Die casting aluminum alloy blackening is a widespread quality issue that degrades product appearance, réduit la résistance à la corrosion, et raccourcit la durée de vie, ce qui pose des défis importants aux constructeurs automobiles, électronique, et industries de l'électroménager. Cette décoloration indésirable provient d'une oxydation incontrôlée de la surface en aluminium., piloté par les processus de production, propriétés des matériaux, facteurs environnementaux, et erreurs de post-traitement. This article systematically analyzes its root causes, provides targeted solutions, and shares practical prevention strategies, helping you eliminate blackening and improve product quality.
1. Core Causes of Die Casting Aluminum Alloy Blackening
Die casting aluminum alloy blackening is not a single-factor issue but the result of multiple interconnected causes. Below is a 总分结构 breaking down the four key categories, supported by causal chains and specific examples:
1.1 Production & Process-Related Causes
Defects in manufacturing processes lay the foundation for blackening by leaving residues or creating structural vulnerabilities. The table below details common process issues and their impacts:
| Process Issue | Technical Explanation | Impact on Blackening |
| Contaminant Adhesion | Residues of release agents, cutting fluids, ou saponification fluids remain on the aluminum surface after casting. These substances contain corrosive components (par ex., fatty acids in cutting fluids) that react with aluminum over time. | Accelerates surface oxidation: A smartphone middle frame with 0.1 mm-thick release agent residue may turn black within 7 days of storage, contre. 30+ days for a clean part. |
| Incomplete Cleaning | Improper cleaning processes (par ex., insufficient rinsing, low detergent concentration) fail to remove contaminants. Gaps in cleaning (par ex., hard-to-reach internal cavities) become “hotspots” for blackening. | Localized blackening: An automotive engine bracket with uncleaned bolt holes develops black spots around the holes within 2 semaines, while the rest of the surface remains intact. |
| Poor Die Casting Parameters | Excessive porosity or loosening caused by low injection pressure (≤50 MPa) or inadequate holding time (<10 secondes) creates micro-pores in the aluminum. These pores trap moisture and contaminants, triggering internal oxidation. | Uniform blackening: A porous LED heat sink (porosité >5%) turns uniformly black after 1 month in a humid environment, as moisture seeps into pores and reacts with aluminum. |
1.2 Propriétés des matériaux & Composition
Aluminum alloy composition and microstructure directly affect its oxidation resistance. Two key material factors drive blackening:
- Excessive Impurities: Alloys with high iron content (>1.3% for ADC12) or impure scrap aluminum (used in smelting) form brittle intermetallic compounds (par ex., Al₃Fe). These compounds are more prone to oxidation than pure aluminum, creating black spots on the surface. Par exemple, an ADC12 part with 1.5% iron content is 2–3x more likely to blacken than one with 0.8% iron.
- Grain Structure & Rugosité de la surface: Alloys with coarse grains (grain size >50 μm) or rough surfaces (Ra >6.3 μm) have larger surface areas exposed to oxygen. Fine-grained (grain size <30 µm) and smooth-surfaced (Râ <1.6 µm) alloys form more uniform oxide films, resisting blackening. A rough aluminum gearbox housing (Râ 12.5 µm) may show blackening within 14 jours, while a polished one (Râ 0.8 µm) stays bright for 60+ jours.
1.3 Environmental Factors
External environmental conditions accelerate oxidation by attacking the aluminum’s natural oxide film. The most impactful factors are:
- Temperature & Humidity: High humidity (>60% RH) provides the moisture needed for electrochemical oxidation. In rainy seasons or coastal areas, the risk of blackening increases by 40–60%. Par exemple, an aluminum air conditioner compressor shell stored in a 70% RH warehouse may turn black in 20 jours, contre. 45 days in a 40% RH warehouse.
- Corrosive Media Contact: Exposure to strong acids, alcalis, or salt spray (par ex., coastal environments, industrial workshops) directly destroys the protective oxide film. A marine-grade aluminum bracket exposed to salt spray (per ASTM B117) turns black within 48 heures, as saltwater breaks down the oxide layer and triggers rapid oxidation.
1.4 Post-traitement & Misuse
Errors in post-treatment or storage amplify blackening risks, even for well-manufactured parts:
- Improper Cleaning Agents: Highly corrosive cleaning agents (par ex., strong alkaline detergents with pH >12) etch the aluminum surface, removing the natural oxide film and exposing the matrix to oxidation. A cleaned aluminum part using a pH 13 detergent may start blackening within 3 jours, contre. 25+ days for one cleaned with a neutral detergent (pH 6–8).
- Negligent Storage Management: Differences in humidity across warehouse heights (par ex., 65% RH at ground level vs. 50% RH at 2m height) cause uneven blackening. Parts stored on the ground without moisture-proof packaging turn black 2–3x faster than those stored on pallets with plastic covers.
2. Targeted Solutions to Die Casting Aluminum Alloy Blackening
Solving blackening requires addressing its root causes across the entire production chain. Below is a linear, actionable breakdown of solutions for each cause category:
2.1 Optimize Production & Cleaning Processes
- Refine Cleaning Protocols:
- Utiliser neutral detergents (pH 6–8) instead of corrosive ones.
- Implement a 3-step cleaning process: Soak (10 minutes in 50°C detergent), rinse (high-pressure water, 0.3 MPa), sec (hot air, 80°C pour 5 minutes).
- Add ultrasonic cleaning for complex parts (par ex., cavités internes) to remove hidden residues.
- Adjust Die Casting Parameters:
- Increase injection pressure to 80–120 MPa and holding time to 15–20 seconds to reduce porosity (target porosity <2%).
- Use vacuum die casting for critical parts (par ex., boîtiers pour dispositifs médicaux) to eliminate micro-pores.
2.2 Improve Material Quality & Composition
- Control Alloy Composition:
- Limit iron content to <1.0% for ADC12 and <0.8% for A380.
- Prioritize high-purity raw materials (99.5%+ pure aluminum ingots) over scrap aluminum for critical parts.
- Add trace rare earth elements (par ex., 0.1–0.3% cerium) to stabilize the oxide film and reduce oxidation rates.
- Optimize Microstructure:
- Use grain refiners (par ex., titanium boride) during smelting to reduce grain size to <30 µm.
- Polish the surface to Ra <1.6 μm after casting to minimize oxidation-prone surface area.
2.3 Strengthen Surface Protection
Surface treatments form a physical barrier between aluminum and the environment, the most effective solutions are:
| Traitement de surface | Process Details | Protection Effect | Applications idéales |
| Anodisation | Create a 10–20 μm thick oxide layer via electrolysis. Type II anodizing (for cosmetics) or Type III hard anodizing (pour la durabilité). | Resists 48–72 hours of salt spray testing (ASTM B117); delays blackening by 6–12 months. | Electronic device shells, garniture extérieure automobile. |
| Spraying/Powder Coating | Apply a 50–100 μm thick polymer coating (par ex., époxy, polyester) via electrostatic spraying. | Isolates aluminum from moisture/oxygen; prevents blackening for 1–2 years. | Home appliance components (par ex., washing machine inner drums). |
| Passivation | Treat with chemical agents (par ex., chromate, trivalent chromium) to enhance oxide film stability. | Improves corrosion resistance by 30–50%; extends blackening-free storage to 60+ jours. | Small precision parts (par ex., boîtiers de capteurs) where thick coatings are impractical. |
2.4 Enhance Environmental & Storage Management
- Controlled Storage Conditions:
- Maintain warehouse humidity at 40–50% RH and temperature at 20–25°C. Use dehumidifiers in rainy seasons or coastal areas.
- Store parts on pallets (≥10 cm above ground) and wrap them in moisture-proof plastic film.
- Isolate Corrosive Media:
- Avoid storing aluminum parts near acid/alkali storage areas or industrial exhaust vents.
- Wear gloves during handling to prevent sweat (contains salts) from contacting the aluminum surface.
3. Practical Prevention: Full-Process Quality Control
Preventing die casting aluminum alloy blackening requires proactive control across the entire production lifecycle. Below is a list of key control points, organized by workflow stage:
3.1 Pre-Production (Matériel & Process Preparation)
- Verify alloy composition via optical emission spectroscopy (OES) to ensure iron content and impurities meet standards (par ex., ADC12: Fe ≤1.3%).
- Test cleaning agents for corrosion: Immerse a sample aluminum part in the detergent for 24 heures; reject detergents that cause discoloration.
3.2 In-Production (Fonderie & Cleaning)
- Monitor die casting parameters in real time (pression d'injection, holding time) using sensors; alert operators if parameters deviate by >10%.
- Conduct 100% visual inspection of cleaned parts: Use a white cloth to wipe the surface—reject parts with visible residue.
3.3 Post-Production (Stockage & Transportation)
- Implement periodic checks: Inspect 5% of stored parts weekly for early signs of blackening (par ex., faint spots); adjust storage conditions if issues arise.
- Use corrosion-resistant packaging for long-distance transportation (par ex., sea shipping): Add desiccants (5g per cubic meter of packaging) to absorb moisture.
Yigu Technology’s Perspective
Chez Yigu Technologie, we see die casting aluminum alloy blackening as a solvable issue—one that requires full-process control rather than post-hoc fixes. Pour les clients automobiles, we optimize cleaning processes (neutral detergents + nettoyage par ultrasons) et ajouter 0.2% cerium to ADC12 to reduce blackening rates by 70%. For electronic clients, we apply Type II anodizing (15 couche de µm) and moisture-proof packaging, ensuring parts stay bright for 12+ mois. We also train clients on storage best practices (40–50% RH, pallet storage) to prevent on-site blackening. Finalement, blackening prevention isn’t just about quality—it’s about protecting brand reputation by delivering consistent, long-lasting products.
FAQ
- Can already blackened die casting aluminum alloy parts be restored?
Oui, but restoration depends on blackening severity:
- Mild Blackening (faint spots): Clean with a neutral detergent (pH 6–8) + soft brush, then apply a passivation agent to restore brightness.
- Severe Blackening (thick oxide layer): Use a dilute acid solution (5% nitric acid) to remove the oxide layer, then re-anodize or spray-coat to prevent re-blackening. Note: Severe restoration may reduce part thickness by 0.01–0.02 mm—avoid for precision parts.
- How long can a well-treated die casting aluminum alloy part stay free from blackening?
With proper treatment and storage:
- Anodized parts (10–20 μm layer): 6–12 months in normal storage (40–50% RH).
- Powder-coated parts (50–100 μm layer): 1–2 years in normal storage.
- Passivated parts: 2–3 months in normal storage—best for short-term use (par ex., temporary components).
- Is scrap aluminum suitable for die casting if blackening is a concern?
Scrap aluminum can be used, but with strict controls:
- Limit scrap content to ≤30% of the total melt (higher scrap ratios increase impurities).
- Use only clean scrap (no oil, peinture, or corrosion); pre-treat scrap with a degreaser before smelting.
- Add grain refiners (par ex., titanium boride) to offset the negative effects of scrap impurities. Pour les pièces critiques (par ex., dispositifs médicaux), avoid scrap aluminum entirely.