Simulation And Experimental Study Of Nanosecond Laser Repair Of Surface Scratches On Materials

In production life,the surface of materials can produce scratches,micro-cracks and other types of defects due to the external environment and human factors,such defects not only affect the aesthetics,but also put some important components at risk of failure,so it is important to study the repair of material surface defects.This paper investigates the effect of nanosecond lasers on the repair of surface defects in metallic materials(304 stainless steel)as well as polymeric materials(acrylic polyurethane),based on laser remanufacturing technology.First,a finite element model of the interaction between the nanosecond laser and the material was established,ignoring the vaporization,melting,heat deformation and chemical changes occurring on the surface during processing,and the relationship between the peak temperature of the material and the gas and liquid phase lines at different energies was simulated to derive the laser energy that can cause a phase change in the material.The variation of temperature at pulse widths of 10ns,50ns and 100ns was simulated and the effect of pulse width on laser energy was found to be small.Since the depth of the molten pool formed after the phase change determines whether the repair can be achieved,the depth and width of the molten pool at different energies were simulated numerically.Secondly,the effect of laser repetition frequency on material temperature at continuous pulses was analysed,with the energy of individual pulses decreasing as the frequency increases,but the resulting thermal build-up effect increasing;Simulation of material temperature changes at different scanning speeds,where the size of the heat-affected zone generated within the material is negatively correlated with the scanning speed and eventually reaches a state of thermal equilibrium.The melt depth of the melt pool at different lap rates by studying the relationship between scanning speed,scanning pitch and spot lap rate.The model was coupled with heat and flow,and the formation of the melt pool and the effect of the distribution of the flow field on the surface morphology of the material were analysed.The flow pattern of the melt pool at different scanning speeds was investigated,and it was concluded that the flow rate of the material in the melt pool has a certain enlargement effect on the size of the melt pool.The temperature change inside the material after considering only the thermal effect and the heat flow coupling was compared,and part of the heat was converted into kinetic energy of the molten material after coupling,resulting in a slightly lower temperature in the coupled field.Finally,repair experiments were carried out using a nanosecond laser on 304 stainless steel and acrylic polyurethane,and the quality of the repair was characterised by the surface morphology of the repair.A better combination of process parameters was derived by carrying out orthogonal experiments to verify the possibility of nanosecond laser repair of defects in different materials.The surface morphology of the repaired 304 stainless steel was good with a laser energy of 22.4 J/cm2,a scanning speed of 8m/s and a longitudinal lap rate of 70%,and the difference between the peak and trough of the material surface profile was 200nm,much smaller than the initial 2um,the acrylic polyurethane was repaired better at laser energy of 2.18 J/cm2,50%transverse lap rate and 80%longitudinal lap rate.In addition,the causes of various types of defects on the surface of the repaired material were analysed.By comparing the repair results of the two materials,it was concluded that the method of repairing defects using the nanosecond laser is more suitable for materials with a large difference between the liquid phase line and the gas phase line.