Fatigue Fracture Behavior Of Hybrid Laser Arc Welded7075-T6Aluminium Joints

The fatigue properties，micro-mechanism of crack initiation and extension of hybridlaser arc welded joints have been investigated by using the fatigue crack propagationtesting，the conventional fatigue testing，the scanning electron micrograph（SEM/EDS），themetallographic observation（OM），the transmission electron microscope（TEM），the electronback-scattering patterns（EBSD） and the Shanghai Synchrotron Radiation Facility（SSRF），the conclusions come as follows：1、The fatigue cracking growth rate of hybrid laser arc welded7075-T6aluminumjoints is：da/dN=3.4×10^-14（K）⁾6.022)（R=0.3），the propagation rate of base metal is：da/dN=4.0×10^-10（K）^2.615（R=0.3） and da/dN=1.1×10^-10（K）^2.902（R=0.1）.2、The fatigue cracking growth rate curve of hybrid welded joints intersects with thatof base metal，that is to say the fatigue cracking growth rate of hybrid welded joints islower than that of base metal especially when the SIF range K is less than15.6MPa.m1/2，which shows that the welding joints can work in good service.3、Under the conditions of same SIF range K，the fatigue cracking growth rate at thehigher stress ratio is larger than that of the lower stress ratio. Furthermore，the stress rangeis more important than the stress ratio （mean stress） to determine the fatigue crackinggrowth rate of welding joints.4、The fatigue strength of hybrid laser arc welded joints was124MPa under stress ratioR=0.2and cycles N=1×106conditions，about44%of its tensile strength.5、All the fatigue and tensile specimens with welded joints break near the fusion linewithout obvious ductility deformation. The fracture surface morphology observation showsthat the hybrid welded joints exhibits as the ductile-brittle mixture fracture. The fatiguecracks frequently initiate at micro-shrinkage porosity and gas pore which created duringweld process.6、The7075-T6base metal had the highest density of {111}<110>texture while thehybrid welded joints was dominated by {111}<110>texture and {011}<100> gauss texture.Like base metal welded joints show some orientation property，though not as obvious asbase metal.7、Large angle boundary is dominant in the different area of welding joint，which morethan90%ï¼›between the weld seam and HAZ there is a fusion line，which is very narrowwith small grain size，is the most high danger area of welded joints. 8、Lost some Zn、Mg and other strengthen elements，severe composition segregationand the growth/melt of Matrix Precipitates during the thermal history of welding processwere the major reason for the soften of hybrid welding joint.9、Combined with SEM/EDS，TEM，EBSD，and SSRF found that near the fusion linethe lost of Matrix Precipitates、more serious segregation（hot crack）than fusion zone centerand the mixed grain distribution always leading to broken in this area.