Anodizing Aluminum Alloy Die Casting Surface Finish

Casting oxidation

Anodic oxidation is a significant process for applications in various die casting products. The anodizing characteristics will affect the features of the final finishing products.

Therefore, this article will give you a detailed overview of anodizing surface finish, you would learn your final product will meet unique anodizing surface finish requirements.

Table of Contents

Anodic Oxidation Definition

Anodic oxidation treatment is the process of obtaining an anodic oxide film on the surface of the workpiece by using electrochemical methods to oxidize the anode with alloy parts as the anode and stainless steel, carbon rod, or aluminum plate as the cathode under a certain voltage current and other conditions, so that the oxide film is porous and can be adsorbed for coloring (sulfuric acid anodic oxidation has the best porosity).

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What are the advantages of anodizing aluminium alloy die casting?

Anti-corrosion

Anodizing can form a protective layer of oxide film on the surface of aluminium alloy die casting, thus increasing the denseness and corrosion resistance of die casting.

Excellent decorative performance

Anodic oxidation not only can get high transparency of the oxide film but also can be dyed with fuel into a variety of bright colours and even special vulgar treatment but also can make ceramic similar to the coating effect

Abrasion resistance

Hard anodized has not only high hardness and thickness but also low roughness. It can greatly improve its wear resistance.

Insulation

Oxide film resistance is very large and belongs to the insulator category. It can play a good role as an insulating layer.

After anodizing, it can still be reprocessed

The anodized film can be used as the base layer of other spraying methods, such as spray paint, electroplating, etc., which can increase adhesion.

Anodizing Die Casting Aluminum Alloy

Cast aluminum alloys and die castings generally contain high silicon content. The anodic oxide film is dark in color, and it is impossible to get a colorless and transparent oxide film. With the increase of silicon content, the color of the anodic oxide film ranges from light gray to dark gray to dark gray.

Commonly divided into using die casting aluminum alloys for three main categories

① Aluminum-silicon alloys containing YL102 (ADC1, A413.0, etc.) and YL104 (ADC3, A360).

② Aluminum-silicon copper alloys, mainly containing YL112 (A380, ADC10), YL113 (A383, ADC12), and YL117 (B390, ADC14).

③ Aluminum and magnesium alloys, mainly containing 302 (5180, ADC5, ADC6)

Aluminum-silicon alloy, aluminum-silicon copper alloy:
For aluminum-silicon alloy and aluminum-silicon-copper alloy, as the name implies, its composition is mainly composed of silicon and copper, in addition to aluminum. The composition shows that it is impossible to oxidize the color of such alloys, and even if de silicon oxidation is used, it isn’t easy to achieve the desired effect. In the case of aluminum-silicon or aluminum alloy with high copper content, the oxide film is more challenging to generate, the generated film is dark and gray, and the glossiness is not good.

Aluminum and magnesium alloys:
For aluminum-magnesium alloy, the oxide film is easy to generate, the quality of the film is better, and it can be oxidized and colored, which is an important feature that distinguishes it from other alloys; however, compared with deformed aluminum alloy, there are also some disadvantages.
1、 An anodic oxide film with duality, larger pores, and uneven distribution, it isn’t easy to achieve a better anti-corrosion effect.
2、Magnesium produces hardening and brittleness, reduces elongation, and increases thermal cracking, such as ADC5, ADC6, etc. In production, it often produces shrinkage and cracking because of its wide range of solidification and shrinkage tendencies. Casting performance is inferior. Therefore, its use range has more significant limitations. It should not produce the structure of a slightly complex workpiece.
3、The market commonly used aluminum and magnesium alloy, because of its complex composition, aluminum purity is too low, sulfuric acid anodic oxidation, it is challenging to produce a transparent protective film, primarily milky white, coloring state is also poor, according to the normal process is challenging to achieve the desired effect.
However, not all die casting aluminum alloys can not achieve the purpose of oxidation colorings, such as aluminum manganese cobalt alloy DM32, aluminum manganese magnesium alloy DM6, etc. The die casting performance and oxidation performance are both excellent.

Limitations of sulfuric acid anodizing on aluminum alloy material

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 thicker, with higher hardness, better corrosion resistance, and better uniformity. Aluminum alloy material, to get a good oxidation effect, to ensure the content of aluminum, usually, to not less than 95% is better.

2、In the alloy, copper will make the oxide film red, destroy the quality of electrolyte and increase the oxidation defects; silicon will make the oxide film gray significantly when the content exceeds 4.5%, the impact is more obvious; iron will exist in the form of black spots after anodic oxidation because of its characteristics.

Aluminum anodizing process flow

Mechanical polishing – oil removal – water washing – chemical polishing – water washing –anodizing –water washing–closing–automatic brightness, chemical polishing goods: aluminum alloy die casting alkaline polishing solution, anodizing goods: Aluminum alloy die casting anodizing solution.

Aluminum alloy dies casting anodizing process

The treated aluminum alloy die casting has beautiful and bright colors, higher corrosion resistance, wear resistance, and insulation than the general aluminum casting. The aluminum casting is suspended in a suitable electrolyte solution and electrolyzed as an anode. In the process of electrolysis, hydroxide ions in water release electrons at the anode to become water and new ecological oxygen, which oxidizes aluminum into a thicker aluminum oxide film because this process is the metal product as the anode is oxidized, so it is called anodic oxidation.

anodic oxidation.png

Process:
1. Pre-treatment:
Aluminum alloy die castings in the process of repeated mechanical processing, stained with more grease, a small number of abrasives, dust, defective oxide film, etc. These substances have poor electrical conductivity and can not be anodized, so they must be pre-treated. The method uses carbon tetrachloride, trichloroethylene, gasoline, or toluene as the cleaning agent. Dip the aluminum alloy die casting into it, brush it with a brush, air dry it, then air dry it dip it into the water, and clean it many times. After the oil is gone, rinse with hot water immediately. If the surface generates a black film, put it in 32% nitric acid solution for 20 seconds to remove the black film, and finally rinse it with cold water. Immerse in distilled water, ready for the production of the oxide film.
Anodic oxidation:
(1) Preparation of sulfuric acid electrolyte: made by mixing sulfuric acid and deionized water, the specific gravity of the solution is about 1.125-1.140. Sometimes a small amount of oxalic acid is added to the sulfuric acid electrolyte to obtain an excellent protective performance of oxygen oxide film.
(2) Oxidation process: Install the line meter, immerse the aluminum alloy die casting into the electrolyte as the anode, then turn on the power and control the process according to the following conditions.
Notice:
The electrolyte temperature is controlled at 12-25℃, the anode current density is 1-2A/dm2, and the voltage in the tank is between 13-23 volts. The time is about 30-40 minutes.
After the above process is finished, take out the aluminum alloy die casting from the electrolyte at any time and rinse the acid stained with clean water. Pay more attention to the low concave part. Otherwise, there will be white spots. After the acid is cleaned, dip it into clean water and set it aside.
1. After the aluminum alloy die casting degreasing treatment, oxidation should be carried out immediately and should not be left for too long. When making the oxide film of aluminum parts, all of them should be immersed in the electrolyte, the tank voltage should be smooth and consistent from beginning to end, and the same batch of products must be entirely consistent, which should be followed even when dyeing.
2. In the process of anodic oxidation, the solution of aluminum, copper, and iron in the electrolyte keeps increasing, which affects the luster of aluminum castings, etc. When the aluminum content is more significant than 24g/l, copper is more excellent than 0.02g/l, and iron content is more significant than 2.5g/l, the electrolyte should be considered for replacement.
3. Buy raw materials to choose products with high purity because the general impurities are slightly more ineffective.
4. Aluminum can not contain impurities: containing more than 2.5% silicon, the bottom film shows gray. Containing magnesium of more than 2%, stained with dull. Containing manganese tone tender but not bright. Containing copper tone with soft iron, nickel, and chromium is too much when the color is dull.

V1 diecast aluminum anodizing solutions

Die casting can complete the forging parts, turning parts / CNC parts challenging to do the structure, angle line, the quality of oxidation, the center of gravity is very critical to the quality of die casting, a small change, the details of the process support control determines the quality of the anode. Manufacturers engaged in die casting oxidation must scientifically support and control the mold runner technology, die casting process, and post-processing methods, with this series of strict control processes can ensure the smooth production of oxidation quality.

Mold runner, gate design, mold temperature control; because the raw material contains a lot of aluminum, poor liquidity, high working temperature characteristics, so the mold runner, the gate to shoot short range design as a guideline to open; transport water is appropriate to use the mold temperature machine to ensure the equilibrium temperature of the mold, to overcome the local over-cooling, too many flow marks.

The use of raw materials, to avoid pollution factors; choose raw materials with low impurity; production use, to eliminate the pollution of silicon, copper, iron, and zinc elements, that is, the need to use high-quality graphite crucible alone, not mixed with other raw materials production.
Die-casting process control, reduce water lines and black water marks; die-casting production using professional mold release agent, scientific spraying, reduce cavity residual water beads, avoid die-casting water lines; control the pressure and speed of die-casting, reduce the local filling overpressure, easy to stick mold.

Pre-machining treatment of blanks; after machining, hand polishing or grinding to remove burrs and oxidation layer according to the demand of products.

Anodic surface treatment plant selection; because the bottom layer of die casting skin contains different degrees of shrinkage and stains, the anodic pre-treatment must be based on the conventional aluminum alloy parts process. Adjust the practice to clean the casting surface oxide layer before the anodic process. That is, the conventional oxidation after the process production can not meet the die casting oxidation process. There should be mass production before the test and audit to verify the suitable professional manufacturers.

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