A Comparison of Thixocasting and Rheocasting

A Comparison of Thixocasting and Rheocasting

Stephen P. Midson The Midson Group, Inc. Denver, Colorado USA

Andrew Jackson Arthur Jackson & Co., Ltd. Brighouse UK

Abstract

The first semi-solid casting process to be commercialized was thixocasting, where a pre-cast billet is re-heated to the semi-solid solid casting temperature. Advantages of thixocasting include the production of high quality components, while the main disadvantage is the higher cost associated with the production of the pre-cast billets.

Commercial pressures have driven casters to examine a different approach to semi-solid casting, where the semi-solid slurry is generated directly from the liquid adjacent to a die casting machine. These processes are collectively referred to as rheocasting, and there are currently at least 15 rheocasting processes either in commercial production or under development around the world.

This paper will describe technical aspects of both thixocasting and rheocasting, comparing the procedures used to generate the globular, semi-solid slurry. Two rheocasting processes will be examined in detail, one involved in the production of high integrity properties, while the other is focusing on reducing the porosity content of conventional die castings.

Key Words

Semi-solid casting, thixocasting, rheocasting, aluminum alloys

22 / 1

Introduction

Semi-solid casting is a modified die casting process that reduces or eliminates the porosity present in most die castings [1] . Rather than using liquid metal as the feed material, semi-solid processing uses a higher viscosity feed material that is partially solid and partially liquid. The high viscosity of the semi-solid metal, along with the use of controlled die filling conditions, ensures that the semi-solid metal fills the die in a non-turbulent manner so that harmful gas porosity can be essentially eliminated. After the die is filled, high pressure (1,000 bar or more) is maintained on the biscuit to feed micro-porosity.

Semi-solid casting has been used commercially for the past 15 to 20 years for the production of near-net shape aluminum components. Until recently, all semi-solid cast components have been produced by thixocasting, a process which re-heats pre-cast billets to the semi-solid casting temperature.

However, commercial pressures have driven casters to examine alternative approaches to semi-solid casting, where the semi-solid slurry is generated directly from the liquid. These processes are collectively referred to as rheocasting. Instead of re-heating a pre-cast billet, rheocasting cools liquid aluminum into the semi-solid range, while simultaneously generating the globular microstructure necessary for semi- solid forming. Creating the semi-solid slurry directly from the liquid eliminates the need for a special (more expensive) feedstock, as well as permitting biscuits, runners and scrap castings to be recycled in-house.

This paper will provide a brief introduction to semi-solid casting, followed by a description of the technical aspects of both thixocasting and rheocasting. A number of rheocasting processes are either in commercial production or under development around the world, and two of these processes will be examined in detail.

Semi-Solid Casting

Most semi-solid casting processes use metal that is between 25-50% solid and 50-75% liquid, utilizing high pressure, cold chamber die casting machines to inject the semi-solid slurry into re-usable, hardened steel dies [1] . For semi-solid casting to be successful, the slurry must contain the globular primary particles shown in Figure 1a. Conventional, dendritic- type microstructures, such as the one shown in Figure 1b, will not work for semi-solid casting. The main advantage provided by all the different semi- solid processes is that the dispersion of the globular solid particles in the liquid produces a highly viscous semi-solid slurry, and controlling the flow of that viscous liquid without splashing or turbulence is much easier than with fully liquid aluminum.

22 / 2

Conventional die casters accept the turbulence associated with high speed filling of filly liquid aluminum. They inject the liquid aluminum into dies using gates speeds of about 30-60 m/sec, and the resulting turbulence produces high levels of residual porosity in the castings.

Aluminum casting processes such as sand and investment casting attempt to fill the die cavity in a non-turbulent manner by limiting the gate speed to a maximum of about 0.25 m/sec [2] . Such low filling speeds may be acceptable when filling ceramic dies, but attempts to fill thin-walled components using un-coated steel dies at such low speeds would result in rapid solidification of the aluminum and non-fills.

The solid particles dispersed in semi-solid metals increase their viscosities as much as 10,000 times greater than those of fully liquid aluminum. Testing has shown that this higher viscosity allows a semi-solid alloy to be injected into a die using gate speeds of as high as 2.5-5.0 m/sec, while still avoiding turbulence [3] . This allows the production of porosity-free, thin walled castings in re-usable steel dies.

Thixocasting

As noted earlier, the thixocasting process was the first semi-solid process to be commercialized. Thixocasting consists of three separate stages: the production of a pre-cast billet having the special globular microstructure, the re-heating of these billets to the semi-solid casting temperature and the casting of the components (see Figure 2).

Thixocasting is capable of producing extremely high quality components having excellent mechanical and functional properties. The billet feed material is typically produced by aluminum companies in batches as large as 50,000 lbs. These pre-cast bars provide billet-to-billet and lot-to-lot chemistry, cleanliness and microstructural repeatability comparable to forging and rolling stock, and far more consistent than is typically achievable when pouring castings from the liquid in single doses [4] . Thus semi-solid components produced by thixocasting tend to have very consistent properties. The main disadvantage associated with thixocasting is higher manufacturing cost, arising both from the premium attached to special feedstock, as well as the inability to easily recycle biscuits and runners.

Rheocasting

Rather than using pre-cast billet, rheocasting generates the special semi- solid microstructure adjacent to the die casting machine directly from the liquid (see Figure 3). The liquid is cooled into the semi-solid range, while simultaneously generating the globular microstructure. Once the metal has been cooled to the correct temperature, the semi-solid slurry is

22 / 3 transferred to the shot sleeve of a die casting machine, and injected into the die using the same type of controlled fill as with thixocasting.

The major advantage of rheocasting is that the semi-solid feed material is produced at the casting machine directly from the liquid. This allows conventional ingot material to be used, eliminating the premium associated with the thixocasting billet. Another advantage is that biscuits and runners can now be recycled directly into the casting stream, again reducing cost (see Figure 3).

Potential disadvantages of rheocasting relate to the consistency of the product and the limited commercial application of the various processes. Questions relating to consistency arise from the fact that rheocasting uses single shot liquid dosing (ie, a single shot of liquid metal is poured to produce each casting), and it is much more difficult to maintain the required levels of metal cleanliness when pouring 5 lbs of metal than when pouring 50,000 lbs [4] . Therefore, it is still unclear whether rheocasting will prove as reliable as thixocasting.

Slurry Generation for Rheocasting

As noted earlier, there are a number of different rheocasting processes in commercial production or under development around the world. These different rheocasting processes generally differ in the manner in which the liquid is cooled and the globular semi-solid microstructure generated. There are four general techniques used to generate the globular, semi- solid microstructure, and most of the different rheocasting processes use some variation of these practices [5] . The techniques are:

Stirring – similar to thixocasting, the liquid aluminum (just enough for one shot) is stirred as it is cooled into the semi-solid temperature range.

Dendrite Fragmentation - a variation to stirring processes is the dendrite fragmentation technique, where the melt is cooled below its liquidus temperature, and the semi-solid alloy is treated in a turbulent manner to break up the dendrites, producing numerous small solid fragments that can be coarsened into globular-shaped aluminum particles.

Pressure Waves – pressure waves generated in the runner system have been shown to generate semi-solid structures.

Numerous solidification nuclei – In this technique, the liquid is poured into a container from a temperature just above its liquidus temperature. The rapid cooling generated during pouring generates a large number of solid nuclei, which prevent the formation of dendrites, instead producing a large number of globular solid particles. Often grain

22 / 4

refining techniques are used to assist the generation of the large number of solid nuclei.

A recent publication [5] identified 15 different rheocasting processes, and these are listed in Table 1, showing the organization that developed each process, and the technique used to generate the slurry. The various rheocasting processes are in different stages of commercial development, with some of the processes being used for the commercial production of components, while other processes are in the early stages of development.

Detailed Description of Two Rheocasting Processes

This section of the paper will describe in more detail two of the rheocasting processes listed in Table 1. One of the rheocasting processes is being used for the production of high integrity, safety-critical type castings, while the other is focusing on producing higher quality, porosity-free die castings.

The New Rheocasting Process

The New Rheocasting (NRC) process [6-8] is shown schematically in Figure 4. The process uses a carousel of crucibles on a circular turntable. At the first position, molten aluminum controlled just above its liquidus temperature is poured into the crucible. It is critical for the success of the process that the superheat of the melt has to be low, so that numerous solid nuclei are produced during pouring - for example, Kaufmann et al [6] report that that superheats of only 20 oC are used. It is these nuclei that generate the globular microstructure. As the carousel indexes, air is blown on the walls of the crucible, allowing the liquid to cool into the semi-solid state in a controlled manner. At the penultimate position, the surface of the metal is re-heated using an induction coil. The objective is obtain a semi-solid slug with a relatively consistent solid fraction from edge to center and from top to bottom. The crucible is then removed from the carousel, and the semi-solid slug is transferred into the shot chamber of a vertical cold chamber die casting machine.

The NRC process was the first of the rheocasting process to be commercialized. However, even at the height of its popularity, it never approached thixocasting in tonnage of parts shipped. Typically the NRC process is used for the production of high integrity castings used in safety- critical, structural applications. In Europe, Stampal announced in 2003 that they would convert all their semi-solid cast parts to the NRC process. Giordano and Chiarmetta from Stampal have provided an example of a automotive suspension part (Figure 5) that was scheduled to enter production in early 2005 at a rate of 20,000 sets per month [9] .

22 / 5

Semi-Solid Rheocasting

The Semi-Solid Rheocasting (SSR TM )[10-12] process was originally developed at the Massachusetts Institute of Technology (MIT), and it uses stirring and dendrite fragmentation techniques for generating the slurry. The MIT researchers found that if solid nuclei are present in sufficient quantities in a melt cooled just below its liquidus temperature, further cooling will cause the nuclei to rapidly spheroidze and grow with a spherical morphology, and that vigorous agitation is unnecessary after the formation of only a small fraction of solid.

In Step 1 of the SSR TM process (Figure 6), a robot dips a coated ceramic crucible into a holding furnace filled with molten aluminum held several degrees above its liquidus temperature and brings it to the SSR station. In step 2, a rotating, cooled graphite rod is inserted into the liquid metal and rapidly cools the melt for a short time, usually within the range of 5-20 seconds. Yurko et al report that the stirring time is controlled by a PLC utilizing a heat transfer algorithm that can account for variables such as furnace and rod temperature and alloy type. The researchers report that this closed loop feedback system is helpful in the foundry environment where furnace temperature normally fluctuates and die casting cycles are frequently interrupted [10] .

Only a small solid fraction is formed during the stirring phase (about 5%), so once the stirring rod is removed (step 3), additional cooling must occur so the semi-solid metal is cooled (without additional stirring) to a solid fraction of about 15-20%. As the melt is cooled, the particles generated in stage two grow to form globular solid particles distributed in the liquid. Once the target solid fraction is reached, the semi-solid alloy is poured from the vessel into the shot chamber of a die casting machine, where it is injected into the die.

One of the commercial focuses of the SSR TM process appears to be the production of higher quality die castings. The inventors of the process note that SSR TM provides the capability to improve the quality of secondary die casting alloys such as AlSi8.5Cu3 (380), whose high eutectic fraction make them difficult or impossible to process by other semi-solid processes at a solid fraction of 50%. Yurko et al suggest that SSR TM can reduce the porosity content, eliminate the need for impregnation, and provide as much as a 25% faster cycle time (as some of the alloy’s latent heat is removed at the SSR TM station).

Summary and Conclusions

A comparison of thixocasting and rheocasting has been presented, reviewing the advantages and disadvantages of each process.

22 / 6

Due to commercial pressures, casters around the world are currently placing more attention on rheocasting. 15 rheocasting processes have been identified, either in commercial production or under development.

TM
Two of the rheocasting processes (NRC and SSR ) are reviewed in more detail. NRC is typically being used for the production of high integrity, safety-critical type castings, while SSRTM is focusing on producing higher quality, porosity-free die castings.

References

1. Fan, Z, Inter. Meter. Rev., Vol. 47, 2002, p 49 2. Campbell, J, Proc. Materials Solutions Conference ’98 on Aluminum Casting Technology, 1998, p3 3. Midson S P, Minkler R B & Brucher H G, Gating of Semi-Solid Castings , Proc 6 th Inter. Conf. on Semi-Solid Processing of Alloys and Composites, Ed. G.L. Chiarmetta & M. Rosso, Turin, Italy, Sept 2000 4. Jorstad, J, Semi-Solid Metal Processing: A Cost Competitive Approach for High Integrity Aluminum Components , Proc. 6 th Inter. Conference on Semi-Semi Processing of Alloys and Composites, Eds. G. Chiametta & M. Rosso, Sept 2000, Turin, Italy, p 227 5. Midson S P, Rheocasting Processes for Semi-Solid Casting of Aluminum Alloys , Die Casting Engineer, January 2006, p48 6. Kaufmann H, Holzl A & Uggowitzer P J, New Rheocasting of High Strength Aluminum Foundry Alloys , Proc. 7 th Inter. Conference on Semi-Semi Processing of Alloys and Composites, Eds. Y. Tsutsui, M. Kiuchi & K. Ichikawa, Sept 2002, Tsukuba, Japan, p 617 7. Adachi M, Sato S, Harada Y & Kawasaki T, US patent number 6,165,411, Apparatus for Producing Metal to be Semimolten– Molded , Dec 26 th , 2000 8. European patent number 745,694Al, “Method and Apparatus for Shaping Semisolid”, 1999 9. Giordano P and Chiarmetta G, New Rheocasting: A Valid Alternative to the Traditional Technologies for the Production of Automotive Suspension Parts , Proc. 8 th International Conference on Semi-Semi Processing of Alloys and Composites, Eds. A. Alexandrou & D. Apelian, Sept 2004, Limassol, Cyprus 10. Yurko J A, Martinez R A & Flemings M C, SSR TM : The Spheroidal Growth Route to Semi-Solid Forming , Proc. 8 th Inter. Conference on Semi-Semi Processing of Alloys and Composites, Eds. A. Alexandrou & D. Apelian, Sept 2004, Limassol, Cyprus 11. Yurko J, Flemings M & Martinez A, Semi-Solid Rheocasting (SSR TM ) – Increasing the Capabilities of Die Casting , Die Casting Engineer, January 2004, p 50 12. Flemings M C, Martinez-Ayers R A, de Figueredo A & Yurko J A, Metal Alloy Compositions and Process , US Patent number 6,645,323, Nov 11, 2003

22 / 7

Technique Used Process Name Organization Location to Generate Slurry Gibbs Gibbs Die Casting USA Stirring Hitachi Hitachi Metals Japan Stirring Honda Honda Japan Stirring Induction CSIR South Africa Stirring Heating/Stirring SEED Process Alcan Canada Stirring Slurry On Mercury Marine USA Stirring Demand Stirring/Dendrite Rheo-Diecasting Brunel University England fragmentation Semi-Solid Stirring + IdraPrince USA Rheocasting numerous nuclei ATM CSIRO Australia Pressure Waves Continuous Worcester Dendrite Rheoconversion Polytechnic USA fragmentation Process Institute Buhler Buhler Switzerland Numerous nuclei Controlled Worcester Diffusion Polytechnic USA Numerous nuclei Solidification Institute Direct Thermal University College Ireland Numerous nuclei Method Dublin New Ube Japan Numerous nuclei Rheocasting Sub-Liquidus THT Presses USA Numerous nuclei Casting

Table 1: Fifteen different rheocasting processes (after 5)

22 / 8

a) b)

100 µm

Figure 1: Aluminum alloy 357 microstructures a) Globular microstructure required for semi-solid processing b) Conventional cast dendritic microstructure

Figure 2: Schematic of the thixoforming process Figure 2: Schematic drawing of the thixocasting process

Figure 3: Schematic drawing of the rheocasting process

22 / 9

Figure 4 Schematic of Ube’s New Rheocasting (NRC) process

Figure 5: Photograph of an automotive suspension arm scheduled to enter production at Stampal in 2005 using the Ube NRC process (after 9)

a) b)

Figure 6: Semi-solid Rheocasting (SSR TM ) (after 10) a) Schematic of the Semi-Solid Rheocasting process b) Photograph of IdraPrince’s SSR TM Station

22 / 10