The Effect Of Longitudinal Vibration On Microstructure And Mechanical Properties Of High Strength Aluminum Alloy Tungsten Inert Gas Welding Joint

High strength aluminum alloy are widely used in aerospace and rail transportation because of its high strength and hardness, lower density. However, the weldability of high strength aluminum alloy is poor due to its physical characteristics. The mechanical properties of welded joint decease significantly when the welding wires with different chemical composition and lower mechanical properties are used. In order to strengthen the welded joint of high strength aluminum alloy, the mechanical vibration was introduced in TIG welding process of high strength aluminum alloy. The mechanical properties and microstructure of high strength aluminum alloy welded joint in different welding parameter, and the influence of mechanical vibration on solidification of welded joint were studied by tensile testing,microhardness, optical microscope(OM), X-ray diffraction(XRD), scanning electron microscope(SEM), energy dispersive spectrometer(EDS), and transmission electron microscope(TEM).Firstly, welding process and welding material of high strength aluminum alloy were studied. The results showed that fracture were located at the center of welded bead when tensile testing for using ER4043. Dimples and micro cleavage surface were founded in tensile fracture. At 115 A welding current, the value of welded joint tensile strength and elongation were σb=285.32 MPa and δ=4.83% respectively. Equiaxed dendrite was observed in the center of welded bead. Many eutectic structure composed of Si were found in grain boundary.Fracture was located at the center of welded bead when ER2319 was tested by tensile testing.A lot of dimples were observed in tensile fracture. At 110 A welding current, the value of welded joint tensile strength and elongation were σb=235.05 MPa and δ=6.3% respectively.Coarse columnar dendrite were observed in the center of welded bead, coarse columnar crystal were found in fusion zone, and network crystal boundary were observed in near weld zone.Many eutectic structure composed of CuAl2 were found in grain boundary in welded bead and near weld zone. Mechanical properties were reduced seriously in some heat affected zone because of overaging. The precipitation strengthening particles were found in some other part of the heat affected zone where reached solution temperature.Secondly, the influence of mechanical vibration on microstructure and mechanicalproperties of welded joint were studied. Fracture was located at the fusion zone when tensile testing were conducted. Tensile properties increased at first, decreased at final with the increasing of the vibration amplitude(0.03-0.06mm) as well as the frequency(30-60Hz).At amplitude H=0.05 mm, the value of welded joint tensile strength and elongation wereσb=283.31 MPa and δ=5.20% respectively; at frequency F=50Hz, the value of welded joint tensile strength and elongation were σb=280.31 MPa and δ=4.96% respectively. Coarse equiaxed dendrite and cellular dendrite were observed in the center of welded bead at lower amplitude and frequency. Fine equiaxed dendrite and cellular dendrite were observed in the center of welded bead at higher amplitude and frequency.Finally, parameters of vibration and weld were optimized by orthogonal experiment. The optimal results of orthogonal experiment were: I=110A, H=0.05 mm, F=50Hz. The value of the optimal welded joint tensile strength and elongation were σb=289.68 MPa and δ=4.95%respectively. Microstructure of optimal weld bead was composed of fine equiaxed dendrite and cellular dendrite which growth direction was in the same direction as the longitudinal vibration.The results of X-ray diffraction showed that microstructures of optimal weld bead were finer than non optimal welded bead.