Study On Microstructures And Properties Of Friction Stir Welding Joints Of 430 Ferritic Stainless Steel

Friction stir welding（FSW）is widely used in aluminum alloys as an efficient new solid-phase joining technology.Due to the welding process in the stirring head undergoing high-temperature dynamic fatigue and wear,limited by the material of the stirring head,there are few studies on high melting point stainless steel FSW.In contrast,the reports on ferritic stainless steel are even less common.This paper uses a tungsten-rhenium alloy with high wear and high-temperature resistance to manufacturing stirring tools.A 4 mm thick 430ferritic stainless steel used in a certain automobile is used as the object of study and connected by FSW technology.Systematic study of the thermal cycle,welding forming,microstructures and mechanical properties of welded joints;and by changing the welding process parameters on the impact of the microstructures and properties of the experimental analysis.Research on the FSW heat generation mechanism,heat source model,and distribution of the welding temperature field.The results of the study showed that each region of the FSW joint experienced different thermal cycles,with the peak temperature being higher and the cooling rate faster the closer to the center of the weld,and the peak temperature on the advancing side（AS）of the joint was higher than that on the retreating side（RS）.Under the process parameters of 600 r/min and 50 mm/min,the average difference between the peak temperature of each temperature measurement point on AS and RS is about 15℃.The joint is well-formed with no macroscopic defects.FSW joints can be mainly divided into base metal（BM）,stir zone（SZ）,thermo mechanical affected zone（TMAZ）,and heat affected zone（HAZ）.Metallographic analysis shows that BM is mainly a heterogeneous band structure,and a large amount of precipitated phase(Cr₂₃C₆)can be found.The closer to SZ,the greater the dissolution degree of Cr₂₃C₆.SZ generated fine and uniform equiaxed crystals with an average grain size of only 5.2μm under heat-mechanical coupling,and a small amount of Cr₂₃C₆ still existed inside the grains.The microhardness distribution curve of the joint cross-section has an overall parabolic pattern,with the lowest hardness value（153 HV）for AS-HAZ and the highest hardness of253 HV for SZ.The tensile strength of the joint is 504 MPa,which is about 89.7%of BM.The fracture location is between AS-HAZ,and the microscopic morphology of the fracture is mainly characterized by tough nests of uneven size and depth.The fractured form is a ductile fracture.The size and depth of the fracture dimples of SZ are more fine and uniform.As the stirring head rotation speed（500 r/min,600 r/min,700 r/min）increased,the peak thermal cycle temperature increased,the average grain size of SZ increased（3.4μm,5.2μm,7.4μm）,and the microhardness decreased（269 HV,253 HV,247 HV）.Poor formation of the beginning part of the joint surface at a speed of 500 r/min.When the speed reached 700r/min,hole defects appeared in the weld,and the joint fractured at the junction of AS-TMAZ and HAZ during the tensile test.The microscopic morphology of the fracture showed dimple and river-pattern characteristics.The fractured form was a mixed tough-brittle fracture.As the welding speed（40 mm/min,50 mm/min,60 mm/min）increases,the peak thermal cycle temperature decreases,the SZ grain size decreases（7.2μm,5.2μm,2.8μm）,and the microhardness increases（249 HV,253 HV,272 HV）.At a welding speed of 40 mm/min,the joint fractured between AS-TMAZ and HAZ with low tensile strength and elongation,and the fracture form was still a mixed tough-brittle fracture.BM with each welding parameter obtained under the joint SZ corrosion resistance from high to low:500-50（rotation speed-welding speed）>600-50>600-40>700-50>BM>600-60.Typical pitting characteristics appeared on the surface of the specimens after corrosion.The diameter of BM pitting pits was about 13.64μm,much larger than the diameter of SZ pitting pits（5.89μm）at the welding parameters（500-50）.Based on the test above,the FSW heat production model was established,and the temperature field was simulated and analyzed by finite element software.The results showed that the temperature field distribution was gradually dispersive semi-elliptical,with the overall trend of small in the front and large in the back,and the degree of dispersion of the temperature field increased with the peak temperature increased.The effect of the change of welding speed on the peak temperature was low,and the simulation results were in good agreement with the actual measurement results.