Fatigue Properties Of Fiber Laservariable Polarity TIG Hybrid Welding Of A7N01 Aluminum Alloy With Filler Wire

A7N01 aluminum alloy belongs to Al-Zn-Mg heat treatment hardening aluminum alloy.It has good weldability and comprehensive mechanical properties,therefore widely used in the field of rail transportation as the main structure of high-speed train car body.In this paper,4 mm-thick AA7N01P-T4 was welded using fiber laser-variable polarity tungsten inert gas?VPTIG?hybrid welding with filler wire.The effects of natural aging on microstructure and mechanical properties of A7N01 aluminium alloy hybrid welded joints were studied systematically.The softening behavior of welded joints was also discussed.The fatigue properties of welded joints with reinforcement and removed reinforcement were compared.The influence of natural aging,microstructure and stress ratio on fatigue crack propagation rate was analyzed,and the relationship between microstructure and fatigue properties was established.The results show that the microstructure of the hybrid welded joints is mainly composed of the equiaxed dendrites,columnar crystals and the fine grains close to the fusion line in the weld center.After 60 days of natural aging,the microstructure of the weld center did not change obviously,the results of TEM analysis showed that the quantity of precipitation MgZn₂ in weld increased after natural aging.MgZn₂ has the obviously strengthening effect on the weld intensity.The microhardness of welded joints is improved in the natural aging condition compared to that of the welded condition.The tensile strength of hybrid welded joints is about 320MPa,which is 75%of that in the parent material.After natural aging for 30 days,the tensile strength of joints increased to 369MPa,which is 83%of that in the parent material and tends to stabilize.The grain orientation and grain boundaries distribution in different regions of welded joints were compared with EBSD.The elemental distribution in different regions of the welded joints was also investigated by EDS.The contrast analysis shows that the grain size is coarse and the grain boundary orientation randomly distributes in weld area.Evaporation and burning of Mg and Zn in the welding process result in the reduction of the precipitation quantity of the weld zone after the solidification.The amount of coarse grains and the precipitation decreases,which results in the apparent softening behavior of welded joints.After natural aging for 30 days for welded joints,the joint fatigue performance of smooth specimens with reinforcement and removed reinforcement are carried out.The fatigue fracture characteristics of welded joints are analyzed in detail by EBSD.With reinforcement welded joints in the cyclic times N=10⁷,fatigue limit is 115MPa.Fatigue cracks lie in the weld toe,which is the site of stress concentration at the initiation.After removing reinforcement,the fatigue limit of welded joints increases to160MPa.The fracture position lies in the weld center softening zone,which is the zone of micro-pores gathering.In order to further evaluate the fatigue fracture resistance of hybrid welded joints,the fatigue crack propagation rate of the base metal,heat-affected zone and weld metal was studied.The fatigue crack propagation rate of welded joints under different stress ratios was compared.By contrast,the microstructure of different regions has great influence on the fatigue crack propagation rate.The base metal has the big fatigue crack propagation resistance ability,therefore,the fatigue crack propagation rate is the slowest.In comparison,the fatigue crack propagation resistance ability of the heat influence zone is smaller,and the weld metal with the fastest fatigue crack propagation rate.This is due to the big grain size in the weld metal And.the weld center is the softening area in the joint,which causing the weak anti-fatigue crack propagation ability.After 30 days of natural aging,the mechanical properties of welded joints reach the stable state.The precipitations in the weld metal and HAZ increased,which in turn hinders the fatigue crack propagation.Therefore,the fatigue crack propagation rate of welded joints decreases relative to that in the welding state.And the anti-fatigue crack propagation ability is increased.The fatigue crack propagation rate is different under different stress ratio.This mainly related to the fatigue crack closure effect,and the fatigue crack propagation rate increases with the increase of stress ratio.