Study On The Welding Process And Mechanical Properties Of The Joint Achieved By Narrow Gap Laser Welding With Filler Wire For SMA490BW Steel

The weathering steel which contains Cr,Ni and other elements are widely used in highspeed train bogies due to its excellent atmospheric corrosion resistance.Traditionally,highspeed train bogies are welded by arc welding methods.However,these welding methods have many disadvantages e.g.high heat input,low production efficiency and large deformation as well as poor joint performance.As a high energy beam welding method,laser welding with filler wire has many advantages such as high welding efficiency,less material filling,small deformation and narrow heat affected zone.It has been used more and more in practical production.In this paper,the 16 mm thick SMA490 BW weathering steel was welded by laser welding with filler wire.Meanwhile,the droplet transfer behavior were studied during narrow gap laser welding with filler wire by means of high-speed photography which demonstrated the influence of welding parameters on droplet transfer.Besides,the influence of different welding parameters on the weld formation is studied by adjusting the parameters of laser power,welding speed,wire feeding speed and defocusing amount,and a welding process window with good welding formation is obtained.After that,the 16 mm thick SMA490 BW weathering steel was welded with optimized welding parameters,the microstructure and mechanical properties of the welded joint were analyzed.High-speed photography test shows that in narrow gap laser welding with filler wire,when the droplet is balanced by the force of gravity,metal vapor spray force,surface tension and plasma resistance,the axial character of the droplet is best.It can form a stable liquid bridge transfer and the welding process is the most stable.The height of laser-wire intersection is the most important parameter which can affect the droplet transfer behavior.When the height of laser-wire intersection are 0mm 1mm and 2mm,the droplet transfer mode transforms from spreading transfer to liquid bridge transfer and sidewall transfer.The maximum size of droplet increases and the transfer period raises.The maximum size of the droplet and transfer period increases with laser power increases.What's more,the droplet transfer mode transforms from sidewall transfer to liquid bridge transfer and spreading transfer with the increase of wire feeding speed.However,the welding speed has little influence on the droplet transfer.As long as the height of laser-wire intersection is 1mm,the droplet transfer mode is always liquid bridge transfer although welding speed changes.With the increase of welding speed,the droplet transfer period first increases and then decreases.Laser power directly determines the penetration of the weld.The greater of laser power,the greater depth and width of the weld can be obtained.However,excessive laser power will cause defects such as pores and undercuts.With the increase of defocusing amount,the weld depth decreases and the weld width increases,but when f=+30mm,the side wall appears undercut and the bottom is not fused.The larger welding speed and wire feeding speed,the smaller penetration depth and the wider weld width are obtained.The effect of wire feeding speed on the weld penetration is more significant.The optimum and reasonable process window of narrow gap laser welding with filler wire is: laser power 4.0-5.0kW,welding speed 0.36-0.48m/min,wire feeding speed 3.0-4.0m/min,the height of laser-wire intersection 1mm.The 16 mm thick SMA490 BW is welded with optimized welding parameters.The total number of welding passes is 5.The weld seam is mainly composed of acicular ferrite,side plate ferrite and a small amount of pearlite.The grain size of overheated zone is large which mainly consisted of Widmannstatten structure and ferrite.The grain size of recrystallized zone is finer and distributes evenly which mainly composed of ferrite and pearlite.The tensile,bending and impact test all meet the requirements.The maximum hardness of the weld appears in the overheated zone,reaching to 290 HV.The minimum hardness is at the center of the weld,about 230-245 HV,and the hardness of the cover welding area is greater than that in the middle and bottom welding area.