Fatigue performance of medical Ti6Al4V alloy after mechanical surface treatments

Abstract

Mechanical surface treatments have a long history in traditional engineering disciplines, such as the automotive or aerospace industries. Today, they are widely applied to metal components to increase the mechanical performance of these. However, their application in the medical field is rather rare. The present study aims to compare the potential of relevant mechanical surface treatments on the high cycle fatigue (R = 0.1 for a maximum of 10 million cycles) performance of a Ti6Al4V standard alloy for orthopedic, spinal, dental and trauma surgical implants: shot peening, deep rolling, ultrasonic shot peening and laser shock peening. Hour-glass shaped Ti6Al4V specimens were treated and analyzed with regard to the material's microstructure, microhardness, residual stress depth profiles and the mechanical behavior during fatigue testing. All treatments introduced substantial compressive residual stresses and exhibited considerable potential for increasing fatigue performance from 10% to 17.2% after laser shock peening compared to non-treated samples. It is assumed that final mechanical surface treatments may also increase fretting wear resistance in the modular connection of total hip and knee replacements.

Fig 1. Example of Ti6Al4V implant failure in cementless total hip prosthesis [4].

Fig 2. Investigated surface treatments: (a) shot peening (SP); (b) deep rolling (DR); (c) ultrasonic shot peening (US-SP); (d) laser shock peening (LSP).

Fig 3. Microhardness measurement distributions in the cross-section.

Fig 4. Setup for fatigue testing up to 10⁷ cycles.

Fig 5. Micrograph of the cross section of the ultrasonic shot peened sample, Kroll &Weck etchant.

Left: surface near region with indication of a thin α-case at the very surface (A), right: core of the sample with α -phase (B) and α +β-phase (C).

Fig 6. Residual stress depths profiles (a) axial direction (b) tangential (hoop) direction.

Fig 7. Full width at half maximum (FWHM) depth profiles (arithmetic average).

Fig 8. Exemplary comparison of microhardness depth profiles of shot peened with laser shot peened state.

Fig 9. Fatigue testing results shown in a Woehler graph.

Fig 10. Mean roughness results of the treated surfaces.