Papers by Author: Masaaki Naka

Abstract: The present study tried to control grain structure in polycrystalline aluminum or titanium by means of a laser spot heating method as well as by a computer simulation technique. Monte Carlo simulation of spot heating was performed utilizing two-dimensional grain structure model composed of 200 x 200 sites with hexagonal cells. Grain growth proceeded preferentially in the higher temperature region and resulted in a large grain surrounded by small grains. This large grain tended to keep on growing during homogeneous heating after the spot heating, suggesting that the spot heating technique can fabricate a peculiar grain structure such as different grain size distribution and texture that are not realized by utilizing conventional uniform heating process. Laser spot heating utilizing a fiber-laser system realized heating of small area such as 30micorn diameter to bring about peculiar grain size distribution as calculated by the computer simulation.

Abstract: When a cylindrical projectile is impact-welded to a flat target, a compound layer is usually observed at the joining interface as a result of the impact welding. In this study, the formation process of the compound layer was formulated as a moving boundary problem, which is a phase transformation technique. The numerical results were compared with the experiment results obtained using an aluminum projectile and stainless steel target. Numerical analysis shows that the melting area is similar to the temperature profile given at the boundary face. The area of the compound layer formed at the joining interface almost agrees with the melting area of the target. The profile of the compound layer is similar to the triangular temperature profile in the given temperature profiles. The mixing ratio of the melting weights of aluminum and stainless steel obtained by the numerical analysis strongly depends on the temperature rise at the interface. The melted weight of aluminum in the experiment is somewhat greater than that in the numerical analysis. The heat conduction analysis including deformation of the projectile and target make the results of the numerical analysis closer to the experimental results.

Abstract: This research introduces a review on the advanced fracture mechanics theory called (MN – rp – σθ) theory, and includes the history, the formulation, the predictions, the uses and applications in the different engineering fields.

Abstract: The ternary system Cu-Si-Ti was investigated using XRD, SEM-EDX, and DTA. The existence of two ternary phases τ1-CuSiTi and τ2-TiSi2-xCux as well as their crystal structures are confirmed. The isothermal section at 800°C is corroborated. From DTA data of equilibrated alloys a reaction scheme linking the isotherm at 700°C with the liquidus surface is derived. Using data on primary crystallisation of as cast alloys a projection of the liquidus surface is proposed.

Abstract: Strength of nanostructured Cr-B and Cr-Ni alloys prepared by sputtering was related with grain size. The alloying of B or Ni to Cr reduces the grain size of the alloys. The increase in volume of grain boundaries or amorphous phase induced by alloying elements causes the departure of strength of nanostructured Cr alloys from the values estimated by the Hall-Petch relation. The formation and microhardness of Cr or Ti base amorphous alloys could be discussed by the chemical bonding between elements. The strength of dissimilar joints was dominated by the thickness and structure of the interfacial reaction zone between SiC and metal. The formation of interfacial reaction zone is discussed by the change of chemical potentials of elements between ceramics and metal.

Abstract: In this work, Ni-Al-Ti-B composite powders with Ni: Al: Ti: B atom ratio of 5.7: 1.9:1:1 were prepared by using mechanical activation (MA) method. The MA processes were performed by vibration ball mill in a water-cooled chamber for 30 h. The ball-to-powder weight ratio was 10:1. In order to reduce the oxidation of the powders during milling process, ball milling was performed under an Argon atmosphere. The evolution of morphology and microstructure of Ni-Al-Ti-B composite powders were examined at different milling stages using scanning electron microscope and X-ray diffraction. Distribution of powders size was analyzed and the results showed that there were three stages for the change of powders size: (1) at the first stage, powders size increased due to more welding than fracture of the powders within 2h of MA, (2) at the second stage, particle size continuously decreased until 16h of MA due to more fracturing than welding, and (3) after 16h of MA, the particle size remained constant when welding and fracture reached an equilibrium. Calculation results according to Scherrer equation from X-ray pattern showed that the effective grain size continuously decreased with MA time. X-ray diffraction pattern confirmed that the formation of Ni and Ti solid solution during MA process of the Ni-Al-Ti-B composite powders.

Abstract: The present paper deals with the control of microstructure of friction stir processed aluminum alloys focusing on grain refinement, thermal stability at elevated temperature and texture development in some aluminum alloys such as 5083, 6061 and 7075 commercial aluminum alloys. 3mm thickness plates of 5083, 6061 and 7075 Al alloys were friction stir processed/welded with several rotation speeds and travelling speeds. Optical microscopy revealed the grain refinement in the stirred zone of each alloy and the average grain size decreased with decreasing rotation speed under various travelling speeds. Annealing of the joints brought about abnormal grain growth at temperatures higher than 773K for 5083 alloy. Critical temperature of the abnormal grain growth tended to decrease as the rotation speed decreased for the fixed travelling speed. Dissimilar joining of 5083 Al alloy to 6061 Al alloy also showed abnormal grain growth when annealed at 773K. A peculiar texture development of 7075 Al joint showing (111)//ND-oriented grains existing throughout the nugget was revealed by EBSP analysis.

Abstract: The reaction between the zinc plate (ZP) and the IF steel with near surface ultra fine grains (NSUFG) structure with grain size of about 89 nm was studied in temperature range of 473K to 623K in order to elucidate the temperature dependence of the reactions and its mechanism, by comparison with the reactions of ZP to coarse grains (CG) sheet, superficial cold rolled CG sheet (CG+R) and superficial cold rolled NSUFG sheet (NSUFG+R). It was found that this NSUFG structure considerably affected reactions between IF steel and ZP. There was almost no effect of superficial cold rolling on their reactions, but the NSUFG structure dramatically enhanced the reactions. The incubation times for appearance of the reaction layer and its layer width of ZP /(NSUFG or NSUFG+R) reactions are shorter and thicker than those of the ZP/(CG or CG+R) ones. The activation energy for reaction was 107kJ/mol, which indicates that the volume diffusion in zinc side and the grain boundary diffusion in the iron side play an important role in the reaction. The layer growth up to the layer thickness less than about 10µm was controlled mainly by the interface reaction and it over about 10µm mainly by the diffusion mass transfer.