Analysis of Solidification Microstructure in Additive Manufacturing by Multi-Phase Field Method

Three-dimensional printing of structural materials, namely, additive manufacturing (AM), has notable advantages in fabricating structurally complex engineering components. These complex components usually endure comprehensive fatigue examination due to their complex stress distribution with varying stress ratios during service. Therefore, it is important to ensure the fatigue reliability of additive manufactured materials across all stress ratios. We found that the AM microstructure itself in a Ti-6Al-4V alloy successfully synthesizes the tripartite advantages of fine prior β grain boundaries, void-free, and fine α grains, which are respectively sensitive to the low, medium, and high stress ratio regions. Under this synergistic effect, the fatigue performance of the natural AM microstructure across all stress ratios not only outperforms all additive manufactured and forged Ti-6Al-4V alloys, but also surpasses other metallic materials. Our finding highlights the potential advantage of additive manufacturing technology in producing complex components with high fatigue resistance, substantially expanding its application scope.

Innovative 3D metal additive manufacturing (AM) techniques are revolutionizing the biomedical industry, since they enable the production of porous structures and patient-customized parts of biomedical-grade materials, such as Ti alloys. Surface treatment via the plasma electrolytic oxidation (PEO) of conventionally manufactured Ti and its alloys has been proved as an outstanding approach to promote the osseointegration of implants. Henceforth, it is of increasing interest to develop PEO treatments for AM Ti alloys. The objective of the present work was to fabricate Ca and P containing thin (~3–10 μm thickness) PEO coatings on a Ti6Al4V alloy manufactured via direct metal laser sintering (DMLS), a laser powder bed fusion AM technique, and to study the electrochemical behavior of the treated specimens in a modified α-MEM solution. The electrical response of the PEO process on the AM alloy was compared to that on wrought mill-annealed Ti6Al4V sheets. The electrochemical behavior of the PEO-treated AM alloy was evaluated via potendiodynamic polarization and electrochemical impedance spectroscopy (EIS) in comparison to the non-treated AM alloy and the PEO-treated conventional counterparts. The surface degradation morphologies were evaluated by electron-optical microscopy and optical profilometry. The effect of the AM microstructure on the PEO process and the microstructure and electrochemical response of the resultant coatings are discussed with the aim to define future research directions relevant to the improvement of the corrosion resistance of AM Ti6Al4V, particularly with regard to pitting corrosion.

An ordered active parameter tracking method for efficient multiphase field simulations

Controlling homogenization length scales and microstructure in additively manufactured Ti-Ta functionally graded materials

Additive manufacturing (AM), particularly laser power bed fusion (LPBF) technique, enables the formation of hierarchical heterogeneous microstructures that markedly enhance the mechanical performance of materials. However, a quantitative, kinetics-based understanding of how AM-induced heterostructures govern the evolution of deformation mechanisms and hetero-deformation-induced (HDI) strengthening during plastic flow remains lacking. Here, we employ metastable 304 L stainless steel, with its intrinsically low stacking fault energy that activates slip, twinning, and martensitic transformation, as a model system to elucidate the interplay between deformation kinetics and hetero-deformation in AM alloys. By integrating strain-rate jump and stress-relaxation experiments with loading–unloading hysteresis measurements and multiscale microstructural characterisation, we quantitatively resolve the evolution of dislocation mobility, dislocation storage, and HDI back stress across different deformation stages. The results reveal a sequential yet overlapping transition of dominant plasticity carriers: from dislocation slip in sub-micron cellular structures and planar slip band, to deformation twinning, and finally to deformation-induced martensitic transformation. This mechanistic evolution is manifested by a continuous reduction of activation volume from over 100 b³ to tens of b³, coupled with a non-monotonic evolution of mobile dislocation density. Throughout deformation, HDI back stress contributes up to ~75% of the flow stress, primarily associated with the massive accumulation of geometrically necessary dislocations at cellular walls, twin boundaries, and martensitic interfaces. These findings establish a quantitative linkage between AM-induced heterogeneous microstructures, deformation kinetics, and HDI strengthening, providing a kinetics-based framework for the mechanistic design of AM-enabled heterostructured materials. • Kinetics-based framework links AM microstructures to plastic deformation. • Activation volume drops from ~120 b³ to ~30 b³ across deformation stages. • Multi-modal deformation mechanisms govern rate control and sustain work hardening. • HDI back stress contributes ~75% of flow stress in LPBF 304 L stainless steel. • Transient tests quantify dislocation mobility, exhaustion, and back-stress evolution.

Electrochemical finishing of metallic surfaces is a scalable, relatively cost-effective technique that is also versatile. One of the main detractions is that the current distribution is uneven and unspecific, often leading to a lack of shape retention and, if it goes too far, inducing corrosion. Pulsing the electrical input creates an unsteady state during electropolishing, controlling the current distribution across the part. The pulse wave prevention of general non-uniform current distribution across the entire geometry instead produces local non-uniformities in the current distribution at given surface features. Many pulse electropolishing parameters can be varied to target individual surface features, including the electrolyte, pulse height, pulse width (equally and unequally distributed), time of polishing as well as the overall waveform.Additive manufacturing (AM) can produce unique, complex structures rapidly. The fast local melting of metal AM parts cool quickly due to the small area, causing unique microstructures, pores, and larger surface roughness compared to cast. AM microstructures change mechanical and chemical properties, such as strength and corrosion, compared to traditionally manufactured parts. Variations in build parameters and composition creates a complicated, feature dependent, corrosion response. By polishing the surface, some of the features, namely the corrosion resistance, wear, and aesthetics can be standardized and enhanced.In this work, we explore the underlying electrochemistry that governs pulse electropolishing. We show that controlling the electrolyte with environmentally benign chemicals ultimately influences the part dimensions that can be polished, the ability to polish, and the propensity to corrode. The pulse height, pulse width (Fig. 1), and total charge passed have an impact on different surface feature removal. We will demonstrate impact of the pulse parameters on the polishing of additively manufactured stainless steel. We go beyond simple planar geometry and apply our understanding to larger scale complex geometry parts, while maintaining near-net shape and minimizing mass loss. Figure 1

Non-acid, alcohol-based electropolishing enables high-quality electron backscatter diffraction characterization of titanium and its alloys: Application to pure Ti and Ti-6Al-4V

Laser and electron beam-based additive manufacturing of Ti-6Al-4V are under consideration for application to aerospace components. A critical concern for these processes is the ability to obtain a consistent and desirable microstructure and corresponding mechanical properties of the deposit. Based on the Rosenthal solution for a moving point-heat source, recent work has developed simulation-based process maps for the thermal conditions controlling microstructure (grain size and morphology) in beam-based deposition of semi-infinite geometries, where a steady-state melt pool exists away from free edges. In the current study, the Rosenthal solution is modified to include the effects of free edges. This is accomplished by the superposition of two point-heat sources approaching one another, with the line of symmetry representing the free edge. The result is an exact solution for the case of temperature-independent properties. Dimensionless results for melt pool geometry are determined, and plotted as a function of distance from the free edge. Results are plotted on solidification maps to predict trends in microstructure for Ti-6Al-4V. Finite element analysis is used to verify results. Results suggest that melt pool geometry is more sensitive to free edges than solidification microstructure.

Analysis of higher-order corrections to the interface kinetics in the multiphase field method

In recent years, there has been widespread adoption of machine learning (ML) technologies to unravel intricate relationships among diverse parameters in various additive manufacturing (AM) techniques. These ML models excel at recognizing complex patterns from extensive, well-curated datasets, thereby unveiling latent knowledge crucial for informed decision-making during the AM process. The collaborative synergy between ML and AM holds the potential to revolutionize the design and production of AM-printed parts. This review delves into the challenges and opportunities emerging at the intersection of these two dynamic fields. It provides a comprehensive analysis of the publication landscape for ML-related research in the field of AM, explores common ML applications in AM research (such as quality control, process optimization, design optimization, microstructure analysis, and material formulation), and concludes by presenting an outlook that underscores the utilization of advanced ML models, the development of emerging sensors, and ML applications in emerging AM-related fields. Notably, ML has garnered increased attention in AM due to its superior performance across various AM-related applications. It is envisioned that the integration of ML into AM processes will significantly enhance 3D printing capabilities across diverse AM-related research areas.

The effect of microstructural morphologies on solidification microsegregation and residual liquid distribution was investigated in Fe-Cr-Ni-C alloy, which forms the basis for austenitic stainless steel with fully γ solidification mode by using the multi-phase field (MPF) method. When the cellular dendrite growth direction became inclined, the growth of the secondary arm became remarkable in the branching part and liquid droplets were formed irregularly around the grain boundary region. A liquid film was formed in the blocking part of inclined cellular growth, where stress concentration occurred continuously. This liquid film can influence solidification cracking susceptibility. Stress concentration occurred discontinuously in the branching part. The effect of varying the interface energy during solidification on the residual liquid morphology was not large, even when 2σL-γ≪σγ-γ, where σL-γ is the interface energy of liquid/γ and σγ-γ is the interface energy of γ/γ.

We proposed a microstructure-based multiscale simulation framework using various commercial simulation software and applied it to analyze the hot rolling of duplex stainless steel. According to the Integrated Computational Material Engineering (ICME) concept, we established a procedure to bridge various simulation software from nano- to macroscopic length scales. Using our framework, first, microstructure evolutions by multiphase field (MPF) simulations coupled with the Calculation of Phase Diagrams (CALPHAD) database were performed. In our application, we simulated the columnar and equiaxed solidification during the continuous casting of duplex stainless steel. In the MPF simulations, the temperature field in the slab was calculated by heat conduction analysis using a finite element method (FEM). Then, the macroscopic elastic and plastic mechanical properties of the microstructure obtained by the MPF simulations were estimated by the virtual material test using a nonlinear FEM based on the homogenization method. Because the elastic constants of single δ-ferrite and γ phases in the microstructure are necessary for the virtual material test, they were calculated by molecular dynamics and first principle calculations. Furthermore, the plastic stress–strain properties of the single phases were estimated on the basis of the results of nano-indentation and uniaxial tensile tests. Subsequently, the hot rolling of the slab was simulated using an elastoplastic FEM with the mechanical properties obtained by the virtual material test. Finally, the static recrystallization in the rolled slab was again simulated by the MPF method.

In this work, molecular dynamics (MD) simulations have been used to study the heterogeneous nucleation occurring at grain boundaries (GBs) during the austenite (FCC) phase to ferrite (BCC) phase transformation in a pure Fe polycrystalline system. The critical nucleus properties (including size, shape, and activation energy) determined by classical nucleation theory are compared with those obtained by using a combination of the multi-phase field method (MPFM) and the nudged elastic band (NEB) method. For nucleation events that exhibit low-energy facets completely embedded within the parent FCC phase, there is a good agreement between the MD and the MPFM result with respect to the critical nucleus size, shape, and nucleation energy barrier. For systems where the emerging nucleus contains facets that cross the GB plane, the MPFM-NEB, when compared to MD, yields a better prediction than the classical approach for the nucleus morphology. New observations from the MPFM-NEB method indicate that the critical nucleus shape may change with volume and therefore depends on the nucleation driving force (undercooling).

Additive manufacturing (AM) technology can potentially disrupt offshore and marine industry by effectively reducing manufacturing processes and leadtime, and enabling new product designs. A review of existing literature has revealed limited knowledge in using AM to process shipbuilding materials, which can otherwise enhance existing shipbuilding manufacturing workflow. This has led to the research motivation to establish the technical feasibility of using selective laser melting (SLM) to process ASTM A131 EH36 shipbuilding steel. SLM process is a widely understood AM technology that has been used extensively to process other type of steels, for example stainless steel 316L. EH36 belongs to a class of high tensile low alloy steel which has traditionally been processed through casting. However, knowledge on using lasers to process EH36 is largely unknown, especially in AM applications. Using SLM to process EH36 is novel and will solve the problem of lack of knowledge in this field. The study thus aims to further the understanding of the mechanical properties and microstructure of SLM processed EH36. A preliminary investigation was first carried out to establish the technical feasibility of using SLM to process EH36. The process parameters obtained were then used to further the investigations. Heat treatment process was applied as a possible post processing technique. Mechanical testing was carried out to characterise its mechanical properties. The fracture surfaces and microstructures were then studied to characterise the material. The results showed that EH36 can be processed using SLM without any visible cracks. The mechanical properties of as built SLM processed EH36 exhibit very high tensile strength, but low ductility. The ductility can be improved through tempering heat treatment process, but at the partial sacrifice of tensile strength. Fractography analysis also affirmed the lack of ductility in the SLM processed EH36 samples. Finally, the microstructure showed that fine grain size and martensitic microstructure were the primary drivers behind its high tensile strength. Tempered samples experience grain coarsening and phase transformation to a mainly ferritic structure, which led to recovery in its ductility but a decrease in the tensile strength. A scaled model was fabricated to validate the results from the microstructure studies, and the findings indicate similar microstructures formed. This demonstrates the repeatability of the developed process parameters on fabrication of complex joints. The study contributes to the scientific knowledge with regard to material and mechanical property characterisation of SLM processed EH36. The findings from the fractography and microstructural analysis will contribute towards building the knowledge and facilitate future work on AM of EH36.

Additive manufacturing (AM) technology has progressed dramatically over the last decade, allowing for rapid manufacturing of components with complex geometries and diverse feedstocks. AM of previously challenging materials such as pure copper has thus been made easier with extended processing routes. However, only powder bed-based AM processes (typically equipped with near IR lasers or an electron beam) have previously been demonstrated for producing bulk pure Cu parts with well-defined geometries, where certain disadvantages and limitations abound. This study demonstrates the first bulk Cu parts with well-defined geometries built by the blown powder, directed energy deposition (DED) process with a blue laser. Near full density (up to 99.6 %) parts with a volume of 1000 mm3 were produced, which is the densest pure Cu part reported so far in laser AM, but at a significantly lower energy density compared to similar volume parts built using near IR lasers. A larger part with a volume of 8000 mm3, the largest volume of pure Cu reported to date in laser AM, was also fabricated using the same build parameters with relative density up to 94.1 %. Microstructure analyses were performed and the influence of different building conditions were examined.