PSI - Issue 42

Yahar Sarafraz et al. / Procedia Structural Integrity 42 (2022) 125–132 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The prior investigations were mostly carried out on the bores, which were friction drilled parallel to the profile thickness t (Heiler, 2018; Dehghan et al. , 2019). In friction drilling parallel to the profile thickness, the bushing height can be up to three times higher than the sheet thickness t, which can increase the thread length (higher number of thread turns) by possible threading process (Heiler, 2018). Innovative friction drilling perpendicular to the profile thickness (front-face drilling) was investigated in recent studies by Biermann and Liu (2014). With this method deep closed bore can be manufactured, which can provide a higher thread length by internal threading as is presented in Fig 1b. Internal thread forming as a chipless process facilitates the manufacturing of the thread profiles through plastic deformation. Forming the threads instead of machining can enhance material strength and productivity (VDI 3334) (Fig 1c and d). Through plastic deformation, work hardening is induced in the subsurface of the formed threads profile (Wittke et al. , 2018; Fernández Landeta et al. , 2015). The microhardness of the subsurface can be enhanced up to 100% in comparison to cast material (Fromentin et al. , 2005; Sarafraz et al. , 2021). In this study, M6 internal threads were manufactured into thin-walled AlSi10Mg cast profiles. The specimens were manufactured with two friction drill diameters and speeds to find the proper process parameters. Pre-study indicated that a decrease in the friction drill speed by 25% at the beginning of the process (penetration of the conical part in specimens) can decrease the oversizing of the created bore. Considering the oversizing of the pilot bore as an influencing factor in forming a complete threads profile, the dimension of the friction drilled bores, as well as the manufactured thread was measured by computed tomography (CT). Mechanical properties of manufactured M6 threads were investigated by subjecting the four initial thread turns to static and cyclic loads. The results from conducted tensile and fatigue tests were compared and correlated to the CT analysis and non-destructive testing (NDT) measurement. The measured change in deformation-induced temperature and alternating current potential drop (ACPD) by the conducted fatigue tests were correlated to plastic strain up to failure corresponding to change in contact surface by thread engagement and occurred softening, hardening, or possible cracks initiation and propagation in thread profiles. The CT scan of the fractured threads provides us with information about the occurred failure mode. 2. Material and methods 2.1. Material The aluminum casting alloy AlSi10Mg containing 9.4% Silicon was prepared from the ingot in thin-walled profiles to manufacture the M6 thread perpendicular to the profile thickness. The optical emission spectrometry (OES) of AlSi10Mg can be observed in Tab. 1 compared to DIN EN 1706.

Tab. 1. Chemical composition of the aluminum casting alloy AlSi10Mg (data in %) (Wittke, 2019)

Elements

Al

Mg

Si

Fe

Cu

Mn

Ni

Zn

Ti

DIN EN 1706 AlSi10Mg

Bal. Bal.

0.2-0.45

9-11

≤ 0.55

≤ 0.05

≤ 0.45

≤ 0.05 0.003

≤ 0.1 0.02

≤ 0.15

0.34

9.4

0.16

0.01

0.04

0.11

2.2. Manufacturing method The manufacturing of the M6 threads was carried out by Institute of Machining Technology (Felinks et al. , 2021a) on thin-walled AlSi10Mg cast profiles using a 4-axis machining center type GROB BZ 40 CS. The horizontally main spindle has a maximum rotational speed of n = 24,000 min -1 . Fig. 2 illustrates three process steps to manufacture the M6 thread in thin-walled casting Al alloy. According to previous studies a hole was pre-drilled before friction drilling to hinder the failure of the bore’s outer wall during the friction drilling process. A conventional uncoated HSS N twist drill was used. A bore was friction drilled in pre-drilled holes using a friction drill (Flowdrill-LF) with a diameter of d fd = 5.4 mm. Finally, the internal thread was formed into the friction drilled bores. The process parameters, which were used for manufacturing the M6 thread, is presented in Tab. 2. Friction drill diameter d fd and speed v f,f d were varied to investigate the their influence on quality of the formed M6 threads.