Research On Process And Properties Of Welded Joint Of Spray Formed 7xxx Series Aluminum Alloys In TIG Welding

Spray formed 7xxx series aluminum alloys have ultra high strength, high toughness, fatigue resistance and corrosion resistance, can be superplastic formed and so on. Welding technology is one of the key technology to the application of 7xxx series aluminum alloy. Based on the advantages of TIG welding technology, this paper adopts TIG welding method to spray formed 7xxx series aluminum alloys.It took account of weldability, especially the welding hot cracking susceptibility and the properties of welded, adopted 4043, 5356 and 7055 three kinds of welding material to the spray formed 7xxx series aluminum alloys in TIG welding. The mechanical properties, microstructures, phase compositions of welded joint and the joints different properties under the action of heat treatment were investigated by optical microscopy (OM), X-ray diffraction (XRD) and scanning electron microscopy(SEM)as well as energy disperse spectroscopy (EDS).The results of study showed that the method of TIG welding can be applied to spray formed 7xxx series aluminum alloy; contrast the selected welding materialsl to the spray formed 7xxx series aluminum alloys in TIG welding, 5356 welding material has larger hot cracking susceptibility, with the condition of high heat input, the penetrated hot crack appeared and expanded in the fusion area; under the condition of low heat input, although the hot cracking sensitivity decrescent, the tensile strength of welded joint is lower than the joints by using 4043 and 7055 wires.While using the wire of 5356 to the TIG welding of spray formed 7xxx series aluminum alloy, there are some microstructures with thick and white island shape in the weld seam beside the fusion area, the phase compositions of island shape microstructure areα-Al, Al_0.403Zn_0.597 and Al₇Cu₃Mg₆.During the weld metal solidifying, due to the solute element segregation between grains or dendrites, low melting point eutectic organization Al₇Cu₃Mg₆ formed, but the amount of the eutectic liquid was insufficient to heal those formed cracks, under the action of thermal cycling peak temperature, 7xxx series aluminum alloys cracked.When using different materials to the spray formed 7xxx series aluminum alloys in TIG welding, the variation of the welded joint hardness curves of pray formed 7475 and 7055 are consistent. When using the wire of 7055, the hardness of welded joint is the highest, the second is 5356, and 4043 is the lowest. The result of SEM and EDS analysis indicated that when using the wires of 4043 and 5356 to the spray formed 7xxx series aluminum alloys in TIG welding, alloying elements such as Si, Fe, Zn and Cu are mainly distributed in the grain boundary, while it is less to distribute in the matrix; the element of Si in wire 4043 is sufficient to heal those formed cracks, so wire 4043 exhibited little weld cracking susceptibility.While using wire 7055 to the spray formed 7xxx series aluminum alloys in TIG welding, the results of study showed that under the heat treatment the welded joint structure is more homogeneous and the grains in some extent have grown, there is no any overheat phenomenon and joints properties have been significantly improved, reach 80% of base metal, the result of study showed that the heat treatment technology is reasonable.