Aluminum casting & mold design services

Introduction

To the executives and procurement managers in the manufacturing sector, achieving the dual goals of “high strength” and “cost reduction” for aluminum cast parts is a critical, unavoidable challenge in today’s fiercely competitive environment. Especially in the transportation equipment sector, including the automotive industry, the growing need for weight reduction demands higher strength and reliability.

One of the fundamental factors that determine the quality of aluminum castings (aluminum chūbutsu), particularly mechanical properties like tensile strength and elongation, is the cooling rate (solidification rate). This cooling rate directly controls how fine and uniform the internal crystal structure (kesshō soshiki) is formed when the molten aluminum alloy solidifies. Scientific evidence shows that a faster cooling rate leads to a finer structure, which consequently improves tensile strength and hardness.

In this article, we will thoroughly explain how the cooling rate in aluminum casting influences the microstructures, such as the dendrite arm spacing (primary and secondary dendrite arm spacing), and how it ultimately determines the product’s strength, using specific numerical data and research examples. Furthermore, based on the expertise of Daiwa Aluminum Vietnam, we will present concrete strategies to leverage this cooling rate control technology for stabilizing quality and strengthening cost competitiveness in overseas procurement. Deeply understanding and properly utilizing this technology is the key to diversifying your supply chain and enhancing the added value of your products.

The “Crystal Structure” Governed by the Cooling Rate

The process where molten aluminum alloy (yōtō: molten metal) is cooled and solidifies within a mold is called solidification (gyōko). The speed of this solidification, or the cooling rate, can be called the “DNA of casting,” as it dictates the final quality of the cast part.

Solidification Process and Dendrites in Aluminum Alloys

As an aluminum alloy cools, the part that solidifies first forms tree-like crystals, commonly known as dendrites (jushijōshō: dendritic crystals). This is a structure where aluminum crystals grow by branching out, forming the backbone of the final cast part.

• Dendrite Arm Spacing (DAS): An index that indicates the spacing between the branches of the dendrite. The Secondary Dendrite Arm Spacing (SDAS), in particular, is widely used as an objective measure of the solidification rate.
• Relationship between Cooling Rate and SDAS: The faster the cooling rate, the shorter the time available for crystal growth, resulting in a narrower SDAS. For example, one study showed that as the cooling rate of ADC12 aluminum alloy increased from 1.5°C/s to 15.5°C/s, the transverse dendrite arm spacing decreased to about 18.28 $\mu \text{m}$. This clearly demonstrates the inverse relationship between the cooling rate and SDAS.

A narrower SDAS, meaning a finer crystal structure, is a fundamental condition for improving the overall uniformity (kin’itsusei) and mechanical properties of the casting.

Comparison of Cooling Rates by Casting Method

The cooling rate varies significantly depending on the casting method (chūzō-hō) employed.

1. Sand Casting (sunagata chūzō): Since the thermal conductivity of the sand mold material is low, the cooling rate is the slowest (generally below a few °C/s). The structure tends to be coarse, making it generally unsuitable for parts requiring high strength.
2. Gravity Die Casting/Permanent Mold Casting (kinagata chūzō): This method uses a heat-resistant metal (such as steel) as the mold. Due to the higher thermal conductivity compared to sand molds, the cooling rate is moderate (generally tens of °C/s). The crystal grains are refined, and mechanical properties are improved compared to sand casting.
3. Die Casting (High-Pressure Casting) (daikasuto): Molten metal is injected into the mold under high pressure, resulting in intimate contact with the mold, which makes the cooling rate the fastest (generally over 100°C/s). This yields an extremely fine crystal structure and achieves high strength. This trend is particularly evident in high-pressure die casting.

Optimization Strategy for Achieving High Strength through Cooling Rate Control

Simply “cooling quickly” is not enough to produce high-quality castings. Controlling the cooling rate requires finding the optimal balance with the occurrence of defects (kekkan) inside the casting.

Quantitative Relationship between Cooling Rate and Mechanical Properties

Grain refinement is known to directly contribute to the increased strength of metals, according to the Hall-Petch relation. This is because an increased number of grain boundaries (ryūkai: boundaries between crystals) enhances the resistance to deformation.

• Impact on Tensile Strength and Hardness: Increasing the cooling rate refines the crystal grains, which improves tensile strength (hippari kyōdo) and hardness (kōdo). The aforementioned study showed that ADC12 alloy achieved a maximum tensile strength of 280.89 MPa and a micro-hardness (HV) of 98.35 HV.
• Impact on Elongation (Ductility): An appropriate cooling rate also improves elongation (nobi), or the material’s ductility. As the dendrite arm spacing narrows, the fracture surface transitions from brittle fracture (zensei hakai) to a ductile/brittle mixed fracture, increasing the number of dimples (indentations) and thereby enhancing the material’s toughness (jinsei).

However, excessively fast cooling, especially for castings that undergo heat treatment (netsushori), increases the risk of residual stress (zanryū ōryoku), leading to distortion or cracking. In heat treatment, the lower the temperature of the quenching water (faster cooling rate), the higher the strength and hardness, but the residual stress risk also increases simultaneously.

Relationship between Casting Pressure, Cooling Rate, and Porosity

In high-pressure casting methods like die casting, the casting pressure (chūzō atsuryoku) is also closely related to the cooling rate and the formation of internal defects in the casting.

• Suppression of Porosity by Pressure: Higher casting pressure helps to crush porosity (ikesu: internal voids) caused by solidification shrinkage, making them less likely to occur. One study showed that as the casting pressure increased from approximately 40 MPa to approximately 70 MPa, the void volume fraction (the proportion of the volume occupied by porosity) tended to decrease from about 4% to below about 2.5%.
• Complex Influence of Cooling Rate: When the casting pressure exceeds 60 MPa, the cooling rate (about 90°C/sec) tends to slow down, but this is presumed to be because the effect of crushing porosity becomes dominant, ultimately leading to higher-quality castings. Thus, the key to achieving high strength lies in the combined optimization of “cooling rate” and “casting pressure.”

The Importance of Cooling Rate Control and Cost Strategy in Overseas Procurement

When Japanese manufacturers consider overseas procurement, their main concerns are “quality stability” and “lead time (delivery)”. The technical capability to appropriately control the cooling rate is crucial for alleviating these concerns and enhancing cost competitiveness.

Contribution to Quality Stabilization: Vietnam’s Advantage

Controlling the cooling rate heavily relies on casting equipment and operational expertise. The following elements are essential for stably executing high-cooling-rate processes like die casting and permanent mold casting:

• Optimal Die Design: Intricate design of cooling channels (water lines, etc.) is necessary to achieve a uniform cooling rate throughout the product. This eliminates inconsistencies in the internal structure and can reduce the defect rate.
• Thorough Temperature Management: Strict control of multiple parameters such as molten metal temperature, mold temperature, injection speed, and pressurization timing is maintained to ensure a highly reproducible cooling profile.
• Investment in Equipment: The adoption of high-performance die casting machines capable of high injection pressure and fast cycle times (time from casting to extraction) is a prerequisite for achieving a stable, high cooling rate.

At Daiwa Aluminum Vietnam, we have introduced advanced die technologies and state-of-the-art equipment to replicate Japanese quality. By thoroughly implementing this cooling rate control, we consistently supply uniform, high-strength castings that are equivalent to or better than those from domestic Japanese factories.

Cost Competitiveness and Supply Chain Diversification

Aluminum casting in Vietnam offers concrete cost benefits to Japanese manufacturers, such as:

Cost Item	Domestic Japanese Factory	Vietnam Factory (Daiwa Aluminum Vietnam)	Estimated Reduction Effect
Labor Costs	High Level	Significantly Lower Level	Approx. 60% to 70% Reduction
Land/Building Costs	High Level	Low Level	Approx. 40% to 50% Reduction
Utility/Energy Costs	High Level	Relatively Low Level	Approx. 15% to 25% Reduction
Logistics Costs	High compared to neighboring countries	Diversification due to geographical advantage	Supply Chain Risk Reduction

Achieving cost reduction, combined with high-strength technology, results in a powerful competitive advantage: “the ability to stably procure high-quality parts at a low price.” Furthermore, geographical diversification of the supply chain directly contributes to the dispersion of geopolitical and natural disaster risks. Securing stable production capacity in an emerging market like Vietnam is extremely important from a Business Continuity Plan (BCP) perspective.

Conclusion

The cooling rate (solidification rate) in aluminum casting is one of the most crucial factors determining product quality, especially mechanical properties such as tensile strength and hardness. By accelerating the cooling rate, the Dendrite Arm Spacing (SDAS) decreases, and the crystal structure is refined, which enhances the casting’s strength. This is the scientific basis for why die casting (fast cooling) and permanent mold casting generally produce higher strength parts than sand casting (slow cooling).

However, the key to achieving the highest strength and elongation while suppressing internal defects (porosity and residual stress) lies not in simple high-speed cooling, but in the combined optimization of casting pressure and mold design. We, at Daiwa Aluminum Vietnam, position this precise cooling rate control technology as a core competence, integrating state-of-the-art equipment with Japanese technical expertise to consistently offer quality equivalent to, or surpassing, that of domestic Japanese factories.

To fundamentally improve your aluminum component cost structure and simultaneously drive product development that meets high-strength and high-reliability requirements, our overseas procurement solution is the optimal choice. We look forward to helping you achieve supply chain diversification, stable supply of high-quality parts, and concrete cost reduction effects. Please share your current challenges and required part specifications with us.