CHAPTER 4 PROPERTIES OF ALUMINIUM ALLOY BASED METAL MATRIX COMPOSITES

Transcription

1 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: 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 Cu Mg 0.3Max Ni 0.5 Max Zn 3.0 Max Mn 0.5 Max Fe 1.3 Max Sn 0.2 Max Pb 0.3 Max Ti 0.2 Max Al Reminder Reminder LM24 aluminium alloy offers excellent casting characteristics and good mechanical properties that make it ideal for engineering and functional

2 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 CuAl 2 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 CuAl 2 (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

3 66 Figure 4.1b Microstructure of the plain LM24 aluminium alloy 3500 Al Al 1000 Si 500 Si Si Al Al Theta, Degrees 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

4 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) 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 E 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.

5 68 Table 4.2 Mean hardness values of the aluminium alloy based MMCs Material Hardness, BHN Plain LM24 alloy 96 LM24 + 1% Al 2 O LM24 + 3% Al 2 O LM24 + 5% Al 2 O LM24 + 1% SiC 104 LM24 + 3% SiC 107 LM24 + 5% SiC 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)). X i = Value of the i th sample. M = Mean of i values = N = Sample size. SD = [ i -M ) 2 /(N-1)] 1/2 SE = SD/(N) 1/2 95% CI = M±(1.96SE) i) / N 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.

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7 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 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.

8 LM24 LM24+1%SiC LM24+3%SiC LM24+5%SiC Figure 4.4 Hardness of SiC reinforced MMCs 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 LM24 + 1% Al 2 O LM24 + 3% Al 2 O LM24 + 5% Al 2 O LM24 + 1% SiC LM24 + 3% SiC LM24 + 5% SiC 2.812

9 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). X i = Value of the i th sample. M = Mean of i values = N = Sample size. SD = [ i -M ) 2 /(N-1)] 1/2 SE = SD/(N) 1/2 95% CI = M±(1.96SE) i) / N 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.

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11 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 LM24 LM24+1%Alumina LM24+3%Alumina LM24+5%Alumina Figure 4.5 Density of alumina reinforced MMCs LM24 LM24+1%SiC LM24+3%SiC LM24+5%SiC Figure 4.6 Density of SiC reinforced MMCs

12 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.