November 22, 2010
The reactive nature of aluminum to the welding arc, which is much more pronounced than that of other base materials, presents many challenges, including excess heat input and burn-through, porosity, poor bead appearance, poor fusion, and cracking. Technology advancements have made tackling these challenges easier by providing a balance of high travel speeds with a narrow, focused arc
The reactive nature of aluminum to the welding arc, which is much more pronounced than that of other base materials, presents many challenges. Some common issues are excess heat input and burn-through, porosity, poor bead appearance, poor fusion, and cracking. Tackling these challenges requires a balance between high travel speed and a narrow, focused arc under a protective inert gas barrier.
The two most common welding processes for effectively joining aluminum are gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). The speed of GMAW, commonly referred to as MIG welding, makes it the most cost-effective and widely used of the two processes. In the past welders’ choices were limited for welding aluminum using GMAW. Today’s processes are more controlled and more forgiving for a wider range of material thicknesses. Creating a high-quality aluminum weld is easy if the welder chooses the right process for the welding application.
Aluminum welding using GMAW initially was performed with constant-current (CC) power sources, a method that has a long and very successful history. The CC output assists in delivering a high-energy axial spray-transfer mode that responds evenly and consistently with the proper preset welding current, despite changes in arc length. CC aluminum welding also can provide a consistent penetration profile throughout the length of the weld; however, unstable arc length at arc initiation poses difficulties when welding aluminum in this mode.
As more advanced power sources became available, operators began using constant voltage (CV), a GMAW process that closely regulates arc length. Made since the 1990s, CV power sources demonstrated more stable output than CC power sources on various materials, including aluminum, by keeping arc length constant during changes in current. The CV process quickly became the standard mode for GMAW.
In recent years advancements in power source technology have led to the development of pulse transfer modes. When applied to aluminum GMAW, these modes simplify the process by controlling heat input, which makes it easier to achieve high-quality aluminum welds. The mode that’s right for the application depends on the material thickness, welding position, and power source capability.
A CV power source delivers constant voltage by varying the current to maintain a constant arc length. In this mode, the operator is able to adjust wire feed speed and arc voltage. CV is considered the conventional way to weld aluminum, and fabricators today still choose CV for its simplicity and lower capital cost over other GMAW methods. Welding aluminum with either CC or CV power sources requires high-energy axial spray transfer to melt the base metal and ensure good fusion.
To obtain spray arc transfer—a steady stream of molten metal that sprays across the arc—the welding current must be above a certain minimum “transition” current. For example, using CV spray transfer with 3⁄64-inch aluminum GMAW wire requires a minimum of 135 amps.
A modification of the traditional CV welding output combines the advantages of CV with those of CC for aluminum welding. In this mode, overall arc energy is regulated to a constant preset level to produce a stable welding current and consistent penetration.
This mode is highly adaptive to arc length fluctuations. It reduces voltage by increasing current in smaller but more frequent increments to maintain preset power output.
Until the development of the pulse transfer mode, it was extremely difficult to weld thin pieces of aluminum with GMAW. The high heat output of the CV spray transfer process burned through thin base materials and required a small-diameter wire, which posed other potential problems, such as feeding difficulty and tangling.
Pulsing provides spray arc transfer across a wider amperage range than CC and CV. The mode controls heat input by pulsing between a peak current that is above the transition current for spray transfer of molten metal and a background current, which is a significantly lower current at which no metal is transferred. This sequence of identical pulses allows spray transfer at a much lower-than-average current and provides the low heat input necessary for welding thin sections. This mode also is capable of providing low deposition rates for out-of-position welding.
When compared to CV welding, the lower heat input of pulse spray transfer decreases distortion, helps prevent burn-through, and enhances arc control (see Figure 1).
An enhanced pulse process was developed specifically for creating a GTAW-like bead appearance when welding aluminum with GMAW (see Figure 2). This mode uses a sequence of low-energy and high-energy current pulses to produce a bead with a stacked dime appearance. The process is forgiving of operator variance, so it is less susceptible to variations in arc length, wire placement, and gun angle—allowing the operator to consistently produce welds with excellent penetration and appearance.
Practical Welding Today® was created to fill a void in the industry for hands-on information, real-world applications, and down-to-earth advice for welders. No other welding magazine fills the need for this kind of practical information. Subscriptions are free to qualified welding professionals in North America.