Corrosion Characterisation of Al-Cu Reinforced In-Situ TiB




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Corrosion Characterisation of Al-Cu Reinforced In-Situ TiB2

R. Rosmamuhamadani1,1, S. Sulaiman2 , M.A. Azmah Hanim3 , M.I.S. Ismail4 , M.K. Talari5, and Sabrina M. Yahaya6

1,2,3,4Mechanical and Manufacturing Department, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia.

5Materials Technology Programme, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia.

6Applied Chemistry Programme, Faculty of Applied Sciences, Universiti Teknologi MARA, 40150, Shah Alam, Selangor, Malaysia

Abstract. Aluminium (Al) based in-situ metal matrix composites (MMCs) have better properties and performance compared to ex-situ MMCs. In this research, aluminium-copper (Al-Cu) alloy was reinforced with 3 and 6wt.% titanium diboride (TiB2). Al-MMCs has been fabricated with salt route reaction process at 800 °C via potassium hexafluorotitanate (K2TiF6) and potassium tetrafluoroborate (KBF4) salts. Hardness vickers tester and Gamry-Electrod Potentiometer were used to characterize the hardness properties and to determine the corrosion rate of Al-Cu alloys. From results obtained, increased TiB2 contents will increase the hardness of Al-Cu alloys. Increased of TiB2 contents also will increase the corrosion rate of Al-Cu alloys. Al-Cu with 3wt.%TiB2 gave the good properties of corrosion when the wear rate recorded the lowest value compare to Al-Cu alloy itself and 6 wt.% TiB2. The corrosion rate of Al-Cu with 3wt.TiB2 was 16.15, while Al-Cu and Al-Cu-6wt%TiB2 were 22.5 and 58.7 mm/y respectively.

1 Introduction

Al is the most popular matrix for MMCs. Al-MMCs has a wide application in the fields of aerospace, automobile and so on, because of their high specific strength, rigidity, wear resistance and good dimensional stability [1].

Recently, in-situ techniques have been developed to fabricate Al-based MMCs, which can lead to better adhesion at the interface and hence better mechanical properties. These in-situ routes provide many advantages such as the in-situ formed reinforcement phases are thermodynamically stable, disperse more uniformly in matrix, free of surface contamination and leading to stronger particle matrix bonding [2].

All metals and alloys undergo corrosion, which is defined as the destructive attack of a metal by the environment, by chemicals, or electrochemical processes [3]. The driving force is the free energy of reaction of the metal to form, generally, a metal oxide. Since corrosion reactions generally occur on the metal surface, they are called interfacial processes. The corrosion process takes place at the metal medium phase boundary and therefore is a heterogeneous reaction in which the structure and condition of the metal surface have a significant role. The corrosive medium must be transported to the surface and the corrosion products removed. Therefore, material transport phenomena, including free convection and diffusion into adjacent surface layers, must also be taken into account. Metallurgical factors that can affect corrosion in an alloy include: crystallography, grain size and shape, grain heterogeneity, impurity inclusions, and residual stress due to cold work.

The influence of nitric acid (HNO3-) on corrosion of the Al-Cu alloys, containing 1.3-30 at.% Cu have been studied [3]. They found the corrosion rates of the Al-1.3 at.% Cu and Al-2.7 at.% Cu alloys in HNO3- were be ~5 and 10 nm min−1 respectively. The corrosion of an Al-30 at.% Cu alloys was less uniform, with a local rate to ~13 nm s−1. Meanwhile [4] have studied the pitting corrosion of ClO4- on pure Al, Al-2.5 wt% Cu and Al-7 wt% Cu alloys in 1.0 M Na2SO4 solution at 25 °C. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al>Al-2.5 wt% Cu>Al-7 wt% Cu.

Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO4- ion concentration and applied step anodic potential, while it decreases with increasing Cu content. The corrosion behavior of pure Al, Al-6% Cu and Al-6% Si alloys in Na2SO4 solutions in the absence and presence of NaCl, NaBr and NaI were studied [5]. The corrosion resistance increases in the order Al
Cu addition on the mechanical properties and corrosion resistance of commercially pure Al have been investigated [6]. They studied the influence of Cu addition to commercially pure aluminum on microstructure, microhardness, grain size, impact energy, flow stress at 0.2 strain, mechanical behavior and corrosion resistance. Three different Al-Cu alloys of 3,6 and 9 wt.% Cu content were prepared and experimentally tested both mechanically and chemically. The results show the addition of Cu resulted in a linear increase of the hardness, and substantial reduction in the grain size, slight reduction the impact energy, substantial increase in the flow stress at 0.2 strains, and improve in the mechanical properties. The potentiostatic measurements showed that the susceptibility of the samples towards corrosion decreases in the order: Al>Al-3 wt% Cu>Al-9 wt% Cu>Al-6 wt% Cu. The corrosion rates of the 3, 6 and 9 wt% Cu alloys in HCl were found to be 0.29, 0.13 and 0.21 nm/s, respectively. The different properties, i.e. impact energy, flow stress at 0.2 strain, mechanical characteristics and corrosion resistance, showed that the 6 wt. % Cu is an optimal composition.

2 Methodology

2.1 Composite Fabrication

Al-6wt.% Cu was respectively melted at 720 °C in an induction furnace. The melts were homogenized for 15 minutes before added the salts which were K2TiF6 and KBF4. These salts then were pre-heated to 250 °C for 1 hour. After melting of the base metals, the two salts were slowly added into the molten Al-6wt.% Cu in an atomic ratio in accordance with TiB2 by using the stirring method.

2.2 Heat Treatment

The aging behaviour of the composites was studied by solutionising the samples at 540 °C for 2 hours followed by quenching in cold water and aging at 170 °C for different intervals. The composites subjected to age were tested their hardness properties within 48 hours regarding to [7] specifications.

2.3 Hardness Test

Vickers hardness is one of a method to measure the hardness of a material. Vickers test procedure as per [7] standard specifies making indentation with a range of loads using a diamond indenter which is then measured and converted to a hardness value.

2.4 Corrosion Characterization

Corrosion test was carried out on each alloy in the as-cast conditions. Potentiostatic polarization measurements were carried out using a Radiometer Analytical model PGZ 100 Potentiostat/Galvanostat with VoltaLab software. The working electrodes employed were the graphite and Al-Cu bars. Anodic and cathodic polarization curves were plotted. The investigated electrodes were cut as cylindrical rods, welded with Cu-wire for electrical connection to contact the test solution. The experiments were performed in a 250 ml volume Pyrex glass cell using Pt wire and a saturated calomel electrode as auxiliary and reference electrodes, respectively. All potentials given in this research are referred to this reference electrode. The experiments were carried out in 0.5 M NaCl solution. The NaCl solution was freshly prepared from analytical grade using doubly distilled water. For each run, a freshly prepared solution as well as a cleaned set of electrodes was used.

3 Results and Discussion

3.1 Hardness Properties

Hardness is one of the most important properties, which is commonly used to give a general indication of the strength and resistance to wear and scratching of a material. It can be defined as the ability of a material to resist permanent indentation or deformation when it is in contact with an indenter under load.
Table-1. Vickers hardness properties of un-aged and aged Al-Cu with different TiB2 contents

Materials

Vickers Hardness Al-Cu alloy

(Hv)

Un-aged

Aged


(540 °C, 2h)

Aged

(150 °C, 48h)


Al-Cu

125

136

174


Al-Cu-3wt.% TiB2

134

142

175


Al-Cu -6wt.%TiB2

141

152

150

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