The Effect of Direct Thermal Method, Temperature and Time on Microstructure of a Cast Aluminum Alloy

City Research Online City, University of London Institutional Repository Citation: Ahmad, A.H., Naher, S. and Brabazon, D. (2014). The effect of direct thermal method, temperature and time on microstructure of a cast aluminum alloy. Materials and Manufacturing Processes, 29(2), pp. 134-139. doi: 10.1080/10426914.2013.822980 This is the unspecified version of the paper. This version of the publication may differ from the final published version. Permanent repository link: http://openaccess.city.ac.uk/3596/ Link to published version: http://dx.doi.org/10.1080/10426914.2013.822980 Copyright and reuse: City Research Online aims to make research outputs of City, University of London available to a wider audience. Copyright and Moral Rights remain with the author(s) and/or copyright holders. URLs from City Research Online may be freely distributed and linked to. City Research Online: http://openaccess.city.ac.uk/ publications@city.ac.uk The Effect of Direct Thermal Method, Temperature and Time on Microstructure of a Cast Aluminium Alloy A.H. AHMAD1, 2, 3, S. NAHER1, 2, 4*, and D. BRABAZON1, 2 1 School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin, Ireland 2 Advanced Processing Technology Research Centre, Dublin City University, Dublin, Ireland 3 Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Pekan, Malaysia 4 School of Engineering and Mathematical Sciences, City University London, UK The direct thermal method is used for the creation of globular microstructures suitable for semi-solid metal forming. In this paper both simulation and experimental results using direct thermal method are presented. ProCAST® software was used to estimate temperature distribution inside the aluminium billet. In validation work, molten aluminium A356 was poured into metallic copper tube moulds and cooled down to the semi-solid temperature before being quenched in water at room temperature. The effect of pouring temperatures of 630°C, 650°C, 665°C, 680°C and holding times of 45s and 60s on the microstructure of aluminium A356 alloy were investigated. The simulation results showed that the average temperature rate within the copper mould, from initial pouring temperature to just before quenching, was approximately 1°C/s. Examination of the solidified microstructures showed that the microstructure was more spherical when lower pouring temperatures and holding periods were used. From the micrographs it was found that the most globular and smallest structures were achieved at processing parameters of 630°C and 45s. Keywords: Aluminium, A356, Direct Thermal Method, Pouring Temperature, Holding Time, Semi-Solid. INTRODUCTION processing was discovered by Flemings and Spenser in Semi-solid metal processing has been commercially used Massachusetts Institute of Technology 40 years ago [3]. In for the casting of various sizes of components on an the initial stage, Flemings and his co-workers discovered industrial scale. Commercial interests in semi-solid metal that the viscosity of the stirred material while in the processing are mainly in the automotive, aerospace, ICT, semi-solid forming was much lower than the viscosity plumbing and military supplier industries. This processing without stirring. It was also found that this lower viscosity technique has become a common casting method for was due to the generation of a spherical grained structure in aluminium and magnesium alloys for producing high the material from the stirred action during solidification. density and high strength products [1]. Some current Interlocking network of dendrite grains in the unstirred research is also investigating ways to make this process material on the other hand generates a high viscosity fluid. feasible for semi-solid steel forming [2]. The effect of Semi-solid metal exhibits not only a shear rate thinning microstructure on fluidity which allows for semi-solid but also a time dependency. This fluid is therefore ________________________________________ * Author to whom correspondence should be addressed. sumsun.naher@city.ac.uk thixotropic in nature. This time dependency is mainly due to disagglomeration of particles clusters after increased superheat pouring with a shear field [7], and the direct thermal method [8]. shear rate processing or due to agglomeration of particles In direct thermal method, liquid metal is poured into a after reduced shear rate processing [3]. The particles have a thin cylindrical metallic mould that has low thermal mass natural affinity for each other and therefore agglomerate and high conductivity [8]. Heat matching between molten during periods of the shearing. However, particle bonds are metal and mould allow for pseudo-isothermal hold within re-melted during deformation as a result of shear stresses, the solidification range. This allows for the alloy to remain which allows the material to flow with greater ease. in a steady semi-solid state where the temperature is held The flow resistance of semi-solid metal is strongly depended between the solid and liquid phases of the alloy. This is upon processing history. The resulting microstructure achieved by the very low rate of heat loss to the (average grain size and shape and the distribution of grain environment through the mould, which is copper tubing with size) in semi-solid metal state and the degree of particle a metal base plate. The advantage of this technique is that agglomeration appear to be the most significant material the process uses the natural solidification principle and is characteristics contributing to this process history dependent low cost. The low superheat of the alloy is extracted into flow behaviour. the mould such that the alloy rapidly cools into the Thermal treatment processing to obtain the required semi-solid zone. The initial rapid cooling gives rise to a high microstructure for semi-solid metal forming is the most rate of nucleation events in the alloy. After a short holding commonly used commercial method to provide the required period this microstructure spherodises. In order to capture globular microstructure. In order to achieve this, the creation this microstructure, the mould is quenched into water at of many small nucleation sites is important. The higher the room temperature while the alloy is still within the number of nuclei, the smaller is the average distance be- semi-solid metal range. Simulation tween nuclei and the smaller are the resultant grains. using finite element method to The new-rheocasting process by UBE technology is one examine the heat distribution during solidification was of the better known examples of this thermal method applied carried out by Wang et al. [9]. This research work showed commercially. In order to generate more nuclei, the molten that the combination of simulation and experimental metal is chilled quickly from above the liquidus to below the techniques were used to optimize the product design in order liquidus and held for a short holding at a semi-solid metal to avoid the casting defects. Publications in recent years temperature. This processes results in a very fine grained have shown that the globular microstructure can be achieved microstructure. the by using direct thermal method [8]. The combination of semi-solid metal temperature is controlled with induction accurate pouring temperature and holding time significantly heating to maintain the desired fraction solid [4]. The treated affect the globular structure creation. It was suggested from semi-solid metal is then injected into the mould within the previous research that a lower pouring temperature would die casting machine. The success of the new-rheocasting create smaller primary and secondary phases within the process technique depends on the initial structure obtained microstructure [10, 11]. Finite element method was seen in in the material. There are few more techniques that use a their work as a useful tool to predict the heat distribution similar principle in order to provide the required starting during solidification process. However the effect of these material. These include the swirled enthalpy equilibrium parameters on the resultant microstructure is still not fully device [5], the continuous rheoconversion process [6], low understood due to the vast range of possible casting In the new-rheocasting process, ________________ * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie conditions in terms of alloy, holding time, pouring Aluminium A356 was used for the ingot (billets) and temperature, mould materials and quenching medium used. copper for the mould. The initial conditions of the mould This work focused on investigating the effect of pouring were set at 680°C for the ingot and 24°C for the mould. temperature and time on the microstructure of A356. Other boundary conditions and thermo-physical properties In particular, simulation and experimental focused on a which were used in the model are summarized in Table 1. pouring temperature range of 630°C to 680°C and specific The moulds were filled via pouring under gravity with the holding times of 45s and 60s. crucible lip position 25 mm above the copper mould during pouring. SIMULATION AND EXPERIMENTAL Simulation TABLE 1. - Boundary conditions and thermo-physical The purpose of the simulation was to investigate the heat properties of A356 used in the simulations. distribution inside the billet during solidification. The Property Value simulation results were used to estimate the temperature of Liquidus Temperature 616°C the billets at various times during the holding period. The Solidus Temperature 556°C solidification process of the alloys was simulated using Pouring Temperature 680°C finite element analysis software ProCAST . Standard heat Mould Temperature 24°C transfer via Fourier modelled was used [9]. The Fourier heat Latent Heat 389kJ/kg conduction equation used is represented as follows: Conductivity 151W/mK Density 2670 kg/m3 Heat Transfer Coefficient 14.26kW/m2 [12] ® ( ) ̇ (1) Experimental where T is the temperature, t is the time, is density, is A 1kg aluminium A356 ingot was placed in a graphite is thermal conductivity, and ̇ is the internal crucible and was heated to a temperature 720°C using power source. The calculations are based on Cartesian induction furnace. Once the desired temperature of the melt coordinates x, y, z of the part geometry such that was obtained, it was poured into a cylindrical copper mould equation (1) can be re-written as follows: of 1 mm wall thickness, 25 mm in diameter, and 75 mm in specific heat, height. ( ) ( ) ( ) ̇ The different pouring temperatures were set at 630°C, (2) 650°C, 665°C and 680°C. After pouring, the molten metal was held in the mould for 45s or 60s, at semi-solid The simulation work began with the development of the mould geometry which was of the same geometry as that used to produce the billets experimentally. These cylindrical temperatures, before quenched into room temperature water. A billet with pouring temperature of 665°C was also allowed to solidify via natural solidification. billets were 25 mm in diameter and 75 mm in height. The model was transferred to the GEOMESH Environment software for the meshing process. The finite element mesh of the ingot consisted of 70,752 nodes and 396,795 tetrahedral elements. ________________ * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie part of the mould. This is may be due to the fact that the bottom part of the mould is farthest away from the hot metal that just transferred from the crucible and also has a longer period of time in contact with the mould walls compared to the melt within the upper regions of the mould. This temperature difference between the top and bottom regions became evident after 21s pouring time (Figure 2 (c)). The effects of pouring temperature on heat distribution were also observed by other in previous work [14]. The temperature continued to drop until 61s with a drop FIGURE 1. - A fraction solid vs temperature curve for A356 at a cooling rate 0.6°C/s [13]. rate of 3°C/s in the simulation and the average temperature drop during the previous experimental findings was 1°C/s The semi-solid temperature range of A356 alloy is [12]. This is come to the agreement that the simulation quoted with different solidus and liquidus temperatures results were close to the previous experimental result, sup- depending on the source of reference. In one previous work porting the accuracy of simulation result. this range was reported as 538°C to 614°C. Figure 1 shows From the simulation, after 45s and 60s, the temperatures the fraction solid vs temperature curve of A356 at 0.6°C/s were approximately 638°C and 622°C respectively. The cooling rate [13]. This fraction solid curve was used to equivalent recorded temperatures from the experimental estimate the amount of solid before quenching. work conducted were 635°C and 620°C at 45s and 60s The samples were sectioned 5 mm from the bottom of respectively. By correlating with Figure 1, these results the cast billets and mounted in Bakelite and microstructure could then be used to estimate the fractions solid of the analysis was performed by optical microscope after etching billets before quenching. the surface of the polished moulded samples with Keller’s In direct thermal method, the temperature cycle was in- reagent. A Reichert light optical inverted microscope with fluenced by the material and wall thickness of the mould Me F2 universal camera connected to a PC was used for [12]. The temperature drops within a copper were higher image capture. Microstructures were examined with Buhler than a steel mould as steel has the higher volumetric heat Omnimet Enterprise software. capacity. Furthermore, the greater selection of a wall thickness is needed for a steel mould. RESULTS AND DISCUSSION Simulation Results The temperature profile within the billet from 2.5s to 61 Experimental Results Effects of Pouring Temperature s after pouring is shown in Figure 2. The simulation results The billet produced with 665°C pouring temperature and show that the mould was fully filled with molten metal 2.5s with natural solidification to room temperature produced a after pouring, see Figure 2 (a). In the initial solidification dendritic microstructure, see Figure 3, as would be expected stage, the temperature started to drop first in the lower under this conventional type of solidification. Even though region of the mould as shown in Figure 2 (b). the molten metal was poured into the copper mould, without In the simulation, when the pouring process completed, the bottom part showed lower temperature than the upper ________________ * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie a controlled period for holding within the semi-solid were also found within this work to be more spherical when temperature produced at lower pouring temperatures as shown in Figure region, a ripened type of dendritic microstructure was produced. The microstructure of the 4 (a). billets with different pouring temperatures and holding times Lower pouring temperatures lead to higher cooling rates are presented in Figure 4 and Figure 5. The lower pouring from above the liquidus to below the liquidus as less temperatures would be expected to have a significant effect superheat has to be extracted. Furthermore, when the not just for the finer grain size but also the morphology of cooling rate is higher, the undercooling of the alloy becomes the grain [11]. As per previous findings, the lower pouring larger. A raise in undercooling increases the amount of temperatures were found to produce more globular and finer nucleation which ultimately results in a smaller grain size microstructures [15]. The primary phase microstructures from (b) (a) this larger crystallization (d) (c) driving force [16]. 680.0°C 671.7°C 663.5°C 655.2°C 646.9°C 638.7°C 630.4°C 622.1°C 613.9°C 605.6°C time : 2.5s (e) time : 11s time : 21s time : 31s (g) (f) 597.3°C 589.1°C 580.8°C 572.5°C 564.3°C 556.0°C time : 41s time : 51s time : 61s FIGURE 2. - Temperature distributions within aluminium billet from (a) 2.5s to (g) 61s after pouring. ________________ * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie temperature was quenched at 570°C, equivalent to 0.65 fraction solid. The corresponding microstructure is shown in Figure 5 (a). For the sample with 650°C pouring temperature, with microstructure shown in Figure 5 (b), was quenched at 590°C (0.4 fraction solid). The microstructure was less globular compare with the sample produced with a pouring temperature of 630°C. This was confirms that size of primary grains was affected with a low fraction solid used [17]. The sample with 680°C pouring temperature was FIGURE 3. - Dendritic microstructure formed within billet quenched at 620°C (Figure 5 (c)). This sample was therefore after solidification from pouring temperature of 665°C under still in the liquid condition upon quenching. The air cooling. microstructure results from this sample showed a primary phase which was too large and irregular compared to what would typically be required for semi-solid metal forming. Effects of Holding Time The role of holding time was to ensure an adequate In order to understand the relationship between pouring fraction solid before quenching. A longer holding time for a temperature and viscosity, the microstructure of the specific constant pouring temperature produced larger respective billets should be related to the material fluidity. primary phase grains. Figure 5 shows the microstructures for Viscosity is an important indicator of the material capability the samples produced with 630°C, 650°C and 680°C to fill the die cavity during casting. Lower viscosity pouring temperatures and with 60s holding time. These have produced better movement of the material and allows the larger grain sizes compared with the microstructures of the semi-solid metal to move into the complex geometries such samples shown in Figure 4 produced with 45s holding time. as thin die cavity sections. Formation of globular microstructure was influenced by fraction solid [17]. The sample with a 630°C pouring (a) (b) (c) FIGURE 4. - Microstructures from a 45s holding time and with pouring temperatures of (a) 630°C, (b) 650°C and (c) 680°C. ________________ * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie (a) (b) (c) FIGURE 5. - Microstructures from a 60s holding time and with pouring temperatures of (a) 630°C, (b) 650°C and (c) 680°C. The two important phases in the microstructure of A356 Relationship between the pouring temperature and are the primary aluminium phase which solidifies first and microstructure formation is determined by the under cooling the secondary aluminium-silicon phase which solidifies temperature. The undercooling is the difference between the second, shown in Figure 6. In general, a microstructure with equilibrium temperature and temperature which the material more globular primary grains and more liquid phase, seen as cools, secondary upon solidification, will have higher fluidity. undercooling is influenced by the solidification rates which before the start of solidification [19]. The depend on the type of a mould material, mould thickness etc. The formation of a microstructure which evolves within secondary phase primary phase material During depends on solidification the degree process, as of undercooling. the undercooling temperature and time increase, the melt potential nucleation decreases resulting in a coarse-grained structure deformation. Higher cooling rate is therefore associated with a finer grain size and a globular microstructure. The higher pouring temperature lead to a higher cooling rate as it need more time to cool from the liquidus to solidus temperature. FIGURE 6. - Microstructure of the sample produced with a For this reason, dendritic microstructure occurs. 45s holding time and a pouring temperature of 630°C. The formation of a globular microstructure in these experiments was influenced by the volume fraction of solid. Pouring temperature is one of the important parameters In semi-solid metal processing, the volume of the that affect the evolution of the primary phase during fraction solid play the important rules in determined the solidification [18]. Low pouring temperature is a key factor successful of the process. The fraction solid which is used in that establishes higher temperature gradients within the semi-solid processing is normally in the range of 0.30 to 0.7 semi-solid metal which encourage the formation of a depending on the type of the material used [20]. During multitude of nuclei and subsequently a more globular rapid solidification, the nuclei start to deform from the microstructure upon holding within the semi-solid state. liquid condition. The nuclei evolved and impinged to each other within the material and later produced a dendritic ________________ * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie microstructure as in typical solidification process. During ACKNOWLEDGMENTS this process (from the liquid to dendritic microstructure The authors would also like to acknowledge the support formation), the fraction solid which occurs inside the from the Ministry of Higher Education, Malaysia and material is increase as it approaching the solidus Dublin City University for funding this work. temperature. The formation of the nuclei which is small at REFERENCES this stage, become larger due to the increment of fraction solid volume. The quenching technique which was applied in this period, captured a finer globular 1. Brabazon D.; Browne D.J.; Carr A.J. Mechanical stir casting of aluminium alloys from the mushy state: shape Process, microstructure and mechanical properties. microstructure. For this reason, the proper selection of a Materials Science and Engineering A. 2002; A326370- holding time in direct thermal method is crucial to allow the 381. formation of a desire microstructure feature due to the 2. Aqida S.; Maurel M.; Brabazon D.; Rosso M. Thermal fraction solid effect. stability of laser treated die material for semi-solid metal forming. International Journal of Material Forming. CONCLUSION 2009; 2(0), pp. 761-764. The simulation of the heat distribution inside the billet and direct thermal method experiment was successfully 3. Spencer showed a fine Effects heat treatment on 5. Tebib M.; Morin J.B.; Chen X.G. Semi-solid processing of hypereutectic A390 alloys using novel rheoforming also more globular than those formed from higher pouring process. Transactions of Nonferrous Metals Society of temperatures. The spherical microstructures were dominant China. 2010; 201743-1748. at the lower pouring temperature (630°C) regardless the 6. Bo X; Yuan Dong L; Ying M; Yuan H; Apelian D. Commercial AM60 alloy for semisolid processing produced larger primary phase particles which were also Effects of continuous rheoconversion process on micro- less globular. The results of this work showed that the best structure. Transactions of Nonferrous Metals Society of pouring temperature and time for the A356 alloy were at combination of proper pouring temperature and holding time and 986-995. pouring temperature and holding time. A lower pouring 630°C and 45s respectively. In direct thermal method, the rheocasting Materials Science and Engineering A. 2011; 528(3): microstructure can be achieved with the correct selection of holding time either 60s or 45s. The longer holding time of microstructure and mechanical properties of A356 alloy. spherical temperature contained more primary particles which were M.C. 4. Bo Chao Liao; Young Koo Park; Hong Sheng Ding. approximately 1°C/s from the initial pouring temperature. work Flemings 31925-1932. quenching. The copper mould was only fully filled 2.5s after experimental R.; crystallization range. Metallurgical Transactions. 1972; about solidification temperature and time prediction before The Mehrabian Rheological behaviour of sn-15 pct pb in the carried out. The simulation provided useful information pouring. The A356 billet temperature decreased at D.B.; China. 2010; 20s723-s728. 7. Guo H; Yang X; Wang J; Hu B; Zhu G. Effects of are necessary in order to produce the microstructure required for semi-solid metal forming. rheoforming properties on of * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie 7075 wrought and mechanical aluminium alloy. Transactions of Nonferrous Metals Society of China. 2010; 20355-360. ________________ microstructures 8. Hussey M.J; Browne D.J.; Brabazon D.; Car A.J. A International Conference on Semi-Solid Processing of Alloys and Composites S2P, 2004, Limassol, Cyprus. direct thermal method of attaining globular morphology th 15. Fan C.H.;Chen Z.H.; He W.Q.; Chen J.H.; Chen D. International Conference on Semi-Solid Processing of Effects of the casting temperature on microstructure and Alloys and Composites. 2002.pp. 575-580. mechanical properties of the squeeze cast al-zn-mg-cu in the primary phase of alloys in Proceedings of the 7 alloy. Journal of Alloys and Compounds. 2010; 9. Jiaqi W.; Paixian F.; Hongwei L.; Dianzhing L; and Yiyi L.; Shrinkage porosity criteria and opimized design of a 100-ton 30Cr2Ni4MoV forging ingot. Materials and 504L42-L45. 16. Jun Wang; Shuxian He; Baode Sun; Qixin Guo; Design. 2012; 35446-456. Mitsuhiro Nishio. Grain refinement of Al–Si alloy 10. Ning Z.L.; Wang H.; Sun J.F. Deformation behavior of (A356) by melt thermal treatment. Journal of Materials Processing Technology 2003, 141(1): 29-34. semisolid A356 alloy prepared by low temperature pouring. Materials and Manufacturing Processes. 2010; 17. Haga T.; Kapranos P. Simple rheocasting processes. Journal of Materials Processing Technology. 2002, 130- (25): 648-653. 11. Hongwei W.; Boa L.; Jinchuan J.; Zunjie W. Influence 131, 594-598. of thermal rate treatment and low temperature pouring 18. Gencalp S.; Saklakoglu N. Semisolid microstructure on microstructure and tensile properties of AlSi7Mg evolution during cooling slope casting under vibration of alloy. Materials and Design. 2011; 322992-2996. A380 aluminum alloy. Materials and Manufacturing Procesess. 2010, 25(9), 943-947. 12. Carr A.J.; Browne D.J.; Hussey M.J.; Lumsden N.; Scanlan M. Modelling and experimental development of 19. Gowri S.; Samuel F.H. Effect of cooling rate on the so- the direct thermal method of rheocasting. International lidification behaviour of Al-7 Pct Si-SiCp metal matrix Journal of Cast Metals Research. 2007; 20(6): 325-332. composites. Metallurgical Transaction A. 1992, 23A, 13. Brabazon D.; Brawne D.; Carr A.J. Processing and properties of rheocast alloys. UCD PhD's Thesis. 2001. 3369- 3376. 20. Chayong S.; Atkinson H.V.; Kapranos P. Thixoforming 14. Bouchard D.; Colbert J.; Pineau F.; Laplume F.; Hamel F.G. Characterization of contact heat transfer coefficients and mathematical modelling of a semi-solid aluminium die casting in Proceedings of the 8 th ________________ * Author to whom correspondence should be addressed . sumsun.naher@dcu.ie 7075 aluminium alloys. Materials Science and Engineering A. 2005, 390, 3-12.