Applications of non-destructive testing techniques for post-process control of additively manufactured parts

This paper presents an overview on the principle of operation for post-process inspection non-destructive testing (NDT) techniques. The techniques include visual inspection, liquid penetrant testing, magnetic particle testing, eddy current testing, ultrasonic testing, and radiography. The applications of these NDT techniques in additive manufacturing (AM) and their suitability for defects detection of additively manufactured parts are reviewed. The sensitivity, and the advantages and disadvantages of each technique are evaluated. The types of defect, and the detectability of these defects by NDT techniques are assessed. The applicability of each NDT technique for different categories of AM process is discussed. The categories of AM are, namely, material extrusion, powder bed fusion, vat photopolymerisation, material jetting, binder jetting, sheet lamination, and directed energy deposition.     

无掩模定域性电沉积三维微结构的工艺研究

在无掩模定域性电沉积增材制造三维金属微结构的工艺试验过程中,针对成型质量、速度与可重复性的基本要求,采用控制单一变量法和正交实验法研究了极间电压、脉冲占空比、极间距等主要工艺参数对实验结果的影响。研究结果表明:极间电压的大小对微区域电沉积速率一致性、脉冲电流占空比对微沉积速率大小、极间距大小对微沉积区域电场线分布均有不同程度的影响。通过正交实验优化得到的工艺参数为:极间电压2 V、脉冲电流占空比0.4、极间距20 μm、脉冲频率2 kHz,三坐标工作台X和Y轴移动速度均为1 μm/s、Z轴移动速度5 μm/s。通过优化工艺参数取得较好的沉积效果,沉积速率可达300 μm³/s,获得的镍质简单三维微结构,最大深宽比12∶1。     

Inconel 625镍基高温合金激光增材制造翘曲变形行为研究

为了明晰激光增材制造翘曲变形行为,获得相关变形规律。对Inconel 625高温合金激光增材制造工艺对零件变形量的影响进行了分析,实验结果表明:扫描路径的长度决定翘曲变形量的大小;不同工艺参数对翘曲变形量的影响程度不同;激光增材制造加工过程一般会同时引起两个方向的翘曲变形;同时翘曲变形量随着激光功率的增加而增加,随着扫描速度的增加而增加,随着送粉速度的增加而减少。     

不同热处理制度下激光增材制造12CrNi2钢组织研究

采用激光增材制造技术制备12CrNi2合金钢核电应急柴油机凸轮轴可以明显降低产品研发周期,优化产品结构.本研究采用光学显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和电子万能试验机对激光增材制造12CrNi2合金钢热处理前后的组织和力学性能进行了研究.微观组织观察发现,试样存在带状特征和柱状晶形貌,圆弧带间为致密的冶金结合,顶部由于无后一道的重熔热作用,柱状晶形貌十分显著.沉积态组织主要由铁素体组成,试样不同部位的铁素体形貌存在一定的差异.经退火处理后,铁素体边界出现大量白色粒状析出物,延伸率显著提高,达到22.9%.经淬火及不同温度回火处理后,带状特征和柱状晶形貌消除,得到马氏体及其回火组织,抗拉强度达到1 000 MPa.与沉积态抗拉强度831 MPa相比,热处理态试样的抗拉强度显著提高.经固溶时效处理后,晶粒明显粗化.     

Microstructure and mechanical behavior of laser additive manufactured AISI 316 stainless steel stringers

Laser additive manufacturing of stainless steels is a promising process for near net shape fabrication of parts requiring good mechanical and corrosion properties with a minimal waste generation. This work focuses on high aspect ratio AISI 316 steel structures made by superposition of sequential layers. A special nozzle for precise powder delivery together with a monomode fiber laser allowed producing high quality steel stringers on AISI 316 steel substrates. Although the stringers average compositions were inside the austenite plus ferrite range, only austenite phase was verified. The cladded structure presented some internal pores and cracks, responsible by the low Young’s moduli. However, the tensile properties were similar to the base material and other literature results. The three-point flexural tests also produced good results in terms of formability. The fabricated structures proved to be useful for use in mechanical construction.     

稀土La2O3对CMT堆焊Inconel 625合金显微组织和抗高温氧化性能影响

目的 研究稀土氧化物对CMT堆焊镍基合金显微组织和性能的影响规律。方法 采用静电自组装工艺制备含有1%(质量分数,下同)La2O3的Inconel 625药芯焊丝,再采用CMT堆焊工艺制备Inconel 625和1% La2O3/Inconel 625镍基合金试样。系统研究添加La2O3对Inconel 625堆焊试样显微组织、显微硬度、拉伸强度及抗高温氧化性能的影响。结果 添加了La2O3的Inconel 625堆焊试样的金相组织明显细化,且显微组织由柱状晶转变为等轴晶。基体组织主要为γ–Ni固溶体和少量Laves相,且添加了La2O3的镍基合金Laves相的尺寸更为粗大。随着稀土氧化物La2O3的添加,显微硬度值从(235.97±2.99)HV0.3增加到(246.53±4.10)HV0.3,抗拉强度从543.12 MPa增加到564.25 MPa。在高温氧化试验条件下,CMT堆焊1% La2O3/Inconel 625试样的氧化速率明显小于未添加La2O3的Inconel 625原始试样的氧化速率。结论 稀土La2O3的添加产生了细晶强化以及柱状晶向等轴晶的转变,使Inconel 625堆焊试样的显微硬度和抗拉强度都有了一定的提升。在高温氧化过程中,稀土La2O3可作为Cr2O3氧化物的形核中心,促进致密Cr2O3氧化膜生长,相应提升了其表面的抗高温氧化性能。     

Effect of laser-scan strategy on microstructure and fatigue properties of 316L additively manufactured stainless steel

The particular roles of grain morphology and defects, controlled using laser-scan strategies, on the mechanical properties and the fatigue behavior of 316L stainless steel are investigated. Microstructural characterization and X-ray tomography analysis was performed to understand the genesis of polycrystalline microstructure and defects. Tensile and fatigue tests were performed to analyze the effect of defect population and microstructural properties on plasticity and damage mechanisms during monotonic and cyclic loading. The effect of the grain-size and shape and type of defect was carefully investigated to evaluate the mechanisms driving the mechanical behavior under quasi-static and fatigue loading. It is shown that the laser-scan strategy determines the anisotropy in the plane perpendicular to the building direction. Moreover, contrary to the existing literature, for 316L obtained by AM, the grain size and shape does not affect the mechanical properties, and LoF defects drive the fatigue life, independent of the defect/grain size ratio.     

工具形状及工艺过程对搅拌摩擦增材成形及缺陷的影响

采用2mm厚的2195-T8铝锂合金作为增材板条,利用5种不同形状的搅拌工具进行搅拌摩擦增材工艺实验。利用金相观察和硬度测试的分析方法,重点探讨搅拌工具形状与工艺过程对增材成形、界面缺陷及硬度分布的影响。结果表明:圆柱状和三角平面圆台状搅拌针下增材界面上下材料无明显混合,偏心圆柱状和三凹圆弧槽状搅拌针有利于增材界面上下材料混合及减小界面钩状缺陷;增材前进侧界面形成致密无缺陷冶金连接,而后退侧界面材料混合不充分,钩状缺陷易伸入焊核区,且弱连接缺陷起源于此。四层增材中,相邻两层焊接方向相反的增材工艺使除顶层增材外其他增材两侧钩状缺陷向焊核区外侧弯曲,弱连接缺陷得到改善;顶层增材后退侧钩状缺陷伸入焊核区。增材焊核区有明显软化现象,但不同增材工艺下焊核区硬度分布均匀,表明搅拌摩擦增材制造可获得性能均匀的增材;相比于单道焊接工艺,来回双道焊接工艺使单层增材焊核区进一步软化;四层增材中,越靠近顶部的增材,其焊核区平均硬度越大。     

热输入对脉冲等离子弧增材制造Inconel 718合金组织与性能的影响

采用基于脉冲等离子弧的增材制造技术在Q235基板上加工了Inconel 718合金试样,通过改变功率和焊接速度研究了不同热输入对试样组织与性能演变规律的影响。借助光学显微镜、扫描电镜、能谱分析、维氏硬度仪等手段对试样晶粒形态、枝晶间距、元素偏析、析出相成分及分布、显微硬度等进行表征,结果表明随着热输入从1.08×106 J/m增大至1.76×106 J/m,晶粒形态从细长的柱状枝晶逐渐转变为粗大的胞状枝晶,枝晶间距从6.34μm增大至9.09μm,Nb、Mo等元素在枝晶间偏析加剧,Laves相由颗粒状、块状逐渐变为长链状,显微硬度不断下降。     

热丝TIG堆焊Inconel 625层组织成形研究

采用热丝钨极惰性气体保护焊,用ENiCrMo-3型焊丝在Q235B钢管内表面进行堆焊试验。利用金相显微镜、扫描电镜、X射线衍射仪对堆焊层微观组织进行观察和元素成分分析,利用显微硬度计测定试样硬度分布。结果表明,堆焊层组织主要为γ-Ni单相奥氏体,在晶界处有碳化物析出;堆焊层组织自熔合线至顶部的基本形态依次为平面晶、胞晶、树枝晶、等轴晶及顶部横向柱状晶;熔合线附近有少量魏氏体组织形成;试样硬度自基材至堆焊层方向呈递增趋势;上层组织在晶界位置贫Cr较下层严重,从金相看其晶粒受到的侵蚀作用更明显。     

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.     

Microstructure and mechanical properties of as-deposited and heat-treated additive manufactured 9Cr steel

Wire arc additive manufacturing technology has been applied to fabricate 9Cr ferritic/martensitic steel. The steel is widely used in the power industries because of good performances. The effects of different heat treatment conditions on microstructure, hardness, tensile properties, and Charpy impact toughness were investigated. The results show that the microstructure of the as-deposited condition consists of untempered lath martensitic with high strength and low toughness. It was found that heat treatment can change the microstructure characteristics. Moreover, samples after heat treatment have been observed with high elongation and impact toughness but relatively low hardness and tensile strength. The better combination of strength, ductility and microstructure were obtained for the normalising temperature of 1323?K and tempering temperature of 1033?K.     

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。     

Laser additive manufacturing of M2 high-speed steel

M2 high-speed steel samples were fabricated by laser additive manufacturing and tempered at different times at a temperature of 560°C. The microstructures of deposited samples were characterised by fine equiaxial grains, dendrites and inter-dendritic network-shape eutectic carbides and were composed of supersaturated martensite, retained austenite and M₂C-type carbides. The content of retained austenite gradually decreased with increasing tempering times. Meanwhile, the micro-hardness of deposited samples was 688?±?10?HV, while the first, second and third tempering times were 833?±?13?Hv, 710?±?6?Hv and 740?±?7?Hv, respectively (standard deviations).Wear resistances of all samples showed an adhesive wear mechanism, and M2 HSS without tempering had a lower friction coefficient with an average of 0.52. M2 HSS after tempering twice at 560°C/2?h had a larger wear volume loss than others.     

Microstructure and mechanical behavior of laser aided additive manufactured low carbon interstitial Fe49.5Mn30Co10Cr10C0.5 multicomponent alloy

Laser aided additive manufacturing(LAAM)was used to fabricate bulk Fe49.5Mn30Co10Cr10C0.5 interstitial multicomponent alloy using pre-alloyed powder.The room temperature yield strength(σy),ultimate tensile strength(σUTS)and elongation(εUST)were 645 MPa,917 MPa and 27.0％respectively.The as-built sample consisted of equiaxed and dendritic cellular structures formed by elemental segregation.These cellular structures together with oxide particle inclusions were deemed to strengthen the material.The other contributing components include dislocation strengthening,friction stress and grain bound-ary strengthening.The high εUTS was attributed to dislocation motion and activation of both twinning and transformation-induced plasticity(TWIP and TRIP).Tensile tests performed at-40℃and-130℃demonstrated superior tensile strength of 1041 MPa and 1267 MPa respectively.However,almost no twinning was observed in the fractured sample tested at-40℃and-130℃.Instead,higher fraction of strain-induced hexagonal close-packed(HCP)ε phase transformation of 21.2％were observed for fractured sample tested at-40℃,compared with 6.3％in fractured room temperature sample.     

Direct fabrication of single-crystal-like structure using quasi-continuous-wave laser additive manufacturing

The synchronously periodic re-melting of molten pool was firstly introduced in additive manufacturing to promote the epitaxial growth of columnar structure using a novel quasi-continuous-wave(QCW)laser.The epitaxial growth of columnar structure was intensified and the single-crystal-like sample with highly oriented "zigzag" columnar grains was produced.The modified molten-pool geometry and the synchronously high-frequency re-melting of the molten pool contribute to the formation of singlecrystal-like structure.This work reports a new route to promote the continuously epitaxial growth of dendrites for fabrication of single-crystal-like sample.     

Toward multiscale simulations of tailored microstructure formation in metal additive manufacturing

Much of the latent promise of metal additive manufacturing (AM) rests in the potential for controlled creation of spatially tailored microstructures, designed to optimize key build-scale properties through systematic variation across a build. Component optimization possibilities and performance potential expand enormously when this becomes possible. However, the extreme conditions created by AM energy sources and the nature of alloy solidification under such conditions are not adequately understood. Modeling and simulation tools capable of quantitatively predicting AM microstructural outputs would enable a major leap forward. We demonstrate through experimental validation that a multiscale (nm/ns to mm/ms) simulation framework coupling CALPHAD thermodynamic models, microstructure scale phase field simulations, and laser track scale multiphysics simulations can quantitatively predict tailored microstructure formation in laser processed Ti-Nb. Extensive simulations and analysis of detailed microstructural predictions over a broad range of conditions reveal scaling laws for characteristic microstructural features that should generalize to a wide range of materials. Several of our findings highlight the central importance of the alloy freezing range $\mathrm{\Delta }{T}_f$, which provides the basis of a generalized strategy for optimizing spatial control of microstructure during AM. This proposed strategy integrates alloy design with process design & control to identify optimal material and process condition combinations. We have also identified conditions and phenomena that appear to require a more complete treatment of far from equilibrium solidification kinetics, highlighting a fundamental need in the field. We anticipate that further development of such methodologies will contribute centrally to realizing the potential of materials with spatially tailored microstructure.     

Prediction of solidification behaviour and microstructure of Ni based alloys obtained by casting and direct additive laser growth

Numerical tools are now used widely in the prediction of material properties necessary in order to gain a better understanding of the relationship between material properties and performance, to improve the reliability of processes and the quality of the final product, and to reduce costs, waste and energy use. In this paper, the solidification properties and the microstructure of some commercial Ni based alloys were analysed and predicted numerically using the ProCAST software. The microstructure of a sample obtained by the direct additive laser growth, a new additive manufacturing technique based on the selective laser melting, is presented and discussed. Numerical approaches and software packages that can be used to model additive manufacturing processes are discussed and critically analysed.