Study On Microstructure Characteristics Of Friction Stir Welded Panels Of 7075 Aluminum Alloy During Stress Aging

In the development of modern aircraft, the performance of aircraft has been put forward higher requirements, traditional riveting assembly structures being gradually replaced by high-performance large-scale aluminum integral panels. Aiming at such problems as lower materials utilization ratio and higher dependence on efficiency CNC equipment in deforming traditional milling integral panels, this paper explores a low-cost manufacturing technology for integral panels based on the hybrid process of friction stir welding(FSW) and creep age forming(CAF). With integral panels as the research object, the relevant basic research work has been conducted accordingly,which plays an important role in CAF development and research expanding. In view of this, a comparative study on creep age formability of aerospace-series 7075 aluminum alloy sheets with and without T-shaped FSW features was performed with the peak aging temperature of 180 oC gained from the previous research results, and the microstructure evolutions during relevant processes were systematically reported by using the optical microscope(OM), scanning electron microscopy(SEM), energy spectrum analysis(EDS), and transmission electron microscopy(TEM) techniques and so on.Results show that, the existing of FSW process will weaken the mechanical performance of 7075-T6 aluminum alloy, affecting its tensile properties and macro hardness, while stress aging treatment will play a positive role in coordinating the uneven characteristics of FSW joints and promoting their tensile properties, and also,the existence of FSW will advance the optimal aging time. For micro-hardness of FSW joints, stress-aging processes have an adverse effect, with the increasing of aging time, the hardness being decreased. OM analysis shows that the grain sizes of the base metal are increased with the increasing of aging time, but the black accumulation decreases, and the precipitations in the base matrix are reduced,resulting in decreased mechanical properties of base metal; furthermore, at the early stage of aging, the black second-phases of heat affected zone are gradually increased,researching maximum up to 12 h, and the Fe element in the matrix is increased with increasing aging time observed by EDS, resulting in incensement of the precipitation strengthening phases of Al7Cu72 Fe and Mg2 Si and thus enhancing the mechanicalproperties of heat affected zone. Analysis from fracture morphology finds that, up to12 h, deeper and large numbers of dimples will account for the enhanced plasticity of samples. And further TEM observations reports an increasing of stress-precipitations in the region of heat affected zone, for which, despite the coarsening trend of the precipitation ofη phase(Mg2Zn), an overall strengthening effect is exhibited during creep age forming process, accordingly reinforcing the mechanical properties of the FSW structures. Stress-aging makes the ηphase of grain boundary precipitates(GBP)growing and coarsening, with an early continuous distribution to later non-continuous one up to 12 h, and an continuously wider precipitate free zone(PFZ) until 16 h.