Suitable Process Window Research Articles

Best practices for machine learning strategies aimed at process parameter development in powder bed fusion additive manufacturing

AbstractThe process parameters used for building a part utilizing the powder-bed fusion (PBF) additive manufacturing (AM) system have a direct influence on the quality—and therefore performance—of the final object. These parameters are commonly chosen based on experience or, in many cases, iteratively through experimentation. Discovering the optimal set of parameters via trial and error can be time-consuming and costly, as it often requires examining numerous permutations and combinations of parameters which commonly have complex interactions. However, machine learning (ML) methods can recommend suitable processing windows using models trained on data. They achieve this by efficiently identifying the optimal parameters through analyzing and recognizing patterns in data described by a multi-dimensional parameter space. We reviewed ML-based forward and inverse models that have been proposed to unlock the process–structure–property–performance relationships in both directions and assessed them in relation to data (quality, quantity, and diversity), ML method (mismatches and neglect of history), and model evaluation. To address the common shortcomings inherent in the published works, we propose strategies that embrace best practices. We point out the need for consistency in the reporting of details relevant to ML models and advocate for the development of relevant international standards. Significantly, our recommendations can be adopted for ML applications outside of AM where an optimum combination of process parameters (or other inputs) must be found with only a limited amount of training data.

High-strength aluminum alloy processed by micro laser powder bed fusion (μ-LPBF): Coordination of laser formability, microstructure evolution, and mechanical properties

As the scale of additive manufacturing process (e.g., laser spot size, powder particle size, powder layer thickness) decreases, the application of micro laser powder bed fusion (μ-LPBF) involves novel mechanisms for process, microstructure and performance coordination. This study provides a systematic view of the processing window and performance enhancement of high-strength Al-Mg-Sc-Zr alloy fabricated by μ-LPBF. The effects of μ-LPBF parameters on defect control and densification activity of the printed parts were analyzed, so as to obtain the suitable processing window. The influence of building orientation and heat treatment on microstructural characteristics and mechanical properties of μ-LPBF processed parts was studied. The μ-LPBF Al-Mg-Sc-Zr exhibited sound surface quality (Ra of 6.088 μm) and considerably refined grains with an average size of 1.102 μm, which was related to the high cooling rate (8.6 × 107 K/s) induced by a small-sized laser beam (25 μm) and a tiny powder particle size distribution (2–20 μm) applied in μ-LPBF. After aging treatment (325 °C/4 h), the superior ultimate tensile strength of 590.24 ± 4.75 MPa combined with the sufficiently high elongation of 11.99 ± 1.17 % was achieved. Due to the significantly decreased scale of μ-LPBF production, the anisotropy caused by the variation of building directions was negligible. These enhanced mechanical properties were attributed to the combined effect of the grain size refinement, the higher number density (1.2 × 1024/mm3) of interior precipitates within grains, and the small-sized molten pool size of μ-LPBF. Computational fluid dynamics (CFD) simulation was applied to reveal the molten pool thermodynamics, indicating that a higher thermal temperature gradient (up to 9.8×107 K/m), a smaller molten pool size (with the width of 38.7–69.8 μm and depth of 8.7–30.0 μm) were generated in μ-LPBF. This work presents great potential in processing high-precision metallic components with fine structural feature size and satisfactory manufacturing quality.

Impact of ultraviolet C treatment on sensory characteristics, amino acid profile, riboflavin content and toxicological activity of native whey

In the fight against fermentation-disrupting bacteriophages, UV-C irradiation is promising due to its capability of damaging nucleic acids and preventing organisms to replicate. The purpose of this study is to investigate the effect of this radiation on whey using three different devices (lab-/technical-scale, batch/continuous) and UV-C doses between 0.4 and 100 J mL−1. In vitro toxicity assays (Comet, WST-1, Ames) proved that irradiated whey does not exhibit any genotoxic, cytotoxic, or mutagenic activity and is presumed to be harmless to health. However, the irradiated product had an intense “cowish/stable-like/manure-like” off-flavor, which was identified as para-cresol by GC-MS-O. Riboflavin decay was observed as well as decreases in cystine and aromatic amino acids, whose degradation products are known to be odor-active. With our current data, we are not able to define a suitable process window or to recommend the application of this technology to treat whey.

Characterization of hot workability of a Cu containing high strength low alloy steel using processing map

Hot workability of an experimental Cu containing martensitic high strength low alloy (HSLA) steel was characterized by conducting isothermal hot compression tests at different temperatures (900–1100 °C) and strain rates (0.0003–1 s−1). Flow curves exhibits predominantly single peak behaviour at temperatures for a strain rate > 0.01 s−1. At temperature > 950 °C and for low strain rates of 0.0003 and/or 0.001 s−1, multiple peaks characterised by oscillations in flow stress was observed. The strain rate sensitivity (m), temperature sensitivity (S) of the material was calculated and the processing map was developed to identify optimum condition for thermo-mechanical processing. The material exhibits three suitable processing windows in temperature – strain rate space viz., (1) 1050–1100 °C; 0.1–0.01 s−1 (2) 980–1020 °C; 0.1–1 s−1 and (3) 980–1020 °C; 0.001–0.0003 s−1. The microstructure of the material deformed under all the above mentioned processing conditions exhibited defect free equiaxed dynamically recrystallized grains with an average grain size of 30–40 μm. Any or combination of these processing conditions can be used by the industry for designing suitable forging process (roughing, finishing etc.) based on the infrastructure available. Further, using Avrami approach, the effect of strain, strain rate and temperature on the kinetics of dynamic recrystallization (DRX) was also evaluated. The results obtained are presented and discussed.

Laser Powder Bed Fusion of Hard‐Phase Containing Co‐Base Alloy

This work considers the processing of the hard‐phase containing CoCrW‐alloy Celsit F using powder bed fusion‐laser beam/metal (PBF‐LB/M), whereby a suitable process window for producing dense parts having a low‐defect density is determined. Several cuboid specimens are manufactured by varying the exposure parameters (laser power, hatch distance, scanning speed). With the help of quantitative image analysis, the samples are evaluated regarding crack density and porosity, and optimal exposure parameters are derived. Dense samples with a low defect density can be produced if the energy density is 60–70 J mm−3 and the base‐plate is preheated to 800 °C. Choosing lower energy densities leads to increased pore formation, whereas higher energy densities result in increased ablation and cracking. With a view to a later application in which additively manufactured hot work tools are made of Celsit F, the possibility of introducing internal cavities as potential cooling channels is considered. It can be shown that the hardness sideways is lower than above and below the cavity due to a changed microstructure and locally different heat dissipation. The results of this work give a first guideline for the PBF‐LB/M‐processing of wear‐resistant CoCrW alloys with a high hard phase volume content to tools for hot working.

The beneficial effect of yttrium microalloying in the solid solution treatment of AISI 321 stainless steel：Grain refinement and inclusions modification

Solid solution treatment is an important means of regulating the microstructure and mechanical properties of austenitic stainless steel, and a suitable solid solution process window is critical for austenitic stainless-steel service. In this paper, the effect of (RE) yttrium and solid solution temperatures on the microstructure and mechanical properties of austenitic stainless steel was investigated by adding trace amounts of RE yttrium to AISI 321 stainless steel with different solid solution temperature treatments. The results show that the austenite grains gradually coarsen with the increase of solid solution temperature, and the addition of RE yttrium effectively inhibits the coarsening of grains and keeps an excellent the comprehensive mechanical properties of AISI 321 stainless steel at 1150 °C for 30 min, this indicates that the addition of RE yttrium broad solid solution process window for AISI 321 stainless steel. In addition, the RE yttrium microalloying has a positive modification effect on the carbonitrides with refinement and spherization, which has a good contribution to the strength and plasticity retention of the material. This research is expected to provide more options for the heat treatment of austenitic stainless steels.

Towards Lunar In-Situ Resource Utilization Based Subtractive Manufacturing

In recent years, space agencies, such as the National Aeronautics and Space Administration (NASA) and European Space Agency (ESA), have expanded their research activities in the field of manufacturing in space. These measures serve to reduce limitations and costs through fairing size, launch mass capabilities or logistic missions. The objective, in turn, is to develop technologies and processes that enable on-demand manufacturing for long-term space missions and on other celestial bodies. Within these research activities, in-situ resource utilization (ISRU) and recycling are major topics to exploit local resources and save transport capacity and, therefore, costs. On the other hand, it is important to carefully consider which items can be brought and which must be manufactured on the Moon. Consequently, on-demand needs in future space missions are considered regarding frequency, raw material and required manufacturing processes according to investigations by ESA and NASA. In conclusion, manufacturing in space state-of-the-art shows a strong focus on additive processes, primarily considering semicrystalline or amorphous plastics. The subtractive processing of metallic or ceramic materials, in turn, currently represents a research gap. Consequently, an approach for in-situ resource utilization-based subtractive manufacturing in space is presented to supplement the existing processes. The latter uses a high-pressure jet of water, with regolith simulate as abrasive in suspension, being directed at the workpiece, which is moved to separate metal and glass. Proof-of-concept results are presented, including suitable process windows, achieved cutting geometries, as well as the effects of parameter variations on the system technology and consumables used. The focus of the investigations supplements the general requirements for the design of machine tools for space applications with inertial process-specific boundary conditions as a step towards higher technology maturity.

Thixomolding of Magnesium - Efficient Process Industrialization by Combining a Digital Twin and Systematic Casting Trials

Besides the realization of the technical specification of a casting, aspects such as resource minimization, cost awareness and robustness of the defined quality requirements are taking major priority, especially in the current global economic situation. Lightweight design, in that respect, is a central topic far beyond the automotive industry and characterizes in particular the ongoing research and development of innovative magnesium cast components. As a resource-efficient alternative to traditional die casting, magnesium-thixomolding offers the benefit of low energy consumption and eschews the necessity of protective gases. The identification of significant influencing factors between process settings, shear and time dependent material properties of semi-solid magnesium, the machine capabilities and the resulting casting quality are the basis for effective product and process design of magnesium thixomolding. Obtaining this knowledge is only possible through methodical process analysis and systematic design of experiments. YIZUMI Germany, together with MAGMA, demonstrates the methodical evaluation of a reliable process window for an AZ91D magnesium “box” component, using a demo mold on a UN1250MGII thixomolding machine. The work shows how a systematic combination of virtual and real casting experiments is used to identify a concrete process layout and generate important knowledge on process variations, robustness as well as control limits. In particular, the comparison of virtual and real casting experiments using statistical methods generates a comprehensive understanding of the impact and limits of different process variables, such as fraction solid, mold temperature and injection speeds. Furthermore it is possible to predict the influence of the gating and component design on the process. The methodological use of qualified casting simulation enables the development of an optimized component design and the determination of a suitable process window with minimized economic or productive risks long before the first component is manufactured – a step towards more resource efficiency and sustainability!

Hot deformation characteristics and processing map of Ti-25Al-14Nb-2Mo-1Fe alloy under hot working process conditions

The hot working process is a proven technique to manufacture special aviation components. However, a suitable working process window for Ti-25Al-14Nb-2Mo-1Fe alloy has not been established. In this work, the hot deformation characteristics of Ti-25Al-14Nb-2Mo-1Fe alloy was systematically studied. In addition, the processing map of the alloy was established to further analyze the inherent machinability properties. Through the processing map analysis and microstructural observation, the instability and stability domains were identified. The optimum working parameters for the alloy were obtained (950–970 ℃/0.001–0.01 s−1, 1015–1100 ℃/0.001–0.01 s−1), which is conducive to optimizing the hot working parameters.

Microstructure and high temperature properties of tungsten processed via electron beam melting additive manufacturing

There is considerable interest in the adoption of additive manufacturing for processing refractory metals. The layer-wise fabrication approach enables opportunities for producing complex geometries which cannot be otherwise be achieved via powder metallurgy. However, the processing science is still in its nascent stages and structure–property relations are relatively unexplored. Fundamental research is needed to further develop the technology and enable the fabrication of refractory metals for high temperature applications. Here we focus on the processing of pure tungsten using electron beam melting additive manufacturing. Experimentally we develop a suitable processing window for achieving high density crack free material. Microstructural analysis reveals that the microstructure generally consists of a columnar structure with a 111 build direction fiber preference, although, fiber switching was observed. Process induced deformation is believed to drive the formation of subgrains whose boundaries exhibit a high dislocation density. High temperature tensile testing reveals that the material exhibits excellent properties closer to that of annealed tungsten. Significant mechanical anisotropy was observed to be present which is likely driven by strong crystallographic texture.

Laser-based monolithic series interconnection of two-terminal perovskite-CIGSe tandem solar cells: determination of the optimal scribe line properties

To achieve a monolithic series interconnection of tandem solar cell devices consisting of a perovskite top cell and a CIGSe bottom cell, a two-terminal interconnection scheme is introduced that includes an additional, fourth patterning step, the so-called iso-cut, which separates the window layer stack between the two solar cells. The implementation of this interconnection scheme requires a process development for a total of four structuring steps, which was achieved by systematically varying the laser parameters. Based on a detailed characterization of the individual scribe line properties with respect to their scribe line depth, morphology, electrical functionality, chemical composition and their influence on adjacent and underlying layers, the optimal patterning parameters and suitable process windows were derived for each step, which is a prerequisite for a loss-free monolithic series interconnection in a tandem module.

Electrically Actuated Shape Recovery of NiTi Components Processed by Laser Powder Bed Fusion after Regulating the Dimensional Accuracy and Phase Transformation Behavior

To develop self-recovery intelligent components based on resistance heating and obtain satisfactory performance in practical applications, this study optimized the forming quality, dimensional accuracy, and phase transformation temperatures of Nickel-titanium (NiTi) alloys by controlling the process parameters. The tensile properties and shape-memory effects of the NiTi alloys prepared using the optimized process were clarified. The relationship between the change in temperature and the shape recovery process of the deformed structure under electrical excitation was investigated. The results show that the suitable processing window for ensuring the forming quality without noticeable distortion and macro cracks depends on the laser parameters. In both the X and Y directions, the measured dimensions increased with an increase in laser power and first decreased and then stabilized with an increase in scanning speed. The XRD results showed that all the as-built samples consisted of B2 austenite and B19’ martensite phases and Ni3Ti. Mechanical tests suggested that excellent tensile properties with a tensile strength of 753.28 MPa and elongation of 6.81% could be obtained under the optimal parameters of 250 W and 1200 mm/s. An excellent shape-recovery rate of 88.23% was achieved under the optimal parameters. Subsequently, chiral lattice structures were successfully fabricated by laser powder bed fusion (LPBF) under the optimal parameters, and a shape-recovery rate of 96.7% was achieved under electrical actuation for a structure with a pre-compressed strain of 20%. This study also found that the temperatures at the grasp regions were always higher than those at other positions because of the generation of contact resistance at the grasp regions. This facilitates the rapid recovery of the structure at the grasp regions, which has important implications for the design iteration of NiTi smart components.

Long-term stable bioprocess-derived Pickering-type emulsions: Identification of key parameters for emulsion stability based on cell interaction at interface

Industrial implementation of highly potent biphasic whole-cell biocatalytic processes is often limited due to the formation of long-term stable Pickering-type emulsions, caused by the presence of cells. State-of-the-art-concepts for phase separation fail or include inefficient and costly strategies (centrifugation/de-emulsifiers). Using the phenomenon of catastrophic phase inversion (CPI), efficient phase separation can be achieved by addition of dispersed phase. To show the industrial applicability of CPI for phase separations of bioprocess-derived Pickering-type emulsions, the mutual influence of the emulsion components and thus emulsion phase behavior and stability has to be known. Characterizing several Pickering-type emulsions (stabilized by E. coliJM101 and P. putidaKT2440 cells), the cell radius, the wettability of cells, and the interfacial tension, were identified to be the most crucial parameters. On this basis, we suggest guidelines and estimation strategies for suitable process (window) selection, assisting in the introduction of these highly potent processes into industrial applications.

Powder‐fed directed energy deposition of soda lime silica glass on glass substrates

AbstractNovel glass processing by powder‐fed directed energy deposition was explored as a method of adding glass décor to glass surfaces and bottles. Consistent, semitransparent, single‐line tracks of soda lime silica glass could be processed onto glass substrates of the same composition, without significant cracks forming in the substrate. A suitable processing window was found with laser power and scan speed showing independent effects on processing. Consideration of processing surface conditions and reduction of laser transmission through transparent substrates was necessary, and the use of an adhesive tape layer aided adhesion of glass feedstock to substrate surfaces. The work demonstrates the potential for a one‐step method of glass bottle decoration for the packaging industry, with scope to create 3D designs of high geometric complexity and customizability on glass substrates, thereby adding value to glass packaging by brand differentiation without the high costs associated with molds and tooling.

Fabrication of micron-sized protrusions on metal surface for metal/polymer easy-disassembly joining by selective laser melting technology

Artificial protrusions fabricated on metal surfaces are adequate for the mechanical interlocking to strengthen the metal/polymer joint. The morphology and its adhesion with the metal substrate of the fabricated protrusions greatly influence the joint strength and joint detachability. However, the large-sized protrusion can cause carbon fiber damage when embedded into the polymer plate, which results in the decline of the joint strength. This paper applied a diode-pumped solid-state laser system to melt stainless powders to fabricate micron-sized protrusions on metal surfaces. The effect of four impacting factors, original powder size, powder bedding thickness, laser power density, and laser irradiation time, on the formation of micro-protrusions was investigated. SEM was used to characterize the morphology and diameter of the formed micro-protrusions. The results showed that the fabricated micro-protrusions present a spherical shape under the stable manufacturing process, and the diameter ranged from 100 µm to 500 µm. The micro-protrusions size positively correlated with the original powder size and powder bedding thickness. Laser power density directly affected the forming of micro-protrusions, and the suitable process window was between 160 W/mm2 to 290 W/mm2. When the laser irradiation time was 10 s, it helped stabilize the fabrication process.

Effect of Laser Remelting Strategy on the Forming Ability of Cemented Carbide Fabricated by Laser Powder Bed Fusion (L-PBF).

Due to the high degree of design freedom and rapid prototyping, laser powder bed fusion (L-PBF) presents a great advantage in the super-hard cemented carbide compared with conventional methods. However, optimizing processing parameters to improve the relative density and surface roughness is still a challenge for cemented carbide fabricated by L-PBF. For this, the effect of the remelting strategy on the forming quality of the L-PBF processed cemented carbide was studied in this article, aiming to explore a suitable process window. The surface quality, relative density, microstructure, and microhardness of the cemented carbide parts fabricated under a single melting and remelting strategy were compared. The results showed that the remelting strategy could efficiently improve the specimens’ surface quality and relative density. Besides, the cracks were not obviously aggravated, and the WC grains could distribute more homogeneously on the binder matrix under the remelting strategy. Therefore, the microhardness showed an improvement compared to the single melting strategy.

Air-based deposition of titanium‑aluminum oxynitride thin films by reactive magnetron sputtering

Quaternary titanium‑aluminum oxynitride (Ti,Al)NxOy films with various nitrogen and oxygen compositions can be applied to many interesting mechanical and technological applications. Conventionally, pure nitrogen and oxygen were required as reactive gases for producing the films by sputtering under a high vacuum environment. In this work, air was employed as a reactive gas instead. Thus, a low vacuum environment was sufficient, which could save a tremendous amount of processing time and cost. (Ti,Al)NxOy films were produced by dc magnetron sputtering at a high base pressure. By merely varying the air/Ar flow ratio in a suitable processing window, X-ray diffraction patterns show that obtained films could turn from a crystalline rock-salt structure to an amorphous phase. X-ray photoelectron spectroscopy revealed the compositions that could be tailored from N-rich to O-rich (Ti,Al)NxOy thin films. With increasing O/N ratios, as-deposited (Ti,Al)NxOy films could turn from metallic-like, semi-conductive, to dielectric-like behaviors. Moreover, the hardness and reduced Young's modulus of obtained (Ti,Al)NxOy films could also be tailored and achieved as high as 39.1 GPa and 331 GPa, respectively. This may be due to solid solution hardening effects.

Bending Strength and Fracture Behaviour of Metal-Ceramic Interpenetrating Phase Composites Manufactured by Using Semi-Solid Forming Technology

Interpenetrating Phase Composites (IPC) belong to a special category of composite materials, offering great potential in terms of material properties due to the continuous volume structure of both composite components. While manufacturing of metal-ceramic IPC via existing casting and infiltration processes leads to structural deficits, semi-solid forming represents a promising technology for producing IPC components without such defects. Thereby, a solid open pore body made of ceramic is infiltrated with a metallic material in the semi-solid state. Good structural characteristics of the microstructure as the integrity of the open-pore bodies after infiltration and an almost none residual porosity within the composites have already been proven for this manufacturing route within a certain process window. On this basis, the following paper focuses on the mechanical properties such as bending strength of metal-ceramic IPC produced by using semi-solid forming technology. Thereby, the impact of the significant process parameters on these properties is analysed within a suitable process window. Furthermore, a fractographic analysis is carried out by observing and interpreting the fracture behaviour during these tests and the fracture surface thereafter.

Experimental investigations on micro-EDM milling of niobium carbide-nickel based cermet using statistical and empirical techniques

Niobium Carbide (NbC) based cermet is a relatively less explored wear resistant material which has potential applications in moulds for ceramic injection moulding as well as cutting tool inserts. The NbC-Ni based cermets exhibit similar hardness as WC-Co cemented carbide but show better wear resistance and chemical stability against iron or steel. The demand for Ni bonded NbC cermets is forecasted to increase due to the rising cost of Co in conventional WC-Co cemented carbides. This poses additional challenges for existing mechanical micromachining processes such as frequent tool failure, higher tool wear and poor surface quality. Therefore, electrically conductive sintered NbC matrix cermets with a Ni binder have been developed which makes it suitable to machine them using electro discharge machining (EDM) and electrochemical machining (ECM) operations.This work presents experimental investigations using micro-EDM milling operation on NbC-Ni workpieces. A combination of empirical and statistical studies is employed to determine the suitable processing window for this material. The empirical study involves microstructural analysis and surface topography characterization using scanning electron microscopy (SEM) and confocal microscopy, respectively. The microstructural features such as micro-cracks, micro-cavities, micro-voids and micro-globules are identified and related to the major material removal mode of melting/vaporization. The statistical study involves parameter optimization and process monitoring. Material removal rate (MRR) of 0.091 mm3/min and tool wear rate (TWR) of 0.048 mm3/min can be achieved to its optimality and tracking the change in sparks frequency, shorts frequency and feed-rate enables to detect the suddenly accumulated debris. In this way, this holistic approach has been demonstrated to achieve an improvement in understanding and optimizing the machining process for novel NbC matrix cermet using micro-EDM technology.

Assessment of Abrasion-Induced Visual Defects in Twin Screw Wet Granulation Using Wall Friction Measurements

Starting point of the presented study were abrasion effects occurring during a twin screw wet granulation (TSG) process of a new chemical entity (NCE) formulation, resulting in gray spots on the final tablets. Several actions and systematic changes of equipment and process parameter settings of TSG process were conducted which reduced the visual defect rate of the tablets, i.e., gray spots on the surface, below the specification limit. To understand the rationale and mechanism behind these improvements, correlations of defect rates and wall friction measurements using a Schulze ring shear tester were evaluated. To check the suitability of the method, a broad range of wall materials as well as powder formulations at various moisture levels were investigated with regard to their wall friction angle. As differences in wall friction angle could be detected, further experiments were conducted using wall material samples made out of different screw materials for TSG. Evaluation of these screw wall material samples gave first hints, which screw materials should be preferred in regard of friction for TSG process. In the finally presented case study, wall friction measurements were performed using the above mentioned NCE formulation with known abrasion issues at TSG processing. The results confirmed that changes which led to a reduced visual defect rate of tablets correlated with a decreased wall friction angle. The results suggest wall friction measurements as a potent tool for equipment selection and establishment of a suitable process window prior to conducting TSG experiments.Graphical abstract