Semi-solid metal (SSM) processing is a technology that resulted from research in the early 1970's at the Massachusetts Institute of Technology. It was found that imposing a shear on a liquid metal before the solidification process began and continuing the shear while the liquid cooled below its liquidus resulted in a non-dendritic microstructure with a shear stress (and corresponding viscosity) nearly three orders of magnitude lower than that of the dendritic material. At rest, the non-dendritic metal slurry behaved as a rigid material in the two-phase region; that is, its viscosity was high enough that it could be handled as a solid. However, when a shear stress was applied, the viscosity decreased dramatically so that the material behaved more like a liquid. Thus, the slurry could flow in a laminar fashion, with a stable flow front, as opposed to the turbulent flow characteristic of molten metal.
A property of semi-solid metal (“slurry”) that renders it superior to conventional casting processes is the non-turbulent (“laminar” or “thixotropic”) flow behavior that results when one enters the “two-phase” field of solid plus liquid. Specifically, shearing of semi-solid slurry leads to a marked decrease in viscosity, so that a partially frozen alloy can be made to flow like a non-Newtonian fluid. Thixotropic flow behavior arises from the ideal SSM microstructure of small, spherical particles (e.g., α-Al) suspended in a liquid matrix. In all semi-solid processes, it is imperative that this microstructure be produced consistently. Moreover, a uniform distribution of this microstructure throughout a volume of slurry is essential for production of high-quality components.
The benefits that semi-solid processing holds over conventional liquid metal casting result from the flow behavior of the partially solidified metal. The way in which a metal fills a mold (or die cavity) directly impacts the solidification of the metal; thus, the properties of the formed part can be enhanced with improved mold filling. Turbulent flow of liquid metal into a die or mold can lead to incorporation of air and mold gases into the melt. This in turn can lead to both macro- and micro-porosity in the final part, which negatively affect its mechanical properties.
There are several reasons that the laminar flow of semi-solid slurries is very advantageous from a casting standpoint. The first major reason is the elimination of gas entrapment, resulting in decreased porosity and oxide content in the formed part. Secondly, since semi-solid metal has lower heat content than superheated molten metal, there is less solidification shrinkage in the casting. Thus, molds can be filled more effectively and uniformly, and less post-casting machining is required. As a result, all semi-solid processes are potentially “near net-shape” processes. The reduced heat content also lowers the thermal stresses of the casting apparatus (typically a steel die) that contacts the metal, leading to longer tool life. Also, since the starting material has the thixotropic microstructure, the microstructure of any part formed with semi-solid processing is always equiaxed and non-dendritic. Therefore, the mechanical properties of the final component are better than a similar part formed from a conventional casting process.
The net result of the above-described advantages is that semi-solid casting can be used to produce intricate components with superior mechanical properties. The typical defects associated with molten metal casting can be circumvented when the microstructure (and thus the flow behavior) of the slurry is controlled. From an economic standpoint, it is expected that due to improved tool life, shorter cycle times, reduced machining, and ability to use less expensive heat treatment schedules, semi-solid processes will ultimately become as cost-effective as conventional casting routes such as high pressure die casting. Perhaps the most attractive attribute of semi-solid forming, however, is that due to the laminar flow of the slurry, very complex shapes can be cast, with thin walled sections on the order of millimeters.
A number of processes have been designed to take advantage of the unique behavior of semi-solid metal slurries. These processes produce the thixotropic microstructure through some method of vigorous agitation during solidification. It was hypothesized that the induced agitation broke up (or facilitated the melting off of) dendrite arms, which then ripened and spheroidized to form a non-dendritic structure.
There are two routes for processing semi-solid metal, i.e. two different ways to arrive at the desired point within the solid-liquid, two-phase region. The first route starts from the solid state (“thixocasting”), and the second starts from the liquid state (“rheocasting”).
Thixocasting processes start out with a solid precursor material (“feedstock”) that has been specially prepared by a billet manufacturer, and then supplied to the casting facility. The feedstock metal has an equiaxed, non-dendritic microstructure. Small amounts or “slugs” of this alloy are partially melted by reheating into the semi-solid temperature range, leading to the thixotropic structure. In most applications, the slug is subsequently placed directly into a shot sleeve of a die casting apparatus, and the part is formed.
During the initial years of SSM process development, mechanical stirring was used in various ways to break up dendrites and produce thixotropic metal structures. The combination of rapid heat extraction and vigorous melt agitation was effected by using different sizes, shapes, and velocities of stirring rods. Various researchers addressed the evolution of the “stircast” structure during this time. Although these methods worked well in that they effectively produced the desired metal structures, erosion of the stirrer became the “weak link” of the process.
Magnetohydrodynamic (MHD) casting process has been utilized to overcome the limitations associated with the use of stirrers. In this approach, dendrites are still formed and then broken from the nuclei by agitation. The source of the agitation is not a mechanical stirrer, but alternating electromagnetic fields. Induction coils are placed around a crucible to induce these forces. The crucible is equipped with a cooling system to initiate freezing in the alloy while the melt is exposed to the electromagnetic forces. Upon cooling down to ambient temperature, the alloy has an equiaxed, non-dendritic microstructure. However, the MHD stirring process requires complicated and expensive machinery.
Thixoforming processes comprise the majority of industrial semi-solid applications used today. Rather than producing a semi-solid slurry directly from a superheated melt, a specially prepared feedstock metal is heated to form the semi-solid slurry. This approach eliminates the need for melting equipment within the SSM casting facility. However, the special feedstock must be purchased from special manufacturers at a premium in the form of metal billets, therefore thixocasting processes are not economical compared to conventional processes. Furthermore, in thixocasting processes, scrap metal must be sent back to the billet manufacturer and cannot be recycled. Most importantly, process control is difficult in thixocasting, because solid fraction (and corresponding viscosity) is sensitive to temperature gradients in the reheated material. Thus, narrow temperature ranges must be achieved consistently for successful operations. This, combined with the time it takes (several minutes on average) to reheat the feedstock to the desired solid fraction, negatively affects productivity.
The development of ideal one-step rheocasting applications is highly preferable to the current two- or three-step applications associated with most thixocasting methods. Current thixocasting approaches are inherently batch processes, in which only small amounts of slurry can be produced during each forming operation. This places limits on the sizes and shapes of parts produced in this manner. A continuous process would circumvent these hindrances, and could be used for a broader variety of applications.
To date, none of these processes has satisfactorily addressed the need for providing a continuous semi-solid casting route. The current need in the SSM field is a relatively simple, easy-to-implement, flexible process that can be used for a wide variety of processing applications. Such a process should use relatively simple methods of melt agitation to avoid the problems associated with the previously discussed approaches.