| Nickel-titanium shape memory alloy(NiTi SMA)has good biocompatibility and shape memory effect,it is widely used in medical,aerospace and other fields.In the connection between NiTi SMA and 304 stainless steel(304 SS),Fe-Ti intermetallic compounds are easily generated in the joint,which leads to the mechanical energy of the joint can’t meet the engineering requirements.High entropy alloy(HEA)has attracted researchers’ attention for its excellent microstructure.Using the "four effects" of HEA is helpful to obtain the joint with good quality,and the joint with simple structure and good mechanical properties can be obtained.In this paper,the practical problems existing in NiTi SMA/304 SS laser self-fusion lap welding joints are studied and analyzed based on first-principles simulation calculation.To solve problems,HEA filling materials were designed with semi-empirical parameter method,and the microstructure and mechanical properties of joints under different filling material composition and laser welding process as well as the regulation behavior of HEA on weld microstructure were studied.The main results are as follows:(1)A large number of cracks and uniformly distributed IMCs such as FeTi,Fe2Ti and Ni4Ti3 are produced in the joint of NiTi SMA/304 SS.The average hardness of the joint reaches 900 HV,the peak load is 865.2 N,and the fracture mode is cleavage fracture.The mechanical,electronic and thermodynamic properties of the compounds were studied by using the first principles.The results show that FeTi and Fe2Ti are brittle and rigid IMCs,which is the main reason for the decrease of mechanical energy of the joint.(2)Considering the compatibility of elements and base metal,the powder composition of HEA was designed by semi empirical parameter method.By evaluating electron concentration,mixing enthalpy,Gibbs free energy and other parameters,HEA filled powders were designed,which have simple FCC structure:CoCrNi,CoCrFeNi,CoCrFeNiMn.(3)The influence of Ni,CoCrNi,CoCrFeNi and CoCrFeNiMn on the weld microstructure and mechanical properties of the joint was studied when the laser power was 1500 W.The results show that the filling powder can effectively refine the microstructure of the joint.Ni powder reduces the generation of brittle and hard FeTi and Fe2Ti in the joint.The width of the transition zone of the joint is 4 mm,the average microhardness is 802 HV,and the peak load is 2058 N.HEA powder improves the distribution of IMCs and the mechanical properties of the joint.CoCrNi and CoCrFeNi not only avoid the Fe2Ti generation in the joint,but also promote the formation of(Fe,Ni)solid solution.The transition zone of HEA powders is 0.83 mm,which is 79%smaller than that of Ni powder.The hardness of the transition zone of CoCrNi,CoCrFeNi and CoCrFeNiMn powder joints are 637.8 HV,965.6 HV and 783.9 HV,respectively.The average hardness of the low hardness zone is 274.2 HV,264.8 HV and 397 HV,and the average peak load is 2434.0 N,2132.4 N and 1652.6 N,respectively.The average peak load of CoCrNi powder joint is the highest,which is about 2.8 times of that of self-fusion welding joint.(4)The effects of laser self-fusion welding and Ni,CoCrNi,CoCrFeNi and CoCrFeNiMn filling powders on weld microstructure and mechanical properties were studied when laser power was 1500 W,1600 W and 1700 W.The results show that the CoCrNi powder joint has the highest peak load under the three laser powers,and the self-fusion welding joint has the lowest peak load.The results of microhardness of CoCrNi powder joint show that the width of transition zone increases gradually with the increase of laser power. |
