Metal matrix composite materials which utilize discontinuous-type reinforcement undergo unit operations associated with ingot, powder or cast metal manufacturing in addition to their own unique operations. An attempt is made to summarize the scope of operations for both cast- and powder-based processing and identify processing issues which cause matrix-reinforcement interface and property variability in discontinuous metal matrix composites.

Processing and properties of metal matrix composites containing discontinuous reinforcement

The effects of vacuum hot pressing temperature and pressure on tensile properties and microstructures of 20 vol.% SiC whisker (SiC w )-2124 Al composites were investigated. The tensile strength of SiC w -2124 Al composite increased rapidly with increasing vacuum hot pressing temperature up to 570 °C and became saturated above 570 °C. The increase in tensile strength with increasing vacuum hot pressing temperature is due to the enhanced densification of composite and reduced damage of whiskers from the softening of 2124 Al matrix with increasing volume fraction of liquid phase. The vacuum hot pressing pressure needed to be higher than 70 MPa to achieve densification of SiC w -2124 Al composite above 99%. A vacuum hot pressing pressure above 70 MPa was not helpful to enhance the tensile strength, since the aspect ratio of whiskers decreased owing to the damage during vacuum hot pressing. Combining the effects of the aspect ratio of whiskers and the density of composite, which are dependent on the vacuum hot pressing parameters, an equation is proposed to estimate the tensile strength of SiC w -2124 Al composite. The effect of porosity on tensile strength of SiC w -2124 Al composite was found to be more sensitive compared with powder metallurgy alloys or ceramics, since the most pores were located at whisker-matrix interfaces and reduced the load transfer efficiency between SiC whiskers and 2124 Al matrix.

Effects of vacuum hot pressing parameters on the tensile properties and microstructures of SiC-2124 Al composites

The effects of extrusion ratio and extrusion temperature on tensile strengths and microstructures of 20 vol.% SiCw-2124Al composites were investigated. The alignment of SiC whiskers along the extrusion direction in SiCw-2124Al composites was improved and the aspect ratio of SiC whiskers was decreased with increasing extrusion ratio from 10:1 to 25:1. The SiCw-2124Al composite exhibited the highest tensile strength at the extrusion ratio of 15:1. The aspect ratio of whiskers and the relative density of composite increased with increasing extrusion temperature from 470 °C to 530 °C. The tensile strength of SiCw-2124Al composite increased with increasing extrusion temperature up to 530 °C. Based on the concept of the load transfer efficiency of misoriented SiC whiskers in the 2124Al matrix, the two microstructural parameters of the alignment and aspect ratio of whiskers, which influenced on the tensile strength in an opposite way, were combined into a single parameter proposed as an effective aspect ratio. The highest tensile strength of SiCw-2124Al composite at the extrusion ratio of 15:1 is explained by the largest effective aspect ratio due to the balance of the alignment and aspect ratio of SiC whiskers. A modified phenomenogical equation describing the tensile strength of SiCw-2124Al composites as a function of microstructural parameters was proposed by introducing the effective aspect ratio as a substitute for the average aspect ratio of SiC whiskers.

Effects of hot extrusion parameters on the tensile properties and microstructures of SiCw-2124Al composites

High strength Al–Al2O3p composites: Optimization of extrusion parameters

Aluminium-glass composites were produced powder metallurgically by extruding Al-glass powder mixture billets. By suitable choice of glass with a low softening temperature and right extrusion parameters the aluminium and glass particles are drawn to fibres. The mechanical and thermal properties of such composites were investigated. In particular the creep properties were determined as a function of structure, heat treatment, temperature and applied stress.

Creep Properties of Powder Metallurgically Produced Aluminium-Glass Composites

Composite materials can be produced by PM means. If both components can be deformed, an atomic bond is formed between the new surfaces generated. The structure, properties, and technological applications of metal-glass composites are illustrated by the Al-glass system. The influence of the starting material and the deformation process on the properties is discussed. For the same starting material and production methods, the hardness, strength, and wear resistance can be increased in excess of those predicted by the law of mixtures, whereas the elastic modulus and physical properties essentially conform to this law. Once formed, the composite materials can be further hot worked, and cold worked to a limited extent. They can also be soldered and glued. It is possible to surface treat them, e.g. anodize or enamel. PM/0248

B. L. Mordike

Papers

Powder Metallurgically Produced Metal—Glass Composites