Last reviewed 16 Sept 2026 · 11 min read
Classification of manufacturing processes
| Group | Processes |
|---|---|
| Casting (solidification) | Sand casting, die casting, investment casting, centrifugal casting |
| Forming (deformation) | Forging, rolling, extrusion, drawing, sheet metal work |
| Joining | Welding, brazing, soldering, riveting, bolting, adhesive bonding |
| Machining (material removal) | Turning, drilling, milling, shaping, grinding; non-traditional (EDM, laser, water jet) |
| Powder metallurgy | Compaction and sintering of powders |
| Additive manufacturing | Layer-by-layer building (3D printing) |
| Finishing and heat treatment | Polishing, coating, galvanising, heat treatment |
Casting
Casting — pouring molten metal into a mould cavity of the desired shape and allowing it to solidify. Suitable for complex shapes, large parts and materials difficult to shape otherwise (cast iron).
Sand casting procedure
- Pattern making (replica of the part, with allowances).
- Mould making — ramming moulding sand around the pattern in a flask (cope and drag); pattern removed.
- Core making and placing (for hollow sections).
- Gating system — pouring basin, sprue, runners, gates; risers feed molten metal to compensate for shrinkage.
- Melting (cupola for cast iron, induction/arc furnaces) and pouring.
- Solidification and cooling.
- Shakeout, fettling/cleaning (removing gates, risers, sand), inspection.
Pattern allowances
| Allowance | Reason |
|---|---|
| Shrinkage (contraction) allowance | Metal contracts on cooling — pattern made larger (e.g. of the order of 1% for cast iron, about 2% for steel) |
| Draft (taper) allowance | Easy withdrawal of pattern from mould |
| Machining (finish) allowance | Extra material for machining |
| Distortion (camber) allowance | Counteracts warping of long thin castings |
| Shake (rapping) allowance | Negative allowance — mould cavity enlarged by rapping; pattern made slightly smaller |
Moulding sand properties
Permeability (venting gases), refractoriness (withstand high temperature), strength/cohesiveness, plasticity, collapsibility (allow contraction), flowability, reusability. Green sand = silica sand + clay binder + water.
Casting defects
| Defect | Cause |
|---|---|
| Blowholes / gas porosity | Trapped gases — poor permeability, excess moisture |
| Shrinkage cavity | Inadequate feeding during solidification |
| Cold shut | Two metal streams meet without fusing — low pouring temperature |
| Misrun | Metal solidifies before filling the mould |
| Hot tears | Hindered contraction — poor collapsibility |
| Scabs, swells, inclusions, mismatch (shift) | Mould erosion, weak mould, misalignment |
Special casting processes
| Process | Features |
|---|---|
| Die casting (gravity/pressure) | Permanent metal dies; non-ferrous alloys (Al, Zn); high production, good finish |
| Investment (lost wax) casting | Wax pattern coated with refractory; intricate precise parts, turbine blades, jewellery |
| Centrifugal casting | Mould rotated — dense pipes (cast iron/ductile iron pipes), cylinder liners |
| Shell moulding | Resin-bonded sand shells — good accuracy |
| Continuous casting | Steel billets/slabs directly from molten metal (steel plants) |
Metal forming
Hot and cold working
| Aspect | Hot working (above recrystallisation temperature) | Cold working (below) |
|---|---|---|
| Force required | Lower | Higher |
| Surface finish, accuracy | Poorer (oxidation/scale) | Better |
| Strength | Refined grains, no strain hardening | Strain hardening — stronger, less ductile |
| Examples | Hot rolling of structural sections, forging | Cold-drawn wires, cold-rolled sheets, cold-twisted bars (historical CTD bars) |
Processes
| Process | Description | Products |
|---|---|---|
| Forging | Shaping by compressive forces — hammering (smith/drop forging) or pressing (press forging); open-die and closed-die | Crankshafts, connecting rods, hooks, tools — strong, with favourable grain flow |
| Rolling | Passing metal between rotating rolls | Plates, sheets, structural sections (I, channels, angles), reinforcement bars, rails |
| Extrusion | Forcing metal through a die (direct or indirect) | Aluminium window sections, tubes, rods |
| Wire/tube drawing | Pulling through a die | Wires, prestressing wires, tubes |
| Sheet metal operations | Shearing, blanking, punching, bending, deep drawing, spinning | Cladding sheets, profiled roofing, ducts, utensils |
FormulaBlanking/punching force
= perimeter of cut; = sheet thickness; = shear strength of material.
Welding and joining
Welding — joining metals by fusion (melting with or without filler) or pressure, producing a permanent joint.
Fusion welding processes
| Process | Heat source / shielding | Features and uses |
|---|---|---|
| Shielded metal arc welding (SMAW / manual metal arc, "stick") | Arc between flux-coated electrode and work; flux gives shielding and slag | Versatile, portable, low cost — site fabrication of steel structures |
| Gas metal arc welding (GMAW / MIG, MAG) | Continuous wire electrode; inert/active gas shielding (Ar, CO₂) | Fast, semi-automatic, workshop fabrication |
| Gas tungsten arc welding (GTAW / TIG) | Non-consumable tungsten electrode; inert gas; separate filler | High quality, thin sections, stainless steel, aluminium |
| Submerged arc welding (SAW) | Arc under a blanket of granular flux; automatic | Thick plates, long straight welds — plate girders, pressure vessels, pipes |
| Flux-cored arc welding (FCAW) | Tubular wire with flux | Site and shop work |
| Gas (oxy-acetylene) welding | Flame of oxygen + acetylene (about 3100–3300 °C) | Thin sheets, repair; also gas cutting of steel |
| Electroslag / electrogas | Vertical thick welds | Heavy sections |
| Laser, electron beam | High-energy beams | Precision |
Oxy-acetylene flame types:
- Neutral (equal O₂ and C₂H₂) — most common, for steel.
- Oxidising (excess O₂) — brass, bronze.
- Carburising / reducing (excess acetylene) — hard facing, some non-ferrous metals.
Resistance and solid-state welding
- Resistance welding — heat from electrical resistance at contact + pressure: spot welding (sheet metal, car bodies, welded mesh), seam welding, projection welding, flash/butt welding (joining rails, bars).
- Solid-state welding — friction welding, forge welding, ultrasonic, explosive welding.
- Thermit welding — exothermic reaction of aluminium with iron oxide — rail joints, repairs.
Brazing and soldering
- Brazing — filler (brass, silver alloys) melts above about 450 °C but below base metal melting point; base metal not melted.
- Soldering — filler melts below about 450 °C (tin–lead or lead-free alloys) — electronics, plumbing joints.
Weld joints and positions
- Joint types: butt, lap, tee, corner, edge.
- Weld types: fillet welds and groove (butt) welds (square, single-V, double-V, U, J, bevel).
- Positions: flat (down-hand) (easiest), horizontal, vertical, overhead (most difficult).
- Welding symbols on drawings — IS 813 / ISO conventions.
Welding defects and inspection
| Defect | Description |
|---|---|
| Porosity | Gas pockets |
| Slag inclusion | Trapped slag between runs |
| Lack of fusion / incomplete penetration | Weld metal not fused with base metal or root |
| Undercut | Groove melted into base metal at weld toe |
| Cracks (hot, cold/hydrogen-induced) | Most serious — stress concentration |
| Overlap, spatter, distortion | Poor technique, excessive heat |
Inspection and testing: visual inspection, dye penetrant (surface cracks), magnetic particle (near-surface in ferromagnetic materials), ultrasonic testing (internal flaws), radiography (X-ray/gamma), destructive bend and tensile tests; welder qualification and welding procedure specifications (WPS).
Structural steel welding — preheating thick sections, low-hydrogen electrodes, sequence to control distortion, qualified welders; design of welds per IS 800.
FormulaFillet weld strength (simplified)
Effective throat thickness (for equal-leg 90° fillet of size )