Double-Sided Arc Welding of AA5182 Sheet in the Lap-Joint Configuration
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Automakers are increasingly using aluminum for structural applications in order to reduce vehicle weight and improve fuel efficiency. However, aluminum bodied cars have largely been confined to lower-volume, niche markets due in part to the number of challenges associated with welding of aluminum in comparison with steel. Therefore, a need exists for new joining processes which can produce high-quality welds in thin aluminum sheet at high production rates and low cost. The recently invented double-sided arc welding (DSAW) process is one such joining process. It has been shown to be capable of producing high quality butt-joint configuration welds in thin aluminum sheet and thick steel plate. In DSAW, an arc is initiated across two torches that are mounted on either side of the workpiece, allowing it to be welded from both sides. The objective of this research was to determine the feasibility and merits of using the DSAW process to produce seam and spot welds in thin aluminum sheet in the lap-joint configuration. The double-sided arc welding (DSAW) apparatus used in the present study was powered by a single square wave alternating current variable polarity power supply to produce an arc across two liquid cooled, plasma arc welding torches. This equipment was used to produce a series of conduction-mode DSAW seam and spot welds in 1 mm thick and 1.5 mm thick AA5182-O and AA6111 sheet in the lap-joint configuration. Metallographic analysis was used to characterize the microstructure of the welds, while microhardness and tensile tests were used to characterize the mechanical properties. Since hydrogen is easily absorbed by molten aluminum, all weld specimens must be cleaned prior to welding in order to produce high quality, pore-free welds. Although previous studies had shown that the specimens could be sufficiently cleaned by degreasing and wire brushing them prior to welding, this cleaning procedure was not found to be adequate for the specimens used in this study and a more aggressive cleaning technique was required. A number of different specimen pre-cleaning techniques were examined, and a combination of degreasing, deoxidizing, and manual wire brushing was found to produce the least amount of porosity in bead-on-plate welds produced in 1.5 mm thick AA5182-O sheet. Further reductions in porosity were accomplished by redesigning the shielding gas cup of the top Thermal Arc torch to promote more laminar gas flow and generate a more evenly distributed shielding gas plume. Using the redesigned shielding gas cup, a shielding gas flow rate of 10 lpm was found to provide good coverage of the weld pool and produce virtually pore-free welds. The feasibility of using the DSAW process to produce spot welds in 1 mm thick AA5182-O sheet in the lap-joint configuration was examined by producing a series of spot welds over a range of welding powers and weld times. Weld nuggets were produced using a welding current as low as 50 A with a cycle time of one second. However, all of the welds exhibited a pinhole at the centre of the nugget which penetrated through the entire thickness of the specimen, regardless of welding current and cycle time used. Solidification shrinkage porosity and crater cracking were also observed near the centerline of the welds. Hydrogen gas porosity and oxide tails were also observed in the welds. These defects were found to decrease the strength and quality of the spot welds made between 1 mm thick AA5182-O sheets in the lap-joint configuration. A series of welds were made to determine if the DSAW process could be used to produce seam welds in 1 mm thick AA5182-O sheet in the lap-joint configuration. Visually acceptable, crack-free welds were produced using welding powers ranging from 2.0 kW to 5.1 kW, at welding speeds between 10 mm/s and 70 mm/s. Welds produced within this range of welding conditions were found to possess excellent cathodic cleaning on both sides of the workpiece, a smooth weld bead, and a columnar-to-equiaxed grain transition. However, transverse cross-sections of the specimens revealed varying amounts of oxide entrainment in the weld metal which was seen most frequently as unbroken interface oxide sheets or tails at the fusion boundaries. Often times, small clusters of porosity were found to nucleate along the oxide tails. This suggested that there was insufficient fluid flow in the weld to disrupt the pre-existing oxide sheets at the interface between the sheets. Careful specimen pre-cleaning using a combination of degreasing, deoxidizing, and manual stainless steel wire brushing was found to reduce, but not eliminate the oxide tails. Microhardness testing revealed that the microhardness was relatively consistent across the weld metal, heat-affected zone (HAZ), and base metal. In a series of tensile-shear tests, all of the welded specimens were observed to fail in the weld metal, within 1 mm of the fusion boundary. Another series of seam welds were produced between 1 mm thick AA6111 and 1 mm thick AA5182 sheets in the lap-joint configuration to explore the nature and intensity of fluid flow in the molten weld pool responsible for breaking oxide tails. The difference in magnesium content between the two alloys produces a different microstructure and response to chemical etching, thereby revealing any effects of fluid motion in the weld pool. Relatively weak buoyancy driven fluid flow was observed when the AA6111 sheet was placed on top of the AA5182 sheet, and some minor stirring was seen between the two sheets. When the slightly less dense AA5182 sheet was placed above the AA6111 sheet, very little fluid flow was observed and the two alloys remained unmixed. Overall, the weld pool formed during DSAW was found to be very quiescent. This suggests that the normally strong Marangoni and Lorentz force induced flow seen in other arc welding processes does not occur in the double-sided arc weld pool. This leaves only a very weak buoyancy driven fluid flow which is incapable of disrupting the pre-existing oxide at the interface between the sheets immediately adjacent to the fusion zone boundary. Overall the DSAW process was found to be capable of producing visually acceptable lap-joint configuration seam welds in AA5182-O sheet over a wide range of welding speeds and welding powers, provided that the pre-existing oxide and any other surface contaminants were chemically removed prior to welding.