Properties of semisolid aluminium matrix composites

Materials Science and Engineering, A 188 (1994) 277-282

277

Properties of semisolid aluminium matrix composites C. J. Q u a a k a n d W. H. K o o l

Delft University of Technology, Laboratory of Materials Science and Technology, Rotterdamseweg 137, 2628 AL Delft (Netherlands) (Received October 18, 1993; in revised form January 31, 1994)

Abstract Steady state viscosities of partially solidified composites have been determined using a high temperature viscometer. The composites are based on the alloy A356, reinforced with SiC particles ( 13/zm; up to 30 vol.%) or SiC short fibres ( 14/~m diameter; 10 vol.%). The average fibre length was 50-100/~m. All slurries are strongly pseudoplastic at a fraction solidified primary phase of 0.3 for the shear rate range investigated. The reinforcement content or morphology does not influence the pseudoplasticity at shear rates lower than approximately 30 s ~and a power law index of between - 1.2 and - 1.4 is found. At higher shear rates, the reinforcing phase causes the slurries to behave in a more newtonian manner. The viscosity of particle-reinforced slurries shows as a function of concentration a minimum at approximately 20 vol.% particles. This is the result of two counteracting effects of the presence of particles: hindrance of the agglomeration process and an increase in the fraction solid. The fibre-reinforced slurries show steady state viscosities which are similar to or lower than corresponding viscosities of particle-reinforced slurries. Short fibres are more effective in hindering agglomeration and coalescence than are particles.

1. Introduction An emerging new technology is the casting of aluminium alloys in the semisolid state. Partially solidified alloys that are stirred in the semisolid range are castable up to a fraction of solidified primary phase of 0.5, owing to the non-dendritic microstructures obtained [1, 2]. Casting of composites in the semisolid state is especially advantageous compared with conventional casting, as the presence of the primary solidified phase prevents the ceramic particles from segregation or agglomeration. Knowledge of the rheological behaviour of partially solidified and stirred slurries is important for the process control during semisolid casting. Semisolid slurries of matrix or composites are strongly pseudoplastic and thixotropic [1-11]. The pseudoplastic behaviour, expressed by the relation between apparent viscosity against shear rate, can be described over a certain shear rate range by a power law function. The strong shear thinning behaviour of semisolid slurries in the steady state condition is attributed to competing agglomeration and deagglomeration processes of the primary phase globules. Strong agglomeration at low shear rates produces a large amount of entrapped liquid and, as a consequence, an increase in the effective fraction of solid, resulting in a high viscosity. At higher shear rates the bonds between primary phase globules are broken by shearing forces and a lower vis(t921-5093/94/$7.0(1 A',~7)I I)921-5(193(94)(19543-6

cosity is found. The presence of non-metallic phases between the globules affects the viscosity of the slurry. The higher total fraction solid should, at first sight, raise the viscosity, but the ceramic particulates hinder the agglomeration of the primary phase, which might result in a decreasing viscosity. In earlier investigations [6, 7, 10, 11] it was indeed found that the presence of SiC particles in semisolid slurries lowers the viscosity. A less pseudoplastic behaviour is observed at high shear rates as the globules are almost completely deagglomerated [5-7, 11]. In this investigation we studied the effect of higher concentrations of up to 30 vol.% 13/~m SiC particles on the steady state viscosity of a semisolid A356 slurry. Also, the effect of the presence of 10 vol.% SiC short fibres on the viscosity is examined.

2. Experimental details

2.1. Materials The materials used are composites of the alloy A356 (Al-7wt.%Si-0.3wt.%Mg) reinforced with SiC particles (SiCp) o r SiC fibres (SiCf). The matrix alloy is a conventional casting alloy with a relatively large semisolid interval. The liquidus temperature of this alloy (and the composites) is 615 °C. The Si content of 7 wt.% prevents chemical reactions between Al and SiC up to a temperature of 750 °C [12]. © 1994 - Elsevier Sequoia. All rights reserved

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Properties of semisolid Al matrix composites

The particle-reinforced composites contain 10, 18 and 30 vol.% SiCp of 13 /~m average size. Standard deviation of the particle size distribution is 6 ~m. The 18 vol.% SiCp material is a commercially available composite, Duralcan F3A20S, for which the SiC content was determined to be 18 vol.% . The 10 vol.% SiC material is fabricated by diluting the Duralcan composite with the matrix alloy. The 30 vol.% 13/~m SiCp composite is fabricated using a compocasting technique, followed by remelting, solidification under pressure and extrusion with an extrusion ratio of 9:1 [13]. (This material was kindly supplied by Dr. M. Surry, Institut National Polytechnique, Grenoble, France.) Fibre-reinforced composites are fabricated, using Nicalon chopped SiC fibres, supplied by Nippon Carbon Co. Ltd., of 14/.tm diameter and 1 mm length. Composites with 10 vol.% SiCf are fabricated using the compocasting route and solidification under pressure. (This material was kindly supplied by Dr. M. Surry, Institut National Polytechnique, Grenoble, France.) The composites are investigated both in the as-cast condition and after extrusion at a ratio of 16:1. Both compocasting and extrusion resulted in a diminution of the fibre length. Dissolution of the matrix and microscopic examination of the fibres revealed that after the introduction and compocasting procedure an average length of about 100 ~ m remained. Extrusion resulted in further fibre breakage to a length of about 50/~m. 2.2. Viscometer measurements

The apparent viscosities of the slurries are determined using a high temperature Searle-type rotation viscometer. The cylindrical stirrer, which acts as the torque-measuring bob, is spiral grooved to prevent segregation of SiC particles. The viscometer is calibrated using silicone oils with known newtonian viscosity. The set-up has been described elsewhere [3, 7, 11]. The average shear rate in the annulus between stirrer and cup is determined by the stirring rate and geometry, whereas the apparent viscosity is calculated from measured torque. The viscometer has an average shear rate range of 0.4-216 s-l, and apparent viscosities between about 0.05 and 1000 Pa s can be determined. The viscometer experiments performed on the fibreand particle-reinforced composites involve three stages, which have been described elsewhere [7, 11]. The procedure is schematically depicted in Fig. 1. After melting, the slurry is continuously cooled at a fixed cooling rate of 0.02 °Cs- ~ from a temperature of 625 °C, while stirred at a fixed shear rate of 105 s-i The apparent viscosity increases strongly after passing the liquidus temperature. Cooling is stopped at a temperature of 598 °C, which corresponds to a frac-

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tion of solidified phase in the matrix of 0.3, according to the Scheil equation for binary AI-7wt.%Si. During the isothermal stage, the partially solidified slurry is kept at this temperature while stirring is maintained for 60 min. The viscosity decreases until a steady state condition is reached in this period. At longer times the apparent viscosity then shows only a limited further decrease owing to continued coarsening of primary globules. In the third stage, the stirring rate is altered instantaneously and, after a certain time, a new steady state condition is reached, which corresponds to the newly imposed shear rate. This step change procedure is repeated provided the slurry is not longer than 3-4 h in the liquid or semisolid state. These step change experiments on a fixed slurry permit the examination of steady state viscosities at different shear rates during one experiment. The apparent steady state viscosities found in these experiments do not differ significantly from the steady state values found when performing full isothermal stirring experiments with various stirring rates [11].

3. Results 3.1. Microstructure

Several slurries were quenched with a cooling rate of 0.5 °C s-1, using compressed air. The microstructures of all slurries show a globular primary phase, surrounded by the remaining liquid, in which most of the SiC particles or fibres are located. The size of the globules is 50-400 pm, depending on the SiC content. Segregation of SiC was observed only in limited amounts at low shear rates. In the microstructures some agglomeration of globules could be distinguished.

C. J. Quaak, W. H. Kool

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279

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A typical microstructure of the fibre-reinforced composite is shown in Fig. 2 for the steady state condition. No preferential orientation of fibres in the flow direction can be observed, which is probably due to reorienration during the relatively slow cooling rate of the slurry during quenching.

3.2. Particle-reinforced composites Viscometer experiments are carried out on slurries of 10, 18 and 30 vol.% 13 /~m SiC particle-reinforced composites. The slurries were prepared at an initial shear rate of 105 s-1, following the procedure described earlier. The fraction solidified in the matrix was 0.3 at the temperature of 598 °C, which results in total solid fractions for the 10 vol.%, 18 vol.% and 30 vol.% composite slurries of 0.37, 0.43 and 0.51 respectively. The shear rate dependence of the apparent steady state viscosities is shown in Fig. 3. The accuracy of the viscosity values is _+ 15%. The strong pseudoplastic behaviour can be quant!fied by fitting the data to the power law relation 7/= k~,m, where ~/, k, y and m denote the apparent viscosity, consistency, shear rate and power law index respectively. Consistencies and power law indices are given in Table 1. No strong

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dependence of the power law index on the particle concentration is found. However, the consistency is dependent on the particle concentration and has a minimum at a SiC particle concentration of about 20%. Figure 4 shows the data, normalized with respect to the matrix result. From this figure it can be seen that the pseudoplastic behaviour of the composite slurries with 18 and 30 vol.% SiC changes at shear rates higher than 30 s-1. These slurries start to behave in a more newtonian manner at these high shear rates. In Fig. 5 the dependence of the apparent steady state viscosity on the SiC content is shown for various shear rates. A minimum of the viscosity is found at an SiC concentration of about 20 vol.%. At lower concentrations the viscosity increases as the hindrance of agglomeration of the primary phase globules becomes lower. At higher concentrations the viscosity increases as the total fraction solid increases. At the high shear rate, the minimum tends to shift to lower SiC concentrations.

C. J. Quaak, W. H. Kool

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Fig. 7. Apparent steady state viscosity vs. shear rate: , o, 10 vol.% SiCp; zX, 10 vol.% SiCf a s cast; . . . . , O, 10 vol.% SiCf as extruded.

3.3. F i b r e - r e i n f o r c e d c o m p o s i t e s

Microstructural investigation of the air-quenched slurries revealed that the longer fibres in the as-cast sample are further broken up to an average length of about 50/~m at the time of steady state, which is comparable with the average length, initially present in the as-extruded sample. However, the length distribution in the as-cast sample is broader. In contrast, breakage of fibres in the as-extruded sample is hardly observed, indicating that further breakage becomes slow at such a length. The average length of 50/~m results in an aspect ratio of the fibres of 3-4. In Fig. 7 the shear rate dependence of the apparent steady state viscosity of the fibre-reinforced slurry is compared with that of the slurry of the composite with 10 vol.% particles. For shear rates lower than about 100 s-[, the viscosity of the as-cast fibre-reinforced composite is similar to that of the particle-reinforced composite. The steady state viscosity of the extruded composite is slightly lower than that of the as-cast or particle-reinforced samples.

(e).

The apparent viscosities of slurries of composites with 10 vol.% Nicalon fibres are determined with the procedure described in Section 2.2. The isothermal temperature is 598 °C, which corresponds with a total fraction solid of 0.37. In Fig. 6 the viscosity evolution during the first and second stages of the slurry preparation is compared with that of the 10 vol.% particlereinforced composite. The shear rate during the experiment was 105 s -1. It is observed that the peak viscosity at the end of the continuous cooling stage is higher for the as-cast composite with the longer fibres than for the as-extruded composite with the shorter fibres. The peak viscosity of the particle-reinforced composite is intermediate. After isothermal stirring for about 1 h, all viscosities are lower and approach each other. The viscosity decrease of the as-cast composite is strongest and, as a consequence, the viscosity of the particle-reinforced composite becomes highest.

c. J. Quaak, W. lt. Kool /

Propertiesof semisolid AI matrix composites

The exponents m in the power law fit are almost identical for shear rates under about 100 s -I. The exponent m changes for shear rates larger than about 100 s- 1 in the case of fibre-reinforced composites. For these shear rates the fibre-reinforced slurries become more newtonian, as in the case of the 18 and 30 vol.% particle-reinforced composites. Both consistencies and power law indices of the viscosity data of the fibre reinforced slurries are given in Table 1.

4. Discussion

4.1. Pseudoplastic behaviour In the stirred semisolid state, the aluminium composite slurries consist of a globular primary phase, a liquid phase and ceramic particles or short fibres, present in the liquid. Pseudoplasticity is mainly due to primary phase interactions. Matrix and composite slurries are strongly pseudoplastic in steady state condition as power law indices between - 1 . 2 and - 1.4 are found. As noted elsewhere [11], strong thixotropic effects play an important role. A simple agglomeration-deagglomeration model, consisting of a dynamic competition between build-up and breakdown of agglomerates, does not sufficiently explain the strong thixotropy. An additional factor has been suggested [11], namely the rigidity of the bonds in the core region of the agglomerates, which is dependent on time. After a shear rate jump to lower shear rates a loosely bonded agglomerate structure is quickly formed from single globules or existing smaller agglomerates. Immediately after such a shear rate jump a power law index m of about - 0.8 is found [7, 11]. Then, the inner globules of the agglomerates are in long contact with each other, and strong necks are formed by diffusion. The rigidity of the agglomerate increases and the liquid phase between the inner globules is more and more immobilized, leading to a further increase in the viscosity and highly negative rn values. No major influence of the presence of SiC particles or fibres on the power law index is found at shear rates lower than about 30 s-J. Apparently, the power law index is not influenced by the size of the agglomerates after evolution, provided that the size does not become too small. Of course, this result is inherent to the power law equation concept. Note that the ceramic phase suppresses the bonding processes within the agglomerate, but in the steady state it does not significantly alter the fluid flow between the agglomerates, as similar rn values are found. Such an independence of the power law index is also observed for the rheology of partially remelted composites at low (~ < 0.1 s -j ) shear rates [13].

281

At high shear rates, the agglomeration state becomes low, the slurry becomes more newtonian and the absolute value of the power law index decreases. This effect is observed here for the particle-reinforced (18 and 30 vol.%) and fibre-reinforced composite slurries. Similar behaviour is observed by other investigators for matrix slurries at still higher shear rates [14]. The transition to a more newtonian behaviour for a slurry with a lower state of agglomeration will occur at lower shear rates. The results imply that for the 10 vol.% composites the agglomeration state of the primary globules is lower for fibre reinforcement than for particle reinforcement. 4.2. Effect of SiCl, concentration on viscosity The particle content affects the value of the steady state viscosity. Higher particle contents result in a less globular phase and a more effective hindering of globule interactions, leading to a decrease in the viscosity, but also in a larger amount of total solid, leading to an increase in the viscosity. At higher particle concentrations the second contribution will be dominant. A viscosity minimum is expected if the first contribution dominates at lower particle concentrations. In the present study, such a minimum viscosity is found at a concentration of about 20 vol.% SiC particles. A similar phenomenon is encountered for partially remelted composites with sufficient fraction liquid [ 13]. At high shear rates the minimum tends to shift to lower concentrations (see Fig. 5), which is expected as a consequence of the low state of agglomeration. At low shear rate (7 = 2 s ~) there is initially an increase in the viscosity before, at about 20 vol.%, the minimum is reached (see Fig. 5). At this shear rate, the state of agglomeration is high and the effective liquid fraction is low. In ref. 13, also an increase in viscosity with increasing particle content is reported at low liquid fractions. It is thought that the liquid concentration is so low that the amount of liquid is not sufficient to accommodate the particles. Fluid flow will be affected and the viscosity rises. At this low shear rate, more liquid, entrapped between the agglomerates, now becomes available at higher fractions of reinforcement and the viscosity minimum is reached. We expect that, at still lower shear rates, only an increase in viscosity with increasing particle content will be found, without reaching a viscosity minimum, in analogy with the results found after partial remelting I13]. 4.3. Effect of fibres on viscosity The presence of fibres in a liquid composite generally results in a higher viscosity, compared with a composite with the same content of particles, owing to their larger resistance to flow. However, the present results on the steady state viscosities of semisolid slurries with

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Properties of semisolid Al matrix composites

10 vol.% SiC fibres show similar or slightly lower values than for slurries with 10 vol.% particles. If the observation that the viscosity of the initially extruded (fibre length, about 50/~m) slurry is the lowest during the whole measurement period is also taken into account, this indicates that the suppression of agglomeration and coalescence of the primary phase is somewhat more effective for the short fibres of about 50/~m than for the particles. We assume that the flow out of the region between two approaching globules or agglomerates is more difficult for fibres of such a length than for particles and that consequently agglomerate build-up is hindered more effectively by the presence of these fibres. In this case the effect on agglomeration exceeds the primary effect on viscosity. Composite slurries with the longer fibres of the initially as-cast material show an increased peak viscosity. This is attributed to a stronger effect on resistance or viscosity for these longer fibres, also considering that fibre length bcomes comparable with globule size. During stirring, the fibres break up (from 100 to 50/~m average length) and the average fibre length becomes comparable with that in the extruded composite. The observation that the steady state viscosities of both fibre-reinforced slurries are not the same when the average length of the fibres becomes similar is ascribed to the difference in fibre length distribution in both slurries. Fibre breakage is observed during composite preparation and during the experiments. After long stirring in the semisolid state or after extrusion a fibre aspect ratio of 3-4 is reached after which further breakage becomes insignificant. No major advantage of the use of fibres over particles is expected regarding mechanical properties as these aspect ratios are too small for adequate load transfer.

5. Conclusions ( 1 ) Slurries of an A356 matrix alloy and composites with SiC particles or fibres are strongly pseudoplastic. For all slurries, the shear rate dependence of the viscosity can be fitted to a power law equation with power law index m equal to -1.3 + 0.1 for shear rates lower than about 30 s- 1. (2) Composite slurries are less pseudoplastic at shear rates higher than about 30 s- 1. (3) The minimum viscosity is found at a particle concentration of about 20 vol.%. Hindrance of agglomeration dominates over the effect of the higher total solid fraction.

(4) Short fibres are more effective in hindering agglomeration than are particles. As a consequence, slurries with short fibres may have a lower steady state viscosity than slurries with particles. (5) Fibres are broken up during stirring in the semisolid state to an average aspect ratio of 3-4.

Acknowledgments The authors would like to thank Dr. M. Surry (Institut National Polytechnique, Grenoble, France) for providing composite materials and for stimulating discussions. The work was financially supported by the Commission of the European Communities under B R I T E - E U R A M Contract BREU-0151.

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