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.     

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.     

Additive manufacturing of mechanical testing samples based on virgin poly (lactic acid) (PLA) and PLA/wood fibre composites

3D printing in additive manufacturing is considered as one of key technologies to the future high-precision manufacturing in order to benefit diverse industries in building construction, product development, biomedical innovation, etc. The increasing applications of 3D printed components depend primarily on their significant merits of reduced weight, minimum used materials, high precision and shorter production time. Furthermore, it is very crucial that such 3D printed components can maintain the same or even better material performance and product quality as those achieved by conventional manufacturing methods. This study successfully fabricated 3D printed mechanical testing samples of PLA and PLA/wood fibre composites. 3D printing parameters including infill density, layer height and the number of shells were investigated via design of experiments (DoE), among which the number of shells was determined as the most significant factor for maximising tensile strengths of PLA samples. Further, DoE work evaluated the effect of material type (i.e., neat PLA and PLA/wood fibres) and the number of shells on tensile, flexural and impact strengths of material samples. It is suggested that material type is the only predominant factor for maximising all mechanical strengths, which however are consistently lower for PLA/wood fibre composites when compared with those of neat PLA. Increasing the number of shells, on the other hand, has been found to improve almost all strength levels and decrease infill cavities. The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-018-0211-3     

High-Entropy Alloy-Induced Metallic Glass Transformation: Challenges Posed by in situ Alloying via Additive Manufacturing

In situ alloying and fabricating glassy structures through a layer-by-layer fashion approach are challenging but have high potential to develop novel-graded materials. For the first time, this cost-effective approach is applied to additive manufacturing (AM) of a Zr-based bulk metallic glass (BMG) from high-entropy alloys (HEAs). A newly developed composition of Zr₄₀Al₂₀Cu₂₀Ti₂₀ is fabricated through laser powder bed fusion (LPBF). Process parameters are optimized within a wide range of laser power (50–200 W) as well as scanning speed (50–800 mm s⁻¹). In all printed samples, microscopic and compositional examinations reveal no glass formation, but very fine grains and CuTi and AlTi nanocrystals. Some glassy transitions at the interfaces may be encouraged to occur with proper melting and mixing. However, the main reason for not obtaining a glassy matrix is the substantial proportion of unmelted Zr raw powder throughout the structure as spherical particles. Consequently, glass formation can be hindered by a considerable amount of compositional deviation. During LPBF, in situ alloying poses significant challenges to developing BMGs. Hence, the various stages of the process, including raw material specifications, laser settings, and process parameters, should be investigated further.     

Additive manufacturing management: a review and future research agenda

Based on the phenomenal changes that additive manufacturing (AM) has brought to industries and markets, managerial approaches should be re-examined and developed to take advantage of emerging opportunities. This revolutionary technology is dramatically changing business and innovation models, shrinking supply chains and altering the global economy. For example, 3D printing shifts production locations closer to customers and leads to free-form product design as well as sustainable manufacturing. Several studies have been conducted on AM technology, but the research stream on AM management is still developing, with studies dispersed in journals across different research areas. Our study presents both systematic and quantitative analyses of the literature, including co-citation analysis, factor analysis and multidimensional scaling, to explore the structure of the AM research domains in the scope of management, business and economics. We found eight main research streams: AM technology selection, supply chain, product design and production cost models, environmental aspects, strategic challenges, manufacturing systems, open-source innovation and business models and economics. Finally, based on the results of our in-depth analysis of the literature, we found nine promising future research directions.     

Directed energy deposition (DED) additive manufacturing: Physical characteristics,defects, challenges and applications

Directed energy deposition (DED) is a branch of additive manufacturing (AM) processes in which a feedstock material in the form of powder or wire is delivered to a substrate on which an energy source such as laser beam, electron beam, or plasma/electric arc is simultaneously focused, thus forming a small melt pool and continuously depositing material, layer by layer. DED has several unique advantages compared to other AM processes, such as site-specific deposition and repair, alloy design, and three-dimensional printing of complex shapes. Herein, recent advances as well as the main aspects governing laser-material interactions during the DED process, melt pool thermal behavior, advanced in situ monitoring, and interaction mechanisms are critically reviewed. The most critical processing variables and their influence on the deposited material properties, along with defect formation mechanisms and characterization techniques, are also identified and discussed. An overview of high-end applications, current challenges associated with DED processing, and a critical outlook of the technology are presented.     

硬质合金粉末挤出打印中增材制造工艺及其显微结构

粉末挤出打印(PEP)是基于传统金属注塑成型和3D打印相结合的新型增材制造技术,具有打印材料范围广、打印成本低等巨大优势。以WC-13Co硬质合金的PEP增材制造为核心,以热塑性打印材料为重点研究对象,开发打印原料的材料体系,研究打印原料的均匀性、流变性能、成形性能、黏结剂的脱除工艺以及烧结工艺对打印件显微结构及力学性能的影响机制。独立开发了硬质合金PEP打印专用的有机黏结剂材料体系,通过EDS分析黏结剂在打印坯体中分散均匀性。采用两步法脱脂工艺可以完全脱除打印坯体中的黏结剂,并结合真空烧结,在1450 ℃下保温60 min,成功制备高性能硬质合金打印件。研究结果发现打印件线收缩率为17.8%,WC晶粒尺寸分布均匀,维氏硬度1410HV₃₀。本研究采用PEP增材制造技术制备了高性能、打印件尺寸可控的硬质合金材料,为硬质合金的增材制造探索出一条有效的技术路线。     

Additive Manufacturing of Porous Structures for Unmanned Aerial Vehicles Applications

Unmanned aerial vehicles (UAVs) have shown promising benefits in many applications. This has been enabled by the emergence of additive manufacturing (AM), which give the designers a large amount of geometrical freedom. In this paper, a novel design process of fused deposition modeling (FDM) combining both topology and infill optimization is introduced for AM of high performance porous structures. Tensile testing of FDM printed samples is first carried out to study the effect of the build orientation on the mechanical properties of acrylonitrile butadiene styrene (ABS) samples. It is found that samples built perpendicular to the load axis are the weakest with a tensile strength of 29 MPa and Young's modulus of 1960 MPa. The materials properties are fed to the finite elements analysis (FEA) for geometrical topology optimization, aiming to maximize stiffness and reduce weight of those parts. Afterwards, an infill optimization is carried out on the topology optimized parts using different mesostructures such as honeycomb, triangular, and rectangular to achieve high structural performance. The results showed that triangular pattern with 50% infill density had the lowest developed stresses, less mass, and strain energy when compared to other structures. Optimum UAVs parts of a quadcopter are successfully manufactured, assembled, and tested.

     

增材制造复杂流道水冷电机壳体对驱动电机持续功率影响的研究

目的 提高量产铸造电机壳体的换热效率,确保电机在高功率持续工作状态下不会过热,从而提高电机的持续功率。方法 基于增材思维对电机水冷壳体的流道进行优化,改变流道形状以增大流道表面积、消除流道涡流并减小流道与内壁的间距。通过仿真分析,不断优化迭代得到最佳的流道设计方案。运用选区激光熔化（SLM）增材技术及相应的后处理工艺,制造出复杂流道结构的电机壳体。结果 采用SLM增材技术制造的AlSi10Mg铝合金壳体在x、xz、z 3个方向上的屈服强度均大于230 MPa,即使在较小壁厚的条件下,壳体强度仍满足设计要求。采用该壳体后,电机的持续功率从原量产电机的45 kW提升到50.7 kW,且仍能连续稳定运行45 min,同时电机温度未超过130℃。微观组织检测和工业CT测试结果显示,SLM电机壳体结构致密,未见气孔夹杂。该增材制造壳体的质量为6.95 kg,与量产电机壳体相比,减重约19%。结论 通过增材制造技术设计制造的电机壳体整体性能良好,可以有效提高换热效率以及电机的持续功率,并实现了电机的减重。     

Evaluation of rheological properties of lead-free solder pastes and their relationship with transfer efficiency during stencil printing process

Although stencil printing is widely used in surface mount technology, it is believed to be the main source of majority of defects in the final assembly. Such defects, which could lead to major reliability issues, can be controlled and/or minimised through proper understanding and control of the flow and deformation behaviour of solder pastes. This study concerns the characterisation of three different Pb-free solder pastes used for ultra-fine pitch assembly applications. We also investigate the paste transfer efficiency through linearly varying stencil apertures sizes, and correlate the paste rheological properties with their transfer efficiency – to provide further understanding of the effects and interactions of stencil printing process variables on the paste transfer efficiency. Three rheological tests, namely: the viscosity, thixotropic and oscillatory amplitude sweep were employed in the characterisation. The paste samples were printed on Cu substrates using stencil printing (with varying aperture cavity dimensions in the range 0.79 mm³–1.62 mm³). A three-level design on two factors experiment, 3², was used to determine significant level of parameters in terms of main effect and interactions. Our results show that the paste type and volume of stencil aperture interact during printing and that pastes with unique rheological properties produced distinctive transfer pattern. The results also show that the printing process variables and their interactions were significant on transfer efficiency of pastes. These results will be of interest to R&D staff involved in formulation of new Pb-free pastes and the design of stencils for ultra-fine pitch assembly applications.     

Hydration monitoring of cement-based materials with resistivity and ultrasonic methods

Two test setups, the electrical resistivity and ultrasonic techniques, were used to monitor the hydration process of cement-based materials. In the electrical resistivity method, a non-contacting device was used. In the ultrasonic method, a wave was transmitted and measured by the embedded piezoelectric ultrasonic transducers, which had good coupling with the surrounding materials. The focus of the study was to detect the setting and hardening behaviors of cement paste during the first 7 days of hydration using the above techniques. Immediate after placing the cement paste into the mould, the measurement started and continued throughout the hydration process. The obtained resistivity and ultrasonic data were used to interpret the hydration process of the specimens. The correlation of two techniques was also studied. The results illustrated that both electrical resistivity and ultrasonic techniques were effective to accurately monitor the hydration of cement pastes. The resistivity method was able to study both the chemical reaction and physical change during hydration, while ultrasonic method was sensitive to physical change of cement only.     

Penetration of corrosion products and corrosion-induced cracking in reinforced cementitious materials: Experimental investigations and numerical simulations

This paper describes experimental investigations on corrosion-induced deterioration in reinforced cementitious materials and the subsequent development and implementation of a novel conceptual model. Reinforced mortar specimens of varying water-to-cement ratios were subjected to current-induced corrosion (10, 50, and 100 μA/cm²). X-ray attenuation measurements and visual investigations provided both qualitative and quantitative information on the penetration of solid corrosion products into the surrounding cementitious matrix. X-ray attenuation measurements provided time- and location-dependent concentrations of corrosion products averaged through the specimen thickness. Digital image correlation (DIC) was used to measure corrosion-induced deformations including deformations between steel and cementitious matrix as well as formation and propagation of corrosion-induced cracks. Based on experimental observations, a conceptual model was developed to describe the penetration of solid corrosion products into capillary pores of the cementitious matrix. Only capillary pores within a corrosion accommodating region (CAR), i.e. in close proximity of the steel reinforcement, were considered accessible for corrosion products. The conceptual model was implemented into a FEM based cracking model and compared to experimental results provided in the literature and obtained from DIC measurements.     

Agile manufacturing system control based on cell re-configuration

The control of an agile manufacturing system (AMS) is expected to be flexible, open, scalable and re-configurable so as to tackle the more complex and uncertain information flows. To meet these requirements, we propose agent-based control architecture for AMS, under which the functions of task planning, scheduling and dynamic control are integrated seamlessly. First of all, this paper introduces the concept of RMC (re-configurable manufacturing cell), based on which, we construct the control architecture for AMS in compliance with multi-agent system (MAS). The whole control process under the architecture comprises two hierarchies, i.e. the upper one for order planning and RMC forming and the lower one for task scheduling within each RMC. For the upper hierarchy, we establish a linear integer programming (LIP)-based mathematical model and a MAS-based dynamic process model, and present a two-step approach to order planning and RMC forming. For the lower hierarchy, we develop the scheduling model, a method based on the bidding mechanism from contract net, and describe the rescheduling mechanism in the control system. To illustrate the methodology proposed in the paper, a simulation study is thoroughly discussed. Our studies demonstrate that the RMC-based control architecture provides an AMS with an optimal, dynamic and flexible mechanism of responding to an unpredictable manufacturing environment, which is crucial to achieve agility for the whole manufacturing system.     

Mechanical and thermal characterization of epoxy composites reinforced with random and quasi-unidirectional untreated Phormium tenax leaf fibers

The present work investigates tensile and flexural behavior of untreated New Zealand flax (Phormium tenax) fiber reinforced epoxy composites. Two series of laminates were produced using the same reinforcement content (20 wt%), arranged either as short fibers or quasi-unidirectional ones. Composites reinforced using quasi-unidirectional fibers showed higher modulus and strength both in tensile and flexural loading, when compared to neat epoxy resin. Short fiber composites, although still superior to epoxy resin both for tensile and flexural moduli, proved inferior in strength, especially as concerns tensile strength. These results have been supported by scanning electron microscopy (SEM), which allowed characterizing fiber–matrix interface, and by acoustic emission (AE) analysis, which enabled investigating failure mechanisms. In addition, thermal behavior of both untreated phormium fibers and composites has been studied by thermogravimetric analysis (TGA), revealing the thermal stability of composites to be higher than for phormium fibers and epoxy matrix alone.     

Evaluation of eddy current testing for quality assurance and process monitoring of automated fiber placement

Standards in energy and cost efficiency are higher the ever especially in the aerospace industry. While structures made from carbon-fiber reinforced plastics (CFRP) show significant advantages in regards to specific strength and lightweight design, further improvements in their production processes are essential in order for CFRP to be competitive in the future. The authors present eddy current (EC) testing as a means for quality assurance (QA) and process monitoring for CFRP parts produced by automatic fiber placement (AFP), which is one the most prevalent production methods in aerospace industry. Eddy current testing shows the potential for highly automated process monitoring that can reduce error correction and cycle time in AFP.     

电子束粉末床熔融制备钛铝基金属间化合物研究进展

钛铝基金属间化合物是一种理想的高温结构材料,但因存在室温塑性差、加工困难等不足而限制了其发展与应用。电子束粉末床熔融(Electron Beam Powder Bed Fusion, EB?PBF)技术能够实现近净成形,其加工中的低热应力特点适宜脆性材料的制备,是近年来广受关注的新型钛铝基金属间化合物成形方法。对用电子束粉末床熔融制备的钛铝基金属间化合物进行了介绍,并对近年来发表的以EB?PBF钛铝材料为研究对象的相关文献进行了综述。从工艺、后处理和性能表征等角度对目前的研究现状进行了分析总结,并对未来的研究工作提出了展望。     

Alloy design via additive manufacturing: Advantages,challenges, applications and perspectives

Additive manufacturing (AM) has rapidly changed both large- and small-scale production environments across many industries. By re-envisioning parts from the ground up, not limited to the challenges presented by traditional manufacturing techniques, researchers and engineers have developed new design strategies to solve large-scale materials and design problems worldwide. This is particularly true in the world of alloy design, where new metallic materials have historically been developed through tedious processes and procedures based primarily on casting methodologies. With the onset of directed energy deposition (DED) and powder bed fusion (PBF)-based AM, new alloys can be innovated and evaluated rapidly at a lower cost and considerably shorter lead time than has ever been achieved. This article details the advantages, challenges, applications, and perspectives of alloy design using primarily laser-based AM. It is envisioned that researchers in industry and academia can utilize this work to design new alloys leveraging metallic AM processes for various current and future applications.     

Additive manufacturing of concrete in construction: potentials and challenges of 3D concrete printing

Additive manufacturing is gaining ground in the construction industry. The potential to improve on current construction methods is significant. One of such methods being explored currently, both in academia and in construction practice, is the additive manufacturing of concrete (AMoC). Albeit a steadily growing number of researchers and private enterprises active in this field, AMoC is still in its infancy. Different variants in this family of manufacturing methods are being developed and improved continuously. Fundamental scientific understanding of the relations between design, material, process, and product is being explored. The collective body of work in that area is still very limited. After sketching the potential of AMoC for construction, this paper introduces the variants of AMoC under development around the globe and goes on to describe one of these in detail, the 3D Concrete Printing (3DCP) facility of the Eindhoven University of Technology. It is compared to other AMoC methods as well as to 3D printing in general. Subsequently, the paper will address the characteristics of 3DCP product geometry and structure, and discuss issues on parameter relations and experimental research. Finally, it will present the primary obstacles that stand between the potential of 3DCP and large-scale application in practice, and discuss the expected evolution of AMoC in general.