Study On Laser Lap Welding Of Titanium Alloy And Stainless Steel

Titanium alloy（Ti alloy）and stainless steel（SS）are widely used in industrial field.By adjusting the location and amount,the composite structure of titanium alloy and stainless steel can easily meet the design requirements and performance requirements.Welding,as a reliable and low-cost technology,is commonly used to make composite structures of titanium alloy and stainless steel.However,brittle Ti-Fe intermetallics and post-weld residual stress seriously deteriorate the mechanical properties of the joint.Therefore,it has important research value to solve the welding problem of titanium alloy and stainless steel and obtain a high-strength Ti-SS joint.In this paper,the finite element software was used to simulate the temperature field,stress field and deformation of the joint with Ni interlayer or Cu interlayer.According to the temperature field cloud map and thermal cycle curve,the temperature distribution characteristics of the joint and the temperature change history of each interface were studied.Under the condition that the SS plate was not fully penetrated,the interface between the interlayer and the base metal can reach the eutectic reaction temperature by adjusting the welding process parameters.The interface temperature gradient of the joint with Cu interlayer was greater than that of the joint with Ni interlayer.With the increase of the distance from the weld centerline,the residual stress first increased and then decreased.The longitudinal residual stress was significantly larger than the transverse residual stress.The post-weld residual stress at the Cu interlayer was significantly lower than that at the Ni interlayer.Welding deformation was mainly concentrated near the weld.With the increase of laser power,the welding deformation tended to increased.When Ni interlayer was added,the effects of interlayer thickness,laser power and welding speed on the micro-structure and mechanical properties of the joint were studied by combining the temperature field and stress field.With the decrease of the thickness of the interlayer,the heat input corresponding to the good joint decreased and the process window decreased.With the decrease of the thickness of the interlayer,the maximum tensile shear force of the joint tended to increased.With the laser power increase,the connection form of the joint changed.When the laser power was 710W and 740W,the SS board did not penetrate.The SS/Ni interface realized the diffusion connection,and the Ni/TC4 interface realized the eutectic connection.When the laser power was 770W,the SS board also did not penetrate.A small amount of SS and all the Ni interlayer were dissolved in the eutectic liquid phase,and the two base metals and the interlayer were connected after cooling and solidification.When the laser power was 810W,the SS-Ni-TC4 realized the fusion connection.The phase composition of the reaction zone also varied with the laser power.When the laser power was 710W,the reaction zone was mainly composed of（β-Ti+Ti₂Ni）eutectic structure.When the laser power was 740W,in addition to the eutectic phase,TiNi phase was also generated.When the laser power was 770W and 810W,the reaction zone had both Ti-Ni phase and Ti-Fe phase.The reaction zone was the position with the highest hardness in the whole joint.The types of phases in the reaction zone were different,and the hardness values were also different.When the laser power was 740,the maximum tensile shear force of the joint was 2698 N（the shear strength was269.8 MPa）.When the laser power was 710W,the joint was fractured at the SS/Ni interface.At other powers,the joints were broken in the reaction zone.With the welding speed decrease,the thickness of the reaction zone gradually increased.The structure in the reaction zone would change with the change of welding speed.When Cu interlayer was added,the effects of interlayer thickness laser power and welding speed on the micro-structure and mechanical properties of the joint were studied by combining the temperature field and stress field.The maximum shear strength of the joint increased gradually with the decrease of the thickness of the interlayer.Different laser powers had different connection forms of connectors.When the laser power was 770W,the SS board was not penetrated.Solid phase bonding based on diffusion was realized at SS/Cu interface,and eutectic bonding was realized at Cu/TC4 interface.When the laser power was 800W,the Cu interlayer was completely dissolved in the eutectic liquid phase.The SS/TC4 realized diffusion bonding based on eutectic connection.When the laser power was 830W,a small amount of SS dissolved in the eutectic liquid phase,and the two base metals and the interlayer were connected after cooling and solidification.When the laser power was880W,the SS-Ni-TC4 achieved fusion bonding.The phase composition of the reaction zone also varied with the laser power.When the laser power was 770W and800W,the reaction layer was mainly composed ofα-Cu,Cu₄Ti and（CuTi₂+CuTi）eutectic structure.When the laser power was 830W,the reaction layer was mainly composed of Fe Ti,CuTi₂ and CuTi.When the laser power was 880W,Cu cannot preferentially react with Ti,resulting in the reaction zone mainly composed of Ti-Fe intermetallic compounds.Theα-Cu was mainly distributed at the boundary of theγ-Fe.The reaction zone was the location of the highest stiffness in the entire joint.The types of phases in the reaction zone were different,and the hardness values were also different.When the laser power was 800W,the maximum tensile shear force of the joint was 3670 N（the shear strength was 367 MPa）.When the laser power was 770W,the joint was fractured at the SS/Cu interface.At other powers,the joints were broken in the reaction zone.With the welding speed decrease,the thickness of the reaction zone gradually increased.The structure in the reaction zone would change with the change of welding speed.