What Causes Shrinkage Cracking in Aluminum Alloy Die Casting?

Shrinkage cracking is one of the most destructive defects in aluminum alloy die casting—manifesting as thin, linear cracks that often penetrate the entire part. Unlike surface blemishes (НАПРИМЕР., оспины) that affect aesthetics, shrinkage cracking directly compromises structural integrity: parts with such cracks may fail under load, leading to safety hazards (НАПРИМЕР., automotive component failure) or costly scrapping (scrap rates can reach 8–15% for thick-walled parts). But what exactly triggers this defect? Is it a material issue, a mold design flaw, or a process parameter mistake? This article breaks down the multi-dimensional causes of shrinkage cracking and provides actionable solutions to prevent it.

1. What Is Shrinkage Cracking in Aluminum Alloy Die Casting?

Before diving into causes, it’s critical to define the defect clearly—avoiding confusion with other crack types (НАПРИМЕР., cold cracks from rapid cooling).

2. Multi-Dimensional Causes of Shrinkage Cracking

Shrinkage cracking is not caused by a single factor—it results from the synergy of material properties, дизайн плесени, process parameters, and operational errors. Below is a detailed breakdown of each core cause:

А. Material Factors: The “Foundation” of Crack Susceptibility

Aluminum alloy composition and microstructure directly determine its ability to resist shrinkage stress.

Беременный. Mold Design Defects: The “Structural” Trigger

Poor mold design creates physical constraints that amplify shrinkage stress.

1. Unreasonable Part/ Mold Structure

• Sudden Wall Thickness Changes: A thickness ratio >3:1 (НАПРИМЕР., 9mm → 3mm) forms “hot spots”—thick areas solidify slowly, while thin areas solidify first. The thin, rigid sections constrain the thick section’s shrinkage, generating tensile stress (>250MPa for Al-Si alloys) that causes cracking.
• Sharp Corners & Lack of Transitions: Corners without rounded edges (radius <1мм) concentrate stress (stress concentration factor >3). Shrinkage stress here exceeds the alloy’s tensile strength (typically 200–300MPa for aluminum), приводя к растрескиванию.

2. Mold Temperature Control Failure

• Local Overheating: Insufficient cooling in thick-walled mold areas (НАПРИМЕР., no cooling channels near 10mm-thick cavities) delays solidification. The prolonged liquid state means the alloy shrinks more, while surrounding solidified metal resists—creating cracks.
• Severe Temperature Gradients: A temperature difference >40°C between the upper and lower mold halves (НАПРИМЕР., 220° C против. 180° C.) disrupts solidification order. Metal in hot areas shrinks later, but cold areas are already rigid, forcing cracks.

3. Gating System Deficiencies

• Too-Thin Inner Gates: A gate thickness <1мм (for parts with 5mm walls) limits pressure transmission. The far end of the casting doesn’t receive enough pressure to compensate for shrinkage, leading to voids and subsequent cracking.
• Poor Runner Layout: Turbulent flow from misaligned runners (НАПРИМЕР., 90° bends without gradual transitions) causes cold separation. These weak, partially fused areas become crack initiation points when shrinkage occurs.

В. Process Parameter Mismatches: The “Operational” Catalyst

Incorrect die casting parameters exacerbate shrinkage stress and reduce the alloy’s ability to resist cracking.

Дюймовый. Operational & Maintenance Errors: The “Human” Factor

Even with good materials and design, mistakes during operation can trigger shrinkage cracking.

• Punch Oil Contamination: Excessive drip lubrication ( >5 drops per cycle) introduces unburned oil into the molten alloy. The oil forms hard carbon inclusions (size 5–10μm) that weaken the alloy—shrinkage stress propagates along these inclusions.
• Over-Spraying Release Agent: Thick release agent layers (>10μm) clog exhaust grooves, trapping gas. Gas expands during solidification, pushing against the alloy and creating cracks.
• Delayed Mold Opening: Mold opening time >60 seconds (Для толстостенных частей) keeps the casting in the mold while it continues to shrink. The mold’s rigidity prevents natural shrinkage, building up stress that cracks the part when demolded.
• Uneven Ejection Force: Ejector rods with misaligned positions (偏差>0.1mm) or unsynchronized movement apply local pressure (>300MPa) to the casting. This extra force, combined with shrinkage stress, causes cracking at the ejection points.

3. Solution Framework: Предотвращать & Fix Shrinkage Cracking

Resolving shrinkage cracking requires a holistic approach—addressing material, дизайн, процесс, and operational issues. Below is a step-by-step solution:

А. Material Optimization: Enhance Crack Resistance

1. Control Alloy Composition:

• Fe ≤0.9%, Zn ≤0.5%, Cu ≤1.0% (for Al-Si-Cu alloys like ADC12).
• Add 0.1–0.2% titanium (Из) to refine grains (reduces grain size to <50мкм), improving tensile strength by 15–20%.

2. Purify the Alloy:

• Use a three-stage degassing process: rotary blowing (400об/мин) → ceramic foam filtration (20-ppi filters) → online slag removal. This reduces oxide content to <0.1%.

3. Preheat Raw Materials:

• Preheat ingots to 300–400°C before melting to remove moisture and oil—avoids inclusions.

Беременный. Mold Design Improvement: Reduce Shrinkage Constraints

1. Optimize Part Structure:

• Limit wall thickness ratio to ≤2:1 (НАПРИМЕР., 6mm → 3mm).
• Add rounded corners (radius ≥2mm) Чтобы уменьшить концентрацию стресса.
• Для толстостенных деталей ( >8mm), add “shrinkage compensation” ribs (width 3–5mm) to absorb shrinkage stress.

2. Improve Temperature Control:

• Install conformal cooling channels (distance from cavity: 5–8mm) for uniform temperature.
• Use a mold temperature controller with ±5°C tolerance to eliminate gradients.

3. Redesign Gating Systems:

• Inner gate thickness: 1.5–2× the part’s wall thickness (НАПРИМЕР., 6mm gate for 3mm walls).
• Use fan gates or spiral runners to ensure laminar flow (Reynolds number <2000).

В. Process Parameter Fine-Tuning

Дюймовый. Операционная стандартизация

• Punch Oil Control: Limit to 2–3 drops per cycle; use high-temperature-resistant oil (stable at >300°C).
• Release Agent Application: Распылите тонкую, равномерный слой (5–8μm) with an airbrush; clean exhaust grooves after 50 цикл.
• Ejection System Check: Align ejector rods monthly (偏差 ≤0.05mm); test synchronization with a force gauge (ensure uniform pressure <200МПА).

4. Yigu Technology’s Perspective on Aluminum Alloy Die Casting Shrinkage Cracking

В Yigu Technology, we see shrinkage cracking as a “systemic signal”—it reveals gaps in material control, дизайн, or process. For automotive clients producing thick-walled engine brackets, our alloy optimization (Фе ≤0,8%, Ti refinement) and conformal cooling molds reduced shrinkage cracking from 12% к <1.8%. For 3C clients with thin-walled parts, our AI-driven process control (real-time pressure/speed adjustment) eliminated cracking caused by parameter mismatches.

We’re advancing two key innovations: 1) A “shrinkage stress simulation tool” that predicts cracking risks in mold design (cuts trial-and-error time by 40%); 2) Wear-resistant mold coatings (Тилн) that maintain uniform temperature, reducing local overheating. Our goal is to help manufacturers shift from “post-repair” to “prevention”—turning shrinkage cracking from a costly defect into a controllable factor.

Часто задаваемые вопросы

1. Can shrinkage cracks in aluminum alloy die castings be repaired, or must the part be scrapped?

Minor surface cracks (глубина <0.5мм) can be repaired with aluminum alloy welding (Тиг сварка) followed by heat treatment. Однако, internal or deep cracks (>0.5мм) require scrapping—repairing hides structural weaknesses that may fail under load. We recommend X-ray testing to assess crack depth before deciding.

2. Does shrinkage cracking affect only thick-walled parts, or can thin-walled parts also be affected?

Thick-walled parts ( >5мм) are more prone, but thin-walled parts can also crack if there are sudden thickness changes (НАПРИМЕР., 2mm → 5mm) or poor mold cooling. Например, thin-walled LED heat sinks with 3mm-thick mounting bosses often crack at the boss-base transition—fix this by adding a 1mm transition fillet.

3. How can I quickly diagnose if a crack is caused by shrinkage or cold cracking?

Check two factors: 1) Edge oxidation: Shrinkage cracks (formed during solidification) have oxidized edges (brown/black), while cold cracks (formed after solidification) have bright, unoxidized edges. 2) Расположение: Shrinkage cracks occur at thick areas/transitions; cold cracks occur at sharp corners or parting lines. For confirmation, use X-ray to see if the crack follows solidification paths (усадка) or is straight (холодный).