Purdue University material engineers have created a patent-pending process to develop ultrahigh-strength aluminum alloys that are suitable for additive manu­facturing because of their plastic deforma­bility. Haiyan Wang and Xinghang Zhang lead a team that has introduced transition metals cobalt, iron, nickel and titanium into aluminum via nanoscale, laminated, deformable intermetallics. “Our work shows that the proper intro­duction of hetero­genous micro­structures and nanoscale medium-entropy inter­metallics offers an alternative solution to design ultrastrong, deformable aluminum alloys via additive manu­facturing,” Zhang said. “These alloys improve upon tradi­tional ones that are either ultrastrong or highly deformable, but not both.” 

Lightweight, high-strength aluminum alloys are used in industries from aerospace to automobile manu­facturing. “However, most commer­cially available high-strength aluminum alloys cannot be used in additive manu­facturing,” graduate student Anyu Shang said. “They are highly susceptible to hot cracking, which creates defects that could lead to the deterioration of a metal alloy.” A traditional method to alleviate hot cracking during additive manu­facturing is the introduction of particles that strengthen aluminum alloys by impeding the movements of dis­locations. “But the highest strength these alloys achieve is in the range of 300 to 500 megapascals, which is much lower than what steels can achieve, typically 600 to 1,000 mega­pascals,” Wang said. “There has been limited success in producing high-strength aluminum alloys that also display beneficial large plastic deformability.”

The researchers have produced inter­metallics-strengthened additive aluminum alloys by using several transition metals including cobalt, iron, nickel and titanium. Shang said these metals traditionally have been largely avoided in the manufacture of aluminum alloys. “These intermetallics have crystal structures with low symmetry and are known to be brittle at room tempera­ture,” Shang said. “But our method forms the transi­tional metal elements into colonies of nanoscale, intermetallics lamellae that aggregate into fine rosettes. The nano­laminated rosettes can largely suppress the brittle nature of inter­metallics.”

Wang said, “Also, the hetero­geneous micro­structures contain hard nanoscale intermetallics and a coarse-grain aluminum matrix, which induces significant back stress that can improve the work hardening ability of metallic materials. Additive manufacturing using a laser can enable rapid melting and quenching and thus introduce nanoscale inter­metallics and their nano­laminates.” The research team has conducted macroscale compression tests, micro­pillar compression tests and post-deformation analysis on the Purdue-created aluminum alloys.

“During the macroscale tests, the alloys revealed a combination of prominent plastic deforma­bility and high strength, more than 900 megapascals. The micropillar tests displayed signi­ficant back stress in all regions, and certain regions had flow stresses exceeding a gigapascal,” Shang said. “Post-defor­mation analyses revealed that, in addition to abundant dislocation acti­vities in the aluminum alloy matrix, complex dislocation structures and stacking faults formed in monoclinic Al₉Co₂-type brittle inter­metallics.” (Source: Purdue U.)

Reference: A. Shang et al.: Additive manufacturing of an ultrastrong, deformable Al alloy with nanoscale intermetallics, Nat. Commun. 15, 5122 (2024); DOI: 10.1038/s41467-024-48693-4

Link: Nanometal Group, School of Materials Engineering, Purdue University, West Lafayette, USA