İmprovement Of The Fatigue Strength Properties Of En-Aw 6082 Aluminum Alloy By Means Of Deep Rolling

Abstract

In this study, fatigue properties of EN-AW 6082 aluminum alloy were altered using T6 heat treatment and deep rolling application. At first, artificial aging schedules were investigated and most optimal schedule for subsequent deep rolling was determined as 480min-180°C artificial aging preceded by 90min-550°C solution heat treatment and water quench. After artificial aging, axisymmetric specimens were subjected to deep rolling. This treatment allows one to induce compressive residual stresses in surface region of components. Effects of tensile loads during loading cycles can be reduced because of these compressive residual stresses. Deep rolling forces of 125 N, 250 N and 500 N were used with feed rates of 0.1, 0.2 and 0.3 mm/pass. Deep rolling was employed in two different directions. These directions were tangential rolling and longitudinal rolling. Conventional direction for deep rolling is tangential rolling in which rolling direction is tangent to turning direction and feed direction is in the direction of longitudinal axis of component. However, compressive residual stresses due to deep rolling are expected to be different in different directions as shown in literature. Therefore, a direction change in rolling is expected to change fatigue strength enhancement after deep rolling considerably. Surface states; namely: Roughness, hardness and residual stresses were measured for Un-treated, tangentially rolled and longitudinally rolled specimens. In addition, stress-controlled fatigue tests were conducted to determine the effects of DR on fatigue behavior. It was shown that DR resulted in much lower roughness values than Un-treated for both rolling directions. However, increase of deep rolling feed rate affected roughness improvement adversely. Hardness values were shown to increase around surface after deep rolling for both rolling directions. This was especially true for 250 N and 500 N deep rolling forces. Up to 10% increase in hardness was obtained after deep rolling. Compressive residual stresses were shown to develop around the surface. Higher deep rolling forces resulted in higher maximum compressive stresses (more negative stresses) and depths at which maximum compressive stresses occur were shifted toward deeper into specimen. Different residual stress profiles for rolling and feed directions were observed. Higher compressive stress values near surface were detected in rolling direction after deep rolling. Fatigue tests showed that both tangential and longitudinal rolling increased fatigue strengths substantially compared to untreated specimens. However, fatigue strength increases were found to be higher for longitudinally rolled specimens than tangentially rolled ones. Fatigue strength increase of 26% could be achieved using longitudinal rolling procedure. In longitudinally rolled specimens, fatigue loading direction and rolling direction is the same. These results can be attributed to higher observed compressive residual stresses near the surface in rolling direction compared to feed direction. As a conclusion, it can be said that longitudinal rolling is an attractive option to improve fatigue properties of components in industrial applications.