Anodisering av pressgjuten aluminium skapar en skyddande yta. Först rengörs detaljen. Därefter bildar en elektrisk ström i ett syrabad ett oxidskikt. Detta skikt stärker aluminiumet och ökar dess motståndskraft mot slitage. Slutligen kan den anodiserade delen färgas för att få färg. Anodisering ger en produkt en specifik färg och ökar dess estetik. Porositet kan dock påverka den slutliga finishen.

Därför belyser den här artikeln några viktiga parametrar för anodisering av gjutning av aluminium. Detta omfattar kvalitetskontroll, förfaranden och förbehandlingstaktik.

Vad är pressgjutning av aluminium?

Gjutning av aluminium är en process som används mest. Det kan göra skarpa detaljerade delar för alla branscher. Det första steget i denna process är att smälta aluminiumlegeringen genom att värma den till dess smältpunkt.

Sedan går den smälta metallen igenom en injektionsprocess för att fylla stålformen jämnt under högt tryck. Formen ger produktens profilform och efter stelning blir delen redo för borttagning.

Processer för pressgjutning

1. Alla delar med tunna väggar (1-2 mm) kan tillverkas med högtrycksgjutning. Detta beror på att den injicerar metall vid 10.000+ psi och därför fungerar snabbare. Denna process är också lämplig för massproduktion.
2. Lågtrycksgjutning hanterar perfekt tunna till måttligt tjocka väggdelar. Det ger den faktiska formen utan att orsaka många defekter. Det är dock en långsam process och använder 20-100 psi.
3. Vakuumgjutning rensar initialt fångad luft från formen. På grund av detta producerar gjutning inte små hål som så småningom omvandlas till porositet.

Vanliga aluminiumlegeringar

A380:

Denna legering har god flytbarhet på grund av närvaron av kiselelement i den. Tillverkarna använder 660-680°C temperatur för att gjuta den.

A380 passar i princip för tunnväggiga föremål och ger bättre hållfasthet för anodisering än högpresterande legeringar.

ADC12:

ADC12 har högre kiselhalt. Detta är anledningen till att dessa legeringar ger snäva toleranser med 2-3 mm väggar.

Alternativt är de inte mycket mer hanterbara att anodisera. Den injiceras vid en temperatur under 650-670°C.

Viktiga egenskaper

• Styrka: A380 producerar starka delar.
• Korrosionsbeständighet: ADC12 korroderar inte lätt.
• Anodisering: A380 går bra att anodisera. ADC12 är däremot svårare eftersom det finns ett överskott av kisel. Det stör oxidskiktet.

Defekter

Små hål (porositet) bildas när gjutningen misslyckas med att kyla metallen jämnt, vilket försvagar delarna.

Innehållet av smuts eller oxider kan fastna i metallen och orsaka inneslutningar. Dessa problem påverkar också anodiseringsresultatet och förstör på något sätt finishen.

Verktyg

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).

Ytfinish

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.

Vad är anodisering av aluminiumgjutning?

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.

Elektrokemisk 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.

Kemiska ekvationer

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

Typer av anodisering Aluminiumformgjutning

1. Anodisering med kromsyra:

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. Anodisering med svavelsyra:

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. Hård anodisering:

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.

Mikrostruktur i det anodiserade skiktet.

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).

Konventionell anodisering kontra hård anodisering

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).

Föranodisering för pressgjutning av aluminium

Rengöring

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.

Etsning

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.

Mätning av ytjämnhet

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.

Betydelsen av uppehållstider och kemiska koncentrationer

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.

Anodiseringsprocess för pressgjutning av aluminium

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.

Strömtäthet, temperatur och tid

• 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.

Bildande av porstruktur

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.

Anodisering med kromsyra kontra svavelsyra

Kromsyra (typ 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.

Svavelsyra (typ 2/3):

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

Katodens roll

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

Inverkan av legeringssammansättning

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.

Processer och kvalitetskontroll efter anodisering

Förseglingsmekanismer

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.

Färgningsprocess

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.

Provning av tjocklek

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.

Anodisering av aluminiumgjutning Korrosionstestning

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.

Kvalitetsstandarder

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

Betydelsen av pH-kontroll vid tätning

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.

Slutsats

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.