Casting defects and how to prevent them - part 1

Casting HipPing

In the first of two blogs on this theme, we take a look at casting defects and the preventative measures needed to avoid these issues.

Porosity

Porosity is the most common casting defect, which can be caused by problems in the running system. To eliminate these problems, we use bespoke running systems that are tailored to each individual customer requirement.

Trapped air and gasses can also cause porosity, either on the surface or inside the casting. Gas porosity occurs because the molten metal can hold a large amount of dissolved gas, unlike the solid form. As a result, gas bubbles form as the material cools[1]. Turbulence can also introduce gasses during the casting process. This occurs when the molten metal is poured too quickly into the mould.

Gas porosity can be eliminated through good mould design or 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.

Cavities

This type of defect includes any internal or exposed cavities on the surface of the casting and it is often caused by turbulence (see above).

Shrinkage

Shrinkage can appear as a crack or breakage on the surface of the casting. There are two main types of shrinkage – solidification shrinkage and pattern maker’s shrinkage. Solidification shrinkage occurs because metals are less dense as liquids than solids. It can occur when the molten metal flows from a thick to a thin section of the mould. This is addressed through the use of risers or chills.

Pattern maker’s shrinkage occurs due to the natural shrinkage of the molten metal as it cools to room temperature. A shrinkage allowance is therefore factored into the pattern design at the start of the process.

Inclusions

Inclusions are created by unwanted particles, such as dross or slag, which reduce the strength of the material. They can be caused by a filter failure within the running system and also by interaction of the metal with the mould surfaces or the outside atmosphere. Improved mould design and/or pouring techniques can address this issue.

In the second part of our analysis of casting defects, we will take a look at the causes of discontinuities, geometrical inconsistencies and misruns.

If you would like any information about our casting capabilities or anything mentioned in this blog, please email us on sales@haworthcastings.co.uk or call us on +44 (0)1794 512685


[1] Degamo, 2003