Studies on softening behavior and mechanism of heat-affected zone of spray formed 7055 aluminum alloy under TIG welding
Zhengping Lu, Junhua Xu, Lihua Yu, Hao Zhang, Yunze Jiang
Abstract
In this investigation, fine micro-zone specimens in the heat-affected zone (HAZ) of TIG welded spray formed 7055 aluminum alloy were obtained by heat treatment method. The softening behavior and mechanism of HAZ of this alloy under single pass TIG welding were investigated. Severe non-uniformities of microstructures and mechanical properties were found, according to which the HAZ was divided into the solid solution zone and the over-aging zone. The most heavily softened area of HAZ occurred in the over-aging zone at the position corresponding to the peak thermal cycling temperature of 230–350 °C. The lowest microstructures value was 105 HV, the lowest tensile strength was about 410 MPa. The elongation was slightly higher than that of the base material. The main causes of softening in the over-aging zone were the dissolution of the coherent precipitate η′ and the growth and coarsening of the non-coherent precipitate η.Thermal cycle temperature in the solid solution zone was about 350–536 °C. Mechanical properties in this zone were higher than those of the over-aging zone. After natural aging, mechanical properties recovered to more than 90% of the base material. Microhardness reached 180 HV and tensile strength reached 570 MPa when the thermal cycle temperature was 450 °C. At the same time, the elongation in the solid solution zone was higher than that of the base material. The high mechanical properties were the results of the combined action of the increased ratio of large angle grain boundaries, the increased solid solubility and the formation of coherent GPI zone.