摘要
钛合金力学性能优异,但其机加工难度大,在离心力场下,精密铸造成形钛合金薄壁复杂铸件具有十分独特的优势。而深入揭示离心力场下钛合金充型流动规律及铸造缺陷的形成机理,对制备钛合金优质铸件,优化其铸造工艺,降低实验费用,缩短生产周期均具有重要的理论意义和实用价值。故此,本文采用相似模拟和及计算机数值模拟方法,深入系统地研究了离心力场下钛合金充型流动规律及铸造缺陷的形成机理。
本文借助相似模拟流体并依据所推导的离心力场下钛合金充型过程的相似准则,研究了离心力场下,不同浇注系统及不同转速条件下模拟流体的充型流动行为,结果显示:在模拟流体在横浇道中正向(即沿离心方向)充型流动过程中,流体总是沿着横浇道后壁进行充型,到达其末端时,流体冲击型壁,在此过程中流体经历了动能和势能之间相互的转化,一次压头转变为二次压头,并导致液体改变流动方向,沿后续浇道以逐层方式平稳的充入铸型,并且在反向(即沿向心方向)充型过程中,流体的自由液面是以转轴为圆心的规则圆弧面,流体只有径向运动而没有切向运动。对于直充式和顶注式,液流直接反向充型,最后充型部位为入口处;
分析了模拟流体充型过程中,横浇道中向外流动的流股横截面积和充型长度,与转速之间的相互关系,结果显示:横浇道中流股横截面积随着充型长度的增加而减小;随着转速的提高,在相同位置处,流体横截面面积相应减小。
研究了离心力场下,模拟流体充型速度及长度随时间的变化规律,结果显示:充型速度随着时间的增加而减小,充型开始阶段,速度下降的较为迅速,随后逐渐趋于平缓。对于同一类型的浇注系统,随着转速的提高,对应时刻的充型速度相应减小;充型过程中,充型长度随时间增大,充型初期,充型长度的增加迅速,随之增幅趋于平缓。而充型长度随转速的变化规律为:在充型开始阶段,转速对充型长度影响很小,随着充型时间的增加,转速对充型长度影响越来越显著,转速越高,充型长度增幅越小。
此外,本文还推导了离心力场条件下,模拟流体中夹杂物运动的相似准则,并借此分析了离心力场下模拟流体中夹杂物的运动规律,研究表明:密度大于液态金属的夹杂物,将弥散分布在流体中形成两相流,将沿径向向外运动。
计算机数值模拟,对纯钛/氧化锆陶瓷铸型界面反应所引起的反应性气孔进行了研究,结果表明:钛熔体与氧化锆陶瓷铸型界面处由界面化学反应引起的发气量变化规律为:初始发气较快,为增幅较大的线性增加阶段,后
Titanium alloys have excellent mechanical properties but they are difficult to be machined. Investment casts in Titanium alloys with thin wall produced under the centrifugal force field have unique advantages. It has important theory significance and the practical value to the development of Titanium alloys, the optimization of craft; reduce of experimental expense with clarification of the filling rule and the formation mechanism of defect in Titanium alloys under the centrifugal force field.
As the result, the filling rule and the formation mechanism of defect in Titanium alloys under the centrifugal force field have been studied by the way of similar simulation and computer numerical simulation.
In this dissertation, the similar criterion of filling process of Titanium alloys under the centrifugal force field is obtained applying to the similarity theory. The filling activity of alloys at different gating systems and different rotational speeds is simulated by water under centrifugal force field. The results show that fluid fills along the back of cross gate during forward filling. Exchange between the kinetic energy and the potential energy is taken place with crashing of fluid to wall, at the same time the primary pressure head become to the second pressure head. Then fluid stream go on filling along other running channels and stuff the cavity layer by layer. Free surface of liquid during filling backward process is on the circular arc with center is rotation axis. So there is not tangential motion, but only radial motion. Fluid fills backward directly for the gating system without ingate , and the final stuffing place is the gateway.
The change rule of the filling speed and the filling length has been studied under the centrifugal force field. Results show that, the filling speed reduces along with the time increasing, and the decrease is more rapid at the initial stage. And then, the decrease keeps smooth. For the same gating system, the filling speed reduces with the enhancing of the rotation speed in the same time. During the filling process, filling length increases with the increase of the filling time, and the change is obvious at the initial period. Then the change becomes smooth. The influence of the filling speed on the filling length can be ignored at the
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