Systematic Improvement Plan for Surface Finish of 5052 Aluminum Alloy Hot-Rolled Discs

To stably reduce the surface roughness (Ra) of 5052 aluminum hot-rolled discs from 1.0–3.0 μm to below 0.4 μm, or even achieve a mirror finish, a systematic optimization of the following four key areas is required, adhering to the principles of “source control, precision processing, and ultimate treatment.”

1. Hot Rolling Process: Precision Control at Source

This stage aims to obtain a uniform, clean hot-rolled blank, laying the foundation for subsequent finishing.

1.1 Full-Cycle Roll Management

• Surface Quality: Work rolls must undergo high-precision grinding, with an initial surface roughness Ra ≤ 0.3 μm. Establish a strict inspection system; after every 100–200 tons produced or per production cycle, rolls must be inspected and reground/replaced to eliminate pitting, aluminum pickup, and scratches.
• Online Cleaning: Equip the exit side of roughing and finishing mills with high-strength online brush rolls​ (steel or nylon) to continuously remove aluminum fines and oxide films from the roll surface, maintaining uniform and stable roll conditions.
• Roll Material: Preferentially use rolls with high hardness and wear resistance, such as high-chromium cast iron or hard alloy-coated rolls, to extend the duration of high surface finish and reduce aluminum pickup tendency.

1.2 Precise Matching of Process Parameters

• Temperature Window: Strictly control the starting rolling temperature​ between 480–520°C and the finishing rolling temperature​ no lower than 300–350°C. Excessively high temperatures increase pickup and oxidation, while excessively low temperatures raise the risk of surface cracking.
• Deformation Schedule: Adopt a “multi-pass, small reduction”​ strategy, with a single-pass reduction rate recommended ≤15%, to ensure uniform deformation and avoid defects like orange peel and micro-cracks.
• Speed Matching: Rolling speed should be coordinated with lubrication and cooling capabilities. A stable, low speed of 70–100 m/min during roughing​ is recommended to minimize surface chatter marks caused by equipment vibration.

1.3 Synergistic Lubrication and Cooling

• Lubrication Assurance: Use specialized hot rolling emulsion with high extreme pressure performance and low viscosity, with concentration controlled between 3%–5%. Optimize nozzle layout and spray pressure to ensure a continuous, stable lubricant film forms in the roll-material bite zone, effectively reducing friction and pickup.
• Intensive Cooling Control: Employ a segmented, strong convection cooling​ system to quickly and uniformly remove rolling heat, suppressing surface oxidation and secondary pickup caused by high strip temperature.

2. Cold Rolling and Finishing: The Key Improvement Stage

Cold rolling further flattens the surface and refines the microstructure, representing the core step for achieving a leap in Ra reduction.

1. Breakdown Rolling (Leveling): Use EDT-textured work rolls​ (Ra 1.5–2.5 μm) for the first pass with a reduction ratio ≥30%, focusing on eliminating black lines, color differences, and macroscopic unevenness inherited from hot rolling.
2. Finish Rolling (Brightening): Switch to superfinely ground work rolls​ (Ra ≤ 0.3 μm). Achieve a uniform, bright rolled surface with Ra stably controlled within 0.4–0.8 μm​ through a process involving ≥2 passes and a total reduction ratio ≥50%.
3. Tension Leveling: Apply low elongation (0.5%–1.5%)​ stretch bending leveling to improve flatness while further relieving internal stress and smoothing the micro-surface.

3. Subsequent Surface Treatment: Achieving Ultimate Finish

Select the appropriate surface treatment process based on final application requirements to achieve the transition from precision-grade to mirror-grade finish.

3.1 Mechanical Polishing (Widely Used)

• Process: Rough polishing (800#–1200#) → Medium polishing (1500#–2000#) → Fine polishing (polishing compound/cloth wheel).
• Result: Can achieve a satin to mirror​ finish, reducing Ra to 0.1–0.4 μm. Control pressure and speed to prevent overheating and oxidation.

3.2 Electropolishing (High-End Mirror Finish)

• Principle: Levels the microscopic surface profile through electrochemical selective dissolution.
• Result: Produces a stress-free, high-gloss super-mirror​ finish with Ra ≤ 0.1 μm, while enhancing corrosion resistance. Process: Degreasing → Electropolishing → Neutralization → Rinsing → Drying.

3.3 Chemical Treatment (Efficient and Uniform)

• Chemical Polishing: Performed in a phosphoric-sulfuric acid system to quickly obtain a uniform, bright surface with Ra approximately 0.4–0.8 μm.
• Alkaline Etching: Uses a mild alkaline solution (e.g., NaOH 50–80 g/L) as a pre-treatment to uniformly remove the natural oxide film and minor defects.

3.4 Cleaning and Protection (Quality Assurance)

• Deep Cleaning: Use alkaline cleaners combined with ultrasonic​ cleaning to thoroughly remove all oil and residues from previous processes.
• Immediate Protection: Perform chromate-free passivation​ or apply a PE protective film​ immediately after cleaning to prevent oxidation stains or scratches during storage and transportation.

4. Common Defect Diagnosis and Rapid Countermeasures

5. Surface Finish Target System and Inspection Control

Establish clear quality tiers and inspection methods for closed-loop control.

• Industrial Grade: Ra 0.8–1.6 μm​ (Standard Hot Rolling + Cold Rolling)
• Precision Grade: Ra 0.4–0.8 μm​ (Optimized Cold Rolling + Mechanical Polishing)
• Mirror Grade: Ra ≤ 0.2 μm​ (Electropolishing / Superfinishing)

Inspection Methods:

• Roughness: Use contact or non-contact surface roughness testers​ to periodically measure Ra values. Critical products require 100% inspection or increased sampling frequency.
• Gloss: For mirror-finished products with appearance requirements, supplement evaluation with a 60° gloss meter​ for quantitative assessment.