Microstructure And Fatigue Properties Of Friction Stir Welded Joint Of Quenching And Partitioning Steel

In recent years,environmental protection and sustainable development have become the consensus of people all over the world.Vehicle lightweight is to reduce the weight of the vehicle as much as possible on the basis of ensuring the strength and safety of the vehicle,so as to improve the power performance of the vehicle,reduce energy consumption and reduce pollution.Quenching and Partitioning(Q&P)steel is considered as the development direction of lightweight automobile steel because of its good strength and plasticity matching,excellent impact energy absorption capacity and formability.Welding is one of the important processes in automobile manufacturing,which plays an irreplaceable role in the manufacturing of automobile body and parts.With the development of automobile industry,there are many problems in traditional welding methods,such as air hole defect,element burning,hot crack and structure segregation.Therefore,it is necessary to find a new welding method to meet the requirements of miniaturization and lightweight.Friction stir welding(FSW)is a new type of solid-phase welding technology with high efficiency,energy saving and environmental protection.It has the advantages of low welding temperature,which can effectively avoid the solidification structure defects of fusion welding.At the same time,it can also promote the refinement,homogenization and densification of the structure,so as to ensure that the joint has good structure and properties.In this paper,Q&P steel was welded by FSW.The microstructure of FSW joint of Q&P steel was characterized by optical microscopy,X-ray diffraction,scanning electron microscopy and electron backscatter diffraction.The microhardness,tensile properties and fatigue properties of the welded joint were tested according to the microstructure characteristics The effect of FSW on the tensile properties and the fatigue properties of the composites are also discussed.The main conclusions are as follows:At a welding speed of 100 mm/min,comparing the microstructure and performance of 400 rpm and 600 rpm shows that: a relatively low speed is more conducive to the retention of retained austenite and the refinement of grains,and when the speed is 400 rpm The elongation of the joint is the best,reaching 10%.In addition,the microstructures of the high-speed joints SZ and TMAZ have relatively high martensitic structures and show higher microhardness values;the joints SZ and TMAZ have a higher degree of grain refinement under low-speed conditions,and the joints The whole has a higher retained austenite content.HAZ is affected by heat and has a softening phenomenon,which becomes a weak area of mechanical properties of the joint.The higher the speed,the more obvious the softening phenomenon.At a welding speed of 400 rpm,a comparison of the microstructure and properties of welding speeds of 50 mm/min and 600 mm/min shows that: compared with low welding speeds,the content of retained austenite in high welding speed joints is more,which further explains the heat input It has a great influence on the microstructure of the welded joint,and when the welding speed is increased to 600 mm/min,the elongation rate reaches 15.3%,and a welded joint with strong plasticity is obtained.In addition,carbide particles are generated during the decomposition of HAZ retained austenite.The formation of a small amount of carbide particles has an effect similar to the strengthening of the second phase on the joint strength and improves the mechanical properties of the joint.The fatigue experiments of FSW welded joints show that the welded joints have similar fatigue limits compared with Q&P steel base materials,both of which are 560 MPa.Observation by scanning electron microscopy shows that fatigue cracks of Q&P steel originate on the surface of the sample,the crack propagation zone is brittle fracture,and the instantaneous fracture zone is ductile fracture.Under cyclic loading,the stability of retained austenite has a significant impact on the fatigue performance of the material,and the retained austenite is more prone to phase transformation under high stress conditions,which further reduces the fatigue performance of the material.In addition,through fracture analysis,it can be found that intergranular cracks are the main mechanism of fatigue crack initiation in fatigue tests.