Cellular Dendrites Research Articles

Probing microstructural evolution behaviors in laser welding of dissimilar GH4169/GH3039 subjected to varied preheating temperatures

The present study examined the evolution of interfacial microstructure and on the element distribution of GH4169/GH3039 dissimilar joints by laser welding under different preheating temperatures. The results indicated that the microstructure observed in the interface layer on the GH4169 side exhibited distinct layers, namely diffusion layer and transition layer, which varied depending on the varied preheating temperature. Within the weld seam (WS), two types of dendrites were predominantly present: lath-like columnar dendrite and cellular dendrite. In the GH3039 side, the grain structure was mainly dominated by columnar dendrites, exhibits perpendicular growth to the fusion line and demonstrates a certain epitaxial growth morphology. Moreover, excessive preheating temperature intensified the migration of elements and phase transition behavior. As the preheating temperature increased, the low-angle grain boundaries (LAGBs) percentage decreased from 13% to 9.4%. The average grain size of the WS at room temperature, 200°C and 400°C is 44.1 μm, 53.3 μm and 58.2 μm, respectively. This paper provides a basis for tuning microstructure and element behavior via preheating temperature optimization in laser welding of dissimilar GH4169/GH3039 for further engineering application.

Effect of the Laser Cladding Parameters on Microstructure and Elevated Temperature Wear of FeCrNiTiZr Coatings.

In order to prepare coating with good friction and wear resistance at elevated temperature on the surface of hot-working tool steel, by using a CO2 laser, FeCrNiTiZr high-entropy alloy coating with different laser scanning speeds (360, 480 and 600 mm/min, respectively) was successfully fabricated by using laser cladding technology on the surface of H13 steel in this paper. Phase constitutions, microhardness, microstructure, and wear characteristics of FeCrNiTiZr coatings under different laser scanning speeds were analyzed. It was determined that 480 mm/min was the optimal laser scanning speed. The results showed that the coating at the scanning speed of 480 mm/min consists of a BCC phase with significant lattice distortion and high dislocation density; the crystal structure is cellular crystal and dendrite crystal. The coating demonstrates the highest microhardness (842 HV0.2), which is 4.2 times that of the substrate (200 HV0.2). Its average friction coefficients at room temperature and 823 K are approximately one-seventh and one-third of the substrate's, respectively, and its wear volume is reduced by about 98% and 81% under these conditions. Compared to the substrate, the coating underwent slight abrasive wear, adhesive wear, and oxidative wear at both room temperature and 823 K. In contrast, the substrate underwent severe abrasive wear, adhesive wear, oxidative wear, and even fatigue wear.

COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF 9 % Ni STEEL JOINTS WELDED BY SMAW AND GTAW

The microstructure and impact toughness of 9 % Ni steel joints welded by shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) were investigated. The two kinds of weld metal were mainly composed of cellular dendrites and a granular precipitated phase. The grains and cellular dendrites of the GTAW weld metal were smaller than those of the SMAW weld metal. The precipitates in the SMAW weld metal were stripe-shaped while those in the GTAW weld metal were rod-shaped; the number of precipitates in the GTAW weld metal was lower than in the SMAW weld metal. The low-temperature impact toughness of the GTAW weld metal was better than that of the SMAW weld melt.

A Comparative Study on the Performance and Microstructure of 304NG Stainless Steel in Underwater and Air Laser Welding.

In order to facilitate the application of underwater laser welding technology in in situ repairs of nuclear power plants, this study conducted comparative experiments between local dry underwater laser welding and laser welding in air on 304NG nitrogen-controlled stainless steel. The aim was to explore its microstructural evolution and mechanical properties in underwater environments. It was found that, near the fusion line of laser welding in air, columnar dendrites gradually evolved into cellular dendrites toward the weld center, eventually disappearing, resulting in a skeletal ferrite and serrated austenite structure. The underwater laser welding joints exhibited similar characteristics yet with more pronounced alternation between columnar and cellular dendrites. Additionally, the size of cellular dendrites decreased significantly, and needle-like ferrite was observed at the weld center. The hardness of underwater laser welded joints was slightly higher than that of in-air laser welded joints. Compared to laser welding in air, the strength of underwater laser welding joints increased from 443 MPa to 471 MPa, and the displacement increased from 2.95 mm to 3.45 mm, both types of welded joints exhibited a mixed mode fracture characterized by plasticity and brittleness.

Impact of morphological and mechanical components on inconel 625 grinding using common cylindrical grinding wheels

This study used a grinding technique based on Gas Tungsten Arc Welding (GTAW) to create walls composed of Inconel 625 alloy. Mechanical and microstructural (MM) adjustments are structural and mechanical alterations that take place during the additive manufacturing process of Inconel 625 grinding. A thorough examination of the modifications made to the nickel superalloy Inconel 625's (I-625) microstructure was conducted during the grinding process. A circular weave and a stringer bead design were used to construct the wall. Tensile properties and microstructural analyses were assessed for each wall. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), the fracture zones of the tensile specimens were examined. The microstructure is mostly composed of equiaxed dendrites, although a unique combination of discontinuous and continuous cellular dendrites can be observed along the cross-section. In tensile testing, circular woven walls performed better than stringer bead walls. The EDS and AFM results show that Ni and Cr make up the majority of the fracture zone, with traces of Nb and Mo. Because there are no lave phases, the fracture mode is ductile. The elemental mapping, which shows the homogenous dispersion of Ni and Cr inside the fracture zone, provides additional evidence in favor of the ductile failure mode. The UTS of the time-consuming TS samples is somewhat higher and exhibits a steadily rising bias in comparison to the specimens with quick TS. The highest level of the UTS sample is 10 %.

Effect of remelting on cracking of Inconel 939 fabricated via laser powder bed fusion

Laser remelting of each layer during laser powder bed fusion (L-PBF) is an important strategy used for enhancing the quality of components. However, the impact of remelting on internal cracking has not been completely understood. This study investigates the feasibility of employing remelting to mitigate internal cracking using Inconel 939 (IN939) as an example, a high-temperature nickel-based alloy with high susceptibility to cracking during L-PBF. The effects of remelting parameters, strategies, and the number of remelting cycles on microstructure features and defects were studied. The residual stresses in remelted samples and initial melt samples were measured for different laser scanning strategies. Reduction in residual stresses is identified as a key factor in minimizing cracking, with the lowest residual stresses observed in remelted samples using the chess strategy. Additionally, crack density in remelted samples was decreased with reducing cellular dendrite size in the fabricated sample. In comparison to initial melt samples, remelted samples exhibited an 85% reduction in crack density and a 60% reduction in porosity. These findings present a potential approach for defect inhibition during L-PBF processing of IN939 and other high-temperature alloys.

Directed energy deposition of functional gradient material to enhance mechanical properties of heterogeneous stainless steels

In this study, 316L/18Ni300 functional gradient material (FGM) were designed and prepared via directed energy deposition, aiming to achieve synergistic enhancement of strength and toughness between two different lattice-structured steels. Macroscopic analyses showed that the interlayer metallurgical bonding of the 316L/18Ni300 FGM was achieved, and no obvious defects such as unfused and inclusions were observed. Spatially heterogeneous structures cellular dendrite, epitaxially grown columnar dendrites, columnar dendrites were observed along the building direction. The microhardness of the 316L/18Ni300 FGM surface is 329 HV, which is an improvement of 185 % as compared to 316L. The static tensile and three-point bending results show that 316L/18Ni300 FGM combines the toughness of 316L with the strength of 18Ni300. In the reciprocating sliding wear test, 316L/18Ni300 FGM showed an increase in wear resistance along the building direction, with a 35 % increase on the surface. This study not only provides innovative ideas and theoretical guidance for 316L/18Ni300 FGM in terms of mechanical property enhancement, but also opens up a new path for broadening 316L for a wide range of applications.

Effect of WC particle size on the microstructure and tribological properties of high-speed laser cladding Ni/WC composite coatings

Ni/WC composite coatings with WC particle sizes of 30–65, 65–100, and 100–145 μm were prepared on 304 steel using high-speed laser-cladding (HSLC). The phase composition, microstructure, and wear mechanism of the composite coatings were studied. The dendrite-growth mechanism around WC with different particle sizes and the effects of WC with different particle sizes on the microhardness and tribological properties of the composite coatings were analyzed. The results demonstrated that the composite coating consisted of intermetallic compound Ni3Fe and secondary carbides W2C and (Cr, Fe)7C3. The composite coating exhibited good macroscopic morphology and uniform microstructure. The WC particle size obviously influenced the columnar and cellular dendrites in the coating. The microhardness values of the three groups of composite coatings were 419.35, 314.64, and 331.65 HV. The average friction coefficients were 0.4876, 0.5693, and 0.5439. The wear volumes were 0.001, 0.0015, and 0.002 mm3. The wear mechanisms included adhesive, abrasive, and oxidation wear mechanisms. The results verified that the WC coating with a small particle size possessed the best microhardness and tribological properties.

Microstructure and molten pool morphology of TiB2/ AlSi7Mg composites fabricated by infrared-blue hybrid laser powder bed fusion

A hybrid laser composed of infrared and blue laser is applied in fabricating TiB2/AlSi7Mg composites on AlSi7Mg substrate by LPBF. The effect on formability, molten pool morphology, molten pool size and microstructure under infrared, blue and hybrid laser were compared. It was confirmed that hybrid laser can make up for the unbalanced energy distribution of infrared laser and the low energy density of blue laser. The increased energy input improves the molten pool size and cellular dendrites size. Therefore, the hybrid laser can improve the formability and forming stability in the LPBF process of low absorption rate alloys.

Effect of variable laser incident angles on microstructure and mechanical properties of laser-cladded Inconel 718

In the laser cladding process of complex components, laser deflection is a significant issue that greatly affects manufacturing quality. This study investigates the influence of different incident angles on the microstructure and wear resistance of laser-clad layers. Firstly, an Inconel 718 coating was prepared on an Inconel 718 substrate using an IPG fiber laser. Next, the macrostructure and microstructure of the coating were observed using electron microscopy. Finally, the microhardness of the coating was measured, and the underlying mechanisms affecting wear resistance due to incident angle were revealed. The results show that with an increase in the laser incident angle, the laser energy distribution becomes more divergent, leading to a shift in the peak energy density and changes in the macroscopic morphology of the melt pool, exhibiting a non-Gaussian distribution and resulting in increased cellular dendrite count and coarsened grains. Laser deflection causes variations in microstructure and properties between inner and external angles, where higher laser energy density and temperature gradients at the inner angle result in finer grains and more numerous smaller secondary dendrites on the dendritic arms, along with less severe element segregation and higher microhardness at the inner angle.

Significant dislocation strengthening of stainless steel 316L via co-directed energy deposition of silica

This study aims at fully utilizing dislocation strengthening for higher hardness of stainless steel 316 L through two methods: 1) Multi-layer directed energy deposition (DED) is utilized to intensify dislocation generation induced by thermal cycling compared to single-layer DED. 2) Ultra-low thermal expansion coefficient silica particles are incorporated into a 316 L substrate, utilizing thermal misfit stresses between particles and the matrix. Silica particles, dispersed within the substrate, enable both multi-layer DED and surface DED. Nine samples (labeled as 1 L–9 L, respectively) with varying deposition layers from 1 to 9 are prepared. As deposition layers increase, silica particles agglomerate and elongate, promoting micro-crack formation in the matrix. Increasing deposition layers is found to alter gradually the solidification structure from planar to cellular and columnar dendrites, and transform the primary austenite to martensite (approximately 100 % at 9 L). This phase transformation is driven by oxygen-affine Cr and Mn elements diffusing into silica particles. Surprisingly, the hardness is significantly increased from about 170 HV at the base metal, 180–220 HV at 1 L–5 L, and 270 HV at 6 L, to maximum 300–330 HV at 8 L. This improvement is theoretically explained by significant dislocation strengthening mechanism (Δρ ∼ 6.70 × 1014 m−2) amplified by the multi-layer silica DED process. Furthermore, distinct dislocation structures including dislocation entanglement around silica particles, patterned dislocation walls, and nano-lamellae pile-ups of dislocations are found at 8 L from transmission electron microscope analysis. The strengthening origins and the potential of SiO2/316 L bulk composites are briefly explored.

High Temperature Tensile Properties and Microstructure of Ni20Cr Alloy Fabricated by Laser Powder Bed Fusion

Additive Manufacturing (AM) brings about an array of modifications in microstructure with respect to conventional routes transforming mechanical performances. These new microstructure features depend on process parameters and especially on volume energy-density delivered by the laser on powder layer. Among the different alloys manufactured by AM, Ni-alloys exhibit high-strength at elevated temperature opening the way of fabrication of gas turbines and jet-engine parts. Ni-superalloys experience precipitation hardening due to the formation of γ′ and γ′′ phases leading to complex microstructures. To better study the influence of the AM microstructure on Ni-alloys mechanical properties, in particular at elevated temperatures, a theoretically monophasic and binary Ni20Cr-alloy manufactured by laser powder-bed fusion was studied in this work. Remarkable Yield Strength (400 MPa) and Ultimate Tensile Strength (UTS) (600 MPa) were observed at 500°C with hardly any loss of properties from room temperature, owing to the thermal stability of cellular dendrites till 700°C. Ductility drop was reported at 700°C due to anomalous brittle behaviour of Ni-alloys. Hardening behaviour vanished at 900°C signifying the deletion of dendrites, disappearance of dislocations, diffusion of Cr from dendritic walls and growth of oxides.

IDG-SW3 Cell Culture in a Three-Dimensional Extracellular Matrix.

Osteocytes are considered to be nonproliferative cells that are terminally differentiated from osteoblasts. Osteoblasts embedded in the bone extracellular matrix (osteoid) express the Pdpn gene to form cellular dendrites and transform into preosteocytes. Later, preosteocytes express the Dmp1 gene to promote matrix mineralization and thereby transform into mature osteocytes.This process is called osteocytogenesis. IDG-SW3 is a well-known cell line for in vitro studies of osteocytogenesis. Many previous methods have used collagen I as the main or the only component of the culturing matrix. However, in addition to collagen I, the osteoid also contains a ground substance, which is an important component in promoting cellular growth, adhesion, and migration. In addition, the matrix substance is transparent, which increases the transparency of the collagen I-formed gel and, thus, aids the exploration of dendrite formation through imaging techniques. Thus, this paper details a protocol to establish a 3D gel using an extracellular matrix along with collagen I for IDG-SW3 survival. In this work, dendrite formation and gene expression were analyzed during osteocytogenesis. After 7 days of osteogenic culture, an extensive dendrite network was clearly observed under a fluorescence confocal microscope. Real-time PCR showed that the mRNA levels of Pdpn and Dmp1 continually increased for 3 weeks. At week 4, the stereomicroscope revealed an opaque gel filled with mineral particles, consistent with the X-ray fluorescence (XRF) assay. These results indicate that this culture matrix successfully facilitates the transition from osteoblasts to mature osteocytes.

Semi-macrosegregation and carbide banding in high-carbon chromium bearing steels: Characteristics, evolution, and control

Semi-macrosegregation in continuously cast blooms and its associated carbide banding defects in hot-rolled bars were investigated to explore the characteristics, evolution, and control methods in high-carbon chromium bearing steel. It was found that the semi-macrosegregation exists only in the central equiaxed crystal zone of the blooms, and C, Cr solute elements inside show typical spot-like positive segregation. Carbide banding appears as white bright bands along the rolling direction, composed of high number density M3C carbide particles, with a significant effect on the mechanical properties of the rolled products. The hereditary evolution from semi-macrosegregation to carbide banding was established by means of banded segregation. Morphologically, normal semi-macrosegregation and carbide-typed semi-macrosegregation are formed based on whether Cr-rich primary carbides are generated. These two types of semi-macrosegregation evolved into two kinds of carbide banding respectively, that is, dark bands and bright bands defined according to the ability to reflect light under OM. To alleviate the carbide banding defect in rolling products of high-carbon chromium bearing steel, the morphology and area ratio of equiaxed crystals in continuous cast blooms should be modified. It is observed that the equiaxed crystal ratio in the bloom can be decreased from 62.5 % to 28 % by increasing liquid steel superheat and inhibiting superheat dissipation in the mold through casting practices. Additionally, the morphology of equiaxed crystals is related to the as-cast solidification structure, and when the equiaxed crystal ratio is large, the equiaxed crystals turn out to be coarser ones with only cellular primary dendrite; when the columnar crystals are longer, however, the equiaxed crystals appear as a dendrite structure with fine secondary dendrites. As a result, semi-macrosegregation was improved obviously with the fine dendrite structure splitting the local residual liquid phase, leading to the decrease of the maximum width of carbide banding from 268.41 μm to 73.72 μm, and the increase of mechanical properties in the rolled bar of high-carbon chromium bearing steel.

Functionally graded deposition of dissimilar steel (316LSi and ER70S-6) fabricated through twin-wire arc additive manufacturing

Fabrication of dissimilar steel-based (SS316LSi over ER70S-6 steel) functionally graded deposition (FGD) using twin wire arc additive manufacturing (T-WAAM) via a unique deposition strategy is investigated in this work. Optical, SEM, EDS, X-ray diffraction (XRD), tensile, hardness, and fractography analyses were undertaken for complete microstructural and mechanical characterisation. Microstructural analysis revealed columnar, equiaxed, and cellular dendrites in δ-ferrite and austenitic phases. XRD investigation depicted the anisotropic behaviour of specimen textures in various planes. The estimated value of average ultimate tensile strength is 901 MPa, yield strength (YS) is 634 MPa, and percentage elongation (EL%) is 48 %, respectively. This work offers a novel strategy for producing superior-strength dissimilar steel components with excellent quality and varied structural utility.

Dissimilar autogenous TIG joint of Alloy 617 and AISI 304H steel for AUSC application

To reduce costs and improve high-temperature performance in Advanced Ultra Super Critical (AUSC) boilers, it is necessary to weld austenitic steel to Inconel alloy. In this study, the autogenous tungsten inert gas (TIG) welding process was used to join Alloy 617 and an austenitic AISI 304H steel plate of thickness 5 mm. Microstructural analysis showed that the microstructure formation was uneven along the weldments, with columnar and cellular dendrites near the interface while the central area of the weld exhibited a combination of columnar, cellular, and equiaxed dendrites. The use of energy dispersive spectroscopy and electron probe micro-analysis unveiled the presence of an unmixed layer at the interface between the weld and AISI 304H steel. Furthermore, a notable variation in the concentration of alloying elements such as Fe, Cr, Ni, Co, and Mo was observed. Within the weld metal, inter-dendritic areas showed the presence of precipitates rich in Cr, Ti, and Mo. Meanwhile, the heat-affected zone (HAZ) of Alloy 617 exhibited the presence of phases like Cr and Mo-rich M23C6 as well as Mo-rich M6C. Hardness tests showed non-uniform hardness along the weldments, with a hardness of 199 ± 6 HV in the weld metal and 225 ± 4 HV in Alloy 617 HAZ, and 207 ± 7 HV in AISI 304H HAZ. The Mo and Cr segregation in the inter-dendritic spaces led to a decline in the tensile properties of the welded parts and resulted in failure from the region of the weld metal.

The heterogeneous microstructure of laser welded joint and its effect on mechanical properties for dissimilar 9Cr steel and alloy 617

The Fe-rich heterogeneous microstructures were observed in the weld metal (WM) of laser welding on dissimilar metals between 9Cr steel and alloy 617, manifesting as vortex-like and lamellar-like structures. They displayed a face-centered cubic structure and consisted of cellular and columnar dendrites. The vortex-like structure was distributed on the upper side of the WM and exhibited the solidification grain boundary. Due to the higher liquid temperature and insufficient mixing, the vortex-like structure exhibited preferential nucleation during the solidification process. On the other hand, the lamellar-like structure was situated on the lower side of the WM and had a thickness of 50 μm. The lamellar-like structure and WM solidified simultaneously with the rapid solidification rate, resulting in noticeable intergranular chemical segregation. During the tensile test, deformation twins formed in the heterogeneous microstructure, due to the lower stacking fault energy and fine particles. Numerous deformation twins played a crucial role in inhibiting crack initiation, which performed the same orientation. Furthermore, the heterogeneous microstructure generated 89.8% of twin boundaries during the crack propagation, while the WM had 30.4%. This phenomenon facilitated plastic energy absorption and deflected the crack propagation path. Thus, the heterogeneous microstructures contributed to the enhancement of mechanical toughness at room temperature.

Microstructural characterization and mechanical properties of Inconel 625 wall fabricated by GTAW-based WAAM using stringer bead and circular weave pattern

In this work, Inconel 625 alloy was used to manufacture walls using the Wire Arc Additive Manufacturing (WAAM) technology, which is based on Gas Tungsten Arc Welding (GTAW). The wall was fabricated using a circular weave and stringer bead pattern. Microstructural analysis and tensile characteristics were evaluated for both walls. Using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and elemental mapping, the fracture zones of the tensile specimens were examined. The microstructure is mostly made up of equiaxed dendrites, with the rare presence of continuous and discontinuous cellular dendrites along the cross-section. In tensile tests, circular weaved walls performed better than stringer bead walls. The circular weave specimen had an ultimate tensile strength (UTS) of 762 MPa in the horizontal and 722 MPa in the vertical orientations. Also, the Inconel 625 wall showed anisotropic behavior (5.3%) during tensile testing. The fracture morphology analysis revealed that all the specimens were fractured as a result of large plastic deformation, corresponding to ductile failure. Based on the EDS results, the fracture zone mainly consists of Ni and Cr with a small percentage of Nb and Mo. The absence of laves phases makes the fracture mode ductile. The elemental mapping shows uniform dispersion of Ni and Cr within the fracture region, further supporting the ductile failure mode.

Metallurgical Characterization of SS 316L Repurposed by Wire Plus Arc Additive Manufacturing

This research aims to analyze the microstructures and mechanical characteristics of stainless steel (SS) 316L repurposed by wire plus arc additive manufacturing (WAAM). The SS 316L wire is deposited on a SS 316 substrate, which can be repurposed. This deposited material underwent optical microscopy, X-ray diffraction, and tensile test, and the results indicate that it features cellular and columnar dendrites at the bottom and equiaxial grains at the top. The tensile strength of the interface region between the deposited material (DM) and the base material (BM) is the highest (559 ± 4.16 MPa vs. 510 ± 4.93 MPa in DM and 540 ± 2.65 in BM), indicating that the BM and the deposited layers are strongly bonded. All the results from the defect observation, microstructures, and mechanical characteristics confirm the potential of the WAAM process for repurposing.

Effect of the Solidification Mode on the Pore Growth of Gasar Porous Mg‐Ag Alloy

AbstractGasar porous Mg–3 wt% Ag alloy is prepared by metal–gas eutectic directional solidification process. The pore structure is related to the solidification mode of the alloy. The pores grow stably under the solidification conditions of planar, cellular, and columnar dendrites but not under the condition of equiaxed dendrites. The numerical simulation shows that the solidification velocity and temperature gradient decrease rapidly with an increase in the sample height, but their ratio remains unchanged. During the solidification process, the solidification mode of the alloy changes from cellular crystals to columnar dendrites, and finally to equiaxed dendrites. The calculated results of the solidification mode transition are consistent with the microstructure morphologies of the Mg–Ag alloy at different heights.