Titanium alpha-case formation
An undesirable, brittle oxygen- and nitrogen-rich surface layer formed during hot working or heat treatment of titanium materials above critical temperatures, which must be completely removed prior to final mechanical processing.
When you weld or heat-treat titanium and it comes into contact with oxygen or nitrogen from the air, a hard, brittle layer forms on the surface – that's called alpha-case. You absolutely have to remove it before further processing, otherwise the material underneath becomes crack-sensitive and breaks under load. You do this either mechanically by grinding or by pickling in an acid solution. It's important to keep an eye on the shielding zone during welding and to remove the layer completely, otherwise you'll have trouble later.