11-08-2012, 04:45 PM
Metal Casting Methods
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Introduction
Metal Casting Methods- A Brief Review
Casting is a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold but experiences some shrinkage. Casting techniques are employed when the finished shape is so large or complicated that any other method would be impractical, a particular alloy is so low in ductility that forming by either hot or cold working would be difficult, and in comparison to other fabrication processes, casting is the most economical. Furthermore, the final step in the refining of even ductile metals may involve a casting process. A number of different casting techniques are commonly employed, including sand, die, investment, squeeze, lost foam, centrifugal casting, shell mould casting semi continuous casting and continuous casting. Only a cursory treatment of each of these is offered. [1]
Squeeze Casting [2]
Squeeze casting (SC) is a generic term to specify a fabrication technique where solidification is promoted under high pressure within a re-usable die. It is a metal-forming process, which combines permanent mould casting with die forging into a single operation where molten metal is solidified under applied hydrostatic pressure. Generally, the SC-fabricated engineering components are fine grained with excellent surface finish and have almost no porosity. They come in a variety of shapes and sizes. The mechanical properties of these parts are significantly improved over those of conventional castings and more sophisticated casting routes of pressure or gravity die-casting.
Partial Squeeze and Vacuum die casting process [3]
To make defect-free casting products with excellent mechanical properties a feasibility study of the partial squeeze and vacuum die casting process was performed. To combine the squeezing and vacuum effect, the plunger injection system was designed and attached to a chill vent type vacuum machinery system. The combination of the vacuum effect before injection and the squeezing effect after injection resulted in excellent defect-free die casting products. The uniform distribution of fine acicular eutectic and pro-eutectic silicon obtained from the trial process also provided excellent mechanical properties.
Die casting injects the liquid metal into permanent molds at high speed. This means low cycle times and accurate castings of good surface quality. Therefore die casting is ideally suited for producing small near net shape artifacts that require good surface texture without secondary processing. In the die casting process, the injection stroke can cause a jet of liquid to hit the far end of the mold cavity and then splash backwards. This produces highly turbulent conditions, introduces a lot of air, and results in very poor quality castings. Great improvements in quality can be made in die castings if the injection is done in several stages, each of which is optimized to reduce turbulence. However, this requires increased control, and therefore more investment, to produce a good quality product. [3]
Investment Casting [7]
The casting of liquid metals to produce solid objects is a manufacturing process that has been practiced for over 5000 years, with investment casting being one of the oldest known metal shaping methods. The technique itself has tremendous advantages in the production of quality components and the key benefits of accuracy, versatility and integrity. As a result the process is one of the most economic methods of forming a wide range of metal components. Environmental and economic pressures have, however, resulted in a need for the industry to improve current casting quality, reduce manufacturing costs and explore new markets for the process. Optimization of the mechanical and physical properties of the ceramic shell will be fundamental to achieving these aims.
An investment casting mould consists of individual layers of fine refractory material and granular refractory material held together by a binder that has been set to a rigid gel. Flexibility exists in changing the composition of each layer. Different methods can be used to remove the wax pattern, normally steam autoclave, leaving a hollow shell. Shells are fired and filled with molten metal that solidifies inside the shell. After casting, the ceramic shell is removed through mechanical or chemical methods to obtain the parts.[7]