In-process fingerprints of dissimilar titanium alloy diffusion bonded layers from hole drilling force data

The manufacture of components that have specific material properties in subcomponent regions is highly desired in many sectors. However, it is challenging to achieve via conventional ingot-wrought and joining processing routes. Recently, diffusion bonding titanium alloy powder using field assisted sintering technology (FAST) has demonstrated that multi-material billets can be manufactured. Such billets still need to be machined into final net shaped components. The machinability and machining strategy of such components needs to be better understood if manufacturing of multi-material components is to be economically viable. This is the first study where drilling machinability of FAST diffusion bonded titanium alloys has been investigated. Location indexed force and torque feedback in-process fingerprinting is utilised during the drilling of multi-material titanium alloy billets. The in-process fingerprinting enabled rapid identification of the types and layering order of alloys within the FAST billets. In addition to force feedback, the hardness, hole surface topography and subsurface microstructure were characterised. Although hardness was found to contribute to variation in bond to bond machinability, results highlighted how alloy chemistry and bond composition are intrinsic to the machining directionality and significantly influence the machined surface quality and process stability. The work demonstrates that machining strategy of multi-material drilling needs to be tailored with respect to direction and diffusion bonded alloy pairing to avoid undesirable surface and subsurface damage at bond locations.