Alpha-Case Reduction Mechanism of Titanium Powder-Added Investment Molds for Titanium Casting

摘要

The surface alpha-case reaction of Ti casting using Ti powder-added investment molds was investigated. During the curing procedure, Ti powders (0, 10, and 50 mass%) were mixed with three types of investment mold materials (Al_2O_3, ZrSiO_4, and ZrO_2) to form an interstitial TiO_2 phase, which is an alpha-case reaction compound. The microstructure and surface hardness profiles of Ti castings with Ti powder-added investment molds indicated that the alpha-case thickness was significantly reduced from approximately 350 to 50-100 μm, and a remarkable reduction in the maximum micro-Vickers hardness value of the Ti casting surface was also achieved. As observed from the experimental results, the alpha-case reduction mechanism suggested that the phase transformation from TiO_2 to TiO not only acts as an effective barrier to O diffusion, but also reduces Al, Si, and Zr concentrations at the casting surface. This reduction might be caused by contact area reduction between the Ti powder-added investment molds and the Ti melts.