This professor believes anti-gravity casting could overhaul the metal casting process
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 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.”
OUT WITH THE OLD
“The main problems with metals are the entrainment defects of the liquid state: bifilms and bubbles,” Campbell says. “They’re both rather similar, but there’s more air in the bubble. Now, there’s a lot of continuous casting, which is better than how steel was made before, but it’s still not perfect. Special steels are generally cast as an ingot which is top-poured because that is the cheapest way to fill an ingot mould. More expensive ingots, which are said to be better, are poured through a conical pouring basin and filled from the bottom.”
These are currently the two principal ways of casting steel, but according to Campbell these techniques are “fundamentally bad.”
“With the conical pouring basin, the basin takes in air and concentrates it, putting in about 50% air into the metal which maximises the population of bifilms,” he explains.
So, what is the alternative?
“The real way to cast steels is what I call the contact pour situation, where the gap between the basin and the running system is eliminated,” Campbell says. “This is a system of melting and casting in which the liquid metal is never poured. It either moves horizontally or uphill and then fills the ingot, rather than being poured in. With this, you suddenly get a fantastic jump in the steel’s properties: it’s no longer full of bifilms and the steel now achieves its full strength. Plus, it wouldn’t suffer hydrogen embrittlement, as it wouldn’t suffer the corrosion we currently see because there are no bifilms to transport it.”
This begs the question, why isn’t this method widespread throughout metal production? Campbell says he has worked with several reputable companies to install his system, namely Rolls Royce and Cosworth, but there is industry scepticism around the existence of bifilms altogether.
“People look at cracks which have caused failures and they just say, oh yes, it’s a fatigue crack,” he shrugs. “We just give something a name and think that we’ve explained it. The trouble with bifilms is that they’re very, very thin oxide films that come together as closed cracks – they don’t show up on radiology. It’s hard work to reverse decades of thinking.”
And how is it that Campbell is able to identify the existence of bifilms, when others can’t? “It’s easier for me because I’ve realised they’re there,” he answers. “You can design inspection techniques to find them, and then you see them immediately. We have to slim down the metal to make it ultra-sensitive to x-rays, then they start to show up clearly. But most people wouldn’t have bothered to slim down the metal that much.”
Now that Campbell has retired from the University of Birmingham, he has made it his mission to try to change the narrative on bilfims within the metal production space.
“The future is interesting, because we could solve problems with our metals that we’ve lived with for years, like hydrogen embrittlement, in which hydrogen pressure vessels explode unexpectedly, or pipelines which explode due to stress, corrosion and cracking,” he concludes. “These things need not happen, with simple changes to eliminate bifilms we would live in a happier, more efficient world. The whole business of bifilms in metals is affecting our lives all the time, not only because of the huge inefficiencies of working with metals, which are less good than they could be, but some of them are so bad as to be dangerous, and none of them need be. All of them could be better.”
