Die-casting aluminum alloy gun black plating process

With the rapid development of science and technology, the application scope of aluminum alloys has been expanding. Currently, it has been widely used in aircraft, automobiles, motorcycles, instrumentation and film machinery industries. The aluminum alloy not only has excellent strength and rigidity, but also the die-casting of parts with complex geometric shapes can be formed at one time, realizing no cutting processing, simple process, and high production efficiency. We use aluminum alloy die-casting to first plate bright nickel and then plate a layer of gun black plating technology, which not only saves costs, but also obtains a highly decorative surface coating with more features and improves the competitiveness of products in the international market. Aluminum alloy electroplating and general electroplating process have certain differences, because aluminum alloy is a more active metal, reduction, replacement ability, give the electroplating process to bring a lot of difficulties, generally use zincation method for pretreatment. In recent years, electroplating science and technology workers at home and abroad have developed many new electroplating processes for aluminum alloys. On this basis, we have developed a new type of aluminum alloy pretreatment liquid—H·S·F liquid, and aluminum alloy electroplating process For simplicity, the bond strength of the coating is greatly improved, so as to ensure the black plating quality of the aluminum alloy die casting gun.

2 Aluminum Alloy Die Casting Pretreatment

Aluminum alloy die castings contain high silicon content, and often have small pores and gaps on the surface. In order to obtain a high decorative appearance, mechanical polishing is required. Due to the low hardness of the aluminum alloy, the mechanical polishing wheel should be soft and have a certain degree of elasticity so as to avoid distortion of the corners of the parts during mechanical polishing. The pretreatment mainly includes organic solvent degreasing, alkali etching, acid etching, and immersing in H.S.F liquid.

2.1 Degreasing organic solvents

It is generally degreasing with organic solvents such as gasoline and trichloroethylene to dissolve minerals and polishing pastes. It can also be scrubbed with detergent solutions.

2.2 Alkaline erosion

In order to remove the light grease and Al2O3 film on the surface of the part, corrosion is performed in a weak lye to expose the aluminum alloy substrate and produce microscopic roughness. However, the alkalinity of the solution should not be too strong. At the same time, the temperature and alkali etching time of the alkali etching solution must be strictly controlled so as to avoid excessive corrosion. The alkaline etching process conditions are as follows:

Na2CO3 30g/L

Na3PO4 30g/L

Additive 2～4g/L

OP-10 emulsifier 0.5 ~ 1mL/L

Temperature 75～85°C

Time 30～60s

2.3 Etching (ash removal)

When the aluminum alloy die casting is etched in the hot alkaline etching solution, due to the chemical dissolution of aluminum and the insolubility of the alloying element Si, a layer of adhered black film remains on the surface of the part. In order to completely remove the film, the following must be mixed. Acid treatment:

HNO 33 copies

HF 1

Small amount of water

Temperature room temperature

Time 20 to 40s

2.4 Dipping H·S·F Liquid

H.S.F fluid is an improvement of zincizing solution. It is a solution specially developed for aluminum surface pretreatment developed by ourselves. The obtained multi-component alloy layer has a compact structure, fine crystals, small pores, good bonding force, and dipping. After the H·S·F solution can be directly bright nickel plating, simplifying the plating process, the process specification is as follows:

H·S·F Concentrate 500mL/L

Excess water

Temperature 15～30°C

Time 30～40s

3 Plating Intermediate Layer The general use of bright nickel plating solution formulations and process conditions:

Nickel sulfate (NiSO4·7H2O) 250g/L

Nickel chloride (NiCl2·6H2O) 60g/L

Boric acid (H3BO3) 40g/L

Sodium lauryl sulfate 0.05～0 1g/L

Brightener suitable

pH 4 to 4 5

Temperature 52~55°C

Cathode current density 2 5 ~ 4A/dm2

Time 12 to 15 minutes

Cathode movement needs

When electroplating bright nickel, it is better to be charged into the groove, and it is plated with a double current for 1 to 2 minutes, and then it is conventionally plated with nickel.

4 Aluminum Alloy Die Casting Plating Gun Black Process

Aluminum alloy die casting blanks → blank inspection → mechanical polishing → petrol or trichloroethylene degreasing → cooling → upper fixture → chemical degreasing and alkali corrosion → warm water cleaning → cold water washing → flowing water cleaning → etching → washing → running water Cleaning → Dipping H · S · F solution → Washing → Washing with water → Bright nickel plating (preferably charged into the tank) → Washing → Washing with running water → Activated in 5% H2SO4 solution → Washing → Washing with running water → Plating gun black → Washing → Washing in running water → Chemical passivation → Washing → Washing with running water → Drying (5 to 10 minutes) → Lower fixture → Inspection → Impregnation or painting. Domestic gun black plating process is mostly tin-nickel alloy coating, there are tin-cobalt alloy coating. There are three types of plating solutions: fluoride type, cyanide type, and pyrophosphate type. From the viewpoint of environmental safety, we chose the pyrophosphate type gun black plating process.

4.1 Bath Formulations and Process Conditions

SnCl2·2H2O 13～15g/L

NiCl2·6H2O 55～60g/L

K4P2O7·3H2O 230～250g/L

H·S·F-2 additive 5～15g/L

Ammonia or ethylenediamine 5 ~ 10mL/L

pH 8-9

T 45～55°C

t 1～3′

Dk 0.5 to 1.5A/dm2

Anode nickel plate

Cathode movement needs

4.2 plating solution preparation

1 Calculated amounts of stannous chloride, nickel chloride, and potassium pyrophosphate were dissolved in 50-60°C hot water, respectively.

2 The dissolved tin salt and nickel salt solution should be slowly added to the potassium pyrophosphate solution under constant stirring, and then stirred for about 15 minutes. If there is turbidity, stirring should be continued until all are clear.

3 Add the calculated amount of additive and add a small amount of NaOH when dissolved in water.

4 Add the calculated amount of ammonia water, add water to the required volume, and mix well.

5 Measure and adjust the pH to 8-9, warm to 45-55°C, and electroplate with electrolysis.

4.3 The role of various components of the bath

4.3.1 Stannous chloride

It is the main salt that provides tin ions. The content of stannous chloride increases, and the tin content in tin-nickel alloy plating increases. Stannous chloride content changes in a wide range have no significant effect on the color of the coating. When the content of stannous chloride is too high, the color of the coating becomes lighter; when the content is too low, the coating becomes brown. The plating bath is not allowed to add oxidants such as hydrogen peroxide, nor is it allowed to stir with air, and only the cathode can be used.

4.3.2 Nickel chloride

It is the main salt that provides nickel ions. As the content of nickel chloride increases, the nickel content in the tin-nickel alloy coating slightly increases. When the nickel chloride content is too low, the coating is light in color. Chloride ion facilitates anode plate activation and dissolution of nickel plates.

4.3.3 Potassium pyrophosphate

It is a nickel ion, tin ion complexing agent. In addition to complexing nickel and tin, potassium pyrophosphate must have a certain amount of free potassium pyrophosphate. When the potassium pyrophosphate content is low, the coating is rough and the color is uneven. When the content is high, the cathode current density decreases and the deposition rate slows down.

4.3.4 Ammonia or ethylenediamine

The addition of ammonia can reduce the internal stress of the coating and make the coating uniform in color. Due to the heavy odor of ammonia, the solution is more volatile after being heated, so ethylenediamine is used instead.

4.3.5 Additives

H·S·F-2 additive, also known as blackening agent, is a complexing agent of tin and nickel ions and is an indispensable component for gun black plating. Due to the difference in the type and content of blackening agent, a gray, brown and brown appearance of light, medium and deep iron can be obtained.

5 Post-plating treatment

After aluminum alloy die-casting gun black plating, it must be washed immediately, passivated and dried. Passivation can improve the corrosion resistance of the coating. The drying process in the oven is the process of coating the film. This process is indispensable.

5.1 Chemical passivation

Chromium anhydride 40 ~ 60g/L

Acetic acid 1~2mL/L

Temperature room temperature

Time 30～60s

5.2 Hard film

Aging plating improves the corrosion resistance of the plating. After the plating passivation, it is washed with water and placed in an oven for 15-20 minutes at 100°C.

5.3 Painting

The purpose of painting is to extend the life of the gun black coating. According to the level of product quality may be. Some low-grade oil-impregnated products improve the anti-corrosion performance. Products with high quality requirements must be painted.

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