Manufacturing Processes I: Casting, Forming, Joining, Powder and Polymer Processing

Casting processes divide by the mould. Expendable moulds are destroyed to remove the casting: green-sand, dry-sand, CO2 (sodium silicate), shell and investment moulds. Permanent moulds are metal and reused: gravity permanent-mould, pressure die, centrifugal and squeeze casting. Sand casting makes the largest parts in any metal at low tooling cost; permanent moulds give better surface and accuracy at higher volume but only in lower-melting alloys.

Patterns replicate the casting. Types: single-piece (solid), split (two-piece, parting at the largest section), match-plate (both halves on one plate, for machine moulding), cope-and-drag, loose-piece (for projections that would lock in the sand), gated (several patterns joined by the gating), sweep (for axisymmetric shapes) and skeleton (for large, simple shapes). Materials: wood (cheap, easily worked, absorbs moisture), metal (aluminium, cast iron — long runs), plastics and epoxy, wax (investment casting) and expanded polystyrene (the full-mould or lost-foam process, where the pattern vaporises).

Moulds and cores. Moulding sand is silica grains with a clay binder (bentonite) and water; green sand is used moist, dry sand is baked for strength, and CO2 moulds are hardened by gassing sodium silicate. Cores form internal cavities; they are made of sand with organic or resin binders, must be strong when dry yet collapsible so the casting can contract, and are held by core prints or, where needed, metal chaplets. The properties that matter: permeability (gases escape, otherwise blowholes), green and dry strength, refractoriness (withstand the pouring temperature), collapsibility and flowability. Standard tests use a rammed cylindrical specimen 50.8 mm in diameter and 50.8 mm high. The permeability number is PN = VH/(pAt), with V the volume of air passed (cm³), H the specimen height (cm), p the pressure (g/cm²), A the cross-section (cm²) and t the time (min). Other tests measure grain fineness (sieve analysis), moisture, clay content, mould hardness and compressive strength.

Metal enters through a pouring basin, down a sprue, into a sprue well, along a runner and through ingates into the cavity. Treating the sprue by Bernoulli with no losses, metal leaves a sprue of height h at v = √(2gh), so continuity makes the sprue taper: A_top/A_bottom = √(h_bottom/h_top), with heights measured from the free surface in the basin. A straight sprue would let the stream neck away from its walls and aspirate air. The gating ratio (sprue : runner : ingate areas) is pressurised when the ingates are smallest (for example 1 : 0.75 : 0.5, keeping the system full) and unpressurised when they are largest (for example 1 : 2 : 2, slowing the metal for oxidising alloys such as aluminium).

Chvorinov's rule gives the solidification time as t_s = B(V/A)^n with n usually 2: time goes as the square of the modulus V/A. A cube of side a has V/A = a/6, so doubling the side quadruples the time. A riser feeds liquid to the casting as it shrinks and must therefore freeze after it; in practice the riser modulus is taken somewhat larger than the casting's (often about 1.2 times). For a cylindrical riser whose height equals its diameter and whose top, base and curved surface all lose heat to the sand (a blind riser joined by a short neck), the modulus is D/6 — and H = D is the proportion that gives the largest modulus for a given volume. If the riser's base sits on the casting and does not cool, the modulus is D/5 instead. Directional solidification from the thin sections towards the riser, helped by chills (which speed freezing locally) and insulating sleeves or exothermic compounds on the riser, is what keeps the last liquid in the riser.

Solidification and microstructure. Metal shrinks three times: as liquid cooling to the freezing range, during freezing, and as solid cooling to room temperature. The riser compensates the first two; the pattern's shrinkage allowance compensates the third. A pure metal freezes at one temperature with a plane front, while an alloy freezes over a range with a mushy zone of dendrites and liquid. In an ingot or thick casting the cross-section shows a fine equiaxed chill zone at the mould wall, long columnar grains growing inward against the heat flow, and a central equiaxed zone. Faster cooling gives finer grains and higher strength; inoculants add nucleation sites for the same effect.

• Pressure die casting forces molten metal into a steel die at high pressure. In the hot-chamber machine the injection cylinder sits in the melt, suited to low-melting zinc, tin, lead and magnesium alloys; in the cold-chamber machine metal is ladled into a separate cylinder, used for aluminium and copper alloys that would attack a submerged plunger. Fast, thin-walled, accurate, with good surface, but tooling is expensive and entrapped gas makes parts unsuitable for heat treatment.
• Centrifugal casting spins the mould. True centrifugal casting makes hollow cylinders such as pipes with no core, the bore formed by the spinning metal itself; semi-centrifugal makes solid axisymmetric parts such as wheels; centrifuging spins several small cavities around a central sprue. Heavier metal is thrown outward and lighter oxides and slag collect at the bore, where they are machined off — giving dense, sound walls. The spin needed is set by the ratio of centrifugal to gravitational acceleration, G = ω²r/g.
• Investment (lost-wax) casting: wax patterns are assembled on a tree, dipped repeatedly in ceramic slurry and stucco to build a shell, the wax is melted out, the shell is fired and the metal poured. It gives very fine detail, excellent finish and close tolerances in any alloy, including those too hard to machine — turbine blades, jewellery, surgical implants — at a higher cost per part.

Non-destructive testing finds defects without harming the part. Visual inspection and dye penetrant find surface-breaking cracks in any non-porous material. Magnetic particle inspection finds surface and near-surface cracks, but only in ferromagnetic materials. Ultrasonic testing sends a pulse through the part and reads echoes from internal flaws; it needs a couplant and a skilled operator. Radiography (X-ray or gamma ray) images internal voids, porosity and inclusions as density differences and gives a permanent record. Eddy current testing detects surface flaws and conductivity changes in conductive materials.

Elastic deformation is recoverable and linear (Hooke's law, with volume change governed by ν). Plastic deformation is permanent, occurs at constant volume (ε1 + ε2 + ε3 = 0 in true strain, the reason plastic Poisson's ratio is 0.5), and is driven by shear — hydrostatic stress alone never causes yielding. In the plastic range a metal's true stress rises with true strain along the flow curve σ = Kεⁿ, with strength coefficient K and strain-hardening exponent n; in a tension test necking begins when the true strain equals n. The flow stress is the instantaneous stress needed to continue deforming, and forming loads are computed from the average flow stress over the strain applied, σ̄ = Kεⁿ/(1 + n).

Yield under a multiaxial stress is predicted by two criteria. Tresca: yielding begins when the maximum shear stress reaches the value at yield in tension, σ1 − σ3 = σ_y. von Mises: when the distortion energy does, (σ1 − σ2)² + (σ2 − σ3)² + (σ3 − σ1)² = 2σ_y². The two agree in uniaxial tension and in balanced biaxial tension, and differ most in pure shear and plane strain, where von Mises gives a yield stress of (2/√3)σ_y = 1.155σ_y against Tresca's σ_y. Rolling a wide strip and most sheet operations are close to plane strain, so this factor appears in their load formulas.

Ideal work is the minimum work of homogeneous deformation, with no friction and no redundant shearing: per unit volume it is w = σ̄ε, the area under the flow curve. For drawing a wire of original area A0 to Af, ε = ln(A0/Af), so the ideal drawing stress is σ_d = σ̄ ln(A0/Af). The drawing stress cannot exceed the flow stress of the drawn wire, or the wire breaks; for a non-hardening material this caps ln(A0/Af) at 1, a maximum area reduction per pass of 1 − 1/e = 63 %. Friction and redundant work lower the real limit. Slab analysis refines ideal work by taking the equilibrium of a thin slab of material, including friction on its faces, and integrating across the deformation zone — it is how the friction terms in the forging and rolling formulas below are derived.

• Forging: open-die upsetting of a cylinder of diameter d and height h needs an average pressure p = σ_f(1 + μd/(3h)) by slab analysis, so the force rises as the part gets flatter. Closed-die (impression-die) forging adds a flash land that raises pressure and forces the cavity to fill.
• Rolling: draft Δh = h0 − hf; the maximum draft the rolls can grip is Δh_max = μ²R. The contact length is L = √(RΔh) and the roll force is approximately F = σ̄ w L for strip width w; torque per roll is about F L/2 and power for two rolls 2πN F L/60 (with L/2 as the lever arm). At the neutral point the strip and roll surface move at the same speed: before it the strip is slower, after it faster.
• Extrusion pushes a billet through a die. Direct (forward) extrusion moves the billet against the container wall, so friction adds to the load; indirect (backward) extrusion moves the die instead and needs less force. The ideal ram pressure is p = σ̄ ln(A0/Af), the extrusion ratio being A0/Af; real presses use empirical factors on top of this.
• Wire and rod drawing pulls the stock through a converging die, so it is a tensile process — the drawn section itself carries the load, which is why the reduction per pass is limited as above.

Blanking cuts out a part (the blank is the product); punching (piercing) cuts a hole (the slug is scrap). Both shear the sheet between punch and die separated by a small clearance, a few per cent of the sheet thickness per side. The cutting force is F = L t τ_s, with L the cut perimeter, t the thickness and τ_s the shear strength (roughly 0.7 of the tensile strength when only that is given). The size rule: in blanking the die is made to the blank size and the clearance is taken off the punch; in punching the punch is made to the hole size and the clearance is added to the die. Shear on the punch or die face spreads the cut over time and lowers the peak force.

Bending stretches the outer fibres and compresses the inner, with the neutral axis shifted inward to a fraction k of the thickness (k ≈ 0.33 to 0.5). The developed length of the bend is the bend allowance, BA = α(R + kt) with α in radians. On unloading the elastic part recovers — springback — so parts are overbent or bottomed; springback grows with yield strength and with R/t. Too small a bend radius cracks the outer surface, which sets a minimum R/t for each material. Deep drawing forms a cup by pushing a blank through a die while a blank holder prevents wrinkling. For a cup of diameter d and height h with no flange, equating areas gives the blank diameter D = √(d² + 4dh). The drawing ratio D/d is limited, typically to about 2 in one draw, beyond which the wall tears; deeper cups are redrawn in stages.

Classification. Joining processes are fusion welding (the base metal melts, with or without filler), solid-state welding (no melting; pressure, friction or diffusion does the bonding), brazing and soldering (only the filler melts), adhesive bonding, and mechanical fastening. Fusion welding heat sources: the oxy-acetylene flame (neutral, oxidising or carburising depending on the oxygen-to-acetylene ratio); the electric arc; resistance heating, H = I²Rt, concentrated at the contact between sheets (spot and seam welding); the laser, a focused photon beam able to form a narrow keyhole; and the electron beam, normally run in vacuum, giving the deepest, narrowest welds with the smallest heat-affected zone.

Heat transfer and losses. The arc supplies VI watts, but only a fraction reaches the work — the heat-transfer efficiency f1, lost to radiation, convection and spatter — and only part of that melts metal — the melting efficiency f2, the rest conducting into the base metal. The net heat input per unit length of weld is H = f1VI/v for travel speed v; for 20 V, 200 A, 5 mm/s and f1 = 0.8 this is 0.8 × 20 × 200/5 = 640 J/mm. The arc voltage rises with arc length, V = A + Bl. A power source rated at current I_r for duty cycle D_r can carry I at duty cycle D with I²D = I_r²D_r. In GTAW with DC electrode negative about two-thirds of the arc heat goes to the work (the anode), giving deep penetration and a cool tungsten; DC electrode positive heats the electrode but cleans oxide, and AC is used for aluminium.

Solid-state welding. In friction welding one part rotates against the other under axial pressure; the heated interface is forged together once rotation stops — ideal for joining rods and dissimilar metals. Friction stir welding traverses a rotating, non-consumable shouldered tool with a pin along the joint line; the tool stirs plasticised material below its melting point, so aluminium alloys that crack or porosity-prone in fusion welding join soundly. Ultrasonic welding applies high-frequency vibration parallel to the interface under light pressure, scrubbing off oxides — used for foils, wires and electrical connections. Welding defects: porosity (gas, moisture, contamination), slag inclusion (poor cleaning between passes), lack of fusion and incomplete penetration (low heat, fast travel, wrong joint preparation), undercut (excess current or travel speed), hot cracks (solidification, from impurities such as sulphur) and cold or hydrogen-induced cracks (hydrogen, hard heat-affected zone, restraint — prevented by preheat and low-hydrogen electrodes), and distortion. They are inspected by the same visual, penetrant, magnetic-particle, ultrasonic and radiographic methods as castings.

Brazing and soldering join without melting the base metal: a filler with a lower melting point is drawn into a close-fitting joint by capillary action, helped by a flux that removes oxides and promotes wetting. The dividing line is the filler's melting temperature: above 450 °C it is brazing (copper, silver and brass fillers, stronger joints), below it soldering (tin-based fillers, electronics and plumbing). Adhesive joining bonds with a polymer (epoxies, cyanoacrylates, anaerobics); it spreads load over a large area, joins dissimilar and thin materials without heat distortion, and seals, but it is weak in peel and cleavage, needs careful surface preparation and loses strength at elevated temperature — so joints are designed to load the adhesive in shear.

Powder production: atomisation (a molten stream broken up by gas or water jets — the main route for alloy powders; gas gives spherical, water irregular particles), chemical reduction of oxides (iron, tungsten), electrolytic deposition (very pure copper), thermal decomposition of carbonyls (fine iron and nickel), and mechanical comminution and milling, including mechanical alloying. Ceramic powders are made by milling and by chemical precipitation and calcination. Powders are characterised by size, size distribution, shape, flow rate and apparent density, then blended with lubricants and binders.

Compaction presses the powder in a die (single- or double-action) into a green compact of modest strength; density is highest near the moving punch and falls with distance because of die-wall friction, which double-action pressing and isostatic pressing (uniform fluid pressure, cold or hot) reduce. Sintering heats the compact in a protective atmosphere to below the melting point of the main constituent, typically about 0.7 to 0.9 of its absolute melting temperature: atoms diffuse, necks grow between particles, porosity falls and strength rises, with some shrinkage. Controlled residual porosity is a feature, not a defect, in self-lubricating bearings and filters. Ceramic powders are shaped by pressing, slip casting or extrusion and then fired — the same compaction-plus-sintering logic, without any melting.

Moulding of composites. Open-mould methods — hand lay-up and spray-up of glass fibre with polyester or epoxy resin — suit large, low-volume parts such as boat hulls. Vacuum bagging and autoclave curing consolidate prepreg laminates for aerospace quality. Closed-mould resin transfer moulding injects resin into a dry fibre preform. Filament winding wraps resin-wetted fibre around a rotating mandrel for pressure vessels and pipes; pultrusion pulls fibre through a resin bath and a heated die for constant-section profiles; compression moulding of sheet moulding compound (SMC) makes automotive panels. Metal-matrix composites are made by stir casting, infiltration or powder routes, and ceramic-matrix composites by infiltration and sintering.