Background on Semi-Solid Casting
The technological and economical potential of innovative materials and their processing is of increasing interest to technology oriented companies. The demands on cast parts for the automobile and aerospace industries, as well as in broad areas of the mechanical engineering sector are still rising. The demand for higher mechanical properties has to be combined with stringent economical and ecological aspects. These apparently inconsistent requirements have to be accomplished with the development of innovative manufacturing methods and material concepts
The processing of metals in the semi-solid state is an innovative technology for the production of advanced materials. It combines a high productivity with near-net-shape quality.
Research on the processing of metal alloys at temperatures between the solidus and liquidus, normally referred to as semi-solid processing (SSP) began in the early 1970s, in the solidification laboratory of the Massachusetts Institute of Technology (MIT). Aiming at characterizing the strength of developing non-dendritic networks during solidification, the results showed that at relatively high shear rates, stress increase was minimal with decreasing temperature below the liquidus. The semi-solid slurry with a non-dendritic microstructure exhibits a distinct rheological behavior, namely, thixotropy and pseudo-plasticity
The current manufacturing routes involve either of, the use of non-dendritic alloy feedstock or cooling from the liquid to semi-solid state before directly injecting into the die. In the former, the material is cut to slugs of appropriate size before re-heating the slugs to the semi-solid state by carousel of induction heater and once the slugs have reached the required liquid fraction, robots automatically transfer them into the shot chamber of the thixoforming press to inject them into a die. The production route is very similar to diecating, However, the resulting properties of the parts are of much higher quality
Casting and forming of metal alloys in a semi-solid state is by now a well established manufacturing technique, which encompasses rheocasting at low solid fractions and thixoforming at higher solid fractions. The flow of metal in a die cavity in semi-solid metal (SSM) processing is determined primarily by the rheological characteristics of semi-solids which, in turn, depends on many variables, such as temperature, holding time, shape, size of particles in the slurry and connectivity of particles, etc
Thixocasting or semi-solid metal processing SSM is the shaping of metal components in the semi-solid state. In this state the alloy is thixotropic: if it is sheared the viscosity falls and it flows like a liquid. A slug of alloy, heated into the semi-solid state, can be cut with a knife and spread like butter, provided the microstructure is non-dendritic. This extraordinary behavior led to extensive work on the thixotropy of alloy slurries and the exploitation of the process, using the advantages afforded by filling a die with a lower temperature slurry which flows in a non-turbulent manner.
Rheocasting is an emerging technique wherein a high shear rate is applied to a solidifying alloy in order to produce a non-dendritic microstructure. The shear generated by either mechanical or electromagnetic means produces a flow pattern in the processing crucible. The mechanism of formation of this non-dendritic microstructure is frequently associated with fragmentation of dendrite arms to small pieces followed by agglomeration or sintering of these pieces to form large clusters. It is reported that semi-solid forming process takes many advantages over the conventional forming process, such as long die life, good mechanical properties and energy saving .In semi-solid forming process, there are two processes such as thixoforming and rheoforming.
Materials fabricated by continuous casting process with electromagnetic system are used in thixoforming. Therefore, the cost of feedstock material is very high. Also, in mass production of thixoforming product, there are disadvantages such as limited selection of alloy, billet reheating cost, non-destructive control cost, the difficulty of scrap recycling and billet loss during reheating process. On the contrary, the rheoforming process on-demand slurry not only cast alloy but also structural alloy can be used and the recycling of scrap is much easier