Electrically Assisted Joining and Forming of Automotive Stainless Steels

Abstract

Electrically assisted manufacturing (EAM) is a new manufacturing technique that has the potential to improve the cost-efficiency of essentially every metal joining process, forming process, and enable greater process capabilities. In this process, electric current is passed through a metal workpiece. Naturally, the electric current can rapidly increase the temperature of the metals by resistance heating (Joule heating). The effectiveness of resistance heating as a repeat heat source for joining and forming may be accompanied by the benefits of thermal (Joule heating) and the possibility of additional athermal (electroplasticity) with the electric current. Especially, the electric current may induce the electroplasticity, wherein the electric current enhances the mobility of atoms such as the impact of the electrons to dislocations to accelerate the movement of dislocation and scatter of electrons to interfacial defects to increase the resistivity of the metals. As electroplasticity has been widely studied, EAM processes have been increasingly applied. The applications of EAM processes are mainly associated with sheet metal forming, bulk deformation, and electrically assisted (EA) joining for various metal alloys (i.e. Stainless steels, Ultra high strength steels, Aluminum alloys, Magnesium alloys, Titanium alloys). This dissertation presents studies on the feasibility of EA joining and EA forming of stainless steels by applying electric current. Firstly, the electrically assisted (EA) brazing of a ferritic stainless steel (FSS) with nickel-based filler metal is experimentally investigated. Microstructural analysis results strongly suggest that the electric current during EA brazing enhances diffusion between the filler metal and the FSS, thus inducing significantly thicker diffusion zones compared with induction brazing. In the present study, the EA brazing provides the technical advantage that the process time can be significantly reduced without sacrificing the joint strength (or a higher joint strength with a similar process time) in comparison with induction brazing. Next, a two-stage forming process of the selected 316L austenitic stainless steel (SUS316L) with rapid EA annealing is experimentally investigated. A single pulse of electric current is applied to the prestrained specimen with a short duration time right after first deformation (prestrain). The stress-strain curve during reloading shows that the flow stress of the SUS316L significantly decreases, which indicates the occurrence of EA annealing. The electric current also increases the maximum achievable elongation of the SUS316L during reloading. The stress-strain curve during reloading and the microstructural observation suggest that the effects of EA annealing on the post-annealing mechanical behavior and microstructure strongly depend on both the applied electric current density (electric current per unit cross-sectional area) and the given prestrain. Finally, electrically assisted (EA) subsecond annealing of prestrained austenitic 304 stainless steel (SUS304) sheets is carried out. The post-annealing tensile test of two different prestrains is compared with each other by applying the same current densities. The post-annealing stress-strain curve during reloading shows that the flow stress of the SUS304 significantly decreases, while the maximum achievable elongation significant increases with increasing electric current. Microstructural analysis confirms the occurrence of electrically induced annealing and the phase transformations. The results of the present study can be used to improve the formability of the SUS304 in sheet metal forming processes with electric current. The results of the present study will be used as basic data and methodologies for the development of joining and forming processes in manufacturing by using electric current. Specifically, both EA joining and EA forming processes are contributed to the development of brazing exhaust pipe systems and springback reduction bipolar plates for fuel cell in the automotive industry.