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Hot-tearing susceptibility of Mg–9Al–xZn alloy
December 2002
Materials Letters 57 (2002) 929 – 934 www.elsevier.com/locate/matlet
Hot-tearing susceptibility of Mg–9Al–xZn alloy Yeshuang Wang *, Qudong Wang, Guohua Wu, Yanping Zhu, Wenjiang Ding School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 13 September 2001; received in revised form 28 April 2002; accepted 30 April 2002
Abstract Hot-tearing susceptibility of Mg – 9Al – xZn alloys is studied. Crack-ring molds were used to test the hot-tearing susceptibility. In situ thermal analysis, optical microscopy, scanning electron microscopy (SEM) and line scan were used to examine the hot-tearing behavior. Hot-tearing originates along the grains at the end of solidification. With Zn additions, the quantity of phase with low melting point in grain boundaries is increased, its melting point is decreased and the hot-tearing susceptibility is increased. The segregation of Zn element in grain boundaries is the main contribution to the high hot-tearing susceptibility of Mg – 9Al – xZn alloys. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mg – Al Alloy; Zn; Hot-tearing; Hot-tearing susceptibility
1. Introduction Mg – Al –Zn-based alloys are the most commonly used magnesium alloys for structural components. Mg – 9Al – 1Zn alloy AZ91 is the workhorse alloy for casting applications as it exhibits good mechanical and physical properties in combination with excellent castability and salt-water corrosion resistance [1]. Al has the most favorable effect on magnesium of any of the alloying elements. It improves strength and hardness, and it widens the freezing range and makes the alloy easier to cast, when present in amounts in excess of 6 wt.%, and the alloy becomes heat treatable. An aluminum content of 9% yields optimum strength. Zinc is next to aluminum in effectiveness as an alloying ingredient in magnesium. Zinc is often used in *
Corresponding author. Tel.: +86-21-62932164-122; fax: +8621-62932548. E-mail address: [email protected] (Y. Wang).
combination with aluminum to produce improvement in room-temperature strength. Zinc also helps overcome the harmful corrosive effect of iron and nickel impurities that might be present in the magnesium alloy [2]. However, it increases hot shortness when added in amounts greater than 1 wt.%. Casting is developed to be complex and thin in structure. At the same time, the hot-tearing problem is becoming more and more serious during the production of these castings. To a large degree, the tendency for hot-tearing limits the size and shape of the alloy [3]. Therefore, it is necessary to find viable ways to control the hottearing default efficiently.
2. Experimental Mg –9Al– xZn (9% Al, 0,0.2,0.4,0.6,0.8,1.0 Zn, all wt.%) was studied. Hot-tearing susceptibility is investigated using crack-ring molds, which is shown in
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 9 8 - 4
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Y. Wang et al. / Materials Letters 57 (2002) 929–934
was cut from the center of the sample, and the microstructure of the alloys was analyzed by an optical microscopy and an electron probe microanalyzer.
3. Results
Fig. 1. Mold of crack-rings.
Fig. 1. A round mold with diameter of 108 mm is cast. Chills are used to regulate the sequence of solidification. Round steel is located in the center of mold to hamper the solidification shrinkage of alloys, the diameter of which is proportional to the degree of hindrance to alloy. Therefore, the critical diameter, at which the sample of crack-rings begins to crack, is inversely proportional to the hot-tearing susceptibility of magnesium alloys. The diameter of the round steel is at the interval of 5 mm in this study, such as 98,93. . .. Hot-tearing susceptibility coefficient (HSC) is adopted, which can be expressed as: HSC ¼
108 Dcrit 108
The testing results of crack-rings are shown in Fig. 2. It can be seen that hot-tearing susceptibility coefficient (HSC) increases with Zn additions. SEM fractographs of hot-tearing fracture surfaces of Mg – 9Al and Mg –9Al – 0.8Zn are shown in Fig. 3a,b. They suggest that the surfaces are very smooth and the grains are evident. Fig. 4 is the optical microscopy of cracks originating after solute heat treatment (T4). It suggests clearly that hot-tearing originates along the grains. Cooling curve analysis shows that the start-solidification temperature is about 592 jC, and the practical end-solidification temperature is about 430 jC, much lower than the solidus of Mg – 9Al (Fig. 5). With Zn additions, the liqudus has few changes, but solidus evidently decreases (Fig. 6). As-cast microstructure of Mg –9Al and Mg – 9Al – 1.2Zn (Fig. 7a,b) shows that the quantity of phase in grain boundaries of Mg –9Al – 1.2Zn alloy is much more than that of Mg –9Al alloy. The segregation of Mg –9Al and Mg – 9Al –0.8Zn was analyzed with line scanning (Fig. 8). Because of content segregation, the content of Al in grain boun-
ð1Þ
where Dcrit is the critical diameter of the round steel, at which the sample of crack-rings begins to crack. Fracture surfaces of the cracked crack-rings sample were analyzed with scanning electron microscopy (SEM). The part including the crack origin was cut, treated with solute heat treatment (T4), and optically analyzed. Cooling curves were also investigated (samples were cast in sand mold). The specimen
Fig. 2. The hot-tearing susceptibility of Mg – 9Al – xZn alloys.
Y. Wang et al. / Materials Letters 57 (2002) 929–934
931
Fig. 3. SEM fractographs for the surface of the specimen cracked in crack-rings tests (a: Mg – 9Al, b: Mg – 9Al – 0.8Zn).
It is well understood that hot-tearing originates at the end of solidification while there is a thin liquid
film in grain boundaries. The strength has been built up, but the ductility is still zero. Tensile stress caused by solidification shrinkage could separate grains. Hot tears then take place easily under a small strain, when the liquid film in grain boundaries is thin enough to resist feeding of the surrounding liquid through the grain boundaries [4]. During the solidification of Mg – 9Al –xZn alloys, Zn and Al were enriched in grain boundaries caused by inter-crystalline segregation. The enrichment of Al
Fig. 4. Optical microscopy fractographs of tears origin from crackrings sample after solute heat treatment (Mg – 9Al – 0.8Zn).
Fig. 5. Cooling curves analysis of Mg – 9Al and Mg – 9Al – 1.2Zn alloys casted in sand mold.
daries is higher than that in grains. The Zn added is enriched in grain boundaries.
4. Discussion 4.1. Mechanism
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Fig. 6. The end-solidifying temperature of Mg – 9Al – xZn alloys with Zn additions temperature.
Increasing the quantity of low melting-point liquid in grain boundaries must decrease the adhesiveness among the grains (dendrites), and make it easier to be separated by external solidification stress. On the other hand, the remaining liquid in grain boundaries solidifies as eutectic reaction. It must lead to shock shrinkage at the end of solidification. With the increasing quantity of low melting-point liquid (with approximately the eutectic composition), the solidifying shrinkage and solidifying stress must be increased. Decreasing the melting point of liquid film in grain boundaries must prolong the time in the vulnerable region. According to the testing results, the end-solidifying temperature of Mg – 9Al –xZn alloy is lowered
and Zn lowered the melting point of alloys. As a result, the time of liquid film existing in grain boundaries was prolonged. According to the theory of Clyne [5], cracking susceptibility coefficient (CSC) can be expressed as: CSC ¼
tV tR
ð2Þ
where tV is the vulnerable time period and tR is the time available for stress relief processes (Fig. 9). Mass and liquid feeding 0:1 < fL < 0:6 Interdendritic seperation 0:01 < fL < 0:1 where fL is the liquid fraction. The liquid film enriched with Al and Zn lowered the melting point of alloys, and so the period of liquid film existing is prolonged. Therefore, the cracking susceptibility coefficient (CSC) is increased by the inter-crystalline segregation of Zn and Al. 4.2. Effects of Zn additions During the solidification of Mg – 9Al –xZn, there are two principal influences of Zn on the alloys, increasing the quantity of eutectic in grain boundaries and decreasing the end-solidifying temperature of the alloys.
Fig. 7. As-cast structure of (a) Mg – 9Al, (b) Mg – 9Al – 1.2Zn casted in sand mold.
Y. Wang et al. / Materials Letters 57 (2002) 929–934
933
Fig. 8. Elemental distribution graphs by line scanning (a: Mg – 9Al, b: Mg – 9Al – 0.8Zn).
by about 0.7 jC per 0.1 wt.% Zn additions. Under the current experimental conditions (at the cooling rate of about 0.5 jC/s), the time in vulnerable region can be prolonged by 1.4 s per 0.1 wt.% Zn additions.
5. Conclusions
Fig. 9. Graphical outline of the method of determination of the stress relied period, tR, and the vulnerable period, tV.
Hot-tearing of Mg – 9Al – xZn alloys originates along grain boundaries at the end of solidification. The inter-crystalline segregation of Zn and Al is the main contribution to the high hot-tearing susceptibility of Mg – 9Al –xZn alloys. Zn additions increase the hot-tearing susceptibility of Mg –9Al– xZn alloys evidently. The effects of Zn on the solidification of alloys can be considered as two components: decreasing the melting point of metals in grain boundaries, in another words, increasing the time that low melting-point liquid film exists; and increasing the quantity of low-melting-point metal in grain boundaries.
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Y. Wang et al. / Materials Letters 57 (2002) 929–934
References [1] S. Housh, B. Mikuckiand, A. Stevenson, Selection and application of magnesium and magnesium alloys, Metals Handbook, vol. 2, ASM Int., Materials Park, OH, USA, 1990, pp. 455 – 479. [2] M.M. Avedesian, H. Bake, ASM Specialty Handbook, American Society for Metals, Materials Park, OH, 1999.
[3] Y. Wang, B. Sun, Q. Wang, Y. Zhu, W. Ding, Materials Letters, (53) (2002) 35. [4] K. Kim, H.N. Han, T. Yeo, Y. Lee, K.H. Oh, D.N. Lee, Ironmaking and Steelmaking 24 (3) (1997) 249. [5] T.W. Clyne, G.J. Davies, The British Foundryman (74) (1981) 65 – 73.
Materials Letters 57 (2002) 929 – 934 www.elsevier.com/locate/matlet
Hot-tearing susceptibility of Mg–9Al–xZn alloy Yeshuang Wang *, Qudong Wang, Guohua Wu, Yanping Zhu, Wenjiang Ding School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 13 September 2001; received in revised form 28 April 2002; accepted 30 April 2002
Abstract Hot-tearing susceptibility of Mg – 9Al – xZn alloys is studied. Crack-ring molds were used to test the hot-tearing susceptibility. In situ thermal analysis, optical microscopy, scanning electron microscopy (SEM) and line scan were used to examine the hot-tearing behavior. Hot-tearing originates along the grains at the end of solidification. With Zn additions, the quantity of phase with low melting point in grain boundaries is increased, its melting point is decreased and the hot-tearing susceptibility is increased. The segregation of Zn element in grain boundaries is the main contribution to the high hot-tearing susceptibility of Mg – 9Al – xZn alloys. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mg – Al Alloy; Zn; Hot-tearing; Hot-tearing susceptibility
1. Introduction Mg – Al –Zn-based alloys are the most commonly used magnesium alloys for structural components. Mg – 9Al – 1Zn alloy AZ91 is the workhorse alloy for casting applications as it exhibits good mechanical and physical properties in combination with excellent castability and salt-water corrosion resistance [1]. Al has the most favorable effect on magnesium of any of the alloying elements. It improves strength and hardness, and it widens the freezing range and makes the alloy easier to cast, when present in amounts in excess of 6 wt.%, and the alloy becomes heat treatable. An aluminum content of 9% yields optimum strength. Zinc is next to aluminum in effectiveness as an alloying ingredient in magnesium. Zinc is often used in *
Corresponding author. Tel.: +86-21-62932164-122; fax: +8621-62932548. E-mail address: [email protected] (Y. Wang).
combination with aluminum to produce improvement in room-temperature strength. Zinc also helps overcome the harmful corrosive effect of iron and nickel impurities that might be present in the magnesium alloy [2]. However, it increases hot shortness when added in amounts greater than 1 wt.%. Casting is developed to be complex and thin in structure. At the same time, the hot-tearing problem is becoming more and more serious during the production of these castings. To a large degree, the tendency for hot-tearing limits the size and shape of the alloy [3]. Therefore, it is necessary to find viable ways to control the hottearing default efficiently.
2. Experimental Mg –9Al– xZn (9% Al, 0,0.2,0.4,0.6,0.8,1.0 Zn, all wt.%) was studied. Hot-tearing susceptibility is investigated using crack-ring molds, which is shown in
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 9 8 - 4
930
Y. Wang et al. / Materials Letters 57 (2002) 929–934
was cut from the center of the sample, and the microstructure of the alloys was analyzed by an optical microscopy and an electron probe microanalyzer.
3. Results
Fig. 1. Mold of crack-rings.
Fig. 1. A round mold with diameter of 108 mm is cast. Chills are used to regulate the sequence of solidification. Round steel is located in the center of mold to hamper the solidification shrinkage of alloys, the diameter of which is proportional to the degree of hindrance to alloy. Therefore, the critical diameter, at which the sample of crack-rings begins to crack, is inversely proportional to the hot-tearing susceptibility of magnesium alloys. The diameter of the round steel is at the interval of 5 mm in this study, such as 98,93. . .. Hot-tearing susceptibility coefficient (HSC) is adopted, which can be expressed as: HSC ¼
108 Dcrit 108
The testing results of crack-rings are shown in Fig. 2. It can be seen that hot-tearing susceptibility coefficient (HSC) increases with Zn additions. SEM fractographs of hot-tearing fracture surfaces of Mg – 9Al and Mg –9Al – 0.8Zn are shown in Fig. 3a,b. They suggest that the surfaces are very smooth and the grains are evident. Fig. 4 is the optical microscopy of cracks originating after solute heat treatment (T4). It suggests clearly that hot-tearing originates along the grains. Cooling curve analysis shows that the start-solidification temperature is about 592 jC, and the practical end-solidification temperature is about 430 jC, much lower than the solidus of Mg – 9Al (Fig. 5). With Zn additions, the liqudus has few changes, but solidus evidently decreases (Fig. 6). As-cast microstructure of Mg –9Al and Mg – 9Al – 1.2Zn (Fig. 7a,b) shows that the quantity of phase in grain boundaries of Mg –9Al – 1.2Zn alloy is much more than that of Mg –9Al alloy. The segregation of Mg –9Al and Mg – 9Al –0.8Zn was analyzed with line scanning (Fig. 8). Because of content segregation, the content of Al in grain boun-
ð1Þ
where Dcrit is the critical diameter of the round steel, at which the sample of crack-rings begins to crack. Fracture surfaces of the cracked crack-rings sample were analyzed with scanning electron microscopy (SEM). The part including the crack origin was cut, treated with solute heat treatment (T4), and optically analyzed. Cooling curves were also investigated (samples were cast in sand mold). The specimen
Fig. 2. The hot-tearing susceptibility of Mg – 9Al – xZn alloys.
Y. Wang et al. / Materials Letters 57 (2002) 929–934
931
Fig. 3. SEM fractographs for the surface of the specimen cracked in crack-rings tests (a: Mg – 9Al, b: Mg – 9Al – 0.8Zn).
It is well understood that hot-tearing originates at the end of solidification while there is a thin liquid
film in grain boundaries. The strength has been built up, but the ductility is still zero. Tensile stress caused by solidification shrinkage could separate grains. Hot tears then take place easily under a small strain, when the liquid film in grain boundaries is thin enough to resist feeding of the surrounding liquid through the grain boundaries [4]. During the solidification of Mg – 9Al –xZn alloys, Zn and Al were enriched in grain boundaries caused by inter-crystalline segregation. The enrichment of Al
Fig. 4. Optical microscopy fractographs of tears origin from crackrings sample after solute heat treatment (Mg – 9Al – 0.8Zn).
Fig. 5. Cooling curves analysis of Mg – 9Al and Mg – 9Al – 1.2Zn alloys casted in sand mold.
daries is higher than that in grains. The Zn added is enriched in grain boundaries.
4. Discussion 4.1. Mechanism
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Y. Wang et al. / Materials Letters 57 (2002) 929–934
Fig. 6. The end-solidifying temperature of Mg – 9Al – xZn alloys with Zn additions temperature.
Increasing the quantity of low melting-point liquid in grain boundaries must decrease the adhesiveness among the grains (dendrites), and make it easier to be separated by external solidification stress. On the other hand, the remaining liquid in grain boundaries solidifies as eutectic reaction. It must lead to shock shrinkage at the end of solidification. With the increasing quantity of low melting-point liquid (with approximately the eutectic composition), the solidifying shrinkage and solidifying stress must be increased. Decreasing the melting point of liquid film in grain boundaries must prolong the time in the vulnerable region. According to the testing results, the end-solidifying temperature of Mg – 9Al –xZn alloy is lowered
and Zn lowered the melting point of alloys. As a result, the time of liquid film existing in grain boundaries was prolonged. According to the theory of Clyne [5], cracking susceptibility coefficient (CSC) can be expressed as: CSC ¼
tV tR
ð2Þ
where tV is the vulnerable time period and tR is the time available for stress relief processes (Fig. 9). Mass and liquid feeding 0:1 < fL < 0:6 Interdendritic seperation 0:01 < fL < 0:1 where fL is the liquid fraction. The liquid film enriched with Al and Zn lowered the melting point of alloys, and so the period of liquid film existing is prolonged. Therefore, the cracking susceptibility coefficient (CSC) is increased by the inter-crystalline segregation of Zn and Al. 4.2. Effects of Zn additions During the solidification of Mg – 9Al –xZn, there are two principal influences of Zn on the alloys, increasing the quantity of eutectic in grain boundaries and decreasing the end-solidifying temperature of the alloys.
Fig. 7. As-cast structure of (a) Mg – 9Al, (b) Mg – 9Al – 1.2Zn casted in sand mold.
Y. Wang et al. / Materials Letters 57 (2002) 929–934
933
Fig. 8. Elemental distribution graphs by line scanning (a: Mg – 9Al, b: Mg – 9Al – 0.8Zn).
by about 0.7 jC per 0.1 wt.% Zn additions. Under the current experimental conditions (at the cooling rate of about 0.5 jC/s), the time in vulnerable region can be prolonged by 1.4 s per 0.1 wt.% Zn additions.
5. Conclusions
Fig. 9. Graphical outline of the method of determination of the stress relied period, tR, and the vulnerable period, tV.
Hot-tearing of Mg – 9Al – xZn alloys originates along grain boundaries at the end of solidification. The inter-crystalline segregation of Zn and Al is the main contribution to the high hot-tearing susceptibility of Mg – 9Al –xZn alloys. Zn additions increase the hot-tearing susceptibility of Mg –9Al– xZn alloys evidently. The effects of Zn on the solidification of alloys can be considered as two components: decreasing the melting point of metals in grain boundaries, in another words, increasing the time that low melting-point liquid film exists; and increasing the quantity of low-melting-point metal in grain boundaries.
934
Y. Wang et al. / Materials Letters 57 (2002) 929–934
References [1] S. Housh, B. Mikuckiand, A. Stevenson, Selection and application of magnesium and magnesium alloys, Metals Handbook, vol. 2, ASM Int., Materials Park, OH, USA, 1990, pp. 455 – 479. [2] M.M. Avedesian, H. Bake, ASM Specialty Handbook, American Society for Metals, Materials Park, OH, 1999.
[3] Y. Wang, B. Sun, Q. Wang, Y. Zhu, W. Ding, Materials Letters, (53) (2002) 35. [4] K. Kim, H.N. Han, T. Yeo, Y. Lee, K.H. Oh, D.N. Lee, Ironmaking and Steelmaking 24 (3) (1997) 249. [5] T.W. Clyne, G.J. Davies, The British Foundryman (74) (1981) 65 – 73.