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Effects of process parameters on the properties of squeeze-cast SiCp-6061 Al metal-matrix composite
HATERUMS SCIENCE & EMGlMEERlWG
A
Materials Scienceand EngineeringA207 (1996) 135%141
Effects of process parameters on the properties of squeeze-cast Sic,-606 1 Al metal-matrix composite Boq-Kong Hwu a, Su-Jien Lin a, Min-Ten Jahn b a Department
of Materials Science and Engineering, National Tsing b Mechanical Engineering Departnzem, California
Hua Unir;ersity, Hsinchu, State University, Long
Tailvan, People’s Beach, CA, USA
Republic
of China
Received29 March 1995;accepted16 August 1995
Abstract The effects of process parameters on the properties of squeeze-cast high reinforcement volume-fraction Sic,-6061 Al metal matrix composite (MMC) have been examined. The process parameters included cooling rate, reinforcement particle size, addition
of silica binder, and solution treatment temperature. The properties studied were flexure strength, microstructure, variation of hardness, solute content gradient, and aging behavior. It was found that water cooling improved the flexure strength and hardness of MMC significantly when compared with furnace-cooled MMC. For the same volume fraction of reinforcement, lowering the average particle size from 85 to 14 pm increased the flexure strength and hardness of the MMC greatly. A solution treatment temperature very close to the eutectic point was found to improve the strength. Mechanisms leading to the above-mentioned effects and suggestions are discussed. Keywords:
Processparameters; Squeeze-castSic,-6061 Al
1. Introduction Particulate-reinforced metal-matrix composites (MMCs) possess excellent combination of properties such as light weight, high elastic modulus, high specific strength, desirable coefficient of thermal expansion, high temperature resistance, and super wear resistance [1,2]. MMCs have been adopted increasingly for industrial applications, especially in aerospace and defense sectors [1,3,4]. Various methods have been developed for manufacturing metal-matrix composites. The methods are differentiated by the state (solid or liquid) of the matrix before processing. Among the liquid-state processes are liquid metal infiltration [5,6], squeeze casting [7,8] and investment casting [9]. Some of the solid-state processes are powder metallurgy [lo], hot isostatic pressing [l 11, and diffusion bonding [12]. Among the processes, squeeze casting stands out as remarkably practical and economical. The success of squeeze casting to produce high quality, non-porous MMCs is based on appropriate control of many process parameters. A comprehensive study related to the effects of various process parameters such as pressure, cooling 0921-5093/96/$15.000 1996- ElsevierScienceS.A. All rights reserved
rate, particle size, binder addition and solution treatment temperature on the properties of high reinforcement volume fraction MMCs is considered necessary for obtaining good quality products. The application of high pressure during squeeze casting is required not only to enhance the wettability of the molten metal to the reinforcement but also to drive the liquid metal into the spaces left by the reinforcement in a preform. The cooling rate after casting may control the interface reaction and composition distribution of matrix alloy and, therefore, may determine the quality of the MMCs. The particle size of the reinforcement will affect the penetration ability of liquid metal into a preform and also the fracture mechanism of MMCs. The addition of binder not only influences the quality of preforms (and therefore the properties of MMCs) but also the interface reaction between the matrix and the reinforcement. In this study various squeeze casting process parameters such as pressure, cooling rate, binder addition, and solution treatment temperature for manufacturing high reinforcement volume fraction MMCs were examined. New experimental results were obtained, and
136
B.-K. Hw
et n1.j hfatetkls Science and Etzgheeritlg A207 (1996) 135-141
the mechanisms leading to the new results were discussed.
control. Surrounding the mold was a heating element. Pairs of thermocouples were used to measure the temperature.
2. Experimental details
2.4. Squeeze casting
2.1. Materials
A preform was put into the mold and preheated with the mold to the solidus temperature of 6061 Al alloy which is 582 “C. Molten Al alloy at 702 “C was then poured into the mold followed by the push action of the preheated ram under the control of the hydraulic press. In squeeze casting of the composites, a pressure of 100 MPa was applied until the ram could no longer advance which meant that the penetration of the liquid metal into the preform was completed. After casting the composite was either furnace cooled (F.C.) or water cooled (W.C.). For comparison purposes, some composites were cast without using any preform and therefore no silica binder was involved. In this case, the Sic particles were squeezed into the mold directly followed by similar preheating and squeeze casting as described. Unless specified, the term “composite” used in this paper is the composite manufactured by using a preform.
Commercial 6061 aluminum alloy was used as the matrix and particulate SIC as the reinforcement. The chemical composition of the 6061 Al alloy is Al0.90%Mg-0.67%Si0.23%Cu-O.l3%Cr-O.l4%Fe. The Sic, used was an abrasive grade with 99.2% Sic content. Three particle sizes of the SIC, were adopted in this investigation, they were mesh number 180 # (N 85 itm), 400 # (N 30 pm) and 800 # (- 14 /lrn). The density of the Sic, was 3.22 g cm - 3. Silica colloid (30 wt% silica in water) was used as the binder in the preparation of preforms. 2.2. Preforms Appropriate amounts of SIC, were mixed with silica colloid in a container. The mixture was stirred thoroughly and blended and then poured into a mold. After baking at 120 “C for 24 h, the preform was removed from the mold. These were cylinders with 40 mm height and 30 mm diameter. The preforms were then sintered at 1000 “C for 4 h to increase their compressive strength. 2.3. Squeeze-casting apparatus Fig. 1 shows the schematic diagram of the squeezecasting apparatus used in this research. A 50-ton hydraulic press was used as the main driving tool to squeeze cast the composite. The mold system used was a pair of half molds with 165 mm height and 31 mm inner diameter. Water circulation paths (6 mm inner diameter) were drilled inside the mold for cooling rate control. The water circulation paths were connected to a pump and a flow gauge to facilitate the cooling rate
hydraulic
frame
2.5. Solution treatment cd aging After squeeze casting, some composites were solution treated at 557 “C for 2 h followed by water quench. The selection of 557 “C for solution treatment instead of conventional 529 “C will be explained later. Aging was carried out at 160 “C for some hours until peak hardness was reached. 2.6. Mechanical tests An MTS machine was used to determine the flexure strength of the composites in either as cast or peak aged condition. Dimensions of test specimens were 3.0 x 4.0 x 36.0 mm according to JIS specification. Before testing, the specimens were polished up to 3 /lrn diamond powder. Vickers hardness tests were carried out using a load of 30 kg to measure the hardness at different cross section of the composites. 2.7. Optical microscopy)
piston ram Al-melt preform cooling water outlet -cooling water inlet
II
I hydraulic pump
Fig.
I\
II thermocouple
1. Schematic diagram of squeeze-casting equipment.
Optical microscopy (OM) was performed to evaluate the distribution of the particulate reinforcements and the extent of the infiltration of Al alloy. The cracks of Sic particles, damages in the preforms and the pores produced during squeeze casting were checked using optical microscopy. Specimens for OM were prepared following conventional metallographic procedure up to 1 /lrn diamond powder polishing. Keller’s reagent was used when etching was required.
B.-K. Hwu et 01. I Materials Scierzce and Engineering A.207 (1996) 135-141
137
Fig. 3. The grain structure of the 180 # Sic,/6061 Al composite.
(b)
ment. The volume fraction (Vr) of SIC, in all the composites was determined to be about 50%. The distribution of particulate reinforcement is rather uniform. The infiltration of 6061 Al alloy is quite complete. No significant pores were observed at the SiC,/Al alloy interfaces. After etching in Keller’s reagent, the grain structures were observed as shown in Fig. 3. The grain sizes decreased with decreasing particle size. They were about 20, 16 and 10 pm, respectively, for composites with 180 # , 400 # and 800 # particulate reinforcements. Good quality composites could be expected from the appearance of microstructures. 3.2. Flexwe test The flexure strengths of as-cast SIC&Al 6061 composites containing various particle size of reinforcement are compared in Fig. 4. The flexure strengths increased when the particle sizes were reduced. They were 279, 396 and 495 MPa, respectively, for composites containing reinforcement with particle 180 # , 400 # and
(4 Fig, 2. Optical micrographs of the Sic,/6061 Al composites for different particle sizes: (a) 180 # ; (b) 400 # ; (c) 800 # .
2.8. Chemical analysis
After casting, the composite was cut into five roughly equal cross-section pieces from top to bottom of the composite along infiltration direction. The weight percent of Mg in each piece was analyzed using inductively coupled plasma-atomic emission spectrometer to determine whether segregation occurred during solidification.
6ooL 400-
3oo
200
Fig. 2 shows the microstructures of as-cast composites containing various particle sizes of reinforce-
J79232
3. Results and discussion 3.1. Microstuctwe
195
500-
I 180#
800-f+ Mesh Number
Fig. 4. The flexure strengths of the as-cast Sic,/6061 Al composites.
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800 # . The flexure strengths of composites made without the addition of silica- binder are also indicated in Fig. 4, the strengths being about 20% lower than the composites made from preforms where silica binder was added. It is clear that the use of silica colloid to make preform was quite successful in this study. Baking the preforms at 120 “C for 24 h removed the water in silica colloid completely. Sintering the preforms at 1000 “C for 4 h made the preforms strong enough to sustain the pressure applied during squeeze casting. Sintering at 1000 “C also promotes the formation of SiO, on the Sic, surface which can enhance the bonding between the SIC, and the 6061 Al matrix alloy [13] such that higher strength is obtained. The results mentioned above are for composites that were furnace cooled after squeeze casting. When water cooling was applied after squeeze casting, the flexure strength increased about 10%. For example, the flexure strength of composite containing 180 # SIC, increased from 279 MPa to 310 MPa when the composite was water cooled. Fast cooling rate reduces the grain size of the matrix which in turn raises the strength of the composite. The magnitude of pressure exerted during casting was also found to affect the property of composites, in general, the higher the pressure the better the flexure strength. The flexure strengths were 279 and 293 MPa, respectively, when the pressure applied was 100 and 200 MPa. Since the increase in flexure strength was only about 5%, the pressure used was 100 MPa through this study. The flexure strength and hardness of the composites increased as the particle size of reinforcement was reduced as shown in Figs. 4 and 5. The increase in strength of composites due to smaller reinforcement particle size has been reported by many authors [10,14]. Statistically, larger flaws and more defects are more likely to exist in larger particles and, therefore, will deteriorate the strength of composites when compared to the composites containing smaller particles. The smaller grain size in the composites containing smaller reinforcement particles can also contribute to the increase in strength. If the volume fraction of SIC, remains the same, the smaller particle sizes will provide more interface area which serves as the nucleation sites of grain formation. Therefore, smaller grains will be observed in the matrix of composite containing smaller Sic,. For the same volume fraction, the spacing between particles is reduced when the particle size is smaller. Therefore, smaller particles will exert more constraint on grain growth during cooling and more restriction on plastic flow during deformation which can also contribute to the increase in strength. The application of a pressure of 100 MPa during squeeze casting has been a common practice Fc151.
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0 '180# 400# v 800# v 180# 0 180# l
0
9 Distance
SiC6/6061, SiCo/6061, SiCi,/SOSl; SiCp/6061, SiCp/6061,No
18
F!C. F, C. E C. n!C. Binder,
27
I
0 36
From Top of Composite(mm)
Fig. 5. The hardnessdistributions of the as-castcomposites.(F.C. meansfurnacecooling and W.C. meanswater cooling.)
3.3. Vasintion of Iwdness with locatiol2 nid segregation of Mg The as-cast cylindrical composite was cut into five roughly equal disk pieces. The thickness of each disk was about 9 mm. A hardness test was performed on the center area of each disk surface to determine the variation of hardness with respect to the position of the as-cast composite. Fig. 5 shows the variation of hardness with location for various composites with different particle sizes and different cooling rates. It is clear from Fig. 5 that a smaller particle size of reinforcement resulted in higher hardness. This is in agreement with the effect of particle size on the flexure strength of the composites. The hardness dropped continuously from the top of the composite to at least 18 mm below the top, except for the composite that was water cooled right after casting. The decrease of hardness from the top to bottom of composites has been reported by Friend et al. [16] in their study of 6-Al,O,J6061 Al composites. In their opinion, the variation of hardness in composites was due to the declining Mg content from the top to bottom of the composites. During infiltration the Mg atom in molten alloy might react with SiOa such that the solute (Mg) content might be gradually depleted. Since the molten alloy took longer path to reach the bottom, the depletion of Mg would be more pronounced at the bottom of the composites resulting in the drop of hardness from the top to bottom. In order to check this argument, the Mg percentage at different locations in the composite was analyzed. As shown in Fig. 6, the Mg content increases from top to bottom of the composites in our study. Therefore, their explanation [16]
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does not fit our results. It is well established that the solute atoms are moving away from solid to liquid phase during solidification. The content of solute will get richer in the material which solidifies last. At the initial stage of squeeze casting, the temperature of molten alloy increases due to the application of pressure (100 MPa). Solidification started from the top of the composite after complete infiltration since heat dissipation went faster through the piston that was connected to the ram. Therefore, the Mg content increased from the top to the bottom, as shown in Fig. 6. If the mold is water cooled, the molten alloy will solidify starting from the bottom of the mold; therefore, the Mg content will get richer from bottom to top. This is again in agreement with Fig. 6. One can explain why the hardness went down while the Mg content went up from top to bottom of the furnace-cooled composites in the following way. During squeeze casting, the 100 MPa pressure was maintained until mold temperature dropped to 582 “C. During solidification the pressure on the top part of the composites where the matrix solidified first was 100 MPa as applied. However, the pressure on the lower part of the composite was lower than 100 MPa during solidification since part of the pressure was used to overcome the friction between the solidified top portion of composite and the mold wall. Therefore, the bonding between Sic, and matrix will not be as good at the bottom where less pressure was experienced as compared to the top. Fig. 7 presents the microstructure of the composite at the top (Fig. 7(a)) and that at the bottom (Fig. 7(b)). No pores in the matrix were observed in the top portion of the composite (Fig. 7(a)), while many pores existed in the matrix close to the bottom portion of the 1.4 . 18046 ,Binder, F.C. l 800# ,No Binder, F.C. v 18O# ,Binder, W. C.
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(a)
Fig. 7. Microstructures for different locations of the composite: (a) top site; (b) bottom site (27 mm below the top).
composite (Fig. 7(b)). The presence of pores in the matrix clearly makes the composite weaker. If the mold is water cooled, the composite will solidify very fast, and no pressure gradient will exist during solidification resulting in uniform hardness from the top to the area close to the bottom of the composites. This argument is in agreement with our experimental results as shown in Fig. 5. At the very bottom, the composite was very hard and brittle. Cracking took place during hardness testing, making accurate measurement impossible. This is attributed to the concentration of silica during baking of preforms, when silica moved upward slowly, making higher concentration of silica at the top of the preform. The preforms were inverted when being put into the mold of the squeeze-casting apparatus. Therefore, the very bottom of composite contained a high concentration of sintered silica which makes the bottom very brittle. If no preforms are used in the squeeze-casting process, the hardness at bottom will continuously drop. The curve for composite without using silica binder in Fig. 5 confirms our explanation. 3.4. Solution treatment and aging
0.2
0
9
18
27
Distance from Top of Composite (mm) Fig, 6. The Mg distributions
of the as-cast composites.
36
Some composites were solution treated at 557 “C for 2 h followed by water quench. Aging was first carried out at 175 “C. It was found that the peak-aging times were very hard to identify since the aging rate of the
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300: E P ?i VI L 22 .u '
, 250-
1
200"
0 #180 SiCp/6061 #400 SiCp/6061, v #&IO SiCp/6061 l
I 10
I 1
1501 as-quehch
I 100
Time (hr) Fig. 8. The aging curves of the composites for different particle size.
composites was very fast. Aging was then performed at 160 “C, and the results are shown in Fig. 8 which gives the relationship between the hardness of composite and its aging time. The composite reached peak hardness faster when the Sic, size was smaller. The peak-aging time for composite with 180 # and 400 # Sic, was about 5 h, while the peak-aging time for composite with 800 # Sic, was about 3 h. Differential scanning calorimetry (DSC) was conducted on the composite specimens. The results of DSC for composite specimens solution treated at 529 and 557 “C are compared in Fig. 9. Two main differences exist between Figs. 9(a) and 9(b). The area under the exothermic peak at about 242 “C is larger in Fig. 9(b) than in Fig. 9(a). An endothermic peak appears at about 560 “C in Fig. 9(a) but not in Fig 9(b). The peak
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at about 242 “C is believed to be due to the formation of metastable p” and stable ,&’ precipitates [17]. The area under the peak is the total enthalpy of precipitate formation which indicates the total volume of precipitate formed. Comparison between the area under the peak at 242 “C of Figs. 9(a) and 9(b) indicates that more precipitates of ,f?’ and p” were formed during aging in composites solution treated at 557 “C than at 529 “C. The presence of endothermic peak at 560 “C in Fig. 9(a) enables us to explain this phenomenon as will be described in next paragraph. From Al-Mg-Si phase diagram, the ternary Mg,SiAl-Si eutectic point occurs at 558 “C. Therefore, the endothermic peak at 560 “C is believed to be due to the melting of Mg,Si-Al-Si eutectic phase. More solute atoms are available for precipitate formation if the composites are solution treated at 557 “C since at this temperature the solution treatment is more complete than at 529 “C. Therefore, a great volume of precipitates will form if the composite is solution treated at 557 “C. Since 557 “C is very close to the ternary Mg,Si-Al-Si eutectic point (558 “C), the temperature control of the oven must be very accurate (better than & 1 “C); otherwise it would risk occurrence of some localized melting which would weaken the composite drastically. This explains why solution treatment at 557 “C was selected in this study instead of commonly adopted 529 “C [18-201. As shown in Fig. 8, the composites reached peak hardness faster when the Sic, size was smaller. It has been suggested that the dislocations generated owing to the thermal mismatch between the reinforcement and the matrix during quenching after solution treatment may promote the formation of precipitates [20]. It is also believed that the surface of Sic, may provide preferential nucleation sites for intermediate precipitates [21]. At the same volume fraction, composites containing smaller reinforce,ment particles have more particle surface area and less particle spacing so that more dislocations are generated after quenching. Therefore, composites containing smaller particles reached peak-age condition faster. The high volume fraction of reinforcement in this study enables the peak hardness to occur within 5 h under 160 “C.
4. Conclusions
200
300
Temperature
400
500
f
(“C 1
Fig. 9. DSC curves of 180 # SC,/6061 Al composites for different solution treatment temperatures: (a) 529 “C; (b) 557 “C.
Based upon the experimental results and discussion provided above, the following conclusions can be drawn. (1) Good quality composites containing high volume fraction (50%) of Sic, with various particle size can be obtained using the squeeze-casting process adopted in this study. The strength and hardness of the composites can be further improved if water cooling is applied during solidification.
B.-K.
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et al. I Materials
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(2) The flexure strength of the composites increased as the reinforcement particle size was reduced for same volume fraction of SIC,. This is attributed to less defects, smaller grain size and more constraint on plastic flow during deformation in composites with smaller reinforcement particles. (3) The hardness of the furnace-cooled composites dropped gradually from the top of the specimen to some distance below. This is attributed to the appearance of pores in the matrix which is due to lower squeezing pressure experienced at the lower part of the specimen. The variation of hardness was avoided when water cooling was applied. (4) A large quantity of solute atoms is available for precipitate formation if the specimen is solution treated at 557 “C, which is very close to the ternary Mg,SiAl-Si eutectic temperature. Subsequent aging generated high peak hardness in less than 5 h even if the specimens were aged at 160 “C.
Acknowledgments
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[3] T.M.F. Ronald, Adu. Mater. Proc., 144 (1993) 24. [4] M.W. Toaz, Discontinuous Ceramic Fiber MMCs. Engirteering Muteriais Handbook, Vol. 1, 1987, Metals Park, OH, ASM, pp. 903-910. [5] S. Yoda, V. Kurihara, K. Wakashima and S. Umekawa, IMernll. Trans. A, 9 (1978) 1229. [6] A. Mortensen and J.A. Cornie, Metail. Trans. A, 18 (1987) 1160. Sci. Lett., 3 (1984) [71 E. Nakata and Y. Kagawa, J. Muter. 968.
PI C.P. Ju, K.I. Chen and J.H. Chern Lin, J.
Mater.
Sci., 29 (1994)
5127.
A.W.H. Morris and A. Burwood-Smith, High Temperature Turbiues, AGARD-CP-73-71, Jan. 1971. UOI X. Ni, M.S. Maclean and T.N. Baker, Mater. Sri. Technol., 10 (1994) 452. t111J.A. McElman, Continuous Si/icon Carbide Fiber MMCsz Eilgi/leering Materials Handbook, Vol. 1, 1987, Metals Park, OH, ASM, pp. 858-866. Jo& [121W.C. Harrigan, Jr. and R.H. Flowers, TMS-AIME/ASM Composite Materials Committee Symp., Oct. 1977, pp. 24-26. thesis, National Tsinghua University, [I31 Min-Shi Tsai, Masters Hsinchu, Taiwan, ROC, June 1993. v41 M. Finot, Y.L. Shen, A. Needleman and S. Suresh, MetaL Mater. Trans. A, 25 (1994) 2403. [I51 Z. Zhu, in S.G. Fishman and A.K. Dhingra (eds.), Cast Reinforced Metal Composites, ASM, Metals Park, Ohio, 1988, pp. [91
93-99.
We are pleased to acknowledge the financial support for this research by the National Science Council, Taiwan, R.O.C. under Grant NSC-82-0405-E-007-318.
References [l] W.C. Harrigan, Jr. Discontinuous Silicon Fiber MMCs. Engineering Materials Ha&book, Vol. 1, 1987, Metals Park, OH, ASM, pp. 889-895. [2] R.K. Everett and R.J. Arsenault, Metal Matrix Composites: Processing a& lnterj&es, 1991, Boston, Academic Press, pp. 83-57.
[161CM. Friend, I. Horsfall, S.D. Luxton and R.J. Young, in S.G. Fishman and A.K. Dhingra (eds.), Proc. ht. Symp. A~u. Cast Reirzjorced Metal Composites, Chicago, IL, USA, ASM, Sept. 1988, pp. 309-315. [I71 P. Appendino, C. Badini, F. Marino and A. Tomasi, Mater. Sci. Eng., Al35 (1991) 275. P81 T. Das, S. Bandyopadhyay and S. Blairs, J. Mater. Sci., 29 (1994) 5680. [I91 J.P. Unsworth and S. Bandyopadhyay, J. Mater. Sci., 29 (1994) 4645.
1201M. Levin and B. Karlsson, ht. J.
1211I. Dutta and D.L. Bourell, 67.
Fatigue, Mater. Sci.
15 (1993) 377. Eng., All2 (1989)
A
Materials Scienceand EngineeringA207 (1996) 135%141
Effects of process parameters on the properties of squeeze-cast Sic,-606 1 Al metal-matrix composite Boq-Kong Hwu a, Su-Jien Lin a, Min-Ten Jahn b a Department
of Materials Science and Engineering, National Tsing b Mechanical Engineering Departnzem, California
Hua Unir;ersity, Hsinchu, State University, Long
Tailvan, People’s Beach, CA, USA
Republic
of China
Received29 March 1995;accepted16 August 1995
Abstract The effects of process parameters on the properties of squeeze-cast high reinforcement volume-fraction Sic,-6061 Al metal matrix composite (MMC) have been examined. The process parameters included cooling rate, reinforcement particle size, addition
of silica binder, and solution treatment temperature. The properties studied were flexure strength, microstructure, variation of hardness, solute content gradient, and aging behavior. It was found that water cooling improved the flexure strength and hardness of MMC significantly when compared with furnace-cooled MMC. For the same volume fraction of reinforcement, lowering the average particle size from 85 to 14 pm increased the flexure strength and hardness of the MMC greatly. A solution treatment temperature very close to the eutectic point was found to improve the strength. Mechanisms leading to the above-mentioned effects and suggestions are discussed. Keywords:
Processparameters; Squeeze-castSic,-6061 Al
1. Introduction Particulate-reinforced metal-matrix composites (MMCs) possess excellent combination of properties such as light weight, high elastic modulus, high specific strength, desirable coefficient of thermal expansion, high temperature resistance, and super wear resistance [1,2]. MMCs have been adopted increasingly for industrial applications, especially in aerospace and defense sectors [1,3,4]. Various methods have been developed for manufacturing metal-matrix composites. The methods are differentiated by the state (solid or liquid) of the matrix before processing. Among the liquid-state processes are liquid metal infiltration [5,6], squeeze casting [7,8] and investment casting [9]. Some of the solid-state processes are powder metallurgy [lo], hot isostatic pressing [l 11, and diffusion bonding [12]. Among the processes, squeeze casting stands out as remarkably practical and economical. The success of squeeze casting to produce high quality, non-porous MMCs is based on appropriate control of many process parameters. A comprehensive study related to the effects of various process parameters such as pressure, cooling 0921-5093/96/$15.000 1996- ElsevierScienceS.A. All rights reserved
rate, particle size, binder addition and solution treatment temperature on the properties of high reinforcement volume fraction MMCs is considered necessary for obtaining good quality products. The application of high pressure during squeeze casting is required not only to enhance the wettability of the molten metal to the reinforcement but also to drive the liquid metal into the spaces left by the reinforcement in a preform. The cooling rate after casting may control the interface reaction and composition distribution of matrix alloy and, therefore, may determine the quality of the MMCs. The particle size of the reinforcement will affect the penetration ability of liquid metal into a preform and also the fracture mechanism of MMCs. The addition of binder not only influences the quality of preforms (and therefore the properties of MMCs) but also the interface reaction between the matrix and the reinforcement. In this study various squeeze casting process parameters such as pressure, cooling rate, binder addition, and solution treatment temperature for manufacturing high reinforcement volume fraction MMCs were examined. New experimental results were obtained, and
136
B.-K. Hw
et n1.j hfatetkls Science and Etzgheeritlg A207 (1996) 135-141
the mechanisms leading to the new results were discussed.
control. Surrounding the mold was a heating element. Pairs of thermocouples were used to measure the temperature.
2. Experimental details
2.4. Squeeze casting
2.1. Materials
A preform was put into the mold and preheated with the mold to the solidus temperature of 6061 Al alloy which is 582 “C. Molten Al alloy at 702 “C was then poured into the mold followed by the push action of the preheated ram under the control of the hydraulic press. In squeeze casting of the composites, a pressure of 100 MPa was applied until the ram could no longer advance which meant that the penetration of the liquid metal into the preform was completed. After casting the composite was either furnace cooled (F.C.) or water cooled (W.C.). For comparison purposes, some composites were cast without using any preform and therefore no silica binder was involved. In this case, the Sic particles were squeezed into the mold directly followed by similar preheating and squeeze casting as described. Unless specified, the term “composite” used in this paper is the composite manufactured by using a preform.
Commercial 6061 aluminum alloy was used as the matrix and particulate SIC as the reinforcement. The chemical composition of the 6061 Al alloy is Al0.90%Mg-0.67%Si0.23%Cu-O.l3%Cr-O.l4%Fe. The Sic, used was an abrasive grade with 99.2% Sic content. Three particle sizes of the SIC, were adopted in this investigation, they were mesh number 180 # (N 85 itm), 400 # (N 30 pm) and 800 # (- 14 /lrn). The density of the Sic, was 3.22 g cm - 3. Silica colloid (30 wt% silica in water) was used as the binder in the preparation of preforms. 2.2. Preforms Appropriate amounts of SIC, were mixed with silica colloid in a container. The mixture was stirred thoroughly and blended and then poured into a mold. After baking at 120 “C for 24 h, the preform was removed from the mold. These were cylinders with 40 mm height and 30 mm diameter. The preforms were then sintered at 1000 “C for 4 h to increase their compressive strength. 2.3. Squeeze-casting apparatus Fig. 1 shows the schematic diagram of the squeezecasting apparatus used in this research. A 50-ton hydraulic press was used as the main driving tool to squeeze cast the composite. The mold system used was a pair of half molds with 165 mm height and 31 mm inner diameter. Water circulation paths (6 mm inner diameter) were drilled inside the mold for cooling rate control. The water circulation paths were connected to a pump and a flow gauge to facilitate the cooling rate
hydraulic
frame
2.5. Solution treatment cd aging After squeeze casting, some composites were solution treated at 557 “C for 2 h followed by water quench. The selection of 557 “C for solution treatment instead of conventional 529 “C will be explained later. Aging was carried out at 160 “C for some hours until peak hardness was reached. 2.6. Mechanical tests An MTS machine was used to determine the flexure strength of the composites in either as cast or peak aged condition. Dimensions of test specimens were 3.0 x 4.0 x 36.0 mm according to JIS specification. Before testing, the specimens were polished up to 3 /lrn diamond powder. Vickers hardness tests were carried out using a load of 30 kg to measure the hardness at different cross section of the composites. 2.7. Optical microscopy)
piston ram Al-melt preform cooling water outlet -cooling water inlet
II
I hydraulic pump
Fig.
I\
II thermocouple
1. Schematic diagram of squeeze-casting equipment.
Optical microscopy (OM) was performed to evaluate the distribution of the particulate reinforcements and the extent of the infiltration of Al alloy. The cracks of Sic particles, damages in the preforms and the pores produced during squeeze casting were checked using optical microscopy. Specimens for OM were prepared following conventional metallographic procedure up to 1 /lrn diamond powder polishing. Keller’s reagent was used when etching was required.
B.-K. Hwu et 01. I Materials Scierzce and Engineering A.207 (1996) 135-141
137
Fig. 3. The grain structure of the 180 # Sic,/6061 Al composite.
(b)
ment. The volume fraction (Vr) of SIC, in all the composites was determined to be about 50%. The distribution of particulate reinforcement is rather uniform. The infiltration of 6061 Al alloy is quite complete. No significant pores were observed at the SiC,/Al alloy interfaces. After etching in Keller’s reagent, the grain structures were observed as shown in Fig. 3. The grain sizes decreased with decreasing particle size. They were about 20, 16 and 10 pm, respectively, for composites with 180 # , 400 # and 800 # particulate reinforcements. Good quality composites could be expected from the appearance of microstructures. 3.2. Flexwe test The flexure strengths of as-cast SIC&Al 6061 composites containing various particle size of reinforcement are compared in Fig. 4. The flexure strengths increased when the particle sizes were reduced. They were 279, 396 and 495 MPa, respectively, for composites containing reinforcement with particle 180 # , 400 # and
(4 Fig, 2. Optical micrographs of the Sic,/6061 Al composites for different particle sizes: (a) 180 # ; (b) 400 # ; (c) 800 # .
2.8. Chemical analysis
After casting, the composite was cut into five roughly equal cross-section pieces from top to bottom of the composite along infiltration direction. The weight percent of Mg in each piece was analyzed using inductively coupled plasma-atomic emission spectrometer to determine whether segregation occurred during solidification.
6ooL 400-
3oo
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Fig. 2 shows the microstructures of as-cast composites containing various particle sizes of reinforce-
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Fig. 4. The flexure strengths of the as-cast Sic,/6061 Al composites.
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800 # . The flexure strengths of composites made without the addition of silica- binder are also indicated in Fig. 4, the strengths being about 20% lower than the composites made from preforms where silica binder was added. It is clear that the use of silica colloid to make preform was quite successful in this study. Baking the preforms at 120 “C for 24 h removed the water in silica colloid completely. Sintering the preforms at 1000 “C for 4 h made the preforms strong enough to sustain the pressure applied during squeeze casting. Sintering at 1000 “C also promotes the formation of SiO, on the Sic, surface which can enhance the bonding between the SIC, and the 6061 Al matrix alloy [13] such that higher strength is obtained. The results mentioned above are for composites that were furnace cooled after squeeze casting. When water cooling was applied after squeeze casting, the flexure strength increased about 10%. For example, the flexure strength of composite containing 180 # SIC, increased from 279 MPa to 310 MPa when the composite was water cooled. Fast cooling rate reduces the grain size of the matrix which in turn raises the strength of the composite. The magnitude of pressure exerted during casting was also found to affect the property of composites, in general, the higher the pressure the better the flexure strength. The flexure strengths were 279 and 293 MPa, respectively, when the pressure applied was 100 and 200 MPa. Since the increase in flexure strength was only about 5%, the pressure used was 100 MPa through this study. The flexure strength and hardness of the composites increased as the particle size of reinforcement was reduced as shown in Figs. 4 and 5. The increase in strength of composites due to smaller reinforcement particle size has been reported by many authors [10,14]. Statistically, larger flaws and more defects are more likely to exist in larger particles and, therefore, will deteriorate the strength of composites when compared to the composites containing smaller particles. The smaller grain size in the composites containing smaller reinforcement particles can also contribute to the increase in strength. If the volume fraction of SIC, remains the same, the smaller particle sizes will provide more interface area which serves as the nucleation sites of grain formation. Therefore, smaller grains will be observed in the matrix of composite containing smaller Sic,. For the same volume fraction, the spacing between particles is reduced when the particle size is smaller. Therefore, smaller particles will exert more constraint on grain growth during cooling and more restriction on plastic flow during deformation which can also contribute to the increase in strength. The application of a pressure of 100 MPa during squeeze casting has been a common practice Fc151.
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0 '180# 400# v 800# v 180# 0 180# l
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9 Distance
SiC6/6061, SiCo/6061, SiCi,/SOSl; SiCp/6061, SiCp/6061,No
18
F!C. F, C. E C. n!C. Binder,
27
I
0 36
From Top of Composite(mm)
Fig. 5. The hardnessdistributions of the as-castcomposites.(F.C. meansfurnacecooling and W.C. meanswater cooling.)
3.3. Vasintion of Iwdness with locatiol2 nid segregation of Mg The as-cast cylindrical composite was cut into five roughly equal disk pieces. The thickness of each disk was about 9 mm. A hardness test was performed on the center area of each disk surface to determine the variation of hardness with respect to the position of the as-cast composite. Fig. 5 shows the variation of hardness with location for various composites with different particle sizes and different cooling rates. It is clear from Fig. 5 that a smaller particle size of reinforcement resulted in higher hardness. This is in agreement with the effect of particle size on the flexure strength of the composites. The hardness dropped continuously from the top of the composite to at least 18 mm below the top, except for the composite that was water cooled right after casting. The decrease of hardness from the top to bottom of composites has been reported by Friend et al. [16] in their study of 6-Al,O,J6061 Al composites. In their opinion, the variation of hardness in composites was due to the declining Mg content from the top to bottom of the composites. During infiltration the Mg atom in molten alloy might react with SiOa such that the solute (Mg) content might be gradually depleted. Since the molten alloy took longer path to reach the bottom, the depletion of Mg would be more pronounced at the bottom of the composites resulting in the drop of hardness from the top to bottom. In order to check this argument, the Mg percentage at different locations in the composite was analyzed. As shown in Fig. 6, the Mg content increases from top to bottom of the composites in our study. Therefore, their explanation [16]
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does not fit our results. It is well established that the solute atoms are moving away from solid to liquid phase during solidification. The content of solute will get richer in the material which solidifies last. At the initial stage of squeeze casting, the temperature of molten alloy increases due to the application of pressure (100 MPa). Solidification started from the top of the composite after complete infiltration since heat dissipation went faster through the piston that was connected to the ram. Therefore, the Mg content increased from the top to the bottom, as shown in Fig. 6. If the mold is water cooled, the molten alloy will solidify starting from the bottom of the mold; therefore, the Mg content will get richer from bottom to top. This is again in agreement with Fig. 6. One can explain why the hardness went down while the Mg content went up from top to bottom of the furnace-cooled composites in the following way. During squeeze casting, the 100 MPa pressure was maintained until mold temperature dropped to 582 “C. During solidification the pressure on the top part of the composites where the matrix solidified first was 100 MPa as applied. However, the pressure on the lower part of the composite was lower than 100 MPa during solidification since part of the pressure was used to overcome the friction between the solidified top portion of composite and the mold wall. Therefore, the bonding between Sic, and matrix will not be as good at the bottom where less pressure was experienced as compared to the top. Fig. 7 presents the microstructure of the composite at the top (Fig. 7(a)) and that at the bottom (Fig. 7(b)). No pores in the matrix were observed in the top portion of the composite (Fig. 7(a)), while many pores existed in the matrix close to the bottom portion of the 1.4 . 18046 ,Binder, F.C. l 800# ,No Binder, F.C. v 18O# ,Binder, W. C.
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Fig. 7. Microstructures for different locations of the composite: (a) top site; (b) bottom site (27 mm below the top).
composite (Fig. 7(b)). The presence of pores in the matrix clearly makes the composite weaker. If the mold is water cooled, the composite will solidify very fast, and no pressure gradient will exist during solidification resulting in uniform hardness from the top to the area close to the bottom of the composites. This argument is in agreement with our experimental results as shown in Fig. 5. At the very bottom, the composite was very hard and brittle. Cracking took place during hardness testing, making accurate measurement impossible. This is attributed to the concentration of silica during baking of preforms, when silica moved upward slowly, making higher concentration of silica at the top of the preform. The preforms were inverted when being put into the mold of the squeeze-casting apparatus. Therefore, the very bottom of composite contained a high concentration of sintered silica which makes the bottom very brittle. If no preforms are used in the squeeze-casting process, the hardness at bottom will continuously drop. The curve for composite without using silica binder in Fig. 5 confirms our explanation. 3.4. Solution treatment and aging
0.2
0
9
18
27
Distance from Top of Composite (mm) Fig, 6. The Mg distributions
of the as-cast composites.
36
Some composites were solution treated at 557 “C for 2 h followed by water quench. Aging was first carried out at 175 “C. It was found that the peak-aging times were very hard to identify since the aging rate of the
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300: E P ?i VI L 22 .u '
, 250-
1
200"
0 #180 SiCp/6061 #400 SiCp/6061, v #&IO SiCp/6061 l
I 10
I 1
1501 as-quehch
I 100
Time (hr) Fig. 8. The aging curves of the composites for different particle size.
composites was very fast. Aging was then performed at 160 “C, and the results are shown in Fig. 8 which gives the relationship between the hardness of composite and its aging time. The composite reached peak hardness faster when the Sic, size was smaller. The peak-aging time for composite with 180 # and 400 # Sic, was about 5 h, while the peak-aging time for composite with 800 # Sic, was about 3 h. Differential scanning calorimetry (DSC) was conducted on the composite specimens. The results of DSC for composite specimens solution treated at 529 and 557 “C are compared in Fig. 9. Two main differences exist between Figs. 9(a) and 9(b). The area under the exothermic peak at about 242 “C is larger in Fig. 9(b) than in Fig. 9(a). An endothermic peak appears at about 560 “C in Fig. 9(a) but not in Fig 9(b). The peak
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at about 242 “C is believed to be due to the formation of metastable p” and stable ,&’ precipitates [17]. The area under the peak is the total enthalpy of precipitate formation which indicates the total volume of precipitate formed. Comparison between the area under the peak at 242 “C of Figs. 9(a) and 9(b) indicates that more precipitates of ,f?’ and p” were formed during aging in composites solution treated at 557 “C than at 529 “C. The presence of endothermic peak at 560 “C in Fig. 9(a) enables us to explain this phenomenon as will be described in next paragraph. From Al-Mg-Si phase diagram, the ternary Mg,SiAl-Si eutectic point occurs at 558 “C. Therefore, the endothermic peak at 560 “C is believed to be due to the melting of Mg,Si-Al-Si eutectic phase. More solute atoms are available for precipitate formation if the composites are solution treated at 557 “C since at this temperature the solution treatment is more complete than at 529 “C. Therefore, a great volume of precipitates will form if the composite is solution treated at 557 “C. Since 557 “C is very close to the ternary Mg,Si-Al-Si eutectic point (558 “C), the temperature control of the oven must be very accurate (better than & 1 “C); otherwise it would risk occurrence of some localized melting which would weaken the composite drastically. This explains why solution treatment at 557 “C was selected in this study instead of commonly adopted 529 “C [18-201. As shown in Fig. 8, the composites reached peak hardness faster when the Sic, size was smaller. It has been suggested that the dislocations generated owing to the thermal mismatch between the reinforcement and the matrix during quenching after solution treatment may promote the formation of precipitates [20]. It is also believed that the surface of Sic, may provide preferential nucleation sites for intermediate precipitates [21]. At the same volume fraction, composites containing smaller reinforce,ment particles have more particle surface area and less particle spacing so that more dislocations are generated after quenching. Therefore, composites containing smaller particles reached peak-age condition faster. The high volume fraction of reinforcement in this study enables the peak hardness to occur within 5 h under 160 “C.
4. Conclusions
200
300
Temperature
400
500
f
(“C 1
Fig. 9. DSC curves of 180 # SC,/6061 Al composites for different solution treatment temperatures: (a) 529 “C; (b) 557 “C.
Based upon the experimental results and discussion provided above, the following conclusions can be drawn. (1) Good quality composites containing high volume fraction (50%) of Sic, with various particle size can be obtained using the squeeze-casting process adopted in this study. The strength and hardness of the composites can be further improved if water cooling is applied during solidification.
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(2) The flexure strength of the composites increased as the reinforcement particle size was reduced for same volume fraction of SIC,. This is attributed to less defects, smaller grain size and more constraint on plastic flow during deformation in composites with smaller reinforcement particles. (3) The hardness of the furnace-cooled composites dropped gradually from the top of the specimen to some distance below. This is attributed to the appearance of pores in the matrix which is due to lower squeezing pressure experienced at the lower part of the specimen. The variation of hardness was avoided when water cooling was applied. (4) A large quantity of solute atoms is available for precipitate formation if the specimen is solution treated at 557 “C, which is very close to the ternary Mg,SiAl-Si eutectic temperature. Subsequent aging generated high peak hardness in less than 5 h even if the specimens were aged at 160 “C.
Acknowledgments
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[3] T.M.F. Ronald, Adu. Mater. Proc., 144 (1993) 24. [4] M.W. Toaz, Discontinuous Ceramic Fiber MMCs. Engirteering Muteriais Handbook, Vol. 1, 1987, Metals Park, OH, ASM, pp. 903-910. [5] S. Yoda, V. Kurihara, K. Wakashima and S. Umekawa, IMernll. Trans. A, 9 (1978) 1229. [6] A. Mortensen and J.A. Cornie, Metail. Trans. A, 18 (1987) 1160. Sci. Lett., 3 (1984) [71 E. Nakata and Y. Kagawa, J. Muter. 968.
PI C.P. Ju, K.I. Chen and J.H. Chern Lin, J.
Mater.
Sci., 29 (1994)
5127.
A.W.H. Morris and A. Burwood-Smith, High Temperature Turbiues, AGARD-CP-73-71, Jan. 1971. UOI X. Ni, M.S. Maclean and T.N. Baker, Mater. Sri. Technol., 10 (1994) 452. t111J.A. McElman, Continuous Si/icon Carbide Fiber MMCsz Eilgi/leering Materials Handbook, Vol. 1, 1987, Metals Park, OH, ASM, pp. 858-866. Jo& [121W.C. Harrigan, Jr. and R.H. Flowers, TMS-AIME/ASM Composite Materials Committee Symp., Oct. 1977, pp. 24-26. thesis, National Tsinghua University, [I31 Min-Shi Tsai, Masters Hsinchu, Taiwan, ROC, June 1993. v41 M. Finot, Y.L. Shen, A. Needleman and S. Suresh, MetaL Mater. Trans. A, 25 (1994) 2403. [I51 Z. Zhu, in S.G. Fishman and A.K. Dhingra (eds.), Cast Reinforced Metal Composites, ASM, Metals Park, Ohio, 1988, pp. [91
93-99.
We are pleased to acknowledge the financial support for this research by the National Science Council, Taiwan, R.O.C. under Grant NSC-82-0405-E-007-318.
References [l] W.C. Harrigan, Jr. Discontinuous Silicon Fiber MMCs. Engineering Materials Ha&book, Vol. 1, 1987, Metals Park, OH, ASM, pp. 889-895. [2] R.K. Everett and R.J. Arsenault, Metal Matrix Composites: Processing a& lnterj&es, 1991, Boston, Academic Press, pp. 83-57.
[161CM. Friend, I. Horsfall, S.D. Luxton and R.J. Young, in S.G. Fishman and A.K. Dhingra (eds.), Proc. ht. Symp. A~u. Cast Reirzjorced Metal Composites, Chicago, IL, USA, ASM, Sept. 1988, pp. 309-315. [I71 P. Appendino, C. Badini, F. Marino and A. Tomasi, Mater. Sci. Eng., Al35 (1991) 275. P81 T. Das, S. Bandyopadhyay and S. Blairs, J. Mater. Sci., 29 (1994) 5680. [I91 J.P. Unsworth and S. Bandyopadhyay, J. Mater. Sci., 29 (1994) 4645.
1201M. Levin and B. Karlsson, ht. J.
1211I. Dutta and D.L. Bourell, 67.
Fatigue, Mater. Sci.
15 (1993) 377. Eng., All2 (1989)