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The Process of Anodizing Die-cast Aluminum Alloys Is Briefly Described As Follows

Views: 0     Author: Site Editor     Publish Time: 2023-03-15      Origin: Site


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Why Use Die-Cast Aluminum Alloys?

Die-cast aluminum alloys are chosen for specific reasons. CNC (Computer Numerical Control) machined cases often result in high yield rates and excellent surface quality, but they come with high costs, extensive CNC usage, and long processing times. This is a typical case of trading high costs for high quality, as seen in products like the Apple series.

For instance, consider smartphone cases. When using CNC, it takes over 30 minutes just to complete the cutting process. Adding the time for precision machining, it's estimated to take nearly an hour. In contrast, die-casting can form the case in just 20 to 30 seconds. With additional precision machining, the entire process can be completed in 10 to 20 minutes. Die-casting uses molds for shaping, which significantly shortens the processing time and reduces costs. However, it's challenging to perform anodizing on die-cast aluminum alloys.

Why is Anodizing Die-Cast Aluminum Difficult?


Anodizing is an electrochemical process that forms an oxide layer on the surface of alloy parts. In an appropriate electrolyte solution, the alloy part is used as the anode and materials like stainless steel, carbon rods, or aluminum plates serve as the cathode. Under certain voltage and current conditions, oxidation occurs at the anode, resulting in an anodic oxide film on the workpiece's surface. This oxide film is porous, allowing it to absorb color (sulfuric acid anodizing offers the best porosity for coloring).

Limitations of Sulfuric Acid Anodizing on Aluminum Alloys:

  • The presence of alloying elements can decrease the quality of the oxide film. Under similar conditions, the oxide film obtained on pure aluminum is thicker, harder, has better corrosion resistance, and is more uniform. For aluminum alloys, to achieve good oxidation effects, the aluminum content should ideally be no less than 95%.

  • In alloys, copper can cause the oxide film to turn reddish, deteriorating the electrolyte quality and increasing oxidation defects. Silicon can turn the oxide film gray, especially when its content exceeds 4.5%. Iron, due to its inherent characteristics, appears as black spots after anodic oxidation.

Die-Cast Aluminum Alloys:

Die-cast aluminum alloys and castings generally contain higher levels of silicon, resulting in dark-colored anodic oxide films. It's impossible to achieve a colorless and transparent oxide film with these alloys. As the silicon content increases, the color of the oxide film changes from light gray to dark gray and eventually to black-gray. Hence, cast aluminum alloys are unsuitable for anodizing.

The commonly used die-cast aluminum alloys are mainly divided into three categories:

  • Aluminum-silicon alloys, which include YL102 (ADC1, A413.0, etc.), YL104 (ADC3, A360).

  • Aluminum-silicon-copper alloys, which include YL112 (A380, ADC10), YL113 (A383, ADC12), YL117 (B390, ADC14).

  • Aluminum-magnesium alloys, which include 302 (5180, ADC5, ADC6).

Aluminum-Silicon and Aluminum-Silicon-Copper Alloys:

For aluminum-silicon and aluminum-silicon-copper alloys, silicon and copper are the main components aside from aluminum. Typically, the silicon content is between 6-12%, which improves the fluidity of the alloy liquid and reduces shrinkage porosity. Copper is the next significant element, enhancing strength and tensile force. The iron content is usually between 0.7-1.2%, which is the optimal range for effective mold release. From their composition, it's evident that these alloys are unsuitable for anodizing. Even with desiliconization, achieving the desired effect is difficult. Additionally, aluminum-silicon alloys or those with high copper content have difficulty forming an oxide layer, and if formed, the layer appears dark and gray with poor luster.

Aluminum-Magnesium Alloys:


For aluminum-magnesium alloys, the oxide film is relatively easy to form, and the quality of the film is generally better. They are capable of being anodized with color, which is an important distinguishing feature from other alloys. However, compared to wrought aluminum alloys, they also have some drawbacks:

The anodic oxide film has a dual nature and tends to have larger, unevenly distributed pores, making it difficult to achieve optimal corrosion resistance.

Magnesium tends to harden and become brittle, reduce elongation, and increase the tendency for thermal cracking. Alloys like ADC5 and ADC6, for example, often suffer from porosity and cracking due to their wide solidification range and high shrinkage tendency, making their casting performance extremely poor. Therefore, their use is greatly limited, especially for structurally complex components which are unsuitable for production.

Commonly used aluminum-magnesium alloys in the market, due to their complex composition and low purity of aluminum, struggle to produce a clear protective film during sulfuric acid anodizing. The film often appears milky white, with poor coloration, making it difficult to achieve desired results with standard processes.

In summary, common die-cast aluminum alloys are not suitable for sulfuric acid anodizing. However, not all die-cast aluminum alloys are unsuitable for anodizing and coloring. For instance, aluminum-manganese-cobalt alloy DM32 and aluminum-manganese-magnesium alloy DM6 have both excellent casting and anodizing properties.


Die-Cast Aluminum Anodizing Solutions

Die-cast parts can achieve structures, edges, and lines that are challenging for forged or CNC machined parts. The quality of anodization heavily relies on the quality of the die-cast parts; even a minor variation or a detail in the process can significantly impact the quality of the anodizing. Manufacturers involved in anodizing die-cast parts must rigorously control the mold's flow channel technology, die-casting process, and post-processing methods. A stringent control process ensures the successful production of high-quality anodized products.

Mold Flow Channel and Gate Design, Temperature Control: Due to the high aluminum content and poor fluidity at high working temperatures, the mold's flow channels and gates should be designed with short shooting distances. Temperature controllers should be used to maintain a balanced temperature in the mold, overcoming issues like local overcooling and excessive flow marks.

Raw Material Usage, Avoiding Contamination: It's crucial to choose raw materials with low impurity content. During production, contamination from silicon, copper, iron, and zinc must be avoided. This necessitates the use of high-quality graphite crucibles dedicated to each specific material to prevent cross-contamination.

Die-Casting Process Control, Minimizing Water Marks and Black Stains: Use professional release agents during die-casting, apply them scientifically to reduce residual water droplets in the cavity, and prevent water marks from die-casting. Control the die-casting pressure and speed to alleviate overpressure in localized filling, which can lead to sticking to the mold.

Pre-Machining of the Blank: After machining, depending on the product's requirements, manually polish or grind to remove burrs and oxidation layers.

Choice of Anodic Surface Treatment: Since die-cast parts often contain different degrees of shrinkage pores and stains on the surface, pre-treatment before anodizing must be adjusted from the conventional aluminum alloy process. The surface layer of the casting must be cleaned before undergoing the anodic process. This means that conventional anodizing processes might not suffice for die-cast parts, and trials and reviews should be conducted before mass production.

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