Unveiling cold cracking and grain refinement mechanisms governed by alloy composition in additive manufacturing of high-strength Mg-Gd(-Zn)-Zr alloys

Additive manufacturing (AM) of Mg alloy components with intricate geometries has garnered increasing interest for achieving lightweight structure and material simultaneously. However, cracking and coarse-grained microstructure remain critical challenges, severely compromising the strength and ductility of the as-built Mg alloys. Here we leverage multi-scale experimental characterizations and high-fidelity simulations of melt pool dynamics and thermal stresses to understand the crucial role of alloy composition in mitigating cold cracking, enhancing grain refinement and improving mechanical properties in AM of Mg-Gd(-Zn)-Zr alloys, aiming to establish comprehensive alloy composition design guidelines. We uncover an unusual paradox between reducing cold cracking susceptibility and refining grains when tailoring the alloy compositions. The solidification growth restriction factor ( Q value) plays a pivotal role in grain refinement through the constitutional supercooling effect, with a recommended Q value exceeding 32 K. Cold cracking occurs when accumulated solidification shrinkage and thermal contraction stresses surpass the ultimate tensile strength of the material. To quantitatively evaluate this, we introduce a novel cold cracking index ( CCI ) that incorporates the sample size, as well as the height and width of the first crack. Mg alloys with intrinsic properties such as high ductility, high thermal conductivity, and low thermal expansion coefficient are identified as more suitable for AM process to avoid cold cracking. These guidelines are validated by successful fabrication of Mg-11Gd-2Zn-0.3Zr (wt.%, GZ112K) alloy, which turns out to be cracking-resistant, fine-grained, and strength-ductility synergistic. This work provides deeper insights into the cold cracking and grain refinement mechanisms and presents AM design guidelines for Mg alloys with improved printability, microstructure and mechanical properties.