In this study, we investigated the coating effect of a piercing punch on the properties of the pierced surface, using 1180-MPa-class ultrahigh-strength steel sheets as workpiece materials. TiN was coated on the piercing punch by physical vapor deposition for this study. The analysis clarified the following facts: i) Holes pierced using noncoated and coated punches present almost the same hole-expansion ratio, ii) the fatigue strength of the holes pierced by the coated punch is superior to those of holes pierced by the noncoated punch. The work hardening, roughness, and residual stress on the pierced surface explain the mechanisms of these results. The application of the coating little increased the work hardening which deteriorates the hole-expansion ratio in general, but this increase was too small to expose the effect on the holeexpansion ratio. On the other hand, the application of the coating clearly improved the roughness by suppressing galling between the punch and the workpiece material. This improvement probably contributes to the superior fatigue properties. The suppression of galling also compressed the residual stress on the pierced surface and contributed to the superior fatigue properties in the case of the coated punch.
This paper deals with the effect of shot peening on the bending strength of an AZ31 magnesium alloy pipe. Magnesium alloy has a wide range of application prospects in the automobile and electronic industries. AZ31 alloy is the most widely used commercial magnesium alloy. In our experiment, extruded pipes of 22 mm O.D., 18 mm I.D., and 2 mm wall thickness were used. Shot peening treatments were applied to the surface of the workpiece using an impeller-type or an air-type peening machine. Tensile and compressive tests were carried out under axial load at a crosshead speed of 10 mm/min, and bending strength tests were performed at a crosshead speed of 100 mm/min under lateral load using an Instron-type testing machine. In the bending tests, the peened workpiece could withstand higher bending yield load. The surface layer of the peened workpieces was also observed by electron backscatter diffractometry (EBSD). It was found that multiple deformation twins were formed during shot peening. The results of the present study revealed that the compressive yield stress of shot-peened pipes is strongly related to their bending strength.