Hot cracks are cracks that occur when the metal is in the semi-solidification stage. They are also often referred to as hot tears or solidification cracks. They are caused by internal stress, improper alloy composition, or thermal imbalance, which will weaken the structural strength and cause deformation, leakage, or cracking during use. Such defects can be effectively prevented by selecting the right alloy, optimizing die casting mold design, and controlling the cooling process.
This article will introduce the types of hot cracks, their effects, root causes, detection methods, prevention strategies, and professional solutions of CEX Casting. Let’s continue to learn more about how to effectively prevent this defect in your next aluminium alloy die casting project.
Types of Hot Cracks in the Die Casting Process
Hot Tearing
Hot tearing occurs when the metal cannot withstand the internal tensile stress during the solidification process.
Due to different cooling rates and inconsistent shrinkage in various areas, the semi-solid area will be torn apart.
This type of hot tearing is common in die casting parts with complex structures and thick walls.
Hot Tear
Hot Shortness
Hot shortness refers to the lack of plasticity of the material at high temperatures, which is usually caused by poor alloy composition.
Cast aluminum alloys with too high iron content will form brittle phases at the grain boundaries and crack under slight stress.
This defect often occurs during the ejection or cooling post-processing stage.
Hot Shortness
Impact of Hot Cracks
Reduced Mechanical Strength
Thermal cracking will form a crack path inside the die casting products, destroying the continuity of the metal, causing it to break or deform prematurely under stress.
Especially under dynamic load or vibration conditions, aluminium die casting components are more prone to fatigue failure and cannot meet normal structural strength requirements.
Increase Scrap and Rework Rate
Once a casting has thermal cracking, it usually cannot pass the size, performance, or sealing inspection and is directly judged as a defective product.
In most cases, it cannot be repaired and can only be scrapped. In severe cases, it will cause batch rework, resulting in reduced production efficiency and increased material costs.
Potential Risks
Internal thermal cracking is mostly invisible to the naked eye. If it is not screened out by non-destructive testing, it will expand into catastrophic damage during use due to pressure, heat, or mechanical shock.
For safety-sensitive components such as automobiles and aviation, this hidden risk may cause equipment failure, customer complaints, and even large-scale recalls.
Main Causes of Hot Cracking
Improper Alloy Composition
Some aluminum alloys are prone to long-term semi-solid state during solidification due to their wide solidification temperature range.
If the iron content in the alloy is too high or the impurity content exceeds the standard, brittle phases distributed along the grain boundaries will be formed.
These brittle structures cannot withstand internal tensile stress during cooling and shrinkage, making them more prone to cracking.
Mold Design Defects
If the die casting die structure has sharp internal angles, sudden wall thickness changes, or an unbalanced pouring system, the local area will be subjected to concentrated stress.
During the solidification and shrinkage process, these stress concentration points cannot be released smoothly, which can easily induce hot cracking, especially in complex geometric or asymmetric structures.
Uneven Cooling Rate
Due to different contact surfaces, structural thickness, or cooling channel distribution, the cooling rate of each area of the casting is prone to unevenness.
This thermal asymmetry will cause some areas to solidify prematurely, while other areas that are still in the semi-solid state will be pulled, forming thermal stress inside, causing cracking between grains or structural fracture.
Too Much Mold Constraint
During the solidification process, the casting needs to shrink freely to release thermal stress.
If the mold structure restricts the casting too much, such as rigid positioning or unreasonable clamping force distribution, it will hinder the natural shrinkage path and cause stress concentration and accumulation.
When the stress exceeds the material’s bearing capacity at high temperature, thermal cracking will occur.
Ejection Error
If the ejection system is activated before the casting is fully solidified, or the ejection force is uneven, additional mechanical stress will be applied to the local area.
These additional loads will be superimposed on the internal stress generated by solidification shrinkage, inducing the formation or expansion of cracks in originally fragile areas, especially in thin walls and restricted areas.
Detection Method
Visual Surface Inspection
Conventional visual inspection can quickly identify surface cracks on edges, corners, and parting lines.
However, small or partially closed cracks, especially in complex structural parts, may be missed. Suitable for daily inspection, but not enough to support critical quality control.
Nondestructive Testing (NDT)
Nondestructive testing technologies such as X-ray, ultrasonic, and dye penetrant can identify internal or potential cracks without destroying the part.
These methods are particularly important in structural or pressure-bearing parts to verify their integrity before the product leaves the factory.
X-Ray Detection of Internal Cracks
Metallographic Analysis
By grinding and etching the cross-section of the casting, the grain boundaries, segregation areas, and crack morphology can be observed under a microscope.
Although this method is time-consuming, it can provide in-depth insights into the causes of defects and help with subsequent process improvements.
Preventive Measures
Alloy Selection and Adjustment
The key to preventing hot cracking is to select aluminum alloys with a narrow solidification temperature range and low iron content.
The iron content should be controlled below 0.12%, and manganese or chromium should be added if necessary to neutralize the brittle phase and improve the plasticity of the grain boundaries.
Spectral analysis before and after smelting must be performed in production to ensure that the composition is always in a stable range.
Mold Design Optimization
The mold structure should avoid sharp corners, sudden changes in wall thickness, and eccentric layout.
The use of rounded corner transitions, balanced flow channels, and symmetrical filling structures in the design can significantly reduce local stress concentration.
The gate and venting system should be verified by mold flow analysis to ensure smooth filling and uniform heat distribution.
Cooling Strategy Control
The cooling system should be designed according to the structural zoning, with water channels or cooling blocks in the thick-walled area, and the thin-walled area should avoid excessive cooling to prevent premature solidification.
The mold temperature fluctuation should be controlled within ±5°C. If necessary, a temperature sensor should be installed to monitor the hot zone to maintain the overall thermal balance.
Process Parameter Setting
A multi-stage injection speed strategy should be adopted, with low-speed filling at the beginning and high-speed pressurization at the end to improve filling stability.
The mold temperature is maintained at 180–250°C, and the aluminum liquid is controlled between 660–700°C to ensure that the fluidity matches the solidification rhythm and reduces thermal cracking caused by parameter imbalance.
Ejection System Optimization
Ejection should be performed after the casting is completely solidified, and the time is set according to the wall thickness and cooling curve to avoid stress cracking in the unsolidified area.
The ejectors need to be evenly distributed on the stress surface. For complex or thin-walled structures, it is recommended to use slow ejection or segmented ejection to reduce local impact and ensure smooth demolding.
Die Casting Mold Ejection Design
CEX Casting Hot Cracks Prevention Solutions
Mold Flow Simulation Analysis
CEX performs mold flow simulation at the early stage of mold development to predict the entire process of melt flow path, solidification sequence, and stress concentration area.
By adjusting the gate size, position, and cooling layout, potential thermal crack points can be avoided in advance, greatly reducing the mold trial cycle and initial defect rate.
Mold Cooling System Optimization
We design differentiated cooling solutions for different geometric features, such as independent cooling channels in thick-walled areas, controlling heat loss rate in thin-walled areas, and optimizing the overall mold thermal field in combination with structural symmetry.
All solutions are verified by thermal simulation before mass production to improve the dimensional consistency and internal stress stability of the finished product.
Process Parameter Control
Each high-pressure die casting production line is equipped with a real-time data acquisition module to continuously monitor key parameters such as mold temperature, metal temperature, injection speed, and injection pressure.
All products have their process windows, and any parameter deviation will automatically alarm and suspend production to ensure that thermal cracks will not occur due to process fluctuations.
Crack Detection System
We have established a multi-level defect detection process: visual screening after the mold, X-ray and ultrasonic spot checks in the middle, and helium leak and tensile testing in the final inspection stage to ensure that both surface and internal cracks are identified and eliminated.
Defective products do not enter the downstream process, control points are placed in advance, and the closed loop is thorough.
Squeeze Casting Process
For structural parts with a higher risk of hot cracking, CEX can use squeeze casting technology, combining low-speed filling and high-pressure pressurized solidification to significantly reduce shrinkage and hot cracking.
This process is suitable for pressure-bearing parts and thick-walled parts, and can effectively improve density and mechanical properties.
CEX Squeeze Casting Workshop
Conclusion
Hot cracking can be effectively prevented through alloy control, mold optimization, and solidification process management.
Early intervention in the design and mold opening stages can help reduce internal stress and improve casting reliability.
As a professional aluminum die casting manufacturer, CEX Casting provides die casting services including simulation-based engineering design, online thermal control systems, and full-process defect detection solutions.
Contact us now to see how our tailor-made solutions can improve the performance of your next aluminium die casting product.