Fabrication and Corrosion Behaviour of Aluminium Alloy ( LM-13 ) Reinforced with Nano-ZrO 2 Particulate Chilled Nano Metal Matrix Composites ( CNMMCs ) for Aerospace Applications

This research presents the results obtained and the discussions made from a series of corrosion experiments involving aluminum alloy (LM 13) reinforced with Nano-ZrO2, size of the particles dispersed varies from 100 200 nm and amount of addition varies from 3 wt% to 15 wt% in steps of 3 wt%. The resulting CNMMCs are solidified under the influence of copper chill of 25 mm thickness to study the effect of corrosion behavior. The corrosion test employed was the electrochemical polarization method according to ASTM G59-97 (2009) standards. Corrosion resistance was found to increase significantly with increase inZrO2 content in chilled CNMMCs. Nevertheless, even with high ZrO2 content corrosion attack i.e., pitting was found to be most severe during the initial stages of each test but it invariably decreased to a very low value in the later stages, due to the formation of an adherent protective layer on the CNMMCs developed. SEM studies of the corroded surface were also examined to study the mechanism of corrosion. Present research is undertaken to develop a corrosion resistant composite for aerospace application where aluminum is highly suspected to corrosion.


Introduction
As aerospace technology continues to advance, there is a rapidly increasing de-How to cite this paper: Hemanth, J. and Divya, M.R. (2018) Fabrication and Corrosion Behaviour of Aluminium Alloy (LM-13) Reinforced with Nano-ZrO 2 Particulate Chilled Nano Metal Matrix Composites (CNMMCs) for Aerospace Applica-mand for advanced materials with enhanced mechanical properties and environmental capabilities for such ultrahigh applications [1].Its application also stretched to automobile, electronic, computer and shipping industries to replace the existing materials [2].The early 1990s are considered to be the renaissance for Al as structural material due to environmental concerns, increasing safety and comfort levels.A significant improvement in the properties of Al alloys, reduced fuel consumption because of its light weight and hence it has created a huge demand from the aerospace industry [3] [4].A recent industrial review revealed that there are hundreds of components from structural to engine in which aluminum alloy is being developed for a variety of applications [5].This growing requirements of materials with high specific mechanical properties with weight savings have fueled significant research activities in recent times targeted primarily for further development of Al based composites that are corrosion resistant [6] [7] [8].It is noticed that the limited mechanical properties (strength and hardness) of Al and its alloys adversely affect its applications in automobile and corrosion in aerospace and marine applications [9] [10].This remains one of the major concerns in its fabrication of ceramic based nano composites to suit its application in recent days.
It is well known that an alloy can corrode due to the presence of localized galvanic cells [11].Eckel [12] has shown that oxygen is also necessary for such corrosion and this is sustained by Logan et al. [13].Swann and Pickering [14] further reported that deep pits are formed in such corrosion.The corrosion behavior of each group of aluminum alloy is very different from that of others groups.It was shown that improvement in corrosion resistance of aluminum composites can be achieved only by addition of alloying elements and not merely by heat treatment [15].For instance, aluminum alloy which contains Fe, Mo, Ni and Mg undergoes a special form of corrosion which is highly dependent on their content.Molybdenum was found to contribute substantially towards improving the corrosion resistance of aluminum alloys [16].Shalaby et al. [17], concluded from their investigations that high-silicon content aluminum alloy composites were susceptible to localized corrosion, which occurred at Si needles embedded in the aluminum alloy.Sung and Was [18] showed that neither grain boundary chromium depletion nor inter-granular boundaries promoted inter-granular cracking by corrosion.At the same time low level addition of Nd (Neodymium) to Al-5%Mg in the acid media test revealed that addition of Nd were effective in decreasing the amount of subsequent inter-granular corrosion attack [19].Nevertheless, experiments on corrosion of aluminum alloy composites containing Ni, Mo and Ti as alloying elements tested in chloride solution indicate that corrosion rate was decreased with increasing the alloy content [20] [21] [22].
It is well known that Al alloys that freeze over a wide range of temperature are difficult to feed during solidification [23].The dispersed porosity caused by the pasty mode of solidification can be effectively reduced by the use of chills [24] [25] [26].Chills extract heat at a faster rate and promote directional solidifica-tion.Therefore chills are widely used by foundry engineers for the production of sound and quality castings.There have been several investigations [27] [28] [29] [30] on the influence of chills on the solidification and soundness of alloys.With the increase in the demand for quality composites, it has become essential to produce Al composites free from unsoundness.Hence in the present investigation copper end chill was employed to achieve the above since volumetric heat capacity (VHC) of the chill material has an effect on the properties of the composite developed.Joel Hemanth et al. [31]  with noticeable weight savings along with corrosion resistance [37].

Experimental Procedure
In this research nano-ZrO 2 particles dispersed in Al alloy (LM 13, properties are shown in Table 1), fabricated by stir casting technique, solidified under the influence of copper chill of 25 mm thick (arrangement shown in Figure 1).The size of the nano-ZrO 2 particles dispersed varies from 100 to 200 nm and the amount addition varies from 3 wt% to 15 wt% in steps of 3 wt%.Synthesis of the composite involved heating of Al alloy in a graphite crucible up to 740˚C using resistance furnace to which the preheated (to 450˚C) reinforcement was added carefully using a graphite spoon and stirred well by an impeller which rotates at 450 rpm to create vortex to get uniform distribution of the reinforcement.This treated Al alloy containing nanoZrO 2 particles were made to solidify under the influence of copper chill set in AFS standard dry sand mold.Finally the samples were heat treated (aging at 500˚C) to relieve all the internal stresses.
Properties of the reinforcement (Nano-ZrO 2 ) are as follows: Melting point: Microstructural characterization was conducted on polished CNMMC specimens using high magnification OLYMPUS metallographic microscope to investigate morphological characteristics of grains, reinforcement distribution and interfacial integrity between matrix and the reinforcement.Corrosion test was conducted using electrochemical potentio-dynamic polarization corrosion method (ASTM G 59 -97, 2009 standard) and the apparatus is as shown in Figure 2.  In practice, all electrodes are polarizable to certain extent i.e., when a direct current is passed through the electrode-solution-interface, the structure of the electrode and the electrode potential changes with respect to the equilibrium value.This is the method used in the present investigation that, a change in the electrode potential occurs when a direct current is passed through the electrode.
Polarization may occur either at the cathode (cathodic polarization) or at the anode (anodic polarization) but cathodic polarization is common.
In addition, corrosion rate mainly depends on the surface conditions of the samples therefore, the samples are to be finely polished.

Results and Discussion
Results of the investigation on solidification, microstructure and corrosion behavior reveal that adding reinforcement content up to 12 wt% has uniform distribution and addition above this limit causes cluster formation.Also, 25 mm thick copper chill used has a pronounced effect on solidification, microstructure and corrosion behavior because of its high volumetric heat capacity.Hence present discussion is focused mainly on CNMMCs containing 12 wt% dispersoid cast using 25 mm thick copper chill block.
For all the tests, specimens were taken from the chill end (CNMMCs near the chill end are sound and defect free) because it was observed that, farther away from the chill (riser end) the specimen is taken, the defects are more.This could be because, the farther away from the chill the specimen is, the lower is the rate of chilling.

Effect of Chilling on Solidification, Microstructure and Corrosion Behavior
The optical photomicrographs in Figure 3 shows the matrix microstructure of chilled CNMMC containing 12 wt% dispersoid cast using copper chill of 25 mm thick.The specimens of chilled CNMMCs are dictated by the microstructure of fine grains due to chilling effect during solidification.Microstructure of CNMMCs containing 15 wt% dispersoid revealed cluster formation (Figure not shown) and hence, 12 wt% addition is treated as the optimum limit of addition of the dispersoid.Figure 4 shows microstructure of un chilled as cast composite containing 12 wt% reinforcement.When the melt of the CNMMC solidifies  Microstructural characteristics of chilled nano-composites are discussed in terms of distribution of reinforcement and reinforcement matrix interfacial bonding.Microstructural studies conducted on the nano-composites containing 12 wt% dispersoid revealed uniform distribution of the reinforcement with limited extent of clusters with good reinforcement-matrix interfacial integrity and significant grain refinement with minimal porosity (Figure 3).This is due to gravity of ZrO 2 associated with judicious selection of stirring parameters (vortex route), good wetting of pre heated reinforcement by the matrix melt.Metallography studies of the heat treated samples also revealed that the matrix is recrystallized completely.Grain reinforcement in case of nano composites can primarily be attributed to capability of nano-ZrO 2 particulates to nucleate Al grains during directional solidification and restricted growth of recrystallized Al grains because of presence of finer reinforcement and chilling.Interfacial integrity between matrix and the reinforcement was assessed using scanning electron mi-Journal of Materials Science and Chemical Engineering croscope of the fractured surfaces to analyze the interfacial de-bonding at the particulate-matrix interface.Here also, the result revealed that a strong bond exists between the interfaces as expected from metal/oxide systems.Therefore, micro tests reveal that, rate heat transfer during solidification (chilling) of the composite in this investigation leads to strong bonding of the dispersoid and the matrix.The result of microstructural studies of CNMMCs however did not reveal presence of any micro-pores or shrinkage cavity or there was no evidence of any microstructural defects.This may be one of the main reasons for increase of corrosion resistance of the composite developed.

Effect of Alloying Elements and Dispersoid Content on Corrosion Behavior
Alloying elements present in LM 13 Al alloy especially 2-wt% Ni, 1-wt% Mn and 12-wt.Si does have an effect on corrosion behavior along with the dispersoid content.In addition to the dispersoid, the addition of Ni, Si along with Mn serves to increase further resistance to corrosion.The observations made so far agree with those of Muthukumara Swamy et al. [16] who concluded from their investigation that the corrosion rate can be varied more significantly by alloying elements than by any heat treatment method.Ni and Si is the most effective element which results in the formation of NiO 2 and SiO 2 which are corrosion resistant.However Fe and Mn level is kept low to avoid structural heterogeneities.
Thus corrosion behavior of CNMMCs is greatly affected by the combined effect of Ni, Si and disperosid ZrO 2 .High silicon content (12 wt%) of LM 13 Al alloy that forms strong needle like structure (see Figure 3) in Al is once again hard, stable and corrosion resistant.It is evident therefore, that a particular alloying present element has a unique effect on the structure and corrosion behavior of CNMMCs.Hence the Ni and Si content combination present in CNMMC along with chilling and the ceramic dispersoid (ZrO 2 ), all contribute to superior corrosion resistance.

Electrochemical Corrosion Behaviour of CNMMCs
The experimental results of the polarization corrosion test done on different CNMMCs (containing 3 wt%, 6 wt%, 9 wt% and 12 wt% dispersoid content) and the resulting polarization curves obtained are shown in Figures 5-8 respectively.
Figures 5-8 indicate that, in every single corrosion test carried out, corrosion rate decreased gradually with time, reaching a stable saturation value after certain duration of testing.This implies that no matter how mild or severe the corrosion was originally, the rate of corrosion will stabilize after a certain time.
Nevertheless, even with high dispersoid content, some pitting was detected in the initial stages of most of the tests.Generally, corrosion attack was found to be most severe during the initial stages of the tests but it invariably decreased to a very low value in the later stages, probably due to the formation of an adherent protective layer on the metal surface.Journal of Materials Science and Chemical Engineering    Hence it is concluded from the results obtained that, polarization of an electrode at the beginning of the test is due to formation of a diffusion layer (corrodent) adjacent to the electrode surface where there is a gradient of the ion concentration.But in the later stages, the diffusion of the ions through the layer already formed controls the corrosion process.Since concentration within the diffusion layer changes (initial to final stages of the test), the electrode potential also changes within the diffusion layer.This is actually the measure of the concentration polarization rate of corrosion.

SEM Analysis of the Corroded Surface
It is observed through the necked eye that the electrodes underwent sporadic pitting after the test.The greatest difference between the corrosion rates of 3 wt% and12 wt% dispersoid content composite is seen after many hours of corrosion testing.It can be seen from the SEM photomicrographs in Figures 9-12 that composite containing 3 wt% dispersoid, has experienced the severest corrosion, as indicated by the large swollen blisters with shallow pitting on the surface.These blisters were found to peel off easily and contained a network of pits.Composite containing 6 wt. % dispersoid has smaller blisters on the surface.When the scale layer was removed by immersion in Clark's solution and after light polishing, small cavities were found in the metal.Composite containing 9 wt% dispersoidis seen to suffer some small pitting attack on the surface, whereas composite containing 12 wt% dispersoid suffered only some localized attack on the surface.To the naked eye, composite containing 9 wt% and 12 wt% dispersoid were seen to be bright and shiny, and free of any visible form of corrosion damage, whereas composites containing 3 wt% and 6 wt% dispersoid were observed to contain some whitish patches.
It is observed from SEM photomicrographs in Figures 9-12 that the electrodes underwent sporadic pitting in the case of 3 wt% dsipersoid content composite followed in that other.Journal of Materials Science and Chemical Engineering    2) Increasing the dispersoid content has increased resistance to corrosion.
3) The copper chill used has increased the rate of heat extraction due to its higher volumetric heat capacity produced fine grained structure and thereby increased resistance to corrosion near the intergranular interface and thereby increased resistance to corrosion.
4) Microstructural studies of CNMMCs developed however did not reveal presence of any micro-pores or shrinkage cavity or there was no evidence of any microstructural defects.This may be one of the main reasons for increase of corrosion behavior of the composite developed.
5) Polarization of the electrode at the beginning of the test is due to formation of a diffusion layer (corrodent) adjacent to the electrode surface where there is a gradient of the ion concentration.But in the later stages, the diffusion of the ions through the layer already formed controls the corrosion process.
6) Finally, it is observed that no matter how mild or severe the corrosion will be originally, it always settles down to a stable corrosion rate.

Figure 1 .
Figure 1.Experimental set up (AFS standard mold containing copper end chill block).

The
SEM photographs of the corroded electrode surface of different CNMMC electrodes after polarization in 3.5% normal NaCl (electrolyte used which is almost equal to sea water normality) with a pH value of 6 are shown in Figures 9-12.It is clear from the results that the composite containing 3 wt% dispersoid is the most susceptible to corrosion followed by other CNMMCs.This means that the alloying elements present in LM 13 Al alloy such as Ni, MnSi and dispersoid ZrO 2 does confer resistance to corrosion of the composite developed.

Table 1 .
Chemical composition of the matrix alloy (LM 13).
1860˚C, UTS: 425 MPa, VHN: 150, Young's Modulus: 98 GPa, If the surface has roughness then the electrochemical corrosion is prominent.Samples with 100 mm 2 exposed area per side are connected to the working electrode.The other portion is covered with heat shrinkable teflon tape and connected to the main electrode.Before the actual polarization test, the electrode is cathodically pola- rized to a potential of 600 to 650 mv, more negative than the open circuit potential.Then the potentio-dynamic polarization test was carried with a scan rate of 40 mv/min.All the potentials were measured with respect to saturated calomel electrode (SCE).Finally, Hitachi S4100 field emission scanning electron microscope was used to analyze the corroded surfaces.