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<p>While molten thermoplastics require high shear-rate processing to reduce viscosity for extrusion, molten metals are incompatible due to very low viscosity. Instead, we can use semi-solid alloys containing a thixotropic microstructure to achieve the desired flow behavior for extrusion. However, large globular grains can clog small nozzle openings, limiting resolution. Current methods to achieve thixotropic microstructure rely on slow globularization mechanisms, resulting in relatively large grains in the semi-solid state (&gt; 50&#xa0;μm). Processing using instability mechanisms can reduce the time required for globularization and produce smaller globules (&lt; 10&#xa0;μm). Through this study, we demonstrate that during the rapid thermal processing of drawn wires, the instability mechanisms accelerate when the wire radius decreases. We stretched drawn wires of a Zn–Al alloy until necking and performed heat treatments in a molten salt bath at two different temperatures (400&#xa0;°C and 450&#xa0;°C). We observed that globularization proceeded faster as the sample diameter reduced and the salt bath temperature increased. The earlier onset of Rayleigh instability also decreased the initial grain size. The result was that the tip of the tensile samples heated to 450&#xa0;°C produced ~5&#xa0;μm size globular grains within 1 second. A small grain size can enable smooth material flow through smaller dies and nozzles.</p>

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Stretching Oriented Microstructures to Accelerate Rayleigh Instability During Rapid Thermal Processing for Generating Thixotropic Material

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年5月16日

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  • Sathyanarayan Sairam Jaishankar,
  • Donggang Yao,
  • Jack G. Zhou

摘要

While molten thermoplastics require high shear-rate processing to reduce viscosity for extrusion, molten metals are incompatible due to very low viscosity. Instead, we can use semi-solid alloys containing a thixotropic microstructure to achieve the desired flow behavior for extrusion. However, large globular grains can clog small nozzle openings, limiting resolution. Current methods to achieve thixotropic microstructure rely on slow globularization mechanisms, resulting in relatively large grains in the semi-solid state (> 50 μm). Processing using instability mechanisms can reduce the time required for globularization and produce smaller globules (< 10 μm). Through this study, we demonstrate that during the rapid thermal processing of drawn wires, the instability mechanisms accelerate when the wire radius decreases. We stretched drawn wires of a Zn–Al alloy until necking and performed heat treatments in a molten salt bath at two different temperatures (400 °C and 450 °C). We observed that globularization proceeded faster as the sample diameter reduced and the salt bath temperature increased. The earlier onset of Rayleigh instability also decreased the initial grain size. The result was that the tip of the tensile samples heated to 450 °C produced ~5 μm size globular grains within 1 second. A small grain size can enable smooth material flow through smaller dies and nozzles.