PSI - Issue 42

Lukas Lücker et al. / Procedia Structural Integrity 42 (2022) 368–373 Lukas Lücker/ Structural Integrity Procedia 00 (2019) 000 – 000

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Table 1. Chemical composition of the investigated AISI 5115 steel according to manufacturer test certificate, all data in wt%. Material C Si Mn P S Cu Sn Al Mo Ni Cr Fe AISI 5115 0.15 0.17 1.17 0.01 0.02 0.15 0.01 0.03 0.04 0.13 1.03 bal.

An almost constant strain on the centerline at the same extrusion strain is created by the forward rod extrusion forming process. Different stress states are caused by varying the shoulder opening angle, which lead to different ductile pre-damage levels. In previous studies this has been quantified in SEM, Hering et al. (2020). Also, the variation of shoulder opening angle has no detectable influence on the strain in the area around the central axis. Consequently, a comparable texture and grain size can be expected. It can be assumed that the influence of residual stress can be neglected due to preparation of specimens and the reduction during the ejection process. Especially for shoulder opening angles of 2 α = 30° and 2 α = 90° and a constant extrusion strain φ = 0.5 it has been shown in Samfaß et al. (2020) that the difference in ductile damage is particularly noticeable. Therefore, in this study forward rod extrusion formed components with the two given forming parameters were compared, Fig. 1. In addition to the shoulder opening angles, the qualitative degree of damage A Void based on a pore area on the centerline is mentioned. These values were examined in SEM analyses over representative areas of A = 1.29 mm². A Void = 302 µm 2 was determined for 2 α = 90° and 76 µm 2 for 2 α = 30°, in Hering et al. 2020, which equals a ratio A void /A = 0. 059‰ for 2  = 30° and A void /A = 0. 234‰ for 2  = 90°. Round specimens with the

Fig. 1. Forward rod extrusion components with shoulder opening angles of 30° and 90° and qualitative degree of damage.

dimensions in Fig. 2 were turned from the components.

2.2. Direct current potential drop measurements For direct current potential drop (DCPD) measurements a newly developed experimental setup was used, allowing sensitive reproducible measurements due to constant contact pressure and constant measuring point distance. The position of contact points of electrical current input (red) and electrical voltage measurement (blue) are shown besides the specimen dimensions in Fig. 2. The electric direct current was applied by a Keithley 2602B system source meter and the electric voltage was measured by a Keithley 2182A nanovoltmeter. The conjunction of both systems enables the so-called “ delta mode ” . In delta mode, the current source applies a highly constant current for a defined time. Thereafter the signal is polarity reversed and then triggers the nanovoltmeter reading at each polarity. As a result, any thermoelectrical offsets, caused e.g. by contacting, can be avoided. Data acquisition and control of the systems were performed by a software specifically programmed for this experimental setup and measurement application. Within DCPD measurements, high currents are known to improve the

Fig. 2. Measurement contacting points for current input and voltage tap in high-resolution DCPD measurements.

data accuracy but simultaneously promote specimen heating. Thus, a compromise has to be chosen for reliable measurements. All electric resistance measurements were applied with a current of I = 0.8 A. This value was defined after a statistical test design in which all important adjustable measurement parameters were optimized so that the measurement scatter was minimized. By doing so, the measurement scattering was reduced by a factor of more than 100 compared to initial measurements.