Mechanisms For Ultrasonic Assisted Wetting And Bonding Of Aluminum Based Composites By Liquid Filler And Their Applications

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.