Pressure Welding of Thin Sheet Metals: Experimental Investigations and Analytical Modeling

Abstract

Emerging applications, such as fuel cell, fuel processor, heat exchanger, microreactors, etc., require joining of thin metallic plates in confined places with small dimensions and minimal damage to the surrounding areas. In this study, the feasibility and modeling of pressure welding (solid state bonding) process are investigated, specifically for bonding of thin sheet metals. The effects of material type (e.g., copper, nickel, and stainless steel) and initial plate thickness (51–254 μm) as well as process conditions (e.g., welding pressure and temperature, 25–300°C) on the minimum welding pressure and the final bond strength are experimentally studied. A pressure welding apparatus was developed for testing of different materials and process conditions. Based on the experimental results, the effects of material and process conditions on the final bond quality are characterized. At room temperature, copper and nickel blanks were successfully bonded, while stainless steel blanks could only be joined at elevated temperature levels (150°C 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 be carried out at elevated temperature levels. In this study, the bond strength of the welded blanks was characterized with uniaxial testing. The tensile test results showed that the bond strength could be increased by increasing the welding pressure or temperature. However, the increase in bond strength by increasing the 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 was found to be dependent on the material and process conditions. In addition, bond formation mechanisms for different materials were studied through microscopic analyses. The microscopy images of the weld spots showed that for a successful 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. Finally, a model for bond strength prediction in pressure welding was developed and validated. This model includes the sheet thickness parameter, which is shown to be a critical factor in bonding thin sheet metals with the sheet thickness in the range of a few hundred micrometers.