Aluminum and Aluminum Alloys Casting Problems

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Introduction

The evolution of aluminum casting dates back to the late 19th century when the first commercial products showcased aluminum's novelty and functionality. These initial applications, which included cooking utensils and decorative items, rapidly expanded to meet various engineering specifications. Alloy development and a deeper understanding of aluminum's physical and mechanical properties have underpinned continuous advancements in casting technology.

Today, aluminum castings are produced in hundreds of compositions using diverse methods such as green sand, dry sand, plaster mold, investment casting, permanent mold, counter-gravity casting, and pressure die casting. Advances in foundry techniques have blurred the constraints that once limited alloy and process selection, enabling innovative applications and more efficient production methods.

Melting and Metal Treatment

Aluminum and its alloys are melted using various furnace types, including coreless and channel induction furnaces, crucible furnaces, and open-hearth reverberatory furnaces. The furnace charge may consist of prealloyed ingots or low-grade scrap, with each material choice influencing casting outcomes.

Key Degradation Factors

Even under optimal conditions, molten aluminum is susceptible to:

• Hydrogen Absorption: Prolonged exposure at high temperatures increases hydrogen solubility, leading to porosity.
• Oxidation: Alloying elements like magnesium exacerbate oxidation, forming complex oxides.
• Transient Element Loss: Reactive elements such as magnesium, sodium, and strontium diminish over time, affecting mechanical properties.

Turbulence and elevated holding temperatures accelerate these degradation processes, necessitating careful control of melting parameters to maintain metal quality.

Hydrogen Influence on Aluminum

Hydrogen Solubility and Sources

Hydrogen is uniquely soluble in aluminum, with solubility increasing with temperature and pressure. Moisture in the atmosphere is a primary hydrogen source, dissociating at the molten surface to allow hydrogen diffusion into the metal. Alloying elements like magnesium can exacerbate this issue by reducing resistance to hydrogen absorption.

Types of Hydrogen Porosity

Two forms of hydrogen porosity occur:

• Interdendritic Porosity: Results from high hydrogen levels during solidification
• Micron-Sized Porosity: Forms at lower hydrogen levels and produces smaller voids

While hydrogen porosity often compromises mechanical properties, in some cases, controlled hydrogen levels can improve superficial soundness.

Hydrogen Removal Methods

Reducing hydrogen content involves fluxing with nitrogen, argon, chlorine, or hexachloroethane. These agents diffuse hydrogen out of the melt, promoting uniformity and reducing void formation during solidification.

Oxidation and Inclusion Management

Oxide Formation

Aluminum oxidizes readily in molten and solid states, forming a self-limiting barrier. However, turbulence disrupts this barrier, introducing oxide particles into the melt that resist separation due to their density.

Inclusion Removal

Fluxes containing chlorine, fluorine, or boron are used to separate oxides and other inclusions. Covering fluxes protect magnesium-rich alloys, while carbon and graphite retard oxidation. Proper fluxing improves casting integrity by removing impurities that otherwise weaken mechanical properties.

Effects of Inclusions

Inclusions reduce effective cross-sectional strength, acting as stress concentrators and compromising fatigue resistance. Their removal is critical for achieving high-performance castings.

Grain Structure and Refinement

Importance of Grain Control

A fine, equiaxed grain structure improves mechanical properties, reduces defects, and enhances overall casting quality. Grain refinement involves adding master alloys containing titanium and boron, which introduce nucleation sites for uniform grain formation.

Benefits of Grain Refinement

• Improved feeding characteristics
• Enhanced tear and crack resistance
• Increased pressure tightness
• Better response to thermal treatments
• Reduced shrinkage and hydrogen porosity

Control of Dendritic Structures

Dendrite arm spacing and cell size are critical for casting soundness. Fine structures minimize shrinkage voids and improve intermetallic distribution, enhancing feeding and mechanical properties.

Advanced Casting Techniques

Process Versatility

Modern advancements have enabled the use of alloys previously considered unsuitable for specific casting methods. Techniques like permanent mold casting now accommodate a broader range of alloys, improving design flexibility and expanding application possibilities.

Structural Control

Controlling microstructural features such as dendrite cell size and intermetallic phase distribution is vital for meeting engineering specifications. Enhanced solidification rates and advanced alloying techniques contribute to more consistent properties and higher-quality castings.