Grain Size Effect in the Micro-V-Bending Process of Thin Metal Sheets

Abstract

The aim of this study is to develop a micro-V-bending testing system and to investigate the grain size effect of 99.5% Ferrum sheet plate in the micro-V-bending process. In this study, two experiments have been conducted. They attempt to explain the characteristic of thickness variation at a constant average grain size of 29.93 m with a standard deviation (SD) of 2.4 and the different average grain sizes at a constant mean thickness of 490.63 m with a SD of 11.3. A laser displacement sensor is employed to monitor the real position of punch movement and to verify the effective stroke. In the micro-V-bending test, the thickness/average grain size (T/D) ratio is used to discuss material behavior such as punch force, springback, and spring-forward. According to the results, the conventional concept of V-bending deformation cannot be applied to different average grain sizes at a constant thickness in the micro-V-bending test particularly when the T/D ratio is less than 2. IntroductionMiniaturization is a trend in product development [1,2]. This trend encourages researchers to verify the feasibility of applying conventional macroscale analysis methods to microscale metal forming process. To date, many studies have focused on first-order size effects which can be accounted for by using conventional models. Studies have also explored second-order size effects that cannot be accounted for using conventional models. In the studies of first-order size effects, the influencing factors include grain size effect, grain boundary effect, free surface effect, grain statistics effect, and processing induced size effect [3,4]. For example, the contribution of frictional force to total force is significant when miniaturizing the deep drawing process. In a study on second-order size effects, all important process dimensions were scaled down due to the observance of geometric similarity. A length scale was designated corresponding to sheet thickness between 1 and 0.1 mm in the tensile test. As a result, the occurrence of second-order size effects was found to be expressed by the decrease in flow stress, vertical strain anisotropy, and ductility [5].It has been demonstrated that grain size effect influences deformability in tensile and air bending testing [6]. Tensile tests have also shown that yield strength decreases with the decrease of specimen thickness at a constant grain size. The material used was Al 2S. However, the experimental results of bending show that yield strength increases with decreasing grain size. The studies mentioned above show the influence of grain size effect on the constitutive behavior

Influence of size effects on material properties and springback behavior of metal foils in micro bending: A review

“…This indicates that the grain size effect obviously affects the flow stress of this kind of material. In addition, some prior researches also verified the grain size effect in the copper [25], C2680 brass [23], CuZn37 brass [19], Fe [26], et al…”

“…Chen and Jiang [26] adopted T/D ratio to investigate the springback behavior in micro V-bending test and found that for the constant grain size, the springback angle decreases with an increase in T/D ratio. For the constant thickness, nevertheless, the springforward occurs when T/D ratio is less than 2, as shown in Fig.…”

Preliminary Investigation of Micro-V-Bending

“…Besides, an increasing of elastic anisotropy of surface grains with the reduction of grains along the thickness direction will cause the springback angle scattering [4,5]. Chen et al [6] discovered that yield strength and maximum punch force decrease with a decrease of t/d ratio. It was also found that when the t/d ratio is smaller than 2, spring-forward occurs.…”

Effect of Grain Size on Springback and System Energy in Micro V-Bending with Phosphor Bronze Foil

“…Previous studies [11,16,18] have proved that the ratio T/D is a wide-applied indicator evaluation parameter for studying the relationship between the the grain size effect and springback. However, it is also found that there is a lack of research on grain size effect when the ratio is around 1 (greater than 1, less than 1 and equal to 1).…”

“…Chen et al [16] conducted micro V-bending with different T/D ratios' ferrum specimens and discussed the relationship between the T/D and bending behaviours. Similarly, Wang et al [17] investigated the grain size effect on springback in micro U-bending.…”