A STUDY ON IMPROVEMENT OF STRENGTH IN WELDS OF ULTRA HIGH STRENGTH BETA TITANIUM ALLOY.

采用高次固溶处理、两级时效处理和形变热处理对β钛焊接热影响区的强度进行了改善，并提出了强化机理。1.β钛合金经固溶处理后再进行时效处理，具有较高的强度。显微组织由α相和β相组成。随着时效硬化的β钛合金焊接，热影响区显示硬度下降区域。骨折发生在这个区域。α相固溶于β相中。焊后进行时效处理。热影响区的硬度高于母材，时效硬化特性与固溶处理温度有关。与1073K固溶处理相比，1573K固溶处理试样的时效硬化速率加快。2.为了获得较高的强度和硬度，采用673K+773K两级时效处理，固溶处理后冷加工，再进行673K或773K时效处理。由于空位和位错浓度的增加，使硬度明显提高，加速了时效硬化。3.研究了显微组织和氢对氢致开裂的影响。固溶处理的试样由β相组成，氢固溶于β相中，氢化物几乎不析出。对于由α + β相组成的时效硬化β钛合金，氢在β相中扩散，在α相中扩散。氢化物以α相沉淀。这些证据证实了内耗测量和透射电子显微镜。

The improvement of strength of weld heat-affected zone of beta titanium was carried out, and strengthening mechanism was proposed by higher solution treatment, two stage aging treatment and thermo-mechanical heat treatment. 1. Beta titanium alloy has high strength by the aging treatment after solution treatment. The microstructure has consisting of alpha phase and beta phase. As the age hardened beta titanium alloy was welded, the decreasing area of hardness revealed in the heat-affected zone. The fracture occurred in this area. The alpha phase solutionized in the beta phase in the decreasing area. Aging treatment after welding was carried out. The hardness in the heat-affected zone became higher than that of the base metal.The age hardening characteristics depended on the solution treatment temperature. The age hardening rate accelerated for the specimen solutiontreated at 1573K, in comparison of the normal solution treatment at 1073K. 2.In order to obtain the higher strength and hardness, two stage aging treatment such as 673K+773K aging treatment,and the cold worked after solution treatment and then aging treatment at 673K or 773K were carried out. The increase of hardness was recognize and age hardening accelerated, because of the increasing of the vacancy and dislocation concentration. 3.Effect of the microstructure and hydrogen on the hydrogen-induced cracking was investigated. For the specimen solution-treated, consisting of beta phase, hydrogen solutioned in the beta phase and hydride hardly precipitated. For the age hardened beta titanium alloy, consisting of alpha+beta phase, hydrogen diffused in the beta phase and the diffused in the alpha phase. the hydride precipitated in the alpha phase. These evidences were confirmed by internal friction measurement and transmission electron microscopy.