Semi-solid casting of pure magnesium

Transcription

1 Semi-solid casting of pure magnesium CURLE Ulyate A. a* and WILKINS Jeremias D. b Light Metals, Materials Science and Manufacturing, Council for Scientific and Industrial Research Pretoria, South Africa a ucurle@csir.co.za, b jdwilkins@csir.co.za Keywords: Rheo-processing, High pressure die casting, R-HPDC, solidification time interval. Abstract. Semi-solid processing works on the principal of a solidification temperature interval of a substance. The substance is heated to a temperature within this interval so that there exists a related solid-liquid fraction ratio. The substance with this phase structure is then shaped by a forging or casting process. It has been stated before that it is impossible to semi-solid process and cast pure metals or eutectic alloys due to their thermodynamic temperature invariance, meaning that there is no temperature interval. It was demonstrated recently that it is possible to semi-solid casting high purity aluminium (Curle UA, Möller H, Wilkins JD. Scripta Materialia 64 (2011) ) and the Al-Si binary eutectic (Curle UA, Möller H, Wilkins JD. Materials Letters 65 (2011) ). The working principal is that there exists a time interval during thermal arrest during which solidification takes place with a solid-liquid fraction ratio until all the liquid is consumed upon cooling. The aim with this work is to demonstrate that pure magnesium can also be rheo-high pressure die cast (R-HPDC) with the system developed at the CSIR in South Africa. Magnesium is notoriously difficult to cast due to the thermal properties of magnesium. The metal was poured into a cup, processed for about 6 seconds after which it was HPDC into a plate. The microstructure of the casting consists of a structure that was solid and a structure that was liquid during thermal arrest at the time of casting. Introduction Semi-solid metal processing is based on the principal of cooling liquid metal from above its liquidus temperature, called rheo-processing, to the two phase region between the liquidus and the solidus of the alloy. By so doing it results in a mixture of solid and liquid. Commonly the liquid is subjected to large convection forces in order to prevent heterogeneous nucleation of solid on the cold surfaces of the container and stirring mechanism. A semi-solid mixture can also be achieved by heating a solid metal to this two phase region and is called thixo-processing. The semi-solid billet is formed in some manor after the semi-solid preparation has been completed. Forging is normally used after thixo-processing because of the higher solid fraction, around 80%; while casting is normally used for rheo-processing because of the lower solid fraction, around 30%. One can quickly see the problem with semi-solid processing of invariant point metals and alloys like pure metals and eutectic compositions there is no two phase region on the thermodynamic equilibrium phase diagram. And it has been claimed before that it is not possible to semi-solid process pure metals and eutectic alloys due to the lack of this solidification temperature interval [1-3]. Curle et al. has refuted these claims by rheo-processing and high pressure die casting (R-HPDC) high purity aluminium [4] and the unmodified Al-Si eutectic alloy [5]. The works were based on the existence of a time interval associated with solidification. The aim of the current work was to demonstrate that it is also possible to semi-solid metal R-HPDC commercial purity magnesium.

2 Experimental procedure R-HPDC equipment. The CSIR has industrial scale and research scale R-HPDC cells. The R-HPDC cell basically consists of a dosing furnace, a cup transfer mechanism, the CSIR-RCS and a HPDC machine. The dosing furnace is used to melt and hold the metal, and for controlled pouring of the metal into the processing cup. In the industrial scale cell the processing cup with a capacity of around 2 kg aluminium alloy is robotically transferred between cell components. The processing cup is manually transferred by hand within the research scale cell. The CSIR-RCS is an induction coil connected to an induction generator. The coil induces a magnetic field inside the processing cup which causes non-contact stirring. The coil is integrated with a forced air cooling coil which air flow can be adjusted to result in controlled cooling inside the processing cup. This integrated coil is a patent of the CSIR [6]. Processing takes place for a predetermined time until it is ready to be transferred for casting. The cold chamber HPDC machine in the industrial scale cell has a claiming force of 630 ton, while the research scale cell has a cold chamber HPDC machine with a 130 ton clamping force. Both machines are shot-controlled to give control over the injection process. R-HPDC pure magnesium. The research scale cell was used for this study. Fig. 1 shows the research scale cell and the components thereof. Metal and melting. Commercial purity (2N) magnesium was used in the study. Around 3 kg of 2N Mg was melted in the manual dosing resistance heated furnace with a steel crucible. SF 6 was used as a cover-gas to protect the liquid Mg from reacting with the ambient air. The pouring temperature was controlled at 660 C, 10 C above the melting point of pure magnesium. Rheo-processing. Around 250 g 2N Mg was poured into a 1.6 mm wall thickness stainless steel cup from the dosing furnace. The cup was quickly transferred to the coil of the rheo-processing unit. The forced air was closed for trials due to the fast solidification of the specific volume of Mg. The Mg was processed for 6 s in the coil at which point it was again quickly transferred to the HPDC. The surface of the cup was also protected from the ambient atmosphere with SF 6 gas during rheo-processing. High pressure die casting. The semi-solid billet was emptied from the cup into the shot sleeve of the HPDC machine. The injection piston was initiated and the semi-solid billet filled the cavity of the die. The die temperature was set at 180 C while the injection speed was set to 1 m/s. Casting geometry. The geometry chosen for casting was a plate of 100 mm x 55 mm x 6 mm. The runner has a 90 smooth transition gooseneck that runs out into the gate of the plate component. Results and Discussion Fig. 2 shows the microstructure of the Mg the solidified in the gooseneck runner of the casting, while Fig. 3 shows the microstructure at the top of the plate. The difference is apparent in that it can be seen that the microstructure in the runner consists only of a single phase while the microstructure from the top of the plate seems to consist of two phases. On the one hand, Fig. 2 shows an equiaxed grain structure with relatively fine grains. Such a microstructure is consistent with turbulence during solidification and a high cooling rate. On the other hand, Fig. 3 shows a globular grains in what seems liquid a past existing liquid phase. The explanation by Curle et al. [4] for high purity aluminium is followed for explanation. Solidification of pure substances does not happen instantaneous but over a certain amount of time during thermal arrest. There isn t a solidification temperature interval but there is a solidification time interval during which heat has to be extracted by the system.

3 Control computer 130 ton HPDC machine CSIR-RCS Dosing furnace Figure 1. Layout of the CSIR rheocasting system research scale cell. The dosing furnace, CSIR-RCS, and HPDC machine is shown.

4 Figure 2. Low magnification image of the microstructure from the runner of the casting showing a single phase structure. Figure 3. Low magnification image of the microstructure from the top of the plate casting showing a seemingly two phase structure.

5 One could think of the more familiar example of water and ice. The temperature of a mixture of ice and water will drop to 0 C and will stay so if well mixed. The ice does not melt instantaneously and the water also does not freeze instantaneously. Thermal arrest occurs at 0 C during which time, depending on the heat characteristics of the system, either the ice or the water is consumed into the other phase. Coming back to the commercially pure Mg. The liquid Mg is poured at a 10 C superheat above its melting point of 650 C. Solid phase embryos are nucleated heterogeneously as the liquid metal runs over the surface of the furnace spout into the processing cup. These embryos grow during rheo-processing under the induced magnetic field inside the processing coil. Now the material in the cup consists of a mixture of both pure solid and liquid Mg. Upon HPDC the liquid phase is captured by a very high cooling rate inside the dies and has a very fine structure which is visually different from the coarse structure of the externally equilibrium solidified solid phase globules. Fig.3 is a high magnification image around the globule grain boundary. Parts of globular grains are visible in the top left- and right-hand corners. In the middle of the image there is an area with a fine structure and also consisting of grey spots which are most likely impurity elements that were ejected into the liquid fraction during solidification inside the cup upon rheo-processing. The fine structure is characteristic of the high cooling rate associated with HPDC. Figure 3. High magnification of the rheo-processed and HPDC microstructure of the top of the plate, showing the globules and liquid fraction areas. Summary It has been demonstrated that commercial purity Mg can be rheo-processed and high pressure die cast into a shape by using the research scale CSIR rheocasting cell. The microstructure of the plate had a two phase appearance which can be explained by the fact that heterogeneous nucleation took place on the pouring spout surface while the low superheat metal was poured from the furnace into the cup. Induction stirring kept the solid and liquid phases mixed without solidification from the inside of the processing cup. The high cooling rate of HPDC rapidly solidified the liquid and solid fractions inside the dies at the time of final solidification. The structure is unintuitive with equilibrium and non-equilibrium solidification characteristics.

6 Acknowledgements The authors would like to thank the Department of Science and Technology (DST) for past funding through the Light Metals Development Network (LMDN) under the Advanced Metals Initiative (AMI) in South Africa. References [1] A. Alexandrou, F. Bardinet, W. Loué. J. Mater. Process. Technol. 96 (1999) 59. [2] J.B. Patel, Y.Q. Liu, G. Shao, Z. Fan. Mater. Sci. Eng. A 476 (2008) 341. [3] S. Nafisi, R. Ghomashchi. J. Alloys Compd. 436 (2007) 86. [4] U.A. Curle, H. Möller, J.D. Wilkins. Scripta Mat. 64 (2011) 479. [5] U.A. Curle, H. Möller, J.D. Wilkins. Mat. Let. 65 (2011) [6] R. Bruwer, J.D. Wilkins, L.H. Ivanchev, P. Rossouw, O.F.R.A. Damm. US Patent 7,368,690 (2008).