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CHAPTER 3

PRODUCTION AND CHARACTERIZATION OF Al-TiB2 MMC

3.1 INTRODUCTION

The development of metal matrix composites (MMC) has reflected the need

to achieve property combinations beyond those attainable in monolithic metals.

Thus, tailored composites resulting from the addition of reinforcements to a metal

may provide enhanced specific stiffness coupled with improved fatigue and wear

resistance, or perhaps increases the specific strength combined with desired thermal

characteristics (for example, reduced thermal expansion coefficient and

conductivity) in the resulting MMC.

The melting point of most aluminium alloys is near that of pure aluminium,

approximately 660°C; this relatively low melting temperature, in comparison to

most other potential matrix metals, facilitates processing of aluminium based

MMCs. Among various processes used for fabrication of Al MMCs, stir casting

process is a highly potential and low cost process. Stir casting is a liquid state

method for fabricating composite materials, in which a dispersed phase (ceramic

particles, short fibres) is mixed with a molten matrix metal by means of mechanical

stirring. The liquid composite material is then cast by conventional casting methods

and could also be processed by conventional metal forming technologies.

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In this chapter, details about the in-situ stir casting process used to produce

the proposed aluminium (Al6061-T6) metal matrix composite reinforced with

titanium boride (TiB2) are presented. The mechanical and metallurgical

characterizations of the cast MMCs are also detailed.

3.2 EXPERIMENTAL PROCEDURE

3.2.1 Production of Al-TiB2 MMC

The matrix material was chosen as Al6061 and the reinforcing material was

chosen as TiB2. K2TiF6 and KBF4 salts were used in a proper proportion to obtain

the required volume fraction of TiB2 in the composite through in situ process. Table

3.1 shows the chemical composition of Al6061-T6 alloy.

Table 3.1Chemical Composition of Al6061-T6 Alloy

Element Mg Si Fe Mn Cu Cr Zn Ni Ti Al

wt.% 0.95 0.54 0.22 0.13 0.17 0.09 0.08 0.02 0.01 Balance

The procedure for deriving the amount of chemical salts required to get the

estimated volume fraction of TiB2 in Al MMC is given below:

1) The atomic weight of each chemical that are in the salts is evaluated.

2) The ratio of the chemicals in the particular composition is calculated.

3) Percentage of chemical composition of the salt is calculated.

4) The percentage of chemical required for producing the required volume of

TiB2 is estimated.

5) The weight of each salt required to obtain the required weight percentage of

TiB2 in the composite is found.

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Using the above procedure, the weight of each salt that is used to extract the

required weight percentage of TiB2 Particles was estimated and presented in Table

3.2.

Table 3.2 Calculated Weights of Salts per Casting

Sl.No. % Reinforcement Weight of Salts

TiB2 (wt. %) K2TiF6 (g) KBF4 (g)

1. 10 34.55 36.24

2. 11 38.00 39.86

3. 12 41.45 43.48

4. 13 44.91 47.11

5. 14 48.36 50.73

The casting was done using the resistance type electric furnace at the

Welding Research Cell, Coimbatore Institute of Technology, Coimbatore (Figure

3.1). The melting was carried out in a graphite crucible. The chemicals used were

highly corrosive in nature. Therefore a primer coating with Wulfrakot was applied

on the crucible and stirrer blade. The crucible was preheated to 200 °C. Six Al6061

T6 treated rods of size 150 mm were placed in the crucible at 450 °C. The weight of

the six rods was 1235 gms. The furnace was heated up to 850 °C. Both the

chemicals as per the quantity mentioned in Table 3.2 were added and mixed

thoroughly.

The following is the chemical reaction taking place in the in-situ process:

3K2TiF6 + 22Al + 6KBF4 3Al3Ti + 3AlB2 + 9KAlF4 + K3AlF6 + Heat (3.1)

3Al3Ti + 3AlB2 12Al + 3TiB2 (3.2)

The salt K2TiF6 reacts with Al to form Al3Ti and gases. The other salt KBF4

reacts with Al to form AlB2 and gases. The formed Al3Ti and AlB2 react together

and yield TiB2 as the product. The theoretical mole ratio of inorganic salts required

to produce TiB2 can be obtained from equations (3.1) and (3.2). One mole of

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K2TiF6 and two moles of KBF4 are required to produce one mole of TiB2. It was

reported in a study that when these salts were added to produce the MMC, a large

amount of Al3Ti was present in MMCs (Zhang et al 2008, Zhao et al 2007).

Figure 3.1 Stir Casting Setup

This might be due to the non-availability of sufficient AlB2 to convert all

Al3Ti into TiB2 as the reaction specified in equation (3.2) was incomplete. The

shape of Al3Ti was reported to be needle like structure which is obviously

detrimental to the strength of the composite (Zhao et al 2007, Zhao et al 2005 and

Varin 2002). Therefore here the salts were added such that KBF4 was slightly in

excess of theoretical mole ratio to eliminate the formation of Al3Ti.

The molten slurry was kept as such at 850 °C for about 10 minutes for the

reaction to take place completely. After the formation of TiB2 the slag in the molten

metal (emerging out of the chemical reaction) was removed completely. Then the

stirrer was switched on and stirred for about 10 minutes so that the TiB2 particles

are completely dispersed into the molten metal. Then the molten metal was brought

to the pouring temperature of around 750 °C. The crucible was taken out and

poured into the preheated die cavity of size 100 mm x 50 mm x 50 mm. Manual

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stirring was done even while pouring using a graphite rod. Composites containing

percentages of Al-TiB2 varying from 10% to 14% reinforcement were fabricated.

Figure 3.2 shows a typical Al-TiB2 composite that was fabricated by in situ stir

casting process. The cast composite is then sliced to a size of 10 mm x 50 mm x 6

mm using a wire cut EDM machine.

Figure 3.2 A Typical In-situ Stir Cast Al-10 wt. % TiB2 MMC

3.2.2 X-ray Diffraction

Five specimens of size 10 mm x 5 mm x 3 mm were prepared from castings

having different content of TiB2 particles. The X-ray diffraction patterns (XRD)

were obtained using Panalytical X-ray diffractometer with Cu- -ray radiation to

confirm the formation of TiB2 particles.

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3.2.3 Evaluation of Tensile Strength

From each sliced plate, three samples were cut according to ASTM E08

Standard as shown in Figure 3.3 for testing the tensile strength of the cast MMC.

Figure 3.4 shows the typical tensile specimen samples. The tensile strength was

evaluated using a computerised universal testing machine (HITECH TUE-C-1000)

for all the three samples and the average UTS and % Elongation were calculated.

Figure 3.3 Dimensions of Tensile Specimen in mm as per ASTM E08 Standard

Figure 3.4 Typical Tensile Test Specimens

3.2.4 Evaluation of Wear Resistance of MMCs

For conducting the wear test on the MMCs, specimens of size 6 mm x 6 mm

x 20 mm were cut from the castings. The wear rate was measured using a pin on

disk (POD) wear and friction monitoring machine (DUCOM TR20) at room

temperature. ASTM G99-05 standard test was conducted on all the samples and the

results were tabulated.

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Figure 3.5 shows the experimental setup which was used for this study. The

polished sample was made to slide on disk made of EN35 steel alloy. Based on trial

experiments, the parameters for evaluating the dry sliding wear were fixed. The

fixed parameters are given below.

1. Sliding velocity = 1.2 m/s

2. Sliding distance = 1500 m

3. Normal load = 15 N

The wear in micron and frictional force in N were calculated using the

sensors and the results were plotted using a computerised data acquisition system.

Figure 3.5 Photograph of Pin-on-disc Apparatus

3.2.5 Evaluation of Corrosion Resistance of MMCs

Electrochemical pitting corrosion test was conducted on the MMCs to

evaluate the corrosion resistance of the MMCs. ACM Gill 5500 Potentiostat &

Galvanostat instrument was used to conduct the tests according to ASTM G5

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standard. The experimental setup has a flat cell in three electrode configuration as

shown in Figure 3.6. The working electrode comprises of the specimen prepared

from the cast MMC. A saturated calomel electrode (SCE) and platinum gauze were

used as reference electrode and counter electrode respectively. The MMC specimen

was held on the flat cell using an adjustable shoe assembly. The shoe made an

electrical contact with the back surface of the specimen and also secured the

specimen against a Teflon gasket. The Teflon gasket exposed 1 cm2 of the specimen

to the cell solution. The test solution was prepared by dissolving 8.75 g of reagent

grade NaCl salt in 250 ml distilled water to form 3.5% (by weight) sodium chloride

solution. The flat cell was filled with 250 ml of this solution and the electrodes were

connected to the respective ports in the corrosion measuring instrument through the

lead provided in the flat cell.

Figure 3.6 ACM Gill 5500 Potentiostat and Galvanostat Instrument

After the specimen had been immersed for 50 minutes to allow for the rest

potential to settle, the test was commenced by measuring the open circuit potential.

A potential scan was applied at -250 mV and scanned in more noble direction to

250 mV at a rate of 100 mV/min. The current density was measured continuously

59

using the data acquisition system connected to the computer using a RS232 port.

The Tafel graph with current density in logarithmic scale along x axis and potential

along the y axis was obtained using the analysis software. The Tafel graph was

further processed using the analysis module in the software and the values of anodic

slope (Ba), cathodic slope (Bc), corrosion current (ICorr), corrosion potential (ECorr),

pitting potential (EP), Polarization resistance (RP) and corrosion rate (VCorr) were

estimated.

3.2.6 Microstructural Analysis

The cast composite plates were cross-sectioned at its mid-point and

specimens of size 20 mm x 10 mm x 6 mm were obtained for metallographic study.

The samples were prepared as per standard metallographic procedure. These

specimens were observed using a scanning electron microscope (SEM) (JEOL-

JSM-6390). The specimens were then etched with a colour etchant containing 2 3 g

sodium molybdate, 5 ml HCl (35%) and 1 2 g ammonium bi fluoride in 100 ml

distilled water for microstructure analysis (Beraha et al 1977). Colour

metallographic study was carried out using a metallurgical microscope (Olympus

Microscope BX51 M).

3.2.7 Microhardness Test

The microhardness of the cast composite was measured using a

microhardness tester (MITUTOYO-MVK-H1) shown in Figure 3.7. The samples

were clamped to the vice available in the machine and using a diamond indenter the

specimen were tested at 50 g load applied for 10 seconds at fifteen different

locations transversely. The test was repeated for three different regions and the

average value of microhardness was calculated and presented in section 3.3.6.

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Figure 3.7 Microhardness Tester

3.3 RESULT AND DISCUSSION

3.3.1 X-ray Diffraction Analysis

Figure 3.8 shows the XRD patterns obtained from the developed composite

revealing the presence of TiB2. The peaks of TiB2 are distinctly clear and they

increase with the increase in percentage reinforcement of TiB2 particles. It is

evident from the XRD pattern that TiB2 is the only phase present without any other

intermetallic phases which indicate the thermodynamic stability of in-situ formed

TiB2. TiB2 did not produce any other inter metallic compounds by reacting with

molten Al6061. Absence of other compounds in significant quantity indicates that

the interface between Al6061 and TiB2 is free from contamination.

3.3.2 Tensile strength

Figure 3.9 shows the effect of the amount of TiB2 particles on the tensile

strength of the produced MMCs. The ultimate tensile strength (UTS) increases as

the amount of TiB2 particles increase. Figure 3.10 shows the effect of TiB2 particles

on the % elongation of MMCs. From the figure it is found that the % elongation

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linearly decreases as the % reinforcement of TiB2 particles increases. Al6061-14 wt.

% TiB2 MMC exhibits 52% higher UTS compared with cast Al6061. The %

elongation of the same is 25% lower than that of cast Al6061.

Figure 3.8 XRD Results of the fabricated composites

Figure 3.9 Effect of % of TiB2 on UTS

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The reason for the increase in UTS in the Al-TiB2 MMC is explained below.

Al6061 and TiB2 has different coefficient of thermal expansion. The average

thermal expansion coefficient of Al6061 is 24 x 10-6/°C while that of TiB2 is 8.1 x

10-6/°C. This difference in thermal expansion between matrix and reinforcement

tends to reduce the dislocations around TiB2 particles during solidification (Han et

al 2002). The interaction between TiB2 and dislocations strengthens the composites

during loading. Increase in content of TiB2 results in increased interaction which

enhances UTS. The uniform distribution of TiB2 particles in the aluminium matrix

also plays a vital role in enhancing UTS. The shape of TiB2 particles which are

formed during the in-situ reaction are mostly spheroid in shape which also

contribute to higher UTS by reducing notch effect. The clear interface between the

matrix and the reinforcement increases the load bearing capacity of the composite

which means higher stress is needed initially to initiate cracking (Kumar et al

2010a). The in-situ formed TiB2 particles are also free from flaws assisting

enhancement of UTS.

Figure 3.10 Effect of % of TiB2 on % Elongation

As the % of reinforcement increases, there is an overall reduction in ductile

matrix content. This leads to the reduction in the % elongation of Al6061-TiB2

MMC when TiB2 content is increased in the MMCs. Figures 3.11 and 3.12 reveal

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the fracture morphology of the Al6061 and Al6061-14wt. % TiB2 composite

respectively. Figure 3.11 shows the presence of larger and uniform distribution of

voids which suggest that the matrix alloy Al6061 has undergone ductile fracture.

On the other hand smaller size voids are seen in Figure 3.12 which indicates

Al6061-14wt. % TiB2 composite underwent brittle fracture macroscopically and

ductile fracture microscopically. The TiB2 particles are intact at many places due to

strong bonding with the matrix.

Figure 3.11 Fracture Morphology of Al6061

Figure 3.12 Fracture Morphology of Al6061-14wt. % TiB2 MMC

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3.3.3 Wear Resistance of MMCs

Figure 3.13 shows the effect of the amount of TiB2 particles on the wear rate

of Al6061-TiB2 MMC. It is evident that as the % reinforcement of TiB2 particle

increases, the wear rate decreases.

Figure 3.13 Effect of % of TiB2 on Wear Rate

The trend shows a non linear curve indicting the complexity in

understanding the sliding wear behaviour of the composite. This nonlinear

behaviour is due to the addition of TiB2 particles in the aluminum alloy. These TiB2

particles tend to increase the hardness of the aluminum alloy which reduces the rate

of material removal.

Figures 3.14 and 3.19 show the worn surface morphology of Al6061 and

Al6061-14 wt. % TiB2 MMC respectively. Figure 3.14 shows that the grooves are

deeper and non-uniform, depicting excess wear. The grooves and pits in the wear

morphology reduce progressively as depicted in Figures 3.15 to 3.19, indicating the

high resistance offered by the MMC to wear when the amount of TiB2 increases.

Presence of TiB2 particles in the aluminium matrix can be attributed to the

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reduction in the depth of the grooves in the wear morphology whereas there is no

such resistance offered by pure AA6016.

Figure 3.14 Worn Surface Morphology of Al6061

Figure 3.15 Worn Surface Morphology of Al6061-10 wt. % of TiB2 MMC

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Figure 3.16 Worn Surface Morphology of Al6061-11 wt. % of TiB2 MMC

Figure 3.17 Worn Surface Morphology of Al6061-12 wt. % of TiB2 MMC

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Figure 3.18 Worn Surface Morphology of Al6061-13 wt. % of TiB2 MMC

Figure 3.19 Worn Surface Morphology of Al6061-14 wt. % of TiB2 MMC

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Figure 3.19 also indicates that the grooves and ridges are parallel and aligned

along the direction of sliding exhibiting lower plastic deformation at the edges of

the grooves. The wear debris is in powder form and not sticking to the surface due

to hard TiB2 particles. The reduction in the plastic deformation and depth of

grooves from the worn surface indicate that the wear rate of the matrix alloy is

reduced when TiB2 particles are reinforced (Kumar et al 2010b, 2010c).

3.3.4 Corrosion Resistant Property

Figure 3.20 shows the effect of TiB2 on the corrosion rate of Al6061-TiB2

MMC. The trend shows that corrosion rate increases with the increase in the wt. %

of TiB2 in the MMC. The reason is attributed to the presence of TiB2 particles

which has dislodged the aluminium matrix with their presence. The TiB2 particles

respond to the chemical reaction of the test solution and get dislodged.

Figure 3.20 Effect of % of TiB2 on Corrosion Rate of Al6061-TiB2 MMC

The interface between aluminium matrix and TiB2 particles gets oxidised and

the particles come out of their position creating voids in the surface thereby

0

1E-05

2E-05

3E-05

4E-05

10 11 12 13 14

Cor

rosi

on R

ate

(mils

/yr)

wt. % of TiB2

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initiating pitting corrosion. As the amount of TiB2 particles increase the pitting

locations increase making the MMC prone to corrosion. Figure 3.21 shows the

corroded surface of MMC with 10 wt. % of TiB2 and it is found that the size of the

pit is very small and it is of the order of about 10 microns. Figure 3.22 shows that

the size of the pit occurred are in the order of about 30 microns and in a number of

places the pitting has been initiated.

Figure 3.21 Corroded Surface of Al6061-10 wt. % TiB2 MMC

Figure 3.22 Corroded Surface of Al6061-14 wt. % TiB2 MMC

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3.3.5 Microstructural Analysis

Figures 3.23 to 3.34 show the optical micrographs and SEM micrographs of

Al6061 and Al6061-TiB2 MMC with various percentage of reinforcement. Figure

3.23 shows the microstructure of cast Al6061 alloy. It exhibits a typical dendritic

Al which is formed due to the super cooling

during solidification. Secondary intermetallic phases are seen around the dendrites

which comprises of the alloying elements like Mg and Si forming Mg2Si (Rao et al

2009). Figures 3.24 to 3.28 show the microstructure of Aluminium MMCs

containing different amount of TiB2. The in-situ formed TiB2 particles have refined

Al grains because TiB2 Al during

solidification. TiB2 particles itself act as nucleus on which the aluminium grains

solidify (Han et al 2002).

Figure 3.23 Optical Micrograph of Al6061 Cast Alloy

Figures show that TiB2 particles are uniformly distributed in the matrix in all

MMCs. The reaction of K2TiF6 and KBF4 with molten aluminium produced TiB2

particles of varying shapes and sizes. Most of the TiB2 particles exhibit spherical

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shape. Some particles show elliptical shape and a few irregular grains are also

visible. The appearance of single particle can be considered as a cluster of particles

because in-situ reaction produces fine particles up to nano scale. The in-situ formed

particles join together to form clusters depending upon synthesis temperature,

holding time, reaction rate and cooling rate (Tjong et al 2000). Kumar et al (2010a)

reported that average reinforcement particle size increased with increased

percentage of reinforcement particles and the coarsening of the particles was due to

high reaction rate. The shape, size and spatial distribution of TiB2 particles

influence the mechanical properties of the composites to a larger extend.

Figure 3.24 Optical Micrograph of Al6061-10wt. %TiB2 MMC

The distribution of TiB2 particles in the matrix is related to the solidification

process of the matrix alloy. The density difference between melt and a particle is

over 1.8 g/cm3. Hence the in-situ formed TiB2 particles will be suspending in the

melt (Han et al 2002). As the wetting effect between particles and melt retards the

movement of the TiB2 particles, it can be distributed in the melt for a long time. The

TiB2 Particle

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Al crystallizes as the primary phase at the beginning of the solidification while

TiB2 particles are extracted to the solidification interface. The microstructure also

reveals some small pits in Figures 3.25 to 3.28. These pits are formed because of the

etchants which have removed the aluminium alloy surrounding the particles thereby

removing the particles from the location leaving a pit on the surface.

Figure 3.25 Optical Micrograph of Al6061-11wt. %TiB2 MMC

Figures 3.24 to 3.28 show that TiB2 particles are well bonded with the matrix

because of the exothermic reaction which induces an increase in local melt

temperature and enhances the wetting of the particles. Neither voids nor reaction

products surround the particle which indicates good interfacial integrity between the

particle and the matrix. Synthesizing TiB2 particles within the melt itself reduces

the opportunity for oxidation thus a cleaner and stronger interface is obtained.

Figures 3.29 to 3.33 show the SEM photomicrographs of Al6061-TiB2 MMCs

containing 10-14 wt. % TiB2.

TiB2 Particle

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Figure 3.26 Optical Micrograph of Al6061-12wt. %TiB2 MMC

Figure 3.27 Optical Micrograph of Al6061-13wt. %TiB2 MMC

TiB2 Particle

TiB2 Particle

Al3Ti

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Figure 3.28 Optical Micrograph of Al6061-14wt. %TiB2 MMC

The micrographs show a uniform distribution of TiB2 particles. The variation

in the amount of reinforcement particles is visible with increased content of TiB2.

SEM micrographs reveal that each single particle present is a cluster of particles

which consists of numerous fine TiB2 particles.

Figure 3.29 SEM Micrograph of Al6061-10wt. %TiB2 MMC

TiB2 Particle

Agglomeration

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Figure 3.30 SEM Micrograph of Al6061-12wt. %TiB2 MMC

Figure 3.31 SEM Micrograph of Al6061-14wt. %TiB2 MMC

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Figure 3.32 SEM Micrograph of Al6061-13wt. %TiB2 MMC

Figure 3.33 SEM Micrograph of Al6061-10wt. %TiB2 MMC

Many particles are in sub-micron size. The Figures 3.29 and 3.30 show

particle of the order of 1 micron to 3 micron in size. Some ultra-fine particles are

also observed in the matrix. The distribution of TiB2 particles in the melt is

influenced by convection current in the melt, movement of solidification front

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against particles and buoyant motion of particles (Bauri et al 2011). It is evident

from SEM micrographs that TiB2 particles are distributed in both transgranular and

intergranular regions in MMCs (Figures 3.29 to 3.32).

3.3.6 Microhardness Survey

Figure 3.34 shows the effect of TiB2 on the microhardness of the MMCs.

The microhardness of the fabricated composites tends to increase with the increase

in TiB2 content. Al6061-10wt. % TiB2 composite exhibits 49.7% higher

microhardness and Al6061-14wt. % TiB2 shows 63.7% higher microhardness when

compared to Al6061 alloy which is attributed to the presence of TiB2 in the

composites. During solidification TiB2 particles increase the dislocation density

which in turn creates more resistance to plastic deformation resulting in enhanced

microhardness.

Figure 3.34 Effect of wt.% of TiB2 on Microhardness

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3.4 SUMMARY

The following are the summary of the results of Chapter 3:

i. Aluminium MMCs containing 10 to 14 wt. % TiB2 were successfully

fabricated by in situ reaction.

ii. The presence of TiB2 and the absence of intermediate phases in the Al

MMCs were confirmed by XRD analysis.

iii. The presence of TiB2 particles increased the tensile strength, wear resistance

and microhardness of the MMCs.

iv. TiB2 particles were found to have uniform distribution, spherical in shape,

good bonding and clear interface resulting in enhanced mechanical and wear

properties.

v. But addition of TiB2 particles reduced the % elongation and corrosion

resistance of the MMCs.

vi. The maximum content of TiB2 was limited to 14wt. % due to the formation

of excessive slag.