Study On Joining Mechanism And Corrosion Behavior Of Titanium/Steel Cold Metal Transfer Welded Joints

Titanium/steel dissimilar metal welds are widely used for the seawater heat exchangers,the joint between the turbojet compressor and the combustion chamber.However,a large residual stress is formed in the welded joint due to the major difference in physical properties between titanium and steel,and brittle Ti-Fe intermetallics are easily formed due to the reaction between titanium and iron,which seriously deteriorates the mechanical properties of the titanium/steel joint.In addition,a large potential difference is formed due to the formation of intermetallics in the joint,which severely affects the corrosion resistance of the t itanium/steel joint.Therefore,the affecting mechanism of microstructure on mechanical properties and corrosion properties of titanium/steel dissimilar metal welded joints with different filler metals has important theoretical significance and practical value in this paper.Firstly,Fe-based filler metal was used to weld pure titanium and galvanized steel,and the influence mechanism of the microstructure on mechanical properties of pure titanium/iron/galvanized steel joint was investigated.The results showed that the zinc coating evaporates under the action of arc,and the rest of zinc coating does not participate in the reaction between Ti and Fe,the formation of the brittle Ti-Fe intermetallics makes the joint crack under the action of large welding residual stress,which leads to poor plasticity of the joint,and seriously deteriorates the comprehensive mechanical properties of the pure titanium/iron/galvanized steel joint.In order to realize the effective joining between pure titanium and galvanized steel,the wetting behavior and interface characteristics of the Cu-based filler metal on the base metals surface were investigated.The results showed that the Cu-based filler metal has good wettability on the surface of galvanized steel,stainless steel,pure titanium and titanium alloy base metals.The wettability becomes better with the increase of heat input,and all of them belong to typical temperature-dependent wetting system.Because the mixture enthalpy of Cu-Ti system is negative than that of Cu-Fe system,and an obvious interfacial layer is formed due to the reaction between Cu and Ti,the wettability of Cu-base filler metal on the surface of pure titanium and titanium alloy is better than that on th e surface of galvanized steel and stainless steel.However,the interfaial temperature decreases because the heat was taken away by the volatilization of zinc on surface of the galvanized steel,which results in the wettability of the Cu-based filler metal on the galvanized steel surface is worse than that on the surface of stainless steel.Based on the good wettability of Cu-based filler metal on the surface of both pure titanium and galvanized steel,the Cu-based filler metal was used to weld the pure titanium and galvanized steel,the influence mechanism of different lapped sequence of base metals on the microstructure and properties of pure titanium/copper/galvanized steel joints was studied.The results showed that a large amount of Ti-Cu intermetallics are formed at the pure titanium/copper weld metal interface of the lapped joint with top pure titanium to bottom galvanized steel,the tension-shear strength of the joint reaches 280 N/mm.However,for the lapped joint with top galvanized steel to bottom pure titanium,due to the dispersion strengthening effect of Ti Fe2 phase close to the side of pure titanium base metal at the galvanized steel/copper weld metal interface,the tension-shear strength of the joint increases to325 N/mm.The corrosion mechanisms of two joints with different lapped sequence in artificial seawater solution are the reduction reaction of oxygen and hydrogen evolution reaction at the cathode,uniform corrosion of galvanized steel,the oxidation of weld metal and the formation of Ti O2 oxide film on the surface of pure titanium at the anode.The Cu-based filler metal was used to weld titanium alloy and stainless steel.It is found that the corrosion resistance of titanium alloy/copper/stainless steel joints is obviously improved.The corrosion mechanism of titanium alloy/copper/stainless steel joint is similar to that of pure titanium/copper/galvanized steel joints.In the titanium alloy/copper/stainless steel joint,galvanic corrosion occurs between Cu-Al-Fe-Ni-Ti intermetallics and copper matrix phase in the weld metal.With the increase of welding heat input,the intermetallics in the weld metal grow,the corrosion potential at the center of the weld metal area slightly decreases,and the corrosion resistance of the joint becomes worse.Due to the formation of a large number of Ti-Cu intermetallics and brittle Ti-Fe intermetallics at the titanium alloy/copper weld metal interface,the tension-shear strength of titanium alloy/copper/stainless steel joint is reduced to 315 N/mm.In order to further improve the strength and corrosion resistance of the titanium alloy/stainless steel joint,the Cu-Ni composite filler metal was used to weld titanium alloy and stainless steel.It is found that Ti-Ni intermetallics with relatively low microhardness can be formed by the reaction between the molten pure nickel intermediate layer and the titanium alloy base metal under the high welding heat input,which reduces the formation of Ti-Cu intermetallics,and makes the tension-shear strength of the titanium alloy/copper-nickel/stainless steel joint increase to 348 N/mm.The corrosion mechanism of titanium alloy/copper-nickel/stainless steel joint is similar to that of titanium alloy/copper/stainless steel joint.In the titanium alloy/copper-nickel/stainless steel joint,no obvious corrosion occurred on the Ti-Ni reaction layer close to the side of nickel at the stainless steel/copper-nickel weld metal interface and the Ti-Cu reaction layer close to the side of titanium alloy.The grain boundary corrosion occurred on the Al-Ni-Ti-Fe-Cu intermetallics in the weld.