gun eliminates the possibility of birdnesting by putting a 4-inch (1-lb.) spool on the gun, so the wire only feeds a few inches.

However, a spool gun needs to have the roll changed after every pound of wire is used, compared with the 8- to 20-lb. spool on a push-pull system. If the operator is using several pounds of aluminum per day, the few minutes need to change spools can add up. Also, the chance of burnback exists when the end of a spool is reached, so many operators stop when a few turns are left on the spool. For any fabricator using several spools of aluminum per day, a push-pull system makes better economic sense.

With a push-pull gun, a motor in the gun pulls the wire through the liner, while the motor in the welder or feeder control becomes an assist motor. By maintaining consistent tension on the wire, the push-pull system helps eliminate birdnesting. It is more ergonomic than the spool gun, since the weight of the spool is not in the operator’s hands. Also, the spool needs to be changed less often than on a spool gun and allows the purchase of larger spools.

The only disadvantage to a push-pull system is their relatively higher initial purchase price, but the increased productivity and the financial advantage of buying larger spools usually provide a quick return on investment. When you compare the cost of 20 1-lb. spools of .030-inch 4043 aluminum filler wire with one 20-lb. spool of the same diameter wire, plus the time to change 20 spools versus one spool, you’ll see that for high volume use, a push-pull system makes financial sense.

A welding distributor, picked at random, lists a price for a 1-lb spool of .030-inch aluminum wire as $5.23. A 20-lb. roll lists for $92, which works out to a difference of $0.63 per pound. Now, add the time for 20 spool changes (and possibly burnbacks) for the spool gun as compared with one change for the larger spool. At 5 minutes to change a spool, that’s 95 minutes of extra time for each 20 lbs. of filler wire used.

A New Gun for Aluminum

In addition to welding machine advances, new advances exist with wire feeders for aluminum. For example, Miller’s new XR-Aluma-Pro gooseneck-style MIG gun optimizes feeding performance for 4000 and 5000 series aluminum through a two-position, series-specific tension control. Proper drive roll tension solves problems related to wire burnback, erratic feeding and arc fluctuations, helping aluminum fabricators improve productivity.

Miller factory tests and pre-sets the tension on every XR-Aluma-Pro gun for 4000 series aluminum (a softer wire requiring a lighter setting) and 5000 series aluminum (a harder, more abrasive wire requiring more tension). When switching wire types, operators change tension using a screwdriver to turn a control on the side of the gun to one series-specific tension setting or the other.

Other MIG guns give operators free reign to fully adjust tension settings. Typically, operators think, “tighter is better.” Unfortunately, too much tension deforms the wire and creates metal savings. Shavings create feeding problems by clogging the drive rolls and working their way into the head tube, which leads to erratic feeding, “micro arcing” within the contact tip and wire burn-back. These problems increase downtime, maintenance time and consumables cost.

The air-cooled XR-Aluma-Pro is the first push-pull gun rated for welding at 300 amps/100 percent duty cycle, 100 amps more than competitive guns. A higher duty cycle can eliminate the need for a water-cooled gun when welding aluminum up to 3/8-in. thick, thus reducing equipment acquisition costs by about $1,100. For welding thicker aluminum at higher amperages, Miller offers the water-cooled XR-Aluma-Pro, rated for welding at 400 amps/100 percent duty cycle. Features common to both guns include a 3-3/4 turn potentiometer for wire feed speed (WFS) control that provides better resolution and is easy to adjust while wearing welding gloves.

An easy-to-rotate, self-seating head tube allows better access to tight spots, while a replaceable feed cable liner reduces parts cost and service time.

In conclusion, MIG welding aluminum is challenging, but it does not have to be a headache. Just use the right equipment and processes. Expecting good results on aluminum with a system designed for carbon steel is like asking a carpenter to frame a house with a wrench instead of a hammer. Both tools will make a weld or drive a nail, but only one optimizes productivity, profitability, quality and operator satisfaction.