Effect Of UIT On Microstructure And Properties Of7A52Aluminum Alloy Welded Joint

As for low strength of joint of7A52aluminum alloy by double wire MIG, ap-plyingultrasonic impact treatment on superficial weld joint under the condition of2A presetcurrent, the impact velocity of0~1.25min/cm2.Observing and analyzing the microstructure,hardness and wear resistance of7A52aluminum alloy welded joint before and afterultrasonic impact treatment by means of optical microscope, XRD, scanning electronmicroscopy, transmission electron microscope, microhardness tester and so on. It wasfound that nanocrystalline organization could be got in7A52aluminum alloy welded jointsurface, meanwhile, hardness and wear resistance of the joint were improved afterultrasonic impact treatment.After ultrasonic impact treatment, weld surface changed relatively flat, andcontinuous, uniform and smooth transition arc was formed in weld toe. As a result of XRD,with the growth of impacting time, the diffraction peak was widened, surface grain wasrefined. Observation by optical microscope and scanning electron microscopy, plasticdeformation layer was formed in the surface layer of welded joint after ultrasonic impacttreatment, and the thickness of it was increased with the time of impact treatment going on.Deformation layer was composed by three parts of strong plastic deformation area,transition area and the primary organization. The crystal boundary has not been obvious innear superficial layer, organization characteristics of transition area between strong plasticdeformation area and matrix organizations. When ultrasonic impact treatment time was1.25min/cm2, the thickness of surface deformation could reach60μm in weld and heataffected zone and50μm in base metal. Observation by transmission electron microscopeshowed that the grain size was in the range of70nm to300nm in the welding joint surfaceregion,50nm to500nm in base metal.Superficial microhardness of7A52aluminum alloy welded joint was improved afterultrasonic impact treatment. The increase of hardness was gradually descended in a certainrange from the weld surface to primary region. When the ultrasonic impact time was1.25min/cm2, the maximum microhardness could be increased by21.7%of the weld zone,27.5%of heat affected zone,22.6%of base metal after ultrasonic impact treatment.Wear test confirmed that wear resistance of7A52aluminum alloy welded joint was improved with the increase of ultrasonic impact treatment time. When ultrasonic impacttime was1.25min/cm2, the weightlessness rat was decreased by0.33%of weld,0.30%ofbase metal under the condition of10N load. The weightlessness rat was decreased by0.24%of weld,0.59%of base metal under the condition of30N load.