钛合金螺纹管宏微观尺度热挤压成形机理研究

It is urgent to develop an internal thread heat exchanger pipe with high corrosion resistance and high performance to reduce the leakage accident caused by marine environmental corrosion. Titanium alloy, it is well known as a kind of marine metal, has the advantages of high specific strength, good thermal stability and heat transfer effect. Therefore, titanium alloy is an ideal material to replace the existing copper or stainless steel internal thread pipe. However, it is difficult to form titanium pipe by traditional method of ball spinning and cold drawing due to the poor plasticity and large deformation resistance of titanium alloy. Based on this, this project is put forward by a new technique that use the rotary threaded core during the hot extrusion of titanium alloy. The forming of high quality and performance internal thread teeth is the basis for obtaining high-performance threaded pipe products. Because the thread tooth is submillimeter, it belongs to the micro-forming and has the size effect. To this end, basised on the effect of size effect on the forming process of threaded teeth, the construction of constitutive relation of plastic deformation behavior and microstructure evolution at macro and micro scale, the macroscopic flow behavior and microstructure evolution of inner thread tube wall, and the micro-forming properties and mechanism of inner threaded teeth will be carried out. Clarifying the relationship between the scale effect and the deformation behavior and the microstructure evolution of the inner threaded teeth during the micro-forming process. Revealling the mechanism of hot extrusion of titanium alloy inner threaded pipe at macro and micro scale. The implementation of this project will lay a theoretical foundation for the development of new extrusion technology of titanium internal thread tubes with high performance.

开发高耐腐蚀、高性能的热交换螺纹管，减少因海洋环境腐蚀的泄露事故，是当前该领域亟待解决的问题。钛合金被誉为海洋金属，是替代现有铜合金或不锈钢螺纹管的理想材料。但是钛合金塑性差、变形抗力大，传统内螺纹管的滚珠旋压法与冷拔法很难成形，为此本项目提出了采用旋转螺纹模芯热挤压成形钛合金内螺纹管。成形高质量和性能的内螺纹齿是获得高性能螺纹管制品的基础，由于螺纹齿为亚毫米级，属于微塑性成形，具有尺寸效应。为此围绕尺寸效应对螺纹齿成形过程的影响，主要开展宏微观尺度塑性变形行为与组织演变的本构关系构建；内螺纹管壁金属宏观流动行为及显微组织演变规律；内螺纹齿微塑性成形性能及机理研究等研究工作。阐明内螺纹齿在微塑性成形过程中的尺度效应与变形行为及微观组织演变本质关系，揭示钛合金内螺纹管宏微观尺度热挤压成形机理，为开发高性能钛合金内螺纹管挤压新技术的奠定理论基础。

船舶和海洋工业中涉及热换器、蒸发器、冷凝器的热交换管主要为铜合金和不锈钢螺纹管，铜合金耐腐蚀性能差，不锈钢螺纹管传热效率低，两者都很难满足需求。钛合金被誉为海洋金属，同时具有热稳定性与传热效果好等优点，但钛合金塑性差、变形抗力大，成形困难，开发其螺纹管的成形技术，揭示成形机理，是当前亟待解决的问题。. 主要研究成果，开发设计了基于500T液压机的内螺纹管挤压模具装置，其中模具的芯轴采用滚动式推力轴承结合联轴器的被动动力源模式，由坯料流动产生的驱动力。优化后的模具结构参数，内螺纹芯轴长度11mm、螺纹升角20°，迎角96°，齿顶角为60°，螺纹齿高1mm， 内螺齿数为30，模具圆角半径R=0.5mm、模拟工作带长度H=3mm。. 提出了基于套筒+试样的一种高温微压缩实验方法，获得了试样尺寸的逐渐减小时，流动应力逐渐降低的尺寸效应。流动应力随着应变速率高低表现出高低敏感性。基于双曲正弦函数构建了TA1纯钛的微观高温压缩本构关系。. 研究了当挤压温度为600℃-800℃时，挤压速度为0.5mm/s-3mm/s对内螺纹管成形过程温度场、应变场、流场及挤压力的影响，获得了合理的挤压工艺，坯料挤压温度700℃，挤压速度为1mm/s，芯轴温度为30℃，模具温度为500℃。. 挤压变形过程中，螺纹管齿部组织主要以亚结构组织为主。随着挤压温度的升高，齿部晶粒取向由<0001>方向向<1 ̅21 ̅0>方向和<011 ̅0>方向发生偏转，小尺寸晶粒的占比逐渐提高，织构强度先升高后降低。织构强度最大值为750℃时的32mud。随着挤压速度的增加，再结晶组织占比提高，在2mm/s时，达到最大值14.4%。晶粒取向随着挤压的进行由<0001>方向向<011 ̅0>方向和<1 ̅21 ̅0>方向发生了偏转。