Anodization of die casting alloys: a challenge in the automotive sector

In the industrial sector, particularly in the automotive sector, the use of die casting processes is constantly growing. This increase is linked to the possibility of producing, currently, components in large volumes and dimensions with a high production rate. The secondary alloys of the Al-Si-Cu system are among those most used in the die casting foundry. To increase their surface properties, in terms of resistance to corrosion and wear, an anodic treatment for automotive alloys can be used. However, this process presents difficulties when applied to alloys with a high concentration of elements, such as die casting alloys, or in components with surface imperfections. In this work, the main advantages and critical points of the anodization process of die casting alloys will be presented. In particular, the results of the research conducted on anodization treatments of AlSi9Cu3(Fe) die casting alloys by varying the Fe and Mn content will be illustrated.

Aluminum in the production of automotive components: an overview

From an industrial point of view, aluminum has been, in recent years, the material at the center of general interest; one of the sectors that has elected aluminum as the material of the present and the future is the automotive sector. This interest is linked to the need to reduce the weight of the components, increase the safety and performance of the vehicle and, at the same time, pay attention to fuel consumption and emissions.

Commercially pure aluminum is produced through an electrolytic process. The weak point of aluminum production is energy consumption; for each ton of so-called primary aluminum, an electrical power of about 17,000 kWh is required and in 2020, one billion tons of CO2 equivalent was exceeded, equal to about 2% of total emissions for the year. For this reason, in recent years there has been an attempt to focus more on the production of secondary alloys, that is, those deriving from the recycling of production waste or end-of-life components. Aluminum recycling is extremely advantageous from an energy point of view since the energy requirement required for the production of a secondary alloy is 10-15 times lower than that required for the production of primary alloys. To produce 1 kg of recycled aluminum, an average of 9.2 MJ is needed compared to 144.6 MJ needed for the same amount of primary aluminum.

There are various methods for processing aluminum alloys but, currently, one of the most used is high pressure die casting. The process consists of injecting molten metal into a mold at high speeds. The advantages, however, do not end with the processing speed since die casting allows for near-net shape components with an excellent surface finish and optimal dimensional control. All this fits into the concept of savings and sustainability since the post-processing phases are significantly reduced.

A specific sector such as the automotive one requires targeted properties and performance. To achieve this goal, using aluminum, the anodization process is used since it allows for obtaining a layer of high hardness that is also capable of providing considerable resistance to wear and corrosion. The oxide formed is an ordered layer of hexagonal cells closed at the base, each of which has a central pore that extends from the bottom to the surface of the cell itself. Observing the section of the layer, two distinct areas can be seen: the protective layer at the base and a porous layer that constitutes the walls of the cell.

Among aluminum alloys, the suitability for anodic treatment for automotive alloys is variable; in particular, secondary die casting alloys are among those that present more critical issues due to the high presence of intermetallic compounds and surface defects that act as antagonists to the optimal development of the oxide layer.

The need of car manufacturers to have access to high-strength, low-weight aluminum components together with the growing use of secondary alloys that comply with sustainability criteria, have pushed several research groups to focus on the optimization of the anodizing treatment in order to allow obtaining satisfactory results even when applied to die casting components. This paper aims to illustrate the strengths and critical issues present in the anodizing process of die casting alloys for the production of aluminum automotive components.

Critical factors in the anodic treatment of automotive alloys

There are many parameters that influence the response of an alloy to the anodizing treatment under the same process conditions. Those that show the most significant impact are: the chemical composition of the alloy, especially from the point of view of the presence of impurities, the size and morphology of chemical compounds, mechanical processing. The presence of alloy elements in solid solution generally does not modify the result of the anodizing treatment, while precipitates and intermetallics present in the microstructure of the material can affect the integrity of the protective layer that has developed.

Anodizing is an electrochemical process that involves the transfer of electrons between the anode and cathode, operated by passing an electric current in the circuit. This implies that, if an electrochemical inhomogeneity develops in the material, the result will be the development of an advancing front of the oxide layer according to preferential directions, corresponding to the areas in which the lowest electrical resistance is present.

One of the main elements present in the alloy that are classified as undesirable is iron, since it makes the oxide layer less resistant to corrosion and hard; it also compromises its uniformity. The most deleterious microstructural phase is the acicular phase β-Al5FeSi which acts as a stress concentration zone and as a preferential zone for the formation and propagation of cracks. In the literature it is common to find studies relating to the influence of iron and how its effect on the properties of the material varies with the size and morphology of the intermetallics rich in this element. In the case of secondary aluminum alloys, the presence of iron is inevitable since the scrap has a significant content of impurities, especially if obtained from products that have reached the end of their life; in particular, in castings produced by die casting the presence of iron is necessary because it counteracts the phenomenon of metallization and, consequently, increases the life of the molds.

Another critical element for the development of a compact oxide layer is copper. The latter, when incorporated into the anodized thickness, leads to the development of internal cracks and porosity; furthermore, as the copper content increases, a progressive decrease in the hardness and wear resistance of the oxide layer formed is noted. Silicon is inserted into secondary alloys to increase the castability of the alloy itself and reduce its volumetric shrinkage during the solidification phase. However, silicon induces the formation of an Al-Si eutectic structure that limits the obtainable oxide thickness and favors the onset of cracks, porosity and non-anodized areas due to its slower oxidation in relation to the oxidation speed of aluminum.

In addition to the chemical composition of the alloy, the mechanical processing of the component plays an important role. The anodic treatment for automotive alloys of a raw component rather than sandblasted or mechanically worked generates different responses. In particular, the most evident effect is the increase in thickness obtainable by removing the cortical layer in which segregative phenomena of element enrichment are evident. Mechanical machining by chip removal, above all, allows a greater quantity of α-aluminum matrix to be exposed to the interface, on which the treatment takes place; this substrate is the most suitable for the purposes of developing a compact, hard and highly corrosion-resistant oxide layer.

Case study: anodization of die casting alloys for automotive components

One of the secondary alloys that is gaining the greatest consensus in the automotive industry is the AlSi9Cu3(Fe) alloy designated as EN AC-46000 according to the EN 1706:2020 standard. Its wide use is linked to its excellent fluidity combined with good mechanical properties.

Given the high presence of polluting elements, it is necessary to modify the alloy with more or less targeted additions of alloying elements, among all Mn. This last element, in fact, is able to improve the mechanical properties both at room temperature and at high temperatures; furthermore, it can mitigate the deleterious effects induced by the presence of iron in the alloy by reducing the size of the β-Al5FeSi phase or replacing it with phases that are less harmful to the performance of the alloy.

In this research project, working on a die-casting alloy of the AlSi9Cu3(Fe) series, systematic variations of the iron and manganese content were carried out on two levels, obtaining four different experimental compositions. To verify the influence of mechanical processing, half of the plates produced with the different alloys obtained, were analyzed after having removed, by milling, 1.5±0.1 mm from the cortical area. The anodizing treatment was performed using an industrial plant.

In order to evaluate the differences between the different formulations and between the oxide layers obtained from the different substrates, both a microstructural and a mechanical characterization were performed. Specifically, the microstructure of the substrate in the die-cast state and in the machined layer was analyzed; the thickness of the oxide obtained was analyzed and this was characterized mechanically through micro-hardness tests, wear tests and scratch tests.

From the microstructural analyses it emerges that varying the iron and manganese content corresponds to a variation in the morphology of the iron-rich compounds. In particular, if only the iron content is increased, an increase in the presence of the acicular β-Al5FeSi phase is observed; while the increase in manganese favors the development of compact iron intermetallics. Where the iron and manganese concentrations are maximum, the absence of the β phase is observed, demonstrating how manganese acts in the microstructural changes of the alloy. It is also evident that there is no evident effect of the variation of the Fe and Mn content on the thickness of the anodic layer; on the contrary, the effect is significant when anodizing a machined component rather than a raw one.

The Vickers micro-hardness tests were conducted along the cross-section of the oxide layer obtained from a machined substrate; the applied loads and the holding time are in accordance with the indications in the ASTM E384-17 standard. From the analysis of the results it is clear that an increase in the iron content in the alloy allows to obtain a harder oxide layer compared to the starting alloy, especially when accompanied by an increase in the manganese content. The wear tests were performed in ball-on-disc configuration, on circular specimens, using an alumina sphere as an antagonist body. From the wear track it was possible to trace the volume of removed material and calculate the wear rate values ​​using the formula:

w = V / (L * N)

where V is the removed volume, L is the sliding distance traveled and N is the load applied during the test. Lower wear rate values ​​correspond to higher wear resistance.

It is possible to note how the wear rate value decreases as the content of the alloy elements increases; in particular, manganese seems to be able to contain the deleterious effects of iron intermetallics. The alloy with the Fe1.3%-Mn0.55% formulation, in which the wear resistance is maximum, presented the highest hardness value, confirming the fact that wear, although dependent on many factors, is strictly linked to the hardness of the tested material. Finally, from the scratch resistance tests, no significant differences emerge in the critical load values ​​obtained; the interesting data is the greater resistance of the anodized specimens starting from the die-cast substrate. This result is probably linked to the surface roughness values ​​which, after anodization, are lower on the specimens treated in the die-cast state.

Perspectives for the optimization of anodization in die-casting alloys

From the research and analyses carried out in this work, it is clear that die-casting aluminum alloys can be subjected to post-casting anodization treatments with good results. Specifically, it can be concluded that:

• By increasing the manganese content in the alloy, it is possible to balance the negative effects related to the presence of iron;
• Mechanical machining by chip removal allows to obtain greater anodic oxide thicknesses;
• An increase in the iron content allows the development of a harder and more wear-resistant anodic layer, especially when balanced by a high manganese content.

To further optimize the anodizing process of die casting alloys for the production of aluminum automotive components, it is necessary to carry out additional tests useful for evaluating the influence of the variation of chemistry on corrosion resistance. Furthermore, it is necessary to evaluate the influence of parameters related to the anodic oxidation process such as: the electrolyte used, the residence times in the bath and the current density.

Source: E. Giansante, G. Scampone and G. Timelli for In Fonderia – Il magazine dell’industria fusoria italiana