Marmara University

Engineering Faculty

Metallurgy & Materials Engineering

MSE 432 Foundry Technology

Semi Solid Metal Casting

Project Prepared By
İsmail Elmadağlı
Çağlar KELEŞ
Göktürk Bayram İskender

Submitted To

Prof. Dr. Altan Türkeli

Introduction

Semi-solid metal casting (SSM), also known as thixocasting, rheocasting, thixoforming or thixomolding, is a near net shape process in the production of parts out of non-ferrous metals, such as aluminium, copper, or magnesium. The process combines the advantages of casting and forging. The process is named after the fluid property thixotropy, which is the phenomenon that allows this process to work. Simply, thixotropic fluids shear when the material flows, but thicken when standing. The potential for this type of process was first recognized in the early 1970s. SSM is done at a temperature that puts the metal between its liquidus and solidus temperature. Ideally the metal should be 30 to 65% solid. The metal must have a low viscosity to be usable, and to reach this low viscosity the material needs a globular primary surrounded by the liquid phase. The temperature range possible depends on the material and for aluminum alloys is 5-10oC, but for narrow melting range copper alloys can be only several tenths of a degree. Semi-solid casting is typically used for high-end castings. For aluminum alloys typical parts include engine suspension mounts, air manifold sensor harness, engine blocks and oil pump filter housing. For magnesium alloys, semi-solid casting is typically used to produce extremely thin walled castings, such as computer and camera bodies.

Process There are a number of different techniques to produce semi-solid castings. For aluminum alloys the more common processes are thixocasting and rheocasting. Other process such as strain induced melt activation (SIMA) and RAP can also be used with aluminum alloys, although are less common commercially.With magnesium alloys, the most common process is thixomolding.

SSM forming is done at a temperature between the liquidus and solidus temperatures, with the fraction solid being in the approximate range of 30 to 65%. The semi-solid billet (Figure 1) maintains its shape and is convenient for loading into the shot sleeve of a conventional die casting machine. For the semi-solid metal to have sufficiently low viscosity, the structure at the forming temperature must consist of a globular primary solid phase surrounded by the liquid phase. The technical and economical feasibility and success of the SSM process are determined to a large extent by the approach used to produce the starting stock with the essentialprecursor structure. [pic] Thixocasting utilizes a pre-cast billet with a non-dendritic microstructure that is normally produced by vigorously stirring the melt as the bar is being cast. Induction heating is normally used to re-heat the billets to the semi-solid temperature range, and die casting machines are used to inject the semi-solid material into hardened steels dies. Thixocasting is being performed commercially in North America, Europe and Asia. Thixocasting has the ability to produce extremely high quality components due to the product consistency that results from using pre-cast billet that is manufactured under the same ideal continuous processing conditions that are employed to make forging or rolling stock. Unlike thixocasting which re-heats a pre-cast billet, rheocasting develops the semi-solid slurry directly from the liquid, normally adjacent to the die casting machine. There are a large number of rheocasting processes that have been proposed over the past 10 years or so, and they generally differ in the method used to generate the semi-solid slurry. 18 different rheocasting techniques were documented in a recent publication For magnesium alloys, thixomolding uses a machine similar to injection molding. In a single step process, room temperature magnesium alloy chips are fed into the back end of a heated barrel through a volumetric feeder. The barrel is maintained under an argon atmosphere to prevent oxidation of the magnesium chips. A screw feeder located inside the barrel feeds the magnesium chips forward as they are heated into the semi-solid temperature range. The screw rotation provides the necessary shearing force to generate the globular structure needed for semi-solid casting. Once enough slurry has accumulated, the screw moves forward to inject the slurry into a steel die . In the SIMA method the material is first heated to the SMM temperature. As it nears the solidus temperature the grains recrystallize to form a fine grain structure. After the solidus temperature is passed the grain boundaries melt to form the SSM microstructure. For this method to work the material should be extruded or cold rolled in the half-hard tempered state. This method is limited in size to bar diameters smaller than 37 mm (1.5 in); because of this only smaller parts can be cast.

Benefits of SSM Forming

As an innovative casting process, SSM casting offers great potential to save costs, energy and material, and reduce the environmental impact of casting.

The benefits of SSM forming can be summarized as follows: • Net or near net shape processing. • Less thermal fatigue heat, less mold or die wear, and less solidification shrinkage due to reduced temperature of feedstock • Much reduced gas entrainment, porosity, shrinkage, hot tearing and other solidification defects due to less turbulent die filling, resulting in better mechanical properties and pressure tightness with reduced section thickness. • Finer, more uniform, microstructures leading to higher mechanical performance. • Increased casting speed relative to liquid processing due to lower thermal demands on the dies. • As substitute for sand casting, SSM cast parts production is relieved of the costs and environmental problems of reclamation and disposal of lead-contaminated sands. • Allows for the lead content of red brasses to be much reduced, and combined with a semi-solid charge shouldenable alloys normally prone to hot tearing to be die cast.
Innovative Semi-Solid Metal (SSM) Processing
| Semi-solid metals can be widel usedto produce aerospce, automotive,marine, and light and heavy equipmentcomponents. However, the |
|current state of the art production of SSMfeedstock, either in practice or underdevelopment, is based on the MHDand the slurry-on-demand |
|approaches.These approaches require additionalprocessing and melting steps in orderto produce either rheocast feedstockbillets (MDH) for |
|shipment or SSMslurries (slurry on demand) in house.These avoidable processing stepsconsume more energy yet they yieldinconsistent results. |
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|Researchers at Worcester |
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|Polytechnic Institute have investigatedmore economical and consistent SSM production Technologies that reduce unneccessary processing |
|steps.These researchers have discovered significant promise in the |
|researchers have discovered significant promise in the ContinuousRheoconversion Process (CRP)approach. CRP has the potential toconsiderably |
|reduce process costsand energy consumption by directlyproducing the desirable SSMmicrostructure. The proposed projectwill serve as a building |
|block to thedevelopment of optimum alloys andoptimum heat treatment schedulesthat are specifically designed for theSSM/CRP process. |
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Conclusion
Due to the lower pressures and temperatures required to die cast semi-solid metal the die material doesn't need to be as exotic. Oftentimes graphite or softer stainless steels are used. Even non-ferrous dies can be used for one time shots. Because of this the process can be applied to rapid prototyping needs and mass production. This also allows for the casting of high melting point metals, such as tool steel and stellite, if a higher temperature die material is used. Other advantages include: easily automated, consistent, production rates are equal to or better than die casting rates, no air entrapment, low shrinkage rates, and a uniform microstructure.
Bibliography
1. Thixomat, Inc., http://www.azom.com/Suppliers.asp?SupplierID=3362, retrieved on 2008-10-14 . 2. Young, p. 1. 3. Lowe, Anthony; Ridgway, Keith; Atkinson, Helen (September 1999), "Thixoforming", Materials World 7 (9): 541-543, http://www.azom.com/details.asp?ArticleID=1373 . 4. P. Kapranos, Proc. 10th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Aachen, Germany & Liege, Belgium, 2008 5. Stephen P. Midson, Rheocasting Processes for Semi-Solid Casting of Aluminum Alloys, Die Casting Engineer, Jan. 2006 6. S. LeBeau & R Decker, "Microstructural Design of Thixomolded Magnesium Alloys", Proc. 5th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Golden, Colordao, 1998 7. Stephen P. Midson, Semi-Solid Casting of Aluminum Alloys: An Update, Die Casting Engineer, Sept. 2008 8. Stephen P. Midson, Robert K. Kilbert, Stephen E. Le Beau & Raymond Decker, "Guidelines for Producing Magnesium Thixomolded Semi-Solid Components used in Structural Applications", Proc. 8th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Limasol, Cyprus, 2004 9. Young, p. 2. 10. Stephen P. Midson, NADCA Semi-Solid & Squeeze Casting Conference, Rosemont, Illinois, 1996