Research On Laser Cleaning Process And Technology Of Aluminum Alloy For High-speed Railway Car Body

With the lightweight of high-speed railway body materials,aluminum alloy occupies 100%of the domestic high-speed railway market due to its lightweight,high strength,and strong corrosion resistance.After 600000 km operation,high-speed EMUs need to undergo three-level shop repair,including cleaning the car body,cleaning the stress concentration parts before flaw detection,derusting,and other cleaning work,in which the cleaning quality of high-speed railway car body skin is particularly important.If the paint on the car body is worn or there is stubborn dirt on the car body,it is necessary to remove the car body first,remove the topcoat and primer on its surface,and then maintain and repaint the car body.Therefore,the oxidation film and oil stain on the surface of aluminum alloy needs to be removed during the welding pretreatment of the current high-speed aluminum alloy body maintenance process,and the black ash near the welding seam of aluminum alloy should be removed.In this paper,the laser cleaning technology is used to deal with 5083 aluminum alloy before welding and post welding,and the laser cleaning technology of aluminum alloy is studied in depth.The temperature field of the laser cleaning process was simulated by ANSYS software,and the initial cleaning process parameters were obtained.Furthermore,the removal effect of the oxide film and dirt,friction and wear performance,residual stress,hardness,and corrosion resistance on the surface of aluminum alloy and its weldment before and after laser cleaning were analyzed,and the best cleaning process parameters were obtained.The main contents of this paper are as follows:（1）ANSYS software was used to simulate the temperature field distribution in the process of pulsed laser cleaning of aluminum alloy,and to reveal the influence of oxide film and substrate temperature on the surface morphology of aluminum alloy under the action of the laser.The temperature field changes of the oxide film and aluminum alloy substrate under the laser spot diameter of 0.5mm-2mm,a pulse width of 50ns-200ns,a repetition rate of 8k Hz-11k Hz,and an overlap rate of 20%-80%were simulated.The pre parameters of the cleaning process in the simulation process were obtained:the optimal laser spot diameter was 0.5mm,the optimal laser pulse width was 50ns,the optimal repetition rate was 8k Hz,the optimal spot overlap rate was 8k Hz,and the best spot overlap ratio was 40%.（2）Aiming at the typical aluminum alloy high-speed rail car body,5083 aluminum alloy plate and its weldment were selected as experimental materials,and the Nd:YAG laser was used for cleaning to seek the best cleaning process parameters.By comparing the aluminum alloy morphology after cleaning under the laser power of 50W-110W and the cleaning speed of1mm/s-20.7mm/s,combined with the surface roughness and surface element test,it is concluded that when the laser power is 98W and the cleaning speed is 4.1mm/s,the removal effect of oxide film on the aluminum alloy surface is the best,and the smoothness is improved.This paper mainly analyzes the surface characteristics of aluminum alloy weldment under the laser cleaning speed of 1mm/s-20.7mm/s.With the continuous slowing down of the cleaning speed,the welding slag such as Mg O and Al₂O₃ is gradually removed,and the oxygen content in the weld area reaches the lowest value at 5.2mm/s.（3）The reciprocating dry friction and wear,microhardness,and residual stress of the heat-affected zone of aluminum alloy plate and weldment cleaned by different laser power and different laser cleaning speed were tested,and the influence of laser cleaning on the tribological characteristics of aluminum alloy plate and weldment was analyzed.The results show that when the laser power reaches 98W,the wear scar depth and friction coefficient on the surface of the aluminum alloy sheet is the minimum.Under this laser power,the tribological characteristics of the heat-affected zone can be improved when the cleaning speed is in the range of 5.2mm/s-20.7mm/s.Besides,laser cleaning can form a hardened layer on the surface of aluminum alloy,release and redistribute the residual stress on the surface of aluminum alloy,resulting in residual tensile stress.For aluminum alloy weldments,laser cleaning can eliminate residual stress and improve the strength of welded joints.（4）The aluminum alloy sheets cleaned by laser power of 50W-110W were subjected to neutral salt spray corrosion test at room temperature for 72h and 5%salt spray concentration.The effect of laser cleaning on the corrosion resistance of aluminum alloy surface was analyzed by electrochemical and XRD tests.The results show that the original oxide layer composed of Mg O+Mg Al₂O₄ on the surface of aluminum alloy is removed by a laser,and a new layer of Al₂O₃ oxide film is formed,which greatly improves the corrosion resistance of the aluminum alloy.However,the surface of aluminum alloy is seriously melted and ablated.