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Investigation of the influence of process parameters on hot extrusion of magnesium alloy tubes
Journal of Materials Processing Technology 192–193 (2007) 292–299
Investigation of the influence of process parameters on hot extrusion of magnesium alloy tubes S.H. Hsiang ∗ , Y.W. Lin Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC
Abstract During the hot extrusion of magnesium alloy, the change of process parameters will affect the mechanical properties of extruded products. In this study, Taguchi method and analysis of variance (ANOVA) are applied to analyze the influence of process parameters on the hot extrusion of magnesium alloy tubes under extrusion ratio of 21.05. The experiments are arranged by orthogonal array method in which magnesium alloys AZ31and AZ61 are used as outer arrays, the factors selected as inner arrays are the billet heating temperature, the initial extrusion speed, the container temperature and the lubricants. The extruded tubes will be carried out tensile test and flattening test, the test results are analyzed by the quality measurement of Taguchi method to find the relationship between the process parameters and mechanical properties of the products, and to acquire the optimal combination of parameters. Then based on the results obtained from the additive model, confirmatory experiments are performed. Besides, the microstructures of extruded tubes are observed to clarify the influence of process parameters on the grain size. Finally, with the same extrusion condition, the variations in tensile strength and flattening strength due to different compositions are discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Hot extrusion tube; Magnesium alloy; Mechanical properties; Taguchi method; Orthogonal array
1. Introduction Magnesium alloy has low specific gravity, high specific strength, good ventilation and shock absorption. It is also opaque to electromagnetic waves and recycle. In recent years, magnesium alloy has begun to replace steel, aluminum and plastic as products have become increasingly miniaturized. Magnesium alloy is 35% and 77% less dense than aluminum alloy and steel, respectively. Therefore, as lightweight products, environmental protection and recyclability become increasingly important, the demand for magnesium alloy has increased and been extensively applied to the automobile industry, the bicycle industry, the aerospace and national defense industries, the civic product manufacturing industry and the 3C (computer, communication and consumer electronic) electronic industry. At present, die casting is the main method for forming and processing magnesium alloy. Wu and Chang [1] used the Taguchi method in the die casting of magnesium alloy to produce a cover for a personal data assistant (PDA), and thus to identify the optimal process parameters. Takuda et al. [2] adopted the finiteelement method to analyze the formability of AZ31 magnesium
∗
Corresponding author. Tel.: +886 2 2737 6448; fax: +886 2 2737 6460. E-mail address: [email protected] (S.H. Hsiang).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.063
base material during deep drawing. Finite element analysis can be used to predict cracking position, which can then be verified experimentally. Chen et al. [3] applied an AZ31 magnesium alloy sheet to study the deep drawing of a square cup. Following deep drawing at room temperature, the final sheet had defects, but at over 200 ◦ C, the formability is improved. In a related study of magnesium alloy, Mwembela et al. [4] studied the microstructure of AZ31 magnesium alloy during hot working. A high temperature of 300–450 ◦ C was firstly employed for rolling, which was followed by forging. As the temperature increased over 300 ◦ C, the grains grew, and the strength of the alloy decreased. In a study of the extrusion of magnesium alloy, Gouveia et al. [5] used the finite-element method to simulate forming by cold forward extrusion. Chandrasekara and Shyan John [6] investigated three magnesium alloys, AZ31, AZ61 and ZK60, as well as the effect of temperature on formability during forward extrusion. They showed that these magnesium alloys cannot be formed at between room temperature and 175 ◦ C, and that these magnesium alloys can be extruded smoothly at 200 ◦ C, but AZ31 and ZK60 magnesium alloys exhibit cracking. When the temperature was increased to 300 ◦ C, the AZ31 magnesium alloy could be well formed. Mural et al. [7] empolyed AZ31B magnesium alloy billets formed by casting, as well as homogenized materials to perform extrusion.
S.H. Hsiang, Y.W. Lin / Journal of Materials Processing Technology 192–193 (2007) 292–299
The microstructure and mechanical properties of the final products were then studied. Using the Taguchi method for plastic processing does not reduce the accuracy of the experimental results, but significantly reduces the number of times the experiments must be performed. It is therefore regarded as a more effective study method. In this study, two magnesium alloy billets, AZ31 and AZ61, are applied to carry out hot extrusion into a tube with a thickness of 2 mm. The Taguchi method is initially applied in the experimental planning and the process parameters of hot extrusion are set to optimize the mechanical properties for AZ31 and AZ61 tube. ANOVA is then employed to analyze the effect of the process parameter on the tube. Finally, the effects of the initial extrusion speed and the lubricant on the mechanical properties of the tube are investigated. 2. The principles of quality design—Taguchi method
(1) Sum of squares of the factor effect vector, SSfactor L
SSfactor =
n × r (yk − ym )2 L
(2)
k=1
where n is the number of times the experiment is performed, r is the number of times the experiment is repeated, L is the number of factor levels, yk is the response value of that factor at level k, and ym is the average of all experimental data. (2) Sum of squares of the total variation vector, SStotal ⎤ ⎡ n r SStotal = ⎣ yij2 ⎦ − n × r × ym (3) where yij stands for the j experiment data of the its set of experiment. (3) Error sums of squares, SSe SSe = SStotal − SSfactor
(4)
(4) Degree of freedom of the factor effect vector, DOFfactor DOFfactor = L − 1
(5)
(5) Degree of freedom of the total variation vector noises, DOFtotal DOFtotal = n × r − 1
(6)
(6) Variance of the factor, Vi
2.1. Experimental planning method The Taguchi method uses the signal-to-noise ratio (S/N) as a quality benchmark, based on the ideal function, different measures are used for the various quality properties. S/N ratios are one of the three forms – the normal-the-best, the smaller-thebetter and the larger-the-better. The mechanical property discussed herein is tensile strength. It is the larger-the-better characteristic. Therefore, an S/N ratio that is the larger-the-better is used, and is given in Eq. (1).
S/N = −10 log i=1 n
level of experimental errors and every factor response. The equations used in the ANOVA statistical analysis in this study are as follows [9,10].
i=1 j=1
The experiments can be planned in four ways: (1) trial-anderror; (2) one-factor-at-a-time experiments; (3) full-factorial experiments; and (4) Taguchi’s orthogonal arrays (OA) [8]. Orthogonal arrays can eliminate the bias produced by one-factorat-a-time experiments, and improve the experimental efficiency of full-factorial experiments. Based on the principle of maintaining the accuracy of experiment results, the use of orthogonal arrays can considerably reduce the time required to perform the experiments, and increases the reproducibility of the experiment results. This method can also be employed to optimize the process parameters. Therefore, orthogonal arrays are applied herein to perform experimental planning.
n
293
Vi =
(1)
in the equation, n stands the total number of measurements, and yi the measured quality value. The i in the suffix refers to the number of the experiment in the plan based on orthogonal arrays, and the calculated S/N ratio is used in the factor response statistical analysis, and the optimal combination of process parameters can be obtained. 2.2. Application of analysis of variance The statistical analysis of variance (ANOVA), used in the Taguchi method, is adopted mainly to evaluate the significance
(7)
(7) Distribution of the predicted values of the two variances, F Ffactor =
Vfactor Verror
(8)
where Verror stands for the variance caused by errors. (8) Contribution, σ i σi =
1 yi2
SSi DOFi
SSi − Verror × DOFi × 100% SStotal
(9) Experimental error, S SSerror S= = Verror DOFerror
(9)
(10)
3. Experimental planning This study involves (1) the hot extrusion process, in which the tubes are analyzed by the Taguchi method, and the optimal combination of process parameters for a best mechanical properties can be obtained; (2) the effect of the initial extrusion speed and the lubricants on the mechanical properties of the tubes; (3) the effect of the initial extrusion speed and the lubricants on the microstructure of the tubes; and (4) the use of ANOVA to analyze the degree of influence of the process parameters on the tubes. Fig. 1 shows the Taguchi experimental planning procedure.
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Fig. 2. Schematic of hot extrusion processing.
where Ao represents the cross-sectional area of the billets and Af is the crosssectional area of the final product. This study involves direct hot extrusion processing. The pre-process involves putting the lubricant on the die and the billets, and then putting them in a furnace to be heated. The container is simultaneously preheated for 5.5 h, before extrusion forming. Fig. 1. Procedure of Taguchi experimental planning.
3.2. Experimental planning of L9 orthogonal array Before the planning orthogonal array (OA) can be implemented, trial and error is applied to determine the control factors of the hot extrusion. This method seeks the levels of the control factor during hot extrusion. Its factor level is set as shown in Table 2, and the final extrusion speed is 1 mm/s. Billets of AZ31 and AZ61 magnesium alloy are the outer OA, and the control factors of the inner OA are billet heating temperature, initial extrusion speed, container temperature and lubricant, each control factor has three levels. Their inner OA is an L9 orthogonal array, and the orthogonal array is set as shown in Table 3.
3.1. Extrusion process and billet Extrusion forming involves placing billet into a container and then applying pressure to them to cause plastic deformation. They are then extruded along the holes of the extrusion die, giving the final product a long stripe with either a concrete or a vacuum center, and an evenly distributed cross-section. Fig. 2 schematically depicts the hot extrusion process. However, during the hot extrusion forming of the tubes, the billet flows into the die from three channels at the extrusion end of the die, and then passes through the welding chamber of the die to be connected to form good tubes. The geometric variance associated with the extrusion process is the ratio between the cross-sectional area of the billets and that of the final product. This is known as the extrusion ratio, and is given by Eq. (11). The billets used in this paper are Ø80 mm × 150 mm, and their elemental composition is as shown in Table 1. After extrusion, the outer diameter of the tubes is Ø40 mm; the inner diameter is Ø36 mm, the thickness is 2 mm, and the extrusion ratio is 21.05. Extrusion ration =
Ao Af
3.3. Test of mechanical properties The tube flattening test, as specified by CNS 13868 (JIS H4090), is used for reference. A 50 mm trial piece is cut from the tube, which is then pressed between the two plates of a testing machine that weighs 30 tonnes. The welded zone is placed at the position of the pressing direction, where it forms a perpendicular angle. During the flattening test, the flattening speed is 7.16 mm/min. When the plates are at H = 0.75 D, the wall of the tube is observed to identify any breaking or cracking. The tensile test is performed according to specifications CNS 2111 and CNS 2112, with a tensile speed of 16.5 mm/min.
(11)
Table 1 Composition of magnesium alloy (wt%) Alloy
AZ31B AZ61A
Magnesium
Remainder Remainder
Composition (Wt.%) Al
Zn
Mn
Ca
Si
Cu
Ni
Fe
Other
2.5–3.5 5.8–7.2
0.6–1.4 0.4–1.5
>0.20 >0.20
<0.04 <0.04
<0.10 <0.10
<0.05 <0.058
<0.005 <0.005
<0.005 <0.005
<0.30 <0.30
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Table 2 Factors and level setting
Table 3 Orthogonal array
4. Results and discussion The magnesium alloys used in this study are AZ31 and AZ61, which are used to carry out hot extrusion into a tube with a thickness of 2 mm. The final of the experiment based on the orthogonal array was performed must then undergo tensile and flattening tests, to understand the mechanical properties of the tube. Also, the tensile strength from the tensile test was analyzed by measuring the quality characteristics and by performing ANOVA, to determine the optimal combination of process parameters and the importance of each factor. Finally, a confirmatory experiment is performed to verify its accuracy. 4.1. Analysis of flattening test The flattening test is conducted with reference to the relevant specifications. When the distance between the two plates is H = 30 mm, the wall of the tube is examined to see whether any breaking or cracking has occurred. Table 4 shows that when H = 30 mm, no breaking is observed in the tubes made from
two set of billets AZ31 and AZ61, the bearable loading of the AZ31 tube is between 1765.80 and 2313.20 N, and the bearable loading of AZ61 is 1827.60 and 2109.15 N. When the flattening test is continued until the tube breaks, the loading of the AZ31 tube is 2416.20 N or above, while that of the AZ61 tube is at least 2820.38 N. Comparing these two sets of billets reveals that when H = 30 mm and they start to break, the more AZ61 tube can bear a higher loading than the AZ31 tube. 4.2. Optimal process parameters for maximizing the tensile strength of the tube Table 5 presents the results of the tensile strength experiment for the tube. This results in this table show that the mean tensile strength of the AZ31 tubes is distributed from 261.08 to 296.20 MPa, and that of the AZ61 tube is distributed from 263.97 to 299.44 MPa. The tensile strengths of the AZ31 billets and the AZ61 billets before extrusion are 239.70 and 230.21 MPa. The comparison shows that the tube that underwent hot extrusion has a higher tensile strength.
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Table 4 Analysis of flattening test (unit: N) Run
1 2 3 4 5 6 7 8 9
Factor (from Table 3)
AZ31
AZ61
A
B
C
D
Loading when H = 30 mm
Loading during breaking
Loading when H = 30 mm
Loading during breaking
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
2313.20 1912.95 2060.10 2133.68 1790.33 1888.43 1765.80 1980.64 1793.27
3463.91 2416.20 2544.71 2808.60 3084.26 2899.84 2850.79 2746.80 2943.00
2109.15 2060.10 2097.38 2133.68 2097.38 1827.60 1839.38 2035.58 2048.33
2820.38 3495.30 3078.38 3507.08 2967.53 2943.00 2833.13 2943.00 3588.50
Table 5 Tensile strength and S/N ratio of tubes Run
1 2 3 4 5 6 7 8 9
Factor (from Table 3)
AZ31 Tensile strength (MPa)
S/N Ratio
A
B
C
D
y1
y2
y3
Average
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
297.95 269.72 275.93 278.31 265.59 261.53 261.06 269.56 262.01
296.20 267.97 273.54 272.75 261.61 260.98 260.82 270.52 264.40
294.45 269.96 266.78 277.12 265.59 261.85 261.37 271.16 262.41
296.20 269.22 272.08 276.06 264.26 261.45 261.08 270.41 262.94
49.4314 48.6019 48.6914 48.8191 48.4401 48.3479 48.3356 48.6405 48.3969
AZ61 tensile strength (MPa)
S/N ratio
y1
y2
y3
Average
300.76 290.16 291.03 289.84 277.91 264.00 289.05 283.48 289.29
300.97 293.42 294.22 299.78 268.37 263.12 287.93 292.63 292.78
296.60 296.20 292.63 297.00 272.75 264.79 288.73 295.41 292.23
299.44 293.26 292.63 295.54 273.01 263.97 288.57 290.51 291.43
49.5257 49.3441 49.3260 49.4097 48.7209 48.4310 49.2050 49.2591 49.2904
The greater tensile strength of a tube corresponds to higher quality. Therefore, the S/N ratio is calculated as a larger-is-better value, using Eq. (1), it is as indicated in Table 5. Figs. 3 and 4 show the results analyzed by the factor effect diagram that is constructed from the S/N values in Table 5. The largest S/N value from this figure yields the optimal combination of process parameters. The process parameters considered in this study are billet heating temperature, initial extrusion speed, container temperature and type of lubricant. The factor effect figure, Fig. 3, reveals that the optimal combination of process parameters for the hot extrusion of AZ31 tube is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 300 ◦ C and a BN lubricant.
The factor effect figure, Fig. 4, reveals that the optimal combination of process parameters for AZ61 tube is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 350 ◦ C and a graphite lubricant. In the quality property analysis of a magnesium alloy tube, the tensile strength must be considered. The extruded tube using the aforementioned optimal combination of process parameters yields a product with the highest tensile strength.
Fig. 3. Factor effect diagram for tensile strength of AZ31 tube.
Fig. 4. Factor effect diagram for tensile strength of AZ61 tube.
4.3. Confirmation experiment The aforementioned analyzed optimal combination of process parameters must be further verified experimentally, to confirm its accuracy. In the verifying experiment for the AZ31
S.H. Hsiang, Y.W. Lin / Journal of Materials Processing Technology 192–193 (2007) 292–299
297
Table 6 Tensile strength of confirmation experiment Run
Material
Billet heating temperature (◦ C)
Initial extrusion speed (mm/s)
Container temperature (◦ C)
Lubricant
Tensile strength (MPa)
1 2 3
AZ31 AZ31 AZ31
320 320 320
2 3 4
300 300 300
BN BN BN
296.60 283.08 286.26
4 5 6
AZ61 AZ61 AZ61
320 320 320
2 2 2
350 350 350
BN Graphite MoS2
295.01 301.37 288.65
tube, only the initial extrusion speed is changed (2, 3 and 4 mm/s), while all other factors remain unchanged. In the verifying experiment for the AZ61 tube, only the lubricant is changed (BN, graphite and MoS2 ), while all other factors remain unchanged. Finally, the tensile strength and microstructures of the extruded tubes are compared. Table 6 presents data on the tensile strength obtained in the verifying experiment. The first set of data concern the extrusion of the AZ31 tube. The initial extrusion speed is varied, and the tensile strength is highest at an initial extrusion speed of 2 mm/s, followed by that at 4 mm/s and then at 3 mm/s. This result is consistent with Fig. 3. The microstructure shown in Fig. 5 reveals that the sizes of the grains of the billets all exceed those of the tube after extrusion. Extrusion considerably reduces the sizes of the grains, which is beneficial to the strengthening of the tube. In the verifying extrusion experiment on the second set of AZ61 tubes, only the lubricant is changed. The results show that when the lubricant is graphite, the tensile strength is the highest, followed by that using BN and then MoS2 . This results are consistent with Fig. 4. The microstructure displayed in Fig. 6 shows
that the size of the grains in the billet of AZ61 differs considerably from that of the grains in the tube. Extrusion reduces and homogenizes the sizes of the grains. 4.4. Analysis of variance ANOVA analysis adopts Eqs. (2)–(10). ANOVA is used to evaluate the tensile strengths of the AZ31 and AZ61 tubes, and the results shown in Tables 7 and 8. With 99.9% confidence, for both AZ31 and AZ61 tubes, the F value exceeds 10.49 (F = FINV(0.001,2,18) = 10.39), and therefore, the control factors A, B, C and D have a strong influence. Also, for both AZ31 or AZ61 tubes, the strengths of the effects of the control factors follow the same order – (A) billet heating temperature; (B) initial extrusion speed; (D) lubricant; and (C) container temperature. This order is also obtained in Section 4.2. And in ANOVA analysis, billet heating temperature has the strongest effect. Therefore, the most important factor that governs the tensile strength of the tube is the billet heating temperature, such that during the extrusion, the change in the billet heating temperature considerably affects the tensile strength of the tube.
Fig. 5. Microstructure of AZ31 billet and tube under different initial extrusion speed.
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Fig. 6. Microstructure of AZ61 billet and tube under different lubricants.
Table 7 ANOVA for tensile Strength of AZ31 Factor
SS
A – Billet heating temperature B – Initial extrusion speed C – Container temperature D – Lubricant
1061.49 760.49 482.99 581.21
Error Total
DOF
Var
F
Contribution
2 2 2 2
530.75 380.24 241.50 290.61
110.64 79.27 50.34 60.58
86.35
18
4.80
–
2972.53
26
–
–
Confidence
Significance
35.39% 25.26% 15.93% 19.23%
>99.9% >99.9% >99.9% >99.9%
YES YES YES YES
4.20%
S = 2.19
100%
Note: At least 99.9% confidence
Contribution
Confidence
Significance
43.07% 19.50% 12.75% 15.28%
>99.9% >99.9% >99.9% >99.9%
YES YES YES YES
9.41%
S = 3.5
F = FINV(0.001,2,18) = 10.39 at least 99.9% confidence.
Table 8 ANOVA for tensile Strength of AZ61 Factor
SS
A – Billet heating temperature B – Initial extrusion speed C – Container temperature D – Lubricant
1483.92 685.13 456.62 542.14
Error Total
DOF
Var
F
2 2 2 2
741.96 342.57 228.31 271.07
60.50 27.93 18.62 22.10
220.75
18
12.26
3388.57
26
–
– –
100%
Note: At least 99.9% confidence
F = FINV(0.001,2,18) = 10.39 at least 99.9% confidence.
5. Conclusions In this study, AZ31 and AZ61 magnesium alloy tubes were used to carry out hot extrusion processing. After undergoing measure of quality and ANOVA analysis of the mechanical properties of the final product obtained by extrusion yields the following conclusions.
1. In the flattening test, the tubes reach the relevant specifications. The breaking loads of the AZ31 and AZ61 tubes exceed 2416.20 and 2820.38 N, the breaking load of AZ61 tube generally exceeds that of the AZ31 tube. 2. The average tensile strength of the AZ31 tube is distributed between 261.08 and 296.20 MPa, whereas that of the AZ61 tube is distributed between 263.97 and 299.44 MPa.
S.H. Hsiang, Y.W. Lin / Journal of Materials Processing Technology 192–193 (2007) 292–299
3. The optimal combination of process parameters that maximizes the tensile strength of the AZ31 tube formed by hot extrusion is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 300 ◦ C and a BN lubricant. The optimal combination of process parameters that maximize the tensile strength of the AZ61 tube formed by hot extrusion is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 300 ◦ C and a graphite lubricant. 4. Two sets of billets of AZ31 and AZ61 were used, and the strengths of the effects of the factors on the tensile strength of the tube follow the order, heating temperature of billet, initial speed of extrusion, type of lubricant and container temperature. 5. Regardless of the billet used (AZ31 or AZ61), extrusion processing considerably reduces the sizes of the grains, increasing the strength of the tube. Acknowledgement The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract No. NSC-94-2212-E-011020.
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References [1] D.H. Wu, M.S. Chang, Use of Taguchi method to develop a robust design for the magnesium alloy die casting process, Mater. Sci. and Eng. A. 379 (2004) 366–371. [2] H. Takuda, T. Yoshi, N. Hatta, Finite-element analysis of the formability of a magnesium-based alloy AZ31 sheet, J. Mater. Process. Technol. 89-90 (1999) 135–140. [3] F.K. Chen, T.B. Huang, C.K. Chang, Deep drawing of square cups with magnesium alloy AZ31 sheets, Int. J. Mach. Tools Manuf. 43 (2003) 1553–1559. [4] A. Mwembela, E.B. Konopleva, H.J. McQueen, Microstructural development in Mg alloy AZ31during working, Scripta Mater. 37 (11) (1998) 1789–1795. [5] B.P.P.A. Gouveia, J.M.C. Rodrigues, N. Bay, P.A.F. Martins, Finite-element modeling of cold forward extrusion, J. Mater. Process. Technol. 94 (1999) 85–93. [6] M. Chandrasekaran, Y.M. Shyan John, Effect of materials and temperature on the forward extrusion of magnesium alloys, Mater. Sci. Eng. A 381 (2004) 308–319. [7] T. Murai, S. Matsuoka, S. Miyamoto, Y. Oki, Effects of extrusion conditions on microstructure and mechanical properties of AZ31B magnesium alloy extrusions, J. Mater. Process. Technol. 141 (2003) 207–212. [8] W.Y. Fowlkes, C.M. Creveling, Engineering Methods for Robust Product Design, Addison Wesley, MA, 1998. [9] G. Taguchi, Introduction to quality engineering: designing quality into products and processes, Asian Productivity Organization, Tokyo, 1990. [10] H.H. Lee, Taguchi methods: principles and practices of quality design, Gau Lih Book Co. Ltd., Taiwan, 2000.
Investigation of the influence of process parameters on hot extrusion of magnesium alloy tubes S.H. Hsiang ∗ , Y.W. Lin Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC
Abstract During the hot extrusion of magnesium alloy, the change of process parameters will affect the mechanical properties of extruded products. In this study, Taguchi method and analysis of variance (ANOVA) are applied to analyze the influence of process parameters on the hot extrusion of magnesium alloy tubes under extrusion ratio of 21.05. The experiments are arranged by orthogonal array method in which magnesium alloys AZ31and AZ61 are used as outer arrays, the factors selected as inner arrays are the billet heating temperature, the initial extrusion speed, the container temperature and the lubricants. The extruded tubes will be carried out tensile test and flattening test, the test results are analyzed by the quality measurement of Taguchi method to find the relationship between the process parameters and mechanical properties of the products, and to acquire the optimal combination of parameters. Then based on the results obtained from the additive model, confirmatory experiments are performed. Besides, the microstructures of extruded tubes are observed to clarify the influence of process parameters on the grain size. Finally, with the same extrusion condition, the variations in tensile strength and flattening strength due to different compositions are discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Hot extrusion tube; Magnesium alloy; Mechanical properties; Taguchi method; Orthogonal array
1. Introduction Magnesium alloy has low specific gravity, high specific strength, good ventilation and shock absorption. It is also opaque to electromagnetic waves and recycle. In recent years, magnesium alloy has begun to replace steel, aluminum and plastic as products have become increasingly miniaturized. Magnesium alloy is 35% and 77% less dense than aluminum alloy and steel, respectively. Therefore, as lightweight products, environmental protection and recyclability become increasingly important, the demand for magnesium alloy has increased and been extensively applied to the automobile industry, the bicycle industry, the aerospace and national defense industries, the civic product manufacturing industry and the 3C (computer, communication and consumer electronic) electronic industry. At present, die casting is the main method for forming and processing magnesium alloy. Wu and Chang [1] used the Taguchi method in the die casting of magnesium alloy to produce a cover for a personal data assistant (PDA), and thus to identify the optimal process parameters. Takuda et al. [2] adopted the finiteelement method to analyze the formability of AZ31 magnesium
∗
Corresponding author. Tel.: +886 2 2737 6448; fax: +886 2 2737 6460. E-mail address: [email protected] (S.H. Hsiang).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.063
base material during deep drawing. Finite element analysis can be used to predict cracking position, which can then be verified experimentally. Chen et al. [3] applied an AZ31 magnesium alloy sheet to study the deep drawing of a square cup. Following deep drawing at room temperature, the final sheet had defects, but at over 200 ◦ C, the formability is improved. In a related study of magnesium alloy, Mwembela et al. [4] studied the microstructure of AZ31 magnesium alloy during hot working. A high temperature of 300–450 ◦ C was firstly employed for rolling, which was followed by forging. As the temperature increased over 300 ◦ C, the grains grew, and the strength of the alloy decreased. In a study of the extrusion of magnesium alloy, Gouveia et al. [5] used the finite-element method to simulate forming by cold forward extrusion. Chandrasekara and Shyan John [6] investigated three magnesium alloys, AZ31, AZ61 and ZK60, as well as the effect of temperature on formability during forward extrusion. They showed that these magnesium alloys cannot be formed at between room temperature and 175 ◦ C, and that these magnesium alloys can be extruded smoothly at 200 ◦ C, but AZ31 and ZK60 magnesium alloys exhibit cracking. When the temperature was increased to 300 ◦ C, the AZ31 magnesium alloy could be well formed. Mural et al. [7] empolyed AZ31B magnesium alloy billets formed by casting, as well as homogenized materials to perform extrusion.
S.H. Hsiang, Y.W. Lin / Journal of Materials Processing Technology 192–193 (2007) 292–299
The microstructure and mechanical properties of the final products were then studied. Using the Taguchi method for plastic processing does not reduce the accuracy of the experimental results, but significantly reduces the number of times the experiments must be performed. It is therefore regarded as a more effective study method. In this study, two magnesium alloy billets, AZ31 and AZ61, are applied to carry out hot extrusion into a tube with a thickness of 2 mm. The Taguchi method is initially applied in the experimental planning and the process parameters of hot extrusion are set to optimize the mechanical properties for AZ31 and AZ61 tube. ANOVA is then employed to analyze the effect of the process parameter on the tube. Finally, the effects of the initial extrusion speed and the lubricant on the mechanical properties of the tube are investigated. 2. The principles of quality design—Taguchi method
(1) Sum of squares of the factor effect vector, SSfactor L
SSfactor =
n × r (yk − ym )2 L
(2)
k=1
where n is the number of times the experiment is performed, r is the number of times the experiment is repeated, L is the number of factor levels, yk is the response value of that factor at level k, and ym is the average of all experimental data. (2) Sum of squares of the total variation vector, SStotal ⎤ ⎡ n r SStotal = ⎣ yij2 ⎦ − n × r × ym (3) where yij stands for the j experiment data of the its set of experiment. (3) Error sums of squares, SSe SSe = SStotal − SSfactor
(4)
(4) Degree of freedom of the factor effect vector, DOFfactor DOFfactor = L − 1
(5)
(5) Degree of freedom of the total variation vector noises, DOFtotal DOFtotal = n × r − 1
(6)
(6) Variance of the factor, Vi
2.1. Experimental planning method The Taguchi method uses the signal-to-noise ratio (S/N) as a quality benchmark, based on the ideal function, different measures are used for the various quality properties. S/N ratios are one of the three forms – the normal-the-best, the smaller-thebetter and the larger-the-better. The mechanical property discussed herein is tensile strength. It is the larger-the-better characteristic. Therefore, an S/N ratio that is the larger-the-better is used, and is given in Eq. (1).
S/N = −10 log i=1 n
level of experimental errors and every factor response. The equations used in the ANOVA statistical analysis in this study are as follows [9,10].
i=1 j=1
The experiments can be planned in four ways: (1) trial-anderror; (2) one-factor-at-a-time experiments; (3) full-factorial experiments; and (4) Taguchi’s orthogonal arrays (OA) [8]. Orthogonal arrays can eliminate the bias produced by one-factorat-a-time experiments, and improve the experimental efficiency of full-factorial experiments. Based on the principle of maintaining the accuracy of experiment results, the use of orthogonal arrays can considerably reduce the time required to perform the experiments, and increases the reproducibility of the experiment results. This method can also be employed to optimize the process parameters. Therefore, orthogonal arrays are applied herein to perform experimental planning.
n
293
Vi =
(1)
in the equation, n stands the total number of measurements, and yi the measured quality value. The i in the suffix refers to the number of the experiment in the plan based on orthogonal arrays, and the calculated S/N ratio is used in the factor response statistical analysis, and the optimal combination of process parameters can be obtained. 2.2. Application of analysis of variance The statistical analysis of variance (ANOVA), used in the Taguchi method, is adopted mainly to evaluate the significance
(7)
(7) Distribution of the predicted values of the two variances, F Ffactor =
Vfactor Verror
(8)
where Verror stands for the variance caused by errors. (8) Contribution, σ i σi =
1 yi2
SSi DOFi
SSi − Verror × DOFi × 100% SStotal
(9) Experimental error, S SSerror S= = Verror DOFerror
(9)
(10)
3. Experimental planning This study involves (1) the hot extrusion process, in which the tubes are analyzed by the Taguchi method, and the optimal combination of process parameters for a best mechanical properties can be obtained; (2) the effect of the initial extrusion speed and the lubricants on the mechanical properties of the tubes; (3) the effect of the initial extrusion speed and the lubricants on the microstructure of the tubes; and (4) the use of ANOVA to analyze the degree of influence of the process parameters on the tubes. Fig. 1 shows the Taguchi experimental planning procedure.
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Fig. 2. Schematic of hot extrusion processing.
where Ao represents the cross-sectional area of the billets and Af is the crosssectional area of the final product. This study involves direct hot extrusion processing. The pre-process involves putting the lubricant on the die and the billets, and then putting them in a furnace to be heated. The container is simultaneously preheated for 5.5 h, before extrusion forming. Fig. 1. Procedure of Taguchi experimental planning.
3.2. Experimental planning of L9 orthogonal array Before the planning orthogonal array (OA) can be implemented, trial and error is applied to determine the control factors of the hot extrusion. This method seeks the levels of the control factor during hot extrusion. Its factor level is set as shown in Table 2, and the final extrusion speed is 1 mm/s. Billets of AZ31 and AZ61 magnesium alloy are the outer OA, and the control factors of the inner OA are billet heating temperature, initial extrusion speed, container temperature and lubricant, each control factor has three levels. Their inner OA is an L9 orthogonal array, and the orthogonal array is set as shown in Table 3.
3.1. Extrusion process and billet Extrusion forming involves placing billet into a container and then applying pressure to them to cause plastic deformation. They are then extruded along the holes of the extrusion die, giving the final product a long stripe with either a concrete or a vacuum center, and an evenly distributed cross-section. Fig. 2 schematically depicts the hot extrusion process. However, during the hot extrusion forming of the tubes, the billet flows into the die from three channels at the extrusion end of the die, and then passes through the welding chamber of the die to be connected to form good tubes. The geometric variance associated with the extrusion process is the ratio between the cross-sectional area of the billets and that of the final product. This is known as the extrusion ratio, and is given by Eq. (11). The billets used in this paper are Ø80 mm × 150 mm, and their elemental composition is as shown in Table 1. After extrusion, the outer diameter of the tubes is Ø40 mm; the inner diameter is Ø36 mm, the thickness is 2 mm, and the extrusion ratio is 21.05. Extrusion ration =
Ao Af
3.3. Test of mechanical properties The tube flattening test, as specified by CNS 13868 (JIS H4090), is used for reference. A 50 mm trial piece is cut from the tube, which is then pressed between the two plates of a testing machine that weighs 30 tonnes. The welded zone is placed at the position of the pressing direction, where it forms a perpendicular angle. During the flattening test, the flattening speed is 7.16 mm/min. When the plates are at H = 0.75 D, the wall of the tube is observed to identify any breaking or cracking. The tensile test is performed according to specifications CNS 2111 and CNS 2112, with a tensile speed of 16.5 mm/min.
(11)
Table 1 Composition of magnesium alloy (wt%) Alloy
AZ31B AZ61A
Magnesium
Remainder Remainder
Composition (Wt.%) Al
Zn
Mn
Ca
Si
Cu
Ni
Fe
Other
2.5–3.5 5.8–7.2
0.6–1.4 0.4–1.5
>0.20 >0.20
<0.04 <0.04
<0.10 <0.10
<0.05 <0.058
<0.005 <0.005
<0.005 <0.005
<0.30 <0.30
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Table 2 Factors and level setting
Table 3 Orthogonal array
4. Results and discussion The magnesium alloys used in this study are AZ31 and AZ61, which are used to carry out hot extrusion into a tube with a thickness of 2 mm. The final of the experiment based on the orthogonal array was performed must then undergo tensile and flattening tests, to understand the mechanical properties of the tube. Also, the tensile strength from the tensile test was analyzed by measuring the quality characteristics and by performing ANOVA, to determine the optimal combination of process parameters and the importance of each factor. Finally, a confirmatory experiment is performed to verify its accuracy. 4.1. Analysis of flattening test The flattening test is conducted with reference to the relevant specifications. When the distance between the two plates is H = 30 mm, the wall of the tube is examined to see whether any breaking or cracking has occurred. Table 4 shows that when H = 30 mm, no breaking is observed in the tubes made from
two set of billets AZ31 and AZ61, the bearable loading of the AZ31 tube is between 1765.80 and 2313.20 N, and the bearable loading of AZ61 is 1827.60 and 2109.15 N. When the flattening test is continued until the tube breaks, the loading of the AZ31 tube is 2416.20 N or above, while that of the AZ61 tube is at least 2820.38 N. Comparing these two sets of billets reveals that when H = 30 mm and they start to break, the more AZ61 tube can bear a higher loading than the AZ31 tube. 4.2. Optimal process parameters for maximizing the tensile strength of the tube Table 5 presents the results of the tensile strength experiment for the tube. This results in this table show that the mean tensile strength of the AZ31 tubes is distributed from 261.08 to 296.20 MPa, and that of the AZ61 tube is distributed from 263.97 to 299.44 MPa. The tensile strengths of the AZ31 billets and the AZ61 billets before extrusion are 239.70 and 230.21 MPa. The comparison shows that the tube that underwent hot extrusion has a higher tensile strength.
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Table 4 Analysis of flattening test (unit: N) Run
1 2 3 4 5 6 7 8 9
Factor (from Table 3)
AZ31
AZ61
A
B
C
D
Loading when H = 30 mm
Loading during breaking
Loading when H = 30 mm
Loading during breaking
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
2313.20 1912.95 2060.10 2133.68 1790.33 1888.43 1765.80 1980.64 1793.27
3463.91 2416.20 2544.71 2808.60 3084.26 2899.84 2850.79 2746.80 2943.00
2109.15 2060.10 2097.38 2133.68 2097.38 1827.60 1839.38 2035.58 2048.33
2820.38 3495.30 3078.38 3507.08 2967.53 2943.00 2833.13 2943.00 3588.50
Table 5 Tensile strength and S/N ratio of tubes Run
1 2 3 4 5 6 7 8 9
Factor (from Table 3)
AZ31 Tensile strength (MPa)
S/N Ratio
A
B
C
D
y1
y2
y3
Average
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
297.95 269.72 275.93 278.31 265.59 261.53 261.06 269.56 262.01
296.20 267.97 273.54 272.75 261.61 260.98 260.82 270.52 264.40
294.45 269.96 266.78 277.12 265.59 261.85 261.37 271.16 262.41
296.20 269.22 272.08 276.06 264.26 261.45 261.08 270.41 262.94
49.4314 48.6019 48.6914 48.8191 48.4401 48.3479 48.3356 48.6405 48.3969
AZ61 tensile strength (MPa)
S/N ratio
y1
y2
y3
Average
300.76 290.16 291.03 289.84 277.91 264.00 289.05 283.48 289.29
300.97 293.42 294.22 299.78 268.37 263.12 287.93 292.63 292.78
296.60 296.20 292.63 297.00 272.75 264.79 288.73 295.41 292.23
299.44 293.26 292.63 295.54 273.01 263.97 288.57 290.51 291.43
49.5257 49.3441 49.3260 49.4097 48.7209 48.4310 49.2050 49.2591 49.2904
The greater tensile strength of a tube corresponds to higher quality. Therefore, the S/N ratio is calculated as a larger-is-better value, using Eq. (1), it is as indicated in Table 5. Figs. 3 and 4 show the results analyzed by the factor effect diagram that is constructed from the S/N values in Table 5. The largest S/N value from this figure yields the optimal combination of process parameters. The process parameters considered in this study are billet heating temperature, initial extrusion speed, container temperature and type of lubricant. The factor effect figure, Fig. 3, reveals that the optimal combination of process parameters for the hot extrusion of AZ31 tube is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 300 ◦ C and a BN lubricant.
The factor effect figure, Fig. 4, reveals that the optimal combination of process parameters for AZ61 tube is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 350 ◦ C and a graphite lubricant. In the quality property analysis of a magnesium alloy tube, the tensile strength must be considered. The extruded tube using the aforementioned optimal combination of process parameters yields a product with the highest tensile strength.
Fig. 3. Factor effect diagram for tensile strength of AZ31 tube.
Fig. 4. Factor effect diagram for tensile strength of AZ61 tube.
4.3. Confirmation experiment The aforementioned analyzed optimal combination of process parameters must be further verified experimentally, to confirm its accuracy. In the verifying experiment for the AZ31
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Table 6 Tensile strength of confirmation experiment Run
Material
Billet heating temperature (◦ C)
Initial extrusion speed (mm/s)
Container temperature (◦ C)
Lubricant
Tensile strength (MPa)
1 2 3
AZ31 AZ31 AZ31
320 320 320
2 3 4
300 300 300
BN BN BN
296.60 283.08 286.26
4 5 6
AZ61 AZ61 AZ61
320 320 320
2 2 2
350 350 350
BN Graphite MoS2
295.01 301.37 288.65
tube, only the initial extrusion speed is changed (2, 3 and 4 mm/s), while all other factors remain unchanged. In the verifying experiment for the AZ61 tube, only the lubricant is changed (BN, graphite and MoS2 ), while all other factors remain unchanged. Finally, the tensile strength and microstructures of the extruded tubes are compared. Table 6 presents data on the tensile strength obtained in the verifying experiment. The first set of data concern the extrusion of the AZ31 tube. The initial extrusion speed is varied, and the tensile strength is highest at an initial extrusion speed of 2 mm/s, followed by that at 4 mm/s and then at 3 mm/s. This result is consistent with Fig. 3. The microstructure shown in Fig. 5 reveals that the sizes of the grains of the billets all exceed those of the tube after extrusion. Extrusion considerably reduces the sizes of the grains, which is beneficial to the strengthening of the tube. In the verifying extrusion experiment on the second set of AZ61 tubes, only the lubricant is changed. The results show that when the lubricant is graphite, the tensile strength is the highest, followed by that using BN and then MoS2 . This results are consistent with Fig. 4. The microstructure displayed in Fig. 6 shows
that the size of the grains in the billet of AZ61 differs considerably from that of the grains in the tube. Extrusion reduces and homogenizes the sizes of the grains. 4.4. Analysis of variance ANOVA analysis adopts Eqs. (2)–(10). ANOVA is used to evaluate the tensile strengths of the AZ31 and AZ61 tubes, and the results shown in Tables 7 and 8. With 99.9% confidence, for both AZ31 and AZ61 tubes, the F value exceeds 10.49 (F = FINV(0.001,2,18) = 10.39), and therefore, the control factors A, B, C and D have a strong influence. Also, for both AZ31 or AZ61 tubes, the strengths of the effects of the control factors follow the same order – (A) billet heating temperature; (B) initial extrusion speed; (D) lubricant; and (C) container temperature. This order is also obtained in Section 4.2. And in ANOVA analysis, billet heating temperature has the strongest effect. Therefore, the most important factor that governs the tensile strength of the tube is the billet heating temperature, such that during the extrusion, the change in the billet heating temperature considerably affects the tensile strength of the tube.
Fig. 5. Microstructure of AZ31 billet and tube under different initial extrusion speed.
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Fig. 6. Microstructure of AZ61 billet and tube under different lubricants.
Table 7 ANOVA for tensile Strength of AZ31 Factor
SS
A – Billet heating temperature B – Initial extrusion speed C – Container temperature D – Lubricant
1061.49 760.49 482.99 581.21
Error Total
DOF
Var
F
Contribution
2 2 2 2
530.75 380.24 241.50 290.61
110.64 79.27 50.34 60.58
86.35
18
4.80
–
2972.53
26
–
–
Confidence
Significance
35.39% 25.26% 15.93% 19.23%
>99.9% >99.9% >99.9% >99.9%
YES YES YES YES
4.20%
S = 2.19
100%
Note: At least 99.9% confidence
Contribution
Confidence
Significance
43.07% 19.50% 12.75% 15.28%
>99.9% >99.9% >99.9% >99.9%
YES YES YES YES
9.41%
S = 3.5
F = FINV(0.001,2,18) = 10.39 at least 99.9% confidence.
Table 8 ANOVA for tensile Strength of AZ61 Factor
SS
A – Billet heating temperature B – Initial extrusion speed C – Container temperature D – Lubricant
1483.92 685.13 456.62 542.14
Error Total
DOF
Var
F
2 2 2 2
741.96 342.57 228.31 271.07
60.50 27.93 18.62 22.10
220.75
18
12.26
3388.57
26
–
– –
100%
Note: At least 99.9% confidence
F = FINV(0.001,2,18) = 10.39 at least 99.9% confidence.
5. Conclusions In this study, AZ31 and AZ61 magnesium alloy tubes were used to carry out hot extrusion processing. After undergoing measure of quality and ANOVA analysis of the mechanical properties of the final product obtained by extrusion yields the following conclusions.
1. In the flattening test, the tubes reach the relevant specifications. The breaking loads of the AZ31 and AZ61 tubes exceed 2416.20 and 2820.38 N, the breaking load of AZ61 tube generally exceeds that of the AZ31 tube. 2. The average tensile strength of the AZ31 tube is distributed between 261.08 and 296.20 MPa, whereas that of the AZ61 tube is distributed between 263.97 and 299.44 MPa.
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3. The optimal combination of process parameters that maximizes the tensile strength of the AZ31 tube formed by hot extrusion is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 300 ◦ C and a BN lubricant. The optimal combination of process parameters that maximize the tensile strength of the AZ61 tube formed by hot extrusion is heating temperature of billet = 320 ◦ C, initial speed of extrusion = 2 mm/s, container temperature = 300 ◦ C and a graphite lubricant. 4. Two sets of billets of AZ31 and AZ61 were used, and the strengths of the effects of the factors on the tensile strength of the tube follow the order, heating temperature of billet, initial speed of extrusion, type of lubricant and container temperature. 5. Regardless of the billet used (AZ31 or AZ61), extrusion processing considerably reduces the sizes of the grains, increasing the strength of the tube. Acknowledgement The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract No. NSC-94-2212-E-011020.
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