Anisotropic yielding in AM Ti-6Al-4V: Experimental methods, models, and case study justifications

The advantages of additive manufacturing (AM) over traditional subtractive methods are well known. Exciting opportunities will be enabled by AM. Efficient structures with complex external and internal features are made possible in AM, allowing for broader design envelopes and optimised systems. Paramount to the adoption of both AM materials and the advanced designs that they promote is an understanding of how materials deform and fail in complex loading configurations. At present, the experimental studies that underpin predictive models are (often) limited to uniaxial conditions. Multiaxial behaviours can be inferred from these observations. However, the results are susceptible to large levels of uncertainty. This, in turn, limits the degree to which anisotropy can be represented in the predictive models.

Unacceptable levels of uncertainty in predictive material models limit confidence in novel designs enabled in AM, thereby undermining the potential impacts of AM. What is required is both an intelligent testing methodology that explores anisotropy in AM materials and a general modelling approach that allows accurate predictions to be made at the design stage.

The project proposed here will develop these approaches using new testing capabilities at UNOTT. Generic multiaxial test coupons will be established. These will be built in Ti-6Al- 4V (as a demonstration material) through collaborations with Renishaw and tested using novel yield locus probing techniques (enabled by the new multiaxial test frame at UNOTT). Hill's anisotropic yield criterion (or a modification of it) will then be used to develop generic plasticity models for the Ti-6Al-4V material. Results will be verified through additional testing along non-standard loading axes. Finally, the value of the work will be quantifiably demonstrated through a design case study of an AM bracket. Conventional (with large but justifiable safety factors resulting from modelling uncertainty) and advanced (developed in the project) material models will be used to analyse an AM bracket component to determine safe working loads. The value will be demonstrated through a greater structural capacity in the case of advanced deformation representations.