Evaluation of Mechanical Properties of High Strength 7075 & 7050 AluminumAlloys after Single and Multi-Stage Precipitation Hardening Heat Treatments

Aluminum is the most widely used material for civil transport airframe constructions, consists of components such as the wing upper, wing lower, fuselage, spars, frames, ribs and control surfaces. In these parts, resistance to applied loads, providing an aerodynamic shape and protecting passengers, payload and equipment from the external environmental conditions are necessary. At present work, high strength 7075 and 7070 aluminum alloys (AA) were subjected to single and multistage precipitation hardening (known also as age-hardening) heat treatments.The extruded slab of AA7075 with the dimension of 60cm×10cm×7cm heat treated as T6, while the rolled plate of AA7050 with the dimension of 200cm×100cm×11cm was subjected to multistage T73-651 heat treatment by the manufacturer. The over-aged T7 temper provides the best resistance to exfoliation corrosion and SCC, but significant sacrifice in tensile properties compared with the single-stageT6 temper. It also enhances resistance to fatigue crack growth and imparts dimensional and thermal stability to the part. The chemical composition of the investigated alloys were determined by atomic adsorption. The precipitates were studied using optical and scanning electron microscope.To identify grain size distribution and image analysis of the precipitates, the Clemex image analysis software was used. The specimens for hardness measurement and tensile test were extracted from the heat treated plates. The hardness of the specimens was measured in S-T, L-S and L-T planes using Brinell hardness tester. Tensile tests were conducted on the cylindrical specimens prepared only in L direction in accordance with ASTM B-577. According to results, very fine precipitates (MgZn ) are evenly distributed throughout the microstructure, while the coarse precipitates (particles) are evident along the grain boundaries. Considering the SEM fractographs, it seems that the intermetallic particles actas void nucleation and damage initiation sites.

کلیدواژه ها:

High strength aluminum alloys ، Single and multi-stage precipitation hardening ، Microstructure ، Mechanical properties