Columnar Dendrites Research Articles

Parallel-GPU-accelerated adaptive mesh refinement for three-dimensional phase-field simulation of dendritic growth during solidification of binary alloy

In the phase-field simulation of dendrite growth during the solidification of an alloy, the computational cost becomes extremely high when the diffusion length is significantly larger than the curvature radius of a dendrite tip. In such cases, the adaptive mesh refinement (AMR) method is effective for improving the computational performance. In this study, we perform a three-dimensional dendrite growth phase-field simulation in which AMR is implemented via parallel computing using multiple graphics processing units (GPUs), which provide high parallel computation performance. In the parallel GPU computation, we apply dynamic load balancing to parallel computing to equalize the computational cost per GPU. The accuracy of an AMR refinement condition is confirmed through the single-GPU computations of columnar dendrite growth during the directional solidification of a binary alloy. Next, we evaluate the efficiency of dynamic load balancing by performing multiple-GPU parallel computations for three different directional solidification simulations using a moving frame algorithm. Finally, weak scaling tests are performed to confirm the parallel efficiency of the developed code.

Effects of Welding Wire Composition on Microstructure and Mechanical Properties of Welded Joint in Al-Mg-Si Alloy

The effects of welding wire composition on microstructure and mechanical properties of welded joint in Al-Mg-Si alloy were studied by electrochemical test, X-ray diffraction (XRD) analysis and metallographic analysis. The results show that the weld zone is composed of coarse columnar dendrites and fine equated grains. Recrystallized grains are observed in the fusion zone, and the microstructure in the heat affected zone is coarsened by welding heat. The hardness curve of welded joint is like W-shaped, the highest hardness point appears near the fusion zone, and the lowest hardness point is in the heat affected zone. The main second phases of welded joints are: matrix α-Al, Mg2Si, AlMnSi, elemental Si and SiO2. The addition of rare earth in welding wire can refine the grain in weld zone obviously, produce fine grain strengthening effect, and improve the electrochemical performance of weld.

Hybrid wire - arc additive manufacture and effect of rolling process on microstructure and tensile properties of Inconel 718

• Rolling process is integrated into the WAAM to refine the grains. • A new heat treatment is applied for Inconel 718 to further enhance the strengths. • The columnar dendrites change to equiaxed grains with interlayer rolling process. • Finer grains and isotropic mechanical properties are obtained in warm rolling. • The strengthening mechanism of the hybrid WAAM and rolling process is revealed. Wire - arc additive manufacture (WAAM) is suitable for Inconel 718 components due to its high deposition efficiency. However, large columnar dendrites decrease the mechanical properties and can cause severe mechanical anisotropy. Cold rolling and warm rolling through flame heating have been investigated to analyze their effects on microstructure and tensile properties compared to as-deposited WAAM material. Standard solution and double aging (SA), as well as homogenization followed by solution and aging (HSA) heat treatments were compared. The results show that the large columnar dendrites change to finer equiaxed grains 16.4 μm and 26.2 μm in size for warm and cold rolled alloy, respectively. This increases to 22.5 μm and 30.1 μm after HSA treatment. The microhardness and strength of rolled material increase significantly and the warm rolled material after HSA treatment exceeds that of the wrought alloy. While the as-deposited and cold rolled samples both show significant anisotropy, isotropic tensile properties are obtained for warm rolled plus HSA heat treated samples. Finer equiaxed grains with more dispersive distributions of γ' and γ" strengthening precipitation contribute to the superior mechanical properties for warm rolled material. For both the cold and warm rolled material, there was an elongation decrease due to precipitated particles, which also led to a trans-granular ductile fracture mode. The strengthening mechanism of the hybrid rolling process was analyzed and found to be related to work hardening, grain boundary strengthening, precipitated strengthening phases and the δ phase.

Effect of Rapid Solidification Processing on the Microstructure and Corrosion of 316L Austenitic Stainless Steel

Austenitic stainless steels processed by rolling are widespread in technological applications, since they have excellent mechanical and corrosion properties. This study investigated the effect of the cooling rate, microstructure and properties of 316L austenitic stainless steel under cold rolled conditions and by rapid solidification. The microstructure of the cold rolling processing steel was composed of austenite and a low percentage of delta ferrite. For the rapid solidification condition, the microstructure evolved from columnar and acicular dendrites to equiaxed dendrites with decreasing cooling rates, without the presence of delta ferrite due to the high cooling rate. Furthermore, thermal analyses in both routes revealed that oxidation kinetics was slower after rapid solidification in synthetic air. The microhardness in the cold rolling condition was lower than in the rapid solidification condition since the microstructure under the solidification condition is more refined. The sample in the rapid solidification condition region RS1 presented the highest corrosion resistance considering the pit potential. The passivation current density in the cold rolled condition was 5.72x10-5A/cm2, while under the rapid solidification condition, regions RS1 and RS2 were 2.24x10-5 A/cm2 and 3.72x10-6 A/cm2, respectively, and region RS3, did not present a passivation region in a broad range of potentials.

Development of Data Assimilation System for 3D Columnar Dendrite Growth Using Phase-field Method

Accurate evaluation of dendrite growth is essential to improve the quality of cast products. The objective of this study is to develop a data assimilation system to integrate X-ray imaging observations and phase-field simulations of three-dimensional columnar dendrite growth in a thin film. Here, solid fraction in the thickness direction of the thin film is used as observation data.

Formation Mechanism of "Fractured Columnar Dendrite" in Commercial Purity Al During Solidification with a Forced Flow Field Induced by Pulsed Magnetic Field

Formation Mechanism of "Fractured Columnar Dendrite" in Commercial Purity Al During Solidification with a Forced Flow Field Induced by Pulsed Magnetic Field

Phase-field study on an array of tilted columnar dendrites during the directional solidification of a binary alloy

An array of columnar dendrites is important to the microsegregation and permeability of interdendritic liquid flow. In this study, an array of tilted columnar dendrites growing during the directional solidification of a binary alloy was investigated via large-scale phase-field simulation. The main conclusion is that the hexagonal array is the dominant array for reasonably tilted dendrites as well as dendrites that grow along the temperature gradient. It is also concluded that the array ordering gradually improves with growth for all tilt angles, and the ordering rate is faster for a small tilt angle.

Preliminary investigation on the effect of coaxial injection of TiC powder in GMAW

ABSTRACT Stable powder injecting into a weld pool in gas metal arc welding (GMAW) process cannot be guaranteed by a regular by-pass-feed method due to the strong repelling from the shielding gas. A novel trial of co-axial through-the-arc powder injecting is thus proposed to study its effect on the bead formation, microstructure, and surface hardness of aluminium-alloy welds. The weld depth and bead width could be both increased. Cellular grains grow on most of the surface of TiC particles, some of which even grow into columnar grains and dendrites. Effective grain refinement of the melted zone (MZ) is observed. The surface hardness of the cross section is significantly increased. These positive results would be beneficial for arc welding and additive manufacturing processes.

Modeling of microstructure formation with gas porosity growth during columnar dendritic solidification of aluminum alloys

In this paper, microstructure formation with gas bubble growth during columnar dendritic solidification has been numerically investigated by using the coupled lattice Boltzmann-cellular automaton-finite difference (LB-CA-FD) model, which combines the physical mechanisms of the redistribution of hydrogen components at the solid–liquid interface and the transportation of hydrogen atoms in liquid. Hydrogen transportation, bubble nucleation and growth are computed by the LB method while the growth of columnar dendrite and the transport of solutes are described by the CA-FD model. Predicted results show that the bubbles begin to nucleate at the roots of primary dendritic trunk and secondary arms, and elongated irregular bubbles are sandwiched between neighboring dendrites after merging and moving during the growth stage. Columnar dendritic change from coarse and ordered to fine and disordered as the cooling rate is increased. Meanwhile, the cooling rate has a significant effect on the incubation time for gas bubbles and the maximum growth rate of porosity. It was found that the misorientation angle has limited effects on the solid fraction and porosity percentage but strongly influences on the morphologies of the columnar dendritic arrays, resulting in the different distributions of bubbles.

A Study on the Fiber YAG Laser Welding of 304L Stainless Steel

This work aims to optimize the main YAG fiber laser parameters to weld 304L stainless steel plates of 3 mm thick. Different laser powers (2500, 2000, and 1500 W) and speeds (60, 40, and 20 mm/s) were used and merged in heat input, maintaining the defocusing distance at –2 mm to get full penetration. The weld quality and the effect of the laser heat input on the microstructures of the weld and heat-affected zones were investigated. Besides, the fracture strength of the welded joints and hardness distribution through the cross-sections were evaluated. The weld width has a direct relationship with heat input. The laser power of 2800 W produced full penetration joints without any macro defects while reduction in laser power pronounced partial penetration defects. The size of the heat-affected zone in all the processing parameters was very small. The microstructure of the weld zone shows columnar dendrite austenite grains with small arm spacing in most of the welded zone. The size of the dendrites became finer at lower heat input. At a higher heat input, a reasonable amount of lathy equiaxed grains with some delta ferrite occurred. A small amount of delta ferrite was detected in the heat-affected zone, which prevented the crack formation. The hardness of the weld metal was much higher than that of the base metal in all processing parameters and it has a reverse relationship with the heat input. The fracture strength of the welded joints was very close to that of the base metal in the defect-free samples and it increased with decreasing the heat input.

A EFFECT OF CeO2 DOPING ON THE MICROSTRUCTURE AND CORROSION BEHAVIOR OF CoCuNiTi HIGH-ENTROPY ALLOY COATINGS

CoCuNiTi high-entropy alloy coatings with an equal molar ratio were prepared on 45 steel substrates using the laser-cladding method. The effect of CeO2 doping on phase structure, microstructure and corrosion behavior of CoCuNiTi coatings were investigated by X-ray diffraction, optical microscope, scanning electron microscope, and electrochemical workstation. The results show that the phase structure of CoCuNiTi coating doped with 1 w/% CeO2 is transformed from the original dual-phase structure of FCC main phase and BCC phase to the dual-phase structure of BCC main phase and FCC phase, mainly because CeO2 addition helps to improve the temperature gradient and solidification rate during solidification, reduce the nucleation resistance and the diffusion distance of the alloying elements, and provide a liquid environment with longer time, lower viscosity and higher diffusion rate. The microstructure of the two coatings is composed of BCC-phase dendrite and FCC-phase interdendrite. The widths of the primary dendrites of the columnar dendrites in CoCuNiTi cladding layer before and after CeO2 doping are about 8.10 µm and 6.51 µm, respectively. The CoCuNiTi coating doped with 1 w/% CeO2 has the smallest corrosion current density, the largest capacitive reactance arc radius and polarization resistance, and the best corrosion resistance in 3.5 w/% NaCl solution, which is mainly due to making the alloy structure refined and the element distribution uniform after the CeO2 addition.

Effect of Solidifying Structure on Centerline Segregation of S50C Steel Produced by Compact Strip Production

Medium-high carbon steels having a high quality are widely used in China. It is advantageous to produce high value-added hot-rolled plates with the crystal refined and chemical composition homogenized in the casting slabs. However, element segregation occurs easily during high-medium carbon steels’ production. Generally, the centerline segregation is improved by enlarging the equiaxed zone with low-superheat casting and electromagnetic stirring (EMS). Studies were conducted on centerline segregation of S50C steel slabs with a thickness of 52 mm produced by the compact strip production (CSP) process in China without EMS equipped. By sampling along the width at different position, the secondary dendrite arm spacing (SDAS) was measured after etching and picture processing, based on which the cooling rate was calculated. It was found that the cooling rate increased from the center to the surfaces of the slabs ranging in 1~20 K/s, 10 times faster than that of a conventional process. The faster cooling rate led to a refined solidifying structure and columnar dendrite through the center of the slabs. The SDAS tended to increase from surfaces to the center, ranging only 32~120 μm smaller than that of a conventional process in 100~300 μm, indicating a finer solidifying structure by the CSP process. Results by EPMA indicated that elements C, Si, and Mn distribute in dispersed spots, increasing towards the center, and the centerline segregation changed in a narrow range: for C mainly in 1.0~1.1, Si in 0.98~1.08, Mn in 0.96~1.02, respectively, meaning a more chemical homogenization than that of thick slabs. Elements’ segregation originated from solute redistribution between solid and liquid. According to thermodynamic calculation, δ region of S50C is so narrow that the solute redistribution mainly occurred between γ-Fe and liquid during solidification. As the equilibrium partition coefficient of element C was the smallest, it was easy for C to be rejected to the residual liquid in the inter-dendritic space, leading to obvious segregation, relatively. Besides, as a result of high-cooling intensity, the solidifying structure became so fine that the Fourier number increased and the volume of the residual liquid decreased, making centerline segregation alleviated effectively both in volume and degree. Although bulging was observed during the industrial experiment, the centerline segregation was still inhibited obviously as the refining solidifying structure with permeability ranged only in 0.1~2.3 μm2 from the surfaces to centerline, which showed a good resistance on the residual flow towards the centerline.

Effect of Freezing–Corrosion Treatment on Wear Resistance of Laser-Cladded Cobalt-Based Coatings on EH32 Steel

Laser cladding on polar icebreaking ship steels that can work under low-temperature conditions has rarely been investigated. Herein, three types of the cobalt (Co)-based coatings with different chemical compositions and structures were fabricated on EH32 steel substrate by laser cladding. The influence of corrosion treatment at room temperature and freezing–corrosion treatment at a low temperature of −80 °C on the wear resistance of the EH32 substrate and the Co-based coatings was investigated. Results showed that the as-cladded Co-based coatings were composed of Co-based solid solutions and hard carbides such as Cr7C3, Cr23C6, Mo2C, WC, and/or W2C. Columnar dendrites with a 3D honeycomb-like structure and equiaxed dendrites occurred in the coatings containing WC and/or W2C. The Co-based coatings could significantly improve the hardness and wear resistance of the EH32 substrate, and the coating with the highest hardness possessed the best wear resistance. Both the corrosion and freezing–corrosion treatments could enhance the wear resistance of the Co-based coatings but weakened the wear resistance of the EH32 substrate. The Co-based coatings after corrosion treatment had superior wear resistance compared to the untreated coatings and the coatings after freezing–corrosion treatment.

Microstructure and oxidation resistance of CoNiCrAlY coating manufactured by laser powder bed fusion

In this study, an IN718 superalloy matrix and its CoNiCrAlY coating were prepared using the laser powder bed fusion (LPBF) technique. The microstructure and isothermal oxidation properties of the CoNiCrAlY coating at 1100 °C were studied. The results showed that the CoNiCrAlY coating prepared by LPBF was dense, uniform, and crack-free. The coating was mainly composed of the γ phase, the β phase, and some Y2O3 microparticles. The microstructure along and perpendicular to the building direction consisted of columnar dendrites and equiaxed grains, respectively, with a strong (100) 〈001〉 cubic texture. The coating showed a low thermal growth oxide layer growth rate and a thin oxide layer during isothermal oxidation for 1–100 h. Cr2O3 oxides were initially formed on the surface of the coating. With an increase in the oxidation time, the α-Al2O3 scale and (Co,Ni)Cr2O4 and CoNiO2 mixed oxides were formed on the surface of the coating.

Parametric effect on the microstructure of direct metal deposited Inconel 718

ABSTRACT Direct metal deposition (DMD) technique results in unique microstructure when compared with the traditional manufacturing processes because of rapid solidification and the combined effect of input process parameters. However, parametric effect on the microstructure evolution is not thoroughly understood. This paper deals with the investigations carried to understand the effect of process parameters on the microstructure characteristics of Inconel 718 deposits fabricated by DMD process. The microstructure observations revealed columnar dendrites at the bottom, middle portions, and equiaxed grains in the top portion of the deposits. The different grain morphologies obtained are primarily due to the interaction effect of thermal gradient and solidification velocity. Furthermore, analysis is carried on the microstructural characteristics, and cooling rates during solidification were evaluated. It was observed that the cooling rates are varying systematically with input parameters. In addition, the grain size is observed to be a function of cooling rate. EDS analysis performed on the deposits, where the elemental mapping revealed long chains of Nb-rich phases at the interdendritic regions. The microhardness measurements done using Vickers indentation method show a minimal variation with process parameters; the hardness followed a linear relationship with the reciprocal square root of grain width.

Coarse Columnar Dendrites of GH3039 Nickel-Based Alloy at Diverging Grain Boundaries during Wire and Laser Additive Manufacturing: A Phase Field Simulation

Coarse columnar dendrite greatly reduced the mechanical performance of GH3039 nickel-based alloy in the additive manufactured parts, which limited its application in the engineering fields. This study provides a comparison of overgrowth behaviors at diverging grain boundaries through two-dimensional phase field simulation, and the effect of dendrite orientation on overgrowth behavior was analyzed. Moreover, our results show that the primary spacing becomes larger as the increasing of dendrite orientation. The columnar dendrites branch new dendrites near grain boundaries to refine the primary arm spacing in the process of wire and laser additive manufacturing (WLAM).

Microstructure and mechanical properties of melt-grown alumina-mullite/glass composites fabricated by directed laser deposition

Melt-grown alumina-based composites are receiving increasing attention due to their potential for aerospace applications; however, the rapid preparation of high-performance components remains a challenge. Herein, a novel route for 3D printing dense (< 99.4%) high-performance melt-grown alumina-mullite/glass composites using directed laser deposition (DLD) is proposed. Key issues on the composites, including phase composition, microstructure formation/evolution, densification, and mechanical properties, are systematically investigated. The toughening and strengthening mechanisms are analyzed using classical fracture mechanics, Griffith strength theory, and solid/glass interface infiltration theory. It is demonstrated that the composites are composed of corundum, mullite, and glass, or corundum and glass. With the increase of alumina content in the initial powder, corundum grains gradually evolve from near-equiaxed dendrite to columnar dendrite and cellular structures due to the weakening of constitutional undercooling and small nucleation undercooling. The microhardness and fracture toughness are the highest at 92.5 mol% alumina, with 18.39±0.38 GPa and 3.07±0.13 MPa·m1/2, respectively. The maximum strength is 310.1±36.5 MPa at 95 mol% alumina. Strength enhancement is attributed to the improved densification due to the trace silica doping and the relief of residual stresses. The method unravels the potential of preparing dense high-performance melt-grown alumina-based composites by the DLD technology.

Microstructure and corrosion resistance of Ni-Cu alloy fabricated through wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) technology is a promising low-cost process for fabricating or restoring marine structures with Ni-Cu alloy. In this work, single pass multi-layer wall of Monel FM60 was deposited with WAAM process. The micrographs revealed the presence of equiaxed and elongated columnar dendrites with minor amount of Cu segregation in Ni-Cu matrix. Due to the complex cyclic thermal history, the microstructure varied along the building direction (BD) and the secondary dendrite arm spacing (SDAS) ranged between 5 and 15 µm. Energy Dispersive X-Ray Analysis (EDS) results confirmed the existence of tiny Ti-rich particles within the Ni-Cu matrix. The WAAM processed Monel FM60 specimens (0.40–0.62 mpy) exhibited better corrosion resistance compared to traditional Monel 400 alloy (0.57–0.67 mpy) in 3.5% NaCl solution. The enhanced corrosion resistance is corroborated to the retaining of strengthening elements in WAAM specimens and the pit size ranged between 40 and 80 µm.

Microstructure and properties of laser cladding coating at the end of L415/316L bimetal composite pipe

This article attempts to use the laser cladding to realize the metallurgical combination of the ends of L415/316L bimetal mechanical lined pipe. For the convenience of the tests on mechanical properties, two kinds of nickel-based alloy powders, self-fluxing alloy powder (SFAP) and non-self-fluxing alloy powder (NSFAP), were cladded on substrate L415 sheet. The microstructure of two cladding coatings was analyzed by optical microscopy (OM) and scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and the mechanical properties and corrosion resistance of two cladding coatings were determined. The results show that two cladding coatings do meet the requirements of both low hardness and high corrosion resistance, and that two cladding coatings which mainly contain γ-Ni solid solution are composed of planar dendrites, cell dendrites, columnar dendrites and equiaxed dendrites. In addition, SFAP coating exhibits high tensile strength and low toughness, showing that SFAP coating is brittle. However, NSFAP coating exhibits good plastic toughness, high tensile strength and excellent impact Charpy energy, and excellent bending performance. SFAP coating is a quasi-cleavage fracture, which is a brittle fracture. The fracture of NSFAP coating is composed of a large number of dimples, which is a ductile fracture. Considering the three aspects of microstructure, chemical composition, and mechanical and corrosion properties, NSFAP is suitable for laser cladding at the end of mechanically lined bimetal pipe.

Investigation on in-situ laser cladding 5356 aluminum alloy coating on 5052 aluminum alloy substrate in water environment

In this paper, underwater dissimilar aluminum alloy laser cladding has been investigated for the first time through a laser nozzle. The influences of water environment on laser cladding of dissimilar aluminum alloy were researched under optimized process parameters. With the transfer of cladding environment from air to water, the height of cladding zone (CZ) and the cladding angle increased, but the width of CZ and the depth of melting zone (MZ) decreased. The microstructure of MZ and CZ were columnar dendrites and equiaxial dendrites, but the underwater microstructure was much smaller than that of the in-air. Both underwater and in-air CZ were composed of Aluminum phase and Al0.5Fe3Si0.5 phase, while the formation of intermetallic compound increased in the in-air CZ. The water environment was beneficial to reduce the magnesium burning loss in the CZ. In the underwater CZ, although the decrease of hard phase compound (Al0.5Fe3Si0.5) formation made microhardness value go down, the microhardness value went up with the increase of magnesium content and the refinement of microstructure. Finally, under the combined action of these three factors, the average microhardness value of underwater cladding layer was higher than that of the in-air cladding layer.