Peritectic titanium alloys for 3D printing

ARTICLE DOI: 10.1038/s41467-018-05819-9 OPEN Peritectic titanium alloys for 3D printing 1234567890():,; Pere Barriobero-Vila1, Joachim Gussone1, Andreas Stark Guillermo Requena1,3 2, Norbert Schell2, Jan Haubrich1 & Metal-based additive manufacturing (AM) permits layer-by-layer fabrication of near netshaped metallic components with complex geometries not achievable using the design constraints of traditional manufacturing. Production savings of titanium-based components by AM are estimated up to 50% owing to the current exorbitant loss of material during machining. Nowadays, most of the titanium alloys for AM are based on conventional compositions still tailored to conventional manufacturing not considering the directional thermal gradient that provokes epitaxial growth during AM. This results in severely textured microstructures associated with anisotropic structural properties usually remaining upon post-AM processing. The present investigations reveal a promising solidification and cooling path for α formation not yet exploited, in which α does not inherit the usual crystallographic orientation relationship with the parent β phase. The associated decrease in anisotropy, accompanied by the formation of equiaxed microstructures represents a step forward toward a next generation of titanium alloys for AM. 1 Institute of Materials Research, German Aerospace Center (DLR), Linder Höhe, 51147 Cologne, Germany. 2 Helmholtz-Zentrum Geesthacht, Max-PlanckStraße 1, 21502 Geesthacht, Germany. 3 Metallic Structures and Materials Systems for Aerospace Engineering, RWTH Aachen University, 52062 Aachen, Germany. Correspondence and requests for materials should be addressed to P.B.-V. (email: ) NATURE COMMUNICATIONS | (2018)9:3426 | DOI: 10.1038/s41467-018-05819-9 | www.nature.com/naturecommunications 1 ARTICLE M NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05819-9 etal-based additive manufacturing (AM)—colloquially termed metal 3D printing—is resulting in a paradigm change across multiple industries, such as the aerospace, biomedical, and automotive sectors. One of its key strengths is the fabrication of near net-shape metallic components with complex geometries providing, e.g., inner channels for cooling fluids, or bionic and load-optimized structures of minimal weight not achievable with conventional production methods like casting or machining. Layer-by-layer AM production from a 3D computer-aided design provides design freedom, increased product customization, and shorter time to market1,2. For titanium-based components, these advantages account for estimated production savings up to 50%, by basically missing out exorbitant machining costs and material loss3. In aerospace, this focuses on parts with high buy-to-fly ratio (BTF): the weight of the purchased stock material to that of the finished part. Typical aerospace components can have the BTF of 10:1, 20:1, and even 40:1 using conventional manufacturing processes. AM is capable to reduce it close to 1:1. For instance, 50% reduction of production costs has been reported for a wrought Ti-6Al-4V engine bracket using AM4. AM also allows the repair of expensive titanium-based components (e.g., flanges, fan blades, vanes, and landing gears) at 20–40% of new parts cost1. AM weightoptimized components can imply a progress of environmental targets. Previous studies concluded on saving 3.3 million litres of fuel over the aircraft’s life, obtained by a 55% weight reduction using AM Ti-6Al-4V seat buckles1. A critical issue for acceptance and certification of AM parts is the degree of isotropy of their microstructure derived from the solidification conditions during AM and eventual posttreatments1. A deep-rooted drawback during AM of Ti-alloys is the steep, directional thermal gradient in the molten metal pool, which prevents nucleation ahead of the solidification front, provoking epitaxial growth across solidified layers1,5. This is particularly relevant for powder-bed AM techniques, such as selective laser melting (SLM). The typical resulting microstructures are coarse, columnar prior β grains with strong <100 > β orientation along the building direction, normal to synthesized powder layers5,6. This effect is well known to occur in the popular α + β Ti-6Al-4V alloy, which accounts for more than 50% of the titanium market7 and leads—by far—AM of Ti alloys8. Owing to the complicated thermal history undergone by materials during SLM, namely sharp cycles of steep heating (~106–107°C s−1) and cooling (>103°C s−1) rates9, brittle martensitic microstructures unsuitable for structural applications are usually obtained via diffusionless transformation of parent β grains (primary high temperature phase) in the as-built condition of α + β Ti alloys. For instance, α′ martensite formation in Ti6Al-4V occurs for cooling rates above ~410 °C s−110. Though in Ti alloys both α′ and the stable α phase present a hexagonal closepacked (hcp) lattice, the low ductility ( < 10%) and fracture toughness exhibited by martensitic microstructures upon AM manufacturing is mainly a consequence of high density of defects (e.g., dislocations, twins) present in the α′ phase11,12. Differently, the brittleness resulting from martensite formation in steels is associated with the distorted body-centered cubic tetragonal lattice containing ordered arrangements of interstitial C atoms13. Post-thermal and/or thermomechanical treatments are commonly applied to the as-built AM components to improve the strength–ductility trade-off. This can include supertransus or subtransus heat treatments14,15, as well as hot isostatic pressing16 inducing formation of stable α and β via decomposition of metastable microstructures. Subtransus treatments have limited impact on the microstructure and columnar morphologies derived from epitaxial growth are usually maintained. During supertransus treatments, rapid growth of β takes place, leading to 2 excessive grain growth and coarsening15. Apart from representing a costly methodology that reduces the economical attractiveness of AM, these post-treatments do not represent an alternative to mitigate crystallographic texture and its effect on mechanical performance of the alloys17–19. Approaches to tackle epitaxial growth in AM Ti alloys include B addition to α + β and β compositions. For instance, the effect of B on powder blends of Ti-20V, Ti-12Mo20, and Ti-6Al-4V powder alloy21,22 can result in grain refinement. Microstructure globularization preserving the Burgers orientation relationship (OR) between α and β phases has been reported20. The use of B leads to formation of ceramic TiB needles. Thus, the presence of TiB has been associated with strength increase at expenses of ductility, as well as localized plastic flow and damage caused by inhomogeneous distributions of the TiB needles21,23. Other investigations with a SLM-produced Ti-1Al-8V-5Fe alloy showed globularization and formation of small β grains along the building direction, ow (...truncated)