Laser cladding is a process that is used to improve the properties of a metal surface. The properties in question may include hardness, wear-, corrosion- and/or fatigue-resistance. The process involves fusing a thin layer of additional metal to the original surface, using a laser as the heat source. Unfortunately, residual stresses are generated due to the rapid and highly localised thermal expansion and contraction that occur during the heating-melting-solidification-cooling cycle. These residual stresses can have a detrimental effect on the final performance of the clad component, especially with respect to corrosion resistance. Detrimental tensile residual stresses can be mitigated through the use of post-processing techniques such as laser shock peening (LSP). LSP is a process that uses a pulsed laser to generate intense spots of recoil pressure on a surface, thereby introducing compressive residual stresses. Post weld heat treatment (PWHT) is another process that could be also used in laser cladding in order to relieve tensile residual stresses. In this work, laser cladding was carried out by depositing a clad layer of AISI grade 316L stainless steel on to either a S275 steel substrate or an AISI grade 316L stainless steel substrate, using different process parameters. The hardness and residual stresses in the overlay and substrate were assessed for each laser clad sample before and after being treated with LSP and PWHT. The corrosion rate and microstructure were also assessed in each case. The novelty of this work is two-fold. Firstly, to the author's knowledge, it is the first study that attempts to link process parameters to both the residual stresses and the corrosion performance of austenitic stainless steel overlays deposited by laser cladding. The second novel aspect is based on the application of both LSP and PWHT to the deposited overlay in order to investigate whether an improvement in the mechanical properties and the corrosion resistance can be realised. In this study, tensile residual stresses were generated in the clad layers. However, the magnitude of the residual stresses did not appear to be particularly sensitive to the deposition parameters. Indeed, it was found that the number of layers that is deposited is more important than the choice of process parameters. LSP was effective in reducing the tensile residual stresses and in fact it introduced compressive stresses to all the samples that were treated. In contrast, PWHT only led to satisfactory stress relief when the AISI grade 316L stainless steel was deposited on to a matching substrate material. This was related to the fact that a difference between the thermal expansion coefficients of the overlay and substrate led to the development of significant tensile residual stresses on cooling down after PWHT. The corrosion tests on the clad coupons led to the development of pits and cracks. However, after LSP only pits were found, without any sign of cracking, for the test durations that were investigated owing to the fact that compressive stresses were generated. Similar results were found after PWHT for the clad samples in which the overlay material matched the substrate material. However, signs of cracking were observed after PWHT in samples where AISI grade 316L stainless steel was deposited on to an S275 steel substrate due to tensile residual stresses remaining within the overlay. This result suggests that there may be little benefit in carrying out PWHT for components in which grade 316L stainless steel is deposited on to a steel substrate. In contrast, there appear to be clear benefits associated with carrying out LSP in order to mitigate the residual stresses and retard the onset of cracking.

<p>This study is concerned with investigating the effects of warm laser shock peening (wLSP) on the enhancement of mechanical performance of laser welded joints. A 3-D finite element model is presented which predicts the surface indentation geometry and in-depth compressive residual stresses generated by wLSP. To define the LSP pressure on the surface of the material, a 1-D confined plasma model is implemented to predict plasma pressure generated by laser-coating interaction in an oil confinement regime. Residual stresses predicted by the finite element model for wLSP reveal higher magnitude and depth of compressive residual stresses than room temperature laser shock peening. A novel dual laser wLSP experimental setup is developed for simultaneous heating of the sample, to a prescribed temperature, and to perform wLSP. The heating laser power is tuned to achieve a predefined temperature in the material through predictive analysis with a 3-D transient laser heating model.</p><p>Laser welded joints of AA6061-T6 and TZM alloy in bead-on-plate (BOP) and overlap configurations, created by laser welding with a high power fiber laser, were post processed with wLSP. To evaluate the strength of the welded joints pre- and post-processing, tensile testing and tensile-shear testing were carried out. To understand the failure modes in tensile-shear testing of the samples, a 3-D finite element model of the welded joint was developed with weld regions’ material strength properties defined through microhardness testing. The stress concentration regions predicted by the finite element model clearly explain the failure regions in the experimental tensile testing analysis. The tensile tests and tensile-shear tests carried out on wLSP processed AA6061-T6 samples demonstrate an enhancement in the joint strength by about 20% and ductility improvement of about 33% over as-welded samples. The BOP welds of TZM alloy processed with wLSP demonstrated an enhancement in strength by about 30% and lap welds demonstrated an increase in joint strength by 22%.<br></p><p></p>

In this report, the feasibility of a massively parallel, nanoscale laser shock peening process is investigated. This report will give a fundamental background on laser shock peening processes in general. The background will include a description of the mechanisms associated with laser shock peening, and the theory behind laser shock peening. The experiments that were performed to develop a nanoscale laser shock peening process will also be described in detail. The problems associated with different experiments and the results will be presented. / text

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