Rapid Thermal Treatment of Oriented Microstructures to Generate Fine Globular Grains

Metal additive manufacturing through fused filament fabrication (FFF) has recently been gaining traction owing to its simplicity, cost benefit, and suitability to a wide range of starting materials, as compared to alternative technologies. The generation of a thixotropic (globular) microstructure is fundamental to obtaining the desired flow behavior in the semi-solid material. Although numerous studies have explored the possibility to print extruded filaments, a scalable process for manufacturing 3D-printable filaments is yet to be established. Drawn filaments possess similar microstructures to extruded filaments and are commercially produced. In this study, we demonstrate that small globular grains (~10 µm) can be generated in a cold-drawn filament of a Zn-Al alloy using rapid thermal treatment. The filaments were heated to different temperatures (375°C, 385°C, 400°C, 420°C, and 450°C) using a molten salt bath. The larger heat transfer coefficient of the molten salt bath, compared to convective heating, drastically reduced the time required to achieve the target temperature to a few seconds. Higher set temperatures led to quicker transformation into globular microstructure due to Rayleigh instability and resulted in smaller initial grain sizes. Longer treatment times promoted grain coarsening due to Ostwald ripening. Such a pretreatment may enable fine resolution printing using thixotropic filaments.