Gas Control and Porosity Prevention in Aluminum Alloy Die-Casting

Gas Control and Porosity Prevention in Aluminum Alloy Die-Casting

In the low-pressure Aluminum Alloy Casting industry, porosity is a hidden killer that leads to casting scrap and performance degradation. At the very least, it affects product sealing and appearance, while at worst, it causes mechanical failure, directly resulting in order rework and cost losses. For die-casting companies targeting the international market, precisely controlling gas content and eliminating porosity defects is essential for meeting the stringent quality standards of Europe, the United States, Japan, and South Korea, and is also a core competitive advantage for earning the trust of professional buyers and securing long-term orders. This article will systematically analyze gas management solutions for aluminum alloy die-casting from three perspectives: gas sources, control technologies, and testing and verification, providing companies with a practical path to quality improvement.

I. Understanding the Root Cause: The Three Core Sources of Gas in Aluminum Alloy Die-Casting

To address porosity, it is first necessary to identify the gas generation process. In the aluminum alloy die-Casting Process, gas primarily originates from three main processes: melting, die-casting, and die-casting. The types of gas and the degree of impact at each stage vary significantly.

Melting: Aluminum alloys, a major source of virgin gas, react easily with oxygen and water vapor during melting (especially when using traditional equipment like coke ovens and gas furnaces). For example, contact between molten aluminum and water vapor generates hydrogen (2Al + 3H₂O → Al₂O₃ + 6H₃). The solubility of hydrogen in molten aluminum increases dramatically with temperature (from approximately 0.4ml/100g at 700°C, dropping sharply to 0.01ml/100g upon cooling to room temperature). Failure to promptly expel hydrogen can form dispersed pores in the casting during solidification. Furthermore, the decomposition of fluxes (such as chlorides and fluorides) and the combustion of oil residue on the surface of scrap aluminum can also generate harmful gases such as carbon monoxide and nitrogen.

Die-casting Process: The "Critical Node" of Secondary Air Intake: The shot-casting stage of low-Pressure Die Casting is the core stage of secondary gas intrusion. When molten metal fills the mold cavity at high speed, excessive injection speeds and improper runner design can prevent air from escaping quickly, leading to "air entrapment." Furthermore, worn seals and hydraulic oil leaks in the shot cylinder can also allow air to enter the cavity along with the molten metal, ultimately forming concentrated air pores in thick walls and corners of the casting.

Molds and Coatings: Easily Overlooked "Hidden Sources" Excessive surface roughness and clogged vents in the mold cavity can hinder the smooth escape of air. Incompletely dried mold coatings (such as water-based release agents) rapidly vaporize at high temperatures, generating large amounts of gas. If these gases cannot be exhausted through the exhaust system, they can form subcutaneous pores or pinholes on the casting surface, directly affecting the surface finish (such as anodizing and spray coating).

II. Precision Control: Four Core Technical Solutions to Eliminate Air Pores at the Source

To address these gas sources, a comprehensive gas management system must be established through a dual approach of "prevention and control," combining process optimization, equipment upgrades, and material control. The following four technical solutions have been proven effective within the industry and can be directly applied in actual production.

1. Melting Stage: Inert Gas Degassing to Reduce Native Hydrogen Content
Technical Principle: Argon (Ar) or nitrogen (N₂) is used as the degassing medium. Inert gas is injected into the molten aluminum in the form of tiny bubbles through a rotating nozzle. As the bubbles rise, they absorb hydrogen from the molten aluminum (the solubility of hydrogen in inert gas is much lower than in molten aluminum), eventually escaping to the surface along with the bubbles.
Practical Key Points: Control the inert gas purity (≥99.99%) to avoid the introduction of impurities; adjust the nozzle rotation speed (300-500 rpm) and gas flow rate (0.5-1.5 L/min) to ensure even bubble distribution; and allow the molten aluminum to rest for 10-15 minutes after degassing to allow any remaining bubbles to completely float. This can keep the hydrogen content of the molten aluminum below 0.15 ml/100 g (the industry's high-quality standard).

2. Die-casting Process: Vacuum Die-casting + Optimized Parameters to Prevent Secondary Air Intake
Vacuum die-casting technology: A vacuum system is installed between the mold cavity and the injection chamber. Prior to die-casting, the cavity is vacuumed to below 50 mbar (some high-end equipment can reach 10 mbar). This significantly reduces the total amount of air in the cavity and fundamentally avoids the problem of "air entrapment." This technology is particularly suitable for the production of products with high sealing requirements (such as automotive hydraulic components and new energy battery housings), reducing porosity by over 60%.
Injection Parameter Optimization: A segmented injection process using "low-speed filling - high-speed pressurization" is employed. During the initial filling phase (when the molten metal enters the runner), the injection speed is controlled at 0.3-0.5 m/s to prevent air impact. During the mid-to-late filling phase (when the molten metal fills 80% of the cavity), the injection speed is increased to 0.8-1.2 m/s to ensure rapid molding. Furthermore, the injection pressure is controlled at 50-80 MPa to enhance the density of the molten metal and suppress any remaining bubbles.

3. Mold Design: Enhance the Exhaust System to Improve Gas Exhaust Efficiency
Venting Groove Layout: Add venting grooves to gas-collecting areas, such as the ends, corners, and thick-walled areas of the mold cavity. The recommended venting groove width is 3-5mm, and the depth is 0.05-0.1mm (to prevent molten metal from overflowing). The grooves should extend to the outside of the mold to ensure direct exhaust of gases. For complex cavities, special features such as "venting needles" and "venting inserts" can be used to enhance localized exhaust.
Mold Temperature Control: Use a mold temperature controller to maintain a stable operating temperature of 180-220°C (adjusted according to the aluminum alloy grade) to prevent localized low mold temperatures from causing rapid solidification of the molten metal and the formation of "gas traps." Ensure uniform spraying of the mold coating and a drying time of at least 5 minutes (at 80-100°C) to eliminate gases generated by coating vaporization.

4. Materials and Auxiliary Materials: Strictly Control and Reduce the Introduction of Hidden Gases
Raw Material Selection: Prefer virgin aluminum ingots with a purity of ≥99.7%. Avoid using scrap aluminum with severe oil contamination or rust. If scrap aluminum is used, it must be degreased (300-400°C) and pickled for rust removal prior to melting to reduce gas generation during smelting.
Flux and Coating Selection: Use environmentally friendly, low-volatility fluxes (such as sodium-free fluxes) to reduce harmful gases produced by flux decomposition. Choose oil-based or semi-aqueous release agents (water content ≤10%) to reduce vaporized gases. Regularly clean paint residue from mold vents to prevent clogging.

III. Testing and Verification: Two Core Methods to Ensure Porosity Control

Effective gas control requires precise testing methods. Through a dual approach of "process testing + finished product verification," we ensure that every batch of castings meets quality standards, providing traceable quality assurance to international buyers.

In-Process Monitoring: Online Hydrogen Content Monitoring: Install an "aluminum liquid hydrogen content analyzer" (e.g., vacuum decompression method or inert gas melting method) at the smelting furnace outlet to monitor the hydrogen content in the molten aluminum in real time. If the hydrogen content exceeds the standard (>0.2ml/100g), pouring is immediately stopped and degassing is repeated. This method can avoid batch scrapping due to molten aluminum quality issues and reduce production costs.

Finished Product Verification: Non-destructive Testing + Metallographic Analysis
Non-destructive Testing (NDT): X-ray RT and ultrasonic testing (UT) are used to detect internal pores in castings. For critical components subjected to high pressure (such as automotive transmission housings), the internal pore diameter must be ≤0.5mm and the number of pores must be ≤1 per square centimeter. Penetrant testing (PT) is used to inspect the casting surface for pinholes to ensure that there are no visible pores.

Metallographic Analysis: Randomly sample finished castings, prepare metallographic specimens, and observe the pore distribution in the microstructure under a microscope. High-quality castings should have a microporosity of ≤1% and no concentrated porosity, ensuring that mechanical properties (such as tensile strength and elongation) meet international standards (e.g., ASTM B179 and EN 1706).

IV. Value Realization: How Does Gas Control Help Companies Win International Orders?

For export-oriented die-casting companies, precise gas control not only improves product quality but also directly translates into market competitiveness, attracting the attention and inquiries of international professional buyers (such as automotive parts manufacturers, medical device companies, and aerospace suppliers).

Reducing costs and improving delivery capabilities: By reducing the scrap rate caused by porosity (from 15% to below 5%), rework costs and the risk of delivery delays are reduced, meeting the core requirements of international buyers for "on-time delivery and zero defects."

Breaking through technical barriers and entering the high-end market: Buyers in Europe, the United States, Japan, and South Korea have stringent requirements for the porosity of castings (for example, German automakers require a 100% pass rate for X-ray inspection). Companies that master core gas control technologies can overcome these technical barriers and enter the high-end supply chain.
Building trust and fostering long-term partnerships: Providing buyers with a "liquid aluminum hydrogen content test report + finished product non-destructive testing report" verifies product quality, strengthens buyer trust, and lays the foundation for subsequent volume orders and long-term partnerships.