Exploring reverse-μEDM capability for the fabrication of microgrooves on pinless friction stir processing tools

Existing microfabrication techniques for producing featured friction stir processing (FSP) tools are restricted to certain alloys, and generating precise microfeatures on the shoulder of a tungsten carbide (WC) FSP tool is particularly challenging. Conventional microfabrication methods such as micromilling and microturning require costly tooling, which limits their use on hard-to-cut materials. In this work, the capabilities of an emerging nonconventional microfabrication technique, reverse micro-electrical discharge machining (reverse-μEDM), are exploited to produce precise triple-spiral microgrooves on a WC FSP tool. The highly precise featured tool plate that is integral to the reverse-μEDM setup was first prepared using two different technologies: laser micromachining of titanium sheets, and micromilling of copper and brass sheets (chosen in view of their higher reflectivity). Reverse-μEDM was then performed using each tool plate, and the material removal rate, tool wear rate, and surface roughness of the fabricated microgrooves were measured as the output responses. The titanium tool plate performed best in generating the FSP tool microfeatures because of its favorable physical properties. The copper and brass tool plates had higher wear ratios than the titanium tool plate but produced poorer surface characteristics and lower geometrical precision. The grooves produced with the titanium tool plate had a precise depth of 150 μm with negligible wall taper along their entire length. To validate the feasibility of the fabrication, a pilot FSP experiment was also conducted on a cold-sprayed Inconel 718 substrate.