10 - Thermoplastic brazing of dissimilar materials

Abstract

A new joining method called Thermoplastic brazing (TPB) was proposed and successfully applied in the joining of dissimilar materials with bulk metallic glass (BMG) as filler metal. The bonding process was divided into two stages wherein, in the first stage, the sandwiched samples with the BMG as the interlayer were heated to the BMGs superliquid region for superplastic deformation. The viscosity of the BMG dropped dramatically and the cracks and holes in the interfacial regions of base metals were completely filled by viscous flow of the BMG, as a result, an equivalent effect to wetting was obtained. In the second stage, the samples were heated to a higher temperature which was about 0.8Tm, where Tm is melting point of the BMG, for atomic diffusion. Metallurgical bonding was realized in this stage. This new joining strategy displays advantages of low requirement for contacting surfaces and low joining temperatures over diffusion bonding and brazing. Furthermore, since there is no liquid-solid transformation, the detrimental effect caused by fusion welding such as quenching effect, hot cracks can be avoided. More importantly, the properties of the joint serving at elevated temperatures can be further improved by selecting a high-melting-point BMG as the filler metal. TiAl-based alloy and Ni-based superalloy, TC4 and 316L stainless steel were selected as the base materials and the bonding was carried out at different temperatures holding for different time to investigate the influence of the heating temperature and holding time on the microstructure and strength of the joint. The microstructure of the joint and the fracture surface were analyzed by means of scanning electron microscopy (SEM), x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) as well as the transmission electron microscope (TEM). Mechanical properties of the joints were tested by shearing test.