Anodizing die casting aluminum creates a protective surface. First, the part is cleaned. Next, an electrical current in an acid bath forms an oxide layer. This layer strengthens the aluminum, boosting its resistance to wear. Finally, the anodized part can be dyed for color.  Anodizing gives a product a specific color and increases its aesthetics. However, porosity can impact its final finish.

Therefore, this article highlights some important parameters of anodizing aluminum die casting. This covers quality control, procedures, and pretreatment tactics.

What is Aluminum Die Casting?

Aluminum die casting is a mostly used process. That can make sharply detailed parts for every industry. The first step in this process is to melt the aluminum alloy by heating it to its melting point.

Then, this molten metal goes through an injection process to fill the steel mold evenly under high pressure. The mold provides the product profile shape and after solidification, the part becomes ready for removal.

Die Casting Processes

1. Every part with thin walls (1-2 mm) can be made with high-pressure die casting. This is because it injects metal at 10,000+ psi and, therefore, works faster. This process is also suitable for mass production.
2. Low-pressure die casting perfectly deals with thin to moderately thick wall parts. It gives the actual shape without causing many defects. However, it’s a slow process and uses 20-100 psi.
3. Vacuum die casting initially clears captured air from mold. Because of this, casting does not produce tiny holes that eventually convert into porosity.

Common Aluminum Alloys

A380:

This alloy has good fluidity because of the presence of silicon elements in it. Manufacturers apply 660-680°C temperature to pour it.

A380 basically suits thin wall items as well as gives better strength for anodizing than high-performance alloys.

ADC12:

There is higher silicon content in ADC12. This is why these alloys make tight tolerances with 2-3 mm walls.

Alternatively, they are not much more manageable to anodize. It is injected at below 650-670°C temperature.

Key Properties

• Strength: A380 produces strong parts.
• Corrosion resistance: ADC12 does not corrode easily.
• Anodizing: A380 goes well with anodizing. Meanwhile, ADC12 is harder as there is excess silicon. That disrupts the oxide layer.

Defects

Small holes (porosity) form when the casting fails to cool metal evenly, weakening parts.

The content of dirt or oxides may trap into the metal, causing inclusions. These issues also impact the anodizing results and somehow ruin the finish.

Tooling

Tooling means mold(die). It is usually made with steel metal to have enough strength. That, therefore, can work under high heat (over 600°C) and pressure (10,000+ psi).

Surface Finish

Die-cast parts may already have better and neater surfaces, but there is a need to remove minor flaws. Their present roughness ranges between 1.6 and 3.2 µm (like fine sandpaper). Polishing these surfaces can make them smoother.

What is Anodizing aluminum die casting?

Anodizing refers to an electrochemical technique. It’s like the further enhancement of the natural oxide layer on aluminum. After anodizing, parts do not corrode easily. They hold paint well and last longer.

Automotive, aerospace, and consumer electronics use it to make their functional and aesthetic components.

Electrochemical Process

To anodize a part, a bath containing an acid electrolyte (e.g., sulfuric acid) is prepared. The personnel then submerge aluminum inside it.

The process makes a hard, porous oxide layer by taking an electric current. This result becomes possible because of the formation of oxygen ions. That bond with the aluminum.

The layer thickness could be around 5 to 100 micrometers (µm) according to the process.

Chemical Equations

• The chemical equation at the anode (aluminum part) is 2Al+3H₂O→Al₂O₃+6H⁺+6e⁻.
• Inside the bath-like cathode is 6H⁺ + 6e⁻ → 3H₂.

Types of Anodizing Aluminum Die casting

1.   Chromic Acid Anodizing:

Most of the aerospace parts need sufficient durability and less weight. In that case, the chromic acid anodizing works well. It becomes easier with it to form a thin oxide layer, typically 2-5 µm thick.

2.   Sulfuric Acid Anodizing:

It falls in the category of common anodizing type. The process involves sulfuric acid use. It can create much thicker oxide layers fluctuating between 5 and 25 µm.

Generally, this anodizing creates balanced properties of durability and aesthetics in parts. That can be consumer products like smartphones, cookware,  and architecture.

3.   Hard Anodizing:

This is another type of anodizing that similarly uses sulfuric acid. However, it is different because of its ability to create a thicker and harder oxide layer. That can be 25 to 100 µm.

Thicker layers are important for heavy-duty parts to set barriers against wear. Examples are hydraulic systems, military equipment, and industrial machinery.

Microstructure of the Anodized Layer.

There are pores in the anodized layer. That resembles a honeycomb-like structure. Their pores absorb paint well and add a protective layer against corrosion and wear.

The formation of pore size depends on what process is used. For example, voltage (12-24 volts) and temperature (18-22°C for Type II).

Conventional vs. Hard Anodizing

Conventional anodizing (sulfuric acid) is best suited for parts that need an aesthetic touch or medium wear resistance.

In case of extreme durability and hardness, hard anodizing (type 3) is ideal. It can achieve hardness up to 350-500 Vickers Hardness (HV).

Pre-Anodizing Preparation for Aluminum Die Casting

Cleaning

The aluminum parts get cleaned through multiple cleaners. Whereas, alkaline cleaners (pH 10-12) are common ones.

They work at 50-70°C. It takes 5-10 minutes to remove dirt, grease, or debris. Cleaners break down oil contents via saponification, converting it into soap form.

Additionally, there are solvent-based cleaners too. They are too quick and degrease without water.

Etching

A kind of slightly rough texture without oxides usually comes from undergoing an etching process. It involves caustic soda (sodium hydroxide, 50-100 g/L). The process occurs at 50-70°C and needs 1 to 5 resting minutes.

The roughness of parts gets improved from 0.8 µm to 1.5-3 µm. So that the part can hold or adhere well with paint. Over-etching is not necessarily important because it causes pitting.

Desmutting

The desmutting process is good to use for easy removal of black smut. Smut (leftover oxides and alloying elements) occurs after the end of the etching step.

That’s why they need a kind of solution (nitric acid with a 10-30% proportion) that dissolves their layers.

The acid release reaction dissolves residual oxides, taking 1-3 minutes. It makes aluminum surfaces totally clean.

Surface Roughness Measurement

Surface roughness varies but can be tracked using profilometry measures. It uses a stylus to analyze peaks and valleys.

Ideal roughness parameters range between 0.5 and 2.5 µm. Overly slippery parts do not hold coatings well, while high roughness does not end in an even finish.

Importance of Dwell Times and Chemical Concentrations

Concentrating on dwell time aids in getting the desired result, either from cleaning or etching.

Otherwise, the part may be damaged.

For example, giving more time (exceeding 5 minutes ) to the caustic soda creates over-etching thin walls.

Anodizing Process for Aluminum Die Casting

Electrolyte Composition in Anodizing aluminum die casting

The bath is prepared by mixing sulfuric acid with water. However, to control pore formation or make better absorption, additives like organic acids work best.

Also, cooling water is important to maintain bath temperature around 18-22 °C.

Current Density, Temperature, and Time

• Current density: DC power source and current limiter manage it. It must be around 1.5–3 A/dm². The higher current accelerates the process but does not provide uniformity.
• Voltage: The 12-24 volts are sufficient for general purpose. It, however, can be modified according to alloy or thickness.
• Time: Anodizing takes 30-60 minutes. More time creates thicker layers.

Pore Structure Formation

We already discussed the pore formation via electric current during anodizing. So agitators distribute chemicals evenly in them. Dyes fill them so correctly that they would not corrode easily.

Chromic Acid vs. Sulfuric Acid Anodizing

Chromic acid (Type I):

Uses 3–10% chromic acid in the bath. Produces thinner layers (2–5 µm) with excellent corrosion resistance but limited dyeability.

This process uses 3-10% chromic acid in the bath. It works best to make thinner walls and stop corrosion. However, it does not apply to every coating.

Sulfuric acid (Type 2/3):

It is better to use for making thicker layers that can hold dye strongly. Wherein, hard anodizing offers extreme durability.

Role of the Cathode

The circuit ends at the cathode (leads or steel). At this point, it forms hydrogen gas to enable anode reaction with oxygen ions.

Impact of Alloy Composition

Higher silicon particles in ADC12 block reactions, because of which the oxide layer does not stick well.

Meanwhile, lower silicon in A380 absorbs paint uniformly.

Post-Anodizing Processes and Quality Control

Sealing Mechanisms

It is important to seal the porous layer. This will improve the protection barrier. You can seal them using hot water (90–100°C for 15–30 minutes). It creates hydrated aluminum oxide.

Dyeing Process

Organic or inorganic dyes cover the porous oxide layer. In this, manufacturers put the parts under dye baths at 50–60°C for 5–20 minutes. Also, spraying and dip coating distribute color equally.

Thickness Testing

Cross-sectional microscopy measures thickness. As shown in the image, the label oxide represents the oxide layer, and A1 is for the aluminum surface.

The thickness level must be according to standards like ISO 7583. The demand lies between 5–25 µm from type 2 or 25–100 µm from type 3. Also, incorporating Eddy’s current test ensures a non-destructive fundamental.

Anodizing aluminum die-casting Corrosion Testing

The process that can identify whether parts can corrode or not is salt spray testing (ASTM B117). The manufacturers spray the parts and leave them for 100–1000 hours.

The electrochemical impedance spectroscopy test analyzes the capability of the oxide layer regarding resistance to electrical currents.

Quality Standards

Quality generally comes first. So, the anodized parts must meet standards like MIL-A-8625. That guide about the acceptable porosity, thickness, and bonding.

Importance of pH Control in Sealing

To maintain the sealing bath’s pH, picking a pH of 5.5–6.5 (for nickel acetate) or neutral pH (hot water) is valuable. Because of this, there are fewer chances of incomplete sealing.

Conclusion

Proper anodizing methods will cause the cast aluminum parts to become more strong. They can resist corrosion more effectively and look beautiful. Optimized casting techniques, suitable alloys, and precise anodized concentration generate an output with quality and durability. There are also standards like MIL-A-8625 from which manufacturers can get proper insights on finishing.