DEVELOPMENT OF THE RHEO-DIECASTING PROCESS FOR Mg- ALLOYS AND THEIR COMPONENTS

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1 Title of Publication Edited by TMS (The Minerals, Metals & Materials Society), 2004 DEVELOPMENT OF THE RHEO-DIECASTING PROCESS FOR Mg- ALLOYS AND THEIR COMPONENTS Z. Fan, S. Ji, G. Liu, E. Zhang BCAST (Brunel Centre for Advanced Solidification Technology) Brunel University, Uxbridge, Middlesex, UB8 3PH, UK Keywords: Semisolid, Mg-alloys, Rheo-diecasting, Microstructure, Mechanical properties. Abstract Research on rheology of semisolid slurries suggests that an ideal semisolid slurry for semisolid metal (SSM) processing is one in which a suitable volume fraction of fine and spherical particles dispersed uniformly in a liquid matrix. Such ideal semisolid slurry can be obtained by enhancing the effective nucleation and promoting spherical growth during solidification. Experimental investigation and theoretical analysis of solidification under forced convection allowed us to identify the conditions for achieving enhanced effective nucleation and promotion of spherical growth. Such conditions can be summarised as uniform temperature and chemistry throughout the whole volume of liquid alloy, high shear rate and high intensity of turbulence during the solidification process. Based on such understandings, a new SSM processing technology, rheodiecasting (RDC), has been developed for the production of components with high integrity. AZ91D Mg-alloy was used to optimise the rheo-diecasting process and component production trials. The experimental results indicate that the rheo-diecast samples have close-to-zero porosity, fine and uniform microstructure and much improved strength and ductility. Rheo-diecasting process is particularly suitable for production of high-safety, airtight and highly stressed components in the automotive industry. Introduction Due to increasing environmental concerns and tightening government regulations on CO 2 emission, vehicle weight and fuel economy are becoming increasingly important in the automobile industry. For instance, the European and North American car producers are committed to reducing fuel consumption by 25% and thereby achieving 30% reduction in CO 2 emission by the year 2010 [1]. Magnesium alloys, as the lightest structural materials, are very suitable for applications in the car industry to assist the realisation of such goals. In the past few years we have seen a 20% average annual increase in Mg usage in the automobile industry. Although the automobile industry will continue to be the dominant driving force for the future growth in magnesium application, other areas, such as aerospace, electronics and health care, will take a substantial share of the magnesium market in the near future. Currently, nearly all these applications are achieved by high pressure diecasting (HPDC) and are limited to a few cast alloys, such as AZ91 and AM60. Further growth in Mg applications will largely depend on the successful development of new processing technologies capable of producing high quality and low cost components and new alloys with higher operating temperatures [2]. In the previous publication [3], we presented the rheomoulding process for processing Mg-alloy components. In this paper we present the development of a new semisolid processing technique, 1

2 rheo-diecasting (RDC) process. Microstructure and mechanical properties of rheo-diecast AZ91D Mg-alloy will be presented and compared with those produced by other processes. The development of rheo-diecasting process for Mg-alloys The most important objective of semisolid metal processing is to achieve laminar mould filling to avoid gas entrapment by increasing the viscosity of the feed material while maintaining adequate fluidity for a complete mould filling. Therefore, it is important to have a good understanding of the rheological behaviour of semisolid slurries. Intensive experimental investigations have confirmed the effects particle morphology on the flow behaviour of semisolid slurries, as illustrated schematically in Fig. 1. Such effects have been modelled successfully through effective solid fraction by Chen and Fan [4]. Rheological understandings achieved through both experimental investigations and theoretical modelling allow one to have a clear target for process development, i.e., the ideal semisolid slurry for semisolid processing, which can be described as a suitable volume fraction of solid particles with fine particle size and spherical morphology dispersed uniformly throughout a liquid matrix. 2. Rosettes 1. Dendrites 3. Spheres η s γ f S Figure 1. Schematic illustration of the effects of particle morphology on the steady state viscosity of semisolid slurries. The ideal semisolid slurry can be achieved by controlling the nucleation and growth processes during solidification. In the conventional casting processes, overheated liquid metal is poured into the relatively cold mould. Heterogeneous nucleation takes place in the undercooled liquid close to the mould wall. The majority of the nuclei are transferred to the overheated liquid region and disappeared, only a small proportion (as low as 0.3% [5]) of the nuclei survived and contributed to the final microstructure, giving rise to a coarse and non-uniform microstructure (Fig. 2). It is clear that an important step towards microstructural refinement is to make sure that every single nucleus formed during nucleation can survive and contribute to the final microstructure. Through both theoretical and experimental studies [6], the following conditions have been identified to achieve 100% nucleus survival rate: (1) uniform temperature and 2

3 chemical composition throughout the entire volume of the liquid alloy during the continuous cooling process; (2) well-dispersed heterogeneous nucleation agents. Under such conditions, nucleation will occur throughout the entire volume of liquid and every nucleus will survive and contribute to the final solidified microstructure, giving rise to a fine and uniform microstructure, as illustrated in Fig. 3. Figure 2. Schematic illustration of the nucleation process in the conventional diecasting processes and its consequences on the final solidified microstructure. 200µ Figure 3. Schematic illustration of the concept of effective nucleation under uniform temperature, uniform chemistry and well-dispersed heterogeneous nucleation agents throughout the entire volume of the liquid alloy during the continuous cooling process. The next step towards the ideal semisolid slurry is to ensure that the survived nuclei grow into spherical particles, rather than dendrites or rosettes. Our theoretical analysis on the morphological evolution during solidification has revealed that with increasing shear rate and the intensity of turbulence, the growth morphology changes from dendrites to spheres via rosettes due to the change in the diffusion geometry in the liquid around the growing solid phase [7]. This theoretical prediction of the morphological change from dendritic growth to spherical growth with increasing shear rate and intensity of turbulence has been verified by our experimental results [8]. The effect of fluid flow conditions on the growth morphology during solidification is 3

4 schematically illustrated in Fig. 4. To sum up, the conditions to achieve spherical crystal growth are high shear rate and high intensity of turbulence. Here we would like to emphasis the importance of turbulent flow on achieving spherical growth. The conventional melt stirring can only provide largely laminar flow and give rise to rosette morphology, which is not adequate for achieving the ideal semisolid slurry. Increasing shear rate and intensity of turbulence Dendrites Rosettes Spheres Figure 4. Schematic illustration of the effects of fluid flow condition on the growth morphology during solidification. Once the conditions to achieve ideal semisolid slurry are understood, we need to identify a physical mechanism to realise such conditions. After intensive search and comparison of available mechanisms, we adapted the well-established high shear dispersive mixing action of the twin-screw device to the task of in situ creation of SSM slurry with fine and spherical particles. The twin-screw device has been named the twin-screw slurry makers [9]. The twinscrews have specially designed profiles to achieve co-rotating, self-wiping and fully intermeshing characteristics. The fluid flow inside the twin-screw device is characterised by high shear and high intensity of turbulence. The shear dispersive mixing power of the twin-screw device ensures the uniformity of both temperature and chemical composition during the continuous cooling process. In addition, there is a large and ever-renewing interface area between the cooling melt and the screws and barrel, which is ideal for enhancing the efficiency of heat exchange during solidification, resulting in a highly efficient slurry making process. For a given charge to the slurry maker, the slurry maker can transform the liquid Mg-alloy into SSM slurry within seconds. A combination of the twin-screw slurry maker and a conventional HPDC machine forms the rheo-diecasting (RDC) process. The RDC process not only provides components with high quality, but also maintains the unique advantages of HPDC process, such as high efficiency, low cost and high volume production. For a more detailed description of the rheo-diecasting equipment and process please refer to another paper in this proceeding [10]. Microstructures of rheo-diecast AZ91D Mg-alloy AZ91D (Mg-8.8Al-0.67Zn-0.22Mn-0.03Si, in wt.%) Mg-alloy supplied by MEL (Manchester, UK) was used for optimisation of the RDC process and evaluation of the rheo-diecast microstructure and mechanical properties. AZ91D alloy ingot was melted at 675 o C under the protection of N vol.% SF 6 gas mixture and fed into the slurry maker at 630 o C. The slurry maker usually works in a temperature range corresponding to a solid fraction range of A 4

5 280ton cold chamber die casting machine was used for casting the standard tensile test samples (Fig. 5). 60mm Figure 5. Photograph of RDC samples for evaluation of mechanical properties. During rheo-diecasting, it is important to ensure a laminar mould filling. To check this, the mould was deliberately filled half way so the conditions at the flow front can be examined. The half filled sample and the microstructure at the flow front are presented in Fig. 6. Fig. 6 revealed that the flow front during the mould filling is parabolic and smooth, indicating that the mould filling was laminar under the optimised conditions. It should be pointed out that the micrographs in Fig. 6 have 30% solid particles (the larger ones), while the smaller ones were formed inside the die through secondary solidification of the remaining liquid in the semisolid slurry. Figure 6. Photograph and micrographs showing the laminar flow front during mould filling. Fig. 7 shows the microstructure of rheo-diecast AZ91D alloy on the entire cross-section of the 6mm tensile test bar. The primary particles are distributed uniformly throughout the entire sample, no segregation was observed under the optimised processing conditions. The detailed microstructural examination and EDX analysis of the RDC samples have revealed the following microstructural characteristics of rheo-diecast Mg-alloys: Porosity is well below 0.5 vol.%. Pores are rarely observed in the rheo-diecast samples. Occasionally observed pores are small in size (at micron level). 5

6 Micro-porosity due to solidification shrinkage can be very much reduced or even eliminated by rheo-diecasting. Oxide particles are fine (a few microns), spherical, well-dispersed and uniformly distributed throughout the casting, reducing the harmfulness of oxide particle clusters and oxide film in cast components. There is no chemical segregation throughout the entire casting including the runners and biscuits. Primary particles have a fine size (around 50µm), spherical morphology and uniform distribution throughout the entire casting. The remaining liquid in the SSM slurry solidifies under high cooling rate in the die resulting in the formation of extremely fine α-phase (<10µm). Figure 7. Microstructure of the rheo-diecast AZ91D alloy through the entire cross-section of the Ф6 tensile test bar. The RDC process has a large processing window. It is capable of working with semisolid slurries with a solid fraction ranging between 0 and 0.5. Semisolid slurries with different solid fractions are achieved by controlling the barrel temperature, which in turn is achieved by counter-balancing the heating power and cooling intensity to ensure a high accuracy and efficiency over temperature control. Fig. 8 shows the microstructures of rheo-diecast AZ91D alloy produced at different processing temperatures. The SEM images of the rheo-diecast AZ91D alloy are presented in Fig. 9, which shows the detailed morphology of primary particles formed through secondary solidification inside the die cavity (Fig. 9a). Such primary particles have an equiaxed morphology and fine particle size (less than 10µm). The detailed morphology of the Mg 17 Al 12 β-phase formed through the final eutectic solidification is presented in Fig. 9b. It has an irregular morphology and is located at the grain boundaries of the fine primary phase. There is no evidence of coupled eutectic growth. In fact, it is difficult to distinguish between the fine primary α- phase and the eutectic α-phase. Therefore, it is more likely that the final eutectic solidification occurred in a divorced manner. Mechanical properties of rheo-diecast AZ91D alloy A special die was manufactured to cast standard tensile test samples for mechanical testing. Initially, the tensile test pieces produced by the rheo-diecasting process were used for process optimisation. Processing parameters, such as screw rotation speed, shearing time, shot velocity, shot pressure, intensifying pressure and die temperature, were systematically varied. The effects of such processing parameters were assessed against sample quality in terms of microstructure and mechanical properties. Under optimised conditions, samples were produced for evaluation of mechanical properties. Table 1 presents the mechanical properties obtained by the rheodiecasting process in comparison with those of the same alloy obtained by different processing technologies reported in the literature [11-14]. Table 1 shows that the rheo-diecast samples have much improved mechanical properties over the samples produced by other processing 6

7 technologies. In particular, the ductility of rheo-diecast samples is substantially increased, twice as much as those by conventional HPDC process. The improved mechanical properties of rheodiecast samples can be attributed to the fine and uniform microstructure, much reduced casting defects, such as pores and solidification shrinkage, refined and well dispersed oxide particles, and fine and divorced eutectic structure. (a) (b) (c) (d) 200µ Figure 8. Microstructures of rheo-diecast AZ91D alloy produced at different processing temperatures. (a) 585 o C; (b) 589 o C; (c) 593 o C; (d) 600 o C. (a) (b) Figure 9. SEM micrographs of rheo-diecast AZ91D alloy showing (a) the morphology of primary particles formed through the secondary solidification of the remaining liquid and (b) the detailed morphology of the Mg 17 Al 12 β-phase formed through eutectic solidification. 7

8 Table 1. Comparison of mechanical properties of AZ91D alloy produced by different processing technologies. Process Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Reference HPDC [11] Thixocasting [12] Thixomoulding [13] New rheocasting (NRC) [14] Rheo-diecasting (RDC) This work Component production trials Trials of component production have been conducted to confirm the reliability of the slurry maker and consistency of the rheo-diecasting process. One of our project partners supplied a component die (the identity of the component is omitted here), which has two cavities and four sliding cores. The component die was originally designed for Al-alloys, and was used here for rheo-diecasting Mg component without any modification. A production trial was carried out in our laboratory using this component die. The results indicate that rheo-diecast components have very good surface finish, close to zero porosity and very fine and uniform microstructure throughout the entire casting, including runners and biscuit. Fig. 10 shows a photograph of the casting with runners and biscuit and micrographs showing the microstructures at different positions in the casting. 200µ Figure 10. Optical micrographs showing the microstructure of rheo-diecast LM24 components at different locations. 8

9 Advantages of the rheo-diecasting process Based on our experiments on process optimisation and component production trials, we have identified the following advantages of the rheo-diecasting process over the conventional HPDC process: Fine and uniform microstructure resulted from enhanced effective nucleation and spherical growth during solidification under high shear rate and high intensity of turbulence. Close-to-zero porosity (well below 0.5 vol.%) due to the elimination of the entrapped air by laminar mould filling. Rheo-diecast components can be subjected to full heat treatment for enhancing mechanical performance without compromising the surface quality and dimensional control. Well-dispersed oxide particles with fine size and spherical morphology achieved by the intensive shearing action of the twin-screw slurry maker. The extensively sheared liquid in the semisolid slurry solidifies in the die forming very fine α-phase and a divorced eutectic structure, which are favourable to the property enhancement. Much improved mechanical properties, elongation in particular, due to structural refinement and uniformity, reduced or even eliminated porosity and other cast defects. Lower scrap rate and higher materials yield. Lower overall component production cost due to higher productivity, lower scrap rate and higher materials yield. Rheo-diecasting can be achieved by simply attaching the slurry maker to a cold chamber HPDC machine. 9 Summary In this paper we have presented the ideas behind the development of the rheo-diecasting process. Study on the rheological behaviour of semisolid slurries allowed us to set up the goal for process development, i.e., the ideal semisolid slurry for SSM processing. To achieve laminar mould filling and fine and uniform solidified microstructure, a semisolid slurry needs to be a suitable volume fraction of fine and spherical particles dispersed uniformly in a liquid matrix. Such ideal semisolid slurry can be achieved by enhancing the effective nucleation and promoting spherical growth. Research on solidification under forced convection allowed us to identify the conditions for achieving 100% nuclei survival rate and spherical growth. Such conditions can be summarised as uniform temperature and chemistry throughout the whole volume of liquid alloy, high shear rate and high intensity of turbulence during the solidification process. Based on such understandings, a new semisolid metal processing technology, rheo-diecasting (RDC), has been developed for the production of Al components with high integrity. Rheo-diecasting can be easily achieved by combination of a twin-screw slurry maker with the existing cold chamber diecasting machine. AZ91D Mg-alloy was used to optimise the rheo-diecasting process and component production trials. The experimental results indicate that the rheo-diecast samples have close to zero porosity, fine and uniform microstructure and are free from other casting defects. Compared with high pressure diecasting or any available semisolid processing techniques, rheodiecasting offers components with improved strength and ductility. Rheo-diecasting process is particularly suitable for production of high-safety, airtight and highly stressed components in the automotive industry.

10 Acknowledgement The authors acknowledge the financial support from EPSRC (UK), Ford Motor Co and MEL (UK). References [1] H. Fridrich, and S. Schumann: in Proc. 2 nd Israeli Inter. Conf. Mg Science and Technology, Dead Sea, Israel, (2000), 9. [2] H. Fridrich and S. Schumann: in Proc. IMA 2001 Magnesium Conf., Brussels, Belgium, (2001), 8. [3] S. Ji, Z. Fan, G. Liu, X. Fang and S.H. Song, in in Proc. 7 th Inter. Conf. Semisolid Metal Processing, Tsukuba, Japan, Sept , 2002, eds. Y Tsutsui et al, pp [4] J.Y. Chen and Z. Fan: Mater. Sci. Tech., 18 (2002), 237, 243, 250, 258. [5] P.D. Lee, Private communication, Department of Materials, Imperial College, London, [6] A Das and Z Fan, Mat. Sci. Tech., 19 (2003) [7] A. Das, S. Ji and Z. Fan, Acta Materialia, 50 (2002), [8] S. Ji and Z. Fan, Met. Mater. Trans. 33A (2002), [9] S. Ji, Z. Fan and M. J. Bevis: Mater. Sci. Eng., A299 (2001), [10] Z. Fan, S. Ji, X. Fang: in this proceedings, [11] C. Pitsaris, T.Abbott, C.H.J. Davies and G. Savage, in Magnesium: Proc. 6 th Inter. Conf. Magnesium Alloy and Their Application, ed. By K.U. Kainer, (Weinheim, Wiley-VCH, Verlay GmbH & Co. KGaA, 2003), 694. [12] J. Aguilar, T. Grimming and A. Bührig-Polaczek, in Magnesium: Proc. 6 th Inter. Conf. Magnesium Alloy and Their Application, ed. By K.U. Kainer, (Weinheim, Wiley-VCH, Verlay GmbH & Co. KGaA, 2003), 767. [13] F. Czerwinski, et al., Acta Materialia, 49 (2001), [14] H. Kaufmann and P.J. Uggowitzer, in Magnesium Alloy and Their Application, ed. By K.U. Kainer, (Weinheim, Wiley-VCH, Verlay GmbH & Co. KGaA, 2000),