The GTAW process is quite often a viable option for welding aluminum. It was developed in 1944 and is still used to weld aluminum alloys today. Some of the highest quality welds used in critical applications, such as full penetration pipe welds on cryogenic pressure vessels, are almost exclusively made with this welding process. Alternating current (AC) is used for most applications. Direct current (DC) power is employed for some specialized applications. The GTAW process was developed earlier than the Gas Metal Arc Welding process (GMAW) For a time, was used to weld aluminum of all metal thicknesses and joint types. The GTAW process has since been replaced by the gas metal arc welding (GMAW) process. This is used for many aluminum welding applications, because of the increased speed of the GMAW process to weld thicker sections.
However, GTAW still has an important place in the aluminum welding industry. GTAW, with alternating current (AC) and pure argon shielding gas, is now most often used to weld thinner aluminum gauges (up to ¼ inch) and for applications where aesthetics are most important. Alternating current (AC) is the most popular gas tungsten arc welding aluminum method. A balanced wave AC arc provides cleaning action for most applications. It divides the arc heat evenly between the electrode and base material. GTAW power sources for AC welding, which allow for adjustment of the balance between polarities. This enables the user to choose either enhanced arc cleaning or greater penetration capabilities. We can find GTAW used in the direct current electrode negative mode (DCEN) for more specialized applications.
This method provides an arc concentration of about 80% heat at the base material and about 20% at the electrode. This results in relatively deep and narrow weld penetration. There is minimal, if any, significant arc cleaning during the welding operation. It uses pure helium shielding gas. This welding method is capable of welding much greater material thicknesses (up to 1 inch). It is most often used in automatic seam welding applications. The third mode of GTAW is the direct current electrode positive (DCEP). With this method, we have about 20% of the heat generated at the base plate and 80% at the electrode. We create excellent cleaning action but very shallow penetration. This is probably the least used method of GTAW.
Oxyfuel gas welding is a gas welding process. It achieves coalescence by using the heat from an oxygen-fuel gas flame and, for aluminum, an active flux to remove the oxide and shield the weld pool. With this process, very thick joints have been welded in the past, but the most common applications have been for sheet metal. One of the problems with this welding process is the flux used during the process is hygroscopic. This means it absorbs moisture from the surrounding atmosphere. When moist, the flux becomes corrosive to aluminum. Therefore, the flux must be removed after welding to minimize the chance of corrosion. Because it can be challenging to ensure all flux traces have been removed, finishing the operation with an acid dip to neutralize any flux residue.
Other disadvantages of using this process for welding aluminum are mechanical strengths tend to be lower and heat-affected zones wider than with arc welding. In addition, welding is only practical in the flat and vertical positions, and distortion can tend to be extreme. Most of the problems are caused by corrosive flux and excessive heat input associated with this process. The oxyfuel gas welding process has been widely used in the history of welding aluminum. This was before the development of the inert gas welding process but has limited use today.
Before the development of the inert gas welding process (GTAW & GMAW), the arc welding of aluminum was mainly restricted. It is restricted to the Shielded Metal Arc Process (SMAW), sometimes referred to as the Manual Metal Arc Process (MMA). This welding process uses a flux-coated welding electrode. The electrodes are straight lengths of aluminum rod coated with flux. The flux acts to dissolve the aluminum oxide on both the base alloy and the rod during welding, which is necessary if coalescence is to occur. Some of the flux components vaporize in the arc to form shielding gases. This helps to stabilize the arc and shield both it and the weld pool from the surrounding atmosphere.
One of the main problems with this welding process was corrosion caused by flux entrapment, particularly in fillet welds where the flux could be trapped behind the weld and promote corrosion from the back of the weld. Other problems were that welds from this process are prone to gross porosity. There are no electrodes available for the high magnesium content base alloys. Once exposed to the air, electrodes absorb moisture into the flux. Eventually it corrodes the aluminum core and produces excessive porosity problems. It was soon found that this process was not the most suited for welding aluminum. Current welding codes and standards for aluminum structures do not recognize this process as suitable for production welding applications.
Without a doubt, the breakthrough for aluminum as a welded structural material occurred with the introduction in the 1940s of the inert gas welding processes. With the introduction of a welding process that used an inert gas to protect the molten aluminum during welding, it became possible to make high-quality, high-strength welds.