Microwave Welding: How It Happened to Me

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I put my tea in the microwave this morning then went to sit on the sofa. I heard some crackling noises, similar to the sound that metal makes when in a microwave.

I ran over, and hit stop then opened the microwave. The microwave tray was over to one side. I took the mug off but the tray didn't even move. I look and the wheels are still on the spinner.

I try to move the tray back and it doesn't budge. The tray was welded to the side of the microwave. :/

So I pried it off and drank my tea.

How did this happen? The tray is supposed to be able to resist heating by microwaves. I've seen that metal in a microwave will have electrical arcs shooting into the tray. Was this something similar? Except with such a large current through such a small contact area that the heat caused the welding?

I wonder if this effect could be re-produced using more focused microwaves.
 
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What was the tray made of?
You can certainly heat thin metal in a microwave enough to melt it.
You can also melt a lot of plastics, especially if you heat certain foods. It's possible to locally heat liquids in a microwave to higher than their boiling points so if the plastic is normally good to 120C it might reach 150C in a small area.
 
It's made of some kind of metal. I have no clue what kind though. The microwave also is an oven.

What actually causes metal so spark in microwaves? I asked my physics teacher and he said he thought it was something to do with the ionization of the air in the microwave that caused some parts of the object to become positively and negatively charged.

Usually that tray is isolated from the metal interior of the microwave by the plastic wheels that it sits on.