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Vol. 4. Issue 2.
Pages 171-179 (April - June 2015)
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Vol. 4. Issue 2.
Pages 171-179 (April - June 2015)
Original Article
DOI: 10.1016/j.jmrt.2014.10.017
Open Access
Effect of Al–5Ti–1B grain refiner on the microstructure, mechanical properties and acoustic emission characteristics of Al5052 aluminium alloy
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Amulya Bihari Pattnaika,
Corresponding author
, Satyabrat Dasa, Bharat Bhushan Jhab, Nedumbilly Prasantha
a Light Weight Metallic Materials Group, CSIR-Advanced Materials and Processes Research Institute, Bhopal, Madhya Pradesh, India
b Surface Engineering Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India
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Table 1. Chemical composition of Al5052 alloy.
Table 2. Tensile test results of unmodified and modified Al5052 alloys tested at a strain rate of 10−2s−1.
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Abstract

In the present investigation, the effect of Al–5Ti–1B grain refiner on the microstructure, mechanical properties and acoustic emission characteristics of Al 5052 aluminium alloy have been studied. Microstructural analysis showed the presence of primary α solid solution. No Al–Mg phase was found to be formed due to the presence of magnesium in the solid solution. The results indicated that the addition of Al–5Ti–1B grain refiner into the alloy caused a significant improvement in ultimate tensile strength (UTS) and elongation values from 114MPa and 7.8% to 185MPa and 18% respectively. The main mechanisms behind this improvement were found to be due to the grain refinement during solidification and segregation of Ti at primary α grain boundaries. Acoustic emission (AE) results indicated that intensity of AE signals increased with increase in Al–5Ti–1B master alloy content, which had been attributed to the combined effect of dislocation motion and grain refinement. The field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) analysis were used to study the microstructure and fracture surfaces of the samples.

Keywords:
Microstructure
Tensile strength
Grain refinement
Mechanical properties
Acoustic emission
Full Text
1Introduction

Al–Mg alloys are a potential candidate for automobile industries owing particularly to their high strength to weight ratio, good corrosion resistance, weldability and formability [1]. The microstructure of Al 5052 aluminium alloy mainly consists of primary α-Al phase and magnesium in the solid solution. The refinement of grains results in the formation of fine equiaxed grain structure, which not only improves the mechanical properties but also the quality and efficiency of the castings [2,3].

The Al–Ti–B ternary master alloys have been commonly used as grain refiners for most aluminium alloys [4]. After several decades of research on this field, no clear consensus has been reached yet on the mechanism of grain refinement in aluminium due to the addition of Al–Ti–B master alloys. Easton and Stjohn [5] classified the mechanism of grain refinement as nucleant and solute paradigms. The nucleant paradigm relates to the heterogeneous nucleation of primary α-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. [6] 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 α-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 [7]. They suggested that both nucleants 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 α grains. Though a number of theories has been proposed to explain the grain refinement in aluminium alloys, none of these could clearly explain the exact mechanism.

Acoustic emission (AE) technique is a non-destructive evaluation procedure in which stress waves are generated from within the material due to the dynamic events occurring inside by the application of load [8]. The stress waves are generated by a number of dynamic events like, dislocation motion, inclusion, fracture, phase transformation, etc. [9]. Online monitoring of deformation behaviour has the potential to provide real time information in order to predict the onset of failure by AE parameters variation. Most of the previous studies on deformation of various materials have related the AE activities to the dislocation kinetics [10]. Scruby et al. [11] systematically studied the AE during deformation of aluminium alloys with various microstructures. They observed that the AE activities in the age hardened alloys were mainly due to shearing of Guinier–Preston zones by the dislocations. Jalaj Kumar et al. [12] in their recent investigation on AE during tensile deformation of near alpha titanium alloys identified twinning as a potential source of acoustic emission. They showed that AE signals were generated mainly due to combined effect of twinning and dislocation motion. In our recent work on a low stacking fault energy material (brass), we found that the AE signal intensity increased with increase in twinning density. We found a strong correlation of microstructural features (in terms of twinning density) with that of AE signal intensity [13].

In case of Al–Mg alloys, the presence of magnesium in the solid solution improves the grain refining capability. According to Birch et al. the addition of magnesium improves the wettability of aluminium melt with the nucleating sites by reducing the surface tension [14]. A great number of investigations have been carried out to study the grain refinement in Al–Mg alloys [15,16]. However, no work has been reported so far to study the grain refinement and its subsequent effect on microstructure, mechanical properties and acoustic emission characteristics of Al5052 aluminium alloys.

The present paper aims to study the effect of Al–5Ti–1B grain refiner on microstructure, mechanical properties and acoustic emission characteristics of Al5052 aluminium alloy in the light of grain refinement.

2Experimental

Al5052 alloy was prepared using pure aluminium (99%) and magnesium (99.9%) as the starting material. First of all, pure Al was melted in an electrical resistance furnace at 750–800°C using a 30kg SiC crucible. Then pure magnesium was added on to the melt. Degassing of the melt was carried out using nitrogen gas for 5min. After stirring and removal of dross, part of the molten metal (10kg) was poured into the cast iron moulds to prepare cast fingers and plates of Al5052 alloy. The chemical composition of Al5052 alloy is shown in Table 1. The remaining part of the molten metal was grain refined using Al–5Ti–1B master alloy to produce two different grain refined alloys with 0.2 and 0.5wt% Al–5Ti–1B respectively.

Table 1.

Chemical composition of Al5052 alloy.

Element  Si  Fe  Cu  Mn  Mg  Zn  Cr  Al 
Wt%  0.25  0.40  0.1  0.1  2.5  0.1  0.15  Rem. 

Microstructural characterizations were carried out using LEICA 5000M optical microscope and NOVA NANO FESEM 430 field emission scanning electron microscope (FESEM) which was equipped with EDX detector. The samples for microstructural characterization were taken from different position of cast ingots. These samples were polished using conventional metallographic techniques and etched with Keller's reagent. The average grain size of the specimens was measured using an image analyzer attached to the optical microscope according to ASTM E 12. The different phases present in the microstructures were identified through X-ray diffraction studies using a Bruker D8 Advance diffractometer (40mA and 40kV) with a scanning speed of 2°/min with a CuKα (λ=1.54Å) target. The hardness values of the polished samples were measured using a Universal hardness tester. The average values of five readings for each sample are reported.

Cylindrical tensile specimens of dimensions of 20mm diameter, 8mm gauge width and 40mm gauge length were cut from the cast ingots of both the as cast and grain refined alloys. Tensile tests were carried out on a floor model 8801instron servo hydraulic universal testing machine of 100kN capacity. These tests were carried out at a strain rate of 10−2s−1. Impact tests of the as cast and grain refined alloys were carried out using an INSTRON make impact testing machine. Charpy V-notch impact specimens with dimension 10mm×10mm×55mm were prepared according to ASTM A370. The damage mechanisms during plastic deformation were studied using scanning electron microscopic analysis of fracture surfaces.

Acoustic emission testing was carried out using a two channel PCI-DiSP with an AEwin version E2.32 AE system (Physical Acoustic Corporation, Princeton, NJ, USA). AE pulse detection was carried out using a resonant piezoelectric transducer with a peak frequency of 150kHz. A preamplifier with a gain of 40dB and a compatible filter (10kHz–2MHz) were used to capture the AE signals. A threshold of 45dB was set to eliminate the background noise by applying the Kaiser effect. The dummy specimens were repeatedly loaded and unloaded below the yielding region to avoid the interference of any noise during the actual experiment. A total system gain of 100dB was used the capture the AE signals.

3Results3.1Microstructural characterization

The FESEM microstructures of Al5052 aluminium alloy before and after grain refinement is shown in Fig. 1. It is clear from the figure that addition of Al–5Ti–1B master alloy resulted in grain refinement of Al5052 alloys. From Fig. 1a it was found that the microstructure of unrefined Al5052 alloy consists of interconnected and coarse dendritic microstructure. The addition of Al–5Ti–1B to Al5052 alloy resulted in changes in the morphology of the phase from coarse interconnected dendrites to fine equiaxed microstructure with homogenous distribution of primary α-phase Fig. 1c. It was observed that grain refinement of the melt resulted in an increase in the grain boundary area per unit volume. This ensures the uniform distribution of insoluble substrates in the matrix, which acts as sites for primary α-Al nucleation. Fig. 2 shows the variation of average grain size with addition of Al–5Ti–1B master alloy. From the figure average grain size decreased from 100μm in unrefined Al5052 alloy to 62μm in grain refined alloy.

Fig. 1.

FESEM images of (a) unmodified Al5052 alloy (b) modified with 0.2wt% Al–5Ti–1B (c) modified with 0.5wt% Al–5Ti–1B.

(0.29MB).
Fig. 2.

The variation of average grain size of the Al5052 alloy with different Al–5Ti–1B contents.

(0.08MB).

Fig. 3 shows the X-ray diffraction (XRD) plots of unrefined Al5052 and grain refined Al5052 alloys. From Fig. 3a it was observed that the unrefined Al5052 alloy was found to be consisting of single primary α-phase, as magnesium remains in the solid solution and does not form any phase. Fig. 3b and c shows the XRD plots of grain refined Al5052 alloys with 0.2 and 0.5wt% Al–5Ti–1B respectively. The grain refined alloys were found to be consisting of two recognizable phases i.e. Al3Ti and TiB2. Fig. 4a and b shows the FESEM microstructure and the corresponding EDX spectrum of unrefined Al5052 and 0.5wt% Al–5Ti–1B grain refined Al5052 alloy. From the EDX spectrum of the grain refined alloy it was observed that Ti segregates at the tip of the primary-α dendrites.

Fig. 3.

XRD plots of (a) unmodified Al5052 alloy (b) modified with 0.2wt% Al–5Ti–1B (c) modified with 0.5wt% Al–5Ti–1B.

(0.18MB).
Fig. 4.

FESEM image and corresponding EDX spectrum of (a) unmodified Al5052 alloy (b) modified with 0.5wt% Al–5Ti–1B.

(0.34MB).
3.2Hardness, impact toughness and tensile properties

Fig. 5a and b shows the hardness and impact toughness values of unmodified and Al–5Ti–1B modified alloys. The plots in Fig. 5 are the average values of five test specimens made from single castings. The absorbed energy of the unrefined Al5052 alloy was found to be 33J. After the addition of 0.5wt% Al–5Ti–1B the absorbed energy values increased to 44J. The tensile test results of unmodified and modified Al5052 alloys tested at a strain rate of 10−2s−1 are shown in Table 2. The UTS and elongation values of the alloy increased from 114.7MPa and 7.8% to 185MPa and 18% respectively.

Fig. 5.

Variation of (a) Hardness (b) Impact toughness values with Al–5Ti–1B content.

(0.12MB).
Table 2.

Tensile test results of unmodified and modified Al5052 alloys tested at a strain rate of 10−2s−1.

Microstructural condition  0.2% offsetYS (MPa)  UTS (MPa)  Total elongation (%) 
Al5052 alloy (unmodified)  55±0.40  115±0.79  8±0.87 
Al5052 – 0.1wt% Al–5Ti–1B (modified)  58±0.30  160±0.24  14±0.7 
Al5052 – 0.5wt% Al–5Ti–1B (modified)  58±0.40  185±0.92  18±0.5 
3.3Acoustic emission during tensile deformation

The variation of stress and AE counts with strain of unmodified and Al–5Ti–1B modified Al5052 alloy is shown in Fig. 6. More copious AE signal was found to be generated in the region where material transform from elastic to plastic regime of deformation. Fig. 7 shows the variation of stress and amplitude of AE signals with strain of unmodified and Al–5Ti–1B modified Al5052 alloy. The AE amplitude in the yielding region for unmodified Al5052 alloy lies in the range of 45–65dB, and the same for Al–5Ti–1B modified alloy was found to be in the range of 45–85dB. Fig. 8 shows the variation of AE cumulative counts during deformation of unmodified and modified Al5052 alloys. This is a more quantitative way of representing the AE data.

Fig. 6.

Variation of stress and AE counts with strain of (a) unmodified and (b) 0.5wt% Al–5Ti–1B modified Al5052 alloy.

(0.17MB).
Fig. 7.

Variation of stress and amplitude of AE signals with strain of (a) unmodified and (b) 0.5wt% Al–5Ti–1B modified Al5052 alloy.

(0.17MB).
Fig. 8.

Variation of AE cumulative counts during deformation of unmodified and Al–5Ti–1B modified Al5052 alloys.

(0.11MB).
4Discussion4.1Effect of grain refiner on microstructure

The FESEM micrographs, Fig. 1, clearly show that the microstructural morphology changes from coarse interdendritic structure to fine equiaxed microstructure due to the addition of Al–5Ti–1B grain refiner. The X-Ray diffraction plots confirm the presence of Al3Ti and TiB2 phases, which acts as potential nucleant for primary-α phase nucleation. 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 [17], phase diagram/peritectic theory [18,19], peritectic hulk theory [20,21], duplex nucleation theory [22,23], and solute theory [7,24]. Cibula et al. [19,24] 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 α-phase nucleation. In comparison to TiB2, Al3Ti was found to be a better nucleant mainly due to its good orientation relationship with aluminium [20]. 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 [7]. The EDX spectrum of Al–5Ti–1B modified alloy confirms the presence of solutal titanium at the tip of the primary-α grain boundary. Thus, the mechanism of grain refinement of Al5052 alloy by the addition of Al–5Ti–1B master alloy can be attributed to the formation of Al3Ti and TiB2 phase, which provides more nucleation sites for heterogeneous nucleation [5,21,22]. It can also be attributed to the presence of solutal titanium at the tip of the primary-α dendrites which restricts the growth of primary α-phase, hence providing more nucleation sites [23].

4.2Effect of grain refiner on mechanical and acoustic emission properties

The hardness and impact toughness values were found to increase with increase in Al–5Ti–1B content, which is mainly attributed to the refinement of grains. The low impact toughness value in unrefined alloy is mainly attributed to the presence of coarse primary-α dendritic structure. The improved impact toughness due to the addition of Al–5Ti–1B master alloy is mainly due to the change in morphology of primary α-phase from coarse dendritic structure to fine equiaxed grains. The energy absorption increases mainly due to the increase in grain boundary area per unit volume. The energy during the dynamic impact test is mainly absorbed by the grain boundaries. Thus, the increase in the grain boundaries increases the energy absorption during impact [23,24]. From the microstructural observation, it is evident that the addition of Al–5Ti–1B to Al5052 alloy resulted in finer grain size and improvement in morphology from coarse dendritic structure to fine equiaxed grains. The reduction in grain size resulted in improvement in tensile properties of Al5052 alloy. The presence of magnesium in the solid solution and solutal titanium at primary-α grain boundary contributed to the solid solution strengthening of modified Al5052 alloy.

The intensity of AE counts was found to increase with increase in Al–5Ti–1B master alloy addition. Most of the acoustic emission signals were generated in the micro plastic and yielding region of the stress-strain curve. Sudden bursts in the AE signals were detected at the time of fracture. The AE cumulative counts were found to increase with increase in Al–5Ti–1B master alloy addition. The AE signals generated in the micro plastic and yielding stage could be attributed to the generation and motion of dislocations from Frank-Read and grain boundary sources [13,25]. The increase in AE activity (counts and amplitude) due to the addition of Al–5Ti–1B master alloy is mainly attributed to the change in morphology of primary-α grain from coarse dendritic to fine equiaxed structure. This fine equiaxed microstructure increases the grain boundary dislocation interaction which results in higher strengthening with increased AE signal intensity. The presence of solute titanium at the primary-α grain boundaries and the presence of magnesium in the solid solution contributes significantly towards pinning and unpinning of dislocations during deformation thus making it inhomogeneous. This fact has been clearly demonstrated by the presence of serrations in true-true strain curve of these modified alloys. Higher numbers of AE counts and their large amplitudes could possibly be attributed to inhomogeneous plastic deformation of modified Al5052 alloy with 0.2% and 0.5% Al–5Ti–1B grain refiner. The increase in AE activity in the whole range of deformation could therefore be due the combined effect of dislocation motion, solid solution strengthening and inhomogeneous plastic deformation.

4.3Fracture surface observation

Fig. 9a shows the fracture surface of the unrefined Al5052 alloy associated with large voids and macro cracks resulting in interdendritic failure, which corresponds to brittle mode of fracture. The coarse dendritic structure acts as a source of stress concentration and crack initiation. Fig. 9b and c illustrates the fracture surfaces of the alloy with 0.2 and 0.5wt% Al–5Ti–1B respectively. The fracture surfaces were found to be associated with fine dimples or tear ridges, which correspond to ductile mode failure. The grain refinement and the improvement in the morphology of the primary α-phase due to the addition of Al–5Ti–1B grain refiner results in a transition of fracture surface from brittle to ductile mode of failure.

Fig. 9.

SEM micrographs of fracture surfaces of (a) unmodified Al5052 alloy (b) modified with 0.2wt% Al–5Ti–1B (c) modified with 0.5wt% Al–5Ti–1B.

(0.41MB).
5Conclusions

In the present work, the effect of Al–5Ti–1B grain refiner on microstructure, mechanical properties and acoustic emission characteristics of Al5052 alloy was studied. The following conclusions can be drawn based on the experimental results.

  • (1)

    The addition of Al–5Ti–1B master alloy reduced the grain size of Al5052 alloy from 100μm to 62μm.

  • (2)

    The morphology of primary α-phase changes from coarse dendritic structure to fine equiaxed grains which is attributed to the grain refinement due to the addition of Al–5Ti–1B and segregation of Ti at the tip of primary α dendrites, which restricts the growth of primary α grains.

  • (3)

    The mechanical properties of Al5052 alloy were improved by the addition of Al–5Ti–1B master alloy. The ultimate tensile strength and elongation values were increased from 114MPa and 7.8% to 185MPa and 18% respectively.

  • (4)

    The increase in AE activity in the whole range of deformation is mainly attributed to the combined effect of dislocation motion, solid solution strengthening and inhomogeneous plastic deformation.

  • (5)

    Fracture surface analysis of both unmodified and Al–5Ti–1B modified alloy showed a transition from brittle to ductile mode of failure.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors would like to thank Director, CSIR-Advanced Materials and Processes Research Institute, Bhopal, India for his constant encouragement and support. The authors would also like to acknowledge the financial support provided by CSIR under the project ESC 0101.

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Copyright © 2014. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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