Property-Control of Titanium Alloys by Spark Sintering

研究了脉冲数与钛合金压坯比电阻率的关系，揭示了在火花烧结初期粉末间产生的现象。压块的电阻率随脉冲数的增加而减小。脉冲放电时这些值降低的原因考虑如下：(1)金属接触面积是由于粉末表面氧化膜的介电击穿而产生的。(2)热的产生是由于初始压实或预先脉冲放电在粉末颗粒之间已经形成的金属接触区有高电流密度，金属接触区被扩散。粉末颗粒间介质击穿的原位稳定性和脉冲放电过程中金属接触区产生的热量是提高颗粒烧结性能的主要原因。对火花烧结法生产的Ti、Ti- 6al - 4v、TiAl、Ti_3Al等钛合金进行了性能控制研究。通过控制烧结温度、烧结压力和烧结时间，获得了晶粒细的烧结坯。晶粒细的压坯具有良好的拉伸、冲击和断裂韧性等力学性能。与常规烧结相比，火花烧结可以在更短的时间内获得高密度和接近平衡态的试样。这种较短的高温暴露对防止晶粒生长很重要。

The relationship between the number of pulses and the specific resistivity of titanium alloy compacts has been investigated to reveal the phenomena which are caused between powders in the early stage of the spark sintering process. The specific resistivity of the compacts decreased with the number of pulses. The reasons for the decrease of these values during pulse discharge are considered as follows : (1) The metallic contact area is created due to the dielectric breakdown of an oxide film on the powder surface. (2) The heat is generated due to a high current density in the metallic contact area which was already created between powder particles by initial compaction or the prior pulse discharges, and the metallic contact area is spread. It is suggested that the in situ sbility of dielectric breakdown between powder particles and the heat generation in the metallic contact area during pulse discharge process leads to improved particle sinterability.The investigations for property-control have been performed on titanium alloys such as Ti, Ti-6Al-4V,TiAl, Ti_3Al produced by spark sintering technique. Fine grained compacts are obtained by controlling of resistance sintering temperature, pressure and time. The mechanical properties such as tensile, impact and fracture toughness properties are revealed on the compacts with fine grains. Furthermore, specimens with high densities and approaching the equilibrium state can be obtained in a shorter time by spark sintering than conventional sintering. Such shorter high temperature exposure is important to prevent grain growth.