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.     

An integrated design methodology for components produced by laser powder bed fusion (L-PBF) process

Laser powder bed fusion (L-PBF) is an additive manufacturing (AM) process that allows to build full dense metal complex part. However, despite the obvious benefits of L-PBF process, it is affected by specific technological drawbacks and it suffers from issues regarding design support tools. In order to fully exploit the advantages of L-PBF, it is necessary to know the technological constraints, such as material availability and the need to minimise the support structures. In this paper, an integrated design procedure that involves topology optimisation, design for laser powder bed fusion rules and finishing requirements is presented in order to define practical guidelines for successful AM of metal parts. The procedure is tested using a case study to prove the effectiveness of the proposed approach.     

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.     

Evolution of Creep Damage of 316L Produced by Laser Powder Bed Fusion

The damage mechanisms of metallic components produced by process laser powder bed fusion differ significantly from those typically observed in conventionally manufactured variants of the same alloy. This is due to the unique microstructures of additively manufactured materials. Herein, the focus is on the study of the evolution of creep damage in stainless steel 316L specimens produced by laser powder bed fusion. X-ray computed tomography is used to unravel the influence of the process-specific microstructure from the influence of the initial void distribution on creep damage mechanisms. The void distribution of two specimens tested at 600 °C and 650 °C is analyzed before a creep test, after an interruption, and after fracture. The results indicate that the formation of damage is not connected to the initial void distribution. Instead, damage accumulation at grain boundaries resulting from intergranular cracking is observed.     

Selective laser melting raw material commoditization: impact on comparative competitiveness of additive manufacturing

The paper analyses the impact of cheaper metal powder supplies on the comparative competitiveness of additive manufacturing (AM). By utilising two case studies, we compare the economic impact of an innovative titanium extraction method on Selective Laser Melting (SLM) and conventional methods of machining and casting. A switch-over analysis identifies the production quantities above which conventional manufacturing is more cost competitive than additive manufacturing. This analysis is performed for current raw material as well as cheaper raw material supply. The results illustrate the improved comparative competitiveness of SLM as the titanium supply is commoditised and more readily available in powder form. The responsiveness of the supply chain is improved as the switch-over point between SLM and conventional methods increases. Moreover, as the raw material supply chain for titanium is transformed through the use of this novel extraction method, the manufacturing supply chain is simplified.     

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。     

Simultaneous topology and build orientation optimization for minimization of additive manufacturing cost and time

The ever-present demand for increased performance in mechanical systems, and reduced cost and manufacturing time, has led to the adoption of computational design tools and innovative manufacturing methods. One such tool is topology optimization (TO), which often produces designs that are impracticable to manufacture. However, recent developments in additive manufacturing (AM) have made production of such complex designs feasible. Therefore, integration of these technologies has the potential to innovate the design and manufacture of mechanical components. This work presents a novel mathematical methodology for multiobjective minimization of structural compliance and AM cost and time, in simultaneous build orientation and density-based TO. Component surface area and support volume were implemented in this method as the physical factors influencing AM cost and time. A new methodology was produced to approximate support volume throughout TO with variable build orientation, enabling direct minimization of support volume in the proposed optimization. The methodology allows derivation of sensitivity expressions, thereby permitting the use of efficient gradient-based optimization solvers. Three numerical examples demonstrated that the proposed methodology can efficiently produce optimum build orientations and topologies, which significantly reduce structural compliance and AM cost and time.     

A tensor-based hierarchical process monitoring approach for anomaly detection in additive manufacturing

Additive manufacturing (AM) is a technology that enables the creation of complex shapes with advanced structural and functional properties. It has transformed the traditional manufacturing operations into a more flexible and efficient process, reshaping the whole value chain and allowing new levels of product customization. AM is a layer-by-layer manufacturing process, in which materials are deposited in each layer to create the object of interest. Due to the layer-wise nature of the process, anomalies and defects might occur within each layer, across several layers or throughout the whole sample. An accurate and responsive detection strategy that enables the detection of various types of anomalies is essential for ensuring the quality and integrity of the manufactured product. In this paper, a hierarchical in situ process monitoring approach, namely, a three level monitoring strategy, is proposed to detect local, layer-wise, and sample-wise anomalies using thermal videos acquired during the manufacturing process. The proposed approach integrates hierarchical low-rank tensor decomposition methods with statistical monitoring techniques to effectively detect anomalies at different levels, namely, the within-layer level, the layer level, and the sample level. Simulations are used to evaluate the performance of the method and compare with existing benchmarks. The proposed approach is also applied to thermal videos acquired during the laser powder bed fusion (L-PBF) process to illustrate its effectiveness in practice.     

Design and experimental validation of self-supporting topologies for additive manufacturing

ABSTRACT

Incorporating additive manufacturing (AM) constraints in topology optimisation can lead to performance optimality while ensuring manufacturability of designs. Numerical techniques have been previously proposed to obtain support-free designs in AM, however, few works have verified the manufacturability of their solutions. Physical verification of manufacturability becomes more critical recalling that the conventional density-based topology optimisation methods will inevitably require post-processing to smooth the boundaries before sending the results to a 3D printer. This paper presents the smooth design of self-supporting topologies using the combination of a new Solid Isotropic Microstructure with Penalisation method (SIMP) developed based on elemental volume fractions and an existing AM filter. Manufacturability of selected simulation results are verified with Fused Deposition Modeling (FDM) technology. It is illustrated that the proposed method is able to generate convergent self-supporting topologies which are printable using FDM.     

Powder-Based Additive Manufacturing: A Critical Review of Materials,Methods, Opportunities,and Challenges

As a 0D material, powder particles can be used to create almost any complicated engineering component by utilizing the high-performance manufacturing capabilities of additive manufacturing (AM). Although powder-based AM methods provide an outstanding practical value and development for modern manufacturing world, they continue to face challenges such as a lack of accessible categories, temperature restrictions, and poor performance of molded components. Therefore, researching for new AM materials and procedures has become an extremely necessary endeavor. For this purpose, a firm grasp of the current state of the art of powder-based AM technologies is imperative. Hence, herein, a comprehensive review is presented on the most widely used powder-based AM methods, and the materials used by these methods. For each method, the development and current state, operating principles, limitations, and future prospects are summarized. In contrast, for materials, their classifications, properties, and preparation methods are explored in great detail, while also commenting on the specific compatibilities between powder materials and powder methods. Industrial and commercialized applications of powder-based AM are also presented in this work. Finally, the limitations of the current powder-based technologies are highlighted, with comments regarding the future of this field.     

陶瓷材料增材制造技术研究进展

增材制造技术是20世纪90年代出现的,以高能束为基础通过逐层叠加材料得到终产品的快速成形技术。以选择性激光烧结/熔融为主线,综述了陶瓷材料增材制造技术的发展历程,概述了间接法和直接法的原理、特点以及其局限性,指出要解决陶瓷制品增材制造存在的问题,必须加强相关理论研究,优化粉末质量和后处理工艺,以及探索合适的工艺参数。     

机制砂砂浆干缩性能的研究

研究了砂灰比对石灰岩机制砂砂浆干缩率、不同类型砂对砂浆干缩率、不同石粉含量对砂浆干缩率的影响。结果表明,随着砂灰比的减小,即水泥浆量的增加,干缩率呈增大的趋势;但在水泥浆未填充满砂子空隙之前,随着水泥浆的增加,干缩率的增幅很小,当填充满之后继续增加,增幅明显变大。在早期(≤7d),石灰岩机制砂砂浆的干缩率大于花岗岩和石英岩机制砂,在后期(>7d),花岗岩和石英岩机制砂砂浆的干缩率大于石灰岩机制砂;在各个龄期,3种机制砂砂浆的干缩率均大于河砂的。随着石粉含量的增大,机制砂砂浆的收缩率先减小后增大。在早期(1d、3d),石粉含量为10%时砂浆干缩率最小;在后期(≥7d),石粉含量为15%时砂浆干缩率最小。     

Effect of tellurium on the microstructure and mechanical properties of Fe-14Cr oxide-dispersion-strengthened steels produced by additive manufacturing

Conventionally,Te has primarily been used to improve the machinability of steel and its alloys.In this work,Te was used to refine the grains of an oxide-dispersion-strengthened (ODS) steel produced by ad-ditive manufacturing (AM) with fixed processing parameters.Addition of Te to the raw powder produced an ODS steel with a fine-grained microstructure,in contrast to the ODS steel manufactured without Te.Moreover,the addition of Te resulted in superior yield strength and ultimate tensile strength,which was attributed to the combined effects of grain refinement and the finer nanoparticles (NPs) composed of Te-rich composite NPs and Cr-rich NPs.For the first time,the AM technique was used to obtain grain and nanoparticle sizes of ～3A μm and 6 nm,respectively,from the Te-added ODS steel     

Topology optimization of continuum structures for natural frequencies considering casting constraints

A method for the topology optimization on the natural frequency of continuum structures with casting constraints is proposed. The objective is to maximize the natural frequency of vibrating continuum structures subject to casting constraints. When the natural frequencies of the considered structures are maximized using the solid isotropic material with penalization (SIMP) model, artificial localized modes may occur in areas where elements are assigned with lower density values. In this article, the topology optimization is performed by the bi-directional evolutionary structural optimization (BESO) method. The effects of different locations of concentrated lump mass, different volume fractions and meshing sizes on the final topologies are compared. Both two and four parting directions are investigated. Several two- and three-dimensional numerical examples show that the proposed BESO method is effective in achieving convergent solid–void optimal solutions for a variety of frequency optimization problems of continuum structures.     

Modeling and Experimental Validation of the Electron Beam Selective Melting Process

Electron beam selective melting (EBSM) is a promising additive manufacturing (AM) technology. The EBSM process consists of three major procedures: ① spreading a powder layer, ② preheating to slightly sinter the powder, and ③ selectively melting the powder bed. The highly transient multi-physics phenomena involved in these procedures pose a significant challenge for in situ experimental observation and measurement. To advance the understanding of the physical mechanisms in each procedure, we leverage high-fidelity modeling and post-process experiments. The models resemble the actual fabrication procedures, including ① a powder-spreading model using the discrete element method (DEM), ② a phase field (PF) model of powder sintering (solid-state sintering), and ③ a powder-melting (liquid-state sintering) model using the finite volume method (FVM). Comprehensive insights into all the major procedures are provided, which have rarely been reported. Preliminary simulation results (including powder particle packing within the powder bed, sintering neck formation between particles, and single-track defects) agree qualitatively with experiments, demonstrating the ability to understand the mechanisms and to guide the design and optimization of the experimental setup and manufacturing process.     

A review on powder-based additive manufacturing for tissue engineering: selective laser sintering and inkjet 3D printing

Since most starting materials for tissue engineering are in powder form, using powder-based additive manufacturing methods is attractive and practical. The principal point of employing additive manufacturing (AM) systems is to fabricate parts with arbitrary geometrical complexity with relatively minimal tooling cost and time. Selective laser sintering (SLS) and inkjet 3D printing (3DP) are two powerful and versatile AM techniques which are applicable to powder-based material systems. Hence, the latest state of knowledge available on the use of AM powder-based techniques in tissue engineering and their effect on mechanical and biological properties of fabricated tissues and scaffolds must be updated. Determining the effective setup of parameters, developing improved biocompatible/bioactive materials, and improving the mechanical/biological properties of laser sintered and 3D printed tissues are the three main concerns which have been investigated in this article.     

A review on powder-based additive manufacturing for tissue engineering: selective laser sintering and inkjet 3D printing

Abstract

Since most starting materials for tissue engineering are in powder form, using powder-based additive manufacturing methods is attractive and practical. The principal point of employing additive manufacturing (AM) systems is to fabricate parts with arbitrary geometrical complexity with relatively minimal tooling cost and time. Selective laser sintering (SLS) and inkjet 3D printing (3DP) are two powerful and versatile AM techniques which are applicable to powder-based material systems. Hence, the latest state of knowledge available on the use of AM powder-based techniques in tissue engineering and their effect on mechanical and biological properties of fabricated tissues and scaffolds must be updated. Determining the effective setup of parameters, developing improved biocompatible/bioactive materials, and improving the mechanical/biological properties of laser sintered and 3D printed tissues are the three main concerns which have been investigated in this article.     

Effect of particle morphology on the flowability of HDH Ti powders treated by high temperature ball milling

The metal powder flowability determined by powder characteristics significantly influences on the final parts manufactured by additive manufacturing (AM). Among the characteristics, the individually effect of morphology on its flowability still is not comprehensively evaluated. In this study, the effect of morphology on the flowability of hydride-dehydrate titanium (HDH Ti) powder treated by high temperature ball milling (HTBM) was investigated systematically using the fractal dimension and shape factors, i.e., circularity, elongation, compactness and convexity. The results of HTBM show that the modified HDH Ti powder with a mass flow rate of 48.1 ± 0.3 s/50 g and the angle of repose (AOR) of 38.88 ± 0.5°. The fractal dimension of powder plays a main role on the flowability, which increase in the decrease of it. The box plot and Pearson correlation coefficient of shape factors indicate that only circularity is strong correlation with flowability, which increases with it. The regression model of flowability expressed with fractal dimension and circularity shows the adjusted R² value of 0.991, which is good agreement with the experiment. Taken together, this study provides a general guide to evaluate the powder flowability based on the morphology.     

Additive Design and Manufacturing of Jet Engine Parts

The additive design (AD) and additive manufacturing (AM) of jet engine parts will revolutionize the traditional aerospace industry. The unique characteristics of AM, such as gradient materials and micro-structures, have opened up a new direction in jet engine design and manufacturing. Engineers have been liberated from many constraints associated with traditional methodologies and technologies. One of the most significant features of the AM process is that it can ensure the consistency of parts because it starts from point(s), continues to line(s) and layer(s), and ends with the competed part. Collaboration between design and manufacturing is the key to success in fields including aerodynamics, thermodynamics, structural integration, heat transfer, material development, and machining. Engineers must change the way they design a part, as they shift from the traditional method of “subtracting material” to the new method of “adding material” in order to manufacture a part. AD is not the same as designing for AM. A new method and new tools are required to assist with this new way of designing and manufacturing. This paper discusses in detail what is required in AD and AM, and how current problems can be solved.     

Additive Manufacturing of Metal Structures at the Micrometer Scale

Currently, the focus of additive manufacturing (AM) is shifting from simple prototyping to actual production. One driving factor of this process is the ability of AM to build geometries that are not accessible by subtractive fabrication techniques. While these techniques often call for a geometry that is easiest to manufacture, AM enables the geometry required for best performance to be built by freeing the design process from restrictions imposed by traditional machining. At the micrometer scale, the design limitations of standard fabrication techniques are even more severe. Microscale AM thus holds great potential, as confirmed by the rapid success of commercial micro‐stereolithography tools as an enabling technology for a broad range of scientific applications. For metals, however, there is still no established AM solution at small scales. To tackle the limited resolution of standard metal AM methods (a few tens of micrometers at best), various new techniques aimed at the micrometer scale and below are presently under development. Here, we review these recent efforts. Specifically, we feature the techniques of direct ink writing, electrohydrodynamic printing, laser‐assisted electrophoretic deposition, laser‐induced forward transfer, local electroplating methods, laser‐induced photoreduction and focused electron or ion beam induced deposition. Although these methods have proven to facilitate the AM of metals with feature sizes in the range of 0.1–10 µm, they are still in a prototype stage and their potential is not fully explored yet. For instance, comprehensive studies of material availability and material properties are often lacking, yet compulsory for actual applications. We address these items while critically discussing and comparing the potential of current microscale metal AM techniques.