Thermal Analysis

THERMAL ANALYSIS OF Al 2014 REINFORCED WITH

ALUMINA (Al2O3) PARTICLE COMPOSITES

M.N. Mazlee
School of Materials Engineering
Universiti Malaysia Perlis (UNIMAP)

Malaysia

mazlee@unimap.edu.my

The precipitation hardening of the matrix alloys was controlled by the presence of the ceramic reinforcements. The addition of a brittle ceramic reinforcement to a precipitation hardenable alloy can significantly alter the nucleation and growth kinetics of precipitation in the matrix as compared to the unreinforced alloy (1). An acceleration of the precipitation sequence in metal matrix composites (MMCs) was observed as a result of enhanced precipitation nucleation on dislocations due to the coefficient of thermal expansion (CTE) mismatch (2,3).

The thermal analysis studies have been carried out on the Al 2014 matrix composites reinforced with 10 volume percent (hereinafter noted as Composite 1) and 15 volume percent (hereinafter noted as Composite 2) of alumina (Al2O3) particles respectively. The effect of alumina particles on the precipitation phases and phases transformation of Al 2014/Al2O3 MMCs was studied using differential scanning calorimetry (DSC). DSC thermograms in Figures 1 and 2 have shown the formation and dissolution phases in Composite 1 and 2.

Composite 1 and Composite 2 were found to have 4 phases and 3 phases of zone formation respectively and 2 phases each of dissolution zone. From DSC thermogram, it was revealed that the sequence of precipitation hardening for Composite 2 was similar to that of Al-Cu alloys; supersaturated alpha -- Guinier-Preston (GP) zone -- theta" -- theta' -- teta. Comparatively, the precipitation of GP zone and metastable phases for Composite 1 and 2 was faster compared to that of Al-Cu alloy (4).

From the microstructure, the deformation banded structure in Composite 2 was shown in Figure 3. The deformation banded structure is believed to be one of the factors which contributes to the accelerated precipitation process in Composite 2. The dilatometer is used to measure and record the change of the length of the composites. The CTE data was acquired using the LINSEIS L75 dilatometer. From the thermal expansion test (Table 1), it was found that the values of CTE of Composite 1 and Composite 2 were not much different. However, the CTE of the composites was lower than the unreinforced Al 2014 alloys (23.20 x 10-6/°C).

References
1. Suresh S. and Chawla K. (1991), Fundamentals of Metal Matrix Composites, Oxford: Butterworth Heinemann.
2. Shamsul J.B. (1998), The Characterisation and Properties of Aluminium Matrix-SiC Composite, PhD Thesis, University of Leeds, UK.
3. Arakawa S., Hatayama T., Matsugi K. and Yanogisawa O. (2000), Scr. Mater., 42, 755-760.
4. Hong S.K., Hwang S.H., Choe J.C., Park I.M., Tezuka H., Sato T. and Komino A. (1997), Material Science Forum, 242, 165-172.

Fabrication of Aluminium Composite...

FABRICATION OF ALUMINIUM COMPOSITE FROM
RECYCLED AUTOMOTIVE COMPONENT

M.N Mazlee
mazlee@unimap.edu.my


ABSTRACT

The fabrication of aluminium composite has been carried out by using a recycled aluminium automotive component. The recycled aluminium engine blocks (Al-Si alloy) have been chosen as a matrix for the composite. The commercial silicon carbide (SiC) particles have been used as the reinforcement. The composition of recycled aluminium alloy and SiC particles were melted via turbulence casting technique using oil-fired non-ferrous melting furnace. The molten aluminium composite was cast into the shape of brake disc via sand casting process. The recycled aluminium composite brake disc has been found to have a significant weight reduction which is about only one-third of the conventional cast iron brake disc comparatively.
Keywords : recycled aluminium engine block, SiC particles, turbulence casting, sand casting, brake disc.


INTRODUCTION

Aluminium has been recycled since the days it was first commercially produced and today recycled aluminium accounts for one-third of global aluminium consumption worldwide. Recycling is an essential part of the aluminium industry and makes sense economically, technically and ecologically [1]. Currently, aluminium is used for structural, automotive components and aerospace fuselage. The others main markets are engineering, packaging and building. The use of aluminium in transportation sector especially for automotive applications is expected to grow in Malaysia for the future years. There is a possibility remelting recycled aluminium from automotive component and combined with reinforcement to produce aluminium matrix composite with better desired mechanical properties.

Aluminium matrix composites reinforced with ceramic particulate are well known for their higher specific modulus, strength and wear resistance as compared with conventional alloys [2,3]. One of the major driving forces for the technological development of aluminium matrix composites reinforced with ceramic particles is a result of these composites posses superior wear resistance and is hence potential candidate materials for a number of tribological applications. Applications in which materials are subjected to mechanical wear include pistons and cylinder liners in car engines and automotive disk brakes in vehicles [2,4]. Aluminium based metal composites offer a very useful combination of properties for brake system applications in replacement of cast iron. Specifically, the wear resistance and high thermal conductivity of aluminum metal composites enable substitution in disk brake discs and brake drums, with a significant weight savings on the order of 50 to 60%. The weight reduction will reduce the inertial forces thus providing an additional benefit in fuel economy. In addition, lightweight metal composite brake discs provide increased acceleration and reduced braking distance. It is reported that, based on brake dynamometer testing, metal composite reduce brake noise and wear, and have more uniform friction over the entire testing sequence compared to conventional commercial cast iron brake disc.

A number of automobiles now use MMC brake components. The Lotus Elise used four discontinuously reinforced aluminium brake discs per vehicle from 1996 to 1998, and the specialty Plymouth Prowler has used DRA in the rear wheels since production started in 1997. Discontinuously reinforced aluminum brake discs are particularly attractive in lightweight automobiles and are featured in the Volkswagen Lupo 3L and the Audi A2. In addition, a number of electric and hybrid vehicles, such as the Toyota RAV4, Ford Prodigy, and the General Motors Precept, are reported to use MMC brake components [5].

Previous researches related to the recycled aluminium composites were such as recycled aluminium matrix composites reinforced with Inconel 601 fibres [6], recycled of AlSiMg–SiCp composite [7], recycling of aluminium alloy and aluminium composite chips AA6061/Al2O3 [8] and recycled aluminum-alloy scrap with Saffil ceramic fibers [9]. There is no research concerning aluminium composite from the combination of recycled engine block and SiC particle. The objective of this research is to fabricate the aluminium composite brake disc by using the recycled aluminium engine blocks with the addition of commercial SiC particles.


MATERIALS AND EXPERIMENTAL PROCEDURE

The recycled aluminium engine blocks (Al-Si alloy) have been used as a matrix material. The commercial SiC particles size ranging from 10 to 20 mm have been used as the discontinuous reinforcement.

The recycled aluminium engine blocks were cut into pieces to facilitate the placing in the graphite crucible. Then, SiC particles were preheated at 500 ± 5°C before put together with the recycled aluminium engine blocks before casting process can be carried out. Magnesium has been added to the composition to increase the wettability of the SiC particles and aluminium matrix. Prior to casting process, sand casting mould was prepared by using silica casting sand and sodium silicate which acts as a binder. Both materials were mixed by using the electric mixer. Then, the sand mould was compacted manually and hardened by using a carbon dioxide (CO2) gas as shown in Figure 1. Polystyrene foam has been used as the pattern materials and cut into the shape of brake disc together with the design of sprue, riser and runner.



Melting process was carried out in the self-design and self-made oil-fired non-ferrous melting furnace. Turbulence casting technique was used to ensure the homogeneity of the SiC particles in the aluminium composite. The molten composite was heated until the melting process has reached the superheating condition. After that, the molten recycled aluminium composite was poured into the sand mould as shown in Figure 2. Finally, the cast aluminium composite product was machined by lathe machining to produce the recycled aluminium composite brake disc to the same size of commercial conventional cast iron brake disc as shown in

Figures 3 and 4.

RESULTS AND DISCUSSION

Figures 3 and 4 show top and bottom view of the recycled aluminium composite brake disc and commercial cast iron brake disc respectively. From the observation, the surface of aluminium composite brake disc has a bright and shiny appearance compared to commercial cast iron brake disc. In general, sound casting has been produced throughout the aluminium composite brake disc due to the effective turbulence casting technique that been applied during the melting process. Less porosity also has been observed when the cross section of the produced composite was analysed. Uniformly distributed porosity at the bottom surface (Figure 4) of the recycled aluminium composite brake disc was a unique feature that was attributed by the entrapped air in the polystyrene foam which has been released during the casting process. The aluminium composite brake disc that has been produced has a significant weight reduction which is about only one-third of the conventional cast iron brake disc comparatively. Quantitavely, the weight of recycled aluminium composite brake disc and conventional cast iron brake disc were 0.85 kg and 2.5 kg respectively.

CONCLUSION

In our present fabrication, we can conclude that the aluminium composite brake disc derived from recycled aluminium engine blocks has been found to have a significant weight reduction which is about only one-third of the conventional cast iron brake disc comparatively.


REFERENCES

[1] Aluminium the Material. Retrieved on 23th February 2003 from http://www.world-aluminium.org/production/recycling/index.html.
[2] Garcia-Cordovilla C., Narciso J. & Louis E., Abrasive wear resistance of aluminium alloy/ceramic particulate composites, Wear, 1996; 192: 170-177.
[3] Peng Yu, Cheng-Ji Deng, Nang-Gang Ma & Dickon H.L. Ng, A new method of producing uniformly distributed alumina particles in Al-based metal matrix composite, Materials Letters, 2003.
[4] Iwai Y., Honda T., Miyajima T., Surappa M.K. & Xu J.F., Dry sliding wear behaviour of Al2O3 fiber reinforced aluminium composites, Composite Science and Technology, 2000; 60: 1781-1789.
[5] Automotive Applications of Metal-Matrix Composites. Retrieved on 24th September 2003 from http://www.asm-intl.org/pdf/spotlights/AutoApp.pdf
[6] J. Lapin & T. Pelachova´, Microstructure and mechanical properties of wrought aluminium alloy prepared by recycling of aluminium matrix composites reinforced with Inconel 601 fibres, Materials Science and Engineering, 1999; A271: 266–274.
[7] L. Ceschini, C. Bosi, A. Casagrande & G.L. Garagnani, Effect of thermal treatment and recycling on the tribological behaviour of an AlSiMg–SiCp composite, Wear, 2001; 251: 1377-1385.
[8] J.B. Fogagnolo, E.M. Ruiz-Navas, M.A. Simón & M.A. Martinez, Recycling of aluminium alloy and aluminium matrix composite chips by pressing and hot extrusion, Journal of Materials Processing Technology, 2003.
[9] M. Samuel, Reinforcement of recycled aluminum-alloy scrap with Saffil ceramic fibers, Journal of Materials Processing Technology, 2003.

Crack Profile

CRACK PROFILE OF Al 2014/Al2O3/15p TIG WELDED JOINT

M.N. Mazlee

School of Materials Engineering

Universiti Malaysia Perlis (UNIMAP)

Jejawi, 02600 Arau, Perlis

Malaysia

mazlee@unimap.edu.my

ABSTRACT

An investigation on the welded joint has been conducted to study the crack profile due to impact loading of aluminium 2014 reinforced with 15 vol. % of Al2O3 reinforcement particles. Tungsten Inert Gas (TIG) arc welding technique has been used to weld the joint. The testing of impact was done by means of Charpy impact test (V-notched specimen). The parameters have been used in TIG arc welding were 60 A current and 10 l/min flow rate. Three crack mechanisms were observed in the welded specimen namely interface separation between matrix and Al2O3 reinforcement particles, voids formation along the fracture surfaces and secondary crack formation.

INTRODUCTION

Aluminium metal matrix composites (AMMCs) is the potential advanced materials to be used in structural applications which attributed by the high specific stiffness and strength, good wear resistance, weight saving and also good weldability. Aluminium alloys reinforced with ceramic reinforcement particles can be welded by both fusion processes and solid state processes. The appropriate welding technique to be applied on the AMMCs plays an important role in terms of the structural integrity between the structural members or components.

Among the TIG research on aluminium alloys and AMMCs that have been conducted were concerning crack propagation [1], liquation crack [2], interfacial chemical reactions between the ceramic reinforcements and the molten matrix alloy [3,4] and welding parameter [5,6]. The objective of this paper is to study the crack profile due to impact loading on aluminium 2014 reinforced with 15 vol. % of Al2O3 reinforcement particles.

MATERIALS AND METHOD

The material used was Al 2014 (Al-Cu) matrix alloy reinforced with 15 vol. % of Al203 reinforcement particles in the form of rectangular bar. The composite was produced by casting method followed by hot extrusion process. The material was supplied by Duralcan Inc., San Diego, California, USA.

Tungsten Inert Gas (TIG) arc welding technique has been used to weld the square butt joint by using Lincoln Arc Welder (Idealarc) 250 equipment. Al-Si rod has been used as a filler material and tungsten thoria as an electrode. Figure 1 shows an X-ray fluorescene (XRF) analysis of Al-Si filler material. The testing of impact was done by means of Charpy impact test (V-notched specimen). The parameters have been used in TIG arc welding were 60 A current and 10 l/min flow rate. Optical microscope was used to identify and analyse the crack profile resulted from Charpy impact test.

RESULTS AND DISCUSSION

Figure 2 shows a crack profile at the edge of welded specimen. From Figure 2, it clearly shows that crack growth and crack propagation were only took place in the fusion zone which has a lower macro Vickers hardness reading (53.9 kg/mm2) relatively. Particles separation can be clearly observed in between a fusion zone and a heat affected zone. Particle clustering feature can be seen in a heat affected zone indicated by a rounded dotted line.

Figure 3 shows a secondary crack at the edge of welded specimen. A secondary crack was happened in almost straight crack line from the initiation of primary crack to the voids formation area. Sub crack in the matrix was clearly observed in the area which adjacent to the crack initiation along the interface separation between matrix and Al2O3 reinforcement particles. Crack branching feature was shown by crack branching out from the voids formation area. Crack branching can be explained as a crack which moved out from the secondary crack and produced sub crack. Further propagation of secondary crack was in the form of crack deflection. Crack deflection means a crack that avoids the particles along the crack route. However, a crack will stop at the end when a crack met a particle in the crack route which acts as a crack stopper.

CONCLUSION

Three crack mechanisms were observed in the welded specimen namely interface separation between matrix and Al2O3 reinforcement particles, voids formation along the fracture surfaces and secondary crack formation.

ACKNOWLEDGEMENT

The authors are grateful to MOSTE for financial support under IRPA Short Term Grant.

REFERENCES

[1] Krishnakumar, Shankar. & Weidong, Wu. (2002). Effect of Welding and Weld Repair Crack Propagation Behaviour in Aluminium Alloy 5083 Plates. Materials and Design, 23, 201-208.

[2] Michael Ellis, Michael Gittos and Isabel Hadley. (1998). Significance of Liquation Cracks in Thick Section Welds in Al-Mg-Si Alloy Plate. In Seventh International Conference Joints in Aluminium (INALCO ’98), Ed. by Ogle, M.H., Maddox, S.J. & Threadgill, P.L. (Victoire Press, Cambridge, England) pp. 320-331.

[3] Peng, H.X., Fan, Z., Mudher, D.S. & Evans, J.R.G. (2002). Microstructures and Mechanical Properties of Engineered Short Fibre Reinforced Aluminium Matrix Composites, Materials Science and Engineering, A335, 207-216.

[4] Ureňa, A., Escalera, M.D. & Gil, L. (2000). Infuence of Interface Reactions on Fracture Mechanisms in TIG arc-welded Aluminium Matrix Composites. Composites Science and Technology, 60, 613-622.

[5] Norman, A.F., Drazhner, V., Woodward, N. & Prangnell, P.B. (1998). Effect of Welding Parameters on the Microstructure of Al-Cu-Mg Autogeneous TIG Welds. In Seventh International Conference Joints in Aluminium (INALCO ’98), Ed. by Ogle, M.H., Maddox, S.J. & Threadgill, P.L. (Victoire Press, Cambridge, England) pp. 26-37.

[6] Liu, H.J., Fujii, H., Maedaa, M. & Nogi, K. (2003). Tensile Properties and Fracture Locations of Friction-Stir-Welded Joints of 2017-T351 Aluminum Alloy. Journal of Materials Processing Technology, 142, 692–696.

Microstructure and Tensile Strength

The Studies of Microstructure and Tensile Strength of
Recycled Aluminium Alloys from Automotive Component

M.N. Mazlee

School of Materials Engineering,
Universiti Malaysia Perlis (UNIMAP)

Jejawi, 02600 Arau, Perlis

Malaysia

mazlee@unimap.edu.my

ABSTRACT

The research has been carried out using recycled petrol (denoted as Alloy A) and diesel (denoted as Alloy B) aluminium engine blocks respectively. Sand casting route has been employed to fabricate the specimens. Five specimens have been cast were 100 wt. % Alloy A (specimen V), 100 wt. % Alloy B (specimen W), 50 wt. % Alloy A : 50 wt. % Alloy B (specimen X), 70 wt. % Alloy A : 30 wt. % Alloy B (specimen Y) and 30 wt. % Alloy A : 70 wt. % Alloy B (specimen Z). Light microscope was used to identify and analyse the microstructure of cast alloys. Keller’s reagent was used as an etchant in order to reveal the microstructure of the alloys. Generally, dendritic cells for all specimens are large resulted from slow cooling in sand mould comparatively. The irregular arrangements of grain boundaries were due to the unmodified process.
Keywords : recycled aluminium, light microscope, Keller’s reagent, dendritic cells.

INTRODUCTION

For many years, the biggest end-use market for aluminium is transportation sector. More than a quarter of all aluminium is used in this sector. Originally indispensable for its lightweight for the aerospace industry, aluminium is now widely used in cars, coaches, lorries, trains, ships, ferries and aircraft (1).

Cast aluminium alloys are used extensively in various applications requiring a high strength-to-weight ratio such as aerospace, automotive and other structural components. The mechanical properties of the cast alloys are determined primarily by the secondary dendritic arm spacing and the morphology of interdendritic phases in their microstructure. In addition, the amounts of porosity in the casting and the inclusion concentration have a strong influence on fracture, fatigue and impact properties (2,3).

In most cast aluminium alloys, solidification begins with the development of a dendritic network of primary (a) aluminium. The secondary dendritic arm spacing is essentially determined by alloy composition, cooling rate, local solidification time and temperature gradient. Among the primary alloying elements, silicon has been reported to have an influence on dendritic arm spacing (2). The objective of this paper is to study the microstructure and tensile strength of recycled aluminium alloys from automotive component.

EXPERIMENTAL PROCEDURE

The recycled petrol (denoted as Alloy A) and diesel (denoted as Alloy B) aluminium engine blocks have been used for remelting process. Five specimens have been cast were 100 wt. % Alloy A (specimen V), 100 wt. % Alloy B (specimen W), 50 wt. % Alloy A : 50 wt. % Alloy B (specimen X), 70 wt. % Alloy A : 30 wt. % Alloy B (specimen Y) and 30 wt. % Alloy A : 70 wt. % Alloy B (specimen Z). The mixture is remelted in oil fired furnace and cast into the sand mould. The cast specimens were machined according to the ASTM Standard E557 for tensile test by Instron machine.

Light microscope (LECO LIM 50 model) was used to identify and analyse the microstructure of cast aluminium alloys from automotive component. Prior to use light microscope, the specimens were cut into small pieces and then mounted using hot mounting technique. Then the specimens were ground via wet grinding technique using silicon carbide paper (240, 360, 400, 600, 800, 1000, 1500 grit). Keller’s reagent was used as an etchant to reveal the microstructure of cast aluminium alloys. The tensile test was carried out to determine the yield strength of remelted aluminium alloys by using Instron Machine.

RESULTS AND DISCUSSIONS

It is expected that alloy A from petrol engine is Al-Mg-Si alloy whereas alloy B from diesel engine is Al-Cu-Si alloy as indicated in the microstructure of Figure 1 and 2 respectively. Figure 1a and 1b show equiaxed grain structure with fine intermetallic inclusions decorated at the grain boundaries. Some porosity also found in the microstructure of specimen V. Figure 2a and 2b show combination of elongated and equiaxed grain structure thick intermetallic inclusions decorated at the grain boundaries. Less porosity observed in this specimen.









From Figure 3, it shows combination of microstructure indicating the fine intermetallics and thick intermetallics decorated at the grain boundaries. This structure shows combination of 50 percent each alloy A and B with different intermetallic inclusion solidified at the grain boundaries. The details of intermetallic inclusions in alloy A and B will be investigated at the later stage of the research.



Figure 4 shows clear fine and elongated intermetallic inclusion embedded in the matrix which is indicating the higher percentage of alloy A. On the contrary, clear fine and elongated intermetallic inclusion embedded in the matrix which is indicating the higher weight percentage of alloy B as shown in Figure 5.





Generally, dendritic cells for all specimens are large resulted from slow cooling in sand mould comparatively. The irregular arrangements of grain boundaries were due to the unmodified process (4).

Tensile Test

Tensile test result in term of yield strength presented in Table 1 from the remelted combination alloys.

From Table 1, it was observed that the yield strength of specimen V (100 percent petrol powered engine engine block) is lower compared to specimen W (100 percent diesel powered engine engine block). It might be due to the difference composition in diesel powered engine which can withstand higher temperature during internal combustion process in the engine comparatively (5). Besides that, the porosity has influenced the strength of specimen V.

However, the strength of the specimen Y indicates the intermediate value of strength between specimen V and W due to the combination of the weight percent of alloys A and B. Specimen Y indicates a value of 168 MPa which is almost similar with specimen V. The strength of specimen Z indicates the lowest value and the reason of this behaviour is difficult to clarify. However, from the visual observation of the cast product, it showed incomplete solidification occurred in this specimen. Besides that, pores also found in the specimen Z. It is assumed that the above reason may lower the strength but the details mechanism of solidification study still under investigation.

CONCLUSIONS

1) Based on the microstructure analysis, it is expected that alloy A from petrol engine is Al-Mg-Si alloy whereas alloy B from diesel engine is Al-Cu-Si alloy, however needs to be further study.
2) The irregular arrangement of grain boundaries was due to the unmodified process.
3) Specimen W gave the highest yield strength whereas specimen Z gave the lowest yield strength.

REFERENCES

1) http:/www.eaa.net/pages/Markets/transportation, Markets and Products: Aluminium in Transportation.
2) Shivkumar, S., Wang, L. & Apelian, D., (1991), Molten Metal Processing of Advanced Cast Aluminium Alloys, Journal of the Minerals, Metals and Materials Society, 43, 26-32.
3) Scott, G.D., Cheney, B.A. & Granger, D.A., (1988), Technology for Premium Quality Castings, (Dunn, E. & Durham, D.R. ed.), TMS, 123 -149.
4) Smith, W.F. (1993), Structure and Properties of Engineering Alloys, McGraw-Hill.
5) http://auto.allstuffworks.com/diesel1.html, Internal Combustion.

The Study of Fracture Behaviour of Recycled Aluminium Alloys from Automotive Component

The Study of Fracture Behaviour of Recycled Aluminium Alloys

from Automotive Component

M.N. Mazlee
School of Materials Engineering
Universiti Malaysia Perlis (UNIMAP)
Jejawi, 02600 Arau, Perlis
Malaysia
mazlee@unimap.edu.my

ABSTRACT

The research has been carried out using recycled petrol (denoted as Alloy A) and diesel (denoted as Alloy B) aluminium engine blocks respectively. The specimens have been fabricated via sand casting process. Two specimens have been chosen were 100 wt. % Alloy B (specimen W) and 30 wt. % Alloy A : 70 wt. % Alloy B (specimen Z). Both specimens have fractured by tensile test. The fracture surface was studied using scanning electron microscope in order to characterise the fracture behaviour of the alloys. It was found that specimen W showed the ductile behaviour which exhibited void formations, dimples and shear fractures whereas specimen Z showed the brittle behaviour such as cleavage fractures.
Keywords: aluminium engine block, sand casting, tensile strength, ductile behaviour, brittle behaviour.

INTRODUCTION

Aluminium has been recycled since the days it was first commercially produced and today recycled aluminium accounts for one-third of global aluminium consumption worldwide. Recycling is an essential process of the aluminium industry and makes sense economically, technically and ecologically. The recycling of aluminium requires only 5 percent of the energy to produce secondary metal as compared to primary metal and generates only 5 percent of the green house emissions (1,2).

It is possible to recycle aluminium alloys based on automotive component. In our research, the diesel and petrol engine blocks have been taken as recycled aluminium. In addition, there is no systematic investigation of fracture mechanism on recycled engine component of aluminium.

Fracture mechanism of aluminium alloy is a very important study in determining the behaviour of that material under applied loads. Therefore, this paper presents the fracture mechanism of recycled aluminium alloys from automotive component.

EXPERIMENTAL PROCEDURE

The recycled petrol (denoted as Alloy A) and diesel (denoted as Alloy B) aluminium engine blocks have been used for remelting process. Two alloys W and Z were taken from diesel and petrol engine blocks. Both alloys were mixed according to the weight percent (wt. %) i.e 100 wt. % Alloy B (specimen W) and 30 wt. % Alloy A : 70 wt. % Alloy B (specimen Z). The mixture is remelted in oil fired furnace and cast into the sand mould. The cast specimens were machined according to the ASTM Standard E557 for tensile test. The micrographs of fractured surfaces were captured and analysed by using a scanning electron microscope (Leica Cambridge S360 model).

RESULTS AND DISCUSSION

Referring to the Figure 1, it shows small and shallow dimples morphology, less plastic deformations in the matrix, intermetallic inclusions (white coloured phase) form linkages network and decohesion phenomenon at the matrix alloy on the surface of specimen W alloy. It also shows small void associated with small dimples (marked by triangular line) and shear mode fracture (marked by black rounded line). The involvement of inclusions in the crack extension process is made evident by their presence within the voids which give the fracture surfaces the characteristic ‘dimpled’ appearance (3). Figures 3 and 5 show fracture surfaces of specimen W at 500 x and 2000 x magnification respectively. In Figure 3, it clearly shows the shear mode fracture (marked by black rounded line) besides linkages of intermetallic inclusions. Meanwhile, Figure 5 shows void formation and coalescence surrounding by linkages of intermetallic inclusions.

However, in contrast observed that many cleavages at matrix alloy and ‘semi’ shear fractures have appeared in Figure 2. In general, Figure 2 shows a brittle morphology. It was also observed that a small crack (marked by SC) also appeared in between the intermetallic inclusions. A same cleavage with large size around 100 mm has been identified and being marked in Figure 2 and Figure 4 by white rounded line. Figure 6 shows a multi layer cleavage surfaces. The cleavage surfaces appear somehow that fracture happens at preferential slip plane.

Based on the appearances, it shows that the ductile fracture has taken place in specimen W meanwhile brittle fracture had been happened in specimen Z. In monolithic metals, ductile fracture follows the sequence of void nucleation, void growth and void coalescence (4,5). Intermetallic inclusions (white coloured phase) between the dimples and cleavage surfaces in specimen Z will decrease the cohesiveness bonding in the matrix thus tensile strength property will be weaken further.

CONCLUSIONS

1) The ductile behaviour of remelted aluminium alloys showed the appearance of void formations, dimples and shear fractures.
2) The brittle behaviour of remelted aluminium alloys showed the appearance of cleavage fractures.


REFERENCES

1) http:/www.world-aluminium.org/production/recycling/index.html, Aluminium the Material.

2) Mazlee Mohd. Noor, (2003), Aluminium Kitar Semula; Perspektif Sektor Pengangkutan, Dewan Kosmik, Karangkraf Sdn. Bhd.

3) Derby, B., (1995), Microstructure and Fracture Behaviour of Particle-Reinforced Metal-Matrix Composites, Journal of Microscopy, 177, 357-368.

4) Hahn, G.T. & Rosenfield, A.R., (1975), Metallurgical Factors Affecting Fracture Toughness of Aluminium Alloys, Metall. Trans. A, 6(A), 653-683.

5) Shivkumar, S., Wang, L. & Apelian, D., (1991), Molten Metal Processing of Advanced Cast Aluminium Alloys, Journal of the Minerals, Metals and Materials Society, 43, 26-32.

Phase Evolution in A356 Alloy

Phase Evolution in A356 Alloy

M.N. Mazlee

School of Materials Engineering, Universiti Malaysia Perlis (UNIMAP)
P.O Box 77, Pejabat Pos Besar, 10007 Kangar, Perlis
mazlee@unimap.edu.my

Introduction

Solution treatments have became prime important in determining the successful for the entire heat treatment of precipitation hardening of aluminium alloys. In process, the alloy is quenched to room temperature just after solution treatment at a rate sufficient to inhibit the formation of precipitates which resulting in non-equilibrium supersaturated solution (SSS) [1]. The purpose of solution treatment is to dissolve maximum practical amount of hardening solutes such as Mg, Cu and Zn into the solid solution in an aluminium matrix [2].
The precipitation hardening in Al-Si-Mg alloys has been extensively studied [3-6]. The precipitation sequence in Al-Si-Mg alloys can be presented as follows [1]:

α (SSS) --- GP zone --- β'' --- β' --- β phase

A356 alloy is one of the most widely applied Al-Si-Mg alloys in the commercial industries due to its good castability and can be strengthened by precipitation hardening [1,3,5-6].
This study has focused on the differential scanning calorimetry (DSC) thermograms to trace the process of formation and dissolution of GP zones, metastable phases and precipitated equilibrium phase.

Experimental Methods

Materials
The materials used in this present work were cast ingot A356 (Al-7%Si-0.3%Mg) alloy. Four different treatments of specimens were studied namely untreated as-received A356 alloy (AR) and three solution treated and as-quenched A356 alloys at 540ºC (ASQ540), 550ºC (ASQ550) and 560ºC (ASQ560) respectively.

Apparatus and Procedures
DSC analysis was carried out using SDT Q600 DSC Instrument. Solution treatments were carried out at heating rate of 10ºC/minute for 7 hours at 540ºC, 550ºC and 560ºC respectively in a normal atmosphere followed by quenching in iced water. A scanning rate of 10ºC/min was used for all temperature ranges except for the all as-quenched between 400ºC to 600ºC which used a scanning rate of 5ºC/min.

Results and Discussion

DSC thermograms in Figure 1 can be described as follows:

1) A ‘low temperature’ section ranging from room temperature to 293ºC corresponds to the formation (A1) and dissolution (A2) of GP zones as shown in Figure 1a. Peak temperature at formation and dissolution of GP zones was observed to be faster when solution treatment temperatures decreased as in Table 1.

2) Figure 1a also illustrates an ‘intermediate temperature’ section between 293ºC and 362ºC which corresponds to the formation and dissolution of metastable phases where 3 exhothermal peaks (B1) and 1 endothermal peak present in solution treated and as-quenched specimens. ASQ550 was found to be the fastest in reaching peak temperature at formation phase as shown in Table 2.

3) A ‘high temperature’ section where an exhothermal peaks C1 and an endothermal peak C2 (Figures 1b and 1bi) corresponding to the formation and dissolution of equilibrium phases as shown in Table 3. In general, the peak temperatures at both formation and dissolution of equilibrium phases were almost similar with a small variation.

In general, a faster formation of GP zone and metastable phases were observed in as-quenched Al-10%Si-0.4Mg which had undergone solution treatment at 525ºC for 12 hours [3] comparatively. This faster trend is may be due to higher both Si and Mg contents which contribute to the formation of Mg2Si precipitates towards the acceleration of age hardening process and a longer solutionising time where the solute atoms dissolved more to form a single phase solid solution.

Conclusions

1) ASQ550 has achieved peak temperature in a shorter time compared to other specimen believed due to the enhanced formation of metastable phase.
2) A faster formation of metastable phases in ASQ550 has contributed to the faster formation of equilibrium phases comparatively.
3) A faster formation of GP zone and metastable phases in A356 alloys is believed can be achieved by prolong the solution treatment time.

References

1. W.D. Callister, Materials Science and Engineering; An Introduction, (1994), Wiley, 783.
2. Hatch J.E., Aluminium: Structure and Physical Metallurgy, Metal Park, OH: ASM International, (1998), 150-160.
3. R.X. Li , R.D. Li, Y.H. Zhao, L.Z. He, C.X. Li, H.R. Guan & Z.Q. Hu, Age-Hardening Behavior of Cast Al-Si Base Alloy, Mater. Lett., 58, (2004), 2096-2101.
4. C.M. Estey, S.L. Cockcroft, D.M. Maijer & C. Hermesmann, Constitutive Behavior of A356 During The Quenching Operation, Mater. Sci. Eng., A 383, (2004), 245-251.
5. Y.J. Li, S. Brusethaug & A. Olsen, Influence of Cu on the Mechanical Properties and Precipitation Behavior of AlSi7Mg).5 Alloy During Aging Treatment, Scripta Mater., 54, (2006), 99-103.
6. Choong Do Lee, Damping Properties on Age Hardening of Al-7Si-0.3Mg Alloy During T6 Treatment, Mater. Sci. Eng., A 394, (2005), 112-116.