Effect of Shot Peening on Surface Characteristics and Fatigue Properties of T5-Treated ZK60 Alloy

Abstract

The effects of shot peening on surface characteristics and high cycle fatigue (HCF) performance of T5-treated high-strength magnesium alloy ZK60 (named after ZK60-T5) were investigated. The glass bead with an average diameter of 0.35 mm was adopted for shot peening and the Almen intensity was arranged from 0.02 to 0.40 mmN. The surface microstructure and texture of ZK60-T5 are greatly changed by shot peening, and residual compressive stress is produced in the surface deformation layer. The magnesium alloy ZK60-T5 shows a pronounced overpeening effect. A marked improvement in fatigue life is obtained at low Almen intensities, namely the fatigue strength (at 10 7 cycles) increases from 150 to 195 MPa at the optimum Almen intensity of 0.05 mmN. The fatigue crack nucleation site of ZK60-T5 is also found from surface regions to the subsurface.

Surface Characteristics and High Cycle Fatigue Performance of Shot Peened Magnesium Alloy ZK60

“…Similarly, the fracture surfaces of the peened specimens can also be subdivided into three stages (see Figures 10(c) and 11(c)). The characteristics of Regions 2 and 3 in the peened specimens were similar to those of the EP specimens and so are not repeated here [16,18].…”

“…Fatigue tests were performed under a rotating beam loading (R = −1) at a frequency of 100 Hz in air. The surface properties of the peened specimens were determined by roughness measurements using profilometry, measurements of the microhardness/depth profiles, and residual stress measurements by means of a successive surface layer removal from the top of the peened specimen [18]. The microstructures of EP and peened specimens were observed by optical microscope (OM) and JEM-2010 (TEM, JEOL, Tokyo, Japan) transmission electron microscope.…”

“…One can see that the fatigue life depends on the Almen intensity at all tested stress amplitudes, especially at the low stress amplitude; the dependence is more evident, that is, the fatigue life first dramatically increased with the increase in the Almen intensity, then decreased drastically as the Almen intensity further increased. Obviously, the significant increase in surface defects such as microcracks and surface spalls at higher Almen intensities outweigh the beneficial effects produced by work hardening and compressive residual stresses [18]. The sensitive response of the magnesium alloys to Almen intensity can be explained by the limit deformability of magnesium alloys at room temperature.…”

Effect of Fine Particle Shot Peening on Very High Cycle Fatigue Properties of Extruded Magnesium Alloys

“…き裂の発生と進展が抑制されるために，Mg-Al 系合金 AZ31，AZ80 (29), (30) や Mg-Zn-Zr 系合金 ZK60 (31) の疲労強度が 向上するとする報告がある．一方，SP 処理によって生じた試験片表面の粗さの増加によって AZ61 展伸材の疲労 強度は大幅に低下し，その疲労限度（10 7 回疲労強度）は未処理材のそれに比べて 30 %低下するとする実験結果 (32) や， 鋳造 Mg 合金 A8 では表面粗さの影響を受けず疲労強度は改善されるとする実験結果 (33) が報告されている． 表面粗さを軽減させるために粒子径 20～30 μm のセラミックス粒子を用いた微粒子ショットピーニング，マイク ロピーニング処理やバニシ処理を行うと疲労強度は向上するとする研究結果も報告されている (32), (34), (35) ．また，鋳 造材および展伸材ともに疲労強度の向上はアークハイト値（Almen intensity）に大きく依存し，適切な SP 処理条 件の選定が必要であるとする指摘がある (36) ．これまでの研究結果から，Mg 合金展伸材の疲労強度特性に及ぼす (28) ．表より明らかなように，ISLUT から得られ…”

“…片との距離を 100 mm とし， 試験片を 10 rpm で回転し 3 周照射した． なお， 疲労強度はアークハイト値に依存し， Mg-10Gd-3Y 合金では 0.04～0.2 mmN (35) ，ZK60-T5 合金では 0.05～0.1 mmN (31) 程度が最適な処理条件であること， …”