MATERIALS • PROCESSES • FINISHES THE FUNDAMENTAL FLAW
Could anti-gravity casting prove to be the silver bullet for crack-free metal casting? Despite industry-wide scepticism, this professor believes so...
“
No one has really understood why metal castings are unreliable when compared to forging, until recently. What I’m about to reveal
today is being denied by millions,” Professor John Campbell, professor emeritus of casting technology at the University of Birmingham, states to a room full of journalists. We’re at a First Friday Club media briefing hosted by Cadence, and Campbell has been invited to share his decades of work on anti-gravity metal casting. “The fundamental issue of making
good metal is simply this: the surface of the liquid metal itself,” Campbell continues. “On the surface of liquid metal is a very thin film created by contact with the air around it. When the metal is poured turbulently during casting, the slightly drier surface is folded over itself and you have two oxide surfaces which will never bond. So, what you now have going off into the liquid metal is a crack, as these two surfaces comes apart extremely easily.” This crack in the liquid state is what
Campbell calls a ‘bifilm’ and, while they require the right technology and equipment to identify, they are apparently prevalent in many traditionally cast metal structures. Bifilms can be fairly harmless if they result in a convoluted crack, Campbell says, but in the cases where
Top-poured turbulent casting process Surface turbulence generating bifilm cracks in the liquid metal
gas diffuses into the solid metal they can become expanded cracks up to 10 times the size of what they were originally, which can lead to significant consequences. “With this knowledge, we can go on
to understand for the first time many things which have bugged metallurgists, such as hydrogen embrittlement,” Campbell continues. “Because hydrogen
can diffuse into the interior of a metal – against all the rules – through these bifilms which act as open pathways for the hydrogen to move throughout the structure. So, when you have a situation where a structure has lots of bifilms and therefore lots of diffusion paths for the hydrogen, the tensile strength can open up these cracks further until the metal fatigues.”
The contact pour casting method explained
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