Microstructural investigations of laser welded dissimilar Nickel-Titanium-steel joints

Abstract
Nickel-titanium based shape memory alloys are used in a variety of applications. These applications desire joining techniques that provide only local degradation of the functional properties and are compatible to the special mechanical properties of shape memory materials. Laser welding of nickel-titanium based shape memory alloys is an established process in medical application. Similar and especially dissimilar joints of nickel-titanium shape memory alloys, in particular joints of nickel-titanium and steel, are requested combinations to open additional fields of applications. A detailed investigation of the microstructural changes of laser welded parts is necessary to provide a good weld quality and to consider the related changes of the mechanical properties in the design process for certain applications.
The formation of phases in the heat-affected zone as well as in the fusion zone was investigated by means of optical microscopy, scanning electron microscopy, electron backscatter diffraction and X-Ray diffraction. Focused ion beam was used for aim preparation of transmission electron microscopy samples at the interface nickel-titanium/fusion zone, the center of the fusion zone and the interface fusion zone/steels.