Over Xmas my boat building BIL cave me an interesting piece of metal.

It's cut from a 35 mm thick by 100 mm wide bar and consists of a 19mm thick layer of MS bonded to a 16 mm thick layer of Al.
They also use a similar bar that is 150 mm wide bar as well
It's used quite often to join (weld) structure is steel to Al structures on boats as there is no galvanic corrosion involved

When I polish it up I can see a secondary layer at the interface of Al and MS.

If it was a result of the manufacturing process one might think it would not be a clearly distinct layer but more of a graded affect.
I wonder if they have used a separate layer of Al or Al alloy as a transition material.
Here you can clearly see that layer marked as "?"
I will polish it up a bit more so I can see if I can get a clearer picture.
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