Study On Microstructure And Properties Of A7N01 Aluminum Alloy As-welded MIG Joints And Their Post-welded Heat Treatment Joints

Welded joints of A7N01 aluminum alloy will be prone to grain growth, dissolution of strengthen phase, inhomogeneity of chemical composition and microstructure, deterioration of welded joints by the effect of welding thermal cycles, which may lead to the degradation of hardness, corrosion resistance, impact toughness and tensile properties of welded joints. The microstructure and properties of as-welded joints and three post-weld heat treatments(PHWT) joints such as T6-treated joints, T7-treated joints and RRA-treated joints are carefully investigated.The pulsed MIG welding of A7N01 aluminum alloy was carried out to obtain sound welded joints with ER5356 filler metal. Tests of microstructure, hardness, impact toughness, mechanical tensile, anodic corrosion to base metal, as-welded joints PHWT joints are conducted to analysize changes of microstructure and properties of welded joints before and after heat treatments. Advanced analysis methods, such as the metallographic technique(OM), X-ray diffraction(XRD), scanning electron microscope(SEM) and energy spectrum(EDS), are also utilized to study the microstructure, fracture mechanism and fracture morphology and physical phase component of welded joints.Results of microstructure and hardness tests show that the inhomogeneity of microstructure and chemical composition exists in MIG welded joints. The coarse equiaxed grains in welding zone(WZ) are refined to some degree and their hardness is improved after heat treatment. The coarse strengthen phases in heat affected zone(HAZ) are dissoluted and fine strengthen phases are deposited under the effect of heat treatments so that the strength and hardness of welded joints are recovered. T6 ranks the first in enhancing hardness of joints, while T7 is helpful in recovering their microstructure. RRA is able to improve both hardness and microstructure.The results of impact tests show that the toughness of weld zone is higher than that of base metal, the toughness of fusion zone is lower than that of base metal, and the toughness of heat affected zone is worst. The impact toughnesses of all three regions in welded joint are improved to some degrees after the effect of heat treatment. T7 is the best way to enhance the impact toughness. To sum up, the HAZ has the greatest improvement in impact toughness among the three regions. As a result, the impact toughness of welded joints is improved.Results of tensile tests show that the tensile strength of base metal has the maximum strength. On the contrast, the as-welded joints has the minimum value which is 74.6% of the tensile strength of base metal. The tensile strengths of welded joints are improved to some degrees after heat treatment. T6 is the best way to enhance tensile strength. The improvements of tensile strength of T7 and RRA are lower than that of T6, but the improvement of RRA is higher than that of T7. The tensile strengths of T6-treated joints, RRA-treated joints and T7-treated joints are respectively 87.39%, 85.85% and 83.56% of that of base metal.The polarization tests show that the corrosion resistance of as-welded joints, whice is far lower than that of base metal, has the worst corrosion resistance. The corrosion resistances of T7-treated joints and RRA-treated joints have more remarkable improvements than that of T6-treated joints. The corrosion resistance of RRA-treated joints is a little higher than that of base metal.. Hence it can be concluded that RRA can improve the corrosion resistance of welded joints effectively.