改善钛合金棒材和锻件低倍和高倍组织的研究

On Elimination of Defects in α+β Titanium Alloy Bar and Forging

本文的目的是探索通过控制锻造热力学参数消除α+β两相钛合金锻件及棒材中存在的低倍组织缺陷的方法。研究结果表明,原始棒材中的低倍粗晶往往是形成锻件低倍粗晶的原因。在β再结晶原理基础上,利用正交试验方法,调整热力学参数,找到了消除低倍粗晶、高倍魏氏组织的途径。当β退火后采用水冷时,β退火加热温度建议取在(α+β)/β转变点以上10～20℃;采用空冷时,取在(α+β)/β转变点以上5～15℃。锻方拔成(成品锻)前加热温度建议取在(α+β)/β转变点以下20～30℃,β退火保温时间以0.6～0.8min/mm为宜。变形程度应大于50％～60％。

The defects that occur in α+β titanium alloy bar, pancake and forging need to be eliminated. Around 1980 such elimination became possible with the appearance of new techniques for fining structure. Starting from past research on defect elimination, this paper presents three new findings as follows: 1. Heating temperature for β annealing should be as low as possible and dwell time for β annealing should be as short as possible. In the past, it was known that heating temperature should be higher than β transus by 30℃ to 40℃ . Our viewpoint is that it should be higher than β transus by just approximately 20℃ . When temperature is too high, nucleation exciting energy and grain-boundary migration exciting energy increase greatly and nucleation becomes too fast, and rapid coalescence and growth of nucleated grain result. Such rapid coalescence and growth are highly undesirable. A similar situation exists when dwell time for β annealing is too long, though, in the authors' opinion, the undesirable growth of nucleated grain is somewhat less serious than is the case for too high a temperature. 2. Air cooling shouid be used after β annealing for large billet. In both laboratory tests and industrial production, poor quality large billets have been reported, but to the authors best knowledge, no workable schemes for guaranteeing good quality large forgings have been proposed. In the authors' opinion there is a limit to quenching depth under water cool- ing. When a bar is more than 60mm in diameter, water cooling can not reach the inner portion of the bar and as a result, β structure is transformed into α' structure in the outer portion of the bar, and into acicular α structure in the inner portion; such undesirable nonhomogeriety persists even after finish-forging. 3.Deformation degree of finish-forging in α+β region should be as heavy as possible for air cooling and 60% is the minimum that can be accepted. A minimum of 50% proposed in the past is believed to be inadequate. After air cooling, β structure is transformed into acicular a which requires heavy deformation to obtain the desired equiaxed α structure.