Effects of Shrinkage Porosity on Mechanical Properties of
a Sand Cast Mg-Y-RE (WE54) Alloy
Jilin Li1, a, Yuequn Ma1,b, Rongshi Chen1,*and Wei Ke1,c
1State Key Laboratory for Corrosion and Protection, Institute of Metal Rearch, Chine Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
a jlli@imr.ac,
冬字开头的成语b yqma@imr.ac, *rschen@imr.ac,
c wke@imr.ac Keywords: Shrinkage porosity, Mechanical property, We54 alloy, San
d cast
Abstract. The distribution of shrinkage porosities in sand cast Mg-Y-RE (WE54) alloy castings was char
acterized through density measurement and calculated by Archimedes’s principle. The effect of porosity on mechanical properties of sand cast WE54 alloy was investigated through tensile tests and microstructure obrvation. It was found that the shrinkage porosities distributed mainly in the middle of the plate where the liquid feeding was quite inconvenient. And the porosities were formed along grain boundaries when condary phas formed at the end of solidification. Hardness tests showed that the vikers hardness declined linearly with increasing porosity volume fraction. While the tensile strength and nominal yield strength declined exponentially as the porosity volume fraction incread. Microstructure obrvation showed that the fracture cracks propagated along the grain boundaries where porosities and condary phas gathering together in as-cast WE54 alloy. The tiny porosities distributed in the condary phas were obrved, which could reduce the tensile strength of cast specimens significantly. The heat treatment strengthening effects were significantly weakened by porosities, and even no heat treatment strengthening effect was detected when the porosity volume fraction was higher than 1%. The microstructure obrvation also proved that no heat treatment strengthening effect existed in samples containing porosities.
Introduction
Magnesium alloys exhibit low densities and high specific strengths, which benefit the application of
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magnesium alloys in automotive and aerospace industries. Up to now, Mg alloys containing heavy rare earth elements have the best combination of mechanical properties and corrosion resistance. As a typical reprentative of the alloys, Mg-Y-RE (WE54) alloy posss excellent mechanical properties at both room temperature and elevated temperature. However, casting defects like shrinkage porosity significantly limits the application of WE54 alloy on industrial products.
Currently, casting is still the main industrial forming method for magnesium alloys. Although magnesium alloys have been applied widely in aerospace, automobile and tele-communication industries, the lag of rearch and development on casting technology is still a bottleneck for their further application [6]. Shrinkage porosity is a common and vere defect occurring during solidification in castings and it is perhaps the pivotal issue determining an alloy’s castability. The formation of shrinkage porosity has been studied extensively in steel and Al alloys, and various methods have been developed for its characterization [7-10]. Lee et al investigated the effects of micro-voids on the tensile property of die-cast AM60B and AZ91D magnesium alloy, and found that the yield strengths of the AM60B and AZ91D alloys exhibited a linear dependence upon the variation of microporosity, while the UTS and elongation of both alloys had a strong dependence upon the variation of microporosity with an inver parabolic relationship [11, 12].
In the prent work, the distribution of shrinkage porosities in sand cast WE54 alloy castings was characterized through density measurement and calculated by Archimedes’s principle. And the effect of porosity on mechanical properties and hot treatment respons of sand cast WE54 alloy was investigated through tensile tests and microstructure obrvation. And the reliability of resin sand casting process on WE54 alloy castings production was discusd.
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Experiment procedures
The WE54 alloys was prepared by melting high purity Mg (>99.95%), Gd (>99%), Y (>99%), Nd (>99%) and a Mg-30wt%Zr master alloy in an electric resistance furnace at 780℃~800℃ under protection of an RJ6 flux. The melt was then poured into resin sand moulds at about 780℃ to cast the plate ingots with size of 300*150*40mm3 and 200*150*40mm3, respectively. The casting process was schematically shown in fig.1. The chemical composition of the ingots was determined by inductively coupled plasma atomic emission spectroscopy (ICP) and was shown in table 1. Speci
mens cut from the cast ingots were solution treated at 525 ℃ for 6 h, quenched into water at about 25℃ and then subquently aged at 250℃ for 16h.
Table 1 Chemical composition of WE54 alloy
Element Y Nd Gd Zr Mg
Content (wt. %) 5.02 1.92 2.13 0.45 Balance.
耳目一新什么意思Content (at. %) 1.46 0.35 0.34 0.13 Balance.
(b)the resin sand moulds (c) physical image of the castings.
Then the porosity volume fraction distribution in the plate castings was characterized through density measurement and calculated by Eq. 1 and Eq. 2 [10]:
()00l A A B ρρρρ=−+− (1)
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t
P t f ρρρ−= (2) Wh
ere ρ, ρ0, ρl and ρt reprent the density of the sample, density of the air (0.0012g/cm 3), density of the liquid (alcohol) and the theoretical density of the WE54 alloy (the maximum measured density was chon as the theoretical density in the prent rearch), A and B reprent the sample mass measured in the air and the sample mass measured in the alcohol, f P is the porosity volume fraction of the sample.
Microstructures were obrved by optical microscope (OM) and scanning electron microscope (SEM, Philips XL30 ESEM-FEG/EDAX). Samples for optical microscopy were etched in a solution of 5vol. % HNO3 in ethanol after mechanical polishing. No chemical etching was applied to specimens for SEM investigations. Vickers hardness testing was performed using 500 g load with a holding time of 15s. The samples for tensile tests with a gauge length of 10 mm, width of 3.5 mm and thickness of 2 mm were cut by an electric-sparking wire-cutting machine from cast ingots. Tensile tests were conducted at room temperature with an initial strain rate of 10−3 s −1 on a universal testing machine. Four specimens were ud for each test condition to ensure the reliability of data.
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Results and discussions健康饮食的英文
Porosity distribution investigation. The distribution of shrinkage porosities in sand cast Mg-Y-RE (WE54) alloy castings was characterized through density measurement and calculated by Archimedes’s principle. And the calculated porosity volume fraction results were shown in fig.2. It was found that the shrinkage porosities distributed mainly in the middle of the plate where the liquid feeding is quite inconvenient. While some differences existed between the two plates, the porosities distributed much more concentrated in the 200*150*40mm3 plate, and the maximum porosity volume fraction was 0.06. While the porosity distributes in a larger zone in the 300*150*40mm3 plate with a maximum porosity volume fraction of 0.05, which was lower than that of the 200*150*40mm3 plate.
Fig. 2 Color map showing measured porosity distribution in the two casting plates
(rir located on the left side of the plate)
Fig. 3 Locations of metallography samples ctioned from the 200*150*40mm 3 plate.
The samples for microstructure obrvation were taken from the plates along their centerlines, and f
our samples referred as A, B, C and D were ctioned from different parts of the plates as marked in Fig.3. The two plates exhibited basically same microstructure. Fig.4 shows the microstructures for the different positions of the 200*150*40mm 3 plate. It was found that the microstructure of sand cast WE54 alloy was basically compod of equiaxed grains and a small amount of condary pha
distributed along grain boundaries. The microstructure changed significantly along the centerline of the plate. On the edge of the plate, the structure was much finer than in the middle of the plate owing to the higher cooing rate on the edge. And at the center of the plate, a certain amount of shrinkage porosities were obrved, as shown in Fig.4(C). The porosities distributed mainly along grain boundaries and always stayed in the condary pha as shown in Fig.5, indicating that the shrinkage porosities were formed along grain boundaries when condary phas formed at the end of solidification procedure.
Fig. 4 Microstructures of samples taken from the 200*150*40mm 3 plate along the centreline: (a) on the left edge (fp=0), (b) right under the rir (fp=0), (c) on the most vere part of the plate (fp=0.06),
(d) on the right edge (fp=0).
Fig.5 Shrinkage porosity distributed in the condary pha.
Mechanical properties of sand cast WE54 alloy. In order to investigate the effects of porosity volume fraction on the mechanical properties of sand cast WE54 alloy, specimens with different porosity volume fractions were chon from the casting plates. And the micro-hardness and tensile properties were tested on the specimens. And the effect of porosity on mechanical properties of sand cast WE54 alloy was investigated through tensile tests and microstructure obrvations. Besides, specimens with different porosity volume fractions were heat treated by T4 (525 ℃×6 h, water quenched) and T6 (525 ℃×6 h, water quenched, subquently aged at 250℃ for 16h) treatments in order to investigate the heat treatment effect of the specimens. And the fracture surface of the samples was obrved through SEM as well.
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V i c k e r s h a r d n e s s Porosity volume fraction (%) U T S (M p a )Porosity volume fraction (%)E l . (%)
Fig.6 Mechanical properties of sand cast WE54 alloy with different
shrinkage porosity volume fractions.
The micro-hardness and tensile test results of as-cast alloy with different porosity volume fractions were shown in fig.6. Hardness tests showed that the vikers hardness declined linearly from 85HV to 65HV with increasing porosity volume fraction. While both the tensile strength and nominal yield strength declined exponentially as the porosity volume fraction increas, which is different with results of die-cast AZ91 and AM60 alloys [11, 12]. The tensile strength and yield strength of porosity-free alloy were 195MPa and 163MPa, respectively. And the tensile strength and yield strength of sand cast WE54 alloy dropped to 150MPa and 140MPa as the porosity volume fraction incread to about 1.0%. The elongation also declined exponentially from 2.0% to about 0.7% with increasing porosity volume fraction.
Microstructure obrvation in fig.7 showed that the fracture cracks propagated along grain boundaries where porosities and condary phas gathered together in cast WE54 alloy, left broken bridgings on the fracture surface and fragments of condary pha near the fracture surface. And the tiny porosities distributed in the condary pha shown in fig.5 were believed to reduce significantly the tensile strength of cast specimens.
Fig.7 Image (SEM) showing (a)broken bridgings on the fracture surface and (b)fragments of
condary pha near the fracture surface.
After T4 treatment, the tensile strength of porosity-free alloy incread to 225MPa, while the yield strength decread to 151MPa. And as the porosity volume fraction incread, both the tensile strength and the yield strength decread significantly. When the porosity volume fraction incread to 1.0%, the tensile strength and yield strength of solution-treated WE54 alloy dropped down to 160MPa and 130MPa, indicating that solution treatment did not strengthen the as-cast alloy. Besides, the elongation of solution–treated alloy also declined exponentially from 10% to about 2% as the porosity volume fraction incread. After aging treatment, the tensile strength and yield strength of
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porosity-free alloy incread to 261.7MPa and 208.9MPa, and decread rapidly as the porosity volume fraction incread. When the porosity volume fraction incread to 1.0%, the tensile strength and yield strength of solution-treated WE54 alloy dropped to 163.6MPa and 146.5MPa, which showe
d no age hardening respon either. The elongation of peak-aged alloy also declined exponentially from 2.3% to about 1.2% as the porosity volume fraction incread. All the above results indicated that the heat treatment strengthening effects were significantly weakened by shrinkage porosities and even no heat treatment strengthening was detected when the porosity volume fraction was higher than 1%.
U T S (M p a )Porosity volume fraction (%)
E l . (%) U T S (M p a )Porosity volume fraction (%)E l . (%)
Fig.8 Effect of shrinkage porosity volume fraction on hot treatment respon of WE54 alloy.
Fig. 9 Secondary electron images showing the fracture morphlogy:(a) as-cast, without porosity,
(b) solution treated, without porosity, (c) peak-aged, without porosity, (d) as-cast, with porosity,
(e) solution treated, with porosity, (f) peak-aged, with porosity.
The SEM appearance of fractures were obrved and shown in fig.9. It was en that the fracture characteristics of porosity-free samples changed significantly with different hot treatment conditions, while no such changes were obrved in the samples containing a certain amount of porosities. Fig.10 showed the backscatter electron images of the fracture surface. Homogeneously distributed condary pha can be detected on the fracture surface of as-cast WE54 alloy in Fig.10 (a), and only a few condary pha was obrved on the fracture surface of solution treated and peak-aged WE54 alloy, as shown in Fig.10 (b) and (c). In contrast, no such change was obrved in samples containing shrinkage porosities, as shown in Fig.10 (d), (e) and (f). The results also suggested tha
t the heat treatment strengthening effects were significantly weakened by shrinkage porosities. (a)
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