Int. J. Microstructure and Materials Properties, Vol. 7, No. 6, 2012 491 Pressure and die temperature effects on microstructure and mechanical properties of squeeze casting 2017A wrought Al alloy Mohamed Ben Amar and Slim Souissi* National Engineering School of Sfax (ENIS), University of Sfax, B.P 599-3038, Tunisia E-mail: benamarmohamed@yahoo.fr E-mail: slim.souissi@ymail.com *Corresponding author Najib Souissi Faculty of Sciences of Sfax (FSS), University of Sfax, B.P 802-3038, Tunisia E-mail: souissi.nejib@yahoo.fr Chedly Bradai National Engineering School of Sfax (ENIS), University of Sfax, B.P 599-3038, Tunisia E-mail: chedly.bradai@enis.rnu.tn Abstract: Squeeze casting process was carried out on an industrial 2017A aluminium alloy conventionally used for wrought products. In this research, the effects of applied pressure and die temperature on the microstructure and mechanical properties were investigated. The results show that the finer microstructure was achieved through the squeeze casting. Furthermore, higher pressures improved the fracture properties and decreased the percentage of porosity of the cast alloy. It was found that a direct squeeze casting pressure and die temperature gave a good combination of tensile strength, yield strength and elongation. Keywords: squeeze casting; pressure; die temperature; 2017A Al alloy; microstructure; mechanical properties. Reference to this paper should be made as follows: Ben Amar, M., Souissi, S., Souissi, N. and Bradai, C. (2012) Pressure and die temperature effects on microstructure and mechanical properties of squeeze casting 2017A wrought Al alloy, Int. J. Microstructure and Materials Properties, Vol. 7, No. 6, pp.491 501. Copyright 2012 Inderscience Enterprises Ltd.
492 M. Ben Amar et al. Biographical notes: Mohamed Ben Amar received his PhD in Mechanical Manufacturing from ENIT University in 2004. Currently, he is working as an Assistant Professor at the Department of Mechanical Engineering in the National Engineering School of Sfax, Tunisia. He has 33 years of experience in teaching and research in the field of cast aluminium alloy. Slim Souissi received his PhD in Mechanical Engineering from National Engineering School of Sfax in 2012 and obtained his Master Diploma in Mechanical Engineering from the same school in 2007. His research area is on development of microstructure and mechanical properties in aluminium alloys. Currently, he is working as an Assistant Professor at the Department of Mechanical Engineering in the National Engineering School of Sfax, Tunisia. Najib Souissi is a PhD student at the Faculty of Science, University of Sfax, Tunisia. He obtained his Master of Inorganic Chemistry in 2011 from the same university. His main research interest includes the effects of squeeze casting parameters of aluminium alloy. Chedly Bradai is a Professor in the Mechanical Engineering Department of National Engineering School of Sfax (ENIS), Tunisia. He is interested in electrochemical research. He is the Director of Mechanical Physical Unit of Materials (UPMM-LASEM) and the Supervisor of many research projects. The scientific activity is documented by more than 50 publications on various aspects of mechanical behaviour of different materials and polymers. His experimental and numerical studies are performed on the material behaviour characterisation under static and dynamics solicitations. 1 Introduction Compared to casting aluminium alloys, wrought aluminium alloys have been widely used in the mechanical automotive and aerospace industry due to their good properties (Guo and Yang, 2007). One of the best promising technologies capable of making better use of wrought aluminium alloys is the squeeze casting process which has been developed for the purpose of counteracting the disadvantages and further extending the advantages of die casting in the casting technique in order to produce better quality cast components (Zhong et al., 2003). Squeeze casting is regarded as a combination of gravity die-casting and closed die forging in a single process (Aweda and Adeyemi, 2009a). The liquid molten metal is compressed under pressure inside the mould cavity of a re-usable metal mould. The process is one of the improved casting techniques used for the production of engineering components through the application of pressure on the cast metal to minimise defects associated with shrinkage cavities and porosity formation (Aweda and Adeyemi, 2009b; Masoumi and Hu, 2011). Several authors have reported the superiority of properties of squeeze cast products over the conventional casting process. This is brought about by the flexibility of the squeeze cast parameters. Hajjari and Divandari (2008) have shown that squeeze casting caused the refinement of the microstructure and reduction in the dendrite arm spacing (DAS) of the cast structure possibly due to increasing the cooling rate (Hajjari and Divandari, 2008). Increasing the squeeze pressure also led to the formation of finer
Pressure and die temperature effects 493 microstructure. Furthermore, higher pressures decreased the percentage of porosity and increased the density of the cast alloy (Hajjari and Divandari, 2008; Kurz and Fisher, 1984). The effect of squeeze casting parameters on the microstructure and mechanical properties of aluminium alloys has been investigated to a large extent (Fan et al., 2010; Maleki et al., 2009). Despite the manufacturing and component property advantages of squeeze casting near-net-shape parts from wrought Al alloys, few studies have investigated the effect of processing the resultant microstructure. The effects of applied pressure and die temperature (T d ) on microstructure and mechanical properties of squeeze cast 2017A Al alloy have been investigated in this work. The results are compared with the microstructure and the mechanical properties of the wrought material in reception state (as extruded). 2 Experimental procedure 2.1 Material Alloy 2017A provides average tensile strength but good machinability. It is widely used in mechanical applications (as extrusions or thick plates) (MIL-HDBK-5H, 1998; Vargel et al., 2004). In this work, the alloy is received as extruded bar of diameter 20 mm and has the composition shown in Table 1. Table 1 Chemical composition (wt%) of the alloy used in this work Si Fe Cu Mn Mg Zn Al 0.7 0.65 4.45 0.9 0.4 0.2 Rest 2.2 Squeeze casting method The squeeze casting experiments were performed on a hydraulic press (Figure 1) consisting of steel mould. The device allows the moulding of vertical specimens under a pressure up to 100 MPa applied by the punch until material solidification. In gravity casting, molten metal was poured directly into the mould without external pressure. The device is also equipped with a system which provides temperature regulation of the mould. The punch-and-die set were made of hot-die steel and the cast billets were hexagonal in shape, 8 mm in side length and 120 mm in length. The selection of the preheating temperature 200, 250, and 300 C is carried out in a way to avoid severe thermal shock, which can damage the piece by a sudden change in the temperature of the material surface. The squeeze casting parameters that were prepared for the specimens are shown in Table 2. Table 2 Process conditions of the specimens Pouring temperature ( C) Die preheating temperature T d ( C) Applied pressure (MPa) 750 200 Gravity 250 50 300 100
494 M. Ben Amar et al. Figure 1 Experimental setup with steel mould used in the squeeze casting process (see online version for colours) 2.3 Microstructural analysis In order to investigate the effect of applied pressures on the microstructure, a series of photos was performed using an optical microscope LEICA DMLP with a digital camera JVC. Each sample was prepared and etched with Keller s reagent (Metallography and Microstructures, 1995). Scanning electron microscopy (SEM) was used to analyse the tensile fracture surface. 2.4 Measuring of density and porosity The variation of density and porosity on the gravity and squeeze cast samples, manufactured in various applied pressures, were assessed for their specific weight using the Archimedes principle. The porosity of specimen was calculated by the following equation (Masoumi and Hu, 2011): Dt Da % Porosity = 100% Da (1) where D a is the density of alloy squeeze cast under 100 MPa and D t is the experimental density. 2.5 Mechanical properties In order to evaluate tensile properties of the gravity and squeeze cast specimens, the tests were carried out in an Instron-5567 tensile materials testing machine. The tests were
Pressure and die temperature effects 495 performed under displacement control with a strain rate starting at 2.6.10 3 s 1. A strain-gage (gage length of 12.5 mm, Mod. 2620-601, Instron Corp.) was attached to the central part of the specimen. All of the specimens were machined and taken from the middle of cast billets. For each casting condition four specimens were tested and the mean results were reported. The geometry of the tensile specimens is shown in Figure 2 with the standard (ISO 6892-1:2009) form. Figure 2 Geometry of tension test specimen [mm] 3 Results and discussion 3.1 Microstructure Figure 3 illustrates the microstructure of the portion of an extruded, gravity die cast and squeeze cast under die temperature fixed at 200 C. Figure 3(a) shows the microstructure of the portion of an extruded bar with a uniform and homogenous microstructure with fine α (Al) grains generated by the extrusion process. Figure 3(b) illustrates the microstructure of the gravity cast sample that contains coarse grain size, high porosity and inhomogeneity. Micrographs 3(c) and 3(d) show the microstructure of squeeze cast specimens that were produced under 50 and 100 MPa pressures, respectively. These micrographs show that the microstructures prepared under higher applied pressures are much finer, homogenous and smaller in size. It is clear that the squeezing pressure has significant influence on the microstructure of the alloy (Vijian and Arunachalam, 2005). The results show that the grain size of the alloy decreases with the increase of the squeezing pressure as shown in Table 3. Furthermore, the inter-metallic phases in the alloy with no applied pressure are coarser than those under high squeezing pressure. This effect is a result of the change in phase diagram according to the Clausius-Clapeyron equation (Ghomashchi and Vikhrov, 2000): ( ) dt T V V = dp ΔH f f l s f where T f is the equilibrium freezing temperature, V l and V s are the specific volumes of the liquid and solid respectively, and ΔH f is the latent heat of fusion. Substituting the appropriate thermodynamic equation for volume, the effect of pressure on freezing point may roughly be estimated by: (2)
496 M. Ben Amar et al. ΔH f P = P0 exp (3) RTf The above equation shows that an increase in the freezing point (T f ) of the alloy is caused by the increase in pressure (P). In this equation P 0, ΔH f and R are constants. For pure aluminium, the calculated value of equation is 0.068 K/MPa (Yue, 1997), which means the liquidus would rise 6.8 K at the pressure of 100 MPa. Increasing the freezing point causes undercooling in the alloy that is already superheated. The higher freezing point brings about the larger undercooling in the initially superheated alloy and thus elevates the nucleation frequency, resulting in a more fine-grained structure. Apart from the changes in undercooling of the molten alloy caused by applied pressure, greater cooling rates for the solidifying alloy can be realised due to reduction in the air gap between the alloy and the die wall and thus larger effective contact area. Obviously, the increase of cooling rate and heat-transfer coefficient will result in the refinement of the grain size of squeeze casting alloy. Table 3 Average grain size of the cast specimens under various pressure levels (T d = 200 C) Applied pressure (MPa) Average grain size (µm) Gravity 64 50 38 100 31 Figure 3 Optical micrographs at T d = 200 C, (a) as extruded, (b) gravity die cast, (c) squeeze cast under 50 MPa, (d) squeeze cast under 100 MPa (see online version for colours)
Pressure and die temperature effects 497 3.2 Alloy densification Density and porosity measurements of the specimens corresponding to different processing conditions are shown in Table 4. The results show that the gravity die cast specimens contained about 8% porosity. However, the squeeze cast specimens under 50 MPa contained about 2% porosity. Nevertheless, the squeeze cast specimens under 100 MPa is almost free of porosity. Consequently, the alloy becomes highly dense with considerably low amount of porosity. Table 4 Density measurements of the cast specimens under various pressure levels (T d = 200 C) Applied pressure (MPa) Density (kg/m 3 ) Porosity (%) Gravity 2,568 8 50 2,732 2 100 2,786 0 3.3 Mechanical properties Figures 4, 5 and 6 show, respectively, the ultimate tensile strength (UTS), yield strength (YS) and percentage of elongation of the gravity and squeeze cast specimens prepared with different die temperature. It can be seen from Figure 4 that at a die temperature of 300 C and 100 MPa, a squeeze cast alloy has better UTS at 312 MPa. Figure 5 presents the better YS value 243 MPa at low die temperature (200 C) and 100 MPA. It is observed in Figure 6 that elongation increases around 7% with the rise in die temperature (300 C) and in high squeeze cast pressure. The extruded specimens have a good UTS, YS and elongation because of good mechanical properties provided by the extrusion process. Figure 4 UTS of 2017A Al alloy manufactured in various conditions (see online version for colours)
498 M. Ben Amar et al. Figure 5 YS of 2017A Al alloy manufactured in various conditions (see online version for colours) Figure 6 Elongation of 2017A Al alloy manufactured in various conditions (see online version for colours) The increase in the elongation, and UTS with increasing applied pressure could be attributed, in part, to the reduction of the grain size and, in part, to materials densification. The increase in the YS might be attributed to the higher dislocation density (Masoumi and Hu, 2011). Conversely, with lengthy solidification time, the alloy exhibits shrinkage problems, higher levels of gas porosity, or large grain size. In addition, all exhibit reduced tensile properties, particularly reduced ductility (Campbell, 1991). 3.4 Tensile fracture surface analysis The fracture surfaces provide useful information on the effect of microstructure on the mechanical response of the alloy. Some of the SEM pictures are selectively presented in
Pressure and die temperature effects 499 the paper. High magnification observations of tensile fracture surface of the extruded specimens reveal fine voids of varying sizes [Figure 7(a)]. The small dimples are evidence for the highest energy absorption due to plastic deformation. This is the feature of ductile failure. The tensile fracture surface of the gravity die cast specimen, as shown in Figure 7(b), is brittle in nature. Porosity can be seen easily in the alloy when the alloy is cast with no applied pressure. It is evident that an internal discontinuity due to the presence of porosity serves as the initiation point of cracks in the gravity die cast specimens. Initiation of the crack normally occurs at small flaws which cause concentration of stress. The cracks take an inter-granular path, particularly when segregation or inclusions weaken the grain boundaries (Donald and Pradeep, 2002). Figure 3(b) shows that the grain size is coarse which results in a bad ductility. The tensile fracture surface of the squeeze cast specimen under 50 MPa revealed that the specimen have failed in a brittle inter/trans-granular manner as shown in Figure 7(c). The failure is in a mixed-mode fracture comprising inter-granular fractures and quasi-cleavage planes. The grain size becomes finer as a result of squeezing pressure. Figure 7(d) shows the fracture surface of the squeeze cast specimen under 100 MPa. It is indicated that failure is in trans-granular fracture. By increasing the applied pressure, the formation of dimples increases which is characterised in a ductile fracture. It is also worth mentioning that the effect of pressure and the resulting higher cooling rate on the grain size have to be added to its impact on the refining of shrinkage porosities (Campbell, 1991). Figure 7 Tensile fractographs (SEM) of different investigated specimens at T d = 200 C, (a) as extruded, (b) gravity die cast, (c) squeeze cast under 50 MPa, (d) squeeze cast under 100 MPa
500 M. Ben Amar et al. 4 Conclusions The effect of pressure and die temperature on the microstructure and mechanical properties of the squeeze cast and the gravity die cast 2017A Al alloy are investigated in this work. The pressure applied in squeeze casting promotes rapid solidification and a refined grain structure. Increasing the applied pressure up to 100 MPa was sufficient to eliminate all traces of shrinkage and gas porosity within the casting which is confirmed by Maleki e al. (2009). Density of the specimens increased with application of 50 MPa pressure. It is further increased steadily for higher applied pressure (up to 100 MPa) above which it approached its theoretical value. It is postulated that the 100 MPa applied pressure was able to fully eliminate gas and shrinkage porosities. The die temperature and pressure gave a good combination of mechanical properties of the gravity cast and squeeze cast 2017A wrought Al alloy. References Aweda, J.O. and Adeyemi, M.B. (2009a) Determination of temperature distribution in squeeze cast aluminium using the semi-empirical equations method, Journal of Materials Processing Technology, Vol. 209, Nos. 17, pp.5751 5759. Aweda, J.O. and Adeyemi, M.B. (2009b) Experimental determination of heat transfer coefficients during squeeze casting of aluminium, Journal of Materials Processing Technology, Vol. 209, No. 3, pp.1477 1483. Campbell, J. (1991) Castings, 1st ed., Butterworth Heinemann, Oxford, UK. Donald, R.A. and, Pradeep, P.P. (2002) The Science and Engineering of Materials, 4th ed., Thomson, Canada. Fan, C.H., Chen, Z.H., He, W.Q., Chen, J.H. and Chen, D. (2010) Effects of the casting temperature on microstructure and mechanical properties of the squeeze-cast Al-Zn-Mg-Cu alloy, Journal of Alloys and Compounds, Vol. 504, No. 2, pp.42 45. Ghomashchi, M.R. and Vikhrov, A. (2000) Squeeze casting: an overview, Journal of Materials Processing Technology, Vol. 101, Nos. 1 3, pp.1 9. Guo, H. and Yang, X. (2007) Preparation of semi-solid slurry containing fine and globular particles for wrought aluminium alloy 2024, Transactions Nonferrous Metals Society of China, Vol. 17, No. 4, pp.799 804. Hajjari, E. and Divandari, M. (2008) An investigation on the microstructure and tensile properties of direct squeeze cast and gravity die cast 2024 wrought Al alloy, Materials & Design, Vol. 29, Nos. 9, pp.1685 1689. Kurz, W. and Fisher, D.J. (1984) Fundamentals of Solidification, Trans Tech Publications, Switzerland. Maleki, A., Shafyei, A. and Niroumand, B. (2009) Effects of squeeze casting parameters on the microstructure of LM13 alloy, Journal of Materials Processing Technology, Vol. 209, No. 8, pp.3790 3797. Masoumi, M. and Hu, H. (2011) Influence of applied pressure on microstructure and tensile properties of squeeze cast magnesium Mg-Al-Ca alloy, Materials Sciences Engineering, Vol. A 528, Nos. 10 11, pp.3589 3593. Metallography and Microstructures (1995) ASM Hand Book, 9th ed., Vol. 9, pp.334 435, Materials Park, Ohio. MIL-HDBK-5H (1998) Metallic materials and elements for aerospace vehicle structures, US Department of Defense, pp.3 64.
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