Characterization of pressure welding process of thin sheet metals in cold and warm temperature conditions

The effects of material and process conditions in the pressure welding process of thin sheet metals on the minimum welding pressure and the final bond strength are investigated in this work. The studied parameters include the material type (copper, aluminum, nickel, and stainless steel), initial blank thickness (0.051-0.254 mm), welding pressure, welding temperature (25-300°C), surface condition (wet, dry, and brushed), and indenter size. Two sets of pressure welding apparatus were developed for testing of different materials and process conditions. Based on the experimental results, copper, aluminum, and nickel blanks were successfully bonded at room temperature ("cold welding"), while stainless steel blanks could only be joined at elevated temperature levels (150 and 300°C). The material type (i.e. strength) and thickness were shown to have significant impact on the welding pressure; in that more pressure is required to bond the blanks with higher strength or thinner. To reduce the required welding pressure, the process can either be carried out at elevated temperature levels or by scratch brushing the surfaces to be joined. In this study, the bond strength of the welded blanks was measured using tensile testing. The tensile test results showed that the bond strength could be increased by either scratch brushing the surfaces or by increasing the welding pressure or temperature. However, the increase in bond strength by increasing welding pressure was shown to have an optimal point, after which the bond strength would decrease with further increase in pressure. This critical pressure value appeared to be dependent on the material and process conditions. The width of the straight line indenter showed no significant impact on the minimum welding pressure. Finally, the bond formation mechanisms for different materials were studied through microscopic analyses. The microscopy images of the weld spots showed that for the bonding to take place, the contaminant layers at the surfaces must be removed or broken to allow the virgin metal underneath to be extruded through. The metallic bonds only form at these locations where both surfaces are free of contaminant layers.