2124Al composites

Journal of

Materials

Processing

ELSEVIER

Journal of Materials Processing Technology 48 (1995) 349-355

Technology

The Effects of Processing Parameters on Mechanical Properties of SiCw/2124AI Composites Soon H. Hong and Kyung H. Chung Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Taejon, Korea The aluminium alloy matdx composites with discontinuous ceramic reinforcements are regarded as potential materials for high-performance structural parts in aerospace and automobile industries. The processing parameters related with complicated fabdcation process of metal matdx composite need to be strictly controlled to produce composites with reproducible and uniform mechanical properties for industrial application. The understanding of the relationships among processing parameters - microstructures - mechanical properties is very important to produce reliable metal matdx composites. In this paper the effects of processing parameters, i.e., volume fraction of whiskers, vacuum hot pressing temperature, vacuum hot pressing pressure, extrusion temperature and extrusion ratio, on mechanical properties of SIC,/2124AI composites fabricated by powder metallurgy process were investigated using a statistical technique known as Taguchi method. The vacuum hot pressing temperature was found to be the most sensitive parameter to the tensile strength. As the vacuum hot pressing temperature increased up to 570°C, the tensile strength increased due to the enhanced densification of composite and increased aspect ratio of SiC whiskers, which were resulted from the reduced strength and increased amount of liquid phase in matdx with increasing vacuum hot pressing temperature. The optimum vacuum hot pressing pressure was considered to be 70MPa, below which the remaining voids reduced the tensile strength of SiC,J2124AI composite. The tensile strength increased with increasing volume fraction of SiC whiskers. However, the volume fraction of whiskers more than 20% was not helpful to increase the tensile strength since the whiskers tend to be non-uniformly distributed. The extrusion temperature need to be higher than the solidus temperature of 2124AI matrix to reduce the damage of whiskers and to improve the alignment of whiskers. The extrusion ratio of 15:1 resulted in the highest tensile strength. The alignment of whiskers improved with increasing extrusion ratio, while the aspect ratio of whiskers decreased due to the damage of whiskers. 1. INTRODUCTION Metal matrix composites (MMCs) are attractive engineering matedals since their properties can be enhanced or tailored through an addition of selected reinforcement. In particular, SiC/AI composites have been focused by many researchers due to their high specific strength and specific modulus at room or elevated temperatures[I-5]. However, the applications of SiC/AI composite are still limited due to large vadation in their properties. The variation of the properties is odginated mainly from the difficulties to control the complicated processing parameters of SiC/AI composites. The optimization of processing parameters is

very important to produce reliable composites with uniform and reproducible properties. The powder metallurgy process is generally known to give better mechanical properties even though the fabdcation cost is higher than the casting process[6-7]. However, the optimization of powder metallurgy process is not easy due to various processing parameters related to complicated fabrication steps. For the optimization of powder metallurgy process, the effects of each processing parameters on mechanical properties of SiC/AI composites need to be clarified. Some researchers[8-9] investigated the effect of the processing parameters, however, the range of parameters investigated were limited and the data base was insufficient.

*Thisresearchwassupportedbythe KoreaScienceandEngineeringFoundationunderContractNo.900401.

0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0924-0136(94)01668-Q

350 S.H. Hong, K.H. Chung / Journal of Materials Processing Technology 48 (1995) 349-355

In this study, the effects of major processing parameters related with the powder metallurgy processing of SiCw/2124AI composite are investigated using a statistical analysis technique known as Taguchi method[10]. The effects of vacuum hot pressing parameters, e.g., volume fraction of whiskers, vacuum hot pressing temperature and vacuum hot pressing pressure, and extrusion parameters, e.g., extrusion temperature and extrusion ratio, were investigated and the relationships between processing parameters microstructure properties of SiCJ2124AI composite were discussed. 2. EXPERIMENTAL PROCEDURES The 2124AI powders were purchased from Chang-sung Co. with average particle size of 20~m. The chemical compositions of powders are shown in Table 1. The J~-SiC whiskers were used as reinforcements, purchased from Tokai

Carbon Co., with average diameter of 1.51~m and length ranged 40~50t~m, average aspect ratio of 30. The 2124AI powders and SiC whiskers were wet mixed in ethyl alcohol of pH 9 with stirring and the mixture were dded at 70°C for 12hrs. The dried powders were consolidated into billets by vacuum hot pressing. The mixed powders were heated to vacuum hot pressing temperature with degassing up to 10.5 Torr, and then consolidated into billets by applying pressure. The consolidated billets were hot extruded to align the whiskers parallel to extrusion direction. The major processing parameters investigated in this study were volume fraction of whiskers, vacuum hot pressing temperature, vacuum hot pressing pressure, extrusion temperature and extrusion ratio.To investigate the effects of processing parameters with minimum experiments, the SIC/2124AI composites were fabricated by a combination

Table 1. The chemical compositions of 2124AI powders. Elements AI Cu Mg Mn Fe wt. % bal. 4,30 1.41 0.63 0.21

Si 0.08

Table 2. The processing condition for each experimental fabrication of SiC_~/2124AI composites determined by L18 orthogonal array for 1 parameter of 6 levels and 4 parameters of 3 levels. Fabrication VHP VHP Vol. % of Extrusion Extrusion Number Temperature Pressure Whisker Temperature Ratio 1 485oc 50MPa 10% 470°C 10:1 2 485oc 70MPa 20% 5O0°C 15:1 3 485oc 90MPa 30% 530°C 25:1 4 500oc 50MPa 10% 500°C 15:1 5 500oc 70MPa 20% 530°C 25:1 6 500oc 90MPa 30% 470oc 10:1 7 515°C 50MPa 20% 530°C 10:1 8 515°C 70MPa 30% 470°C 15:1 9 515°C 90MPa 10% 500°C 25:1 10 530oc 50MPa 30% 500°C 25:1 11 530oc 70MPa 10% 530°C 10;1 12 530oc 90MPa 20% 470°C 15:1 13 545oc 50MPa 20% 470°C 25:1 14 545oc 70MPa 30% 500°C 10:1 15 545oc 90MPa 10% 530°C 15:1 16 560oc 50MPa 30% 530°C 15:1 17 560oc 70MPa 10% 470°C 25:1 18 560oc 90MPa 20% 500°C 10:1

S.tI. Song, K.H. Chung / Journal of Materials Processing Technology 48 (1995) 349-355 of certain levels of parameters which were predetermined by an orthogonal array from Taguchi method. The L18 orthogonal array was used for 1 parameter of 6 levels and 4 parameters of 3 levels and the fabrication conditions are shown in Table 2. Additional fabrication of composites was conducted to investigate the effects of vacuum hot pressing temperature ranged 570~600°C above those in orthogonal array with the other parameters fixed as the vacuum hot pressing pressure of 70MPa, the volume fraction of whiskers of 20%, the extrusion temperature of 500°C and the extrusion ratio of 15:1. The extruded bars were T6 heat treated with solutionizing at 4930C for 3hrs and followed by aging at 190°C for 8hrs, The tensile specimens with gauge length of 5.7mm and gauge diameter of 2.54mm were machined. The tensile tests were conducted with an initial strain rate of 1.66x103/sec. The densities of extruded composites were measured by ASTM standard test procedure[11]. The alignment of SiC whiskers in matrix were analyzed by the pole figures from X-ray diffraction, 3. RESULTS AND DISCUSSION The results of the tensile tests of composites fabricated with the processing conditions as the orthogonal array in Table 2 were analyzed by ANOVA(Analysis of Variance) method[10]. The effects of each processing variables on tensile strength were investigated to determine the level of each parameters which lead to optimum processing condition. In order to analyze the effect of one specific processing parameter, the average value of tensile strengths was compared to compensate the variation of other processing parameters. 3. 1. Temperature and Pressure of Vacuum Hot Pressing The variation of tensile strength and density of composites with varying vacuum hot pressing temperature was shown in Fig. 1. The tensile strength rapidly increased with increasing vacuum hot pressing temperature up to 570°C, and became saturated above 570°C. The increase of tensile strength with increasing vacuum hot pressing temperature up to 570°C

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0 ]. . . . . . . . . . . . . . [95 480 520 56O VHP Temperature (o~{)0 Fig. 1 The variation of tensile strength and relative density of SiCw/2124AI composites with varying vacuum hot pressing temperature. can be explained by the following two reasons. Firstly, as the vacuum hot pressing temperature increases, the strength of 2124AI matrix decreases and the volume fraction of liquid phase, which penetrate easily into clusters of whiskers, in matrix increases. Thus, the amount of remaining voids in composite decreased with increasing vacuum hot pressing temperature. Fig.l also shows that the relative density increased with increasing vacuum hot pressing temperature up to 560°C and became theoretical density above 560°C. The vadation of relative density with varying vacuum hot pressing temperature agrees well with the variation of tensile strength as shown in Fig.1. Secondly, as the vacuum hot pressing temperature increased, the aspect ratio of whiskers increased. Since the volume fraction of liquid phase in 2124AI matrix increases with increasing vacuum hot pressing temperature, the matrix becomes more deformable and the damage of SiC whiskers would be reduced during vacuum hot pressing. The reduced damage of whiskers resulted in larger aspect ratio at higher vacuum hot pressing temperature as shown in Fig.2. The tensile strength of SIC/2124AI composite increases with increasing aspect ratio of SiC whiskers due to the enhanced load transfer from matrix to whiskers. Therefore, the increase of tensile strength of SIC/2124AI composite with increasing vacuum hot pressing temperature up

352S.H. Hon~

K.H. (:hung / Journal of Materials Processing Technology 48 (1995) 349-355

to 570°C is considered due to the increase of both the density of composite and the aspect ratio of SiC whiskers.

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3. 2. Volume Fraction of R e i n f o r c e m e n t s The variation of tensile strength of SIC/2124AI composite with varying volume fraction of SiC whiskers is shown in Fig.4. The variation of tensile strength of SIC/2124AI composite showed highest tensile strength for 20vo1.% of SiC whiskers, while the tensile strength decreased when the amount of SiC whiskers increased to 30vo1.%. The relative density of composite decreases with increasing volume fraction of SiC whiskers as shown in Fig .4. 500. ~ 100

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. . . . . . 95 10 20 30 40 Volume Fraction of Whiskers (%) Fig. 4 The vatiation of tensile strength and relative density of SiCw/2124AI composites with varying volume fraction of whiskers. As the volume fraction of SiC whiskers increased, the inter-whisker spacing decreased and the whiskers were more likely to form clusters. The density of composite decreased with increasing volume fraction of whiskers due

S.H. Hon~ K.H. Chung / Journal of Materials Processing Technology 48 (1995) 349-355 to the void formation at whisker clusters from incomplete infiltration of matrix. At the same time the whiskers were more easily damaged from direct contact with neighboring whiskers during vacuum hot pressing and extrusion with increasing volume fraction of whiskers. This damage of whiskers lead to the decrease in aspect ratio of whiskers with increasing volume fraction of whiskers, as shown in Fig.5. 7-

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Volume Fraction of Whiskers (%) Fig. 5 The variation of aspect ratio of whiskers in SiCw/2124AI composites with varying volume fraction of whiskers. The increase of tensile strength up to 20vo1.% of SiC whiskers is considered from the contribution of rule-of-mixture. However, the decrease of tensile strength at 30vo1.% of SiC whiskers can be explained by the decrease of both the relative density of composites and the aspect ratio of SiC whiskers. 3. 3. Extrusion Temperature and Extrusion Ratio The variation of tensile strength and relative density with varying extrusion temperature are shown in Fig.6. The variation of aspect ratio of whiskers with varying extrusion temperature is shown in Fig.7. The extrusion at 470°C, much lower than solidus temperature of 502°C, results in a low relative density below 98%, but the extrusion above solidus temperature gives almost theoretical density of above 99%. The density of composite and aspect ratio of whiskers increased with increasing extrusion temperature from the enhanced densification of composite with less damage of whiskers due to

200' 560 7 460 • Extrusion Temperature (°C) Fig. 6 The variation of tensile strength and relative density of SiCw/2124AI composites with varying extrusion temperature. 6.5.

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354 S.1-1. Hong, K,H. Chung / Journal of Materials Processing Technology 48 (1995) 349-355

tensile strength of composites were observed for the extrusion ratio of 15:1. 500.

whiskers decreased with increasing extrusion ratio as shown in Fig.10. These two microstructural changes during extrusion influence on the tensile strength in opposite way. The alignment of whiskers enhances the tensile strength, while the reduced aspect ratio of whiskers decreases the tensile strength. When the extrusion ratio was lower as 10:1, the inadequate alignment of whiskers resulted in lower tensile strength. While, when the extrusion ratio was higher as 25:1, the reduced aspect ratio of whiskers resulted in lower tensile strength.

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rr 5.5The whiskers were aligned parallel to extrusion direction during hot extrusion of vacuum hot pressed ingot. The XRD pole figures from SiC whiskers in Fig.9 show that the degree of whisker alignment sharply increased with increasing extrusion ratio. At the same time, the whiskers were damaged during the extrusion process. Thus the aspect ratio of

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(~) Extrusion Direction Fig. 9 <111> pole figure from SiC whiskers of 20 vol.% SIC/2124AI composites along extrusion direction with varying extrusion ratio. The 30, 50 and 70% intensity traces of SiC <111> peak relative to the peak intensity along extrusion direction are shown. The outer trace is for 30% intensity, center trace is for 50% intensity and the inner trace is for 70% intensity. (a)Extrusion ratio of 10:1, (b) extrusion ratio of 15:1, (c) extrusion ratio of 25:1.

S.H. Hong, K.H. Chung / Journal of Materials Processing Technology 48 (1995) 349-355 355 4. CONCLUSIONS The effects of powder metallurgy processing parameters on mechanical properties of SIC,/2124AI composites were investigated and the main results are summadzed as following: 1. The tensile strength of composites increased with increasing vacuum hot pressing temperature due to the local melting and softening of 2124AI matrix. The vacuum hot pressing temperature need to be higher than 560°C and the vacuum hot pressing pressure need to be higher than 70MPa to achieve full densification of composite. 2. The addition of SiC whiskers more than 20% was not helpful to improve the tensile strength due to the inadequate densification from local clustering and reduced aspect ratio of whiskers. 3. The increased extrusion temperature and extrusion ratio resulted in improved alignment of SiC whiskers with full densification. However, the tensile strength decreased at higher extrusion ratio of 25:1 due to the reduced aspect ratio of SiC whiskers from the significant damage of whiskers during extrusion. The extrusion temperature need to be slightly above the solidus temperature of 2124AI matrix in order to avoid fir-tree cracking of composite. REFERENCES

1. A. P. Divecha, S. G. Fishman and S. D. Karmarkar, J. Met., 9(1981)12 2. R. J. Arsenault, Mater. Sci. Eng., 64(1984)171 3. J. R. Pickens, J. Mater. Sci., 16(1981)1437 4. J. C. Bittence, Adv. Mater. Proce.,(1987)45 5. S. Abkowitz and P. Weihrauch, Adv. Mater. Proce., (1989)31 6. F. Kloucek and R. F. Singer, SAMPE Prec., 31 (1986)1701 7. D. L. Erich, Int. J. Powder Metall., 23(1987)45 8. R. B. Bhagat, Metall. Trans., 16A(1985)623 9. R. B. Bhagat, A. H. Clauer, P. Kumar and A. M. Ritter (Eds.), Proc. TMS Annual Meeting '90, Anaheim, 1990, TMS, Warrendale, 1990, p.327

10. R. Ross, Taguchi Technique for Quality Engineering, McGraw-Hill, New York, 1988, p.1-169 11. "Standard Test Method for Density and Interconnected Porosity of Sintered Metal Powder Structural Parts", ASTM B 328, ASTM, Philadelphia, 1983 12. S. Kalpakjian, Manufacturing Processes for Engineering Materials, Addison-Wesley, Massachusetts, 1984, p.365