November 5, 2013
Welding aluminum and stainless steel can be challenging. Pulsed gas metal arc welding (GMAW-P), can help meets those challenges, while boosting productivity and reducing weld defects.
Welding operators want equipment that helps them get the job done faster while still producing high-quality welds.
In some applications, particularly those involving aluminum and stainless steel, pulsed gas metal arc welding (GMAW-P) offers high productivity; the ability to weld on thin and thick metals; to help reduce weld defects, such as burn-through and distortion; and the capability to use the process in all positions.
Popular materials in many welding applications, aluminum and stainless steel vary in numerous ways, but both can be prone to distortion with welding processes that use high heat. Because of its good weld pool control and decreased heat input, the GMAW-P process may help make welding operations more efficient by decreasing distortion in aluminum and stainless weldments.
GMAW-P allows you to tailor arc characteristics to specific welding applications. With some power sources that offer the process, you can adjust the width of the arc cone by manipulating certain pulse data variables, such as peak and background currents, pulse width, and frequency. Doing this helps you tailor the bead profile to the application. For example, you could create wider weld beads that improve tie-in on both sides of a joint, or achieve a narrower bead and increase travel speed.
GMAW-P also can help you run a tight and clean arc that generates minimal spatter.
GMAW-P is a modified spray-transfer process in which the power source switches between a high peak current and a low background current from 30 to 400 times per second. During this switch, the peak current pinches off a droplet of wire, propelling it to the weld joint. At the same time, the background current maintains the arc at a lower energy level that is not designed to transfer material across the arc.
This action differs from a standard spray-transfer process, which continuously transfers tiny droplets of molten metal into the weld joint.
GMAW-P runs cooler for the same wire feed speed, when compared to the standard GMAW spray process, and it allows the weld pool to freeze slightly to help prevent burn-through. The lower heat input the process generates offers benefits similar to those found with gas tungsten arc welding (GTAW), though GMAW-P is much faster. It also offers the good penetration and fusion associated with a traditional spray-transfer process.
Aluminum continues to be a staple in many industries, including aerospace, automotive, and shipbuilding. It offers corrosion resistance and strength, making it good for applications subjected to moisture and those that require lighter weights.
Because aluminum is very thermally conductive, it transfers heat away from the weld at a faster rate than many other materials do. This makes it more difficult to establish a weld pool. One of the biggest difficulties in welding aluminum is providing enough heat to create fusion in the root while controlling the heat input to prevent problems like burn-through.
Aluminum applications that are prone to weld defects, such as lack of fusion and porosity, or problems like burn-through, spatter, and warping, are good candidates for GMAW-P, which offers lower heat input and better weld pool control than constant-voltage GMAW.
Because GMAW-P is a modified spray-transfer process, it works well with both thin and thick sections. On thicker sections, it helps minimize downtime for repositioning parts, since the process generates a cooler weld pool, giving it good weld pool control when welding out of position. On these sections, however, the lower temperature of the arc and weld pool may make it more difficult to gain the desired penetration. On thinner sections of aluminum, the process minimizes the opportunity for burn-through and reduces the risk of warping by lowering heat input and allowing for a more concentrated arc.
Additionally, GMAW-P works well for bridging gaps in the weld joint and can aid in the cleaning action required to remove the oxide layer on the surface of aluminum.
The peak current pulse of GMAW-P also helps ensure good fusion and provides faster travel speeds, while the background current lowers overall heat input.
Note that GMAW-P on aluminum works best with the use of a push/pull gun, as the filler metal used to complete the weld is quite soft and prone to birdnesting. With a push/pull system, a motor in the gun pulls the wire through the liner, while another motor in the power source or feeder control serves as an assist motor.
Stainless steel has become an increasingly common material across multiple applications, including those in the petrochemical and food and beverage processing industries. The material offers corrosion resistance and relatively high tensile strength. It also possesses good ductility to withstand a range of service conditions and temperatures.
Welding stainless steel does come with some challenges. It is a poor conductor of heat and is prone to thermal expansion during the welding process, two factors that can lead to distortion. Stainless steel also generates a sluggish weld pool that doesn’t flow out of the weld toe as easily as carbon steel.
Using GMAW-P with stainless steel, you can run arcs tighter at the weld pool, which allows the bead to fill faster at the weld toe. The process also lets you tailor an arc to give a softer, more fluid weld pool for improved tie-in at the weld toes. The faster travel speeds and cooler temperatures GMAW-P offers are additional benefits for stainless steel.
GMAW-P also can help reduce the problem of carbide precipitation, which is sometimes associated with welding stainless steel. Carbide precipitation results when chrome, which has been added to the steel to generate its corrosion-resistance properties, combines with carbon to form chrome carbides. It occurs at temperatures between about 800 and 1,400 degrees F. Carbide precipitation lowers the material’s resistance to corrosion. Controlling heat input and maintaining steady travel speeds are two key factors in preventing this problem and gaining quality stainless steel welds with GMAW-P.
While using GMAW-P for aluminum and stainless steel is advantageous, simply choosing the process isn’t a substitute for good prewelding practices. For example, it’s still critical to clean the materials properly, including removing the oxides on aluminum with a designated stainless steel wire brush or another approved means before welding.
Use a slight push technique on the welding gun to get the best results with GMAW-P. It is not recommended that you drag the gun when welding with this process.
GMAW-P typically requires more training upfront, as well as a higher investment cost. However, the system can provide a quick return on investment through increased productivity, reduced downtime, and better weld quality. As an economic benefit, the process allows for the use of larger filler wire diameters to weld thin-gauge material, which can improve feeding performance and save money, because the larger wires are less prone to feeding problems and help increase the deposition rate.
The various challenges associated with welding aluminum and stainless steel often can be addressed by using GMAW-P.