Die damage is one of the problems occurring in the forging of steel. In cold forging, a high mechanical load causes cracking and galling of a die. In warm and hot forging, repetitive high thermal load softens a die, and causes wear, heat cracking, and galling. In many research reports, better lubricants and forging conditions for reducing these problems have been proposed. There are some studies in which the relationship between the internal temperature of a die and die damage under mild friction conditions was investigated. However, under severe friction conditions such as those in a backwardextrusion-type test, this relationship has not been quantitatively reported yet. In this study, friction coefficients between dies and billets were measured using the die torque and die load values obtained by die rotation in a backward-extrusiontype test. The temperature of a die surface was also estimated. Then, the relationship between friction coefficients and the maximum temperatures of die surfaces was quantified under cold, warm, and hot forging conditions, and it was compared with a formerly reported relationship measured under mild friction conditions by a ring compression test. Results indicate that the temperature of a die surface was increased by heat generation through plastic deformation in cold forging affecting the friction coefficient. On the other hand, the initial temperature of a billet determined the friction coefficient in warm and hot forging. The maximum temperature of the die surface was determined by the initial temperature of a billet in warm and hot forging. These results show that the maximum temperature of a lubricant while testing is the main factor that determines the friction coefficient in all forging temperature ranges under mild and severe friction conditions.

An internal fixation plate is used to repair a mandible damaged owing to injury or tumor resection. In the clinical use of the plate, a surgeon must bend a conventional flat plate to fit the shape of the mandible, however, the bending process requires a long time and the dimensional accuracy is insufficient for fixation. In this study, the authors constructed a numerical simulation model of the mandible, examined various factors affecting the deformation and stress state of the mandibular fixation plate, and then investigated the effects of the process parameters of additive manufacturing on the mechanical properties and corrosion resistance of prototype plates, and the biological safety. As a result, the following was found. (1) The mechanical properties and corrosion resistance of fabricated commercially pure titanium (C.P.Ti) plates are the same as those of conventionally wrought C.P.Ti and they can be used as internal fixation plates for mandibular bone defects. (2) We designed and fabricated a prototype fixation plate for a resected mandible, and confirmed that it could be fixed on the mandible model with good compatibility. (3) The additively manufactured plate was acid heat-treated and implanted in the rat skulls to confirm osseointegration. The resulting bonding forces were sufficient for assisting the fixation force of screws.