Why can titanium shavings from milling or machining combust when exposed to open flame in air?

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I did military engineering for some time so we worked with a lot of Titanium, but when machining it we were always told to remove scraps ASAP and bag them up because if a single spark from a welding torch got on the pile then it would go up in flames and can even explode. To demonstrate this the instructor got a pile of titanium shavings and lit them on fire. We were never instructed why this happens and googling it I could only find stuff about titanium powder in pure oxygen or the melting point of titanium.

In: Physics