Abstract— cast specimen also showed an increase.

Abstract— In the
present investigation, the effect of Al–5Ti–1B grain refiner on the microstructure and mechanical properties of
heat treated Al 336 aluminium alloy have been studied. Microstructural analysis
showed the transition of needle like silicon to globular silicon after the
addition of the grain refiner. The results indicated that the addition of
Al–5Ti–1B grain refiner into the alloy caused a significant improvement in
ultimate tensile strength (UTS) values from 330.16MPa to 363.69MPa. The
Rockwell hardness values of the cast specimen also showed an increase. The main
mechanisms behind this improvement were found to be due to the grain refinement
during solidification of the melt.

Keywords—microstructure, Al-5Ti-1B, grain refinement,
tensile strength, mechanical properties

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I.           
INTRODUCTION

Al–Si alloys are excellent substitutes for cast iron
used in automobile industries. Addition of silicon to aluminium gives high
strength to weight ratio, low thermal expansion coefficient and high wear
resistance 1,2. These alloys also show improved strength and wear properties
as the silicon content is increased beyond eutectic composition.

The microstructure of hypo-eutectic aluminium alloys
mainly consists of primary ?-Al dendrites and eutectic silicon in the solid
solution. The microstructure of hyper-eutectic Al-Si alloy consists of primary
silicon in the eutectic matrix. The refinement of grains of the hypo-eutectic
alloys results in the formation of fine equiaxed ?-Al dendrites and the
improvement of the mechanical properties.

Experiments on the grain refinement of Al-Si alloys were
conducted using many refiners like Al-Ti master alloy, strontium, Al-Ti-B
master alloy, etc. The Al–Ti–B ternary master alloys have been commonly used as
grain refiners for most aluminium alloys owing to their low costs and excellent
results 3. The mechanism of grain refinement of Al-Si alloys by Al-Ti-B
master alloy is quite complex and after several decades of research, no clear
consensus has been reached yet on the mechanism. Easton and Stjohn 4
described the mechanism of grain refinement as nucleant and solute paradigms.
The nucleant paradigm refers to the heterogeneous nucleation of primary ?-Al
grains on insoluble substrates, which acts as nucleation sites. The solute
paradigm includes the role of solute elements on grain refinement process.
Mohanty et al. 5 studied the mechanism of grain refinement in aluminium
alloys by directly adding TiB2 crystals into the aluminium melt. They
observed that the TiB2 particles were found in the grain
boundaries and the Ti atoms segregate at TiB2/melt interface
resulting in the formation of a thin layer of TiAl3. This
undergoes a peritectic reaction to form primary ?-Al. Johnsson et al.
introduced the solute theory to explain the grain refinement of aluminium
alloys due to the addition of Al–Ti–B master alloys 6. They suggested that
both nucleant and solutes particles influence the grain refinement. The solute
titanium atoms segregates and restricts the growth of nucleant particles thus
making available larger number of nucleating sites for nucleation of primary ?
grains.

Though a number of theories have been proposed to
explain the grain refinement in aluminium alloys, none of these could clearly
explain the exact mechanism.

Some recent trends in grain refinement of Al-Si alloys
include reduction of grain size under the influence of a travelling magnetic
field (TMF) 15. The
formation of a fine equiaxed structure was obtained by both the addition of
grain refining AlTi5B1-particles and electromagnetic stirring. Friction stir
processing (FSP) provides micro-structural modification and control in the near-surface
layer of metal components 16. FSP of cast Al and Mg alloys resulted in the
break-up of coarse dendrites and secondary phases, refinement of matrix grains,
dissolution of precipitates and elimination of porosity, thereby improving the
mechanical properties of the castings significantly. Improvements in the
traditional Al-Ti-B grain refiner in the recent times have also improved the
capability of the grain refinement process. New grain refiners, such as Al–3B
and Al–3Ti–3B master alloys with excess-B have been developed with well
documented advantages for Al–Si alloys 17.

 

 

 

TABLE I.

CHEMICAL COMPOSITION OF Al
336

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Al 336

 

 

 

 

 

Constituents

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Si

Fe

Cu

Mn

Mg

Zn

Ni

 

Cr

 

Pb

 

Ti

Sn

 

Al

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Constituents percentage

11 – 13

0.6

1.5

0.1

1

0.3

1.5

 

0.02

 

0.018

 

0.03

0.011

 

Bal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE II.

CHEMICAL COMPOSITION OF Al 6061

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Al 6061

 

 

 

 

 

Constituents

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Si

Fe

Cu

Mn

Mg

Zn

Ni

 

Cr

 

Others

 

 

 

Al

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Constituents percentage

0.6

0.7

0.3

0.15

1

0.3

0

 

0.04 – 0.3

 

0.05 – 0.15

 

 

Bal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The present paper aims to study the effect of Al–5Ti–B
grain refiner on microstructure and mechanical properties of Al 336 aluminium
alloy on the account of grain refinement. Al 336 is eutectic Al-Si alloy.

                                                                                                                           
II.           
EXPERIMENTAL PROCEDURE

The chemical composition of Al 336 alloy is shown in
Table I. Al 336 alloy was prepared using Al 6061, Al-50%Cu, Al-50%Si and
Al-10%Ni master alloys as the starting material. The chemical composition of Al
6061 alloy is shown in Table II. The required weights of each alloy are
calculated before melting.

 Next, the master
alloys were melted in a diesel fired tilting furnace at 750–800 ?
C using a graphite crucible. After stirring, the molten metal was poured into
the pre-heated cast iron moulds to prepare cast rods of Al 336 alloy.

Two moulds were used to pour the molten metal. The
geometry of the moulds were, square prism of side 35mm and height 300mm and a
cylindrical mould of diameter 40mm and height 350mm.

                     
Fig 1.           
As cast specimens, from left to right, unrefined Al
336, Al 336 with 0.5 wt.% Al-5Ti-B, Al 336 with 1 wt.% Al-5Ti-B

 

         The procedure
was repeated to prepare castings of two different grain refined alloys with 0.5
and 1 wt% Al–5Ti–1B grain refiner respectively. The as cast specimens are shown
in Fig 1.

          The cast
specimens were then heat treated using a 30kW electrical furnace. The heat
treatment process carried out was T6 process. The T6 heat treatment includes
solution treatment and aging treatment. The solution treatment was firstly
carried out at 470 °C for 6 h, and then quenched into water. The aging
treatment was then performed at 225 °C for 8 h.

          Microstructural characterizations
were carried out of the heat treated samples using LEICA 5000 M optical
microscope. The metallographic samples for microstructural characterization
were cut from the centre of the cast ingots. These samples were etched with
Keller’s reagent after polishing.

                     
Fig 2.           
Rockwell hardness test specimens of diameter 40mm

          Hardness was measured using the
Rockwell hardness tester on “B” scale with 1/8″ steel ball indenter with minor
load of 10 kg, and major load of 100 kg on the cast specimens. The sample was
placed on anvils and major load of 100 kg was applied up to 6 seconds. The
average hardness values of inner and outer regions for each sample are
reported. The hardness test specimens were cut from the cylindrical cast
ingots, Fig 2.

          Cylindrical tensile specimens of
dimensions of 4 mm diameter 50 mm gauge length were cut from the cast ingots of
both the as cast and grain refined alloys according to ASTM E08 standards, Fig
3. Tensile tests were carried out using universal testing machine of capacity
30kN at room temperature with a 10 mm/min stretching rate.

                     
Fig 3.           
Tensile test specimens, cut according to ASTM E8
standards

                                                                                                                                                       
III.           
RESULTS

A.        
Microstructural
analysis

The microstructures of heat treated Al 336 aluminium
alloy before and after grain refinement is shown in Fig. 6. It is clear from
the figure that addition of Al–5Ti–1B master alloy resulted in grain refinement
of Al 336 alloys. From Fig. 6a and Fig. 6b it was found that the microstructure
of unrefined Al 336 alloy consists of long needle like silicon. The addition of
Al–5Ti–1B to Al 336 alloy resulted in the transformation of needle like silicon
to globular silicon Fig. 6c. The eutectic matrix in grain refined alloy was
also uniformly distributed and finely spaced. This ensures the uniform
distribution of insoluble substrates in the matrix, which acts as sites for
primary ?-Al nucleation.  The average
size of Si needles in unrefined alloy is 61µm while in 1% Al-5Ti-B refined
samples, it is 31µm. Grain refinement lead to the breaking down of the long Si
needles. This results in the formation of globular Si, Fig 6f. The average
diameter of the globular Si is 15µm.

The microstructure of non-grain refined Al 336 Fig. 6a
showed the presence of primary silicon plates. This may be due to change in the
casting conditions which may have resulted in the shifting of the eutectic
point.

B.   
Mechanical
Properties

Table IV shows the hardness values of unrefined and
Al–5Ti–1B refined alloys. The hardness values of the outer regions of the
castings were better than the inner regions. Fig. 4 shows the comparison of
hardness vales of all the specimens.

The stress strain curves of unrefined and refined Al 336
alloys are shown in Fig 7. The tensile test resulted in brittle fracture of the
cast specimens with very little elongation. The UTS values of the alloy
increased from 330.16MPa to 363.69MPa for refined Al 336. The tensile test
results are provided in Table III.

                     
Fig 4.           
Rockwell hardness of A) unrefined Al 336 (B) Al 336
with 0.5 wt.% Al-5Ti-B (C) Al 336 with 1 wt.% Al-5Ti-B

                     
Fig 5.           
UTS of A) unrefined Al 336 (B) Al 336 with 0.5 wt.%
Al-5Ti-B (C) Al 336 with 1 wt.% Al-5Ti-B

                                                                                                                                                   
IV.           
DISCUSSION

A.   
Effect of
Grain Refiner on Microstructure

The
micrographs, Fig. 6, clearly show that transition of needle like silicon to
globular silicon. Several researchers have explained grain refinement in
aluminium alloys due to the addition of Al–5Ti–1B master alloy in terms of
different theories such as carbide/boride theory 7, phase diagram/peritectic
theory 8,9, peritectic hulk theory 10,11, duplex nucleation theory 12,13,
and solute theory 6,14. Cibula et al. 9,14 observed that the use of
Al–5Ti–1B as grain refiner, introduces both titanium and boron in to the melt
in the form of AlB2, TiB2 and Al3Ti.
They suggested that TiB2 particles act as insoluble substrates
for primary ?-phase nucleation. In comparison to TiB2,
Al3Ti was found to be a better nucleant mainly due to its good
orientation relation-ship with aluminium 10. Johnsson and Bakrued proposed
the solute theory, which suggested that both addition of solute atoms and
nucleant particles are vital for grain refinement of aluminium alloys.

 

 

 

 

 

                     
Fig 6.           
Microstructures of (a, b) unrefined Al 336 (c, d) Al
336 with 0.5 wt.% Al-5Ti-B (e, f) Al 336 with 1 wt.% Al-5Ti-B

 

TABLE III.

Tensile Test Results

 
 
 
 
 

 

 

 

 

 

 

 

 

Tensile Test

 

 

Sample

 

 

 

 

 

 

 

 

0% grain refiner

0.5% grain refiner

1% grain refiner

 

 

 

 

 

Tensile Strength(MPa)

 

330.16

353.11

363.69

 

 

 

 

 

 

 

 

 

Maximum

 

 

1.008

0.994

0.958

Elongation(%)

 

 

 

 

 

 

 

 

 

 

Load at peak(kN)

 

4.15

4.44

4.62

 

 

 

 

 

 

TABLE IV.

Hardness Values

 

 

 

 

 

 

 

 

 

Hardness test

 

 

Sample

 

 

 

 

 

 

 

 

0% grain refiner

0.5% grain refiner

1% grain refiner

 

 

 

 

 

Middle (HRB)

 

57

57

59

 

 

 

 

 

 

 

 

Outer (HRB)

 

57

61

63

 

 

 

 

 

 

 

 

 

(b)

 

(a)

 

(c)

 

                                  
Fig 7.           
Stress-Strain diagram of (a) unrefined Al 336 (b) Al
336 with 0.5 wt.% Al-5Ti-B (c) Al 336 with 1 wt.% Al-5Ti-B

B.    Effect of Grain Refiner on Mechanical
Properties

The hardness was found to increase with
increase in Al–5Ti–1B content, which is mainly attributed to the refinement of
grains. From the microstructural observation, it is evident that the addition
of Al–5Ti–1B to Al 336 alloy resulted in improvement in morphology of Si
needles to globular Si. Long Si needles are a source of stress concentrations.  These changes lead to an improvement in
tensile properties of refined Al 336 alloy.

The increase in mechanical properties was found
to be decreasing with the increase in concentration of the grain refiner
Al-5Ti-1B. This may be due to the formation and settling of inter-metallic
compounds formed during the melting process.

                                                                                                                                                   
V.           
CONCLUSION

      In the present work, the effect of
Al–5Ti–1B grain refiner on microstructure and mechanical properties of Al 336
alloy was studied. The following conclusions can be drawn based on the
experimental results.

The morphology of Al 336 changes on the addition of
Al-5Ti-B grain refiner. The needle like silicon in the eutectic matrix
gets transformed to globular silicon. The eutectic matrix is also
uniformly distributed for refined Al 336.
The mechanical properties of Al 336 alloy were improved by
the addition of Al–5Ti–1B master alloy. The ultimate tensile strength
values were increased from 330.16MPa to 363.69MPa for refined Al 336.
The hardness was also found to increase on the addition of
the grain refiner for refined Al 336.

ACKNOWLEDGMENT

        The authors are grateful for the
research fund sanctioned by Centre for Engineering Research and Development
(CERD), Government of Kerala for the work, letter no. KTU/Research_3/1431/2 016
dated 25th October, 2016 and from the support from the staffs of
Kerala Technological University and Government Engineering College, Thrissur,
Kerala, India.

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