In the manufacturing process of zinc alloy die-casting, laminations are a serious problem because they cause blister defects and the deposition of zinc on the mold surface. Thus, it is very important to analytically predict the risk of lamination formation before determining the mold design. In this study, we proposed a criterion for predicting the locations of laminations. The criterion value is estimated from the result of a molten metal flow simulation and can indirectly predict the locations of laminations. As the analytic method in the mold-filling simulation, we adopted a gas-liquid two-phase flow model with a finite element method, and the Cahn-Hilliard equation was adopted to determine the interface between the gas and the liquid. We predicted the locations of laminations for two different types of gating system using the mold-filling simulation. The validity of the proposed criterion for laminations was confirmed by comparison of the results of the simulation and the observation of laminations after practical die-casting.

Mater. Trans. 60 (2019) 793-801に掲載

Relationship between microstructural changes and hydrogen permeabilities of rolled and annealed Nb₃₀Ti₃₅Co₃₅ alloys has been investigated. In the as-cast state, the alloy has a lamellar structure consisted of bcc-(Nb, Ti) and B2-TiCo phases. Hydrogen permeability was decreased with increasing rolling reduction. Granule (Nb, Ti) phase was formed in the TiCo matrix after annealing between 1173 K and 1373 K. Hydrogen permeability of non-rolled alloys was almost constant after microstructural change by annealing. On the contrary, hydrogen permeability of rolled alloys was increased with increasing annealing temperature, recovered to 90％ of the as-cast alloy by annealing at 1373 K, which is quite larger than the value expected by the mixing rule consisted of both two phases. Therefore, rolling and annealing technique is effective for industrial preparation of Nb-TiCo alloy foil to obtain large hydrogen flux for hydrogen production.

Resistance spot welding is used in the assembly of automobile bodies. The use of high tensile strength steel plates in automobile bodies is expanding, and there is a need to develop a welding process that improves the joint strength per resistance spot weld point. To improve the joint strength per resistance spot weld, resistance spot welding can be combined with other joining methods, such as the WeldBond method. As an additional joining method around the nugget, other than using adhesives as in the WeldBond method, brazing can be used to obtain a strong joint without melting the base metal. In this paper, it was proposed that the joint strength can be enhanced by using resistance spot welding and brazing or soldering based on the rule of mixture. And, based on this concept, effect of additional joint around a nugget, such as brazing or soldering, on rotational deformation of joint part and tensile shear strength of resistance spot welded lap joint was examined.

First, three types of lap joint specimens that were using only resistance spot welding, using only brazing and using both joining methods (called WeldBraze) were prepared. A high-strength steel sheet HT590 with a thickness of 1.0 mm was used. For the brazing filler metal, lead-free solder (composition: Sn-0.3Ag-0.7Cu) was used. Next, tensile shear tests were conducted using all the test specimens. In general, rotational deformation of the joint occurs due to the offset of the load axis for the tensile shear test of the lap joint. It is necessary to investigate the effect of the rotational deformation on joint strength. Therefore, a tensile shear test was performed using a test specimen in which the welded joint part was sandwiched between two steel blocks to suppress rotational deformation.

As a result, it was confirmed that the new joining method using both brazing and resistance spot welding increased the joint strength and the amount of displacement up to fracture as compared with the case of brazing or resistance spot welding only.

So, it was clarified that the factor of the strengthening of the joint was the rule of mixture, and the increase of displacement up to the fracture was affected by the suppression of rotational deformation.

In conclusion, it was found that the joining method using both brazing and resistance spot welding was effective in improving the joint strength.

Flux-free brazing of aluminum alloys was carried out under high purity nitrogen gas with ultra-low oxygen partial pressure prepared by zirconia oxygen pump. The flux-free brazability of the aluminum alloys was improved by adding a small amount of magnesium into the sample, which was more effective in the core alloy than in the brazing filler alloys. Higher heating rate of the sample increased a fillet length. When the oxygen partial pressure of atmospheric gas was much reduced to the order of 10⁻²⁵ Pa through a zirconia oxygen pump, a long fillet was obtained, comparable to that formed by flux brazing. The reason for the improvement of the brazability was discussed from the viewpoint of the fine segmentation of the oxide film caused by the combined effects of reduction of the oxide film, thermal expansion at melting the filler alloy, and gas phase formation reaction from the molten filler alloys.

A high temperature recycling process of neodymium magnets using B₂O₃ flux has been reported. However, the B₂O₃ flux shows a poor fluidity of the slag. For this purpose of solving the problem, present work considers the use of Na₂B₄O₇ with lower viscosity than the B₂O₃ flux.

The phase equilibria for the Nd₂O₃-Na₂B₄O₇ pseudo binary system was investigated in the temperature range from 1460 K to 1780 K. We found that this pseudo-binary system forms the homogeneous melt in the concentration range of from 16 mass％Na₂B₄O₇ to 20 mass％Na₂B₄O₇ and over 55 mass％ Na₂B₄O₇ at 1673 K. Based on the determined this diagram, the feasibility study of the recycling process of the neodymium magnets using Na₂B₄O₇ flux was carried out 20 kg and 100 kg high frequency induction furnace. When neodymium magnets and an EV rotor were melted with Na₂B₄O₇ in a carbon crucible, the rare earth elements of Nd, Pr, Dy and Tb in the magnets and rotor were enriched to the Na₂B₄O₇ slag phase and the iron in the magnets and the rotor were contributed to the Fe-C alloy phase. The oxalate precipitation method was used for the recovered Nd₂O₃-Na₂B₄O₇ slag, more 99 mass％ purity ratio of RE_xO_y was recovered.