Aleksander,

Titanium in a relatively un-alloyed state can transition from a HCP to BCC crystalline structure at high temp and low pressure. If the welding process you describe as "explosive" had pressures in excess of 3GPa, then the titanium crystals could have moved from the HCP structure to an Omega crystalline structure, a martensitic transformation, even at room temperature. This would take place through the material at or near the acoustic velocity, as the shock wave propagates through.

I have attached a link below to a very interesting paper on the subject, publicly available from the Ohio State University Physics department.

https://etd.ohiolink.edu/rws_etd/document/get/osu1070481734/inline

This paper deals mainly with changes at the atomic level, but may lead you in a direction concerning phase transition and crystalline structure that is helpful.

Changes in the phase or crystalline structure of the Titanium can produce stresses in the final weldment, in particular if the bimetal plate to which you have fused has residual stresses of another value or in a different vector.

I hope that you find this helpful,

J. Cole Zimmermann

Dear John,

thank you for your replay. But, Ti Grade 12 is a titanium grade alloyed with molybdenum (0.3%) and nickel (0.8%). I also investigated pure alfa-Ti grade (after explosive welding) and residual stresses are in tension state without phase transition. I attach micrograph of Ti grade 12 after explosive welding. Unfortunately, I do not have micrograph before welding process (I hope to get it).

Best regards,

Alek