Simulation And Experimental Research On The Morphology Of Melting-solidification Zone In Laser-polishing Ti-6Al-4V Alloy

Ti-6Al-4V alloy is widely used in marine engineering,construction,chemical and other fields because of its significant advantages such as high strength and corrosion resistance,but the material is difficult to process and the processing efficiency is low,so the modification and strengthening of its surface has become a hot spot for research all over the world nowadays.As a new non-contact polishing method,laser polishing melts a thin layer of metal surface by laser radiation to obtain a smooth surface,which has significant advantages over traditional polishing methods,such as high processing flexibility,economy and environmental protection.However,the study of the dynamics,transient and steady state of the molten solidification zone during laser polishing is still a challenge,both based on the existing theoretical knowledge of laser polishing and laser polishing simulation.Therefore,in this paper,laser polishing experiments are conducted on Ti-6Al-4V alloy using continuous laser,and the changes of the morphology of the melt coagulation zone under dynamic,transient and steady state are analyzed by comparing the experimental and simulation results.In this paper,we firstly analyze the influence of laser polishing surface shallow melting mechanism(SSM)and surface over melting mechanism(SOM)on the morphology of titanium alloy melt condensation zone,simulate laser single line scan to establish a 3D simulation model,analyze the laser melting depth,width and surface profile morphology under three different cross sections,and compare with the experimental results,the error of melting depth and width is 1.7% and 9.7% respectively,the model is more accurate.The accuracy of the model is high,so that the experimental scheme of single-line scanning can be developed,and the analysis shows that the influence on the melting width is in the order of laser power,defocusing amount and scanning speed.Secondly,in order to analyze the influence of different process parameters on the morphology of the laser surface scanning melting zone,a three-dimensional surface scanning simulation model was established based on the experimental and simulation studies of single line scanning,and it was analyzed that with the continuous laser single line scanning,the temperature gradient of the melt pool at the end of laser scanning decreases and the molten metal increases,which prolongs the convection of the melt pool and reduces the cooling rate of the melt pool,so that the molten metal in the melt pool first flows and spreads from the center of the melt pool to the surrounding area and increases the area of the melt pool,and finally the morphology and temperature of the melting zone stabilize when the heat accumulation stabilizes.Then the laser surface scanning model was optimized and deflated,and the laser surface scanning simulation experiment was designed to analyze the influence law of laser power,scanning speed and scanning spacing on the morphology and surface roughness of the melting zone,and the laser surface scanning experiment was designed based on this.At 4.5 m/min,the surface roughness decreases up to 93%,but at 5m/min,the surface roughness rises further;at 25 μm and 50 μm,the laser linear scanning trajectory is obvious,and the morphology of the fusion zone is prone to oxidation and blackening,even causing surface wear and burn,which increases the surface roughness,and at 75 μm,the irregular surface morphology of the laser fusion zone,such as scanning traces,micro-convex peaks and pits,is significantly reduced,and the surface becomes smooth and polished.The surface becomes smooth and flat,and the polishing effect is better.Finally,a BP neural network prediction model was established to compare the real values of surface roughness,melt width and depth with the predicted results,and the maximum relative errors were 5.11%,2.09% and 1.22%,respectively.In this paper,we analyze the influence of laser power,scanning speed and scanning spacing on the morphology of laser polishing melting zone by establishing a simulation model,and compare it with the experimental results to verify that the model error is small,which can provide some guidance for laser polishing experiments,thus effectively reducing the experimental time,saving the experimental cost and improving the experimental efficiency of laser polishing.