Case Studies and Countermeasures for Aluminum Casting Defects: Causes and Solutions for Porosity, Cracks, and Dimensional Defects

◆Table of contents

Introduction

In Japan’s manufacturing industry, lightweight and high-strength aluminum castings are essential core components, used in everything from automotive parts to industrial machinery and construction materials. However, their manufacturing process is complex, and casting defects such as “porosity,” “cracks,” and “dimensional defects” can become serious business challenges, leading to additional costs in later processes and even product recalls. Amid demands for supply chain stabilization and enhanced cost competitiveness, understanding how to suppress these defects to improve quality and productivity is a pressing issue for procurement and purchasing managers.

This article focuses on the three major defects that are particularly problematic in aluminum castings. We will provide a comprehensive explanation, from their occurrence mechanisms to specific countermeasures, incorporating the latest data and case studies. By understanding the root causes and implementing appropriate solutions, we hope this article will assist your company in improving quality, reducing costs, and achieving a stable supply of components.

The 3 Major Defects Frequent in Aluminum Castings and Their Impact

While there are various defects in aluminum casting, “porosity,” “cracks,” and “dimensional defects” have the most significant impact on quality, cost, and delivery. These defects not only significantly reduce the mechanical strength of the product but can also be fatal flaws in components requiring airtightness and pressure resistance (e.g., engine blocks, valve bodies). Some studies report that the defect rate in the casting process can exceed 5%, with the majority attributed to these three defects. The occurrence of defects leads not only to direct cost increases from re-melting, repairs, and additional inspections but also to lost business opportunities due to delivery delays and damage to customer trust. Therefore, a systematic approach to prevent them is essential.

Defect Case ①: Porosity (Blowholes & Pinholes) – Causes and Countermeasures

What is “Porosity”? – Cavity Defects that Impair Internal Quality

Porosity is a general term for cavity-like defects that occur inside a casting. They are broadly classified into two types based on their cause. One is “gas porosity (blowholes, pinholes),” which occurs when gas contained in the molten metal is released during the solidification process. Aluminum is particularly prone to gas porosity because it readily dissolves hydrogen gas, and its solubility decreases sharply with temperature (e.g., dropping to about 1/20th when cooling from a molten state at 660°C to a solid state at 550°C). The other is “shrinkage porosity,” which occurs when the supply of metal cannot keep up with the volume contraction during solidification. Since the solidification shrinkage rate of aluminum alloys can reach 6-8%, controlling shrinkage porosity is extremely important.

Top 5 Causes of Porosity

Hydrogen Gas in Molten Metal: Moisture in the atmosphere or combustion gases from the melting furnace (natural gas, LPG, etc.) react with the molten metal, which is then absorbed as hydrogen gas.
Gas from Molds and Cores: Moisture in the molding sand or binders in the cores (sand molds used to create hollow sections) thermally decompose upon contact with the high-temperature molten metal, generating gas. It is recommended to manage the amount of resin-based binder to be 1.0% or less.
Turbulence and Entrapment: If the flow of molten metal (metal flow) is turbulent when pouring it into the mold, it can entrap air, causing porosity.
Improper Solidification Control (Shrinkage Porosity): If the product has non-uniform wall thickness, the supply of molten metal cannot reach the thick-walled section that solidifies last, resulting in a cavity (shrinkage porosity) due to contraction.
Improper Use of Release Agents or Coatings: In die casting, if too much mold release agent is applied or if the air blow-off after application is insufficient, it can vaporize upon contact with the molten metal and cause defects. For water-based coatings, drying for at least 2 hours at 180-200°C may be required.

Specific Countermeasures to Prevent Porosity

The basic strategy to counter porosity is to eliminate gas sources, facilitate gas evacuation, and control solidification properly.

Thorough Molten Metal Management: Strictly control the melting temperature to 750°C or below to suppress gas absorption due to overheating. Additionally, performing a degassing treatment (fluxing) with argon or nitrogen gas for about 8-10 minutes to reduce the hydrogen gas concentration in the molten metal to 0.1 cc/100g or less is effective.
Optimization of Casting Plan: Utilize CAE (Computer-Aided Engineering) for flow and solidification simulations to design gates and gas vents that suppress turbulence and allow gas to escape smoothly.
Mold and Core Management: Thoroughly dry molding sand and cores before use and select low-gas-emission binders.
Adoption of Vacuum Die Casting: By filling the mold with molten metal after evacuating the cavity to a vacuum of about 50-100 hPa, gas entrapment can be fundamentally prevented, resulting in high-quality castings.
Hot Isostatic Pressing (HIP): As a post-casting treatment, pressurizing the product with an inert gas at 100-200 MPa (approx. 1,000-2,000 atm) at a high temperature of around 500°C can effectively close and eliminate internal microporosity.

Defect Case ②: Cracks (Solidification Cracking & Hot Tears) – Causes and Countermeasures

Types and Mechanisms of Cracks

Cracks in castings are critical defects that fundamentally undermine product reliability. They are classified by the temperature range in which they occur: “solidification cracking (hot tears),” which occurs in the final stage of solidification when the metal is hot and brittle, and “cold cracks,” which occur after cooling or during heat treatment. Solidification cracking, in particular, is caused by tensile stress that develops when the solidifying casting is constrained by the mold, preventing it from shrinking. It is more likely to occur in alloys like Al-Si and Al-Cu. Since these cracks can become starting points for fatigue failure, which significantly shortens product life, 100% detection and prevention are required.

Top 4 Causes of Cracks

Improper Product Geometry: Areas with abrupt changes in wall thickness or sharp corners (small fillet radii) create stress concentrations, which can become starting points for cracks.
Solidification Shrinkage Stress: In complex-shaped castings, solidification timing varies between sections, leading to non-uniform shrinkage stress and the accumulation of internal strain.
Mold Constraint: The sand mold, die, or cores can restrain the casting as it attempts to shrink, generating tensile stress that leads to cracking.
Flaws in Ejection, Cooling, or Heat Treatment: Forcible ejection at high temperatures, excessively rapid cooling, or improper heat treatment (e.g., rapid heating/cooling) can generate significant thermal stress and cause cracks.

Specific Countermeasures to Prevent Cracks

The key to preventing cracks is a design that mitigates stress concentration and a casting plan that suppresses stress generation during solidification and cooling.

Review of Product Design: To avoid stress concentration, use the largest possible fillet radius (R) at corners. Make changes in wall thickness gradual, and consider adding ribs to increase rigidity. Generally, a radius of 2 to 4 times the wall thickness is recommended.
Improvement of Casting Plan: Place risers (reservoirs of molten metal to compensate for shrinkage) or chills (metal pieces embedded in the mold for forced cooling) at thick sections (hot spots) that solidify last to promote directional solidification (a technique to make the casting solidify sequentially from one direction).
Optimization of Mold Design: In die casting molds, optimize the placement of ejector pins to ensure that stress is applied evenly during part ejection. Also, properly manage the mold temperature with cooling water at 40-50°C to prevent seizure or heat checking.
Management of Casting Conditions and Post-Processing: Control solidification time by optimizing pouring temperature and injection speed. Adhere strictly to the ejection timing and, as a rule, cool the casting gradually after it is removed from the mold.

Defect Case ③: Dimensional Defects – Causes and Countermeasures

Problems Caused by Dimensional Defects

Dimensional defects directly lead to assembly problems in later processes and failure to meet product performance requirements. As modern products demand high-density mounting and precision fitting, the requirements for dimensional accuracy are becoming increasingly stringent. The typical dimensional tolerance for aluminum die casting is about ±0.1-0.2 mm, but higher-precision methods (e.g., semi-solid die casting) can aim for ±0.02 mm. To achieve this level of accuracy consistently, a deep understanding of the physical phenomenon of shrinkage and integrated management across all processes from design to manufacturing is required.

Top 4 Causes of Dimensional Defects

Accuracy of Die/Pattern Design and Manufacturing: Flaws in design drawings, loose machining tolerances, and misalignment of mold halves (mold shift) are direct causes.
Prediction Error in Casting Shrinkage Rate: The shrinkage rate for aluminum alloys is 0.6-0.8%, but it varies slightly depending on the alloy type, casting shape, and casting conditions. Misjudging this rate leads to dimensional errors.
Thermal Deformation of the Mold: The high temperature during casting (600-700°C) causes the mold to expand thermally. If this deformation is not accounted for, it will result in inaccuracies in the dimensions of the cooled product.
Post-Casting Deformation: The casting can warp or twist due to forces during ejection or the release of residual stress during heat treatment, shot blasting, etc.

<div class=”data-box”>Key Data: Dimensional Accuracy Targets in Aluminum Casting- General Die Casting Tolerance: ±0.1mm to ±0.2mm- Target Tolerance for High-Pressure Die Casting: ±0.05mm- Target Tolerance for Semi-Solid (Thixoforming) Method: ±0.02mm- Target Tolerance for Post-Casting CNC Machining: ±0.01mm- Solidification Shrinkage Rate for a typical aluminum alloy (ADC12): Approx. 0.6%

Source: CEX Casting, Daiwa Ketal Vietnam</div>

Countermeasures for Achieving High Dimensional Accuracy

High-Precision Mold Design and Manufacturing: Utilize 3D CAD/CAM for an integrated design and manufacturing process covering the product model, mold model, and electrode model. The accuracy of the “compensation model,” which accounts for shrinkage and thermal deformation, is particularly critical.
Use of Casting Simulation (CAE): Simulate the casting process in advance to predict molten metal filling behavior, temperature distribution, solidification process, and casting deformation. This allows for the identification and correction of potential problems at the design stage before the mold is manufactured.
Strict Management of Casting Conditions: Standardizing parameters such as injection pressure (which can reach 100-200 MPa in high-pressure die casting), injection speed, mold temperature, and cooling time, and consistently producing under the same conditions, is key to dimensional stability.
Systematic Quality Control: Regularly inspect product dimensions and the presence of internal defects using coordinate measuring machines (CMMs) and CT scanners. This requires continuous improvement activities incorporating statistical quality control (SQC) methods, such as maintaining a process capability index (Cpk) of 1.33 or higher.

Conclusion

While “porosity,” “cracks,” and “dimensional defects” that undermine the quality of aluminum castings may seem like separate issues, their root causes are deeply interconnected with fundamental manufacturing processes such as “molten metal management,” “casting plan,” “mold design,” and “temperature control.” To eradicate these defects, it is essential to take a systematic approach that looks at the entire process—from product design to casting and post-processing—rather than relying on a single countermeasure.

In particular, the use of CAE simulation is a powerful tool for finding optimal manufacturing conditions while reducing costs, as it significantly contributes to reducing the number of prototypes and shortening development time. We encourage you to use the causes and countermeasures explained in this article to re-examine your own challenges and deepen technical discussions with your casting partners, aiming to build a more robust supply chain that achieves both quality and cost competitiveness.

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Struggling with porosity, cracks, or dimensional defects in your aluminum castings? This article provides a thorough explanation of the causes of the three major defects and introduces specific countermeasures and solutions with case studies. Find the hints to improve quality and reduce costs here.

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