Fundamentals of Casting — Condensed Notes

11.1 Introduction to Materials Processing

• Purpose: convert materials into shapes with properties suited to service environment; key steps include design, material selection, process selection, manufacture, inspection, and feedback.
• Five basic families of shape-producing processes:
  • Casting: molten material flows into mold, solidifies to form shape.
  • Material removal: remove material from an oversized piece (machining and related techniques).
  • Deformation: shape formed by plastic deformation (forming, forging, etc.).
  • Consolidation: join small pieces (welding, brazing, soldering, adhesives, mechanical fasteners).
  • Powder metallurgy and direct digital manufacturing/additive manufacturing.
• Casting advantages/limitations: enables complex geometries and hollow sections but can exhibit solidification defects (shrinkage, porosity).
• Pattern terminology and design influence: patterns define casting size; allowances compensate for shrinkage and other dimensional changes.

11.2 Introduction to Casting

• Six basic steps in most casting processes:
  1) Mold/cavity creation with proper shrinkage allowance and detail; single-use vs. reusable molds.
  2) Melting to proper temperature, quantity, and quality.
  3) Pouring technique to fill mold and vent gases; avoid entrapment.
  4) Solidification control to minimize internal porosity and restraint-induced cracks.
  5) Mold removal from casting.
  6) Cleaning/finishing/inspection of the finished part.
• Prevalence: ~90% of manufactured goods contain at least one metal casting.
• Casting capabilities: suitable for complex shapes, hollow parts, large sizes, and materials hard to machine.

11.3 Casting Terminology

• Pattern: duplicated part used to form mold cavity; pattern materials include wood, metal, foam, plastic (urethane common).
• Allowances: pattern dimensional adjustments for shrinkage, draft, finish, and distortion.
• Mold components and features:
  • Parting line: interface separating mold halves (cope and drag).
  • Draft: taper on pattern surfaces to aid withdrawal from mold.
  • Core: internal features (holes/passages) formed by inserted cores; core box forms core prints.
  • Riser: extra mold cavity that feeds liquid metal during solidification to compensate shrinkage.
  • Gating system: network delivering molten metal to mold cavity; includes pouring cup, sprue, runners, gates, and vents.
  • Mold cavity: final hollow shape where metal solidifies.
  • Core print: locates/supports a core inside the mold.
• Pattern/draft considerations:
  • Draft is needed on surfaces parallel to withdrawal direction to prevent damage.
  • Draft requirements depend on pattern, mold material, and molding procedure.

11.4 The Solidification Process

• Solidification: two-stage process – nucleation (formation of stable solid particles) and growth (solidification front advances).
• Undercooling: solidification begins below the equilibrium melting point; energy considerations drive nucleation.
• Nucleation sites: walls, impurities, or inoculants promote nucleation and refinement of grain size.
• Grain structure zones:
  • Chill zone: very near mold walls; fine grains from rapid cooling.
  • Columnar zone: elongated, directionally oriented grains due to preferential growth.
  • Equiaxed zone: interior grains that are more isotropic; promoted by inoculants and slower cooling.
• Cooling curves: temperature vs time during solidification reveal key events.
  • Pouring temperature, superheat (above liquidus), cooling rate, thermal arrest (plateau during fusion), total and local solidification times.
• Solidification metrics:
  • Liquidus: highest temp where material starts to be all liquid.
  • Solidus: lowest temp where material is all solid.
  • Freezing range: range between liquidus and solidus.
• Practical implications: faster cooling/shorter solidification times yield finer structures and better properties; mold material and cooling conditions influence rates.
• Gas porosity and dross:
  • Gas/elements dissolved in molten metal form porosity; dross/slag can be skimmed if managed properly.
  • Degassing methods: vacuum degassing, gas flushing (e.g., nitrogen), ultrasonic degassing, and fluxing.
  • Dross/slag removal also aided by filtration and skim operations.
• Fluidity and pouring temperature:
  • Fluidity affects castability: minimum section thickness, fill length, detail capture, and filling of extremities.
  • Pouring temperature/superheat increases fluidity but raises risk of reactions and dissolved gas.
  • Penetration defect occurs when liquid metal too fluid fills pores between sand particles.
• The gating system and filling behavior:
  • Gating design aims to minimize turbulence, gas absorption, and erosion.
  • Short sprues and rounded channels reduce heat loss and promote smooth flow.
  • Choke controls metal flow rate; gate location affects filling dynamics and defect risk.
• Solidification shrinkage:
  • Three shrinkage stages: liquid contraction before solidification, solidification shrinkage, and final solid-state contraction to room temperature.
  • Variations in shrinkage are alloy-dependent; directional solidification helps feed the casting and relocate shrinkage to risers.
  • Some alloys expand on solidification (e.g., gray cast iron).
• Riser and riser design:
  • Riser serves as a feeding reservoir; it should solidify after the casting to feed shrinkage.
  • Directional solidification from casting extremities toward the riser is desired.
  • Riser size and location affect yield and feasibility; smaller riser cross-section and proper connection help maintain heat transfer and feeding.
• Risering aids:
  • External chills: high-thermal-conductivity masses placed near casting to accelerate solidification and promote directional feeding.
  • Internal chills: embedded blocks within mold; must be compatible with the casting alloy.
  • Heat management strategies to slow riser solidification or accelerate casting solidification (sleeves, exothermic materials).
• Casting process classes (brief):
  • Expendable-mold processes require new molds per casting.
  • Permanent-mold processes reuse molds; often faster and more economical for high-volume runs.

11.5 Patterns

• Pattern allowances (essential): shrinkage, draft, finish, distortion.
• Shrinkage allowance: compensate for contraction during cooling; typical rule uses a length increase by a factor related to
  $ext{A length} = ext{length} imes \big(1 + \frac{ ext{α} ext{ΔT}}{1}\big)$
  where $\big( ext{ΔL} = ext{α} L ext{ΔT} \big)$.
• Shrink rules: measurement devices larger than standard rules to predefine shrinkage allowances (e.g., brass shrinkage ~1.5%).
• Other pattern allowances:
  • Finish allowance: extra metal for machining to achieve final surface finish.
  • Distortion allowance: accounts for shape changes due to differential shrinkage during solidification.
  • Multi-use molds require adjustments to compensate for mold-temperature rise over successive casts.
• Parting line and draft: the most favorable parting line is flat for ease of mold release; draft on surfaces parallel to withdrawal reduces damage during pattern removal.
• Pattern design impact: draft and allowances influence final part weight, machining needs, and wall thickness consistency.
• Example: six options for ring with drafting at parting line; evaluating for flat side surfaces, parallel inner/outer diameters, machining needs, and wall-thickness uniformity.

11.6 Design Considerations in Castings

• Goals: high quality at low cost; small design changes can reduce defects and ease manufacturing.
• Parting line location considerations:
  • Affects number of cores, core support, gating efficiency, final weight, dimensional accuracy, and molding ease.
  • Reorienting parting line can eliminate cores and reduce weight (and sometimes draft).
  • Examples show multiple parting-line arrangements to balance draft, ease of core usage, and appearance.
• Solidification control and geometry:
  • Regions with high surface area-to-volume cool faster and become stronger; thick sections cool slower and risk shrinkage/porosity and coarse grains.
• Fillets and hot spots:
  • Generous interior radii distribute stress and reduce shrinkage cracks but excessive fillets increase material and potential hot spots.
• Thick sections and hot spots:
  • Intersections of thick sections can created localized hot spots and shrinkage; stagger ribs to minimize contraction and hot spots.
• Parting line and appearance:
  • Flash at parting line may be visible; moving line to a corner can hide defects.
• Thin-walled castings:
  • Desired for weight reduction but challenging for mold filling; consider minimum section thickness varies by geometry, material, process, and foundry practice.
• Typical minimum thickness guidelines (illustrative):
  • Aluminum (Sand) ≈ 3.18 mm; Permanent Mold ≈ 2.36 mm; Die Cast ≈ 1.57 mm
  • Magnesium (Sand) ≈ 3.96 mm; Permanent Mold ≈ 3.18 mm; Die Cast ≈ 2.36 mm
  • Steel (Sand) ≈ 4.75 mm; Die Cast ≈ 2.36 mm; Investment Cast ≈ 1.57–2.36 mm
  • Zinc die casting: can achieve walls as thin as ≈ 0.5 mm
• Computer simulation: used to predict mold filling (fluid flow) and solidification (heat transfer) via finite element/finite difference methods; enables design changes before pattern/mold manufacture.
• Example: computer model shows directional solidification toward the riser in a steel mining shovel adapter.

11.7 The Casting Industry

• Industry scale: US metal casting shipments exceed 14 million pounds annually; value > $18 billion.
• Major casting materials: gray iron, ductile iron, aluminum alloys, copper-base metals.
• Applications: agricultural/industrial machinery, valves, pumps, railroad equipment, power tools, housing for appliances, etc.
• Additive manufacturing note: direct digital manufacturing is included as a newer process option alongside traditional casting and forming methods.

Key Equations and Concepts

• Shrinkage compensation (pattern shrinkage):
  
  \Delta L = \alpha L \Delta T
  
  where $\alpha$ is the coefficient of thermal expansion and $\Delta T$ is the temperature change from melting to room temperature.
• Solidification time (Chvorinov's Rule):
  \ t_s = B \,\left(\frac{V}{A}\right)^n, \quad n \in [1.5, 2.0] \n
• Riser sizing (approx. using n = 2 and a 25% slower solidification for the riser):
  $t<em>{riser} = 1.25 \, t</em>{casting}$
  and for equal mold constants B and same material, approximately:
  \left(\frac{Vr}{Ar}\right)^n = 1.25 \, \left(\frac{Vc}{Ac}\right)^n \Rightarrow \frac{Vr}{Ar} = (1.25)^{1/n} \frac{Vc}{Ac} \approx 1.118 \frac{Vc}{Ac} \quad (n=2).
  
• Cylinder riser geometry (illustrative):
  $V = \frac{D^2 H}{4}, \ A = D H + 2\left(\frac{D^2}{4}\right) = D H + \frac{D^2}{2}$
  (riser geometry used to estimate V/A ratio for sizing).