Question

Classify the types of porosity that occur in ingots and cast parts and list the important factors that determine their occurrence. List the practical steps which must be taken if porosity is to be kept to a minimum in aluminium alloy with a wide freezing range cast in a sand mould.

Answer 1

Porosity is the most common casting defect, which can be caused by problems in the running system.

In general, there are three basic classifications of casting porosity:

1. Blind Porosity: From one surface only and therefore not forming a continuous passage for liquid.
   
2. Through Porosity: Stretching from one side of a casting to another thereby causing a leak path.
   
3. Fully Enclosed Porosity: Enclosed within the casting, and has no passage to the surface.

Causes of porosity

• The design of the mold and cast parts
• The purity of the metal or alloy being used
• Pressure and shot speed of the machines
• Shrinkage of the material wall thickness
• Too much lubricant in the die
• Sharp corners in the mold
• Low metal temperatures
• Air trapped in the metal

Prevention of porosity

Porosity varies in severity. Sometimes it is acceptable, but most often it’s best to limit it as much as possible.

The most direct way to control for porosity is b y creating an efficient process for die casting while ensuring the material, working with is of a high quality. Process monitoring should focus on equipment maintenance and stability, which can help guarantee an even and adequate amount of pressure throughout the casting.

Gas porosity, which is the formation of air bubbles inside of a casting as it cools, can be avoided by melting the material in a vacuum or in an environment of low-solubility gases, including argon. This porosity occurs because liquids can naturally hold in dissolved gas. Sometimes this can be addressed by exposing the melted material to another gas — the two gases react and pull each other out of the liquid. Gas porosity can also be eliminated by introducing nitrogen into the aluminium metal before the liquid pour. HiPping can also be used to eliminate small amounts of porosity inside the casting. This involves placing the casting in a high temperature furnace, which is enclosed in a pressurised container. The casting is then heated in an inert gas, commonly argon, which applies ‘isostatic’ pressure to eliminate the porous elements.

If oxide formation is the cause of porosity, then the materials will also benefit from being properly degassed after melting or filtered before using the metal for making the casting.

Hot tears and hot spots are metallurgical defects that occur because of problems during cooling. If this occurs, it is essential to make sure that the casting is properly being cooled in that local area of the part. If hot spots persist, then it's necessary to adjust the cooling practices by either more die spray or adding more localized cooling channels to that problem area.