Ultrastrong pure aluminum structure with gradient nanocrystals via selective pulsed laser melting: Computation framework and experiments

Abstract

Controlling the transient temperature field on the selective laser melting (SLM) process is important not only for quality assurance, by reducing the occurrence of voids, cracks, and excessive deformations, but also for controlling the microstructure within the heat-affected zone. Microstructures with grains sizes gradients are known to have a synergy between the high yield strength of its fine-grained volume and the high ductility of its coarse-grained volume. This synergy attributes to the geometrically necessary dislocations present at the interface between these volumes. This work presents a systematic philosophy that uses temperature field control via pulsed lasers to induce gradient heterostructures on a pure aluminum part fabricated by SLM. Pulsed lasers have the advantage of enabling rapid and localized heating and cooling, which makes the formation of ultrafine grains possible. Therefore, it is a valuable tool to locally melt and re-solidify a metal into a refined microstructure and generate the desired gradient nanostructure in additive manufacturing of metal parts. In this work, we developed a multiscale computational framework to simulate the laser processing, by determining a transient temperature field induced by each laser pulse, microstructure evolution through the thermal history of the pulse over the treated surface, and heterostructure's mechanical properties, for the first time. This framework consists of modules of pulsed laser interaction with Aluminum parts, including Finite Element Analysis (FEA), microstructure evolution modeling via Molecular Dynamics (MD) simulations, and mechanical property prediction via MD by simulating stress-strain tests and nanoindentation tests. These modules determine how the pulsed laser-induced transient temperature field affects the heterostructure formation and mechanical strength. This comprehensive simulation framework provides means to determine selective laser melting parameters to enhance material properties. EBSD revealed that ultrafine resulting surface microstructure was formed, as predicted by the modeling framework, and this microstructure is due to the high cooling rates after short pulse durations, at the nanosecond scale. The process employed in this work can produce nanograined structures with yield strengths as high as 250.8 MPa and indentation strengths as high as 725 MPa, which are comparable to some steel's mechanical properties. This work provides a valid computational framework to build gradient heterostructure in many 3D metallic parts with superior mechanical properties using pulsed laser-induced selective laser melting.