Metal-Rolling Processes and Equipment Study Notes

Metal Forming Processes: Overview and Classification

• Metal forming involves several distinct processes used to reshape materials:
  • Rolling
  • Forging
  • Extrusion
  • Wire Drawing
  • Piercing/Blanking
  • Bending
  • Roll Forming
  • Deep Drawing

Hot and Cold Working of Metal

• Most metal working (forming) processes can be categorized into hot or cold working based on the temperature relative to the melting point of the metal.
• Hot Working Processes:
  • Temperature: Performed above $50\%$ of the melting point.
  • Work Hardening: No work hardening occurs during the process.
  • Failure Risk: No fear of component failure during forming.
  • Disadvantages: Causes slag, porosity, and inclusions.
  • Quality: Results in poor surface finish and inaccurate dimensions.
• Cold Working Processes:
  • Temperature: Performed below $30\%$ of the melting point.
  • Work Hardening: Significant work hardening takes place.
  • Internal Stress: Residual stresses are usually introduced into the material.
  • Quality: Produces repeatable, accurate components with a good surface finish.
  • Forces: Cold working forces are significantly higher than the forces required during hot forming.

Metal Rolling Processes

• Definition: Rolling is the process of changing the cross-sectional shape and dimensions of a metal section by passing it between two flat or shaped rotating rolls.
• Initial Material: Almost all metals and their alloys begin their life cycles as castings. Cast metal usually requires reshaping via rolling to reach a usable form.
• Rolling Mechanism:
  • The gap between the rotating rolls is strictly less than the initial thickness ($h_0$) of the entering bar.
  • A friction force is necessary to "bite" the bar and pull it through the rolls.
• Flat and Shape Rolling:
  • The majority of steel products are converted from ingot form through rolling.
  • Flat rolling involves producing flat plates and sheets.
• Product Categories:
  • Primary Rolling Products:
    • Slabs: Thick, flat plates.
    • Blooms: Large rectangular bars.
    • Billets: Large square bars.
  • Secondary Rolling: Used to produce sheets and bars with various cross-sections, including round, square, and hexagonal shapes.
  • The choice between hot and/or cold rolling depends on the physical properties required of the finished material.

Rolling Mechanics and Analysis

• Deformation Principle: As a metal bar passes through the rolls, it is squeezed and elongates as its cross-sectional area decreases.
• Volume Conservation: $L_0 \times h_0 \times W = L_i \times h_i \times W$
• Thickness Reduction (R): The percentage of thickness reduction is calculated as:
  • $R = \frac{h - h_0}{h_0} \times 100\%$
• Absolute Reduction ($\Delta H$): The difference between the initial and final thickness.
  • $\Delta H = H_1 - H_2$
• Relative Reduction: The ratio between absolute reduction and the initial part thickness/area.
• Contact Angle ($\alpha$):
  • At the start of contact, rolls act with a normal force ($N$) and a resulting friction force ($F$).
  • $\cos(\alpha) = 1 - \frac{\Delta H}{D_{roll}}$
  • Where $D_{roll}$ is the roll diameter.
• Biting Conditions:
  • Normal force analysis defines force components: $S$ (squeezing force), $Q$ (pulling force), and $P$ (resisting force).
  • $S = N \cos(\alpha) + F \sin(\alpha)$
  • $Q = F \cos(\alpha)$
  • $P = N \sin(\alpha)$
  • Rolling occurs if $Q > P$, which translates to $F \cos(\alpha) > N \sin(\alpha)$.
  • Since $F = \mu N$, the biting condition is satisfied when: $\mu > \tan(\alpha)$

Rolling Stand Arrangements

• Various configurations exist for rolling equipment:
  • 2-High: Two horizontal rolls.
  • 2-High Reversible: Rolls can change direction for multiple passes.
  • 3-High: Three rolls; the workpiece passes through the bottom two in one direction and back through the top two in the opposite direction.
  • 4-High: Includes two smaller work rolls and two larger backup rolls to prevent deflection.
  • Cluster (Sendzimir): Multiple backup rolls support work rolls for high-precision or high-strength rolling.

Forging Processes

• Definition: Forging produces products near their final shape by applying compressive forces. It improves the strength and toughness of components.
• Applications: Highly stressed components such as crankshafts and connecting rods for car engines.
• Open Die Forging:
  • Hot forging involves heating the workpiece far above its recrystallization temperature ($1250^{\circ}\text{C}$ for steels).
  • The workpiece is squeezed using manual or power hammers, or presses for higher accuracy.
  • Specific operations include edging, fullering, and drawing.
  • Advantages: Simple, inexpensive dies, suitable for small quantities and a wide range of sizes ($15\,kg$ to $500\,kg$).
  • Limitations: Simple shapes only, difficult to hold close tolerances, low production rate, high skill required, and requires subsequent machining.
• Impression Die (Closed Die) Forging:
  • Components are formed between two carved dies. Flash (excess metal) is usually produced.
  • Advantages: Good material utilization, better properties than open-die, good dimensional accuracy, high production rates, and high reproducibility.
  • Disadvantages: High die cost, machining often necessary afterward, and economical only for large quantities.
• Upset and Precision Forging:
  • Upsetting involves applying lengthwise impact pressure to one end of a blank (common for bolts and gears).

Forging Equipment

• Mechanical Presses: Convert motor rotation into linear ram motion.
  • Crank presses.
  • Knee joint (Knuckle) presses.
  • Screw presses.
  • Orbital presses.
• Hydraulic Presses: The ram is actuated directly by a hydraulic cylinder/piston.
• Industry examples of forging include companies like Sparco, Mahle, Phi-Tool, Presswerk Krefeld, and Schulte-Ufer.

Extrusion Processes

• Definition: Compressive force is exerted on a ductile billet (hot or cold) to force it through a steel die hole of a specific shape.
• Materials: Limited to ductile non-ferrous metals like aluminum ($Al$), zinc ($Zn$), and copper ($Cu$) alloys.
• Types of Extrusion:
  • Direct Extrusion: Metal flows in the same direction as the ram motion. It is more popular because it is easier to set up.
  • Indirect Extrusion: Metal flows in the opposite direction of the ram. It requires lower force than the direct process.
  • Hydrostatic Extrusion: The billet is surrounded by fluid to reduce friction.
  • Lateral Extrusion: The metal is extruded at an angle to the ram movement.
• Impact Extrusion:
  • Used for ductile materials deformed at room temperature by a high-speed punch.
  • Direct (Forward) Impact: Metal moves in the punch direction.
  • Indirect (Backward) Impact: Metal moves in the opposite direction of the punch between the die and punch walls.
  • Combined Impact: Metal flows in both directions.
  • Advantages: High reduction ratios, quick setup, mass production, and final product achieved in a single stage.
• Automotive Applications: Roof rails, bumpers, seat tracks, engine mounts, airbag housings, and fuel distribution pipes.

Wire and Tube Drawing

• Definition: Drawing involves pulling a wire through a series of dies to reduce its cross-section.
• Process Detail: Employs a series of dies with slightly reduced sizes in each subsequent stage.
• Ideal Drawing Force ($F$):
  • $F = \bar{Y} \times A_2 \times \ln\left(\frac{A_1}{A_2}\right)$
  • Where $\bar{Y}$ is the average flow stress of the wire material.
• Die Zones:
  • A) Entry zone.
  • B) Drawing/reduction zone.
  • C) Sizing zone.
  • D) Exit zone.
• Die Materials: Tungsten Carbide ($WC$) or synthetic diamonds.
• Performance Metrics:
  • Reduction ratio: $15\text{--}25\%$ per stage.
  • Speed: $10\text{--}100\,m/min$ for large diameters; up to $1500\,m/min$ for fine diameters.
  • Forces: Can reach up to $1.5\,MN$ in initial stages.

Sheet Metal Forming and Die Cutting

• Die Cutting Operations:
  • Punching (Piercing): Producing a hole where the slug is discarded.
  • Blanking: Cutting the part (the blank) out of the sheet.
  • Other Operations: Parting, notching, slitting, lancing, and perforating.
• Punch Force ($F$):
  • $F = 0.7 \times t \times L \times \text{UTS}$
  • Where $t = \text{sheet thickness (mm)}$, $L = \text{sheared length or perimeter (mm)}$, and $\text{UTS} = \text{Ultimate Tensile Strength}$.
• Blank Details: Characteristics include rollover depth, burnish depth, fracture depth, and burr height.

Bending Operations

• Simple Bending:
  • Includes wiping dies, U-dies, and V-dies.
  • Bending Force ($P$):
    • $P = \frac{k \times Y \times L \times t^2}{W}$
    • Constants for $k$: $0.3$ (wiping), $0.7$ (U-die), $1.3$ (V-die).
• Advanced Bending:
  • Channel forming, joggle, hemming (flattening), offset forming, and two-stage lock seams.
  • Example: Hemming of aluminum outer panels often recommends a radius $R = 0.75 \times t$.
• Roll Forming: Continuous, incremental bending of a strip as it passes through successive pairs of rolls.

Deep Drawing

• Definition: A cold metal forming process where a flat blank is forced into a die cavity by a hydraulic punch to form a cup-like cylindrical part with a depth greater than its diameter.
• Process Stages:
  1. Shearing a blank.
  2. Clamping the blank on the die.
  3. Stretching the blank with the punch.
  4. Returning the punch and removing the part.
  5. Trimming excess "ears."
• Materials: Mild steel, high-strength steel, stainless steel, pure copper, pure aluminum, 70/30 brass, and cupro-nickel alloys.
• Applications: Car bodies, kitchen sinks, beverage cans, cartridge cases, and bullet envelopes.