PSI - Issue 13

Daisuke Sasaki et al. / Procedia Structural Integrity 13 (2018) 1006–1009 Author name / Structural Integrity Procedia 00 (2018) 000–000

1007

2

which can reduce weight and working time. On the other hand, steel sheets for automobiles are pickled in sulfuric acid or hydrochloric acid to remove rust on a surface. During the pickling, hydrogen penetrates into the material, causing hydrogen embrittlement which degrades the mechanical properties of used materials. Hydrogen embrittlement is promoted by high stress concentration. Mechanical clinching causes work hardening and makes stress concentration part. It indicates that mechanical clinching increases the susceptibility to hydrogen embrittlement. However, there are no reports about the influence of hydrogen which causes delay fracture and hydrogen embrittlement. In this study, to investigate the influence of hydrogen on crack formation during mechanical clinching, we conducted mechanical clinching after pickling a steel sheet. Materials were A5052-H34 and SPCC270. The plate thicknesses of A5052-H34 and SPCC270 were 1.5 mm and 1.6 mm, respectively. Specimen width and length were 30 mm and 100 mm, respectively. A joining method was a mechanical clinching. The die diameter and depth were 9.3 mm and 1.9 mm, respectively. The punch diameter was 5.2 mm. The joining load was 30 kN. Before clinching test, SPCC270 was charged with a solution consisting of 3 [g / l] NH 4 SCN and 3%NaCl. The current density was 19.6 A / m 2 , 19.6 x 10 − 4 A / m 2 . A counter electrode was a platinum wire. The test temperature is room temperature. The immersion time was determined based on a hydrogen di ff usion rate at 300 K. The hydrogen di ff usion rate was D Fe = 1.27 x 10 − 8 m 2 / s. Based on the di ff usion rate, the di ff usion distance is ( D Fe T ) 1 / 2 ( T : di ff usion time). At approximately 3.61 s, the di ff usion distance is 1.61 mm. Cathode charging time was 5 hours. After the cathode charging, the clinching test was finished within 10 minutes. A cross head speed was 1 mm / min. 2. Experimental Method

Table 1. Chemical composition of SPCC270. C Si

Mn

P

S

0.04

0.01

0.19

0.15

0.11

3. Result and Discussion

3.1. Joining Test Result and Cross Sectional Observation

The load-displacement diagram of the clinching tests is shown in Fig.1. At both specimens without charging and with 19.6 x 10 − 4 A / m 2 charging, the load increases as the cross head displacement increases. At specimen with 19.6 A / m 2 charging, the load once falls at displacement of 2.6 mm and increases as the cross head displacement increases. The cross section of the joint is shown in Fig.2. Figure 2 (a) shows the definition of a minimum thickness ∆ t and a interlock ∆ x . These parameter are important to evaluate a joining strength and reliability. As shown in Fig.2 (b), a minimum thickness ∆ t and a interlock ∆ x of the specimen without charging are 0.37 mm and 0.17 mm, respectively. As shown in Fig.2 (c), at the specimen with 19.6 x 10 − 4 A / m 2 charging, the minimum plate thickness ∆ t and the interlock ∆ x are 0.41 mm and 0.20 mm, respectively. At both specimens, cracks are not confirmed near minimum plate thickness parts and the interlocks form. At the specimen with 19.6 x 10 A / m 2 charging, cracks form and the joining part of SPCC270 fractures. According the load-displacement diagram and the cross sectional diagram, cracks form on the joining part at load drop point in Fig.1. It suggests that when the current density is large, hydrogen in the SPCC270 promotes the crack formation and makes joining di ffi cult.

3.2. Fractrography

Fracture surface analysis was performed using an optical microscope (OM) and an electron microscope (SEM). The OM observation result is shown in Fig.3. As shown in Fig.3 (a), it was observed that the specimen with 19.6 x