Obtention of Hard Coating Using Electrochemical Process in Aluminum-Silicon Alloys for Automotive Vehicles

Automotive industry is searching for new ways to improve vehicles’ energy efficiency through mass reduction, using aluminum alloys. This change requires a surface protection to extend the life cycle of the components and aluminum anodization is the most used solution. This research is focused on the intake and exhaust ducts’ surfaces of aluminum internal combustion engines cylinder head, which are subject to chemical agents and temperature variation. To extend the working life of this component it is necessary to obtain a covering protective layer. The process targeted the anodization of an internal surface of a much larger part of a cast aluminum-silicon alloy cylinder head. The anodization was obtained using a H2SO4 solution (184 g.L-1) and a DC voltage starting at 20 V. The Al2O3 layer obtained, inside the cylinder head´s ducts, has an average thickness of 120 µm in accordance with the proposal of providing a suitable surface protection.


Introduction
The aluminum anodization processes related to the formation of protective alumina (Al 2 O 3 ) layers, were first used on 1854.Ever since, new aluminum alloys were developed and this electrochemical process is widely used for most surface protection applications.The automotive industry is searching for new ways to improve vehicles' energy efficiency and the main initiative is mass reduction.The replacement of heavy materials is crucial and the aluminum alloys are being widely used as a substitute.Several steel parts are being replaced by cast aluminum parts on all major subsystems such as engines, suspension, body structure and others.This material replacement in many cases requires a surface protection to extend the life cycle of the components and aluminum anodization is the most used solution.
Internal combustion engines are subject to a huge amount of heat and require a large cooling system to keep the operational temperature under control.The combustion heat is transferred directly to the aluminum alloy on the cylinders' heads combustion chambers and exhaust ducts inside the cylinder head.To increase the energy efficiency, this heat transfer needs to be lowered as much as possible.Cylinder heads are, in almost every engine, produced with cast aluminum alloys from the AA 3XX series, which has silicon as its most significant alloy element.Al-Si alloys are widely used in castings due to their low melting temperature, high fluidity and low trend to show solidification cracks, and high mechanical strength after heat treatment [1][2][3][4][5] .
The anodization process requires an electrical direct or alternate current power source, an electrolyte solution, a cathode (a conductive material that is inert in contact with the electrolyte), and the anode, which is the part from which the anodic oxide will grow [6][7][8][9][10][11][12] .
When the part and the cathode are both in direct contact with the electrolyte, an electrochemical process occurs when an electric current is forced in the system promoting the transformation of the metallic aluminum into an oxide layer (Al 2 O 3 ), according to Equation 1.This oxide layer grows from the anode surface in a so-called duplex structure composed by a barrier layer and a porous layer.This structure is shown on Figure 1.
After the anodization process, there is an additional stage required do close the pores, called sealing.The sealing process can be done using steam, hot water or salt solutions 6,9,[12][13][14] .This additional process promotes the closing of all pores, increases the oxide layer corrosion resistance and reduces the thermal cracking, and any chemical retention inside the pores 14 .This research´s aim was to obtain an aluminum oxide layer performing a hard anodization on the inner surfaces of the exhaust ducts of a cylinder head.The oxide layer should achieve a compact structure with thicknesses exceeding 100 μm, large cells and the smallest possible pores diameters.Several anodization runs were required to adjust the process parameters to obtain the target configuration.This layer is meant to reduce the heat transfer from the exhaust gas to the cylinder head body, causing a reduction of the engine´s cooling requirements and therefore, the cooling system´s mass.The application is shown on Figures 2 and 3.
Oxide layers on Al-Si alloys do not present a regular thickness due to silicon particles that act as a physical barrier to the oxide growth.Given this scenario, experimental parameters were stablished to be pursued as a goal.They were the obtention of the most regular possible thickness layer and an average thickness value of 120 µm.

Materials and Methods
This topic addresses the anodization process development stages including materials, parameters, fixtures and equipment.

Specimen
A regular production cylinder head cast with a modified AA 380 alloy, developed by General Motors -GM (nominal chemical composition: Al-7.5 Si-3.5 Cu mass%), was manufactured using permanent mold casting.The component alloy composition and a comparison with the standard AA 380 alloy can be seen on Table 1.Prior to anodizing, the samples and ducts' surfaces were cleansed, stripped in a 10 mass % sodium hydroxide plus 3 mass % sodium fluoride solutions for 10 minutes, rinsed in demineralized water and air-dried.The EDS measured phase's mass percentage values can be seen on Table 1.

Anodic oxidation
The ducts' surfaces were anodized at an electric voltage from 20 V to 40 V (DC), with a constant current density of 4.2 A.dm -2 (until reaching 40 V) in a 184 g.L -1 H 2 SO 4 solution at 4 ºC to 6 ºC for 480 minutes.To reach the required process temperature the electrolyte was cooled on an external cooling unit and pumped directly through the cylinder head ducts.It was necessary to develop a fixture that allowed this cooled electrolyte flow to reach only the anodization targeted surfaces.The electrolyte laminar flow of 5 L. min -1 improved temperature distribution throughout the target area during the anodization process.A sketch of the fixture schematics is shown on Figure 4.After anodizing, the part was thoroughly rinsed in demineralized water at 10 °C, followed by a rest time to get back to room temperature and then submersed in hot water at 30 °C, then heated to 98 ºC and kept for 120 minutes to promote the pores sealing, followed by air drying at room temperature.
The anodization results (i.e.oxide layer porosity, thickness and regularity) are driven by the process parameters as electric voltage, current density, electrolyte temperature and process time.The development stages required several steps with parameters adjustments before reaching a set of parameters that resulted on an obtained oxide layer with a sufficient thickness average and uniformity according to the stablished values.It started with the anodization of samples in a format of blocks that were taken from a cylinder head cast with dimensions of 10x10x5 mm 3 and ended with the localized anodization of a full cylinder head exhaust ducts.The process parameters' developments are summarized on Table 2.The cathodes were prepared according to the specimens' geometry, i.e., for the block samples the cathodes were two 100x100x3 mm 3 aluminum 99% pure plates positioned on each opposite side of the sample in a standard beaker.For the cylinder head ducts, the cathodes were prepared using cylindrical bars of a 99% pure aluminum, with a nominal diameter of 15 mm and 10 mm according to the available space inside the ducts.
The anodization parameters (up to stage 9) were determined on a cast only cylinder head #1.After this, a new machined functional regular production cylinder head #2 had its four exhaust ducts anodized and was submitted to a usage test on a dynamometer at GM.This test simulates an actual everyday usage of an engine for approximately 85,000 km 15 , to assure the proper operation of the engine while testing the oxide layer resistance and durability when submitted to an actual thermal cycling of an internal combustion engine for a long usage period.
As far as the research through the literature and the patent databases could find, this developed process is unique and limits the required resources for the target surface anodization.It allows a localized anodization of an internal surface on an Al-Si cast alloy obtaining a surface protection just where it is needed 16 .

Results and Discussion
The oxide layers obtained on each stage are shown on Figures 5 to 13 and a detailed description of the incremental changes made during the anodization process parameters optimization is shown after each stage.On stage #1 the anodization parameters used were taken from Tajima 6 .The obtained oxide layer had an average thickness of 25 µm that did not achieve the target thickness and showed that silicon indeed impairs oxide growth over the entire surface.The oxide layer had cracks in almost all of its extension, see Figure 5. On stage #2 the temperature was lowered to get a more compact porous layer and process time was increased to get a thicker layer.A small increase of 15 µm to 20 µm on the thickness was observed but the cracks were still present, see Figure 6.
On stage #3 the process time was further increased to achieve a thicker layer.The electric voltage varied to maintain a constant current density that allows a more consistent layer growth.This was done until the power source limit of 40 V was achieved.A significant increase on the thickness was observed, achieving an average of 75 µm but the cracks were still present, see Figure 7. On stage #4 the process temperature was lowered and the electric voltage varied to maintain a constant current density.This was done until the power source limit of 40 V was achieved.Also, the sealing temperature was raised to 98 ºC.No significant increase on the thickness was observed and the cracks were still present, see Figure 8.
On stage #5 the process temperature was lowered to the recommended values for the "hard anodization" type, that are -2 ºC to +2 ºC.It was necessary to add an additional cooling unit to achieve 0 ºC on the electrolyte temperature during the anodization process.Another small increase on the thickness was observed and also significant reductions on the cracks extension, due to a more compact oxide layer, see Figure 9. On stage #6 the anodization of an entire duct inner surface was performed.Process parameters of stage #5 were kept, with the exception of temperature.The cooling units were not suitable to the size of the entire cylinder head.There was an increase of 4 ºC on the process temperature.The oxide layer average thickness was maintained, and the cracks extension was still an issue, see Figure 10.
On stages #7 and #8 the anodization was performed on ducts #2 and #3 and the results were similar to the first duct, see Figures 11 and 12.After stage #8 a series of considerations took place to investigate the reasons behind the cracks.It was observed that the inlay process, or resin mounting, could be the cause.This is performed on a device that applies high pressure over the sample, immersed on a phenolic resin powder.
The pressurized content was heated up to 150 ºC for 8 minutes and then cooled with water.The resin powder under high pressure and temperature molds into a solid block containing the part sample inside it.The resin bonds to the sample surfaces during the molding process and the assessment was that the resin contraction during the cooling step was pulling the oxide layer and provoking cracks The next stage samples were protected with an aluminum foil prior to the inlay process and the resin cooling time was increased.Figure 13 shows the resin, the sample with an oxide layer, the aluminum foil in the middle, and a distance between the contracted resin and the sample surface that did not present cracks.
On stage #9 the anodization was performed on duct #4.The thickness results were similar to the first three, without the occurrence of cracks.The stage #9 surface appearance after the anodization process is shown on Figure 14.The uniform oxide layer covers the entire surface of the exhaust duct.
After the anodization parameters' optimization stages, the electrolyte cooling units were optimized to achieve the required process temperature range.After that, a regular production cylinder head was disassembled, prepared and the ducts were anodized.With the anodization process conclusion (stage #10) on all four exhaust ducts, the cylinder head was assembled on a working internal combustion engine to be tested on a dynamometer that simulated an actual everyday usage of the engine for approximately 85,000 km 15 .This test was conducted to assure the oxide layer resistance and durability.The oxide layer after the durability test is shown on Figure 15.
The results show an average thickness of 120 µm and an acceptable thickness and structure uniformity that are characteristic of Al-Si alloys anodization, free of cracks and other harmful defects for the component's function.The oxide layers were intact without signs of degradation and spalling, and will provide a suitable surface protection that is the main objective.

Conclusion
This research presents the results of an internal surface protection of an internal combustion engine's cylinder head's gases' exhaust ducts.The development of a unique anodization process in which the entire part does not need to be submersed on a container was key to the feasibility of this research's proposal.This drawback was solved with the development of a lean process that performs the anodization only on the required duct's inner surfaces making it economically viable saving resources such as electrolyte volume, cooling capacity and electric power.The oxide layer obtained with this research presents an average thickness of 120 μm with an excellent adhesion to the base aluminum alloy and has shown no degradation after an 85,000 km actual usage test on an internal combustion engine.

Figure 1 .
Figure 1.Aluminum oxide layer hexagonal order structure showing Al 2 O 3 cells with both barrier layer and porous layer.Source: Castro and Sillos 13 (adapted).Reprint with permission (Castro and Sillos 13 ) copyright 2012 SurTec.

Figure 4 .
Figure 4. Anodization process fixture sketch showing a section of a cylinder head exhaust duct, tubes to pump the electrolyte through the duct and the cathode bar positioned inside the duct.Source: author

Figure 7 .Figure 8 .Figure 9 .
Figure 7. Oxide layer thickness obtained with process parameters of stage #3.Source: author Figure 8. Oxide layer thickness obtained with process parameters of stage #4.Source: author

Figure 10 .
Figure 10.Oxide layer thickness obtained with process parameters of stage #6.Source: author

Figure 14 .Figure 15 .
Figure 14.Section of the cylinder head on the exhaust duct showing the internal duct's surface after anodization.Source: author

Table 2 .
Anodization parameters used on each development stage until reaching the target thickness average value and uniformity.The used electrolyte throughout the stages was H 2 SO 4 184 g.L -1 .
E v = electric voltage; i = current density; -= not measured