【作者】 周阳；

【导师】 胡乾午；

【作者基本信息】 华中科技大学 ， 光学工程， 2017， 硕士

【摘要】 大型铝合金铸件在航空、航天和汽车等领域广泛使用,但铝合金自身硬度较低,耐磨性差,且铸造过程中难以避免地会产生气孔、疏松等缺陷,严重影响了工件质量。采用激光熔覆方法,可显著提高材料表面的综合性能,能够对铝合金件的缺陷进行修复。然而,现有的铝合金激光熔覆工艺尚为欠缺,送粉困难、表面成形质量差、气孔和裂纹难解决成为制约铝合金激光熔覆进展的主要问题。对此,本文进行了如下研究。本文首先研制了一种新型的复合运动式送粉器,采用螺旋叶轮推进和转盘式抽吸相结合的运动送粉方式实现送粉;并对实验室已有的送粉器进行改装优化,通过数值模拟,对出口流道进行改装,实现了粉量均匀且在0-50g/min范围内连续可调的送粉过程,解决了铝合金激光熔覆中的送粉难题。其次,在2A12铝合金基板上进行工艺实验,从粉末基材预处理,光束模式及质量,粉末送给方法,保护气氛设计等方面对铝合金激光熔覆工艺进行优化,获得了铝合金激光熔覆平整光滑的成形表面,并初步提高了熔覆层质量,将气孔率控制在1%左右。为进一步提高铝合金激光熔覆层质量,本文通过合理的磁场设计,开发了交变磁场辅助激光熔覆技术,利用交变磁场对熔池金属产生的洛伦茨力作为电磁驱动力,实现了对气孔/夹杂物的选择性分离,成功地将气孔率控制在0.5%以内,根据“GB/T22087-2008”,该结果达到了B级焊接质量标准。通过FLUENT软件对熔覆熔池进行数值模拟,观察电磁力作为单驱动力作用下熔池的流场状况。模拟结果表明,在电磁力作用下,熔覆熔池表面的液态金属由边缘流向熔池中心,使熔池加深;熔池中心地带,金属熔体向下流动,在两翼形成涡流;这说明在电磁驱动力的作用下,熔池金属受到向下的体积力作用,能将气孔和非金属夹杂物推向熔池顶部与边缘。最后,通过添加10%Fe₂O₃+90%AlSi10Mg的混合粉末进行验证实验,结果表明,电磁驱动力作用下,Fe₂O₃与Al在高温下反应生成的Al₂O₃,被推动到熔池顶端。验证实验和数值模拟均解释了交变磁场对铝合金熔覆熔池电磁净化作用的机理。交变磁场在熔池表面产生的电磁力,对高电导率粒子产生下压作用,由此产生的提升力将低电导率的粒子（气孔/低导电率夹杂物）推向熔池表面。更多还原

【Abstract】 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.更多还原