MECHANICAL PROPERTIES AND FRACTURE STUDIES ON UFO 7075 AL ALLOY UNDER DIFFERENT LOADS (EXPERIMENTAL AND SIMULATION STUDIES)

Abstract

The present work tells about experimental evaluation of Mechanical and Fracture properties and simulation studies (by FEM and XFEM) to analyse fracture behaviour of ultrafine-grained 7075 Al alloy produced by cryorolling technique. The results were compared with the above said properties of starting material i.e. wrought 7075 al alloy. The precipitation hardenable Al 7075 alloy (Al-Mg-Zn-Cu) plates were solutionised at 490°C for six hours and then cryorolling (rolling at liquid nitrogen temperature) done to achieve different thickness reductions (40%, 70% and 90%). It was observed that at 70% thickness reduction fully formed UFG (ultrafine-grained) structure produced for the 7075 alloy. Some of the specimens were. then artificially aged after giving short annealing treatment for a while at different temperatures to optimize the peak ageing condition. The effect of cryorolling and cryorolling + peak ageing conditions on mechanical properties and fracture characteristics under different loads were investigated by using vicker's microhardness measurements, tensile testing, Charpy impact testing, High cycle fatigue testing, fatigue crack growth rate testing and fracture toughness testing. Microstructural characterization done using SEM and FE-SEM. XRD was used to analyse the formation of different phases during cryorolling of 7075 Al alloy and the phases present in the starting bulk alloy. The severe strain induced during plastic deformation of Al 7075 alloy, given by cryorolling produces ultrafine-grained structures with improved mechanical properties such as Yield strength, hardness, impact toughness, fatigue strength which are mainly due to grain size effect, higher dislocation density and suppression of dynamic recovery. Improved fracture toughness obtained in case of ufg alloy, mainly due to higher fracture stress of the said alloy what can be explained similarly like the well established Hall-patch effect. The cryorolled specimens after peak ageing treatment shows marked increase in strength, ductility, fracture toughness was due to the combined effect of grain refinement, precipitation hardening and partial recovery. FEM and XFEM simulations was carried out to show_ improved fracture behaviour and crack arrest capabilities of the ufg 7075 alloy. Fracture surface morphology analysed at different loading conditions such as tensile, impact, fatigue and monotonic loading shows significant differences to each other for both starting bulk alloy and its ufg form.