Fundamentals of materials processes wherein a reinforcing phase is combined with bulk molten metal are reviewed. Capillary phenomena which govern the incorporation of the reinforcement into the matrix are discussed first, with focus on thermodynamics, mechanics, and chemistry of wetting. Engineering strategies that have been adopted to increase reinforcement wettability are also addressed. Relevant transport phenomena are covered next, with focus on the infiltration process and the rheology of composite slurries. The last section of this review is concerned with solidification of the matrix after it has been combined with the reinforcement, including nucleation and growth of the matrix, and particle pushing by the solidifying metal. This review places emphasis on the physical phenomena that govern the processing and the microstructural development of cast metal matrix composites.

Solidification Processing Of Metal Matrix Composites

Wetting of ceramic materials by liquid silicon, aluminium and metallic melts containing titanium and other reactive elements: A review

One of the major problems in the fabrication of aluminum-ceramic composites is the wettability of ceramic reinforcements by liquid aluminum. In this article, the wettability at the liquid metal/ ceramic particulates interface has been evaluated by means of an infiltration technique that permits a determination of the threshold pressure and the infiltration kinetics. The aim of the present article is to check the reliability of the experimental technique. The work was carried out on pure aluminum and A12O3, SiC, and TiC particulates of average diametersD in the range 7.5 to 150 μm. An Al-1 wt pct Pb alloy was also used to investigate the effects of surface tension. All infiltrations were carried out in air. The results for the infiltrated heightvs applied pressure or time follow Darcy’s law. On the other hand, threshold pressures are proportional to 1/D (capillary law) and some deviations can be ascribed to differences in the shape of the particulates (especially TiC). More than 75 pct of the reduction in the threshold pressure promoted by 1 wt pct of lead can be related to the decrease in the surface tension of pure Al. The results indicate that effects due to changes in wetting should be very small, as expected when aluminum is covered with an oxide layer.

Evaluation of the wettability of liquid aluminum with ceramic particulates (SiC, TiC, AI 2 O 3 ) by means of pressure infiltration

The production of ceramic particle-reinforced metal-matrix composites by the melt-stirring technique involves two different steps: firstly, the establishment of particle-melt contact, and secondly, the wetting of the particles by the melt. The wetting behaviour of metal-ceramic systems has been studied extensively, whereas the contacting problem has been largely neglected. In this paper, a novelin situ thermal method has been used to investigate the kinetics of particle-melt contacting, based on melt cooling during and immediately after the particle addition. An enthalpy balance relates the amount of cooling and the fraction of the particles contacting the melt. For the aluminium alloy (A356) with 15 vol% silicon carbide (17 μm diameter), it was found that only 16% of the particles contacted the melt after mixing times of 15 min, but contact could be improved to 36% by adding magnesium to the melt. The fraction contacting the melt agreed well with the particle volume fraction calculated independently. Sampling of the froth layer on top of the melt revealed that a large portion of the particles resides there prior to incorporation into the melt.

Particle incorporation by melt stirring for the production of metal-matrix composites

Pressure infiltration of Al–12 wt.% Si–X (X = Cu, Ti, Mg) alloys into graphite particle preforms

SiC-ceramic materials, either as flat substrates or porous fiber preforms, are spontaneously wetted by aluminum at 700 °C to 800 °C when they have been pretreated with an aqueous solution of K2ZrF6. The wetting enhancement effect results from exothermic chemical reactions occurring at the SiC/Al interface. The first phenomenon thought to occur is a disruption of the alumina film covering liquid aluminum due to a reaction of A12O3 with K2ZrF6. Then alumina is totally dissolved at low temperatures by potassium/aluminum mixed fluorides, giving rise to a very fluid cryolitic liquid spreading out on the surface of liquid aluminum and to the true SiC/Al interface. Simultaneously, a large evolution of heat occurs, mainly due to the reduction of K2ZrF6 by aluminum and the formation of Al3Zr. The impregnation of 2D-SiC/SiC preforms by aluminum is modeled, and the effect of both the contact angle decrease and local temperature rise on the impregnation of the preforms, e.g., by gravity casting, is established.

J. M. Quenisset

Papers

The K2ZrF6 wetting process: Effect of surface chemistry on the ability of a SiC-Fiber preform to be impregnated by aluminum