HEAT TREATMENT EFFECT ON MECHANICAL PROPERTIES AND SEM ANALYSIS OF AL6061/WC/B4C/TI/CR METAL MATRIX COMPOSITES

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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 7, Issue 2, March-April 2016, pp. 232–243, Article ID: IJMET_07_02_025

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HEAT TREATMENT EFFECT ON

MECHANICAL PROPERTIES AND SEM

ANALYSIS OF AL6061/WC/B4C/TI/CR

METAL MATRIX COMPOSITES

Sivakumar.M

Assistant professor, Mechanical Engineering,

Maharaja Institute of technology, Coimbatore, India

Vignesh.M, Sridhar.R, Sri Hari.K, RamaSubramanian.R

Students, Mechanical Engineering,

Maharaja Institute of technology, Coimbatore, India

ABSTRACT

Recent developments in material technology help to find and fabricate new

materials which may replace existing materials for various applications.

Among those, composite materials play a vital role which is combination of

two or more materials with different physical and chemical properties.

Aluminum plays a vital role in the automotive, aerospace and marine

industries this for its strength, less weight, flexibility and cost. The present

study is to cast a metal matrix composite of al 6061 with the selected

reinforcing materials. The reinforcing material used here are titanium (Ti),

tungsten carbide (Wc), boron carbide (B4C) and chromium (Cr) in the form of

particles. The casting is done using the stir casting method and the

mechanical properties and SEM analysis of the hybrid composites are tested

with the ASTM standard’s and are compared with the pure Al6061. The

mechanical properties of the aluminum metal matrix composites such as

impact, tensile and hardness are tested. The tensile specimens hardened using

the annealing and normalizing process and the tests are taken with the

different specimens and these are compared with Al6061.

Key words: Al6061-Ti-B4C, Al6061-Wc-Cr metal matrix composite, tensile

strength, impact, hardness, x-ray diffraction, scanning electron microscopy.

Cite this Article Sivakumar.M, Vignesh.M, Sridhar.R, Sri Hari.K,

RamaSubramanian.R, Study of Process Parameters of Gravity Die Casting

Defects. International Journal of Mechanical Engineering and Technology,

7(2), 2016, pp. 232–243.

http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=7&IType=2

http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=7&IType=2

Heat treatment effect on mechanical properties and SEM analysis of

Al6061/Wc/B4C/Ti/Cr metal matrix composites


1. INTRODUCTION

These days engineering need modern materials to develop materials for most

compound applications, since aluminum is most commonly used material in the

automotive and aerospace fields the composite metals based on aluminum has

increased. Among the aluminum materials al6061 is widely used due to its corrosion

resistance, stability, and less cost. Due to the need of more strength, thermal

resistance for these applications leads a way to the fabrication of the aluminum metal

matrix composites. Here the al6061 is reinforced with the titanium, boron carbide,

chromium and tungsten carbide materials. Among these materials titanium and

chromium possessed with high strength. Boron carbide and tungsten carbide has the

high thermal resistance. These materials were combined and reinforced with the stir

casting process currently practiced continuously. In general stir casting of MMCs

involves producing a melt of the selected matrix material followed by introducing

reinforcement material into the melt, obtaining a suitable dispersion through stirring.

Its advantages lie in its simplicity, flexibility and applicability to large quantity

production.

2. LITERATURE REVIEW

V. L. Kingston et al [3]- -Aluminum based matrix composites remain the most

explored metal matrix material for the development of MMCs. In the present study,

the effect of Silicon carbide, Boron carbide on Stir cast Aluminum Metal Matrix

Composites is discussed. Graphite is used as a lubricant. Aluminum Metal Matrix

Composites with Silicon carbide and Boron carbide particle reinforcements are

finding increased applications in aerospace, automobile, space, underwater, and

transportation applications. The hybrid metal matrix composite which consists of

aluminum and other constituents such as graphite, silicon carbide and boron carbide

are to be casted in three different compositions varying the boron carbide and silicon

carbide content and are further employed to different testing such as hardness test,

compression test, tensile test, impact test, micro hardness and micro structural

analysis. Conventional stir casting process has been employed for producing

discontinuous particle reinforced metal matrix composites.(with the mixture of Ti, Wc

and Cr).

3. EXPERIMENTAL PROCEDURE

Materials used

The materials used for the fabrication of the composites are titanium, boron carbide,

tungsten carbide and chromium. Each material has its own unique mechanical

properties. We have selected two composition to form two types of MMC. The

compositions are as follows.

i) 1000gAl6061+10%wt Ti+5%wt B4C.

ii) 1000gAl6061+10%wt Wc+3%wt Cr.

The above two compositions are fabricated and their mechanical properties are

compared with that of the pure aluminum. The material selected are described below

with their mechanical properties in a tabular column.

3.1. ALUMINUM

Aluminum is remarkable for the metal's low density and for its ability to resist

corrosion due to the phenomenon of passivation. Commercially pure aluminum has a

Sivakumar.M, Vignesh.M, Sridhar.R, Sri Hari.K, RamaSubramanian.R


tensile strength of approximately 120 MPa and can be improved 180 MPa by cold

working. The heat treatable grades can develop a tensile strength of around 570 MPa

and even higher in some alloys. The wear of MMCs depends on the particular wear

conditions, but there are many circumstances where Al based composites have

excellent wear resistance. The coefficient of thermal expansion of aluminum alloys is

affected by the nature of their constituents: example the presence of silicon and

copper reduces expansion while magnesium increases it. The chemical composition of

the Al 6061 is shown in the table below.

Table 1 composition of Al6061.

3.2. BORON CARBIDE

Boron Carbide particulate reinforced aluminum composites possess a unique

combination of high specific strength, high elastic modulus, good wear resistance and

good thermal stability than the corresponding non-reinforced matrix alloy system. A

limited research work has been reported on AMCs reinforced with B4C due to higher

raw material cost and poor wetting. B4C is a robust material having excellent

chemical and thermal stability, high hardness and low density (2.52 g/cm3) and it is

used for manufacturing bullet proof vests, armor tank, etc. Hence, B4C reinforced

aluminum matrix composite has gained more attraction with low cost casting route.

3.3. TITANIUM

Titanium metal is a strong material when reinforced with aluminum composite they

possess heavy strength for the composite. Since the aluminum is lighter and this

titanium composite gives the strength to that of the aluminum composite and increase

the hardness of the material. The tensile strength of the titanium metal is 250 Mpa as

it has a density of 4.56g/cm3 and it is used in the aerospace materials.

3.4. TUNGSTEN CARBIDE

As we know the tungsten carbide is the stronger material, with a melting point of

2830oc and with the highest tensile strength when the material is mixed with the

aluminum the properties of the materials are shared with that of the aluminum and this

will give the composite the high melting point and the tensile strength. As the density

of the material is concerned the density of the material is 15.6g/cm3 with a highest

density of the selected materials so the mixing of the material takes some time but the

mixture will be strong enough than anything due to the high density.

3.5. CHROMIUM

Chromium is the shiniest of all the metals and they are used in the process of making

chrome plating. The car bumpers and other parts are made of chromium for their

shine property but the cost of the material cost them more. But when the metal is

mixed with the aluminum the property of the metal is adapted by the composite and

the composite material can be used in the automotive fields. The melting point of the

material is 1907oc and the young’s modulus of the material is 279 GPa. The density of

the material is 7.19g/cm3.

ELEMENTS Si Cu Mg Fe Mn Al

PERCENTAGE 0.63 0.32 1.08 1.72 0.52 rem




Table 2 Mechanical properties of the materials

3.6. Fabrication method

Aluminum alloys was melted in a graphite crucible inside a high temperature furnace

for a temperature for about 600 to 650oc until the aluminum is melted. The reinforcing

materials such as titanium and boron carbide are mixed and are preheated at a

temperature of 450oc in a pre-heater furnace for 15 to 20 min. then the aluminum is

lifted off from the furnace and placed in the stir-casting furnace and the pre heated

reinforce materials are then mixed with the aluminum alloy where the temperature of

the furnace is maintained at a temperature of 760oc. the mechanical stirrer is inserted

into the crucible and the composition are stirred at a rpm of 400 for 10mins until the

materials are mixed the molten aluminum is poured in the die. The same process is

repeated with the composition of tungsten carbide and chromium.

Figure 1 stir casting furnace

4. HEAT TREATMENT’S

The heat treatments are done in a material to increase its grain structure and to make

the machinability of the material easy, the heat treatment also increases the

mechanical properties of the material, there are many types of heat treatment process

they are,

Annealing

Normalizing

Quenching

Case hardening.

MATERIALS Density

g/cm3

Tensile

strength MPa

Hardness

(rock well)

Melting point oc

Al 6061 2.70 125 30-33 580

Titanium 4.56 250 34 1668

Tungsten carbide 15.6 344 60 2830

Boron carbide 2.52 569 30 2763

Chromium 7.19 103 68 1857



Here we have done two heat treatment process of the above mentioned processes,

we have choose the process of annealing and quenching and the tensile test specimens

of the hybrid composites and pure aluminum are heat treated by the above two

mentioned process.

4.1. ANNEALING

Annealing is heat treatment process in which the material is heated form 2100 to 450

0c

for a duration of 0.5 to 2 hours and the material is cooled down in the furnace at a

temperature of 200c for 3 hours. This process increases the grain structure and

machinability of the material.

4.2. QUENCHING

Quenching is the process of heating the material up to the critical temperature of

about 6000c for about 3 hours and rapidly cooling it down to the temperature using the

water or oil bath or by using the vacuum. This process increases the grain structure

hardened the material and improve the machinability of the material.

5. TESTINGS DONE

There are several tests taken to determine the mechanical properties of the hybrid

composites. The test taken are the

Tensile test

Hardness(Rockwell)

Charpy test(impact test)

X-ray diffraction

Scanning electron microscopy.

5.1. Tensile test

The tensile test is done on the pure al 6061 and the other two hybrid composite with

three different specimens. The test specimens were heat treated before testing to

determine the heat treatment effect on the mechanical properties of the composite

materials. There are about three specimens taken from each composition. The

specimens were heat treated with different treatment methods such as “annealing” and

“quenching”. The specimens were tested with the universal testing machine, to

determine the point of break, elongation etc.,

Figure 2 tensile test specimen




5.2. Impact test (CHARPY METHOD)

The impact test is taken the charpy test method as per the ASTM dimensions. The test

is taken for all the composition. The impact test results are compared between the

pure al 6061 and the hybrid composites.

Figure 3 impact test specimen

5.3. Hardness test (ROCKWELL HARDNESS)

Hardness test are taken for the samples of all compositions and these hardness are

compared with the hardness of the al6061. Here the hardness test is taken form the

“Rockwell hardness” methods.

Figure 4 The specimen for the hardness test

5.4. XRD ANALYSIS

The X-ray diffraction is a technique that is used to identify the materials composition

and the materials crystal structure and crystallography. Here in the X-ray diffraction

method a beam of X-ray is made to fall into the materials and this X-ray penetrates

the material and the report form a series of graphs with different profiles these profiles

shows a series of peaks, these peaks indicates the crystallite size and the lateral strain

on the material.

5.5. SEM ANALYSIS

The scanning electron microscopy is a method that is used to view the microstructure

of the material. In this method a strong beam of electron is projected into the material

and this electron beam is used examine the materials inner microstructure. The result

of the beam is viewed in the computer and the microstructure of the material is

identified.

6. RESULT DISCUSSIONS

6.1. IMAPCT TEST (CHARPY METHOD)

Impact test is done on the impact testing machine with the charpy method according

to the ASTM dimensions and standard’s. The specimen is placed in the charpy test



machine and the pendulum is fixed in the stand. The pendulum is released and they

are made to strike the specimen. The energy absorbed be the material during its

breakage is noted in the pointer and the calculation is done. The results obtained by

the charpy test is calculated and they are displayed in the table.

Table 3 Impact test results

The graph for the impact strength and toughness of the samples are graphed

below.

Figure 5 toughness in bar chart

6.2. HARDNESS (ROCKWELL HARDNESS)

The hardness of the material is calculated by the results obtained by the hardness scale

of the machine. The specimen is placed in the vice and the tester is made to touch the

surface of the material. This will show the hardness of the material in the scale

attached to the machine. The process is done for three trails and the average value of

the material is calculated and the calculated value is tabulated and the graph is drawn.

Table 3 hardness values

S.NO Composition Observed values,

HRB Average, HRB

1 Pure Al6061 30 31 30 31

2 Al6061+Ti+B4C 85 86 80 84

3 Al6061+Wc+Cr 78 75 80 78

S.NO Samples Toughness (MPa)

1 S1 29

2 S2 38

3 S3 37




Figure 6 hardness in bar chart

6.3. TENSILE TEST

The tensile test of the materials are tested using the universal testing machine with a

capacity of 10 KN the materials are placed in the jaws of the machine are a load is

applied this load makes an elongation in the material this elongation of the material is

noted and the yield strength and the point of breaking are noted. This process is done

for the material of both composition and as well as for the pure Al6061 .The material

that heat treated are also tested. The result of the materials tensile load and elongation

and their point of breaking are calculated and a graph is drawn the results are

displayed in the table and the graph is showed that has the load applied in the

materials during the test.

The table shows the tensile strength of the three composition and their yield stress

at normal temperature.

Table 5 tensile strength at normal temperature

S.NO Composition Yield stress

N/mm2

Tensile strength

N/mm2

1 Al6061 120.124 133.073

2 Al6061+Ti+B4C 89.161 112.386

3 Al6061+Wc+Cr 114.222 142.885

The table below shows the tensile strength of the material after heat treated

(annealed specimen).

Table.6. tensile strength of the annealed specimens


N/mm2

Tensile strength

N/mm2

1 Al6061 83.575 104.529

2 Al6061+Ti+B4C 102.54 131.162

3 Al6061+Wc+Cr 70.63 88.693

The table below shows the tensile strength of the materials and their yield stress

after heat treated (quenched).



Table 7 tensile strength of quenched specimens


N/mm2

Tensile

strength N/mm2

1 Al6061 83.46 119.476

2 Al6061+Ti+B4C 76.076 95.839

3 Al6061+Wc+Cr 71.23 124.660

The graphs have been shown below comparing the tensile strength of the

materials.

Figure 7 tensile strength at normal temprature bar chart

Figure 8 tensile strength of annealed specimens bar chart

Figure 9 tensile strength of quenched specimens bar chart




XRD ANLYSIS

The XRD analysis for the specimens are done in the x ray diffraction machine by

passing through a beam of xray in to the specimen and by identifying the amount of

the diffraction light emmited the graphs are drawn with the peaks in them this shows

the crystallite size and the lattice strain of the structure, the crystallite size and the

lattice strain of the specimens are calculated by the scherrer equation shown below, DP = 0.94 λ /β0.5cosθ

The lattice strain of the specimens are graphed by the values that we obtain from

the peaks of the xrd analysis

Figure 10 XRD peak graph of specimen 2

Figure 11 lattice strain chart

SEMB ANALYSIS

The SEM analysis results shows the crystal bonding between the base material and

the reinforcemnts shown in the SEM images, the SEM images of these specimens are

shown below,

Figure 12 SEM analysis of sample 1

Figure 13 SEM analysis of sample 2



Figure 14 SEM analysis of sample3

7. CONCLUSION

The significant conclusion of the studies are followed below,

1. The hybrid composite with the composition of al6061+Ti+B4C and al6061+Wc+Cr

are prepared by the stir casting process.

2. The mechanical properties of the cast composite materials are tested.

3. As far as we studied the heat treatment process have changed the physical and the

chemical properties of the materials.

4. The tensile strength values of the heat treated specimen varies as the heat treatment

process alters the grain structure of the material.

5. The lattice strain of the material is identified using the XRD testing and this shows

the sample 2 have the least strain and have the highest of hardness values.

6. The SEM analysis shows the boinding of the reinforcement materials with that of the

base material. And by this analysis we can be sure that the bonding between the

materials are very good in this method than the other methods.

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