| name | casting-defects |
| description | Casting defects — shrinkage porosity, gas porosity, cold shuts, misruns, hot tears, inclusions, detection methods, remedies, ASTM E446 (radiography), sand/die/investment casting. |
| metadata | {"priority":7,"promptSignals":{"phrases":["casting defects","casting porosity","shrinkage defect","casting inspection","sand casting defects","die casting defects","investment casting defects"],"minScore":3}} |
Casting Defects — Complete Skill
Defect Classification
Porosity
Shrinkage porosity:
Origin: metal shrinks as it solidifies; insufficient liquid feed → voids in final solidified region
Location: at last-to-solidify hot spots; inside thick sections; at feeding cut-off zones
Morphology: irregular, dendritic-network, non-spherical; interconnected in severe cases
Prevention:
- Risers (feeders): liquid reservoir provides metal as shrinkage occurs; design to solidify last
- Riser design: Chvorinov's rule V/A ratio; riser V/A > casting V/A × 1.2
- Directional solidification: taper casting → solidification from thin to thick toward riser
- Exothermic riser compounds: maintain riser liquid longer
Gas porosity:
Origin: dissolved gases (H, N, CO) released during solidification; trapped as spherical bubbles
Location: throughout casting; more at hot spots (last solidified)
Morphology: spherical or rounded; isolated bubbles
Causes (by metal):
- Aluminum: hydrogen absorption from moisture in sand, flux, mold wash; H₂ solubility drops at solidification → porosity
- Steel: CO from SiO₂ + C reaction; also nitrogen from atmosphere
- Iron: hydrogen; blow holes from mold gas
Prevention:
- Degassing: rotary degassing with inert gas (Al); vacuum degassing (steel)
- Dry molds and cores; bake before pour
- Rapid pour (minimize gas absorption time)
- Deoxidation: Al/Si addition to steel (binds O₂)
Chvorinov's Rule (solidification time):
t_s = B × (V/A)^n [s; B = mold constant; V/A = volume-to-surface area ratio; n ≈ 2 for sand casting]
Cold Shut
Origin: two streams of metal meet but don't fully fuse; low temperature at junction
Appearance: visible line or seam on casting surface; like a crack but rounded
Cause: low pouring temperature; slow pouring speed; poor gating (two streams from different gates)
Prevention: increase pouring temperature; redesign gates for single-fill pattern; top gates for thin sections
Misrun
Origin: metal solidifies before filling mold completely; short pour
Location: typically at furthest point from gate, thin sections, extremities
Prevention: increase pouring temperature; improve gate design; reduce section changes
Hot Tear (Hot Cracking)
Origin: tensile stress during solidification (shrinkage); occurs at grain boundaries when metal is semi-solid
Location: acute corners, abrupt section changes, where mold constrains free contraction
Morphology: branching, intergranular cracks with rough oxidized surfaces
Prevention:
- Design: round corners (min radius t/5); reduce section changes; avoid constrained geometry
- Processing: reduce mold/core rigidity; increase knockout temperature
- Alloy selection: alloys with wide freezing range (e.g., 7075 Al → prone; 380 → less prone)
Inclusions
Sand inclusions: mold erosion → loose sand carried into casting; common in ferrous castings
Oxide films (bifilms): entrained oxide layer (Al₂O₃ in Al); severe effect on elongation and toughness
Slag inclusions: oxidized flux/slag entrapped in metal stream
Prevention:
- Filter systems: ceramic foam filters in gating; removes inclusions > filter pore size
- Quiet pouring: avoid turbulence (Fr < 1 for Al filling)
- Slag removal before pour
- Inert atmosphere for reactive metals
Blow Holes
Origin: gas evolved from mold/core binders during pour → large spherical voids at mold surface
Mold gas sources: organic binders in resin-bonded sand; moisture
Prevention: bake molds/cores; vent design; dry materials; use gas-permeable mold
Shrinkage Pipes (Centerline Shrinkage)
Solidification from outside in: liquid core shrinks → pipe forms along centerline
Common in: ingots, large cylindrical sections
Prevention: risers at top of casting; top-pouring to feed downward
Detection Methods
| Method | Detects | Sensitivity |
|---|
| Visual inspection | Surface defects, cold shuts, misruns | Qualitative |
| X-ray radiography | Internal porosity, inclusions | ≥ 1–2% wall thickness |
| Computed tomography (CT) | 3D porosity map, internal cracks | ≥ 0.1 mm voids |
| Dye penetrant (PT) | Surface-connected cracks | ≥ 0.1 mm |
| Magnetic particle (MT) | Surface + near-surface cracks (Fe alloys) | ≥ 0.5 mm |
| Ultrasonic (UT) | Internal defects; thick sections | Varies; ≥ 1–5 mm |
Radiographic grading (ASTM E446):
Radiograph of casting → compare to reference standards (E155 for Al, E186 for steel)
Grade 1 (best) to Grade 5 (worst); specify acceptance (e.g., "Grade 3 or better for Class A zones")
Porosity measurement:
Reduced pressure test (Al): freeze sample under vacuum; visible gas porosity relative index
Archimedes density test: bulk density vs. theoretical; Δρ/ρ_theoretical ≈ porosity volume fraction
Remediation
Impregnation: vacuum impregnate with polymer sealant (Loctite Resinol 90C); seals micro-porosity in hydraulic components
Pressure test after impregnation; accepted practice for pressure-tight castings
NOT acceptable for structural (mechanical) load-bearing applications
Welding repair: fill surface-connected shrinkage by welding; pre-heat; PWHT
Weld repair procedure per ASTM A370 or MIL-STD-278; requires NDT after repair
HIP (Hot Isostatic Pressing):
Eliminates internal porosity in premium castings (aerospace, medical)
Al alloys: 520°C / 1000 bar / 4 hr; Ti alloys: 920°C / 1750 bar / 2 hr
Porosity closure → improved fatigue life 50–100%
Standards
| Standard | Scope |
|---|
| ASTM E446 | Radiographic reference standards for steel casting |
| ASTM E155 | Reference radiographs for Al and Mg castings |
| ASTM E186 | Heavy steel casting radiography |
| ASTM A802 | Surface acceptance criteria for steel castings |
| ISO 4986 | Radiographic examination of castings |
Output
Provide: defect type and likely cause, detection method and sensitivity, ASTM E446/E155 acceptance grade specification, remedy (process change vs. impregnation vs. HIP vs. weld repair), prevention measures (riser sizing [V/A ratio], gate design, pouring temperature [°C], degassing procedure), required NDT type, and applicable standard (ASTM E446, E155).
