Failure analysis of formed internal threads in flow drilled bores of AlSi10Mg cast profiles

Abstract

Manufacturing the internal thread in lightweight components using chipless flow drilling and internal thread forming processes can reduce material and time consumption. Furthermore, using these methods imposes a further work-hardening into the subsurface of the formed thread profiles and consequently improves the thread strength. While the required core hole diameter for conventional (cut) tapping depends on the thread dimension, this diameter depends either on the material properties such as ductility for the thread forming process (form tapping). This work investigates the effect of the used process parameters in flow drilling (friction drill speeds and diameter) on the quality of the drilled bores and the formed threads. A total metallographic analysis was performed according to the formed regions by the flow drilling process and was correlated to the measured hardness profile conducted along the bore thickness. The CT scan was chosen to measure the dimension deviation of the manufactured bores and their effects on the formed threads. Correlating the deviation to the quasi-static and dynamic strengths declared that decreasing the tool diameter by 0.1 mm decreased the inner volume of the friction drilled bore, which enhanced the geometry of the formed thread and increased the percentage of the thread engagement and the strengths accordingly. Although the friction drilling with an adaptive speed reduced the oversizing of the created bore in comparison to the flow-drilled bores with a constant speed, measuring the volume of the formed threads into mentioned bores indicated an opposite trend. While the formed M6 threads into the flow drilled bores with constant speed showed a better strength by subjecting to a quasi-static load, manufactured threads with an adaptive speed indicated a higher dynamic strength from the load increase fatigue test. S-N curves declared that the adapted flow drilling speed led to higher dynamic strength of the formed thread subjecting to the fatigue tests in the HCF (High Cycle Fatigue) range. The measured results from NDT monitoring by the ACPD method were correlated to plastic strain up to failure. The in-situ quasi-fatigue test in the CT scan clarified the fatigue fracture according to cyclic softening and hardening, or possible crack initiation and propagation in thread profiles during the fatigue test.