Bringing SSM casting to the masses.
* Two SSM casting processes, rheocasting and sub liquidus casting, ate profiled.
* While SSM has a reputation as a high-cost casting process, these two innovations present opportunities improve its cost effectiveness.
While semi-solid metal (SSM) casting has been practiced across the U.S. for approximately 30 years, the process continues to grow and redefine itself. Recently, two new approaches to SSM casting have been developed, enhancing the potential that the process could be increased in use. These two processes, rheocasting and sub liquidus casting (SLC), have taken SSM to a new cost-conscious level.
Conventional SSM processing involves the reheating of billets to a semi solid state and subsequently casting them in a diecasting machine. The process is capable of the dimensions, details and thin-wall sections of conventional high-pressure die castings while accomplishing the high integrity generally ascribable only to squeeze castings of the highest quality gravity and low-pressure permanent mold castings. Made from heat treatable premium alloys such as A356/357, 354 and C355, SSM can provide high levels of strength and ductility.
Several advantages of the SSM process include:
* product complexity, close dimensional tolerances, near net shape, thin walls and excellent surface finish compared to conventional die castings;
* exceptional soundness--in most cases SSM castings contain less than 0.1% porosity, better than any other mass production casting process;
* ability to utilize a variety of alloys, including A356/357, 354 and C355;
* low process temperature resulting in short cycle times and low stress on tooling;
* ability to undergo a T-5 heat treatment without losing ductility. This allows the castings to achieve required mechanical properties without the dangers of blistering, distortion of quench stress associated with the full %6 heat treatment required or other structural casting processes.
The search for low-cost routes for producing parts by semi-solid forming directly from the liquid state (or rheocasting) is quickly progressing in today's foundry industry. The new rheocasting process developed by the Japanese firm UBE 1ndustries Ltd., features liquid metal poured into a container approximately the size of a billet, which is subsequently red to the shot chamber of the forming machine
Unlike traditional SSM casting, the rheocasting process eliminates reheating of billets by producing "slurry on demand." This is achieved by solidifying the alloy to a semi-solid state in a special container (either steel of ceramic) to the L+S region. The SSM "slug" produced is then fed to a diecasting machine.
The basic sequence of the process is comprised of five major steps:
Cleaning, Coating and Preheating--The entire sequence of cleaning, coating and preheating of the vessel is performed on carousel 2 of the slurry producing unit (Fig. 1). After the cleaning step is performed on "J" and "K," the vessel is coated at "L" and at "M" it is preheated. After these procedures, the vessel is transported to spot "1" (the carousel turns clockwise) where it can be taken by the robot and placed on carousel 1, spot "A."
[FIGIRE 1 OMITTED]
Pouring the Molten Alloy--The second step comprises filling the ladle with molten alloy from the holding furnace and pouring it into the preheated and coated vessel (shown in Fig. 1 at carousel 1, spot "A"). The vessel is inclined by a specified angle so that the molten alloy is gently poured in along its sidewall.
Cooling Down--The alloy is held within the insulated vessel for a period of time and cooled down to a temperature where a specified fraction solid is reached (partial solidification), whereby a large number of fine spherical primary crystals are generated,
Reheating--Due to the temperature difference between the molten alloy and the preheated vessel, a chilled zone is formed on the outer surface of the slug. To obtain a uniform microstructure throughout the entire vessel, it is mandatory to remove this zone before the slug is formed. Thus, the vessel is inserted into an induction-heating furnace (carousel 1, spot "G") for a specific period of time. Thereafter, the vessel is transported to spot "H", where it can be accessed by the robot.
Diecasting--Depending on the mode the operator selects on the casting unit, the vessel taken by the robot from spot "H" can be placed either on the sample holder (spot "S") or it is red into a forming mold, where it is shaped under pressure. Thereafter, the empty vessel is positioned on carousel 2, spot "I," and the procedure starts again. If the operator wants to take a sample, the vessel is positioned at spot "S," and an empty vessel is taken from spot "T." The sample holder can be moved outside of the casting unit and the slug can be quenched or sliced.
The rheocasting process is not as sensitive to variations in process parameters such as reheating power input. The distribution of the alpha-phase is relatively uniform across the crucible and no segregation of silicon is observed. This is essential to producing high quality parts where the mechanical properties have to be uniform throughout the entire part.
Also, entrapped liquid, which is observed in some SSM processes, is not observed in rheocast parts. The amount of liquid phase that is provided for the viscosity of the slug is directly proportional to the fraction solid. It can therefore be deduced that the flowability of the prepared slurry will be better.
The SLC Process
The economics of SSM casting have improved with the introduction of sub liquidus casting (SLC), a slurry approach recently developed by THT Presses, Inc., Dayton, Ohio. The SLC process uses normal foundry ingot, primary or secondary, and requires no processing equipment extraneous to the casting machine of processing time outside of the normal diecasting cycle.
The SLC machines (Fig. 2) have a vertical shot and a horizontal die parting configuration. While the machine is small (requiring no excavation for installation), it offers an equivalent shot capability of a larger, more conventional machine. Also, because of their unique shot sleeve and piston design, the machines are capable of larger shots than machines of higher tonnage.
[FIGURE 2 OMITTED]
The SLC process employs a large diameter, short shot approach. This feature allows for larger shots and enables tight control of the metal temperature required for SSM slurry processing. This reduces plunger speed necessity, drastically reducing impact pressures at the conclusion of each shot. This also provides the opportunity for multiple-cavity gating.
The indexing table feature enables pouring molten metal into a shot tube at one station, making appropriate temperature adjustments and achieving required slurry ripening, then making the actual shot in the next shot tube and removing the biscuit in the last station. Unlike other SSM slurry approaches, the SLC process requires virtually no slug pre-preparation equipment or processing time outside of the casting machine.
If a slurry is developed in a normal configuration shot sleeve (large length to diameter ratio) of either vertical or horizontal orientation, the portion of the shot that is suitable for injection into a die cavity amounts to only a small fraction (less than 20%) of the total volume poured. To accomplish greater useful volumes of correct temperature/structure, the metal requires rigid controls on a relatively high shot tube temperature, which significantly slows the casting cycle.
The SLC slot sleeve design naturally provides both the time necessary in the semi-solid temperature regime to gain the globular structure desired and a major portion of the poured shot suitable to enter the die cavity (more than 60%). This all can occur within the normal machine cycle.
Suitable SSM structures can be achieved in the SLC process. When starting with normally grain refined melt, the process provides globule sizes of [+ or -] 75 microns. When starting with a super-refined melt, the process can provide cell sizes of [+ or -] 25 microns. Mechanical properties achieved in castings from those slurries are comparable to, and sometimes higher than, those realized from MHD stirred billet-based castings.
Can SSM Be Cost Effective?
In spite of the many economical benefits offered by SSM processing, its penetration into the foundry marketplace has been severe]y inhibited by the high cost of the billet (raw metal). In the year 2000, it is estimated that worldwide use of SSM billet was approximately 25,000 tons, meaning that the actual product made from such billet was about 10,000-15,000 tons. Those products represent only a small fraction of the global use of aluminum castings.
If the cost of the alloy were similar to the raw material for other aluminum casting processes, the economics and desirability of using SSM casting would improve dramatically. Traditional stirred billet can sell for as much as 35-50% more than the cost of primary foundry alloy purchased as ingot for re-melting.
However, there ate several areas where SSM can actually help a foundry cut down costs, making the investment in the process easier to swallow.
Section Thinness--With its ability to achieve section thin ness, detail and dimensional control comparable to diecasting, SSM minimizes the material used and can provide components with the lowest possible weight. Material cost is minimized and, in automotive applications, fuel economy is maximized.
Near-Net-Shape--SSM's ability to cast components with near-net-shape either eliminates machining entirely or greatly minimizes the required machining stock. With steel tooling and pressure exerted on the metal during fill and solidification in the filled cavity, whatever complex detail exists in the component is reproduced in the final cast part. Also, while the SSM fills rapidly, its viscous nature prevents turbulence, which leads to a reduction in casting-related defects.
Heat Treatment--Heat treatment is an area where all versions of SSM provide significant cost advantage over all liquid metal casting processes. Castings poured from liquid A356/357 type alloys have good ductility in the as cast condition, but if they are given a T-5 heat treatment, they lose up to hall of that material property. To achieve reasonable levels of both strength and ductility, those castings must be given a full T-6 heat treatment, during which they may blister, warp or retain high levels of residual stress.
Because of their unique microstructure and semi-solid processing conditions, SSM castings can be given that T-5 heat treatment and still retain their high as-cast ductility. This feature provides a distinct cost advantage in terms of capital furnace capacity and energy requirements. SSM cast components can be quenched in hot water directly from the die and then artificially aged to achieve increased strength and hardness with no loss of ductility, thus avoiding the risk of defects such as blisters and casting distortion.
The Thixomolding Process
Another SSM casting variation is the thixomolding process, which offers magnesium castings with high strength and low wall thickness. Thixomolding combines conventional diecasting and plastic injection molding into a one-step process for the net shape molding of magnesium alloys.
The process requires no investment in molten metal process and handling equipment and eliminates the safety hazards of handling molten magnesium. The injection system, which is similar to plastic injection molding machines, consists of a high temperature screw and barrel coupled to a high speed shot system that drives the reciprocating screw.
In the process, magnesium alloy feedstock is thermally processed by the rotating screw and then injected into a die cavity. The temperature is then raised to a semi-solid region and, after determining the desired temperature and shot size, is injected into a preheated metal mold. The screw is driven forward, filling the die cavity.
The small amount of turbulence allows thixomolded components to have low levels of porosity and gives designers dimensional stability with precision and repeatability. Also, since the process does not require any external foundry or material handling, it is environmentally friendly.
John L. Jorstad is the president of J.L.J. Technologies, Inc., Richmond, Virginia; Mike Thieman is the president and CEO and Rick Kamm is the vice president of engineering at THT Presses, Inc., Dayton, Ohio; M. Lukasson is part of the Foundry Institute at RWTH, Aachen, Germany; Diran Apelian is the director and professor at Worcester Polytechnic Institute, Worcester, Massachusetts; and Rathindra DatGupta is the chief scientist as SPX Contech Div., Dowagiac, Michigan.
For More information
"Sub Liquidus Casting (SLC): Process, Concept and Product," J. Jorstad, M. Thieman, R. Kamm, M. Loughman and T. Woehlke, 2003 AFS Transactions, No. 03-i62.
"Alloy Characterization for the New UBE Rheocasting Process," M. Lukasson, D. Apelian, R. DasGupta, 2002 AFS Transactions, No. 02-032.
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|Title Annotation:||Technology in Progress; semi-solid metal; Technology in Progress; semi-solid metal|
|Date:||Oct 1, 2003|
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