Recently, bars for office automation and automobile industry have been required to achieve high fatigue strength. Generally, tensile residual stress is generated at the surface layer of a drawn bar after the drawing process. The tensile residual stress at the surface layer reduces fatigue strength. Therefore, decreasing in tensile residual stress is required. In this study, the effects of approach geometry and bearing length on the residual stress were investigated. Next, the residual stress of the bar drawn using a new die, which had a protrusion in the bearing area, was investigated. Consequently, the effective approach geometry for decreasing tensile residual stress was determined. Tensile residual stress decreased with increasing bearing length. Additionally, it was clarified that the compressive residual stress is generated during drawing with a protrusion die.
Recently, smooth bar and wire surfaces for office automation and automobile industry have been required to achieve high fatigue strength. In a conventional process, the drawn bar surfaces are smoothed by additional processes, such as grinding and polishing. However, these processes results in a higher production cost and a lower productivity. To produce ultrasmooth bars by only drawing, a new die was developed and evaluated. The new die had protrusions with hights of 20-60 μm in the bearing area. The protrusion die induced high contact pressure and shearing strain on the surface of the drawn bar. In this research, the carbon steel and aluminum bars were drawn using the new die. As a result, surface roughness under the maximum height of irregularities 1 μm was achieved using the new designed die.
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