What is hard oxidation of aluminum alloy and what is its function？

Hard anodizing of aluminum profile is a kind of thick film anodizing method, which is a special anodizing surface treatment process. The maximum thickness of the anodic oxide film prepared by this process can reach about 250 microns. The microhardness oxide film of 1500 kg / mm2 can be obtained on pure aluminum, while the microhardness oxide film of 400 ~ 600 kg / mm2 can be obtained on aluminum alloy. The hardness value of the inner layer of the oxide film is greater than that of the outer layer, that is, the barrier layer is greater than that of the oxide film with pores. Because there are loose pores in the oxide film, it can adsorb various lubricants and increase the antifriction capacity. The thermal conductivity of the oxide film is very poor. Its melting point is 2050 ℃, the resistance coefficient is large, and the breakdown voltage after sealing treatment (impregnating insulation or paraffin) can reach 2000V, which has high corrosion resistance in the atmosphere, It has high wear resistance, is also an ideal thermal insulation film, has good insulation, and has a series of advantages such as strong combination with base metal. Therefore, it has been widely used in national defense industry and mechanical parts manufacturing industry.

Specifically, hard anodizing is mainly used in aluminum alloy parts with high wear resistance, heat resistance and good insulation. Such as all kinds of inner wall of cylinder, piston, steam plug, cylinder, bearing, floor of aircraft cargo compartment, roller and guide rail, hydraulic equipment, steam impeller, leveling machine, gear and buffer pad. So, the question is, what kind of oxide film is a hard oxide film?

The characteristics and detection methods of hard anodic oxide film can be summarized as follows:

1. The qualified oxide film is uniform dark black, blue black or brown.

2. The thickness of oxide film is about 50 μ m。

3. Hardness of oxide film HV ≥ 300.

4. If the oxide film is uniform gray black, it is still qualified as long as the thickness and hardness meet the design requirements.

5. Although the oxide film is uniform dark black, blue black or brown, and the thickness and hardness meet the design requirements, it is unqualified if there are many white or gray white points.

6. Although the oxide film is uniform dark black, blue black or brown, there are regular uneven phenomena, which is the fault of the base metal. It is also qualified as long as the thickness and hardness meet the design requirements.

7. It is not allowed to have loose film that is easy to be wiped off due to scorching, bright spots that are corroded by local heating, and film layers falling off at edges and corners on the oxide film. Except for the clamp seal, there shall be no place without oxide film on the local surface of the whole part. Small cracks on the oxide film of aluminum clad sheet metal parts are allowed.

8. Thickness measurement of oxide film. Cut the transverse test piece from the part or test piece and measure the thickness of the oxide film under the metallographic microscope, or directly measure the thickness of the oxide film with the eddy current thickness gauge.

9. Hardness measurement of oxide film: the hardness (microhardness) of oxide film can be measured on the transverse test piece with microhardness value, which shall not be less than 300kgf / mm2, and lyl2 alloy shall not be less than 250kgf / mm2.

2、 If the oxide film is not uniform and complete due to the composition of the alloy in the hard anodizing process of aluminum alloy, how to deal with it?

Hard anodizing is difficult for aluminum alloys such as aluminum copper, aluminum silicon and aluminum manganese; When the copper content in the alloy exceeds 5% or the silicon content exceeds 7.5%, DC hard anodizing is not suitable.

Treatment method: when AC / DC superposition method is adopted, the content range can be relaxed.

3、 What effect will improper current density of hard anodizing have on film hardness? How?

influence:

1. When the current density exceeds 8A / DM2, the hardness decreases due to the influence of calorific value;

2. The current density is too low and the voltage rises slowly. Although the calorific value decreases, the chemical dissolution time of the film by sulfuric acid is long, so the hardness is low.

Treatment method: the relationship between current density and film hardness is complex. In order to obtain the film with ideal hardness, it is necessary to select the appropriate current density according to different materials, usually 2 ~ 5A / DM2.

4、 How to deal with low hardness when the temperature of hard anodizing solution is low?

When pure aluminum is hard anodized, its hardness decreases when the temperature is close to 0 ℃.

Treatment method:

1. In order to obtain high hardness oxide film, sheet metal parts with aluminum coating shall be hard anodized at 6 ~ 11 ℃;

2. Generally speaking, if the temperature decreases, the wear resistance increases, which is due to the decrease of the dissolution rate of electrolyte to the membrane.

5、 After adjusting the total concentration of hard anodizing bath solution once, the total equivalent concentration increases, and how to deal with it if there is no black hard film?

Reason: malic acid may be decomposed into low molecular organic acids (such as acetic acid).

Treatment method: add an appropriate amount of water glass dilution solution.