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 (e.g., pockmarks) that affect aesthetics, shrinkage cracking directly compromises structural integrity: parts with such cracks may fail under load, leading to safety hazards (e.g., 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 (e.g., cold cracks from rapid cooling).
| Feature | Shrinkage Cracking | Cold Cracking (for Comparison) |
| Appearance | Thin, branching cracks; often follows solidification paths (e.g., along wall thickness transitions). | Straight, brittle cracks; no branching; edges show no oxidation. |
| Formation Time | Occurs during solidification (when the alloy shrinks but is constrained). | Forms after solidification (due to rapid cooling and thermal stress). |
| High-Incidence Areas | Thick-walled sections (e.g., engine block ribs), wall thickness transitions (e.g., 10mm → 3mm), and near cores/inserts. | Sharp corners, mold parting lines, and areas with high residual stress. |
| Detection Method | X-ray flaw detection (for internal cracks); dye penetrant inspection (DPI) for surface cracks. | Ultrasonic testing (UT); visible to the naked eye for severe cases. |
2. Multi-Dimensional Causes of Shrinkage Cracking
Shrinkage cracking is not caused by a single factor—it results from the synergy of material properties, mold design, process parameters, and operational errors. Below is a detailed breakdown of each core cause:
A. Material Factors: The “Foundation” of Crack Susceptibility
Aluminum alloy composition and microstructure directly determine its ability to resist shrinkage stress.
| Material Issue | Technical Details | Impact on Shrinkage Cracking |
| Harmful Element Overload | – Iron (Fe) content >1.2%: Forms hard β-Al₅FeSi phases that weaken grain boundaries.- Zinc (Zn)/Copper (Cu) excess in Al-Si-Cu alloys: Increases brittleness (tensile strength drops by 15–20%).- Magnesium (Mg) excess in Al-Mg alloys: Causes intergranular corrosion, reducing crack resistance. | Hard phases act as stress concentration points; brittle alloys break easily when shrinkage is constrained. |
| Impurities & Oxides | Raw materials with >0.5% oxide slag or foreign particles (e.g., dust, oil residues). | Oxides hinder metal flow during solidification, creating voids that evolve into cracks under stress. |
| Abnormal Grain Structure | Coarse grains (grain size >100μm) or non-dendritic structures from uneven cooling. | Coarse grains reduce grain boundary bonding force—shrinkage stress pulls grains apart, forming cracks. |
B. 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 (e.g., 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 <1mm) concentrate stress (stress concentration factor >3). Shrinkage stress here exceeds the alloy’s tensile strength (typically 200–300MPa for aluminum), leading to cracking.
2. Mold Temperature Control Failure
- Local Overheating: Insufficient cooling in thick-walled mold areas (e.g., 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 (e.g., 220°C vs. 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 <1mm (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 (e.g., 90° bends without gradual transitions) causes cold separation. These weak, partially fused areas become crack initiation points when shrinkage occurs.
C. Process Parameter Mismatches: The “Operational” Catalyst
Incorrect die casting parameters exacerbate shrinkage stress and reduce the alloy’s ability to resist cracking.
| Parameter Issue | Technical Details | Quantitative Thresholds | Impact on Cracking |
| Excessive Pouring Temperature | Temperature >720°C (for Al-Si alloys) prolongs solidification time. | Each 10°C increase beyond 700°C extends solidification by ~15%, increasing shrinkage volume. | Longer shrinkage time = more stress accumulation; overheated alloy also becomes more brittle. |
| Insufficient Mold Preheating | Mold temperature <180°C (for Al-Si alloys) causes rapid surface cooling. | A mold at 150°C cools the alloy’s surface to solid in <1 second, while the core remains liquid. | The rigid surface layer traps the shrinking core, creating “pulling” stress that cracks the part. |
| Low Injection Specific Pressure | Pressure <50MPa (for cold chamber die casting) fails to compensate for shrinkage. | Pressure <40MPa: The far end of the casting has a shrinkage porosity rate >5% (ASTM E446 standard). | Porous areas are weak; shrinkage stress turns small pores into large cracks. |
| Improper Filling Speed | – Too fast (>5m/s): Causes turbulence and gas entrainment.- Too slow (<2m/s): Prolongs solidification, increasing shrinkage. | Speed >6m/s: Gas content in the alloy doubles (from 0.2 to 0.4cc/100g Al). | Gas bubbles act as stress concentrators; slow filling amplifies shrinkage constraints. |
D. 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 (for thick-walled parts) 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: Prevent & Fix Shrinkage Cracking
Resolving shrinkage cracking requires a holistic approach—addressing material, design, process, and operational issues. Below is a step-by-step solution:
A. Material Optimization: Enhance Crack Resistance
- 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 (Ti) to refine grains (reduces grain size to <50μm), improving tensile strength by 15–20%.
- Purify the Alloy:
- Use a three-stage degassing process: rotary blowing (400rpm) → ceramic foam filtration (20-ppi filters) → online slag removal. This reduces oxide content to <0.1%.
- Preheat Raw Materials:
- Preheat ingots to 300–400°C before melting to remove moisture and oil—avoids inclusions.
B. Mold Design Improvement: Reduce Shrinkage Constraints
- Optimize Part Structure:
- Limit wall thickness ratio to ≤2:1 (e.g., 6mm → 3mm).
- Add rounded corners (radius ≥2mm) to reduce stress concentration.
- For thick-walled parts ( >8mm), add “shrinkage compensation” ribs (width 3–5mm) to absorb shrinkage stress.
- 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.
- Redesign Gating Systems:
- Inner gate thickness: 1.5–2× the part’s wall thickness (e.g., 6mm gate for 3mm walls).
- Use fan gates or spiral runners to ensure laminar flow (Reynolds number <2000).
C. Process Parameter Fine-Tuning
| Parameter | Optimal Settings (Al-Si Alloys, Cold Chamber) | Monitoring Method |
| Pouring Temperature | 680–700°C | Digital thermocouple (±2°C accuracy) |
| Mold Preheating Temperature | 200–220°C | Infrared thermal imager |
| Injection Specific Pressure | 50–70MPa | Pressure sensor (real-time curve monitoring) |
| Filling Speed | 3–4m/s (staged: slow start → fast middle → slow end) | Speed encoder (±0.1m/s precision) |
| Holding Time | 10–15 seconds (1.5× solidification time) | Timer linked to mold temperature |
| Mold Opening Time | 30–45 seconds (for 8–10mm thick parts) | Proximity sensor (triggers opening when core temperature <300°C) |
D. Operational Standardization
- Punch Oil Control: Limit to 2–3 drops per cycle; use high-temperature-resistant oil (stable at >300°C).
- Release Agent Application: Spray a thin, uniform layer (5–8μm) with an airbrush; clean exhaust grooves after 50 cycles.
- Ejection System Check: Align ejector rods monthly (偏差 ≤0.05mm); test synchronization with a force gauge (ensure uniform pressure <200MPa).
4. Yigu Technology’s Perspective on Aluminum Alloy Die Casting Shrinkage Cracking
At Yigu Technology, we see shrinkage cracking as a “systemic signal”—it reveals gaps in material control, design, or process. For automotive clients producing thick-walled engine brackets, our alloy optimization (Fe ≤0.8%, Ti refinement) and conformal cooling molds reduced shrinkage cracking from 12% to <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 (TiAlN) 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.
FAQ
- Can shrinkage cracks in aluminum alloy die castings be repaired, or must the part be scrapped?
Minor surface cracks (depth <0.5mm) can be repaired with aluminum alloy welding (TIG welding) followed by heat treatment. However, internal or deep cracks (>0.5mm) require scrapping—repairing hides structural weaknesses that may fail under load. We recommend X-ray testing to assess crack depth before deciding.
- Does shrinkage cracking affect only thick-walled parts, or can thin-walled parts also be affected?
Thick-walled parts ( >5mm) are more prone, but thin-walled parts can also crack if there are sudden thickness changes (e.g., 2mm → 5mm) or poor mold cooling. For example, 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.
- 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) Location: 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 (shrinkage) or is straight (cold).