Semi-Solid Processing of Metals
Semi Solid Metal (SSM) processing is gaining momentum as an effective alternative to the classical manufacturing processes of casting and forging.
This category takes advantages of both liquid and solid forming processes. In the absence of shear forces, the semisolid slug is similar to a solid, e.g. self standing, while with application of shear, the viscosity is reduced appreciably and the material flows like a liquid, i.e. thixothropic behavior: “dual characteristics where the SSM billet has solid-like characteristics, but flows like a liquid under shear stress”.
Such unique characteristics have made SSM routes attractive alternatives to conventional casting, even on an industrial scale. Knowledge of different routes and understanding the formation of slurries is a necessity for choosing the optimum and most appropriate process for any application. The review article published recently by SMG director elucidates the most widely utilized SSM processes and discusses the basic concepts.
Technologies for SSM Processing
Technologies for SSM processing can be generally divided into two basic groups:
- Rheo- routes; transfer of the prepared SSM slurry directly into a die or mould for component shaping
- Thixo-routes; preparation of a feedstock material having an equiaxed or globular structure, reheating of the feedstock material to temperatures between solidus and liquidus (mushy zone) followed by shaping of the mush (liquid + solid).
For rheo-casting, there is a wide range of processes patented or under investigation within research and development centers worldwide. In spite of technical and technological differences amongst the available rheo-routes, they may be categorized within the following groups:
- Mechanical stirring, MS
- Magneto Hydro Dynamic (MHD) stirring / Electro Magnetic Stirring (EMS)
- New Rheo-Casting NRC™ or UBE process
- Liquid mixing process, LMP
- The new MIT process, Semi Solid Rheocasting SSRTM Casting
- Ultrasonic treatment
- Hitachi Process
- Low pouring or superheat casting, LSC
- Sub Liquidus Casting (SLC®)
- Swirled Enthalpy Equilibration Device (SEED)
The research activities embarked on by SMG cover different angles ranging from process mechanics to alloy structure and improvement. Some of our researchers have worked intensively and researched in details the capabilities and drawbacks of a good number of these routes with main emphasis on MS, EMS, LSC and SEED. We have widely published in scientific journals and presented at international conferences. Interested parties may contact the SMG to initiate a systematic approach towards SSM manufacturing routes, whether in production or research and development.
One of the most important characteristics of SSM billets is their rheological behavior during high pressure die casting and injection into the mold. The main parameter to study the rheological behavior of semi-solid metal, SSM, slurries is the viscosity and its importance is equivalent to that of fluidity for liquid metals or tensile modulus in the solids. The viscosity value is an indication of materials capability in filling the die cavity during casting operations. It is also a measure of the magnitude of force required for deformation of materials. A lower viscosity value induces easier movement of SSM slurry through the die and allows an intricate thin wall component to be cast with lesser machine pressure and metal scrap, i.e. greater material yield.
The viscosity of SSM slurries is dependent on metallurgical parameters including the fraction solid, and its morphology, e.g. dendritic or globular, solid particle size and distribution, chemical composition of the alloy and pouring temperature. A new parallel plate compression viscometery test machine was designed and fabricated to study the rheological behavior of SSM billets.
For further information on all aspects of Semi-Solid-Metal processing and particularly about the SMG “know-how” and its implication to your industry, please contact us.