Forging processes include various steps to attain favorable material properties such as heat treatment, rapid quench, cold work stress relieving, and artificial aging. These steps, however, also contribute to bulk residual stress. Excessive bulk residual stresses cause a wide of problems, including part distortion during machining and in use, reduced crack initiation life, increased crack growth rates, and an overall reduction in part life. This paper summarizes recent work aimed at measurement-based assessment of bulk residual stresses in cold-compressed aluminum die forgings. The results show that forging process induced residual stress is a repeatable phenomenon with RMS repeatability less than 5% of yield.

Undesired distortion can arise during machining of metals from two main mechanisms: 1) release of bulk residual stress in the pre-form, and 2) deformation induced by the cutting tool. The interaction between these two mechanisms is explored herein using aluminum plate-shaped samples that have a large surface with variations of bulk residual stress (BRS), where that surface is subsequently milled and we observe milling-induced residual stress (MIRS) and distortion. Plate samples are cut from two kinds of large blocks, one kind stress-relieved by stretching and a second kind that had been solution heat treated, quenched, and aged. MIRS is measured following milling using hole-drilling with fine depth increments. Distortions of thin wafers cut at the milled surfaces are used to show how the interactions between BRS and MIRS change milling-induced distortion. Data from the study show that the directions of MIRS and distortion relative to the milling direction are changed when milling in samples with high BRS magnitude (roughly ±100 MPa), with the direction of maximum curvature rotating toward or away from the milling direction depending on the sign and direction of BRS. High magnitude BRS increased distortion, nearly doubling the amount found compared to milling in samples free of BRS.