铝压铸件后处理: 铸造完美

1. 介绍

铝 die casting is a high-efficiency, 广泛应用于汽车领域的近净成形制造工艺, 电子产品, 航天, and household appliance industries due to its ability to produce complex components with high dimensional accuracy and excellent mechanical properties.

然而, as-cast aluminum die castings often contain inherent defects such as flash, 毛刺, 孔隙率, surface oxides, 和残余应力.

Post-processing is therefore an indispensable link in the aluminum die casting production chain—it not only eliminates defects and improves surface quality but also optimizes mechanical performance, 增强耐腐蚀性, and ensures compliance with end-use requirements.

2. Why post-processing matters for die-cast aluminum

铸造 is a highly productive near-net-shape process, but the as-cast component is a starting point, not a finished engineering part.

Post-processing is essential because the as-cast condition carries characteristic microstructural features, surface conditions and defects that affect function, 可靠性, appearance and downstream manufacturability.

What the as-cast state leaves you with — the root causes for post-processing

• Near-surface and internal porosity. Hydrogen porosity (球形) and shrinkage/interdendritic porosity (irregular) form during solidification.
  Even low porosity volumes (fractions of 1%) can provide leakage paths, stress concentrators or initiation sites for fatigue cracks.
• Residual stresses and microstructural inhomogeneity. 高压铸造 (HPDC) cools rapidly and unevenly; this produces local residual stresses and non-uniform mechanical properties that can relax unpredictably during machining or in service.
• Surface discontinuities and excess metal. 大门, 跑步者, parting lines and flash are inherent to the process and must be removed or finished for function and safety.
• As-cast surface chemistry and contamination. Die lubricants, oxides and soluble residues remain on surfaces and interfere with coating adhesion, plating continuity and corrosion resistance.
• Insufficient dimensional accuracy for functional features. 交配面孔, sealing surfaces and threaded holes generally require machining to achieve the tolerances and finishes needed for assemblies.
• Low as-cast mechanical performance in critical zones. Typical die-cast Al-Si alloys have moderate as-cast strength and limited ductility; tailored heat treatment or aging can stabilize dimensions and improve mechanical properties where required.

3. Core Classification and Technical Principles of Aluminum Die Casting Post-processing

Aluminum die casting post-processing can be categorized into four core modules based on functional objectives: defect removal, 表面修饰, performance optimization, 和精确完成.

Each module adopts targeted technologies with distinct technical principles and application scenarios.

缺陷去除: Eliminating Inherent Casting Imperfections

Defect removal is the primary post-processing step, focusing on eliminating flash, 毛刺, 孔隙率, 收缩腔, and oxide inclusions generated during the die casting process.

These defects not only affect the appearance of components but also reduce structural integrity and fatigue life.

Trimming and Deflashing

Flash and burrs are inevitable in aluminum die casting, resulting from molten aluminum seeping into the gap between die halves.
Trimming and deflashing aim to remove these excess materials to meet dimensional specifications.

• Mechanical Trimming: The most widely used method, utilizing hydraulic or pneumatic presses with custom-designed trimming dies.
  It offers high efficiency (到 100 每分钟零件) and consistent precision, suitable for mass production.
  The principle is to apply concentrated pressure along the parting line to shear off flash.
  Key parameters include trimming force (determined by part thickness and aluminum alloy type) and die clearance (typically 0.05–0.15 mm to avoid part deformation).
• Cryogenic Deflashing: Suitable for complex-shaped components with hard-to-reach burrs (例如。, 内部频道).
  The process involves cooling the part to -70°C to -100°C using liquid nitrogen, which embrittles the burrs (aluminum alloy burrs lose ductility at low temperatures), then removing them via high-pressure air blasting or mechanical vibration.
  This method avoids part deformation but has higher operational costs than mechanical trimming.
• Thermal Deflashing: Uses high-temperature (500–600°C) molten salt or hot air to burn off burrs.
  It is suitable for small burrs (≤0.2 mm) but requires strict control of temperature and time to prevent part oxidation or dimensional changes.
  This method is gradually being phased out due to environmental concerns about molten salt waste.

Porosity and Shrinkage Cavity Treatment

Porosity in aluminum die castings (caused by trapped air or dissolved gases during solidification) severely impairs corrosion resistance and mechanical performance. Common treatment methods include:

• Impregnation Sealing: The most effective method for sealing surface and subsurface porosity.
  It involves immersing the part in a low-viscosity resin (例如。, 环氧树脂, 酚类) under vacuum or pressure, allowing the resin to penetrate into pores, then curing to form a impermeable seal.
  Per ASTM B945, impregnated parts can achieve leak rates as low as 1×10⁻⁶ cm³/s, making them suitable for hydraulic components and fluid-carrying parts.
• Welding Repair: Used for large shrinkage cavities or surface defects. 提格焊接 (钨惰性气) with matching aluminum alloy fillers (例如。, ER4043 for A380 die castings) is preferred to minimize heat input and avoid thermal deformation.
  然而, welding may introduce new stresses and requires post-weld heat treatment to restore mechanical properties.

Surface Modification: Enhancing Corrosion Resistance and Aesthetics

Aluminum die castings have poor natural corrosion resistance (due to the presence of alloying elements like silicon and copper).
Surface modification not only improves corrosion resistance but also provides decorative or functional surfaces (例如。, 电导率, 戴阻力).

化学转换涂层

Chemical conversion coatings form a thin (0.5–2 μm) adherent film on the aluminum surface via chemical reactions, enhancing corrosion resistance and serving as a primer for painting. 常见类型包括:

• 镀铬转化涂层: Traditional method using hexavalent chromium compounds, 提供出色的耐腐蚀性 (salt spray test ≥500 hours) and paint adhesion.
  然而, hexavalent chromium is highly toxic, and its use is restricted by REACH (欧盟) and RoHS directives. It is only permitted in specialized aerospace applications with strict waste treatment.
• Non-Chromate Conversion Coatings: Environmentally friendly alternatives, including trivalent chromium, cerium-based, and zirconium-based coatings.
  Trivalent chromium coatings (per ASTM D3933) provide salt spray resistance of 200–300 hours, comparable to hexavalent chromium, and are widely adopted in automotive and electronics industries.
  Cerium-based coatings (无机) offer good corrosion resistance but have lower paint adhesion, suitable for non-painted components.

阳极氧化

阳极氧化 creates a thick (5–25μm) 氧化膜 (al₂o₃) on the aluminum surface via electrolysis, significantly improving corrosion resistance and wear resistance.
For aluminum die castings, two types are commonly used:

• Type II Sulfuric Acid Anodizing: 最常见的类型, producing a porous oxide film that can be dyed in various colors.
  It offers salt spray resistance of 300–500 hours and is used in decorative components (例如。, 设备外壳, 汽车装饰).
  然而, die castings with high porosity may have uneven film formation, requiring pre-sealing with nickel acetate.
• Type III Hard Anodizing: Uses lower temperatures (-5°C to 5°C) and higher current densities to produce a dense, 难的 (HV 300–500) 氧化膜.
  It is suitable for wear-resistant components (例如。, 齿轮, 活塞) but may cause dimensional changes (film thickness must be accounted for in design).
  Aluminum die castings with high silicon content (例如。, A380, Si=7–11%) may form a brittle film, limiting its application.

有机涂料

Organic coatings (绘画, 粉末涂料) provide additional corrosion protection and aesthetic effects, often applied after chemical conversion coating.

• 粉末涂料: Uses electrostatically charged powder (聚酯纤维, 环氧树脂) that adheres to the aluminum surface, then cures at 180–200°C.
  It offers excellent durability (salt spray resistance ≥1000 hours) and is free of volatile organic compounds (VOC), making it environmentally friendly. Suitable for outdoor components (例如。, 汽车保险杠, 建筑固定装置).
• 液体绘画: Includes spray painting and dip coating, suitable for complex-shaped parts with intricate details.
  High-solids polyurethane paints are preferred for their corrosion resistance and gloss retention, but they require proper ventilation to control VOC emissions.
• 电子涂层 is a liquid-based electro-deposition process in which aluminum die-cast parts are immersed in a waterborne bath containing charged polymer particles.
  When an electric current is applied, these particles migrate and uniformly deposit onto all conductive surfaces, including complex geometries, corners, 和凹进.
  It provides excellent corrosion protection, 统一的覆盖范围, and strong adhesion to pre-treated or conversion-coated surfaces. Typical salt spray resistance can exceed 500 hours on properly prepared aluminum die castings.

性能优化: Adjusting Mechanical Properties and Residual Stresses

Aluminum die castings often have residual stresses (from uneven cooling during solidification) and limited mechanical properties. Post-processing techniques like heat treatment and stress relief are used to optimize performance.

热处理

Unlike wrought aluminum alloys, aluminum die castings have limited heat treatability due to porosity and alloy composition (high silicon content).
然而, certain alloys (例如。, A380, A383) can undergo specific heat treatments:

• T5 Heat Treatment: 溶液热处理 (480–500°C) followed by air cooling and artificial aging (150–180°C for 2–4 hours).
  This process improves tensile strength by 15–20% (A380 T5: tensile strength ≥240 MPa, yield strength ≥160 MPa) without significant dimensional changes. It is widely used in automotive structural components (例如。, 发动机支架).
• T6 Heat Treatment: 溶液热处理, 水淬火, 和人工衰老. It provides higher strength than T5 but may cause part deformation and porosity expansion (due to rapid cooling).
  T6 is only suitable for low-porosity die castings (例如。, those produced by vacuum die casting).

尤其, heat treatment of aluminum die castings must strictly control temperature uniformity to avoid thermal cracking. Per SAE J431, the maximum heating rate should not exceed 5°C/min for thick-walled parts.

压力缓解

Residual stresses in aluminum die castings can cause dimensional instability during machining or service. Stress relief methods include:

• Thermal Stress Relief: Heating the part to 200–250°C for 1–2 hours, then slow cooling.
  This reduces residual stresses by 30–50% without altering mechanical properties. It is a common pre-machining step for precision components (例如。, 电子外壳).
• 振动应力消除: Applying low-frequency vibration (10–100 Hz) to the part to induce microplastic deformation, relieving residual stresses.
  It is suitable for parts sensitive to heat (例如。, those with organic coatings) and offers shorter processing time (30–60分钟) than thermal stress relief.

Precision Finishing: Achieving Dimensional Accuracy and Surface Roughness

Although aluminum die castings have high dimensional accuracy (±0.05–0.1 mm), some critical surfaces (例如。, 配合面, threaded holes) require additional precision finishing to meet strict tolerances.

加工

数控加工 is the primary precision finishing method, including milling, 转动, 钻孔, 和敲击. Key considerations for machining aluminum die castings include:

• 工具选择: Carbide tools with sharp cutting edges are preferred to minimize cutting forces and avoid chip adhesion (aluminum has high ductility). Coated tools (例如。, 蒂恩) improve wear resistance and tool life.
• 切割参数: 高切割速度 (1500–3000 m/min) and moderate feed rates (0.1–0.3 mm/rev) are used to reduce heat generation and prevent workpiece deformation.
  冷却液 (emulsified oil or synthetic coolant) is essential to lubricate the cutting zone and flush chips.
• Porosity Impact: Porous areas can cause tool chatter and uneven surface finish. Pre-machining inspection (例如。, 超声测试) helps identify high-porosity regions, which may require repair or scrapping.

抛光和抛光

抛光 and buffing are used to improve surface roughness (Ra ≤0.2 μm) for decorative or optical components.
Abrasive polishing (using silicon carbide or aluminum oxide abrasives) is followed by buffing with a soft wheel and polishing compound (例如。, rouge) to achieve a mirror finish.
For die castings with porosity, a filler (例如。, polyester putty) may be applied before polishing to ensure a smooth surface.

3. Quality Control and Testing Standards for Post-processing

质量控制 (QC) is critical to ensuring consistency and reliability of post-processed aluminum die castings. QC measures cover each post-processing stage and adhere to international standards to maintain credibility.

维度检查

Dimensional accuracy is verified using tools ranging from basic gauges to advanced metrology equipment:

• 协调测量机 (CMM): Used for complex components to measure 3D dimensions with accuracy up to ±0.001 mm.
  Per ISO 10360, CMM calibration is required annually to ensure measurement reliability.
• Vision Inspection Systems: High-speed optical inspection for surface defects (例如。, 划痕, 凹痕) and dimensional deviations. Suitable for mass production, with detection rates up to 99.9% for defects ≥0.1 mm.
• 硬度测试: Brinell or Vickers hardness testing (per ASTM E140) to verify heat treatment effectiveness. For A380 T5 die castings, the typical hardness is 80–95 HB.

耐腐蚀性测试

Corrosion resistance of surface-treated parts is evaluated using standardized tests:

• 盐喷雾测试 (ASTM B117): The most common test, exposing parts to a 5% NaCl spray at 35°C.
  The duration of corrosion-free performance (例如。, 500 hours for anodized parts) is used to qualify surface treatments.
• 电化学阻抗光谱 (EIS): A non-destructive test to evaluate the integrity of surface coatings.
  It measures the coating’s impedance to assess corrosion resistance and predict service life.

非破坏性测试 (NDT) for Defects

NDT methods detect internal and surface defects without damaging the part:

• X-Ray Inspection (ASTM E164): Used to detect internal porosity, 收缩腔, and welding defects.
  Digital radiography (DR) provides real-time imaging and improved defect detection accuracy compared to traditional film radiography.
• 超声测试 (ASTM A609): Evaluates subsurface porosity and bond integrity of coatings.
  High-frequency sound waves (2–10 MHz) are transmitted through the part, and reflections from defects are analyzed to determine their size and location.
• 染料渗透剂测试 (ASTM E165): Detects surface cracks and porosity. A colored dye is applied to the part, penetrates into defects, then excess dye is removed, and a developer is applied to reveal defects.

4. Industry-Specific Applications of Post-processing

Post-processing requirements for aluminum die castings vary by industry, depending on functional needs, 环境条件, 和监管标准. Below are key applications in major industries:

汽车行业

汽车 aluminum die castings (例如。, 发动机块, 传输外壳, 悬架组件) require strict post-processing to meet durability and safety standards:

• 发动机块: T5 heat treatment to improve strength, impregnation sealing to prevent oil leakage, and CNC machining of mating surfaces (tolerance ±0.01 mm).
• Exterior components (保险杠, 修剪): Trivalent chromium conversion coating + powder coating to resist corrosion from road salt and environmental factors (salt spray test ≥1000 hours).

电子行业

Electronic 成分 (例如。, 智能手机外壳, 散热器) demand high surface quality, 维度的准确性, 和电磁兼容性 (EMC):

• 智能手机外壳: Precision CNC machining, polishing to mirror finish, and anodizing (II型) for corrosion resistance and color customization.
• 散热器: Chemical conversion coating to enhance thermal conductivity, and CNC drilling to create cooling channels (公差±0.02 mm).

航空业

Aerospace aluminum die castings (例如。, aircraft brackets, 液压成分) require rigorous post-processing and quality control to meet aerospace standards (SAE AS9100):

• 液压成分: Impregnation sealing (per SAE AS4775) to ensure leak tightness, and T6 heat treatment for high strength.
• 结构支架: Vibratory stress relief to eliminate residual stresses, and ultrasonic testing to detect internal defects.

Household Appliance Industry

Appliance components (例如。, refrigerator compressor housings, washing machine drums) focus on corrosion resistance and aesthetics:

• 压缩机外壳: Powder coating to resist moisture and corrosion, and thermal stress relief to prevent dimensional changes during operation.
• 装饰面板: 抛光 + anodizing or painting to achieve a visually appealing finish.

5. 结论

Aluminum die-cast post-processing is not a single operation but a tailored sequence chosen to meet mechanical, 泄漏, cosmetic and assembly requirements.

Early collaboration between design, foundry and finishing suppliers yields the best balance of cost and performance: 设计可制造性 (均匀的壁厚, adequate draft, boss geometry for inserts), minimize post-processing where possible, and specify clear acceptance tests.

For critical pressure, 密封, or high-fatigue applications, plan for vacuum impregnation, X-ray inspection and controlled heat treatment.

For appearance and corrosion resistance, select conversion pre-treatment compatible with chosen final coating, and avoid restricted chemistries when possible.

 

常见问题解答

When should I specify vacuum impregnation?

When parts are required to be leak-tight (hydraulic housings), when plating or painting will be compromised by through-porosity, or for parts subject to fluid sealing. Impregnation is a standard remedy for through-porosity.

Can all die-cast aluminum be anodized?

Not effectively. High-Si die-cast alloys often give poor anodize finish. If anodizing is required, use a compatible alloy or specify special pre-treatments and acceptance criteria.

What thread insert is best for die-cast bosses?

For high pull-out strength and durability use solid inserts (例如。, M4–M12) installed by press or thermal insertion; Helicoil is common for smaller diameters. Specify boss thickness and insert type in design.

Is post-casting heat treatment always beneficial?

并不总是. T5 ageing can improve properties and stability for many die-cast alloys.

Full solution + 年龄 (T6) may be impractical or ineffective on some die-cast alloys and can increase distortion.

How do I control costs while ensuring quality?

Reduce the number of critical machined features, design for minimal porosity risk (even wall thickness), specify only necessary tests (例如。, sample X-ray vs 100% 检查), and choose common, compliant coating systems. Early supplier involvement is the most effective lever.