| Subassemblies, which are linked by stainless steel and titanium alloy, have both materials’ excellent mechanical properties, and save rare resources. Such subassemblies complement each other’s advantages of base metals in behavioral and economic aspects,which make them have extensive application prospects in petrochemical industry, aerospace and transportation. But it is very easy to produce hard and brittle Fe-Ti intermetallic compounds because of metallurgical incompatibility between Fe and Ti. There is great residual internal stress produced in the welding process because of the difference of physical properties, which harms the mechanical properties of joints. Choosing Cr, V as the elements of powder intervention layers, which were evenly put on the surfaces of base metal with specific qualities. Exploring a resistance welding method of TC4 and 304 SS with powder layers’ intervention. The metallurgical reactions were researched with the changes of powder intervention methods, technology parameters, additive amount of powders. The influencing mechanism of Cr and V powder layers intervention on the micro-structure and mechanical property in the resistance brazing joints of titanium alloy and stainless steel. The preliminary conclusions are as follows:(1) Four kinds of joints were designed according to the different methods of powder intervention. The joints without the powder layer intervening was named as I joints. The joints,in which V or Cr powder layer was intervened on the surface of titanium alloy or stainless steel, were named II or III joints. The joints with Cr and V powders intervening meanwhile on the both sides were named as IV joints. According to the results of shear strength tests and microscopic analysis, such as SEM and XRD, Cr,V powders layers intervention make the number of Fe-Ti IMCs in the brazing structure decreased greatly, and the shear strength of II,III, IV joints is clearly higher than I joints. But clusters are not dissociated completely because of high melting point of V. Expect the part of V powder participate in the formation of Ti-V diffusion reaction layer on the surface of titanium alloy and some σ-FeV brittle phases on the surface of stainless steel, the most part of V is still inlaied in the joints as the shapes of granular clusters. Because the gaps between clusters are a bit larger, the hindering effect to Ti atoms is poor, which makes the number of Cu-Ti, Cr-Ti, etc IMCs in III joints more than II joints with Cr powder layers intervention, and the shear strength of III joints obviously lower than II joints. In the IV joints, Cr-Fe and Ti-V diffusion reaction layers are severally formed on the sides of titanium alloy and stainless steel. The production of Ti base and σ-FeV brittle phases is far lower than II, III joints, so the improvement to the shear strength of joints is more prominent when Cr, V powder layers intervene simultaneously. But in the structures of II, III, IV joints, the residually granular or lath-like Cr, V powder clusters coarsen thestructures, destroy the uniformity of weld structures, limit the further improvement of joints’ mechanical properties.(2) The technology parameters of resistance brazing mainly include welding current,welding time, tip pressure, thickness of brazing filler, tip section dimension, and spreading area of filler. The influence rules of 3 major technology parameters, including welding current,welding time and tip pressure, to structures and properties of joints of titanium alloy and stainless steel were mainly investigated. By analyzing the structures of joints in the cases of changing only 1 parameter of IV joints, finding that structures of joints couldn’t be improved when parameters are too large or too small. When the parameters are too small, the atoms’ ability of diffusion is poor, and the effective Cr-Fe, Ti-V diffusion reaction layers couldn’t be formed on the near-surface of base metals. The welding seams are mixed structures consisting Ag45 CuZn filler metal and residual Cr, V powder clusters. When the parameters are too large,the diffusion of atoms in IV joints is drastic. In the diffusion reaction layers, Cu-Ti, Cr-Ti brittle phases also exist other than Cr-Fe and Ti-V structures. When parameters are chosen rightly, Cr, V atoms from powder intervention layers diffuse and form Cr-Fe and Ti-V diffusion reaction layers obviously on the near-surfaces of base metals. The Fe-Ti, Cu-Ti,Cr-Ti, ect IMCs nearly disappear. Through the mechanics performance testing of joints with different technology parameters, the results show that the shear strength of IV joints could relatively higher be 186.92 MPa when the welding current is 3.5kA, the welding time is 2.0s,the tip pressure is 0.4MPa.(3) The study indicates that the powder layers with the different adding amount have remarkable effect on the diffusion of atoms from base metal. When the adding amount is small(like m=0.01g), the migration distance of powder clusters increases and the atomic concentration of Cr, V added elements decreases. The gaps between clusters enlarge, and the diffusion roads of Ti, Fe atoms increase, which lead diffusion reaction layers on the near-surfaces mainly are made up of Cu-Ti, Cr-Ti brittle phases, and become the weak links of the joints. When the adding amount is large(like m=0.04g), the formation of Ti-V, Cr-Fe diffusion reaction layers on the near-surfaces of base metal could reduce the production of Ti base brittle phases. But the diffusion ability of added Cr, V powder is limited, and there are a lot of residual clusters without participating in the welding process. So the uniformity of weld micro-structure is destroyed, the quality of joints is deteriorated. The suitable adding amount of powder layers is suitable couldn’t only ensure diffusion of atoms, but the reaction of Cr, V atoms and Fe, Ti atoms could also be adequate, and there are new formations of Cr-Fe, Ti-V structures on the near-surface of base metal, which reduces the production of Ti base brittle phases obviously. At the same time, the number of residual cluster in the joints is small, which is beneficial to the improvement of joints’ structures, and obtain the joints with relatively higher mechanical properties. When the adding amount of Cr and V powder is 0.02 g and0.03 g respectively, the shear strength of joint can be 204 MPa. The result of contrast experiment of 400 and 2000 mesh shows that the powder doesn’t dissociate completely, the metallurgical reaction within powder is insufficiency, and the residual clusters destroy the homogeneity of weld metal structure when the mesh number is too large. When the mesh number is too small, the phases produced by powder free in the space among other phases,which cannot generate clear reaction layers to hinder brittle phases’ production. Those powder with two mesh number doesn’t work with the obvious improvement of joints’ mechanical properties. So the mesh number’s influence to the joints’ structures and properties need be further studied. |
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