Mokume Mechanics Illustrated

[Chinobi]

Hey all, thought I would share a partial failure that illustrates some of the mechanics involved in forging mokume billets to provide some more visual reference to the concepts that get slung around as if they are everyday things.

 

I started this billet with 4 English 20 pence coins (they are 84% copper and 16% nickel, unlike the other English 'silver color' coins, which are 75% copper and 25% nickel) sandwiching 3 English 2 pence coins (bronze: 97% copper, 2.5% zinc, 0.5% tin per royal mint spec's, they switched to copper plated steel in late 1992 so consider yourself warned)

 

These were all heavily circulated and pretty far from clean, which I normally avoid or clean, but it was a billet of opportunity so I just went ahead with it.  the stack was bolted together in my torque plates which can be seen elsewhere, no flux or containment box, just some whiteout on the plates as an antibond.  it is also worth mentioning that the coins in question are NOT the same diameter!  the bronze 2P is something like 1/32 or so larger than the nickel 20P.

 

That said, the first thing to point out could be the result of two things:

one, the nickel alloy is significantly stiffer than the bronze, when the billet was compressed I suspect that enough bronze was extruded to cause expansion of the outside circumference of the coins which generated enough tension to cause the bronze to crack, as shown.

two, could just be a property of the bronze alloy being brittle and crumbly at that high a temperature

ultimately I think I am glad that I had the softer metal as a wider disk than the stiffer, because if it was reversed that could lead to a very deep cold shut where the bronze would be pressed flat against the nickel without any bonding.

 

the second and third thing reference the same pic

this is after grinding everything down to get rid of the excess copper and some minor bond failures at the very edge, the HUGE delamination was failed from the first heat as I was able to later peel that layer almost entirely off and only about a quarter of the width of the stack was actually bonded, which I was able to chisel off.

the two things to look at are again the visible extrusion of the bronze from between the nickel layers, so when working metals of highly dissimilar stiffness you should plan for much greater losses in the softer metal when you have to grind the edges, and start with a thinner gauge of the stiffer metal, which will allow it to deform a little more readily and move with the softer metal a little more, but also it will resist thinning more than the softer metal, so when the billet has been worked down the widths of the two layers will be closer to equal, as the thicker layer of soft metal has been squished down to be closer to the thinner layer of stiff metal.

the second thing to look for is something that Ian Ferguson in his book points out as 'unconfined compression stress', which, while an accurate description of the physics is kind of a brain teaser in result.  the source of the problem is indeed unconfined compression stress, but this actually is generating tension in the extreme edge of the billet, pulling the layers apart and can cause bond failure.  the edges of that billet were ground totally flat, more or less perpendicular to the two faces before the next heats and forgings.  through the forging the height of the stack was reduced, which by necessity means that the width must increase, however the top and bottom are locked in place at the moment of impact by friction to the faces of the hammer and anvil and are restrained from expanding at the very face.  the sides of the stack are unconfined, so when they are compressed they bulge out in the direction of least resistance.  this causes the previously vertical face to bend (look past the extruded bronze, though those layers demonstrate the same phenomenon in their shape as well), this can be most readily seen in the outermost layers, which are now laid back at almost 35-40 degrees from vertical.  this bending causes tension to develop along the surface of the edge because the length has increased, as the curved edge is now longer than the straight edge, and that tension can be sufficient to break even solid bonds between layers.

moral of the story being to watch your edges when you forge mokume, don't let them bend out too far, and if the billet is large enough you can grind a slight concavity into it to extend the amount of forging time between grindings, as the edge then has to extrude to vertical before it can progress to bulging out and become problematic.

 

I hope this will be helpful to anybody that is getting their feet wet with mokume, I found the concepts quite interesting and when I shot pics of this one in progress I figured it would be beneficial to put pictures to the words as the visuals were pretty extreme in this case.