Microstructural Changes of OFC according to the Processing Number of Multi-Axial Diagonal Forging (MADF)

다축대각단조(MADF) 가공횟수에 따른 OFC의 미세조직 변화

Abstract

This study investigated the effects of the processing number of multi-axial diagonal forging (MADF) on the microstructural changes of OFC fabricated by MADF processes. The as-extruded OFC was cut to $25mm^3$ cube for the MADF processes. The MADF process consists of plane forging with a thickness reduction of 30% and diagonal forging with a diagonal forging angle of $135^{\circ}$. In order to analyze the microstructural evolutions according to the number of repetitions, 1, 2, 3 and 4 cycles of the MADF process were performed. OFC specimens were successfully deformed without surface cracking for up to 4 cycles of MADF. The grain size, average misorientation and average grain orientation spread (GOS) of MADF processed materials were analyzed using EBSD technique and their Vicker's hardness were also measured. The results showed that MADF process effectively refined the microstructure of OFC with initial average grain size of $84.2{\mu}m$. The average grain sizes of specimens MADF processed for 1, 2, 3, 4 cycles were refined to be $8.5{\mu}m$, $2.2{\mu}m$, $1.5{\mu}m$, $1.1{\mu}m$, respectively. The grain refinement seemed to be saturated when OFC was MADF processed over 2 cycles. In the case of specimens subjected to two or more cycles of MADF, the degree of decrease in average grain size was drastically reduced as the number of cycles increased due to softening phenomena such as dynamic recovery or dynamic recrystallization during processing. The degree of increase in average Vicker's hardness was also dramatically reduced as the number of cycles increased due to the same reason.

Fig. 1 Schematic diagram of forging processes in Multi-Axial Diagonal Forging(MADF)

Fig. 2 Schematic diagram showing MADF process

Fig. 3 Multi-axial diagonal forged OFC specimens

Fig. 4 Photos showing the analyzed part

Fig. 5 IPF Map(Z) of initial as-extruded OFC

Fig. 6 Inverse Pole Figure Map(Z) of MADF processed OFC

Fig. 7 The average grain size of MADF processed OFC for various cycles

Fig. 8 The average Misorientation of MADF processed OFC for various cycles

Fig. 10 The average Vickers hardness of MADF processed OFC for various cycles

Fig. 9 The average GOS of MADF processed OFC for various cycles

Table 1 The average grain size of the MADF processed OFC for various cycles

Table 2 The average Vickers hardness of the MADF processed OFC for various cycles