Technical Information: Surface Integrity
The surface of titanium alloys is thought to be easily damaged during some traditional machining operations. Damage appears in the form of micro-cracks; built-up edge; plastic deformation; heat-affected zones; and tensile residual stresses. In service, this damage can lead to degraded fatigue strength and stress corrosion resistance. In a study of grinding effects on Ti-6Al-4V alloy, gentle or low-stress grinding parameters displayed no readily identifiable changes at the surface, while conventional and abusive practices altered the surface layer noticeably. There was an appreciable drop in hardness in the gently ground specimen, but very good high-cycle-fatigue values were noted.
Titanium has an endurance limit of 372 MPa (54 ksi) for the gentle grinding and values of 83 and 97 MPa (12 and 14 ksi) for conventional and abusive conditions, respectively. As can be seen, in operations like end mill cutting or turning, the same sensitivity to abusive conditions was not observed, possibly due to residual surface compressive stresses.
Machinists and companies specializing in the machining of aerospace materials generally will have developed techniques to maximize surface integrity of titanium alloys. Thus optimum properties usually are achieved during the production machining of titanium. In those areas of application where maximum fatigue strength is required, not only are appropriated machining parameters used, but also selected surface areas of components may be glass bead blasted to restore, or to retain, a high level of favorable compressive surface stress.
References: Titanium: A Technical Guide (1988), ASM International, Materials Park, OH, 44073-0002, page 75-85