Experimental study on influence of section thickness on mechanical behavior of die-cast AM60 magnesium alloy

Experimental study on influence of section thickness on mechanical behavior of die-cast AM60 magnesium alloy

Materials and Design 38 (2012) 124–132 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/lo...

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Materials and Design 38 (2012) 124–132

Contents lists available at SciVerse ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

Experimental study on influence of section thickness on mechanical behavior of die-cast AM60 magnesium alloy Gongyao Gu ⇑, Shaoting Lin, Yong Xia, Qing Zhou State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, PR China

a r t i c l e

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Article history: Received 29 November 2011 Accepted 9 February 2012 Available online 17 February 2012 Keywords: A. Non-ferrous metals and alloys E. Fracture F. Plastic behavior

a b s t r a c t Influence of section thickness on mechanical behavior of die-cast AM60 magnesium alloy has been experimentally studied. Tension, compression and shear tests with this material were performed on a universal test machine at strain rates from 5  10 4 s 1 to 5  10 2 s 1. Specimens were cut from plates with five as-cast section thicknesses of 6.5 mm, 5.2 mm, 3.9 mm, 2.6 mm and 1.3 mm. According to the test results, flow stress becomes less sensitive to section thickness with larger section thickness, and the influence of strain rate on flow stress is also decreasing with larger section thickness. At different stress states, the tested material follows the von-Mises yield criterion. And stress state is found to be the main factor influencing the fracture behavior. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Due to the increasing requirements of fuel economy and environment protection on vehicle products, magnesium alloys have become more attractive for application in automotive industry [1–3] due to their high strength-to-weight ratio in comparison with aluminum and steel [4]. Die-casting, including high-pressure die-casting (HPDC) [5], is one of the dominant techniques for manufacturing magnesium components because of the high castability of this material. There are extensive applications of die-cast magnesium alloys on vehicle components such as instrument panel. As an important tool for vehicle structure design, finite element (FE) simulation needs reliable input of material mechanical properties. Therefore, it is necessary to characterize the mechanical behavior of the die-cast magnesium alloys based on experimental study [6–9]. The influence of section thickness on the mechanical behavior of die-cast magnesium alloys has been reported in some literature [10–18]. A large part of these contributions studied the die-cast AZ91 magnesium alloy [10–15], however, the experiment results with this material seem not fully consistent among these studies. Rodrigo et al. [10] reported that with larger section thickness the yield stress of their tested AZ91 magnesium alloy became lower and the fracture elongation increased. Reverse trend about the relationship between fracture elongation and section thickness was found in other research on the same material [11–14]. And this discrepancy on the effect of section thickness was also noted by

⇑ Corresponding author. Tel./fax: +86 10 62788689. E-mail address: [email protected] (G. Gu). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2012.02.015

Prakash and Regener [14]. Although there were relatively less similar research on other types of die-cast magnesium alloy such as AM60 [10,16], the experiment result of influence of section thickness on the mechanical properties was also found to be controversial among these studies. According to the previous studies on die-cast magnesium alloys [10–18], the effect of section thickness on the mechanical behaviors is indeed caused by the discrepancies of microstructure due to different cooling rates with various thicknesses. It was reported by Hu et al. [17] that due to the higher cooling rate during the casting process, thinner section led to finer microstructure and lower porosity level, which was considered as the reason for higher yield stress, higher ultimate strength and larger fracture elongation. Besides those previous studies, the relationship between microstructure characteristics and mechanical behavior of die-cast magnesium alloys were also investigated by other literature [19–22], among which Cáceres et al. [19] and Ning et al. [22] carried out an experimental study on the effect of cooling rate with AZ91 alloy, respectively. However, as most of these studies focuses on the quasistatic mechanical behavior, there was not much detailed discussion about the relationship between the microstructure and the strain rate sensitivity of the die-cast magnesium alloy. An ongoing investigation in our laboratory related to lightweight structure design is focusing on application of die-cast magnesium alloy AM60 and requires accurate characterization of its mechanical behaviors, in which it is necessary to identify the influence of section thickness on the test material as fully as possible. Tests at different stress states and different strain rates were carried out in this study to verify whether the influence of section thickness was consistent among various loading conditions. Microstructure observation with the method of scanning electron

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microscope was applied in attempt to correlate the mechanical behavior with the microstructure characteristics of the tested material. Section 2 of this paper introduces the test material and the corresponding manufacturing process; test method and specimen design are included in Section 3; test result and corresponding discussions are presented in Sections 4 and 5, respectively; Section 6 summarizes the conclusions of this study. 2. Material Die-casting AM60 magnesium alloy was chosen as the subject material of this study. A Yizumi 350-ton die-casting machine was employed for manufacturing the original plate of AM60 alloy. The initial melt temperature was 680 °C and the die temperature was 250 °C. The casting pressure was 60 MPa. Mold steel H3 was used to fabricate the mold. The coating was Acheson Deltacast 333 and the spray took about 1.5 s. A specially designed casting die was applied and shown in Fig. 1. Specimens for this study were cut from the central part of the five finger-like rectangular plates, which were of five different thicknesses including 6.5 mm, 5.2 mm, 3.9 mm, 2.6 mm and 1.3 mm, respectively. The length and width of each as-cast rectangular plates are the same to each other, as is shown in Fig. 1. The area of the flow gate is 93 mm2 and the volumetric flow rate is 5.58  10 3 m3s 1. The cooling rate was around 2500 °C/s at the surface of specimen. 3. Test method Quasi-static tension, compression and shear tests were performed on a universal test machine with loading capacity of 100 kN. For all these different loading conditions such as shear test, it was relatively difficult to measure the deformation information with traditional equipments like extensometer. A noncontact measurement method, digital image correlation (DIC), was employed in this study. Since this method could provide full-field deformation measurement with satisfactory accuracy during the test process, it has been widely used in the past few years [23]. Its reliability has already been verified based on the comparison of the measurement result with other techniques [23]. A CCD camera was used for image capturing during the test. And the software Vic-2D was employed for calculating the deformation based on DIC method. Both the universal test machine and the industrial camera are shown in Fig. 2. Fig. 3 shows the specimen designs in this study. All the three types of tests were performed with specimens of all the five section thicknesses as summarized in the previous section. The standards ASTM E8M [24] and ASTM E9 [25] were used as references for designs of tension specimen in Fig. 3a and compression one in Fig. 3b, respectively. The current shear specimen in Fig. 3c took the design

Fig. 1. As-cast sample.

Fig. 2. Setup of tension test.

suggested by Bao [26] for reference. To identify the influence of strain rate on the mechanical behavior, both tension and compression tests were carried out at strain rates of 5  10 4 s 1, 5  10 3 s 1 and 5  10 2 s 1. Shear tests were performed at strain rate of 5  10 4 s 1. Five repeated tests were performed for each test type and strain rate. 4. Test results 4.1. Results of tension test Fig. 4 shows the true stress vs. true strain curves of tension tests with 6.5 mm thick specimens at strain rate of 5  10 4 s 1. Results of all the five repeated tests and also the mean curve are included in this figure. It is to be noted that each curve shown in the rest part of this paper is the average result of the five repeats at the corresponding loading condition. Similarly, result of tension tests with 1.3 mm thick specimens at the same strain rate is shown in Fig. 5. It has been noted that the variation of fracture strain is obviously larger in Fig. 4 than that in Fig. 5. However, this difference between specimens of various thicknesses is to be further checked with tests results at other strain rates, which is shown in Fig. 13 in the following part of this section. Fig. 6 compares the true stress vs. true strain curves of tension tests with specimens of all the five different thicknesses at strain rate of 5  10 4 s 1. Each curve ends at the average fracture strain of all the five repeated tests with the same specimen thickness. Among specimens of all the five thicknesses, the mean fracture strain of the 3.9 mm thick specimens is the smallest, which is roughly 0.03. Before the strain of 0.03, the flow stress increases with the specimen thickness reduced from 3.9 mm to 1.3 mm, which is consistent with some of the literature [16]. And this is considered to be the result of faster solidification during the casting process and finer microstructure with smaller section thickness. However, before the strain of 0.03, the curves for 3.9 mm, 5.2 mm and 6.5 mm thick specimens are quite close to each other. It seems that in this range of section thickness the flow stress of the tested die-casting AM60 magnesium alloy is less sensitive to this geometric parameter than that with specimens thinner than 3.9 mm. For clearer illustration of this trend, the values of flow stress at the strain of 0.01, 0.02 and 0.03 are shown in Fig. 7. Exponentially-fitted curve with true stress vs. section thickness for each strain is also presented in this figure. Report of such an obviously convergence relationship was not found in the literature summarized in the previous section, and further discussion on it is included in the following sections. Similar relationship between flow stress and section thickness can be also observed from the results of tension tests at strain rates of 5  10 3 s 1 and 5  10 2 s 1, as shown in Figs. 8 and 9.

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Fig. 3. Specimen designs (a: tension; b: compression; c: shear).

Fig. 4. True stress vs. true strain curves of tension tests with 6.5 mm thick specimen at 5  10 4 s 1.

Fig. 5. True stress vs. true strain curves of tension tests with 1.3 mm thick specimen at 5  10 4 s 1.

Fig. 6. True stress vs. true strain curves of tension tests at 5  10

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Fig. 7. Flow stress vs. section thickness for tension tests at 5  10

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Fig. 8. Flow stress vs. section thickness for tension tests at 5  10

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Fig. 9. Flow stress vs. section thickness for tension tests at 5  10

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Figs. 10 and 11 compare the true stress vs. true strain curves of tension tests at the three strain rates with 6.5 mm and 1.3 mm thick specimens, respectively. In Fig. 10, the three curves are quite close

to each other. In other words, the flow stress of the 6.5 mm thick specimens is not sensitive to strain rate from 5  10 4 s 1 to 5  10 2 s 1. However, for the 1.3 mm thick specimens, the flow stress is increasing with strain rate as shown in Fig. 11. The

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Fig. 10. True stress vs. true strain curves of tension tests with 6.5 mm thick specimen.

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Fig. 12. True stress vs. strain rate curves of tension tests at true strain of 0.02.

Fig. 13. Fracture strain of tension tests. Fig. 11. True stress vs. true strain curves of tension tests with 1.3 mm thick specimen.

situation with the 2.6 mm thick specimens is similar to that with 1.3 mm thick ones, and the situation with the 5.2 mm or 3.9 mm thick ones is similar to that with 6.5 mm thick ones. According to these results summarized above, the flow stress of this material in tension tests is sensitive to the strain rate only in the range of section thickness from 1.3 mm to 2.6 mm, but it seems insensitive to the strain rate with section thickness larger than 3.9 mm. This phenomenon is clearly illustrated in Fig. 12. It is also observed in Fig. 12 that with the same incremental strain rate the increased flow stress for 1.3 mm thick specimens is larger than that for 2.6 mm thick ones. Although there were already some experimental studies on the strain rate effect of mechanical behaviors of die-cast AM60 magnesium alloy [27–30], theses literature did not offer much detailed discussion on the relationship between the microstructure and the strain rate sensitivity. And also there were few published test results on the variation of strain rate sensitivity with different section thicknesses like the results summarized in this section. The mean fracture strain and its standard deviation of tension tests are summarized in Fig. 13. At each strain rate, the fracture strain of 3.9 mm thick specimens is the smallest among all the five section thicknesses. And there is no significant monotonic relationship between fracture strain and section thickness, which is inconsistent with either the conclusion by Rodrigo et al. [10] or the one by Chadha et al. [16]. There is also no clear relationship between fracture strain and strain rate as shown in Fig. 13. According to Chadha et al. [16], the fracture elongation of die-cast AM60 magnesium alloy decreases with larger porosity of the tested material. Based on their studies on die-cast AZ91 magnesium alloy, Prakash et al. [20,21] pointed out that crack initiation and growth from shrinkage pores is one of the major microstructure failure modes of this material. Supplier of our tested die-cast magnesium alloy reported that the porosity of the specimens was no more than 5%. And a further discussion on the results shown in Fig. 13 is presented in the following section based on the microstructure observation.

It is to be noted that most of the standard deviation shown in Fig. 13 is quite large, which is nearly half of the mean value of the corresponding fracture strain. This also implies that the fracture behavior is partly dependent on the stochastic characteristics of this material such as the porosity, or even the size and shape of the pores [20,21]. Similar conclusion was obtained by Dørum et al. [9], and they have developed a fracture criterion including stochastic parameter to describe the fracture behavior of the die-casting AM60 magnesium alloy. 4.2. Results of compression test Fig. 14a shows the result of in-plane compression tests at the strain rate of 5  10 4 s 1. In the strain range from 0.02 to 0.05, the flow stress becomes higher with the section thickness decreased from 3.9 mm to 1.3 mm. Also in this strain range, the result curves for the section thickness ranging from 6.5 mm to 3.9 mm are relatively close to each other, among which the flow stress with 6.5 mm thick specimens is slightly higher than that with 5.2 mm ones. These results of compression tests above are similar to those of tension tests. A zoom-in plot of the curves between the strain of 0.02 and 0.05 is shown in Fig. 14b for clearer illustration of this trend. For strain larger than 0.05, there is no obvious relationship between the section thickness and the flow stress. A possible reason for this is that at relatively large deformation stage of the inplane compression, there is some out-of-plane deformation in the gage section of the specimen due to the increase of cross-section area or the buckling deformation, and the stress state there is no longer pure uniaxial compression. However, the corresponding part of stress–strain curve shown in Fig. 14 is still considered to represent the result at pure uniaxial compression. Similar situation is found for comparison of results of compression tests among different section thicknesses at other strain rates. Figs. 15 and 16 compare the true stress vs. true strain curves between tension and compression tests at 5  10 4 s 1. In Fig. 15, the two curves for 6.5 mm thick specimens are quite close to each

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Fig. 17. Fracture strain of compression tests.

Fig. 14. True stress vs. true strain curves of compression tests at 5  10 4 s whole range of deformation; b: in strain range between 0.02 and 0.05).

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Fig. 18. Fracture surface of tension and compression specimens (a: tension; b: compression).

Fig. 15. True stress vs. true strain curves of tension and compression at 5  10 with 6.5 mm thick specimens.

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Fig. 16. True stress vs. true strain curves of tension and compression at 5  10 with 1.3 mm thick specimens.

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other. But the curves for 1.3 mm thick specimens in Fig. 16 are slightly different from each other. For the specimens of the other thicknesses the comparison is similar to that in Figs. 15 and 16. Therefore, it is reasonable to conclude that before the occurrence of fracture in tension tests, this material shows symmetrical mechanical behavior in tension and compression. This conclusion is also confirmed with the study by Dørum et al. [8].

Fracture strain of compression tests are summarized in Fig. 17. Since significant buckling occurred during the compression tests with the thinnest specimens of 1.3 mm thickness at 5  10 3 s 1 and 5  10 2 s 1, the test results under these two loading conditions are not included in Fig. 17. Similar to the results of tension tests reported in Fig. 13, at each strain rate of compression tests there is no monotonic relationship between fracture strain and section thickness. For each of the specimen thicknesses from 3.9 mm to 6.5 mm, the mean fracture strain of compression tests is increasing with the strain rate. However, this trend is not very significant due to the existing standard deviation. It is to be noted that the fracture strain of compression tests is obviously larger than that of tension tests for each section thickness and each strain rate. The former one could be roughly three or four times of the latter one. This difference between the two types of tests is probably due to the corresponding different fracture mechanisms as illustrated in Fig. 18. The fracture surface of tension specimen shown in Fig. 18a is perpendicular to the loading direction, and some small pores can be found on this surface. It has been mentioned above that the pores could be a major source of crack initiation and growth in tension tests [20,21], and a fracture model under this loading condition was suggested by Weiler and Wood [31,32]. The fracture surface of compression specimen shown in Fig. 18b is much smoother than that of tension one in Fig. 18a. Few pores can be found on the fracture surface in Fig. 18b, and it is possible that the original pores inside the specimen were closed during the compression process. The fracture surface of compression specimen is approximately 45° to the loading direction. According to Bao [26], this type of fracture shown in

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Fig. 18b is called ‘‘shear fracture’’, and the crack develops along the closest packed sliding planes. As shown in Fig. 17, there is no clear relationship between this fracture mechanism and the section thickness.

4.3. Results of shear test In Fig. 19, the relationship among shear stress vs. shear strain curves of shear tests at 5  10 4 s 1 with specimens of different thicknesses is quite similar to the one of tension tests shown in Fig. 6: for specimens of thickness from 6.5 mm to 3.9 mm, the stress–strain curves are quite close to each other; for specimens of thickness from 3.9 mm to 1.3 mm, the flow stress becomes higher with smaller section thickness. Fracture shear strain is summarized in Fig. 20. As the shear test was carried out by compressing loading along the longitudinal direction of the specimen shown in Fig. 3c, buckling occurred in the tests with the thinnest specimen and therefore the corresponding fracture shear strain is obviously smaller than those of the other thicker specimens in Fig. 20. It can be observed that at this loading condition there is no clear relationship between fracture strain and section thickness. In comparison with the result in Figs. 13 and 17, the value of fracture strain of shear tests is close to that of tension tests, which is much smaller than that of compression

tests. Small pores can be found on the fracture surface of shear specimen shown in Fig. 21, which is similar to the situation with the fracture surface of tension tests in Fig. 18a. However, the fracture surface in Fig. 18a is not so smooth as that in Fig. 21. A hybrid work combining test and finite element simulation was carried out to identify the yield criterion of the tested material. The stress–strain curve of tension test with 2.6 mm thick specimen at 5  10 4 s 1 was processed as the input parameters for  MAT_024 model in the finite element software LS-DYNA [33], and fracture was not considered in this material model. Both tension and shear specimens were modeled with solid elements of 1 mm mesh size. The tension test at 5  10 4 s 1 was first simulated to verify the accuracy of the input parameters of the material model as shown in Fig. 22a. Then the verified material model was used for simulating the shear test with specimens of the same thickness at the same strain rate, and good correlation can be observed in Fig. 22b. Since the material model MAT_024 follows the von-Mises yield criterion and it was shown above that the tested material showed approximately symmetrical behavior in tension and compression tests, it is deduced that this yield criterion is suitable to describe the yielding behavior of this material. Similar conclusions can be obtained for specimens of other thicknesses. Therefore, the yield criterion of the material seems unaffected by the section thickness.

5. Discussions As plenty of literature correlated the mechanical behavior of die-cast magnesium alloys with their microstructure characteristics [10–22], the microstructure of the tested die-cast AM60 alloy in this study was also observed with scanning electron microscope (SEM). Photos of specimens of the five different section thicknesses are shown in Fig. 23. The magnification ratio is 500  for each of the five SEM photos. And it is to be noted that all these photos were taken at the core part of those specimens. According to Chadha et al. [16], the gray regions in each of the photos in Fig. 23 are the a-Mg dendrites, which forms the largest Fig. 19. Shear stress vs. shear strain curves of shear tests at 5  10

Fig. 20. Fracture shear strain of shear tests at 5  10

Fig. 21. Fracture surface of shear specimen.

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Fig. 22. Simulation of tests at 5  10 b: shear).

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with 2.6 mm thick specimens (a: tension;

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Fig. 23. SEM photos of specimens of different thicknesses (a: 1.3 mm thick; b: 2.6 mm thick; c: 3.9 mm thick; d: 5.2 mm thick; e: 6.5 mm thick).

portion of the microstructure; the while regions around the dendrites are the intermetallics of Mg17Al12, which is the b-phase of the microstructure; and the darker regions are the pores distributed relatively more randomly inside the microstructure than the two phases mentioned above. Inclusions were not found on the observed surface, and this is consistent with the result by Prakash et al. [20,21], who reported that the area fraction of inclusions in their tested die-cast AZ91 magnesium alloy was extremely low compared with other constituents. As Prakash et al. [21] showed that the b-phase was located at the grain boundary region of a-Mg, it is possible to qualitatively compare the grain size among specimens of different section thicknesses based on the microstructure shown in Fig. 23. It is obvious that the microstructure becomes finer with section thickness reduced from 3.9 mm to 1.3 mm. However, the difference in the estimated grain size is not so clear among the specimens of section thickness from 3.9 mm to 6.5 mm. Cáceres et al. [19] analyzed the grain boundary strengthening mechanism in detail and verified that the Hall–Petch equation could be a good description of the relationship between yield stress and grain size. Therefore, the variation of the estimated grain size above could be part of the reason for the convergence relationship of flow stress with section thickness increased from 1.3 mm to 6.5 mm shown in Fig. 7. And this implies that under the current boundary condition of die-casting,

the average grain size may not increase proportionally with the section thickness. According to Meyers [34], the strain rate sensitivity of mechanical behavior of magnesium alloy is due to its hexagonal close packed microstructure. However, based on this statement only, it is difficult to explain the reason for the variation of strain rate sensitivity of flow stress with different section thicknesses as shown in Fig. 12. It is supposed that this phenomenon may be due to the increased total area of contact surfaces between neighboring grains or between a-Mg grains and b-Mg17Al12 particles with finer microstructure. During plastic deformation of the tested material, the resistance caused by the relative motion on these contact surfaces may contribute to the overall flow stress. If the resistance on the contact surface is viscous, then finer microstructure with larger contact surfaces will lead to higher strain rate sensitivity of flow stress. However, it should be noted that this deduction is quite preliminary and still needs further verification. It has been shown in Figs. 13, 17 and 20 that there is no clear relationship between the fracture behavior and the section thickness of the tested material in each of the three test types, and this is inconsistent with the conclusions in literature [10,16], which obtained a monotonic relationship between fracture elongation and section thickness. Furthermore, in this study the standard deviation

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of fracture strain is relatively large, especially in tension and shear tests. Although the detailed porosity of the tested specimens is not available at present, the test results summarized above about the fracture behavior can be partly explained by the SEM photos shown in Fig. 23. As the photos were taken at the core part of the fracture surface of each specimen, the pores observed on these surfaces are supposed to have contributed to the crack initiation and growth. Hu et al. [17] concluded that thinner section led to finer microstructure and lower porosity, however, in Fig. 23a there is obviously more pores on the fracture surface of 1.3 mm thick specimen than that of 5.2 mm thick one in Fig. 23d. This is just an example to show the random distribution of pores inside the diecast magnesium alloy. It is to be noticed that the pore shown on the left side of Fig. 23b is much larger than the other pores in Fig. 23, which illustrates that the size and the shape of pores could be also stochastic characteristics of the microstructure. And this is considered as the reason for the unclear relationship between fracture strain and section thickness and also the relatively large variation of fracture strain, as Prakash et al. [20,21] showed that besides the porosity both the size and the shape of pores affected the fracture of die-cast magnesium alloy. Therefore, more repeated tests are necessary to identify the relationship between the fracture behavior of the die-cast magnesium alloy and its casting boundary conditions.

6. Conclusions According to the test result and discussion in the sections above, the following conclusions are derived for the studied diecasting AM60 magnesium alloy: (1) The flow stress becomes less sensitive to the section thickness with larger section thickness. For specimens thicker than 3.9 mm, the flow stress keeps roughly constant among different section thickness. The reason is considered that the grain size which affects the flow stress does not increase proportionally with the section thickness. (2) The influence of strain rate on flow stress is related to the section thickness. With section thickness larger than 3.9 mm, the flow stress seems unaffected by strain rate from 5  10 4 s 1 to 5  10 2 s 1, and the strain rate sensitivity of flow stress becomes higher with section thickness decreased from 2.6 mm to 1.3 mm. A preliminary explanation for this is supposed that finer microstructure with smaller section thickness leads to larger contact surfaces among neighboring a-Mg grains or between a-Mg grains and b-Mg17Al12 particles, and the strain rate sensitivity of flow stress increases with smaller section thickness if the resistance of relative motion on these contact surfaces is viscous. (3) For specimens of each section thickness, the studied material shows symmetrical behavior of plastic deformation between tension and compression tests, and it is verified to follow the von-Mises yield criterion. (4) The fracture strain in compression tests is much larger than those in tension or shear tests, which implies that the fracture behavior depends on the stress state. However, the fracture behavior is not well correlated with either the section thickness or the strain rate according to the test results obtained in this study. The random distribution of microscopic pores inside the material, a dominant factor affecting the macroscopic fracture, has not shown regular trend for different section thicknesses.

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Acknowledgments The authors would like to acknowledge the State Key Laboratory of Automotive Safety and Energy of China for their funding support. And this research is also funded by the MOST (Ministry of Science and Technology) of China under the contract of No. 2010DFA72760. References [1] Friedrich H, Schumann S. Research for a ‘‘new age of magnesium’’ in the automotive industry. J Mater Process Technol 2001;117:276–81. [2] Mordike BL, Eber T. Magnesium properties–applications–potential. Mater Sci Eng A 2001;302:37–45. [3] Kulekci MK. Magnesium and its alloys application in automotive industry. Int J Adv Manuf Technol 2008;39:851–65. [4] Easton M, Song WQ, Abbott T. A comparison of the deformation of magnesium alloys with aluminium and steel in tension, bending and buckling. Mater Des 2006;27:935–46. [5] Laukli HI. High pressure die casting of aluminium and magnesium alloys-grain structure and segregation characteristics. PhD thesis, Norwegian University of Science and Technology, Trondheim, Norway; 2004. [6] Aune TK, Westengen H, Ruden T. Mechanical properties of energy absorbing magnesium alloys. SAE paper no. 930418, SAE International Congress, Detroit; 1993. [7] Wood JT, Klassen RJ, Gharghouri MA, Maire E, Wang G, Berkmortel R. Mechanical properties of AM60B die casting: a review of the AUTO21 program on magnesium die-casting. SAE paper no. 2005-01-0725, SAE International Congress, Detroit; 2005. [8] Dørum C, Hopperstad OS, Lademo OG, Langseth M. Aluminium and magnesium castings-experimental work and numerical analyses. Int J Crashworthiness 2003;8:455–70. [9] Dørum C, Hopperstad OS, Berstad T, Dispinar D. Numerical modelling of magnesium die-castings using stochastic fracture parameters. Eng Fract Mech 2009;76:2232–48. [10] Rodrigo D, Murray M, Mao H, Brevick J, Mobley C, Chandrasekar V, et al. Effect of section size and microstructure features on the mechanical properties of die cast AZ91D and AM60B magnesium alloy test bars. SAE paper no. 1999-010927, SAE International Congress, Detroit; 1999. [11] Aghion E, Moscovitch N, Arnon A. The correlation between wall thickness and properties of HPDC magnesium alloys. Mater Sci Eng A 2007;447:341–6. [12] Cáceres CH, Poole WJ, Bowles AL, Davidson CJ. Section thickness, macrohardness and yield strength in high-pressure diecast magnesium alloy AZ91. Mater Sci Eng A 2005;402:269–77. [13] Cáceres CH, Griffiths JR, Pakdel AR, Davidson CJ. Microhardness mapping and the hardness–yield strength relationship in high-pressure diecast magnesium alloy AZ91. Mater Sci Eng A 2005;402:258–68. [14] Prakash LDG, Regener D. Micro–macro interactions and effect of section thickness of hpdc AZ91 Mg alloy. J Alloys Compd 2008;464:133–7. [15] Sumitomo T, Cáceres CH, Veidt M. The elastic modulus of cast Mg–Al–Zn alloys. J Light Met 2002;2:49–56. [16] Chadha G, Allison JE, Jones JW. The role of microstructure on ductility of diecast AM50 and AM60 magnesium alloys. Metall Mater Trans A 2007;38A: 286–97. [17] Hu H, Zhou M, Sun Z, Li N. Tensile behavior and fracture characteristics of die cast magnesium alloy AM50. J Mater Process Technol 2008;201:364–8. [18] Yang K, Nagasekhar AV, Cáceres CH. Section thickness and the skin effect in a high-pressure die cast Mg–12%Al alloy. In: Magnesium technology 2010. Warrendale: The Mineral, Metals & Material Society; 2010. [19] Cáceres CH, Davidson CJ, Griffiths JR, Newton CL. Effect of solidification rate and ageing on the microstructure and mechanical properties of AZ91 alloy. Mater Sci Eng A 2002;325:344–55. [20] Prakash LDG, Regener D, Vorster WJJ. Microscopic failure modes of hpdc AZ91HP magnesium alloy under monotonic loading. Mater Sci Eng A 2008;488:303–10. [21] Prakash LDG, Regener D, Vorster WJJ. Effect of position on the tensile properties in high-pressure die cast Mg alloy. J Alloys Compd 2009;470:111–6. [22] Ning Z, Cao P, Wang H, Sun J, Liu D. Effect of cooling condition on grain size of AZ91 alloy. J Mater Sci Technol 2007;23:645–9. [23] Sutton MA, Oreu JJ, Schreier HW. Image correlation for shape, motion and deformation measurements-basic concepts, theory and applications. 1st ed. New York: Springer; 2009. [24] ASTM E8M. Standard test methods for tension testing of metallic materials. ASTM International; 2004. [25] ASTM E9. Standard test methods of compression testing of metallic materials at room temperature. ASTM International; 2000. [26] Bao Y. Prediction of ductile crack formation in uncracked bodies. PhD thesis, Massachusetts Institute of Technology, Cambridge, USA; 2003. [27] Lee CD. Dependence of tensile properties of AM60 magnesium alloy on microporosity and grain size. Mater Sci Eng A 2007;454–455:575–80. [28] Carlson BE. The effect of strain rate and temperature on the deformation of die cast AM60B. SAE paper no. 950425, SAE International Congress, Detroit; 1995. [29] Aune TK, Albright D, Westengen H, Johnsen T, Andersson B. Behavior of die cast

132

G. Gu et al. / Materials and Design 38 (2012) 124–132

magnesium alloys subjected to rapid deformation. SAE paper no. 2000-011116, SAE International Congress, Detroit; 2000. [30] Song WQ, Beggs P, Easton M. Compressive strain rate sensitivity of magnesium–aluminum die casting alloys. Mater Des 2009;30:642–8. [31] Weiler JP, Wood JT. Modeling fracture properties in a die-cast AM60B magnesium alloy I – analytical failure model. Mater Sci Eng A 2009;527: 25–31.

[32] Weiler JP, Wood JT. Modeling fracture properties in a die-cast AM60B magnesium alloy II – the effects of the size and location of porosity determined using finite element simulations. Mater Sci Eng A 2009;527: 32–7. [33] LS-DYNA keyword user’s manual. Livermore Software Technology Corporation; 2007. [34] Meyers MA. Dynamic behavior of materials. 1st ed. Berlin: Springer; 1994.