Surface Modification of Magnesium AZ31 by Using Scanning Cyclic Press and the Improvement Effect on Fatigue Properties

Abstract

A new surface modification technique, scanning cyclic press (SCP), was developed. SCP scans a metal surface with a vibrating indenter under precise loading control based on servo fatigue testing machine and can apply a variable cyclically compressive load. This study applied SCP to magnesium alloy AZ31 to investigate the effect on fatigue properties. After applying SCP, the surfaces of specimens were observed by using a laser scanning microscope and uniaxial push-pull fatigue tests were conducted. As a result, surface roughness of SCP-treated specimens slightly increased; however, the fatigue life became longer than that of untreated specimens. To clarify the reason for the improvement effect, fracture surfaces of the specimens were investigated in detail on the basis of SEM observation. SEM observation showed differences between the fracture surfaces of the untreated and SCP-treated specimens. The origin of fracture was at the surface in the untreated specimen. In the SCP-treated specimen, however, the fracture origins were sub-surface and a band-like layer just beneath the surface was observed on fracture surface, whereas the layer did not exist on that of untreated specimen. The surface hardness of SCP-treated specimen increased to almost twice as much after applying SCP. The result suggests that SCP modified surface layer of specimen and this layer suppressed crack initiation from surface.

Surface modification of machine-finished magnesium alloy AZ31 using a scanning cyclic press

“…This indicated that the fatigue strength of the surface fracture of machine-finished AZ31 was around this stress level. In mirror-polished specimens, the fracture origin located at the surface except the specimen unbroken at 80 MPa (Fujimura et al, 2017). It showed that the fatigue strength of the surface fracture of the machine-finished specimen was higher than that of the mirror-polished specimen.…”

“…This improvement effect is the result of SCP modifying the surface layer of the machined specimen. In our previous study, we applied SCP to the surface of a mirror-polished specimen (Fujimura et al, 2017). The microstructure observation at the cross-section of the SCP-treated specimen showed that the mesh-patterned region, which was finer than the matrix (grain size: 45 µm) as shown in Fig.…”

“…Because the specimen rotates during the indenter contacts the surface, a streaked pattern is formed by slightly scraping the surface, and the abrasion powder is also generated. Due to the SCP condition of this research, the displacement of the indenter during the first cycle was 2.1 µm in the rotation direction and 0.05 µm in the axial direction (Fujimura et al, 2017). Therefore, the ellipse-shaped indentation formed during the second cycle widely overlapped that of the first cycle.…”

“…Figure 4 shows the S-N plots of the machine-finished specimens with and without SCP. In the figure, the fatigue test results of mirror-polished specimens and its SCP-treated specimens (Fujimura et al, 2017) are also shown together for comparison. Focusing on the surface finish of untreated specimens, the fatigue life of the machine-finished specimen (white diamonds) was longer than that of the mirror-polished specimen (white circles) except the stress level of 150 MPa.…”

“…Unlike S 2 PD, SCP uses a precise loading control based on servo fatigue testing machines. Our previous study showed that SCP modifies the surface Fujimura, Ooga, Takahashi, Nakamura and Wajima, Mechanical Engineering Journal, Vol.7, No.4 (2020) [DOI: 10.1299/mej.20-00007] layer of a mirror-polished specimen of an extruded magnesium alloy, AZ31, and improves its fatigue properties (Fujimura et al, 2017); however, the general surface finish of actual machine parts is not mirror-polished but machined, and a work-hardened layer is usually formed at the machined surface (Hikiji et al, 2004;Kakiuchi and Uematsu, 2019). To investigate whether SCP improves the fatigue properties of AZ31 regardless of the surface finish, SCP was applied to machined specimens.…”