Research On Microstructure And Fatigue Property Of 7075 Aluminum Alloy By Laser Welding

With the growing environmental and energy issues,it is imperative to carried out the requirement of lightweight.As a high-strength aluminum alloy with 570MPa of tensile strength,7075 aluminum alloy have advantages,low density,corrosion resistance,no low temperature brittleness,excellent plasticity,easy machining performance and other characteristics,which is a promising lightweight materials.Besides,laser welding with filler wire is an advanced welding method,large heat source energy density,high welding speed,good quality welded joints,reducing the defects in welding metallurgy,so it is very promising to combinated 7075 aluminum alloy with laser for reducing the total weight of the vehicle.In this paper,7075 aluminum alloy was welded by laser welding with filler wire.The macro-morphology,microstructure,phase structure of joints were observed and analyzed,and measured hardness joints,tensile properties,fatigue properties The effects of microstructure and microstructure on the microhardness and tensile properties of the joint were analyzed.The ratcheting effect and cyclic hardening and softening properties of the joint and the base metal were analyzed.Finally,the fatigue fracture and fatigue test of the joint and the base metal were carried out,and surface damage were analyzed.Under 4.5kW of laser power,the 5m/min of wire feed speed,the+1mm of defocusing amount and the 4m/min of welding speed,the welded joint with good shape can be obtained.The welded joints of 7075 aluminum alloy consist of the fusion zone(FZ),heat affected zone(HAZ),base metal(BM).The edge of the FZ is mainly columnar dendrite grains,and the center of the FZ is mainly equiaxial grains.The microstructure in the HAZ was fibrous along the rolling direction,and the grains in the HAZ slightly coarsened compared with those in the BM.The phase of BM is mainly composed of?-Al solid solution,the precipitation of?-MgZn₂ and S-Al₂MgCu,but no precipitation was found in the FZ.The hardness of the FZ is the lowest,which is 95HV¹20HV.The microhardness of heat affected zone is higher than that of weld zone,and the microhardness value of HAZ increases gradually from the FZ to the BM.With the increase of welding speed,the microhardness values of FZ and HAZ have a tendency to increase.At different welding speeds,tensile test specimens of welded joints were fractured in the FZ.Tensile strength of the joints is 345MPa³77MPa,as a base material of 60.4%⁶6.1%.There is a necking phenomenon and the surface of fracture is mainly the equiaxed dimples,which is a typical characteristic of ductile fracture.There is no necking phenomenon in the tensile test specimen of base metal.and the fracture surface is composed of a few small dimples and a large cleavage plane,which is a ductile-brittle mixed fracture.The strength and plasticity of the joint are lower than the base metal,which is mainly related to the low strength of the FZ.Under conditions of 10⁵cycles,the fatigue strength of base metal and welded joints for 7075 aluminum alloy are respectively 260.59MPa and 176.36MPa.The fatigue samples of joints and base metal of 7075 aluminum alloy are both prone to cyclic softening phenomenon in the low stress level,but they are both prone to cyclic hardening phenomenon in the high stress level.Ratchet strain of base metal and joint for 7075aluminum alloy can be divided into three intervals:the initial transient phase,the relative stability phase and failure phase.The relatively stable phase is the largest proportion of whole fatigue among them,which determines the fatigue life.For the joint fatigue specimen,with the increase of the stress level,the ratcheting effect becomes more obvious in the relatively stable phase and the fatigue life decrease correspondingly.Being similar to the fatigue sample of joint,the ratchet effect of the base metal in the relatively stable phase is more obvious at the high stress level.The macro-fracture of base metal and joint fatigue consist of fatigue crack source area(fatigue source),fatigue crack propagation area(propagation area)and transient breaking area.The fatigue sources in fatigue fracture for joints are mainly loose and mixed particles on the surface of the fatigue specimen.The secondary cracks appear in the propagation area.The transient breaking area is similar to the fracture morphology of static tensile,which is a typical equiaxed dimple.The fatigue sources in fatigue fracture for the BM are mainly the inclusions and micro-notch in the surface.The cleavage step,the secondary crack and the fatigue strip appear in the propagation area.To the further,the second phase particles in the propagation area hinder the crack to propagate.In fatigue loading process,fatigue damage of joint fatigue specimens is mainly concentrated in the FZ.The fatigue failure of the FZ is caused by the shearing stress,which causes the microcracks in the grain boundary during the sliding process,and the microcracks will be connected with the other microcracks to get the further propagation.The surface crack of the BM fatigue test specimen is tortuous,and the second phase particles will change the direction of crack propagation.When the front of the crack encounters fine second phase particles during the process of crack propagation,the crack tends to bypass it.When the front of the crack encounters the large and long second-phase particles,the crack tends to bypass or pass through the second phase particles.During the fatigue cycle,secondary cracks generated in the plastic zone of the crack tip,and the main fatigue crack is liable to spread to these secondary cracks.