Influence Mechanism of Process Parameters on Nanosecond Laser Polishing Quality of Ti6Al4V Titanium Alloy

Ti6Al4V (TC4) titanium alloy is widely used in aerospace, biomedicine, and other high-precision applications due to its excellent specific strength, corrosion resistance, and biocompatibility. However, its surface quality directly affects the fatigue life and service performance of parts, and traditional polishing methods suffer from low efficiency and high pollution. As a non-contact, controllable surface treatment technology, nanosecond laser polishing has demonstrated unique advantages in balancing processing efficiency and surface quality. This study systematically discussed the influence of key process parameters (spot overlap rate, laser power, and scanning times) on nanosecond laser polishing of TC4 titanium alloy. It revealed the internal physical mechanism by analyzing the temperature and velocity fields and vortex dynamics during molten-pool evolution. It is found that the polishing effect is determined by the process parameters, which adjust the thermal-fluid coupling physical field (temperature distribution, melt flow, and vortex structure) in the molten pool. There is an optimal combination of parameters (spot overlap rate of 79%, laser power of 0.8W, scanning speed of 5m/min, scanning 3 times) that can place the molten pool in an optimal dynamic balance state and achieve effective flatness. The experimental results show that under this parameter, the surface roughness of the specimen with an initial roughness of 1.223 μm is reduced by about 32%. The research further clarified the mechanism by which the initial roughness of the base metal influences the molten pool: the greater the initial roughness, the more pronounced the "peak shaving and valley filling" effect. Under the same parameters, the improvement rate of the specimen with the initial roughness of 1.623 μm could reach about 40%. This study not only establishes the optimized process window, but also reveals the essential relationship between "process parameters - bath behavior - surface quality" from the level of the physical field of the molten pool, which provides an essential theoretical basis and practical guidance for the laser polishing process of TC4 titanium alloy high-precision surface.