Phân tích nguyên nhân và biện pháp cải thiện độ xốp trung tâm trong đĩa nhôm cán nóng

Hot-rolled aluminum discs are critical intermediate products in the aluminum processing industry chain, widely used in high-end fields such as cookware, thiết bị điện, ô tô, and aerospace. Their internal quality directly determines the mechanical properties, khả năng định hình, and service life of the final products. Center porosity​ is one of the most common internal defects in hot-rolled aluminum discs, manifested as fine, scattered pores or loose structures in the central area. It significantly reduces the material’s density, sức mạnh, and plasticity, and can easily become a source of cracks in subsequent processes like stamping, vẽ, and welding, leading to increased product rejection rates and production costs. Vì thế, a systematic analysis of the formation mechanism of center porosity and the development of scientific improvement measures are of great engineering value for enhancing product quality and strengthening the market competitiveness of enterprises.

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1. Analysis of the Causes of Center Porosity in Hot-Rolled Aluminum Discs

Center porosity is the result of the combined effects of processes and microstructural evolution throughout the entire production chain, bao gồm melting/casting, cán nóng, và làm mát. Its causes can be summarized into the following four categories:

1.1 Inheritance and Retention of Original Defects in the Ingot

The feedstock for hot-rolled discs is semi-continuously cast aluminum ingots. Original porosity within the ingot is the primary source:

1. Insufficient Feeding for Solidification Shrinkage

   Aluminum alloys undergo a volume contraction of approximately 6%–7% during solidification. If the final solidification of the ingot’s center is hindered by the already solidified shell, the residual liquid between the dendrites becomes isolated, and the shrinkage cavities cannot be filled, hình thành shrinkage porosity—the most dominant form.

2. Gas Evolution and Entrapment

   Molten nhôm readily absorbs hydrogen during melting and holding. Upon solidification, hydrogen solubility drops sharply, and supersaturated hydrogen precipitates as bubbles. If bubbles cannot float out in time and are blocked by the dendritic network, gas porosity​ forms, exacerbating defects when combined with shrinkage porosity.

3. Non-Uniform Solidification Structure

   During semi-continuous casting, the ingot surface cools rapidly while the center cools slowly, forming a structure of “fine grains on the surface, coarse grains in the center.” The coarse grains and developed dendrites in the center hinder feeding and gas venting, and lead to difficulty in porosity healing during hot rolling due to uneven deformation.

4. Effects of Inclusions and Segregation

   Bao gồm (ví dụ., alumina) in the molten aluminum can act as nucleation sites for bubbles and impede melt flow. sự tách biệt (ví dụ., solute enrichment) in the central region alters the local solidification behavior, further increasing the tendency for porosity.

1.2 Unreasonable Hot Rolling Process Parameters

Hot rolling is the key process for healing porosity. Improper parameters can not only fail to eliminate original defects but also induce new ones:

1. Insufficient Total Reduction

   A total rolling reduction that is too low (tiêu biểu <60%) results in insufficient deformation in the center, inadequate metal flow, and prevents the original porous cavities from being compacted and healed, leading directly to their retention.

2. Unbalanced Distribution of Pass Reductions

   Excessive reductions in early passes and insufficient ones later, or deformation concentrated only on the surface, prevent the center from receiving adequate triaxial compressive stress; insufficient reductions in later passes can also leave porosity unhealed.

3. Improper Rolling Temperature Control
   • Temperature too low: Aluminum alloy plasticity decreases, deformation resistance increases, making deformation in the center difficult, leading to poor healing effects and a tendency to cause work hardening and cracking.
   • Temperature too high: Grain coarsening occurs, and excessive metal fluidity may lead to “đốt cháy” or structural inhomogeneity in the center, which is detrimental to porosity repair.
4. Unreasonable Rolling Speed and Lubrication

   Excessive rolling speed shortens deformation time, preventing sufficient flow in the center; insufficient lubrication increases friction, causing greater surface deformation than the center, aggravating deformation inhomogeneity.

1.3 Defects in Cooling and Heat Treatment Processes

Post-rolling cooling and subsequent heat treatments directly affect the healed state and stability of the structure:

1. Non-Uniform Cooling Rate

   Excessive cooling (ví dụ., direct water quenching) causes rapid surface contraction while the center lags, generating high internal stresses that may re-open healed pores. Slow cooling can lead to grain coarsening, reducing density.

2. Insufficient Homogenization Annealing

   Homogenization annealing of the ingot before hot rolling aims to eliminate dendritic segregation and improve microstructural uniformity. If the annealing temperature is too low or the holding time is insufficient, non-equilibrium phases are not fully dissolved, and original porosity and segregation are inherited by the hot-rolled disc.

3. Improper Cooling After Annealing

   Rapid cooling generates internal stresses and provides insufficient time for atomic diffusion; excessively slow cooling may cause grain coarsening.

1.4 Equipment and Operational Factors

Equipment precision and operational standardization indirectly affect porosity control:

1. Insufficient Mill Rigidity

   Low rigidity of the rolling mill stand leads to significant elastic deformation during rolling, resulting in uneven slab thickness and insufficient deformation in the center.

2. Uneven Heating of the Slab

   Temperature control deviations in the reheating furnace or improper slab placement cause temperature gradients across the ingot cross-section, leading to uneven deformation during rolling.

3. Non-standardized Operations

   Issues such as slab wandering during rolling, excessive temperature loss between passes, or uneven application of lubricant can all exacerbate deformation non-uniformity, affecting the improvement of porosity.

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2. Systematic Improvement Measures for Center Porosity in Hot-Rolled Aluminum Discs

A comprehensive improvement plan is required, addressing the entire process from melting/casting source, hot rolling process optimization, cooling/heat treatment improvement, to equipment and management.

2.1 Melting and Casting Stage: Reducing Original Ingot Porosity at Source

The core objective is to improve melt cleanliness, optimize the solidification process, and enhance feeding and degassing.

2.1.1 Optimize Melt Refining Process

• Enhanced Degassing: Sử dụng rotary inert gas (Ar/N₂) injection degassing, controlling time (15-25 phút), rotor speed (200-300 rpm), and gas flow (0.5-1.0 m³/h) to ensure hydrogen content is reduced to below 0.12 mL/100g. Add efficient degassing agents (ví dụ., hexachloroethane) nếu cần thiết.
• Strict Dross Removal and Filtration: Let the melt settle for ≥30 min​ after melting; sử dụng ceramic foam filters (30-50 ppi)​ or deep bed filtration to remove non-metallic inclusions.
• Control Melting and Holding Parameters: Nhiệt độ nóng chảy: 720-750oC; thời gian nắm giữ: ≤2 h; Use flux cover or inert gas protection throughout.

2.1.2 Optimize Casting Process

• Control Casting Temperature and Speed: Casting temperature: 50-80℃ above the liquidus; Adjust casting speed according to ingot size (slower for larger ingots).
• Optimize Cooling System: Nhận nuôi uniform cooling technology​ to minimize the cooling rate difference between surface and center. For large ingots, segmented coolingcó thể được sử dụng.
• Enhance Feeding Design: Sử dụng insulating or exothermic risers, following the principle of “directional solidification”. Electromagnetic stirring​ can be used to fragment dendrites and promote melt flow.
• Add Grain Refiners: Thêm vào Al-Ti-B or Al-Ti-C refiners, controlling Ti content to 0.05-0.25%.

2.1.3 Perfect Ingot Homogenization Annealing

• Nhiệt độ ủ: 0.9-0.95 of the solidus temperature​ (ví dụ., ~580-600℃ for 1050 hợp kim).
• Thời gian nắm giữ: 4-8 h​ (depending on ingot size and alloy type).
• Phương pháp làm mát: Furnace cooling or air cooling​ after annealing.

Bàn 1: Key Control Points in the Melting and Casting Process

2.2 Giai đoạn cán nóng: Optimizing the Process for Effective Porosity Healing

The core is to apply sufficient triaxial compressive stress to the center through reasonable reduction, nhiệt độ, and speed control.

2.2.1 Rational Distribution of Reduction Rate

• Tổng mức giảm: Đảm bảo ≥70%​ (ví dụ., from 200mm ingot to ≤60mm disc). For 7XXX series alloys, ≥75%​ is recommended.
• Pass Reduction Optimization: Adopt the principle of “small initially, large in the middle, stable at the end“:
  • Initial Passes: 10–15%, to break surface coarse grains and reduce resistance.
  • Middle Passes: 20–30%, to apply strong deformation to the center, promoting healing.
  • Final Passes: 5–10%, to control dimensional accuracy and surface finish.
• High-Reduction Rolling: Increase single-pass reduction where equipment permits to enhance hydrostatic pressure in the center.

2.2.2 Precise Control of Rolling Temperature

• Initial Rolling Temperature: 450–500℃​ (adjusted per alloy, ví dụ., 460–480℃ for 3XXX series).
• Finishing Rolling Temperature: 300–350℃​ to avoid work hardening (too low) or grain coarsening (too high). Reheating between passes is needed to maintain uniform cross-sectional temperature.

2.2.3 Optimize Rolling Speed and Lubrication

• Rolling Speed Strategy: “Low speed for biting, medium speed for rolling, high speed for delivery”.
• Bôi trơn: Sử dụng efficient hot rolling lubricants​ sprayed evenly to reduce friction and ensure uniform deformation.

Bàn 2: Optimization of Core Hot Rolling Process Parameters

2.3 Cooling and Heat Treatment: Stabilizing the Structure, Preventing Porosity Recurrence

2.3.1 Control Post-Rolling Cooling Rate

• Nhận nuôi slow and uniform cooling​ (air cooling or stacking), avoiding direct water/quench cooling to minimize thermal stress that could re-open healed pores.

2.3.2 Perfect Subsequent Heat Treatment

• Ủ (ví dụ., 350-400℃ for 3XXX series) can be applied as needed to relieve stress, stabilize the structure, and further heal residual porosity. Cool slowly after annealing.

2.4 Equipment and Management: Ensuring Stable Process Execution

• Equipment Maintenance & Nâng cấp: Regularly inspect mills, furnaces, hệ thống làm mát. Upgrade to high-precision mills, intelligent furnaces if necessary.
• Standardized Operations & Process Monitoring: Develop SOPs. Implement online inspection (ví dụ., kiểm tra siêu âm) for real-time internal quality monitoring.
• Đào tạo nhân sự & Kiểm soát chất lượng: Enhance operator training. Establish a full-process quality sampling system.

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3. Verification of Improvement Effectiveness and Quality Control

Establish a scientific quality inspection and verification system to ensure the effectiveness of improvement measures:

1. Macrostructural Examination

   Section, etch, and observe the central area. Rate the porosity level according to national standards (ví dụ., GB/T 3246.1), targeting Cấp 1 or lower.

2. Ultrasonic Testing (UT)

   Trình diễn 100% ultrasonic inspection to ensure no defects exceeding standards.

3. Mechanical Property Testing

   Test tensile strength, sức mạnh năng suất, and elongation to verify improvement.

4. Process Parameter Traceability

   Establish a production parameter database to trace key parameters for each batch, enabling continuous process optimization.

Bàn 3: Quality Inspection Methods and Standards for Center Porosity

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4. Phần kết luận

Improving center porosity in hot-rolled aluminum discs is a systematic project focusing on three key aspects:

1. Control Defects at the Ingot Source: Strengthen melt refining, optimize solidification and feeding, perfect homogenization annealing.
2. Core Optimization of Hot Rolling Process: Ensure sufficient total reduction (≥70%), distribute passes rationally, and precisely control temperature and speed.
3. Stabilize the Structure in Subsequent Cooling: Use uniform slow cooling, combined with appropriate heat treatment to prevent internal stresses and structural defects.

Enterprises should develop customized process plans based on their own equipment, alloy types, and product specifications. Through continuous inspection, tối ưu hóa, and full-process fine management, the issue of center porosity can be fundamentally resolved, enabling the production of high-quality, highly stable hot-rolled aluminum discs to meet the increasingly stringent quality requirements of downstream industries.