Development of a Friction Spot Joining Tool for 980 MPa Tensile Strength Steels Joining

摘要

Car manufacturers have been increasing the use of ultra-high tensile strength steels with strengths exceeding 980 MPa. This is to improve collision safety and reduce vehicle weight, thereby reducing CO_2 emissions. Due to their high carbon content, these steels cannot achieve sufficient joining strength when joined using the current resistance spot welding technology. To achieve high joining strength in high-carbon content steels, we focused on the friction spot joining (FSJ) method, a solid-phase joining method that employs the frictional heat of the joining tool to soften and ultimately join the sheets through a plastic flow process. FSJ has already been commercialized for the joining of aluminum sheets, but its poor tool life has prevented its application to steel sheet joining. In order to make the FSJ method practically applicable to steel joining, we developed a cemented carbide tool base material with good resistance to breakage, plastic deformation and thermal cracking, a PVD coating that provides hardness and oxidation resistance, as well as a tool geometry. We also determined appropriate joining conditions to reduce tool wear. By integrating all these elements, we have successfully developed an FSJ tool with superior tool life (7,000 spots) for ultra-high tensile strength steel joining.