TMS QUICK LINKS:
  • TMS ONLINE
  • MEMBERS ONLY
  • MEMBERSHIP INFO
  • MEETINGS CALENDAR
  • PUBLICATIONS

JOM QUICK LINKS:
  • JOM HOME PAGE
  • MORE HTML ARTICLES
  • COMPLETE ISSUES
  • BOOKS REVIEWS
  • SUBSCRIBE

In the search for a single-step manufacturing process for components with various shapes, sound structural integrity, and properties comparable to the wrought state at a low cost similar to casting, semi-solid processing may provide the solution. Although semi-solid processing is already a viable manufacturing method, it is still under intensive development and a critical breakthrough is still expected. This article provides the characteristics of semi-solid processing, including historical and technical backgrounds, major concepts, and commercialization examples.

INTRODUCTION

Today’s engineer can select from numerous techniques of manufacturing net-shape components using metals and their alloys. The majority of these techniques, in principle, could be classified into two conventional routes restricted to either the solid or liquid state. The liquid-state methods involve casting with a variety of modifications: gravity, high-pressure die casting, squeeze casting, etc.

In contrast, the solid-state techniques generally require multi-step operations after casting, such as homogenization of chemistry, hot working, cold working, forming, machining, or heat treatment. As a result, the properties of wrought components are predominantly superior to castings. The number of manufacturing steps and their complexity, however, contribute to a significantly higher cost of the final product (Figure 1). The economy factor represents the downside of many non-conventional manufacturing techniques (e.g., powder metallurgy). Thus, there is a continuous quest for a technology that would reduce costs and at the same time improve properties.

The ultimate goal in this search is the single-step manufacturing of components with various shapes, sound structural integrity, and properties comparable to the wrought state at a low cost similar to casting. It is believed that the emerging technology of semi-solid processing may satisfy these requirements. Although semi-solid processing is already established as a viable method of manufacturing, it is currently under intensive development and a critical breakthrough is still expected. This report provides the basic characteristics of semi-solid processing, including historical and technical backgrounds, major concepts, and commercialization examples. The literature data are complemented by original findings in clarifying existing discrepancies, thus allowing for a proper understanding of the principles of this modern technology.

Semi-solid metal processing (SSP), sometimes referred to as semi-solid metallurgy, was initiated at the Massachusetts Institute of Technology in 1971 by Spencer, Flemings, and co-workers. During experiments with the Sn-15%Pb alloy, they found that applying shear during solidification substantially reduced the stress measured (Figure 2). In fact, the stress at a given temperature below the liquidus was orders of magnitude less than when the alloy was cooled to that temperature without shear. Decreasing temperature leads to a rapid increase in viscosity but, as seen in Figure 3, the higher the shear rate the lower the maximum viscosity value and the shorter the time to reach its steady state.

However, the key for semi-solid processing, the phenomenon of thixotropy, was discovered about a half century earlier in 1923 by Schalek and Szegvari in non-metallic systems. They found that aqueous iron oxide gels would become completely liquid through gentle shaking to such an extent that the liquefied gel was hardly distinguishable from the original sol. Previously, these kinds of physical changes had only been known to occur by modifying the temperature when gels would melt on heating and then re-solidify on cooling. The term thixotropy was introduced by Peterfi in 1927 as a combination of two Greek words: thixis, meaning stirring or shaking, and trepo, meaning turning or changing.

In later years, various systems were studied, including clays, oil suspensions, creams, drilling mud, flour doughs, flour suspensions, fiber greases, jellies, paints, and starch pastes.

A system is described as thixotropic when a reduction in magnitude of its rheological properties, such as elastic modulus, yield stress, and viscosity occurs reversibly and isothermally with a distinct time dependence on application of shear strain. The most frequent structural changes that produce thixotropy are those where the structure breaks down under high shear rate but recovers under low shear rate or when at rest. Therefore, thixotropy arises from changes in floc structural arrangement due to forces acting between suspended particles and breakdown due to the shear rate. It was revealed in the 1930s that thixotropy was more pronounced in systems containing non-spherical particles because rotation and movement allowed for their alignment and dis-alignment in a three-dimensional structure (Figure 4).

Semi-solid metal processing represents one of the many engineering applications that utilize thixotropy. It should be pointed out that although metallic alloys sheared below their melting point are described as thixotropic, in fact, they exhibit mainly shear-thinning behavior. Shear thinning, also referred to as pseudoplasticity, describes a decrease of viscosity with an increase in shear rate. Both terms are frequently confused since in real systems it is difficult to separate the time-dependent and time-independent behavior of the non-Newtonian fluid. An explanation of pseudoplastic behavior is the formation of agglomerates/clusters of favorably oriented solid particles at low shear rates which increase viscosity. It is assumed that changes are reversible and high shear rates break down agglomerates, reducing viscosity.

The key difference of semi-solid processing is the reduced temperature when compared to the casting of superheated melts (Figure 5). The generally accepted advantages of hardware performance and energy economy, achieved due to reduced operating temperatures, are universally positive for all alloys and include the following: lower energy consumption, no handling of liquid metal, longer die life, better yield from the raw material due to lower oxidation and evaporation, and fewer other losses related to melt overheating. While these benefits are related to hardware performance, the reduced processing temperature also replaces a molten alloy with a semi-solid slurry. Then, further reduction in temperature below the liquidus changes the solid/liquid ratio and slurry properties which affect the final product, not only in terms of its internal integrity, but also its microstructure. Unfortunately, the present literature, while assessing the improvement in properties, does not distinguish between microstructural factors and the component’s integrity factors.

As depicted in Figure 5, the influence of the semi-solid slurry on the component’s integrity is complex. The common assumption that the benefits of semi-solid processing arise exclusively from the flow behavior of the partially solidified metal is, apparently, a simplification. In general, the slurry affects product integrity through a reduction in porosity. The turbulent flow of a liquid alloy into a mold can result in the entrapment of air and mold gases into the melt, which in turn may translate into micro- and macro-porosity (voids or oxides). Smooth flow of the semi-solid slurry minimizes these defects. Similarly, a lower liquid content within the semi-solid slurry reduces shrinkage porosity. Both the flow behavior and solidification shrinkage are improved with a reduction in liquid content (i.e., the alloy’s temperature). The temperature cannot be reduced indefinitely since it simultaneously lowers the heat content, which adversely affects the mold filling and potentially the product’s integrity.

As was the case for the benefits regarding hardware performance, the higher components integrity has a universally positive effect for all alloys. The improvement in a component’s integrity not only increases the material properties and allows the achievement of near-net shape capabilities but also makes it heat treatable. This is in contrast to superheated melt castings where heat treatment is, in most cases, not possible due to blistering.

To distinguish between a component’s integrity and microstructure, differences are schematically emphasized in Figure 6. The key microstructural change, observed after a slurry’s solidification, is a replacement of dendritic forms by globular morphologies. The new features, unmelted globules with a size less than 0.1 mm, are surrounded by the solidification product of the remaining liquid fraction. Their presence leads to reduced segregation, typical for coarse dendrites, but also the selective partition of chemical elements between solid and liquid which affects the phases and microstructural components. A magnitude of latter changes depends on the particular alloy and its phase diagram: the higher solid fraction causes the larger change in the chemistry of the remaining liquid.

The enrichment of a liquid alloy in certain chemical elements can lead to increased precipitation of phases and modifications in their distribution pattern. In extreme cases, the phases not present during complete liquid casting may be formed. If the phases are of a brittle nature, their location at boundaries between the primary solid and matrix or secondary magnesium grains may lead to a reduction in an alloy’s ductility. Thus, excluding the macro-segregation factor, the influence of globular solids on the direction of changes of alloy properties is not universally positive and should be evaluated for individual alloy chemistry. An advantage of the component’s integrity, which allows for heat treatment, may be used to dissolve brittle phases, thus improving properties. The effectiveness of heat treatment depends, however, on the particular alloy system. It is generally known that heat treatment is more efficient for aluminum based face-centered-cubic structures than for alloys with magnesium-based hexagonal-close-packed matrix.

Understanding the difference between the integrity and microstructure is not only important for the proper selection of the processing parameters to achieve the maximum with existing alloys, but also for the development of new alloys designed for semi-solid techniques.

A number of technologies have been developed over the last three decades, mainly in a laboratory environment, to take advantage of the unique behavior of semi-solid slurries. The progress in the commercialization of many of them seems to indicate the beginning of the large-scale acceptance of semi-solid processing by major industries. All the technologies can be divided into two fundamentally different basic routes: rheo-processing and thixo-processing. As shown in Figure 7, there are also hybrids that combine features of both routes; injection molding is considered to be an example of such hybrids.

When utilizing rheo-processing, the starting fully molten precursor is presolidified under controlled conditions and then transferred into the mold. In a majority of applications it dominates a slurry-on-demand concept, where a single dose of slurry is separately prepared for each shot. During the new rheocast process the melt is poured into the crucible-receiver and then cooled to a semi-solid-state temperature before transfer to the shot tube. The semi-solid rheocasting represents the effective means to the grain-refined slurry at the onset of solidification by a spinning cold finger which is applied to the upper surface of melt while it cooled below the liquidus.

The continuous rheoconversion employs the liquid mixing technique in a specially designed reactor that provides copious nucleation and forced conversion during the initial stages of solidification. During the sub-liquidus casting, the pre-grain-refined material is poured into the shot sleeve at temperatures just above liquidus and cooled to a semi-solid state before transfer to the mold. The swirl enthalpy equilibration device consists of extracting a controlled quantity of enthalpy to generate the slurry and then draining away the excess liquid to form a compact slug ready for casting.

The thixo-route involves two stages: first, billet preparation which consists, in fact, of a portion of the rheo-route and, second, billet re-heating and component forming. If the forming process is conducted in a closed die, it is called thixocasting. When a mold is open during this operation, the process is termed thixoforging. The purpose of preheating the previously prepared billet prior to forming is to create the material with the precisely controlled solid fraction of fine spherical particles. The particles are uniformly distributed in a liquid matrix which has a lower melting temperature. Both conventional and induction heating methods are used with success.

The most common process of billet manufacturing is based on magneto-hydrodynamic stirring during semi-continuous casting. Another alternative is applying special grain-refining techniques to generate a fine-grained rosette-like structure. According to the stress-induced melt-activation idea, a conventionally cast billet of generally limited size is cold or warm deformed, frequently by extrusion. The rapid slug cooling technology explores a cooling control to produce a fine and homogeneous microstructure of globular dendrites suitable for thixoforming. Although rheomolding was discovered first, thixoforming methods were the first to be implemented commercially. However, due to their cost, these methods have been dominated by rheocasting of several modifications.

A distinct method of semi-solid processing is represented by injection molding. Similar to the rheo-route, it consists of a single step, but like the thixo-route it starts from a solid feedstock. In addition to thixomolding, other unique processing methods were developed, including near-liquidus molding or semi-solid extrusion molding. The one-step processing is possible due to the specific characteristics of a coarse particulate feedstock, termed as chips, granules, or pellets, created during their manufacturing.

The cold-deformed structure of mechanically comminuted chips has common features with that produced by the stress-induced melt activation method. Similarly, the structure of rapidly solidified granules with fine dendritic forms behaves during re-melting in the same way as structures produced by magneto-hydrodynamic stirring or grain refining. As a result of the unique microstructure, the particulates transform into thixotropic slurries under the sole influence of heat. The structural features, deliberately created during the manufacturing of billets for thixoforming, are obtained here as a side effect of comminuting bulk ingots into small particulates. Injection molding is at present the leading commercial technique of the semi-solid processing of magnesium alloys.

Frank Czerwinski is a chief metallurgist of Development Engineering at Husky Injection Molding Systems Ltd., in Bolton, Ontario, Canada.

For more information, contact Frank Czerwinski, Husky Injection Molding Systems Ltd., 560 Queen Street South, Bolton, ON, L7E 5S5 Canada; (905) 951-5000, ext. 3263; fax (905) 951-5365; e-mail fczerwinski@husky.ca.