Investigation On The Microstructure And Properties Of Friction Welded Joints Between Cold Working Die Steel And Carbon Steel

Cold working die steel is the most widely used in mould industry,its performance and application value directly determine the level of die production.As a new type of cold working die steel,5Cr5 MoV is one of the important materials of automobile large panel die.The connection between 5Cr5 MoV with other carbon steel materials such as 1045 steel plays an important role in the application of the materials.Because friction welding is an efficient,environmentally friendly,and widely used solid-state bonding technology for dissimilar steel welding,the friction welding of 5Cr5 MoV and 1045 steel has been studied to control the cost of die and optimize the structure of die.It is of great significance to promote the application of 5Cr5 MoV die materials.In this paper,continuous drive friction welding was used to weld 5Cr5 MoV and 1045 steel,and the effect of force and heat on the microstructure and mechanical properties of the steel was studied with the upsetting pressure as a single variable.The continuous drive friction welding can achieve the high quality and high efficiency connection between cold working die steel and carbon steel dissimilar steel,and can obtain the welded joint with good forming and mechanical properties.The influence of upsetting pressure on macroscopical shapes and microstructures of the welded joint was studied.When the upsetting pressure increases gradually,the joint flanging amount increases and the width of the welding heat affected zone on both sides of the weld decreases.Because of the difference in composition and properties of the two matrix metals,the flanging amount and the width of the heat affected zone of the 1045 steel side are always larger than that of the 5Cr5 MoV die steel.The weld is a transition zone formed by mechanical mixing and metallurgical reaction of both sides of high temperature metals.The microstructure of the weld is composed of ferrite,pearlite,martensite and retained austenite,and the width of weld decreases with the increase of upsetting pressure.Alloying elements have a significant diffusion phenomenon at the transition between the weld and the5Cr5 MoV die steel.The microstructure distribution characteristics of heat affected zone(HAZ)on both sides of weld and the influence of upsetting pressure on microstructure evolution of different parts were studied.Because the effect of welding heat cycle on each part of welded joint is different,the side HAZ of 5Cr5 MoV die steel can be divided into complete quenching zone and incomplete quenching zone,the HAZ of 1045 steel are divided into superheated zone,phase change recrystallization zone and incomplete recrystallization zone.The microstructure of 5Cr5 MoV complete quenching zone is composed of a large amount of martensite,a small amount of residual austenite and cementite,and the size of the martensite needle blade becomes small and keeps an certain direction with the increase of the pressure.The microstructure of incomplete quenching zone is composed of some martensite,residual austenite and ferrite.The content of martensite increases with the increase of upsetting pressure,and the banded structure appears.The microstructure of 1045 steel side superheat zone consists of a large amount of pearlite and ferrite as well as a small amount of martensite and residual austenite.The grain size changes from coarse to fine with the increase of upsetting pressure,the microstructure of phase change recrystallization zone is fine pearlite and ferrite,and a small amount of twins are formed with the increase of upsetting pressure,while the microstructure of the incomplete recrystallization region has no obvious change due to the short heating time,and the ferrite size decreases with the increase of the pressure.The influence of upsetting pressure on the strength and hardness of welded joints and the distribution of strength and hardness on welded joints were studied.The weld has a high hardness,up to 918 HV,and the hardness decreases gradually to both sides of the base metal.With the increase of upsetting pressure,the high hardness zone of die steel side increases,while the high hardness zone of 1045 steel side becomes narrower.The overall tensile strength of the welded joint increases first and then tends to stabilization with the increase of the upsetting pressure,and the strength decreases slightly when the pressure is too high.When the upsetting pressure in the range of 170-210 MPa,the maximum tensile strength of the welded joint can reach 698 MPa,in this range,the intensity of the two positions is almost equal at the axis and 2/3 R from the axis.The tensile strength distribution at the cross section of the welded joint under 170 MPa upsetting pressure has good uniformity.The tensile fracture occurred in the side quenching zone of 5Cr5 MoV die steel,and all show brittle fractures,and the crack diffusion mode was slightly different under different upsetting pressure.