Microstructures And Mechanical Properties Of CO₂ Laser Welded Of Spray Formed 7055 Aluminum Alloy

Due to the excellent performance of ultra specific strength, high toughness, fatigue resistance and corrosion resistance and processability, Spray formed 7055 aluminum alloys is widely used as high strength structure in aircraft, railway vehicl es, nuclear industry, rockets, cars and shipbuilding manufacture.. Welding technology is one of the key technology to the application of 7055 aluminum alloy. Based on this reason, the CO2 laser welding method was adopted to the 2 mm thick spray formed 7055 aluminum alloys plate butt welding test in this paper, two kinds of welding filler material of ER5356 and 7055 were chosed to the laser welding of spray for med 7055 aluminum alloys The microstructures of welded joint were investigated by OM, XRD, SEM, TEM, EDS as well as EBSD. And the change in mechanical properties and corrosion resistance of welded joints under the action of heat treatment(475 ℃/4 h+120 ℃/24 h) were also investigated. The results are as follows:Under the optimum condition for the laser power of 1200 W, welding speed of 400 mm/min and use the material of 7055,welded joint with welding with no defects such as hot cracks and large pores can be obtained.It is showed here that there are obvious weld zones(heat-affected zone, fusion zone, and weld zone) in the laser welding of 7055 aluminum alloy. From an analysis of the EBSD of the weld zone, we can conclude that the HAZ produces equiaxed recrystallized grains, and the grain orientations of the HAZ are same to the base metal, they are all <111>and<001>.There is EQZ in the weld-fusion line nearby, which does not inherit the grain orientations of the parent metal.The weld joint forms a typical nonunion growth mode. The weld zone near the fusion line is columnar dendrite, the center is cellular dendrite. The base metal and the heat affected zone obviously take on {112}<111>S copper texture and {123}<634>S texture. The texture of the fusion zone is not obvious, there is only a small amount of {112}<111> copper texture. The weld joint has a weak {001}<110> cube texture and the residual base material {123}<634>S texture; Based on the study of the XRD of the weld joint, the main composition of the welds were the α(Al)and the(Mg Zn2)strengthening phases. The joint EDS analysis shows that the joints are composed of α(Al)+T(Al Zn Mg Cu) eutectic, α(Al)+S(Al2Mg Cu)Zn eutectic, and the Cu element produced microsegregation at grain boundaries. By the way of TEM, we analyze the different zones in weld joint, the grains in the heat affected zone slightly larger than the base material, however, for the η′ phase, the heat affected zone is more diffuser than the base metal. Consequently,compared with the base material, the hardness of the HAZ is not obvious softening. The columnar dendrite in weld joint is very coarse, and it is not found precipitated phase in grains, so the weld joint became the weakest link.Microhardness test for the joint shows that the hardness is symmetrical distribute about weld joint in the weld joint as different welding materials were added to the 7055 aluminum alloy in the process of laser welding, the heat affected zone, the base metal hardness successively increased, without aging softening zone. After adding the ER5356 welding material, the hardness of the weld joint and fusion zone are approximately 120~140 HV and 140 ~ 150 HV,after adding the 7055 welding material, the hardness of the weld joint and the fusion z one are approximately 130 ~ 40 HV and 140 ~ 150 HV, the hardness of the weld zone is significantly higher than the welding seam which adds the ER5356.After solution and aging treatment, weld grain boundary segregation are eliminated, some grain boundaries disappear, precipitates of grains are fine and uniform; The microhardness of EQZ grain has increased from 139.8 HV to 174.8 HV compared with the prior heat treatment. The microhardness of the weld center has increased from 125.5 HV to 173 HV. Joint strength has significantly increased, reaching 68% of the strength of the base metal. After heat treatment, the fracture of the weld seam is dominated by ductile transcrystalline fracture. The size of the fracture dimple surface is obviously smaller than that of before the heat treatment. Compared with the prior heat treatment, the fracture of the weld is increased and the toughness of the weld is increased. The polarization curves show that the corrosion resistance of the welded joint which after solid solution and aging treatment is hig her than that of the non heat treatment.