Rheo-Diecasting of Al-Alloys

Transcription

1 Proceedings of the 9 th International Conference on Aluminium Alloys (2004) 1092 Edited by J.F. Nie, A.J. Morton and B.C. Muddle Institute of Materials Engineering Australasia Ltd Rheo-Diecasting of Al-Alloys Z. Fan Brunel Centre for Advanced Solidification Technology, Brunel University, Uxbridge, Middlesex UB8 3PH, UK Keywords: semisolid, processing, rheo-diecasting, microstructure, mechanical properties. Abstract A new technology, rheo-diecasting (RDC), has been developed for manufacturing near-net shape components of high integrity directly from liquid Al-alloys. The rheo-diecasting process innovatively adapts the well-established high shear dispersive mixing action of the twin-screw extruder to the task of in situ creation of semisolid slurry followed by direct shaping of the semisolid slurry into a component using the existing cold chamber diecasting process. The rheo-diecast component has close to zero porosity, fine and uniform microstructure throughout the entire component. Compared with those produced by conventional high-pressure diecasting, rheo-diecasting samples have much improved tensile strength and ductility. 1. Introduction Al-alloys, as lightweight structural materials, are playing an important role in achieving vehicle weight reduction and improving fuel economy in the automotive industry. Since 1990, the use of Al has been doubled in cars and tripled in the light truck market. Currently, 85% of all Al castings are used by the automotive and mass transport industry, and a large proportion of such castings are produced by high-pressure diecasting (HPDC) process. However, the quality of components manufactured by the HPDC process is limited by the presence of a substantial amount of porosity, which not only excludes the application of HPDC components in high-safety and airtight systems, but also denies the opportunity for further property enhancement by heat treatment. It is clear that further increase in Al application in the transport industry will require a major advance in processing technologies. The new processes need to be capable of producing components of high integrity and improved performance while being comparable with the HPDC process in terms of production cost and efficiency. Porosity due to turbulent mould filling could be reduced or even eliminated if the viscosity of the melt could be increased to reduce the Reynolds number sufficiently so that trapped air is minimised [1]. This is the concept of semisolid metal (SSM) processing. Since early 1970s, a number of SSM processing techniques have been proposed [2]. One of the most popular SSM processes is thixocasting, in which non-dendritic alloys pre-processed by electromagnetic stirring are reheated to the semisolid region prior to the shaping process. As a new processing technique, thixocasting does improve component integrity and performance, but proves to be cost intensive, low efficiency and less flexible. After 30 years of extensive R&D, thixocasting is currently experiencing a decline in acceptance as a viable production technology [2].

2 1093 Under such circumstances, a new processing concept, rheo-diecasting process, has been developed at the Brunel University. In this paper we present the rheo-diecasting process and the microstructure and mechanical properties of rheo-diecast samples. 2. The Rheo-diecasting Process The rheo-diecasting process is an innovative one-step SSM processing technique to manufacture near-net shape components of high integrity directly from liquid Al-alloys. The process innovatively adapts the well-established high shear dispersive mixing action of the twin-screw extruder (originally developed for polymer processing) to the task of in situ creation of SSM slurry with fine and spherical solid particles followed by direct shaping of the SSM slurry into a near-net shape component using the existing cold chamber diecasting process. Figure 1: Schematic illustration of the rheo-diecasting (RDC) process. Figure 1 schematically illustrates the rheo-diecasting equipment for Al-alloys. It consists of two basic functional units, a twin-screw slurry maker (the key technology) and a standard cold chamber HPDC machine. The rheodiecasting process starts from feeding predetermined dose of liquid metal from the melting furnace into the slurry maker where it is rapidly cooled to the SSM processing temperature while being mechanically sheared by a pair of closely intermeshing screws converting the liquid into a semisolid slurry with a pre-determined volume fraction of the solid phase dictated by the barrel temperature. The semisolid slurry is then transferred to the shot chamber of the HPDC machine for component shaping. In order to prevent Al-alloy from oxidation, nitrogen gas is used as the protective environment during the slurry-making process. The fluid flow inside the twin-screw slurry maker is characterized by high shear rate, high intensity of turbulence and cyclic variation of shear rate. 3. Microstructures A number of commercially available and newly developed Al-alloys, including LM24 (Al9.3Si-3.3Cu-1.6Zn) (all the alloy compositions are in wt.% unless stated otherwise), A357 (Al-7Si-0.6Mg), Al-6Si-2Mg-0.5Fe, AS1241 (Al-12Sn-4Si-1Cu) and Al-6Si-6Pb, were used in this work. Alloys were usually melted at 700 o C and fed into the slurry maker at a temperature 40 o C above their melting point. In the case of Al-6Si-6Pb alloy, preliminary mixing of the separated Al and Pb phases was necessary before feeding. The slurry maker usually works in a temperature range corresponding to a solid fraction range of A 280ton cold chamber die casting machine was used for casting the standard tensile test samples. Figure 2 shows the typical microstructures of rheo-diecast alloys. Detailed microstructural characterisation of various rheo-diecast samples has revealed the following microstructural characteristics: 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).

3 1094 Primary particles have a fine size, 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 Al-phase (<10 m). The rheo-diecast microstructure is bimodal, larger particles (few tens of microns) were formed inside the slurry make and fine particles (few microns) inside the die. Oxide particles are fine (a few microns), spherical and well dispersed and uniformly distributed, reducing the harmfulness of oxide particle clusters and oxide film in cast components. Sn-rich and Pb-rich phases can be dispersed uniformly without any macro-segregation. 100µ 50µ 100µ 100µ Figure 2: Microstructures of the rheo-diecast Al-alloys. (a) A357; (b) A-6Si-2Mg-0.5Fe; (c) A1241 (Al-12Sn- 4Si-1Cu); (d) Al-6Si-6Pb, the dark particles are Pb Mechanical Properties A special die was made 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 UTS (MPa) NRC (T5) Thixocasting (T5) RDC (F) RDC (T5) Elongation (%) Figure 3: Comparison of mechanical properties of A357 alloy from different processes. Thixocast & NRC data [3].

4 1095 mechanical properties. Under optimised conditions, samples were produced for evaluation of mechanical properties. Figure 3 shows the tensile strength and elongation of the rheodiecast (RDC) samples of A357 alloy in comparison with those of the same alloy produced by thixocasting and new rheocasting (NRC) processes (data after Ref [3]). Also shown in Figure 3 are the mechanical properties under T5 heat treatment condition. Figure 3 demonstrated clearly that rheo-diecasting process offers much better mechanical properties than thixocasting and NRC processes. Although A357 has been extensively used for SS processing, it is not really suitable for SSM processing due to its high sensitivity of solid fraction to processing temperature [4]. We have developed a new alloy, Al-6Si-2Mg-0.5Fe, which is designed specially for realising the full potential of the RDC process. Table 1 summarises the mechanical properties of the rheo-diecast Al-6Si- 2Mg-0.5Fe alloy in comparison with those of A357 alloy produced by permanent mould and thixocasting processes. Rheo-diecast Al-6Si- 2Mg-0.5Fe alloy has much improved tensile strength and acceptable ductility. Table 1: Mechanical properties of rheo-diecast Al-6Si-2Mg- 0.5Fe alloy in comparison with thixocast A357 alloy. Alloy (wt.%) Processing conditions σ 0.2% (MPa) UTS (MPa) Elonga. (%) Al6Si2Mg0.5Fe HPDC (This work) HPDC + T RDC RDC + T Al7Si0.6Mg PM + T (A357) PM + T Thixocast Thixocast + T Component Production Trials Trials of component production have been conducted to confirm the reliability of the slurry make and consistency of the rheo-diecasting Figure 4: Rheo-diecast component (LM24) with runner and biscuit. The micrographs showing the microstructures of the component at different locations. 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. A production trial was carried out in our laboratory using this component die. The results indicate that rheodiecast components have very good surface finish, close to zero porosity and very fine and uniform microstructure throughout the entire casting, including runners and biscuit. Figure 4 shows a photograph of the casting and microstructures at different positions in the casting. Extremely low porosity and uniform microstructure are favourable for higher fatigue properties. Extensive fatigue tests and component evaluation are under the way. 6. Discussion 6.1. Solidification Behaviour during the Rheo-Diecasting Process 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 flow into the overheated liquid region and remelt, only

5 1096 a small proportion (as low as 0.3%) of the nuclei survive and contribute to the final microstructure giving rise to a coarse and non-uniform microstructure. Under the intensive mixing action in the twin-screw slurry maker, both the temperature and composition fields in the melt are extremely uniform. During the continuous cooling under forced convection, heterogeneous nucleation takes place throughout the whole volume of the undercooled liquid. Compared with conventional solidification, the actual nucleation rate may not be increased but all the nuclei will survive resulting in an increased effective nucleation rate [5]. In addition, the intensive mixing action is likely to disperse the clusters of potential nucleation agents, giving rise to an increased number of potential nucleation sites. However, it seems that a laminar flow is much less effective for such purposes. Consequently, laminar flow is less powerful for structural refinement compared with turbulent flow. It has been long believed that the nondendritic particles are developed from the initial dendritic morphology under dynamic agitating conditions through the following mechanism. The initial dendrites are fragmented through dendrite arm detachment by either a shear force or remelting at the dendrite arm roots. With increasing shearing time, those fragmented dendrite arms change gradually to spheroids via stages of dendrite growth, rosettes and ripened rosettes [6]. However, our theoretical analysis of the morphological evolution during solidification revealed that the above mechanism might be only applicable to the case of a simple shear flow with low shear rate. With increasing shear rate and the intensity of turbulence, the growth morphology changes from dendritic to spherical via rosette due to a 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 recent experimental results in rheo-diecast Sn-15wt.%Pb alloys [8]. The remaining liquid in the SSM slurry will solidify in the die cavity without any intensive shearing during its solidification. However, this remaining liquid has been intensively sheared in the twin-screw-slurry maker. It has a uniform temperature and composition throughout the liquid. According to the previous analysis, nucleation would occur throughout the entire remaining liquid, and every single nucleus would survive and contribute to the final microstructure. However, different from the nucleation in the twin-screw slurry maker, nucleation in the die cavity will occur with a much higher nucleation rate due to the high cooling rate provided by the metallic die (in the order of 10 3 K/s). Under such conditions, each nucleus would not have much chance to grow before the remaining liquid is completely consumed, giving rise to a very fine structure [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 twin-screw 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 achieved by the viscous semisolid slurry Well dispersed oxide particles with fine size and spherical morphology achieved by the intensive shearing action of the twin-screw slurry maker Much more tolerant to Fe contents. Rheo-diecast samples can tolerant up to 1.5 wt.% Fe as impurity without any severe damage to mechanical properties. This is because

6 1097 intensive shearing changes the morphology of Fe-containing compounds from long needles to equiaxed particles. Therefore more scraps can be used in the melting furnace Much improved mechanical properties, elongation in particular, due to structural refinement and uniformity, reduced or even eliminated porosity and other cast defects Capable of processing wrought alloys, alloys with large narrow freezing range (e.g., Al- Sn) and alloys based on the immiscible systems (e.g., Al-Pb) Longer die life Lower scrap rate and higher materials yield Lower overall component production cost Rheo-diecasting can be achieved by simply attaching the slurry maker to a cold chamber HPDC machine. 7. Summary A new semisolid metal processing technology, rheo-diecasting, has been developed for the production of Al-alloy components with high integrity. Rheo-diecasting can be easily achieved by adding a twin-screw slurry maker to the existing cold chamber diecasting machine. 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, rheo-diecasting offers components with improved strength and ductility, which can be attributed to microstructural refinement and uniformity, much reduced or eliminated porosity and other casting defects, and refined and dispersed oxide particles. Other advantages of the rheo-diecasting process include flexible with alloy compositions, more tolerant to Fe contents and lower overall production cost. Rheo-diecasting process is particularly suitable for production of highsafety, airtight and highly stressed components in the automotive industry. Acknowledgement The author acknowledges contributions to this work from Dr S Ji, Dr X Fang, Mr J Patel, Mr M Hitchcock and Mr G Liu, and the financial support from EPSRC (UK) and Ford Motor Co. References [1] S. A. Metz and M. C. Flemings, AFS Trans., 78 (1970), 453. [2] Z. Fan: Inter. Mater. Rev., 47 (2002), [3] P. Giordano and G. L. Chiatmetta, Proc. 7 th S2P, Tsukuba, Japan, Sept , 2002, eds Y Tsutsui et al, pp [4] Y. Q. Liu and Z. Fan: Materials Science Forum, (2002), 717. [5] A Das and Z Fan, Mat. Sci. Tech., 19 (2003) [6] M. C. Flemings: Metall. Trans., 22A (1991), [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, A Das and Z. Fan, Scripta Mater., 46 (2002),