Meta-analysis of literature data in metal additive manufacturing: What can we (and the machine) learn from reported data?

Obtaining in-depth understanding of the relationships between the additive manufacturing (AM) process, microstructure and mechanical properties is crucial to overcome barriers in AM. In this study, database of metal AM was created thanks to many literature studies. Subsequently meta-analyses on the data was undertaken to provide insights into whether such relationships are well reflected in the literature data. The analyses help reveal the bias and what the data tells us, and to what extent machine learning (ML) can learn from the data. The first major bias is associated with common practices in identifying the process based on optimizing the consolidation. Most reports were for consolidation while data on microstructure and mechanical properties was significantly less. In addition, only high consolidation values was provided, so ML was not able to learn the full spectrum of the process - consolidation relationship. The common identification of process maps based on only consolidation also poses another bias as mechanical properties that ultimately govern the quality of an AM build are controlled not only by the consolidation, but also microstructure. Meta-analysis of the literature data also shows weak correlation between process with consolidation and mechanical properties. This weak correlation is attributed to the stated biases and the non-monotonic and non-linear relationships between the process and quality variables. Fortunately, trained ML models capture well the influence and interactions between process parameters and quality variables, and predicts accurately the yield stress, suggesting that the correlation between process, microstructure and yield strength is well reflected in the data. Lastly, due to the current limitation in the process map identification, we propose to identify the process map based on not only the consolidation, but also mechanical properties

Alloy design against the solidification cracking in fusion additive manufacturing: an application to a FeCrAl alloy

This study developed a design methodology against liquid-state cracking by combining the Scheil–Gulliver solidification simulations and Machine Learning analysis to design alloys for Fusion Additive Manufacturing. Applying this design approach resulted in a Fe–20Cr–7Al–4Mo–3Ni. The alloy was successfully printed with relative densities of over 99%. Microstructure of printed material was extensively characterised through scanning and transmission electron microscopy, energy dispersive spectroscopy and x-ray diffraction, confirming a single-phase material with low texture and negligible chemical segregation. Neither solidification nor liquation cracks were detected, supporting the validity of the methodology, however, the alloy suffered from solid-state cracking, hindering the ductility

Microstructural Study of Cold-Sprayed CoCrFeNiMn High Entropy Alloy

The rapid development of cold spraying technology for additive manufacturing of engineering components has made it a viable option for developing thick deposits from high-entropy alloys (HEAs). The microstructure of cold-sprayed CoCrFeNiMn deposit was investigated in this study using electron backscattered diffraction, scanning electron microscopy, and finite element analysis (FEA). The limited studies on the impact deformation behavior of the HEA during cold spraying, limiting our understanding of impact phenomena, and interactions between the HEA particles under ultra-high strain rate deformation motivated this study. From the microstructural characterization, heterogeneous microstructure appears to be formed in the cold-sprayed HEA deposit, comprising of equiaxed ultrafine grains at the particle–particle interfacial regions and coarse grains at the particle interiors. The FEA reveals large strain (> 250%) and temperature (> 90% of the alloy solidus temperature), mainly at the splat’s interfaces. Adiabatic shear instability and rotational dynamic recrystallization resulting from heat accumulation and high strain are believed to be responsible for these observations during the ultra-high strain rate deformation of the HEA. The large deformation and grain refinement experienced by the HEA resulted in greater deposit hardness when compared with the sprayed powder, with the nanohardness increasing from 1.16 GPa in the powder to 5.14 GPa in the deposit. This study explores and provides an understanding of the deformation behavior of the HEA and the resulting microstructure during cold spraying

Cyclic plasticity and fatigue damage of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion

The CrMnFeCoNi high-entropy alloy is highly printable and holds great potential for structural applications. However, no significant discussions on cyclic plasticity and fatigue damage in previous studies. This study provides significant insights into the link between print processes, solidification microstructure, cyclic plasticity and fatigue damage evolution in the alloy fabricated by laser powder bed fusion. Thermodynamics-based predictions (validated by scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX)) showed that Cr, Co and Fe partition to the core of the solidification cells, whilst Mn and Ni to the cell boundaries in all considered print parameters. Both dislocation slip and deformation twinning were found to be responsible for plastic deformation under monotonic loading. However, the former was found to be the single dominant mechanism for cyclic plasticity. The surface finish helped to substantially delay the crack initiation and cause lack-of-fusion porosity to be the main source of crack initiation. Most significantly, the scan strategies significantly affect grain arrangements and grain dimensions, leading to noticeable effects on fatigue crack propagation; in particular, the highest resistance crack propagation was seen in the meander scan strategy with 0° rotation thanks to the most columnar grains and the smallest spacing of grain boundaries along the crack propagation path

Assessing the printability of alloys in fusion-based additive manufacturing: towards criteria for alloy selection

Fusion-based Additive Manufacturing (AM) has recently gained much attention due to the process's flexibility and the possibility of fabricating complex metal components. The technology offers tremendous advantages in production time, geometrical flexibility and material savings. However, the process is quite unique and distinctive from traditional manufacturing methods, which brings a lot of unsolved metallurgical issues during processing and post-processing. Moreover, the limited materials portfolio restricts the broader adoption of the technology. The purpose of this study was to assess the printability of alloys in fusion-based AM, where the “printability” is defined as “the ability of a material to be consolidated layer-upon-layer to form a designed part with desired microstructure and properties that ensure the performance of the printed material over its intended service life”. This PhD thesis gains a deeper understanding of the interplay between the process characteristics, material physical properties and the printability aspects of the alloys during fusion-based AM. Out of the fusion-based technologies, laser powder bed fusion (LPBF) is arguably the most common. It has quite similar process characteristics to electron beam powder bed fusion (EB-PBF) and direct energy deposition (DED) methods, so LPBF was chosen as a representative technology for fusion-based AM. It is, however, recognised that slightly different processing conditions in EB-PBF and DED might require some modifications of the developed theories. Liquid-state cracks are one of several defect types commonly observed in fusion-based AM, limiting the reliability of components. Cracks tend to create high stress concentration locations that can significantly limit mechanical performance, particularly in fatigue. It is of paramount importance to prevent liquid-state cracking in LPBF to enable the reliable long-term performance of alloys. The tendency of alloys to form solidification cracks was correlated with the material solidification behaviour using fusion welding concepts, particularly solidification gradient. Solidification gradient served as a good assessment indicator to explain why 316L steel is immune to solidification cracking, while IN718, HEA and Hastelloy-X contain some degree of cracks. Although the solidification gradient alone could not rank different alloys according to the crack density, it could explain the difference in the number density of cracks between Hastelloy X variants with slightly different chemical compositions. It was found that minor compositional changes in Si and Mo content in Hastelloy X can drastically influence the solidification behaviour and cracking tendency, hence the printability. The solidification cracking assessment criteria was then used to develop a design methodology against liquid-state cracking by combining the Scheil–Gulliver solidification simulations and Machine Learning analysis to design alloys for Fusion Additive Manufacturing. Applying this design approach resulted in a Fe–20Cr–7Al–4Mo–3Ni. The alloy was successfully printed with relative densities of over 99 %. The microstructure of printed material was extensively characterised through scanning and transmission electron microscopy, energy dispersive spectroscopy and X-ray diffraction, confirming a single-phase material with low texture and negligible chemical segregation. Neither solidification nor liquation cracks were detected, supporting the validity of the methodology; however, the alloy suffered from solid-state cracking, hindering the ductility and reliability in structural applications. Another key aspect of printability is the ability to achieve desired microstructure and mechanical properties. Process parameters, in particular, scanning strategy, can significantly influence the consolidation, solidification microstructure, and mechanical properties of the alloy. A range of print parameters was used for a comprehensive assessment of printability of CoCrFeMnNi high entropy alloy, providing a basis to establish the relationship between process, microstructure, and mechanical properties. The study demonstrates a high relative density of the alloy fabricated with energy density in the range 62.7–109.8 J/mm3. It is shown that the scan strategy plays an important role in consolidation. For the same energy density, the rotation of 67° between two consecutive layers tends to yield higher consolidation than other considered strategies. Moreover, the scan strategy is found to be most influential in microstructure development. The scan strategy rotation angle controls the extent to which epitaxial growth can occur, and hence the crystallographic texture and the grain morphology. Amongst four considered strategies, the 0°- and 90°-rotation meander led to the strongest preferred texture while the 67°-rotation resulted in a weaker texture. The 67°-rotation strategies led to broadened grains with lower aspect ratios. The understanding of texture and grain size explains the observed mechanical properties (such as flow stress and plastic anisotropy) of the alloy. Mechanical properties of the printed alloy can be further tailored via post-processing, particularly heat treatment. The microstructural development and strengthening mechanisms of IN718 manufactured by fusion-based AM was investigated and compared to a wrought variant. Two different heat treatments were performed, a standard solution heat treatment (SHT) commonly used for conventional IN718 and a modified SHT developed for LPBF IN718 from prior literature. CALPAHD tools were used to gain insights into the dynamics of precipitate formation, while characterisation techniques were used to compare the morphology. It was found that LPBF processed IN718 had superior mechanical properties compared to the wrought variant, both heat-treated with the conventional SHT. By deconvoluting the strengthening contributions of various strengthening mechanisms, it was identified that primarily the strengthening in heat-treated IN718 comes from order and coherence strengthening form γ’’. However, despite the underaged size of the precipitates in LPBF IN718 after conventional heat-treatment, the contributions from prior solidification cells and increased dislocation density had a high enough impact to raise the strength of the alloy above the strength of conventional IN718 with optimal precipitate size. Additionally, it was found that the oxide scale on the surface of LPBF processed IN718 after the heat treatment had a much lower impact on the mechanical properties of the alloy in comparison to near-surface porosity. Hence, to enhance printability, a reliable method to optimise process parameters has to be developed. To accelerate the search for an optimal printability window during the fusion-based AM, an approach incorporating the analytical Rosenthal model and machine learning based on prior literature results was developed. Over 2000 observations of process parameters and resulting sample properties were collected to train the machine learning models. First, a Gaussian regression model was trained to predict the laser absorptivity in Fe-based, Ni-based and Al-based alloys depending on the process parameters. This process-dependent absorptivity was incorporated into the analytical Rosenthal model to accurately calculate the melt-pool depth depending on the alloy, laser power, scan speed and laser spot size. A neural network ensemble was developed to predict the melt-pool depth as a function of the aforementioned process parameters. These models were used to assess the printability of 316L, Hastelloy X, IN718 and Ti6Al4V, in particular, the consolidation, based on the ratios between melt-pool depth and layer thickness; and melt-pool width and hatch spacing. Such consolidation plots identify regions of optimal printability based on the process parameters without the need for lengthy experimental trials.Open Acces

Twinning induced plasticity in austenitic stainless steel 316L made by additive manufacturing

Additively manufactured (AM) 316L steel exhibits extraordinary high yield strength, and surprisingly good ductility despite the high level of porosity in the material. This detailed study sheds light on the origins of the observed high yield strength and good ductility. The extremely fine cells which are formed because of rapid cooling and dense dislocations are responsible for the macroscopically high yield strength of the AM 316L (almost double of that seen in annealed 316L steel). Most interestingly, twinning is dominant in deformed samples of the AM316. It is believed that twinning-induced plasticity (TWIP) behaviour to be responsible for the excellent ductility of the steel despite the high level of porosity. The dominant twinning activity is attributed to Nitrogen gas used in 3D printing. Nitrogen can lower the stacking fault energy of the steel, leading to the disassociation of dislocations, promoting the deformation twinning. Twinning induces large plasticity during deformation that can compensate the negative effect of porosity in AM steel. However, twinning does not induce significant hardening because (1) the porosity causes a negative effect on hardening and (2) twinning spacing is still larger than extremely fine solidification cells

Printability and microstructure of the CoCrFeMnNi high-entropy alloy fabricated by laser powder bed fusion

The CoCrFeMnNi high-entropy alloy is a promising candidate for metal additive manufacturing. In this study, single-layer and multi-layer builds were produced by laser powder bed fusion to study microstructure formation in rapid cooling and its evolution during repeated metal deposition. CoCrFeMnNi showed good printability with high consolidation and uniform high hardness. It is shown that microstructure in the printed alloy is governed by epitaxial growth and competitive grain growth. As a consequence, a bi-directional scanning pattern without rotation in subsequent layers generates a dominant alternating sequence of two crystal orientations

The role of side-branching in microstructure development in laser powder-bed fusion

In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (i.e. 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67 rotation between layers

Comprehensive assessment of the printability of CoNiCrFeMn in laser powder bed fusion

This study assesses the printability including the consolidation, solidification microstructure, and mechanical properties of the CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion. A range of print parameters was used for a comprehensive assessment of printability, providing a basis to establish the relationship between process, microstructure, and mechanical properties. The study demonstrates a high relative density of the alloy fabricated with energy density in the range 62.7-109.8 J/mm3. It is shown that the scan strategy plays an important role in consolidation. For the same energy density, the rotation of 67° between two consecutive layers tends to yield higher consolidation than other considered strategies. Moreover, the scan strategy is found to be most influential in microstructure development. The scan strategy rotation angle controls the extent to which epitaxial growth can occur, and hence the crystallographic texture and the grain morphology. Amongst four considered strategies, the 0°- and 90°-rotation meander led to the strongest preferred texture while the 67°-rotation resulted in weaker texture. The 67°-rotation strategies led to broadened grains with lower aspect ratios. The understanding of texture and grain size provides explanations to the observed mechanical properties (such as flow stress and plastic anisotropy) of the alloy