Abstract
High electrical and thermal conductive metals (HETCM) play a key role in smart electronics, green energy, modern communications and healthcare, however, typical HETCM (e.g., Ag, Au, Cu) usually have relatively low mechanical strength, hindering further applications. Selective laser melting (SLM) is a potentially transformative manufacturing technology that is expected to address the issue. Ag is the metal with the highest thermal conductivity, which induces microscale grain refinement, but also leads to high internal stresses by SLM. Here, we select Ag7.5Cu alloy as an example to demonstrate that multi-scale (micro/meso/macro) synergies can take advantage of high thermal conductivity and internal stresses to effectively strengthen Ag alloy. The mimicry of metal-hardened structures (e.g., large-angle boundary) is extended to the mesoscale by controlling the laser energy density and laser scanning strategy to manipulate the macroscale internal stress intensity and mesoscale internal stress direction, respectively, to form mesoscale large-angle "grains", resulting in multiple mutual perpendicular shear bands during fracture. The presented approach achieved a significant enhancement of yield strength (+ 145%) and ductility (+ 28%) without post-treatment. The results not only break the strength-ductility trade-off of conventional SLM alloys, but also demonstrate a multi-scale synergistic enhancement strategy that exploits high thermal conductivity and internal stresses.
Highlights
High electrical and thermal conductive metals (HETCM) play a key role in smart electronics, green energy, modern communications and healthcare, typical HETCM (e.g., Ag, Au, Cu) usually have relatively low mechanical strength, hindering further applications
The results reveal a remarkable hierarchy of microstructures clarifies the relationships amongst different features and provides guidance for future structural manipulation of materials produced by additive m anufacturing[17]
The Selective laser melting (SLM) process leads to grain refinement and residual stress increase of the Ag alloy, which in turn, triple the Vickers hardness of the Ag alloy component when compared to the Vickers hardness obtained from the casting process[21]
Summary
High electrical and thermal conductive metals (HETCM) play a key role in smart electronics, green energy, modern communications and healthcare, typical HETCM (e.g., Ag, Au, Cu) usually have relatively low mechanical strength, hindering further applications. The results break the strength-ductility trade-off of conventional SLM alloys, and demonstrate a multiscale synergistic enhancement strategy that exploits high thermal conductivity and internal stresses. The SLM process control of high thermal conductivity metallic materials is challenging and is not conducive to mechanical enhancements at different s cales[19,20]. Xiong et al.[21] selected a 1 μm wavelength laser device and spherical Ag alloy powders They have demonstrated it can overcome the challenges of high reflectivity and thermal conductivity in producing Ag parts at the various process parameters. Like many high thermal conductive metals, silver alloy powder is exposed to a series of processing steps, layer-by-layer rapid melting and solidification during SLM, which leads to inevitable high thermal gradients and internal stresses and associated d efects[28–31]. A unidirectional scanning strategy can cause internal stress defects induced by the concentration of stress d irections[19,20]
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