Can die-cast aluminum alloy be anodized?
CNC+ anodized shells often have high yield and good appearance and texture, but they have high cost, large amount of CNC, and long processing cycle. It is a typical case of high cost in exchange for high quality, such as the Apple series.
Taking the case of a smart phone as an example, when CNC is used, it takes more than 30 minutes to complete the cutting, plus finishing operations, it is estimated that it will take nearly an hour. The die-casting process can be formed in only 20-30 seconds, plus the finishing process, the operation can be completed in 10-20 minutes. The die-cast processing shell is formed by molds, so the processing time is short and the cost is relatively low. However, die-cast aluminum is difficult to anodize.
"Why is die-cast aluminum difficult to anode?"
01. Anodizing
Anodizing is an electrochemical method. In an appropriate electrolyte, alloy parts are used as anodes, and stainless steel, chromium, or conductive electrolyte itself is used as cathodes. Under certain voltage and current conditions, the anode is oxidized, thereby The process of obtaining anodized film on the surface of the workpiece requires sulfuric acid anodization in the process of anodizing and coloring.
02. Limitations of sulfuric acid anodizing on aluminum alloy materials
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 the thickest, the highest hardness, the best corrosion resistance, and the best uniformity. For aluminum alloy materials, in order to obtain a good oxidation effect, the content of aluminum must be ensured. Under normal circumstances, it is better to not less than 95%.
2. In the alloy, copper will make the oxide film reddish, destroy the quality of the electrolyte, and increase the oxidation defects; silicon will make the oxide film gray, especially when the content exceeds 4.5%, the effect is more obvious; iron is due to its own characteristics. After anodizing, it will be in the form of black spots.
03. Die-cast aluminum alloy
Cast aluminum alloys and die castings generally contain high silicon content. The anodic oxide film is dark, and it is impossible to obtain a colorless and transparent oxide film. As the silicon content increases, the color of the anodic oxide film varies from light gray to gray. Dark gray to black gray. Therefore, cast aluminum alloy is not suitable for anodizing.
Commonly used die-cast aluminum alloys can be divided into three main categories:
One is aluminum-silicon alloy, mainly including YL102 (ADC1, A413.0, etc.), YL104 (ADC3, A360);
The second is aluminum silicon copper alloy, mainly including YL112 (A380, ADC10), YL113 (A383, ADC12), YL117 (B390, ADC14);
The third is aluminum-magnesium alloy, mainly including 302 (5180, ADC5, ADC6).
Aluminum silicon alloy, aluminum silicon copper alloy
For aluminum-silicon alloys and aluminum-silicon-copper alloys, as the name implies, in addition to aluminum, silicon and copper are the main components; usually, the silicon content is between 6-12%, which mainly plays a role in improving the fluidity of the alloy liquid ; The copper content is second only, mainly to enhance the strength and tensile force; the iron content is usually between 0.7-1.2%, within this ratio, the demolding effect of the workpiece is the best; it can be seen by its composition It is concluded that this type of alloy is impossible to oxidize and color, and it is difficult to achieve the desired effect even if desiliconization is used. For aluminum-silicon alloys or aluminum alloys with a high copper content, the oxide film is more difficult to form, and the resulting film is dark, gray, and poor in gloss.
Aluminum-magnesium alloy
The oxide film of aluminum-magnesium alloy is easy to form, and the quality of the film is better. It can be oxidized and colored. This is an important feature that distinguishes it from other alloys. However, there are some disadvantages in comparison.
1. The anodic oxide film has duality, with large pores and uneven distribution, which makes it difficult to achieve the best anti-corrosion effect;
2. Magnesium has the tendency to produce hardening and brittleness, reduce elongation, and increase hot cracking, such as ADC5, ADC6, etc. In production, due to its wide solidification range and large shrinkage tendency, shrinkage porosity and cracks often occur, and casting performance Extremely poor, therefore, there is a big limitation in its scope of use, and the workpiece with a slightly complicated structure is not suitable for production at all;
3. The commonly used aluminum-magnesium alloys on the market have complex composition and low aluminum purity. When sulfuric acid anodizes, it is difficult to produce a transparent protective film. Most of them are milky white and the coloring state is also poor. It is difficult to achieve the desired effect according to the normal process.
In summary, it can be seen that the commonly used die-cast aluminum alloys are not suitable for sulfuric acid anodization; however, not all die-cast aluminum alloys cannot achieve the purpose of oxidation and coloring, such as aluminum-manganese-cobalt alloy DM32, aluminum-manganese-magnesium alloy DM6, etc. Die casting performance and oxidation performance are both excellent.
"Die-cast aluminum anodizing solution"
Die-casting parts can complete the structure, edges and corners, and oxidation quality that are difficult to achieve for forging parts, car parts/CNC parts, and the quality of the die-casting parts is very important. A small change and detailed process support determines the quality of anodes. . Manufacturers engaged in the oxidation of die-casting parts must scientifically control the flow channel technology, die-casting process and post-processing methods of the mold. With this series of strict control processes, the smooth production of oxidation quality can be ensured.
The design of mold runners and gates, and the control of mold temperature; due to the characteristics of large aluminum content in raw materials, poor fluidity, and high working temperature, mold runners and gates are set up based on the design of short range; the water channel is appropriate Use a mold temperature controller to ensure the equilibrium temperature of the mold, overcome local overcooling and excessive flow marks;
Use raw materials to avoid pollution factors; choose low-impurity raw materials; eliminate the pollution of silicon, copper, iron and zinc in production and use, that is, high-quality graphite crucibles must be used alone, and cannot be mixed with other raw materials for production;
Process control in the die casting process to reduce water marks and black watermarks; use professional release agents and scientific spraying during die casting production to reduce residual water droplets in the cavity and avoid die casting water marks; control die casting pressure and speed to reduce local filling overpressure, Easy to stick mold;
Pre-processing of the blank; after machining, according to the needs of the product, manual polishing or grinding to remove burrs and oxide layers;
The choice of anode surface treatment plant; because the bottom layer of the die casting surface contains different degrees of shrinkage and stains; therefore, the anode pretreatment must be adjusted on the basis of the conventional aluminum alloy part process to make the surface oxide layer of the casting clean before the anode process can be carried out. That is to say, the conventional oxidation process production cannot satisfy the oxidation process of die castings. There should be tests and audits to verify suitable professional manufacturers before mass production.
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