The correlation between wall thickness and properties of HPDC Magnesium alloys

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1 Materials Science and Engineering A 447 (2007) The correlation between wall thickness and properties of HPDC Magnesium alloys E. Aghion a,, N. Moscovitch b, A. Arnon a a Ben-Gurion University of the Negev, Department of Materials Engineering, Beer-Sheva 84105, Israel b Magnesium Research Institute, Beer-Sheva 84100, Israel Received 21 May 2006; received in revised form 4 October 2006; accepted 21 October 2006 Abstract A systematic study was carried out to evaluate the correlation between high pressure die casting (HPDC) parameters and the mechanical properties of Magnesium alloys. The tested alloys included two conventional alloys: AZ91D and AM50A and a newly developed alloy, MRI153M. The nature of the above correlations was semi-empiric, based on computer simulation of the die casting process using MAGMA simulation software and actual measurements of mechanical properties. This was combined with microstructure analysis with the aim of understanding the interaction between high pressure die casting and the obtained properties. Special attention was also given to the complex effect of the wall thickness of die cast specimens and their subsequent porosity level Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Die casting; Solidification; Mechanical properties; Casting simulation 1. Introduction The development of magnesium applications for the automotive industry has received significant attention due to its light weight and consequent potential to reduce both fuel consumption and green house effect [1 3]. Magnesium applications were also developed for the electronics industry, where weight reduction and electro-magnetic shielding are of primary importance. In both cases, the dominant process for producing cost effective components is high pressure die casting (HPDC) [4,5]. However, the increasing demand for HPDC magnesium components requires proper understanding of the correlation between HPDC parameters and properties of the end product [6]. This is particularly important for newly developed Magnesium alloys where limited HPDC experience exists [7]. The aim of the present study is to evaluate the correlation between HPDC parameters and the properties of conventional and new Magnesium alloys for die casting applications. This is required for part design as well as for optimizing the die cast- Corresponding author at: Department of Materials Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel. Tel.: ; fax: address: egyon@bgu.ac.il (E. Aghion). ing process in order to obtain adequate properties in service conditions. The tested alloys in this study include the newly developed MRI153M, and AZ91D and AM50A as reference materials. The chemical compositions of the selected alloys AZ91D and AM50A met ASTM standards. The chemical composition of MRI153M is % Al % Mn 0 0.8% Zn 0.03% Si 0.003% Cu 0.001% Ni 0.004% Fe % Ca 0 0.5% Sr balance Mg. The typical mechanical properties are shown in Table 1 [8]. It should be noted that the castability of MRI153M is similar to that of AZ91D. However, it has a significant advantage in terms of creep resistance at high temperature applications [9]. The scientific tools of this study include a computer simulation model which was combined with measurements of mechanical properties and microstructure assessments. 2. Experimental Semi-empiric correlations between HPDC parameters and properties of the tested alloys: MRI153M, AZ91D, and AM50A were separately obtained by computer simulation modeling, mechanical testing, and microstructure analysis. The computer simulation was carried out using MAGMA simulation software for evaluating the solidification features. The thermo-physical /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.msea

2 342 E. Aghion et al. / Materials Science and Engineering A 447 (2007) Table 1 Typical mechanical properties of the tested die cast Mg alloys (separately die cast specimens) Properties AZ91D AM50A MRI153M Tensile yield strength [MPa] 20 C C Ultimate tensile strength [MPa] 20 C C Compression yield strength [MPa] Elongation in 60 mm [%] Impact strength [J] Young s modulus [GPa] Hardness [Brinell] Fatigue strength [MPa] properties used for the purpose of this simulation are presented in Table 2 [10]. For the purpose of the simulation, the parameters provided to the MAGMA software for the MRI153M alloy were those of the AZ91D alloy with the correct non-equilibrium solidification range for MRI153M. Mechanical testing in terms of ultimate tensile strength, yield strength, impact, hardness, and elongation were carried out on rectangular die cast tensile specimens with a width of 12 mm and thicknesses between 1.5 and 12 mm as shown in Fig. 1. The tensile specimens were obtained using a Hydra OL-320 cold chamber die casting machine. According to the process optimization, the melt and die temperatures for MRI153M were and C, respectively. As for AZ91D, the melt and die temperatures were and C, respectively, while for AM50A the melt and die temperatures were and C, respectively. Tensile specimens were tested by X-ray radiography to ensure soundness. Only sound and high-quality specimens with reasonable porosity levels were used for the detailed study. In general, the porosity was measured using Archimedes principle only in the gauge area (the area to be studied). The microstructure assessment, mainly in terms of grain size, was carried out on a cross-section of the rectangular die cast specimens. The technical method of measurement was based on counting the number of grains along arbitrary lines drawn on the metallographic images. Fig. 1. Tensile specimen drawings and their associated dimensions in mm. 3. Results and discussion A schematic illustration of the rectangular die cast tensile specimens showing the cross-section and X-direction used for the following analysis is introduced in Fig. 2. The X-direction indicates the direction of heat transfer during solidification which affects the cross-sectional properties of the specimen. For the purpose of this study, several assumptions were made: (1) One dimension geometry, including heat transfer solidification (only in the X-direction); (2) uniform die temperature in a thin layer near the die surface; (3) parallel layers of similar microstructure; (4) symmetry conditions are being applied. The soundness of the specimens versus their wall thickness in terms of porosity for AZ91D, AM50A, and MRI153M is shown Table 2 Thermo-physical properties used in MAGMA simulation [10,11] Property AZ91D AM50A MRI153M Density at 20 C [g/cm 3 ] Linear thermal expansion coefficient [ m/mk] Thermal conductivity at 20 C [W/Km] Specific heat [kj/kg K] Latent heat of fusion [kj/kg] Non-equilibrium solidification range [ C] Fig. 2. Specimen transverse cross-section and the direction of heat flow from the cavity.

3 E. Aghion et al. / Materials Science and Engineering A 447 (2007) Table 4 Tensile properties of die cast AM50A vs. specimen thickness Specimen thickness [mm] Yield strength [MPa] Ultimate tensile strength [MPa] Elongation [%] ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 2.6 Fig. 3. Porosity percentages according to specimen wall thickness. Table 3 Tensile properties of die cast AZ91D vs. specimen thickness Specimen thickness [mm] Yield strength [MPa] Ultimate tensile strength [MPa] Elongation [%] ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.6 in Fig. 3. In general, it was evident that increased wall thickness resulted in reduced porosity. This can mainly be explained by the fact that die casting of thin walled specimens encounters high turbulent flow of the molten metal, during the first phase of die casting, inducing volume porosity. The volume porosity in this case can be identified as micro-cavity porosity that is obtained from the combination of gas porosity and shrinkage. In addition, the increased porosity in thin walled specimens can be attributed to the significantly lower effectiveness of the third phase (pressure phase) of the die casting process compared to the effectiveness of this phase in thick specimens. It should be pointed out that the aim of the third phase is to reduce the cavity porosity resulting from the metal shrinkage, as well as to reduce gas porosity which is generated from the dissolved gas in the molten metal [12]. The correlation between specimen thickness and mechanical properties, namely yield strength, ultimate strength, and elongation, is shown in Tables 3 5 for AZ91D, AM50A, and MRI153M, respectively. This has shown that increased Table 5 Tensile properties of die cast MRI153M vs. specimen thickness Specimen thickness [mm] Yield strength [MPa] Ultimate tensile strength [MPa] Elongation [%] ± ± 15 3 ± ± ± 9 3 ± ± ± 8 3 ± ± ± ± ± ± ± 0.4 wall thickness results in decreased yield and ultimate strength, while elongation was increased for all the selected alloys. The combined results of all the alloys in terms of YTS are shown in Fig. 4. This revealed relatively similar behavior of AZ91D and MRI153M compared to AM50A. The similarity in yield strength behavior of AZ91D and MRI153M probably originates from their similarity in chemical composition. In order to evaluate the mechanical strength of the three selected alloys, hardness tests were performed. The results obtained for AZ91D, AM50A, and MRI153M according to the specimen wall thickness are shown in Fig. 5. This clearly shows that in both AZ91D and MRI153M, similar mechanical strength was achieved. The differences in mechanical properties of the surface, compared to the mid-section, can be explained through microstructure analysis of the relevant specimen. Grain size of AM50 at the surface was significantly smaller compared to the grain size at the mid-section (Fig. 6) due to the differences in solidification rate between the surface and the mid-section. Hence, the increased strength at the surface can be explained in terms of the Hall Patch equation. In general, it should be pointed Fig. 4. The correlations between TYS of Mg alloys and their casting thickness.

4 344 E. Aghion et al. / Materials Science and Engineering A 447 (2007) Fig. 5. Hardness results of AZ91D, AM50A, and MRI153M. out that there is a clear correlation between the grain diameter and the thickness of the specimen. As the wall thickness of the die cast specimen increases, the grain size also increases, as shown in Fig. 7, and the solidification time lengthens as shown in Fig. 8. Fig. 9 illustrates the solidification time versus grain time and from the values of n it is evident that there is a strong dependence of the grain size on the solidification time in AZ91D alloy, a very low dependence in AM50A and a medium dependence in MRI153M. This behavior is due to the differences between the castability of the various alloys. In fact, AZ91D has the highest castability while AM50A has the lowest one. In general, the castability is attributed to the formation of MgO particles in the melt during the filling of the die cast cavity. Since there is a strong connection between the melt ability to form MgO particles and the Al content in the alloy (the higher it is, the lower its ability to form oxides will be), the content of MgO particles in the flowing path of AM50A will be higher compared to that of AZ91D. The increased content of MgO particles generates a high level of nucleation sites and consequently a reduced grain size. For example, solidi- Fig. 7. Average grain size (across the entire cross-section) for all specimens and alloys as acquired by optical microscopy. fication time of 3 s will generate a grain size of 7.8 m in AM50A while the same solidification time will result in a 10.1 m average grain size in AZ91D. This characteristic can explain the significant differences in the n values depicted in Fig. 9. The semi-empiric correlations of the above properties with solidification rate were obtained using computer simulation in the form of MAGMA s simulation software. The relationships between the solidification rates in terms of solidification time versus tensile yield strength are illustrated in Fig. 10. This indicates that reduced solidification rate result in increased grain diameter and consequently reduced yield point. Similar results are shown in Figs. 11 and 12, which indicate that increased grain diameter and longer solidification time results in reduced impact energy. Fig. 6. Microstructure of die cast AM50 with 12 mm transverse cross-section (T = distance from surface) with measured local grain diameter.

5 E. Aghion et al. / Materials Science and Engineering A 447 (2007) Fig. 8. A typical cooling curve for AM50A alloy as received from MAGMA simulation software. Fig. 9. Solidification time vs. average grain size simulated for AZ91D, AM50M, and MRI153M, and fitted according to the relationship: grain size = (solidification time) n. Fig. 10. The correlation between TYS of die cast Mg alloys and associated solidification time.

6 346 E. Aghion et al. / Materials Science and Engineering A 447 (2007) Fig. 11. Impact vs. grain diameter of the selected alloys. Fig. 12. Impact vs. solidification time of the selected alloys. 4. Conclusions Semi-empiric correlations between high pressure die casting parameters and mechanical properties of AZ91D, AM50A, and MRI153M were established. These correlations highlight the close relationship between the microstructure, properties, and process parameters. In fact, it is evident that proper computer simulation of the casting process can be combined with experimental data to obtain sound and high quality die cast Magnesium alloy components. References [1] A. Tharumajah, P. Koltun, Life cycle assessment of Magnesium components supply chain, in: IMA 62nd Annual World Magnesium Conference, Berlin, Germany, May 22 24, 2005, pp [2] E. Aghion, B. Bronfin, D. Eliezer, Mater. Process. Technol. 117 (3) (2001) [3] E. Aghion, B. Bronfin, H. Friedrich, Z. Rubinovich, The environment impact of new Magnesium alloys on the transportation industry, in: TMS Annual Meeting, Magnesium Technology 2004, Charlotte, North Carolina, USA, March 14 18, 2004, pp [4] B.J. Coultes, J.T. Wood, G. Wang, R. Berkmortel, Mechanical properties and micro-structure of Magnesium high pressure die casting, in: TMS Annual Meeting, Magnesium Technology 2003, San Diego, California, USA, March 2 6, 2003, pp [5] B. Landkof, Magnesium applications in the electronic industry, in: E. Aghion, D. Eliezer (Eds.), Second Israeli International Conference of Magnesium Science and Technology, Dead Sea, Israel, February 22 24, 2000, pp [6] K. Peterson, P. Bakke, D. Albright, Magnesium die casting alloy design, in: TMS Annual Meeting, Magnesium Technology 2002, Seattle Washington, USA, February 17 21, 2002, pp [7] E. Aghion, B. Bronfin, F. von Buch, S. Schumann, H. Friedrich, JOM (November 2003) [8] F.V. Buch, S. Schumman, H. Friedrich, New die casting alloy MRI153 for powertrain applications, in: TMS Annual Meeting, Magnesium Technology 2002, Seattle, Washington, USA, February 17 21, [9] S.M. Zhu, B.L. Mordike, J.F. Nie, Creep studies of MRI153M Magnesium alloy castings, in: TMS Annual Meeting, Magnesium Technology 2005, San Francisco, California, February 13 17, 2005, pp [10] Hydro-Magnesium, Die cast Magnesium alloys, Data sheet, August [11] Dead Sea Magnesium, State of the art Magnesium products, Data Sheet, February [12] P.R. Beeley, Foundry Technology, Halsted Press Division: Wiley, New York, 1972, p. 473.