Hard anodizing of AK9ch high silicon aluminum alloy

This work is devoted to anodic oxidation at low temperatures in the sulfuric acid electrolyte of the foundry aluminum alloy AK9ch, with a high silicon content. Optimal anodizing conditions were chosen for obtaining coatings with a hardness of more than 300 HV and a thickness of about 40 microns. With an increase in the thickness of the coatings by increasing the current density or anodizing time, their hardness begins to decrease. The resulting hard coatings are planned to be used on large parts obtained by casting aluminum alloy AK9ch.


Introduction
Aluminum alloys, due to their properties, including the strength-to-weight ratio,corrosion resistance, are among the most common structural materials.Forged or rolled aluminum alloys are widely used in the construction, automotive, aviation and space industries.However, along with forged alloys, foundry alloys are also widely used due to their high fluidity, relative cheapness, and ease of manufacturing complex large parts.One of the most common foundry alloys are silumins.Silumins are aluminum alloys based on silicon (2-12 wt%) and a small (less than 0.1 wt%) amount of iron, copper, magnesium and several other elements that give it unique properties, such as plasticity and corrosion resistance, which make them promising, for example, for the manufacture of aircraft components and parts of various machines.The chemical composition of silumins and the features of the smelting process are the main factors determining the mechanical characteristics of the material.Alloys have: greater brittleness compared to duralumin, which can lead to destruction during processing and loading, high density, and relatively low microhardness.
The most common method of finishing the surface of aluminum and its alloys is anodic oxidation, which is used as a protective coating with various properties, including anticorrosive [1], decorative with or without dyes, as well as hard anodizing coating to improve the wear resistance of the surface [2].Anodizing is an electrochemical process of the growth of an oxide film on the surface of aluminum, which is used as an anode in an electrolytic cell, due to the introduction of oxygen ions into the metal under the effect of electric voltage.The growth of the oxide film is based on ion exchange, in which aluminum cations (Al 3+ ) migrate through the resulting oxide towards the electrolyte, and anions (O 2-, OH -) move from the surface of the electrolyte to the aluminum anode.Under certain conditions [3], there is a transition from the growth of a dense thin film to the formation of a porous oxide structure.Electrolytes based on various acids (sulfuric, phosphoric, chromic, oxalic) are used for anodic oxidation of aluminum and its alloys, the most common is an electrolyte based on sulfuric acid [4].
During the hard anodizing process, the thickness of the oxide coating ranges from 20 to 100 microns [5,6], while a number of studies have noted that with increasing thickness, its microhardness decreases, which is associated with a longer or active exposure to an electrolyte based on sulfuric acid [6,7].To reduce the activity of the electrolyte, a thick oxide coating is usually produced at low temperatures, less than 100°C [4,7].When anodizing Al-Si cast alloys, there are a number of problems.During their anodizing, eutectic silicon particles are not completely oxidized, forming only a thin film of silicon oxide, which leads to an uneven distribution of the electric field during the oxidation process, and prevents the growth of a homogeneous oxide coating.During the anodizing process, the presence of eutectic Si particles can also cause the formation of defects in the anode layer, such as voids filled with gaseous oxygen, non-anodized zones and cracks [8][9][10].The purpose of this work was to work out the production of solid anode coatings on the AK9ch alloy, for further use on large parts obtained by casting.

Materials and methods
Rectangular parts made of 20×40 mm AK9ch alloy with a thickness of 5mm were used as samples.The samples were pre-sanded and polished on one side using Pikal Paste for Metal Polishing.In the electrochemical cell, the samples were used as an anode and placed with the polished side to the lead plate of the cathode.To obtain solid anode coatings, the electrochemical cell was forcibly cooled to an electrolyte temperature of 4-5°C.An aqueous solution of 25 g/l of sulfuric acid was used as an electrolyte.The amount of sulfuric acid was selected based on taking into account the maximum conductivity of the electrolyte, taking into account the cooling temperature [11].
The surfaces of the samples were washed and thoroughly degreased with alcohol, the preparation did not include preliminary etching and clarification of the alloy, due to the accumulation of silicon particles on the surface of the alloy.During anodizing, a galvanostatic method was used, which, taking into account the maintenance of a constant current, makes it possible to predict the linear growth rate of the oxide film.
The morphology of the surface of the obtained samples was studied using an optical and scanning electron microscope (SEM) Hitachi SU1510.Qualitative and quantitative analysis of the coating composition was performed using the SEM EDX spectrometer.Quantitative analysis of the coating composition was carried out by the Proza (Phi-Rho-Z) method.The thickness of the resulting coating was measured on SEM using elemental oxygen mapping EDS on a sample slice.The hardness measurement on the samples before and after the coating was obtained by using the Vickers method with using a Shimadzu HMV-G21DT microhardness meter.

Result and discussion
Anodizing of the samples was done in galvanostatic mode at current densities of 1-4 A/dm 2 for 40-80 min.Before oxidation, the sample and the electrolyte were cooled to a temperature of 4-5°C, the electrolyte temperature was maintained as much as possible during the anodizing process.Figure 1 shows examples of kinetic dependences of voltage growth over 40 min at different current densities.At the beginning of the dependencies, a rapid increase in voltage is observed, which is accompanied by the formation of a barrier layer of oxide, the rate of formation of which increases with increasing current density.At the second stage, the rate of voltage growth decreases significantly, forming a bend, and goes to a smooth linear growth of the porous coating layer.It should be noted that when using low electrolyte temperatures, there is no significant local voltage reduction during the transition of growth from a barrier to a porous layer, as when anodizing at room and higher temperatures.The absence of a voltage drops and smooth growth at the stage of formation of a porous layer may indicate a minimum rate of pore etching by reducing the aggressiveness of the electrolyte using a reduced temperature.Figure 2 shows the SEM image of the sample surface before anodizing in secondary electrons (SE) (figure 2a), as well as SEM images after anodizing in secondary (figure 2b) and backscattered electrons (BSE) (figure 2c).SEM images after anodizing are accompanied by the accumulation of charge on the non-conductive surface of aluminum oxide.The process of charge accumulation can be observed in the mode of secondary electrons in the form of light areas of partial illumination of the image.In this case, dark areas are formed in those places where a silicon matrix is present near the surface, allowing the electric charge to drain.In the backscattered electrons, the accumulation of charge has less effect on the image and makes it possible to better observe the surface, as well as cracks located in areas without silicon.
During optical examination of the surface morphology, it can be noticed that after anodizing (figure 2 d), silicon inclusions in the transparent coating layer become visible.At the same time, cracks are observed on the oxide surface in electronic and optical images, which can form due to the expansion of the oxide matrix material with silicon inclusions, as observed in [9].  Figure 3 shows the energy dispersion spectrum (EDS) of the coating surface on a logarithmic scale, on which lines of aluminum, oxygen, silicon, and other components of Fe and Cu, which are part of the AK9ch alloy, are observed.Carbon is usually associated with organic pollutants that may be present on an unprepared surface.It should be noted separately that there is a clear presence of sulfur, which appears due to anodizing in a sulfuric acid electrolyte.Table 1 shows a quantitative analysis of the surface composition, from which it can be seen that the atomic percentage of the oxygen amount of the surface is more than 50 at%, while 42 at% of oxygen is sufficient for the binding of aluminum in Al2O3, residual oxygen can be on the surface as part of organic contamination, or as a residue of SO4 2-anions.
To measure the thickness of the obtained coatings, element mapping was performed using EDS sections of the sample.Figures 4a, b show the mapping of oxygen (green) and sulfur (blue) on the sample slice, it can be seen from the images that the penetration depth of silicon and sulfur is the same.The presence of background dots in the lower part of the images is associated with Bremsstrahlung (or braking X-ray) radiation, which can be observed in Figure 4.The presence of sulfur, taking into account its valence (S 6+ ) in the entire depth of the anodic coating, is apparently due to the residual presence of SO4 2-anions in the pores.Thus, simultaneous mapping of the slice by oxygen and sulfur allows to determine the thickness of the anode layer with high reliability.Figures 4c, d show the overlay of element mapping on the SEM image in reflected electrons: silicon and oxygen on the sample slice (Figure 4c), silicon on the sample surface (Figure 4d).
Measurement of the mechanical properties of an uncoated alloy by micro-indentation at loads from 2 N, gives results of the order of 60-70 HV according to the Vickers method, and 55-65 HB according to Brinell.Measuring the hardness of coating by Brinell method was difficult due to the absence of a visible imprint on the surface.The absence of a circular indenter imprint may be due to the elastic properties of the coating-alloy structure, in which the aluminum alloy is elastically deformed under load, while no inelastic deformations with material removal are observed on the coating.The Vickers pyramidal indenter provides inelastic deformation by which the size of the print can be traced.The results of measuring the thickness and hardness depending on the parameters of obtaining the coating are shown in Table 2.
At optimal parameters, the hardness of the coating was observed above 300 HV, which is 4-5 times higher than the initial hardness of the aluminum alloy AK9ch.At the same time, due to the heterogeneous structure of the coating due to large silicon inclusions, the hardness of the samples had a spread of up to 15-20%.However, despite this, a number of major trends can be identified.With an increase in the time of obtaining the coating or the current density, the process of reducing the hardness of the samples also begins, which is not related to the thickness of the coating, but to its properties.A high current density is usually associated with an increase in the local temperature in the porous coating layer.As the temperature increases, the chemical activity of the electrolyte increases, leading to more intensive etching of the oxide in the pores.An increase in the anodizing time also leads to a significant etching result in the pores, reducing the volume ratio of the oxide in relation to the pores.A similar decrease in hardness with an increase in time or current was also observed in [7,8].Thus, it is necessary first of all to take into account the current density and anodizing time to obtain a hard coating with an optimal ratio of hardness and thickness.In our case, to obtain a coating in a sulfuric acid electrolyte (without additives of other acids) on the AK9ch alloy, we consider the current density of about 2A/dm 2 to be optimal parameters at an anodizing time of 30-50, which makes it possible to obtain coatings of sufficient thickness, about 40 microns, with a hardness of about 300 HV.

Conclusions
In this work, solid anode coatings on aluminum alloy AK9ch were obtained at low temperatures in a sulfuric acid electrolyte.Optimal anodizing conditions have been selected for obtaining coatings with a hardness of more than 300 HV and a thickness of about 40 microns.With an increase in the thickness of the coatings by increasing the current density or anodizing time, their hardness begins to decrease.These parameters are planned to be used to obtain hard wear-resistant coatings on large parts obtained by casting aluminum alloy AK9ch.

Figure 1 .
Figure 1.Kinetic curve U(t) of galvanostatic anodizing at different current densities.

2 .
SEM images of the sample surface before (a) and after anodizing in secondary electrons (b), and in backscattered electrons (c), optical image after anodizing (d).

Figure 4 .
Figure 4. Elemental mapping of the sample slice by oxygen (a) and sulfur (b), oxygen and silicon (c) on the background of the SE image of the slice, as well as silicon on the background of the BSE image (2 A/dm 2 , 40 min) of the sample surface (d).

Table 2 .
Thickness and microhardness of the coating on the main anodizing parameters.