A metal matrix composite (MMC) is a composite material that has two or more constituent parts, with at least one of the constituent parts being a metal. This means that the rest of the materials can be different such as organic or ceramic compounds. Whenever three or more materials are present, the mixture is called hybrids composite. As such, MMCs have always been seen to be complementary to cermets. Aluminum metal matrix composites is also a form of the MMC, given that it focuses on the Discontinuously Reinforced Aluminum (DRA) materials which are seen as forms of metal matrix composites. In these composites, the aluminum matrix is reinforced with the ceramic particles, short fibers and whiskers.
Conversely, aluminum metal matrix can also be made using the cold isostatic pressing. In the cold isostatic pressing, pressure is applied from multiple directions so as to bring about uniformity of compaction (also known as high quality parts) and the increase of shape capability. This is the case when compared to uniaxial pressing where pressure is applied but not from multiple directions.
Sintering on the other hand, refers to the process of consolidating performed parts- a process which can be achieved through the use of either vacuum or gas pressure.
Two methods of carrying out the isostatic pressure exist: the wet bag isolating pressure and the dry bag isolating pressure.
The Wet Isostatic Pressing of Aluminum (CIP)
In the wet bag isostatic pressure, the powder is incased in a rubber sheath, and this rubber being immersed in a liquid. The pressure bag is to be filled with water. The rubber bag, also known as the mold is to be removed so that each cycle is refilled. This type of press will nevertheless be applicable when large and complex amounts are needed (Hammer, 25).
As far as the dry bag version of CIP is concerned, the bag of aluminum has to be an integral part of the vessel and is to be used when there are a lot of similar parts to be used. Unlike wet bags, dry bag presses are always limited to simple shapes and smaller sizes that can be easily removed. It is always important to automate dry bag presses from the powder filling to the stage of part removal. The part is usually held in shape during the cold pressing of aluminum metals with binders. These binders are usually burnt out of the part of the aluminum in the furnace, during sintering (Das and Zedalis, 64). The liquid in this case is to transmit the pressure to the powder in a very uniform manner. It is also important t to note that the wet bag version's cycles have to usually take tens of minutes, compared to the uniaxial pressing methods which may take very little amount of time.
Although isostatic processes are able to use cold, hot or warm temperatures to accomplish a task, the cold isostatic processes can be used for performing powders before further densification takes place. Cold Isostatic Presses are normally divided into two: the wet bag and the dry bag.
This above method whereby simply, powder is contained in a flexible (usually) polyurethane mould which is then immersed in water and then pumped to very high pressure has been known to be the most viable procedure to follow, in the processing of aluminum metal matrix.
For instance, given that the powder is compacted with uniform pressure from all directions, and that no lubricants are used, aluminum metals with high and uniform density can be produced. At the same time, the process of wet isostatic pressing of aluminum is known as being important in that it eliminates the constraints which limit the geometry of parts that from compacting in unidirectional rigid manner. In almost the same wavelength, isostatic pressing is recommended for its uniform density which is always seen as being important in producing repeatable and predictable shrinkage after sintering (Ferrando and Divecha, 25).
On the converse, CIP is also important because of the processes it undergoes, given that the process produces cylinders that are thin-walled. Similarly, the cylinder undercuts are known not to carry any setbacks or geometries which produce difficulties that come with the traditional pressing techniques (Nair and Tien, 66).
Chemical engineers also explain that the use of isostatic pressing is totally established as a form of production tool in both the manufacturing of ceramics and in the production of powder metallurgy.
The Hot Isostatic Pressing of Aluminum
In the dry bag application of isostatic pressure, instead of immersing the tooling fluid as in the case of the wet bag isostatic pressing, the tooling is itself allowed to build both internal channels. It is this internal pressure into which the high pressure fluid is supposed to be pumped.
The aforementioned consolidation of performed parts that involves the use of gas or vacuum pressure may involve the use of a gas type that may bring about the desired reaction with the part of the material (which in this point is aluminum). Sintering is normally carried out at temperature that is close to the melting point of the aluminum. The temperature nevertheless will have to be hot enough to allow densification so as to allow powder particles to diffuse together. Given that sintering can produce parts with high density (94% - 99% density), sintering of aluminum materials or metals is easy given that densification is all that is needed, so that the rest of the processes are needless. This helps save time and other resources.
That sintering is a process more comfortable and convenient is underscored by the presence of the binder which falls off in its own accord after being burnt out of the part off the aluminum during the sintering process. That the piece that is being produced will come out in the desired shape is also made tractable by the presence of the rubber binder that holds the part in shape. This happens during the exposure of the part to very high temperature. At this juncture, the shape of the part is very fluid and mobile (Reiterer, Kraft and Janosovits, 75).
The use of isostatic pressing over other conventional processes or methods is known as being important lies in the concept of uniformity and non-uniformity. Unlike uniaxial die press, CIP gives uniform density. On the contrary, the uniaxial die press indeed can produce acceptable parts, but most importantly, in the large complex parts and in the long tubes, the die friction brings about non uniformity. This gives isostatic presses a competitive edge over other conventional means of processing aluminum such as the previously mentioned uniaxial die press. Adding this advantage to the fact that CIP has seen the emergence of stronger aluminum materials really confirms the viability of using CIP-manufactured aluminum products or components, or sintered aluminum and its components. It is against this backdrop that such aluminum products are making inroads in lucrative manufacturing industries such as aviation and motor industry. Some of these products include: Chevrolet corvette and GM pick up trucks, F-16 fighter aircraft ventral fins, F-16 fighter jet fuel access covers, Toyota diesel engine pistons, the Plymouth Prowler brake rotors, the Pratt and Whitney 4000 series engine fan and exist guide vanes, GM EV-1 brake drums, bicycle components, Motorola's Iridium Satellites and even golf clubs.