An Experimental Study On The Removal Of Surface Oxide Layer From Hot Rolled Steel With Picosecond Laser

Hot rolled steel is widely used in ships,automobiles,bridges,construction,machinery,pressure vessels and other manufacturing industries due to its high strength,good toughness,easy processing and good weldability.As the hot rolled steel would produce a certain thickness of oxide layer during the rolling process and affect the surface quality of the steel,the surface quality of the workpiece after using traditional methods like high-pressure water or mechanical methods is unsatisfactory and even damage the environment.The laser removal method is a novel,environmental friendly rust removal method and is an ideal alternative.According to the situation above,a laser processing system using ultra-fast picosecond laser as light source is designed and built in this thesis.A series of experimental studies are carried out on the removal of the oxide layer on the hot rolled steel surface,The mechanism influence of each process parameter on the removal quality of oxide layer is obtained and then an innovative removal strategy is proposed,The detail description is listed as follows:(1)The type and composition of the hot-rolled steel oxide layer are introduced,the removal method adopted at the present stage is introduced,and expounded the research status of using laser to remove the metal oxide layer.Beginning with the design of optical and hardware systems,a set of picosecond laser processing platform was built which is capable of laser cleaning and micro-nano processing of hard and brittle materials.(2)The laser removal mechanism on metal oxide layer is analyzed,and then pointed out that the laser gasification and thermal expansion are the main removal mechanisms.The evaluation index of laser rust removal effect is introduced and the surface quality evaluation criteria of laser rust removal was proposed..(3)The mechanisms of laser average power density,pulse repetition frequency,spot overlapping rate and scan track overlap rate,and the number of repeated scans on the surface quality of the workpiece using Orthogonal and single-variable test method are investigated,and optimized processing parameters are obtained.(4)An innovative composite derusting strategy was proposed,ie,a higher surface quality can be obtained by using a combination of high and low spot overlap ratios and laser energy density alternately.The oxygen content reduced about 92% compared to the oxide surface.The rust removal efficiency of each rust removal strategy is calculated.(5)The mechanisms of the effect of laser treatment on the surface hardness and corrosion resistance of the workpiece are preliminarily explored,the surface hardness of the workpiece after laser irradiation become larger and the corrosion resistance was improved compared with the metal matrix.