Rule
No. 8:
"Severe Segregation"
All freezing will cause some segregation of alloying elements.
Most casting alloys segregate their elements to only a small
extent, so that the problem is not noticeable (for instance
as in most Al-Si-Mg alloys).
However, some alloys segregate seriously, to the point at
which parts of the casting will be well outside chemical specification.
For instance Al-4.5Cu alloys might have specification limits
of 4.0 and 5.0 per cent copper. Although the average percentage
of the copper in the casting will be normally nicely inside
this limit, parts of the casting can easily rise to 5.5 per
cent or more in a chilled area, perhaps causing that locality
to be too hard, strong or brittle. Conversely, other hotter
parts will decrease to 3.5 % Cu or less, causing those regions
to be too soft and weak. Such problems occur naturally at
abrupt changes in section (which is a concern since such locations
are usually also regions of stress concentration), and at
other parts where freezing patterns are altered (such as under
feeders or under chills).
(Click here to view diagram
of segregation effects in a Cu alloy casting with variable
section thickness.)
Most foundries are unaware of the problem. A designer and
his buyer need to proceed therefore with caution. In principle
the problem can be predicted by computer packages, and therefore
allowed for in the design (more information in "CASTINGS").
|
