The quality of aluminum alloy melt directly determines product performance, precision, and yield, affecting the stability of casting, rolling, and extrusion processes, as well as mechanical properties, corrosion resistance, and surface quality. Melting, alloying, and refining temperatures, along with post-refining holding time, are key process parameters. Proper optimization of these parameters ensures melt purity, uniform composition, and minimal gas and inclusion content, significantly enhancing melt quality and final product performance.
1. Introduction
Aluminum alloys, because of their low density, high specific strength, corrosion resistance, and ease of processing, are widely applied in aerospace, rail transportation, automotive manufacturing, electronics, electrical appliances, and other fields. With continuous industrial technology upgrades, the market’s performance requirements for aluminum alloy products are increasingly stringent. Melt quality as the “source” of aluminum alloy production directly determines the upper limit of the quality of subsequent processed products. In the aluminum alloy smelting process, the control of process parameters is the key factor affecting melt quality, among which the control of smelting temperature (including melting temperature, alloying temperature, and refining temperature) and the holding time after refining is particularly important.
Melting temperature determines the melting efficiency of aluminum ingots and scrap, and the initial state of the melt. Alloying temperature affects the dissolution rate of alloying elements and composition uniformity. Refining temperature directly relates to the effectiveness of refining agents and the efficiency of removing gases and inclusions in the melt; while the holding time after refining, as the link between smelting and casting, directly influences the flotation and separation of residual gases and fine inclusions, thus affecting melt purity. Currently, in the field of recycled aluminum smelting, due to complex raw material composition and diverse scrap types, the regulation of smelting temperature and holding time is more difficult. If parameters are set improperly, the melt may have excessive hydrogen content, increased inclusions, and composition segregation, ultimately leading to defects in products such as shrinkage, porosity, cracks, and inclusions, reducing product yield and service life.
This paper, based on the basic principles of aluminum alloy smelting and combined with the actual operating conditions of recycled aluminum production, systematically studies the influence mechanisms of melting temperature, alloying temperature, refining temperature, and holding time after refining on aluminum alloy melt quality, analyzes the synergistic relationship between these parameters, and proposes targeted optimization and control schemes, aiming to provide a reference for precise control of aluminum alloy smelting processes, help enterprises improve melt quality, reduce production costs, and enhance product competitiveness.
2. Basic Principles of Aluminum Alloy Smelting and Melt Quality Evaluation Indicators
2.1 Basic Principles of Aluminum Alloy Smelting
Aluminum alloy smelting is the process of heating aluminum ingots, recycled aluminum scrap, and other raw materials to a certain temperature, making them transform from solid to liquid, and through alloying, refining, degassing, and inclusion removal operations, obtaining a liquid aluminum alloy with uniform composition, high purity, and stable properties. The entire smelting process mainly includes six core stages: charging, melting, alloying, refining, holding, and casting. These stages are interconnected and influence each other, and the control of smelting temperature and holding time pervades the melting, alloying, refining, and holding stages.
The core objective of aluminum alloy smelting is: on the premise of ensuring sufficient raw material melting and uniform distribution of alloy elements, to maximize the removal of hydrogen (mainly from moisture in raw materials, water vapor in furnace gas, and hydrogen-containing components in refining agents) and non-metallic inclusions (mainly aluminum oxide Al2O3, silicon oxide SiO2, carbides, etc., originating from the oxide film on raw material surfaces, refractory erosion, and residual refining agents), while avoiding excessive oxidation of the melt and loss of alloy elements, ultimately obtaining a high-quality melt that meets casting requirements.
2.2 Melt Quality Evaluation Indicators
Melt quality evaluation mainly revolves around three core dimensions: purity, composition uniformity, and fluidity, including the following key indicators:
- Gas content: refers to the hydrogen content in the melt (usually expressed as hydrogen partial pressure or mL per 100 g aluminum). Hydrogen is the main cause of defects such as shrinkage, porosity, and pinholes. Generally, melt hydrogen content should be controlled within 0.10 to 0.20 mL per 100 g aluminum (varies slightly by alloy grade).
- Inclusion content and size: refers to the number, size distribution, and type of non-metallic inclusions in the melt. Inclusions destroy continuity in aluminum alloys, reducing mechanical performance and corrosion resistance. In high-quality melts, inclusion size should be controlled below 50 um, and the fewer inclusions per unit volume the better.
- Composition uniformity: refers to the uniform distribution of alloy elements (such as Si, Mg, Cu, Mn) in the melt. Composition segregation leads to performance differences in different parts of the product, affecting product stability and consistency. Generally, the deviation between actual and target element content should not exceed +/-0.1% (except for special alloys).
- Fluidity: refers to the ability of the melt to fill molds under its own weight. Poor fluidity leads to underfilling and cold shuts. Fluidity is mainly influenced by melt temperature, composition, gas content, and inclusion content.
- Oxidation loss rate: refers to the proportion of aluminum and alloy elements lost due to oxidation during smelting. Excessive oxidation loss increases production cost and produces oxide slag, increasing melt inclusions. Typically, oxidation loss rate should be controlled below 2%.
3. Effects of Smelting Temperature on Aluminum Alloy Melt Quality
Smelting temperature is one of the most critical process parameters in aluminum alloy smelting. The temperature settings for different smelting stages (melting, alloying, refining) directly influence the initial state, composition uniformity, purity, and subsequent processing performance of the melt. The following analyzes the effects from three dimensions: melting temperature, alloying temperature, and refining temperature.
3.1 Effects of Melting Temperature on Melt Quality
Melting temperature refers to the minimum temperature required to fully melt solid aluminum raw materials (ingots, recycled scrap), and it needs to balance melting efficiency and melt quality. Both excessively high and excessively low temperatures negatively affect melt quality.
3.1.1 Effects of Low Melting Temperature
When the melting temperature is below the melting point of aluminum (pure aluminum melts at 660 deg C; aluminum alloy melting points vary slightly, usually between 600 and 650 deg C) or only slightly above it, raw material melting proceeds slowly and local unmelted areas occur, forming a “solid-liquid coexistence” state. In this situation, unmelted solid particles remain in the melt as inclusions, increasing inclusion content. These unmelted particles also disrupt melt continuity, reduce melt fluidity, and impair mold filling, resulting in casting defects such as underfilling and cold shuts.
In addition, low melting temperature increases melt viscosity, hindering the detachment and flotation of oxide films. Oxide films, as non-metallic inclusions, remain suspended in the melt, reducing melt purity. Furthermore, at low temperatures, atomic mobility is weak, and alloy elements added in the subsequent alloying stage dissolve slowly, leading to insufficient dissolution and composition segregation, thus affecting melt composition uniformity.
For recycled aluminum smelting, raw materials often contain oil, oxide skins, coatings, and other contaminants. When the melting temperature is too low, these contaminants cannot fully decompose and volatilize, mixing into the melt and increasing carbides, oxides, and hydrogen content, further deteriorating melt quality.
3.1.2 Effects of High Melting Temperature
Appropriately increasing melting temperature can speed up melting, shorten melting time, and reduce melt viscosity, facilitating oxide film detachment and flotation. However, when the melting temperature is too high (usually above 700 deg C, and for some high-strength aluminum alloys above 720 deg C), it negatively impacts melt quality in the following ways:
- It accelerates oxidation loss. Aluminum is a reactive metal, and at high temperatures quickly forms aluminum oxide Al2O3 with oxygen in the air. Higher melting temperature increases oxidation, raising oxidation loss rate and producing significant oxide slag. If oxide slag is not removed in time, it can re-enter the melt, increasing inclusions. At the same time, alloy elements such as Mg and Zn may significantly volatilize and be lost, causing melt composition to deviate from target values and reducing product mechanical performance.
- It increases melt hydrogen content. High temperatures increase the solubility of hydrogen in the aluminum melt. Hydrogen from moisture in raw materials and water vapor in furnace gas decomposes faster and dissolves readily into the melt, leading to excess hydrogen content. In addition, at high temperature, the evaporation of volatile elements such as Mg and Zn increases, aggravating composition segregation.
- It reduces melt stability. Excessively high melting temperature leads to excessive superheat in melt. The intensified thermal motion of atoms enhances melt convection, reintroducing floated oxide slag and inclusions into the melt, making them difficult to remove. In subsequent holding and casting, the overheated melt can cause coarse grain structure, shrinkage, and porosity in castings, negatively affecting microstructure and mechanical properties.
3.1.3 Optimal Melting Temperature Range
Optimal melting temperature must be determined comprehensively according to alloy grade, raw material type (primary ingots or recycled scrap), and smelting equipment characteristics. For pure aluminum and low-alloy aluminum alloys (such as 6061, 6063), melting temperature is usually controlled between 660 and 680 deg C; for medium-high strength aluminum alloys (such as 2A12, 7075), due to higher alloying content and slightly lower solidus temperature, melting temperature can be controlled between 650 and 670 deg C; for recycled aluminum smelting, considering complex scrap composition and higher contaminant content, melting temperature can be moderately increased but should not exceed 700 deg C, typically 670 to 690 deg C.
In actual production, real-time adjustments to melting temperature based on melt status are necessary to avoid unmelted regions or excessive superheat. At the same time, furnace atmosphere should be kept stable to reduce air ingress, minimize oxidation loss, and prevent hydrogen absorption, laying a solid foundation for subsequent process stages.
3.2 Effects of Alloying Temperature on Aluminum Alloy Melt Quality
Alloying refers to adding specific types and amounts of alloy elements (such as Si, Mg, Cu, Mn, Cr) to the molten aluminum after melting, heating and stirring to fully dissolve and uniformly distribute the elements in the melt, forming an aluminum alloy melt that meets the target composition. Alloying temperature is a key parameter affecting the dissolution efficiency of alloying elements, composition uniformity, and melt quality. The temperature must ensure full dissolution of alloy elements while avoiding excessive oxidation loss and gas absorption.
3.2.1 Effects of Low Alloying Temperature
When the alloying temperature is too low, the melt viscosity is high and atomic diffusion is slow, making it difficult for added alloy elements to dissolve quickly. This can result in “undissolved particles.” For example, producing 6061 aluminum alloy at an alloying temperature below 680 deg C causes Mg particles to remain in solid form, becoming inclusions, reducing melt purity and fluidity, and causing Mg content to be below target, affecting product strength and corrosion resistance.
Low alloying temperature also leads to uneven diffusion of alloy elements, resulting in composition segregation. For Al-Si alloys, low temperature slows Si diffusion, causing some regions to have excessive Si content while others are deficient. This leads to hardness and strength inconsistencies in castings.
For recycled aluminum, existing alloy elements and fluctuating composition in scrap can worsen composition segregation if the alloying temperature is too low. Undissolved particles can combine with impurities to form complex inclusions, deteriorating melt quality.
3.2.2 Effects of High Alloying Temperature
Moderately increasing alloying temperature lowers melt viscosity, accelerates dissolution and diffusion of alloy elements, and improves composition uniformity. However, if alloying temperature is too high (usually above 750 deg C depending on alloy grade), it negatively impacts melt quality:
- Accelerated oxidation loss: In addition to aluminum oxidation, alloy elements such as Mg, Zn, Mn react with oxygen, causing high loss. For instance, Mg oxidation loss can exceed 15% above 750 deg C, far above normal, preventing alloy from meeting composition requirements. Oxides of alloy elements also become new inclusions.
- Increased hydrogen content: High temperature increases hydrogen solubility in melt. Convection enhances hydrogen absorption, further increasing melt hydrogen content.
- Melt overheating: High alloying temperature increases superheat, reduces stability, promotes reintroduction of oxide slag during holding, and may lead to coarse grains and casting defects such as cracks and shrinkage.
3.2.3 Optimal Alloying Temperature Range
The optimal alloying temperature depends on alloy grade, type, and amount of added elements. Principle: ensure full dissolution and uniform distribution of alloy elements while minimizing oxidation loss and gas absorption.
- Low-alloy alloys (6063, mainly Si and Mg): 680–700 deg C. Ensures sufficient dissolution and keeps Mg oxidation loss below 5%.
- Medium-high strength alloys (2A12, mainly Cu, Mg, Mn): 700–720 deg C. Ensures full dissolution with inert gas protection to reduce oxidation loss.
- High-alloy alloys (7075, mainly Zn, Mg, Cu): 720–740 deg C with inert gas protection to reduce Zn and Mg loss.
In production, staged heating and gradual addition of alloy elements are recommended. Sufficient stirring (mechanical or electromagnetic) improves element dissolution and uniformity. Post-alloying composition should be checked and adjusted as necessary.
3.3 Effects of Refining Temperature on Aluminum Alloy Melt Quality
Refining removes gases and inclusions from the melt using refining agents (such as chlorinated compounds, argon, nitrogen) or by vacuum refining, rotary injection, etc. Refining temperature affects agent activity and the efficiency of removing gases and inclusions.
3.3.1 Effects of Low Refining Temperature
Low refining temperature increases melt viscosity and decreases agent activity:
- Gas removal efficiency decreases. Large bubbles reduce hydrogen adsorption, making it difficult to reach required hydrogen limits.
- Inclusion removal efficiency decreases. Poor fluidity slows inclusion flotation, leaving residual inclusions in the melt.
- Residual refining agents may remain unreacted, forming new inclusions or reacting with alloy elements.
3.3.2 Effects of High Refining Temperature
High refining temperature reduces viscosity and accelerates agent activity:
- Hydrogen reabsorption increases due to higher solubility.
- Oxidation loss accelerates due to air exposure.
- Refining agent consumption increases, raising production costs.
3.3.3 Optimal Refining Temperature Range
Depends on refining method and alloy grade. Principles: ensure refining efficiency while minimizing hydrogen reabsorption and oxidation loss.
- Gas refining (argon/nitrogen): 700–720 deg C for low-alloy alloys; 720–740 deg C for medium-high strength alloys.
- Vacuum refining: 680–700 deg C.
- Solid agents (e.g., chlorinated compounds): 700–720 deg C; for recycled aluminum, 720–740 deg C with extended refining time.
Temperature and refining time should be adjusted together. Keep furnace atmosphere stable to avoid hydrogen reabsorption.
4. Effects of Holding Time After Refining on Melt Quality
Holding after refining allows small bubbles and fine inclusions to float to the surface and be removed, improving melt purity. Holding time directly affects removal of residual gases and inclusions.
4.1 Effects of Short Holding Time
Holding less than 10 minutes can lead to:
- Excess hydrogen: fine bubbles remain in melt, dissolving hydrogen and causing defects during casting.
- Residual inclusions: small inclusions (10–50 um) cannot float, reducing melt purity and flow.
- Reduced composition uniformity: density differences cause segregation of elements like Cu and Mn, leading to inconsistent casting performance.
4.2 Effects of Long Holding Time
Holding more than 30 minutes can lead to:
- Increased oxidation: thickening oxide film may re-enter melt, increasing inclusions.
- Temperature drop and reduced fluidity: lower filling efficiency, coarse grains, reduced mechanical properties.
- Hydrogen content rebound: reactions with air/water produce hydrogen, reducing refining effectiveness. Element segregation may worsen.
4.3 Optimal Holding Time Range
Depends on alloy grade, melt temperature, refining efficiency, and casting requirements:
- Low-alloy alloys: 15–20 minutes.
- Medium-high strength alloys: 20–25 minutes.
- Recycled aluminum: 20–30 minutes, but not exceeding 30 minutes.
Keep melt insulated to reduce heat loss and avoid air ingress. Remove surface oxide before casting. Adjust holding time based on real-time melt conditions.
5. Synergistic Control Strategies for Smelting Temperature and Holding Time
Improving melt quality requires coordinated control of melting, alloying, refining temperatures and holding time. Parameters are interrelated; optimization requires matching all parameters.
5.1 Alloy Grade-Based Control
- Low-alloy alloys (6061, 6063): melting 660–680 deg C, alloying 680–700 deg C, refining 690–710 deg C, holding 15–20 minutes. Alloying slightly higher than melting ensures full dissolution; refining slightly higher improves removal; holding not too long to avoid Mg loss.
- Medium-high strength alloys (2A12, 7075): melting 650–670 deg C, alloying 700–720 deg C, refining 710–730 deg C, holding 20–25 minutes. Use inert gas to reduce element loss; higher refining temp ensures fine inclusion removal; longer holding removes residual bubbles/inclusions.
- Recycled aluminum: melting 670–690 deg C, alloying 690–710 deg C, refining 700–720 deg C, holding 20–30 minutes. Higher melting ensures scrap melting; longer refining improves degassing; holding time flexible based on impurity content.
5.2 Raw Material-Based Control
- Primary ingots: melting 660–670 deg C, alloying 680–700 deg C, refining 690–710 deg C, holding 15–20 minutes. Avoid unnecessary high temperature; shorter refining time; holding time moderate.
- Recycled scrap: melting 670–690 deg C, alloying 690–710 deg C, refining 700–720 deg C, holding 20–30 minutes. Higher melting for impurity removal; staged alloying with stirring; increased refining agent and time; longer holding for inclusion flotation.
5.3 Production Efficiency and Quality Balance
- Melting: ensure full melting, shorten time via optimized charging, heating power, control temperature to avoid overheat.
- Alloying: staged heating, batch addition, improve element dissolution, reduce oxidation.
- Refining: optimize agent type and amount, improve process, shorten time while maintaining efficiency.
- Holding: adjust time based on refining effect; shorter holding increases efficiency; longer if refining insufficient.
6. Conclusions and Prospects
6.1 Conclusions
- Low melting temperature causes incomplete melting, high viscosity, increased gas/inclusion content; high temperature increases oxidation and hydrogen absorption. Optimal 650–700 deg C depending on alloy grade and raw material.
- Low alloying temperature causes incomplete dissolution and segregation; high temperature increases oxidation and gas absorption. Optimal 680–740 deg C based on element type and content.
- Low refining temperature reduces refining efficiency; high temperature increases hydrogen reabsorption and oxidation. Optimal 690–730 deg C based on method and alloy.
- Short holding time leaves bubbles/inclusions; long holding increases oxidation and reduces fluidity. Optimal 15–30 minutes based on alloy and refining.
- Improving melt quality requires coordinated control of melting, alloying, refining temperatures and holding time, considering alloy grade, raw material, and production efficiency.
6.2 Prospects
- Intelligent control: IoT, big data, AI for real-time monitoring and automatic optimization of melt temperature, composition, gas and inclusion content.
- New refining techniques: efficient, eco-friendly refining processes and agents to reduce temperature/time effects on quality and costs.
- Recycled aluminum smelting optimization: remove harmful impurities like Fe and Si effectively.
- Research on multi-parameter synergy: study interaction with stirring, furnace atmosphere, establish melt quality prediction models, support precise process control.
Post time: Dec-20-2025
