Fabrication, characterization, and tribological performance of micro dent arrays produced by laser shock peening on Ti-6Al-4V alloy

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University of Alabama Libraries

Ti-6AL-4V is a widely used structural material in aerospace, biomedical, and power generation industries due to its low density, good corrosion resistance and high strength properties. However, Ti-6Al-4V is not a suitable material for wear applications due to its tendency to gall. Improving the tribological performance by reducing frictional coefficient and decreasing wear rate may result in significant savings. Laser shock peening (LSP) is a surface treatment designed to induce compressive residual stress and improve fatigue performance. This study presents a novel micro LSP based surface patterning technique to fabricate micro dent arrays. Comprehensive 3D dent topography was measured to evaluate the effect of laser intensity on dent geometry, process repeatability, and pile-up. Surface integrity has been characterized with respect to dent profile, surface residual stress, surface hardness, and microstructure. Post-peening polishing process has been conducted to eliminate the potential pile-up. Tribological performance of dented arrays with and without pile-up and two peening densities was tested at low and high viscosity lubrication conditions using pin-on-disk setup. Real time measurements of the coefficient of friction (CoF) and acoustic emissions were made. Finally, correlations between dent density, CoF, wear rate, and AE signal were investigated. 2D dent profile showed an increase in dent depth and diameter with laser power. 3D surface topography revealed good process repeatability. Pile-up region develops around the dent containing about 40% of the total material volume displaced from the peened region. Consequently, the remaining 60% is compressed within the dented region. Peened surfaces with micro-dent arrays have high compressive residual stress and increased hardness by 37% in the peened zone. Pin-on-disc tribology tests have shown a CoF reduction by 18% on peened surfaces with 10% dent density and pile-up when compared to polished smooth surface under high viscosity conditions. An increase in CoF was found for a surface with 20% dent density and pile-up when compared to the smooth surface. It was found that removing pile-up reduces CoF with only 2% to 3% for both peening densities. Correlations between wear rate, absolute energy, amplitude, and counts was established. RMS signal has a weak correlation with frictional coefficient.

Electronic Thesis or Dissertation
Mechanical engineering