A R C H I V E S

o f

F O U N D R Y E N G I N E E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310) Volume 7

Issue 1/2007 65 – 68

13/1

Discontinuities in metal composite cast

materials

K. GawdzińskaInstitute of Basic Technical Science, Maritime University of Szczecin, ul. Podgórna 51/53, 70-205 Szczecin, Poland

e-mail: kasiag@am.szczecin.pl

Received 08.02.2007; Approved for print on: 12.03.2007

Abstract While examining the quality of composite alloys it is impossible to neglect defects connected with the structural continuity of these materials. This article briefly describes a mechanism of fracture formation in metal composite alloys with reinforcement saturated by a liquid alloy matrix. These defects are illustrated by metallographic study results (macroscopic and microscopic examinations were performed with a scanning electron microscope). Key words: metal composites, defects, discontinuities.

1. Introduction Depending on the manufacturing method, discontinuities in

metal composite casts may form at various stages of the technological process. As regards casts made by in situ methods, defects of this type are similar to those found in casts from traditional materials, such as cast steel, cast iron, non-ferrous metal alloys [1]. The reinforcing phase in these materials is created in the metallurgic process or during a controlled and directional process of alloy crystallization [2, 3, 4]. It does not affect significantly possible occurrence of defects such as discontinuities in a finished product, as is the case in composites produced by ex situ methods (e.g. composites made by infiltration of the reinforcing structure with liquid metal) [5, 6, 7]. Defects classed as discontinuities may in this case be the same as defects of casts from traditional materials, or completely different due to the presence of the reinforcing phase and the manner of its infiltration into the matrix structure. During the infiltration process, first the initiation pressure is overcome and the metal flows into spaces (ducts) that have the best flowing conditions (these spaces are located in areas of locally smaller packing density of fibres); the reinforcement area with increased hydraulic resistance is filled up [8]; finally, the critical stage comes up,

namely the fulfilement of capillary spaces (Fig. 1), created by fibres adjacent to or located close to each other [8, 9].

Figure 1 presents the behaviour of metal in a wedge-shaped capillary with improper wetting (angle θ > 90).

Fig. 1. Behaviour of a liquid in a wedge-shaped capillary with the vertical angle α (wetting angle θ > 90) [10]

The value of capillary pressure for this case can be

determined from the Young-Laplace equation [8]. Therefore, in the case of poor wetting of fibre surface by

liquid metal, capillaries formed in disordered fibre

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7 , I s s u e 1 / 2 0 0 7 , 6 5 - 6 8

65

reinforcements, where numerous areas of fiber interface occur, we observe spaces which require high pressure to be filled up, the pressure that theoretically tends to infinity. It is not therefore possible to fill spaces between fibres. Consequently, each infiltrated composite with reinforcement from disordered fibres is porous [8].

2. Discontinuities in metallic composite materials formed during the manufacturing process

Applying a pressure higher than necessary for the required (incomplete) filling (±40 MPa – Fig. 2c) of capillaries is not desired, as it may cause deformations or displacements of the reinforcement shape [8, 11, 12]. Such pressure also causes excessive stresses in fibres within front areas of liquid metal, which may lead to fractures, breaks, discontinuities of the reinforcement structure. Examples of these defects are shown in Figure 2a,b,d.

Also, during the cooling of a solidified composite there occurs a contraction in the solid phase of the matrix metal and reinforcement. Table 1 provides examples of the linear coefficient of thermal expansion of saturated composite components.

Table 1. Values of the linear coefficient of thermal expansion of selected saturated composite components (based on [3, 11, 13, 15])

Component Temperature [K]

Linear expansion coeff. α × 106 ×10–1

Matrix Al Cu Mg Ti

1000 1200 800

1200

37.8 23.8 35 11.95

Reinforcement C-graphite Al2O3

Fe B

glass

1000 800

1200 658

300 – 400

29.2 7.99

22.5 8.3

35 – 113

The comparison of these values indicates that the linear

thermal expansion coefficient of composites is generally lower than that of the matrix. As a result, contraction stresses in a cast are reduced, consequently, deformations are smaller and susceptibility to cast fractures [11].

However, stresses in the reinforcement appear. These may be causes of reinforcement element fractures or fractures at the matrix-reinforcement interface. This kind of defect – fracture at the matrix-reinforcement interface is illustrated in Figure 3.

The above consideration refers to the specific cooling saturated composite casts. This does not exclude phenomena similar to those occurring during cooling of casts made of traditional materials and the effects caused by these phenomena (e.g. cast deformation). These may include thermal contraction or structural stresses equivalent in macro-, micro- and submicroscopic areas.

a)

b)

c)

d)

Fig. 2. Fractures of reinforcement elements; a,b (>50MPa), c (30 MPa) – reinforcement: aluminosilicate fibre, matrix: AlSi11; d (>50 MPa) – reinforcement: aluminosilicate fibre, matrix: Wood’s alloy (SEM)

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7 , I s s u e 1 / 2 0 0 7 , 6 5 - 6 8

66

Fig. 3. Fracture at the matrix – reinforcement interface; composite – reinforcement: boron fibre – long, matrix: AlSi 11 (SEM)

Deformations and fractures may also be similar, e.g. fracture

of a hot cast (examinations have confirmed the occurrence of such deformations; macroscopic examination – Fig. 4 and SEM – Fig. 5) and a fracture of a cold cast visible as narrow gaps running through, mostly with metallic sheen, in some places with colourful oxide tarnish, visible on the cast surface as regular scratches, as shown in Fig. 6. It is very difficult to distinguish fractures in ‘hot casts’ and in ‘cold casts’ for aluminum alloys (or composites with a matrix of these alloys).

a)

b)

Fig. 4. Fractures of a hot cast: a – material specimen (composite: reinforcement – aluminosilicate fibre, matrix – AlSi11); b – the same specimen after deep etching

Fig. 5. Fracture of a hot cast; narrow, irregular gap visible on the cast surface as a zigzag scratch (SEM)

Fig. 6. Fracture of a cold cast; composite: reinforcement – aluminosilicate fibre, matrix – AlSi11

Defects such as fractures, discontinuities running along grain

boundaries (or between reinforcement fibres) in pure metal are matrix fractures (Fig. 7).

3. Conclusions It may be concluded from the foregoing analysis that the

defects in the form of discontinuities in cast composites include fractures:

– in the matrix structure; – in composite reinforcement elements; – at the reinforcement – matrix interface.

These defects can be detected only by microscophic examination. Besides, there are fractures occurring in ‘hot’ and ‘cold’ casts which are of macroscopic character. The detection of macroscophic fractures can be facilitated by deep etching.

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7 , I s s u e 1 / 2 0 0 7 , 6 5 - 6 8

67

Fig. 7. Matrix fractures; composite – matrix: aluminosilicate fibre, matrix: AlSi 11 (SEM)

References [1] Polish standard norm PN-85/H-83105. Casts. Division and

Terminology of Defects (in polish). [2] E. Fraś, A. Janas, A. Kolbus, E. Olejnik, Matrix-

Particle interphase Boundaries of the selected ”in situ” and “ex situ” composites MMCS, (in polish) Archiwum Odlewnictwa, 2006, vol. 6, Nr 18(1/2), s. 297-304.

[3] M. Ottmuller, C. Korner, R.F. Singer, Influence of the fiber-matrix interface on the strength of unidirectional carbon fiber reinforced magnesium composites, “Metal-Matrix Composites and Metallic Foams”, Euromat 99-Vol. 5.

[4] A. Dolata-Grosz, M. Dyzia, J. Śleziona: Solidification and structure of heterophase composite, Journal of Achivements in Materials and Manufacturing Engineering, Vol. 20, Issues 1-2, 2007, p.103-106.

[5] A. Dolata-Grosz, M. Dyzia, J. Śleziona, J. Myalski, Influence of ceramic reinforcement kind on the solidification process of aluminim matrix, (in polish) Archiwum Odlewnictwa, 2006, vol. 6, Nr 22, s. 145-151

[6] S. Wong, E. Neussl, D. Fettweis, O.R. Sahm, H.M.L. Flower, High strength Al-Zn-Mg matrix-alloy for continuous fibre reinforcement, [w:] Metal matrix composites and metallic foams, Euromat – vol. 5, Wiley-Vch Verlag Gmbh 2000, s. 119-127.

[7] K. Gawdzińska, J. Grabian, A. Dolata-Grosz, Selected usable properties of Al11/1H18N9T suspension composite, (in polish) Archiwum Odlewnictwa, 2006, vol. 6, Nr 22, s. 192-199.

[8] J. Grabian, The saturation of reinforcement with ceramic disordered fibres during the production of cast of metal composites, (in polish) WSM w Szczecinie, 2001.

[9] J. Grabian, J. Jackowski, M. Szweycer, Infiltration of AK11 alloy into composite reinforcement of aluminosilicate fibres, (in polish) Archiwum Technologii Maszyn i Automatyzacji PAN O/Poznań, 1999, vol. 19, nr 1.

[10] M. Szweycer, Surface phenomena in metal-matrix cast composites technology, Cast Composites, Commission 8.1, CIATF 1998.

[11] K. Gawdzińska, Structure Defects Classification of Casts from Saturated Metal Composites, (in polish) Doctor Thesis, Technical University of Szczecin 2003.

[12] K. Gawdzińska, A. Janas, B. Głowacki, Defects in reinforcement in Metal Casting Made of Composite Materials, (in polish) Archiwum Odlewnictwa, 2006, vol. 6, Nr 18(1/2), s. 345-352.

[13] J. Piątkowski, The analysis of solidification and micro-structure of Al-Si allos, (in polish) Archiwum Odlewnictwa, 2006, vol. 6, Nr 18(1/2), s. 364-369.

[14] K. Gawdzińska, Possibilities of Defects in Metal-Matrix Composite Casts in the area of Matrix Matal-Reinfocement Connection, (in polish) Archiwum Odlewnictwa, 2006, vol. 6, Nr 18(1/2), s. 353-359.

[15] O.M. Byalik, L.V. Golub, A.N. Doniy, A.J. Sheloval, Primenenie komputerowoj sistemy kontrola kachestva rasplavov, Liteynoe Proizvodstvo 1996, nr 2.

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7 , I s s u e 1 / 2 0 0 7 , 6 5 - 6 8

68