Lightweight cellular solids have been used as an energy-absorbing material for automotive and aerospace applications. Additive manufacturing is a useful process for the design and manufacture of cellular solids. The present study focuses on the relationship between the energy-absorbing property and the cell structure of additively manufactured porous aluminum alloys. Disordered cell structures with different regularity are designed by the technique of 3D-Voronoi division. Open-cell structures with a porosity of 90% and a strut diameter of 1 mm are generated using commercial 3D CAD software. Explicit FEM analysis including a Johnson-Cook failure model successfully simulates the compressive stress-strain curves of disordered porous aluminum alloys. Experimental results of compression tests were in good qualitative agreement with the FEM analysis. The ordered cell structure with high regularity is disadvantageous for energy-absorbing applications because of the formation of a macroscopic shear band. A slightly low cell regularity in a additively manufactured cellular solid is effective for increasing energy absorption.
A new method of rotary plate forging with spiral grooved tools was proposed for the width expansion of a plate. This process consists of two stages: (1) plate width expansion with a pair of spiral grooved tools and (2) surface finishing rolling with a pair of camel-crown rolls. We performed finite element analysis to investigate the effect of tool geometry on the width expansion ratio, strain distribution in the plate and forming load. Compared with conventional rolling with straight cylindrical rolls, the effect of width expansion was observed in the proposed process with the spiral grooved tools. Four process conditions were examined to confirm the efficacy of the method; the pitch of the convex parts of the spiral grooved tools, process number and back tension in the traveling direction were changed in the analysis. From the results, it was found that the width expansion ratio was larger than that of the conventional rolling process. The back tension decreased the forming load and the magnitude of induced strain was generally larger than that of conventional rolling.
The unmelted regions in a material made by additive manufacturing degrade its mechanical properties. First, the effect of processing conditions in additive manufacturing with selective laser melting (SLM) on the mechanical properties of maraging steel was investigated under constant productivity, keeping the volume energy density constant. Various combinations of hatching pitch and scan rate of the laser were considered. Four blocks were fabricated under different processing conditions, and micro specimens cut out from the blocks were used in tensile tests. The microstructure of the blocks was observed and the void area fraction was obtained. The fractured equivalent strain was the largest in the case of the lowest void area fraction. The effect of the void area fraction on the ductility was confirmed. Second, single bead experiments under each processing condition were conducted, and the cross-sectional shapes of beads and molten pools were observed. These cross-sectional shapes were approximated by elliptic arcs and the conditions required for reducing unmelted defects were investigated geometrically. In this study, the molten overlap height in the middle of the neighboring scanning lines was a function of hatching pitch, and the optimal processing conditions for maximizing the molten overlap height were found.