Research On Friction Stir Welding Of DH590 High Strength Steel For Automobile

Friction stir spot welding process is a relatively new solid phase connection technology that has proven to be a cost effective connection process.The new demand for high strength and high melting point materials such as high strength steel for welding technology has promoted the development of high strength steel Friction stir spot welding and Friction stir welding research.This article takes DH590 dual phase steel as the test object,and adopts FSSW and FSW to conduct the welding test.In the Friction stir spot welding test,through the change of process parameters,the method of improving the strength of FSSW joints and the evolution of microstructure were studied.At the same time,the formation and elimination of "Hook" connection interface of DH590 high strength steel during FSSW are analyzed.And analyze the welding sample fracture mode and cause.In the FSW welding test,the microstructure,mechanical properties and electrochemical corrosion resistance of the air cooled FSW joint and the water cooled FSW joint were compared and studied by changing the welding conditions for the softening of the heat affected zone caused by martensite tempering.In order to explore effective methods to suppress the softening of the joint heat affected zone and improve the mechanical properties of the joint.The main conclusions are as follows:In the FSSW test of DH590 high strength steel,a 1.2 mm thick steel plate was welded using a PCBN ceramic mixing tool,and a spot welded joint with no internal defects and thermal deformation and good welding quality was obtained.The stir zone is composed of lath martensite,primary ferrite and part of acicular ferrite.The increase in the speed of the welding rotation rate promotes the increase of the welding temperature and causes the grain coarsening.In the case where the amount of depression is constant and the rotation speeds are 600 rpm,800 rpm,and 1000 rpm,respectively,the average grain size of the stir zone is 8.2?m,8.8?m,and 10.0?m,which are higher than the base grain size of 6.7?m material.The Thermo mechanically affected zone of the joint is mainly composed of grain refined ferrite and martensite.The hardness test results show that there is softening in the heat affected zone of the joint.Because the martensite phase in the dual phase steel was a metastable phase,it is easily transformed by the influence of heat input,and the formation of tempered martensite causes the local area hardness to decrease,forming a soften zone.The rotation speed of the welding rotating and the amount of penetration depth the rotating tool are important parameters for improving the strength of the DH590 high-strength steel FSSW joint.With the increase of the penetration depth and rotation speed of the rotating tool,the input of welding heat increases,the flowability of the plasticized material increases,resulting in an increase in the bonded width of the rotating tool and improved mechanical properties.when the penetration depth of the rotating tool is 0.1mm or 0.2mm,a "Hook" is formed in the spot welding joint,and the joint fails in the interface fracture mode.When the penetration depth of the rotating tool is 0.3mm,the "Hook" interface disappeared to form a linear connection interface,and the joint breaks and fails in the plug failure mode.Among them,when the rotating speed is 800 rpm and the penetration depth is 0.3 mm,the mechanical property of the FSSW joint is optimally 15.56 KN.This is because the increase in the amount of depression can effectively increase the full width of the joint.The crack source is transferred from the "Hook" connection interface in the stir zone to the area outside the Stir Zone,which greatly improves the mechanical properties of the joint.In the case of 500 rpm for the rotation rate,the moving speed was 50mm/min DH590 high strength steel FSW joints transverse tensile test results showed that,under good welding conditions,air cooling and the water cooling joint FSW joint tensile strength of 564 MPa and 581 MPa,respectively,the joint efficiency up to 0.93 and 0.96.The elongation decreased from 33.5% of the base material to 8.15% and 16.46%,respectively.At the same time,the failure locations of FSW joints are all located in the base metal outside the weld.The refinement of the grains in the stir zone and the formation of martensite are the main reasons for the increase in the tensile strength of the joint.Among them,the grain size of the stirring zone of the air cooled FSW joint and the water cooled FSW joint are 3.57?m and 1.55?m,respectively.The air cooled FSW joint stir zone is mainly composed of lath martensite and primary ferrite.The water cooled FSW joint is composed of uniformly distributed,fine grained ferrite and a small amount of martensite,and has good strength and toughness.The elongation of the joint is improved,and the comprehensive mechanical properties of the joint are effectively improved.The electrochemical test results of air cooled FSW joints and water cooled FSW joints in 3.5wt.% Na Cl solution showed that the corrosion potential and corrosion current density of FSW welded samples are reduced compared with the base material.This indicates that the FSW joints are prone to corrosion,and the corrosion products form a film at a higher rate than the base metal.At the same time,the dense corrosion product film reduces the corrosion rate of the sample.Among them,the water cooled FSW joint has the best resistance to electrochemical corrosion and the degree of corrosion is relatively light.