【作者】 许志武；

【导师】 杨士勤； 闫久春；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 铝基复合材料(AlMMCs)具有比一般合金材料优良的比强度、比刚度、尺寸稳定性和耐磨损等性能,尤其是近年来该种材料制备技术的快速发展,成本大为降低,在航空航天、电子和汽车等领域的应用将越来越广泛。但是,由于该类材料增强相与基体物理、化学性能的巨大差异,焊接性较差。这使加工具有一定复杂形状、难以通过一次成型的功能结构件尤为困难。因此,高效、可靠的连接技术是铝基复合材料扩大应用亟待解决的关键问题。制约铝基复合材料接头质量及其应用的两个基本问题是实现填充材料同时与基体材料和增强相材料的物理润湿复合和复杂构件在非真空条件下的可靠连接。结合超声波的特点,以Al2O3、SiC颗粒增强铝基复合材料为主要对象,通过研究超声波作用下液态钎料与AlMMCs的润湿复合行为,创新性地提出了该种材料非真空、无钎剂条件下的超声波毛细焊接工艺,并为了优化接头性能,研究了液态焊缝中增强相颗粒运动、迁移和分布规律以及陶瓷颗粒/钎料复合体的凝固行为,获得了可控的、复合化的焊缝组织,接头性能显著提高,为铝基复合材料可靠、高效地焊接开辟了一条新途径。主要研究工作及结论包括:超声波作用下液态钎料与AlMMCs的润湿行为研究发现,大气环境中、从母材导入超声波的条件下,超声振幅大于临界值10μm后,液态钎料可在铝基复合材料表面实现良好的润湿铺展,当超声振幅小于10μm时,钎料润湿铺展的情况与钎料放置的位置有关。采用有限元的方法对润湿界面的声压场的数值模拟结果表明,只有当超声振幅达到10μm后,润湿界面才可产生较大的声负压,而当超声振幅小于10μm时,只有局部的润湿界面产生较大的声负压,而其它润湿界面处的声负压值接近零。计算的结果初步说明了实验观察到的结果。Zn-Al钎料可通过铝基复合材料表面氧化膜的(裂缝)通道潜入到氧化膜/基体界面并沿基体表面发生铺展,形成“皮下潜流”现象,而且表现出“线性铺展”的行为。线性铺展的速率随加热温度、母材表面粗糙度及吸气量的增加显著提高,而与钎料量无关。当潜流发生时,母材表面的氧化膜首先被潜流金属剥离后在超声波作用下破碎;当潜流现象不发生时,钎料首先通过氧化膜裂缝与母材基体发生扩散,导致基体液化,液化区表面的氧化膜在超声波作用下破碎。钎料与母材润湿结合后,钎料成为陶瓷颗粒增强的复合材料:亚微米级的颗粒以颗粒团的形式存在,而微米级的以单个颗粒的形式存在。基于超声润湿行为的基础上,探讨了超声辅助毛细填缝焊接过程的可行性。研究发现,在不润湿的基础上,超声波作用可实现钎料的水平填缝过程,钎料的液-气界面初始为凸界面,当界面润湿改善后转变为凹界面。较佳的焊接工艺为:超声波振幅10~25μm,作用时间大于3s,预留间隙100~400μm。接头中残留的氧化膜往往是断裂源,导致接头强度低。钎料填缝结束后,焊缝各个区域氧化膜的破除并不同步,需延长超声波作用时间使润湿界面的氧化膜彻底破除。无氧化膜及陶瓷颗粒增强的接头强度取决于钎料的强度。为了提高接头性能,有必要形成复合焊缝,但需保证其中的增强相均匀分布。为了获得复合焊缝,可将复合材料共晶液相层中的陶瓷颗粒通过超声作用,分散、混合到液态钎料层中,适宜的超声波振幅和作用时间范围分别为10~25μm和2~10s,振幅过小、作用时间过短不足以使颗粒分布均匀,振幅过大容易导致熔体发生雾化现象,作用时间过长熔体中出现颗粒聚集现象。透明介质中颗粒运动迁移的动态观察揭示了超声波作用下介质中的声流模式,以及超声振幅、熔体粘度对声流模式、颗粒分散的影响。为了控制陶瓷颗粒在液态焊缝中的上浮现象,通过实验及Stokes理论计算了颗粒的上浮速率。实验结果表明随着温度和颗粒的尺寸的增加,颗粒的上浮速率急剧增加;颗粒的体积份数提高时,颗粒的上浮速率几乎线性降低。理论计算值的变化规律与实验结果一致,但在数值上有所偏差,在此基础上提出了修正系数。修正后的上浮速率为:温度的影响:Vm = ( 0.0116Tm?4.2902)Vh;体积份数的影响: Vm = 0.414Vh(C<10%) ,Vm = ( 0.6591?0.0236C)Vh (C>10%)。常规凝固过程中基体晶粒的过度生长是导致增强相颗粒发生偏聚的主要原因。利用超声波对SiCp/Zn-Al复合体凝固过程中的晶粒细化及颗粒分布控制研究表明:超声波的作用方式对基体组织的细化以及颗粒的分布有非常重要的影响,采取适当的恒温处理,在一定的固相份数范围内(30~45%),利用先结晶的固相的“原位钉扎”作用,可避免颗粒宏观的偏聚,实现颗粒的均匀分布,同时又可获得组织细化的效果。超声波作用“破碎已形成的枝晶”在组织细化过程中起到了主导的作用。在超声波振幅不变的条件下,熔体的粘度决定了超声波细化的效果,当粘度超过1.0×10-4Pa·s后,组织细化不明显。经过晶粒细化及颗粒分布控制的复合体,强度提高近25%。焊缝中陶瓷颗粒分布控制的物理模拟的规律同样适用于实际焊接过程,经过优化控制的焊缝中颗粒分布均匀,基体组织细化,接头强度提高近20%。初步的工程应用研究表明,提出的铝基复合材料超声辅助焊接工艺,工艺过程简捷,实用性强,成功实现了工程构件的焊接,接头满足了高钎透率、高强度以及具有复合结构的要求。更多还原

【Abstract】 Aluminum metal matrix composites (AlMMCs), which possess higher specific strength, stiffness, dimensional stability and wear resistance compared with unreinforced aluminum alloys, are of great potential uses in aerospace, electronic and transportation industries due to the lowered cost associated with the rapid development in material fabrication technology. However, these materials own poor weldability because of the great difference in the physical and chemical properties between the reinforcement and the matrix. This makes it difficult to fabricate complex components which can not be made in one processing step. So, efficient and reliable welding technology is the key problem that widing the use of AlMMCs must face to.The two basic problems that hinder the quality and application of AlMMCs joint are the wetting of both the reinforcement and the matrix by filler metal at the same time and joining complex component under vaccum-free condition. With these in mind, the ultrasonic assisted wetting of AlMMCs by liquid filler in air is investigated and an ultrasonic assisted capillary joining technology is firstly proposed. Furthermore, for obtaining a weld reinforced with particles (composite wled) and thus increasing the joint property, migration of particles in the liquid filler metal and solidification of particle/filler composite metal under ultrasonic action are investingated. As a result, a composite technology of particle-reinforced bond region is obtained and the property of bonded joint is significantly increased. The major research efforts and results of the present study include:The investigation of ultrasonic assisted wetting of AlMMCs in air shows that, with imposing ultrasonic on the base metal, satisfied spreading of liquid filler is obtained when ultrasonic vibration amplitude is higher than 10μm and the spreading of liquid filler is influenced by the droplet location when the ultrasonic vibration is less than 10μm. The acoustic pressure field at the wetting interface is calculated by using the acoustic analysis component enclosed in the finite element software ANSYS. The simulating results show that, only when the applied ultrasonic vibration amplitude exceeds 10μm, is the acoustic pressure at the wetting interface high enough to cause the cavitation effect; when the applied ultrasonic vibration amplitude is lower than 10μm, the acoustic pressure at local wetting interface is nearly equal to zero, demonstrating that acoustic cavitation would not occur in this case. These simulating results preliminarily explain the observed wetting experiment results.Zn-Al filler can penetrate to the substrate oxide film/substrate metal interface through the crack in the oxide and spreads on the bare substrate, leading to the formation of undermining phenomena. It is found the the propagation of undermining is linear with time before the depletion of the filler droplet. The undermining velocity at the‘linear’spreading stage is independent of the volume of the filler droplet, but increases with temperature, surface roughness and gaseous products absorbed by the substrate. When undermining occurs during wetting, the oxide film is firstly detached from the base metal surface by the undermining filler, and subsequently broken by the ultrasonic cavitation. When undermining is not present during wetting, liquid filler firstly diffuses into the base metal through the crack of the surface oxide, causing melting of the base metal. Consequently, the bond between the surface oxide film and the base metal is weakened and the oxide film can be eliminated easily by the ultrasonic cavitation. During wetting of the AlMMCs, sub-micron particles in the form of particle block, while micron particles individually transfer into the liquid filler and the liquid filler becomes a particle-reinforced material ultimately.Based on the ultrasonic assisted wetting result, the possibility of ultrasonic assisted infiltrating of joint gap in air is investigated. It is found that horizontal infiltration of a“non-wetting”capillary does occur under ultrasonic induction, during which the liquid-gas interface is convex at the beginning and transits into concave one after the interface wetting is improved. An optimum bonding technology is: ultrasonic vibration amplitude 10~25μm, vibration time >3s and joint gap 100~400μm. The oxide film remaining in the bond region is conventionally the fracture source. So, it is necessary to prolong ultrasonic vibration time to insure the thorough removal of the oxide film after joint filling, as the oxide film at the wetting interfaces could not be broken simultaneously. To improve the joint strength, forming composite weld (i.e. particle reinforced weld) is needed because the strength of the weld without particle reinforcement is strongly restricted by that of the filler metal.One way to form composite weld is by incorporating the particles in the eutectic liquid layer of the base metal into the liquid filler. The optimum processing parameters are: ultrasonic vibration amplitude 10~25μm and viration time 2~10s. The dynamic observations of particle migration in the transparent medium illustrate the styles of acoustic stream under ultrasonic vibration, as well as the effect of ultrasonic vibration amplitude and viscosity of medium on the acoustic streams and the associated particle dispersing result.In order to avoid the particle floating in the liquid filler, particle floating velocity is calculated by both the experimental and theoretical methods. Experimental results show that particle floating velocity increases significantly with heating temperature and paticle size, and decreases linearly with increasing particle volume fraction. The changing trend of theoretically calculating values is similar as those of the experimental, however, deviations still exist and corrections are proposed. The corrected floating velocities are: regarding T~ Vm = ( 0.0116Tm?4.2902)Vh; regarding particle volume fraction~ Vm = 0.414Vh(C<10%), Vm = ( 0.6591?0.0236C)Vh (C>10%).Particle pushing by the solid-liquid interface due to the over growth of matrix grain is mostly responsible for particle segregation in the traditional solidification. The results on control of grain refining and particle distribution in SiCp/Zn-Al melt show that the ultrasonic processing style has significant effect on the solidified microstructure. When proper isothermal ultrasonic processing is applied (the solid grain fraction is in the range of 30~45%), SiC particles are mechanically locked by solid grains, thus avoiding severe particle migration in the melt. In this case, grain refinement as well as uniform particle distribution is obtained.Disrupting the dendritics by the ultrasonic cavitation and streams is the main source of grain refining. Under constant ultrasonic vibration, the degree of grain refinement is dependent on the viscosity of the melt, which is rather low as the melt viscosity is higher than 1.0×10-4Pa·s. The strength of SiCp/Zn-Al composite subjected to proper treatment is increased closely up to 25%.The optimization law of grain refinement and particle distribution related to SiCp/Zn-Al composite also works in the case of joining of AlMMCs and the joint strength can be increased nearly up to 20%. The preliminary investigation on engineering use of the proposed ultrasonic assisted bonding technology for AlMMCs shows that this joining technolgy can successfully produce components including joints with high bonding area, high strength and composite structure similar as the base metal and is characterized by brief operation and high practicability, which meets the requirement of the engineering use.更多还原