Titanium has a tendency to gall when in rubbing contact with itself and other surfaces. A number of techniques have been developed to engineer the titanium surface and overcome this problem:
• Ion Implantation
The Shot Peening Process
Shot peening is a cold working process in which the surface of a component is bombarded with small spherical particles of metal, glass or ceramic – the shot. Each piece of shot striking the metal surface acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. To create this dimple the surface layer must yield in tension. Below the surface, the bulk metal in attempting to regain its original shape generates a compressive stress in the cold worked surface. The maximum compressive residual stress produced at of just under the surface of a titanium part is at least 50% of the tensile strength of the material being peened. The surface may be smoothed or textured, and surface hardness increased due to the cold working effect of the peening.
Figure 1. Stress distribution at a shot peened surface.
Note: SS = Surface compressive stress
CS max. = maximum induced compressive stress
d= effective depth of compressive stress effect
TS max. = maximum induced tensile stress. This is controlled to maintain stress equilibrium in the part. It must not be so large that internal failure become possible.
Applications for Shot Peened Titanium
Shot peening of titanium sheet is used to produce a range of attractive finishes for architectural panels and personal items, such as watches. Compressive stresses are beneficial in increasing resistance to fatigue, but only when this initiates from the component surface. Shot peening is regularly and widely used on titanium components for aerospace and industrial applications to provide enhanced fatigue resistance and reduced susceptibility to galling, fretting and stress corrosion cracking. Shot peening alone confers the benefits indicated, but may also be used beneficially in conjunction with other surface treatments.