【作者】 王伟；

【导师】 陶杰；

【作者基本信息】 南京航空航天大学 ， 材料学， 2015， 硕士

【摘要】 TC4钛合金广泛应用于航空航天领域,常以退火状态作为最终使用状态,然而钛合金由于弹性模量低,钛合金结构件初始应力复杂,在退火过程中产生较大变形。为控制TC4钛合金在热处理过程中的变形,本文采用数值模拟和试验结合的方法对TC4钛合金退火变形及其控制进行研究。通过研究钛合金退火变形规律,结合工艺参数对钛合金退火变形及热校形的影响,以退火后变形最小化为目标,对钛合金退火工艺参数和热处理夹具进行优化,从而控制钛合金薄壁件退火变形。首先,本文采用MSC.marc分别建立了热粘塑性和热弹塑性钛合金退火模型,模拟TC4钛合金退火过程,并试验验证。研究结果表明,蠕变是预测变形准确性的关键因素,热粘塑性模型预测薄板退火变形更加准确,误差小于10%。残余应力是薄板发生整体变形的根本原因,残余应力在蠕变作用下产生的蠕变应变及弹性应变是退火变形的直接原因。残余应力以弹性应变的形式释放,在降低时存在滞后现象。其次,研究了不同退火温度对钛合金力学性能的影响,确定了退火温度范围;在模型优化的基础上,采用数值模拟研究了不同退火温度、保温时间、加热速度和冷却速度对薄板退火变形的影响,并得到试验验证。结果表明,钛合金在所选退火温度范围内经退火后的强度比未经退火的均有提高,断后伸长率差别不大。随着退火温度升高,蠕变速率增加,退火变形量增大;随着保温时间延长及加热速度、冷却速度降低,蠕变时间延长,退火变形量增加。最后,研究了工艺参数对钛合金薄板热校形的影响,同时分析了重力对退火变形的影响。结果表明,随着退火温度升高,热校形最终变形量越大,热校形效果越好,并且最终变形量增长速率随着温度升高而先增加后降低,保温时间延长、加热速度降低和冷却速度降低也导致热校形最终变形量的增加。重力及夹具对钛合金退火变形影响显著,夹具形式影响着热处理过程中重力的影响效果,决定零件最终变形。在薄板热校形及退火变形的研究基础上,结合T形件简支撑退火变形规律,对退火工艺参数及热处理夹具进行优化,优化后T形件退火变形在±0.06mm范围内,绝对变形小于0.08mm,针对初始变形分别为0.025mm和0.250mm的T形件,夹具均可有效控制热处理变形。更多还原

【Abstract】 Ti6Al4V alloy, widely applied in aviation and aerospace fields, is regularly used in annealed state. However, titanium alloy parts often have large annealing distortion due to their low elastic modulus and complex initial stress distribution. Therefore, annealing distortion of Ti6Al4 V sheets was investigated using the combination of numerical simulation and experimental methods. Meanwhile, the influence of process parameters on annealing distortion and hot sizing of titanium sheets were researched. Then, the process parameters and fixture were optimized to control the annealing distortion of thin-walled Ti6Al4 V parts.Firstly, thermal-viscoplastic and thermal-elastic-plastic annealing models were established respectively with MSC.marc to simulate Ti6Al4 V annealing process. The results indicated that creep was the key factor to accurately predict distortion. And thermal-viscoplastic model was more sophisticated than thermal-elastic-plastic model in predicting annealing distortion. Meanwhile, the internal residual stresses, which changed strain distribution and forced the sheet to bend, were the fundamental reason to cause distortion. Besides, residual stresses released in form of elastic strain exhibiting hysteresis in annealing process.Secondly, we studied the influence of process parameters on annealing distortion by numerical simulation and experimental verification after the optimization of annealing temperatures. The results showed that annealing treatment improved the strength of titanium alloy, but had no obvious effect on the elongation. The increase of annealing temperature and holding time, along with the reduction of heating rate and cooling rate, led to the rise of annealing distortion.Finally, the effect of process parameters on hot sizing and annealing distortion were investigated respectively. The results showed that the increase of annealing temperature and holding time, along with the decrease of heating rate and cooling rate, resulted in the rise of the final deformation of hot sizing. Meanwhile, gravity and fixture had significant effect on annealing distortion of titanium alloy. Fixture form, influencing the effect of gravity on parts deformation in annealing process, decided the final deformation. On the basis of the above studies, the absolute final deformation of T shaped parts was less than 0.08 mm in the fixture constraint with the optimization of annealing process parameters and fixture. The final deformation of T parts was in the range of ±0.06 mm. The fixture design was effective in controlling heat treatment distortion.更多还原