Hardening Particulate Ti Media Through Controlled Oxidation
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Shot peening is a well-established surface treatment commonly used to impart a compressive residual stress on a part to improve fatigue life.[1–3] The possible drawbacks of this surface engineering process, depending on the final application, include increased surface roughness from the indentations caused by shot media and the potential for contamination of the workpiece surface from material transfer of the shot residue. Shot contamination can have deleterious effects on properties. Surfaces shot peened with iron-based shot (a common shot media) tend to have poorer corrosion resistance when compared to their untreated counter part; such as the case when Fe-based peening media is used on Al[4] and Mg alloys.[5] Clemens et al. reported that the Fe concentration in shot peened AZ91 was as high as 1.5 wt pct on the peened surface. Other researchers[6] have attempted using ceramic peening media for shot peening, and using this media results in no measurable corrosion or fatigue deficit; however, Schuh et al. showed that ZrO2 shot still results in contamination of the surface when used to shot peen Ti-64.[7]
D.A. BRICE, R.M. RAHIMI, and D.F. BAHR are with the School of Materials Engineering, Purdue University, West Lafayette, IN, 47907. Contact e-mail: [email protected] Manuscript submitted April 1, 2019. Article published online June 27, 2019 3980—VOLUME 50A, SEPTEMBER 2019
One route to circumvent surface contamination would be to use Ti-based shot to peen Ti alloys; this would require Ti-based shot that is harder than the target alloys. Titanium displays a large solubility for oxygen in the a-Ti phase[8] and the addition of oxygen to a-Ti is a potent hardener.[9,10] It is reported in literature that the hardening in Ti from oxygen additions is due to the distortion of the lattice parameters and the increase of the critical resolved shear stress of pyramidal and basal slip systems allowing for prismatic slip to be activated preferentially.[11] Oxidizing (distinct from oxidation) of titanium should produce a Ti-based shot media that is both hard and will avoid surface contamination because of the large solubility of oxygen in a-Ti before producing an oxide layer. Minimizing oxide formation would decrease the potential for incorporating Ti-oxides in any media transfer. This study proposes a method to produce Ti-based shot peening media that exploits solid solution strengthening of Ti with oxygen. Pure titanium powder (99.8 pct metal basis) was obtained from Atlantic equipment engineers (AEE) with an initial composition presented in Table I. The powder had a particle size range between 50 and 150 lm. In order to harden the powder, there has to be a controlled diffusion of O into the powder particles. Surface engineering of Ti alloys via case hardening procedures are well established, but often the goal is to incorporate a case with a thickness on the order of 100’s of lm’s.[12] Previous researchers developed a hardening mechanism for Ti structural parts where the material is oxidized at high temperature to produce a distinct
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