Papers by Author: Kojiro F. Kobayashi

Abstract: We synthesized polymorphic diamond directly from highly oriented pyrolytic graphite (HOPG) using femtosecond laser driven shock wave without catalyst. A femtosecond laser pulse (wavelength: 800 nm, pulse width: 120 fs, intensity: 2×1015 W/cm2) was irradiated onto the HOPG surface in air. Crystalline structures of HOPG after the laser irradiation were analyzed using the synchrotron X-ray at the BL13XU in the SPring-8. We found that the hexagonal diamond exists in the HOPG which was irradiated by the femtosecond laser normal to the basal plane.

Abstract: For the improvement of a surface wear resistance of aluminum alloys, iron-based alloys were clad on the surface of an aluminum alloy by laser. By investigating the effect of CO2 and diode laser irradiation conditions on the formability of Fe-Cr-C clad layers, it was confirmed that the application of diode laser made it possible to obtain stable beads in low heat input compared with CO2 laser, which has been conventionally used for laser cladding. Besides, by optimizing the laser parameters, the dilution ratio of clad layers by Al substrate was minimized less than 10%. At the clad interface, the reaction layer consisting of Fe2Al5 and FeAl3 formed which caused cracks at the clad interface. Even by controlling laser conditions, this brittle layer formation could not suppressed. In this research, by using the immiscible Fe-Cu-Cr alloys as clad materials, Fe based clad layers were formed on Al substrate without any cracks at clad interface. By the effect of Marangoni motion of decomposed duplex liquid phases, Cu segregated between Al substrate and Fe based clad layers and suppressed the reaction between them.

Abstract: Dissimilar joining of Al alloys and steel was carried out using diffusion bonding process. The effects of Si and Mg contents of Al alloys and bonding parameters on the interfacial reaction were fundamentally investigated. While the reaction layers consisting of Fe-Al type intermetallic compounds (IMCs) formed at the interfacial region, in the joint with Al alloys including 1mass% Si or more a ternary Fe-Al-Si IMC layer formed at the Al alloy side. The growth of the reaction layers followed the parabolic growth low. A maximum joint strength was obtained at an average reaction layer thickness ranging from 0.5 to 1μm. The thicker reaction layer caused the fracture of the joints at a lower stress because of brittleness of the IMCs, and the thinner reaction layer including un-bonded interface also resulted in a low joint strength. As a result a thin and uniform reaction layer including less un-bonded interface can realize a high joint strength. Since the Fe-Al-Si IMC layer uniformly formed more rapidly than the binary Fe-Al IMCs in the joint with Al alloys including 1mass% Si or more, a higher joint strength was obtained at a thinner average reaction layer. As a result, it was found that the chemical compositions of 6000 series Al alloy controlled to be Mg (0.6 to 1.0mass%) and Si (more than 1.0mass%) were appropriate to obtain the better bonding characteristics.

Abstract: Dependence of the femtosecond laser ablation depth on the laser pulse energy was investigated for Zr55Al10Ni5Cu30 bulk metallic glass. Investigation of the femtosecond laser ablation of bulk metallic glasses has not been reported. Femtosecond laser pulses (wavelength of 800 nm, pulse width of 100 fs, pulse energies of 2 – 900 μJ) were focused and irradiated on the polished surface of metals in air. The ablation depth of the metallic glass is deeper than that of its crystallized metal at a pulse energy in the strong ablation region. We suggest that the energy loss at grain boundaries of hot electrons which is accelerated by the laser electric field influence the ablation depth in the strong ablation region.

Abstract: We propose a novel bonding process using Ag metallo-organic nanoparticles, of which the average particle size is around 11 nm. In this paper, Al, Ti, Ni, Cu, Ag and Au disc joints were made using the Ag metallo-organic nanoparticles in order to investigate the bondability of the various metals. These joints are evaluated based on measurement of the shear strength, and the observation of the fracture surfaces and the cross-sectional microstructures. The shear strength of the various metal joints increased in the following order: Al, Ti, Ni, Cu, Ag and Au. This corresponds to the order of the standard-free energy value of the oxide formation for each metal. In particular, while the strengths of the Cu, Ag and Au joints, in which the oxides can be reduced by carbon, were the same level, those of the Al and Ti joints, of which the oxides were more stable than carbon oxides, were extremely low. This result suggests that the carbon atoms or organic elements generated by the decomposition of the organic shell of the Ag metallo-organic nanoparticles may play a role in deoxidizing the oxide film on the metal surface. This can promote chemical bonding between the Ag nanoparticles and metals at low temperatures.

Abstract: Sn-3.5Ag (Sn-Ag) and Sn-3.5Ag-0.75Cu (Sn-Ag-Cu) solder balls were reflowed on electroless Ni-P/Au plated Cu pad with varying thickness of Au layer (0 to 500nm). In the Sn-Ag solder joint, a P-rich layer including voids, which resulted from Ni diffusion from the Ni-P plating to form Ni3Sn4 interfacial reaction layer, formed at the interface regardless of Au plating thickness. This caused the degradation of the joint strength. On the contrary, the Sn-Ag-Cu solder joint had no continuous P-rich layer formed and showed a higher joint strength than the Sn-Ag solder joint in the case of Au plating of 50nm or less. Cu alloying to the solder promote the formation of (Cu, Ni)6Sn5 instead to Ni3Sn4 as the interfacial reaction layer. The (Cu, Ni)6Sn5 reaction layer can suppress the diffusion of Ni from the N-P plating and thereby inhibit the formation of the P-rich layer. However, in the case of thick Au plating of 250nm or more, a thin P-rich layer formed at the interface even in the Sn-Ag-Cu solder joint and the joint strength was degraded. Au dissolving into the solder from the Au plating during the reflow process may encourage the diffusion of Ni from the Ni-P plating into the solder. As a result, the Sn-Ag-Cu solder joints with 50nm Au coating provided the best joint strength, although its joint strength considerably degraded after the aging treatment at 423K.

Abstract: The synthesis of the high-pressure ε phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. The lower pressure and temperature α phase (bcc) transforms to the γ phase (fcc) at higher temperatures and to the ε phase (hcp) at higher pressures. A shock induced α to ε phase transition in iron is one of the most famous transitions under high pressure. The induced high-pressure ε phase by a conventional shock loading returns to the α phase and it is not quenched after the shock release because this transition is considered to be diffusionless. Crystalline structures in a recovered iron sample after the femtosecond laser (800 nm, 120 fs, 1014 W/cm2) irradiation were determined using the electron diffraction and the synchrotron X-ray diffraction methods. These results show the existence of the ε phase and the fcc structure in the recovered iron. The femtosecond laser-driven shock wave may have the potential to synthesis high-pressure phases of other materials that has not been done using the conventional shock wave.

Abstract: This paper presents laser beam welding (LBW) of an age-hardened aluminum alloy and a bake-hardening (BH) steel, which are promising materials for lightweight car body, compared with conventional arc welding. The LBW of the aluminum alloy had a significantly narrower fusion zone and heat affected zone (HAZ) than the arc weld and the hardness of the softened zones was almost completely recovered to the base metal hardness after a post weld aging treatment. This advantage of LBW is due to a low overall heat input resulting from its high energy density. This is also beneficial for welding of the BH steel. In the LBW of the pre-strained BH steel, both the fusion zone and HAZ were bake-hardened after a heat-treatment and the fracture occurred in the base metal in the tensile test. In contrast to this, the arc weld had a large softened zone in the HAZ and fractures in this region.

Abstract: The microstructure and strength for the micro joints of Pb free Sn-Ag based solders with Au/Ni alloy platings were investigated. For the joint using Sn-Ag solder, Ni3Sn4 reaction layer formed at the solder/pad interface and also P-rich layer formed in Ni-P plating. The P-rich layer was confirmed to be composed of Ni-P-Sn ternary compound layer and crystallized Ni3P layer. Both of them introduced defects, which degraded the joint strength. Addition of Cu to Sn-Ag solders suppressed the formation of such a P rich layer while the (Cu, Ni)6Sn5 reaction layer was formed at the solder/pad interface. These different interfacial reactions would affect the changes in the joint strength during heat exposure at 423K. On the contrary, addition of Co to Ni platings enhanced the interfacial reaction and the Sn-Ag solder completely transformed to the intermetallic compounds under higher melting temperature even by heating to 543K. The addition of Co in Ni could change the interfacial reaction layer from Ni3Sn4 to (Ni, Co)Sn2 with higher diffusivity of Ni which enhanced formation of intermetallic phases. The control of interfacial reaction by the alloying elements is important to obtain ideal micro joints.

Abstract: Quenching of the nanocrystalline high-pressure e phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. A small quantity of the g phase of iron also existed. We found that the e phase was induced by the shock itself but not the g phase. The g phase was suggested to be induced as an intermediate structure between the a-e transition. The femtosecond laser driven shock may have the potential to quench high-pressure phases which has not been attained using conventional methods.