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Slurry-Based Semi-Solid A new process for the fabrication ofhigh-quality, semi-solid formed components offers advantages over components made via traditional semi-solid manufacturing methods.

Chris 5. Rice* and Patricio F. Mendez* Fig. 1 - Automotive fuel rails are being direct slum; formed by Gibbs Die Casting Semi-Solid Technologies Inc. Corp. T11e process is compatible with existing equipment, reduces costs, and provides Somerville, Massachusetts high quality, pressure-tight components.

with liquid, I while ired slurry (DSF) is a semi-solid - forming process that eliminates capital t ~" cost expenditures, reduces the number L of steps required, and hence reduces the D r: S+L cost of semi-solid formed components relative to s other semi-solid . Semi-solid forming continues to make inroads in the manu­ s fachu·e of mechanically demanding and pressure­ Start witlt solid, tight aluminum components because of its ability partially remelt to provide near-net-shape components with prop­ erties far exceeding those of other casting tech­ 100% AI 100% alloying elements nologies. For these reasons, semi-solid formed com­ ....,__Composition~ ponents have increasingly become a preferred Fig. 2 - This diagram shows the two approaches to semi­ alternative for , components machined solid Janning. One is to begin with a liquid metal and cool it completely from billet, and investment castings. to the semi-solid state while applying shear forces. T11e other is Traditional semi-solid metal processing typically to take billet with a globular microstructure and reheat it to involves continuously casting metal bar stock with the semi-solid state. a special microstructure, storing and/ or shipping Semi-solid metal behavior this material, cutting it to length, reheating to the At a vety basic level, a "semi-solid" material is semi-solid state, transporting to the forming press, simply a mixture of a liquid and solid phase. In the and forming the product. context of semi-solid metal processing, a semi-solid The DSF process eliminates many of these steps is a mixture of rounded solid phase particles sus­ by producing a large quantity of semi-solid slurry pended in a liquid matrix. These metal mixtures from molten metal, and by maintaining a constant are slurries, much like slush, ketchup, or sand cas­ supply at the fomring press. Process complexity is tles - mixtures of dispersed solids within a liquid. reduced, capital eqtripment is eliminated, and spe­ In a metal , thermodynamics determine at cialty raw material is avoided. DSF provides the what temperatures a metal is solid, liquid, or par­ high-density, heat-treatable, pressure-tight com­ tially solid and liquid. For most alloy compositions, ponents possible with traditional semi-solid pro­ a freezing range exists between the states of solid cessing, but at lower cost. The process, enabled by and liquid where both liquid and solid persevere accurate temperature control and thorough mixing together in the semi-solid state. of the slurry, is fully integrated with existing Semi-solid slurries may be created in two ways. vacuum die casting equipment and is currently in The simplest is to begin with a liquid metal and service for the manufacture of automotive fuel rails cool it into the semi-solid state by applying shear (Fig. 1). forces (mechanical, electromagnetic, or otherwise) This article describes the behavior of semi-solid as the metal cools. These shear forces break up den­ metals, details the process steps, and compares it drites as they form during solidification, and create with die casting and conventional semi-solid rounded solid particles in the melt. This rounded processes. structure is still apparent in the solidified metal and *Member ofA SM International is the microstructural characteristic of semi-solid ADVANCED MATERIALS & PROCESSES/OCTO BER 2001 49 formed components. The second method is to take a solid piece of metal that has a globular micro­ Two factors structure (formed previously from the liquid state - and allowed to solidify) and reheat it into the semi­ heavily solid state, where the liquid matrix remelts. The influenced two methods are illustrated in Fig. 2 for a hypo­ the design thetical binary aluminum alloy. Conventional semi-solid processing of alununum of the slurry has followed the second path, which will be desig­ furnace­ nated here as the billet method. Bars of stirred, con­ the required tinuously cast metal are subsequently cut into bil­ lets, which can be reheated to the semi-solid state heat removal at a later time for forming. When the billets are re­ rate, and the heated, they are transferred to the shot sleeve of a need to die casting press and forced under high pressure into a mold cavity. maintain a The advantage of the billet process is that it Fig. 3 - The semi-solid state is a mixture of rounded solid­ near­ achieves high solid fractions: when formed into phase particles in a liquid matrix. components, the billets are often at upwards of 50% constant to 60% solid. Assuming that the bar stock is sup­ of heat extraction were needed in the most extreme temperature plied by an outside vendor, it allows the parts case. Additionally, because an average of 90 kg (200 of the bath. maker to avoid handling molten metal. lb) of molten aluminum is added every ten min­ The disadvantages are that a premium must be utes, the bath temperature may not vary more than paid for the bar stock, the bar must be cut to length ±1 oc (±2°F). Each of these requirements dictated a depending on the part size, the billets must be re­ furnace with large volume and high surface area. heated from room temperahrre in what most often A cylindrical furnace one meter in diameter and is a multi-station apparatus, and one meter tall could contain 2000 lb of slurry and the heated billets must then be transferred either remove 7 kW /m2. With a temperahrre difference manually or robotically to the casting press. It is of soac (90°F) between liquid and slurry, specific these additional steps and equipment that are elim­ heat of 60 kJ /kg for liquid aluminum, and latent inated by Direct Slurry Forming. heat removal of 77 kJ I kg, this configuration could meet the design needs. Process description The second step of the process is maintaining the Gibbs Die Casting Corp., a major automotive alu­ slurry in a homogeneous, isothermal state as it minum die caster, needed a process that could pro­ awaits forming. ill addition to the thermal require­ vide the advantages of semi-solid processed com­ ments for creating a shrrry discussed above, it is ponents, but at a cost much closer to that of die necessary to provide adequate shearing forces to casting. Additionally, given the high volume of the the mixture to create the rounded solid particles automotive business, process throughput was iden­ and prevent agglomeration of the solids. tified as being very important. A third requirement ill the arrangement described here, where heat was compatibility with existing die casting equip­ is removed through the walls of the furnace, alu­ ment. To meet these needs, it was decided that the minum alloy would typically form dendrites at the process must be capable of at least a 550 kg/hr (1200 outer walls, and these dendrites would grow in­ lb /hr) production rate. It also had to accommodate w ard as solidification progressed. If the average the Gibbs vacuum ladling process. temperature of the bath were maintained constant The process developed in response to these re­ the result would be complete segregation: a solid quirements can be separated into three steps: for­ shell of low-alloy aluminum surrounding a high­ mation of semi-solid metal, maintenance of the alloy liquid center. sluny suspension, and transport of the semi-solid However, the goal is a unifom1 mixhrre of fine­ mixture to the die casting . Die casting op­ grained solid particles in a liquid matrix. This re­ erations typically have furnaces for melting ingot, sult can be achieved by regularly scraping the walls and the molten metal is transported to holding fur­ of the furnace during solidification, breaking naces at the casting press. This metal can be as hot growing dendrites from the wall, and moving them as 650"C (1200°F) at the holding furnaces. Producing into the bulk of the bath. With sufficient shearing 550 kg/hr of partially solidified slurry is a matter action, these dendritic structures are moved through of thermodynamics: h1coming molten metal must the melt and coarsened into rmmded structures by be cooled and stirred sufficiently fast to remove the flow of liquid around them. Figure 3 illustrates enough heat to lower the temperature to the re­ the viscous consistency of the resulting slurry. quired slurry temperahrre-typically about 600°C Equally important to creating the solid portion (1100°F). of the slurry is keeping jt uniformly distributed Two factors heavily influenced the design of the within the mixhrre. Because of density differences slurry fumace- the reqt.illed heat removal rate, and between solid and liquid phases, the solid particles the need to maintain a near-constant temperature in aluminum alloy A356 (a low-iron, aluminum­ of the bath. Based on the production rates and tem­ silicon casting alloy not typical in die-casting) will perature differences, it was found that about 20 kW settle downward over time, resulting in a high con- 50 ADVANCED MATERIALS & PROCESSES/OCTOBER 2001 gee-shaped rotor

Anchor­ shaped scraping rotor

cooling hannels

Fig. 4 - This diagram shaws the design ofthe slunyJonning Fig. 5 - These are a sampling of parts made l1y direct sluny forming. The parts fumace. The impeller has inclined rotating blades that force the vasry from thin-walled to thick-walled, pressure-tight to mechanically demanding. flow of material upward in the melt. The scraping rotor shears growing.dendrites off the walls and into the melt. Actual practice closely matched the predicted re- - centration of solid particles at the bottom of a fur­ suits. nace, and ve1y few of them at the top. To overcome It was also found that temperature control of the this, it was necessary to introduce a method of ladle tube is important to prevent freezing of the agitation along a third axis. slurry. The tube must be kept at or above the liq­ The answer was to agitate the melt by means of uidus temperature of the alloy. Vacuum ladling an impeller with inclined rotating blades that forced technology was key to the success of this process, the flow of material upward in the melt. The final because it eliminates the need to have a p erson or mixing system in the furnace consists of an anchor­ robot dedicated to the transfer of slurry from fur­ shaped rotor with vertical shearing rods located nace to casting press. proximal to the furnace walls for the scraping of solidifying material, and an auger located within Comparison with die casting the anchor shape to promote vertical homogeneity. The DSF technique shares many similarities with Figure 4 depicts the furnace and mixing configu­ conventional die casting operations. The same ration. presses and raw material are suitable for DSF and This original mixing arrangement is the main dif­ conventional die-casting; therefore, no significant ference between DSF and the original implemen­ modifications must be made to existing die-casting tations of rheocasting, and it is what enabled, for equipment when implementing DSF. Die materials the first time, commercial production of semi-solid are the same, and DSF achieves the same dimen­ aluminum components from slurry without billets. sional tolerances fow1d in die castings. A more detailed depiction of this arrangement can In contrast, the resulting product has significant be found in U.S. Patents 5,881,796 and 5,887,640. differences. Components manufactured by DSF are The final step of the process is to transfer the consistently leak-proof, even for difficult compo­ slurry from the furnace to the casting machine shot nents that cannot be made reliably by conventional sleeve. As mentioned earlier, the objective was for die casting methods. Because of reduced porosity, the existing vacuum ladling method to become the mechanical properties of DSF products are supe­ h·ansfer mechanism. This mechanism consists of a rior to their die cast equivalent. High-temperature vacuum that draws liquid aluminum through a heat treatments, such as T6 heat treatment, are pos­ tube into the shot sleeve of a cold chamber die sible with DSF because of reduced gas entrapment casting press. Figure 4 shows the transfer tube, in semi-solid castings. Finally, low-iron aluminum which is connected to a cold chamber shot sleeve. alloys not normally considered suitable for die Because liquid aluminum has significant superheat, casting because of soldering and die wear, including the metal flows through the tube without freezing, A356 and 357, can be formed via DSF because of and when the vacuum is removed, the metal lower temperature and latent heat. quickly flows back to the bath. However, a semi-solid slurry contains no super­ Comparison with billet methods heat, and because it is partially solidified, it is likely The DSF process has a number of advantages to freeze. Another concern is the increased viscosity over conventional semi-solid forming techniques, of the slurry, because the thickness of the slurry will some of which are outlined below: prevent its flow through a tube. Therefore, the flow • Less energy is required during processing be­ of slurry through a tube was modeled. Given a vis­ cause metal is heated only once. No reheating is cosity of 0.2 Pascal-seconds for a slurry of 30% solid, necessary, as the semi-solid is formed only one time. it was predicted that not only would the slurry flow • Complexity is reduced because no multi­ through the tube under vacuum, but also the ladle station heating systems are needed, and no heated time for 10 lb (a typical shot weight) of slurry would billets must be handled. increase by less than 10% over liquid aluminum. • Flexibility is increased because alloy modifica- ADVANCED MATERIALS & PROCESSES/OCTOBER 200 1 51 tions can be made in-h ouse, and the solid fraction Fig. 6 - The Ford can be tailored to the application. Zetec SE 13 DISI Capital cost • Metal supply is less restricted because it is not engine is a prototype - 1.1 liter, 3-cylimder, necessary to cast only the special semi-solid is reduced direct fuel injection processed feed bar available from only a few sup­ engine designed for because no . reducing emissions sawmg • Capital cost is reduced because no sawing and improuingfuel equipment, special induction heating systems, or economy in equipment, robotic handling is necessary. congested urban special • Scrap cost is reduced because scrap can be re­ environments. Fuel induction cycled in house simply through remelting, and it rails on the Zetec does not have to be sold and converted back to billet engines for the Ford heating stock. Focus are slum; cast. systems, or Each of these differences contributes to a process robotic that can provide semi-solid components at reduced improved mechanical properties are required over handling is cost. die castings, or where costs m ust be lower than other processes such as forgings or welded assem­ necessary. Industrial implemen tation blies. The near-net-shape capabilities of semi-solid The DSF process has been in production at Gibbs forming combined with DSF offer an exciting new Die Casting for over 18 months. Automotive fuel cost competitive manufacturing solution. • rails that can be found on the Zetec engines (Fig. 6) of Ford Focus automobiles are slurry cast. Many For more information: Dr. Patricio Mendez, President, other parts have b een prototyped, including Semi-Solid Technologies, Inc., 223 Somerville Ave. #3, compressors, compressor heads, motor mounts, Somerville, MA 02143; tel: 617 /629-2376; fax: 801/729- pistons, and clutch covers. The parts vary from thin­ 6929; e-mail: [email protected]. walled to thick-walled, pressure-tight to mechani­ cally demanding, and single to m ulti-cavity. Figure Haw useful did you find the information 5 shows a variety of parts sampled with this presented in this article? Very useful, Circle 274 process. Of general interest, Circle 275 DSF lends itself well to high-volume automotive Not useful, Circle 276 applications where leak tightness is required, wh ere

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