Study On Microstructure And Residual Stress Regulation Of AISI 304 Steel-Niobium Laser Joint

High-quality and high efficiency steel-niobium welding technology is a key process in the manufacturing process of aircraft engine combustion chambers.However,due to the large differences in metallurgy and thermophysical properties of these two materials,there are intermetallic compounds and large residual stress at the welded joints,which seriously affects the quality of the welded joint.Therefore,this subject studies the microstructure and stress regulation of AISI 304 steel-Nb laser welding joint.The weldability of stainless steel and niobium is very poor.After laser welding,the joint cracked directly.Through fracture analysis,the continuous distribution of Fe₂Nb and Fe₇Nb₆ intermetallic compounds is the main reason for the joint fracture.Therefore,in order to reduce the formation of Fe-Nb brittle compounds in the weld,the energy control method is used to shift the laser beam by 0.5mm to the steel side,and a well-formed joint can be obtained.The tensile strength of the obtained welded joint is221MPa,and it breaks at the IMC layer.Because of the hardness and brittleness of the IMC,the microhardness in the horizontal direction of the weld cross section is the highest,reaching 1143.22HV.Through the orthogonal test method,it is analyzed that the laser beam offset has the largest influence on the joint performance,followed by the welding speed,and the laser power has the smallest influence.Therefore,in order to improve the performance of the welded joint,a welding method with a low welding speed and a beam deviation to the steel side can be used.In addition to energy control,the metallurgical control introduced into the intermediate layer also has a significant effect on the joint structure and performance.Preset six materials of Ag,Cu,V,Mo,Ta,and W as the intermediate layer for laser welding.The three materials of V,Mo and Ta will form an intermetallic compound with Fe,causing the joint to break;W due to the difference between the melting point and the base material is too large,the joint fusion is poor,the joint fracture after welding;Ag and Cu can be used as intermediate layer materials to obtain well-formed joints.Among them,the strength of the Cu intermediate layer joint is relatively high,which is 250MPa,indicating that the Cu intermediate layer welding effect is the best.In order to further improve the strength of the joint,a two-pass welding process was used to study the joint structure and performance of Cu intermediate layers with different thicknesses.The research proves that the thickness of the Cu intermediate layer is the key to obtain a good weld.When the thickness of the Cu intermediate layer is 1 mm,the welding sample fractures in the weld,and the brittle phase Fe₇Nb₆ in the weld is the main factor affecting the joint performance.When the thickness of the Cu intermediate layer is 1.5 mm,the welding sample is broken on the Nb side of the base material,and no Fe—Nb intermetallic compound is detected in the weld.The solid solution strengthening of Fe to Cu,the mixing of Nb-rich particles and dendrites in the Cu matrix makes the welded joint strengthened,and the tensile strength is higher than that of the base material Nb.When the thickness of the Cu intermediate layer is 2 mm,the welding sample is also broken in the weld,and the unmelted Cu intermediate layer reduces the joint strength.The temperature field and stress field of AISI 304 steel-Nb partial steel welding and the preset1.5mm thick Cu intermediate layer are calculated.The results show that after the intermediate layer is preset,the temperature field distribution characteristics are basically unchanged,but the peak temperature of the molten pool decreases,The temperature gradient on both sides is reduced.The distribution characteristics of the stress field are basically unchanged too,the longitudinal residual stress peak distribution is on the stainless steel side,and the transverse residual stress peak distribution is on the Nb side,but the joint residual stress is higher than that of direct partial steel welding.The increase in joint strength indicates that the key to improving the joint performance of the Cu intermediate layer is the optimization of the weld microstructure,and the stress state of the joint is secondary.