Effect Of Ultrasonic Impact Treatment On Very High Cycle Fatigue Properties Of A7N01P Aluminum Alloy Welded Joints

Fatigue studies were conducted under load control with a ultrasonic fatigue testing machine with smooth specimens of the aluminum wrought alloys A7N01P.Very-high cycle fatigue properties of base metal,original welded joints,non-excess welded joints and ultrasonic impact treatment(UIT)welded joints were studied.Specimens are tested at approximately 20 kHz frequency and load ratios R=-1.The fatigue studies were analyzed and compared by fitting S-N curve with fatigue test data.The scanning electron microscope was used to distinguish fracture characteristics and compare failure mechanism of different specimens.The effects of ultrasonic impact treatment on the surface of welded joints were studied through Metallographic test,Hardness test and Residual stress test.The transmission electron microscopy was done for the surface deformation and grain refinement mechanism of welded joints after UIT.The test results are as follows:(1)A7N01P aluminium alloy welded joint was a typical casting structure,which mainly consists of fine grains in the fusion zone,columnar grains growing along the edge of welded metal and equiaxed grains in the middle of weld metal.The heat affected zone was divided into quenching zone and softening zone.The grain growth and recrystallization occurred in the quenching zone.The softening zone was less affected by heat conduction,and the grain structure was similar to base metal.(2)The Microhardness of welded joint was lower than the heat affected zone and base metal.The Microhardness of solid solution zone was the highest,followed by base metal zone and softening zone of HAZ.The average tensile strength of A7N01P aluminum alloy welded joints was 332.7MPa,and the elongation was about 7.2%.(3)The S-N curves were obtained by fitting fatigue data as follows.Cylinder base metal:lgN(10)19.11lg(35)?(28)48.14(10~6?N?1.356×10~8);lgN(10).463lg(35)?(28)17.76(1.356×10~8?N?10~9);Plate base metal:lgN(10)17.84lg(35)?(28)44.34(2.05×10~6?N?10~9);The S-N curve of welded joint:lgN(10)14.22lg(35)?(28)32.48(2.47×10~5?N?10~9);The S-N curve of plate welded joint by UIT;lgN(10)15.19lg(35)?(28)36.51(3.95×10~5?N?10~9);The S-N curve of plate welded joint by mechanical polishing treatment;l gN(10)18.46lg(35)?(28)43.32(3.10×10~5?N?10~9);The S-N curve of cylinder welded joint:l gN(10)16.98lg(35)?(28)40.51(7.52×10~5?N?10~9).(4)Under the same fatigue life,the fatigue strength of base metal specimens is higher than welded joints.The fatigue strength of welded joints by UIT increased about 41.5%and44.1 compared with as-weld at different cycle of 1×10~7 cycle and 5×10~8.UIT can effectively improved the fatigue life of the welded joints.(5)Shear fracture was the main reason for cylindrical base metal.The fracture of plate base metal was quasi-dissociation fracture.The fatigue crack sources of as-welded specimens were mostly located at the surface and sub-surface defects.The fatigue cracks of UIT were mostly initiated from the surface defects,but a few cracked from the weld toes.In this case,Several cracks originated on the inner surface of the hardened layer of weld toes.Cracks of plate welded joints with no excess height originated from the weld surface processing marks,surface inclusions and surface pore,The cracks of cylindrical welded joints originated from surface defects,secondary surface inclusions and internal pore.(6)Obvious plastic deformation was occurred in the deformed layer after UIT,forming a streamlined structure with compacted grains.Different groups of ultrasonic impact parameters were adopted:1A/2min,1A/5min,1A/10min,1.5A/2min,1.5A/5min and 1.5A/10min.The corresponding thickness of layer were 175?m,260?m,380?m,340?m,460?m,540?m.(7)The surface was affected by work hardening after UIT.When the impact parameters are 1.5A/2min,1.5A/5min and 1.5A/10min,the maximum microhardness of the deformation layer were 167.7HV,171.1HV and 194.3HV.The impact parameters are 1A/2min,1A/5min and 1A/10min.The corresponding maximum microhardness of the surface layer were161.4HV,178.5HV and 190.3HV.(8)When the impact parameters are 1A/2min,1A/5min and 1A/10min,the average grain size of the surface layer is 940~970nm,490~520nm and 280~300nm.When the impact parameters are 1.5A/2min,1.5A/5min and 1.5A/10min,the average grain size of the surface layer is 530~580nm,300~320nm and 130~160nm.The effect of grain refinement on welded joints was obvious.