增材制造金属材料的疲劳性能研究进展

金属增材制造作为前沿热点制造技术之一,近年来在各种重要工业领域的研究和应用日益广泛。利用增材制造技术制备金属材料的过程中,不可避免会造成材料表面粗糙、气孔、未熔合等缺陷,虽然工艺技术的改进可以在一定程度上减小缺陷程度,但至今仍无法完全消除这些缺陷。增材制造金属材料的过程中,缺陷部位通常会成为应力集中源诱发疲劳裂纹的形核,造成金属材料的疲劳寿命下降。首先从表面质量、内部缺陷及微观结构等方面阐述了增材制造金属材料疲劳性能的影响因素;其次从宏观与微观角度概括了疲劳裂纹萌生/扩展机理的研究现状与进展;总结了热处理、表面优化、电磁辅助以及超声辅助等疲劳延寿技术的研究进展;最后讨论了基于机器学习技术的疲劳寿命评估模型,同时展望了机器学习和人工智能技术在增材制造金属材料领域的应用前景,为推动增材制造金属材料的发展和应用提供了借鉴与参考价值。     

Effect of surface roughness on fatigue performance of additive manufactured Ti–6Al–4V

Additive manufacturing is increasingly considered for production of high quality, metallic, aerospace parts. Despite the high potential of this manufacturing process to reduce weight and lead time, the fundamental understanding of additive manufactured Ti–6Al–4V material is still at an early stage, especially in the area of fatigue and damage tolerance. This paper covers the effects of inherent surface roughness on the fatigue life. In the as built condition, metallic parts have a poor surface texture, which is generally removed in fatigue critical areas. It is shown that the fatigue properties of Ti–6Al–4V samples, produced by direct metal laser sintering and electron beam melting, are dominated by surface roughness effects. A simple model based on an equivalent initial flaw size is formulated.     

增材制造Ti-6Al-4V钛合金低周疲劳性能研究进展

金属增材制造技术可用于大型、复杂高性能钛合金结构件的制备,在航空航天等领域具有显著的优势和巨大的发展潜力。虽然增材制造Ti-6Al-4V合金构件的强度已经能够超过锻件,但它仍存在内部孔隙、熔合不良、粗大的柱状晶及残余拉应力等问题,使其在疲劳性能上与锻件具有一定的差距。本文在介绍直接能量沉积、选区激光熔化和电子束选区熔化3种代表性增材制造技术的原理及特点的基础上,简述了3种工艺制备Ti-6Al-4V合金构件的微观组织、静态力学性能及低周疲劳性能的研究进展,重点讨论了打印方向、缺陷、显微组织和表面处理对低周疲劳性能的影响。分析了增材制造Ti-6Al-4V合金构件低周疲劳性能、拉伸性能与微观组织之间的内在关系,并对提高构件低周疲劳性能的方法和推动其广泛应用的发展方向进行展望。     

Fatigue behaviour of additive manufactured materials: An overview of some recent experimental studies on Ti‐6Al‐4V considering various processing and loading direction effects

Although additive manufacturing (AM) has gained significant attention due to the advantages it offers and is currently a focus of much research, design of critical load carrying components utilizing such processes is still at its infancy. This is due to the fact that most of the load carrying components made by AM processes are subjected to cyclic loads, and fatigue behaviour of AM metals is far less understood as compared with those made by conventional methods, such as wrought and cast metals. To better understand the fatigue behaviour of AM metals, a wide range of issues that affect the behaviour in a synergistic manner must be considered. These include the effects of defects, residual stresses, surface finish, geometry and size, layer orientation, and heat treatment. Additionally, due to the multiaxial nature of the loading and/or complex geometries typically manufactured by AM processes, the stress state is often multiaxial including both normal and shear stresses. In this paper, the aforementioned effects influencing the fatigue resistance of AM parts, including torsion and multiaxial fatigue behaviour, are briefly discussed using some recently generated experimental data on Ti‐6Al‐4V by the authors.     

Evaluating fatigue crack propagation properties using miniature specimens

Standard fatigue crack propagation (FCP) test placed strict requirement on specimen size. FCP rate in subsize specimen was found to be slightly but consistently slower than that in the standard specimens. Based on a critical study on two aluminum alloys and two steels, we found the lower rates can be attributed to the plane stress state in the miniature specimens being different from that in the standard specimens. By taking account of crack closure, the miniature specimen data are brought in line with the standard specimen results and may serve as an upper bound estimate of FCP properties.     

SPECIMEN SIZE DEPENDENCE OF LOW FREQUENCY FATIGUE TESTS IN HIGH-PRESSURE HYDROGEN

Abstract— Fatigue data required for estimates of cracked component lifetimes are conventionally obtained by cyclic loading of specimens manufactured to a specific geometry. Crack growth in the specimen results in an increase in the stress intensity factor range and crack growth curves are calculated from the variation of crack length with time. An environmental fatigue study of the effect of high pressure hydrogen on the low cycle fatigue of a medium strength steel has shown that, due to effects of elapsed time in the environment and effects of specimen size, in certain circumstances this procedure may not yield geometry-independent results which can be applied with confidence to cracked components. It is concluded that to obtain useful crack growth data in cases where fracture is influenced by diffusion or other strongly time dependent processes might require a modified approach to fracture mechanics testing procedures.     

Computed tomography for characterization of fatigue performance of selective laser melted parts

Components manufactured by maturing additive manufacturing techniques like selective laser melting (SLM) find potential competence in several applications especially in automotive and aerospace industries as well as in medical applications like customized implants. The manufactured parts possess better, or at least comparable, yield strength and tensile strength values accompanied with a reduced fracture strain. Though their fatigue performance in the as-built condition is impaired due to surface roughness, it can be sufficiently improved by post-process surface treatments. Even then, there exists a high fatigue scatter due to remnant porosity. Characterization of remnant porosity is necessary for a reliable component design to be employed for cyclic applications. Computed tomography has been used in this study to evaluate the influence of porosity-incited stress concentration on the corresponding fatigue scatter. Microscopic analysis, tensile tests, fatigue tests with continuous load increase and constant amplitudes as well as finite element analysis have been used for this purpose. Critical pore characteristics and a modification in the process scanning strategy have been recommended so that the components can be reliably used in fatigue-loaded applications.     

Post‐treatment selection for tailored fatigue performance of 18Ni300 maraging steel manufactured by laser powder bed fusion

This study investigates the fatigue behaviour of additively manufactured 18Ni300 maraging steel. Specifically, the surface and material parameters impacting fatigue performance are analysed through various post‐treatment combinations. Vertically built miniaturised test samples produced by laser powder bed fusion are tested in as‐built and age‐hardening heat‐treated conditions. To utilise the potential of using additive manufacturing for complex‐shaped parts in which conventional machining tools could have limited access, vibratory finishing and sand blasting are employed. The fatigue results show that in as‐built microstructural condition, both the surface treatments significantly enhanced the fatigue performance, with vibratory finishing outperforming sand blasting owing to better surface finish. After heat treatment, sand‐blasted samples performed better than vibratory‐finished ones because of higher residual stresses. This competing interaction between post‐treatments sheds light on identifying the relative influence of various factors. With systematic postfracture and microstructural analyses highlighting the fatigue influencing factors, recommendations are drawn to select post‐treatments to achieve the desired fatigue performance.     

Fatigue life prediction of additively manufactured material: Effects of surface roughness,defect size,and shape

In this paper, the effects of process‐induced voids and surface roughness on the fatigue life of an additively manufactured material are investigated using a crack closure‐based fatigue crack growth model. Among different sources of damage under cyclic loadings, fatigue because of cracks originated from voids and surface discontinuities is the most life‐limiting failure mechanism in the parts fabricated via powder‐based metal additive manufacturing (AM). Hence, having the ability to predict the fatigue behaviour of AM materials based on the void features and surface texture would be the first step towards improving the reliability of AM parts. Test results from the literature on Inconel 718 fabricated via a laser powder bed fusion (L‐PBF) method are analysed herein to model the fatigue behaviour based on the crack growth from semicircular/elliptical surface flaws. The fatigue life variations in the specimens with machined and as‐built surface finishes are captured using the characteristics of voids and surface profile, respectively. The results indicate that knowing the statistical range of defect size and shape along with a proper fatigue analysis approach provides the opportunity of predicting the scatter in the fatigue life of AM materials. In addition, maximum valley depth of the surface profile can be used as an appropriate parameter for the fatigue life prediction of AM materials in their as‐built surface condition.     

Influence of the build orientation on the fatigue strength of EOS maraging steel produced by additive metal machine

This paper presents a research dealing with the dependence of the fatigue strength of maraging steel parts, manufactured by direct selective laser sintering, on the production build orientation. Three sets of specimens have been manufactured according to the ISO 1143 Standard (2010) by EOSINT M280 additive manufacturing machine, with the following heat and mechanical treatments, in agreement with the recommendations by the material manufacturer and current literature. The expected outcomes are the Fatigue Limit values of the material and the maximum number of cycles observed at different stress levels for three different build orientations (three different angles, 0°, 45° and 90°, between the build direction and the longitudinal axis of the samples). The results have been processed and compared by statistical methods in order to determine the fatigue curves in the finite life domain and the fatigue limits, along with their confidence bands and intervals, and to investigate the significance of the build orientation factor.     

Critical assessment of the fatigue performance of additively manufactured Ti–6Al–4V and perspective for future research

To realize the potential benefits of additive manufacturing technology in airframe and ground vehicle applications, the fatigue performance of load bearing additively manufactured materials must be understood. Due to the novelty of this rapidly developing technology, a very limited, yet swiftly evolving literature exists on the topic. Motivated by these two points, we have attempted to catalog and analyze the published fatigue performance data of an additively manufactured alloy of significant technological interest, Ti–6Al–4V. Focusing on uniaxial fatigue performance, we compare to traditionally manufactured Ti–6Al–4V, discussing failure mechanisms, defects, microstructure, and processing parameters. We then attempt to identify key knowledge gaps that must be addressed before AM technology can safely and effectively be employed in critical load bearing applications.     

Microstructural features of Ti-6Al-4V manufactured via high power laser directed energy deposition under low-cycle fatigue

Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically com-plex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the microstructural features of a Ti-6Al-4 V alloy manufactured via high power laser directed energy deposition subjected to low-cycle fatigue loading were studied.Before fatigue loading,the microstructure of the as-deposited parts was found to exhibit a non-homogeneous distribution of columnar prior-β grains(200-4000 μm)at various scanning velocities(300-1500 mm/min)and rela-tively coarse α-laths(1.0-4.5 μm).Under cyclic loading,fatigue microcracks typically initiated within the aligned α phases in the preferred orientation(～45° to the loading direction)at the surface of the fatigue specimens.Fatigued Ti-6Al-4V exhibited a single straight dislocation character at low strain amplitudes(＜0.65％)and dislocation dipoles or even tangled dislocations at high strain amplitudes(＞1.1％).In addition,dislocation substructure features,such as dislocation walls,stacking faults,and disloca-tion networks,were also observed.These findings may provide opportunities to understand the fatigue failure mechanism of additive manufactured titanium parts.     

Development of a stochastic approach for fatigue life prediction of AlSi12 alloy processed by selective laser melting

Parts manufactured by selective laser melting (SLM) process possess unique features in terms of surface roughness, microstructure, residual stresses as well as defect distribution. These defects are responsible for failure of the parts in functional applications. When fatigue loading is applied, these defects are the dominant cause of crack initiation, resulting in scatter of fatigue properties. This scatter occurs due to interacting phenomena like defect size, location as well as the magnitude and type of load. For the purpose of investigating the effect of defects on fatigue life performance of AlSi12 manufactured by selective laser melting, a procedure was developed based on the weakest-link theory and Weibull's probability density function. Using various destructive and non-destructive techniques, defects, including remnant porosity and surface roughness, have been characterized in amount, size and location. Therefore fatigue life prediction, relying on equations constituted from crack propagation properties, was carried out. Predicted fatigue life and Weibull's statistical parameters were used to compare the effect of both defect types on fatigue reliability of AlSi12 produced by SLM. The most probable fatigue life for a sample was interpreted based on Weibull probability density function with respect to maximum probability of occurrence. The prediction of numerous possible values enabled an estimation of fatigue scatter to be made. Thus, the findings of this novel approach enabled conclusions about strength and reliability of different SLM AlSi12 configurations and gave a prelude towards application-oriented design of SLM components.     

高精度数显试样标距划线与测量仪的研制

采用机械装置与电子精密测量仪器组合的方法,将Huafei CQ61180X300Vario微型精密车床、SINO KA-300系列光栅尺和SDS3-1数显表有机地装配在一起,通过设计、加工连接零部件和划线装置,研制出一种新型的高精度数显试样标距划线与测量仪。该装置操作简单、快捷、直观、性价比高,划线准确、测量精度高。近一年来的实际应用结果表明,该仪器对拉伸试样标距的划线、断后标距的测量、断裂韧度试样断口裂纹长度的测量以及对高温疲劳试样标距的确定等均可满足相关标准的要求。     

Overview of non-destructive evaluation techniques for metal-based additive manufacturing

Three-dimensional printing/digital or additive manufacturing is an area that is taking off with considerable rapidity and magnitude. In the same time, non-destructive evaluation (NDE) is playing an important role in the acceptance of additively manufactured parts, in order to provide the required confidence in the quality of the part and its expected safety and performance while in service. This article represents a summary addressing the subject of applicable NDE techniques to detect manufacturing anomalies and service-induced flaws. The topic is relatively new, attracting much research attention and funding, while in the meantime manufacturing processes are continuously improving. The number of publications covering additive manufacturing is increasing exponentially, and everyday new articles, conferences, and workshops are bringing out new information.     

核电用316L不锈钢粉末增材制造研究现状

增材制造技术是一种无须模具、近净成形的先进制造工艺。不锈钢是一种在核电行业广泛应用的结构材料。实现不锈钢结构件的增材制造将进一步推动增材制造技术的发展,也可为核行业带来革命性改变。以核电用316L不锈钢为例,系统阐述了不锈钢粉末增材制造研究现状,包括粉末制备工艺现状、增材制造成形工艺现状以及成形件的组织性能研究现状。目前,增材制造用316L不锈钢粉末的制备工艺主要为雾化法,粉末的物化性能受制粉工艺参数的影响。在激光粉末床熔融增材制造技术、电子束选区熔化技术和等离子增材制造技术中,尤以激光粉末床熔融增材制造不锈钢的应用最为广泛。增材制造316L不锈钢的组织与性能存在各向异性,但各向异性可通过增材制造的后处理技术消除。目前增材制造最为常用的后处理技术为热处理。与锻造316L不锈钢相比,经热等静压处理的增材制造316L不锈钢的力学性能与辐照性能更优。目前,核用不锈钢的增材制造技术还处于起始阶段,后续应重点关注增材制造的成形机理及成形材料中子辐照性能等内容。     

Application of data envelopment analysis for cell performance evaluation and process improvement in cellular manufacturing

This paper proposes a methodology for cell performance evaluation and improvement which considers multiple cell inputs and outputs. A specific technique in data envelopment analysis called 'window analysis', which captures the cell efficiency changes over time, is modified and utilized in the methodology. The evaluation is performed by considering the local part families (manufactured in single cells) and the infrequent parts (manufactured in multiple cells) processed by a cell. A major contribution of this research is in proposing a new 'modified window analysis' technique for cell performance evaluation, and in demonstrating its effectiveness over the 'traditional window analysis'. Another contribution is in using the cross efficiency matrix to identify periods of best cell operating practices which aid management in cell process improvement.     

Investigation of fatigue and fracture properties of asphalt mixtures modified with carbon nanotubes

Load‐induced cracking is one of the primary forms of distress in asphalt pavements at intermediate temperatures. Binder modification is a good alternative to promote the cracking resistance of asphalt mixtures. In the current research study, the effects of carbon nanotubes as a binder modifier on the fatigue and fracture performance of asphalt mixtures are investigated. The carbon nanotubes are added at five different percentages ranging from 0.2% to 1.5% to the base binder to study their effects on the fracture resistance and fatigue life of the asphalt mixtures. Using the cracked semi‐circular bend specimen, the critical value of J‐integral (Jc) was obtained for the investigated modified asphalt mixtures. Also, the fatigue behaviour of asphalt mixtures was studied using flexural beam fatigue test specimen. By employing the ratio of dissipated energy change approach, the plateau value of tested mixtures was determined as a measure of fatigue performance. Results showed that the carbon nanotubes can enhance both fracture resistance and fatigue performance of tested asphalt mixtures especially at higher percentages of the carbon nanotube.     

超高周疲劳的影响因素及疲劳机理的研究进展

超高周疲劳的研究可以满足某些特殊零部件极高循环周次的要求。综述了近年来超高周疲劳的研究进展,从S-N曲线的特征、断面上的鱼眼形貌以及裂纹的萌生与扩展特征等方面介绍了超高周疲劳的典型特征。分析了影响超高周疲劳的若干因素,如氢的作用、加载频率、应力比和晶粒尺寸等。进而提出了一些今后超高周疲劳的研究方向:超高周疲劳裂纹扩展的微观机理、扩展速率尤其是微观、宏观上的控制参量的研究以及确定鱼眼与ODA区边缘的应力强度因子范围对内部裂纹扩展门槛值的影响作用。     

非金属夹杂物特性对钢铁材料疲劳性能影响的研究进展EI北大核心CSCD

本文综述了非金属夹杂物对钢铁材料疲劳性能的影响及研究现状,从夹杂物的角度出发,首先介绍非金属夹杂物特征提取的最新研究进展,分别从实验测量方法和数学公式科学统计方法两方面进行论述;其次根据夹杂物对于疲劳损伤的主要原理,介绍5种应用较为广泛的定量化分析夹杂物特征参数与钢材疲劳性能的数学模型;然后以夹杂物的形貌特征、力学性能以及与基体之间的相互作用为出发点,探究非金属夹杂物的特性对重载零件钢材疲劳性能的影响。最后指出从多角度解析非金属夹杂物对钢材疲劳性能的主要作用机理,构建非金属夹杂物对钢材疲劳寿命预测模型是未来该领域的研究重点。