Strength enhancement and analysis using laser shock peening on Inconel 738LC and silicon nitride

Abstract

In this dissertation, laser shock peening (LSP) was applied on metal and ceramic material to enhance the fatigue strength and bending strength, respectively. The research consisted of two parts: an investigation of the effects of LSP on the properties, especially the fatigue strength at high temperature, of Inconel 738 low carbon (LC) (Chapter 2), and an examination of the effects of LSP on the properties of silicon nitride ceramic with sintering additives (Chapter 3). The effectiveness of laser shock peening (LSP) at both room and elevated (850°C) temperatures were examined for Inconel 738LC, a material widely used for gas turbine blade. LSP was performed with a Nd:YAG laser (532 nm, 8 ns, 10 Hz, top-hat profile) using aluminum foil (100 μm thick) as the protective coating at the laser intensity of 10 GW/cm2 and the number of pass of 10. As the result of LSP, surface hardness increased by 38% and compressive residual stress increased about 8 times from those of the unpeened samples. Low cycle fatigue test was carried out at room temperature and elevated temperature (850°C) under strain control. For the room temperature condition, the fatigue life of laser shock peened samples increased by about 2.4 times from that of unpeened samples. At the high temperature of 850°C, the fatigue life of laser shock peened samples was overall similar to that of the unpeened samples, either slightly higher or lower depending on applied total stress level. The results showed that LSP is beneficial in enhancing fatigue strength at low temperature but its effectiveness largely disappears at a temperature as high as 850°C, possibly due to severe thermal relaxation. Silicon nitride ceramic (Si3N4) for industrial applications is conventionally manufactured with sintering additives, and the properties of Si3N4 change significantly based on the contents of these additives. In this study, we investigate the effects of laser shock peening (LSP) on Si3N4 sintered with varying ratios of sintering additives. The Si3N4 samples were sintered with a mixture of Y2O3, MgO, and SiO2 sintering additives at 5, 7, 9, and 11 wt%. A Nd:YAG laser (wavelength = 532 nm, maximum pulse energy = 1.4 J, repetition rate = 10 Hz, pulse duration = 8 ns, beam diameter = 11 mm, top-hat profile) was used to irradiate the Si3N4 samples. The samples were coated with a protective layer (100 μm thick aluminum foil) and a water layer to confine plasma. LSP of Si3N4 with 5% sintering additives resulted in a slight change in surface hardness but a 77% decrease in surface compressive residual stress. In contrast, LSP of Si3N4 with 11% sintering additives led to a simultaneous increase in surface hardness (7.3%) and surface compressive residual stress (67%), indicating a significant difference in the effectiveness of LSP depending on the ratio of sintering additives. Additionally, the surface of Si3N4 with 11% sintering additives showed evidence of grain refinement after LSP. It was demonstrated that the bending strength of Si3N4 with 11% sintering additives increased by 15.2%, and the depth of the fracture origin was significantly deepened.