Prediction of Residual Stress Random Fields for Selective Laser Melted A357 Aluminum Alloy Subjected to Laser Shock Peening

Abstract

Residual stress (RS) is a major processing issue for selective laser melting (SLM) of metal alloys. Postprocessing by way of heat treatment or hot isostatic pressing is usually required for acceptable mechanical properties. In this work, laser shock peening (LSP) treatment on both SLM and cast aluminum A357 alloys are compared with regard to the development of beneficial near-surface compressive RS. Experiments are conducted using high energy nanosecond pulsed laser, together with a fast photodetector connected to a high-resolution oscilloscope and high-speed camera to identify detailed temporal and spatial laser pulse profiles to improve numerical predictions. Constitutive modeling for SLM A357 alloy is performed using finite element simulation and data obtained from X-ray diffraction (XRD) measurements. Since XRD-RS measurements are accompanied with significant machine-reported error, an effective method is introduced to quantify the material constitutive model uncertainty in terms of a joint probability mass function. Conventionally, most constitutive behavior research for LSP involves deterministic material modeling. Predicted RS using deterministic approaches fail to reflect real-world variations in the materials, laser treatment, or RS measurements. A discretized Bayesian inference is used to quantify the rate-dependent plasticity material model parameters as a joint probability function. RS are then characterized as random fields, which provides far greater insight into the practical ability to attain desired residual stresses. Moreover, for identical LSP treatments, it is determined that the material models are significantly different for the SLM and the conventional cast A357 aluminum alloys, resulting in much lower magnitude of compressive RS in the SLM alloy.

Simulation and Experimental Comparison of Laser Impact Welding with a Plasma Pressure Model

“…Measured spatial and temporal profiles of the laser pulse are shown in Figure 4 and Figure 5, respectively. [37,39], with permission from ASME, 2019.…”

“…Since a large amount of energy (2.5-3 J) is released in a very short amount of time (~17 ns) over a small area (0.08 cm 2 ), the ablative layer reaches extremely high temperatures (above 10,000 °C [40]) and vaporizes instantaneously creating the plasma depicted in Figure 5. Measured laser pulse temporal profile [37,39], with permission from ASME, 2019.…”

“…where Z 1 and Z 2 are shock impedances of glass and aluminum, respectively, and α is the fraction of internal energy that dissipates into heat. Coupling Equations (2) and (3) gives Equation (4) [37].…”

“…Table 2. Parameter values used in plasma pressure modeling [37,42], with permission from ASME, 2019.…”

“…LIW and laser characterization setup. reproduced from[37], with permission from ASME, 2019.Metals 2019,9, x FOR PEER REVIEW 7 of 26…”