What is Anodizing
Anodizing is an electrochemical method. In an appropriate electrolyte, alloy parts are used as anodes, stainless steel, chromium, or conductive electrolyte itself is used as cathodes, and the anodes are oxidized under certain voltage and current conditions. Thus, the process of obtaining anodized film on the surface of the workpiece requires sulfuric acid anodization during the anodizing and coloring process.
Limitations of sulfuric acid anodizing on aluminum alloy materials
1. The presence of alloying elements will reduce the quality of the oxide film. Under the same conditions, the oxide film obtained on pure aluminum is the thickest, the highest hardness, the best corrosion resistance, and the best uniformity. For aluminum alloy die-casting materials, to obtain a good oxidation effect, the content of aluminum must be ensured. Under normal circumstances, it is better to not less than 95%.
2. In the alloy, copper will make the oxide film reddish, destroy the electrolyte quality, and increase oxidation defects. Silicon will make the oxide film gray, especially when the content exceeds 4.5%, the effect is more obvious. Due to its characteristics, iron will exist in the form of black spots after anodizing.
Die cast aluminum alloy
Cast aluminum alloys and die castings generally contain high silicon content, and the anodic oxide film is dark, and it is impossible to obtain a colorless and transparent oxide film. As the silicon content increases, the color of the anodized film ranges from light gray to dark gray to dark gray. Therefore, cast aluminum alloy is not suitable for anodizing.
Commonly used die-cast aluminum alloys can be divided into three main categories:
- Aluminum-silicon alloy, which mainly includes YL102 (ADC1, A413.0, etc.), YL104 (ADC3, A360);
- Aluminum silicon copper alloy, mainly including YL112 (A380, ADC10), YL113 (A383, ADC12), YL117 (B390, ADC14);
- Aluminum-magnesium alloy, mainly including 302 (5180, ADC5, ADC6).
Aluminum silicon alloy, aluminum silicon copper alloy
For aluminum-silicon alloys and aluminum-silicon-copper alloys, as the name suggests, in addition to aluminum, silicon and copper are the main components. Under normal circumstances, the silicon content is between 6-12%, which mainly plays a role in improving the fluidity of the alloy liquid. The copper content is second only, mainly to enhance the strength and tensile force. The iron content is usually between 0.7-1.2%. Within this proportion, the demolding effect of the workpiece is the best. It can be seen from its composition that this type of alloy is impossible to oxidize and color, and it is difficult to achieve the desired effect even if desiliconization is used. For aluminum-silicon alloys or aluminum alloys with high copper content, the oxide film is more difficult to form, and the resulting film is dark, gray, and poor in gloss.
The oxide film of aluminum-magnesium alloy is easy to form, and the quality of the film is better. It can be oxidized and colored. This is an important feature that distinguishes it from other alloys. But in comparison, there are some shortcomings.
1. The anodic oxide film has duality, with large pores and uneven distribution, which makes it difficult to achieve the best anti-corrosion effect.
2. Magnesium tends to produce hardening and brittleness, reduce elongation, and increase thermal cracking, such as ADC5, ADC6, etc. In die casting process, due to its wide solidification range and large shrinkage tendency, shrinkage porosity and cracks are often generated, and the casting performance is extremely poor. Therefore, there are greater limitations in the scope of its use, and workpieces with a slightly complex structure are not suitable for production at all.
3. The commonly used aluminum-magnesium alloys on the market have complex composition and low aluminum purity. When sulfuric acid anodizes, it is difficult to produce a transparent protective film. Most of them are milky white and have poor coloring. It is difficult to achieve the desired effect according to the normal process.
In summary, it can be seen that common die-cast aluminum alloys are not suitable for sulfuric acid anodization. However, not all die-cast aluminum alloys can not achieve the purpose of oxidation and coloring, such as aluminum-manganese-cobalt alloy DM32, aluminum-manganese-magnesium alloy DM6, etc., which have excellent die-casting performance and oxidation performance.