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Effect of position on the tensile properties in high-pressure die cast Mg alloy
Journal of Alloys and Compounds 470 (2009) 111–116
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of position on the tensile properties in high-pressure die cast Mg alloy D.G. Leo Prakash a,∗ , Doris Regener b , W.J.J. Vorster a a b
Department of Engineering Science, University of Oxford, Parks Road, OX1 3PJ Oxford, UK Institut f¨ ur Werkstoff- und F¨ ugetechnik, Otto-von-Guericke-Universit¨ at Magdeburg, PF 4120, 39016 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 29 December 2007 Received in revised form 17 February 2008 Accepted 18 February 2008 Available online 26 March 2008 Keywords: Metals and alloys High-pressure die casting Mechanical properties Microstructure Effect of position
a b s t r a c t Tensile tests were performed at different locations of high-pressure die cast (HPDC) Mg alloy and the effect of position on the tensile properties such as yield strength (YS), ultimate tensile strength (UTS), ductility and fracture strain (FS) are explained. Additionally, the significance of micro-failure mode of the material is presented. The average size, area fraction and clustering tendency of pores and Mg17 Al12 () particles as well average grain size are correlated with the mechanical properties and found their influences. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are the lightest metallic structural materials and having a unique combination of properties, which are very attractive in automobile applications [1–3]. Most magnesium alloys are now produced by HPDC method [4] and the most common HPDC magnesium alloy is AZ91 [5]. This is due to its good combination of strength and ductility [6] as well as excellent castability. The microstructure of HPDC AZ91 alloy reveals dendrite microstructure with primary ␣-Mg solid solution, surrounded by a supersaturated divorced eutectic region. The intermetallic  phase (Mg17 Al12 ) particles are embedded in this supersaturated ␣-Mg solid solution. Inclusions, pore bands and microporosity are the processing defects of HPDC castings. The complex geometry of these microstructural features, their locations and arrangements are often non-uniform and usually strong spatial correlations exist. These results a multi-length scale micro-features, which cause multiple fracture micro-mechanisms and, affect the fracture path and mechanical properties of this material. The spatial arrangement, size distribution and shape of different microstructural feature also alter the mechanical properties. Various studies have reported the effect of section thickness on the tensile properties of HPDC and sand cast magnesium-based alloys [7–11]. Bowles et al. [12] reported the skin effect in HPDC cast Mg–Al alloys. However, there is no contribution which explains
∗ Corresponding author. E-mail address: [email protected] (D.G. Leo Prakash). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.02.051
the effect of position on the tensile properties in the high-pressure die castings. This highlights the need of systematic microstructural quantitative analysis to obtain the variation in micro-quantities with respect to the position in the castings and their relation to the tensile properties. Additionally, understanding the microscopic failure modes of the material is also equally important to predict the influence of the micro-features on fracture behavior, which is strongly related to the mechanical properties. Therefore, the aim of this work was to perform the same by quantitative characterization of microstructure and correlating the micro-quantities with the macro-properties of AZ91 alloy. Such an analysis is mandatory for the structural applications of this material. 2. Experimental procedure The AZ91 plates were produced using the cold chamber HPDC machine with the dimension of 200 mm × 53 mm × 10 mm. The HPDC conditions are given below: • • • •
• • • •
Melt temperature in furnace = 680 ◦ C. Initial die temperature = 230 ◦ C. Maximum die temperature = 400 ◦ C. Filling (piston speed). ◦ First phase of casting (filling at the gate), v = 0.4 m/s. ◦ Second phase (cavity fill), v = 3 m/s. ◦ Third phase (including solidification): rapid retardation of the piston with pressure increasing to 1200 bar. Protective gas used = SF6 in nitrogen. Fluidizing gas used = nitrogen. Cooling time in the mould = 20 s. Cooling temperature of mould = 280 ◦ C.
The time needed to transfer and pour the shot charge from the furnace in to the chamber is generally about 2 s. The liquid metal flow is from one side, along the
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Fig. 1. The schematic view of the cut-out of middle and edge specimens from the HPDC plate. Fig. 2. Stress vs. strain results of the tensile experiments.
Table 1 Chemical composition of HPDC AZ91 Mg alloy Alloy
AZ91
% Al % Mn % Zn % Si % Cu % Ni % Fe
9.3 0.12 0.79 0.02 0.0007 0.0006 0.0046
castings. As the scope of the present study was micro–macro correlations, all the castings were produced with the same conditions. In order to obtain the effect of position tensile specimens of a cross section of 10 mm × 10 mm and 50 mm gauge length were machined from the edge and middle of the plates (as shown in Fig. 1). The skin regions of the edge specimens were machined out (3 mm) in the gauge region (along the width) as shown in Fig. 1. This kept on similar area fraction of skin region for the middle and edge specimens. Uniaxial tensile test was performed on the edge and middle specimens at a constant strain rate of 10−4 s−1 in a computer controlled servohydraulic test machine at room temperature. The chemical composition of the investigated HPDC AZ91 Mg alloy is presented in Table 1. The specimens from each tensile test were taken and polished by standard methods for optical microscopy. A microstructural area of 25 mm2 (a quarter of the cross-section) was grabbed with an optical microscopy at 100× as continuous microstructural frames from the unetched cross-section. This microstructural frames were used to create the microstructural montage and it was further introduced to image processing to quantify the gas and shrinkage microporosity. The nearest neighbour distance limit is used to separate the gas and shrinkage porosity and the detailed procedure is presented elsewhere [13–16]. The unetched crosssection was further etched (with a combination of picric acid (6 g), water (10 ml), acetic acid (5 ml) and ethanol (100 ml)) to reveal the  phase. The microstructural area of 1.86 mm2 was grabbed from the etched surface at 1000× for the quantification of  phase by image processing. The clustering tendency of these features was explained by comparing the spatial arrangement (nearest neighbour distance) of them from the present montage with the expected random arrangement. The values of clustering tendency below 1 indicate clustering, equal to 1; random, equal to 2.15; uniform distributions. The details of montage creation and the quantification procedure of microporosity and  phase are documented elsewhere [13–17]. In addition, fracture surface analysis of failed tensile specimens and in situ tensile analysis coupled with SEM was performed to understand the microscopic failure mode of the material.
3. Results and discussion The variations in the tensile properties and microstructural quantities with respect to different position in the casting are described in this section to explain the effect of position in HPDC castings. In addition, the microscopic failure modes of the material and the influence of microstructure on the macro-properties of the HPDC AZ91 magnesium alloy are discussed. 3.1. Macro-properties Fig. 2 shows the difference between the stress versus strain curves obtained from the tensile tests of the edge and middle specimens of the HPDC castings. Around 20 specimens were investigated in each case and no necking was observed in the deformed and failed specimens. This alloy exhibit no yield point and the 0.2% proof strength was taken as an indication of the yield point. Both the edge and middle cases showed a similar flow of deformation except the variations in the strain hardening behavior. The edge specimens show a higher strain hardening rate compared to the middle specimens. Additionally, significant difference in the value of strain to fracture is observed between the edge and middle specimens, and this could be of the variations in the arrangement of microporosity and the brittle  particles. The average and variations of YS (Fig. 3a) and UTS (Fig. 3b) of edge and middle specimens are shown in Fig. 3. Significant difference in these quantities with respect to the position in the castings is observed. The YS and UTS are found to decrease considerably in the middle of the casting compared to the edge region and the variations in the magnitude of YS and UTS in different cases show a similar trend. The YS decreases from an average of 131.65 N/mm2 for edge of the castings to 125.72 N/mm2 for the middle region and in the same trend, UTS decreases from an average of 184.59 to 168.04 N/mm2 . The strain hardening also increases significantly
Fig. 3. Average and variation in YS and UTS of edge and middle specimens in the castings.
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113
Fig. 4. Average and variations of FS values of edge and middle specimens in the castings.
in the edge compared to the middle specimens of the casting and this increase in the amount of strain hardening and strain to fracture cause the higher UTS for the edge specimens of the casting. Fig. 4 shows the average and variations of FS values of edge and middle of the castings. The effect of position on the FS is high compared to YS and UTS (see Figs. 3 and 4). The early failure of middle compared to the edge specimen indicating that the elongation of the middle specimen is low compared to other. The average elongation of the edge and middle specimens of the HPDC casting is 3.42% and 2.89%, respectively. The present study concludes that the strength and ductility are decreasing in the middle compared to the edge specimens. Form et al. [18] suggested that the local solidification time alters the microstructural development of the castings. Thus microstructural variation is expected in different positions of the castings due to the partial solidification in the shot sleeve and faster solidification of skin and edge region compared to the middle of the casting. This would be the reason for the obtained changes in mechanical properties with respect to the position in the casting. The comparison of the obtained tensile properties with the reported literature data of 10 mm thick HPDC castings is presented in Fig. 5 [19–23]. The overall YS and UTS values presented in the present work are in good agreement with Regener et al. [21], Stich et al. [22] and Schindelbacher et al. [7]. The elongation results also matched with Stich et al. [22] and Schindelbacher et al. [7]. However it is worth to note that the property comparison was made without considering the processing conditions as those are different or not informed in different works. The correlation of property variations between middle and edge specimens with the literature data
Fig. 6. Typical microstructure of HPDC AZ91 magnesium alloy which shows the shrinkage pores (A) and  particles (B).
is not possible as there is no reported work which addresses this issue. 3.2. Microstructure and failure modes The HPDC castings show a distinct fine-grained surface layer and a coarse-grained interior. A typical example of this grain size variation is presented elsewhere [17]. A microstructure of HPDC AZ91 alloy which shows the shrinkage pores and  particles of the material is shown in Fig. 6. It is concluded earlier that AZ91 follows intergranular brittle failure and, cleavage and quasi-cleavage are the most common fracture modes [24]. Additionally, insufficient independent slip systems in HPDC magnesium alloys causes less deformation due to the reduced dislocation motion between the neighbouring grains which, leads to a grain boundary cleavage and grain size play an important role here. The in situ tensile analysis confirms that the crack initiation and growth from shrinkage pores, damage of brittle  particles and grain boundary cleavage are the primary failure modes of the present material. Examples of these micro-mechanisms are shown in Fig. 7. The intergranular arrangement of shrinkage pores and  particles of HPDC AZ91 alloy also makes the intergranular failure more predominant [24]. The effect of inclusion on fracture was found to be negligible due to its scarce presence in the present material. Regener et al. [25] also observed the  particle cracking in Mg alloys during the in situ tensile analysis. In addition, Yoo et
Fig. 5. Comparison of tensile properties: (a) strength and (b) elongation with literature data.
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Fig. 7. Microscopic failure modes: (a) crack initiation and growth from shrinkage pore and grain boundary fracture, (b) damage of  particles and (c) grain boundary failure.
al. [26] documented that damages in massive  phase in the grain boundaries supports the grain boundary fracture. The different failure modes of HPDC AZ91 alloy under tensile loading and the extent effect of shrinkage pores and  particles on failure are explained in the previous contribution of authors [24]. The microstructural damages or cracks have strong effect on the macro-behavior of the material. The above explanations notice the greater influence of pores,  particles and grain size on fracture behavior of HPDC AZ91 alloy. Particularly, the size, shape and arrangement of these inhomogenities are the important parameters which influence the mechanical properties by altering the fracture behavior. 3.3. Micro-quantities and their effects The microstructural quantification results of the specimens from the edge and middle of castings and their corresponding tensile properties are shown in Table 2. The results confirms the significant variation in the area fraction, average size and clustering tendency of microporosity and  particles, and average grain size with respect
to the position in the casting. Increased grain size in the middle compared to edge of the castings is observed and this is due to the slow cooling rate of middle region compared to edge region of the casting during solidification. In addition, the partially solidified liquid metal from the shot sleeve occupies the middle of the casting, which also increases the grain size in the middle of castings. The arrangement of finer  particles in the edge region of casting compared to the coarse particles in the middle region is confirmed and this is due to the higher cooling rate in the edge region during solidification. The area fraction of microporosity is also high in the middle of castings compared to the edge region and this is also due to the high probability of shrinkage in the middle region during solidification. Particularly, the area fraction of clustered shrinkage pore is around 2/3 out of the total pore area fraction in all the castings. It is also confirmed from Table 2 that the clustering nature of pores and  phase is high in the middle of castings compared to the edge region. Besides, pores are hardly present in the skin region compared to other regions. With respect to the changes in micro-quantities a significant difference in UTS, YS and FS between edge and middle of the cast-
Average size
5.91 8.82 25.9 33.5 1.78 2.18 0.66–0.70 0.59–0.63 0.65–0.70 063–0.65 Edge Middle
2.9–4.1 3.1–5
1–1.27 1.3–1.8
Pores
Clustering tendency
 Pores 
Area fraction (%) Casting type
Table 2 Comparison of micro-quantities and macro-properties
 (m2 )
Pores (m2 )
Grain (m)
171–190 165–177
UTS (N/mm2 )
128–135 122–131
YS (N/mm2 )
FS (%)
1.41–1.9 1–1.48
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115
ings is observed. The edge specimens of the castings hold better properties due to the fine grain size, finer  particles and, low area fraction and clustering nature of microporosity. Additionally, the stress required in activating the twinning is sensitive to grain size [7] and twinning is likely to be restricted to the larger grains. This is also the reason for reduced strain to fracture value in the middle specimens of the castings. The large grains in the centre are softer due to decrease in Al content [8,27] compared to the small grains in the edge region. The small grain size provides a high fraction of grain boundary region inhibiting transmission of slip across the boundaries. The soft and coarse grains of the middle specimen in the castings cause early crack initiation and growth from the clustered shrinkage pores arranged in the grain boundary and the grain boundary arrangements of  particles also provide additional support to this process. The high area fraction and clustering nature of shrinkage pores and coarse  phase particles and their arrangement in the grain boundary regions may speed up the failure by early crack initiation and growth in the middle specimen of the castings. Particularly, crack initiation and growth in the microlevel is strongly related to the clustering nature of shrinkage pores and  particles which, gives a strong impact on FS quantities. This exactly reflects from the results of middle specimen of the castings where high clustering nature of pores and coarse  particles strongly reduces the values of FS by early failure.  particles not only supports the fracture process by getting itself damaged [24] but also increases the strength of HPDC AZ91 alloy by the arrangement of high density finer  in the edge region. In respect to tensile properties, porosity could have the effect of altering the stress field to initiate fracture, affecting crack propagation and reducing the effective load bearing nature of the material. When a tensile load is applied to a brittle alloy, the stress is concentrated around the pores and the fracture begins at this point. Once the crack has formed, pores can be regarded as pre-existing elements of crack and hence lead to rapid crack propagation. Shrinkage porosity initiates fracture more rapidly by virtue of their sharper root radius; however, clustering of this pores leads to an easy link up between neighbouring pores which deteriorate the mechanical properties, especially to a loss of ductility. Crack initiation from the shrinkage pores at strains around half of the fracture strain (strain at failure) observed with the same material [24] also strengthens these explanations. The UTS of middle casting is low compared to edge castings due to softening in middle casting by higher area fraction of microporosity and high probability of crack initiation and damage of  particles. Besides, the high strain hardening rate in edge specimens is also due to the arrest in dislocation motion by the highly dense fine  particles. 4. Summary and conclusions In the present study, tensile analysis of edge and middle specimens of HPDC AZ91 alloy is performed and the effect of position on the properties is obtained. The improved properties such as YS, UTS, FS and ductility of the edge compared to middle region of the casting are confirmed. The area fraction, average size and clustering nature of microporosity and  particles, and grain size were quantified and their strong effects on tensile properties are confirmed. These lower micro-quantities and clustering nature of these features provide better YS, UTS, FS and ductility for the edge region of the castings compared to the middle of castings. The FS values are more sensitive to the position of the castings due to the high clustering nature of shrinkage pores and  particles and their intergranular arrangement. In addition to the reduction of FS,  particles also increase the strength of the material. The lower grain size and finer  particles provides better strength to the edge of castings compared to the course-grained middle region.
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References [1] D. Magers, J. Willekens, in: B.L. Mordike, K.U. Kainer (Eds.), Magnesium Alloys and Their Applications, Wiley-VCH, Wolfsburg, Germany, 1998, pp. 105–112. [2] S. Schumann, F. Friedrich, in: B.L. Mordike, K.U. Kainer (Eds.), Magnesium Alloys and Their Applications, Wiley-VCH, Wolfsburg, Germany, 1998, pp. 3–13. [3] D. Brungs, in: B.L. Mordike, K.U. Kainer (Eds.), Magnesium Alloys and Their Applications, Wiley-VCH, Wolfsburg, Germany, 1998, pp. 113–117. [4] D. Magers, J. Willekens, in: B.L. Mordike, K.U. Kainer (Eds.), Proceedings on Magnesium Alloys and Their Applications, Wolfsburg, Germany, WerkstoffInformations GmbH, 1998, pp. 105–112. [5] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [6] A.L. Bowles, T.J. Bastow, C.J. Davidson, J.R. Griffiths, P.D.D. Rodrigo, Magnesium Technology 2000, The Minerals, Metals and Materials Society, Nashville, USA, 2000, pp. 295–300. [7] G. Schindelbacher, R. Rösch, in: B.L. Mordike, K.U. Kainer (Eds.), Magnesium Alloys and Their Applications, Wiley-VCH, Wolfsburg, Germany, 1998, pp. 247–252. [8] W.P. Sequeira, G.L. Dunlop, M.T. Murray, Proceedings of the 3rd International Magnesium Conference, Institute of Materials, Manchester, 1996, pp. 63–73. [9] W.L. Orchan, J.E. Gruzleski, Trans. Am. Foundry. Soc. 100 (1992) 415–424. [10] A. Couture, J.W. Meier, Trans. Am. Foundry. Soc. 74 (1966) 164–173. [11] D.G.L. Prakash, D. Regener, J. Alloys Compd 464 (2008) 133–137.
[12] A.L. Bowles, J.R. Griffiths, C.J. Davidson, in: J. Hryn (Ed.), Magnesium Technology 2001, The Minerals, Metals and Materials Society, Warrendale, PA, USA, 2001, pp. 161–168. [13] D.G. Leo Prakash, D. Regener, Prakt. Metallogr. 42 (2005) 555–575. [14] J.P. Anson, J.E. Gruzleski, Mater. Charact. 43 (1999) 319–335. [15] J.P. Anson, J.E. Gruzleski, Trans. Am. Foundry. Soc. 107 (1999) 135–142. [16] P. Baudhuin, M.-A. Leroy-Houyet, J. Quintart, P. Berthet, J. Microsc. 115 (1979) 1–17. [17] D.G.L. Prakash, D. Regener, J. Alloys Compd 461 (2008) 139–146. [18] G.W. Form, P.J. Ahearn, J.F. Wallace, Trans. Am. Foundry. Soc. 67 (1959) 64–69. [19] D. Rodrigo, M. Murray, H. Mao, J. Brevick, C. Mobley, V. Chandrasekar, R. Esdaile, SAE International Congress and Exposition, paper-1999-01-0927, Detroit, USA, 1999, pp. 785–789. [20] D. Regener, G. Dietze, H. Heyse, Materialwiss. Werkst. 35 (2004) 73–78. [21] D. Regener, G. Dietze, H. Heyse, Materialwiss. Werkst. 33 (2002) 606–613. [22] A. Stich, H.G. Haldenwanger, Magnesium 2000, 2nd International Conference on Magnesium Science and Technology, Dead Sea, Israel, 2000, pp. 27–34. [23] M. Easton, T. Abbott, C. Caceres, Mater. Sci. Forum 419–422 (2003) 147–152. [24] D.G. Leo Prakash, D. Regener, W.J.J. Vorster, Mater. Sci. Eng. A 488 (2008) 303–310. [25] D. Regener, E. Schick, I. Wagner, H. Heyse, Materialwiss. Werkst. 30 (1999) 525–532. [26] M.S. Yoo, Y.C. Kim, S. Ahn, N.J. Kim, Mater. Sci. Forum 419–422 (2003) 419–424. [27] J.P. Weiler, J.T. Wood, R.J. Klassen, R. Berkmortel, G. Wang, Mater. Sci. Eng. A 419 (2006) 297–305.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of position on the tensile properties in high-pressure die cast Mg alloy D.G. Leo Prakash a,∗ , Doris Regener b , W.J.J. Vorster a a b
Department of Engineering Science, University of Oxford, Parks Road, OX1 3PJ Oxford, UK Institut f¨ ur Werkstoff- und F¨ ugetechnik, Otto-von-Guericke-Universit¨ at Magdeburg, PF 4120, 39016 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 29 December 2007 Received in revised form 17 February 2008 Accepted 18 February 2008 Available online 26 March 2008 Keywords: Metals and alloys High-pressure die casting Mechanical properties Microstructure Effect of position
a b s t r a c t Tensile tests were performed at different locations of high-pressure die cast (HPDC) Mg alloy and the effect of position on the tensile properties such as yield strength (YS), ultimate tensile strength (UTS), ductility and fracture strain (FS) are explained. Additionally, the significance of micro-failure mode of the material is presented. The average size, area fraction and clustering tendency of pores and Mg17 Al12 () particles as well average grain size are correlated with the mechanical properties and found their influences. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are the lightest metallic structural materials and having a unique combination of properties, which are very attractive in automobile applications [1–3]. Most magnesium alloys are now produced by HPDC method [4] and the most common HPDC magnesium alloy is AZ91 [5]. This is due to its good combination of strength and ductility [6] as well as excellent castability. The microstructure of HPDC AZ91 alloy reveals dendrite microstructure with primary ␣-Mg solid solution, surrounded by a supersaturated divorced eutectic region. The intermetallic  phase (Mg17 Al12 ) particles are embedded in this supersaturated ␣-Mg solid solution. Inclusions, pore bands and microporosity are the processing defects of HPDC castings. The complex geometry of these microstructural features, their locations and arrangements are often non-uniform and usually strong spatial correlations exist. These results a multi-length scale micro-features, which cause multiple fracture micro-mechanisms and, affect the fracture path and mechanical properties of this material. The spatial arrangement, size distribution and shape of different microstructural feature also alter the mechanical properties. Various studies have reported the effect of section thickness on the tensile properties of HPDC and sand cast magnesium-based alloys [7–11]. Bowles et al. [12] reported the skin effect in HPDC cast Mg–Al alloys. However, there is no contribution which explains
∗ Corresponding author. E-mail address: [email protected] (D.G. Leo Prakash). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.02.051
the effect of position on the tensile properties in the high-pressure die castings. This highlights the need of systematic microstructural quantitative analysis to obtain the variation in micro-quantities with respect to the position in the castings and their relation to the tensile properties. Additionally, understanding the microscopic failure modes of the material is also equally important to predict the influence of the micro-features on fracture behavior, which is strongly related to the mechanical properties. Therefore, the aim of this work was to perform the same by quantitative characterization of microstructure and correlating the micro-quantities with the macro-properties of AZ91 alloy. Such an analysis is mandatory for the structural applications of this material. 2. Experimental procedure The AZ91 plates were produced using the cold chamber HPDC machine with the dimension of 200 mm × 53 mm × 10 mm. The HPDC conditions are given below: • • • •
• • • •
Melt temperature in furnace = 680 ◦ C. Initial die temperature = 230 ◦ C. Maximum die temperature = 400 ◦ C. Filling (piston speed). ◦ First phase of casting (filling at the gate), v = 0.4 m/s. ◦ Second phase (cavity fill), v = 3 m/s. ◦ Third phase (including solidification): rapid retardation of the piston with pressure increasing to 1200 bar. Protective gas used = SF6 in nitrogen. Fluidizing gas used = nitrogen. Cooling time in the mould = 20 s. Cooling temperature of mould = 280 ◦ C.
The time needed to transfer and pour the shot charge from the furnace in to the chamber is generally about 2 s. The liquid metal flow is from one side, along the
112
D.G. Leo Prakash et al. / Journal of Alloys and Compounds 470 (2009) 111–116
Fig. 1. The schematic view of the cut-out of middle and edge specimens from the HPDC plate. Fig. 2. Stress vs. strain results of the tensile experiments.
Table 1 Chemical composition of HPDC AZ91 Mg alloy Alloy
AZ91
% Al % Mn % Zn % Si % Cu % Ni % Fe
9.3 0.12 0.79 0.02 0.0007 0.0006 0.0046
castings. As the scope of the present study was micro–macro correlations, all the castings were produced with the same conditions. In order to obtain the effect of position tensile specimens of a cross section of 10 mm × 10 mm and 50 mm gauge length were machined from the edge and middle of the plates (as shown in Fig. 1). The skin regions of the edge specimens were machined out (3 mm) in the gauge region (along the width) as shown in Fig. 1. This kept on similar area fraction of skin region for the middle and edge specimens. Uniaxial tensile test was performed on the edge and middle specimens at a constant strain rate of 10−4 s−1 in a computer controlled servohydraulic test machine at room temperature. The chemical composition of the investigated HPDC AZ91 Mg alloy is presented in Table 1. The specimens from each tensile test were taken and polished by standard methods for optical microscopy. A microstructural area of 25 mm2 (a quarter of the cross-section) was grabbed with an optical microscopy at 100× as continuous microstructural frames from the unetched cross-section. This microstructural frames were used to create the microstructural montage and it was further introduced to image processing to quantify the gas and shrinkage microporosity. The nearest neighbour distance limit is used to separate the gas and shrinkage porosity and the detailed procedure is presented elsewhere [13–16]. The unetched crosssection was further etched (with a combination of picric acid (6 g), water (10 ml), acetic acid (5 ml) and ethanol (100 ml)) to reveal the  phase. The microstructural area of 1.86 mm2 was grabbed from the etched surface at 1000× for the quantification of  phase by image processing. The clustering tendency of these features was explained by comparing the spatial arrangement (nearest neighbour distance) of them from the present montage with the expected random arrangement. The values of clustering tendency below 1 indicate clustering, equal to 1; random, equal to 2.15; uniform distributions. The details of montage creation and the quantification procedure of microporosity and  phase are documented elsewhere [13–17]. In addition, fracture surface analysis of failed tensile specimens and in situ tensile analysis coupled with SEM was performed to understand the microscopic failure mode of the material.
3. Results and discussion The variations in the tensile properties and microstructural quantities with respect to different position in the casting are described in this section to explain the effect of position in HPDC castings. In addition, the microscopic failure modes of the material and the influence of microstructure on the macro-properties of the HPDC AZ91 magnesium alloy are discussed. 3.1. Macro-properties Fig. 2 shows the difference between the stress versus strain curves obtained from the tensile tests of the edge and middle specimens of the HPDC castings. Around 20 specimens were investigated in each case and no necking was observed in the deformed and failed specimens. This alloy exhibit no yield point and the 0.2% proof strength was taken as an indication of the yield point. Both the edge and middle cases showed a similar flow of deformation except the variations in the strain hardening behavior. The edge specimens show a higher strain hardening rate compared to the middle specimens. Additionally, significant difference in the value of strain to fracture is observed between the edge and middle specimens, and this could be of the variations in the arrangement of microporosity and the brittle  particles. The average and variations of YS (Fig. 3a) and UTS (Fig. 3b) of edge and middle specimens are shown in Fig. 3. Significant difference in these quantities with respect to the position in the castings is observed. The YS and UTS are found to decrease considerably in the middle of the casting compared to the edge region and the variations in the magnitude of YS and UTS in different cases show a similar trend. The YS decreases from an average of 131.65 N/mm2 for edge of the castings to 125.72 N/mm2 for the middle region and in the same trend, UTS decreases from an average of 184.59 to 168.04 N/mm2 . The strain hardening also increases significantly
Fig. 3. Average and variation in YS and UTS of edge and middle specimens in the castings.
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Fig. 4. Average and variations of FS values of edge and middle specimens in the castings.
in the edge compared to the middle specimens of the casting and this increase in the amount of strain hardening and strain to fracture cause the higher UTS for the edge specimens of the casting. Fig. 4 shows the average and variations of FS values of edge and middle of the castings. The effect of position on the FS is high compared to YS and UTS (see Figs. 3 and 4). The early failure of middle compared to the edge specimen indicating that the elongation of the middle specimen is low compared to other. The average elongation of the edge and middle specimens of the HPDC casting is 3.42% and 2.89%, respectively. The present study concludes that the strength and ductility are decreasing in the middle compared to the edge specimens. Form et al. [18] suggested that the local solidification time alters the microstructural development of the castings. Thus microstructural variation is expected in different positions of the castings due to the partial solidification in the shot sleeve and faster solidification of skin and edge region compared to the middle of the casting. This would be the reason for the obtained changes in mechanical properties with respect to the position in the casting. The comparison of the obtained tensile properties with the reported literature data of 10 mm thick HPDC castings is presented in Fig. 5 [19–23]. The overall YS and UTS values presented in the present work are in good agreement with Regener et al. [21], Stich et al. [22] and Schindelbacher et al. [7]. The elongation results also matched with Stich et al. [22] and Schindelbacher et al. [7]. However it is worth to note that the property comparison was made without considering the processing conditions as those are different or not informed in different works. The correlation of property variations between middle and edge specimens with the literature data
Fig. 6. Typical microstructure of HPDC AZ91 magnesium alloy which shows the shrinkage pores (A) and  particles (B).
is not possible as there is no reported work which addresses this issue. 3.2. Microstructure and failure modes The HPDC castings show a distinct fine-grained surface layer and a coarse-grained interior. A typical example of this grain size variation is presented elsewhere [17]. A microstructure of HPDC AZ91 alloy which shows the shrinkage pores and  particles of the material is shown in Fig. 6. It is concluded earlier that AZ91 follows intergranular brittle failure and, cleavage and quasi-cleavage are the most common fracture modes [24]. Additionally, insufficient independent slip systems in HPDC magnesium alloys causes less deformation due to the reduced dislocation motion between the neighbouring grains which, leads to a grain boundary cleavage and grain size play an important role here. The in situ tensile analysis confirms that the crack initiation and growth from shrinkage pores, damage of brittle  particles and grain boundary cleavage are the primary failure modes of the present material. Examples of these micro-mechanisms are shown in Fig. 7. The intergranular arrangement of shrinkage pores and  particles of HPDC AZ91 alloy also makes the intergranular failure more predominant [24]. The effect of inclusion on fracture was found to be negligible due to its scarce presence in the present material. Regener et al. [25] also observed the  particle cracking in Mg alloys during the in situ tensile analysis. In addition, Yoo et
Fig. 5. Comparison of tensile properties: (a) strength and (b) elongation with literature data.
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Fig. 7. Microscopic failure modes: (a) crack initiation and growth from shrinkage pore and grain boundary fracture, (b) damage of  particles and (c) grain boundary failure.
al. [26] documented that damages in massive  phase in the grain boundaries supports the grain boundary fracture. The different failure modes of HPDC AZ91 alloy under tensile loading and the extent effect of shrinkage pores and  particles on failure are explained in the previous contribution of authors [24]. The microstructural damages or cracks have strong effect on the macro-behavior of the material. The above explanations notice the greater influence of pores,  particles and grain size on fracture behavior of HPDC AZ91 alloy. Particularly, the size, shape and arrangement of these inhomogenities are the important parameters which influence the mechanical properties by altering the fracture behavior. 3.3. Micro-quantities and their effects The microstructural quantification results of the specimens from the edge and middle of castings and their corresponding tensile properties are shown in Table 2. The results confirms the significant variation in the area fraction, average size and clustering tendency of microporosity and  particles, and average grain size with respect
to the position in the casting. Increased grain size in the middle compared to edge of the castings is observed and this is due to the slow cooling rate of middle region compared to edge region of the casting during solidification. In addition, the partially solidified liquid metal from the shot sleeve occupies the middle of the casting, which also increases the grain size in the middle of castings. The arrangement of finer  particles in the edge region of casting compared to the coarse particles in the middle region is confirmed and this is due to the higher cooling rate in the edge region during solidification. The area fraction of microporosity is also high in the middle of castings compared to the edge region and this is also due to the high probability of shrinkage in the middle region during solidification. Particularly, the area fraction of clustered shrinkage pore is around 2/3 out of the total pore area fraction in all the castings. It is also confirmed from Table 2 that the clustering nature of pores and  phase is high in the middle of castings compared to the edge region. Besides, pores are hardly present in the skin region compared to other regions. With respect to the changes in micro-quantities a significant difference in UTS, YS and FS between edge and middle of the cast-
Average size
5.91 8.82 25.9 33.5 1.78 2.18 0.66–0.70 0.59–0.63 0.65–0.70 063–0.65 Edge Middle
2.9–4.1 3.1–5
1–1.27 1.3–1.8
Pores
Clustering tendency
 Pores 
Area fraction (%) Casting type
Table 2 Comparison of micro-quantities and macro-properties
 (m2 )
Pores (m2 )
Grain (m)
171–190 165–177
UTS (N/mm2 )
128–135 122–131
YS (N/mm2 )
FS (%)
1.41–1.9 1–1.48
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ings is observed. The edge specimens of the castings hold better properties due to the fine grain size, finer  particles and, low area fraction and clustering nature of microporosity. Additionally, the stress required in activating the twinning is sensitive to grain size [7] and twinning is likely to be restricted to the larger grains. This is also the reason for reduced strain to fracture value in the middle specimens of the castings. The large grains in the centre are softer due to decrease in Al content [8,27] compared to the small grains in the edge region. The small grain size provides a high fraction of grain boundary region inhibiting transmission of slip across the boundaries. The soft and coarse grains of the middle specimen in the castings cause early crack initiation and growth from the clustered shrinkage pores arranged in the grain boundary and the grain boundary arrangements of  particles also provide additional support to this process. The high area fraction and clustering nature of shrinkage pores and coarse  phase particles and their arrangement in the grain boundary regions may speed up the failure by early crack initiation and growth in the middle specimen of the castings. Particularly, crack initiation and growth in the microlevel is strongly related to the clustering nature of shrinkage pores and  particles which, gives a strong impact on FS quantities. This exactly reflects from the results of middle specimen of the castings where high clustering nature of pores and coarse  particles strongly reduces the values of FS by early failure.  particles not only supports the fracture process by getting itself damaged [24] but also increases the strength of HPDC AZ91 alloy by the arrangement of high density finer  in the edge region. In respect to tensile properties, porosity could have the effect of altering the stress field to initiate fracture, affecting crack propagation and reducing the effective load bearing nature of the material. When a tensile load is applied to a brittle alloy, the stress is concentrated around the pores and the fracture begins at this point. Once the crack has formed, pores can be regarded as pre-existing elements of crack and hence lead to rapid crack propagation. Shrinkage porosity initiates fracture more rapidly by virtue of their sharper root radius; however, clustering of this pores leads to an easy link up between neighbouring pores which deteriorate the mechanical properties, especially to a loss of ductility. Crack initiation from the shrinkage pores at strains around half of the fracture strain (strain at failure) observed with the same material [24] also strengthens these explanations. The UTS of middle casting is low compared to edge castings due to softening in middle casting by higher area fraction of microporosity and high probability of crack initiation and damage of  particles. Besides, the high strain hardening rate in edge specimens is also due to the arrest in dislocation motion by the highly dense fine  particles. 4. Summary and conclusions In the present study, tensile analysis of edge and middle specimens of HPDC AZ91 alloy is performed and the effect of position on the properties is obtained. The improved properties such as YS, UTS, FS and ductility of the edge compared to middle region of the casting are confirmed. The area fraction, average size and clustering nature of microporosity and  particles, and grain size were quantified and their strong effects on tensile properties are confirmed. These lower micro-quantities and clustering nature of these features provide better YS, UTS, FS and ductility for the edge region of the castings compared to the middle of castings. The FS values are more sensitive to the position of the castings due to the high clustering nature of shrinkage pores and  particles and their intergranular arrangement. In addition to the reduction of FS,  particles also increase the strength of the material. The lower grain size and finer  particles provides better strength to the edge of castings compared to the course-grained middle region.
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