液氮低温切削钛合金Ti-5Al-5V-5Mo-3Cr刀具失效界限和加工特性研究

Near-beta metastable titanium alloys like Ti-5Al-5V-5Mo-3Cr (Ti-5553) are increasingly used in aeronautics in some critical applications. However, Ti-5553 has a poor machinability rating due to its high strength, high toughness and high hardness. In cryogenic(liquid nitrogen) cutting Ti-5553 operations, the issues focus on severe tool wear, cutting instability and surface defects due to the large thermal gradient. Accordingly, the aim of this study is to understand the boundary of tool failure and the characteristics of cryogenic(liquid nitrogen) cutting Ti-5553. The research work is shown as following in detail: (i) According to the analysis of the process of liquid nitrogen penetrating capillary in cutting zone and the cooling effect of liquid-vapor phase change,investigation of friction model at the chip-tool-workpiece interface in cryogenic cutting Ti-5553 process. (ii) Influence of cryogenic cooling on the evolution mechanism of tooe wear and the boundary of tool failure when considering the friction model and the chemical performance match between cutting tools and Ti-5553. (iii) Considering the effect of the chip and liquid-vapor phase change and tool wear on dynamic cutting force, construction of model of cutting stability and determination of the limits of stability in cryogenic cutting Ti-5553 operations with presence of boundary of tool failure. (iv) Investigation of the relationship between the micro-structure of subsurface and surface defects in cryogenic cutting of Ti-5553, construction of the model among the cryogenic cutting conditons and the micro-structure of subsurface and surface defects based on the evaluation of cutting stability. The results of above research work will provide systematic theoretical support in the field of optimizing process planning in cryogenic cutting and high performance machining Ti-5553.

航空用钛合金Ti-5Al-5V-5Mo-3Cr（Ti-5553）是近β相亚稳态高强度、高韧性和高硬度钛合金，切削加工性极差。同时，在液氮低温切削时陡峭的工件表面温度梯度，会加剧加工表面缺陷的形核与破坏，降低已加工表面质量。针对上述技术难题，本项目进行低温切削Ti-5553时液氮渗入切削区毛细管润滑和液-汽相变冷却行为研究，建立切屑-刀具-工件间的摩擦模型；基于化学匹配性和摩擦模型，进行刀具磨损演变机制研究，建立低温切削刀具失效界限；分析切屑形态、液-汽相变和刀具磨损演变对动态切削力影响规律，构建切削稳定性预报模型，提出低温切削过程稳定性评价标准；进行表层微观结构演变机制和表面缺陷映射关系的研究，建立基于切削稳定性评价的低温切削条件-表层微观结构-表面缺陷关系描述模型，为优化液氮低温切削工艺条件，实现Ti-5553的低温高效切削提供系统的理论支持。

航空用钛合金Ti-5Al-5V-5Mo-3Cr（Ti-5553）是近β相亚稳态高强度、高韧性和高硬度钛合金，切削加工性极差。针对上述技术难题，本项目进行液氮低温切削Ti-5553刀具失效界限和加工特性研究，其主要成果包括：.（1）基于毛细管理论研究液氮渗入切削区毛细管行为，建立液氮渗入毛细管动力学模型和切屑-刀具-工件间液氮边界润滑层模型；通过实验与建模相结合的方法确定对流传热系数，分析液氮的微尺度液-气相变对切屑-刀具间切削温度的影响；建立Ti-5553的Johnson-Cook（J-C）本构模型和切屑-刀具-工件间的摩擦模型，实现液氮低温切削Ti-5553有限元数值仿真，揭示液氮低温冷却润滑状态下切屑-刀具-工件间的摩擦特性；.（2）进行高温氧化试验和元素扩散实验，研究刀具材料（硬质合金、涂层硬质合金、陶瓷、PCBN）与钛合金Ti-5553的化学匹配性；基于化学匹配性进行液氮低温刀具磨损切削实验，检测刀具磨损形貌，分析刀具磨损机理，建立基于热-力载荷效应的液氮低温切削Ti-5553刀具失效界限；.（3）研究低温切削时切屑的形态变化，分析其对切削力动态特性的影响规律，考虑刀具磨损对瞬时切削力的影响作用，建立动态切削力模型；基于功率谱均方根法建立切削稳定性预报模型；基于热-力载荷刀具失效界限进行实验测试，建立液氮低温切削Ti-5553切削过程稳定性边界条件；.（4）基于低温切削稳定性进行工件已加工表层微观结构的演变机制研究，进行表面缺陷检测与分析，揭示表层微观结构演变对表面缺陷产生的影响规律，获得表面缺陷与表层微观结构之间的映射关系；研究低温切削条件与表面缺陷映射规律，建立钛合金 Ti-5553 基于切削稳定性评价的低温切削条件-表层微观结构-表面缺陷关系描述模型。.上述成果为Ti5553等航空航天用典型难加工材料的高效绿色切削加工工艺、飞机典型零部件的高疲劳寿命加工工艺提供理论与试验依据。