Rapid Solidification Joining Of Metastable Materials

In this paper,the rapid solidification joining condition with the characteristics of high energy density and high cooling rate was built by using the device of micro-type capacitor discharge welding machine.Three different types of metastable alloy foils of Ni-Si eutectic, Fe-Cu pefitectic and Zr-based amorphous were welded separately.The morphological characteristics and formation mechanism of joint were analyzed,and nugget cooling rate was calculated theoretically.Furthermore,the mechanical properties of joint were also tested and measured..The results indicate that:Under rapid solidification joining condition,the metastable alloy foils can be well welded by a CDW machine and the joints obtained having typically characteristics of the rapid solidification.A regular oblate spheroid nugget with 102μm in diameter and 48μm in height was formed in the lapped joint of Ni-Si alloy foils with the thickness of 35-48μm.The microstructure of nugget is consists of fine and homogeneous equiaxed grains,the bonding zone is rather narrow and its width is only 2.0～3.0μm.Even more,there is no microstructural coarsening in original foils near the nugget.The cooling rate of nugget reaches to 7.82×10~6K/s.The cooling rate of rapid solidification joint for Fe-Cu peritectic alloy is up to 1.65×10~6K/s, the microstructure of nugget as well as its peripheral zone is fine and homogeneous and the grain size is merely 1～2μm.The liquid separation takes place within nugget during rapid solidification joining process.The Fe-rich droplets move towards to the center of the nugget resulting from Marangoni migration driven by temperature gradient,colliding and growing continuously,as a result,leading to the formation of Fe-rich phase segregation at nugget center. With the increase of the temperature gradient,the growth velocity of the droplets increases linearly.Eelectrode force and weld voltage have obviously influence on the shearing strength of joint.When U=75V,F=14N,the shearing strength of joint has the maximum which is up to 241MPa.XRD and TEM analyses results indicate that the Zr-based amorphous alloy also can be well welded by CDW method,and the joint obtained still remains the amorphous state and there is no sign of crystallization.Nugget solidification occurrence mainly concentrates on the previous 100μs of the welding process and its cooling rate is up to 8.08×10~6K/s.Owing to having much higher resistivity,the welding energy needed for metal glass foils is much less than that of crystalline alloys and the proper welding process parameters for Zr-based amorphous alloy foils are as follows:U=52V,F=24N.Comparing with crystalline alloy,the mechanical properties of amorphous alloy increase a lot due to no lattice defects such as dislocation and fault.The shearing strength of Zr-based amorphous joint is up to 1123MPa.