Key Engineering Materials Vol. 964

Abstract: Pores are the inevitable concomitant in the current state of laser powder bed fusion (PBF-LB/M) of AlSI10Mg components. Various pore characteristics, such as pore size and pore shape, influence the quality and affect the intended functionality of the component. Today, the experimental effort to find the appropriate process parameters for additive manufacturing (AM) results in high costs and long time-to-market. Pore formation is highly dependent on the applied process parameters. Consequently, pores can also be seen as an individual process fingerprint. Computed tomography is a commonly used measurement tool for AM components and can be used to comprehensively investigate process-induced defects. Furthermore, X-ray data allows an accurate categorisation of pores and provides a large amount of labelled data for supervised learning applications. The applied classification method classifies the pores into six classes (A-F) according to their shape and size. A total number of 3,066,249 pores detected in cylindrical samples were categorised and used for machine learning. The purpose of this work is to demonstrate an approach for predicting AM process parameters depending on the resulting pore distribution using supervised learning methods. The result is an expandable machine learning model based on an artificial neural network.

Abstract: The properties of AlSi10Mg produced by Laser Powder Bed Fusion (PBF-LB) are defined by a multitude of different machine and laser parameters. This multi-parameter space presents the challenge of optimizing the material properties for a given application by the sheer amount of possible parameter combinations. Characterizing this multi-parameter space empirically is limited by time and resources and thus yields an incomplete picture of the process capabilities and local optima, respectively. To improve on this situation, machine learning to map the process parameters on the tensile properties of AlSi10Mg was used. The Hybrid Neural Network (HNN) used in this study consisted of a Convolutional Neural Network (CNN) to process the micrographs and a Dense Neural Network (DNN) to process the LPBF process parameters as well as the output of the CNN. The micrographs given to the CNN part of the network were printed with the same parameters given to the DNN part to include the information of the bulk microstructure as it strongly influences the tensile properties of the material. With the HNN, we observed good accuracy of the predicted tensile properties, given the small amount of training data. Furthermore, we explore which features of the micrographs were extracted by the CNN.

Abstract: The need to reduce the cost and, therefore, the processing time of metallic materials has pushed academia and industry toward the use of additive manufacturing (AM) technologies. This paper aims to study the effectiveness of a green electropolishing treatment of AlSi10Mg aluminium alloy components produced using Selective Laser Melting (SLM) technology. The influence of treatment duration in relation to specimen surface polishing and the effect on corrosion resistance were evaluated. Morphological characterizations, roughness measurements and electrochemical tests were performed. Specifically, the study identified a set of parameters to achieve a significant reduction in roughness and an increase in the electrochemical characteristics of the components. Green electropolishing could be a viable post-processing treatment substitute to the classical treatment in which environmentally harmful acids are used.

Abstract: Electron-Beam Powder Bed Fusion (EB-PBF) is one of the most important metal additive manufacturing (AM) technologies. In EB-PBF, a focused electron beam is used to melt metal powders in a layer by layer approach. In this investigation two pre-alloyed steel-based powders, stainless steel 316L and V4E, a tool steel developed by Uddeholm, were used to manufacture functionally graded materials. In the proposed approach two powders are loaded into the feeding container, V4E powder on top of 316L one, preventing their mixing. Such type of feeding yields components with two distinct materials separated by a zone with gradual transition from 316L to V4E. Microstructure and local mechanical properties were evaluated in the manufactured samples. Optical Microscopy, Scanning Electron Microscopy and EDX on the polished cross-sections show a gradual microstructural and compositional transition from characteristic 316L at the bottom of the specimens to the tool steel towards the top. Nanoindentation experiments confirmed a consequent gradient in hardness and elastic modulus, which gradually increase towards the top surface of the samples. The achieved results provide great possibilities to tailor the composition, microstructure, mechanical properties, and wear resistance by combining different powders in the powder bed AM technology. Potential applications include the tooling industry, where hard and wear-resistant materials are demanded on the surface with tougher and more ductile materials in the core of the tool.

Abstract: Modified PH 13-8Mo alloy exhibits a good combination of corrosion resistance and mechanical properties for demanding applications in aerospace, petrochemical, and tooling industries. Additive manufacturing, specifically the laser metal deposition process with powder as feedstock (LMDp), has the potential to be utilized in these industries. However, very limited knowledge on the LMDp of this alloy currently exists. The aim of this work was, therefore, to deposit a multi-track single layer of modified PH 13-8Mo alloy as a first step towards 3D geometries, and to analyze the resulting microstructure by using Optical Microscopy, Scanning Electron Microscopy, X-Ray Diffraction, Electron Backscatter Diffraction, and micro-hardness. It was found that the multi-track single layer was free from major defects. The microstructure was heterogeneous, and it consisted of a martensitic matrix and small amounts of δ ferrite, austenite, and AlN. The results of this research will be used to tailor the microstructure and properties of future 3D additively manufactured components.

Abstract: Sheet-based gyroids with different unit cell size, wall thickness, porosity gradients and manufacturing modalities were manufactured using electron beam- based powder bed fusion (E-PBF) using ‘melt’ and ‘wafer’ themes. Aim of the research is to understand the challenges of the designing, manufacturing and post-processing of such structures and their characteristic features.

Abstract: Additive Manufacturing (AM) brings about an array of modifications in microstructure with respect to conventional routes transforming mechanical performances. These new microstructure features depend on process parameters and especially on volume energy-density delivered by the laser on powder layer. Among the different alloys manufactured by AM, Ni-alloys exhibit high-strength at elevated temperature opening the way of fabrication of gas turbines and jet-engine parts. Ni-superalloys experience precipitation hardening due to the formation of γ′ and γ′′ phases leading to complex microstructures. To better study the influence of the AM microstructure on Ni-alloys mechanical properties, in particular at elevated temperatures, a theoretically monophasic and binary Ni20Cr-alloy manufactured by laser powder-bed fusion was studied in this work. Remarkable Yield Strength (400 MPa) and Ultimate Tensile Strength (UTS) (600 MPa) were observed at 500°C with hardly any loss of properties from room temperature, owing to the thermal stability of cellular dendrites till 700°C. Ductility drop was reported at 700°C due to anomalous brittle behaviour of Ni-alloys. Hardening behaviour vanished at 900°C signifying the deletion of dendrites, disappearance of dislocations, diffusion of Cr from dendritic walls and growth of oxides.

Abstract: Metamaterials, including materials with regularly distributed porous structures, are currently a very intensively developing area of the technology industry. They bring a number of advantages compared to components produced in the classic way. The primary focus of such porous structures is to lighten the product and at the same time preserve its physical or mechanical properties, which subsequently conveys benefits in the form of saving material for the production of the device, and when used in aeroplanes or cars, they also save the amount of fuel consumed, so it can be said that such products and equipment are more user-friendly and environmentally friendly. There are many types of structures with different configurations, different types of basic cells, and different distributions of pores or their topology, so it is very important for the designer to know and decide which type of structure is most advantageous to use in the proposed product that will be subjected to a specific load. The article deals with the investigation of the mechanical properties of porous structures produced by the Direct Laser Metal Sintering (DLMS) method. It is focused on experimentally tested samples made of AlSi10Mg alloy with the Neovius structure, which were produced with four different relative weights. Results of quasi-static pressure testing at a crossbar speed of 10 mm/min (testing machine 250 kN Instron 8802 servo-hydraulic machine) point out that the trend of the influence of the relative weights on the First Peak Local Maximum best described by a second-order polynomial function.

Abstract: This work examines the feasibility of joining two dissimilar metals, vanadium (V) and wrought Nitronic 40 stainless steel, through electron beam additive manufacturing (EBAM). Depositing V on Nitronic 40 led to mixed results with some builds exhibiting microcracking and other builds exhibiting severe cracking resulting in delamination. These build failures are thought to be caused by a large coefficient of thermal expansion (CTE) mismatch and solubility issues, demonstrating the challenges associated with this material combination. The large melting temperature discrepancy between Nitronic 40 and V was thought to exacerbate the issues with CTE mismatch and solubility. Four strategies could be employed by EBAM to mitigate the observed issues to successfully deposit V on Nitronic 40: (1) adjust wire feed speed, (2) use dual wire feeders, (3) use different wire feedstocks to control composition, and (4) create a transition layer known as buttering to accommodate CTE mismatch.

Abstract: The laser powder bed fusion (L-PBF) technique was utilized to manufacture a hybrid M789-N709 alloy by depositing M789 steel on wrought N709 steel. The tensile strength of the M789-N709 interface generated during the process has been established to be higher than that of the base materials. In the previous work of the current authors, extensive characterization of the M789-N709 interface (before and after heat treatment) was performed by means of electron backscatter diffraction, electron probe microanalysis, transmission electron microscopy with energy dispersive spectroscopy, and atom probe tomography analyses, to understand the mechanisms associated with its superior strength. In the present work, since the application of the hybrid alloy is targeted towards an elevated temperature environment, the individual high-temperature mechanical properties of M789 and N709 steels were acquired at various temperatures and strain rates using a Gleeble 563 thermomechanical system. Then, based on the flow curves, phenomenological-, and physical-based constitutive material models were established. These constitutive models can be utilized to accurately assess the high-temperature response of the hybrid alloy system using finite element analysis programs. This work demonstrates the application of thermomechanical processing and constitutive modeling in the field of metal additive manufacturing.