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Tensile behaviour and fracture characteristics of die cast magnesium alloy AM50
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
journal homepage: www.elsevier.com/locate/jmatprotec
Tensile behaviour and fracture characteristics of die cast magnesium alloy AM50 Henry Hu a,∗ , Ming Zhou a , Zhizhong Sun a , Naiyi Li b a
Department of Mechanical, Automotive & Materials Engineering, University of Windsor, Windsor, Ontario, Canada b Manufacturing & Process Department, Ford Research and Advanced Engineering, Ford Motor Company Dearborn, MI 48121, USA
a r t i c l e
i n f o
a b s t r a c t
Keywords:
Most of automotive magnesium applications are presently produced by high-pressure die
Magnesium alloy AM50
casting processes. Understanding of tensile and fracture behaviours of die cast AM50 is
Die casting
critical for proper design of different applications. In the present study, magnesium alloy
Tensile and fracture behaviours
AM50 was high pressure die cast into rectangular coupons with section thicknesses of 2, 6 and 10 mm. Tensile and fracture behaviours of the die cast AM50 were characterized. The microstructure of die cast AM50 alloy was evaluated by optical microscopy. The results of tensile testing indicate that the tensile properties including yield strength (YS), ultimate tensile strength (UTS) and elongation (Ef ) decreases with increasing section thicknesses of die cast AM50. The analysis of tensile behaviour shows that the straining hardening rates during the plastic deformation of the alloy increase with decreasing the section thicknesses. The observation via SEM fractography illustrates that the fracture behaviour of die cast AM50 is influenced by section thicknesses. As the section thickness increases, the fracture of AM50 tends to transit from ductile to brittle mode due to arising porosity content and coarsening microstructure. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Magnesium usage in automobiles has arisen significantly due to consumer demands for increased performance and fuel economy of vehicles. Most magnesium applications presently used in the automotive industry are high-pressure die cast (HPDC), and have relatively good strength and high ductility at room temperature. Applications of HPDC magnesium alloys AM50, such as front-end support assemblies, steering wheel armatures and steering column support brackets (Powell et al., 2002), have not only complex shapes but also cross sections with various thicknesses. Very often, under normal die casting conditions, thick sections have a higher tendency to
∗
Corresponding author. Tel.: +1 519 253 3000; fax: +1 519 973 7007. E-mail address: [email protected] (H. Hu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.11.275
solidification shrinkage and porosity caused by inclusion of gas than thin walls. It has been indicated (Zhou et al., 2004; Gjestland et al., 2003; Zhou, 2004; Chadha et al., 2004; Dahle et al., 2001; Wang et al., 2003) that the porosity level of components can influence mechanical properties, such as ultimate tensile strength (UTS), 0.2% yield strength (YS) and elongation (Ef ). An increase to 10 mm from 2 mm in the section thickness of magnesium die castings reduces their properties significantly. This is attributed to the presence of a large amount of porosity and coarse microstructure resulting from high tendency of gas entrapment in thick magnesium coupons and low solidification rate during the high-pressure die casting process. However, detailed information on tensile and
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
365
Fig. 1 – Microstructure in the skin region of the die cast AM50 alloy with the section thicknesses: (a) 2, (b) 6, and (c) 10 mm.
fracture behaviour of die cast AM50 alloy with different section thicknesses is limited, which is essential to design magnesium components properly. This paper presents an in-depth analysis tensile and fracture behaviour of die cast AM50 alloy with section thicknesses of 2, 6 and 10 mm. The influence of section thicknesses on the tensile behaviour of the alloy was studied based on the analysis of true stress–strain relation. The fracture behaviour of the die cast AM50 affected by section thicknesses was characterized by using SEM fractography.
2.
Experimental procedures
2.1.
Alloy and casting preparation
The magnesium alloy selected in this study was die casting alloy AM50 (Mg–4.9 wt.%Al–0.39 wt.%–0.2 wt.%Zn). Flat rectangular coupons of 0.125 m × 0.027 m with different section thicknesses of 2 mm, 6 mm and 10 mm were die cast on a 700 tonnes cold chamber horizontal high-pressure die casting machine. Detailed die casting conditions were given in Zhou (2004).
2.2.
Tensile testing
The mechanical properties of the die cast AM50 alloys were evaluated by tensile testing, which was performed at ambient temperature on an Instron machine equipped with a computer data acquisition system. Following ASTM B557, subsize flat tensile specimens (25 mm in gage length, 6 mm in width, and 10 mm in as-cast thickness) were machined from the die cast coupons. The tensile properties, including 0.2% yield strength (YS), ultimate tensile strength (UTS), and elongation
to failure (Ef ), were obtained based on the average of three tests.
2.3. Characterization of microstructure and fractured surface Specimens were sectioned, mounted, and polished from the centre of the die cast coupons and prepared following the standard metallographic procedures. A Buehler optical image analyzer 2002 system was used to determine primary characteristics of the specimens. The fractured surfaces of tensile specimens were analyzed to ascertain the nature of fracture mechanisms by A JSM-5800LV scanning electron microscope (SEM) with a maximum resolution of 100 nm in a backscattered mode/1 m in X-ray diffraction mapping mode, and maximum useful magnification of 30,000.
3.
Results and discussion
3.1.
Microstructure
The microstructures of the skin region of the investigated high-pressure die cast AM50 alloy for 2, 6 and 10 mm section thicknesses are shown in Fig. 1. For all the three thicknesses, there is a fine microstructure skin in the specimens. However, the number of relatively large primary ␣-Mg solid solutions (bright phases) appeared in the thick specimens (6 and 10 mm) are higher than that in the 2 mm thick. Examination of the skin region reveals that there is little to no porosity and a fine and dense microstructure, that resulted from partial-solidification of the primary ␣-Mg phase in the shot sleeve prior to cavity filling and rapid cooling during solidification. Fig. 2 presents the optical microstructure in the center area of the investigated
Fig. 2 – Microstructure in the centre area of the die cast AM50 alloy with the section thicknesses: (a) 2, (b) 6, and (c) 10 mm.
366
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
Fig. 3 – . Typical true strain vs. stress curves for the die cast AM50 alloy.
alloy. It can be seen that the average size of the primary ␣-Mg solid solutions (dendrite) in the center of 6 and 10 mm thick specimens is large compared to that of 2 mm coupons due to rapid solidification taking place in the 2 mm sample. The microstructure observation is consistent with the findings by Dahle et al. (Dahle et al., 2001).
3.2.
Tensile behaviour
Fig. 3 shows representative true stress and strain curves of the die cast AM50 alloy. For all three-section thicknesses of specimens, the stress variation with the strain follows almost the same pattern. Under tensile loading, the alloy deformed elastically first. Once yield points reached, plastic deformation of the alloy set in. However, the fracture of 2 mm-thick specimens occurs at a much higher stress and elongation than that for 6 and 10 mm thick specimens. The variation of engineering tensile properties including UTS, YS and Ef with section thicknesses are compiled in Table 1. The UTS and YS decreases to 112 and 82 MPa for 10 mm thick specimens from 240 and 134 MPa for the 2 mm coupons, which implies over 50% reduction in UTS and almost 40% decrease in YS, respectively. Moreover, the elongation values, 11%, 6% and 2% for 2, 6, 10 mm, respectively, indicate evidently that a significant decrease in elongation occurs when the section thickness of specimens increases. The results of the current study are in good agreement with the relationship between tensile properties and section thicknesses for different types of die casting magnesium alloys reported in the literature (Mao et al., 1999; Schindelbacher and Rosch, 1998). As mentioned above, differences in the porosity level and the microstructure of die cast AM50 should be responsible for the deviation in strengths and elongation. The fine microstructure
Fig. 4 – Strain hardening rate vs. true strain for plastic deformation of the die cast AM50 alloy.
and low porosity level of thin specimens enhances their tensile properties. The relatively low strengths and elongations of thick specimens result from the coarse microstructure, thin skin layer, high porosity level in the center, and the presence of large pores. This experimental observation is consistent with the finding presented in Winston Sequeira (2000). Sequeira et al. (Winston Sequeira, 2000) investigated the skin effect in flat die castings by removing of the skin from 1 mm thick flat die cast AZ91D flat tensile specimens. The skin removal led to a considerable drop in yield strength from 185 to 159 MPa, which indicates the skin, at least partially responsible for its high tensile properties. The strain-hardening behaviours of the die cast AM50 alloys are illustrated in a plot of strain-hardening rate (d/dε) versus true plastic strain (ε) during the plastic deformation as shown in Fig. 4, which is derived from Fig. 3. It is evident that, despite of decreasing with an increase in true strain, the strain-hardening rates during the plastic deformation of the die cast alloys vary with their section thicknesses. As the section thickness decreases, the strain-hardening rates increase. This observation implies that, compared to the 6 and 10 mm-thick samples, the die cast AM50 alloy with the thin cross-section (2 mm) is capable of spontaneously strengthening itself increasingly to a large extent, in response to extensive plastic deformation prior to fracture. The low porosity level and the even dispersion of fine intermetallic particles inside grains and around ground boundaries, which resist slip in the primary phase should be responsible for the relatively high strain-hardening rate of the thin alloy in the early stage of plastic deformation, i.e., instantly after the onset of plastic flow as indicated in Fig. 4.
Table 1 – Engineering tensile properties of the die cast AM50 alloy at room temperature Section thickness (mm) 2 6 10
Ultimate tensile strength (UTS) (MPa) 240 188 112
Yield strength (YS) (MPa)
Elongation (Ef ) (%)
133 113 82
11 6 2
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
367
Fig. 5 – SEM fractographs showing the morphology of fractured surfaces: (a) 2, (b) 6, and (c) 10 mm-thick die cast coupons.
Fig. 6 – Optical micrograph showing microstructure underneath fractured surface of (a) 2, and (b) 10 mm thick coupons.
3.3.
Fracture characteristics
Examination of the fracture surfaces of tensile specimens via SEM manifests the fracture behaviour of die cast AM50 with three different thicknesses, which is shown in Fig. 5. The high magnification reveals detailed features of fracture surface and determines the manner where the primary crack originated. The analysis of SEM fractography shows that the fracture behaviour of die cast AM50 is influenced by the section thicknesses. As the section thickness increases, the fracture of AM50 tends to transit from ductile to brittle mode. The fracture surface of the 2 mm thick specimen illustrated in Fig. 5 is primarily ductile in nature, which is characterized
by the presence of deep dimples. The fractograph as portrays the dimples with extensive deformation marking along the walls of individual craters shown in Fig. 5(a). A considerable amount of energy is consumed in the process of the formation of microvoids and microvoid-sheet, eventually leading to the creation of cracks. Thus, this type of fracture failure results from the coalescence of microvoids under the tensile stress. It seems, however, that the failure of the 10 mm-thick specimen is caused by a combined brittle fracture mechanism of void coalescence and intergranular fracture. The similar mechanism for the fracture of die cast magnesium alloy AZ91D has also been reported in Luo et al. (1996). The initiation point of cracks began with the internal discontinuity due to the pres-
368
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
ence of porosity. The final fracture results from the growth and coalescence of the cracks. The brittle eutectic -Mg17 Al12 segregation along the grain boundaries should be the main cause of the intergranular fracture. The damaged microstructure underneath the fractured surfaces presented in Fig. 6, at least in part, supports this interpretation. Overall, the SEM observations on the fractured surfaces show a good agreement with the ductility data given in Fig. 3 and Table 1.
4.
Conclusions
The microstructure of the high-pressure die cast magnesium alloy AM50 is influenced by its section thickness based on optical metallography and SEM/EDS analysis. Low porosity levels, fine primary ␣-Mg dendrites and pores are present in the die cast AM50 with relatively thin cross-sections. The results of tensile testing indicate that the mechanical properties, UTS, YS, and Ef , increase significantly with a reduction in the section thickness of the alloy. The analysis of tensile behaviour reveals that, an increase in high strain-hardening rates of the alloy with decreasing the section thickness enables the alloy to spontaneously strengthen the materials increasingly to a large extent, in response to extensive plastic deformation prior to fracture. It seems that the section thicknesses determine porosity level and microstructure characteristics, and consequently dictate tensile behaviour of die cast magnesium alloy AM50. The observation via SEM fractography illustrates that the fracture behaviour of die cast AM50 is influenced by the section thickness. As the section thickness increases, the fracture of AM50 tends to transit from ductile to brittle mode. Cracks primarily initiate at the internal discontinuities due to the presence of porosity.
Acknowledgements The authors would like to take this opportunity to thank the Natural Sciences and Engineering Research Council of Canada for supporting this work. In addition, one of the co-authors, Naiyi Li, would like to thank the generous support from the management in Manufacturing Systems Department, Ford
Research and Advanced Engineering Laboratory, Ford Motor Company.
references
Chadha, G., Allison, J.E., Jones, J.W., 2004. The Role of Microstructure and Porosity in Ductility of Die Cast AM50 and AM60 Magnesium Alloys Magnesium Technology 2004. TMS, pp. 181–186. Dahle, A.K., Sannes, S., John, St.D.H., Westengen, H., 2001. Formation of defect bands in high pressure die cast magnesium alloys. J. Light Metals 1, 99–103. Gjestland, H.T., Sanners, S., Westengen, H., Albright, D., 2003. Effect of casting temperature section thickness and die filling sequence on microstructure and mechanical properties of high pressure die castings. NADCA Transactions, Indianapolis, pp. T03–036. Luo, A., Hu, H., Lo, S.H.J., 1996. Microstructure and mechanical properties of squeeze cast AZ91D magnesium alloy. In: Light Metals Symposium, CIM, Montreal, pp. 375–387. Mao, H., Brevick, J., Mobley, C., Chandrasekar, V., 1999. Effects of Section Size and Microstructural Features on the Mechanical Properties of Die Cast AZ91D and AM60B Magnesium Alloy Test Bars, SAE Technical Paper Series, 1999-01-0927. Powell, B.R., Rezhets, V., Balogh, M.P., Waldo, R.A., 2002. Microstructure and Creep Behavior in AE42 Magnesium Die-casting Alloy, JOM, TMS, 34–38. Schindelbacher, G., Rosch, R., 1998. Mechanical Properties of Magnesium Die Casting Alloys at Elevated Temperatures and Microstructure in Dependence of Wall Thickness Magnesium Alloys and Their Applications. Werkstoff-Informationsgesellschaft mbH, pp. 247–252. Wang, R.M., Eliezer, A., Gutman, E.M., 2003. An investigation on the microstructure of an AM50 magnesium alloy. Mater. Sci. Eng. A335, 201–207. Winston Sequeira, 2000. The Microstructure and Mechanical Properties of High Pressure Diecast Magnesium Alloy AZ91D, The University of Queensland, Brisbane, Australia, PhD Thesis, pp. 142. Zhou, M., 2004. An Experimental Study of Die and Squeeze Cast Magnesium Alloy AM50, Master Thesis, University of Windsor, Windsor, Ontario, Canada. Zhou, M., Li, N., Hu, H., 2004. Effect of section thicknesses on tensile behavior and microstructure of high pressure die cast magnesium alloy AM50. Mater. Sci. Forum 476, 463–468.
journal homepage: www.elsevier.com/locate/jmatprotec
Tensile behaviour and fracture characteristics of die cast magnesium alloy AM50 Henry Hu a,∗ , Ming Zhou a , Zhizhong Sun a , Naiyi Li b a
Department of Mechanical, Automotive & Materials Engineering, University of Windsor, Windsor, Ontario, Canada b Manufacturing & Process Department, Ford Research and Advanced Engineering, Ford Motor Company Dearborn, MI 48121, USA
a r t i c l e
i n f o
a b s t r a c t
Keywords:
Most of automotive magnesium applications are presently produced by high-pressure die
Magnesium alloy AM50
casting processes. Understanding of tensile and fracture behaviours of die cast AM50 is
Die casting
critical for proper design of different applications. In the present study, magnesium alloy
Tensile and fracture behaviours
AM50 was high pressure die cast into rectangular coupons with section thicknesses of 2, 6 and 10 mm. Tensile and fracture behaviours of the die cast AM50 were characterized. The microstructure of die cast AM50 alloy was evaluated by optical microscopy. The results of tensile testing indicate that the tensile properties including yield strength (YS), ultimate tensile strength (UTS) and elongation (Ef ) decreases with increasing section thicknesses of die cast AM50. The analysis of tensile behaviour shows that the straining hardening rates during the plastic deformation of the alloy increase with decreasing the section thicknesses. The observation via SEM fractography illustrates that the fracture behaviour of die cast AM50 is influenced by section thicknesses. As the section thickness increases, the fracture of AM50 tends to transit from ductile to brittle mode due to arising porosity content and coarsening microstructure. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Magnesium usage in automobiles has arisen significantly due to consumer demands for increased performance and fuel economy of vehicles. Most magnesium applications presently used in the automotive industry are high-pressure die cast (HPDC), and have relatively good strength and high ductility at room temperature. Applications of HPDC magnesium alloys AM50, such as front-end support assemblies, steering wheel armatures and steering column support brackets (Powell et al., 2002), have not only complex shapes but also cross sections with various thicknesses. Very often, under normal die casting conditions, thick sections have a higher tendency to
∗
Corresponding author. Tel.: +1 519 253 3000; fax: +1 519 973 7007. E-mail address: [email protected] (H. Hu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.11.275
solidification shrinkage and porosity caused by inclusion of gas than thin walls. It has been indicated (Zhou et al., 2004; Gjestland et al., 2003; Zhou, 2004; Chadha et al., 2004; Dahle et al., 2001; Wang et al., 2003) that the porosity level of components can influence mechanical properties, such as ultimate tensile strength (UTS), 0.2% yield strength (YS) and elongation (Ef ). An increase to 10 mm from 2 mm in the section thickness of magnesium die castings reduces their properties significantly. This is attributed to the presence of a large amount of porosity and coarse microstructure resulting from high tendency of gas entrapment in thick magnesium coupons and low solidification rate during the high-pressure die casting process. However, detailed information on tensile and
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
365
Fig. 1 – Microstructure in the skin region of the die cast AM50 alloy with the section thicknesses: (a) 2, (b) 6, and (c) 10 mm.
fracture behaviour of die cast AM50 alloy with different section thicknesses is limited, which is essential to design magnesium components properly. This paper presents an in-depth analysis tensile and fracture behaviour of die cast AM50 alloy with section thicknesses of 2, 6 and 10 mm. The influence of section thicknesses on the tensile behaviour of the alloy was studied based on the analysis of true stress–strain relation. The fracture behaviour of the die cast AM50 affected by section thicknesses was characterized by using SEM fractography.
2.
Experimental procedures
2.1.
Alloy and casting preparation
The magnesium alloy selected in this study was die casting alloy AM50 (Mg–4.9 wt.%Al–0.39 wt.%–0.2 wt.%Zn). Flat rectangular coupons of 0.125 m × 0.027 m with different section thicknesses of 2 mm, 6 mm and 10 mm were die cast on a 700 tonnes cold chamber horizontal high-pressure die casting machine. Detailed die casting conditions were given in Zhou (2004).
2.2.
Tensile testing
The mechanical properties of the die cast AM50 alloys were evaluated by tensile testing, which was performed at ambient temperature on an Instron machine equipped with a computer data acquisition system. Following ASTM B557, subsize flat tensile specimens (25 mm in gage length, 6 mm in width, and 10 mm in as-cast thickness) were machined from the die cast coupons. The tensile properties, including 0.2% yield strength (YS), ultimate tensile strength (UTS), and elongation
to failure (Ef ), were obtained based on the average of three tests.
2.3. Characterization of microstructure and fractured surface Specimens were sectioned, mounted, and polished from the centre of the die cast coupons and prepared following the standard metallographic procedures. A Buehler optical image analyzer 2002 system was used to determine primary characteristics of the specimens. The fractured surfaces of tensile specimens were analyzed to ascertain the nature of fracture mechanisms by A JSM-5800LV scanning electron microscope (SEM) with a maximum resolution of 100 nm in a backscattered mode/1 m in X-ray diffraction mapping mode, and maximum useful magnification of 30,000.
3.
Results and discussion
3.1.
Microstructure
The microstructures of the skin region of the investigated high-pressure die cast AM50 alloy for 2, 6 and 10 mm section thicknesses are shown in Fig. 1. For all the three thicknesses, there is a fine microstructure skin in the specimens. However, the number of relatively large primary ␣-Mg solid solutions (bright phases) appeared in the thick specimens (6 and 10 mm) are higher than that in the 2 mm thick. Examination of the skin region reveals that there is little to no porosity and a fine and dense microstructure, that resulted from partial-solidification of the primary ␣-Mg phase in the shot sleeve prior to cavity filling and rapid cooling during solidification. Fig. 2 presents the optical microstructure in the center area of the investigated
Fig. 2 – Microstructure in the centre area of the die cast AM50 alloy with the section thicknesses: (a) 2, (b) 6, and (c) 10 mm.
366
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
Fig. 3 – . Typical true strain vs. stress curves for the die cast AM50 alloy.
alloy. It can be seen that the average size of the primary ␣-Mg solid solutions (dendrite) in the center of 6 and 10 mm thick specimens is large compared to that of 2 mm coupons due to rapid solidification taking place in the 2 mm sample. The microstructure observation is consistent with the findings by Dahle et al. (Dahle et al., 2001).
3.2.
Tensile behaviour
Fig. 3 shows representative true stress and strain curves of the die cast AM50 alloy. For all three-section thicknesses of specimens, the stress variation with the strain follows almost the same pattern. Under tensile loading, the alloy deformed elastically first. Once yield points reached, plastic deformation of the alloy set in. However, the fracture of 2 mm-thick specimens occurs at a much higher stress and elongation than that for 6 and 10 mm thick specimens. The variation of engineering tensile properties including UTS, YS and Ef with section thicknesses are compiled in Table 1. The UTS and YS decreases to 112 and 82 MPa for 10 mm thick specimens from 240 and 134 MPa for the 2 mm coupons, which implies over 50% reduction in UTS and almost 40% decrease in YS, respectively. Moreover, the elongation values, 11%, 6% and 2% for 2, 6, 10 mm, respectively, indicate evidently that a significant decrease in elongation occurs when the section thickness of specimens increases. The results of the current study are in good agreement with the relationship between tensile properties and section thicknesses for different types of die casting magnesium alloys reported in the literature (Mao et al., 1999; Schindelbacher and Rosch, 1998). As mentioned above, differences in the porosity level and the microstructure of die cast AM50 should be responsible for the deviation in strengths and elongation. The fine microstructure
Fig. 4 – Strain hardening rate vs. true strain for plastic deformation of the die cast AM50 alloy.
and low porosity level of thin specimens enhances their tensile properties. The relatively low strengths and elongations of thick specimens result from the coarse microstructure, thin skin layer, high porosity level in the center, and the presence of large pores. This experimental observation is consistent with the finding presented in Winston Sequeira (2000). Sequeira et al. (Winston Sequeira, 2000) investigated the skin effect in flat die castings by removing of the skin from 1 mm thick flat die cast AZ91D flat tensile specimens. The skin removal led to a considerable drop in yield strength from 185 to 159 MPa, which indicates the skin, at least partially responsible for its high tensile properties. The strain-hardening behaviours of the die cast AM50 alloys are illustrated in a plot of strain-hardening rate (d/dε) versus true plastic strain (ε) during the plastic deformation as shown in Fig. 4, which is derived from Fig. 3. It is evident that, despite of decreasing with an increase in true strain, the strain-hardening rates during the plastic deformation of the die cast alloys vary with their section thicknesses. As the section thickness decreases, the strain-hardening rates increase. This observation implies that, compared to the 6 and 10 mm-thick samples, the die cast AM50 alloy with the thin cross-section (2 mm) is capable of spontaneously strengthening itself increasingly to a large extent, in response to extensive plastic deformation prior to fracture. The low porosity level and the even dispersion of fine intermetallic particles inside grains and around ground boundaries, which resist slip in the primary phase should be responsible for the relatively high strain-hardening rate of the thin alloy in the early stage of plastic deformation, i.e., instantly after the onset of plastic flow as indicated in Fig. 4.
Table 1 – Engineering tensile properties of the die cast AM50 alloy at room temperature Section thickness (mm) 2 6 10
Ultimate tensile strength (UTS) (MPa) 240 188 112
Yield strength (YS) (MPa)
Elongation (Ef ) (%)
133 113 82
11 6 2
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
367
Fig. 5 – SEM fractographs showing the morphology of fractured surfaces: (a) 2, (b) 6, and (c) 10 mm-thick die cast coupons.
Fig. 6 – Optical micrograph showing microstructure underneath fractured surface of (a) 2, and (b) 10 mm thick coupons.
3.3.
Fracture characteristics
Examination of the fracture surfaces of tensile specimens via SEM manifests the fracture behaviour of die cast AM50 with three different thicknesses, which is shown in Fig. 5. The high magnification reveals detailed features of fracture surface and determines the manner where the primary crack originated. The analysis of SEM fractography shows that the fracture behaviour of die cast AM50 is influenced by the section thicknesses. As the section thickness increases, the fracture of AM50 tends to transit from ductile to brittle mode. The fracture surface of the 2 mm thick specimen illustrated in Fig. 5 is primarily ductile in nature, which is characterized
by the presence of deep dimples. The fractograph as portrays the dimples with extensive deformation marking along the walls of individual craters shown in Fig. 5(a). A considerable amount of energy is consumed in the process of the formation of microvoids and microvoid-sheet, eventually leading to the creation of cracks. Thus, this type of fracture failure results from the coalescence of microvoids under the tensile stress. It seems, however, that the failure of the 10 mm-thick specimen is caused by a combined brittle fracture mechanism of void coalescence and intergranular fracture. The similar mechanism for the fracture of die cast magnesium alloy AZ91D has also been reported in Luo et al. (1996). The initiation point of cracks began with the internal discontinuity due to the pres-
368
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 364–368
ence of porosity. The final fracture results from the growth and coalescence of the cracks. The brittle eutectic -Mg17 Al12 segregation along the grain boundaries should be the main cause of the intergranular fracture. The damaged microstructure underneath the fractured surfaces presented in Fig. 6, at least in part, supports this interpretation. Overall, the SEM observations on the fractured surfaces show a good agreement with the ductility data given in Fig. 3 and Table 1.
4.
Conclusions
The microstructure of the high-pressure die cast magnesium alloy AM50 is influenced by its section thickness based on optical metallography and SEM/EDS analysis. Low porosity levels, fine primary ␣-Mg dendrites and pores are present in the die cast AM50 with relatively thin cross-sections. The results of tensile testing indicate that the mechanical properties, UTS, YS, and Ef , increase significantly with a reduction in the section thickness of the alloy. The analysis of tensile behaviour reveals that, an increase in high strain-hardening rates of the alloy with decreasing the section thickness enables the alloy to spontaneously strengthen the materials increasingly to a large extent, in response to extensive plastic deformation prior to fracture. It seems that the section thicknesses determine porosity level and microstructure characteristics, and consequently dictate tensile behaviour of die cast magnesium alloy AM50. The observation via SEM fractography illustrates that the fracture behaviour of die cast AM50 is influenced by the section thickness. As the section thickness increases, the fracture of AM50 tends to transit from ductile to brittle mode. Cracks primarily initiate at the internal discontinuities due to the presence of porosity.
Acknowledgements The authors would like to take this opportunity to thank the Natural Sciences and Engineering Research Council of Canada for supporting this work. In addition, one of the co-authors, Naiyi Li, would like to thank the generous support from the management in Manufacturing Systems Department, Ford
Research and Advanced Engineering Laboratory, Ford Motor Company.
references
Chadha, G., Allison, J.E., Jones, J.W., 2004. The Role of Microstructure and Porosity in Ductility of Die Cast AM50 and AM60 Magnesium Alloys Magnesium Technology 2004. TMS, pp. 181–186. Dahle, A.K., Sannes, S., John, St.D.H., Westengen, H., 2001. Formation of defect bands in high pressure die cast magnesium alloys. J. Light Metals 1, 99–103. Gjestland, H.T., Sanners, S., Westengen, H., Albright, D., 2003. Effect of casting temperature section thickness and die filling sequence on microstructure and mechanical properties of high pressure die castings. NADCA Transactions, Indianapolis, pp. T03–036. Luo, A., Hu, H., Lo, S.H.J., 1996. Microstructure and mechanical properties of squeeze cast AZ91D magnesium alloy. In: Light Metals Symposium, CIM, Montreal, pp. 375–387. Mao, H., Brevick, J., Mobley, C., Chandrasekar, V., 1999. Effects of Section Size and Microstructural Features on the Mechanical Properties of Die Cast AZ91D and AM60B Magnesium Alloy Test Bars, SAE Technical Paper Series, 1999-01-0927. Powell, B.R., Rezhets, V., Balogh, M.P., Waldo, R.A., 2002. Microstructure and Creep Behavior in AE42 Magnesium Die-casting Alloy, JOM, TMS, 34–38. Schindelbacher, G., Rosch, R., 1998. Mechanical Properties of Magnesium Die Casting Alloys at Elevated Temperatures and Microstructure in Dependence of Wall Thickness Magnesium Alloys and Their Applications. Werkstoff-Informationsgesellschaft mbH, pp. 247–252. Wang, R.M., Eliezer, A., Gutman, E.M., 2003. An investigation on the microstructure of an AM50 magnesium alloy. Mater. Sci. Eng. A335, 201–207. Winston Sequeira, 2000. The Microstructure and Mechanical Properties of High Pressure Diecast Magnesium Alloy AZ91D, The University of Queensland, Brisbane, Australia, PhD Thesis, pp. 142. Zhou, M., 2004. An Experimental Study of Die and Squeeze Cast Magnesium Alloy AM50, Master Thesis, University of Windsor, Windsor, Ontario, Canada. Zhou, M., Li, N., Hu, H., 2004. Effect of section thicknesses on tensile behavior and microstructure of high pressure die cast magnesium alloy AM50. Mater. Sci. Forum 476, 463–468.