CNC machined housings paired with anodic oxidation typically yield high-quality finishes and superior aesthetic textures, though they suffer from high production costs and lengthy lead times.
Take a smartphone chassis as an example: using CNC milling, the initial cutting alone requires over 30 minutes; when including finishing operations, the total cycle time approaches one hour. In contrast, the die-casting process requires only 20 to 30 seconds for molding, and with finishing included, the entire task can be completed within 10 to 20 minutes. Die casting service uses molds for high-speed forming, significantly reducing both processing time and costs. However, die cast aluminum is notoriously difficult to anodize.
Why is Die Cast Aluminum Difficult to Anodize?
Anodic oxidation is an electrochemical process. The alloy part serves as the anode, while stainless steel, chromium, or a conductive electrolyte acts as the cathode. Under specific voltage and current conditions within an electrolyte, the anode undergoes oxidation to form an anodic oxide film on the workpiece surface. For color anodization, sulfuric acid anodizing is typically required.
Material Constraints in Sulfuric Acid Anodizing
The presence of alloying elements can degrade the quality of the oxide film. Under identical conditions, pure aluminum yields the thickest, hardest, most corrosion-resistant, and most uniform oxide film. To achieve optimal results, the aluminum content of the material should generally remain above 95%.
- Copper: Causes the oxide film to take on a reddish tint, degrades electrolyte quality, and increases oxidation defects.
- Silicon: Turns the oxide film gray, particularly when the content exceeds 4.5%.
- Iron: Due to its inherent characteristics, iron appears as black spots on the surface following the anodizing process.
Die Cast Aluminum Alloys
Cast aluminum alloys and die cast parts generally contain high levels of silicon, which results in dark-colored oxide films. Obtaining a colorless, transparent film is virtually impossible; as silicon content increases, the color of the film transitions from light gray to dark gray and eventually to blackish-gray. Consequently, standard cast aluminum alloys are unsuitable for decorative anodization.
Commonly used die cast aluminum alloys can be categorized into three main groups:
- Al-Si Alloys: Primarily including YL102 (ADC1), YL104 (ADC3, A360).
- Al-Si-Cu Alloys: Primarily including YL112 (A380, ADC10), YL113 (A383, ADC12), YL117 (B390, ADC14).
- Al-Mg Alloys: Primarily including 302 (5180, ADC5, ADC6).
Al-Si and Al-Si-Cu Aluminum Alloys
As the names imply, silicon and copper are the primary constituents besides aluminum. Silicon content typically ranges from 6% to 12%, primarily serving to improve the fluidity of the molten alloy. Copper is the second most prevalent element, used to enhance strength and tensile properties. Iron content is usually maintained between 0.7% and 1.2% to optimize the demolding process.
Given this composition, these alloys cannot be effectively color-anodized. Even with desiliconization treatments, the results are rarely ideal. Furthermore, Al-Si or high-copper alloys struggle to form a stable oxide film; the resulting layer is often dull, gray, and lacks luster.
Al-Mg Aluminum Alloys
Al-Mg alloys are distinct in that they can form high-quality oxide films capable of being color-anodized. This is a critical differentiator from other die cast alloys. However, they possess several drawbacks:
- The anodic film exhibits non-uniformity and high porosity, making it difficult to achieve optimal corrosion resistance.
- Magnesium tends to cause work hardening and brittleness, reducing elongation and increasing the propensity for hot cracking. Alloys like ADC5 and ADC6 have a wide solidification range and high shrinkage, often leading to shrinkage porosity and cracks. Their poor castability limits their use for workpieces with complex geometries.
- Commercial Al-Mg alloys often have complex trace compositions and low aluminum purity. During sulfuric acid anodizing, they tend to produce an opaque, milky-white protective film rather than a transparent one, resulting in poor dyeing performance.
Conclusion
In summary, commonly used die cast aluminum alloys are generally unsuitable for sulfuric acid anodization. However, this is not a universal rule for all die-cast materials. Specialty alloys, such as Al-Mn-Co (e.g., DM32) and Al-Mn-Mg (e.g., DM6), offer an excellent balance of die-casting performance and anodizing capability.
How Getzshape Can Help
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