Study On Microstructure And Properties Of Ultrasonic-induced Welded Joints Of 441 Ferritic Stainless Steel

With the successful application of refining technologies such as AOD,VOD and SS-VOD in the smelting process of stainless steel,the grade system of modern ferritic stainless steel is increasingly perfect,meeting the needs of automobile manufacturing,railway vehicles and stainless steel products and other industries.Ferritic stainless steel is second only to austenitic stainless steel in annual output,gradually replacing the application of austenitic stainless steel in some fields.These include automotive exhaust systems,where 400-series ferritic stainless steels replace austenitic stainless steels that are prone to corrosion cracking.However,ferritic stainless steel has poor weldability,coarse grains and severely insufficient toughness of the weld structure,which cannot be effectively solved based on the optimization of the welding process.Therefore,it is urgent to develop a welding method that is not limited by material composition,content and welding process in order to improve the weldability of ferritic stainless steel.Therefore,in this study,it is carried out that the microstructure and properties of ultrasonic-induced welded joints of 441 ferritic stainless steel by means of the refining effect of ultrasonic waves on the solidification of the molten pool,in order to solve the problem of insufficient toughness in the forming process of ferritic stainless steel after welding.In this paper,a self-built ultrasonic-assisted welding test platform is used to apply low-power ultrasonic vibration to the base metal in the form of point-to-surface contact.With the excitation of a small ultrasonic power,a standing-wave field is formed in the base metal,and the strong local area vibration of the base metal is realized,rather than relying on a large-power ultrasonic generator to force the entire base metal to vibrate strongly.According to the requirements of fixed-point welding and continuous welding,a welding torch-arc joint control system is designed to precisely control the welding process.It is equipped with a vibration fluctuation measurement device to monitor the vibration state at the location of the molten pool.A COMSOL solution calculation model is established to simulate the propagation mode of ultrasonic waves and the distribution of ultrasonic energy field in the base metal.The research shows that the ultrasonic vibration is centered on a point-shaped sound source and propagates in the base metal in the form of an antisymmetric Lamb wave,forming a stable standing wave field in the base metal.The wave front is spherical in propagation of ultrasonic.The vibration of the base metal shows the characteristics of standing wave fields such as nodal lines and antinodes in the time domain,forming the vibration attenuation areas and enhancement areas,revealing the general influence of ultrasonic input parameters on the distribution of ultrasonic energy field.The ultrasonic input amplitude will not affect the distribution state of the ultrasonic energy field,but it will change the intensity of the particle vibration in the base metal.The larger the amplitude,the greater the vibration intensity.The position of excitation point affects the distribution state of ultrasonic energy field and changes the position of attenuation zone and enhancement zone which is beneficial to select the appropriate excitation point position according to the weld seam trajectory.By measuring the ultrasonic vibration distribution state and wave force,it is found that the measurement results are consistent with the simulation results,which verifies the reliability and accuracy of the simulation results.Under the condition of fixed-point remelting welding,the running speed of the welding torch was set to 0,and the changes of the microstructure of the weld were observed under the action of ultrasonic waves.The results show that the coarse columnar grains are transformed into fine equiaxed grains under the action of ultrasound,and the grain size is significantly improved.During the columnar crystal transformation,cliff refinement occurs in the weld region close to the fusion zone.At the same time,it is found that when the position of the molten pool is in the enhanced region(antinode),the refinement trend of the weld structure is larger than that in the attenuation region(nodal line).When the ultrasonic input amplitude is 10.0?m and the action time is 3s,the grain refinement effect of the weld at the antinode is the best.The main reason for the grain refinement induced by ultrasound is discussed,which is that the ultrasonic cavitation effect increases the degree of supercooling during the solidification of the molten pool and reduces the heterogeneous nucleation work based on the second phase particles.Under the action,the nucleation rate in the solidification and crystallization process is improved,and the effect of grain refinement is achieved.Under the condition of continuous remelting welding,the transformation of microstructure of weld metal from columnar grains to equiaxed grains is realized.On the basis that the structure determines the performance,the microhardness of the weld area has been greatly improved.The tensile test results show that the tensile strength of the weld area in the ultrasonic-induced welding joint is improved,and it exhibits better mechanical properties than the base metal in the process of plastic deformation,and is no longer a weak link in the subsequent forming process.By strengthening the weak link in the weld area,the weldability of 441 ferritic stainless steel is improved,and the toughness reserve of 441 ferritic stainless steel weldment is improved.