Surface microstructural alterations and grain refinement mechanisms of tungsten heavy alloys by thermo-mechanical coupling: A laser-assisted milling case

Ultrafine-grained (UFG) tungsten heavy alloys possess potential applications in precision instruments and nuclear protection. In the present study, a comprehensive investigation of machined surface microstructural alterations and grain refinement mechanisms in laser-assisted milling (LAM) 95W-3.5Fe-1.5Ni alloy was demonstrated by recognizing its surface microstructures and mechanical properties under different combinations of the laser power and cutting depth. Graded microstructural characteristics along the surface degenerating layer and subsurface deformed layer of tungsten heavy alloy were identified for the first time, dramatically changing from workpiece bulk. Results showed that the adaptability between the thickness of the laser heat-affected layer and the depth of cut greatly influenced surface recrystallization layer microstructures regarding the thermal-mechanical coupling effect. An ordered, continuous, and equiaxed refined grains distribution layer was recognized when a particular machining condition was adopted, resulting in a substantial improvement of mechanical hardness. It could be indicated that a redundant heat-affected area would induce strain accumulation and promote recrystallization in LAM. Moreover, a dislocation density-based recrystallization model was applied to complement the experimental results considering the temporal transformation of grain refinement and recovery phenomena. Clarifying the above aspects is crucial to control the tungsten heavy alloys' surface microstructures and service performance during high-performance machining operations.