Electric arc welding is the most prevalent welding type in industry. It creates two main groups of health hazards for workers; fumes and gases, and radiant energy. Shielded Metal Arc (SMA) welding is the most widely used welding method in industrial plant welding shops. The main chemical health hazards associated with this type of welding are fumes. Fumes are particles formed when the electrode and base metal constituents are vaporized and condensed in the welding area. Potential health problems can be anticipated by measuring the concentration of fumes in the welding space and comparing these data to established exposure standards. If high concentrations of these fumes are present, control measures should be undertaken to reduce the potential toxic effect to workers. Most of the studies have been done on mild (carbon) steel welding where it is generally necessary to monitor only the total amount of fumes. Stainless steel welding differs from carbon steel welding in that it generates considerable fume concentrations of chromium and nickel, which are suspected human carcinogens. The first part of this study evaluated the health risks posed to workers exposed to chromium and nickel fumes from routine stainless steel welding procedures. All the welding was performed in an industrial plant welding shop by one experienced welder. The welded piece was a three-part stainless steel cylinder. The whole period of welding lasted almost three weeks, although the actual welding was done in eleven days during that period. All sampling was performed with filter cassettes connected to personal air pumps. Sampling was performed in welder's breathing zone, in the general area (background sample), and at conveniently located points outside the breathing zone for evaluation of ratios of chromium and nickel to total fumes. The results indicated that at this particular industrial plant, exposure levels did not exceed the Occupational Safety and Health Administration (OSHA) Permissible Exposure Limits (PELs) and the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs). The results also indicated that it was not necessary to monitor the general area because of very low concentrations of chromium and nickel fumes. Rather, it is suggested that the monitoring focuses on the welder's breathing zone where it is important to sample hexavalent chromium (chromates) because of its proven carcinogenic effect and therefore very low TLV. Also, it was found that if TLV for chromates is not exceeded, then levels of total chromium and nickel are also likely to be below limits. The second part of the study sought to devise a simplified method of monitoring of welding operations. The results suggested that it is not always necessary to sample for all the components (total fumes, total chromium, chromates, and nickel) when estimating worker's exposure. Rather, it is possible to simplify the process by establishing the ratios of fume constituents during a period of heavy welding, thus enabling the industrial hygienist to make a reasonable estimate of exposure that occurs at other times. The estimate can be made by sampling either the main constituent (chromates) or total fumes, and predicting the exposure to remaining constituents of interest from these data. In addition, and in contrast to previous studies, it has been concluded that when fume concentrations are low, a welder's helmet does not provide any additional protection against fumes. Additional protection can be provided with the use of proper local ventilation, such as with a flexible hose, to reduce exposure well below suggested limits. / Graduation date: 1994

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 109-116).

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.

Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 1999. / Word processed copy. Summary in English. Includes bibliographical references (leaves 66-68). Also available online.

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.

Thesis (PhD (Metallurgical Engineering))--University of South Australia, 1997

Thesis (PhD (Metallurgical Engineering))--University of South Australia, 1997

Thesis (M.S.)--University of Missouri--Rolla, 2007. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed February 5, 2008) Includes bibliographical references (p. 29-31).

Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1999. / Bibliography: leaves 191-198.

Thesis (Ph.D.)--University of Wollongong, 2005. / Typescript. Includes appendices. Bibliographical referennces: leaf 196-206.