Influence of pre-drilling on hardness and tensile failure of formed internal threads in thin-walled AZ91 cast alloys

Abstract This study investigates the influence of pre-drilling diameter on hardness and tensile failure of the formed internal threads (M6), manufactured perpendicular to the profile thickness of thin-walled AZ91 magnesium casting alloys. Previous investigations have shown that providing a pre-drilling before friction drilling prevents the failure of the bore’s outer wall during the friction drilling process. Considering the oversizing of core bore (friction drilled bore) as an effective factor that can contribute to an incomplete formation of the threads profile through thread forming process, the influence of the pre-drilling diameter was investigated on the hardness and tensile failure of the formed M6 threads. Computed tomography of the friction drilled bores indicated an increase in oversizing of the friction drilled bore with increasing the pre-drilling diameter. Metallographic investigations of the manufactured samples by friction drilling and thread forming process revealed a rhombus-shaped deformation region around the manufactured thread, which provides an enhanced hardness especially at the subsurface of threads profile. The resulting hardness was decreased insignificantly in thickness direction with an increase in pre-drilling diameter. The failure mode of the M6 threads was analysed by means of quasistatic tensile tests. Due to incomplete formation of the threads profile, variation of the pre-drilling diameter did not declare a concrete trend in measured tensile strengths. Fractographic failure analysis by scanning electron microscopy on threads before and after tensile tests showed that the cold-formed threads were mostly sheared off in load direction (thread stripping) by tensile tests implying an internal thread failure caused by shear stresses because of the incomplete formed threads profile, especially along the cross section corresponding profile thickness direction.