DESIGN OF MOULD IN PODWER METALLURGY TECHNIQUE FOR METAL MATRIX COMPOSITES

The demand for materials of superior properties is increasing day by day. Due to this materials scientist are developing new materials to meet the demand of growing need of materials.

The material scientist for the development of metal matrix composite has been given much attention, which are continuously replacing traditional materials. There are several fabrication techniques for the production of MMC. Among the fabrication processes of MMCs of recent development, powder metallurgy is one of the most widely used fabrication techniques. 

Powder Metallurgy (P/M) offers designers and users a versatile and efficient method of producing components. The process is versatile because it can be used for simple and complex shapes, and a full range of chemical, physical and mechanical properties is possible to obtain. P/M is efficient because it produces moderate to high-volume net or near-net shapes, with very little raw material loss. 

In general, the process has very good potential to improve performance through uniform properties, fine grain structures, and chemical homogeneity. During the process, the matrix material powders and the reinforcement particles are blended to produce a homogeneous distribution and fed into a mould of desired shape, hot press to a desired level of compaction and final consolidation by extrusion, forging, rolling or some other hot working method. The powder metallurgy attracted attention of the parts manufacturer during the last decades based on progress in materials, process and equipment. 

The conventional powder metallurgy process consists of three main-steps: powder mixing, compacting (sintering) and extrusion. To carry out the processing of MMC through powder metallurgy, development of powder metallurgy set up is essential. Powder metallurgy set up consists of several main parts such as die, mould, in which the powder is contained, punches, which are used to apply compacting pressure and heater for hot compaction as well as hot extrusion of MMCs. The main aim of the article is to discuss model design, process planning and fabrication of powder metallurgy set up as well as heating system for hot compacting /extrusion (direct and indirect).

The fabrication methodology of a composite part depends mainly on three factors:

 (i) the characteristics of constituent matrices and reinforcements.

 (ii) the shapes, sizes and engineering details of products.

(iii) end uses. 

The composite products are too many and cover a very wide domain of applications ranging from an engine valve, or a printed circuit board laminate, or a large-size boat hull or to an aircraft wing. The fabrication technique varies from one product to the other. The matrix types (i.e., whether they are plastics, metals or ceramics) play a dominant role in the selection of a fabrication process.

FABRICATION PROCESSES FOR METAL MATRIX COMPOSITES

Aluminium, magnesium, titanium and nickel alloys are commonly used as metal matrices, although several other matrix materials including super alloys have also been used. Both metal and ceramic reinforcements are employed. The choice of a particular matrix-reinforcement system is mainly controlled by the end use of the fabricated composite part. Several parameters influence the selection of a particular fabrication process. These are (i) types of matrices and reinforcements, (ii) the shape, size, orientation and distribution of reinforcements, (iii) the chemical, thermal and mechanical properties of reinforcements and matrices, (iv) shape, size and dimensional tolerances of the part and (v) finally the end use and cost-effectiveness. 

Compared to standard metallurgical processes, fabrication methods for metal matrix composites are much more complex and diverse. Some problems that are of major concern are the densification of the matrix while maintaining its purity, the control of reinforcement spacing and proper chemical bonding between the matrix and reinforcements. Based on the physical state of the matrix i.e., solid phase and liquid phase, fabrication processes can be grouped under solid phase processing and liquid phase processing.

Diffusion Bonding 

   The diffusion bonding employs the matrix in the solid phase, in the form of sheet or foil. Composite laminates are produced by consolidating alternate layers of precursor wires or fibre mats and metal matrix sheets or foils under temperature and pressure. The precursor wires are collimated filaments held together with a fugitive organic binder. This is achieved either by winding binder-coated filaments onto a circular cylindrical mandrel or by spraying the binder on the filaments that are already wound on a mandrel. When the solvent is evaporated, the fibre-resin combination forms a rolled fibre mat on the mandrel surface. The binder resin in precursor wires and fibre mats decomposes at a high temperature without leaving any residue. Under temperature and pressure metal sheets or foils melt and diffuse through fibre layers to form a laminate.

Powder Metallurgy Process

  Almost all metals and their alloys can be converted into powder form. Metal powders are commonly produced by atomization techniques. A stream of molten metal is disrupted either by impacting another fluid (gas or water) jet under high pressure or by applying mechanical forces and electrical fields leading to formation of fine liquid metal droplets which then solidify resulting in fine powder particles. 

The inert gases, argon and nitrogen, are used in the gas atomization, and the resulting powder particles are smooth and spherical.

Powder metallurgy is a versatile process but its application to fabrication of metal matrix composites may not be straight-forward, especially because of the presence of reinforcement phase.

There are quite a few composite fabrication techniques using continuous fibres of which two processes that use hot pressure bonding need special mention. In one process (known as powder cloth process), metal powder filled clothes are first produced by mixing metal matrix powders with an organic binder and then blending with a high purity Stoddard solution.

In the thermal spray processes, metal powders, are deposited on the fibre substrates using either plasma spray or arc spray techniques and composites are subsequently produced by consolidating these metal matrix coated fibres under heat and pressure. The plasma spray technique is employed to deposit spherical metal powders that are injected in the plasma stream (the temperature is about 10,000K and the traveling speed is around Mach 3) within the throat of the gun. The powder particle size is very critical, because the powder should melt, but not vapourise before it reaches the substrate. The arc spray technique uses continuous metal matrix wires of 0.16-0.32 cm diameter instead of metal powders. Two wires of opposite charge are fed through an arc spray gun.