Influence of Grinding on Residual Stress Measurements of Shot Peened Aluminium Alloy AA7075

Abstract

Aluminium alloys series 7xxx containing zinc, magnesium and copper, as the main alloy elements, are characterised by high ultimate tensile strength (UTS) between 350 MPa and 650 MPa, depending on the state of the alloy. For comparison, the extruded profiles from 6xxx alloys after heat treatment show the UTS at levels of 160 MPa to 340 MPa. Due to its high specific strength, formability, corrosion resistance, resistance to stress corrosion cracking, lightweight and wide application, the aluminium alloy 7075 is widely used in automotive and aerospace industries [1] to [4]. However, these alloys generally have poor ductility and low fracture strength in the as-cast condition, and extensive processing, which includes a combination of heat treatment and hot-cold working, is required to improve the mechanical properties [5]. González et al. [6] investigated different shot peening treatments with conventional and severe parameters, which were performed on an aluminium 6063 alloy in order to assess the differences induced in the microstructure of the surface layer and to evaluate their effects on fatigue behaviour. The obtained results evidence the notable influence of shot peening parameters on the surface layer microstructure, which simultaneously influence fatigue behaviour. Jamalian and Field [7] carried out a microstructural analysis of an AZ31 Mg alloy, which revealed a direct relation between the thickness of the ultra-fine grained layer and severe shot peening parameters, with each of them having a distinct effect on grain size. Furthermore, microhardness tests demonstrated how pressure and shot size control fine grains at the surface. Tensile test results revealed that the best mechanical properties were obtained by maximum shot size and pressure at minimum processing time. Nam et al. [8] investigated the effects of four peening parameters on microhardness and residual stress of AA 2124-T851. To verify the validity of the optimal conditions obtained from experimental results, metallurgical analyses of the shot-peened aluminium alloy were conducted with respect to hardness, residual stress, surface morphology, X-ray diffraction (XRD) analysis and surface roughness. They concluded that shot peening induces plastic deformation, increases surface hardness and introduces significant levels of compressive residual stress. Under optimal peening conditions, the average microhardness and compressive residual stress are ~13 % higher than that of the unpeened sample. Žagar and Grum [9] studied two types of aluminium alloys, EN AW 2007 and EN AW 6082, treated by shot peening, in which the surfaces of the metals were subjected to cold deformation Influence of Grinding on Residual Stress Measurements of Shot Peened Aluminium Alloy AA7075 Žagar, S. – Šturm, R. Sebastjan Žagar* – Roman Šturm University of Ljubljana, Faculty of Mechanical Engineering, Slovenia