Effect of friction stir forming parameters on mechanical properties and plastic flow of material in the mechanical interlocking of optical fiber and SP-700 superplastic titanium alloy

Abstract. The present study discusses the fluidity of titanium alloy during friction stir forming (FSF) and investigates the effect of process parameters on the mechanical properties of titanium alloy in the mechanical interlocking of optical fiber and SP-700 superplastic titanium alloy. In this study, a guide slit is provided in a titanium alloy plate, an optical fiber is placed inside the slit, and FSF is performed on the surface of the titanium alloy. By performing FSF, titanium softens and material plastically deforms inside the slit, which results in mechanically joining optical fiber and titanium alloy. Experimental results of evaluating the mechanical properties after FSF revealed that the hardness value of the material that flowed into the slit was significantly higher than that of the base material, which confirms that the stirring zone has flowed into the slit around the fiber. It was also confirmed that the temperature becomes unstable during the process for a travel speed of 50 mm/min or less, also above 800 mm/min. This resulted in insufficient material flow. As a result, inhomogeneous structures were confirmed. Tensile test results after FSF showed that the strength was lower than that of the base metal (67% to 55% of the base metal) for most process parameters. Although no significant changes in strength were observed by changing the travel speed, it was confirmed that the strength has an increasing tendency as the rotation speed increases, which is considered to be related to the grain refinement of the material. It was also concluded that the limitations in the present study such as insufficient material flow and reduced strength and also damage to the embedded optical fiber can be improved by pre-heating the workpiece and controlling the FSF process parameters which require further experiments and temperature measurements during FSF. In this study, the developed composite can give us hope for embedding FBG sensors inside high melting point alloys to simultaneously measure deformation, strain, pressure, and temperature and create new functional smart materials.