Progress and perspectives in cold plasma-enabled hybrid machining of difficult-to-machine materials

Difficult-to-machine materials commonly suffer from severe tool wear, unstable interfacial friction and surface/subsurface damage during precision machining. In hybrid machining, cold plasma acts mainly as a surface- and interface-regulation method rather than a bulk-softening method, because it can modify surface chemistry, alter interfacial conditions and improve wettability under low thermal load. Based on this role, this review discusses the development of cold plasma toward in-process application, typical cold plasma jet configurations, excitation methods and the integration of cold plasma with hybrid machining processes. It further reviews the material-dependent responses of cold plasma hybrid machining in hard and brittle inorganic materials, metal alloys and composite materials, with emphasis on how plasma-induced surface changes are translated into machining performance and surface-integrity improvements. The available studies show that the effectiveness of cold plasma strongly depends on coupling mode, material system and the match between the plasma-modified layer and the actual removal conditions. Current limitations mainly include insufficient direct mechanistic evidence, limited studies on multiphase composites and incomplete understanding of multiphysics-coupled plasma machining. Future work should therefore focus on broader and more stable hybrid integration, deeper mechanism validation, improved modeling and simulation and more reliable process control.