Die casting cold cracks are cracks that occur after the casting is fully solidified due to thermal or mechanical stress. They are usually caused by uneven cooling, improper die casting mold design, or excessive ejection force. Unlike hot cracks, cold cracks form at lower temperatures and can be internal or hidden. Prevention measures include maintaining uniform mold temperature, optimizing injection and ejection parameters, using high-quality alloys, and performing appropriate post-heat treatment.
This article will provide a comprehensive examination of how to identify cold cracks, their causes, prevention methods, repair techniques, key design recommendations, and why CEX Casting is the ideal partner to resolve aluminium die casting cold crack defects.
Characteristics and Identification of Cold Cracks
Characteristics
Cold cracks often appear as jagged or hairline cracks and usually appear after the part is fully solidified.
They may appear on the surface, contour lines, or edges, and are small, irregular, and sometimes difficult to detect with the naked eye.
Cold cracks often occur in stress concentration areas such as thick wall structures, rib intersections, sharp internal corners, and ejector marks, especially when the wall thickness transition is uneven or cooling is unbalanced.
Identification Method
Visual inspection is suitable for finding obvious surface cold cracks, and dye penetration can identify subtle surface cold cracks.
X-rays are suitable for detecting internal cold cracks, especially in areas with uneven wall thickness.
Ultrasonic waves identify the location and depth of cold cracks through sound wave reflection, and are suitable for key parts detection.
Cold Cracks
Causes of Die Casting Cold Cracks
Thermal Stress
Uneven Cooling Rate
During the solidification process of castings, differences in thickness or shape lead to different cooling rates in different areas.
The slow cooling area continues to expand, while the fast cooling area has begun to shrink, forming an internal stress concentration.
When this stress exceeds the tolerance limit of the material, cracks will appear after cooling.
Poor Cooling Channel Design
If the cooling channel layout is unreasonable, the number is insufficient, or it is too far away from the key area, local overheating areas (hot nodes) will be formed.
These areas solidify late, resulting in an expansion of the thermal gradient, which makes it impossible to release the thermal stress in time, and it is very easy to form cold cracks during demolding or cooling.
Mechanical Stress
Excessive Ejection Force
If the die casting sticks due to insufficient draft angle, rough die surface, or die wear, a larger ejection force needs to be applied.
This additional mechanical stress will concentrate at stress weak points such as rib roots, corners, and thin-walled areas, causing cracks and even expanding into structural fractures during subsequent CNC processing or transportation.
Premature Processing or Handling
Processing, trimming, or handling of castings before they are completely cooled and thermally stable will disrupt the internal stress distribution that has not yet been released.
This disturbance is very likely to induce cold cracks in castings with high precision requirements or asymmetric structures.
Mold Design Defects
Sharp Corners and Sudden Changes
If there are sharp internal corners, rib intersections, or sudden changes in cross-section in the mold, the aluminum liquid will not flow smoothly, the cooling will be uneven, and local stress concentration will occur.
These stress hot spots become typical cold crack starting points and are very easy to expand during the use of the casting.
Large Wall Thickness Variation
If there is a large difference in wall thickness of the casting, the thick area will cool slowly, and the thin area will shrink quickly, resulting in strong stretching in the boundary area.
This stress unevenness accumulates in the transition area and eventually forms cold cracks that develop in the direction of tensile stress, which is common in shell and frame structures.
Temperature Control Problems
Low Alloy or Mold Temperature
If the molten aluminum temperature or mold temperature is too low, it will accelerate the solidification rate, shorten the stress relief time, and make the internal thermal gradient of the metal too high.
Rapid cooling “locks” the internal stress, and a slight disturbance in the later stage may release it in the form of cold cracks or deformation.
Thermal Shock Effect
When the mold is not fully preheated and high-temperature aluminum liquid is injected, the drastic temperature difference between the mold and the aluminum liquid will cause surface hardening and rapid volume shrinkage.
The initially formed microcracks may expand into complete cold cracks during subsequent cooling, processing, or use.
Alloy-Related Issues
Low Ductility Alloys
Some aluminum alloys with high silicon and high iron content have poor tensile strength and ductility, and weak stress absorption ability.
It is difficult to relieve internal tension during cooling or ejection, and cold cracks are prone to occur under temperature or external force disturbances.
Impurities and Recycled Material
Recycled aluminum that has not been fully degassed and refined often contains oxides, inclusions, and pores.
These internal defects will weaken the local structure and become stress concentration points. During cooling and shrinkage, these areas are more prone to cold cracking.
Best Practices for Preventing Cold Cracking
Mold Thermal Management
Uniform Mold Preheating
The mold needs to use a mold temperature controller or heating rod to preheat each cavity surface to 180–250°C (adjusted according to alloy type and wall thickness).
Keep the temperature consistent before starting the die casting process. Pay special attention to the temperature of areas such as ribs and thick walls.
Design of Equidistant Cooling Channels
During the mold design stage, the number, diameter, and distribution of cooling channels should be planned through mold flow analysis tools (such as MAGMASOFT, AnyCasting).
Key practices include:
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Add cooling water channels at the rib root, thick wall, and transition area. The channel diameter is generally φ8–φ12mm, and the coverage is maintained at equal distances.
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The water temperature is controlled at 20–30°C, and the flow rate is ≥1.5m/s.
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For areas where it is inconvenient to open water channels, heat pipes or oil cooling are used for auxiliary cooling.
Cooling System
Optimize Injection Parameters
Control Filling Speed and Pressure
The primary and secondary injection speeds should be adjusted according to the trial mold data. Recommended parameters:
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Primary speed: 0.1–0.3m/s, for slow advancement.
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Secondary speed: 1.0–2.5m/s, to ensure that the cavity is filled quickly and prevent cold shut.
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Dynamic pressure: 30–80MPa (adjusted according to wall thickness) to ensure good fusion.
Appropriate Holding Time
Set the holding time to 4–6 seconds according to the wall thickness of the casting (such as 4mm) to ensure that the metal is evenly filled during solidification.
Adjustment Strategy:
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Observe whether there is shrinkage or collapse at the end of the mold cavity. If so, extend the holding pressure for 1-2 seconds.
Ejection Process
Uniform Ejection Force
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The number of ejectors should cover the entire part area and should not be concentrated in a certain area.
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The diameter of each ejector is recommended to be ≥φ6mm, and the length is the same.
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The spring force or cylinder ejection system is set within the specified upper limit (such as 0.3-0.5MPa) to prevent violent ejection.
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Before using the die casting die, apply a release agent and check whether the ejector is stuck.
Reasonable Draft Angle
When designing the draft angle, refer to the following standards:
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Outer wall: 1.5-2°, inner wall: 2.5-3°.
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Add 0.5-1° to the reinforcing ribs, deep cavities, or column areas.
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All draft surfaces should be polished Ra<1.6μm to reduce friction resistance.
Avoid Premature Ejection
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The casting should be cooled in the mold until the temperature difference between the mold cavity is ≤20°C before ejection.
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For die casting products with a thickness of ≥5mm, ejection should be delayed by 5-8 seconds.
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Use water cooling or air cooling to assist cooling and accelerate balanced cooling.
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Avoid artificial rush to produce and cause early mold opening.
Material Control
Use High-Quality Aluminum Ingots
When purchasing raw materials should ensure:
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Oxide content <0.002%, hydrogen content <0.15ml/100g.
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Sampling spectral analysis is performed before smelting to ensure the stability of alloy composition.
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Uncertified recycled materials or mixed aluminum slag are prohibited.
Degassing and Refining Treatment
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During the smelting process, use a rotary degasser to remove hydrogen with argon or nitrogen, with a recommended speed of 300-400rpm and a time of ≥6 minutes.
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Add refining agents during the smelting process and stir thoroughly to remove inclusions.
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After cleaning the slag, let it stand for 3-5 minutes before pouring.
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Before pouring, a double filter residue treatment is performed. Ceramic foam filter (pore size 20-30ppi) is recommended.
Heat Treatment Process
T5/T6 Heat Treatment
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T5 treatment process: direct artificial aging (160-180°C, 4-8h) after demoulding.
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T6 treatment process: solution treatment (520-540°C, 2-4h) + water cooling + aging (170°C, 6-8h).
T6 heat treatment can change the alloy phase structure, improve yield strength and fracture toughness, and reduce the probability of cold cracking in stress concentration areas.
Stress Relief Annealing
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Recommended annealing temperature: 280-320°C, keep warm for 2-3 hours, and then cool slowly.
Annealing can release residual stress caused by uneven cooling and avoid delayed cracking during machining or use.
Repair Methods After Cold Cracks Occur
TIG Welding
For cold cracks with a length of no more than 10mm, a width of less than 0.2mm, and located in non-load-bearing areas, TIG welding can be used for repair.
The repair process is as follows:
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Use a grinding tool to make a 1-2mm guide groove at both ends of the crack to prevent crack expansion.
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Use welding wire that matches the original alloy (such as ER4045 for A356) to fill the crack layer by layer, and control the welding current in the range of 60-90A to avoid thermal stress in the welding area again.
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After welding, CNC machining is used to restore dimensional accuracy, and local heat treatment (such as artificial aging) is performed to relieve new stress.
This method is limited to appearance or secondary parts, and shall not be used in areas such as structural connections, stress-bearing ribs, and pressure cavities.
TIG Welding
Sealing Filler
When the cold crack is located in the shell, non-pressure-bearing cavity, and other areas, and the crack penetrates but does not affect the overall rigidity, epoxy resin, polyurethane sealant, or metal-filled sealant can be used for sealing.
The specific steps are as follows:
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Clean the crack area and use acetone or alcohol to remove oil stains.
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Fill the sealing material into the crack channel by vacuum injection, brushing, or needle injection.
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Cure at room temperature or heat, and control it to be completed within 2-12 hours, depending on the material.
This method is suitable for static shells, electronic boxes, non-fluid channels, and other scenarios, but it is strictly forbidden to use it in pressure-bearing, vibration, or high-temperature areas.
Design Suggestions: How to Avoid Cold Cracks from the Source
Use Fillets and Smooth Transitions
All internal sharp corners should be designed as fillets ≥1.5mm, and it is recommended to use a fillet radius of ≥2mm at the intersection of ribs to avoid sharp stress concentration.
The transition from thick to thin wall thickness should be controlled within a gradient ratio of 1:3 to prevent uneven thermal shrinkage and fracture.
If space is limited, chamfers or bevels can be added locally to buffer geometric transitions.
Balancing Gate Design and Ventilation
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Runners and gates should be arranged in symmetrical areas to ensure that molten aluminum is filled synchronously from multiple directions.
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Overflow grooves are set at the end or thin-walled area to guide the cold front.
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Venting grooves are arranged at high ribs or closed areas, and the groove depth is generally 0.01-0.03mm to ensure the timely discharge of gas.
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All runners and gates should avoid the main stress surface to prevent fracture marks from forming in the stress area.
Alloy Selection
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Before selecting materials, their crack resistance should be confirmed through tensile, impact, hardness, and other sample tests.
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Preferably select aluminum alloys with an elongation of ≥8% and good aging responsiveness, such as A356, ADC12 modified versions.
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Ensure that the selected alloy is suitable for the T5/T6 heat treatment curve to avoid heat treatment deformation or cracking risks.
DFM Design Collaboration
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Provide suppliers with 3D models (STEP/STP format) and functional requirements at the beginning of the project.
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Suppliers conduct mold flow analysis and thermal simulation to identify potential hot nodes, cold shut areas, and stress concentration areas.
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Optimize rib layout, venting grooves, and gate positions based on the simulation results.
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DFM should be fed back to designers to adjust the structure in time to avoid the features of cold cracks in the design.
DFM Report
How Does CEX Casting Prevent Cold Cracks?
Centralized Smelting System
CEX Casting uses a centralized smelting system with automatic temperature control and online degassing and refining throughout the process to effectively control hydrogen content and oxide inclusions.
Stable alloy composition and low impurity level greatly reduce stress concentration and cold crack risks during the cooling stage.
In-House Mold Development
All molds are designed by the internal team. Before mold opening, DFM, mold flow, and thermal simulation analysis are combined to focus on optimizing hot spots, sharp corners, and wall thickness transition structures.
By eliminating the high-incidence points of cold cracking in advance during the design stage, we ensure that the product has good thermal balance and crack resistance from the first mold.
Patented Squeeze Casting Technology
CEX offers patented squeeze die casting services, which apply shrinkage pressure during the solidification stage of aluminum liquid to compact the internal voids of the metal and eliminate pores.
This process is particularly suitable for thin-walled, rib-shaped, closed cavities and other structures that are sensitive to cold cracking, significantly improving the overall crack resistance.
Full Process Defect Detection
We are equipped with a variety of detection methods, such as X-ray, helium leak detection, tensile strength, and spectrum, focusing on high stress areas, thick-thin transition areas, and sealing surfaces.
Ensure that all aluminium die casting components with potential cold cracks, shrinkage, or abnormal structures are discovered and eliminated before leaving the factory, and the structural stability of the parts can be traced.
One-Stop Service
CEX provides a complete production chain from mold design, high-pressure die casting, CNC machining, heat treatment, to surface treatment.
This avoids transportation, handling, and repeated clamping between multiple suppliers, reduces additional mechanical stress generated during transportation and processing, and reduces the risk of cracking of castings due to external forces or residual stress during processing.
Conclusion
Cold cracks are a serious problem in die casting quality control, but they can be effectively prevented through reasonable design, material control, and precision process management.
Avoiding sharp transitions, maintaining thermal balance, and selecting high-quality suppliers are the keys.
As a trusted one-stop aluminum die casting manufacturer, CEX Casting has in-house mold design, patented squeeze casting, and full-process testing capabilities to help you effectively eliminate cold crack hazards.
Contact us now to get a customized defect-free aluminium alloy die casting solution for you.