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Forgeability and die-forging forming of direct chill casting Mg–Nd–Zn–Zr magnesium alloy
Materials Science and Engineering A 527 (2010) 3690–3694
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Forgeability and die-forging forming of direct chill casting Mg–Nd–Zn–Zr magnesium alloy Xingwei Zheng a , Jie Dong a,∗ , Dongdi Yin a , Wencai Liu a , Fenghua Wang a , Li Jin a , Wenjiang Ding a,b a b
National Engineering Research Centre of Light Alloy Net Forming, Shanghai Jiaotong University, 200240 Shanghai, PR China Key State Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, 200240 Shanghai, PR China
a r t i c l e
i n f o
Article history: Received 9 October 2009 Received in revised form 19 January 2010 Accepted 2 March 2010
Keywords: Direct chill casting Forgeability Microstructure Mechanical properties
a b s t r a c t Mg–3.0Nd–0.2Zn–0.4Zr (wt.%, NZ30K) is a newly developed magnesium rare earth alloy with middle strength and high toughness in as-cast state. In this paper the forgeability of the as-cast NZ30K alloy prepared by direct chill (DC) casting was studied. The forgeability of the DC casting NZ30K alloy was firstly modeled by using uniaxial tensile and compression tests at temperatures between 250 and 400 ◦ C and strain rates ranging from 0.001 to 10 S−1 . The results show that the optimum forging temperature of the as-cast NZ30K alloy is between 350 and 400 ◦ C and strain rate ranges from 0.01 to 1 S−1 . Finally, a flange of automobile clutch was tentatively and successfully forged by using the DC casting NZ30K billet. The forged flange possesses desired microstructure and high mechanical properties. The mechanical properties of the product are further enhanced by T5 heat treatment for precipitate strengthening. The maximum ultimate tensile strength, yield strength and elongation of the T5 state alloy are 323.5 MPa, 318.2 MPa and 11.2%, respectively. The creep strain rates for the NZ30K alloy are 2.0 × 10−8 and 1.2 × 10−7 at the 200 and 250 ◦ C, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys have demonstrated significant potential application in aerospace and transportation fields due to their high specific strength, low density and good stiffness in comparison with commonly used aluminum alloys, steel engineering structure materials [1,2]. Die casting is a dominate forming method for magnesium alloy parts until now, however due to its unavoidable casting defect, porosity, which makes the mechanical properties of the magnesium alloy parts produced by die casting cannot meet the requirement of the industry for a wide range [3]. Forging can not only improve the mechanical properties of magnesium alloy, but also produce a part reaching or closely approaching the size and shape of the finished one. Therefore, it is a potential better way for producing magnesium alloy parts. However poor deformability of magnesium alloy at room temperature (RT) for its closed-packed hexagonal (HCP) crystal structure and scarce information on the mechanical properties of magnesium alloy at elevated temperature are still two main aspects which inhibit the development of magnesium alloy forging
∗ Corresponding author at: Shanghai Jiaotong University, School of Material Science and Engineering, Dongchuan Road No. 800, Shanghai 200240, China. Tel.: +86 21 34203052; fax: +86 21 34202794. E-mail address: [email protected] (J. Dong). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.002
technology [4,5]. Consequently researchers are focusing on investigating the forging technology of magnesium alloy. Progresses have been made and some sound products are produced [6,7]. However, in their study only the commercial magnesium alloy, AZ31 and AZ70, are used to produce magnesium alloy forging parts, while investigations on forging technology of the magnesium alloys containing rare earth elements are still limited. It is well known that magnesium alloys containing rare earth elements have great potential application in the future transportation and aerospace fields for their better mechanical properties and heat resistance than that of commercial magnesium alloys. Therefore it is meaningful to investigate the forging technology of magnesium alloys containing rare earth elements. Mg–3.0Nd–0.2Zn–0.4Zr (wt.%, NZ30K) alloy is a newly developed magnesium alloy with middle strength and good toughness in as-cast state in our research group. Elongations of the as-cast, T4 and T6 state NZ30K alloys prepared by permanent mould at RT are 14%, 22% and 11%, respectively [8], which indicates that NZ30K alloy is a potential wrought magnesium alloy. Furthermore the corrosion resistance of as-cast, T4 and T6 alloy is much better than that of AZ91D [9,10]. On the other hand, the NZ30K alloy is cheap for little addition of rare earth. Therefore the NZ30K alloy owns great potential application market in the future. In order to promote the application of NZ30K alloy, the forging process parameters of the NZ30K alloy is determined and a sound flange of automobile clutch is forged according to the determined parameters in this paper.
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Fig. 1. The microstructure of the as-cast billet. Fig. 2. The mechanical properties of the NZ30K alloy at different temperatures.
2. Experimental material and procedures An alloy of nominal composition NZ30K magnesium alloy was prepared from pure Zn, Mg and Mg–30Nd, Mg–30Zr master alloys (wt.%) by melting in an electrical resistance furnace under the mixture atmosphere of SF6 , CO2 and air in this research. A billet with diameter of 180 mm was prepared on a DC casting equipment. The casting speed and casting temperature of the DC casting are 100 mm/min and 720 ◦ C, respectively. The microstructure of the as-cast billet is shown in Fig. 1, Microstructure of the as-cast billet is identified by the presence of equiaxed grain with the average grain size of 50 m. Tensile samples were cut into rectangular ones with dimension of 10 mm width, 2 mm thickness and 30 mm gauge length by an electric-sparking wire cutting machine and all the test samples were taken from the centre of the billet along the billet axis. Tensile tests were carried out at temperatures ranging from 200 to 400 ◦ C with a constant initial strain rate of 0.01 S−1 on a Zwick/Roell-20KN material machine. And compression testing was performed at temperatures ranging from 250 to 400 ◦ C and the strain rates ranging from 0.001 to 10 S−1 on a Gleeble 1500D simulator. The dimension of the compression sample is 10 mm × 12 mm. Graphite flakes were placed on the end surfaces of the compression samples in order to improve lubrication. The testing data were recorded automatically. All specimens were immediately quenched in water after compression. The tested samples for the microstructure observation were etched with a phosphoric–picral solution (0.7 ml phosphoric acid, 4.2 g picric acid and 100 ml ethanol). Average grain sizes were inspected by the intercept method given in the literature [11]. The creep tests were conducted in a purified argon atmosphere. The test sample was heated at 0.33 K/s and maintained for 600 s at the test temperatures before test. The stress reached the test values within 0.5 s and then creep measurements started immediately. The creep temperatures are 200, 250 ◦ C and the creep stress is 100 MPa. 3. Result and discussion It is vital important to determine appropriate forging parameters such as the optimum forging temperature and strain rate ranges in order to produce a sound forged product. The appropriate forging parameters for NZ30K alloy will be discussed in the following sections. 3.1. Tensile tests at different temperatures Forging temperature is one of the most important parameters for magnesium alloy forging technology [7], which can be
demonstrated by the mechanical properties tested at different temperatures. The mechanical properties of the NZ30K alloy are shown in Fig. 2, which demonstrates the elongations and tensile strengths at the different temperatures with constant strain rate of 0.01 S−1 . It can be seen that the elongation of the NZ30K alloy increases and the tensile strength decreases with the increasing deformation temperature. It is worth to point out that the elongation of the NZ30K alloy increases slowly as the deformation temperature increases from room temperature to 300 ◦ C, while the elongation of the NZ30K alloy increases remarkably with the increasing deformation temperature as the deformation temperature is higher than 300 ◦ C. The reasons may be attributed to the following aspects: (1) the amount of the secondary phase decreases with the increasing deformation temperature. (2) The number of slip system increases with the increasing deformation temperature. (3) The dynamic recrystallization is accelerated with the increasing temperature. The forgeability of alloy can be evaluated by Eq. (1) according to the results in the literature [12]. The forgeability of alloy is good as the Kı value is range from 1 to 2 and the forgeability of the alloy is excellent as the Kı > 2. Kı =
ı b
(1)
where ı is the elongation; b is the ultimate tensile strength; Kı values of as-cast NZ30K alloy prepared by DC casting at different temperatures are listed in Table 1. It can be seen that the forgeability of NZ30K alloy significantly depends on the forging temperature. The forgeability of the NZ30K alloy is good as the deformation temperature is higher than 350 ◦ C. However, magnesium alloy may cause alloy oxidation, grain coarsening and irreparable damage of the microstructure due to incipient fusion as the deformation temperature is higher than 400 ◦ C [6]. Therefore the appropriate forging temperature range of the NZ30K alloy is range from 350 to 400 ◦ C. The appropriate forging temperature range of AZ70 is range from 325 to 400 ◦ C reported in Ref. [6]. The appropriate forging temperature range of the NZ30K alloy is narrower in comparison with AZ70 alloy, which can be attributed to that the addition of rare Table 1 The Kı values of NZ30K alloy at different temperatures. Temperature (◦ C)
Kı
Forgeability
RT 200 250 300 350 400
0.078 0.112 0.212 0.344 1.252 6.277
Not Worse Worse Worse Good Excellent
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Fig. 3. True stress–strain curves of NZ30K alloy at 400 ◦ C.
earth element Nd in magnesium alloy can enhance the mechanical properties and improve the heat resistance of magnesium alloy [13,14], which in turn to minimize the Kı value according to Eq. (1). Consequently, the lowest forging temperature is increased. 3.2. Compression tests at different strain rates The compression tests were carried out on Gleeble 1500D in order to assess the influence of strain rate on the forgeability and forging pressure of NZ30K alloy. The strain rate ranges from 0.001 to 10 S−1 and the deformation temperature is between 200 and 400 ◦ C. The aim of the compression tests at different temperatures is to check the rightness of forging temperature range determined in Section 3.1. The true stress–strain curves of NZ30K alloy were shown in Fig. 3. The true stress increases with the increasing strain rate (as shown in Fig. 3). The peak flow stress is practically independent of strain rate as the strain rate ε˙ ≤ 0.1; no significant work hardening was observed for any specimen. The results indicate that dynamic recovery take places at lower strain rate during the deformation. However obvious work hardening was observed as the strain rate ε˙ > 0.1, which indicates the NZ30K alloy is not suitable for forging as the strain ε˙ > 0.1. On the other hand, high strain rate will result in intense heat effect and crack occurs during forging forming [6]. Therefore, the lower strain rate, the better for the forging deformation, however, too low strain rate will lead to lower productivity. Taken both quality of the product and the productivity into account, strain rate between 0.01 and 1 S−1 for NZ30K alloy is acceptable during forging deformation.
Fig. 4. The macrostructure of the sample compression at different strain rate and temperature (the numbers in the left side of the deformed samples represent deformation temperature/◦ C and the number at the bottom of the deformed samples represent strain rate/S−1 ).
Table 2 The mechanical properties of NZ30K alloy. Strain rate
200
250
300
350
400
450
0.001 0.01 0.1 1
210 219 238 246
206 211 225 242
142 180 194 198
70 101 131 168
40 61 97 142
28 45 54 67
The macrostructure of the samples compressed at temperatures between 250 and 400 ◦ C and at strain rates ranging from 0.001 to 10 S−1 is shown in Fig. 4. It can be seen that no crack can be observed after compression test as the deformation temperature is higher than 350 ◦ C in any strain rate. And crack can be observed in all the test samples as the deformation temperature is lower than 250 ◦ C. These results also mean that the appropriate forging temperature of NZ30K alloy should be above 350 ◦ C, which agrees with the Kı values calculation results in Table 1 in Section 3.1. 3.3. Die-forging forming of flange of automobile clutch According to the results in Sections 3.1 and 3.2, it is vital important to choose optimum forging parameters in order to produce a sound forged product. The above results show that the optimum forging temperature for as-cast NZ30K alloy ranges from 350 to 400 ◦ C and the optimum forging strain rate for as-cast NZ30K alloy ranges from 0.01 to 1 S−1 . The NZ30K billets were preheated to 400 ◦ C before forging, and the mould for the forging was preheated
Fig. 5. The flange of automobile clutch of NZ30K produced by forging: (a) vertical view; (b) front view.
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Fig. 6. The microstructure of the flange of automobile clutch. Fig. 8. The typical stress and strain curves of NZ30K alloy at different states.
to 250 ◦ C. In order to assess the deformation pressure for the forging of NZ30K alloy, the mechanical properties of the NZ30K at different strain rate and temperature were shown in Table 2. The constitute equation of the NZ30K alloy was developed according to the Ref. [15].
= 100 sinh−1 6.67 × 10ε˙ exp
−207367 RT
0.1052 (2)
where ε˙ is the strain rate; p is the peak tensile stress; R is the molar gas constant; T is the absolute temperature. According to Eq. (2), the forging load of the flange of automobile clutch is about 409 kN as the forging temperature is 400 ◦ C and the maximum strain rate of 1 S−1 . Therefore, an oil hydraulic press with the capacity of 630 kN can meet the requirement of mould forging as the mechanical properties of the NZ30K are higher than that of AZ31 alloy. The flange of automobile clutch is shown in Fig. 5. No crack is observed on the surface of the flange of automobile clutch prepared base on parameters determined in Sections 3.1 and 3.2. 3.4. Microstructure and mechanical properties of the flange of automobile clutch
The effect of aging time on the hardness of the forging NZ30K alloy and the microstructure of the peak aging NZ30K alloy is shown in Fig. 7. It can be seen that the forged NZ30K alloy exhibits hardening response. The Hv increases with the increase of the aging time, before reaching the peak hardness (PHv). In this work, the Hv reaches to the PHv at the aging time about 2 h and the corresponding PHv is about 71. The aging time of PHv is shorten in comparison to the T4 state NZ30K alloy reported in Ref. [8], which is about 15 h. However, the PHv of the NZ30K alloys in these two different states is similar. The microstructure of the peak aging NZ30K alloy is shown in Fig. 7b. The Mg12 Nd not only distributes along the grain boundary, but also dispersive distribute in the grain after T5 heat treatment. 3.4.2. The mechanical properties of the flange of the automobile clutch Yield ratio is defined as yield strength divides ultimate tensile strength, which represents the reliability of structural material as shown in Eq. (3). It shows that the reliability of the structural material increases with the increasing of the yield ratio. Yield ratio =
3.4.1. Microstructure of the flange of automobile clutch The microstructure of the flange of automobile clutch is shown in Fig. 6. Microstructure of the flange is identified by the presence of equiaxed grain. It was found that the grain size of the equiaxed grain is about 50 m. Furthermore small amount of compounds are observed along the grain boundary in the alloy. According to the precious investigations [8,13,14], the compound is Mg12 Nd.
0.2 b
(3)
where 0.2 is the yield strength; b is the ultimate strength. The typical stress–strain curves of the as-cast, forging, T5 state NZ30K alloys are shown in Fig. 8. The curves indicate that forging has significant influence on the yield ratio of the NZ30K alloy. The corresponding yield ratio of the as-cast, forged and T5 state NZ30K alloys are 0.57, 0.96 and 0.98, respectively. It indicates that
Fig. 7. The aging hardness and the microstructure of the peak aged NZ30K alloy: (a) aging hardness response; (b) peak aging microstructure.
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Table 3 The tensile properties of the NZ30K alloy at different states (average deviation is given in parenthesis). Alloy state
0.2 (MPa)
b (MPa)
ε (%)
As-cast Forging T5
112.6 (5.3) 269.3 (6.5) 318.2 (8.3)
195.6 (4.2) 278.8 (6.1) 323.5 (9.4)
14.6 (0.6) 12.2 (0.6) 11.2 (0.6)
the steady-state creep rate for the AZ91 alloy is 1.7 × 10−7 at 200 ◦ C. In comparison with the commercial AZ91 alloy, the results shows that the high temperature creep properties of the NZ30K alloy is substantially better than those of the AZ91 alloy. 4. Summary and conclusions 1. The optimum forging parameters for the DC casting NZ30K alloy are determined by deformation experiment at temperatures between 250 and 400 ◦ C and at strain rates ranging from 0.001 to 10 S−1 . The results show that the optimum forging temperature range is between 350 and 400 ◦ C. And the optimum strain rate ranges from 0.01 to 1 S−1 . 2. The flange of automobile clutch produced by DC casting NZ30K alloy possesses desired microstructure and good mechanical properties. The mechanical properties are further improved by T5 heat treatment. The maximum UTS, YS and elongation of the T5 state alloy are 323.5 MPa, 318.2 MPa and 11.2%, respectively. 3. The high temperature creep tests show that the creep properties of NZ30K alloy are substantially better than those of the AZ91 alloy. Acknowledgements
Fig. 9. The creep curves of the NZ30K alloy tested at 200 and 250 ◦ C under an applied stress of 100 MPa.
the reliability of the NZ30K alloy is enhanced by forging. However the subsequent T5 heat treatment has little influence on the yield ratio of NZ30K alloy. The tensile mechanical properties including ultimate tensile strength (UTS), yield strength (YS) and elongation (ε) of the as-cast, forged and T5 state NZ30K alloys are listed in Table 3. The mechanical properties of the forged NZ30K alloy are greatly enhanced compared with the as-cast NZ30K alloy. And the mechanical properties of the forged NZ30K alloy are further improved by T5 treatment. The T5 state NZ30K alloy shows the maximum tensile strength after aging at 200 ◦ C for 2 h. The detail UTS and YS are 323.5 MPa and 318.2 MPa, respectively. Nevertheless the elongation of the different states NZ30K alloys is similar. The tensile results show that the T5 heat treatment improves the mechanical properties of the NZ30K alloy (as shown in Table 2). According to Figs. 6 and 7b, it can be found, comparing the as-forged microstructure with T5 treated microstructure of the forged alloy, that the Mg12 Nd not only distributes along the grain boundary, but also in the grain after T5 heat treatment. And the precipitate Mg12 Nd exhibits semi-coherent structure with matrix, which could effectively inhibit slip along the basal plane [16]. Consequently, the UTS and YS of the NZ30K alloy are enhanced by T5 heat treatment. The Similar strengthening mechanism is reported in Mg–Y–Nd system [17]. Creep resistance is an important index for application of alloys in the automobile industry. Creep curves of the forged NZ30K alloy are shown in Fig. 9 under the creep condition of 200, 250 ◦ C and 100 MPa. The steady-state creep rates for the NZ30K are 2.0 × 10−8 and 1.2 × 10−7 at the 200 and 250 ◦ C, respectively. As shown in [18],
Financial support by the National Ministry of Science and Technology (Grant Nos. 2007CB613703 and 2009AA03Z521) and the Shanghai Council of Science and Technology (Grant No. 07ZR14051) are gratefully acknowledged. References [1] X.W. Zheng, L.D. Wang, J.L. Wang, Y.M. Wu, Z.L. Ning, J.F. Sun, L.M. Wang, Materials Science and Engineering A 515 (2009) 98–101. [2] Z. Horita, K. Matsubara, K. Makii, T.G. Langdon, Scripta Materialia 47 (2002) 255–260. [3] S.K. Guan, L.H. Wu, L.G. Wang, Transaction of Nonferrous Metals Society of China 18 (2008) 315–320. [4] Z. Trojanova, R. Kral, A. Chatey, Materials Science and Engineering A 462 (2007) 202–205. [5] V. Gartnerova, Z. Trojanova, A. Jager, P. Palcek, Journal of Alloys and Compounds 378 (2004) 180–183. [6] S.K. Guan, L.H. Wu, P. Wang, Materials Science and Engineering A 499 (2009) 187–191. [7] P. Skubisz, J. Sinczak, S. Bednarek, Journal of Materials Processing Technology 177 (2006) 210–213. [8] P.H. Fu, L.M. Peng, H.Y. Jiang, J.W. Chang, C.Q. Zhai, Materials Science and Engineering A 486 (2008) 183–192. [9] J.W. Chang, L.M. Peng, X.W. Guo, A. Atrens, P.H. Fu, W.J. Ding, X.S. Wang, Journal of Applied Electrochemistry 38 (2008) 207–214. [10] J.W. Chang, X.W. Guo, P.H. Fu, L.M. Peng, W.J. Ding, Electrochemical Acta 52 (2007) 3160–3167. [11] GB/T 4296-2004, Inspection Method for Microstructure of Wrought Magnesium Alloy [S], Standards Press of China, Beijing, 2004. [12] W.H. Qi, Shangdong Metallurgy 20 (1998) 16–19. [13] Q. Li, Q.D. Wang, Y.X. Wang, X.Q. Zeng, W.J. Ding, Journal of Alloys and Compounds 427 (2007) 115–123. [14] B. Liu, M.L. Zhang, R.Z. Wu, Materials Science and Engineering A 487 (2008) 347–351. [15] I.A. Maksoud, H. Ahmed, J. Rodel, Materials Science and Engineering A 504 (2009) 40–48. [16] Q.M. Peng, H.W. Dong, L.D. Wang, Y.M. Wu, L.M. Wang, Material Characterization 59 (2008) 983–986. [17] J.F. Nie, B.C. Muddle, Acta Materialia 48 (2000) 1691–1703. [18] M. Regev, A. Rosen, M. Bamberger, Metallurgical and Materials Transactions A 32 (2001) 1335.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Forgeability and die-forging forming of direct chill casting Mg–Nd–Zn–Zr magnesium alloy Xingwei Zheng a , Jie Dong a,∗ , Dongdi Yin a , Wencai Liu a , Fenghua Wang a , Li Jin a , Wenjiang Ding a,b a b
National Engineering Research Centre of Light Alloy Net Forming, Shanghai Jiaotong University, 200240 Shanghai, PR China Key State Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, 200240 Shanghai, PR China
a r t i c l e
i n f o
Article history: Received 9 October 2009 Received in revised form 19 January 2010 Accepted 2 March 2010
Keywords: Direct chill casting Forgeability Microstructure Mechanical properties
a b s t r a c t Mg–3.0Nd–0.2Zn–0.4Zr (wt.%, NZ30K) is a newly developed magnesium rare earth alloy with middle strength and high toughness in as-cast state. In this paper the forgeability of the as-cast NZ30K alloy prepared by direct chill (DC) casting was studied. The forgeability of the DC casting NZ30K alloy was firstly modeled by using uniaxial tensile and compression tests at temperatures between 250 and 400 ◦ C and strain rates ranging from 0.001 to 10 S−1 . The results show that the optimum forging temperature of the as-cast NZ30K alloy is between 350 and 400 ◦ C and strain rate ranges from 0.01 to 1 S−1 . Finally, a flange of automobile clutch was tentatively and successfully forged by using the DC casting NZ30K billet. The forged flange possesses desired microstructure and high mechanical properties. The mechanical properties of the product are further enhanced by T5 heat treatment for precipitate strengthening. The maximum ultimate tensile strength, yield strength and elongation of the T5 state alloy are 323.5 MPa, 318.2 MPa and 11.2%, respectively. The creep strain rates for the NZ30K alloy are 2.0 × 10−8 and 1.2 × 10−7 at the 200 and 250 ◦ C, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys have demonstrated significant potential application in aerospace and transportation fields due to their high specific strength, low density and good stiffness in comparison with commonly used aluminum alloys, steel engineering structure materials [1,2]. Die casting is a dominate forming method for magnesium alloy parts until now, however due to its unavoidable casting defect, porosity, which makes the mechanical properties of the magnesium alloy parts produced by die casting cannot meet the requirement of the industry for a wide range [3]. Forging can not only improve the mechanical properties of magnesium alloy, but also produce a part reaching or closely approaching the size and shape of the finished one. Therefore, it is a potential better way for producing magnesium alloy parts. However poor deformability of magnesium alloy at room temperature (RT) for its closed-packed hexagonal (HCP) crystal structure and scarce information on the mechanical properties of magnesium alloy at elevated temperature are still two main aspects which inhibit the development of magnesium alloy forging
∗ Corresponding author at: Shanghai Jiaotong University, School of Material Science and Engineering, Dongchuan Road No. 800, Shanghai 200240, China. Tel.: +86 21 34203052; fax: +86 21 34202794. E-mail address: [email protected] (J. Dong). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.002
technology [4,5]. Consequently researchers are focusing on investigating the forging technology of magnesium alloy. Progresses have been made and some sound products are produced [6,7]. However, in their study only the commercial magnesium alloy, AZ31 and AZ70, are used to produce magnesium alloy forging parts, while investigations on forging technology of the magnesium alloys containing rare earth elements are still limited. It is well known that magnesium alloys containing rare earth elements have great potential application in the future transportation and aerospace fields for their better mechanical properties and heat resistance than that of commercial magnesium alloys. Therefore it is meaningful to investigate the forging technology of magnesium alloys containing rare earth elements. Mg–3.0Nd–0.2Zn–0.4Zr (wt.%, NZ30K) alloy is a newly developed magnesium alloy with middle strength and good toughness in as-cast state in our research group. Elongations of the as-cast, T4 and T6 state NZ30K alloys prepared by permanent mould at RT are 14%, 22% and 11%, respectively [8], which indicates that NZ30K alloy is a potential wrought magnesium alloy. Furthermore the corrosion resistance of as-cast, T4 and T6 alloy is much better than that of AZ91D [9,10]. On the other hand, the NZ30K alloy is cheap for little addition of rare earth. Therefore the NZ30K alloy owns great potential application market in the future. In order to promote the application of NZ30K alloy, the forging process parameters of the NZ30K alloy is determined and a sound flange of automobile clutch is forged according to the determined parameters in this paper.
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Fig. 1. The microstructure of the as-cast billet. Fig. 2. The mechanical properties of the NZ30K alloy at different temperatures.
2. Experimental material and procedures An alloy of nominal composition NZ30K magnesium alloy was prepared from pure Zn, Mg and Mg–30Nd, Mg–30Zr master alloys (wt.%) by melting in an electrical resistance furnace under the mixture atmosphere of SF6 , CO2 and air in this research. A billet with diameter of 180 mm was prepared on a DC casting equipment. The casting speed and casting temperature of the DC casting are 100 mm/min and 720 ◦ C, respectively. The microstructure of the as-cast billet is shown in Fig. 1, Microstructure of the as-cast billet is identified by the presence of equiaxed grain with the average grain size of 50 m. Tensile samples were cut into rectangular ones with dimension of 10 mm width, 2 mm thickness and 30 mm gauge length by an electric-sparking wire cutting machine and all the test samples were taken from the centre of the billet along the billet axis. Tensile tests were carried out at temperatures ranging from 200 to 400 ◦ C with a constant initial strain rate of 0.01 S−1 on a Zwick/Roell-20KN material machine. And compression testing was performed at temperatures ranging from 250 to 400 ◦ C and the strain rates ranging from 0.001 to 10 S−1 on a Gleeble 1500D simulator. The dimension of the compression sample is 10 mm × 12 mm. Graphite flakes were placed on the end surfaces of the compression samples in order to improve lubrication. The testing data were recorded automatically. All specimens were immediately quenched in water after compression. The tested samples for the microstructure observation were etched with a phosphoric–picral solution (0.7 ml phosphoric acid, 4.2 g picric acid and 100 ml ethanol). Average grain sizes were inspected by the intercept method given in the literature [11]. The creep tests were conducted in a purified argon atmosphere. The test sample was heated at 0.33 K/s and maintained for 600 s at the test temperatures before test. The stress reached the test values within 0.5 s and then creep measurements started immediately. The creep temperatures are 200, 250 ◦ C and the creep stress is 100 MPa. 3. Result and discussion It is vital important to determine appropriate forging parameters such as the optimum forging temperature and strain rate ranges in order to produce a sound forged product. The appropriate forging parameters for NZ30K alloy will be discussed in the following sections. 3.1. Tensile tests at different temperatures Forging temperature is one of the most important parameters for magnesium alloy forging technology [7], which can be
demonstrated by the mechanical properties tested at different temperatures. The mechanical properties of the NZ30K alloy are shown in Fig. 2, which demonstrates the elongations and tensile strengths at the different temperatures with constant strain rate of 0.01 S−1 . It can be seen that the elongation of the NZ30K alloy increases and the tensile strength decreases with the increasing deformation temperature. It is worth to point out that the elongation of the NZ30K alloy increases slowly as the deformation temperature increases from room temperature to 300 ◦ C, while the elongation of the NZ30K alloy increases remarkably with the increasing deformation temperature as the deformation temperature is higher than 300 ◦ C. The reasons may be attributed to the following aspects: (1) the amount of the secondary phase decreases with the increasing deformation temperature. (2) The number of slip system increases with the increasing deformation temperature. (3) The dynamic recrystallization is accelerated with the increasing temperature. The forgeability of alloy can be evaluated by Eq. (1) according to the results in the literature [12]. The forgeability of alloy is good as the Kı value is range from 1 to 2 and the forgeability of the alloy is excellent as the Kı > 2. Kı =
ı b
(1)
where ı is the elongation; b is the ultimate tensile strength; Kı values of as-cast NZ30K alloy prepared by DC casting at different temperatures are listed in Table 1. It can be seen that the forgeability of NZ30K alloy significantly depends on the forging temperature. The forgeability of the NZ30K alloy is good as the deformation temperature is higher than 350 ◦ C. However, magnesium alloy may cause alloy oxidation, grain coarsening and irreparable damage of the microstructure due to incipient fusion as the deformation temperature is higher than 400 ◦ C [6]. Therefore the appropriate forging temperature range of the NZ30K alloy is range from 350 to 400 ◦ C. The appropriate forging temperature range of AZ70 is range from 325 to 400 ◦ C reported in Ref. [6]. The appropriate forging temperature range of the NZ30K alloy is narrower in comparison with AZ70 alloy, which can be attributed to that the addition of rare Table 1 The Kı values of NZ30K alloy at different temperatures. Temperature (◦ C)
Kı
Forgeability
RT 200 250 300 350 400
0.078 0.112 0.212 0.344 1.252 6.277
Not Worse Worse Worse Good Excellent
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Fig. 3. True stress–strain curves of NZ30K alloy at 400 ◦ C.
earth element Nd in magnesium alloy can enhance the mechanical properties and improve the heat resistance of magnesium alloy [13,14], which in turn to minimize the Kı value according to Eq. (1). Consequently, the lowest forging temperature is increased. 3.2. Compression tests at different strain rates The compression tests were carried out on Gleeble 1500D in order to assess the influence of strain rate on the forgeability and forging pressure of NZ30K alloy. The strain rate ranges from 0.001 to 10 S−1 and the deformation temperature is between 200 and 400 ◦ C. The aim of the compression tests at different temperatures is to check the rightness of forging temperature range determined in Section 3.1. The true stress–strain curves of NZ30K alloy were shown in Fig. 3. The true stress increases with the increasing strain rate (as shown in Fig. 3). The peak flow stress is practically independent of strain rate as the strain rate ε˙ ≤ 0.1; no significant work hardening was observed for any specimen. The results indicate that dynamic recovery take places at lower strain rate during the deformation. However obvious work hardening was observed as the strain rate ε˙ > 0.1, which indicates the NZ30K alloy is not suitable for forging as the strain ε˙ > 0.1. On the other hand, high strain rate will result in intense heat effect and crack occurs during forging forming [6]. Therefore, the lower strain rate, the better for the forging deformation, however, too low strain rate will lead to lower productivity. Taken both quality of the product and the productivity into account, strain rate between 0.01 and 1 S−1 for NZ30K alloy is acceptable during forging deformation.
Fig. 4. The macrostructure of the sample compression at different strain rate and temperature (the numbers in the left side of the deformed samples represent deformation temperature/◦ C and the number at the bottom of the deformed samples represent strain rate/S−1 ).
Table 2 The mechanical properties of NZ30K alloy. Strain rate
200
250
300
350
400
450
0.001 0.01 0.1 1
210 219 238 246
206 211 225 242
142 180 194 198
70 101 131 168
40 61 97 142
28 45 54 67
The macrostructure of the samples compressed at temperatures between 250 and 400 ◦ C and at strain rates ranging from 0.001 to 10 S−1 is shown in Fig. 4. It can be seen that no crack can be observed after compression test as the deformation temperature is higher than 350 ◦ C in any strain rate. And crack can be observed in all the test samples as the deformation temperature is lower than 250 ◦ C. These results also mean that the appropriate forging temperature of NZ30K alloy should be above 350 ◦ C, which agrees with the Kı values calculation results in Table 1 in Section 3.1. 3.3. Die-forging forming of flange of automobile clutch According to the results in Sections 3.1 and 3.2, it is vital important to choose optimum forging parameters in order to produce a sound forged product. The above results show that the optimum forging temperature for as-cast NZ30K alloy ranges from 350 to 400 ◦ C and the optimum forging strain rate for as-cast NZ30K alloy ranges from 0.01 to 1 S−1 . The NZ30K billets were preheated to 400 ◦ C before forging, and the mould for the forging was preheated
Fig. 5. The flange of automobile clutch of NZ30K produced by forging: (a) vertical view; (b) front view.
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Fig. 6. The microstructure of the flange of automobile clutch. Fig. 8. The typical stress and strain curves of NZ30K alloy at different states.
to 250 ◦ C. In order to assess the deformation pressure for the forging of NZ30K alloy, the mechanical properties of the NZ30K at different strain rate and temperature were shown in Table 2. The constitute equation of the NZ30K alloy was developed according to the Ref. [15].
= 100 sinh−1 6.67 × 10ε˙ exp
−207367 RT
0.1052 (2)
where ε˙ is the strain rate; p is the peak tensile stress; R is the molar gas constant; T is the absolute temperature. According to Eq. (2), the forging load of the flange of automobile clutch is about 409 kN as the forging temperature is 400 ◦ C and the maximum strain rate of 1 S−1 . Therefore, an oil hydraulic press with the capacity of 630 kN can meet the requirement of mould forging as the mechanical properties of the NZ30K are higher than that of AZ31 alloy. The flange of automobile clutch is shown in Fig. 5. No crack is observed on the surface of the flange of automobile clutch prepared base on parameters determined in Sections 3.1 and 3.2. 3.4. Microstructure and mechanical properties of the flange of automobile clutch
The effect of aging time on the hardness of the forging NZ30K alloy and the microstructure of the peak aging NZ30K alloy is shown in Fig. 7. It can be seen that the forged NZ30K alloy exhibits hardening response. The Hv increases with the increase of the aging time, before reaching the peak hardness (PHv). In this work, the Hv reaches to the PHv at the aging time about 2 h and the corresponding PHv is about 71. The aging time of PHv is shorten in comparison to the T4 state NZ30K alloy reported in Ref. [8], which is about 15 h. However, the PHv of the NZ30K alloys in these two different states is similar. The microstructure of the peak aging NZ30K alloy is shown in Fig. 7b. The Mg12 Nd not only distributes along the grain boundary, but also dispersive distribute in the grain after T5 heat treatment. 3.4.2. The mechanical properties of the flange of the automobile clutch Yield ratio is defined as yield strength divides ultimate tensile strength, which represents the reliability of structural material as shown in Eq. (3). It shows that the reliability of the structural material increases with the increasing of the yield ratio. Yield ratio =
3.4.1. Microstructure of the flange of automobile clutch The microstructure of the flange of automobile clutch is shown in Fig. 6. Microstructure of the flange is identified by the presence of equiaxed grain. It was found that the grain size of the equiaxed grain is about 50 m. Furthermore small amount of compounds are observed along the grain boundary in the alloy. According to the precious investigations [8,13,14], the compound is Mg12 Nd.
0.2 b
(3)
where 0.2 is the yield strength; b is the ultimate strength. The typical stress–strain curves of the as-cast, forging, T5 state NZ30K alloys are shown in Fig. 8. The curves indicate that forging has significant influence on the yield ratio of the NZ30K alloy. The corresponding yield ratio of the as-cast, forged and T5 state NZ30K alloys are 0.57, 0.96 and 0.98, respectively. It indicates that
Fig. 7. The aging hardness and the microstructure of the peak aged NZ30K alloy: (a) aging hardness response; (b) peak aging microstructure.
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Table 3 The tensile properties of the NZ30K alloy at different states (average deviation is given in parenthesis). Alloy state
0.2 (MPa)
b (MPa)
ε (%)
As-cast Forging T5
112.6 (5.3) 269.3 (6.5) 318.2 (8.3)
195.6 (4.2) 278.8 (6.1) 323.5 (9.4)
14.6 (0.6) 12.2 (0.6) 11.2 (0.6)
the steady-state creep rate for the AZ91 alloy is 1.7 × 10−7 at 200 ◦ C. In comparison with the commercial AZ91 alloy, the results shows that the high temperature creep properties of the NZ30K alloy is substantially better than those of the AZ91 alloy. 4. Summary and conclusions 1. The optimum forging parameters for the DC casting NZ30K alloy are determined by deformation experiment at temperatures between 250 and 400 ◦ C and at strain rates ranging from 0.001 to 10 S−1 . The results show that the optimum forging temperature range is between 350 and 400 ◦ C. And the optimum strain rate ranges from 0.01 to 1 S−1 . 2. The flange of automobile clutch produced by DC casting NZ30K alloy possesses desired microstructure and good mechanical properties. The mechanical properties are further improved by T5 heat treatment. The maximum UTS, YS and elongation of the T5 state alloy are 323.5 MPa, 318.2 MPa and 11.2%, respectively. 3. The high temperature creep tests show that the creep properties of NZ30K alloy are substantially better than those of the AZ91 alloy. Acknowledgements
Fig. 9. The creep curves of the NZ30K alloy tested at 200 and 250 ◦ C under an applied stress of 100 MPa.
the reliability of the NZ30K alloy is enhanced by forging. However the subsequent T5 heat treatment has little influence on the yield ratio of NZ30K alloy. The tensile mechanical properties including ultimate tensile strength (UTS), yield strength (YS) and elongation (ε) of the as-cast, forged and T5 state NZ30K alloys are listed in Table 3. The mechanical properties of the forged NZ30K alloy are greatly enhanced compared with the as-cast NZ30K alloy. And the mechanical properties of the forged NZ30K alloy are further improved by T5 treatment. The T5 state NZ30K alloy shows the maximum tensile strength after aging at 200 ◦ C for 2 h. The detail UTS and YS are 323.5 MPa and 318.2 MPa, respectively. Nevertheless the elongation of the different states NZ30K alloys is similar. The tensile results show that the T5 heat treatment improves the mechanical properties of the NZ30K alloy (as shown in Table 2). According to Figs. 6 and 7b, it can be found, comparing the as-forged microstructure with T5 treated microstructure of the forged alloy, that the Mg12 Nd not only distributes along the grain boundary, but also in the grain after T5 heat treatment. And the precipitate Mg12 Nd exhibits semi-coherent structure with matrix, which could effectively inhibit slip along the basal plane [16]. Consequently, the UTS and YS of the NZ30K alloy are enhanced by T5 heat treatment. The Similar strengthening mechanism is reported in Mg–Y–Nd system [17]. Creep resistance is an important index for application of alloys in the automobile industry. Creep curves of the forged NZ30K alloy are shown in Fig. 9 under the creep condition of 200, 250 ◦ C and 100 MPa. The steady-state creep rates for the NZ30K are 2.0 × 10−8 and 1.2 × 10−7 at the 200 and 250 ◦ C, respectively. As shown in [18],
Financial support by the National Ministry of Science and Technology (Grant Nos. 2007CB613703 and 2009AA03Z521) and the Shanghai Council of Science and Technology (Grant No. 07ZR14051) are gratefully acknowledged. References [1] X.W. Zheng, L.D. Wang, J.L. Wang, Y.M. Wu, Z.L. Ning, J.F. Sun, L.M. Wang, Materials Science and Engineering A 515 (2009) 98–101. [2] Z. Horita, K. Matsubara, K. Makii, T.G. Langdon, Scripta Materialia 47 (2002) 255–260. [3] S.K. Guan, L.H. Wu, L.G. Wang, Transaction of Nonferrous Metals Society of China 18 (2008) 315–320. [4] Z. Trojanova, R. Kral, A. Chatey, Materials Science and Engineering A 462 (2007) 202–205. [5] V. Gartnerova, Z. Trojanova, A. Jager, P. Palcek, Journal of Alloys and Compounds 378 (2004) 180–183. [6] S.K. Guan, L.H. Wu, P. Wang, Materials Science and Engineering A 499 (2009) 187–191. [7] P. Skubisz, J. Sinczak, S. Bednarek, Journal of Materials Processing Technology 177 (2006) 210–213. [8] P.H. Fu, L.M. Peng, H.Y. Jiang, J.W. Chang, C.Q. Zhai, Materials Science and Engineering A 486 (2008) 183–192. [9] J.W. Chang, L.M. Peng, X.W. Guo, A. Atrens, P.H. Fu, W.J. Ding, X.S. Wang, Journal of Applied Electrochemistry 38 (2008) 207–214. [10] J.W. Chang, X.W. Guo, P.H. Fu, L.M. Peng, W.J. Ding, Electrochemical Acta 52 (2007) 3160–3167. [11] GB/T 4296-2004, Inspection Method for Microstructure of Wrought Magnesium Alloy [S], Standards Press of China, Beijing, 2004. [12] W.H. Qi, Shangdong Metallurgy 20 (1998) 16–19. [13] Q. Li, Q.D. Wang, Y.X. Wang, X.Q. Zeng, W.J. Ding, Journal of Alloys and Compounds 427 (2007) 115–123. [14] B. Liu, M.L. Zhang, R.Z. Wu, Materials Science and Engineering A 487 (2008) 347–351. [15] I.A. Maksoud, H. Ahmed, J. Rodel, Materials Science and Engineering A 504 (2009) 40–48. [16] Q.M. Peng, H.W. Dong, L.D. Wang, Y.M. Wu, L.M. Wang, Material Characterization 59 (2008) 983–986. [17] J.F. Nie, B.C. Muddle, Acta Materialia 48 (2000) 1691–1703. [18] M. Regev, A. Rosen, M. Bamberger, Metallurgical and Materials Transactions A 32 (2001) 1335.