Analysis of joint microstructure of melt-spun Ni-Si alloy foils by rapid solidification joining process

The spot welding characteristics of melt-spun Ni-8.24%Si alloy foils with 35-45 μm in thickness were investigated using a mini-type capacitor discharge welding machine.The cooling rate of the nugget was calculated.The joint microstructure and its forming mechanism were also analyzed.The results indicate that the compact energy and instant discharge from capacitance can realize spot welding of rapidly solidified Ni-8.24%Si alloy foils, and a regular oblate spheroid nugget with 102 μm in diameter and 48 μm in thickness is formed.Owing to the momentary welding time hold in welding process, the cooling rate of the joint reaches to 7.82×10⁶ K/s.Thus the microstructure of nugget is characterized by rapid solidification, which consists of fine and homogeneous equiaxed grains with the sizes of 0.5-3.0 μm.The bonding zone is rather narrow and its width is only 2.0-3.0 μm.Even more, there is not microstructural coarsening in base alloy near the nugget.Therefore, the microstructure of welded joint is extraordinary consistent with the microstructure of base alloy.