A Systems Approach for Achieving Stress Free Parts In High Strength Aluminum Alloys

This article presents a systems approach which has been used over the past 50 years for improving the dimensional stability of high strength aluminum alloys. When applied correctly, high strength aluminum parts can be produced with minimum residual stresses and optimum dimensional stability, while at the same time achieving all structural properties desired. This approach started with the severe problems of residual stress in the aerospace industry in the late 1960's. It was evidenced by (1) premature failure of parts in service, including both stress corrosion cracking and early fatigue failures and (2) unwanted part movement - both in service and during final machining which was being used to meet required dimensional tolerances. Unfortunately, the problem continues today as (1) the aerospace and aluminum industries push the envelope toward bigger aircraft requiring larger, high strength aluminum components and (2) extremely tight dimensional stability is required in optical components such as space mirrors. This approach involves a step by step procedure, which has been found to be necessary to produce stress free parts. The article will cover a fundamental understanding of the creation of high residual stress levels in heat treated aluminum alloys as well as the integration of six critical parameters that must be understood and properly applied to achieve stress free parts - cooling rate control, proper application of mechanical, thermal and cryogenic stress relieving methods, proper control of the machining process, and the affect of other outside forces which can impart high levels of stress to the part.