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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 or 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. 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 where 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 it may also be used beneficially in conjunction with other surface treatments.
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Figure 1. Example of residual stress profile created by shot peening.
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Applications Of Shot Peening On Titanium Components
Aero engine fan discs are shot peened in the bore to enhance low cycle fatigue (LCF) performance. Fan blades are peened all over the airfoil using glass beads at low intensity to provide resistance to impact damage from foreign objects entering the engine. Older style fan blades with mid chord snubbers are repaired after some time in service by a plasma sprayed coating on the snubber faces, which have worn away under frictional load. The plasma coating is considerably harder than the base titanium alloy and thus the snubber is shot peened prior to plasma spraying to prevent cracks in the coating propagating through to the snubber and the blade.
Many titanium alloy components are susceptible to fretting and fretting fatigue when they run in contact with each other in service. Shot peening of the blade retaining slots on fan and turbine discs as well as the mating blade roots prevents fretting and loss of fatigue strength.
Several of the higher strength titanium alloys are susceptible to stress corrosion cracking (SCC) in more aggressive media, for example sour environments at elevated temperature encountered downhole in the oil and gas industry. One of the requirements for SCC failure is tensile loading at the component surface. Shot peening counters the effect of applied tensile stresses and effectively prevents stress corrosion cracking.
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