Bill,
I have to correct the last sentence in my post on "Casting details - ...". I
said:
>"Regarding your comment on all-sand castings, I'm not at all sure they would
have inherently
>less strength. If 356 all-sand castings are "cooked" to the same T6 heat treat
spec. as 356
>SPM castings I see no reason why they would have less strength."
I got uncomfortable with that, so I dug out my old metallurgy textbooks for the
facts.
The author states:
"The tensile strengths of aluminum casting alloys range from
about 18 to 48 ksi. Sand-cast aluminum alloys because of their relatively large
dendritic cell size due to slow solidification rates, have lower tensile
strengths
than permanent-mold or die-cast aluminum alloys. Higher strengths are ob-
tained in the two latter methods by higher solidification rates and lesser gas
porosity due to the use of metal molds. In die casting, pressure feeding
reduces
gas porosity also. As an example, alloy 356 has a minimum tensile strength of
30
ksi when sand cast and aged to peak strength (T6 temper), but when it is
permanent-mold-cast it has a peak strength of 33 ksi."
Ref.: Structure and properties of Engineering Alloys
by William F. Smith
It is clear that when an aluminum casting "freezes" rapidly it is stronger than
when it solidifies slowly.
The "dendritic cell size" referred to above is essentially the grain structure
of the aluminum.
Smaller grain size = stronger castings.
I hate it when I'm wrong! At least the record is now set straight. Sorry about
that.
Karl Payne
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