Что вызывает почернение алюминиевого сплава при литье под давлением и как его решить?

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Die casting aluminum alloy blackening is a widespread quality issue that degrades product appearance, reduces corrosion resistance, and shortens service life—posing significant challenges for manufacturers in automotive, Электроника, and home appliance industries. This unwanted discoloration stems from uncontrolled oxidation of the aluminum surface, driven by production processes, Свойства материала, Факторы окружающей среды, and post-treatment errors. 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, подкреплено причинно-следственными цепочками и конкретными примерами:

1.1 Производство & 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 IssueTechnical ExplanationImpact on Blackening
Contaminant AdhesionResidues of release agents, режущая жидкости, или saponification fluids remain on the aluminum surface after casting. These substances contain corrosive components (НАПРИМЕР., 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, против. 30+ days for a clean part.
Incomplete CleaningImproper cleaning processes (НАПРИМЕР., insufficient rinsing, low detergent concentration) fail to remove contaminants. Gaps in cleaning (НАПРИМЕР., 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 недели, while the rest of the surface remains intact.
Poor Die Casting ParametersExcessive porosity or loosening caused by low injection pressure (≤50 MPa) or inadequate holding time (<10 секунды) creates micro-pores in the aluminum. These pores trap moisture and contaminants, triggering internal oxidation.Uniform blackening: A porous LED heat sink (пористость >5%) turns uniformly black after 1 month in a humid environment, as moisture seeps into pores and reacts with aluminum.

1.2 Свойства материала & Композиция

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 (НАПРИМЕР., Al₃Fe). These compounds are more prone to oxidation than pure aluminum, creating black spots on the surface. Например, an ADC12 part with 1.5% iron content is 2–3x more likely to blacken than one with 0.8% железо.
  • Зерновая структура & Шероховатость поверхности: Alloys with coarse grains (grain size >50 μm) или грубые поверхности (Ra >6.3 μm) have larger surface areas exposed to oxygen. Fine-grained (grain size <30 мкм) and smooth-surfaced (Раствор <1.6 мкм) alloys form more uniform oxide films, resisting blackening. A rough aluminum gearbox housing (Раствор 12.5 мкм) may show blackening within 14 дни, while a polished one (Раствор 0.8 мкм) stays bright for 60+ дни.

1.3 Environmental Factors

External environmental conditions accelerate oxidation by attacking the aluminum’s natural oxide film. The most impactful factors are:

  • Температура & Влажность: Высокая влажность (>60% RH) provides the moisture needed for electrochemical oxidation. In rainy seasons or coastal areas, the risk of blackening increases by 40–60%. Например, an aluminum air conditioner compressor shell stored in a 70% RH warehouse may turn black in 20 дни, против. 45 days in a 40% RH warehouse.
  • Corrosive Media Contact: Exposure to strong acids, щелочи, or salt spray (НАПРИМЕР., coastal environments, industrial workshops) directly destroys the protective oxide film. A marine-grade aluminum bracket exposed to salt spray (за ASTM B117) turns black within 48 часы, as saltwater breaks down the oxide layer and triggers rapid oxidation.

1.4 Пост-обработка & Misuse

Errors in post-treatment or storage amplify blackening risks, even for well-manufactured parts:

  • Improper Cleaning Agents: Highly corrosive cleaning agents (НАПРИМЕР., 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 дни, против. 25+ days for one cleaned with a neutral detergent (pH 6–8).
  • Negligent Storage Management: Differences in humidity across warehouse heights (НАПРИМЕР., 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:
  1. Использовать neutral detergents (pH 6–8) instead of corrosive ones.
  2. Implement a 3-step cleaning process: Soak (10 minutes in 50°C detergent), смывать (high-pressure water, 0.3 МПА), сухой (hot air, 80° C для 5 минуты).
  3. Add ultrasonic cleaning for complex parts (НАПРИМЕР., Внутренние полости) 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 (НАПРИМЕР., Корпуса медицинского устройства) to eliminate micro-pores.

2.2 Improve Material Quality & Композиция

  • 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 (НАПРИМЕР., 0.1–0.3% cerium) to stabilize the oxide film and reduce oxidation rates.
  • Optimize Microstructure:
  • Use grain refiners (НАПРИМЕР., titanium boride) during smelting to reduce grain size to <30 мкм.
  • 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:

Поверхностная обработкаProcess DetailsProtection EffectИдеальные приложения
АнодированиеCreate a 10–20 μm thick oxide layer via electrolysis. Type II anodizing (for cosmetics) or Type III hard anodizing (для долговечности).Resists 48–72 hours of salt spray testing (ASTM B117); delays blackening by 6–12 months.Электронные оболочки устройства, Автомобильная внешняя отделка.
Spraying/Powder CoatingApply a 50–100 μm thick polymer coating (НАПРИМЕР., эпоксидная смола, полиэстер) via electrostatic spraying.Isolates aluminum from moisture/oxygen; prevents blackening for 1–2 years.Home appliance components (НАПРИМЕР., washing machine inner drums).
ПассивацияTreat with chemical agents (НАПРИМЕР., хромат, trivalent chromium) to enhance oxide film stability.Improves corrosion resistance by 30–50%; extends blackening-free storage to 60+ дни.Small precision parts (НАПРИМЕР., Корпуса датчиков) 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 (Материал & Process Preparation)

  • Verify alloy composition via optical emission spectroscopy (OES) to ensure iron content and impurities meet standards (НАПРИМЕР., ADC12: Fe ≤1.3%).
  • Test cleaning agents for corrosion: Immerse a sample aluminum part in the detergent for 24 часы; reject detergents that cause discoloration.

3.2 In-Production (Кастинг & Уборка)

  • Monitor die casting parameters in real time (давление впрыска, время выдержки) 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 (Хранилище & Транспорт)

  • Implement periodic checks: Осмотреть 5% of stored parts weekly for early signs of blackening (НАПРИМЕР., faint spots); adjust storage conditions if issues arise.
  • Use corrosion-resistant packaging for long-distance transportation (НАПРИМЕР., sea shipping): Add desiccants (5g per cubic meter of packaging) поглощать влагу.

Перспектива Yigu Technology

В Yigu Technology, we see die casting aluminum alloy blackening as a solvable issue—one that requires full-process control rather than post-hoc fixes. Для автомобильных клиентов, we optimize cleaning processes (neutral detergents + Ультразвуковая чистка) and add 0.2% cerium to ADC12 to reduce blackening rates by 70%. Для электронных клиентов, we apply Type II anodizing (15 мкм слой) and moisture-proof packaging, ensuring parts stay bright for 12+ месяцы. We also train clients on storage best practices (40–50% RH, pallet storage) to prevent on-site blackening. В конечном счете, blackening prevention isn’t just about quality—it’s about protecting brand reputation by delivering consistent, долговечные продукты.

Часто задаваемые вопросы

  1. Can already blackened die casting aluminum alloy parts be restored?

Да, 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% азотная кислота) to remove the oxide layer, then re-anodize or spray-coat to prevent re-blackening. Примечание: Тяжелая реставрация может уменьшить толщину детали на 0,01–0,02 мм — избегайте прецизионных деталей..
  1. Как долго хорошо обработанная деталь из алюминиевого сплава, отлитая под давлением, не почернеет??

При правильном обращении и хранении:

  • Анодированные детали (10слой –20 мкм): 6–12 месяцев при нормальном хранении (40–50% RH).
  • Детали с порошковым покрытием (50слой –100 мкм): 1–2 года при нормальном хранении.
  • Пассивированные детали: 2–3 месяца при нормальном хранении – лучше всего подходит для краткосрочного использования (НАПРИМЕР., временные компоненты).
  1. Подходит ли алюминиевый лом для литья под давлением, если вызывает беспокойство почернение??

Алюминиевый лом можно использовать., но со строгим контролем:

  • Ограничьте содержание лома до ≤30% от общего объема расплава. (более высокий коэффициент лома увеличивает примеси).
  • Используйте только чистый лом (Нет масла, краска, или коррозия); перед плавкой лом предварительно обработать обезжиривателем.
  • Добавить переработчики зерна (НАПРИМЕР., titanium boride) компенсировать негативное воздействие примесей лома. Для критических частей (НАПРИМЕР., медицинские устройства), полностью избегать лома алюминия.
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