Challenges In Additive Manufacturing Research Articles

Selective Laser Melting of Ti-6Al-4V: The Impact of Post-processing on the Tensile, Fatigue and Biological Properties for Medical Implant Applications.

One of the main challenges in additive manufacturing (AM) of medical implants for the treatment of bone tissue defects is to optimise the mechanical and biological performance. The use of post-processing can be a necessity to improve the physical properties of customised AM processed implants. In this study, Ti-6Al-4V coupons were manufactured using selective laser melting (SLM) in two build orientations (vertical and horizontal) and subsequently post-processed using combinations of hot isostatic pressing (HIP), sandblasting (SB), polishing (PL) and chemical etching (CE). The effect of the different post-manufacturing strategies on the tensile and fatigue performance of the SLMed parts was investigated and rationalised by observing the surface topography. Vertically built samples showed higher yield strength (YS) and ultimate tensile strength (UTS) than the horizontal samples, increasing from 760.9 ± 22.3 MPa and 961.3 ± 50.2 MPa in the horizontal condition to 820.09 ± 16.5 MPa and 1006.7 ± 6.3 MPa in the vertical condition, respectively. After the HIP treatment, the ductility was substantially improved in both orientations; by 2.1 and 2.9 folds in the vertical and horizontal orientations, respectively. The vertically built samples demonstrated a superior ductility of 22% following HIP and polishing. Furthermore, chemical etching was found to be the most effective surface post-processing treatment to improve the fatigue performance after HIP, achieving the highest run-out strength of 450 MPa. Most importantly, chemical etching after HIP enhanced the cellular affinity of the surface, in addition to its good fatigue performance, making it a promising post-processing approach for bone implants where tissue integration is needed.

Prediction of selective laser melting part quality using hybrid Bayesian network

Additive manufacturing (AM) is gaining popularity because of its ability to manufacture complex parts in less time. Despite recent research involving designs of experiments (DOEs) to characterize the relationships between some AM process parameters and various part quality characteristics, to date, there seems to be no universally accepted comprehensive model that relates process parameters to part quality. In this paper, to support the goal of manufacturing parts right the first time, a Bayesian network in continuous domain is developed which relates four process parameters (laser power, scan speed, hatch spacing, and layer thickness) and five part quality characteristics (density, hardness, top layer surface roughness, ultimate tensile strength in the build direction and ultimate tensile strength perpendicular to the build direction). A machine learning algorithm is used to train the network on a database mined from a large number of publications with experimental data from parts built using 316L with selective laser melting. Using this Bayesian network, the user is able to enter a value for one or more known nodes or variables, and the network provides predictions on all the remaining nodes in the form of probability distributions. A method is developed whereby the user inputs are checked for reasonableness using an n -dimensional convex hull, and if necessary a recommendation is returned based on user-defined weights. The network is validated by retaining a subset of the training data for testing and comparing the network’s predictions to the known values. Accuracy is optimized by continually re-training the network using parts built with a specific machine of interest. The industrial relevance of this research is outlined with respect to four current challenges in AM, including the length of time to determine optimal process parameters for a new machine, ability to organize relevant knowledge, quantification of machine variability, and transfer of knowledge to new operators.

A photo-reversible crosslinking resin for additive manufacturing: reversibility and performance.

Improving the adhesion between layers and achieving the recycling of resins are challenges in additive manufacturing (AM) technology. In this work, a new type of photo-reversible crosslinking resin based on polyvinyl alcohol (PVA) and coumarin (HMC) was prepared via grafting reaction. The critical idea was to create a coumarin based photo-reversible crosslinking resin by carefully tailoring the photo-crosslinking time and temperature, so that the resin could be extruded through the nozzle and then maintain the proper shape during UV-curing. Photo-reversible crosslinking of AM resin was realized without the use of monomers, photo-initiators or propagating. A reasonable irradiation time with 354 nm (crosslink) or 254 nm UV light (cleavage) of 10 min was critical for photo-reversible crosslinking of PVA-g-HMC at 120 °C. An important result of this work was that the developed photo-reversible crosslinked resin could be reused and the printed resin exhibits excellent adhesion properties, thermal conductivity and oxygen barrier performance.

Toward Additively Manufactured Electrical Machines: Opportunities and Challenges

With the growing interest in electrification and as hybrid and pure electric powertrains are adopted in more applications, electrical machine design is facing challenges in terms of meeting very demanding performance metrics, for example, high specific power, and harsh environments. This provides clear motivation to explore the impact of advanced materials and manufacturing on the performance of electrical machines. This article provides an overview of additive manufacturing (AM) approaches that can be used for constructing electrical machines, with a specific focus on additively manufactured iron core, winding, insulation, and permanent magnet, as well as cooling systems. Since there has only been a few attempts so far to explore AM in electrical machines (especially when it comes to fully additively manufactured machines), the benefits and challenges of AM have not been comprehensively understood. In this regard, this article offers a detailed comparison of multiple multimaterial AM methods, showing not only the possibility of fully additively manufactured electrical machines but also the potential significant improvements in their mechanical, electromagnetic, and thermal properties. The articles will provide a comprehensive discussion of opportunities and challenges of AM in the context of electrical machines.

Processability of Atypical WC-Co Composite Feedstock by Laser Powder-Bed Fusion.

Processing of tool materials for cutting applications presents challenges in additive manufacturing (AM). Processes must be carefully managed in order to promote the formation of favourable high-integrity ‘builds’. In this study, for the first time, a satelliting process is used to prepare a WCM-Co (12 wt.% Co) composite. Melting trials were undertaken to evaluate the consolidation behaviour of single tracks within a single layer. Tracks with continuous and relatively uniform surface morphology were obtained. These features are essential for high-quality AM builds in order to encourage good bonding between subsequent tracks within a layer which may reduce porosity within a 3D deposition. This study elucidates the formation of track irregularities, melting modes, crack sensitivity, and balling as a function of laser scanning speed and provides guidelines for future production of WCM-Co by laser powder-bed fusion.

Adaptation of Full-Reference Image Quality Assessment Methods for Automatic Visual Evaluation of the Surface Quality of 3D Prints

Automatic visual quality assessment of the 3D printed surfaces is currently one of the most demanding challenges in additive manufacturing. Regardless of the applications of the computer vision for the 3D printing process monitoring purposes, a reliable surface quality evaluation during manufacturing may introduce brand new possibilities. The detection of some distortions and their automatic evaluation can be helpful when deciding to stop the process to save time, energy, and filament. In some cases, some further corrections can also be made for relatively small distortions. Since many general-purpose image quality assessment methods have been proposed in recent years, their applications for the quality evaluation in the additive manufacturing are investigated. As most of the metrics are full-reference and require the availability of the original perfect quality image, their direct application is not possible. Therefore, their adaptation is described in the paper together with experimental verification of classification results obtained using various metrics.

3D Printed E-Band Hybrid Coupler

In this letter, an additively manufactured 3dB hybrid coupler is presented. While the geometrical structure is generated by a stereolithography (SLA) printer from UV curable photopolymer resin as one single part, conductive coating is provided by an electroless silverplating process. The utilized slotted waveguide approach improves the solvent flow during metal plating and therefore enables the fabrication of complex waveguide structures. Measured imbalance between the through and coupling ports stays below 1.5 dB over the entire E-band, while the respective phase difference ranges only between 87° and 91° despite the challenges of additive manufacturing in such frequency regions.

Achieving Ti6Al4V alloys with both high strength and ductility via selective laser melting

Improving the ductility of manufactured Ti-based alloys by selective laser melting (SLM) while still maintaining the high mechanical strength is one of the major challenges in additive manufacturing. In this work, a systematic investigation on the process parameters for achieving such a material is performed. By employing the optimal parameters during SLM, the fabrication of strong and ductile Ti6Al4V alloys was achieved. The ultimate tensile strength of SLM-built Ti6Al4V alloy was measured as 1170 MPa, while the elongation to failure remained over 10%. Through analysis of the microstructure and mechanical properties of the SLM-built Ti6Al4V alloy, it was found that the process parameters play an essential role in the tensile behavior, in particular the ductility of SLM-built Ti alloy. The considerable enhancement of the mechanical properties in our work is mainly attributed to the limitation of martensite formation and the minimization of pores.

Study of challenges in Additive Manufacturing in the Automotive Industry

Study of challenges in Additive Manufacturing in the Automotive Industry

Challenges of additive manufacturing in production systems

Additive Manufacturing (AM) has become indispensable in the context of digitalization and Industry 4.0 and is said to be a mega trend of the 21st century. The technology offers immense opportunities to revolutionize the production of parts and components in all industries. Despite of the outstanding technical possibilities, the industry-wide adaptation rate is low. The current approach of looking at AM from a mostly technological view is a major reason for this. The challenge is to efficiently integrate 3D printing and other additive processes into existing manufacturing processes and systems. AM must be perceived as a multidimensional topic and viewed from different perspectives, two of which are the AM technology and the planning and management of production systems. These two perspectives have to be addressed simultaneously and cross-linked. In order to use AM to tackle some of the most challenging problems in modern manufacturing systems like increasing variant diversity, shorter product lifecycles, the demand for digitized processes and cyber-physical systems, it is necessary to develop interdisciplinary approaches and solutions, because none of the disciplines can reach the necessary performance and cost-efficiency alone.

Defect Analysis for Additively Manufactured Materials in Fatigue from the Viewpoint of Quality Control and Statistics of Extremes

Additive manufacturing (AM) is expected to be a promising new manufacturing process for components having complex geometry. However, the disadvantage or challenge of AM is the presence of defects and surface roughness which are inevitably produced by the manufacturing process. Without strict and reliable quality control of components regarding defects and surface roughness, we cannot positively admire the advantages of AM as the new technology.In this paper, the method of quantitative evaluation of defects with complicated shapes and configurations, is explained from the viewpoint of small 3D cracks. First of all, the ideal fatigue strength (goal) to be attained by AM is discussed. Use of Fatigue-Grades from 5 (ideal strength) to 1 (lowest grade) is proposed for mutual comparison of fatigue performance of AM materials. The factors which decrease fatigue strength and degrade tensile properties are quantitatively analyzed. It is verified that the fatigue limit of AM materials is determined by the presence of nonpropagating crack emanating from defects. The reasons are made clear for fatigue fracture from surface defects even by tension-compression fatigue test as due to the difference of population of defects and increase in stress intensity factor for surface crack compared to internal cracks. Practical guides will be presented for the fatigue design and development of high quality AM materials, based on the combination of the 3D defect analysis, the statistics of extremes on defects, and the √area parameter model. It is shown that fatigue notch effect in AM materials is influenced by probability of presence of defects at notch root.

Analysis of Favorable Process Conditions for the Manufacturing of Thin-Wall Pieces of Mild Steel Obtained by Wire and Arc Additive Manufacturing (WAAM)

One of the challenges in additive manufacturing (AM) of metallic materials is to obtain workpieces free of defects with excellent physical, mechanical, and metallurgical properties. In wire and arc additive manufacturing (WAAM) the influences of process conditions on thermal history, microstructure and resultant mechanical and surface properties of parts must be analyzed. In this work, 3D metallic parts of mild steel wire (American Welding Society-AWS ER70S-6) are built with a WAAM process by depositing layers of material on a substrate of a S235 JR steel sheet of 3 mm thickness under different process conditions, using as welding process the gas metal arc welding (GMAW) with cold metal transfer (CMT) technology, combined with a positioning system such as a computer numerical controlled (CNC) milling machine. Considering the hardness profiles, the estimated ultimate tensile strengths (UTS) derived from the hardness measurements and the microstructure findings, it can be concluded that the most favorable process conditions are the ones provided by CMT, with homogeneous hardness profiles, good mechanical strengths in accordance to conditions defined by standard, and without formation of a decohesionated external layer; CMT Continuous is the optimal option as the mechanical properties are better than single CMT.

Defect analysis and fatigue design basis for Ni-based superalloy 718 manufactured by selective laser melting

It is well known that high strength metallic materials with Vickers hardness HV > 400 are very sensitive to small defects. This paper discusses fatigue properties of a Ni-based Superalloy 718 with HV = ∼470 manufactured by additive manufacturing (AM). The advantage of AM has been emphasized as the potential application to high strength or hard steels which are difficult to manufacture by traditional machining to complex shapes. However, the disadvantage or challenge of AM has been pointed out due to defects which are inevitably contained in the manufacturing process.Defects of the material investigated in this study were mostly gas porosity and those made by lack of fusion. The successful application of the area parameter model was confirmed. Although the statistics of extremes analysis is useful for the quality control of AM, the particular surface effect on the effective value of defect size must be carefully considered. Since the orientations of defects in AM materials are random, a defect in contact with specimen surface has higher influence on fatigue strength than an internal defect and has the effective larger size termed as areaeff than the real size, area, of the defect from the viewpoint of fracture mechanics. The guide for the fatigue design and development of higher quality Ni-based Superalloy 718 by AM processing based on the combination of the statistics of extremes on defects and the area parameter model is proposed.

Influence of defects, surface roughness and HIP on the fatigue strength of Ti-6Al-4V manufactured by additive manufacturing

The additive manufacturing (AM) is expected to be a promising manufacturing process for high strength or hard steels such as Ti-6Al-4V for the aerospace industry components having complex shapes. However, disadvantage or challenge of AM is presence of defects which are inevitably contained in the manufacturing process. This paper focuses on the effects of defects, surface roughness and Hot Isostatic Pressing (HIP) process on the fatigue strength of a Ti-6Al-4V manufactured by AM. Defects were mostly gas pores and those made by lack of fusion. Many defects which were formed at subsurface were eliminated by HIP and eventually HIP improved fatigue strength drastically to the level of the ideal fatigue limit to be expected from the hardness. Surface roughness had strong detrimental influence on fatigue strength. The method for estimating the effective size √areaeffmax of irregularly shaped defects and interacting adjacent defects was proposed from the viewpoint of fracture mechanics.

Current status and challenges of Additive manufacturing in orthopaedics: An overview

Additive manufacturing is a rapidly emerging technology which is being successfully implemented in the various field of medicine as well as in orthopaedics, where it has applications in reducing cartilage defects and treatments of bones. The technology helps through systematic collection of information about the shape of the "defects" and precise fabrication of complex 3D constructs such as cartilage, heart valve, trachea, myocardial bone tissue and blood vessels. In this paper, a large number of the relevant research papers on the additive manufacturing and its application in medical specifically orthopaedics are identified through Scopus had been studied using Bibliometric analysis and application analysis is undertaken. The bibliometric analysis shows that there is an increasing trend in the research reports on additive manufacturing applications in the field of orthopaedics. Discussions are on using technological advancement like scanning techniques and various challenges of the orthopaedic being met by additive manufacturing technology. For patient-specific orthopaedic applications, these techniques incorporate clinical practice and use for effective planning. 3D printed models printed by this technology are accepted for orthopaedic surgery such as revision of lumbar discectomy, pelvic surgery and large scapular osteochondroma. The applications of additive manufacturing in orthopaedics will experience a rapid translation in future. An orthopaedic surgeon can convert need/idea into a reality by using computer-aided design (CAD) software, analysis software to facilitate the manufacturing. Thus, AM provides a comprehensive opportunity to manufacture orthopaedic implantable medical devices.

Effect of Hot Isostatic Pressure treatment on the Electron-Beam Melted Ti-6Al-4V specimens

The main advantages of additive manufacturing (AM), including fabrication of complex geometry lightweight objects, lattice structures, chain and gear mechanisms, better environment saving materials and resources and energy, better tool life of component, etc. are already well known. The main challenge in AM of metals is to achieve proper mechanical properties and performance, especially for safety critical parts. Thus, post-processing procedures for AM come to the front.In the framework of this research were prepared specimens by Electron Beam Melting (EBM). In case of the EBM, the greatest problem is porosity, which is especially critical for airspace components. One of the procedures recommended for decreasing porosity is Hot Isostatic Pressure (HIP) treatment, and its effect should be thoroughly studied to widespread its application.To investigate this aspect, Ti-6Al-4V cylinder parts were printed on the ARCAM EBM A2X machine using a 210×210 mm building platform. The microstructure and mechanical properties of the EBM specimens were tested before and after the HIP treatment. High cycle fatigue tests (HCFT), fracture surface examination, as well as micro-CT evaluation were performed.

Automated Process Monitoring in 3D Printing Using Supervised Machine Learning

Quality monitoring is still a big challenge in additive manufacturing, popularly known as 3D printing. Detection of defects during the printing process will help eliminate waste of material and time. Defect detection during the initial stages of printing may generate an alert to either pause or stop the printing process so that corrective measures can be taken to prevent the need to reprint the parts. This paper proposes a method to automatically assess the quality of 3D printed parts with the integration of a camera, image processing, and supervised machine learning. Images of semi-finished parts are taken at several critical stages of the printing process according to the part geometry. A machine learning method, support vector machine (SVM), is proposed to classify the parts into either ‘good’ or ‘defective’ category. Parts using ABS and PLA materials were printed to demonstrate the proposed framework. A numerical example is provided to demonstrate how the proposed method works.

Challenges in Additive Manufacturing of Space Parts: Powder Feedstock Cross-Contamination and Its Impact on End Products.

This work studies the tensile properties of Ti-6Al-4V samples produced by laser powder bed based Additive Manufacturing (AM), for different build orientations. The results showed high scattering of the yield and tensile strength and low fracture elongation. The subsequent fractographic investigation revealed the presence of tungsten particles on the fracture surface. Hence, its detection and impact on tensile properties of AM Ti-6Al-4V were investigated. X-ray Computed Tomography (X-ray CT) scanning indicated that these inclusions were evenly distributed throughout the samples, however the inclusions area was shown to be larger in the load-bearing plane for the vertical specimens. A microstructural study proved that the mostly spherical tungsten particles were embedded in the fully martensitic Ti-6Al-4V AM material. The particle size distribution, the flowability and the morphology of the powder feedstock were investigated and appeared to be in line with observations from other studies. X-ray CT scanning of the powder however made the high density particles visible, where various techniques, commonly used in the certification of powder feedstock, failed to detect the contaminant. As the detection of cross contamination in the powder feedstock proves to be challenging, the use of only one type of powder per AM equipment is recommended for critical applications such as Space parts.

Effects of Defects, Surface Roughness and HIP on Fatigue Strength of Ti-6Al-4V manufactured by Additive Manufacturing

The additive manufacturing (AM) is expected to be the promising manufacturing process for high strength or hard steels such as Ti-6Al-4V for the aerospace industry components having complex shapes. However, disadvantage or challenge of AM is presence of defects which are inevitably contained in the manufacturing process. This paper focuses on the effects of defects, surface roughness and Hot Isostatic Pressing (HIP) process on the fatigue strength of a Ti-6Al-4V manufactured by AM. The guide is presented for the fatigue design and development of high quality and high strength Ti-6Al-4V by AM processing based on the combination of the statistics of extremes on defects and the √area parameter model.Defects were mostly gas porosity and those made by lack of fusion. Many pores which were formed near surface were eliminated by HIP and eventually HIP improved fatigue strength drastically to the level of the ideal fatigue limit to be expected from the hardness. Surface roughness had strong detrimental influence on fatigue strength. The method for estimating the effective size √areaeffmax of irregularly shaped defects and interacting adjacent defects was proposed from the viewpoint of fracture mechanics.Although the statistics of extremes analysis is useful for the quality control of AM, the particular surface effect and interaction effect of adjacent defects must be carefully considered. The effective defect size for adjacent defects is much larger than a single defect. Since the orientations of defects in AM materials are random, a defect in contact with specimen surface has a higher influence (termed as the effective defect size √areaeff) than the real size of the defect from the viewpoint of fracture mechanics. Considering the volume and number of productions of components, the lower bound of the fatigue limit σwl based on √areaeffmax can be determined by the √area parameter model.

Defect Analysis and Fatigue Design Basis for Ni-based Superalloy 718 manufactured by Additive Manufacturing

It is well known that high strength metallic materials with Vickers hardness HV>400 are very sensitive to small defects. This paper discusses fatigue properties of a Ni-based Superalloy 718 with HV=~470 which was manufactured by additive manufacturing (AM). The advantage of AM has been emphasized as the potential application to high strength or hard steels which are difficult to manufacture by traditional machining to complex shapes. However, the disadvantage or challenge of AM has been pointed out due to defects which are inevitably contained in the manufacturing process.Defects of the material investigated in this study were mostly gas porosity and those made by lack of fusion. The √area parameter model was confirmed the successful application. Although the statistics of extremes analysis is useful for the quality control of AM, the particular surface effect on the effective value of defect size must be carefully considered. Since the orientations of defects in AM materials are random, a defect in contact with specimen surface has higher influence and has the effective larger size termed as √areaeff than the real size, √area, of the defect from the viewpoint of fracture mechanics. The guide for the fatigue design and development of higher quality Ni-based Superalloy 718 by AM processing based on the combination of the statistics of extremes on defects and the √area parameter model is proposed.