হট-রোলড অ্যালুমিনিয়াম ডিস্কে কেন্দ্রীয় ছিদ্রের জন্য কারণ এবং উন্নতির ব্যবস্থার বিশ্লেষণ

Hot-rolled aluminum discs are critical intermediate products in the aluminum processing industry chain, বহুল ব্যবহৃত উচ্চ-শেষ ক্ষেত্র যেমন cookware, বৈদ্যুতিক যন্ত্রপাতি, অটোমোবাইল, এবং মহাকাশ. তাদের অভ্যন্তরীণ গুণমান সরাসরি যান্ত্রিক বৈশিষ্ট্য নির্ধারণ করে, গঠনযোগ্যতা, 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, শক্তি, and plasticity, and can easily become a source of cracks in subsequent processes like stamping, অঙ্কন, and welding, leading to increased product rejection rates and production costs. অতএব, 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, সহ melting/casting, গরম ঘূর্ণায়মান, এবং শীতল. 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, গঠন shrinkage porosity—the most dominant form.

2. Gas Evolution and Entrapment

   Molten অ্যালুমিনিয়াম 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

   অন্তর্ভুক্তি (যেমন, alumina) in the molten aluminum can act as nucleation sites for bubbles and impede melt flow. বিচ্ছেদ (যেমন, 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 (সাধারণত <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 “জ্বলন্ত” 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 (যেমন, 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: ব্যবহার করুন rotary inert gas (Ar/N₂) injection degassing, controlling time (15-25 মিনিট), rotor speed (200-300 rpm), and gas flow (0.5-1.0 m³/ঘণ্টা) to ensure hydrogen content is reduced to below 0.12 mL/100g. Add efficient degassing agents (যেমন, হেক্সাক্লোরোইথেন) প্রয়োজন হলে.
• Strict Dross Removal and Filtration: Let the melt settle for ≥30 min​ after melting; ব্যবহার ceramic foam filters (30-50 ppi)​ or deep bed filtration to remove non-metallic inclusions.
• Control Melting and Holding Parameters: গলে যাওয়া তাপমাত্রা: 720-750℃; সময় ধরে রাখা: ≤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: দত্তক uniform cooling technology​ to minimize the cooling rate difference between surface and center. For large ingots, segmented coolingব্যবহার করা যেতে পারে.
• Enhance Feeding Design: ব্যবহার করুন 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: যোগ করুন Al-Ti-B or Al-Ti-C refiners, controlling Ti content to 0.05-0.25%.

2.1.3 Perfect Ingot Homogenization Annealing

• অ্যানিলিং তাপমাত্রা: 0.9-0.95 of the solidus temperatureআমি (যেমন, ~580-600℃ for 1050 খাদ).
• হোল্ডিং টাইম: 4-8 জআমি (depending on ingot size and alloy type).
• কুলিং পদ্ধতি: Furnace cooling or air cooling​ after annealing.

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

2.2 হট রোলিং স্টেজ: Optimizing the Process for Effective Porosity Healing

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

2.2.1 Rational Distribution of Reduction Rate

• মোট হ্রাস: নিশ্চিত করুন ≥70%আমি (যেমন, 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℃আমি (adjusted per alloy, যেমন, 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”.
• তৈলাক্তকরণ: ব্যবহার করুন efficient hot rolling lubricants​ sprayed evenly to reduce friction and ensure uniform deformation.

টেবিল 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

• দত্তক slow and uniform coolingআমি (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

• অ্যানিলিং (যেমন, 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 & আপগ্রেড: Regularly inspect mills, furnaces, কুলিং সিস্টেম. Upgrade to high-precision mills, intelligent furnaces if necessary.
• Standardized Operations & Process Monitoring: Develop SOPs. Implement online inspection (যেমন, হেনান হুয়াওয়ে অ্যালুমিনিয়াম শিল্প অ্যালুমিনিয়াম সার্কেল রপ্তানি উত্পাদন) for real-time internal quality monitoring.
• কর্মী প্রশিক্ষণ & মান নিয়ন্ত্রণ: 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 (যেমন, হেনান হুয়াওয়ে অ্যালুমিনিয়াম শিল্প অ্যালুমিনিয়াম সার্কেল রপ্তানি উত্পাদন 3246.1), targeting শ্রেণী 1 or lower.

2. Ultrasonic Testing (হেনান হুয়াওয়ে অ্যালুমিনিয়াম শিল্প অ্যালুমিনিয়াম সার্কেল রপ্তানি উত্পাদন)

   সঞ্চালন 100% ultrasonic inspection to ensure no defects exceeding standards.

3. Mechanical Property Testing

   Test tensile strength, শক্তি ফলন, 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.

টেবিল 3: Quality Inspection Methods and Standards for Center Porosity

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4. উপসংহার

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, অপ্টিমাইজেশান, 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.