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A: During my attempt to address these questions, I will also try to clarify some of the terms and definitions used.
The GTAW process is quite often a viable option for welding aluminum. It was developed in 1944 (see fig1), and is still extensively used to successfully 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, but direct current (DC) power is employed for some specialized applications. The GTAW process was developed earlier than the Gas Metal Arc Welding process, (GMAW) and 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 The GTAW process has since been replaced by the gas metal arc welding (GMAW) process for many aluminum welding applications, primarily because of the increased speed of the GMAW process to weld thicker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 gauges of aluminum (up to ¼ inch) and also for applications where aesthetics are most important. Alternating current (AC) is the most popular method of gas tungsten arc welding aluminum. A balanced wave AC arc provides cleaning action for most applications and divides the arc heat about evenly between electrode and base material. GTAW power sources for AC welding, which allow for adjustment of the balance between polarities, enable the user to choose either enhanced arc cleaning or greater penetration capabilities. For more specialized applications, we can find GTAW used in the direct current electrode negative mode (DCEN). This method provides arc concentration of about 80% of the heat at the base material and about 20% at the electrode. This results in relatively deep and narrow weld penetration, and very little, if any, significant arc cleaning during the welding operation. Typically used with pure helium shielding gas, this method of welding is capable of welding much greater thicknesses of material (up to 1 inch) and 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. Very thick joints have been welded in the past with this process, but the most common applications have been for sheet metal. One of the problems with this welding process is that the flux used during the process is hydroscopic, meaning it absorbs moisture from the surrounding atmosphere. When moist, the flux becomes corrosive to aluminum. Therefore, after welding, the flux must be removed to minimize the chance for corrosion. Because it can be difficult to be certain that all traces of flux have been removed, it was often necessary to finish 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. 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 was widely used for welding aluminum prior to the development of the inert gas welding process, but has limited use today.
Prior to the development of the inert gas welding process (GTAW & GMAW) the arc welding of aluminum was mainly 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 that help 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 and electrodes, once exposed to the air, begin to absorb moisture into the flux, which eventually 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 welding process as being 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 at high speeds and in all positions, without corrosive fluxes.
Fig 1. 1944 -1994 advertisement celebrating 50 years of Heliarc (The trade name used for the GTAW/TIG welding process that is sometimes still used today). A major breakthrough for aluminum as a structural welded material.
Fig 2. This USA Army water canteen welded with the OFW process and dated 1918 some 25 years prior to the development of the GMAW / MIG and GTAW / TIG inert gas welding processes.