Manufacturing Processes and Materials Fundamentals
1. Manufacturing Basics
Manufacturing is the process of making parts or products by changing their geometry, properties, appearance, and/or by assembly.
- Technological definition: Physical or chemical processes alter material geometry, properties, or appearance.
- Economic definition: Transforms material into items of greater value through processing or assembly.
Manufacturing Process Groups
- Processing operations: Change the shape, properties, or surface of a material. Includes shaping, property-enhancing, and surface processing.
- Assembly operations: Join two or more parts. Includes welding, brazing, soldering, adhesive bonding, and fasteners.
Shaping Processes
- Solidification: Liquid to solid (e.g., casting).
- Particulate processing: Powder pressed and sintered.
- Deformation: Force exceeds yield strength, causing plastic shape change (e.g., rolling, forging, extrusion).
- Material removal: Removing excess material (e.g., turning, drilling, milling).
Net shape refers to parts requiring little or no waste and no machining; near-net shape requires only minimum machining.
2. Materials
Metals
Metals feature high electrical and thermal conductivity, ductility, malleability, and high melting points.
- Ferrous metals: Iron-based (e.g., steel, cast iron).
- Non-ferrous metals: Not iron-based (e.g., aluminium, copper, nickel, tin).
Steel is an iron alloy with carbon:
- Low carbon steel (<0.20% C): Used for sheet metal and plate fabrication.
- Medium carbon steel (0.20–0.50% C): Used for crankshafts, connecting rods, and machine parts.
- High carbon steel (>0.50% C): Used for springs, cutting tools, blades, and wear parts.
Ceramics and Polymers
- Ceramics: Inorganic compounds of metallic/semi-metallic and non-metallic elements. Characteristics: hard, brittle, high melting temperature, chemically stable, insulating, low ductility.
- Polymers: Long-chain molecules made of repeating units. Characteristics: lightweight, low melting temperature, insulating, flexible, easy to process.
- Polymer types: Thermoplastics (reheatable/remeltable), Thermosets (chemically set, rigid), and Elastomers (rubber-like, large elastic deformation).
Composites
Two or more phases bonded together to achieve superior properties. Types include MMC (metal matrix), PMC (polymer matrix), and CMC (ceramic matrix).
3. Mechanical Behaviour
- Elastic deformation: Reversible; material returns to its original shape after the load is removed.
- Plastic deformation: Permanent; atoms move relative to each other.
- Stress: Force per original area.
- Strain: Change in length divided by original length.
Material Properties
- Yield strength: Stress level where plastic deformation begins.
- Ductility: Ability to plastically deform before fracture.
- Malleability: Ability to be rolled or hammered into sheets.
- Toughness: Ability to absorb energy before fracture.
- Hardness: Resistance to indentation, scratching, or wear.
- Brittleness: Fractures with little plastic deformation.
Working Temperatures
- Cold working: Below recrystallisation temperature; increases strength/hardness, reduces ductility, improves surface finish.
- Hot working: Above recrystallisation temperature; lower force, large deformation, less work hardening, rougher surface.
4. Casting
Casting involves pouring molten metal into a mould cavity to solidify into a specific shape.
Casting Methods
- Expendable mould: Destroyed after use (e.g., sand casting, investment casting).
- Permanent mould: Reused (e.g., gravity die casting, pressure die casting).
Common Casting Defects
- Porosity: Gas holes or voids.
- Shrinkage cavity: Metal contracts during solidification.
- Misrun: Metal solidifies before filling the mould.
- Cold shut: Two metal streams meet but do not fuse.
- Hot tearing: Cracks from restrained contraction.
- Inclusions: Trapped slag, oxide, or sand.
Riser purpose: Feeds liquid metal to compensate for shrinkage. Key rule: The riser must solidify after the casting (requires larger volume-to-area ratio).
5. Bulk Deformation
Metal forming uses plastic deformation to change metal shape with little to no waste and improved mechanical properties.
Bulk Deformation Processes
- Rolling: Thickness reduced by compressive forces from rotating rolls.
- Forging: Compressive force shapes metal between dies; provides high strength due to grain flow.
- Extrusion: Billet forced through a die to produce a constant cross-section.
- Drawing: Material pulled through a die to reduce cross-section (e.g., wire, rods, tubes).
6. Sheet Metal Forming
Sheet metal forming involves thin sheets with a high surface-area-to-volume ratio.
- Shearing: Cutting sheet metal using a punch and die.
- Blanking: The cut-out piece is the desired product.
- Punching: The removed piece is scrap; the hole is the desired feature.
- Bending: Plastic deformation around a straight axis.
- Springback: Elastic recovery after bending, causing the final angle to change.
- Deep drawing: Flat sheet blank drawn into a cup shape.
Common defects: Wrinkling, tearing, earing, springback, and surface scratches.
7. Non-Metals
- Injection moulding: Molten polymer injected into a mould; high production rate.
- Extrusion: Polymer forced through a die for constant cross-sections (e.g., pipes).
- Blow moulding: Air expands plastic into a mould (e.g., bottles).
- Thermoforming: Heated sheet formed over a mould using vacuum or pressure.
- Ceramic processing: Powder preparation, forming, drying, sintering, and finishing.
- Sintering: Heating compacted powder below the melting point to bond particles.
8. Additive Manufacturing
Additive manufacturing (AM) creates parts from 3D model data by joining material layer-by-layer.
Key AM Categories
- Material extrusion: Filament melted and deposited (e.g., FDM).
- SLM (Selective Laser Melting): Laser melts a metal powder bed.
- DED (Directed Energy Deposition): Focused energy melts material as it is deposited.
- WAAM (Wire Arc Additive Manufacturing): Uses a welding arc and wire feed.
Advantages: Design freedom, complex geometry, lightweight design, low tooling, and rapid prototyping. Disadvantages: Slow build rate, high cost, anisotropy, and poor surface finish.
9. Welding
Welding is a permanent joining process using heat, pressure, or filler material.
- Fusion welding: Base metal melts.
- Solid-state welding: No melting; uses pressure, diffusion, or friction.
- HAZ (Heat Affected Zone): Base metal not melted, but microstructure and properties are changed by heat.
Common defects: Cracks, porosity, slag inclusion, lack of fusion, lack of penetration, undercut, and distortion. distortion. Safety: Protect against UV radiation, fumes, burns, electric shock, and fire.