Thixocasting

Semi-solid metal processing offers several advantages over conventional technologies such as casting, forging and powder metallurgy. Semi-solid metal processing enables the manufacturing of components with complex shapes, with thin walls, with good mechanical properties and with a high dimensional tolerance and accuracy. The thixocasting process uses stirring of the melt during the solidification of a continuous cast bar to obtain the globulitic microstructure.

The thixocasting process is a semi-solid metal processing route (SSM), which involves forming of alloys in the semi-solid state to near-net-shaped products.

The process of thixocasting offers a number of advantages, such as improved mechanical properties, good surface finish, near net shape and so on. However, the thixocasting process has also a number of disadvantages, such as the need for special feedstock with near spherical primary crystals. In order to cast such special billets for thixocasting one has to pay a more expensive premium than normal. Eliminating this additional specialized casting step leads to savings in both costs and time.

Compared with the conventional casting technologies, thixocasting has a lower forming temperature, significantly longer die life, high part precision, production efficiency and comprehensive mechanical properties. As compared with hot forging technologies, thixocasting has quite a low yield strength, high fluidity, low forming load and low surface roughness. Especially in the thixocasting process, a complex geometry product can be obtained by only one step forming. This technology has been widely applied in nonferrous metal forming and satisfactory results were derived, but not with ferrous metal.

Table 1: Production parameters for thixocasting

In the investigation of Killicli V. et al. it is shown that there are microstructural features of casting defects in AA7075 aluminum alloy produced by thixocasting.

The thixocasting process was conducted using a cold chamber high pressure die casting machine using a medium frequency induction heating generator. Specimens were formed at 611°C which correspond to 50% liquid fraction and different injection speeds at constant die temperature (150°C). The liquid fraction was calculated from DTA data. The experimental set-up of thixocasting process is schematically represented in Figure 1.

Figure 1: Schematic representation of experimental set-up for the thixocasting process

AA7075 alloy part produced by thixocasting illustrates in Figure 2. Specimens were prepared by standard metallographic procedures and polishing with up to 0.1 μm colloidal silica and etching with Keller’s reagent. Microstructural features were studied using a Leica DM400M optical microscope and Jeol JSM 6060LV scanning electron microscope (SEM).

Figure 2: AA7075 alloy part produced by thixocasting

Experimental results indicate that the many casting defects such as microporosity, microshrinkage, dendritic solidification in liquid pool, hot tearing in liquid phase, microsegregation at grain boundaries and liquid segregation in the sharp corner of the die were observed in the microstructure of the AA7075 aluminum alloy produced by thixocasting. The lower mechanical properties of thixocast AA7075 alloys can be attributed to casting defects. To avoid these casting defects the thixocasting process parameters (liquid fraction/semi solid temperature, injection speed, injection pressure and die temperature) must be controlled tightly.