Cellular Dendrites Research Articles

Two-dimensional phase-field simulations of competitive dendritic growth during laser welding

A phase-field model is applied to study competitive dendritic growth between different grains with different crystalline orientations during laser welding of Al-Cu alloy. An unfavorable oriented (UO) grain is set to locate between two favorable oriented (FO) grains to study the dendritic growth behaviors at not only converging grain boundary (GB) but also diverging GB. It is found that the cellular dendrites appear firstly nearby the GBs at initial instability stage. At early competitive growth stage, the UO misalignment angle influences the competitive dendritic growth at both GBs. After completely entering competitive growth stage, the leading dendrites that are much faster than other dendrites at GBs firstly grow in lateral direction. The side branches growing from leading dendrites block growth path of nearby dendrites and change the UO/FO grain widths. The converging GB deflection angle and the side branch development at diverging GB as characteristic competitive dendritic growth behavior at relatively steady growth stage also significantly affect UO and FO grain widths. The numerical results are basically consistent with microstructure obtained by experiment.

Influence of static magnetic field on microstructure and mechanical behavior of selective laser melted AlSi10Mg alloy

In this work, the influence of a static magnetic field on the microstructure and mechanical behavior of AlSi10Mg alloy was studied. Our findings show that the applied magnetic field results in increase of the relative density and decrease of cellular dendrite spacing in the processed material. Moreover, the fraction of grains with a columnar morphology decreases and the fraction of equiaxed grains increases with increasing magnetic field intensity. As a result, the AlSi10Mg alloys fabricated via selective laser melted (SLM) with a superimposed magnetic field exhibited both a high ultimate tensile strength and ductility, which are superior to the AlSi10Mg alloys using identical process parameters without magnetic field. Furthermore, the influence of a static magnetic field on the melt pool scale and mushy zone scale was analyzed and simulated numerically. Our results suggest that the decrease pores density may be attributed to magnetic damping of convection and the volume force imposed on the cellular dendrite reaches 105 N/m3, which is sufficient to fracture the columnar grains and refine the cellular dendrite spacing. The present study provides novel insight into the potential of using a superimposed magnetic field during SLM processing and the associated benefits in terms of materials performance.

Liquid phase separation in AlCrFeNiMo0.3 high-entropy alloy

Buttons of arc melted AlCrFeNiMo0.3 high-entropy alloy were characterized in the as-cast state using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The macrostructure of the arc-melted AlCrFeNiMo0.3 alloy consisted of two regions with different compositions: A top layer of AlNi-rich dendritic microstructure composed of a cellular dendrite core (DC) with a CrFeMo-rich rim and AlNi-rich interior, followed by radiating lamellar microstructure with the same phases at the interdendritic regions (ID). In contrast, the bottom region which is Cr, Fe, and Mo-rich consisted of dendrite elements, spheres, followed by a cellular microstructure with radiating lamellae similar to that observed in the top layer. The presence of two regions with different compositions is indicative of a stable liquid phase separation (LPS). In addition, the presence of spheres in the microstructure in the bottom region with different composition from that of the matrix is indicative of secondary LPS. We attributed the LPS in AlCrFeNiMo0.3 alloy to the significantly more attractive interaction between Al–Ni than that of the other binary combinations. Thermodynamic calculations were carried out on the alloy using SSOL5 and TCHEA thermodynamic databases. The thermodynamic calculation results indicate that the first phase to solidify is the disordered CrFeMo-rich BCC. Upon further cooling, a spinodal decomposition reaction forming disordered BCC (CrFeMo) and precipitates of ordered AlNi BCC B2 phase is accompanied with the formation of an FCC L12 phase. However, the calculations did not yield predictions of stable LPS; instead a very low-lying metastable LPS was predicted.

Weldability, microstructure and mechanical properties of laser-welded selective laser melted 304 stainless steel joints

Laser welding is a promising process for joining small components produced by selective laser melting (SLM) to fabricate the large-scale and complex-shaped parts. In the work, the morphology, microstructure, microhardness, tensile properties and corrosion resistance of the laser welded stress-relieved SLMed 304 stainless steel joints are investigated, as the different sections of stress-relieved SLMed 304 stainless steel are joined. Results show that the SLMed 304 stainless steel plates have a good laser weldability. The microstructure of laser-welded joints consists of the cellular dendrites in austenite matrix within columnar grains, exhibiting a coarser dendrite structure, lower microhardness (∼220 HV) and tensile properties (tensile strength of ∼750 MPa, and area reduction of ∼27.6%), but superior corrosion resistance to those of SLMed plates. The dendrite arm spacing of the joints varies from ∼3.7 μm in center zone, to ∼5.0 μm in fusion zone, to ∼2.5 μm in epitaxial zone. The SLMed anisotropy shows a negligible effect on the microstructure and performance of the laser-welded joints. The laser welding along the building directions of the SLMed base plates can induce a slightly finer dendritic structure and higher tensile properties.

Metallurgical and mechanical investigations of Inconel 625 overlay welds produced by GMAW-hardfacing process on AISI 347 pipes

Inconel 625 is a widely employed material for hardfacing to protect components of aircrafts, oil refinery and power generation, possessing better corrosion resistance behaviour at high temperature applications. These characteristics are related with the hardfacing material composition and microstructure of overlaid welds which is influenced by the process parameters and the substrate composition of the component to be protected. Hardfacing has been extensively used for modifying surface and repairing. In this research, single bead overlays of Inconel 625 were deposited on the surface of AISI 347 pipe by GMAW process. Overlaid welds were deposited at three different heat inputs (0.48 kJ/mm, 0.53 kJ/mm and 0.59 kJ/mm) to synergic mode, while the voltage and wire feed rate are pre-programmed. The overlaid weld thickness varied from 1.5 mm to 2.62 mm on substrate with suitable overlaying parameters. This study was explored with Optical microscopy, Energy-dispersive x-ray spectroscopy (EDS), scanning electron microscopy (SEM), and mechanical tests were used for microstructural and mechanical characterization. The results showed that the overlay welds of Inconel 625 exemplified a noticeable microstructure upon solidification, while the bond interface can be observed and divided into distinct zones. Near the bonding interface, a non-mixed zone with precipitated ferrite of different formations were observed. The surface of overlaid weld microstructure to interface of substrate/weld overlay comprised of cellular, columnar and fine dendrites. An increase in Fe wt% in the partially mixed zone is corroborated to the melting and dispersion of the substrate. Uni-axial tension test and Vickers-micro hardness test were carried out on the specimen sectioned from overlay welds. The mechanical test responses revealed that the tensile strength and micro-hardness of the overlaid welds improved significantly compared to substrate. The mechanical properties of hardfaced area were better than bonding area and superior to substrate, revealing an excellent overlay weld deposition using Inconel 625.

An Analytical Model for Prediction of Solidification Cracking Susceptibility in Aluminum Alloys Taking into Account the Effect of Solidification Rate

Solidification cracking in pulsed laser welding of aluminum alloys has been the subject of a number of researches. The purpose of this study is to analyze the possible effects of high solidification rates involved in pulsed laser welding on the solidification path of aluminum alloys and solidification cracking sensitivity. For the analysis, conditions at the interface are not necessarily considered to be at equilibrium. Accordingly, the solidification path is modified taking into account the effects of solidification rate on the solidus, liquidus, and eutectic temperatures, the liquidus slope, and the interface parameters, including the partition coefficient, the cell/dendrite tip radius, and the liquid composition in the cellular dendrite tip. In line with the previous researches, cracking susceptibility is assumed to be proportional to the range of temperature squared for solidification crack formation ( $$ \Delta T^{2} $$ ). The results of calculations for hypo-eutectic Al-Cu alloys show that cracking susceptibility increases and shifts to a higher copper composition with the increasing solidification rate. The given calculations are evaluated against the reported experimental results.

Mechanical Properties of Tool Steels with High Wear Resistance via Directed Energy Deposition

This study focused on the mechanical and metallurgical characteristics of high-wear-resistance steel (HWS) deposited using directed energy deposition (DED) for metal substrate hardfacing or repairing. As post-deposition heat treatment changes the metallurgical characteristics of deposits, the effect of post-deposition heat treatment on the mechanical properties was investigated via microstructure observation and by conducting hardness, wear, and impact tests. The obtained micro-images showed that the deposited HWS layers exhibit cellular and columnar dendrites, and the microstructure of heat-treated HWS (HT-HWS) transformed its phase during quenching and tempering. The hardness and wear resistance of the HT-HWS deposits were higher than those of the HWS deposited specimen, whereas the latter exhibited a higher fracture toughness. The matrix microstructure and carbide characteristics, which are characterized by the chemical composition of the materials, significantly influenced the mechanical properties.

Temperature field simulation and grain morphology on laser welding-brazing between Ti-6Al-4V and 1050 aluminum alloy

Laser welding-brazing (LWB) technique is considered to offer advantages over fusion welding in terms of dissimilar jointing since it can avoid liquid dissimilar alloys mixing in the molten pool. In this paper, LWB technique is utilized to join 1050 aluminum alloy to Ti-6Al-4V. A 3D finite element model is developed to calculate the temperature field in the LWB process with a combined Gaussian heat source model. The simulated asymmetric temperature field is in good agreement with the experimental metallography, which confirms the validity of the present combined heat source model. The peak temperature of the center of the weld seam increases with time in the welding process. The experimental and simulation results both indicate that the heat-affected zone (HAZ) width of the upper is narrower than that of the bottom. Besides, the growth orientation of coarsened dendrite near the interface at root face is substantially perpendicular to interface. The grain morphology of the molten pool appears as dendrite, cellular dendrite and equiaxed crystal from the fusion line to the center of the weld seam.

Heat treatment of Inconel 718 produced by selective laser melting: Microstructure and mechanical properties

Inconel 718 (IN718) samples have been produced by selective laser melting (SLM). The effects of solution temperature, time and cooling rate as well as aging hardening on the microstructure and mechanical properties of SLMed IN718 have been studied. It is found that the as-fabricated IN718 is characterized with fine cellular dendrites with Laves phase precipitating in the subgrain boundaries, which is profoundly different from cast and wrought materials and needs different heat treatment schedules. The relationship between the minimum solution time and solution temperature is established and it provides a basis for the selection of solution treatment parameters. In addition, decreasing the cooling rate of solution treatment will contribute to the precipitation of strengthening phases. The precipitation temperatures of γ′ and γ″ are about the same for SLMed and wrought IN718, but the former has a faster aging response. The tensile properties of SLMed IN718 can be tuned in a large range by properly varying the microstructure. The highest elongation of 39.1% can be obtained after solution treatment (water quenching) without aging treatment and the highest yield and tensile strength (1374/1545 MPa) can be obtained after the direct aging treatment. The match of strength and ductility is able to be tailored by controlling the amount of strengthening phases, which can be realized by adjusting the cooling rate of solution treatment and aging time.

Study of thermal behavior and solidification characteristics during laser welding of dissimilar metals

Abstract A transient three-dimension model was developed to study the thermal behavior and solidification characteristics during laser linear welding of 304ss and Ni considering the free surface deformation of the melt pool. The evolution of temperature and velocity field, the weld pool configuration and the solidification microstructure were investigated numerically and experimentally. A moving mesh method based on the Arbitrary Lagrangian Eulerian (ALE) was adopted to capture the free surface. Dimensional analysis was carried out to study the different heat transfer mechanisms. The solidification parameters, including the cooling rate (GR) and the morphological parameters (G/R), can be calculated through the transient thermal analysis to predict the solidification structure. The molten pool configuration on both sides changed from circular to elliptical as a result of the change of the dominant heat transfer mechanism. Sharp changes of the curvatures of the weld pool boundary were also observed and explained by the Prandtl number. Because the G/R was bigger at the bottom and the GR was larger at the top surface of the molten pool, the microstructure morphology changed from equiaxed dendrites to cellular dendrites to planar, and grain size increased from the top area to the bottom in 304ss side. The similar phenomenon was also observed in Ni side.

The crystal growth, intercellular spacing and microsegregation of selective laser melted Inconel 718 superalloy

The solidified microstructure of SLM-fabricated IN718 was investigated from macroscopic to microscopic perspective in this study using optical microscope, scanning electron microscope, transmission electron microscopy. Based on the experimental results and theoretical analysis, the solidification behavior, the characteristics of the primary dendrite spacing λ1 and microsegregation were elucidated. In the present SLM-produced IN718, the solidification starts at the molten pool boundaries by the epitaxial growth. The solidification rate is estimated in the range of 0.1–1 m/s. The newly-formed crystals grow into cellular dendrites, the direction of which is either parallel to the original direction or changed by 90°. The primary dendrite spacing λ1 varies over a wide range of 27.6 × 10−2 ∼147.3 × 10−2 μm due to dramatically changeable average cooling rate from 1.1 × 105–1.2 × 107 °C/s. There is a significant microsegregation of Nb in the inter-dendritic region, leading to the formation of Laves phases, the amount of which is about 9.0 ± 1.3%. Meanwhile, a solute trapping of Nb in the dendritic trunk occurs during rapid solidification.

Microstructure and oxidation resistance of a multiphase Mo-Si-B ceramic coating on Mo substrates deposited by a plasma transferred arc process

To improve the oxidation resistance of a Mo substrate, a multiphase Mo-Si-B ceramic coating was deposited using a plasma transferred arc (PTA) process. The phase constituents, microstructure, and oxidation resistance of the coating were investigated. The results show that the microstructure of the Mo-Si-B coating is characterised by rod-like Mo5SiB2 dendrites, a lamellar structure of Mo3Si/Mo5SiB2 binary eutectics, and cellular Mo5Si3 dendrites. Compared with the Mo substrate, the Mo-Si-B coating exhibits a significant improvement in the oxidation resistance at 1300 °C in an air atmosphere. This is mainly owing to a dense and continuous borosilicate layer formed on the surface of the Mo-Si-B coating, which acts as an oxygen diffusion barrier. Moreover, it is observed that the Mo5SiB2 dendrites exhibit a higher oxidation resistance compared to Mo3Si/Mo5SiB2 eutectics during high-temperature oxidation exposure. The oxidation behaviour is discussed based on the oxidation kinetics and a cross-sectional microstructure analysis of the oxidised Mo-Si-B specimen.

Microstructure investigation of Inconel 625 coating obtained by laser cladding and TIG cladding methods

The purpose of this study was to investigate the microstructure of the Ni base Inconel 625 coatings that were fabricated on the Inconel 738 substrate with two processes of laser cladding and TIG cladding. For this purpose, single-pass samples were precipitated to obtain optimal parameters. In the laser cladding method, laser power, laser scanning rate and powder feed rate were considered as variables, and current and its type were considered as variables for the TIG method. Based on the results, using the parameters of optimum single-pass samples that were free of porosity, crack and had minor geometric dilution, coatings were applied in both methods with 50% overlap. In order to microstructural, elemental and phasic characterization, field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were used. Based on the results, the microstructure of coatings from surface to interface of coating/substrate consists of coaxial, columnar and cellular structure dendrites, respectively, and the laser coating has a finer microstructure due to the higher cooling rate. In addition, the austenite, carbide and Laves phases were observed in both coatings.

Laser metal deposition as repair technology for 316L stainless steel: Influence of feeding powder compositions on microstructure and mechanical properties

Laser metal deposition was used to repair grooves on 20 mm thickness 316L stainless steel plates using two different 316L stainless steel commercial powders – Fe-0.15C-11.8Cr-0.15Mn-0.2Ni-0.031P-0.56Si-0.05S (wt.%) powder and Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder. Good comprehensive performance of fine metallurgical bonding with were successfully achieved under optimized processing parameters. The microstructure and mechanical properties (micro-hardness, ultimate tensile strength, bending strength, low-temperature impact toughness) of the repaired specimens were investigated. Results indicated that chemical composition (different element contents) of the powders played an important role in determining the microstructure, phases and properties of the specimens. It was found that the microstructure of the specimens repaired with Fe-0.15C-11.8Cr-0.15Mn-0.2Ni-0.031P-0.56Si-0.05S (wt.%) powder was homogeneous and consisted of Cr-rich martensite while a number of cellular dendrite were presented in microstructure of specimens repaired with Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder and the repaired specimens consisted of ferrite and austenite. Due to solid solution strengthening, the average hardness of the specimens repaired with former powder was higher than that of the specimens with later powder. However, in the aspect of mechanical properties, LMD with Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder had better performance than the specimens repaired with another kind of powder. The relationship between the microstructure characteristics and mechanical performances of the repaired specimens was also discussed.

Cellular/dendritic transition, dendritic growth and microhardness in directionally solidified monophasic Sn-2%Sb alloy

Abstract Horizontal directional solidification experiments were carried out with a monophasic Sn-2%Sb (mass fraction) alloy to analyze the influence of solidification thermal parameters on the morphology and length scale of the microstructure. Continuous temperature measurements were made during solidification at different positions along the length of the casting and these temperature data were used to determine solidification thermal parameters, including the growth rate ( V L ) and the cooling rate ( T R ). High cooling rate cells and dendrites are shown to characterize the microstructure in different regions of the casting, with a reverse dendrite-to-cell transition occurring for T R >5.0 K/s. Cellular ( l c ) and primary dendrite arm spacings ( l 1 ) are determined along the length of the directionally-solidified casting. Experimental growth laws relating l c and l 1 to V L and T R are proposed, and a comparative analysis with results from a vertical upward directional solidification experiment is carried out. The influence of morphology and length scale of the microstructure on microhardness is also analyzed.

Mechanical properties characterisation of dissimilar joint of high-temperature materials using Thermo-Calc Classic (TCC) diagram analysis

ABSTRACTWeldments were fabricated between Incoloy 800HT and P91 steel using ERNiCr-3, and ER505 filler materials by GTAW process. The ERNiCr-3 weld has to interface columnar and equiaxed dendrites whereas the ER505 weld has cellular dendrites. The ER505 weld / P91 steel base interface shows δ ferrite. Unmixed zone (UMZ) on Incoloy 800HT side and Type I and Type II zones were observed on P91 steel side of weldments. It was observed that with an increase in heat input, there was an increase in tensile strength of ERNiCr-3 weldments whereas there was a decrease in tensile strength of ER505 weldments. The impact energy of ERNiCr-3 welds was 96 Joules whereas ER505 welds had the impact energy of 178 Joules. Based on microstructure and mechanical properties, ER505 was found to be the ideal filler for joining of Incoloy 800HT to P91 steel than ERNiCr-3 filler.

Comparative study of IN600 superalloy produced by two powder metallurgy technologies: Argon Atomizing and Plasma Rotating Electrode Process

Two type products of IN600 alloy powders fabricated by Argon Atomizing (AA) and Plasma Rotating Electrode Process (PREP) were compared, in terms of microstructure, sphericity of powder, defects and mechanical properties. The interior morphology of AAed IN600 powder was mainly composed of cellular dendrites, and the dendrites were discontinuous and disordered. While, the interior morphology of PREPed IN600 powder was mainly composed of dendritic dendrites. The granularity of AAed IN600 powder ranged from 40 to 150 μm, in comparison that of PREPed IN600 powder ranged from 70 to 130 μm. The sphericity of PREPed powder particles were better than that of AAed powder particles. Large hollow powders existed in AAed IN600 alloy powder, while PREPed IN600 alloy powder did not exist. The AAed and PREPed IN600 alloy both exhibit equivalent tensile strength and yield strength after hot isostatic pressing and same heat treatment condition, but the section shrinkage rate of PREPed alloy (15.5%) was higher than that of AAed alloy (14.6%), indicating that the plasticity of PREPed IN600 alloy was better than that of AAed IN600 alloy.

Study on uni-axial tensile strength properties of Ytterbium fiber laser welded Hastelloy C-276 sheet

Failure of as-welded Hastelloy C-276 specimen in uni-axial tensile test has been reported to occur at the weld joint with around 50% reduction in elongation length relative to that of as-received Hastelloy C-276 specimen. The proposed reason for such an occurrence is due to the micro-segregation of elements during solidification of the weld zone. Micro-segregation causes formation of brittle inter-metallic phases, mainly at the grain boundaries of the weld zone, leading to a change in deformation behavior of the welded specimen. Reduction in the linear heat input required for welding tends to reduce micro-segregation. Ytterbium fiber laser with its higher melting efficiency, requires lower linear heat input for welding, in comparison to other sources of heat. In the present investigation, Ytterbium fiber laser has been used for welding of 3 mm thick Hastelloy C-276 sheet and the tensile strength properties of the as-welded specimens have been studied. It is observed that the fiber laser as-welded specimens fail at the base metal, and the percent elongation is found to be increase twice as compared to that of previous reported literatures. Analysis of the fiber laser as-welded specimen showed (a) Reduction in micro-segregation within the weld zone due to high cooling rate (=1000 °C/s) of the melt pool; (b) Cooling time through the sensitization temperature range at the heat-affected zone was found to be 1.01 s as compared to 360 s required for precipitation of inter-metallic phase; (c) Ductile nature of the weld zone due to presence of inclined cellular dendrites at the weld centerline facilitating plastic deformation; and (d) Significant increase in elongation of heat-affected zone grains as compared to that of the base metal grains.

Investigations on microstructure and wear resistance of Fe-Mo alloy coating fabricated by plasma transferred arc cladding

A wear-resistant Fe-Mo alloy coating was fabricated on the AISI 1045 steel substrate by plasma transferred arc (PTA) cladding, using pure Mo powders as the precursor material. The microstructure, microhardness and wear resistance of the coating were investigated by scanning electron microscope (SEM) with energy dispersive spectrum (EDS), X-ray diffraction (XRD), Vickers hardness and pin-on-disk wear testers, respectively. It was found that the coating presented rapidly solidified microstructure mainly consisting of cellular R-Fe63Mo37 dendrites and a small amount of lamellar R-Fe63Mo37/α-Fess dendrites uniformly distributed in the α-Fess matrix. Based on the eutectic growth and undercooling theories, the possibility of the R/α peritectic coupled growth in the rapidly solidified peritectic Fe-Mo alloy was discussed. The extensive growth of columnar crystallites into the substrate provided good metallurgical bonding at the interface between the coating and the AISI 1045 steel substrate. The results showed that the microhardness and wear resistance of the coating were much higher than that of the AISI 1045 steel substrate. The transferred oxides layer containing MoO3 formed on the worn surface had a good friction-reducing property and also contributed to the wear resistance of the coating.

Comparisons on microstructure, mechanical and corrosion resistant property of S136 mold steel processed by selective laser melting from two pre-alloy powders with trace element differences

Selective laser melting (SLM) method has a great potential for fabricating injection mold with complex structure. In this study, two S136 mold steel pre-alloy powders with trace element difference were fabricated by SLM, and then comparison of samples on microstructure, mechanical and corrosion resistant property was investigated. Results showed that the sample fabricated with a higher Si+Mn content exhibited the lower mechanical properties, such as micro-hardness of 48.73 HRC, ultimate tensile strength of 1186.7 MPa and elongation of 10.6% due to a few of pores observed on the cross surface. For the SLM sample produced by another powder with a lower Si+Mn content, it showed equiaxed grains and cellular dendrite grains coexisted and exhibited the superior mechanical properties (50.31 HRC, 1467.9 MPa and 11.1%) combined with well corrosion resistance. Then the injection mold insert with conformal cooling channel was successfully processed by this powder. The cooling time was reduced by 30% during the injection cycle. Therefore, it can be seen that trace element differences have an important effect on the SLM S136 mold steel. The results suggest that it is essential for successful fabricating components with a suitable composition of alloy during SLM process.