Local Solidification Conditions Research Articles

Interpenetrating microstructure in laser powder-bed fusion parts using selective rescanning

In-situ microstructural control is desirable in additively manufactured metal parts due to limited post-processing options for net-shaped components. Here, we introduce a novel selective rescanning approach to control the local solidification conditions and the microstructure in metal parts produced by laser powder-bed fusion (LPBF). We show that the melt pool dimensions, grain size, and sub-grain cell structure can be selectively varied in three dimensions to engineer the mechanical response of LPBF parts. The lattice-based rescanning strategy enables the formation of an interpenetrating microstructure comprised of fine and coarse grains. The localized heating and cooling-induced thermal stresses increase the hardness and tensile strength of rescanned specimens. The study shows the potential of selective rescanning strategy as a promising avenue for achieving precise control of microstructure and properties in as-printed LPBF parts without subsequent processing.

Emergence of rapid solidification microstructure in additive manufacturing of a Magnesium alloy

Abstract Bioresorbable Mg-based alloys with low density, low elastic modulus, and excellent biocompatibility are outstanding candidates for temporary orthopedic implants. Coincidentally, metal additive manufacturing (AM) is disrupting the biomedical sector by providing fast access to patient-customized implants. Due to the high cooling rates associated with fusion-based AM techniques, they are often described as rapid solidification processes. However, conclusive observations or rapid solidification in metal AM — attested by drastic microstructural changes induced by solute trapping, kinetic undercooling, or morphological transitions of the solid-liquid interface — are scarce. Here we study the formation of banded microstructures during laser powder-bed fusion (LPBF) of a biomedical-grade Magnesium-rare earth alloy, combining advanced characterization and state-of-the-art thermal and phase-field modeling. Our experiments unambiguously identify microstructures as the result of an oscillatory banding instability known from other rapid solidification processes. Our simulations confirm that LPBF-relevant solidification conditions strongly promote the development of banded microstructures in a Mg-Nd alloy. Simulations also allow us to peer into the sub-micrometer nanosecond-scale details of the solid-liquid interface evolution giving rise to the distinctive banded patterns. Since rapidly solidified Mg alloys may exhibit significantly different mechanical and corrosion response compared to their cast counterparts, the ability to predict the emergence of rapid solidification microstructures (and to correlate them with local solidification conditions) may open new pathways for the design of bioresorbable orthopedic implants, not only fitted geometrically to each patient, but also optimized with locally-tuned mechanical and corrosion properties.

Leveraging the digital thread for physics-based prediction of microstructure heterogeneity in additively manufactured parts

A major limitation of additive manufacturing (AM) processes is that local conditions of material deposition frequently lead to unintentional heterogeneities in microstructure and properties within a single component, despite nominally uniform process conditions. Up to now, there has been no way to a priori determine the distribution of these heterogeneities, requiring expensive trial-and-error approaches to fabrication, testing, and characterization. Here, a physics-based framework for creating a digital representation of the laser powder bed fusion (PBF) process is proposed to predict the variation in solidification behavior that leads to heterogeneous microstructures in an as-built part. By leveraging in situ process data stored in the part’s digital thread, the scan path and process parameters were input into a heat transfer model which predicted solidification data at the melt pool scale. A two-step unsupervised clustering algorithm was used to first cluster the local solidification conditions (12.5 µm3 voxels) and then to cluster the regional behavior on the scale of multiple scan passes and print layers (250 µm3 super-voxels). This process was used to identify regions with similar solidification characteristics for multiple locations in a Stainless Steel 316-L component. The corresponding as-built part was sectioned and characterized using electron backscatter diffraction (EBSD). Quantitative analysis of the pole figures confirmed that the predicted regions of heterogeneity in the solidification conditions corresponded with differences in the observed microstructure. This work shows a viable path for estimating the microstructural heterogeneity for additively manufactured parts to either limit microstructural variation throughout a part or to enable functionality-based variation of the microstructure.

Hot tear prediction in large sized high alloyed turbine steel parts - experimental based calibration of mechanical data and model validation

The main defects in heavy steel castings are related to hot tear formation during solidification. Depending on the steel grade, design, and local solidification conditions, it is possible to predict regions with higher risk of hot tear formation during the casting process. However, steels containing Boron show more complex crack and defect patterns compared to common steel casting alloys. The mechanisms behind the Boron induced hot tearing is investigated in this work to understand the influence of Boron enrichment during solidification and the influence on hot tearing. The experimental work includes the investigation of phase diagrams and the corresponding fractions of the solid and liquid phases depending on temperature using thermal analysis e.g. DSC and HT-LSCM. The hot tearing sensitivity and mechanical properties during solidification are obtained in the Submerged Split Chill Test, SSCT. In addition IMC-B 3-point bending tests are performed to determine high-temperature material properties in the solid state. The work is part of a research project where the final goal is to improve the hot tear predictions based on experimental work and carry out a benchmark simulation of a real sized casting and use it to show the agreement between the numerical results and extensive non-destructive testing from industrial observations.

Encoding data into metal alloys using laser powder bed fusion

Beyond near-net shape manufacturing of parts with complex geometry, additive manufacturing (AM) makes it possible to fabricate materials with distinct, site-specific microstructures. This ability is unique to AM and enables the design of architected materials that were previously unattainable. Here, we leverage this strategy to encode data into metal parts using the microstructure as the medium to store information. We use a novel laser scanning technique to control the local solidification conditions during laser powder bed fusion and embed a linear barcode and Quick Response (QR) code into stainless steel 316 L. The codes use blocks of different crystallographic texture–i.e., regions in which the crystal lattice of most grains is aligned towards a preferred orientation–as the data “bits”. The data may be retrieved by analytical techniques that are sensitive to local microstructure variations. As a demonstration, we decode the barcodes by measuring the scattering of optical light from their etched surface using a technique called directional reflectance microscopy. The resulting texture maps are readable by conventional barcode scanners, such as those found on mobile phones. The ability to embed data has significant potential in fields such as law enforcement, biomedicine, and transportation, where permanent, damage-resistant tracking is essential.

Characterisation of the multiple effects of Sc/Zr elements in selective laser melted Al alloy

The widespread application of selective laser melting (SLM) fabricated high performance lightweight Al components is hindered by the available commercial alloys due to their limited mechanical properties and processability. The introduction of Sc/Zr elements to the Al matrix for SLM provides conspicuous benefits, including grain refinement, precipitation strengthening and high thermal stability. In the present study, the multiple effects of Sc/Zr elements in SLM fabricated Al alloy are systematically investigated, using the recently developed Al-Mn-Sc alloy as an example. The application of Sc/Zr elements generated an unconventional heterogeneous grain structure characterised by the alternative growth of columnar-equiaxed grains in the molten pool, which effectively avoided the solidification cracks and alleviated the mechanical property anisotropy issues. The formation of the heterogeneous grain structure is mainly attributed to the precipitation of the primary Al3(Sc,Zr) particles which dependent on the local solidification conditions. The post heat treatment was also conducted to clarify the role of Sc/Zr additions on stress relieve effect and precipitation hardening effect. Moreover, the room temperature and elevated temperature mechanical properties of the SLMed Al-Mn-Sc alloy were also evaluated. The fundamental understanding of the Sc/Zr benefits in the SLM fabricated Al alloy is expected to shed light on future high performance Al alloy development specifically for the additive manufacturing process.

Microstructure and creep properties of cast near-eutectic Al–Ce–Ni alloys

This study investigates the as-cast and aged microstructures, thermal stability, ambient temperature strengthening, and creep resistance of three ternary Al–Ce–Ni alloys (wt%): near-eutectic Al–10Ce–5Ni (with both eutectic and hypoeutectic regions), hypoeutectic Al-7.5Ce-3.75Ni (with numerous primary Al dendrites), and hypereutectic Al-12.5Ce-6.25Ni (with coarse, blocky primary Al3Ni and Al11Ce3 precipitates and some primary Al dendrites). Depending on the alloy composition and local solidification conditions, the following eutectic morphologies are found: (i) coarse Al–Ce eutectic colonies where Al11Ce3 is in the form of “Chinese script”, (ii) intermingled regions of binary Al–Ce and Al–Ni eutectic colonies, with finer Al3Ni and Al11Ce3 fibers, (iii) large ternary eutectic colonies, where the binary Al3Ni and Al11Ce3 phases are alternating or intertwining within the individual, fine fibers (diameters of ∼60–170 nm, depending on solidification rates), and (iv) ternary eutectic zones (between primary Al dendrites), where fine Al3Ni and Al11Ce3 build up a 3D-interconnected network. The high volume fraction of intermetallic phases and extremely fine eutectic spacing/fiber diameter both contribute to high ambient strengthening (higher as-cast microhardness than binary Al–Ce or Al–Ni), and also provide enhanced creep resistance at 300 and 350 °C. The alloys are coarsening-resistant up to 425 °C for extended periods, with a gradual decrease in microhardness. The alloys aged at 400 °C to 1050 h show fiber fragmentation and coarsening of the resulting particles, with the faster-diffusing Ni driving more rapid coarsening of the Al3Ni particles which engulf finer, more stable Al11Ce3 particles. Severe overaging (performed at 590 °C for 24 h) leads to Al3Ni and Al11Ce3 spheroids which remain submicron-sized in eutectic colonies, but micron-sized at colony boundary and at Al dendrite-eutectic interface. Creep resistance at 300 °C of overaged Al–10Ce–5Ni remains substantial, consistent with load-transfer based composite strengthening being an important strengthening mechanism in these alloys, making them excellent candidates for replacement of heavier steel or titanium parts operating under stress up to 300 °C.

Influence of geometry on columnar to equiaxed transition during electron beam powder bed fusion of IN718

Correlation between spot-melt scan parameters (linear spot-density aka areal energy density), build geometry, and solidification microstructure evolution (columnar vs equiaxed) in a powder bed fusion technology is investigated. It is shown that to maintain the equiaxed solidification microstructure evolution in electron beam powder bed additive manufacturing (AM), the areal energy density per layer needs to be scaled with respect to the 2D cross-sectional area of the layer being melted. Samples with two different cross-sectional areas (40 × 40 mm and 20 × 20 mm) have been fabricated with varying areal energy densities. For a given square cross-section (20 × 20 mm), increasing the areal energy density (4.8 MJ/sq.m to 14.7 MJ/sq.m) transitioned the solidification microstructure from columnar to equiaxed. The observed microstructure data (Electron Back Scattered Diffraction - EBSD) is quantified by calculating the principal component (PC) score using a spatial statistics methodology. The sample with equiaxed grains is found to have a low PC score while the sample with columnar grain had a high PC score. A semi-analytical model is used to simulate the heat transfer and the local solidification conditions as a function of processing parameters (linear spot-density). The result from the heat transfer model is correlated with previously quantified microstructure data. Space-Time analysis of the melt pattern is done and correlated with the observed microstructure. In addition, from the findings, appropriate parameters have been used to additively manufacture a turbine blade with site-specific or hybrid solidification microstructure (traditional fabrication possible via a patented method of localized cold working and heat treatment).

Tailored grain morphology via a unique melting strategy in electron beam-powder bed fusion

This study presents a unique melting strategy in electron beam-powder bed fusion of Alloy 718 to tailor the grain morphology from the typical columnar to equiaxed morphology. For this transition, a specific combination of certain process parameters, including low scanning speeds (400–800 mm/s), wide line offsets (300–500 μm) and a high number of line order (#10) was selected to control local solidification conditions in each melt pool during the process. In addition, secondary melting of each layer with a 90° rotation with respect to primary melting induced more vigorous motions within the melt pools and extensive changes in thermal gradient direction, facilitating grain morphology tailoring. Four different types of microstructures were classified according to the produced grain morphology depending on the overlap zone between two adjacent melt pools, i.e., fully-columnar (overlap above 40 %), fully-equiaxed (overlap below 15 %), mixed columnar-equiaxed grains, and hemispherical melt pools containing mixed columnar-equiaxed grains (overlap ~20–25 %). The typical texture was <001>; however, the texture was reduced significantly through the transition from the columnar to equiaxed grain morphology. Along with all four different microstructures, shrinkage defects and cracks were also identified which amount of them reduced by a reduction in areal energy input. The hardness was increased through the transition, which was linked to the growth of the γʺ precipitates and high grain boundary density in the fully-equiaxed grain morphology.

Analytical Description of the Criterion for the Columnar-To-Equiaxed Transition During Laser Beam Welding of Aluminum Alloys

An equiaxed grain structure in a laser welded seam is beneficial. In current literature the material and process perspective are addressed separately. This paper combined the material and process perspective in one analytical expression in order to access a deeper understanding of the key parameters for an equiaxed solidification during laser beam welding. For this, the steady state model of Hunt for growth of equiaxed dendritic grains was combined with heat conduction calculations and nucleation theory. The derived analytical expression identified the minimum values of the welding parameters required for the growth of equiaxed dendritic grains and describe their interaction with the metallurgically given nucleation density. The theoretically derived analytical description agrees well with experimental results obtained with two different batches of EN AW-6016 over a wide-range of laser welding parameters. The analytical description summarizes the effects of nucleation processes, local solidification conditions and the influence of process parameters on the growth of equiaxed dendritic grains. Furthermore, the equation identifies the key alloy properties, which influence the required process parameter for an equiaxed solidification. For the first time, the full range of phenomena, including metallurgical and thermal effects, was merged in one analytical expression.

Influence of Process Conditions on the Local Solidification and Microstructure During Laser Metal Deposition of an Intermetallic TiAl Alloy (GE4822)

Temperature-time cycles are essential for the formation of microstructures and thus the mechanical properties of materials. In additive manufacturing, components undergo changing temperature regimes because of the track- and layer-wise build-up. Because of the high brittleness of titanium aluminides, preheating is used to prevent cracking. This also effects the thermal history. In the present study, local solidification conditions during the additive manufacturing process of Ti-48Al-2Cr-2Nb with laser metal deposition (LMD) are investigated by both simulation and experimental investigations. Dependencies of the build-up height, preheating temperatures, process parameters and effects on the resulting microstructure are considered, including the heat treatment. Solidification conditions are found to be dependent on the build height and thus actual preheating temperature, process parameters and location in the melt pool. Influences on both chemical composition and microstructure are observed. Resulting differences can almost be balanced through post heat treatment.

A computational study of porosity formation mechanism, flow characteristics and solidification microstructure in the L-DED process

Laser-assisted directed energy deposition is an additive manufacturing process used to manufacture metallic parts. The gas porosity is one of the prominent deposition defects in the processed parts. This influences the mechanical properties which can cause the part failure. In this work, the mechanism of gas porosity formation at low energy density is addressed using computational modeling. An investigation is carried out to capture the powder particles interaction with the melt pool and resulting porosity formation, molten pool hydrodynamics, and solidification microstructure in the L-DED process. The numerical results reveal that the stagnant zone in the melt pool leads to entrapment of bubbles which eventually forms porosity. This bubble entrapment phenomenon is studied by varying the powder mass flow rate, and it is found that increasing the mass flow rate results in rapid bubble formation which increases the chances of gas porosity formation. The temperature gradient and cooling rates are used for solidification analysis and prediction of as-solidified grain morphology. Using the empirical relation, the effect of local thermodynamic solidification conditions on the size of the dendritic microstructure is analyzed. The predicted melt pool geometry and porosity morphology agree with the experimental results.

Role of impinging powder particles on melt pool hydrodynamics, thermal behaviour and microstructure in laser-assisted DED process: A particle-scale DEM – CFD – CA approach

High speed imaging of molten pool free-surface hydrodynamics in laser-assisted directed energy deposition process clearly revealed a highly oscillatory and dynamic melt flow due to impinging powder particles. Surprisingly, most of the reported computational work exclude the injection of powder particles and rather adopt a homogeneous mass and energy addition approach, and therefore provides less accurate predictions. In this work, we develop a coupled multi-physics particle-scale approach utilizing the discrete element method for particle trajectory prediction, the computational fluid dynamics for free-surface thermo-fluidic modelling and the cellular automata method for grain growth evolution. In the model, the governing physical phenomena, such as laser-powder interaction, in-flight particle heating, phase change (melting, vaporization and solidification), free-surface evolution, molten pool hydrodynamics and impinging particles-melt interaction have been considered. Experiments for the deposition of Inconel-625 on an Inconel-625 substrate are carried out, and the model predictions are validated with the experimental measurements. For the first time, the predicted thermo-fluidic simulation results reveal highly oscillatory, chaotic and random melt flow attributed to the impinging powder particles. During the deposition, it is found that the role of the Marangoni convection is less significant as compared to the momentum imparted by the impinging powder particles in the melt pool. Using the simulated thermal undercooling data, cellular automata-based grain growth simulation predicts elongated columnar dendrites in the melt pool that grows epitaxially from the melt pool interface and stretches towards the centre. Using the Kurz-Fisher model, the effect of local thermodynamic solidification conditions on the size of dendritic microstructure is also described. The predicted melt pool geometry, temperature field and grain structure compare well with the experimental measurements.

Fabrication of Single Crystals through a µ-Helix Grain Selection Process during Electron Beam Metal Additive Manufacturing

Selective Electron Beam Melting (SEBM) is a powder bed-based additive manufacturing process for metals. As the electron beam can be moved inertia-free by electromagnetic lenses, the solidification conditions can be deliberately adjusted within the process. This enables control over the local solidification conditions. SEBM typically leads to columnar grain structures. Based on numerical simulation, we demonstrated how technical single crystals develop in IN718 by forcing the temperature gradient along a µ-Helix. The slope of the µ-Helix, i.e., the deviation of the thermal gradient from the build direction, determined the effectiveness of grain selection right up to single crystals.

Effect of Direct Energy Deposition Process Parameters on Single-Track Deposits of Alloy 718

The effect of three important process parameters, namely laser power, scanning speed and laser stand-off distance on the deposit geometry, microstructure and segregation characteristics in direct energy deposited alloy 718 specimens has been studied. Laser power and laser stand-off distance were found to notably affect the width and depth of the deposit, while the scanning speed influenced the deposit height. An increase in specific energy conditions (between 0.5 J/mm2 and 1.0 J/mm2) increased the total area of deposit yielding varied grain morphologies and precipitation behaviors which were comprehensively analyzed. A deposit comprising three distinct zones, namely the top, middle and bottom regions, categorized based on the distinct microstructural features formed on account of variation in local solidification conditions. Nb-rich eutectics preferentially segregated in the top region of the deposit (5.4–9.6% area fraction, Af) which predominantly consisted of an equiaxed grain structure, as compared to the middle (1.5–5.7% Af) and the bottom regions (2.6–4.5% Af), where columnar dendritic morphology was observed. High scan speed was more effective in reducing the area fraction of Nb-rich phases in the top and middle regions of the deposit. The &lt;100&gt; crystallographic direction was observed to be the preferred growth direction of columnar grains while equiaxed grains had a random orientation.

Microstructure Solidification Maps for Al-10 Wt Pct Si Alloys

Hypo-eutectic Al-Si alloys are widely used in both the automotive and aerospace industries; however, they still have limited usage as structural materials, due to the inherent morphology of the Si phase that forms within the eutectic structure. This non-ideal Si morphology can be modified, via alloy additions and/or rapid solidification (RS), but the underlying mechanism(s) behind this is poorly understood. This work focused on understanding the influence of RS on the eutectic structure, for hypo-eutectic Al-10 wt pct Si alloys produced by Impulse Atomization and Differential Scanning Calorimetry. This study found that the eutectic Si forms into four distinct morphologies: (1) flaky, (2) fibrous, (3) globular + fibrous and (4) globular, depending on the solidification conditions. As a result, two solidification maps of the Si morphology are proposed, one based on local eutectic solidification conditions and another based on a solidification continuous cooling diagram (SCCT). Both maps help identify the required conditions for certain Si morphologies to form. Hardness measurements were also carried out and it was found that the Si morphology would influence the alloy hardness, with the highest value being achieved when the eutectic Si was globular. This result indicates that the Si morphology is an important factor that can alter the mechanical properties of hypo-eutectic Al-Si alloys.

Solidification pattern, microstructure and texture development in Laser Powder Bed Fusion (LPBF) of Al10SiMg alloy

A comprehensive analysis of solidification patterns and microstructural development is presented for an Al10SiMg sample produced by Laser Powder Bed Fusion (LPBF). Utilizing a novel scanning strategy that involves counter-clockwise rotation of the scan vector by 67° upon completion of each layer, a relatively randomized cusp-like pattern of protruding/overlapping scan tracks has been produced along the build direction. We show that such a distribution of scan tracks, as well as enhancing densification during LPBF, reduces the overall crystallographic texture in the sample, as opposed to those normally achieved by commonly-used bidirectional or island-based scanning regimes with 90° rotation. It is shown that, under directional solidification conditions present in LPBF, the grain structure is strictly columnar throughout the sample and that the grains' orientation aligns well with that of the α-Al cells. The size evolution of cells and grains within the melt pools, however, is shown to follow opposite patterns. The cells'/grains' size distribution and texture in the sample are explained via use of analytical models of cellular solidification as well as the overall heat flow direction and local solidification conditions in relation to the LPBF processing conditions. Such a knowledge of the mechanisms upon which microstructural features evolve throughout a complex solidification process is critical for process optimization and control of mechanical properties in LPBF.

Microstructure, mechanical and corrosion properties of electron-beam-melted and plasma-transferred arc-welded WCP/NiBSi metal matrix composites

As one of the new additive manufacturing processes, electron beam melting (EBM) has seen its promising potential in the fabrication of metal matrix composites (MMCs) components with complex geometries. In this work, WCP/NiBSi MMCs were fabricated by EBM and plasma-transferred arc welding (PTAW) for a comparative study. The microstructures of both samples were examined using a scanning electron microscope (SEM) equipped with an electron backscattered diffraction (EBSD) detector. The macrohardness was tested using a Rockwell hardness method (Type C), while the microhardness was measured using different loadings (0.5–1.0 N) based on different phases. The anti-abrasion performance was tested as per the ASTM G105 standard. The corrosion behavior of the MMCs was also assessed by potentiodynamic polarization. The results indicate that the EBM bulk and the PTAW cladding MMCs exhibit different microstructures due to the different local solidification conditions. This is believed to lead to the varied mechanical properties and corrosion resistance of the MMCs, and the possible mechanisms were also discussed.

Quantitative Experimental Study of Defects Induced by Process Parameters in the High-Pressure Die Cast Process

A quantitative experimental study of the effects of process parameters on the formation of defects during solidification of high-pressure die cast magnesium alloy components is presented. The parameters studied are slow-stage velocity, fast-stage velocity, intensification pressure, and die temperature. The amount of various defects are quantitatively characterized. Multiple runs of the commercial casting simulation package, ProCAST™, are used to model the mold-filling and solidification events. Several locations in the component including knit lines, last-to-fill region, and last-to-solidify region are identified as the critical regions that have a high concentration of defects. The area fractions of total porosity, shrinkage porosity, gas porosity, and externally solidified grains are separately measured. This study shows that the process parameters, fluid flow and local solidification conditions, play major roles in the formation of defects during HPDC process.

Phase filed simulation of dendritic growth of copper films irradiated by ultrashort laser pulses

A phase field model (PFM) is combined with a two-dimensional two-temperature model (TTM) to simulate the evolution of dendritic growth during re-solidification of ultrafast laser-material interaction. The dynamic solidification conditions at different locations of the melting pool obtained from TTM are fed into the quantitative PFM based on the macro-micro coupled method. A series of simulations are executed to investigate the influence of laser parameters, such as laser influence and pulse duration, on melting pool characteristics and local dendrite morphology. Besides, dendrite structures calculated at different areas of the melting pool were discussed based on local solidification conditions, and the simulated dendrite arm spacing (DAS) for various cooling rates was made comparison with previously published experimental data. The simulated results reveal that the maximum temperature gradient has significant influence on the local dendritic competitive growth, while the laser parameters effect the local microstructure distinctly due to the changes of solidification conditions. This work demonstrates the potential application of PFM to predict the microstructure morphology presented in ultrashort laser-material interaction and other industrially relevant conditions with complex solidification conditions.