Laser-chemical Composite Polishing and Surface Morphology Evolution of Medical TC4 Titanium Alloy
Surface finish is a crucial quality characteristic of medical device components.However,existing surface polishing technologies have some shortcomings,such as difficulty in controlling the polishing effect,poor precision,and inability to manage components with complex shapes.In this study,a laser-chemical composite polishing system was designed and built for the precision polishing of medical-grade TC4 titanium alloy.The material removal mechanism of laser-chemical composite machining was analyzed.The evolution process of the material surface morphology and surface roughness changes at different polishing stages was investigated by conducting laser chemical composite polishing experiments on medical TC4 titanium alloys;thus,clarifying the mechanism of laser chemical composite polishing.The results show that material removal by laser-chemical composite polishing is based on the combined influence of laser thermal-mechanical effects and laser-induced chemical corrosion.Moreover,these two factors have a synergistic effect under certain conditions,which can mutually promote and enhance material removal efficiency and processing quality.Within an appropriate process window,chemical dissolution can completely remove the residues and remelts generated by laser ablation.Laser irradiation causes temperature differences between the peak and valley regions of the material surface,leading to different chemical dissolution rates.By exploiting the difference in the dissolution rates between the peaks and valleys on the surface of the workpiece,improvements in the surface roughness of the laser-irradiated region were achieved.Moreover,the chemical polishing mechanism is based on the atomic-scale dissolution of materials,which results in higher polishing precision compared with laser thermo-mechanical etching.The ratio of the laser etching material removal to the chemical dissolution material removal determines the ultimate roughness limit of the titanium alloy surface.A higher proportion of chemical dissolution resulted in better surface smoothness but a lower polishing efficiency.Therefore,during the final polishing stage,reducing the energy of laser irradiation on the workpiece surface can decrease the laser etching ratio and improve the final polishing effect.Additionally,the uneven distribution of the alloying elements on the surface of the TC4 titanium alloy affected the final surface smoothness.This is because Al and Fe in TC4 titanium alloys exhibit better chemical activity in acidic environments than V and Ti.During the chemical corrosion process,micro-galvanic cell phenomena occur,leading to preferential dissolution of the anode.Therefore,based on the chemical polishing mechanism,it can be understood that the ultimate polishing limit is influenced by the purity and microstructure of the material.The purer the material composition and smaller the microstructure,the better the final polishing effect.The selective removal of laser-chemical composite polishing is based on laser etching and thermochemical dissolution induced by laser activation.During laser-chemical composite polishing,laser irradiation acts as a local and selective heat source,inducing appropriate thermal shock and activating a non-uniform chemical reaction between the chemical solution and metal surface,resulting in temperature-induced chemical corrosion.In the laser irradiation area,the passivation film on the metal surface was stripped and dissolved under the dual action of physics and chemistry,whereas other parts of the workpiece material were protected by the passivation film,and almost no corrosion occurred.Under the continuous action of laser etching and chemical dissolution,the etching rates in the laser-irradiated and non-irradiated areas were significantly different,thereby achieving selective etching of the metallic material.The presence of bubbles during processing significantly affects the laser-chemical composite polishing.The causes of bubble generation were analyzed,and several methods were proposed to overcome bubble disturbances.It was demonstrated through experiments that,after taking appropriate measures,the bubble disturbance during the polishing process can be effectively reduced.Finally,the selective precision polishing of the medical TC4 titanium alloy was achieved using suitable process parameters.The surface roughness of laser irradiation area Ra decreased from 5.230 μm to 0.225 μm,and Sa decreased from 8.630 μm to 0.571 μm,which is a decrease of 95.7％and 93.4％,respectively.These research findings provide a reference for the precision polishing of titanium alloys or other self-passivating metals.
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