Manufacturing Engineering Series Part 2: Casting, Forging & Metal Forming

Casting Processes

Casting is humanity's oldest metalworking process — dating back 6,000 years to Mesopotamia. Today, over 70% of all manufactured goods contain at least one cast component. From engine blocks to turbine blades to hip implants, casting transforms liquid metal into complex shapes that would be impossible or uneconomical to machine from solid stock.

Sand casting is the most common casting process, producing ~60% of all castings by weight. A pattern (usually wood or metal) is pressed into sand mixed with a binder (clay or resin) to create a mold cavity. Liquid metal is poured in, solidifies, and the sand mold is broken away.

Investment casting (lost-wax process) produces the highest-precision castings. A wax pattern is coated with ceramic slurry, the wax is melted out, and molten metal fills the ceramic shell. This process creates turbine blades for jet engines — parts that operate at 1,100°C while spinning at 10,000 RPM, with tolerances of ±0.1mm and internal cooling channels impossible to machine.

Die Casting & Permanent Mold

Die casting injects molten metal at high pressure (10–175 MPa) into reusable steel molds (dies). It's the fastest casting process — cycle times of 30-90 seconds — making it ideal for high-volume production of thin-walled, dimensionally precise parts.

Solidification Defects & Mold Design

Understanding solidification is crucial because most casting defects originate during the liquid-to-solid transition. As metal cools, it shrinks (typically 2-7% by volume), potentially creating voids, porosity, and residual stresses.

import numpy as np

# Chvorinov's Rule: Calculate solidification time for various geometries
# t = B * (V/A)^2

# Mold constant B (depends on mold and metal)
B_sand_aluminum = 3.5       # min/cm² for aluminum in sand mold
B_sand_steel = 2.0          # min/cm² for steel in sand mold
B_metal_aluminum = 1.2      # min/cm² for aluminum in metal mold

shapes = {
    "Cube 10cm": {"V": 10**3, "A": 6 * 10**2},
    "Sphere Ø10cm": {"V": (4/3) * np.pi * 5**3, "A": 4 * np.pi * 5**2},
    "Cylinder Ø10×10cm": {"V": np.pi * 5**2 * 10, "A": 2*np.pi*5**2 + 2*np.pi*5*10},
    "Plate 20×20×2cm": {"V": 20*20*2, "A": 2*20*20 + 4*20*2},
}

print("Chvorinov's Rule - Solidification Time Analysis")
print("=" * 70)
print(f"{'Shape':<22} {'V(cm³)':<10} {'A(cm²)':<10} {'V/A(cm)':<10} {'t_sand(min)':<12}")
print("-" * 70)

for name, dims in shapes.items():
    ratio = dims["V"] / dims["A"]
    t = B_sand_aluminum * ratio**2
    print(f"{name:<22} {dims['V']:<10.1f} {dims['A']:<10.1f} {ratio:<10.3f} {t:<12.2f}")

print(f"\nKey Insight: The sphere has the highest V/A ratio and solidifies LAST.")
print(f"The plate has the lowest V/A ratio and solidifies FIRST.")
print(f"Design risers to be spherical (slow to solidify) to feed castings longer.")

Forging Processes

Forging shapes metal through controlled compressive forces — hammers, presses, or rolls apply pressure to plastically deform the workpiece into the desired shape. Unlike casting, forging does not melt the metal — it works the material in its solid state, producing parts with superior mechanical properties due to refined grain structure and favorable grain flow.

Grain Flow & Residual Stresses

Grain flow is the single most important advantage of forging over casting and machining. When metal is forged, the grain boundaries align along the contour of the part — like wood grain following the shape of a tree branch. This alignment creates a part that is strongest in the direction of primary loading.

Precision & Isothermal Forging

Isothermal forging maintains the workpiece and dies at the same elevated temperature throughout the process. This eliminates die chilling (where metal in contact with cooler dies becomes harder and harder to deform), enabling forging of difficult materials like titanium and nickel superalloys with much lower forces and better dimensional control.

import numpy as np

# Forging Force Estimation for Closed-Die Forging
# Using the forging load equation: F = k * sigma_f * A_projected

# Material flow stresses at forging temperature
materials = {
    "Aluminum 6061 (450°C)": {"sigma_f": 30, "k": 6},   # MPa, complexity factor
    "Steel 4340 (1100°C)": {"sigma_f": 100, "k": 8},
    "Ti-6Al-4V (950°C)": {"sigma_f": 200, "k": 10},
    "Inconel 718 (1050°C)": {"sigma_f": 350, "k": 12},
}

# Part: connecting rod (projected area = 80 cm² = 8000 mm²)
A_projected = 8000  # mm²

print("Closed-Die Forging Force Estimation")
print("=" * 65)
print(f"Part: Connecting Rod (projected area = {A_projected} mm²)")
print(f"\n{'Material':<28} {'σf (MPa)':<12} {'k':<6} {'Force (kN)':<12} {'Force (ton)':<12}")
print("-" * 65)

for name, props in materials.items():
    F = props["k"] * props["sigma_f"] * A_projected / 1000  # kN
    F_ton = F / 9.81  # metric tons
    print(f"{name:<28} {props['sigma_f']:<12} {props['k']:<6} {F:<12.0f} {F_ton:<12.0f}")

print(f"\nConclusion: Titanium and superalloys require 5-10× more force than aluminum.")
print(f"Press selection: Use the next standard press size above the calculated force.")

Bulk & Sheet Metal Forming

Bulk forming processes — rolling, extrusion, and drawing — transform metal billets into long products with uniform cross-sections. These are the highest-volume forming operations in the world: over 1 billion tons of steel are hot-rolled annually, and aluminum extrusion produces everything from window frames to heat sinks to bicycle frames.

Rolling passes metal between rotating rolls to reduce its cross-section. Hot rolling (above recrystallization temperature) produces slabs, plates, bars, and structural shapes. Cold rolling (below recrystallization temperature) produces precise sheet, strip, and foil with excellent surface finish and tight tolerances.

Extrusion forces metal through a shaped die opening, producing continuous profiles of virtually any cross-section. Think of it like squeezing toothpaste — the material takes the shape of the opening.

import numpy as np

# Rolling Force Calculation (Simplified)
# Using the Slab Method: F = L * w * sigma_avg * Q

# Hot rolling parameters for steel strip
w = 1200          # Strip width (mm)
h0 = 25           # Entry thickness (mm)
h1 = 20           # Exit thickness (mm)
R = 300           # Roll radius (mm)
sigma_f = 150     # Flow stress at temperature (MPa)
mu = 0.3          # Friction coefficient (hot rolling)

# Draft and contact length
delta_h = h0 - h1    # Draft (mm)
L = np.sqrt(R * delta_h)  # Projected contact length (mm)

# Average flow stress with friction hill
Q = 1 + (mu * L) / (2 * (h0 + h1) / 2)  # Friction multiplier

# Rolling force
F = L * w * sigma_f * Q / 1000  # kN

# Rolling torque (per roll)
T = F * L / 2  # kN⋅mm

# Rolling power (per roll)
N_rpm = 60       # Roll speed (RPM)
omega = 2 * np.pi * N_rpm / 60  # rad/s
P = 2 * T * omega / (1000 * 1000)  # MW (both rolls)

# Extrusion ratio calculation
D0 = 200          # Billet diameter (mm)
D1 = 50           # Extrusion diameter (mm)
A0 = np.pi / 4 * D0**2
A1 = np.pi / 4 * D1**2
R_extrusion = A0 / A1
epsilon = np.log(R_extrusion)

print("Rolling & Extrusion Analysis")
print("=" * 55)
print(f"\n--- Hot Rolling: Steel Strip ---")
print(f"Draft:             {delta_h} mm ({h0}→{h1} mm)")
print(f"Contact length:    {L:.1f} mm")
print(f"Friction factor Q: {Q:.3f}")
print(f"Rolling force:     {F:.0f} kN ({F/9.81:.0f} tons)")
print(f"Torque (per roll): {T:.0f} kN⋅mm")
print(f"Total power:       {P:.3f} MW")

print(f"\n--- Direct Extrusion ---")
print(f"Extrusion ratio:   {R_extrusion:.1f}:1")
print(f"True strain:       {epsilon:.3f}")
print(f"Ram force:         {sigma_f * A0 * epsilon / 1000:.0f} kN (ideal)")

Deep Drawing & Sheet Metal Forming

Deep drawing transforms flat sheet metal into hollow, three-dimensional shapes — beverage cans, kitchen sinks, automotive body panels, and artillery shell casings. The sheet is held by a blank holder and pushed into a die cavity by a punch.

The critical parameter is the Limiting Drawing Ratio (LDR) — the maximum ratio of blank diameter to punch diameter that can be drawn without tearing. For most steels, LDR ≈ 2.0-2.3, meaning a 200mm blank can be drawn into a cup with a ~90mm diameter in a single operation. Deeper cups require multiple redraws with intermediate annealing.

Hydroforming & Superplastic Forming

Hydroforming uses high-pressure fluid (typically 70-400 MPa water) to form sheet or tube into complex shapes against a single die. Unlike conventional stamping (which requires matched punch and die), hydroforming needs only one hard tool — the fluid acts as a universal "soft punch."

Superplastic forming (SPF) exploits the superplastic behavior of certain fine-grained alloys (notably Ti-6Al-4V and Al-Mg alloys) at elevated temperatures. These materials can achieve elongations of 200-2000% — similar to stretching chewing gum — enabling extremely complex shapes in a single operation.

Advanced & Frontier Forming

The forefront of metal forming combines digital design tools, flexible forming processes, and additive manufacturing to eliminate the need for expensive dedicated tooling. These technologies are transforming metal forming from a high-volume-only proposition into an economically viable option for prototyping and low-volume production.

Simulation-Driven Die Design

Finite Element Analysis (FEA) simulation has revolutionized die design. Instead of building and testing physical dies through expensive trial-and-error ($50,000-500,000 per die iteration), engineers simulate the entire forming process digitally — predicting material flow, thinning, wrinkling, springback, and residual stresses before any steel is cut.

Additive-Assisted Forming

Incremental Sheet Forming (ISF) is a dieless forming process where a CNC-controlled tool progressively deforms sheet metal into a 3D shape. Think of it as "3D printing in reverse" — instead of adding material, a stylus progressively pushes sheet metal into shape, one layer at a time.

ISF eliminates the need for matched dies entirely, making it economically viable for prototypes and production runs under 500 parts. Combined with 3D-printed tooling inserts for specific features, hybrid approaches are emerging that blend the flexibility of additive manufacturing with the speed of traditional forming.