Dissimilar TC4 titanium alloy and 316L stainless steel were successfully joined utilizing thermoplastic bonding (TPB) with a Ti-based bulk metallic glass (BMG) as the filler. Good physical pre-bonding and metallurgical bonding were successively achieved through superplastic flowing in the superliquid region and solid-sate atomic diffusion and reaction. The experimental results indicated that the superplastic flow of BMG in the superliquid region greatly promoted void shrinkage in the interfaces. Hence, a compact contacting was obtained at temperature much lower than the melting point of the filler metal, which is beneficial to the atomic diffusion between the filler metal and base metals as temperature increased. After holding for some time below the melting point of the filler metal, three different reaction layers derived from the atomic diffusion and reaction between the base metal and filler metal were formed in the interfacial regions. A series of irregular bulky and small plate-shaped fine phases were formed and uniformly distributed in the interfacial regions and intermediate region. The shearing tests showed that a high strength of 225 MPa was obtained after diffusing at 695 °C for 60min. The fracture surface of the joint shifted from the 316L interface to then central crystallization layer as temperature increased.

The joining of dissimilar materials such as TiAl- and Ni-based alloys has always been a challenging work due to the larger differences in properties. In this work, a new strategy called thermoplastic bonding (TPB) was proposed and successfully applied in joining of the two alloys. Through utilizing superplastic flow of bulk metallic glasses (BMGs), a good mechanical bonding with no cracks between the base metals and filler was achieved, and after holding a time at a higher temperature lower than the melting point of the filler, the alloys were metallurgical bonded through atomic diffusion. The results showed that the phases in the joint bonded by TPB were very fine (approximately 300 nm) and uniformly distributed at the grain boundaries due to the low temperature and concomitant sluggish diffusion effect, while a series of directional intermetallics detrimental to the joint strength formed in the joint bonded by vacuum brazing. Tension tests showed that the strength of the TPB bonded joint was as high as 415 MPa, while that of the vacuum brazed joint was only approximately 315 MPa. These results verified that this new joining method can obtain a liquidus bonding effect equivalent to that obtained by a solid diffusion method. (C) 2019 The Authors. Published by Elsevier Ltd.

The gamma-TiAl-based alloy and Ni-based superalloy (GH536) were successfully brazed utilizing a low-melting-point amorphous Ti35Zr25Be30Co10 filler below 800 degrees C. Before brazing, the wetting ability of the filler was investigated and it was found that the amorphous filler exhibited a good wetting ability with a wetting angle less than 30 degrees. A sound joint with no cracks and holes was obtained after brazing and dendritic phases formed in the wetting interfacial regions and across the brazed joint. The structural evolution of the wetted interfaces and brazed joints were studied by x-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS). The results show that temperature played an important role in the formation of the dendritic phases. Coarse grains were presented and dendritic phases decreased when the joint was brazed at a higher temperature, which seriously deteriorate the shear strength of the joint. The shear tests showed that the maximum shear strength of the joint was 300 MPa when the brazing temperature and holding time were 770 degrees C and 20 min.