Tooling costs and other capital costs are high due to the cost of designing dies. Operational costs, however, are relatively low, due to the high level of automation and the small number of production steps (i.e. direct pouring into a permanent mould rather than preparing destroyable patterns and/or moulds). The process, therefore, is best suited to mass production.
Die casting is most suitable for non-ferrous metals with relatively low melting points (i.e. around 870°C) such as lead, zinc, aluminium, magnesium and some copper alloys. Casing metals with high melting points, including iron, steel and other ferrous metals, reduces die life.
Dies are usually made from two blocks of steel, each containing part of the cavity, that are locked together while the casting is being made. Retractable and removable cores are used to form internal surfaces. Molten metal is injected into the die and held under pressure until it cools and solidifies. The die halves are then opened and the casting is removed, usually by means of an automatic ejection system.
The die is cleaned between each casting cycle, preheated and lubricated to reduce wear on the die, to improve surface quality and to aid ejection. Mould-coating material can also be used to protect the molten metal from the relatively cool and conductive surface of the mould. Cooling systems are often used to maintain the desired operating temperature.
- Once the capital items such as the die casting machine, the dies and the tooling are in place, operating costs are low relative to most other casting processes. This is due to the reduced number of process steps, the elimination of temporary moulds and patterns from the process, and the lower volume of materials that needs to be handled.
- Dies can sustain very high production rates (i.e. over 400 shots per hour). Total cost of castings can be relatively low at high levels of production.
- High design flexibility and complexity allows products to be manufactured from a single casting instead of from an assembly of cast components.
- Computer monitoring facilitates consistency and real-time quality control.
- Good accuracy, consistency and surface finish are possible, with high metal yields.
- Cleaning, machining, finishing and fabrication costs are low.
- There are low levels of waste due to elimination of refractory material, leading to a cleaner work environment.
- Capital costs for equipment and dies are high.
- Pressure dies are very expensive to design and produce.
- Die casting is not applicable to steel and high-melting-point alloys.
- Casting size is limited to a maximum of about 35 kg.
SEMI-SOLID METAL CASTING PROCESS
Semi-solid casting is a modification of the die-casting process that achieves metallurgical benefits similar to forging. Metal billets are heated to a semi-solid state and pressed under pressure into the die. Prior to moulding, the heated material can be picked up and will hold its shape unsupported. Under pressure it flows like a liquid to take the form of the die accurately, as in die casting. This lower energy-intensive process creates a fine and uniform structure that is virtually free of porosity. In a related process called Rheocasting the metal is melted and, during solidification to a semi-solid state, its morphology is altered using mechanical, electromagnetic or other forces to create a fine microstructure.
- Gives high dimensional accuracy and metallurgical integrity.
- The lower injection temperature extends die life by an order of 10, and produces components to tighter tolerances compared with traditional die casting.
- Gives higher structural integrity, quality and soundness compared with cast parts.
- Castings can be heat-treated to obtain characteristics similar to those of permanent mould castings.
- Can achieve lower cost production than forging and low-pressure / gravity casting processes for most structural parts.
- As for traditional die casting, size is generally limited.
CONTINUOUS PROCESSING AND CASTING
A future objective in this area is to refine the production technique and special purpose production equipment that will enable automotive component manufacturers to produce high strength Al-MMCs that can be used in specialised semi-solid, high-pressure die casting machines to produce near net shape automotive parts and components.
The proposed production method comprises:
- Molten secondary aluminum alloy delivered by road tanker;
- Using the excess heat in the molten metal to re-melt casting scrap;
- Cooling the melt to a semi–solid (mushy) state;
- The addition of fly ash particles to produce ULTALITE® Al-MMC;
- Transferring semi-solid ULTALITE® Al-MMC to a high pressure die casting machine
- Rheocasting of the brake drums;
- Trimming of the drums;
- Quality inspection;
- Recycling of runners and risers;
- Finish machining of the drums;
The proposed production method has many cost saving features including the use of secondary molten metal, in-house recycling of runners, etc, and in-situ mixing of fly ash particles. Competing materials are sold as ingots and cannot be mixed or recycled on site. Semi-solid casting gives improved mechanical properties and increases tool life due to lower metal processing temperatures.
The most challenging technical aspect of any Al-MMC is to develop a lower cost method of introducing the ceramic particles into the aluminium metal with complete wetting and no clumping of the particles.
There has been recent development and commercialization of the “Slurry on Demand” for the Semi-Solid “Rheocast” process. Such a process offers the lowest cost method of adding fly ash particles to molten Aluminium. This is because the process avoids the step of producing and reheating the special thixotropic billets used within the high integrity Semi Solid Thixocasting process.
The proposed production equipment would receive molten metal in bulk delivery tankers, which is then transferred to holding furnaces. Recycled ULTALITE® scrap will be added to the melt. ULTALITE® will be mixed in the Cyco high shear mixing furnace that is fed with measured quantities of semi-solid aluminium and fly ash, and then moved to holding furnaces. The density of fly ash is comparable to aluminium so moderate stirring would be sufficient to stop the fly ash from segregating in the holding furnace.
The Semi-Solid ULTALITE® mix will then be transferred to the shot sleeve of the high pressure die casting machine utilising the “Slurry on Demand” Semi-Solid process.
During the development to date, majority of castings have been produced using the ‘squeeze casting’ method. It was anticipated that using this method would provide a better, more even distribution of flyash particles across the casting and eliminate the forming of any air bubbles that may affect that wear qualities of the brake drum.
As a result of continued trials and modifications to the die and the casting technique, performance of the castings has been gradually improved. The recent dynamometer tests conducted by TRW are a proof of that.
PATENTS AND TRADEMARKS
The ULTALITE® material and process technology are protected by patents and patent applications in Australia, the USA and most other developed countries. The term “ULTALITE®” is an internationally registered trademark and the property of Cyco Systems Corporation.
Details of current patent and trademark registrations are available upon request.