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.