International Research Journal of Engineering and Technology (IRJET) Volume: 04 Issue: 02 | Feb -2017
www.irjet.net
e-ISSN: 2395 -0056 p-ISSN: 2395-0072
Changes in Structural Features of Al-12Si-3Cu Alloy Due to Age Hardening Veena Shivaprasad 1, Purnimaa Sasikumar Dixit 2, Hariharan Nalatore 3 Department of Physics, Sir M Visvesvaraya Institute of Technology, Bangalore- 562157, India Department of Physics, Sir M Visvesvaraya Institute of Technology, Bangalore- 562157, India 3 Department of Physics, Sir M Visvesvaraya Institute of Technology, Bangalore- 562157, India
1,2,3 2
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Abstract - In the current work, the effect of artificial ageing on microstructural features of Al-12Si-3Cu alloy is studied. T6 heat treatment is carried out for this purpose. The experimental alloy is solutionized at 500oC for 8 hours and quenched in water at room temperature. Artificial ageing is carried out at 200oC. Hardness of the test specimen is measured and graph of hardness values Vs ageing time is drawn. The micro structural changes are observed under optical microscope and structural changes are correlated with the age hardening behavior of the test alloy. Key Words: Al-Si-Cu alloy ; Age hardening ; Microstructure.
1.INTRODUCTION Aluminium – Silicon casting alloys are used for automotive applications due to less weight and ability to cast into any complex shapes. The Al-Si alloy group is popular in many applications where copper additions harden the alloy [1]. Copper acts as a strengthening agent in aluminium. Heat treatment improves the microstructure and mechanical properties of these alloys. The most commonly used heat treatment processes are solution treatment and age hardening [2]. Copper additions up to 5 wt.% lead to alloys with very high strength and good toughness when subject to natural or artificial ageing[3]. Typically, T6 heat treatment is applied which involves three steps namely solution treatment, quenching and age hardening and commonly used to improve the mechanical properties of the alloy. [4]. During solution treatment, the alloy is heated to high temperature for relatively long periods of time to dissolve Cu rich particles and to modify the acicular morphology of the eutectic Si to a less detrimental rounded one. [5]. In T6 heat treatment cycle the selection of solution treatment temperature and solution treatment time is critical because if the melting temperature exceeded, the incipient melting takes place at the grain boundaries. This results in reduction in mechanical properties [6]. Wang et al. found 500° C as the suitable solution treatment temperature for alloys with more than 2 wt.% Cu. [7]. After solution treatment the alloys are normally quenched in water at room temperature to obtain a super saturated solid solution and artificial ageing is done at an elevated temperature to obtain a uniform distribution of small precipitates because which gives the high strength [8].
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The present study is part of a larger research study, which was conducted to get the better understanding of the effect of T6 heat treatment on the microstructure and hardness of Al-12Si-3Cu alloy.
2. Experimental Procedure Al-12Si alloy was melted in an electrical furnace under a cover flux (45%NaCl+45%KCl+10%NaF), and the melt was held at 7200 C. Degassing was done with solid hexachloroethane and 12 wt% of pure copper wire pieces were added to the melt packed in an aluminium foil. The melt was stirred for 30 – 45 seconds with zirconia-coated iron rod. Al-12Si-3Cu alloy specimens were swaged to required dimensions (diameter 22 mm and 250 mm length). Chemical analysis of the test alloy was done using optical emission spectroscopy and found to have following composition: Table -1: Elemental composition of the cast alloys Alloy Al-12Si3Cu
Composition (wt %) Si
Cu
Fe
Mn
Mg
Al
12.04
3.08
0.13
0.05
0.08
Bal
The small samples with dimensions of 22 mm diameter and length 12 mm were cut from solidified castings. All the castings were cut in the same dimensions, to avoid the effect of casting size on cooling rate and microstructure, For optical microscopic studies, metallographic samples were prepared and etched with Keller reagent (1.5ml HNO3, 2.5ml HCl, 1.0 ml HF, and 95ml H2O). Experimental test specimens were solution heat treated in muffle furnace at 500oC with holding time of 8 hours and water quenched at room temperature. Artificial ageing was carried at 200oC with different holding time 1, 2, 4, 6, 8, 12, 16, 20, 24, 28, 32 and 36 hours. The surfaces of the test specimens were initially polished on a belt grinder and then on a series of SiC water proof emery papers with increasing grit size. Final stages of polishing were performed on a disc polisher using 75 m Al2O3 powder with water until a scratch free surface is obtained. The samples were then cleaned with soap solution and ethyl alcohol followed with drying. The polished samples were etched using Keller’s reagent (2.5% HNO3 + 1.5% HCl + 1% HF + 95% H2O by volume) for about 75-90s in order to develop ISO 9001:2008 Certified Journal
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