When MIG welding aluminum, obtaining good quality and productivity depends on using the right welding process and system. Because aluminum quickly transfers heat away from the weld area, establishing the weld puddle takes more energy than does 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.
Spray transfer mode is commonly used when MIG welding aluminum. In this mode, molten metal is continually transferred across the welding arc, and it is easy to put too much heat into metal. In addition, in spray transfer mode, because of the heat input, the puddle stays fluid longer, and the operator is limited to welding in the horizontal and flat positions.
For many, the solution is pulsed MIG, a process that precisely controls the amount of heat put into a weld and overcomes the limitations of traditional MIG welding. In pulsed MIG, the current pulses between a high peak current in which metal transfer occurs and a low background current that maintains the arc but is too low for transfer to take place. This pulsed spray process creates a relatively cooler puddle while eliminating the short circuits that can lead to spatter.
The first generation of pulsed MIG machines, however, were so complex that many people shied away from the process. Fortunately, the latest generation of pulsed MIG systems eliminates that complexity, and some machines only require the operator to know the wire type and diameter and gas being used to provide optimal arc quality. In addition, because pulsed MIG has shown benefits in many applications, manufacturers are now offering systems that provide out-of-the-box solutions to meet specific needs, such as short-run fabrication or mass production.
Pulsed MIG benefits
In many aluminum applications, such as trailer, truck body, and components (e.g., cargo boxes), material thicknesses tend to be 3/8-in. and thinner. For thin components, the pulsed MIG welding process solves heat control issues, and eliminates the associated warping and burn-through. For heavy sections of aluminum, the relatively cool puddle of pulsed MIG provides the ability to weld out-of-position. This eliminates the need to reposition the weldment, and saves time (the spray transfer process is suitable only 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.
As an economic benefit, pulsed MIG welding allows using larger filler wire diameters, such as .047 in. instead of .035 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. According to a welding distributor chosen at random, a 16-lb. spool of .035-in aluminum filler wire costs $72, whereas a spool of .047-in. wire costs $67, a difference of $0.31 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.
Taking today's pulse
While the pulsed MIG principle is relatively simple, many factors go into developing the pulse parameters, which vary depending on wire size, wire type, and the shielding gas used. Peak and background current levels, frequency, pulse width, rise, and fall times — all need to be factored into the calculations. With the first generation of pulsed MIG machines, this burden was placed on the operator, and the complexity placed the process out of reach for many who could have benefited from its use.
That has changed. New pulsed MIG systems feature built-in pulsing programs optimized for specific wire types and sizes, such as 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). More importantly, the systems are operator-friendly. In many cases, operators select the wire size and wire type and set wire feed speed to match the application: faster speeds for thicker material and slower speeds for thinner material. The systems automatically adjust all other voltage and pulsing variables.
In this respect, new pulsed-MIG systems actually are easier to use than conventional MIG systems, which require operators to fine-tune both wire feed speed and voltage. With the new systems, operators only need to adjust wire feed speed, although controls are available for those who wish to optimize arc length and arc cone width.
Experienced MIG welders can learn to use pulsed MIG systems with just a few hours of training and 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 who have good work ethics and turned them into productive welders with just two or three weeks of training.
To summarize, pulsed MIG can provide:
- Ease of use
Today's welding operator only needs to know a wire type, size, gas, and preferred arc setting, and then adjust wire feed speed to weld different thicknesses.
- Decreased waste and rework
Pulsed MIG eliminates excess heat that can lead to burn-through or warping on thin materials, especially aluminum, and provides better control for less-than-optimal fit up.
- Out-of-position welding
Pulsed MIG allows the puddle to freeze slightly between current peaks. This means it can be used in out-of-position welding and eliminates the need to reposition weldments.
- More productive aluminum welding with good bead appearance
While it used to require a skilled TIG welder to achieve a good bead appearance on aluminum, Pulsed MIG rivals TIG in appearance and can replace TIG in many applications. Additionally, MIG typically requires less training than does TIG.
- Larger wires — less cost
Pulsed MIG also can allow the operator to use larger diameter wires, which are usually less expensive per pound than the thinner wires. In the case of aluminum wire, they are easier to feed and less prone to birdnesting.
- Decreased training time
Today's pulsed MIG technology enables new operators to start making production-quality welds in less time, and it gives experienced operators even more control over bead appearance.
Custom systems
Just as important, systems are now designed with specific needs in mind. While one unit may be optimized for welding the alloys found in a power plant, another is designed for ease-of-use in a mass production setting. The fabricator who needs to wheel the power source around the weldment or perhaps move it around the shop may benefit from an all-in-one pulsed MIG welder with built-in wire feeder, such as the Millermatic 350P. This system has an output of 25 to 400 amps (300 amps at 60-percent duty cycle), welds aluminum up to 1/2-in. thick in a single pass, and contains programs for the most commonly used aluminum and steel wire types and sizes. It offers the flexibility of working on single- or three-phase power, and it can be easily wheeled around large weldments. When paired with a dedicated push-pull gun, such as the XR-Aluma-Pro, it provides an out-of-the-box welding solution for aluminum fabricators.
In a mass production facility, a pulsed MIG system with a wire feeder in the welding cell and a small power source that can be positioned remotely may be the ideal choice. A pulsed MIG power source, such as the Invision 350 MPa has an output of 5 to 425 amps (350 amps at 60-percent duty cycle) and 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.
In conclusion, pulsed MIG can remove many of the challenges with both welding aluminum and finding skilled welders. Today's machines not only simplify the process, but come as systems optimized for different applications. If you haven't checked into pulsed MIG welding in the last three years, take the time to do so now.