Columnar Dendritic Structure Research Articles

Laser surface cladding of mild steel with 316L stainless steel for anti-corrosion applications

Fiber YAG (Yttrium-Aluminum-Garnet) Laser cladding is used to generate a thin layer of 316L stainless steel over low carbon steel substrate alloy to enhance corrosion resistance. In this work, the cladding technique was used at the laser powers of 2800, 2400 and 2000 W with a fixed travelling speed of 6 mm/s. The work aims to investigate the optimal heat-input for the thin clad in terms of microstructure and bonding quality. The microstructure of both formed layers by cladding and substrate was investigated. On the clad cross-section of the specimens, the micro-Vickers hardness was measured. At laser powers of 2800 and 2400 W, the produced layers were well bonded to the substrate without visually noticeable macro-defects. Some defects associated with cracks appeared at a laser power of 2000 W. The stainless steel clad layer consisted mainly from highly refined mixed columnar and cellular dendritic austenite structure elongated perpendicularly to the substrate surface in the direction of heat flow. At higher powers, the structure showed organized parallel columnar dendrites while at lower powers, it shows fine cellular dendrites in random directions. A narrow dilution zone (less than 10 µm) was obtained where the carbon and chromium elements can stay together and form harmful carbides. The depth of the fused part in carbon steel side has a direct relationship with the laser power, and it consisted mainly from feathery bainite inside large ferrite grains. The hardness of the produced layers was improved than the base metal. Sudden hardness increment at the limited area around the interface was obtained after all the laser powers. Decreasing laser power led to hardness increment at the clad and fusion zone.

The Effect of Rapid Solidification on Microstructure and Corrosion of Advanced Biomaterial Co-Cr-Mo-C Alloy

In this research, the microstructure and corrosion properties of rapidly solidified Co-Cr-Mo-C alloy as an advanced biomaterial alloy were studied. The use of rapid solidification casting method represents significant changes in not only the amount of formed e-HCP phase, which is strongly influenced by rapid solidification, but also in electrochemical behavior and solidified structure. In this research, rapid solidified Co-Cr-Mo-C alloy is studied using OM, SEM, EDS, XRD, and dynamic potentiostate. Co-alloy ingots were melted into an induction furnace filled by argon gas and casted into a V-shape sand and chill copper molds to prepare rapid solidified samples and its properties were measured in different cooling rates. The microstructure examination demonstrating the structure of alloy is mainly consist of columnar dendritic structure with the distribution of carbides within primary and secondary dendrites arms and finer dendritic structure along with modified carbide distribution will be achieved by rapid solidification. This structure will improve alloy’s corrosion behavior and reduces its corrosion rate when it is tested in Ringer’s solution as an electrolyte.

Effect of laser parameters on the properties of high entropy alloys: A preliminary study

This preliminary study investigates the fabrication of Al-Co-Cr-Fe-Ni-Cu and Al-Ti-Cr-Fe-Co-Ni High Entropy Alloys with the Laser Deposition Technique. The effect of the laser parameters on the microstructure and hardness properties of the advanced materials were studied for aerospace applications. The results revealed that significant 300% increment occurs in the microhardness from 200 to 600 HV for the AlCoCrFeNiCu alloy system and 70% increment from 500 to 850 HV for the AlTiCrFeCoNi system as the laser power increases from 600 W to 800 W. However, as the scanning speed increases, the microhardness decreases. There were no notable changes in the microstructure interface of both alloys with a change in process parameters and further analysis showed the AlCoCrFeNiCu alloy had a transitional columnar dendritic structure while the AlTiCrFeCoNi alloy had the equiaxed dendritic microstructure with the direction of the grains observed along with the height of the deposit. The hardness properties of AlTiCrFeCoNi observed was attributed to the Ti content known to improve strength by facilitating a solid laves phase which suggests a potential application for aerospace wear-resistant coatings.

Permeability tensor for various columnar dendrite structures

Permeability is a very important parameter determining fluid dynamics in macroscale casting simulations. For a columnar solidification structure, the permeability has anisotropy and becomes a tensor quantity. In our previous study, we proposed a permeability tensor and confirmed its validity. In this study, the applicability of the permeability tensor is shown through a series of permeability computations that are conducted for some columnar dendritic structures with different morphologies.

Effect of Ca on Microstructure and Mechanical Properties of Directionally Solidified Mg-Zn-Ca Alloys

The effect of Ca on the microstructure and mechanical properties of directionally solidified (DSed) Mg-3Zn-xCa alloys (x=0.2,0.5,0.8wt.%) was investigated in the present work. The results showed that the DSed samples with the growth rate of 120 μm/s had columnar dendritic structures and the primary dendritic arm spacing (PDAS) decreased with the content of Ca increase. The TEM result indicated that the growth orientation of the DSed Mg-Zn-xCa alloys was , which was independent of the content of Ca. The tensile tests at room temperature showed that the mechanical properties of the DSed Mg-Zn-xCa alloys were strongly affected by the content of Ca. The addition of Ca remarkably improved the ultimate tensile strength (UTS) and the yield strength (YS), while dramatically reduced the elongation (El). Prismatic slip and twinning were the main deformation mechanisms in tensile tests.

Improvement of high temperature oxidation resistance of additively manufactured TiC/Inconel 625 nanocomposites by laser shock peening treatment

Additive manufacturing has seen large growth due to its numerous process advantages, yet some undesirable defects in additive manufactured (AM) products include pores and micro-cracks. These defects weaken the high temperature oxidation resistance of the final parts. In this work, laser shock peening (LSP) is used as a post-treatment method to change the surface characteristics of selective laser melted (SLM) nano-TiC particle-reinforced Inconel 625 nanocomposites (TiC/IN625). The effects of LSP on surface morphology, residual stress, microhardness, microstructure, and high temperature oxidation behavior of fabricated parts are studied. The results indicate pores in the as-built sample can be closed by the severe plastic deformation, which is induced by LSP. The maximum hardness is found to reach 462 ± 7 HV with a ∼ 460 μm hardened layer, and the surface stress state transforms from tensile to compressive after LSP. The full width at half maximum (FWHM) values of the (111) and (200) diffraction broaden, which can be attributed to grain refinement and an increase in lattice strain in the LSP samples. It is found that LSP causes a large number of columnar dendritic structures in the as-built sample to transform into cellular dendritic structures. Dislocation walls and dislocation tangles with high dislocation density form in the LSP sample. Compared with as-built sample, the LSP samples exhibit lower mass gain after oxidation at 900 °C for 100 h, indicating that LSP samples have greater oxidation resistance at high temperature. The underlying mechanism governing the high temperature oxidation resistance is proposed based on the experimental results. This study shows that LSP can be used as an effective method to modify the surface characteristics of SLM TiC/IN625.

Permeability Tensor for Columnar Dendritic Structures: Phase-Field and Lattice Boltzmann Study

Permeability is a very important multiscale parameter providing information of dendritic structures in macroscale simulations of solidification with liquid flow during casting. In our previous study, it was demonstrated that the permeability of liquid flow normal to a columnar dendritic structure can be well approximated by the Kozeny-Carman (KC) equation with a coefficient kc = 9. In this study, the permeability of liquid flow parallel to a columnar dendritic structure is investigated using a method that combines both phase-field and lattice Boltzmann simulations. It is shown that the permeability of parallel flow can also be approximated by the KC equation with kc = 3. A permeability tensor is then proposed for a columnar dendritic structure. Validity of the permeability tensor is confirmed by simulations of liquid flow in arbitrary directions of a columnar dendritic structure.

Permeability tensor for columnar dendritic structures: Phase-field and lattice Boltzmann study

Permeability is a very important multiscale parameter providing information of dendritic structures in macroscale simulations of solidification with liquid flow during casting. In our previous study, it was demonstrated that the permeability of liquid flow normal to a columnar dendritic structure can be well approximated by the Kozeny–Carman (KC) equation with a coefficient kc = 9. In this study, the permeability of liquid flow parallel to a columnar dendritic structure is investigated using a method that combines both phase–field and lattice Boltzmann simulations. It is shown that the permeability of parallel flow can also be approximated by the KC equation with kc = 3. A permeability tensor is then proposed for a columnar dendritic structure. Validity of the permeability tensor is confirmed by simulations of liquid flow in arbitrary directions of a columnar dendritic structure.

Microstructure and mechanical properties of high-manganese-containing high-speed twin-roll cast Al-Mn-Si alloy strips and their cold-rolled sheets

Al-Mn based alloys with high-manganese content are expected to have improved mechanical properties due to solid solution hardening and/or dispersion hardening. However, the increase of Mn solubility of the alloy is difficult by using the conventional DC casting. In order to solve this problem, in the present study, we focused on the twin-roll casting method which is characterized by high cooling rates. Several kinds of high Mn-containing Al-Mn-Si alloy strips were fabricated by using a vertical-type high-speed twin-roll caster equipped with a pair of water-cooled copper rolls. Direct temperature measurement of the liquid melt during the casting was also performed. The alloy strips of various compositions containing up to 4 Mn and 2 Si (wt%) were successfully obtained. By observing the microstructure of the cross section of the strip, we found the characteristic solidified structure. The solidified structure consisted of three layers. Two solidified shells with a columnar dendrite structure grew from the roll surfaces toward the strip center. In the mid-thickness region, the band structure consisting of equiaxed dendrites and globular grains was observed between the solidified shells. Very fine primary particles were observed in the matrix near the strip surface, while, relatively coarse particles with blocky and needle-like shape were observed in the central band of the as-cast strip. The electric conductivity measurement was performed for the as-cast strips. Mn solubility in Al matrix was estimated from the obtained values. The estimated Mn solubility in the Al-2Mn-xSi strips was between 1.5 ~ 1.8wt% Mn. It was over 1.43wt%Mn for the Al-4Mn-xSi strips. We found that the Mn solubility of the as-cast strips was considerably high. The strips were cold-rolled to the sheets and then annealed at various conditions. They were subjected to the tensile tests, and the effects of solid solution hardening and dispersion hardening are discussed.

Parametric investigation and characterization on SS316 built by laser-assisted directed energy deposition

In the present work, parametric investigation and characterization of stainless steel 316 (SS316) built by laser-assisted directed energy deposition (L-DED) is performed. Single-track L-DED experiments are carried by varying laser power, scanning speed, and powder feed rate using full factorial experimental design. The effect of L-DED process parameters on the track geometry, deposition rate, and microhardness is investigated, and three different combinations of process parameters yielding maximum deposition rate and hardness are identified for bulk investigation. The identified process parameters are laser power of 1000 W, powder feed rate of 8 g/min, and scanning speed of 0.4 m/min, 0.5 m/min, and 0.6 m/min. The austenitic phase [Formula: see text] is detected at all the conditions. However, ferrite [Formula: see text] peak is observed at 0.6 m/min due to microsegregation and thermal gradients. The minimum crystallite size is estimated to be 24.88 nm at 0.6 m/min. The porosity and microstructure analysis is carried out by optical microscopic images. The fine columnar dendritic structure is observed in L-DED samples at all conditions. An average microhardness of 317.4 HV0.98 N is obtained at 0.4 m/min, and it is observed that microhardness reduces with an increase in scanning speed mainly due to increase in lack of fusion and porosity. Tribology studies are carried out at different values of normal load and sliding velocity. The minimum specific wear rate of 0.02497 × 10−4 mm3/Nm is observed at scanning speed of 0.4 m/min. Scanning electron microscope of the wear tracks analysis shows abrasive wear as the major wear mechanism. This study provides a path for building SS316 components for various engineering applications.

Microstructure and mechanical properties of directionally solidified Mg–Zn alloy as a biomaterial

ABSTRACTDirectionally solidified Mg-4wt-% Zn alloy was prepared and the effect of growth rate on its microstructure evolution and mechanical properties was investigated. A typical cellular structure was observed when the growth rate was lower than 60 µm s−1. The microstructure evolved from cell to columnar dendrite as the growth rate increased. The ultimate tensile strength of the directionally solidified alloy was found to be higher than that of the alloy ingot with the same cooling rate. The ultimate tensile strength of the directionally solidified alloy increased with increasing growth rate but it decreased during the cell–dendrite transition. The results indicate that the mechanical properties of the directionally solidified alloy with fine cellular and columnar dendritic structures meet the requirements of biomaterials.

Resistance element weld-bonding and resistance spot weld-bonding of Mg alloy/austenitic stainless steel

Mg alloy/austenitic stainless steel (ASS) joints were produced by resistance element weld-bonding (REWB) and resistance spot weld-bonding (RSWB) techniques. Both joints consisted of two zones, namely, the adhesive zone and weld zone. The weld zone of the REWB joints was formed through a metallurgical reaction between the rivet and ASS. The microstructure of the fusion zone consisted of austenite and intercrystalline delta ferrite. The weld zone of the RSWB joints was formed via welding-brazing mechanism (involving molten Mg alloy and solid ASS), and the nugget was formed in the Mg alloy. The nugget microstructure consisted of fine columnar dendritic structure. Digital image correlation (DIC) analysis was used to study the deformation behavior and strain evolution in the loading direction during lap-shear test. The REWB and RSWB joints experienced lower degree of out-of-plane displacement, sustained higher peak load, and exhibited higher strain to failure than their conventional counterparts. However, the REWB joints exhibited superior lap-shear performance, with high energy absorption and favorable failure mode (pull out failure in the ASS sheet). Upon the failure of the adhesive zone at peak load, the load was sustained by the weld, and the final failure occurred via pullout mode in the ASS. The RSWB joints failed in a sudden manner, mainly by cohesive failure in the adhesive zone and Mg alloy nugget/ASS interface failure in the weld zone.

Grain Morphology and Orientation Effect on the High-Temperature Tensile Behavior of Directionally Solidified Magnesium Alloy

Columnar-structured Mg97.27Zn2.54Y0.19 alloy with the preferred growth direction of 〈22$$\bar{4}$$5〉 and 〈12$$\bar{3}$$3〉, and Mg98.01Zn1.84Y0.13Zr0.03 alloy with the preferred growth direction of 〈11$$\bar{2}$$0〉 were prepared using the directional solidification technique and then tensile tested at 250–300 °C. The results showed that the Mg97.27Zn2.54Y0.19 alloy with columnar dendritic structure experienced a significant deformation strengthening period when stretched at 250 °C, and a typical dynamic recrystallization stage at 300 °C, which were mainly accounted for its well-developed secondary dendrites, a large number of branch-shaped second phases distributed in the longitudinal grain boundary, and less granular second phases existing in the crystal. While in the whole test temperature range (250–300 °C), the stress–strain curves of Mg98.01Zn1.84Y0.13Zr0.03 alloys with cellular dendritic crystals showed a steady deformation state because of the second phases with strip distributing in the longitudinal grain boundary, which was induced by a balance between the deformation strengthening and the dynamic recrystallization. Especially at 300 °C, this steady deformation state existed in the strain of 5.6%–36%. The good uniform plastic deformation was closely related to the small misorientations between columnar grains, the associated movement of grains by grain boundary slipping, and the less percentage of dynamically recrystallized grains which could do harm to the orientation continuity of the columnar crystals. The directionally solidified Mg98.01Zn1.84Y0.13Zr0.03 and Mg97.27Zn2.54Y0.19 alloys with different orientation and grain morphology show different high temperature mechanical properties at 250 °C and 300 °C.

Microstructure and grain growth direction of SRR99 single-crystal superalloy by selective laser melting

Nickel-based Single Crystal (SX) superalloys have an excellent creep ductility and thermal fatigue resistance at elevated temperatures. But SX structure is difficult to be manufactured under the demand of increasing operating temperature and complex shapes. In this work, the feasibility of manufacturing Nickel-based SX-superalloys by selective laser melting (SLM) was investigated theoretically and experimentally. Firstly, the solidification conditions including temperature field, thermal gradient and solidification speed of the SLMed multi-track samples were calculated by an established finite element model to judge the theoretical feasibility based on the columnar to equiaxed transition (CET). Then, the single-track, triple-track and cuboid samples on a single-crystal SRR99 substrate were deposited to judge the experimental feasibility and explore their grain growth mechanism during SLM. The results showed that CET can be avoided during the whole process of SLM. The unidirectional SRR99 SX structure with a height of 2 mm can be successfully obtained by SLM. On horizontal section, the microstructures can be divided into [001] orientation zone, [010] orientation zone and overlapping zone with a typical secondary dendrite structure. On vertical section, the columnar dendritic structure along the dominant thermal gradient direction is present. The primary dendrite spacing of the SLMed samples (∼1–2 μm) is more than two orders of magnitude lower than that of the cast one (∼320 μm). So, SLM technology possesses the capacity to manufacture SX superalloy. When the deposited height reaches 6 mm, the misorientation angle is about 25° and a crack initiates. Finally, the grain growth mechanism of the SLMed SRR99 single-crystal superalloy was proposed based on the local thermal gradient, the existing well-aligned crystallographic orientation and the preferential grain growth direction.

Effect of grain morphology on the degradation behavior of Mg-4 wt% Zn alloy in Hank's solution.

The degradation behavior of Mg-4 wt% Zn alloy with three different microstructures was examined in Hank's solution at 37 °C by electrochemical measurements and immersion tests in this study. The results show that the sample with cellular structure exhibits a more positive corrosion potential, lower corrosion current density, larger impedance and more protective film than samples with columnar dendritic and equiaxed dendritic structure. The higher corrosion resistance is attributed to the preferred orientation, eliminating susceptible grain boundaries and reduced secondary phases.

Microstructure and Creep Behavior Property of Dissimilar Joints Between Incoloy 800HT and P91 Steel

Dissimilar joints are commonly encountered in the fabrication of high-temperature power plant components. In this current work, dissimilar joints between P91 and Incoloy 800HT have been produced using two different types of filler materials like ER505 and ERNiCr-3 with the GTAW welding process. Microstructure characterization of the weld metal reveals the presence of columnar dendritic structures. The microhardness value is higher in ER 505 filler-based weldment due to the formation of δ ferrite, M23C6 and MX precipitates. Impression creep test of dissimilar weldments has been conducted with the help of flat bottom cylindrical punch. The impression creep test has been carried out at the temperature of 873.15 K and 923.15 K with a constant load of 135 Mpa with 7200 s of holding time by using a vacuum of the order of 1 × 10−6 mbar in the heating chamber. The impression creep test result has revealed that the depth of penetration of weld with ER505 filler metal has a higher value compared to other weldments. The creep strength of the specimen is compared by the calculated activation energy of each specimen. The creep strength of the weld with ER505 filler metal (Exp No 4) is higher than that of all the other weld’s specimens. Hence, the dissimilar weld with ER505 as a filler material by GTAW process will produce better results.

Dissimilar welding of Al0.1CoCrFeNi high-entropy alloy and AISI304 stainless steel

Abstract

Very High Cycle Fatigue Behaviour of the 316L Weldment Fabricated By Laser Butt-Welding

In nuclear industry, 316L weldments are widely used in components fabricating, which usually suffer from very-high-cycle fatigue. Therefore, the very high cycle fatigue (VHCF) behaviour of 316L weldment fabricated by laser butt-welding joint were studied using an ultrasonic fatigue testing system. The microscopic examination indicated that an evident pore is observed in the weld seam and the fine columnar-dendrite structure is formed in the central part of the weld seam. Near the weld interface in the weld seam, the weld microstructure shows larger columnar-dendrite. The hardness distribution of the weldment is to coincide with the characteristic of the microstructure. In the central part of the weld seam, the hardness is the highest, and is the lowest in the base metal. Fatigue failure still occurred in the VHCF regime. The fatigue crack initiated from the internal pores caused by laser butt-welding. Although the fatigue strength of weld seam might be decreased due to the pore, the fatigue strength of the weldment was the same as the base metal in this research. That is, the defect with a certain size in the weld seam does not affect the strength of the weldment.

Epitaxy and new stray grain formation mechanism during epitaxial laser melting deposition of Inconel 718 on directionally solidified nickel-based superalloys

The epitaxy behavior and stray grains (SGs) formation in the deposit during epitaxial laser melting deposition (E-LMD) of directionally solidified (DS) superalloys were investigated. Columnar dendritic structures were obtained by epitaxial solidification on the DS substrate. The deposit also remained the orientation of the substrate. The SGs at the fusion interface, which were hardly eliminated, were attributed to different SGs formation mechanisms. The SGs were divided into GB-SGs and MC-SGs by the distribution characteristics. The GB-SGs at the low-angle and high-angle grain boundaries with a new mechanism of dynamic recrystallization induced by accumulation of thermal strain and stress under repeated spatially variable heating and cooling. The MC-SGs around the carbides were related to the misoriented cellular crystal formation caused by the varied shape of the solid-liquid interface. The columnar to equiaxed transition (CET) was another mechanism of MC-SGs formation.

Effect of Pulse Frequency on Microstructural and Corrosion Properties of Inconel 718 Gas Tungsten Arc Weldments

A study has been carried out to understand the effect of pulse frequencies on the formation of laves phase, the microstructural and corrosion properties of Inconel 718. Bead on welds was made by using gas tungsten arc welding (GTAW) in pulsed mode at different frequencies such as 2, 4, 6, 8 and 10 Hz. Varying frequencies exhibit significant changes in weld surface ripples. Microstructural analysis revealed that the welds at 2, 4 and 6 Hz showed both columnar and equiaxed dendrite structure, while at 8 and 10 Hz, the welds predominantly had equiaxed dendrites. Scanning electron microscopy results showed a reduction in continuous laves phase with the increase in pulsing frequency. Corrosion studies confirmed that the pitting resistance was increased at a higher frequency due to the reduction in laves phase in welds.