The Influence Mechanism Of Ni Layer On The Titanium/SS Microstructure And Properties Of Welded Joints

Titanium and its alloys are light weight, corrosion resistant and has good mechanical properties, gradually replacing some alloys susceptible to severe corrosion of copper alloys and other alloys, but because of the expensive price of titanium and its alloys, titanium and its alloys are extremely limited to be used in the industry. Stainless steel not only has good performance, but it can lower the cost of their applications than titanium and titanium alloys, so linking the two metals are used in aerospace, aviation, and fields. Welding of Titanium and titanium alloy and stainless steel, with unique advantages and good economic returns, however, TC4and304SS dissimilar metal welding process,it is easier form brittle intermetallic compounds and low melting point eutectic in the joint, and the existence of the organization reduces the mechanical properties of joints, resulting in reduced service connector. To solve the problem of deteriorating performance weld between the titanium/steel joints caused by metal compounds,in this test,we use Ag45CuZn as solder,and vacuum brazing method in this study, by plating Ni layer on titanium alloy and stainless steel matching surface, and studing the effect property on the base metal,the diffusion behaviour of the main atoms, microstructure and mechanical properties in the brazing joint when the Ni layer exists in the joint,the test analysis shows that:(1) In TC4titanium alloy and304stainless steel mating surface electrochemical plating Ni layer, by research the microstructure and surface composition show that: the Ni plating layer TC4titanium alloy and304stainless steel surfaces in the form of film still exists, only a few Ni and the elements of the base material atoms are forming Ni-Cr-Fe, and (Cr, Ni) of the aggregates, and when the thickness of the Ni layer exceeds a certain value, the film will be greatly reduced pore diameter; TC4titanium and Ni alloy plated layer contrast, hole diameter stainless steel surface Ni film is almost in the same number of values, and homogeneous.(2) Holes around the organization to do the scanning analysis showed, Fe can through the holes diffuse to the surface,but they can not through the density Ni layer to the surface, the size of the hole diameter is the main effect to limit Fe through Ni layer diffusion to other regions.(3) The test results show that, the Ni layer blocks the atoms diffusion of brazing joint. Moreover, the degree of obstruction are differences among Fe,Ti and other atoms. Further, this differential is in close relationship with physical location of Ni layer in the joint region, Ni-plated on stainless steel side can improved the organization and performance of the brazing joint, but it has the opposite effect when the Ni-plated on titanium side.(4) Ni-plated on stainless steel side, the hinder of Ni layer alters osmotic concentration of the atoms, slows the metallurgical reaction degree, and reduces the content of brittle intermetallic compounds.Consequently, the microstructure and mechanical properties of brazing joint can be effectively improved because of the existence of Ni layer.(5) Since Fe、Ti、Cr and Ni enrichment in the side of titanium alloy, formed FeTi+Cr2Ti+β-Ti eutectic, resulting in nearly the entire region a titanium alloy is the weakness area of the brazing joint.