Michel Suéry

Semi-solid processing of alloys

EVOLUTION AND DESIGN OF MICROSTRUCTURE IN SEMISOLID ALLOYS.- Fundamental Aspects.- Characterization of Microstructure in Semisolid Slurries.- Evolution of Microstructure in Semisolid Alloys During Isothermal Holding (Soaking).- Recent Developments in Slurry Formation.- RHEOLOGY AND MODELING.- and Definitions for Rheology and Modeling.- Experimental Determination of Rheological Behavior.- Modeling of Semisolid Processing.- General Conclusions on Rheology and Modeling.- INDUSTRIAL APPLICATIONS OF SEMISOLID PROCESSING.- to Industrial Applications of Semisolid Processing.- Raw Material.- Process Control in Die Filling and Die Design.- Component Design Rules.- Practical Applications in Use Today.- The Future.

Characterization of Microstructure in Semisolid Slurries

It has been understood for some time that the flow behavior of semisolid metal slurries, and the properties of parts shaped from the slurries, depend both on the fraction solid and on the size, distribution, and morphology of the solid particles within the liquid matrix. The fraction solid f s is essentially determined by the temperature for a given alloy; the other features, however, are a function of the history of preparation and it is known that slurry flow is enhanced by achieving fine spherical particles. Fraction solid (f s) can be determined from phase diagrams for simple alloy systems if equilibrium may be assumed, or from direct measurement on rapidly quenched specimens, using metallographic techniques on sectioned surfaces, such as line intercept: f s = L a, where L a is the total intercept length in the a particle phase per unit length of test line. Likewise, the average particle size (assuming spheres of constant diameter d) may be obtained by the measurement: d = L a/N a, where N a is the number of grains per unit area of sectioned surface. It should be noted that the fraction solid obtained from phase diagrams is strictly a weight fraction and only coincides with the volume fraction obtained from metallographic measurements above when the densities of liquid and solid are considered equal and shrinkage is therefore ignored. Other techniques have been used to obtain these quantities, but are indirect, requiring calibration, and are probably less reliable. However, see in situ X-ray microtomography in Sect. 3.4, where the measurement is carried out in the semisolid condition to avoid quenching artifacts.

Experimental Determination of Rheological Behavior

This chapter will be divided into two main parts. The behavior of alloys during partial solidification will be considered first but only alloys with a globular morphology will be examined. In this case, the alloy is considered as a homogeneous medium and therefore, the viscosity only (and sometimes the fluidity) has been determined. Alloys during partial remelting will thereafter be considered. In this case, the solid fraction can vary in large proportions so that the two approaches mentioned above will be detailed. In each part, the experimental methods will be examined first and then, the experimental results will be presented. It is necessary to mention here that only the most representative results will be presented and not all of the results in the literature. In addition to these two main parts, a comparison between partially solidified and partially remelted alloys will be performed. This comparison is important in view of the latest industrial developments of the rheocasting processes. Finally, the question of yield stress will be addressed.

Process Control in Die Filling and Die Design

Until recently, much of the SSM technology was highly proprietary and unknown to the general casting world. In the last several years, however, sufficient information has become available to make it possible to identify a number of alternative technical solutions that have been implemented around the world for the commercial production of high quality parts. Of particular note, it is now clear that real-time controlled machines are not an absolute pre-requisite for the production of safety critical or other high quality parts. Many parts are being produced in high volume and with excellent results on standard three-phase injection machines ((a) pre-set injection velocity, (b) pre-set ramp and (c) pre-set consolidation time under pressure before final ejection), as indeed were the very first semisolid die castings produced in the MIT laboratories. Rather, the key ingredients of a successful SSM system approach are: A reliable source of consistent quality raw material An appropriate delivery system for semisolid material to the casting machine A diaphragm or alternative approach to strip oxides from the slug’s surface or delivery system and eliminate or minimise the entrapment of oxides within the formed parts (see Figure 11.1) A powerful, repeatable, injection system capable of generating sufficient static pressure to feed solidification shrinkage throughout the freezing cycle Appropriate processes and controls to optimise the heat-treatability of the formed parts Coupled with intelligent die design to facilitate both turbulent-free filling and adequate feeding of shrinkage, these features have a proven record of producing high quality parts over long time periods.

Introduction and Definitions for Rheology and Modeling

The semisolid state in a metallic alloy can be obtained either during solidification from the liquid state, or during partial remelting from the solid state. Metal forming during solidification (rheocasting) has been studied in the early stages of the development of semisolid processing by Flemings and his coworkers, but it was not the main forming method until very recently with the development of the new rheocasting (NRC) process. However, solidification of an alloy with mechanical, passive, or electromagnetic stirring was extensively used to produce globular microstructures. Therefore, knowledge of the rheological properties of alloys subjected to stirring during solidification is of great importance in comparison with non-stirred alloys on the one hand and with partially remelted alloys on the other hand. Metal forming was thus mainly carried out after partial remelting: thixoforming (thixocasting or thixoforging) is then concerned with solid fractions, which can vary in a quite large range, from 10 to 20% in thixomolding to much larger values in thixoforging. Study of the rheological behavior of alloys during such a treatment is therefore also important for better forming conditions and improved properties of the formed components. The alloys studied from a rheological point of view belong essentially to three categories: the model alloys like Sn–Pb with a low melting temperature for which the experiments are relatively easy, the aluminum and magnesium alloys, which are the main thixoformed alloys, and the alloys with a high melting temperature for which only feasibility tests have been carried out.

Introduction to Industrial Applications of Semisolid Processing

Semisolid metal (SSM) processing is a hybrid technology combining features of both casting and forging that enables the production of near net-shape components of superior properties and surface finish. It was developed, from a discovery made at the Massachusetts Institute of Technology in the early 1970s, by Spencer et al. [1] that stirring of alloys during solidification led to a change in the solidifying microstructure resulting not only in a dramatic lowering of the apparent viscosity of the semisolid slurry, but also facilitating two-phase homogeneous flow at quite high fractions solid. More detail of this discovery and the effects of shear rate cooling rate and fraction solid can be found in Part II. The process of stirring alloys during solidification to produce non-dendritic solid within a slurry, and then injecting this slurry directly into a die as in liquid metal die casting, was called “rheocasting” by the MIT researchers and that name has largely stuck. Rheocasting started out as the preferred process route for industrial production and a new company formed byMIT and a group of industrial partners “Rheocast Corp.” designed and built several large-scale rheocasters for production of both aluminium and copper-base alloys. The first major customer of the technology and for the large-scale rheocasters produced by Rheocast Corp. was International Telephone and Telegraph (ITT) Corp.

Component Design Rules

Semisolid Metal Processes generate near net-shape end products with a high degree of precision because they use hard steel tooling and high injection pressures. This combination results in a high level of replication of the die contours and generally the NADCA standards for “precision tolerances” can be achieved on a regular basis. It is important whenever possible to eliminate sharp corners and provide proper draft and radii that assist the smooth and complete filling of the die cavity under speeds associated with high production cycles. Such provisions will tend to extend die life as well as allow the manufacturers to make use of the natural advantages of the SSM processes. Wall thicknesses typically range down as low as 1 mm, depending on the alloy system used, part size and shape and of course the intended application. Apart from the linear dimensional tolerances, other tolerances will be additive, such as “Parting line, Moving die components, Angularity, Concentricity, Parting line shift, Draft requirements, Flat requirements, Cored holes for cut threads, Cored holes for formed threads, Cored holes for pipe threads, and Machining stock allowance” tolerances. The NADCA publication no: 403 on “Product specification standards for die castings produced by the semisolid and squeeze casting processes” provides a full list of tolerances for a variety of alloys systems [NADCA].

Practical Applications in Use Today

Prediction is difficult, especially about the future, said J.D. Barrow; amusing and very true. However, there is no doubt that in today’s manufacturing business world one cannot avoid the fact that the rate of change is accelerating. Apart from conventional incremental innovation, radical innovation has become strategically very important and used as a source of competitive advantage in the global market place. The development of semisolid metal processing is an example of the full spectrum of experiences involved in turning a “research idea” to a successful “commercial product”. Following an unsuccessful attempt to exploit the technology directly by forming an entrepreneurial company, located on the famous Route128 ring road around Boston, the original MIT technology patents eventually were licensed to International telephone and telegraph corporation (ITT Corp.) in the late 1970s. ITT Corporation held the technology very closely, a practise which was also adopted by Alumax Inc., which acquired the technology from ITT Corporation around 1985. The philosophy of both corporations was to maintain tight secrecy around all developments of semisolid processing and to resist request for licensing or joint venture without stringent demands, which effectively eliminated all opportunities for the type of cross-fertilization, which in turn so greatly evolves as technical innovation. However, by around 1985, ITT Teves, a subsidiary of ITT Corporation and a leading manufacturer of braking systems, had established a semisolid manufacturing facility in Northern Germany, which sparked interest in a number of European manufacturers. As a consequence, in the late 1980s, a number of European manufacturers, having been rebuffed by Alumax, began to develop their own independent approaches for both raw material production and parts forming.

Evolution of Microstructure in Semisolid Alloys During Isothermal Holding (Soaking)

In a recent review of coarsening, Flemings [19] states that this process refers to the growth “of solid regions of low curvature at the expense of regions of higher curvature,” and this includes “the growth of larger particles or dendrite arms with the simultaneous dissolution of smaller particles or arms (so-called ‘ripening’), the filling of spaces between particles or dendrite arms (‘coalescence’) and the breakup of dendrites (‘dendrite multiplication’).” This is a view adopted in the present chapter. The driving force is of course always the reduction of total solid–liquid interface area and the reduction in the associated interfacial energy, and the general mechanism by which this is achieved is diffusion of solute atoms through the liquid from concentration gradients established between regions of high and low curvature. Owing to the complex geometries that may exist in coarsening semisolid systems with competing fluxes between different particles, which can change with both time and place, simple kinetic equations to describe the overall coarsening process is probably not possible. However, where the geometry of the solid may be clearly described, for instance in terms of dendrite arms formed early in solidification, simple kinetics can be derived. More complex geometries may develop later during coarsening that are more difficult to provide with an adequate geometrical description.

Recent Developments in Slurry Formation

Despite the proven improvement in the quality of products formed by semisolid processing, providing both better surface finish and mechanical properties after heat treatment, the main obstacle to progress in the industrial use of thixoforming are due to the additional costs involved in. 1. The production of solid billets having the correct internal structure, e.g., by the MHD process 2. The reheating, melting, and soaking of the billet to reformthe semisolid structure 3. The scrap produced by each thixoformed article, which must be returned to the billet manufacturer for reconstitution as appropriate billets These cost problems have led back in recent years to a reconsideration of the rheoforming route (the production of semisolid alloy slurry on site followed by immediate injection of slurry into the die), that was originally used at MIT, thus bypassing casting of solid by MHD. The UBE process [35] has been developed whereby rapid cooling of the aluminum alloy in a cylindrical steel crucible forms a semisolid billet. The semisolid billet and crucible are then transferred to the die casting machine by robot arm, where the billet is dropped into the shot sleeve for immediate injection. The scrap from the operation is collected ready for remelting in the holding furnace on site for further casting. This route clearly obviates the need for MHD stirring to produce a nondendritic structure, and the heating and partial melting of billets prior to injection.