Effect of Picosecond/nanosecond Pulsed-laser Polishing on Surface Integrity of Selective Laser Melting TC4
The TC4 titanium alloy is widely applied in various fields owing to its low density,low thermal conductivity,corrosion resistance,and stable high-temperature mechanical properties.In this study,picosecond and nanosecond pulsed lasers are used in laser polishing to investigate the effects of different laser-polishing methods on the surface properties of additively manufactured titanium alloy.Pulsed-laser polishing,as a novel surface-processing technology,primarily focuses on precision polishing to achieve superior surface roughness and performance while avoiding laser-induced substrate overheating.Researchers have conducted polishing treatments using nanosecond and picosecond pulsed lasers on different materials and confirmed that pulsed-laser polishing significantly reduces surface roughness,refines the grain structure of melted pools,enhances the microhardness of polished surfaces,and alters the mechanical properties of samples.However,comparative studies pertaining to the polishing of the same thin-wall structure material are limited,thus rendering it challenging to analyze the thermal effects of different lasers and their impact on performance.Based on the average surface roughness and depth of the heat-affected zone as targets,after optimizing hundreds of parameter sets,the optimized processing parameters for picosecond pulsed lasers are determined to be as follows:spot diameter,15μm;scanning speed,2 000 mm/s;power,16 W;frequency,500 kHz;pulse width,10 ps;and serpentine scanning path.Meanwhile,the optimized processing parameters for nanosecond pulsed lasers are as follows:spot diameter,25 μm;scanning speed,2 000 mm/s;power,70 W;frequency,200 kHz;scanning spacing,0.08 mm;dot engraving time,0.1 ms;and serpentine scanning path.A comparative study is performed using picosecond and nanosecond pulsed lasers to investigate their effects on the surface morphology,surface roughness,depth of the heat-affected zone,elemental distribution,oxidation degree,and tensile properties of laser-melted TC4 titanium alloy thin-wall structures.The results indicate that both types of lasers can significantly improve the surface quality of additively manufactured TC4 titanium alloy,i.e.,the original roughness of 3.52 μm is reduced to 0.71 and 0.66 μm via picosecond and nanosecond lasers,respectively.The nanosecond pulsed laser exhibits higher thermal accumulation,thus resulting in a higher surface oxidation degree and a deeper heat-affected zone of approximately 34.20 μm,which is approximately 40.9％higher than that afforded by the picosecond pulsed laser.During laser polishing,the thermal accumulation of picosecond pulsed lasers is minimal and residual unmelted powder particles do not melt during polishing,whereas the thermal accumulation of nanosecond pulsed lasers is significant,thus causing residual unmelted particles to melt during laser polishing.The surface layer after nanosecond pulsed-laser polishing exhibits only a few minor cracks,which is attributable to thermal stress caused by high thermal accumulation.Although the thermal accumulation of the picosecond pulsed laser is smaller and the depth of the heat-affected zone is shallower,within the heat-affected zone,the oxidation degree due to picosecond-laser processing is slightly higher than that due to nanosecond-laser processing.Results of mechanism analysis show that the spot diameter of the picosecond pulsed laser is extremely small and that under the action of a Gaussian spot,the energy density at the center of the laser heat source is extremely high.Consequently,the molten pool splashes during polishing,thereby removing protruding material and resulting in a small amount of remelting,which reduces the surface roughness.The nanosecond pulsed laser relies primarily on the thermal accumulation of the overall spot to melt the original surface.Under the effect of Marangoni flow in the melt pool,the molten material flows toward the depression area,thus improving the surface roughness and resulting in a larger depth of the heat-affected zone.Using picosecond and nanosecond pulsed lasers for the laser melting and polishing of TC4 titanium alloy thin-wall structures,this study elucidates two different mechanisms of pulsed-laser polishing,thus providing new insights and methods for preparing high-surface-quality metal components via additive manufacturing.
laser polishingpicosecond lasernanosecond laseradditive manufacturingmicrostructure and properties
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