Keywords:

nickel-titanium, laser welding, digital image correlation, mechanical properties, pseudoelastic

Abstract:

Pseudoelastic Nickel-Titanium shape memory alloys are mainly used in medical applications. The joining of similar shape memory components is often produced by laser welding. Due to severe temperature effects during the welding process the microstructure and the chemical composition may change significantly, bringing about different mechanical properties in the fusion zone, the heat affected zone and the base material. The comparison of the stress-strain curve of the non-welded material and the welded sample shows a significant change in the "elastic region "as well as a shift of the transformation peak to higher strain. After the loading and unloading the residual stresses are much higher than compared to the base materials. In a bend and free recovery test the change to a partial pseudo-plastic behaviour was proven. The results of the bend and free recovery tests were quantified by thermal analyses as well as by digital correlation. The stress-strain-temperature curve shows a change from a mainly pseudelastic to a mixture of pseudoelastic/pseudoplatic behaviour of the welded sample. These results are discussed on the base of metallographic investigations. 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 and EDX. In addition the transition temperatures of the fusion zone were studied by means of DSC-measurement.