This paper describes the difficult machinability of nickel titanium alloy (NiTi alloy) and its mechanism. As a result of examining the difficult cutting machinability via a turning experiment, NiTi alloy cutting showed larger cutting force, higher cutting temperature, and severe tool wear with plastic deformation of the tool compared to Ti-6Al-4V. In addition, the discharged chips were tangled with the jaw chuck and the cutting tool. As a result of investigating the cause of these difficult machining properties by orthogonal cutting, it was found that the progression of severe flank wear is affected by the elastic recovery due to the super elasticity of the material. The verification of the results according to the shear plane theory suggest that the large deformation resistance of the material is the cause of the increase in cutting temperature. Furthermore, because the cutting temperature exceeds the shape memory transformation temperature, the generated chips are shape memory processed. It was also found that because the generated chips are super elastic, chips are not easily broken and they are lengthened, and are easily entangled with a cutting tool and a jaw chuck.

Friction stir welding is a relatively new welding process that provides advantages over conventional fusion welding processes including the possibility of joining non-fusion-weldable alloys, reducing distortion, and improving their mechanical properties. In the present study, the effects of the tool pin surface geometry on heat transfer and material flow during friction stir welding were investigated. Tapered cylindrical tool pins made of H13 hot work tool steel with and without grooves were used in experiments to join AA6061 aluminum alloy plates at a tool rotation speed of 1800 rpm and transverse speed of 50 mm/min. The changes in workpiece temperature during the process were recorded using K-type thermocouples embedded at four locations. From the experiments, it was found that welds made with the grooved tool pin exhibited better mechanical properties such as tensile strength and microhardness owing to enhanced material flow. It was also observed that the grooved tool pin generated less heat; this could be the reason for the superior properties of the welds it produced during joining processes. From the experiments, it was found that welds s formed by the tool pin with grooves showed better mechanical properties than those formed without grooves, owing to the enhanced material flow. Moreover, the welding strength was improved by using the tool pin with grooves. It was also observed that the tool pin without grooves generated more heat than that with grooves, which could be the reason for the inferior properties of the welds produced using the grooved pin.

An optimal surface topography for the use of an environmentally friendly lubricant was elucidated, by making a wide combination of surface roughnesses Ra and RSm representing height and frequency of surface asperity, using the wet blast treatment. A forward conical can - backward straight can extruded friction test method proposed by the present authors was used. Coulomb friction coefficient was evaluated using a calibration curve representing the relationship between the surface friction coefficient of the conical punch and the front/back extrusion height of the workpiece. It was found that the surface topography of workpiece largely affects the lubricating property when the friction coefficient is large. Especially, the surface treatment with larger Ra and smaller RSm is effective for improving the friction. Furthermore, the correlation between the friction coefficient and the proposed parameter for the surface topography Ra/RSm was shown to be better than the Ra.