Lec 6: Casting Defects

Study Notes

Casting Defects

Filling related defects:
- Blowhole
- Sand burning
- Sand inclusion
- Cold lap or cold shut
- Misrun
- Gas porosity
Shape related defects:
- Mismatch defect
- Distortion or warp
- Flash defect
Thermal defects:
- Cracks or tears
- Shrinkage
- Sink mark
Defects by appearance:
- Metallic projection
- Cavities
- Discontinuities
- Incomplete casting
- Incorrect dimension or shape
- Defective surface
- Rat Tail and Buckles

Porosity in Castings

Types of porosity:
- Gas porosity
- Shrinkage porosity
- Microporosity
Effects of porosity on casting properties:
- Reduced ductility
- Rough surface
- Permeable regions affecting pressure tightness

Gas Porosity

Formation:
- Greater solubility of gases in liquid metals than in solids
- Gases expelled during solidification form gas porosity
- Reactions between molten metal and mold material produce gases
Prevention:
- Inert gas flushing (purging)
- Melting and pouring in a vacuum
- Hot isostatic pressing
- Steep temperature gradient for alloys
Causes of gas porosity:
- Precipitation from solution
- Entrained air
- Gaseous decomposition of mold materials

Gas Porosity in Aluminum

Primary concern: Hydrogen from sources such as:
- Melting and handling
- Reaction with the mold during passage
- Reaction with mold and core materials
Control: Effective degassing prior to casting
Characteristics of gas porosity defects:
- Vary in size, distribution, distance below surface, and morphology
- Causes can be deduced from careful examination
Sources of gas:
- Trapped air
- Hydrogen dissolved in aluminum alloys
- Moisture from water-based die lubricants
- Steam from cracked cooling lines

Formation and Prevention of Gas Porosity

Air is compressed as metal fills the cavity
Swirling flow can elongate air spheres, causing defects
Possible causes:
- Low metal pouring temperature
- Insufficient metal fluidity
- Slow pouring
- Slag on metal surface
- Interruption during pouring
- High gas pressure in mold
- Lustrous carbon from molding
- Thin metal sections
- Inadequately pre-heated metallic molds
Remedies:
- Increase pouring temperature
- Modify metal composition for better fluidity
- Pour rapidly without interruption
- Improve mold filling
- Remove slag
- Adjust molding material properties
- Ensure adequate venting
- Eliminate lustrous carbon
- Modify casting design to avoid thin sections
- Pre-heat metal molds and use insulating coatings

Inspection Methods

Steel castings:
- Magnetic particle inspection
- Liquid penetrant inspection
Aluminum castings:
- Radiography
- Ultrasonic inspection
- Liquid penetrant inspection

Shrinkage in Castings

Formation and causes:
- Results from metal contraction during cooling
- Occurs in three states:
  - Liquid
  - Liquid + Solid (solidification shrinkage due to phase transformation)
  - Solid (thermal contraction)
Greatest contraction:
- Occurs during cooling of solidified metal
Solidification process:
- Outer shell freezes first, progressively solidifying towards the center
- Shrinkage cavity forms as a result
- Liquid metal is fed from the riser (feeder) to compensate

Description

This quiz covers the different types of defects that can occur in casting, including filling related defects, shape related defects, thermal defects, and defects by appearance. Learn about the various types of porosity and other defects in castings.