A particular casting plan for a T-shape gravity-casting made of JIS-AC4C-alloy shows hot-tearing at the corner portions. We designed four casting-plans and compared thermal stress distribution when each corner portion has a solid fraction of 40% during solidification. The elastic FEM stress analysis combined with casting simulation revealed that one of the four casting plans shows high tensile stresses at the corners. The casting plan installing the corner portions at the bottom of the casting plan does not cause crack-opening because the upper portion works as a riser at the later stage of solidification. At the initial stage when the surface facing the mold solidifies, the inner portion of the vertical bar part of the T shape has a stage showing compressive stress. The stress changes to tensile stress with the advancement of solidification. It was demonstrated that the casting plan causes hot-tearing at the semi-solid corner when the solidified inner portion enhances tensile stress.

  It is said that surface tension and oxide film affect the flow of melt in aluminum thin casting. Nakae et al. examined the flow pattern of melt in thin plate castings with varied slope angles of the FEM mesh model by direct observation, while Mori et al. examined the flow pattern of melt in thin plate castings with varied atmospheres by direct observation. In this study, our purpose was to reproduce the direct observation results of Nakae et al. and Mori et al. by flow simulation taking into consideration the effects of surface tension and oxide film.

  Analysis results and experimental results more or less agreed, but the simulation could not reproduce a part of unstable flow in the experiment. We think this difference is due to the breakage of thin and uneven oxide film.

  In general, the manufacturing process of NdFeB sintered magnets is as follows : preparing the starting alloys, crushing and pulverizing into fine powder, press-forming in magnetic fields for alignments, sintering, aging and cutting. It is well known that microstructures of starting alloys affect the ease of pulverization, press-forming for alignments in the magnetic field and sintering.

  A single roll casting process (hereinafter referred to as strip-casting) is a beneficial method for obtaining starting alloys with suitable microstructure. For this reason, strip-casting is regarded as a de-facto standard process for the production of starting alloys for Re-Fe-B magnets. With this method, strips of 100-500μm thickness are formed by pulling up from a melt by a single roll. For this reason, it is important to control primary crystal growth.

  In this study, we investigated primary crystal growth in starting alloys.

  First, Nd_15.66Fe_bal.B_5.52 (atomic%) alloys were prepared using a strip-casting furnace. A batch size was 500kg and roll diameter was 500mm. Crystal structures of obtained strip-casting samples were observed by EPMA. In this measurement, we focused on the morphologies of primary crystal growth and the distribution of constituent elements.

  Next we investigated the relations between the surface roughness of the rolls and frequencies of nucleation at the surface contacting a roll. The alloys were prepared using a strip-casting furnace. A batch scale was 5kg and a roll diameter was 300mm. The alloy composition was Nd_10.35Dy_3.88Fe_bal.B_5.96 (atomic%).

  In this study, we also reviewed the method of reducing segregations of chill crystals existing in the microstructures of the starting alloys.