Guide pratique des défauts de gaz en fonderie : Causes, types et prévention

Introduction

In the complex world of precision casting, gas defects in castings are a common and difficult problem. These tiny, hidden flaws not only affect the appearance of a casting but can also severely impact its mechanical properties and reliability, ultimately leading to costly scrap and rework.

Understanding the causes of gas defects and mastering effective prevention methods are essential skills for every casting engineer and quality control professional. This guide serves as a practical resource, guiding you to a deeper understanding of the definition and causes of gas defects, as well as how to use scientific methods to eliminate them.

What are Gas Defects and Porosity Defects in Casting?

Gas defects in casting are generally defined as voids that form inside the solidifying metal or on its surface due to various gases being unable to escape. These defects, including the common porosity defects in casting, can be divided into several main types based on their shape and location:

Porosité This is the most common type of gas defect, often referred to as porosity defect. Porosity typically appears as spherical, elliptical, or irregular cavities inside or on the surface of the casting. They can be isolated or appear in a honeycomb-like cluster.

Trous d'épingle Similar to porosity but much smaller, pinholes are tiny voids, usually less than 2mm in diameter, that often appear in clusters on or just below the casting surface.

Trous d'air These are larger cavities with smooth walls, typically found at the top or sides of the casting. They are usually formed when gas is trapped in the mold cavity.

Causes courantes des défauts de gaz

To effectively prevent gas defects and related porosity defects, it’s essential to first understand their root causes. The formation of pores in a casting is not due to a single factor but is the result of multiple interacting elements. These causes can be broadly classified into the following categories:

Questions relatives au métal en fusion

• Excessive Dissolved Gas: Molten metal can absorb gases, especially hydrogen, at high temperatures. As the metal cools and solidifies, the solubility of the gas decreases, and the excess gas forms pores.

• Moisture and Oil in Materials: If the raw materials or remelted scrap contain moisture, oil, or rust, these substances will decompose during the melting process and produce gas.

Moule à sable et problèmes de noyaux

• Poor Sand Mold Permeability: Sand mold permeability is critical for gas to escape. If the sand grains are too fine, the clay content is too high, or the compaction is excessive, it can lead to insufficient permeability, preventing gases from escaping smoothly.

• Moisture and Binders in Cores: Moisture or organic binders in the core can generate a large amount of gas at high temperatures. If these gases do not have an adequate escape route, they will be trapped within the casting.

Questions relatives au processus de déversement et de coulée

• Improper Gating System: A poorly designed gating system can cause turbulence in the molten metal, leading to air entrapment. Similarly, improperly placed vents or risers can hinder gas from escaping.

• Excessively High Pouring Temperature: A high pouring temperature increases the gas content in the molten metal and prolongs the solidification time, creating conditions for gas formation.

Méthodes efficaces de prévention des défauts de gaz

Fortunately, gas defects are not unavoidable. Through comprehensive control from materials, equipment, and processes, their occurrence can be significantly reduced.

Contrôle et préparation des matériaux

• Pre-melt Treatment: Thoroughly dry all remelted scrap and raw materials to remove moisture and oil.

• Degassing Treatment: Degas the molten metal before pouring, for example, by purging with an inert gas (such as argon).

• Control Pouring Temperature: Strictly control the pouring temperature within the recommended range to avoid overheating.

Conception de moules et de noyaux en sable

• Improve Permeability: Use sand with coarser grains and better permeability, and control the compaction density.

• Create Venting Channels: Design reasonable vents and risers in the mold cavity and core to provide a smooth escape path for gases. To learn more about how core making affects casting quality, you can read our in-depth article, Fabrication de noyaux dans la coulée en sable : Types, procédés et conseils de conception.

Optimisation du système et du processus de traitement des vannes

• Optimize the Gating System: Design an appropriate gating system and runners to ensure the molten metal flows smoothly into the mold cavity, reducing turbulence.

• Control Pouring Speed: Maintain a stable pouring speed to avoid inconsistent flow, which can lead to air entrapment.

Summary and Recommendations

Through comprehensive quality control, the formation of gas defects, especially porosity defects, can be effectively prevented. Every step is critical, from pre-melting preparation to mold design and the final pouring operation.

Preventing these defects is not just about product quality, but also about the bottom line. To understand the economic impact, check out our guide on the Les 5 principaux facteurs de coût dans les projets de moulage de métaux sur mesure.

We recommend that you refer to this guide and adjust it based on your specific casting conditions. If you encounter difficult gas defect problems in your production, our team of professional casting engineers is ready to provide technical support and solutions.