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Development status of hard anodizing of duralumin?

Article source: Editor in charge of the website: Author: popularity: - Time of publication: 14:06:00, April 23, 2015

Hard anodizing is a thick film anodizing process, which is a special anodizing surface treatment process for aluminum and aluminum alloys. The results show that the maximum thickness of the anodic oxide film can reach about 250 μ m, and the micro hardness oxide film of 1500 kg / mm 2 can be obtained on pure aluminum, and 400-600 kg / mm 2 on aluminum alloy. The hardness value of the inner layer of the oxide film is larger than that of the outer layer, that is, the barrier layer is larger than that of the oxide film with pores. Because of the loose pores in the oxide film, it can absorb various lubricants and increase the friction reducing ability. The thermal conductivity of the oxide film is very poor. Its melting point is 2050 ℃, and the resistance coefficient is large. After sealing treatment (impregnated with insulating materials or paraffin), the breakdown voltage can reach 2000V, and it has high corrosion resistance in the atmosphere, It has a series of advantages, such as high wear resistance, ideal heat insulation film, good insulation, and a series of advantages such as strong combination with the base metal. Therefore, it has been widely used in national defense industry and mechanical parts manufacturing industry. It is mainly used in aluminum and aluminum alloy parts with high wear resistance, heat resistance and good insulation performance. Such as the inner wall of cylinder, piston, steam plug, cylinder, bearing, floor of aircraft cargo compartment, rolling bar and guide rail, water conservancy equipment, steam impeller, leveling machine, gear and cushion. Compared with the hard chromium plating process, the hard oxidation process has the advantages of low cost, strong film bonding, convenient treatment of plating solution and cleaning waste water. However, the disadvantage of this process is that when the thickness of the coating is large, the mechanical fatigue strength index of aluminum and aluminum alloy will be affected.

There are many methods of hard anodic oxidation electrolysis, such as sulfuric acid, oxalic acid, propylene glycol, sulfosalicylic acid and other inorganic salts and organic acids. The power supply can be divided into DC, AC and AC / DC superimposed power supply, etc. at present, the following two kinds of hard anodizing are widely used.

(1) Sulfuric acid hard anodizing direct current method;

(2) Oxalic acid hard anodizing AC / DC gravity separation method.

Among them, sulfuric acid method is a kind of hard oxidation method which is widely used at present.

Principle of hard anodizing

The principle of hard anodizing of aluminum alloy is to accelerate the formation of oxide film on the surface of aluminum alloy under the action of electric field, that is, lead plate is used as cathode, aluminum alloy is used as anode, dilute sulfuric acid solution is used as electrolyte

4H2+4e=2H2↑

Oh - moves towards the anode and reacts with the anode

4OH--4e=2H2O+2O↑

When the excess electrons are lost on the anode, the precipitated oxygen is in atomic state. Because the atomic oxygen is more active than the molecular oxygen, it is easier to react with aluminum

2A1+3O→A12O3

The above reaction is carried out uniformly and simultaneously on the surface of aluminum and aluminum alloy parts.

With the increase of electrifying time, the current increases and the oxide film thickens. At the same time, due to the duality of the chemical properties of (Al2O3), it is an alkaline oxide in acidic solution and an acidic oxide in alkaline solution. There is no doubt that the oxide film solution dissolves in sulfuric acid solution. Only when the formation rate of the oxide film is greater than its dissolution rate can the oxide film thicken. When the dissolution rate is equal to the formation rate, the oxide film will not thicken. When the oxidation rate is too high than the dissolution rate, the oxide film with powder is easy to form on the surface of aluminum and aluminum alloy parts.

When the electrolyte is hard anodized, it is electrolyzed at the temperature of - 10 ℃ ~ + 5 ℃. The oxidation film formed by hard anodizing has high resistance, which will directly affect the oxidation effect of current intensity. In order to obtain thicker oxide film, it is necessary to increase the external voltage. The purpose is to eliminate the influence of high resistance and keep the current density at a certain level. However, when the current is large, there will be intense heating phenomenon. In addition, a lot of heat will be released when the oxide film is generated, which makes the electrolyte temperature around the parts rise sharply. The temperature rise will accelerate the dissolution of the oxide film and make the oxide film unable to dissolve Thickening. In addition, the heating phenomenon is the most serious at the contact between the film and the metal. If it is not solved in time, the local surface of the processed parts will be burned out due to the temperature rise.

The solution is the combination of cooling equipment and stirring. The purpose of stirring is to make the electrolyte temperature uniform in the whole tank, so as to obtain high quality hard oxide film.

2. Process requirements for hard anodizing

In order to obtain good quality hard anodic oxide film and ensure the required size of parts, the following requirements must be followed.

2.1 acute corner rounding

The processed parts are not allowed to have sharp corners, burrs and other sharp edges and corners. Because of hard oxidation, the anodizing time is very long, and the oxidation process (a1 + O2 → A12O3 + Q) itself is an exothermic reaction. Moreover, because the corners of the general parts are often the parts where the current is relatively concentrated, these parts are most likely to cause local overheating of parts and burn parts. Therefore, all edges and corners of aluminum and aluminum alloy should be chamfered, and the radius of chamfer y circle should not be less than 0.5mm.

2.2 surface finish

After hard anodizing, the surface finish of the parts is changed. For the rough surface, it can be smoother than the original. For the parts with higher original finish, the brightness of the surface is reduced after this treatment, and the reduction range is about 1-2 levels.

2.3 allowance of part size

Due to the high thickness of hard oxide film, if the aluminum parts need to be further processed or assembled in the future, a certain machining allowance shall be reserved in advance and the clamping position shall be specified.

Because the size of parts should be changed during hard anodizing, the possible thickness and dimensional tolerance of oxide film should be predicted in advance during machining, and then the actual size of parts before anodizing should be determined, so as to meet the specified tolerance range after treatment.

Generally speaking, the increased size of parts is about half of the thickness of oxide film.

2.4 special fixture

Due to the hard anodized parts in the oxidation process, to withstand a high voltage and high current, must make the fixture and parts can maintain a very good contact, otherwise, due to poor contact will cause breakdown or burn the contact parts of parts. Therefore, it is required to design and manufacture special fixture for parts with different shapes and specific requirements after oxidation.

2.5 local protection

If there are both common anodizing and hard anodizing on the same part, the specific process shall be arranged according to the smoothness and precision of the parts. Generally, ordinary anodizing is carried out first. In hard anodizing, the surfaces that do not need hard anodizing are insulated. The insulation method is to use spray gun or brush. Nitro glue or ethylene peroxide adhesive prepared is applied to the surface that does not need to be treated. The insulating layer should be thin and uniform. Each coating should be dried at low temperature for 30-60 minutes, and a total of 2-4 layers can be applied Preparation of insulating adhesive:

(1) 5 parts of nitro solution (q98-1) and 1 part of red nitro liquid (q04-3) (in which a small amount of methyl red can be used to replace the red nitro solution), dilute the solution to industrial viscosity with X-1 diluent, and brush coating for 60-80 s and spraying for 20-30 s (with No. 4 viscosity agent).

(2) 100 g perchloroethylene glue (g98-1), 15-20g red perchloroethylene anticorrosive solution (g52-1), diluent adjusted to appropriate viscosity with x-3, generally brush coating or dip coating.

(3) Polyvinyl alcohol 100g, banana water 500cc. Preparation method: dissolve polyvinyl alcohol into banana water, form glue liquid, brush on the parts that need insulation, cure at room temperature, about 4 hours, such as drying at 60 ~ 70 ℃ for 30 minutes.

3. Hard oxidation process

Chemical degreasing (or ultrasonic degreasing) cleaning and cleaning of aluminum parts hard anodizing cleaning hot water or steam sealing products

4. Electrolyte formula and operation specification for sulfuric acid hard anodizing (see Table 1)

4.1 process formula

Table 1

name
Formula 1
Formula 2
Formula 3
 
Sulfuric acid d = 1.84 / (g / L)
200~300
15%
10%
 
Temperature / ℃
0.5~5
20~25
25~100
 
Cell terminal voltage / V
40~90
25~60L
From 40 to 120 L
 
——
-8~10
zero
-5~5
 
Time / h
2~2.5
two
two
 
stir
Air agitation is required
Air mixing is required
Air mixing is required
 

 

4.2 operation method

(1) First, turn on the cooling equipment, reduce the electrolyte temperature to the temperature range specified by the process, hang the lead plate on the cathode, and then place the mounted parts on the anode conducting rod to clamp tightly, and keep a large distance between the parts and the parts, between the parts and the cathode, and absolutely not contact. Turn on the electrolyte stirring of compressed air (Note: the compressed air must be degreased).

(2) The current is increased gradually from 2.5 to 2.5 DMA. After that, the current density was kept at 2.5 A / DM2, and the current was investigated every 5 minutes. The starting voltage of the current was 8-12v. The final voltage could be determined according to the thickness and material of the film.

(3) In the process of hard anodizing, the voltage and ammeter must be paid attention to frequently. If the phenomenon of sudden increase of current and drop of voltage is found, it indicates that the film layer of the part has been partially dissolved. The power supply should be turned off immediately, and the dissolved parts should be checked and taken out. Other parts can continue to be oxidized, and the current can be sufficient at one time.

4.3 effects of various factors on hardness and growth rate of oxide film

The formation of high-quality hard oxide film on aluminum and aluminum alloy mainly depends on the composition concentration of electrolyte, temperature, current density and the composition of raw materials.

4.3.1 concentration of electrolyte

When sulfuric acid electrolyte is used for hard anodizing, it is generally in the concentration range of 10% - 30%. When the concentration is low, the hardness of oxide film is high, especially for pure aluminum, except for aluminum alloy with high copper content (cy12). Because the aluminum alloy with high copper content is easy to form CuAl2 compound, which dissolves quickly in oxidation and is easy to burn aluminum parts. Therefore, it is generally not suitable to use low concentration sulfuric acid electrolyte, so oxidation treatment or AC / DC superposition treatment must be carried out in high concentration (H2SO4 in 300-400g / L).

4.3.2 effect of temperature on film

The temperature of electrolyte has a great influence on the wear resistance of oxide film. Generally speaking, if the temperature drops, the wear resistance of anodic oxide film of aluminum and aluminum alloy will increase. This is due to the decrease of the dissolution rate of electrolyte for the film. In order to obtain the oxide film with higher hardness. We should master the temperature within ± 2 ℃ for hard anodizing treatment.

 

Aluminum parts gasoline washing, hanging and coating insulating glue, washing out light cleaning, room temperature hard anodizing cleaning, drying, insulating rubber unloading and hanging device sealing finished product

The results show that the quality of the hard oxide film obtained by this process is good; the appearance of the film should be uniform dark black, blue black or brown; the thickness of the film is about 50 μ m; the hardness: hardness: HV > 300

4.4 the process has the following characteristics

(1) This process has a wide range of specifications, including wide range of bath solution concentration, working temperature from 0 to 30 ℃ (wide), allowable current density from 5 to 15 A / DM2, oxidation of 30-90 min. the obtained microhardness can reach hv300-500 and the coating is 50 μ M.

(2) This process is especially suitable for aluminum alloy with copper content less than 5%.

(3) It is suitable for oxidation of inner surface of deep blind hole, and uniform oxide film can be obtained.

(4) Although the price of malic acid is higher than that of sulfuric acid, there is no need for refrigeration and cooling equipment, and the production cost is relatively low.

Sulfurized hard oxidation of 5 aluminum alloy at room temperature

5.1 composition and process specification of sulfo room temperature hard oxidation solution for aluminum alloy (see Table 2)

Table 2

name
Formula 1
Formula 2
 
H2SO4 (g / L)
10~15
15~20
 
Sulfonated anthracene C14H10 (ml / L)
3.5~5
7~9
 
Citric acid (g / L)
——
40~50
 
Lactic acid (g / L)
30~40
30~40
 
Boric acid (g / L)
35~40
——
 
Temperature / ℃
18~30
5~35
 
Anode current density / a / DM2
10~20
15~20
 
Time / min
80~100
ninety
 

Formula 1 is cheaper and cheaper than formula 2 because boric acid is cheaper than citric acid. At the same temperature (30-32 ℃), the quality of oxide film of formula 2 is better than that of Formula 1. No.1 formula contains boric acid, because the solubility of boric acid is small, the working temperature should not be less than 15 ℃, otherwise a large amount of precipitation will be produced.

5.2 preparation and purification of sulfonated anthracene

5.2.1 purification of crude anthracene

Because the crude anthracene contains a certain amount of coal tar and water, they and the crude anthracene penetrate each other to form block and granular materials instead of powder, which must be removed before sulfonation. Otherwise, the quality of oxide film will be affected. In order to ensure the quality of sulfonated anthracene, the crude anthracene must be cleaned before sulfonation

(1) The mixture of kerosene and anthracene was put into a distillation flask, covered with a cork, plugged in a condensate tube, and refluxed in a glycerin bath for 30 minutes;

(2) After distillation, the supernatant is poured into a beaker and cooled to room temperature, and coarse anthracene crystals precipitate at the bottom of the flask;

(3) Filter, pour the filtrate into the original distillation flask, and continue to reflux for 30 minutes;

(4) Pour the distilled solution into a beaker, cool it to room temperature, let the crude anthracene crystallize and precipitate, and repeatedly install 3), 4) process treatment; (5) pour the crude anthracene which is cleaned and filtered for the first time into the distillation bottle, and then repeat the treatment in each process;

(6) The coarse anthracene from the last cleaning and filtering is put into the porcelain bottle to dry.

2.2.5 sulfonation process

(1) Weigh the 1:1 concentrated sulfuric acid and purified crude anthracene respectively, and then put the concentrated sulfuric acid into the beaker, stir with glass rod, add the crude anthracene to the concentrated sulfuric acid, and continue to stir until there is no particle uniform paste.

(2) When heated at 50-80 ℃, the mixture is stirred continuously until the smoke is released. When heated to 120-150 ℃, it is mixed into liquid, and small bubbles appear. After cooling, it will form a hard dark black block.

(3) Add the required amount of deionized water, heat and dissolve into aqueous solution (heating temperature 70-80 ℃).

5.2.3 the composition of oxidation solution and the influence of operation conditions

(1) With the increase of the concentration of sulfonated anthracene, the working voltage and the hardness of the oxide film increase, but the thickness of the oxide film decreases with the increase of the concentration of sulfonated anthracene.

(2) On the other hand, with the increase of boric acid and boric acid concentration, the thickness of boric acid oxide film decreases The quality of oxide film.

However, the solubility of boric acid increases with the increase of temperature. When the temperature is 15 ℃, it can only dissolve about 3%. But in the tank, the boric acid content is more than 4% ~ 5%, which is beneficial to the formation of oxide film, so the bath liquid temperature must not be lower than 18 ℃.

(3) Effect of lactic acid: the organic weak acid with hydroxyl group (30g / L, pH = 2.32) has less corrosion on the oxide film. In addition, due to the existence of hydroxyl group, the electrolyte shows strong reducibility, which can improve the working conditions and prevent the temperature rise of electrolyte in the oxidation process. The influence of lactic acid is also with the increase of concentration, the voltage decreases, and the hardness of oxide film slightly decreases.

(4) The influence of citric acid: citric acid is a ternary organic acid, which is divided into three stages of ionization. The k values of ionization constant are 8.4 × 10-4, 1.8 × 10-5, 4 × 10-6. The degree of ionization is small, belonging to weak acid, and the pH value is larger. When the citric acid is used alone, the film is compact and porous soft under 50 ℃ due to the characteristics of citric acid. In this process, due to the existence of free sulfuric acid and sulfonated anthracene, the hard anodizing can be carried out smoothly below 35 ℃.

(5) Influence of current density: voltage, hardness and thickness of oxide film increase with the increase of current density.

(6) The effect of temperature: the temperature should be strictly controlled within the process range. When the temperature is low, the hardness of the oxide film is higher; when the temperature increases, the thickness gradually increases, and the hardness gradually decreases.

(7) Time effect: the longer the oxidation time, the higher the hardness of the film formed by different aluminum specifications, which is better for cast aluminum and forged aluminum, but it is unfavorable for hard aluminum and aluminum copper alloy.

Design and equipment conditions of hanger for hard anodizing

6.1 hard anodizing hanger

The fixture and fixture of hard anodizing should have enough mechanical strength and rigidity to prevent the parts from being washed down by the torrent solution while stirring the electrolyte. In addition, the hanger should have good contact conductivity, light weight, strong and durable, easy to load and unload products, and appropriate requirements for loading capacity and parts layout. There are two common types of hard anodizing hanger: one is the clamp with compression screw, the other is the clamp connected with bolt. All contacts with the parts are made of aluminum, aluminum magnesium alloy and aluminum silicon magnesium alloy. In addition to the conductive requirements of the contact parts of the products, other parts should be insulated with the hanger to make them non-conductor. In this way, the anodizing process can be concentrated on the parts, improving the production efficiency and saving the metal materials and power consumption of the hanger.

6.2 heating of hard oxidation solution and re dissolution of oxide film

In the process of oxidation, the working surface passes a large current, because the oxide film has a large resistance, most of the heat is concentrated on the part of the oxide film. The calorific value can be calculated by the following formula:

Joule heat Q1 = 0.864 × voltage × current (kcal / h)

Calorific value of oxidation liquid Q2 = 2.334 × current (kcal / h)

Reaction heat of anodic oxidation 2Al + 3 [O] → Al2O3 + 375800 cal

Total calorific value Q = (Q1 + Q2) × 1.1 (kcal / h)

According to the above formula, the refrigeration equipment for cooling can be designed. The heat value of hard oxidation must be quickly exchanged. If it is not cooled in time, the formation of oxide film is not only chemical dissolution, but also electrochemical dissolution due to the addition of electric field. In this way, the surface finish of the film is seriously affected and the thickness is reduced. Therefore, it is necessary to have forced cooling measures to keep the electrolyte at low temperature in order to obtain the oxide film with higher hardness.

6.3 hard anodizing electrical equipment

The sulfuric acid dural DC anodizing process only needs DC generator or rectifier, among which the rectifier is more efficient and requires constant current control and automatic device for voltage rise during film growth.

7 post closure treatment

The sealing of hard anodizing should be carried out in hot ion water or 5% dichromate solution for 15-30 minutes at room temperature, or with anhydrous oil or wax at 80 ℃ for 15-30 minutes.

8 hardness oxidation quality inspection

8.1 appearance

Due to the different aluminum materials and processes, the color of the oxide film is different. The film is brown, dark brown, gray to black. The lower the electrolyte temperature, the thicker the oxide film. It is not allowed to have burnt or easily agitated loose film, and the phenomenon of bright spots of oxidation corrosion and partial film falling off at the edge corner due to local heating are not allowed. Except for the influence of the fixture, the local surface of the whole part surface shall not have the place without oxide film, and small cracks are allowed in the oxide film of the whole part of the aluminum clad plate.

8.2 oxide film thickness test

The thickness of oxide film can also be measured directly by eddy current thickness gauge.

8.3 hardness determination

Microhardness can be measured by microhardness tester in the transverse direction, and should not be less than 300kg / mm2.

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