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The production of aluminium alloy composites using a cold isostatic press and extrusion approach
Journal of Materials Processing Technology, 29 (1992) 245-253
245
Elsevier
The production of aluminium alloy composites using a cold isostatic press and extrusion approach H.F. Lee, F. Boey, K.A. Khor, M.J. Tan, J. Gan and N.L. Loh School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263 (Received June 18, 1991; accepted July 9, 1991 )
Industrial Summary Whilst experimental results indicate that metal-matrix composites possess higher specific mechanical strengths, one of the main difficulties in implementing these materials on a large scale has been the costly and often complicated processes used. This paper presents the preliminary results obtained in developing a powder metallurgy process involving an approach based on the use of a cold isostatic press together with subsequent extrusion. Three systems have been compared: an unreinforced A1 alloy system; an Al alloy-Ni short-fibres composite; and an Al alloyA1203 particulates composite. Mechanical mixing of the matrix powder and fibre reinforcement was followed by a high-pressure isostatic pressure compaction. Pressurization was done in the range of 10-400 MPa. The green compacts were extruded subsequently at 5-10 mm/min, and reduction ratios of 5 and 10, at both 400 and 500°C. Whilst the initial high-pressure compaction step was sufficient to reduce the porosity to as low as 5% when a pre-degassing step was used, both composite systems could not achieve porosity of less than 10% even after sintering at 600°C. However, by using the subsequent extrusion step, reduction to less than 5% porosity was achieved at 500 ° C, for both of the composite systems.
1. Introduction
Metal-matrix composites, in particular those using an aluminium alloy based matrix, have become viable structural materials that possess a high specific strength and modulus. One of the methods commonly used for the processing of such composites is the powder-metallurgy approach [ 1-2 ], which involves the processing and consolidation of fine particles to produce a solid material. One of the advantage of this approach is the relative ease in which the reinforcements can be mixed into the matrix. Compared to ingot metallurgy in which the matrix is melted before the reinforcement is added, powder metallurgy avoids the problem of phase- or precipitation-segregation during cooling. Rapid solidification in powder processing can also achieve a much higher cooling rate for a desired solid solution strengthening, with aluminium alloy powders. The problem of obtaining a homogeneous reinforcement mix is also less 0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
246 tedious for powder metallurgy. However, the conventional method of powder consolidation by simple sintering is usually also prohibitively energy- and timeconsuming. More recently, other alternative processes involving high strains at ambient temperature by cold rolling [3 ] or at elevated temperatures by forging [4] or extrusion [5,6] have been used. To increase the efficiency of such consolidation processes, the green compacts used in the processes should already have minimal percentage porosity. One approach which has been shown recently to reduce significantly the percentage porosity during compaction, and to do so homogeneously throughout the body of the compact, is to use isostatic pressing [ 7]. Cold isostatic pressing using high pressures has gained growing acceptance because of the following advantages: (i) The pressure is applied evently around the entire part, so that a better density can be achieved than in mechanical pressing. Consequently, shrinkage is more uniform and predictable. (ii) There are virtually no residual stresses in the compacted material, since there is no die wall friction. (iii) there is no necessity for additions of binders and lubricants, and therefore the additional step of burning off the binder is eliminated. In addition, this means also a lower pre-sintering porosity. The main advantage in hot extrusion is the relatively short time involved in the consolidation process. Whilst the ability to vary the net shape of the product is limited, the process would be cost-effective for producing large and regular structural materials. This paper thus presents some preliminary results obtained in producing metal composites using an alternative high-pressure cold isostatic pressing followed by a hot extrusion process, which would be less prohibitive in terms of both energy- and time-requirements than a conventional sintering process. 2. Experimental procedure
The cold isostatic press used in this work was a "Dr. CIP" wet-bag press capable of pressurization up to 400 MPa within 30 min. The wet-bag technique used consisted of loading the matrix/reinforcement mixture material into an elastomeric bagging. Given the high compressive stress attained during compaction (up to 400 MPa), an elastomeric material which can withstand this pressure cyclically without severe stress relaxation effects has to be used. Various materials were tested, including high-carbon butadiene, neoprene and silica-reinforced silicone rubbers. The reinforced silicone rubber was found to be most suitable due mainly to its higher resilience, its easy castability and its sealing effect [8]. In particular, its relatively resilient behavior also reduced the effect of stress relaxation, so that the mold could be reused repeatedly [8,9 ]. The bagging finally used was cast using a Dow Coming RTV 3120 reinforced silicone rubber, and cured with Dow Corning "S" catalyst at room tern-
247
perature for at least 48 hours. Before the pre-cured silicone liquid mix was poured into the mold, de-airing in a vacuum chamber (down to 50 mbar) was done to ensure that entrapped air bubbles were kept to a minimum. This was critical, as the voids that existed tended to coalesce into cracks during pressurization, often resulting in premature failure of the bagging. This problem was particularly severe because powder fallout from the bagging could seriously damage the hydraulic pressurization unit by jamming the high-pressure inlet/ outlet valve. For the powder-compaction process, a degassing step prior to compaction was necessary to reduce open porosity and to minimize fissure cracking during decompression. Degassing was performed using a mechanical vacuum pump down to 0.05 bar, for at least 10 rain before being sealing. Compaction was done in a Kobelco "Dr. CIP" press capable of a pressure of 400 MPa. The pressurization medium used was water with soluble oil added as an emulsion to prevent corrosion. The high degree of incompressibility of water enabled the pump time to be relatively short, and also resulted in a lower fluid volume being needed. Preliminary tests indicated that the decompression cycle has to be controlled carefully. If the decompression rate is too great during the time when PRESSURE(MPa) 500
400
300
200
100
0 0
20
HOLDING PRESSURISATION
40
60
DECOMPRESSION
Fig. 1. Pressure-time cycle used in the cold isostatic press.
80
100
120
248 the elastomer bagging is supposed to separate physically from the compacted piece, the springback effect of the elastomer can result in the tearing of the sample, resulting in fissure cracks. Fig. 1 shows a typical cycle t h a t was used for this work, and which avoided the above phenomenon. The extrusion work was performed using a Fogg and Young press, using a 32 mm pre-extruded green compact and a 10 and 15 mm diameter extrusion die, thus giving reduction ratios of 10 and 5, respectively. The ram speeds used were 5 and 10 m m / s at 400 and 500 ° C. Specimens were prepared from the middle third of the extruded rod only, to avoid extrusion end defects. For the purpose of calculating the degree of shrinkage, the free flow density % SHRINKAGE IN VOLUME
5o I
2o1:s• ¸¸¸¸¸ i ¸ 0
100
.
.
.
200
.
.
.
.
.
.
300
400
500
COMPACTING PRESSURE (MPa)
Fig. 2. Plot of percentage shrinkagewith compactionpressure, for different dwelltimes. Circular, square and triangular symbolsrepresent A1alloypowder without reinforcement,A1alloypowder reinforced with 10 wt% A1203and 10 wt% Ni short fibres, respectively.Hollowsymbolsindicate a compactionof 10 min, whilst bold symbolsindicate compactionwas done for 30 min. % DENSITY
O=_.f
i
0
100
L
_
200 300 COMPACTINGPRESSURE(MPa)
Fig. 3. As for Fig. 2, but for percentage of the theoretical density.
400
500
249 % POROSITY LEVEL
25
10
........................... ~
0
....
i 100
0
I 200 COMPACTING PRESSURE (MPa)
i 300
400
Fig. 4. As for Fig. 2, but for the percentage porosity.
10 A
,..I UJ ,W --I I--
or r o
0
0.1 0
i
t
100
200
[]
~
~
i
300 400 500 SINTERING TEMPERATURE (° C)
AI+10 wt % AI203
©
i
600
700
Aluminium
Fig. 5. Plot of the percentage porosity after conventional sintering at different temperatures for 20 rain.
of the powder mixture was determined. This is defined as the density of the powder as poured into a measuring cylinder to a height of 300 ram, the compacted density being determined using a water immersion principle. The shrinkage is thus the percentage reduction in unit volume of the material after compaction compared to the unit free flow volume. The degree of porosity was measured by first preparing the compact surface metallographically, and then
250
measuring the percentage porosity using an image analyzer. At least 16 readings were taken for each sample, with the averaged results being reported. 3. Results and discussions Figure 2 shows the plot of the degree of shrinkage, measured as the percentage of the free flow density versus the applied compaction pressure whilst Fig. 3 shows the corresponding results in terms of percentage of the theoretical density. Figure 4 shows the results of the percentage porosity of the green compacts against the compacting pressure. Both Figs. 2 and 3 indicate that by about 300-400 MPa pressure, the increase in shrinkage or in percentage of the theoretical density obtained by further increase of the pressure reduces significantly. The porosity results in Fig. 4, however, indicate that further significant reduction in porosity can be achieved. The influence of increasing the holding time during compaction, from 10 to 30 min, is also seen to be less at higher
10
10
~
~
-
_
<....
8
i ~6
REDUCTION RATIO 5
5 -[
-....
I
""
~. 4 ......................
" ~ ""~;:
...........
-I~-. . . . . . . . . . . . . .
~
..............
O
2
i/ 0 w 0
.......
~. . . . . . . 5
x.... l0
~
_
10
__ 15
RAM SPEED(MM/S) Al+AI203(400 °C)
~
AI+AI203(500°C)
AI+Ni(400 *C)
-t-
AI+Ni(500oC)
Fig. 6. Plot of the percentage porosity after hot extrusion, for different combinations of reduction ratio (top horizontal scale) and ram speed (bottom horizontal scale). All extrusions were done at 4 0 0 ° C (hollow symbols) and 500°C (bold symbols).
251
Fig. 7. Microstructure of the extruded AI alloy-Ni short-fibre composite, indicating the anisotropic alignment and breakage of the fibres.
2
Fig. 8. Microstructure of the extruded A1 alloy-Al203 particulate composite, indicating partial agglomeration of the A1203 particulates.
252
Fig. 9. Melt fracture phenomenon resulting from extrusion at 600°C: the distortion penetrated into the whole extrudate. ( × 1.5 )
pressures. As expected, the unreinforced system results in the least porosity after compaction, followed by the Ni short-fibre reinforced system, and then the A1203 system. Earlier works have indicated that, in general, porosity or void content of less than 5% is necessary to achieve strengths consistently reaching the optimal strength of the composite [ 10-13]. Using this as an indication, Fig. 4 would show that for the composite systems, the minimum porosity requirement has not been met. Fig. 5 enables a comparison of the results after conventional sintering at up to 500°C for 20 min for the unreinforced A1 alloy material and the A1 alloyA1203 composite, indicating the wide disparity in porosity values. At such a high porosity level of more than 10%, it would have been impractical to use conventional sintering methods to reduce the porosity to less than the required maximum of 5%. The percentage porosity results obtained after extrusion are shown in Fig. 6. For both composite systems tested, the lowest percentage porosity was achieved at 500°C, at a ram speed of 10 m m / s and a reduction ratio of 10. Whilst the lowest achieved porosity for the A1 alloy-Ni short-fibres was less than 3%, for the A1 alloy-Al203 particulates composite it was 4%. Figures 7 and 8 show the microstructure of the extruded A1 alloy-Ni short-fibre and A1 alloy-Al203 particulate composites respectively. The anisotropic alignment of the Ni fibres in the former system is noted, along with some degree of the fibre aspect ratio reduction due to fibre breakage. The partial agglomeration of the A1203 particulates is also noted in the latter figure. Extrusions at temperatures higher than 500 °C were attempted,but this resulted in melt fracture at the surface, indicated in Fig. 9 as a regular ridged distortion, which penetrated into the whole extrudate. Such a phenomenon is triggered when the critical wall stress is exceeded in the die, upon which intermittent slipping occurs to relieve the excess deformation energy absorbed. Further work is in progress to reduce this occurrence by a longer pre-heating time together with simultaneous pre-heating of the die itself.
253
4. Conclusions
High-pressure cold isostatic pressing up to 400 MPa, followed by hot extrusion at 500°C, at 10 mm/s and at a reduction ratio of 10, has been used to produce two A1 alloy composite systems reinforced with Ni short fibres and A1203 particulates, to achieve less than 3 and 4% porosity, respectively.
References 1 D.L. Erich, Metal matrix composites: Problems, applications, and potential in the P / M industry, Int. J. Powder MetaU., 23 (1) (1981) 45-54. 2 A. Saigal and G. Leisk, Mechanical properties of alumina reinforced aluminium metal matrix composites, in: D.R. Durham and A. Saigal (Eds.), Microstructural Development and Control in Material Processing ASME MD-Vol. 14, 1990, pp. 115-119. 3 Y.L. Liu, N. Hansen and D.J. Jensen, Recrystallization microstructure in cold rolled A1 composites reinforced by SiC whiskers, MetaU. Trans. A, 20A (September 1989) 1743-1752. 4 K. Suzuki, Y. Matsuoka and Y. Fujita, Workability of SiCw reinforced Al alloy and its applicability to die forging, Adv. Technol Plast., 1 (1990) 229-234. 5 A.K. Mukhopadhyay, H.M. Flower and T. Sheppard, Development of microstructure in AA8090 alloy produced by extrusion processing, Mater. Sci. Technol., 6 (1990) 461-468. 6 H. Asanuma, M. Hirohashi and O. Hayama, Fabrication of complicated shape metal matrix composites by combined extrusion, Adv. Technol. Plast., 2 (1990) 945-950. 7 P.J. James, Isostatic Pressing Technology, Applied Science Publ., New York, 1983, p. 117. 8 B. Stenberg, Compression relaxation of rubbers, PhD Thesis, KTH, Stockholm, Sweden, 1977, 33. 9 F. Boey, Stress relaxation behaviour of an irradiated silicone elastomer, Radiat. Phys. Chem., 37 (2) (1991) 1203-1207. 10 L.G. Stringer, Correlation between compression strength and void content in hand layed up/ vacuum based CFRP composites, RARDE internal report, Christchurch, UK, 1987. 11 L.G. Stringer, Optimisation of the wet lay up/vacuum bag process for CFRP composites, Composites, 20(5) (1989) 441. 12 F.Y.C. Boey and S.W. Lye, Effects of vacuum pressure in an autoclave curing process for a thermosetting fibre-reinforced composite, J. Mater. Process. Technol., 23 (1990) 121-131. 13 F.Y.C. Boey, Reducing the void content and its variability in composite specimens using a VIM process, Polym. Test., 9(6) (1990).
245
Elsevier
The production of aluminium alloy composites using a cold isostatic press and extrusion approach H.F. Lee, F. Boey, K.A. Khor, M.J. Tan, J. Gan and N.L. Loh School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263 (Received June 18, 1991; accepted July 9, 1991 )
Industrial Summary Whilst experimental results indicate that metal-matrix composites possess higher specific mechanical strengths, one of the main difficulties in implementing these materials on a large scale has been the costly and often complicated processes used. This paper presents the preliminary results obtained in developing a powder metallurgy process involving an approach based on the use of a cold isostatic press together with subsequent extrusion. Three systems have been compared: an unreinforced A1 alloy system; an Al alloy-Ni short-fibres composite; and an Al alloyA1203 particulates composite. Mechanical mixing of the matrix powder and fibre reinforcement was followed by a high-pressure isostatic pressure compaction. Pressurization was done in the range of 10-400 MPa. The green compacts were extruded subsequently at 5-10 mm/min, and reduction ratios of 5 and 10, at both 400 and 500°C. Whilst the initial high-pressure compaction step was sufficient to reduce the porosity to as low as 5% when a pre-degassing step was used, both composite systems could not achieve porosity of less than 10% even after sintering at 600°C. However, by using the subsequent extrusion step, reduction to less than 5% porosity was achieved at 500 ° C, for both of the composite systems.
1. Introduction
Metal-matrix composites, in particular those using an aluminium alloy based matrix, have become viable structural materials that possess a high specific strength and modulus. One of the methods commonly used for the processing of such composites is the powder-metallurgy approach [ 1-2 ], which involves the processing and consolidation of fine particles to produce a solid material. One of the advantage of this approach is the relative ease in which the reinforcements can be mixed into the matrix. Compared to ingot metallurgy in which the matrix is melted before the reinforcement is added, powder metallurgy avoids the problem of phase- or precipitation-segregation during cooling. Rapid solidification in powder processing can also achieve a much higher cooling rate for a desired solid solution strengthening, with aluminium alloy powders. The problem of obtaining a homogeneous reinforcement mix is also less 0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
246 tedious for powder metallurgy. However, the conventional method of powder consolidation by simple sintering is usually also prohibitively energy- and timeconsuming. More recently, other alternative processes involving high strains at ambient temperature by cold rolling [3 ] or at elevated temperatures by forging [4] or extrusion [5,6] have been used. To increase the efficiency of such consolidation processes, the green compacts used in the processes should already have minimal percentage porosity. One approach which has been shown recently to reduce significantly the percentage porosity during compaction, and to do so homogeneously throughout the body of the compact, is to use isostatic pressing [ 7]. Cold isostatic pressing using high pressures has gained growing acceptance because of the following advantages: (i) The pressure is applied evently around the entire part, so that a better density can be achieved than in mechanical pressing. Consequently, shrinkage is more uniform and predictable. (ii) There are virtually no residual stresses in the compacted material, since there is no die wall friction. (iii) there is no necessity for additions of binders and lubricants, and therefore the additional step of burning off the binder is eliminated. In addition, this means also a lower pre-sintering porosity. The main advantage in hot extrusion is the relatively short time involved in the consolidation process. Whilst the ability to vary the net shape of the product is limited, the process would be cost-effective for producing large and regular structural materials. This paper thus presents some preliminary results obtained in producing metal composites using an alternative high-pressure cold isostatic pressing followed by a hot extrusion process, which would be less prohibitive in terms of both energy- and time-requirements than a conventional sintering process. 2. Experimental procedure
The cold isostatic press used in this work was a "Dr. CIP" wet-bag press capable of pressurization up to 400 MPa within 30 min. The wet-bag technique used consisted of loading the matrix/reinforcement mixture material into an elastomeric bagging. Given the high compressive stress attained during compaction (up to 400 MPa), an elastomeric material which can withstand this pressure cyclically without severe stress relaxation effects has to be used. Various materials were tested, including high-carbon butadiene, neoprene and silica-reinforced silicone rubbers. The reinforced silicone rubber was found to be most suitable due mainly to its higher resilience, its easy castability and its sealing effect [8]. In particular, its relatively resilient behavior also reduced the effect of stress relaxation, so that the mold could be reused repeatedly [8,9 ]. The bagging finally used was cast using a Dow Coming RTV 3120 reinforced silicone rubber, and cured with Dow Corning "S" catalyst at room tern-
247
perature for at least 48 hours. Before the pre-cured silicone liquid mix was poured into the mold, de-airing in a vacuum chamber (down to 50 mbar) was done to ensure that entrapped air bubbles were kept to a minimum. This was critical, as the voids that existed tended to coalesce into cracks during pressurization, often resulting in premature failure of the bagging. This problem was particularly severe because powder fallout from the bagging could seriously damage the hydraulic pressurization unit by jamming the high-pressure inlet/ outlet valve. For the powder-compaction process, a degassing step prior to compaction was necessary to reduce open porosity and to minimize fissure cracking during decompression. Degassing was performed using a mechanical vacuum pump down to 0.05 bar, for at least 10 rain before being sealing. Compaction was done in a Kobelco "Dr. CIP" press capable of a pressure of 400 MPa. The pressurization medium used was water with soluble oil added as an emulsion to prevent corrosion. The high degree of incompressibility of water enabled the pump time to be relatively short, and also resulted in a lower fluid volume being needed. Preliminary tests indicated that the decompression cycle has to be controlled carefully. If the decompression rate is too great during the time when PRESSURE(MPa) 500
400
300
200
100
0 0
20
HOLDING PRESSURISATION
40
60
DECOMPRESSION
Fig. 1. Pressure-time cycle used in the cold isostatic press.
80
100
120
248 the elastomer bagging is supposed to separate physically from the compacted piece, the springback effect of the elastomer can result in the tearing of the sample, resulting in fissure cracks. Fig. 1 shows a typical cycle t h a t was used for this work, and which avoided the above phenomenon. The extrusion work was performed using a Fogg and Young press, using a 32 mm pre-extruded green compact and a 10 and 15 mm diameter extrusion die, thus giving reduction ratios of 10 and 5, respectively. The ram speeds used were 5 and 10 m m / s at 400 and 500 ° C. Specimens were prepared from the middle third of the extruded rod only, to avoid extrusion end defects. For the purpose of calculating the degree of shrinkage, the free flow density % SHRINKAGE IN VOLUME
5o I
2o1:s• ¸¸¸¸¸ i ¸ 0
100
.
.
.
200
.
.
.
.
.
.
300
400
500
COMPACTING PRESSURE (MPa)
Fig. 2. Plot of percentage shrinkagewith compactionpressure, for different dwelltimes. Circular, square and triangular symbolsrepresent A1alloypowder without reinforcement,A1alloypowder reinforced with 10 wt% A1203and 10 wt% Ni short fibres, respectively.Hollowsymbolsindicate a compactionof 10 min, whilst bold symbolsindicate compactionwas done for 30 min. % DENSITY
O=_.f
i
0
100
L
_
200 300 COMPACTINGPRESSURE(MPa)
Fig. 3. As for Fig. 2, but for percentage of the theoretical density.
400
500
249 % POROSITY LEVEL
25
10
........................... ~
0
....
i 100
0
I 200 COMPACTING PRESSURE (MPa)
i 300
400
Fig. 4. As for Fig. 2, but for the percentage porosity.
10 A
,..I UJ ,W --I I--
or r o
0
0.1 0
i
t
100
200
[]
~
~
i
300 400 500 SINTERING TEMPERATURE (° C)
AI+10 wt % AI203
©
i
600
700
Aluminium
Fig. 5. Plot of the percentage porosity after conventional sintering at different temperatures for 20 rain.
of the powder mixture was determined. This is defined as the density of the powder as poured into a measuring cylinder to a height of 300 ram, the compacted density being determined using a water immersion principle. The shrinkage is thus the percentage reduction in unit volume of the material after compaction compared to the unit free flow volume. The degree of porosity was measured by first preparing the compact surface metallographically, and then
250
measuring the percentage porosity using an image analyzer. At least 16 readings were taken for each sample, with the averaged results being reported. 3. Results and discussions Figure 2 shows the plot of the degree of shrinkage, measured as the percentage of the free flow density versus the applied compaction pressure whilst Fig. 3 shows the corresponding results in terms of percentage of the theoretical density. Figure 4 shows the results of the percentage porosity of the green compacts against the compacting pressure. Both Figs. 2 and 3 indicate that by about 300-400 MPa pressure, the increase in shrinkage or in percentage of the theoretical density obtained by further increase of the pressure reduces significantly. The porosity results in Fig. 4, however, indicate that further significant reduction in porosity can be achieved. The influence of increasing the holding time during compaction, from 10 to 30 min, is also seen to be less at higher
10
10
~
~
-
_
<....
8
i ~6
REDUCTION RATIO 5
5 -[
-....
I
""
~. 4 ......................
" ~ ""~;:
...........
-I~-. . . . . . . . . . . . . .
~
..............
O
2
i/ 0 w 0
.......
~. . . . . . . 5
x.... l0
~
_
10
__ 15
RAM SPEED(MM/S) Al+AI203(400 °C)
~
AI+AI203(500°C)
AI+Ni(400 *C)
-t-
AI+Ni(500oC)
Fig. 6. Plot of the percentage porosity after hot extrusion, for different combinations of reduction ratio (top horizontal scale) and ram speed (bottom horizontal scale). All extrusions were done at 4 0 0 ° C (hollow symbols) and 500°C (bold symbols).
251
Fig. 7. Microstructure of the extruded AI alloy-Ni short-fibre composite, indicating the anisotropic alignment and breakage of the fibres.
2
Fig. 8. Microstructure of the extruded A1 alloy-Al203 particulate composite, indicating partial agglomeration of the A1203 particulates.
252
Fig. 9. Melt fracture phenomenon resulting from extrusion at 600°C: the distortion penetrated into the whole extrudate. ( × 1.5 )
pressures. As expected, the unreinforced system results in the least porosity after compaction, followed by the Ni short-fibre reinforced system, and then the A1203 system. Earlier works have indicated that, in general, porosity or void content of less than 5% is necessary to achieve strengths consistently reaching the optimal strength of the composite [ 10-13]. Using this as an indication, Fig. 4 would show that for the composite systems, the minimum porosity requirement has not been met. Fig. 5 enables a comparison of the results after conventional sintering at up to 500°C for 20 min for the unreinforced A1 alloy material and the A1 alloyA1203 composite, indicating the wide disparity in porosity values. At such a high porosity level of more than 10%, it would have been impractical to use conventional sintering methods to reduce the porosity to less than the required maximum of 5%. The percentage porosity results obtained after extrusion are shown in Fig. 6. For both composite systems tested, the lowest percentage porosity was achieved at 500°C, at a ram speed of 10 m m / s and a reduction ratio of 10. Whilst the lowest achieved porosity for the A1 alloy-Ni short-fibres was less than 3%, for the A1 alloy-Al203 particulates composite it was 4%. Figures 7 and 8 show the microstructure of the extruded A1 alloy-Ni short-fibre and A1 alloy-Al203 particulate composites respectively. The anisotropic alignment of the Ni fibres in the former system is noted, along with some degree of the fibre aspect ratio reduction due to fibre breakage. The partial agglomeration of the A1203 particulates is also noted in the latter figure. Extrusions at temperatures higher than 500 °C were attempted,but this resulted in melt fracture at the surface, indicated in Fig. 9 as a regular ridged distortion, which penetrated into the whole extrudate. Such a phenomenon is triggered when the critical wall stress is exceeded in the die, upon which intermittent slipping occurs to relieve the excess deformation energy absorbed. Further work is in progress to reduce this occurrence by a longer pre-heating time together with simultaneous pre-heating of the die itself.
253
4. Conclusions
High-pressure cold isostatic pressing up to 400 MPa, followed by hot extrusion at 500°C, at 10 mm/s and at a reduction ratio of 10, has been used to produce two A1 alloy composite systems reinforced with Ni short fibres and A1203 particulates, to achieve less than 3 and 4% porosity, respectively.
References 1 D.L. Erich, Metal matrix composites: Problems, applications, and potential in the P / M industry, Int. J. Powder MetaU., 23 (1) (1981) 45-54. 2 A. Saigal and G. Leisk, Mechanical properties of alumina reinforced aluminium metal matrix composites, in: D.R. Durham and A. Saigal (Eds.), Microstructural Development and Control in Material Processing ASME MD-Vol. 14, 1990, pp. 115-119. 3 Y.L. Liu, N. Hansen and D.J. Jensen, Recrystallization microstructure in cold rolled A1 composites reinforced by SiC whiskers, MetaU. Trans. A, 20A (September 1989) 1743-1752. 4 K. Suzuki, Y. Matsuoka and Y. Fujita, Workability of SiCw reinforced Al alloy and its applicability to die forging, Adv. Technol Plast., 1 (1990) 229-234. 5 A.K. Mukhopadhyay, H.M. Flower and T. Sheppard, Development of microstructure in AA8090 alloy produced by extrusion processing, Mater. Sci. Technol., 6 (1990) 461-468. 6 H. Asanuma, M. Hirohashi and O. Hayama, Fabrication of complicated shape metal matrix composites by combined extrusion, Adv. Technol. Plast., 2 (1990) 945-950. 7 P.J. James, Isostatic Pressing Technology, Applied Science Publ., New York, 1983, p. 117. 8 B. Stenberg, Compression relaxation of rubbers, PhD Thesis, KTH, Stockholm, Sweden, 1977, 33. 9 F. Boey, Stress relaxation behaviour of an irradiated silicone elastomer, Radiat. Phys. Chem., 37 (2) (1991) 1203-1207. 10 L.G. Stringer, Correlation between compression strength and void content in hand layed up/ vacuum based CFRP composites, RARDE internal report, Christchurch, UK, 1987. 11 L.G. Stringer, Optimisation of the wet lay up/vacuum bag process for CFRP composites, Composites, 20(5) (1989) 441. 12 F.Y.C. Boey and S.W. Lye, Effects of vacuum pressure in an autoclave curing process for a thermosetting fibre-reinforced composite, J. Mater. Process. Technol., 23 (1990) 121-131. 13 F.Y.C. Boey, Reducing the void content and its variability in composite specimens using a VIM process, Polym. Test., 9(6) (1990).