Papers by Author: Yuji Kobayashi

Abstract: Nowadays, one of the most crucial focus in the aluminium-foundry sector is the production of high-quality castings. Mainly, High-Pressure Die Casting (HPDC) is broadly adopted, since by this process is possible to realize aluminium castings with thin walls and high specific mechanical properties. On the other hand, this casting process may cause tensile states into the castings, namely residual stresses. Residual stresses may strongly affect the life of the product causing premature failure of the casting. Various methods can assess these tensile states, but the non-destructive X-Ray method is the most commonly adopted. Namely, in this work, the residual stress analysis has been performed through Sinto-Pulstec μ-X360s. Detailed measurements have been done on powertrain components realized in aluminium alloy EN AC 46000 through HPDC processes to understand and prevent dangerous residual stress state into the aluminium castings. Furthermore, a comparison with stresses induced by Rheocasting processes is underway. In fact, it is well known that Semi-Solid metal forming combines the advantages of casting and forging, solving safety and environmental problems and possibly even the residual stress state can be positively affected.

Abstract: In general, shot peening is a surface treatment that improves the performance of engineering components, since application of surface compressive stresses reduces the tensile component of stress. In the shot peening process, the medium consists of small spheres, which are usually made of high-carbon cast steel; the diameter of the spheres is in the range from 0.3 to 1.2 mm. More recently, a new type of microshot has been developed to enhance the peening effect. The diameter of the new spheres is in the range from 0.02 to 0.15 mm. The effect of microshot peening on the residual stress of spring steel was investigated. The projective method of the microshot was of the compressed air type. The microshot of 0.1 mm diameter was high-carbon cast steel and cemented carbide, and the workpiece used was the commercially spring steel JIS-SUP10. The effect that process variables such as shot speed and peening time have primarily on residual stress was studied. The surface layer of the workpieces was sufficiently deformed by microshot peening. The residual stress was observed near the surface. At a large number of cycles to fracture, microshot peening can more effectively enhance the fatigue strength. The use of hard microshots such as cemented carbide was found to cause a significantly enhanced peening effect for spring steel.

Abstract: The butt joining of dissimilar sheets using a shot peening process was investigated. Shot peening is a surface treatment and improves the performance of engineering components. In shot peening, the substrate undergoes a large plastic deformation near its surface due to a hit with many shots. Thus, plastic flow characterized by a shear droop occurs at the edge of the substrate due to shot peening. When the dissimilar sheets with the edge of the notch geometry are connected without level difference and shot-peened the connection, the sheets can be joined due to the plastic flow generated by the large plastic deformation during shot peening. In the experiment, a compressed-air-type shot peening machine was employed. The influences of processing conditions on the joining of the dissimilar sheets were examined. The joint strength increased with the kinetic energy of shots. It was found that the present method using shot peening process was effective in joining dissimilar sheets.