【作者】 涂凯；

【导师】 闫洪；

【作者基本信息】 南昌大学 ， 材料加工工程， 2021， 硕士

【摘要】 7xxx系铝合金因其具有较高的比强度、较高的比刚度和良好的热加工性能等优点而被应用于汽车制造、电子通信及军事等领域。但是,因为现代社会的快速发展,人们对高性能材料的要求越来越高,此时性能优异的铝基复合材料就派上用场了。7075铝合金作为本次实验所需的基体材料,TiO₂@CNTs作为增强体。通过化学镀表面改性的方法在碳纳米管表面包覆一层TiO₂颗粒,旨在提高CNTs与Al基体之间的润湿性,使得CNTs能够顺利的加入7075熔体中。再通过球磨混粉与液压的方法制备了Al-TiO₂@CNTs中间预制块,在高能超声震动的协助下把Al-TiO₂@CNTs中间预制块加入熔融的7075铝合金基体中制备了铸态0.9wt.%7075-TiO₂@CNTs复合材料。但是因为熔融铸造制备的复合材料在凝固的过程中不可避免的会产生元素的偏聚而导致材料的强度和硬度降低的趋势,所以对复合材料进行了T6热处理,探究了时效温度和时效时间对复合材料的机械性能的影响。在摩擦磨损试验机上对铸态和T6态复合材料进行了干滑动摩擦磨损测试,分析了材料的磨损率、磨损系数和磨损表面,并探究了材料的磨损机制。实验结果表明:向7075熔体中加入0.9wt.%TiO₂@CNTs增强体,在高能超声震动的协助下增强体能够较为均匀的分散于基体中。7075-0.9wt.%TiO₂@CNTs复合材料的强度和显微硬度相比于7075基体而言都有较大的提升,铸态复合材料的极限抗拉强度、屈服强度和显微硬度分别为261.79 MPa,201.22MPa,126.02 HV,与7075基体相比分别提高了31.99%、40.93%和32.22%。为了消除铸态复合材料中的元素偏析,进一步优化材料的组织和提高材料的强度和显微硬度,对复合材料进行了T6热处理使得复合材料中沿晶界偏析的第二相元素减少。实验结果表明:经过固溶处理之后复合材料中沿晶界偏析的第二相元素逐渐融入基体中,只有少量高熔点的第二相依然偏聚于晶界处,其中固溶处理起到了固溶强化的作用。固溶处理之后的复合材料的屈服强度、极限抗拉强度和显微硬度分别为240.39 MPa、301.13 MPa和142.61 HV,与铸态复合材料相比较分别提升了19.47%、15.02%和13.16%。固溶处理的主要目的是为了后续的时效处理做准备的,在低温时效的过程中过饱和的固溶体逐渐从α-Al基体中析出,形成尺寸为纳米级的MgZn₂强化相,强化相以弥散分布的形式存在于基体中,起到了弥散强化的作用,并且细小的强化相与位移位错相互作用,抑制了位错的运动,位错密度的增加也使得复合材料的机械性能得到了大幅度的提升。经过T6热处理之后复合材料的断口上存在许多大小不一的韧窝,此时材料主要是以韧性断裂为主。利用摩擦磨损试验机测试铸态及T6热处理之后的复合材料的摩擦磨损性能,其中磨损率和磨损系数等性能被研究,利用扫描电子显微镜观察磨损表面和磨屑的形貌,并分析了摩擦磨损机制。研究结果表明:7075熔体中添加0.9wt.%TiO₂@CNTs增强体可以有效的减小材料的磨损率和磨损系数,因为CNTs可作为固体润滑剂覆盖在样品磨损表面,并且减小磨损表面与摩擦副之间的接触面积,从而极大的提高材料的耐磨性能。经过T6热处理之后复合材料的强度和硬度都得到了极大的提升,材料的硬度越大则材料的耐磨性能越好。在相同磨损条件下,T6热处理之后的复合材料较铸态复合材料的磨损率相比减少了一个数量级,并且材料的磨损系数也降低了许多,通过SEM分析了磨损表面的形貌可知T6态材料主要是以磨粒磨损为主要磨损机制,与铸态材料的磨屑相比,T6热处理之后的复合材料的磨屑变得非常细小,这也充分说明了T6热处理之后的复合材料拥有较好的耐磨性能。更多还原

【Abstract】 7xxx series aluminum alloys are widely used in automobile manufacturing,electronic communications,military and other fields because of their high specific strength,specific rigidity and good thermal processing performance.However,because of the rapid development of society,people’s requirements for high-performance materials are getting higher and higher.At this time,aluminum matrix composite materials with excellent performance come in handy.This article uses 7075 aluminum alloy as the matrix material,and carbon nanotubes as the reinforcement.The surface modification method of electroless plating is used to coat a layer of TiO₂ particles on the surface of the carbon nanotubes to improve the wettability between the CNTs and the Al matrix,so that the CNTs can be successfully dissolved into the matrix.Then the Al-TiO₂@CNTs intermediate preform was prepared by the method of ball milling and powder mixing and hydraulic pressure.With the assistance of high-energy ultrasonic vibration,the Al-TiO₂@CNTs intermediate preform was added to the molten 7075 alloy to prepare the as-cast0.9wt.%7075-TiO₂@CNTs composite material.However,because the composite material prepared by melt casting will inevitably lead to the segregation of elements in the 7075 matrix and weaken the strength and hardness of the material,the composite material was processed with a T6 heat treatment process,and the effect of aging temperature and aging time on the mechanical properties of composite was explored.Dry sliding friction and wear tests were carried out on the as-cast and T6composite materials.The wear rate,wear coefficient and wear surface of the material were analyzed,and the wear mechanism of the material was explored.Dry sliding friction and wear tests were carried out on the as-cast and T6 composites.The wear rate,wear coefficient and wear surface of the composites were analyzed,and the wear mechanism of the composites was explored.The experimental results show that0.9wt.%TiO₂@CNTs reinforcing phase is added to the 7075 melt,and the reinforcing phase can disperse uniformly along grain boundaries in the matrix under the assistance of high-energy ultrasonic vibration.Compared with 7075 matrix,the strength and microhardness of 7075-0.9wt.%TiO₂@CNTs composite have been greatly improved.The ultimate tensile strength,yield strength and microhardness of the composite are 261.79 MPa,201.22 MPa,126.02 HV,respectively.Compared with the matrix material,increased by 31.99%,40.93%,and 32.22%,respectively.In order to eliminate the element segregation in the as-cast material and further improve the strength and microhardness of the composite material,T6 heat treatment was carried out to reduce the grain boundary segregation.The experimental results show that the second phase elements segregated along the grain boundary in the composite material gradually merge into the matrix after solution treatment,and only a small amount of insoluble second phase is still segregated in the matrix.The solution treatment has achieved solid solution strengthening.effect.The yield strength,ultimate tensile strength and microhardness of the composite material after solution treatment are 240.39 MPa,301.13 MPa and 159.3 3HV,respectively.Compared with the as-cast composite material,improved by 19.47%,15.02%and13.16%respectively.The main purpose of the solution treatment is to prepare for the subsequent aging treatment.During the low temperature aging process,the supersaturated solid solution gradually formed the size about the nanoscale MgZn₂strengthening phase.The strengthening phase is in the form of a dispersion distribution,which plays the role of dispersion strengthening,and the interacts between the fine strengthening phase and the displacement dislocations inhibits the movement of the dislocations.The increase of dislocation density also greatly improves the mechanical properties of the composite material.After T6 heat treatment,there are many dimples of different sizes on the fracture of the composite material.At this time,the material is mainly ductile fracture.The friction and wear properties of the as-cast composite materials and composite under heat treatment were tested by the friction and wear tester.The properties,such as the wear rate and the wear coefficient were studied.The friction and wear mechanism were analyzed by observing the wear surface and wear debris via scanning electron microscope.The experimental results show that adding0.9wt.%TiO₂@CNTs reinforcing phase to 7075 melt can effectively reduce the wear rate and wear coefficient of the material,because CNTs can be used as a solid lubricant to cover the wear surface of the sample and reduce wear The contact surface between the surface and the friction pair,thereby greatly improving the wear resistance of the material.After T6 heat treatment,the strength and microhardness of the composite material have been greatly improved.The higher the hardness of the material,the better the wear resistance of the material.Under the same wear conditions,the wear rate of the composite material after T6 heat treatment is one order of magnitude less than that of the as-cast composite material,and the wear coefficient of the material is also reduced a lot.The SEM analysis of the wear surface morphology shows that the material is mainly based on abrasive wear as the main wear mechanism.Compared with the wear debris of the as-cast material,the wear debris of the composite material after the T6 heat treatment becomes very fine,which also fully shows the composite material after T6 heat treatment has better wear resistance.更多还原