Tomomi Shiratori

By Tatsuhiko Aizawa, Tomomi Shiratori and Takafumi Komatsu

Austenitic stainless steel type AISI304 sheets and plates as well as fine-grained type AISI316 (FGSS316) substrates and wires were employed as a work material in the intense rolling, the piercing and the plasma nitriding. AISI304 sheet after intense rolling had textured microstructure in the rolling direction. Crystallographic state changed itself to have distorted polycrystalline state along the shearing plane by piercing, with the strain induced phase transformation. FGSS316 substrates were plasma nitrided at 623 K for 14.4 ks to have two-phase fine nanostructure with the average grain size of 100 nm as a surface layer with the thickness of 30 μm. FGSS316 wires were also plasma nitrided at the same conditions to form the nitrided surface down to the depth of 30 μm. This nitrided wire was further uniaxially loaded in tensile to attain more homogeneously nitrided surface nano-structure and to form the austenitic and martensitic fiber structure aligned in the tensile direction. Each crystallographic structure intrinsic to metals and metallic alloys was tailored to have preferable micro−/nano-structured cells by metal forming and nitrogen supersaturation. The crystallographic change by metal forming in a priori and posterior to nitriding was discussed to find out a new way for materials design.

Part of the book: Electron Crystallography

By Tatsuhiko Aizawa, Tadahiko Inonara, Tomoaki Yoshino, Tomomi Shiratori and Yohei Suzuki

CVD-diamond coated special tools have been widely utilized to prolong their tool life in practical production lines. WC (Co) punch for fine piercing of metallic sheets required for high wear-toughness to be free from chipping and damages and for high product quality to punch out the holes with sufficient dimensional accuracy. The laser trimming process was developed to reduce the surface roughness of diamond coating down to submicron level and to adjust its diamond layer dimensions with a sharp punch edge for accurate piercing. The pulsed laser irradiation was employed to demonstrate that micro-groove was accurately formed into the diamond coating. Less deterioration in the worked diamond film by this laser treatment was proved by the Raman spectroscopy. The femtosecond laser trimming was proposed to sharpen the punch edge down to 2 μm and to form the nano-textured punch side surfaces with the LIPSS (Laser Induced Periodic Surface Structuring)-period of 300 nm. Fine piercing experiments were performed to demonstrate that punch life was significantly extended to continuous punching in more than 10,000 shots and that mirror-shining hole surfaces were attained in every shot by regularly coining the nanotextures. The sharp punch edge with homogeneous edge profile was responsible for reduction of the induced damages into work sheet by piercing. The punch life was extended by the ejection mechanism of debris particles through the nanotextures on the punch side surface. The present laser treatment was useful in trimming and nanostructuring the complex-shaped punch edge for industrial application.

Part of the book: Engineering Applications of Diamond

By Tatsuhiko Aizawa, Tomomi Shiratori, Tomoaki Yoshino, Yohei Suzuki and Takafumi Komatsu

The high-density plasma nitriding at 673 K and 623 K was employed to make 10% of nitrogen supersaturation on AISI316 base austenitic stainless steels. The processing parameters and nitrogen-hydrogen gas flow ratio were optimized to increase the yield of N2+ ion and NH-radical for efficient nitriding. The nitrided AISI316 specimens were prepared for multidimensional analysis to describe the fundamental features of low-temperature plasma nitriding. First, macroscopic evaluation revealed that nitrogen supersaturation induced the γ-lattice expansion and the higher nitrogen content than 4% of mass in depth. The mesoscopic analysis describes the holding temperature and initial grain-size effects on the microstructure changes. Plastic straining, grain-size refinement, and nitrogen zone-boundary diffusion processes advance with nitrogen supersaturation to drive the inner nitriding behavior. The microscopic analysis explains the microstructure refinement, the two-phase structuring, and the microstructure modification. Through this multi-dimensional analysis, the essential characteristics of the low-temperature plasma nitriding of 316 austenitic stainless steels were precisely understood to extend the engineering treatise on the bulk nitrogen stainless steels for surface modification and treatment of stainless steels by nitriding. This plasma nitriding was applied to strengthen and harden the AISI316 wire surfaces toward its application on surgery wires.

Part of the book: Stainless Steels

By Tatsuhiko Aizawa, Tadahiko Inohara, Yohei Suzuki and Tomomi Shiratori

A femtosecond laser micro−/nano-texturing was proposed to fabricate the coated and surface treated dies with the tailored textures for surface decoration and surface property control of metal, polymer and glass products. The polygonal model for microtextures with nanotextures by the LIPSS-effect was utilized to fabricate a DLC-coated SKD11 die with a star-shaped emblem. This die was set up into the cassette die set for directly imprinting this emblem into aluminum alloy and PET sheets. The periodic surface structure was synthesized as a surface geometry model to build up the super-hydrophobic surface on the nitrogen supersaturated AISI316 die. This die was also set up into a hot stamping system to directly imprint the hydrophobic surface onto the phosphorous glass products. Through the femtosecond laser micro−/nano-texturing and CNC-imprinting, the metal, polymer and glass product surfaces were optically decorated to have color grating and plasmonic brilliance and functionally controlled to be hydrophobic.

Part of the book: Terahertz, Ultrafast Lasers and Their Medical and Industrial Applications

By Tatsuhiko Aizawa, Tomomi Shiratori and Yohei Suzuki

A plasma nitriding-assisted 3D printing method was developed to build up the micro-punch and micro-die systems. Two dimensional punch head and core-die cavity geometries were ink-jet printed or screen-printed onto the AISI316 and SKD11 tool substrate surfaces in following their two-dimensional computer-aided design (CAD) data. The low-temperature plasma nitriding process was utilized to make nitrogen supersaturation only into the unprinted substrates. The sand-blasting and chemical etching were utilized to mechanically or chemically remove the printed parts from punch and die substrate. As sand-blasted and chemically etched AISI316 and SKD11 punches and core-dies were simply finished and used as a die set for micro-embossing, micro-piercing and micro-punching processes. In particular, a micro-pump was selected as a miniature mechanical element. Its 3D CAD geometry was sliced to 2D CAD data for each functional AISI304 stainless steel sheet. A pair of punch and die for each 2D CAD geometry for constituent sheet was prepared by the plasma nitriding-assisted 3D printing. Each sheet was punched out by using this set of punch and die to functionalize each sheet unit in correspondence to the sliced CAD data. These constituent sheets were assembled and joined to a structural unit of micro-pump.

Part of the book: Advances in 3D Printing

By Tatsuhiko Aizawa, Kenji Fuchiwaki, Takeshi Kihara and Tomomi Shiratori

Carbon supersaturation (CS) process was developed to prepare the CS-tool steel dies with massive carbon solute content toward the galling-free metal forming. The impinged carbon solutes diffused and agglomerated onto the hot spots at the die-work interface by stress gradient during the metal forming. This in situ formed free-carbon thin film worked as a tribofilm to reduce the friction and adhesive wear on the die-work interface. Titanium and titanium alloys were selected as a work material common to forging, near-net forming and fine blanking processes. The ball-on-disc method was employed to demonstrate the significant reduction of friction coefficient by CS-tool steels against the pure titanium ball. Upsetting process was used to describe the galling-free forging behavior even under the higher reduction of thickness than 50%. Pin-forming process was utilized to prove that taller pins than designed target were extruded and their height was preserved even with increasing the number of strokes. Fine blanking process was used to describe the integrity of CS-punch with higher grade of titanium gears. The in situ solid lubrication by formation of free-carbon tribofilm was discussed in each metal forming. In particular, the initial learning trial was proposed to shorten the incubation time for the free-carbon film coverage onto the hot spots.

Part of the book: Titanium-Based Alloys