What it is
Single-crystal materials are indispensable for high-performance applications, but conventional manufacturing depends on strict quasi-steady thermal conditions, which conflict with the intense cyclic thermal fluctuations inherent to additive manufacturing; this has historically prevented the 3D printing of large or architected single crystals. The authors present a free-form additive manufacturing approach that uses delicately controlled alternating and pulsed heat inputs to govern and direct single-crystal growth. With it they fabricated metre-tall, centimetre-diameter single crystals of the titanium alloy Ti-6Al-4V and, to their knowledge, the first single-crystal lattice structures containing complex zigzag ligaments; they say this effectively eliminates theoretical size constraints for single-crystal components. The bulk single-crystal Ti-6Al-4V showed an order-of-magnitude increase in creep resistance (resistance to slow deformation under sustained load) over standard grain-size scaling predictions, which the authors attribute to the elimination of grain boundaries together with a hierarchical alpha/beta architecture induced by the printing that regulates dislocation density, mobility and void evolution. They also printed single crystals of hard-to-process refractory metals, including tungsten, molybdenum, niobium and molybdenum-niobium alloys, and present the work as a large-scale method for single-crystal additive manufacturing that opens complex geometries previously unattainable by classical methods.
Why it matters
Alternating, pulsed heat let a 3D printer grow metre-tall Ti-6Al-4V single crystals and, to the authors' knowledge, the first single-crystal lattices with zigzag ligaments. Single crystals, materials without grain boundaries, are indispensable for high-performance applications, but they have needed strict quasi-steady thermal conditions that clash with the intense heat cycling of additive manufacturing, which had kept large or architected single crystals out of reach of 3D printing. The printed bulk Ti-6Al-4V crystals showed an order-of-magnitude increase in creep resistance over standard grain-size scaling predictions, and the same approach printed single crystals of hard-to-process refractory metals such as tungsten, molybdenum and niobium.
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Filed underAdditive Manufacturing Materials and Processes, Additive Manufacturing and 3D Printing Technologies, Cellular and Composite Structures
