Metal Casting Process
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Materials and Design
Pressure Die Casting Process
Since the discovery of the earth's minerals, metal casting process has played an important role in society. An integral part of every technological advance, castings have allowed us to build equipment to feed our people, fight for democracy, build infrastructure and manufacture cars, trains and airplanes. In general, castings have been and will continue to be the key ingredient in the recipe for a better way of life. There are various types of casting process, such as sand-casting, permanent-mould casting, investment casting and die-casting. Due to the words limitation, in this investigative study, only the development, technical challenges, recent findings, future direction, range of applications and shortcomings of pressure die-casting process will be discussed.
Pressure Die-casting is similar to permanent mold casting except that the metal is injected into the mold under high pressure of 7-350Mpa (1,000-50,000) psi. This high pressure will not be removed till the solidification is finished. It is called die-casting, because the molds in this casting operation are called dies. Metal is forced into the die cavity by the pressure is the most notable feature that shows this process is different from others in the casting category (Long et al., 2008).
In this technique, the mould will not be destroyed after each cast but is permanent, being made of a metal such as cast iron or steel. There are two types of pressure die casting processes, High pressure die casting and low pressure casting. High pressure die casting is the most widely used, representing about 50% of all light alloys casting production. Low pressure die casting currently accounts for about 20% of production and its use is increasing. Rests are gravity die casting, vacuum die casting and squeeze casting process (AZOM, 2008).
High pressure die casting process
In high pressure die casting process, the liquid metal is injected at high speed and high pressure into a metal mould. A schematic view of high pressure die casting is given in Figure 1.
Above equipment contains two vertical platens on which bolsters are located which hold the die halves. One of the platen is fixed and the other can move to open and close the die. A measured amount of metal is poured into the shot sleeve and then introduced into the mould cavity using a hydraulically-driven piston. Once the metal has solidified, the die is opened and the casting removed (AZOM, 2008).
For high pressure die casting process, special precaution must be considered to avoid too much gas inclusions which cause blistering during the subsequent heat treatment or wielding of the casting product. Since the casting machine and its dies are quite costly, only high-volume production will use the pressure die casting process for economical reason (AZOM, 2008).
Low pressure die casting
In Figure 2, the die is filled from a pressurised crucible below. Low-pressure die casting is especially suited to the production of components that are symmetric about an axis of rotation. Light automotive wheels are normally manufactured by this technique (AZOM, 2008).
Benefits of the process development
Over the last three years the biggest improvement of die casting process is the development of material property data and interfacial hear transfer coefficients. Material property thermal data is the essential part of any simulation program, which has been developed for various mold materials, feeding system and metal alloys, such as aluminum, steel and compacted graphite iron. Furthermore, the work done to understand the fluid mechanics of filters is another development of the die casting process, which include the developed pressure drop data for pressed, extruded and reticulated foam filters and these data is easily available in the industry literature (ALLBUSINESS, 2008).
There are types of the casting defects occur during production of pressure die casting process such as insufficient pouring, cooling separated, crack and shrink. They are formed in the mold filling and solidification processes, which contributed to the final casting performance. The processes of mold filling and solidification are developed in time sequence while the casting defects are forming, which can be reflected by its numerical simulation in order to predict the locations of the casting defects (Minaie and Voller, 1998).
Numerical simulations of mold filling and solidification processes include numerical analysis, heat transfer, fluid mechanics, solidification theory, engineering mechanics, computer aid graph analysis etc. The mold filling process can be described very precisely and the reliable initial temperature field is provided for the next solidification process by using the advantage of numerical simulation on temperature and fluid fields, which benefits to elevate the simulation accuracy of solidification process. Numerical simulations of mold filling and solidification processes, which play the key function in the casting production, are the world known leading area, widening and promoting the development of casting subject by using the advance computer technology. And it also initiates a new way to improve the casting quality (Baicheng and Houfa, 1998).
Technical Challenges & Details
The molten metal flow is the major issue that relates to the mold filling process, heat and mass transfer flow process at changeable temperature accompanied with heat loss and solidification. This process can be presented by the continuity and momentum equations. In addition, the energy balance equation can describe the heat exchange between the molten metal and the casting chamber. The consideration of position and movement of the free surface is the key for the calculation of unstable flow. Also, it is essential to deal with the boundary conditions of the free surface. There for, the mathematical equations can be expressed as follows , , ,  and
where Ï is the density and u is the velocity
The numerical simulation is a non-linear instantaneous thermal analysis in the solidification process. The casting form changes gradually with the decreasing of temperature from liquid state to semi-solid and solid states, in which many physical process and phenomenon played an important role for the casting quality take place, and the temperature field of the casting varies with the time. Therefore, the shrinkage cavity and slack can be predicted with the numerical simulations of the filling and solidification processes (Laurent and Rigaunt, 1992).
Range of Applications
CAD/CAM/CAE is now as an essential part to keep pace with growing technology and demand for quality, low cost, precision and faster delivery in tool engineering. The followings are the few latest methods, which are being used today in die making industry:
Scanning of product to be die cast.
CAD/CAM (Computer added design and manufacturing)
CAPP (computer added process planning)
CAI (computer added inspection)
2D drawing to 3D modeling
3D model to 2D drawing
Black-box designing (conceptual base designing)
Analysis & Simulation (stress, Strain, thermal analysis etc.)
Analysis for Gate, runner, ejector pins and cooling line etc.
NC data generation
Pressure die casting process is widely used in resource producing company, such as world class Aluminum company 'Alcoa' and 'NALCO', and copper alloy giant 'Kennecott'.
The formation of blow-holes in the die casting is a major drawback, which is resulting from the turbulence produced by the high velocity when the liquid metal alloy is injected. Another major drawback is inevitable shrinkage of the casting as it solidifies, and which is proportional to the temperature at which the alloy is injected. Though cheap to make, the poor quality of current die cast components therefore makes the use of finer quality alloys unfeasible (Moschini and Renzo, 1998).
Recent finding & Future direction
In the world of computer simulation time, the pressure die casting process improvements move very fast in the foundry industry today, a lot has changed in the last few years, for example, three years ago the computers that industry companies using to process the die casting process simulation were based on Unix workstations, which was more than $30,000 and the only way to analyze the filling of a mold cavity was using NavierStokes equations, which was extremely slow.
Nowadays, the personal computer (PC) conversion is finished, some of new machines can run 20-30 times faster than the one in three years ago. For example, a very complicated filling and solidification work that used to run days and days can be completed in one hour (ALLBUSINESS, 2008).
The concurrent development of optimization techniques have been capitalized on and incorporated by the software companies into their programs. At first glance, this allows the user to let the computer help optimize such things as riser placement and size and chill locations. However it isn't hard to imagine that this is just the beginning, and that we are about to get on a journey in which die casting process simulation programs soon will perform what is unthinkable today (ALLBUSINESS, 2008).
Die casting molds tend to be expensive as they are made from hardened steel-also the cycle time for building these tend to be long. Also the stronger and harder metals such as iron and steel cannot be die-cast in the past. However, by the developing of the numerical simulations system and optimization techniques, these issues are no longer impede the pace of progress in the modern world. Numerical simulations and optimization techniques can help foster the success and viability of the foundry industry for many years to come. The more capability and accuracy that is built into our simulation tools, the better and more efficient casting can be produced.
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