镁合金板材弯曲工艺冷冲压英文文献和中文翻译(2)

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2. Experimental material and procedure

2.1. The preparation of experimental material

Commercial AZ31B magnesium alloy sheets with a thickness of 0.8 mm, cut into 1000 mm × 100 mm (length × width) pieces, were used in the experiments. Fig. 1 shows the schematic diagram of the RUB process. The radius of the cylindrical support was 1 mm and the bending angle was 90◦. The magnesium alloy sheet was bent on a cylindrical support under a constant force T with a constant speed v. There was six-pass bending, which indicated that there were six bending operations in all at two orientations in the experiment. This meant that after each bending pass, the sheet was turned over and the bending orientation was also changed in the next pass. The RUB processed sheets were annealed at 533 K for 60 min, and then were subjected to tensile tests, deep drawing, and cold stamping of cell phone housing investigation. Previous studies (Song et al., 2010; Huang et al., 2010) have investigated microstructure and texture evolution of AZ31 magnesium alloy sheets underwent RUB. For the as-received sheet, the grains were fine. After the magnesium alloy sheet underwent RUB was annealed at 260 ◦C, the grains near the surface of sheet grew obviously, while those in the central region had little growth. The average grain size of two state sheets was almost the same. Fig. 2 shows {0 0 0 2} pole figures of two state sheets. The asreceived sheets exhibit a strong basal texture, where the majority of grains are oriented with their {0 0 0 2} basal planes parallel to the rolling plane of the sheet. In contrast, the RUB processed sheets exhibit a large inclination of c-axis around the normal direction (ND) towards the RD, which weakens basal texture of the sheet.

2.2. Uniaxial tensile tests

The specimens for tensile tests had a parallel length of 57 mm, a width of 12.5 mm and a thickness of 0.8 mm. The specimens were cut along planes coinciding with at the angles of 0◦ (RD) and 45◦ and 90◦ (TD) to the rolling direction. Prior to testing, all specimens were polished by the abrasive paper to remove major scratches to avoid fracture occurring at an undesired location of the specimen. The uniaxial tensile tests were carried out on a CMT6305-300 KN testing machine with an initial strain rate of 3 × 10−2 s−1 to examine the mechanic properties such as the yield strength, the ultimate tensile strength and the fracture elongation. The strain hardening exponent values (n-value) were obtained by power law regression ( = εn) of the tensile test data within a uniform strain of ε = 15 %. The Lankford values (r-value), r = εw/εt, where the variables εw and εt denote the strains in the tensile specimens’ transverse and thickness directions, respectively, were measured on the specimens at a uniform plastic deformation of ε = 15 %.

2.3. Limiting drawing ratio (LDR) tests

To evaluate the deep drawability of the RUB processed AZ31 magnesium alloy sheet, limiting drawing ratio (LDR) tests were carried out on a 600 kN hydraulic press to examine the stamping formability at room temperature. The schematic diagram and geometry dimension of mold are shown in Fig. 3 and Table 1, respectively. Magnesium alloy sheets were processed into circular specimens with various diameter dimensions using wire-cutting.

Before deep drawing, all circular specimens should be polished by the abrasive paper in order to avoid crack in them. Special positioning ring was adopted to fix the specimens. A rigid blank holder was used on the molds, which can offer sufficient blank holder force to press the blank tightly by adjusting the spring. Consequently, the

blank holder and die were uniformly lubricated with oil. The punch was not lubricated.

2.4. Cold stamping of cell phone housing

The as-received sheets and the RUB processed AZ31 magnesium alloy sheets with a thickness of 0.6 mm were used in these experiments; three sets of stamping dies for cell phone housing manufacture were used, the blanking die, deep drawing die and piercing die. Compared with the blanking and piercing die, the structure of deep drawing die was more complex. The main parameters of deep drawing die were as follows: punch radius rp = 1 mm; die radius rd = 2 mm; die clearance in the straight wall C = 0.6 mm; die clearance in the corner C = 0.66 mm. The three sets of dies driven by the crank press completed the blanking, the deep drawing and the piercing process in turn.