Study On Laser Welding Process Of Thin-Walled Stainless-Steel Natural-Gas Injector For Vehicles

In recent years,natural gas has been widely used in commercial vehicles because of its advantages such as environmental protection,adequate supply,safety and reliability.The Natural-gas injector is the core component of the natural gas engine,in which the valve tube（303Cu austenitic stainless steel）and the valve seat（440C-Nb martensitic stainless steel）are combined in a lapping manner and need to be joined by welding technology.The mechanical and sealing properties after welding are related to the reliability and safety of Natural-gas injectors.Therefore,studying the welding process and achieving good welding performance for practical applications is essential.The natural-gas injector is taken as the study case where its valve pipe and valve seat are laser welded.The laser lap welding process study and numerical simulation results of thin-walled stainless steel materials and the welding mechanism of 303Cu/440C-Nb dissimilar stainless steel materials were presented.The methods of laser preheating and post-heat treatment to improve the welding performance were studied,and the welded joints with good mechanical and sealing properties were obtained.The effect of the process parameters on the weld morphology was studied through single-factor tests.The results showed that the melt depth and width trend is positively proportional to the increase of laser power and inversely proportional to the growth of welding speed.With the amount of defocusing from negative to positive,the top melt width first decreases and then increases,and the melt depth and the melt width at the material connection increase and then decrease.The influence of the process parameters on the welding properties was studied through orthogonal tests,and the results showed that the laser power had the most significant impact on the welding properties.Low power,medium welding speed,and positive defocusing can improve welded joints’overall performance.The optimized process parameter combination is laser power 215W,welding speed 144°/s,and defocusing amount+1.5 mm.The microstructure analysis of the welded joint showed that the top of the weld is mainly columnar crystal,the center of the weld is equiaxed crystal,and the uneven fusion zone appears at the bottom of the weld.The migration of Nb and Mo from the base metal of 440C-Nb to the weld metal results in the formation of lamellar eutectic carbides at the bottom of the weld metal.The central hardness of the weld on the 303Cu side（181.8 HV）is less than that on the 440C-Nb side（266 HV）.The welded joint properties tests showed that the maximum press-off force is 9640 N,and the leakage rate is 3.34×10^-4.A composite heat source composed of a disk surface heat source and a conical heat source was established,and the simulation results were in good agreement with the experimental results.The temperature field analysis results showed a large temperature gradient within the materials at the start of the welding process and cooling process.The analysis of the microstructure evolution process combined with the equilibrium phase diagram showed that the weld metal solidifies with austenite as the initial phase,which easily forms M₇C₃ and some M₂₃C₆ carbides on the bottom 440C-Nb side.The results of the stress field analysis showed that the equivalent stress is mainly generated during the rapid cooling process,to a maximum of 707 Mpa.The residual equivalent stress is mainly concentrated at the joint of two materials in the welded joint.Numerical simulation and experimental analysis were carried out to study the improvement methods of laser preheating process with increasing power and laser post-heat treatment process with decreasing power.The effects of different treatment processes on temperature distribution,stress variation,microstructure and welding properties of welded joints were analyzed.The numerical simulation results showed that the preheating and post-heat treatment process could avoid the sudden increase in material temperature at the beginning of the welding process and the rapid decrease in temperature at the cooling process,and the stress concentration phenomenon at the material connection has been significantly improved.The microstructure analysis showed that the preheating and post-heat treatment could obtain the best weld joint morphology.This method can improve the uneven fusion and crack trend in welded joints,making the microstructure distribution at material joints more uniform.The pressure-off and helium leakage test results showed that the preheating and post-heat treatment could effectively improve the welded joint’s mechanical and sealing properties.Compared to the case without the treatment process,the maximum value of the press-off force increased from 9640 N to 10566 N,and the helium leakage rate decreased from3.34×10^-4 to 1.02×10^-6.