ELECTRICALLY ASSISTED PRESSURE JOINING OF SIMILAR AND DISSIMILAR METAL ALLOYS

Abstract

Metals are the most widely used materials for fabrication of products. Metal joining, as one of the key manufacturing technologies, is used extensively in production and plays predominant roles in manufacturing industry. Electrically assisted pressure joining (EAPJ) is one of the electrically assisted manufacturing (EAM) techniques. EAM is a new concept for manufacturing processes that utilize the electroplasticity of metals and their alloys to improve productivity, efficiency, and quality. EAPJ using resistance heating as a heat source is a solid state joining without melting or solidification. In this dissertation, the feasibility and effectiveness of EAPJ on similar and dissimilar metal materials are investigated. Firstly, EAPJ of a Grade 1 titanium (Ti) alloy sheet with thickness of 1 mm is experimentally investigated. In this study, a custom-made EAPJ apparatus is developed for testing different process conditions. During joining process, an electric current is directly applied to the lap joint specimens during plastic compressive deformation. The optical microscopy (OM) and electron back-scatter diffraction (EBSD) results confirm that defect-free joints are successfully fabricated in the selected Ti alloy. The shear tensile test results show that the joint strength and fracture modes are strongly affected by the EAPJ process conditions, such as the amount of plastic deformation and the electric current intensity. The result of present study shows that the EAPJ is efficiently applicable to joining of Ti alloy sheets. Next, a study on the EAPJ of bulk workpieces, stainless steel 316L (SUS316L) cylindrical specimens, is carried out. An additive manufactured (AM) metal porous interlayer is used in the study to improve the process effectiveness for the EAPJ of cylindrical bulk components. The AM porous interlayer is fabricated using SUS316L powder by metal additive manufacturing (or simply, 3D printing) with selective laser melting. During EAPJ, axial compression and an electric current were simultaneously applied to the specimen assembly. The OM and EBSD results show that any of the EAPJ joints with or without the porous interlayer is successfully fabricated. A lower joining pressure is needed when applying the AM metal porous interlayer with a lower compressive strength and higher electric resistivity. In the present study, the practicability and effectiveness for the EAPJ of cylindrical bulk components are demonstrated by using an AM metal porous interlayer. Finally, EAPJ of dissimilar materials, SUS316L and Inconel 718 (IN718) cylindrical bulk specimens, is experimentally investigated. The specimen assembly for the experiment comprises two cylindrical solid specimens (SUS316L and IN718), with an identical geometry. In EAPJ, electric current and plastic compression are directly applied to the specimen assembly simultaneously. The microstructural analysis (FE-SEM with EBSD and EDS) confirms that defect-free joints are successfully fabricated by EAPJ in the selected dissimilar metal alloys without melting and solidification. Obvious atomic diffusion region at the interface of the EAPJ joint is identified. The results of tensile tests show that all the joints fracture from SUS316L alloy side, which undergoes typical ductile fracture with severe plastic deformation prior to fracture. The result of the present study confirms that the concept of EAPJ is efficiently applicable to solid state joining of dissimilar material combination.