Anomalies in precipitation hardening process of 7075 aluminum alloy extruded by KOBO method

Abstract

In this paper the influence of the initial conditions of KOBO method extrusion of 7075 aluminum alloy on the structural and mechanical parameters of the products is presented; in particular the paper focuses on how heat treatment affects the course of the post deformation process. Moreover, our fundamental purpose is to attempt to answer the question regarding the role of point defects and their potential usage to control the material's properties.The impetuses to conduct this research were the previous experimental studies of the kinetics of metal extrusion by KOBO method and the results of mechanical properties measurements and microstructural observations of the products. They allowed to formulate the thesis that the point defects generated in metals subjected to large and rapid plastic deformation, forming nano-dimensional clusters, constitute the dominant element of deformation mechanism and structural phenomena that take place both during and after the process.In this paper it is shown that, based on the analysis of the experimental results, the clusters of point defects have a significant thermal stability which remains after extrusion, slows down or completely inhibit the process of diffusion of alloying elements at aging temperature (150°C). On the other hand, a very fast decay of clusters occurring at high temperature (annealing/solutioning at 470°C/5min) creates an excess of point defects, which result in increased diffusion during subsequent aging at 150°C.