Residual stress formation and distribution due to precipitation hardening and stress relieving of AA7075

Abstract

Distribution of residual stresses due to precipitation hardening, stress relieving by cold compression and stress relieving by heat treatment of AA 7075 has been studied. The residual stresses were measured by contour method. For understanding the formation of the residual stresses, the quenching step of the precipitation hardening process has been analyzed by FEM. Mechanical properties of the material in different temperatures and also cooling curve due to quenching step have been measured by experimental tests. The Newton's law of cooling is used to determine the heat transfer coefficient. The cooling curves from the experimental test, Newton's law of cooling and FEM showed good agreement which provides evidence on the verification of the FE model. The results of the residual stress measurement indicate that considerable amount of residual stresses forms during the precipitation hardening process of AA 7075. Annealing heat treatment reduces the level of the residual stresses but does not change the distribution, while stress relieving by cold compression was found to both reduce the level of residual stresses and change the distribution.