Sepehr Sadeh

In this study, spatial and temporal profiles of an Nd-YAG laser beam pressure pulse are experimentally characterized and fully captured for use in numerical simulations of laser impact welding (LIW). Both axisymmetric, Arbitrary Lagrangian-Eulerian (ALE) and Eulerian dynamic explicit numerical simulations of the collision and deformation of the flyer and target foils are created. The effect of the standoff distance between the foils on impact angle, velocity distribution, springback, the overall shape of the deformed foils, and the weld strength in lap shear tests are investigated. In addition, the jetting phenomenon (separation and ejection of particles at very high velocities due to high-impact collision) and interlocking of the foils along the weld interface are simulated. Simulation results are compared to experiments, which exhibit very similar deformation and impact behaviors. In contrast to previous numerical studies that assume a pre-defined deformed flyer foil shape with uniform initial velocity, the research in this work shows that incorporation of the actual spatial and temporal profiles of the laser beam and modeling of the corresponding pressure pulse based on a laser shock peening approach provides a more realistic prediction of the LIW process mechanism.

Laser shock peening (LSP) is an advanced surface treatment technique that can extend fatigue life in metallic components by inducing near-surface compressive residual stresses. In this study, LSP was implemented to induce compressive residual stresses and modify material properties of selective laser melted (SLM) aluminum A357 specimens. An initial hypothesis on the effect of LSP during tension testing was formulated and tested using finite element simulation. The hypothesis was that, due to the LSP-induced tensile residual stress field in the middle of the specimen cross sections, yielding was expected to initiate in this region. True stress-strain curves of two as-built (AB) and two laser shock peened samples were obtained through transverse tensile tests. The single explicit analysis using time dependent damping (SEATD) technique was used to simulate LSP process utilizing Johnson-Cook (J-C) constitutive parameters. J-C parameters for the cast A357 alloy were used for preliminary study. This was followed by the simulation of the transverse tensile test. J-C parameters for SLM A357 alloy were then empirically estimated, and simulations were repeated accordingly. It was found that the specific LSP pattern induced tensile residual stresses along the edges as well as the middle of the test specimen’s cross-section. Axial residual stress and yield strength profiles along three different paths on specimen’s cross-section were compared and yield regions were investigated. This supported the initial hypothesis, but also provided for a more detailed understanding of actual tensile test failure in the specific SLM A357 specimens for the given LSP treatment. In addition, the same LSP treatment on SLM A357 alloy resulted in lower magnitude of compressive residual stress than for cast A357 aluminum alloy.

Lasers are widely used as high-accuracy tools for material processing. Different types of lasers such as CO₂, Nd:YAG, and excimer lasers are used in different operating modes such as continuous wave, pulsed or Q-switched. Volumes of materials and their composition, structure, and properties can be controlled or modified by varying laser pulses. In this research, by using laser as a material processing tool, an experimental method was developed for laser induced implantation doping of glass substrates with conductive metals. Experiments were performed on glass samples using Q-switched Nd:YAG lasers. Gold, silver, and copper were used as conductive dopant materials. Initial experiments were performed using nickel as a catalyst. Effect of the catalyst on the composition of implanted dopant material was observed using Large Area Rapid Imaging Analytical Tool (LARIAT). Through further experiment, the effect of several parameters such as beam fluence, scanning speed, pulse repetition frequency, wavelength, substrate temperature, dopant material, and glass substrate material on the morphology of heat affected zones were investigated by optical microscopy (OM). Depth of penetration in doped glass samples was measured for different substrate temperatures by means of a laser displacement sensor. The effect of beam fluence and glass substrate thickness on depth of penetration was investigated. The results of these non-destructive measurements were verified using scanning electron microscopy (SEM). Based on optical observations, morphological characteristics of the heat affected zone were assessed in order to obtain the best parameter settings in different experiments. These settings were defined by factors such as the number and size of cracks in glass substrates, and the quality of the distribution of dopant metal over the scanned pattern. While using a catalyst with substrates at room temperature, the best parameter settings were obtained at wavelength of 532 nm, pulse repetition frequency of 6 kHz, beam fluence of 0.36 J/cm², and scanning speed of 0.10 m/s. By removing the catalyst, these settings were changed to 355 nm, 10 kHz, 0.09 J/cm², and 0.01 m/s for gold sputtered soda-lime glass substrate at 500 °C. For beam fluence values ranging from 0.06 J/cm² to 0.38 J/cm², the obtained values for average depth of penetration were 255 μm and 187 μm in 1 mm and 3 mm thick soda-lime glass substrates respectively. Further development of this implantation method could lead to implantation of electronic circuits in transparent substrates, inspiring the evolution of transparent electronic devices such as transparent smart phones, smart windows and displays, and lighting products in the future.