Nickel-Titanium Shape Memory

Nitinol is a nickel-titanium shape memory alloy that remembers its original shape through a change between martensite and austenite. The crux when welding is the heat input: if you heat the crystal structure too much, you lose the shape memory effect and the material becomes brittle. So you have to keep the heat as low as possible and control the cooling, otherwise you'll get cracks or the part will warp uncontrollably. The transition temperatures can shift, and if you don't get it right, the component is scrap afterwards.