Evaluation of Microstructure, Conductivity and Hardness of Al-6Si-0.5Mg-xCu(x=0, 1 & 2) Casting Alloys at Different Ageing Conditions

ABSTRACT

Factors that influence the microstructure, conductivity and hardness of Al-6Si-0.5Mg-xCu(x=0, 1 & 2) casting alloys were critically analyzed. Experimental improvements of three alloys were studied and the microstructure, conductivity and hardness have been investigated. The addition of Cu to Al-6Si-0.5Mg alloy resulted in the formation of a hard Al2Cu phase and improved the hardness. The Cu-free and Cu-containing Al-6Si-0.5Mg alloys were homogenized (24hr at 500oC), solution treated (2hr at 540oC) and aged at different temperature and time. Formations of different inter-metallic phases were studied by conductivity and hardness measurement in the both Cu free and Cu added Al-6Si-0.5Mg alloys. Inter-metallic phases (hard phases) formed during ageing after solution treatment and they have strong effect on the age hardening and conductivity. An addition of Cu has strong interaction with Al, Si, Mg atoms and vacancies. The increased conductivity in the materials results from the decrease in concentration of the alloying elements in the aluminum matrix due to decomposition of second phase particles and less dislocation density in the formed structure.

Keywords: Al-6Si-0.5Mg alloy, conductivity, age hardening, inter-metallic phases, microstructure.

1. INTRODUCTION

Al-Si-Mg wrought and casting alloys have found extensive use in variety of applications in the automotive, aerospace and defence industries due to their wide range of mechanical and physical properties [1].It is well known that the conductivity is significantly decreased by the addition of alloying elements, whereupon elements in solid solution results in a higher resistance than the same amount of elements forming inter metallic phases. The later typically reduce conductivity proportionally with increasing volume fraction. For Al–Si–Mg and Al–Si–Cu–Mg alloys, a T6 heat treatment consisting of solution treatment, quenching and ageing is often used to increase the strength by precipitating nanometer particles, which provide excellent obstacles for the dislocation movement [2-3]. Among the elements added to Al-Si-Mg alloys for increasing strength and grain-size control, copper has arrested considerable attention. Cu additions reduce the natural aging rate of Al-Mg-Si alloys but generally increase the kinetics of precipitation during artificial aging [4]. In addition to the phases that precipitate in the ternary alloys, the equilibrium precipitate in high Cu Al-Mg-Si-Cu alloys, Q, has been identified as Al5Cu2Mg8Si6 and Al4CuMg5Si4 [5-6].

On the other hand, there are several reports regarding the effects of the Cu addition on the age-hardening behavior of Al-Mg-Si alloys. It was confirmed two types of nano clusters in the both Cu-free and Cu-added alloys [7]. Furthermore, some effects such as enhancement of hardness, refinement of precipitates, precipitation sequence and interaction parameters among Cu atoms and solute atoms are investigated in Al-Mg-Si alloys [8-12]. For Al–Si–Mg–Cu alloys, the precipitation behaviors are rather complicated and several phases such as β(Mg2Si), θ(CuAl2), S (CuMgAl2) or Q (Cu2Mg8Si6Al5) in metastable situations may exist [13–15].

It is well known that the presence of alloying elements or impurities in solid solution increases the electrical resistivity of aluminum alloys. Moreover, the presence of small particles in the structure, such as precipitates or dispersoids, causes significant scattering of conduction electrons and, hence, increases the electrical resistivity of an alloy[16-17].The size and distribution of small particles affect the electrical resistivity of the alloy[18–21].The contribution of precipitates and dispersoids particles could be neglected if the precipitate spacing is greater than the mean free path of the electron, i.e., particle spacings larger than 100 nm in aluminum alloys[17].In the intermediate range of precipitate and dispersoid particle spacings commonly observed in commercial aluminum alloys, the contribution of precipitates to electrical resistivity is less clear. The electrical resistivity analysis can be used as an indirect means of evaluating the volume fraction of dispersoids and the dissolution of constitutive particles during the homogenization of aluminum alloys. There is much research on the effect of stable and metastable precipitates during precipitation hardening on the electrical resistivity of aluminum alloys. All of the researchers have concluded that the presence of elements in the solid solution and small particles possessing coherent inter-faces increases the electrical resistivity or decrease the conductivity. [22–24].

The objective of this research work is to evaluate the effects of Cu and ageing treatment on the evolution of the microstructure, conductivity and hardness of Al-6Si-0.5Mg alloys.

2. EXPERIMENTAL

The Cu-free Al-6Si-0.5Mg alloy and Cu-added Al-6Si-0.5Mg alloy were used in this study. Melting was carried out in a natural gas heating clay-graphite crucible furnace. Several heats were taken for developing the base Al-6Si-0.5Mg alloy. In the process of preparation of the alloys the commercially pure aluminium (99.7% purity) and aluminium-silicon alloy melted in a clay-graphite crucible, Copper in the form of sheet (99.98% purity), was then added by plunging. Magnesium ribbon (99.7% purity) was added into solution duly packed in an Al foil. The final temperature of the melt was always maintained at 900±15oC with the help of the electronic controller. The melt was degassing with solid hexachloroethane (C2Cl6) and homogenized by stirring at 700oC before casting. Casting was done in iron metal moulds preheated to 200oC. Mould sizes were 15mm x 150mm x 300mm. All the alloys were analysed by wet chemical and spectrochemical methods simultaneously. The chemical compositions of the alloys are given in table1.