Microstructure and properties analysis of laser-welded Ni-Ti and 316l sheets using copper interlayer

Dissimilar pulsed laser welding of 1 mm-thick sheets was carried out on Nitinol/S.S joint. In order to modify the chemical composition of weld metal and to restrict the formation of Fe-Ti intermetallic compounds (IMCs) in the weld metal, Copper thin film with different thicknesses, 100 and 150 mu m, was used as an interlayer. The effects of interlayer thickness on microstructure, microhardness profile and chemical composition of weld-metal was investigated. Investigations showed that the interlayer thickness has a great influence on the chemical composition of the weld metal and, consequently, affects all of joint properties such as microstructure and ultimate tensile strength. According to X-Ray diffraction analysis, it was revealed that the increase of interlayer thickness decreases the concentration of Fe-Ti IMCs in weld metal and this makes weld metal softer to some extent. Moreover, since the Copper is a ductile metal of low melting temperature, added Copper into weld metal can compensate thermal stresses. Therefore, cracks originated from thermal stresses as well as brittleness of weld-metal were eliminated when the thickness of interlayer increased. However, there were some restricting factors as to the increase of the thickness of Copper interlayer, namely, the formation of Ti-Cu IMCs as well as the formation of Cu-rich globules which have relatively weak bonds with their surrounding matrix and this turns their boundaries to a suitable place for nucleation and propagation of a crack. (C) 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.