Composites Science and Technology 47 (1993) 51-56

CASTING DEFECTS A N D PROPERTIES OF CAST A356 ALUMINIUM ALLOY REINFORCED WITH SiC PARTICLES

Rong Chen & Guoding Zhang Institute of Composite Materials, Shanghai Jiao Tong University, Shanghai 200030, People's Republic of China

(Received 17 January 1992; revised version received 7 April 1992; accepted 29 May 1992)

Abstract This paper is concerned with casting defects and properties of test bars cast from an A356 aluminium alloy reinforced with SiC particles. The composite materials were produced by the molten aluminium mechanical stirring technique. Particular attention was given to the characteristics of the casting defects which appeared at the tensile fracture surfaces of the cast test bars by means of extensive analytical electron micros- copy. These observations showed that the casting defects on the fracture surface were agglomerates of SiC particles, aluminium and silicon oxides, mixtures of SiC particles with aluminium and silicon oxides, and brittle chemical compounds containing aluminium and silicon elements. These defects either were present in the as-received ingots or were introduced in the casting process during which oxide films were easily incorpor- ated into the liquid metal. The numbers and areas of the defects on the fracture surface have an obvious effect on the mechanical properties of the cast test bars. Appropriate methods of improving the quality of the composite ingots as well as of optimizing the casting technique must be used to minimize these defects.

Keywords: AI/SiC composite materials, casting de- fects, MMC particulate composites, fracture surfaces, defect analysis

liquid metal process may be the first to achieve large-scale production and has great commercial potential.

For the work described in this paper, an ingot of A356 aluminium alloy reinforced with SiC particles produced by the liquid metal stirring process was first remelted, then, after holding for some time above the liquidus temperature, was cast into test bars. The final shaped casting was heat-treated to obtain the optimum properties. The possible problems in this route are interface reaction during remelting, casting fluidity, and final casting defects such as gas porosity, oxides and inclusions within the text bar. Since the silicon content of the A356 alloy is sufficiently high, the SiC particles are stable in the matrix and do not react with aluminium to form AI4C3 at temperatures above the liquidus for certain periods of time. 7 In practice, melt holding times can be many minutes and good casting fluidity is achievable. The main problem is therefore to optimize the casting technique and improve the quality of the composite ingots for casting so as to minimize the number of defects. The aim of this work is to provide some preliminary answers concerning the casting defects seen in the fracture surfaces of cast test bars and their effects on tensile properties.

MATERIALS

INTRODUCTION

Aluminium matrix composite materials reinforced with SiC particles offer higher wear-resistance, higher modulus and better dimensional stability than conventional aluminium alloys.l-5 In the past, however, these materials have been processed by power metallurgy, and, as a result, have only been available in the form of wrought products. 6 The development of the molten aluminium mechanical stirring method enables these materials to be cast into shapes or ingots for further processing. This low-cost

Composites Science and Technology 0266-3538/93/$06.00 © 1993 Elsevier Science Publishers Ltd.

51

The aluminium matrix used in this investigation was an A356 alloy containing about 10% wt Si, 0 . 1 % w t M g and 0 .07%wtMn. SiC particles with about 10/zm mean diameter were used as reinforce- ment in the as-received condition. The composite materials were manufactured by the melted alumi- nium liquid stirring method. This process involves the introduction of SiC particles into molten aluminium alloys followed by agitation to achieve uniform distribution, and subsequent casting and solidifying into the final form. The composite ingots, containing 15% vol. and 20% vol. SiC particles, were then remelted in a coated steel crucible to about 700°C in an electric resistance furnace. After holding for about 10 min while mechanical stirring took place, they were

52 Rong Chen, Guoding Zhang

then cast into investment moulds. The cast test bars were then heat treated for 24 h at 540°C, quenched in hot water at 80-90°C, aged for 24 h at room temperature, and then aged for 24 h at 160°C.

RESULTS AND DISCUSSION

Tensile properties of A356/SiCp composite cast test bar The best ultimate tensile strength (UTS) and the average UTS values were 285 MPa and 234MPa respectively from the heat-treated test bars. The test bar specimens with dimensions of 7mm diameter, 30mm span and 80mm length were made by the conventional gravity investment casting method. Every mould contained six test bar specimens as a group which were used to measure the tensile strength after heat treatment. The UTS values of the composite materials containing 15% vol. and 20% vol. of SiC particles were within the same range.

Visual examination of the fracture surfaces of the test bars showed the presence of a number of defects which were classified into three different types, including 'black' defects, 'white' or 'silver' defects and porosity. These defects were generally irregular in shape, and could be up to about 1.2mm in their largest dimension. They were of different colours from the rest of the fracture surface and were readily apparent to the naked eye. These defects involved agglomerates of SiC particles, aluminium and silicon oxides, mixtures of SiC particles with aluminium and silicon oxides, brittle chemical compounds and some contaminants. The black defects sometimes covered large areas. The white or silver defects were similar in appearance to those found on the surface of the ingot fractures, generally white, grey or silver in colour.

The estimated percentage area of these defects on the surface of the fractures was estimated visually. The total area of the defects is plotted against the ultimate tensile strength (UTS) in Fig. 1 for aluminium composites reinforced with 15% vol. and 20% vol. of SiC particles. As expected, the UTS increases as the incidence of defects on the surface of the test bar decreases.

Distribution of SiC pmlides in the ingot and the cast test bars A photomicrograph of the A356/SiCp ingot is shown in Fig. 2. SiC particles tend to agglomerate along the aluminium grain boundaries and are trapped by converging dendrites in the intercellular regions.

At the lower magnification, the reinforcement particles appear to be uniformly distributed within the metal matrix. However, at higher magnification it can be seen that the grains themselves have a relatively lower density of the particles within them. The distribution of the particles in the test bars is the same

4111.oo

3~O.(X)

300.00

~0~00

IE ~.m

~ lel).lll

lOO.Oo

© • O

Q

[~ 1,qmiL%

©

o

©

©

0 . 0 0 . . . . i . . . . ~ . . . . i . . . . i . . . . i , . ,

o 5 lO 15 20 25 80

Ea~ma~l ~ At~ on Fratlum ~nf~o0% Fig. 1. Ultimate tensile strength of a test bar cast from 15% vol. and 20% vol. A356/SiCp composites versus a visual assessment of the percentage of cross-sectional area of

defects on the fractured surface.

Fig. 2. Photomicrograph of 15% Vf A356/SIC r composite ingot.

as that in the ingot. Because the particles are pushed to the intercellular regions during the solidification process, the distribution of the SiC particles may become more uniform as the solidification rate increases.8

Analysis of the defects in the fracture surfaces of test bars A micrograph of the fracture surface of a cast test bar of A356/SiCp composite material taken from a region free of defects is shown in Fig. 3. In some places can be seen the characteristic feature of ductile dimples in the aluminium with SiC particles at their base. However, other types of defect locations can also easily be seen on the fracture surface, as shown in the following.

Gas porosity Gas porosity is found in some areas of the fracture surfaces of the test bars. Their lengths vary, but most

Cast Al alloy reinforced with SiC 53

Fig. 3. Micrograph of the fracture surface of the test bar cast of A356/SiCp taken from a region free of defect.

of them are about 500 ~m long. In Fig. 4 we see that agglomerates of SiC particles occur inside the gas porosity. The propagation of cracks originating from the gas porosity is also found, as shown in Fig. 5. Gas porosity is very difficult to remove after the gas is introduced into the molten metal, either in the manufacturing or in the casting process. It has serious effects on the properties of the test bars.

Aluminium and silicon oxides White block-shaped inclusions in the fracture surface of a test bar under investigation in the scanning electron microscope (SEM) are shown in Fig. 6. The colour is the appearance in the SEM, and this in- clusion is too small to be resolved with the naked eye. These inclusions are different in shape, separate from the composite matrix, and form a complete block with dimensions much larger than the mean diameter of the SiC particle. The length of the inclusion in Fig. 6 is about 80/~m. Chemical compositions of the block analysed by EDAX at many points, as shown in Fig. 7, reveal that only aluminium and a small percentage

Fig. 5. Gas porosity on the fracture surface showing cracks originating from the gas porosity.

Fig. 6. Aluminium and silicon oxide on the fracture surface of a test bar casting.

Fig. 4. Gas porosity on the fracture surface of cast test bars.

RATE = I065CPS TIME = 10OLSEC FS = 3780CNT PRST = 100LSEC

~ AIK i wt.% at,% %S.E.--

AIK 89.43 89,80 0.40 - -

1.00 2.00 3.00 4.00 l CNT O.OOKEV 10ev ch A EDAX

Fig. 7. EDAX spectrum obtained from the block inclusion shown in Fig. 6.

54 Rong Chen, Guoding Zhang

chemical composition of these spherical particles, as analysed by EDAX, reveals that the magnesium is above 20% wt, whereas in the matrix the magnesium content is only about 0-4% wt. This area is therefore enriched in magnesium. It has been reported that the presence of magnesium is favourable for improving the infiltration of the SiC particles by aluminium. ''~° But if the high magnesium content of the aluminium matrix results in an increased possibility of forming black defects, the magnesium level may have to be controlled to some degree.

Fig. 8. Black spot on the fracture surface of a test bar.

of silicon are present. This kind of inclusion is most likely a compound of aluminium and silicon oxides, such as A1203" SIO2, which was formed when the oxide film was introduced into the matrix in the casting process. These oxides are found only in the fracture surface of the castings, not in the composite ingot.

Black spots Black spots can be seen by the naked eye in the fracture surfaces of test bars, some of them covering a large area. The largest diameter of black spot found by SEM investigation was 1-1.2 mm, as shown in Fig. 8. In the black area there is the porosity with many small spherical particles around it, as shown in the magnified micrograph in Fig. 9. These black spots, different from the normal area, are defects that may be contaminant- or element-rich phase areas. The

Fig. 9. Black spot on the fracture surface shown at higher magnification.

White or silver spots White or silver spots can also be found by the naked eye on the fracture surfaces of cast test bars. They are generally white, grey or silver in colour, and some contained several small light grey areas. The morphologies of these spots revealed by the SEM are very variable. According to their morphological characteristics, they can be divided into three main types, as follows.

(a) Strip-shaped white spots (Fig. 10). Some of these have a distinct boundary between the white spot and the matrix. The chemical composition of this area, as analysed by EDAX, mainly includes aluminium and silicon, the contents ranging from 30 to 60% wt, depending on the point analysed. Figure 11 is a magnified micrograph of strip-shaped area. Compared with the composite matrix, the surface of the defect is smoother. SiC particles can be found inside these defects. Point a in Fig. l 1, for example, is a SiC particle which extends into the defect surface. From their morphology and chemical compositions, we speculate that this kind of defect may be a mixture of SiC particles and aluminium silicon oxides, formed when the oxides in the surface containing SiC particles were introduced into the matrix during the casting process.

S00m

Fig. 10. Strip-shaped white spot on the fracture surface of a test bar.

Cast Al alloy reinforced with SiC 55

Fig. 11. Strip-shaped white spot at higher magnification: point 'a': a SiC particle.

(b) Plane-shaped white spots (Fig. 12). The surface of the plane-shaped white spot seen in Fig. 12 is smooth and can be divided into several regions. From its size it is not an agglomeration of SiC particles. Chemical composition analysis shows that only aluminium and silicon are present in these spots, and we speculate that the defect is a brittle aluminium and silicon inclusion. Since this inclusion is not found in the as-received composite ingot, it is most likely to have come from the casting process.

(c) Block-shaped white spots (Fig. 13). Block- shaped white spots consist of three large block inclusions, as shown in Fig. 13. The alloying element distribution in the areas at the points a and b, as analysed by EDAX, is different. The area at point b includes aluminium and silicon, but that at point a contains only silicon without aluminium. In order to

Fig. 12. Plane-shaped white spot on the fracture surface of a test bar.

~ 0 ~

Fig. 13. Block-shaped white spot on the fracture surface of a test bar.

confirm this phenomenon, we used a scanning auger microprobe (SAM) to improve the elemental analysis. Several points near point a were selected for analysis after 2 rain argon ion etching to remove surface contaminants. A typical SAM spectrum of the element composition in the area near point a is shown in Fig. 14. Since no aluminium was found at point a, and only the three elements of silicon, carbon and oxygen were present in this area, it is estimated that the inclusion at point a is an agglomerate of SiC particles, and that the surfaces of the particles contained silicon oxides as a result of particle oxidation. These agglomerates may be formed when particles did not disperse properly in the composite manufacturing process.

A micrograph at high magnification of the inclusion at point b is shown in Fig. 15. It is a brittle compound containing aluminium and silicon with a cleavage strip in the surface. These inclusions are very harmful to the properties of the test bars on account of their large dimensions and brittle characteristics.

AES SURVEY SF=186.880 DAT=3.46 7

6

5

4 Si

N 2

1

0 200 400 600 800 1000 KINETIC ENERGY, EV

Fig. 14. SAM spectrum obtained from the blocky inclusion at the point a in Fig. 13.

56 Rong Chen, Guoding Zhang

These defects may be agglomerates of SiC particles, mixtures of SiC particles and aluminium silicon oxides, or brittle compounds containing mainly aluminium and silicon.

30pro

Fig, 15. Brittle compound on the fracture surface of a test bar.

CONCLUSION

The best ultimate tensile strength and average UTS values of the A356/SiCp test bar castings were 285MPa and 234MPa, respectively. The UTS increases as the incidence of defects on the surfaces of test bars decreases.

SiC particles were basically clustered in the grain boundaries of the aluminium in the composite ingot and test bars of the A356/SiCp materials. The distribution of SiC may become more uniform as the solidification rate increases.

Gas porosity existed both in the ingot and the cast test bars. Crack propagation was found to have originated from the porosity in the fracture surface of the test bars.

There are block-shaped oxide inclusions, of A1203" SiO2 compound, on the fracture surfaces of the test bars which do not bond well to the composite matrix. These were formed when oxide films on the surfaces of the component materials were introduced into the matrix during the casting process.

Black spots on the fracture surface could be found with the naked eye. They may be contaminants or a magnesium-rich phase. As a result of their large dimensions they are harmful to the properties of the material.

There are several features consisting of white or silver spots on the fracture surface, including strip-shaped, plane-shaped and block-shaped defects.

REFERENCES

1. Selvaduray, Guna, Hichman, Ray, Quinn, David, Richard, Dan & Rowland, Dan, Relationship between microstructure and physical properties of AlzO3 and SiC reinforced aluminum alloys. In Proceedings of the International Conference of Interfaces in Metal Ceramics Composites, eds R. Y. Lin et al. The Minerals, Metals & Materials Society, New York, 1989, pp. 271-89.

2. Lloyd, D. J., Lagace, H. P. & Mcleod, A. D., Interracial phenomena in metal matrix composites. In Proceedings of the Third International Conference of Composite Interface, Controlled lnterphases in Compos- ite Materials, ed. H. Ishida. Elsevier Science Publishing, London, 1990, pp. 359-75.

3. Girot, F. A., Quenisset, J. M. & Naslin, R., Discontinuously reinforced aluminum matrix compos- ites. Composite Science and Technology, 30 (1989) 155-84.

4. Lavernia, E. J., Synthesis of particulate reinforced metal matrix composites using spray atomization and co-deposition. SAMPE Quarterly, 22(2) (January 1991) 2-12.

5. Feiger, A. L. & Walker, T. A., The processing and properties of discontinuously reinforced aluminum composites. Journal of The Minerals, Metals & Materials Society, 43(8) (1991) 8-15.

6. Sargent, M. A., Rensen, C. & Alsem, W. H. M., The structure and properties of particulate SiC/aluminum alloy 6061 metal matrix composite manufactured via hot isostatic pressing. In Proceedings of The International Conference on Metal & Ceramic Matrix Composites, eds R. B. Bhagat et al. The Minerals, Metal & Materials Society, New York, 1990, pp. 137-53.

7. Lloyd, D. J. & Chamberlain, B., Properties of shape cast AI-SiC metal matrix composites. In Proceedings of the International Symposium on Advances in Cast Reinforced Metal Composites, eds S. G. Fishmen & A. K. Dhingra. ASM International, Illinois, 1988, pp. 263-9.

8. Skibo, M., Morris, P. L. & Lloyd, D. J., Structure and properties of liquid metal processed SiC reinforced aluminum. In Proceedings of the International Sympo- sium on Advances in Cast Reinforced Metal Composites, eds S. G. Fishmen & A. K. Dhingra. ASM International, Illinois, 1988, pp. 257-61.

9. Kobashi, Makoto & Choh, Takao, Effects of alloying elements on SiC dispersion in liquid aluminum. Materials Transactions JIM, 31(12) (1990) 1101-7.

10. Kobashi, Makoto & Choh, Takao, Particulate incor- poration process and uniformity of particulate distribu- tion of the aluminum matrix composite manufactured by melt stirring method. J. Japan Inst. Metals, 55(1) (1991) 79-84.