超声能场与搅拌摩擦焊塑性流变材料的耦合作用机理

Ultrasonic energy can decrease the deformation resistance of materials, and is an assisted energy source of low power consumption and high efficiency. Preliminary studies have shown that exerting ultrasonic energy on the metal to be welded in front of the friction stir tool can lower the required friction heat of materials' plastic deformation, and there is obvious process effectiveness of lowering welding loads and improving weld quality. However, the way how ultrasonic energy changes the behavior of the plastically deformed material is still unrevealed. This project will get deep insight into the underlying interaction mechanism between the ultrasonic energy field and the plastically deformed material in FSW. The transmission, coupling and action regularities of the ultrasonic energy within the plastically deformed material in the vicinity of the tool is explored. The describing and characterizing methods of the volume effects and surface effects due to ultrasonic energy in friction stir zone are developed. The physical reasons why ultrasonic energy field changes the materials' yield stress and flow stress are elucidated. The method to modify the constitutive relation is put forward. The coupled model is established to describe the correlations of the ultrasonic field, strain field, fluid flow field and temperature field. The plastic flow behaviors, heat generation characteristics, and temperature distribution features of the material around the tool under the action of ultrasonic energy field are quantitatively analyzed and experimentally measured. The optimization and match between the ultrasonic variables, the welding process parameters and the weld joints' microstructures & properties are realized. It will lay solid foundation for effective guiding the appropriate utilization of ultrasonic energy in FSW process and raising the technological level of welding high strength aluminum alloy structures in China.

超声可降低材料的变形阻力，是一种能耗低、利用率高的辅助能量。前期研究表明，在搅拌头前方待焊工件上施加超声能场，可减少材料塑性流变所需的摩擦热量；降低焊接载荷并改善焊接质量的工艺效果明显。但超声改变摩擦搅拌区塑性变形材料行为的机制，尚未阐明。本项目拟研究搅拌摩擦焊接过程中超声能场与塑性流变材料的相互作用机理。探究超声波在搅拌头附近塑性变形材料中的传输、耦合及作用规律；构建摩擦搅拌区超声体积效应与表面效应的描述和表征方法；阐明超声能场改变材料屈服应力和流动应力的物理机制。提出超声作用下材料本构关系的修正方法，建立"超声能场-应变场-流场-热场"的耦合模型，定量分析和测试超声能场作用下搅拌头附近材料塑性流动行为、产热特点、温度分布特征等；实现超声参量、焊接工艺参数与接头组织性能之间的优化与匹配。为有效地指导超声能量在搅拌摩擦焊接过程中的合理应用、提升我国高强铝合金结构的焊接制造水平奠定坚实基础。

常规的搅拌摩擦焊接（FSW），需要很大的轴向压力和搅拌头转矩来产生足够的摩擦热，导致设备体积庞大、搅拌头磨损、焊接速度较低。超声振动作为一种机械能，在加工过程中可降低材料的变形抗力。在FSW过程中施加超声振动，使得搅拌头周围的材料更容易发生塑性流动，从而可以改善焊接质量、降低焊接载荷并提高焊接速度。然而，超声振动对FSW焊接热过程和塑性材料流动的影响机理仍未阐明。本项目直接将超声振动施加在搅拌头前方的待焊工件上，研制了“超声振动强化搅拌摩擦焊接 (UVeFSW)” 实验系统；从数值分析和实验表征两方面研究了UVeFSW过程中超声能场与塑性流变材料的相互作用机理。. 提出了三种定量表征剪切层内材料流动的实验方法，分别表征了搅拌头周围的塑性变形材料体积、连续流动的材料流速和非连续流动的材料应变和应变速率。基于位错热激活理论，分析了超声振动对塑性变形材料位错运动的影响，修正了材料本构关系。建立了“超声能场-热场-应变场-流场”耦合的UVeFSW数理模型，定量分析了UVeFSW过程中超声声压、声场能量密度、产热、传热、应变/应变速率、粘度和材料流动的动态变化规律。结果表明，搅拌头前方的超声振动能场与塑性变形材料相互作用，降低了材料的塑性变形激活能，产生声致软化效应，提高了搅拌头附近材料的塑性流动性，使得塑性变形材料体积扩大、焊核区晶粒发生细化并改变了晶粒取向、再结晶过程进行得更加彻底。阐明了超声振动改善焊接接头组织与性能的物理机制，为提升高强铝合金结构的焊接制造水平奠定了坚实基础。. 本项目已发表期刊论文31篇（其中，SCI收录28篇，EI收录3篇）。在国际会议做特邀大会报告（Plenary/Keynote）3次、邀请报告（Invited）3次。授权发明专利1项。获得两项国际学术奖励。培养出站博士后1人、博士生6人、硕士生4人；有2篇学位论文被评选为2018和2016年度山东省优秀博士和硕士论文。组织举办国际会议1次。

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