1 Introduction 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 that is more distinctive and improves product competitiveness in the international market. Aluminum alloy electroplating and general electroplating process have certain differences, because aluminum alloy is a relatively active metal, reduction, replacement ability, bring a lot of difficulties to the electroplating process, 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. 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 When the aluminum alloy die casting is etched in the hot alkali 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: 2.4 Dipping H·S·F Liquid H.S.F fluid is an improvement of the zinc dipping solution. It is a solution specially developed for aluminum surface preparation developed by us. The obtained multi-component alloy layer has a compact structure, fine crystals, small pores, and good bonding force. After the H·S·F solution can be directly bright nickel plating, simplifying the plating process, the process specification is as follows: It is better to charge into the groove, use a double current to impact plating for 1 to 2 minutes, and then press the conventional nickel plating. LED Effects,Strobe Wash Led,Hybrid Led Strobe,Led Effect Light Matrix EV LIGHT Guangzhou Co., Ltd , https://www.evlightprofessional.com
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 of the alkali etching solution and the alkali etching time 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)
HNO 33 parts HF 1 parts water a little temperature room temperature time 20 ~ 40s
H·S·F Concentrate 500mL/L
The amount of 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 ~ 4 5
Temperature 52~55°C
Cathode current density 2 5 ~ 4A/dm2
Time 12-15 minutes cathode movement needs
When plating bright 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 → Flow Cleaning → Bright Nickel Plating (Better Charged into Tank) → Washing → Washing in Running Water → Activated in 5% H2SO4 Solution → Washing → Washing in 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 protection and 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, continue stirring with warming 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 cathode movement is allowed.
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 heating, 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 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.