Roll aluminum sheet by casting a flat slab 20-30 mm thick, reheating it to about 400 °C, and passing it through steel rollers, taking 20-30% off the thickness each pass; anneal at 350 °C for an hour if it becomes too hard to work, and continue to 1-3 mm. From 25 mm to 2 mm at 25% per pass takes nine passes, and the rollers’ drive is the hazard: a rolling mill has a nip point and no short stop.
Part of Aluminum Smelting
Flattening cast aluminum into sheet or bar between rollers: temperatures, reduction per pass, annealing and the drive hazard.
Hot Rolling Thins The Slab, Annealing Restores Its Softness
The carrier’s steps: cast a flat slab 20-30 mm thick; reheat to 400 °C (hot rolling temperature); pass through steel rollers, reducing thickness by 20-30% per pass; continue until the desired thickness, 1-3 mm for general sheet; anneal at 350 °C for 1 hour if the sheet becomes too hard to work. Rolling hardens the metal as it deforms, as I recall, which is why annealing is needed. The slab can be cast from remelted scrap (Ingot Casting); a sound, gas-free casting rolls better, so dry the melt carefully.
What You Need
- A rolling mill: two steel rollers, a frame with adjustable gap, and a drive. The corpus has no rolling-mill article. The drive is a hand crank or a water-driven line shaft (Power Transmission), the overhead line-shaft article, not the electrical one.
- A stopping device: a belt shifter or a clutch (Friction Clutch).
- A furnace that holds a slab at 400 °C, and an oven for 350 °C.
- Tongs, gloves, a guard over the rollers.
Method
Step 1: Heat the slab to about 400 °C. Judge it by a test piece, since aluminum shows no glow at this temperature; the carrier gives no colour cue.
Step 2: Roll a pass taking 20-30% off the thickness, feeding the slab square into the nip.
Step 3: Reduce the gap and repeat; reheat when the slab cools.
Step 4: Anneal at 350 °C for 1 hour when the sheet stiffens, then continue.
Step 5: Trim and flatten the finished sheet.
Worked Example: How Many Passes
If each pass keeps a fraction f of the thickness, n passes leave f^n of the original, so n = ln(final / start) / ln(f).
- 25 mm to 2 mm at 25% per pass (f = 0.75): ln(0.08) / ln(0.75) = (-2.526) / (-0.288) = 8.8, so 9 passes.
- Fewest, 20 mm to 3 mm at 30% (f = 0.70): ln(0.15) / ln(0.70) = (-1.897) / (-0.357) = 5.3, so 6 passes.
- Most, 30 mm to 1 mm at 20% (f = 0.80): ln(1/30) / ln(0.80) = (-3.401) / (-0.223) = 15.2, so 16 passes.
So the carrier’s ranges imply roughly 6 to 16 passes, with reheats and anneals between. No throughput or drive power is stated here because the corpus gives no rolling force; do not size a drive from this article.
Common Mistakes
- Reducing too hard on a cold slab. It cracks at the edges.
- Skipping the anneal. The sheet stiffens and tears.
- Rolling a gas-filled casting. Blisters appear in the sheet.
Safety
Rollers, hot slab and the drive
The nip between the rollers draws in fingers, sleeves and hair and crushes a hand in a fraction of a second. The hazard is the drive you cannot stop from where you stand, so fit a stop (belt shifter, clutch or a disconnect) within the operator’s reach, give a second person the job of watching it, and never reach toward the rollers while the drive is engaged. Rollers keep turning after the drive is released because of the stored energy of the line shaft and flywheel; the corpus gives no coast-down time, so measure it and wait it out. A slab at 400 °C burns without glowing; use tongs and gloves and mark hot pieces.
What’s Next
- Learn next: Sulfuric Acid Anodizing - protecting the finished sheet
- Combine with: Ingot Casting - producing the slab