Re-interpreting metallic alloys for additive manufacturing

A transformation path opens up an alternative to avoid the typical coarse anisotropic microstructures obtained upon additive manufacturing of titanium alloys and provides new windows for the alloy design of other metallic systems
Re-interpreting metallic alloys for additive manufacturing

Our paper in Nature Communications: Peritectic titanium alloys for 3D printing

The alloys used nowadays for additive manufacturing (AM) of metals are mostly based on compositions inherited from conventional production techniques, e.g. casting or forging. However, traditional manufacturing usually involves thermo-mechanical treatments for microstructural engineering, i.e. control of grain size and texture, breaking up anisotropy, and thus, the final shape of components is a consequence of forming steps followed by machining, to obtain their final geometry. The attractiveness of AM is the near net-shape fabrication of components with complex geometries or functionalities not achievable using the design constraints of traditional manufacturing. This also accounts savings in machining costs, lead time and material loss. 

Nowadays, the strong anisotropy exhibited by alloys that solidify with cubic fcc or bcc primary phases (e.g. conventional compositions of Ni- or Ti-based alloys) represents a deep-rooted drawback during AM, where thermo-mechanical processing as implemented in traditional manufacturing is not possible. For instance, severe texture associated with anisotropic structural properties usually remains upon AM and even after post-processing, which can be a critical issue for acceptance as well as certification of AM parts. This effect is particularly relevant for powder-bed AM techniques such as selective laser melting (SLM), a technique under investigation at the German Aerospace Center (DLR) for the production of critical components for transportation (air, space, land), as well as for the development of novel alloys tailored to the metallurgical conditions of SLM. In our department Metallic Structures and Hybrid Material systems lead by Prof. Guillermo Requena, we came up with the idea of exploiting metastability around liquid-solid and solid-solid states by adding the solute element La to Ti.

As explained in our publication, the findings obtained reveal a promising phase transformation path not exploited yet to decrease the texture in as-built as well as post-treated AM Ti-alloys. In situ high energy synchrotron X-ray diffraction (HEXRD) permitted to investigate the influence of phase transformation kinetics of solid/liquid as well as solid/solid states on the resulting texture evolution and the influence of kinetic variables (i.e. metastablity) during microstructure formation. HEXRD revealed that α grains can nucleate via a peritectic reaction that breaks the regular Burgers-related β → α transformation responsible for the anisotropic behavior of AM Ti-alloys. As a result, significant texture reduction as well as equiaxed microstructures can be achieved. This opens up an alternative to avoid the typical epitaxial growth that generates coarse columnar β grains with strong <100> β orientation along the building direction during AM of Ti-alloys (e.g. Ti-6Al-4V). 

The addition of peritectic forming elements capable to induce as-built as well as post-processed additive-layer-manufactured microstructures of reduced texture can have a positive impact in commercial titanium compositions. This, accompanied by the formation of equiaxed microstructures represents a step-forward towards a next generation of titanium alloys for AM. 

Moreover, the approach shown in our investigations of adapting alloys to AM using a peritectic reaction opens up windows for target oriented alloy design in other alloy systems.

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