friction stir processing of nitinol (niti) alloy

This research investigates the feasibility of friction stir processing (fsp) for the surface treatment of nitinol (niti) alloy. after establishing a suitable working window for fsp processing of the nickel-titanium shape memory alloy based on the applied thermo-mechanical cycles undergoing the concept of elevated temperature severe plastic deformation, the pseudoelastic behavior of processed alloys was investigated. nickel-titanium alloy with an atomic ratio of 50.8: 49.2 was produced by melting in a vacuum. then, it was subjected to surface changes by friction stir processing. in this respect, the effect of changes in the main parameters of the fsp, such as tool geometry, rotational speed, as well as the traverse speed of the tool, were evaluated. the alloying of nickel-titanium, with the addition of w particles throughout the process as well as surface treatment, was also explored after optimizing the fsp's processing parameters. the results of investigations showed the sections and surfaces without visible defects within the limits of rotation speeds of 630 to 800 rpm and traverse speed of 50 mm/min. afterward, the microstructure and mechanical properties of the samples were investigated, and the results were compared with the base metal. among the most critical changes in the microstructure observed based on microscopic characterizations was the formation of a coaxial fine-grained area with an average size of 20 microns after fsp was visible in the area under the tool. phase changes in nickel-titanium alloy before and after friction stir modification were investigated by x-ray diffraction (xrd) analysis, which shows the formation of cubic austenite phases (b2) and partial monoclinic phases (b19'). the microhardness measurements of the stirring zone (sz) compared to the base metal (bm) and heat-affected zone (haz) showed a noticeable improvement in hardness from ~280 hv up to around 340 hv. as one of the leading mechanical behavior evaluations, the pseudoelastic tensile loading-unloading tests were accomplished at different strain levels on the stir zone, and the amount of elastic strain recovery and pseudoelastic percent in the samples were calculated and compared. the prepared modified samples exhibited up to 90% pseudoelasticity in the stirred region. also, the effects of heat treatment and aging post-processing at a temperature of 500°c for around two hours on the pseudoelastic behavior of base metal and stir zone was investigated, and the results indicated a significant improvement in ductility and yield stress (ys). finally, the sample with a rotational speed of 800 rpm and a traverse speed of 50 mm/min with less generation of stored dislocations displayed the best combinations of pseudoelastic properties and ductility compared to other samples.