Study On The Clad Quality And The Impact Factors In Aluminum Alloy Laser Cladding

Large-scale aluminum alloy castings are widely used in the fields of aviation,aerospace and automotive,besides low hardness and poor wear resistance,the defects of shrinkage blowholes and porosity easily produced in casting process affects the quality of the workpieces seriously.The method of laser cladding is able to improve the overall performance of the material surface significantly,and it can also repair the defects of aluminum parts.However,the current technology of laser cladding for aluminum alloy shows more weaknesses,for instance,sending difficulty of powders,poor quality of surface forming,blowholes and cracks,which is referred to as the primal issues on the development of aluminum alloy laser cladding.Therefore,some researches were conducted as following in this paper.Firstly,a new type powder feeder with compound motion that applies the union assembly with pushing forward spiral impellers and suction turning disc was developed.In addition,a powder feeder existing in laboratory has been modified,its outlet channel was optimized according to the result of FLUENT numerical simulation,and an equal and stable powder delivery system was achieved,which solves the problem of powder delivery in aluminum alloy powder,the amount of feeding powders can be adjusted continuously in the range of 0-50 grams per minute.Secondly,the process experiment of laser cladding on 2A12 aluminum alloy plates was produced.The technical process of aluminum alloy laser cladding,including pretreatment of powders and substrates,mode and quality of the laser beam,method of powders feeding,and design of the protective atmosphere,was optimized.Then,a flat and smooth forming surface was fulfilled by laser cladding for aluminum.The quality of the cladding layers was improved preliminarily,and the porosity was controlled below1%.In order to further improve the quality of the layers of aluminum laser cladding,a technique of alternating magnetic field assisted laser cladding（AMFALC）was developed through rational design of the magnetic field.The selective separation of the blowholes/inclusions was achieved by the Lorentz force caused by the alternating magnetic field that was generated in the metal pool,and the porosity was successfully controlled within0.5%,the results show that the quality meets the B grade welding standards according to the standard document“GB/T22087-2008”.The numerical simulation of the melt pool has been performed by use of the FLUENT software so as to observe the conditions of the flow field under dynamic actions of the single drive of the electromagnetic force.The simulation results show that in the melt pool surface,the liquid metal flows from the edge to the center,and a deeper melt pool is occurred;meanwhile,due to the role of the electromagnetic force,the liquid metal in the internal center of the pool flows downward,and the vortexes are formed on both sides.This indicates that the metal in the melt pool is acted upon with downward volumetric force under action of the electromagnetic force,and the blowholes and non-metallic inclusions could be pushed upon to the top and the edge of the melt pool.By using the mixed powders of 10%Fe₂O₃+90%AlSi10Mg,the verification experiments were produced,the results show that the Al₂O₃,produced by the reaction between Fe₂O₃ and Al at high temperature,is pushed to the top of the melt pool under actions based on the electromagnetic driving force.The numerical simulation and the verification experiments give an account of the mechanism of the electromagnetic cleaning on melt pool of aluminum alloy under actions of the alternating magnetic field.The electromagnetic force generated by the alternating magnetic field on the surface of the melt pool creates a downward pressure on the high-conductivity particles,nevertheless the low-conductivity particles（blowholes/low-conductivity particles）were pushed upon to the surface of the melt pool acted by the resulted lifting forces.