The effect of interfacial alloy formation on the mechanical properties of the additively manufactured Ti6Al4V/Ti1Al8V5Fe microstructurally graded material

Multi-material (MM) structures have been shown in the past to be potential candidates for future high performance high strain rate applications. With the advancement of MM additive manufacturing (AM), there is a renewed push to explore the vast array of materials combinations to deliver advanced protective capabilities. At the same time, many fundamental issues are still plaguing MM assemblies, primarily linked to the often-poor quality of the MM interface. Here, we present a novel approach to obtain microstructural and mechanical properties gradient without interface defects by leveraging the link between the chemical composition and complex grain/phase morphology in titanium alloys. We used additive manufacturing (AM) to combine α+β-titanium alloy Ti6Al4V with metastable β-titanium alloy Ti1Al8V5Fe (which belong to different alloy classes but share common alloying elements), into a microstructurally graded material (MGM). Uniquely, the classic MM interface was replaced by the two additional intermixed alloy layers with unique chemical composition, microstructure and mechanical properties, with both ultimately making a significant contribution to the overall performance. The properties of these interface alloys are affected by many factors, such as thermal properties of the substrate, process parameters, alloying element distribution and post-manufacturing heat treatment. As a result, we showed that a superior combination of the strength and ductility could be achieved in the hybrid material after heat treatment compared to the original materials or the as-built hybrid material, which was ultimately attributed to the formation of the interface alloys. The presented approach is not limited to titanium alloys and could be extended to other materials systems and is expected to contribute to the development of a deeper understanding of the intermixing phenomena and its effects on microstructure and mechanical performance of MGMs, opening the door to a range of unique solutions in alloy and MM structural design for high performance applications.