A research team from the Beijing Institute of Technology (BIT) has achieved additive manufacturing of meter-scale titanium alloy single crystals, a first for industrially produced metal single crystals, according to a study published in Nature on Oct 21.

The paper, titled Additive manufacturing of ultralong and architected single crystals, was co-first-authored by Liu Changmeng, a professor at BIT's School of Mechanical Engineering; Cui Yinan, an associate professor at Tsinghua University's School of Aerospace Engineering; Wang Jiachen, formerly a postdoctoral researcher at the same school of Tsinghua University and now at Nanjing Normal University; and Han Xiao, a doctoral student at BIT's School of Mechanical Engineering. Liu, Cui and Y. Morris Wang of the University of California, Los Angeles were the corresponding authors.

Nature also published a News & Views article alongside the study, saying the work "creates exciting opportunities for future technological and scientific developments". Reviewers of the paper described it as "an important milestone in industrial manufacturing".

The method is the first metal additive manufacturing technique developed in China to appear in Nature, opening a new direction for the field. Liu's team has spent a decade working on the metallurgy of the micro molten pools that serve as the basic crystal growth unit in additive manufacturing, overcoming the four bottlenecks of size, structure, material and cost.

Single-crystal materials are strategic foundational materials for aero-engines, gas turbines, advanced nuclear systems and semiconductors. Conventional preparation relies on seed crystals, moulds and directional solidification, which struggles to combine large size with complex geometry, broad material compatibility and low cost.

The researchers proposed a "deep-squat pulse" heat input method, alternating strong and weak heat input to control how the molten pool evolves and to establish the dynamic stability needed for single-crystal growth under strongly non-equilibrium conditions. No pre-fabricated seed crystal is required: single crystals up to a meter tall and several centimeters in diameter were grown directly from a polycrystalline substrate.

By coupling solidification with the mechanical stability of unsupported molten pools, the team built a mechanics-solidification model and a printability phase diagram, enabling mold-free and support-free manufacturing of complex single-crystal structures, including lattices built from meandering single-crystal struts.

Using the widely used Ti-6Al-4V titanium alloy as a model material, the team showed that the creep resistance of the additively manufactured single crystals improved by orders of magnitude over both wrought and additively manufactured polycrystalline material, far more than conventional grain-size scaling laws would predict. Multi-scale characterization and crystal plasticity modeling revealed that the removal of grain boundaries, together with the internal α/β phase structure, changes dislocation motion and void formation.

The method also proved applicable to other hard-to-machine refractory metals, with the team producing single crystals of tungsten, molybdenum, niobium and a molybdenum-niobium alloy. Built on a wire arc manufacturing platform, it reduces reliance on dedicated moulds and precision temperature-control equipment, and wire feedstock costs less than powder.

The work was supported by the National Natural Science Foundation of China, the National Key R&D Program of China and Tsinghua University's Dushi Program.