This paper discusses the effect of inclusions on internal fracture in skew rolling. The prediction of internal fractures is crucial given that internal fractures cause loss of product strength and are difficult to detect. Our previous report, published in 2019, proposed a new ductile fracture criterion that considers the effect of shear stress on void coalescence in a cylindrical coordinate system. This paper covers the following three points: (1) the evaluation of the hot workability of materials with different inclusions in tensile testing, (2) the relationship between the hot workability and the damage value determined using the proposed criterion, and (3) the effect of inclusions on void evolution in skew rolling. Given that the critical damage value decreased with decreasing hot workability, internal fractures are more likely to occur in a material that contains numerous large inclusions. The experimental results led to the hypotheses that the plastic strain nucleates the voids originating from inclusions and that shear stress allows the voids to coalesce. This study has demonstrated that decreasing the diameter and number density of inclusions is effective toward suppressing internal fractures.

A high-power pulsed microchip laser, which has a pulse duration of sub-nanosecond order, had been developed. A focused microchip laser pulse can induce an effective shock wave for deforming the irradiated metal surface plastically. When a sheet metal surface is scanned by the laser, dieless sheet forming, called laser peen forming, is achieved through the accumulation of such plastic deformations. The authors have applied the method to sheet metal bending. A tamping layer, such as water, on a target surface encourages laser-induced shock waves. Therefore, the sheet metals were irradiated in water. Several materials were bent, and the feasibility of the process was confirmed. Fundamental deformation properties in terms of forming parameters, such as defocusing, pulse energy, material hardness, and thickness, were examined. It was confirmed that, qualitatively, the process had similar deformation modes to those obtained by peen forming. However, when fluence was greatly increased by focusing or increasing the pulse energy, laser absorption by water reduced the bending deformation.