AFS SPONSORED RESEARCH


HEAT TREATMENT EFFECTS ON THE MECHANICAL PROPERTIES AND MICROSTRUCTURE OF PREFORM-BASED SQUEEZE CAST ALUMINUM METAL MATRIX COMPOSITES


A. Loukus and J. Loukus REL Inc., Calumet, MI, USA


Copyright © 2011 American Foundry Society Abstract


Many aluminum alloys are used in the production of metal matrix composites (MMCs). Historically the matrix alloy’s thermal treatment parameters (solution heat treatment temperatures and times, and aging temperatures and times) have been adopted for the MMCs containing that alloy. Previous MMC work indicates that solution heat treatment parameters have a significant effect on the strength and ductility of the cast composite. The current study investigates the effects of solution times on the tensile properties of Al- MMC composites containing two different reinforcement types, SiCp and Al2


O3 short fiber, and volume fractions in INTRODUCTION


Aluminum metal matrix composites (MMCs) have found applications in various industries; aerospace, military, auto- motive electronic packaging and/or sporting goods.1-7


These


composites provide considerably increased flexural strength, fatigue strength, stiffness, wear and heat resistance proper- ties with similar densities when compared to monolithic Al alloys. These attributes make them good candidates for lightweight structural components.


To eliminate the effect of the casting process on the fab- rication of both particulate and short fiber reinforced MMC materials, the research team used a preform based squeeze cast process for the fabrication of all test samples. There are a few different approaches to manufacturing MMCs. The advantages and the disadvantages of com- mercially viable casting methods are listed in Table 1.


Stir casting is the commercial casting process of choice when producing aluminum MMC components reinforced with particulate reinforcements, but is generally limited to a reinforcement level of about 30 volume percent. At greater amounts of particulate, the viscosity of the melt becomes too high to facilitate flow of the liquid into a casting mold. The addition of short fibers into a melt by use of stir casting methods generally furnishes negative results. The stresses generated during the stirring process break the low ductility fibers into short sections. This results in a low (< 4) aspect


International Journal of Metalcasting/Winter 11


two different matrix alloy chemistries, A359 and modified A319. Study goals are to optimize the matrix-reinforcement interface strength and thereby maximize tensile strength and elongation of these Al-MMC composites. Scanning Electron Microscopes (SEM) analysis and tensile data analysis conclude that the solution treatment times must be changed for each composite from the time suggested for conventional alloy heat treatment parameters.


Keywords: metal matrix composites, squeeze casting, MMC heat treatment, ceramic reinforcement, preforms


ratio and increases the melt viscosity greater than that of par- ticulate reinforcment.8,9


Pressure infiltration processing begins with a ceramic pre- form (sponge) of the desired shape, with volume fraction of reinforcements ranging up to 60 volume percent. These preforms may be made from particulates, short fibers or combinations of particulates and short fibers. The preform must have sufficient mechanical stability to withstand in- filtration forces while maintaining sufficient permeability. Pressure infiltration decreases the air entrapment, reduces the number of metal-starved cavities and decreases the pro- cessing time which is critical to avoid the formation of the harmful brittle phases at the interface between matrix alloy and reinforcement.


While there are significant benefits for pressure infiltra- tion of casting aluminum into ceramic preforms, there are also potential pitfalls at the metal-reinforcement interface and the matrix. Thus, no matter which methods are used for MMC production, the microscopic compositional in- homogeneity always exists inside the matrix alloy. These effects, however, are more controllable in pressure infiltra- tion than in the other processing methods due to the faster processing time (less time with molten metal in contact with the reinforcement). The pressure infiltration (squeeze casting) process was selected to manufacture the MMCs for the heat treatment experiments because of the range of reinforcement being studied.


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