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.

This paper presents the preliminary results of experiments designed to mimic typical machining and thermal processing practices for aerospace titanium alloys. The most significant finding is that multiple side mill passes result in lower magnitude compressive stresses than a single side pass, which suggests that successive interactions with the milling tool serves to relieve residual stresses at the surface. The most likely mechanism for this is that Ti exhibits significant springback during machining, and multiple tool passes essentially remove the “springback” layer. Each successive removal of material allows stress relaxation in the remaining surface layer. By contrast, with only a single pass, surface residual stresses did not sufficiently relax.