PSI - Issue 21

Nathaniel Mupe et al. / Procedia Structural Integrity 21 (2019) 73–82 Mupe et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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mm were fabricated, using a micro cutter machine. The as-received material was prepared, as explained in section 3.2, for both micrographs and EBDS analysis. The average grain size of the as-received material calculated by linear intercept method that exempted twin boundaries and Inca software was 10.64 µm. Vickers microhardness test was conducted along the major and minor axis was Hv, ~57.

Table 1.AZ31 technical data sheet Chemical Composition Limits Weight % Al Zn Mn

Si

Cu

Ca

Fe

Ni

Others

Mg

Mg AZ31 3

1 0.2 min 0.05max 0.05max 0.04max 0.005max 0.005max 0.30max bal

Fig. 1. (a) Schematic illustration of NTE process; (b) NTE die twisting channel; (c) I-extruded rod, II-AZ31 after 1 pass at 373K, III-tensile specimen machining. 3. Experimental procedure 3.1. Details of NTE NTE die with effective geometry was designed to allow higher plastic strain while imposing homogenous shear strain to the material without rigid body rotation. The design aims at reducing strain reversal apart from frictional forces exerted on billet upon rotation in the twisting channel. The equivalent strain (ε eq ) introduced in the material by the NTE is determined at the three parts and is influenced by the following variable; a, r, θ and L . The equivalent strains at part I and III are equal , each having ε ~0.47. The value of equivalent strain accumulated at part II is ε ~0.15. More details on kinetics and kinematics of NTE technique are explained by Maulidi et al. (2018) and Yalçinkaya et al. (2019). In NTE, the theoretical equivalent plastic strain after 1 cycle is 1.1. The die was preheated while temperature was measured by a thermocouple connected to die through the cover set and simultaneously displayed on the heater human machine interface panel. Heat resistant lubricant was sprayed on the channel, billets covered with a heat resistant PTFE tape and then slightly greased to reduce the pressing loads as successive billets were fed. A billet was fed into the channel by pushing freely with the hand. A plunger was set directly above the billet and compression force fed using the Shimadzu autograph AG-500KN. The first billet placed inside the preheated die set was held for about 15 minutes prior to pressing [Gzyl et al. (2015)]. The successive billets were sequentially placed after 6 minutes’ interval on the surface of the heated die cover for approximately 15 minutes before pressing. The testing parameters were constant with maximum force set at 80 kN and test speed at 5 mm/min. The total stroke displacement for each billet was 30 mm. The billets were subjected to NTE for one pass each at 373 K, 473 K and 523 K. Pressing at 523 K was repeated with back pressure (BP) module placed beneath the die for comparison. Bryla et al. (2012) demonstrated that temperatures at 473 K and 523 K were effective for improving mechanical properties. Svoboda and Vago (2015) revealed that high temperatures at 623 K were favorable for improving deformation to fracture. Conducting first pass of AZ31 at room temperature, was unsuccessful due to cracks formation along the specimen.