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.