The hybrid micro-machining process employs simultaneous or ensuing action of two or more machining processes or takes aid of some energy assistance in material removal to enhance advantages and minimize potential disadvantages found in individual material removal techniques. There are various examples, such as abrasive electro-discharge grinding, electrochemical discharge machining, laser-assisted turning, and jet-assisted machining. This article presents a novel classification of and reviews the past and present research and applications of the hybrid micro-machining process, highlighting its effects on response variables such as material removal rate, tool wear, and surface roughness. Although hybrid micro-machining has been an active research area in non-conventional and critical machining since few years, insufficient knowledge about capabilities in sizes, materials, machinery, and manipulating devices is still a major constraint in the development of these processes. In the near future, there is ample scope for the hybrid micro-machining process in exploring issues in material removal at micro-scale, effect of residual stresses, and environment.

Together, 316L steel, magnesium-alloy, Ni-Ti, titanium-alloy, and cobalt-alloy are commonly employed biomaterials for biomedical applications due to their excellent mechanical characteristics and resistance to corrosion, even though at times they can be incompatible with the body. This is attributed to their poor biofunction, whereby they tend to release contaminants from their attenuated surfaces. Coating of the surface is therefore required to mitigate the release of contaminants. The coating of biomaterials can be achieved through either physical or chemical deposition techniques. However, a newly developed manufacturing process, known as powder mixed-electro discharge machining (PM-EDM), is enabling these biomaterials to be concurrently machined and coated. Thermoelectrical processes allow the migration and removal of the materials from the machined surface caused by melting and chemical reactions during the machining. Hydroxyapatite powder (HAp), yielding Ca, P, and O, is widely used to form biocompatible coatings. The HAp added-EDM process has been reported to significantly improve the coating properties, corrosion, and wear resistance, and biofunctions of biomaterials. This article extensively explores the current development of bio-coatings and the wear and corrosion characteristics of biomaterials through the HAp mixed-EDM process, including the importance of these for biomaterial performance. This review presents a comparative analysis of machined surface properties using the existing deposition methods and the EDM technique employing HAp. The dominance of the process factors over the performance is discussed thoroughly. This study also discusses challenges and areas for future research.

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