Field-Assisted Sintering Technology processing route of metal-metal composites

A hybrid processing method named AddFAST has been developed for producing novel microstructures and properties in metal parts. The process combines Field-Assisted Sintering Technology (FAST) with Additive Manufacturing (AM) to bond a complex AM lattice structure with a bed of powder, combining them into one composite item. The aim of this project is to assess the viability of this method and investigate the practical knowledge necessary for it to develop into a reliable, applicable industrial process.
AddFAST samples were investigated initially using a Ti-6-4 AM lattice infiltrated with Ti-6-4 powder. The microstructure of the sintered parts showed two clear regions of contrasting grain morphology and texture, in an arrangement based on the AM lattice’s structure. The interface between the regions was dense and well-bonded. A parametric study of the FAST process showed that when sintered above the alloy’s beta transus temperature, the two regions normalise and their distinction is lost. Processing at lower temperatures for long dwell periods produced dense items with the desired microstructure, but also with much higher energy demands. Process temperature was found to be of greater significance than process time in determining the final part quality. Investigation of practical factors found that the process is tolerant of powder size for infiltrating fine AM structures, and that the FAST process stage can compress the original AM structure by a third.
Combining the AM lattice with powder of different alloy composition showed that the AddFAST process can be used to combine multiple alloys into one part, whether alloys of the same system or different ones. Bonding at the composition interface was dense and high-quality, but required the heat and load of the FAST processing to smooth the AM structure’s surface to do so. Without this effect, interfacial bonding was limited. The well-bonded mixed-composition samples showed an interdiffusion region at the interface, but no evidence of intermetallic formation.
Compression testing on single-composition samples found that the AM regions’ presence only impacted mechanical properties when they occupied a sufficient volume fraction. The prior β grains of the AM region caused a decrease in strength, which was more pronounced at high temperature. Mixed-composition samples showed the addition’s impact as a load-bearing reinforcement, even at low volume fractions. Excess difference in mechanical properties, however, led to catastrophic failure.
AddFAST processing has been demonstrated as a viable route to create complex, architectured microstructures in metal parts. This has the potential to produce novel behaviours with a high degree of control in a variety of alloy systems.