16 Questions
What is the primary cause of gas porosity in liquid metals?
Greater solubility of gases in liquid metals
Which of the following is a type of porosity in castings?
Shrinkage Porosity
What is the effect of porosity on casting properties?
Reduced ductility
What is the cause of gas porosity in aluminum?
Hydrogen from sources such as melting and handling
How can gas porosity be prevented?
Inert gas flushing (purging)
What is the term for the reaction between molten metal and mold material that produces gases?
Gaseous decomposition of mold materials
What is the type of defect that occurs due to poor mold filling?
Entrained air
What is the method to control gas porosity in aluminum casting?
Effective degassing prior to casting
What is the primary factor that affects the formation of gas porosity defects?
Trapped air in the cavity
What is the result of metal contraction during cooling?
Shrinkage
Which of the following is NOT a source of gas in aluminum alloys?
Oxygen from the atmosphere
What is the purpose of the riser (feeder) in the solidification process?
To feed liquid metal to the shrinking cavity
Which inspection method is commonly used for aluminum castings?
All of the above
What is the primary cause of gas porosity defects in castings?
All of the above
What is the result of solidification shrinkage due to phase transformation?
Solidification shrinkage
Which of the following is a remedy for gas porosity defects?
All of the above
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
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.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
