3D打印与传统轻金属疲劳损伤机制异同与寿命评估研究

3D printed and conventional light metals have each from special features in applications of lightweight materials. The former has been widely focused in recent years, especial in the applications about the complex structures, light weight and size limit structures etc. It is because that 3D printing technology owns the digital, intellectualized and internet (+) manufacturing characteristics. However, as the additive manufacturing technology, the materials will unavoidably exist some mesoscopic defects including hole, crack, inclusion and interface bonding as well as residual stress etc. in its manufacturing processing. Therefore, to investigate the effects of these defects on the fatigue damage mechanism and fatigue life becomes the necessary issues in its applications. In this work, the fatigue tests about the defects how to influence on the fatigue crack initiation and propagation mechanism or behavior of 3D printed metals will be carried out based on SEM in-situ technology and rotation bending fatigue equipment with environmental boxes in order to understand the fatigue damages of 3D printed metals with some defects, to describe and evaluate the fatigue crack initiation position and crack growth rate formal quantificationally, to forecast the fatigue life of 3D printed structure based on the coupling methods both simulation analysis and experimental data. Then, the fatigue damage hierarchy (system) both 3D printed and conventional light metals can be established, the fatigue damage theory and experimental method of 3D printed metals with some defects will be also obtained through above mention works. Therefore, the 3D printed structures will be used in safety and reliability aspects.

轻金属3D打印技术近年来发展迅猛，特别是在一些复杂结构、特殊尺寸方面，从设计到成型均能实现数字法、智能化或互联网+。但这种材料在物理化学过程制备中不可避免会存在孔洞、裂纹、夹杂物及合金元素缺失等缺陷。这些介观缺陷一般对材料的静强度影响甚微，但在长期服役时，缺陷对材料的疲劳影响不可忽视。传统粉末冶金材料的研究经验表明，类似的介观缺陷对疲劳损伤行为起到显著作用。因此，为保障轻金属3D打印材料的使用安全，对其疲劳损伤机制和裂纹演化规律进行实验，评价方法和模拟表征，并与传统材料的疲劳损伤机制异同展开比较研究十分必要。本项目拟在3D打印航空铝合金、钛合金中，通过SEM原位和旋转弯曲疲劳试验技术，揭示这些材料的疲劳介观损伤机制和疲劳裂纹扩展规律，讨论材料微观组织、残余应力变化对疲劳寿命影响的定量表征方法，模拟计算材料与结构性能间的转换关系及提出逆向设计对策等展开详细研究，为其安全使用提供依据。

本研究以SLM-Ti6Al4V(SLM-TC4)及轧制Ti6Al4V合金为主要研究对象，通过大量实验定量表征了两种制备Ti6Al4V和SLM-AlSi10Mg合金的微观组织、疲劳裂纹扩展速率、扩展门槛值、疲劳极限以及断裂韧性之间的异同。基于寿命数据和SEM原位疲劳裂纹扩展试验认识了微结构对疲劳裂纹萌生与扩展行为的影响，从而跨尺度讨论了同一材料因制备方式的不同导致的上述关键疲劳与断裂性质差异和影响机制；同时，采用EBSD和TEM技术详细研究了SLM-Ti6Al4V和SLM-AlSi10Mg合金不同滑移系对疲劳裂纹扩展行为、裂纹扩展速率以及失效机理的影响，加深了对SLM-Ti6Al4V和SLM-AlSi10Mg合金损伤容限及失效行为的认识。主要研究内容、重要结果、关键数据及科学意义如下：.1..旋转弯曲疲劳实验获得SLM-Ti6Al4V以及轧制Ti6Al4V合金SN曲线，研究表明：增材制造方向对SLM-Ti6Al4V合金疲劳性能影响显著，SLM-Ti6Al4V合金0°样品展现出与轧制Ti6Al4V合金相似的较高疲劳极限。考虑增材制造不可避免的内部缺陷对SLM- Ti6Al4V样品疲劳极限影响，揭示了Ti6Al4V合金长裂纹应力强度因子门槛值与真空压强之间线性关系，提出了Murakami修正公式，预测了含内部缺陷的SLM-Ti6Al4V和SLM-AlSi10Mg合金疲劳（上下）极限，建立了含缺陷的SLM-TC4损伤容限KT图，提供出了合理可靠的疲劳载荷安全区，对SLM- Ti6Al4V合金的推广使用起到积极的促进作用。.2..SEM原位疲劳实验定量获得SLM-Ti6Al4V以及轧制Ti6Al4V合金疲劳裂纹扩展速率（da/dN）数据；研究表明：SLM-Ti6Al4V和SLM-AlSi10Mg合金0°与45°样品展现出较高的裂纹扩展阻力，达到轧制Ti6Al4V合金相同水平。基于样品断口稳定裂纹扩展区尺寸大小，提出了简便预测KIC的方法并得到了实验数据验证。.3..基于EBSD和TEM分析技术定量分析晶体取向对轧制Ti6Al4V合金疲劳裂纹扩展速率、ΔKth影响。揭示轧制Ti6Al4V合金样品断口表面不同类型带状特征形成机理，定量讨论Ti6Al4V合金α-Ti相（hcp晶格）发生解理断裂或塑性断裂时开动的滑移系、Schmid因子与晶体取向间关系。