Investigation On Microstructure And Properties Of 800MPa Grade High-strength Low-alloy Steel Joints By Laser And Laser Arc Hybrid Welding

With the development of thermo mechanical control process(TMCP)and micro alloying process,the mechanical properties of high-strength lowalloy(HSLA)steel is improved and new requirements are put forward for the welding of HSLA steel.The strength and the toughness of the 800 MPa HSLA steel joint by arc welding decreased and there are few systematic studies on laser welding of 800 MPa HSLA steel.Therefore,the present work focuses on the 800 MPa HSLA steel joints with a thickness of 8 mm by laser and laser arc hybrid welding,which provides useful references for the application of HSLA steel welds.The results are as follows:The research on the formation of the joints with a thickness of 8 mm by laser welding and laser arc hybrid welding is carried out.The optimal process parameters of laser welding include laser power of 6 ~ 7 k W,welding speed of 1.0 m./min and defocusing distance of 0 mm.The optimal process parameters of laser arc hybrid welding include arc welding,standoff distance of 2 mm,laser power of 5 ~ 7 k W,welding speed of 1.0 m/min,wire feeding speed of 8 ~ 10 m/min and defocusing distance of 0 mm.The microstructure of the fusion zone(FZ)of laser welded joint is determined by the heat input.The FZ microstructure transforms from acicular ferrite to martensite for a reduction in the heat input.A large number of martensite/austenite constituents distributed in the matrix phase in a long chain morphology are observed at a moderate heat input,i.e.,0.56 k J/mm.Decreasing heat input to 0.39 k J/mm and increasing heat input to 0.78 k J/mm tend to reduce the amounts of the martensite/austenite constituents.The microstructure of the FZ of laser arc hybrid welded joint is significantly influenced by the wire feeding speed.With the wire feeding speed increasing from 8 m/min to 10 m/min,the microstructure of FZ of the top area on the joins is acicular ferrite and the microstructure of bottom area transforms from martensite to acicular ferrite;The heat input of the bottom area on the joint is less than that of the top are when wire feeding speed is 9m/min and below.As wire feeding speed increased to 10 m/min,the microstructure and properties of the joint are consistent along the thickness direction due to higher heat input of arc.The microstructure of the heat affected zone(HAZ)is not sensitive to the welding method and heat input.The microstructure of the HAZ of the joint by laser welding and laser arc hybrid welding is similar,which can be further divided into four sub-zones,namely,coarse grained HAZ(CGHAZ),fine grained HAZ(FGHAZ),intercritical HAZ(ICHAZ)and subcritical HAZ(SCHAZ).The CGHAZ and the FGHAZ contain lath martensite and granular bainite.The IGHAZ contains martensite and polygonal ferrite with some bainite due to partial austenitization.The SCHAZ contains a mix of martensite and bainite as the matrix of the steel,but several fine particles of carbides can be observed in the grains due to tempering.The softened zone of HAZ consists of the ICHAZ and SCHAZ.Low heat input is beneficial to improve the strength of the joint by laser welding due to the formation of martensite in the FZ and a narrow HAZ,and moderate heat input is detrimental to the toughness of the joint.In this work,the strength of the welded joints is enhanced from 748 MPa to 944 MPa and the location of fracture changes from the FZ to the HAZ for a reduction in the heat input from 0.78 k J/mm to 0.39 k J/mm.The large amount of martensite/austenite constituents dispersed with a long chain morphology in the matrix at a moderate heat input are deleterious to the impact toughness of the FZ.In this work,a low impact energy of 44.1 J is obtained at a heat input of 0.56 k J/mm.With a reduction in the heat input to 0.39 k J/mm or an increase in the parameter to 0.78 k J/mm,the impact energy is increased to beyond 54.0 J owing to the matrix phase of acicular ferrite or lath martensite and a reduction in the formation of the martensite/austenite constituents.With the wire feeding speed increasing from 8 m/min to 10 m/min,the strength of the joint by laser arc hybrid welding decreases and the toughness is hardly affected.In addition,the joint by laser welding shows higher strength and toughness than that of laser arc hybrid welding.The finite element simulation of the tensile specimen shows that:(1)the tensile strength of the joint is affected by the size of HAZ.The tensile strength of the joint decreases with the increase of the size of HAZ(0.5~8mm)with the same material properties.The smaller the size,the greater the decrease in the tensile strength of the joint;(2)with the strength ratio of HAZ to the base metal increasing to 0.9 and the ratio of the size of HAZ to the thickness of the base metal dropping to 0.0625,the joint fails in the basematerial zone.When the tensile strength of FZ is lower than or equal to the strength of HAZ,the joint fails in FZ,and the joint fails in HAZ under other circumstances;(3)as the width of HAZ increases,the slope of the tensile strength of the joint changes with the tensile strength of HAZ increases and the slope of that with the tensile strength of FZ decreases.