64

CHAPTER 4

PROPERTIES OF ALUMINIUM ALLOY BASED METAL

MATRIX COMPOSITES

4.1 PROPERTIES OF LM24 ALUMINIUM ALLOY

LM24 aluminium alloy is essentially a pressure die casting alloy and

it is suitable for high volume precision die castings which conforms to BS

1490: 1988. The chemical composition of LM 24 aluminium alloy used in the

present investigation is given in the Table 4.1. It is most widely used for the

aluminium casting alloys manufacturing.

Table 4.1 Chemical composition of %weight of LM24 aluminium alloy

Element LM24 as per standards LM24 developed in the

percent work

Si 7.5-9.5 9.220

Cu 3 - 4 3.625

Mg 0.3Max 0.198

Ni 0.5 Max 0.090

Zn 3.0 Max 1.852

Mn 0.5 Max 0.314

Fe 1.3 Max 1.027

Sn 0.2 Max 0.065

Pb 0.3 Max 0.073

Ti 0.2 Max 0.054

Al Reminder Reminder

LM24 aluminium alloy offers excellent casting characteristics and

good mechanical properties that make it ideal for engineering and functional

65

parts used for the manufacture of thin wall sectioned castings. The plain LM24

aluminium alloy offers excellent pressure retention properties and similar

machining characteristics to other pressure diecasting alloys. Due to this, it is

chosen as a matrix material.

4.2 MICROSTRUCTURE AND XRD STUDIES

The optical microstructure of the plain LM24 aluminium alloy is

presented in Figures 4.1a and 4.1b. The microstructure shows interdendritic

particles of eutectic silicon and CuAl2 in a matrix of aluminium solid solution.

The addition of Cu (3-5 %wt) to hypereutectic Al–Si alloy improves the wear

resistance at high loads due to the precipitation of a hard-phased CuAl2

(Dwivedi, 2006). The X-ray diffraction pattern of the plain LM24 aluminium

alloy is given in Figure 4.2.

Figure 4.1a Microstructure of the plain LM24 aluminium alloy

66

Figure 4.1b Microstructure of the plain LM24 aluminium alloy

0500

100015002000250030003500

10 20 30 40 50 60 70 80

2 Theta, Degrees

SiSi

Al

AlAl

SiAl

Figure 4.2 XRD Pattern of the plain LM24 aluminium alloy

4.3 MECHANICAL PROPERTIES

For engineering applications, it is necessary to know the important

mechanical properties of the newly developed aluminium alloy – aluminium

oxide / silicon carbide composites. Mechanical properties are the foremost

67

important feature in selecting any material for structural machine components.

For any tool, any power transmission device or any wear element, the

properties needed for its serviceability would preferably include strength,

formability, rigidity, toughness and durability. There are many tests such as

tensile and hardness tests to measure the mechanical properties, and these tests

supply the most useful information for most of the applications. (Kenneth G.

Budinski and Michael K.Budinski, 2002).

4.3.1 Hardness Tests

Hardness is probably one of the most used selection factors. The

hardness of materials is often equated with wear resistance and durability. A

number of ways are available to measure the hardness of the sample. The

hardness of the specimen is determined using a Brinell hardness testing

machine as per the standard ASTM E10 - 08. In Brinell hardness testing, a

small diameter ball is pushed into the surface, and an optical measuring device

is used to measure the diameter of the resulting indentation. This diameter is

then used to calculate the Brinell Hardness Number (BHN) (Kenneth G.

Budinski and Michael K.Budinski, 2002).

The LM24 aluminium alloy - aluminium oxide / silicon carbide

reinforced composite specimens are polished and placed on the Brinell

hardness testing machine and then a 10 mm diameter steel ball is pushed with

the loading force of 500 N for 15 seconds. Brinell hardness number has been

calculated by using the standard formula. The hardness of the specimen is

determined using a Brinell hardness testing machine for 5 samples in each type

and the mean value is evaluated. The effect of hardness by reinforcement of

aluminium oxide, and silicon carbide particles of the LM24 aluminium alloy is

shown in Table 4.2.

68

Table 4.2 Mean hardness values of the aluminium alloy based MMCs

Material Hardness, BHN

Plain LM24 alloy 96

LM24 + 1% Al2O3 102

LM24 + 3% Al2O3 105

LM24 + 5% Al2O3 108

LM24 + 1% SiC 104

LM24 + 3% SiC 107

LM24 + 5% SiC 110

4.3.1.1 Effect of Reinforcement on Hardness

The Mean (M), Standard Deviation (SD), Standard Error (SE) and

the upper and lower limits of Confidence Interval (CI) of the hardness in BHN

of the plain LM24 aluminium alloy and the aluminium alloy - aluminium oxide

/ silicon carbide composite are presented in Table 4.3. The formulae used for

the calculation are given as follows (Ronald et al (2002) and David L Streiner

(1996)).

Xi = Value of the ith sample.

M = Mean of i values = i) / N

N = Sample size.

SD = [ i -M )2/(N-1)]1/2

SE = SD/(N)1/2

95% CI = M±(1.96SE)

In all the conditions, the mean of hardness lies within the respective

upper and lower limits of confidence for the plain LM24 aluminium alloy and

the aluminium alloy - aluminium oxide / silicon carbide composite.

69

70

From the results, it is found that the hardness of the plain LM24 aluminium

alloy is good due to the better compaction in pressure die casting and also the

fine grain size of the casting. The hardness of the aluminium alloy - aluminium

oxide / silicon carbide composite increases with the amount of ceramic

reinforcement and is higher than that of the plain LM24 aluminium alloy due to

the particulate reinforcement and higher hardness of the particles. The hardness

of aluminium alloy - silicon carbide composite is higher than that of aluminium

alloy - aluminium oxide composite, because of higher hardness of silicon

carbide. The hardness increases with the increase of percentage weight of

particulate reinforcement of aluminium oxide / silicon carbide. The influence

of alumina and SiC in the hardness of the LM 24 aluminium alloy is also

shown in the Figures 4.3 and 4.4 respectively.

96

102

105

108

80

85

90

95

100

105

110

LM24 LM24+1%Alumina LM24+3%Alumina LM24+5%Alumina

Figure 4.3 Hardness of alumina reinforced MMCs

The improved hardness properties of the aluminium alloy –

aluminium oxide and aluminium alloy - silicon carbide composites have the

advantage of many engineering applications especially in the automobile and

aerospace industries.

71

96

104

107

110

90

95

100

105

110

LM24 LM24+1%SiC LM24+3%SiC LM24+5%SiC

Figure 4.4 Hardness of SiC reinforced MMCs

4.3.2 Density measurements

Density of the aluminium alloy and aluminium alloy - aluminium

oxide / silicon carbide composites are measured by using ‘Archimedes’

principle. The effect of particle reinforcement of aluminium oxide / silicon

carbide of the LM24 aluminium alloy is shown in the following Table 4.4.

Table 4.4 Mean density values of the aluminium alloy based MMCs

Material Density, g/cc

Plain LM24 alloy 2.790

LM24 + 1% Al2O3 2.802

LM24 + 3% Al2O3 2.826

LM24 + 5% Al2O3 2.850

LM24 + 1% SiC 2.794

LM24 + 3% SiC 2.803

LM24 + 5% SiC 2.812

72

4.3.2.1 Effect of Reinforcement on Density

The Mean (M), Standard Deviation (SD), Standard Error (SE) and

the upper and lower limits of Confidence Interval (CI) of the density, in g/cc

for the plain LM24 aluminium alloy and the aluminium alloy - aluminium

oxide / silicon carbide composite are presented in Table 4.5. The formulae used

for the calculation are given as follows (Ronald et al (2002) and David L

Streiner (1996).

Xi = Value of the ith sample.

M = Mean of i values = i) / N

N = Sample size.

SD = [ i -M )2/(N-1)]1/2

SE = SD/(N)1/2

95% CI = M±(1.96SE)

In all the conditions, the mean of density lies within the respective

upper and lower limits of confidence for the plain aluminium alloy and the

aluminium alloy - aluminium oxide / silicon carbide composites.

7473

74

From the results, it is well-known that the density of the LM24 aluminium

alloy based metal matrix composites marginally increases due to the

percentage weight reinforcement of aluminium oxide / silicon carbide

particles. The effect of reinforcement of alumina and SiC in the density of the

LM24 aluminium alloy is also shown in Figures 4.5 and 4.6 respectively.

2.79

2.802

2.826

2.85

2.76

2.78

2.8

2.82

2.84

2.86

LM24 LM24+1%Alumina LM24+3%Alumina LM24+5%Alumina

Figure 4.5 Density of alumina reinforced MMCs

2.79

2.794

2.803

2.812

2.78

2.79

2.8

2.81

2.82

LM24 LM24+1%SiC LM24+3%SiC LM24+5%SiC

Figure 4.6 Density of SiC reinforced MMCs

75

The density of the LM24 aluminium alloy - aluminium oxide /

silicon carbide composite increases with the amount of ceramic reinforcement

and is higher than that of the plain LM24 aluminium alloy due to the higher

density ceramic particulate reinforcement. The density increases with the

increase of percentage weight of particulate reinforcement of aluminium

oxide / silicon carbide. The density of aluminium alloy - aluminium oxide

composite is higher than that of the aluminium alloy - silicon carbide

composite, because of the higher density of aluminium oxide.

The improved properties of these aluminium alloy - aluminium

oxide and aluminium alloy - silicon carbide composites can be used for many

engineering applications especially in the automobile and aerospace

industries.

4.4 SUMMARY

This chapter emphasizes the characteristics of the LM24 aluminium

alloy and aluminium alloy - aluminium oxide / silicon carbide composites.

The distribution of hard ceramic particles is analyzed through optical

microscopic studies. XRD study reveals the phases present in the material.

Hardness and density measurements reveal that the reinforcement of the hard

ceramic particles increases both hardness and density of the LM24 aluminium

alloy. Hardness of the silicon carbide reinforced composite is superior to the

aluminium oxide reinforced composites because of the higher hardness of the

silicon carbide particles. The density of the aluminium oxide reinforced

composite is higher than that of the silicon carbide reinforced composites

because of the higher density of the aluminium oxide particles.