Supersolidus Liquid Phase Sintering Research Articles

Surface functionalization of binder jetted steels through super-solidus liquid phase sintering and electro-spark deposition

This study proposed a surface functionalization strategy for porous binder jet additive manufacturing (BJAM) steels. The methodology involves a sequential process of super-solidus liquid phase sintering (SLPS) followed by electro-spark deposition (ESD). SLPS with varying liquid fractions was used to modify the initial surface quality of BJAM steels, creating a range of surface roughness and bulk porosity. The ability of ESD to address substrate surface roughness and porosity variation was investigated using lower (Inconel 625) and higher (WC-Co) melting point coating materials. The results revealed a clear correlation between substrate surfaces and the surface finish, uniformity, and thickness of ESD deposits. Inconel 625, with its lower melting point, showcased an infiltration behavior and high tolerance to substrate conditions. Conversely, achieving high-quality WC-Co coating necessitated prior SLPS treatment to attain substrates with minimal roughness and sub-surface porosity. This investigation provides insights into optimizing surface modifications for porous BJAM metal parts, considering different coating materials and their compatibility with substrate conditions.

In-situ monitoring of sintering and analytical modeling of densification and shrinkage in binder jetted 316 L stainless steel

In binder jetting, shrinkage and deformation occur during the sintering step, both of which are affected by the green density of the binder jetted materials. The study innovatively introduces a cost-effective, practical, and in-process monitoring system for visualizing shrinkage and deformation on larger samples than conventionally observed using small-scale specimens in dillatometry equipment. The powder characteristics and binder jet printing process itself influence the initial green density. The comprehensive analysis of powder flowability and packing density, densification behavior, and shrinkage reveals that the consolidated parts using virgin powder (with a green density of 55 %) can achieve a relative density above 99.9 % with an anisotropic shrinkage in the Z > X > Y direction. In contrast, the used or recycled powder exhibits a lower green density of ∼48 %, higher shrinkage rate in all three dimensions, and a decreased degree of anisotropy. Using in-process imaging and experimental data on the grain size attained through optical microscopy and electron backscatered diffraction imaging, the material's shear and bulk viscosities were determined. The formation of delta-ferrite and its impact on densification were discussed in the context of solid-state and supersolidus liquid phase sintering. The model relied on the continuum sintering theory formulated by Skorohod and Olevsky. The strain evolution from the in-situ imaging of sintering process is correlated with porosity based on the used feedstock and applied sintering temperatures. The outcomes of this study offer valuable perspectives on anisotropic sintering mechanisms, bridging the knowledge gap regarding the relationships between structures produced through binder jetting and subsequent sintering of materials.

Additive manufacturing of carbon-martensitic hardening ledeburitic cold work tool steels using Fused Filament Fabrication and subsequent Supersolidus Liquid-Phase Sintering

Additive manufacturing of carbon-martensitic hardening ledeburitic cold work tool steels using Fused Filament Fabrication and subsequent Supersolidus Liquid-Phase Sintering

Research of a novel intermetallic compound-precipitation hardened steel bonded TiCN-based ceramic

Traditional steel-bonded TiC/TiCN based ceramics are sensitive to over-tempering and quenching deformation. In this work, a novel intermetallic compound-precipitation hardened steel-bonded TiCN ceramic (70(Fe-Co-Mo)-30TiCN) was designed and prepared using supersolidus liquid phase sintering and hot isostatic press. Specific heat treatment was researched for further refinement of microstructure and improvement of hardness, strength, toughness, machinability and tempering resistance. The results show that the addition of 30% TiCN particles can not only improve the efficiency of ball milling, inhibit the grain growth, but also increase the peak hardness and tempering resistance of the steel. After high- temperature (1250 °C) solution treatment, the primary coarse and reticular μ phase completely dissolved into matrix and then precipitated uniformly in nanoscale after tempered at 900 °C. The ceramic showed low hardness (57.77HRC) after oil quenching, and performed high hardness (66-73HRC) only after short-time tempering at 430–700 °C. Nano strengthening μ-phases ((Fe,Co)7Mo6) were characterized by electron probe micro-analyzer, transmission electron microscope and selected area electron diffraction.

Computational Efficient Modeling of Supersolidus Liquid Phase Sintering in Multi-component Alloys for ICME Applications

One of the challenges in computational design of pre-alloyed powders for sintering is the absence of predictive, efficient, and fast acting models that enable the design space of alloys to be tractable. This study presents an efficient and predictive model to simulate the densification as well as shape distortion of pre-alloyed powder compacts during supersolidus liquid phase sintering (SLPS). The model combines the generalized viscous theory of sintering with microstructural models for diffusional creep accommodated by viscous grain boundary sliding. Critical model parameters are obtained from thermodynamic modeling based on the calculation of phase diagrams (CalPhaD) and simulations of diffusional transformations in metals. The model is validated by comparing simulation results with experimental data from the literature for various types of engineering alloys. In addition, a processing window for defect free sintering of samples is presented by defining a microstructural softening parameter for a sintering body. The model can be used in the design of pre-alloyed powders for SLPS within the context of an integrated computational materials engineering (ICME) frameworks.

Supersolidus liquid phase sintering of water-atomized low-alloy steel in binder jetting additive manufacturing

This work investigates the feasibility of net shape manufacturing of parts using water-atomized (WA) low-alloy steel with comparable densities to conventional powder metallurgy parts via binder jetting additive manufacturing (BJAM) and supersolidus liquid phase sintering (SLPS). In this study, a modified water-atomized powder grade with similar composition as MPIF FL-4405 was printed and pressure-less sintered under a 95% N2–5% H2 atmosphere. Combinations of two different sintering schedules (direct-sintering and step-sintering) and three different heating rates (1, 3, and 5 °C/min) were applied to study the densification, shrinkage, and microstructural evolution of BJAM parts. This study demonstrated that, although the green density of the BJAM samples was ∼42% of the theoretical density, the sintered parts experienced large linear shrinkage up to ∼25% and reached ∼97% density without compromising shape fidelity. This was ascribed to a more homogeneous pore distribution throughout the part before ramping up to the SLPS region. The synergistic effects of carbon residue, the slow heating rate, and the additional isothermal holding stage at the solid-phase sintering region were determined to be the key factors for sintering BJAM WA low-alloy steel powders with resulting minimal entrapped porosity and good shape fidelity.

Deciphering microstructure-defect-property relationships of vacuum-sintered binder jetted fine 316 L austenitic stainless steel powder

Austenitic 316 L stainless steel (SS) powder with a mean particle size of 10.7 µm was binder jetted followed by vacuum sintering to study microstructure-defect-property relationships. It was demonstrated that supersolidus liquid phase sintering (SLPS) facilitated densification to attain a final relative density of 99.4% at 1400 °C for 2 h. Surface topology analysis revealed an average surface roughness of 1.0 µm and a trace number of open-to-surface pores in agreement with the apparent Archimedes density values. Microstructure analysis of the samples sintered at temperatures ≥ 1370 ℃ displayed the formation of equiaxed austenitic grains with a mean size of ∼33 µm in which δ-ferrite (eutectic phase enriched in Cr and Mo) and σ (intermetallic compound of Fe-Cr-Mo) phases formed at the grain boundaries. Anisotropy shrinkage was seen with a maximum linear shrinkage of 18.4%, 17.7%, and 21.5% in X- , Y- , and Z- directions, respectively. Mechanical testing results showed a yield strength of 202 MPa, tensile strength of 574 MPa, elongation of 90%, and microhardness of 132 HV 0.5 at the optimum sintering temperature of 1400 °C. Finally, fractography revealed a brittle-ductile fracture when porosity was more than 10% (sintered at <1370 °C) while a failure containing a combination of deep dimples (sintered at 1400 °C) as well as secondary central cracks and parabolic dimples (sintered at 1430 °C, due to a high shear fracture) was detected in the sintered parts that possess densities above 99%.

Supersolidus Liquid Phase Sintering and Heat Treatment on Atomic Diffusion Additive Manufacturing Produced Ledeburitic Cold Work Tool Steel*

Abstract In this work, the possibility of manufacturing complex-shaped components from a carbon-martensitic hardenable cold-work steel (1.2379; X153CrMoV12; D2) is investigated. For this purpose, cube-shaped samples with an edge length of 10 mm were produced using the fused-filament fabrication process, which were post-compacted after solvent debinding by supersolidus liquid-phase sintering. Using the knowledge of liquid phase volume content as a function of temperature, supersolidus liquid phase sintering experiments were performed. The microstructure formation process was characterized by electron microscopy and X-ray diffraction. The microstructure and hardness of the processed samples were compared in the heat-treated condition with the properties of the same steel 1.2379 (X153CrMoV12; D2) in the as-cast, deformed and heat-treated condition. The results demonstrate effective post-densificationc close to theoretical density of cold-work tool steel samples fabricated by fused-filamet fabrication using supersolidus liquid-phase sintering at 1280 °C. The defect-free microstructure in the heat-treated state is characterized by a martensitic matrix and eutectic Cr-rich M7 C3 and small amounts of V-rich MC carbides. The hardness of the annealed Supersolidus liquid phase sintering samples are 681 ± 5 HV10, which is above the level of the reference material 1.2379 (629 ± 7 HV10) in the as-cast, formed and heat-treated condition.

Multi-scale characterization of supersolidus liquid phase sintered H13 tool steel manufactured via binder jet additive manufacturing

Additive manufacturing (AM) of H13 tool steel by binder jet 3D printing (BJ3DP) followed by pressureless supersolidus liquid phase sintering (SLPS) provides a low-cost alternative manufacturing method for components with intricate geometric features. However, the microstructure-mechanical property relationships for BJ3DP-SLPS produced H13 tool steel are not well understood, which makes it challenging to develop printing and post-processing methods that maximize part performance. In this work, we leverage atom probe tomography and transmission electron microscopy along with thermodynamic calculations to rationalize the microstructure-mechanical property relationships in as-sintered BJ3DP H13 tool steel. We report for the first time, the presence of a continuous eutectic film-like carbide in H13 along with the more commonly observed cuboidal MX carbides in the prior liquid channels of the microstructure. Further, atom probe tomography revealed the interconnected nature of the MX carbides that appear to be discrete in two-dimensional micrographs. These continuous eutectic carbides and interconnected MX carbides result in brittle failure of the material. Characterization of these microstructural features will be critical in developing appropriate post-processing heat treatments for the improved mechanical performance of BJ3DP H13.

Solid-State and Super Solidus Liquid Phase Sintering of 4340 Steel SLM Powders Shaped by Fused Filament Fabrication

4340 steel powders were processed with an additive manufacturing process using the FFF (Fused Filament Fabrication) technique. A composite filament was developed to print samples and study the effect of the bed and nozzle temperatures on its physical and microstructural properties. The printed samples were debinded and sintered by: Solid State (SS) at 1300 °C or SLPS (Supersolidus Liquid Phase Sintering) at 1420 °C. Metallography and scanning electron microscopy (SEM) identified the microstructure and phases. The hardness of the sintered samples was measured with the Vickers method. The SLPS process contributes to better densification and volume contraction; however, it promotes geometrical distortion of the samples compared to the SS samples. The microstructure of the sintered samples consists of ferrite situated in the original austenite grain and bainite. The sintering mechanism significantly influenced the hardness of the samples. Finally, a part was designed, printed, debinded, and sintered with the aim of studying the maximum inclination angle, the minimum vertical and horizontal holes, and the minimum vertical layer thickness, which can be obtained through the whole process.

Computational Alloy Design for Process-Related Uncertainties in Powder Metallurgy

An integrated computational materials engineering approach to the design of alloys for supersolidus liquid phase sintering has been developed. The method aims to minimize the sensitivity of the alloys to variabilities in material (e.g., composition) and process parameters (e.g., temperature) during sintering while also maximizing mechanical properties. This is achieved by developing a fast acting and high throughput design models that can quantify the processability and the resulting mechanical properties. A highly processable alloy is defined as one that is tolerant to both composition and process conditions such that changes in either do not materially affect the alloy properties. The design models are validated using experimental data from the literature and the computational design approach is demonstrated by designing unique high-speed steels with enhanced processability for powder metallurgy.

Supersolidus Liquid Phase Sintering of Water-Atomized Low-Alloy Steel in Binder Jetting Additive Manufacturing

Supersolidus Liquid Phase Sintering of Water-Atomized Low-Alloy Steel in Binder Jetting Additive Manufacturing

Microstructure evolution for isothermal sintering of binder jet 3D printed alloy 625 above and below the solidus temperature

When compared to beam-based melting methods, binder jet 3D printing is less time consuming, and the printed material is less prone to residual stresses. However, binder jet printing often contains remnant porosity that survives pressureless sintering and limits the mechanical properties of metals including fatigue strength and ductility. In this work, gas atomized nickel-based superalloy 625 powders with three different particle size distributions are binder jet printed and subsequently isothermally sintered at subsolidus and supersolidus temperatures. Individual feature measurements were conducted on two-dimensional sections and pore size distribution frequencies were plotted to reveal the difference in microstructural evolution between subsolidus and supersolidus sintering. Supersolidus liquid phase sintering is thought to have facilitated particle rearrangement under the surface tension of the liquid phase, resulting in a homogeneous microstructure that favored subsequent densification and higher final density. Additionally, printed powder with a wide particle size distribution not only reached high green density of ~52%, but also achieved final densities above 99%.

Predicting sintering window during supersolidus liquid phase sintering of steels using feedstock analysis and CALPHAD

Binder jet 3D printing results in carbon pickup from the binder at the inter-particle regions during binder burnout. The excess carbon lowers the solidus temperature of the green part locally during supersolidus liquid phase sintering (SLPS). Currently, thermodynamic calculations rely on the use of bulk powder compositions to predict the sintering window. This can overestimate the sintering temperature since the local increase in carbon on powder surfaces is not accounted for, that can locally lower the solidus temperature during SLPS. Here we show that using the inputs from feedstock and binder analysis, the excess carbon and hence the sintering window for the alloy can be predicted using CALPHAD. We present a case study on applying the proposed approach for pressureless sintering of binder jet additive manufactured (BJAM) of H13 steel via SLPS.

Fixed and Variable Temperature Super-Solidus Liquid Phase Sintering of High Chromium Cast Iron With 25 Wt.%CR and Its Microstructure

A variable temperature super-solidus liquid phase sintering (SLPS) technique is employed in fabrication of high chromium cast iron (HCCI) with 25wt.%Cr to extend its sintering temperature window. Its microstructure evolution, mechanical properties, and abrasive wear behavior are investigated systemically. The results indicate that the variable temperature SLPS can obtain samples with full density plus fine and uniformly distributed carbide particles, and its carbide volume fraction is increased by 4~5% in comparison with the fixed temperature SLPSed one. Meanwhile, its bending strength and impact toughness can be raised by 8.0% and 16.7%, respectively. Finally, the sintering temperature window for variable temperature SLPS of HCCI is extended by 12 °C, reaching 27 °C.

Densification and Shape Distortion of the Al-Cu-Mg Pre-alloyed Powder Compact in Supersolidus Liquid Phase Sintering Process

Densification and Shape Distortion of the Al-Cu-Mg Pre-alloyed Powder Compact in Supersolidus Liquid Phase Sintering Process

UNS S32707 hyper-duplex stainless steel processed by powder injection molding and supersolidus liquid-phase sintering in nitrogen sintering atmosphere

UNS S32707 hyper-duplex stainless steel (HDSS) was prepared by powder injection molding (PIM) and supersolidus liquid-phase sintering from powder prepared by a plasma rotating-electrode process, and the nitrogen content and relative density of the sintered parts were 0.37% and 97.9%, respectively. The densification and nitrogen content of sintered parts at different sintering temperatures in a nitrogen atmosphere were studied. The results indicate that the relative density increased gradually, whereas the nitrogen content of the sintered parts decreased gradually with increasing sintering temperature under the nitrogen pressure of 0.2 bar. The parts sintered at 1400 °C for 1.5 h exhibited ideal sintering relative density and nitrogen content. After solution treatment at 1150 °C for 1 h, the ratio of ferrite to austenite was approximately 46.8:51.8; the measured tensile strength, the yield strength, the elongation and the hardness were 761 MPa, 529 MPa, 14%, and 321.4 HV, respectively. The samples prepared by PIM can achieve full densification followed by hot isostatic pressing and solution treatment, and its tensile strength and elongation are significantly improved, which is better than that of casting. This study provides a new method for the preparation of high-nitrogen-content HDSS with a complex shape and controllable nitrogen content by near-net shaping.

Sintering Preparation of 15 wt% Cr Cast Iron as well as Its Mechanical Properties and Impact Abrasive Wear

15 wt% Cr sintered High Chromium Cast Iron (HCCI) with full density was successfully prepared by Super-solidus Liquid Phase Sintering (SPLS) technique, with water atomized 15 wt% Cr high chromium cast iron powder as initial materials. Its densification behavior and microstructure evolution in SPLS process and mechanical properties were investigated systematically. Additionally, the impact abrasive wear resistance under different impact energies were also analyzed and compared with another sintered HCCI with 20 wt% Cr. The results indicated that sintering temperature has a strong influence on the sintered alloy’s density, hardness, impact toughness and bending strength. The M7C3 type (M is Cr and Fe) carbides were obviously coarsened as temperature increased and their rod-shaped branches were fully developed at the same time, thereby resulting in carbide network formation in the matrix. The reasonable sintering temperature range was 1195–1205 °C, and the optimum mechanical properties had the hardness of 63.9 HRC, bending strength of 2112.65 MPa and impact toughness of 7.92 J/cm2. What is more important impact abrasive wear test results indicated 15 wt% Cr sintered HCCI’s wear resistance could be comparable to 20 wt% Cr sintered HCCI under impact energy 1~3 J/cm2, and it is more cost effective.

Grain growth activation during supersolidus liquid phase sintering in a metal injection molded nickel-base superalloy

State-of-the-art sintering practices for metal injection molded (MIM) nickel superalloys (NiSAs) lead to fine-grain microstructures, reducing creep resistance compared to cast microstructures. MIM uses fine pre-alloyed powders which can increase prior particle boundary (PPB) carbide and oxide formation, restricting grain growth efforts. In the present work, the ability to activate grain growth in a MIM NiSA during supersolidus liquid phase sintering (SLPS) is evaluated using differential scanning calorimetry (DSC). A comparative DSC analysis technique was developed to quantify the SLPS liquid fraction with time and temperature and the grain growth behaviour of the MIM NiSA during SLPS was determined from optical microscopy. SEM-EDS of the sample microstructures revealed that the formation of refractory metal (RM)-Ta mixed MC carbides at the PPB restricts grain growth in the MIM NiSA. Time above the solidus temperature was found to dissolve the RM-Ta mixed MC carbides, enacting grain growth after a short incubation period. The results underscore the difficulty of accessing grain growth under the challenging conditions of MIM while demonstrating that SLPS can increase the grain size in the MIM NiSA.

Hard Cladding by Supersolidus Liquid Phase Sintering: An Experimental and Simulation Study on Martensitic Stainless Steels

Martensitic stainless steels are suitable for diverse structural applications but degrade when subjected to wear-prone activities in service. To enhance their service life, the densification of high Cr, martensitic, X190CrVMo20-4-1 tool steel powder on two different martensitic stainless steel substrates via supersolidus liquid-phase sinter (SLPS) cladding was investigated. The objective was to assess the influence of the difference in compositions of the martensitic stainless steels employed as substrates on the interfacial diffusion, microstructure, hardness and bonding strength of the steel-to-steel claddings. Computational thermodynamics and diffusion simulations were employed to supplement experimental findings. Owing to interdiffusion, a M7C3 carbide-free, banded region exists in the X190 adjacent to the interface with the width dictated by chemical potential gradient of carbon. The hardness of the substrate was lower near the interface region because of carbon enrichment, which promoted the presence of retained austenite. An interfacial strength of 798 MPa was achieved with fairly ductile X190 matrix near the cladding interface as the fracture surface was characterized by mixed fracture modes of dimple rupture and cleavage with localized quasi-cleavage features. Experimental observations and computational simulations are in agreement. The implications of the SLPS cladding technique are discussed in the context of tool development.