How to Solve Sand Holes and Black Spots on the Surface of Aluminum Discs

Solving surface defects like sand holes and black spots on aluminum discs requires a systematic investigation and control of the entire production process.

Core Solution: Trace the root cause of the defects and optimize the process accordingly.

Step 1: Precisely Define the Problem – A “Detective’s Mindset”

Accurate observation is half the battle. Examine the defects closely with a magnifying glass or microscope:

Defect Type Key Characteristics Likely Root Cause (Initial Direction)
Sand Hole (Gas Porosity)​ Irregular small pits, smooth inner walls, mostly round or oval, sometimes pinprick-sized. Melting and Casting Process.​ Caused by gas (mainly hydrogen) evolving during solidification.
Sand Hole (Inclusion)​ Irregular pits, with foreign material at the bottom​ (different color/texture from the aluminum matrix). Raw Materials or Melting Process.​ Could be oxide skin, furnace lining debris, flux residue, etc.
Black Spot (Impression/Embedded)​ Black or dark color, cannot be wiped off, can be picked out with a needle, aluminum substrate underneath. Rolling, Slitting, Handling Processes.​ Foreign hard particles (iron chips, dust, equipment wear debris) pressed into the surface.
Black Spot (Oxidation/Contamination)​ Gray-black color, may smear or diffuse when wiped, sometimes appears as a film. Annealing and Storage Processes.​ Caused by a dirty annealing furnace, residual oil stains, or reaction with elements like sulfur at high temperatures.
Black Spot (Process Lubricant Residue)​ Greasy, may have a distribution pattern, possibly accompanied by color variation. Rolling and Cleaning Processes.​ Caused by degraded rolling oil, insufficient air-knife blow-off, or ineffective degreasing in the cleaning line.

Step 2: Systematic Root Cause Tracing and Solutions – “Follow the Clues” Along the Process

1. Raw Materials & Melting/Casting – Tackling Sand Holes at the Source

  • Root Cause:​ Hydrogen gas and inclusions formed in the cast slab.
  • Checklist & Solutions:
    • Aluminum Ingots & Scrap:​ Are they wet, oily, or heavily oxidized? Is the scrap ratio too high? Is pre-treatment (e.g., shot blasting, preheating) adequate?
    • Melting Process:
      • Refining:​ Are the flux type, amount, and timing correct? Are more efficient methods like rotary degassing​ or in-line degassing​ used?
      • Holding/Dwelling:​ Is there sufficient holding time before casting for inclusions to float?
      • Tools:​ Are stirring tools, degassing rods, etc., preheated and dry?
    • Casting Process:
      • Temperature:​ Are pouring temperature and cooling rate optimal? Too fast cooling traps gas.
      • Launder & Filtration:​ Is the launder clean? Are ceramic foam filters used to filter the molten metal?

2. Hot Rolling/Casting-Rolling – Defect Transfer and Amplification

  • Root Cause:​ Defects in the slab are elongated and spread; new contamination can be introduced.
  • Checklist & Solutions:
    • Slab Surface:​ Is the scalping depth sufficient to completely remove the oxide layer and segregation layer?
    • Rolling Oil & Coolant:​ Is it clean and well-filtered? Changed regularly? Aged oil can carbonize and cause black spots.
    • Work Rolls & Guide Rolls:​ Is the surface worn, aluminum-pickup, or spalling? This gets imprinted onto the sheet.

3. Cold Rolling & Intermediate Annealing – The “High-Risk Zone” for Black Spots

  • Root Cause:​ Rolling contamination, poor lubrication, annealing oxidation.
  • Checklist & Solutions:
    • Rolling Oil Management: This is critical.​ Check oil viscosity, ash content, filtration fineness (recommended ≤5μm). Replace aged oil.
    • Air-Knife & Cleaning:​ Are the air-knives at the mill exit effective in removing excess oil?
    • Annealing Furnace:
      • Furnace Atmosphere/Internals:​ Is there lining spalling, rust, or re-condensation of evaporated residues?
      • Protective Atmosphere:​ For bright annealing, is the purity and dew point of the protective gas (e.g., nitrogen) adequate?
      • Circulation Fans:​ Are they clean and balanced to avoid stirring up dust?

4. Finishing & Stamping – The Final “Checkpoint”

  • Root Cause:​ Secondary damage during slitting, packaging, handling, and stamping.
  • Checklist & Solutions:
    • Slitting Equipment:​ Are the rotary shears, recoiler shafts clean and burr-free? Is the lubricant dedicated and clean?
    • Shop Floor Environment:​ How is dust controlled in the stamping area? Are there covers over the dies?
    • Die Condition: Core of the stamping process.​ Is the die surface polished and free of scoring? Is the die clearance correct? Is regular cleaning and maintenance performed? Dirt on the die is stamped onto the disc.
    • Cleaning & Packaging:​ Is the final cleaning line effective in degreasing and drying? Is the packaging material suitable (e.g., VCI paper, desiccant)?

Step 3: Establish Long-term Preventive Measures – From “Firefighting” to “Fire Prevention”

  1. Standard Operating Procedures (SOPs):​ Develop and strictly enforce detailed SOPs for key processes: melting, refining, rolling, annealing, cleaning.
  2. Key Parameter Monitoring:​ Implement daily checks and logging for critical parameters (melt temp, refining time, rolling oil specs, annealing curve, protective gas dew point).
  3. Preventive Maintenance (TPM):​ Perform regular, scheduled cleaning, maintenance, and part replacement for furnaces, filtration systems, mills, annealing furnaces, and filters—don’t wait for breakdowns.
  4. Incoming & In-Process Inspection:​ Strengthen inspection of incoming materials (ingots, oil, packaging). Set inspection points after key processes (e.g., after casting, cold rolling, annealing) to catch defects early.
  5. Root Cause Analysis (RCA):​ When problems recur, use tools like “5 Whys” to find the underlying cause, not just treat symptoms.

Summary & Recommendation:

Start by observing the specific morphology of the defects. Prioritize investigating the melting process (for sand holes)​ and the cold rolling/annealing processes (for black spots). A simple controlled experiment—like using a batch of fresh, dry rolling oil or thoroughly cleaning the annealing furnace—can quickly verify the source of the problem.

Resolving such surface quality issues requires patience and systematic thinking. Once the root cause is identified and controlled, product quality and consistency will improve significantly.