Research On Ultrasonic Assisted Laser Welding Technology And Service Performance Of Welded Joints Of CrCoNi Medium Entropy Alloy

Medium and high entropy alloys are one of the new material design concept materials,which are prepared by three or more principal elements with equal atomic ratio or approximately equal atomic ratio.They showed excellent mechanical properties and greatly enriches the selection of high-performance alloy materials in design and application field and have becoming ane of the hot topic materials in research field.As one of the most maturely researched alloys,CrCoNi medium alloy exhibits excellent strength,toughness,radiation resistance and corrosion resistance,which is expected to become a new next generation structural material.The good weldability,mechanical properties and deformation behavior of welded joints under different environments（cryogenic temperature,alternating load and corrosive medium）are the basis of engineering application.Therefore,this paper studies the laser welding processes of CrCoNi medium alloys and the service behavior of the joints under different environments.The main conclusions are as follows:（1）The mechanical properties,microstructure and phase structure of laser joints of as-cast CrCoNi medium alloys were analyzed through different experiments combined with molecular dynamics calculation method.The results showed that the as-cast CrCoNi medium alloy have good weldability and there were no cracks,pores and other defects are found in the joints.The microstructure of the fuse zone was coarse and contains a lot of dendrites and the average crystal arm spacing was about 9μm,which was smaller than that of the base metal（36μm）.The joint have excellent plastic performance,the elongation reached respectively 107%and 76.5%in 293K and 77K,and the tensile strength were525MPa and 828MPa respectively.The reason for the increase of tensile strength at low temperature is that a large number of deformation twins appear during the tensile process which impeded the dislocation movement and enhanced the plastic hardening ability.（2）The paper has built an ultrasonic assisted welding system,which introduced ultrasonic wave of 20KHz into the weld pool during the welding process to regulate the microstructure of the weld zone.Meanwhile,nitrogen was used as the protective gas.This new welding processes successfully introduced CrCoNi medium-alloy joints with high strength and toughness,which made the tensile strength were 686MPa and 1017MPa at293K and 77K,respectively.The strength is superior to the joints of as-cast medium entropy alloy.This paper analyzed the influence of ultrasonic on the solidification behavior of molten pool,it is found that the width of weld increases and the size of dendrite in the fusion zone decreased with the increase of ultrasonic power.Ultrasonic assistance can effectively introduce N atoms into the fusion zone and form compound Cr₂N.At the same time,a part of N atoms dissolve into the matrix in the form of solute,which increases the dislocation slip resistance and increase of joint strength.（3）The high cycle fatigue perpormance in stress ratio R=0.1 of CrCoNi medium-alloy base and its ultrasonic assisted laser joints were tested,and then the S-N curve was drawn,the fatigue limit of base metal and joint were 440MPa and 405MPa,and K（ratio of the fatigue limit and tensile strength）were 0.5 and 0.59 respectively.Compared with the traditional high-performance structural materials,the fatigue properties of CrCoNi medium-alloy and joints were better than those of most traditional materials.The deformation mechanism of the base metal under fatigue load is mainly slip and deformation twins.Uniformed distribution of deformation twins can cause effect of"pinching"to dislocation movement and has a positive effect on the fatigue property,which cause plastic strain more uniform in the grain and postpones the initiation of crack.The crack nucleation mechanism of the welded joints under fatigue load is dislocation slip,the dislocation movement easier and its density increases continuously under fatigue load,then the stress concentration is generated at the intersection of multiple grain boundaries to form micropores.Finally,the size of micropores becomes larger and the micropores converge to form fatigue cracks.（4）Tensile test was carried out on CrCoNi medium-alloy and its joint under different"hydrogen environment"of pre-hydrogen charging（hydrogen charging current is40m A/cm²）and in-situ hydrogen charging（hydrogen charging current is 1m A/cm²）by means of electrochemical hydrogen charging,then the tensile fracture morphology,microstructure near the fracture were analyzed.The mechanical behavior and deformation mechanism of CrCoNi medium-alloy and its joints in hydrogen environment were clarified clearly.The"hydrogen embrittlement"phenomenon of CrCoNi medium-alloy and its joints were obvious when the tensile rate is 10^-5s^-1,the plasticity of base metal and joints decreases by 75%and 77.8%,respectively.As a solute,hydrogen enters the lattice and strengthens the lattice distortion and increases the flow stress,result of in the strain hardening ability improved and yield strength of the base metal increases from 527MPa to 577MPa.The fracture mode of both base metal and joint is hydrogen-induced intergranular cracking in hydrogen environment.The interaction between hydrogen and dislocation enhances the driving ability of dislocation migration and improves the activity of dislocation,rapidly aggregates at grain boundaries to form stress concentration points finally,which promotes the occurrence of hydrogen embrittlement.