Equiaxed Dendrites Research Articles

Microstructure evolution and compressive property variation of AlxCoCrFeNi high entropy alloys produced by directional solidification

The AlxCoCrFeNi (molar radio, x = 0.3, 0.6, 0.9 and 1.2) high entropy alloys (HEAs) were directionally solidified at the withdrawal rate of 150 μm/s. The effects of Al content on microstructure evolution, element segregation as well as compressive property were investigated. It was found that the increase of Al content resulted in the formation of BCC crystal structure. Meanwhile, the microstructure of AlxCoCrFeNi HEAs transformed from dendrite to equiaxed dendrite. The Al0.3CoCrFeNi HEA formed a single FCC solid solution phase, the Al0.6CoCrFeNi HEA consisted of CoFeCr-enriched FCC solid solution phase + NiAl-enriched BCC solid solution phase, the Al0.9CoCrFeNi and Al1.2CoCrFeNi HEAs were composed of NiAl-enriched B2 solid solution phase + CoFeCr-enriched BCC solid solution phase. Morphology of solid-liquid interface was coarse dendrite or typical dendrite. As the Al content increased, the compressive strain of directionally solidified AlxCoCrFeNi HEAs decreased from 21.26% to 18.05%, and the ultimate compressive strength increased first and then decreased. The improvement of strength results from crystal structure transition and solution strengthening. As the Al contents increased from 0.6 to 0.9, the crystal structure of AlxCoCrFeNi HEAs transformed from CoFeCr-enriched FCC structure + NiAl-enriched BCC structure to CoFeCr-enriched BCCstructure + NiAl-enriched B2 structure. The strain of Al0.3CoCrFeNi HEA was higher than others, and Al0.9CoCrFeNi HEA exhibited the maximum ultimate compressive strength of 2114.57 MPa.

Influence of heat input on intermetallic formation in dissimilar autogenous laser welding between Inconel 718 and AISI 316L steel

Intermetallic formation during dissimilar welding of Inconel and stainless steel significantly hinders producing sound-quality weld. In the present work, a high-power CO2 laser welding system is employed to join Inconel 718 and AISI 316L stainless steel without using any filler materials. The finite element based thermal model provides the temperature distribution in the weld zone and consequently the solidification parameters (G/R and G.R). The micro-segregation and intermetallic formation in laser welding are correlated with these solidification parameters. The segregation of principal alloying elements triggers the formation of intermetallic phases during solidification. The presence of various phases like Laves, NbC, and TiC in the fusion zone reduces with increase in heat input. The amount of these intermetallic phases present in the weld zone is estimated by XRD analysis. With an increase in heat input, the mode of solidification changes from columnar to equiaxed dendrites that is characterized at reduced value of solidification parameter (G/R). The intermetallic Laves particle diminishes significantly (more than 90%) with a reduction of G/R parameter. In equiaxed dendrites, the segregation and formation of these secondary particles reduces to more than 40% compared to the columnar dendritic structure. Overall, the reduction in the intermetallic particles improves the joint efficiency up to 100% with an elongation of 14.2% for the equiaxed structure of the laser welding system.

Laser‐Directed Energy Deposition Additive Manufacturing of Nickel–Titanium Coatings: Deposition Morphology, Microstructures, and Mechanical Properties

To overcome the disadvantages of comparatively poor surface hardness and deprived resistance against wear of Ti–6Al–4V alloy, the protective NiTi coatings are fabricated at different processing parameters by laser‐directed energy deposition (LDED). This work presents a comprehensive study of deposition morphology, phase constituents, microstructural evolution, composition distribution, and mechanical properties of as‐fabricated NiTi deposition tracks. The relationship among processing parameters and deposition morphology, phase constituents, and mechanical properties is established. The microstructural evolution mechanism and the composition distribution on the cross section of deposition tracks are revealed. Results show that the deposition tracks are successfully formed under all laser processing parameters, and the dilution ratio increases with the increase in laser power and scanning speed. The deposition tracks consist of NiTi2, NiTi, and α–Ti. According to the microstructural analysis, planar crystals are formed between the substrate and the deposition tracks, and columnar and equiaxed dendrites are found in the deposition tracks. The average microhardness increases continuously with the increase in laser power. The deposition track prepared at laser power of 1400 W has the highest average microhardness of 732.45 HV0.2. The deposition track at this laser power exhibits excellent wear properties, with a wear rate of 2.25 × 10−6 mm3 Nm−1.

Metastable nanostructures with enhanced hardness of Ti-Ni-Al alloy through electrostatic levitation processing

• Rapid solidification of Ti-Ni-Al alloy was investigated by ESL technology. • ESL plus quenching process led to formation of multiple nanostructures. • Solute cluster, distortion phase and other nanoprecipitate were achieved. • Ti-Ni-Al alloy with metastable nanostructures was conspicuous hardened. We prepared a Ti-Ni-Al alloy comprised of metastable equiaxed (βTi) dendrites and interdendritic Ti 2 Ni phase by means of electrostatic levitation plus quenching. It exhibited an impressive improvement in hardness compared with the sample through DTA processing. By employing the double Cs corrector transmission electron microscope characterization, the coherent solute cluster and semi-coherent nano-distortion (βTi) were found to form within the metastable (βTi) dendrites, and (αTi) nanoprecipitates generated inside interdendritic Ti 2 Ni phase matrix. These dispersive metastable nanostructures and the novel (αTi) + Ti 2 Ni biphase boundary contributed cooperatively to pinning dislocation. The harmonic cooperation of precipitation strengthening, solution strengthening and fine-grained strengthening were realized to enhance the alloy.

Improving mechanical properties of austenitic stainless steel by the grain refinement in wire and arc additive manufacturing assisted with ultrasonic impact treatment

In this study, the ER321 stainless steel is fabricated by wire and arc additive manufacturing (WAAM) assisted with ultrasonic impact treatment (UIT). The mechanical properties and microstructure characterizations of ER321 stainless steel with and without UIT are investigated. It is found that grain structure of ER321 stainless steel is reduced to equiaxed dendrites (with UIT) from coarse columnar dendrites (without UIT). The UIT effectively refines grain sizes that decreases by ∼150% and homogenizes the grain structure of the deposition layers. Simultaneously, the UIT also facilitates the recrystallization that leads to the reduction of dislocation and texture densities. Furthermore, the improvement of grain structure enhances yield strength (∼10.5%), ultimate tensile strength (∼3.7%), microhardness (∼12.5%) and elongation of ER321 stainless steel. The grain boundary strengthening is the main strengthening mechanism, which leads to the yield strength increment of ER321 stainless steel under UIT condition. A way to effectively control grain structure in additive manufacturing-fabricated metal products using an UIT technique is provided in this work.

Data assimilation with phase-field lattice Boltzmann method for dendrite growth with liquid flow and solid motion

Integrating phase-field simulations and in situ observation experiments is a promising approach to better understand dendrite growth during alloy solidification. To integrate simulations and experiments, we developed a data assimilation system using an ensemble Kalman filter for dendrite growth with liquid flow and solid motion. In this system, we used the phase-field lattice Boltzmann (PF–LB) model as the simulation model. Through twin experiments in which an isolated equiaxed dendrite grows with sedimentation owing to the density difference between the solid and liquid phases, we validated that the developed data assimilation system can effectively incorporate the observation data into the PF–LB simulation. In addition, we discuss the accuracy of data assimilation in different conditions by comparing the results of columnar dendrite growth with natural convection.

Effect of high-frequency beam oscillation on microstructures and cracks in laser cladding of Al-Cu-Mg alloys

This paper provides a new method to suppress solidification cracks in laser cladding of Al-Cu-Mg alloy by high-frequency beam oscillation without additional auxiliary measures. The effects of oscillation frequency on crack suppression were investigated by analyzing the microstructure evolution. The results show that the intercrystalline precipitated phase changed from coarse and continuous line to spiderweb-like morphology with the oscillation frequency (f) increased from 50 Hz to 500 Hz, and the thermal cracking tendency was completely reduced when the density of coarse intergranular precipitates was <300 μm/mm2 (f ≥ 200 Hz). The coarse Al2Cu and Al0.92Cu1.08Mg low-melting eutectic phases formed by the Cu element segregation between the columnar dendrites constituted the crack source and crack expansion channel. The rapid and periodic motion of a high-frequency oscillating laser can promote the uniform energy distribution and drive the melt flow, which will induce the columnar to equiaxed transition. The web-like eutectic liquid film between equiaxed dendrites has a smoother reflux channel and coordinated deformation ability. When the solidification shrinkage force pulls the local liquid film, the rest of the liquid phase can return to heal the crack in time, which is the main mechanism for the solidification cracks to be reduced.

Microstructural and mechanical characterization of pre-used SA 335 P91 super-heater pipe weldment used in 70 MW CFBC boilers

The enhanced SA 335 P91 steels are specially designed to be used in the super-heaters in thermal power plants which are commonly operated at an elevated temperature. The present study involves an analysis of microstructure and the tensile property of the used P91 tubular steel of such heat exchanger in as-received conditions. The joints were fabricated by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) by employing ErNiCrMo-3 and E9018 B9 fillers. The microstructure showed the presence of flakes of martensite, formation of Nb-rich phases with an unmixed zone in the interface of the weld followed by the columnar and equiaxed dendrites in the centre of the weld zone, while employing the ERNiCrMo-3 filler in GTAW. Whereas in SMA welding, no such unmixed zone was observed while employing E9018 B9 filler except the precipitation of carbides. The metallurgical and mechanical characterization of welded joints was performed by optical microscope, scanning electron microscopy with energy dispersive spectroscopy (EDS), hardness, toughness and tensile tests. From the SEM/EDS analysis in the fusion zone of joints inferred the uniform distribution of the element with Nb phases and oxide formation in the pre used P91 steel for both the weldments irrespective of the fillers employed. Alongside the hardness of the SMA weld zone showed a higher hardness by the precipitation of M23C6 and martensitic structure compared to the GTAW weld zone which is found to be lower owing to the presence of elements such as Mo, Nb, Al and C. In both the joints, the tensile failure occurred away from the fusion zone in the parent metal and SMAW joints imparted better strength than GTAW joints. The GTAW joints showed better toughness by ensuring the ductile mode of failure in fractography studies.

Heat treatment and processing route consequences on the microstructure and hardness behavior of tungsten-containing high-entropy alloys

In this work, the effect of metallurgical changes due to heat treatment on the properties of high entropy alloys has been explored. Moreover, the effect of annealing temperature and time on the final microstructure has to be investigated thoroughly. Thus, AlCrFeMnNiWx (X = 0, 0.05, 0.1, 0.5 mol) HEAs have been examined at 800 °C, 1000 °C, and 1200 °C to understand the effect of heat treatment. The HEAs were prepared by vacuum arc melting (VAC) and powder metallurgy route. After that, X-ray diffraction and scanning electron microscopy were utilized to comprehend the microstructure transformation. Furthermore, The HEAs' mechanical characteristics and microstructural evolution were studied after annealing at various temperatures.Interestingly, a dramatic change has been observed in the microstructure in the case of arc melted HEAs. The columnar and equiaxed dendrite was observed after solidification dissolved entirely after the heat treatment at 1200 °C and left dendritic behavior. However, HEA with maximum tungsten content formed some lamellar structure additionally. On the other hand, SPSed HEAs have shown significant phase coarsening and appeared with augmented grain size. In contrast, the evidence of the temperature range of existing of the sigma phase has been verified. Furthermore, energy dispersive spectroscopy has also been employed to confirm the chemical composition and distribution of phases present in the HEAs and presented schematically. Eventually, the composition-microstructure- properties correlation has been established mean by CALPHAD results and comparing the predicted and existing phases for further understanding of phase formation.

Effect of Ag alloying on the microstructure and mechanical properties of laser welded-brazed Ti/Al dissimilar joints

A novel method to fabricate Ag alloyed aluminum weld and Ti/Al interface zone by adding silver mesh interlayer in TA2/AA5150 dissimilar joints was proposed. Microstructures and mechanical properties of the Ti/Al joints with and without silver mesh were investigated. Aluminum grains in the weld without Ag was composed of the primary aluminum and the surrounding aluminum. It was resulted from the redistribution of Mg element during aluminum solidification. As laser power decreased, the undercooling and temperature gradient at the front of solidification interface increased and the nucleation rate of aluminum also increased. It caused that the aluminum grains transformed from the equiaxed dendrites to the columnar dendrites. Mg element in the aluminum weld with Ag was enriched by the MgAg phase and the MgAgAl phase. It made the primary aluminum disappear and the active Mg element decrease. Aluminum grains with Ag element was finer than grains without Ag. The growth of aluminum dendrites was inhibited by the MgAg phase. The texture in the aluminum weld with Ag was weakened and the dendrites were not fully developed compared with the weld without Ag. Mismatch of mechanical properties between Al3Ti and Al was decreased by Ag alloying. The optimal average shear strength of the joints increased from 147.7 MPa to 171.4 MPa assisted with the silver mesh.

Numerical Analysis Growth Kinetics of Dendrite Tip during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Expedient Favorite of Undercooling-Limited Epitaxy

When multicomponent molten pool nonequilibrium solidification, the interrelationship of location-dependent dendrite tip undercooling on low heat input and optimal growth crystallography is progressively discussed over planar interface morphology stability range to unidirectionally facilitate epitaxial growth by single-crystallinity control during laser repair of nickel-based superalloy to inhibit microstructure heterogeneity. Suppression of disoriented dendrite growth and crystallography orientation deviation along columnar interface is necessary for crackless repair. Axis-symmetrical (001)/[100] welding configuration kinetically reduces dendrite tip undercooling, nucleation and subsequent disoriented dendrite growth rather than unsymmetrical (001)/[110] welding configuration. When comparison between low heat input, within which laser power is limited and welding speed is rapid, and high heat input, within which laser power is considerable and welding speed is insignificant, the former attenuates dendrite tip undercooling and morphology transition between columnar and equiaxed dendrites to stabilize epitaxy and ameliorate dendrite growth with advantageous solidification conditions, especially drastic temperature gradient and small dendrite growth velocity. Axis-symmetrical growth crystallography and low heat input are favored to mitigate size of high-undercooling region, where stray grain formation are dominant, for homologous single-crystallization of epitaxial growth with satisfactory growth kinetics of dendrite tip, and are capable of elimination of undercooling-induced overgrowth for high quality weld, instead of aggressive unsymmetrical growth crystallography and high heat input. Additionally, the achievement of low heat input with axis-symmetrical welding configuration possesses stronger resistance to unstable interface morphology and solidification cracking. When comparison between growth regions of [100] and [010] crystalline orientation, where identical heat input is kept on both sides, wider dendrite tip undercooling is mainly located on the right side than left side to insidiously exacerbate crack-vulnerable dendrite growth, which is a ubiquitous phenomenon in the adverse (001)/[110] welding configuration. The effect of low heat input on dendrite tip undercooling is spontaneously smaller than growth crystallography. Hence, during nonequilibrium solidification of weld pool, the important mechanism of crystallography-induced microstructure heterogeneity obviation due to undercooling-limited epitaxial growth is consequently provided. The theoretical predictions cogently explain the experiment results in a concise way to properly illustrate microstructure degradation phenomena in the both sides of weld by reproducible calculation of mathematical modeling.

Numerical Analysis of Nucleation and Growth of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Solidification Cracking Diminution through Single-Crystallinity Control

The effect of thermo-metallurgical factors, such as heat input and welding configuration, on solidification cracking driving forces nearby dendrite tip, such as solidification temperature range and columnar/equiaxed transition (CET) was thermodynamically and kinetically discussed with aid of comprehensive numerical analysis for multicomponent melt-pool solidification during laser processing under non-equilibrium solidification conditions to better understand problematical solidification cracking phenomena. By using (001)/[100] welding configuration, axisymmetrical distributions of columnar/equiaxed transition and solidification temperature range alongside solidification interface are homogeneously produced on both sides of weld pool. By using (001)/[110] welding configuration, nonaxisymmetrical distributions are heterogeneously produced, and are able to bring about infelicitous microstructure degradation. Unidirectional region of [001] columnar dendrite is more prone to epitaxial growth without morphology transition to conservatively better crystallography-assisted single-crystal growth. Unidirectional epitaxial growth is collapsed, and onset of stray grain nucleation and solidification cracking eventuates in [100] region of equiaxed dendrite growth. Low heat input relatively broadens portion of unidirectional columnar dendrite, where stray grain is infrequently nucleated and grown, and thus morphology transition seldom happens, as long as undercooling and solidification temperature range alongside dendrite tip are sufficient low to challengingly develop crackless dendrite growth and high-quality weld by thermometallurgy-aided single-crystallinity control. Auspicious (001)/[100] welding configuration simultaneously abates overall stray grain nucleation and constricts solidification temperature range nearby fusion boundary to wane microstructure heterogeneity. Conversely, plenteous stray grain formation is kinetically attained and extensive solidification temperature range nearby fusion boundary is thermodynamically obtained by problematical (001)/[110] welding configuration to metallurgically induce pernicious equiaxed dendrite and disintegrate dendrite growth. Moreover, the mechanism of solidification cracking diminution as consequence of appropriate optimization of thermo-metallurgical determinants during multicomponent nickel-based single-crystal superalloy melt-pool non-equilibrium solidification is also proposed. The potent consistency between the predicted and experimented results is exceedingly tenable.

On the hot isostatic pressing of Inconel 625 structures built using laser powder bed fusion at higher layer thickness

This paper reports the effect of hot isostatic pressing (HIP) on the porosity, microstructure and mechanical properties of laser powder bed fusion (LPBF) IN625 structures built at a higher layer thickness of 100 µm. It is observed that the process-induced pores/voids of volume fraction (Vf) 0.43% in as-built IN625 structures are reduced significantly to ~ 0.01% after HIP treatment. The microstructure is changed from fine columnar dendrites to coarse equiaxed dendrites. The microstructural analysis of as-built structures reveals the presence of cellular/dendritic growth along with elemental segregation of Nb, Si and C and precipitation of Nb-rich carbides, whereas coarse recrystallized microstructure along with elemental segregation of Si and precipitation of Nb, Mo and Cr rich carbides is observed in hot isostatic pressed (HIP) samples. HIP structures exhibit lower tensile strength, higher ductility and lower anisotropy as compared to LPBF built structures. There is a reduction in the Vickers micro-hardness of IN625 samples after HIP, and the values are observed to be similar to their conventional counterparts. Further, an increase in the energy storage capacity of the material is observed after HIP treatment through Automated Ball Indentation (ABI®) studies. The study paves a way to develop ~ 100% dense, defect-free and isotropic engineering components using LPBF.

An experimental investigation on the effect of gas tungsten arc welding current modes upon the microstructure, mechanical, and fractography properties of welded joints of two grades of AISI 316L and AISI310S alloy metal sheets

In this investigation, dissimilar welded joints of AISI 316 L and AISI 310S stainless steels were produced using continuous and pulsed modes current of the gas tungsten arc welding process. A filler metal type ER309L was used to strengthen the welded joints. The fracture mode of the tensile and Charpy impact test samples was studied using a field emission scanning electron microscope (FE-SEM). Results showed that the welded joints were broken in the 316 L steel side during the tensile test due to the presence of lower alloying elements in this steel compared with the AISI 310S stainless steel. As well, microhardness and Charpy impact tests results showed that changing the welding current from continuous to the pulsed one increased the values of these two mentioned attributes. Fractography analysis, performed on the fracture surfaces of both joints, showed a completely ductile fracture under both tensile and Charpy impact tests. Moreover, microstructural observations showed that the weld metal (WM) structure was austenitic-ferritic (AF) and contained columnar and equiaxed dendrites. Changing the welding current from the continuous to the pulsed one led to the transformation of the columnar dendrites to the very fine equiaxed dendrites. This welding current variation reduced the dendrite size of the WM and decreased the area of the unmixed zone (UMZ). Finally, XRD results indicated that austenite was the predominant phase in the welded joints.

Multi-phase field lattice Boltzmann model of columnar-to-equiaxed transition in entire welding molten pool

Columnar-to-equiaxed transition (CET) often occurs during the solidification of the weld. Moving molten pool and strong fluidity are two notable characteristics in this process. The former causes the deflective epitaxial growth of columnar dendrites, while the latter makes the equiaxed dendrites grow asymmetrically and continuously move, collide and coalesce in the melt. This paper proposed a multi-phase field lattice Boltzmann model to simulate the CET in the entire molten pool. The model can characterize the polycrystalline growth, compute the fluid flow, and simulate the solid motion, collision, and coalescence. It also combined the transient growth condition model and the continuous nucleation model to reproduce the moving molten pool and define heterogeneous nucleation respectively. It further combined the initial grain structure of the base metal to provide a more realistic condition for epitaxial growth. The proposed model was verified by simulating the pure motion of dendrite in lid-driven flow, the free fall of single dendrite in static flow, and the single-dendrite growths in lid-driven and channel flow. The CET in the entire molten pool was simulated under different conditions, including considering or ignoring the fluid flow and gravity, the columnar and equiaxed dendrites growing simultaneously or sequentially. In these simulations, settlement, collision, and coalescence of equiaxed dendrites were reproduced successfully. Clear asymmetric growth, dendrite aggregation effect, and solute vortex were found. Significant differences in the size, symmetry, dendrite spacing, dendrite morphology, and solute distribution were found when the melt flow was considered or ignored. The dense equiaxed dendrite belt was found in the later stage of CET. They gathered in front of columnar dendrites and dominated the following solidification. The typical mixed structure that consists of columnar and equiaxed grains was obtained after complete solidification.

Effects of Al/Zn interlayer on the solidification path and liquation cracking susceptibility of AZ31/ZK61 dissimilar magnesium alloy resistance spot welding joints

In this study, Al and Zn interlayers are added separately between AZ31 and ZK61 workpieces to investigate the method of eliminating liquation cracking in dissimilar magnesium alloy resistance spot welding (RSW). Calculation of phase diagram technique was used to analyze the effect of Al or Zn element addition on the metallurgical reactions in the nugget. The T-fS curves reveal that the (fS)nugget is higher than (fS)ZK61 during the cooling stage of the AZ31/ZK61 RSW, which triggers the liquation cracking. Adding Al or Zn elements into the nugget can change the T-fS curve of the fusion zone, thereby restraining the liquation cracking at different degrees. Microstructural analysis at the nugget/ZK61 interface shows that the cracks become discontinuous in the AZ31/Al/ZK61 samples, while the cracks are eliminated in AZ31/Zn/ZK61 samples. With the addition of interlayers, the microstructure of the nugget consisted of fully equi-axed dendrites, while the microstructure of the nugget without interlayer consisted of a mixture of columnar and equi-axed dendrites. Lap-shear test of the dissimilar magnesium alloy RSW joints shows that the mechanical properties of the samples can be improved significantly when the liquation cracking is restrained. The AZ31/ZK61, AZ31/Al/ZK61, and AZ31/Zn/ZK61 samples exhibit nugget-drop-off, partial-interfacial-partial-nugget-drop-off, and fully-interfacial failure modes, respectively.

Time invariance of three-dimensional morphology of equiaxed dendrite: A phase-field study

Dendrite morphology has a significant effect on solute segregation and fluid flow in bulk metallic materials. Therefore, the detailed morphological evolution of dendrites is important to better understand these processes. Recently, three-dimensional (3D) dendrite morphology has been analyzed using interface shape distribution (ISD) maps that are spanned by the curvedness and shape factor of the local solid−liquid interface in an Al−Cu alloy. Data were collected through in-situ observations using synchrotron radiation imaging techniques [J.W. Gibbs et al.: Sci. Rep., 5 (2015) 11824]. This methodology is quite effective for describing 3D dendrites. In this study, we thoroughly investigated the morphological evolution and the related ISD map of free-growing equiaxed dendrites in an Al−3mass%Cu alloy using a quantitative phase-field model. The ISD was found to differ depending on the degree of undercooling. Importantly, the results indicate the presence of a time-invariant feature after sufficient branching and growth of secondary arms, when the degree of undercooling is substantial enough to produce a bunch of branching. The time invariance is considered a universal feature of equiaxed dendrite growth.

The microstructure and mechanical properties of the additive manufactured AlCoCrFeNi high entropy alloy

The equimolar AlCoCrFeNi high entropy alloy (HEA) was successfully fabricated by direct energy deposition (DED), and its phase composition, microstructure and mechanical properties were discussed systematically. The DED samples showed a duplex BCC crystal structure, which is consistent with the original powder state. The microstructure of the DED samples transferred from columnar grain structure into equiaxed dendritic structure along building direction from bottom to top. What's more, the crystallographic orientation in different positions of the DED samples showed an apparent difference. The columnar grain structure presented a strong texture in building direction at the bottom, while equiaxed dendritic structure exhibited random crystallographic orientations at the top. The equiaxed dendrites structure in the DED samples is comprised of dendrites (DR) and interdendrites (ID) regions due to elemental segregation. Meanwhile, the fine microstructure of DR and ID regions is discovered to be nano-sized spherical precipitates morphology and weave-like morphology, respectively. Furthermore, compared with AlCoCrFeNi HEA produced by conventional casting method, the mechanical properties of the DED samples display a superior combination of strength and ductility. Therefore, this research could provide an effective theoretical guidance for the preparation of high strength-ductility of HEAs.

Laser-cladding and robotic hammer peening of stainless steel 431 on low alloy steel 4140 for surface enhancement and corrosion protections

Surface enhancement and coating are distinct methods that have long been developed for protecting metallic components from corrosion, erosion, and wear in aggressive working environments. The former is to engineer the surface and near-surface of the base material towards desired performance improvements. In comparison, a coating is to deposit extrinsic materials with better-desired performance direct on the surface of the base material. Recent developments in additive manufacturing rendered increased interest in the hybrid process of surface enhancement and coating. Here, we report laser-cladding (LC) and robotic hammer peening (RHP) of stainless steel (SS431) layers on 4140 alloy steel substrates for corrosion protection. Materials and structural studies reveal that the LC-SS431 layers are dominated by crack-free equiaxed dendrites. Their hardness and compressive residual stress have been apparently improved by the RHP process without introducing any interface degradations. Corrosion test in an aqueous NaCl (3.5 wt.%) electrolyte shows that proper RHP processes can enhance the corrosion resistance of the SS431 coatings, and the enhancement is more than that of the substrate.

Study of microstructure and wear properties of laser borided Inconel 718

• Effect of laser power density on microstructure and wear properties of laser borided Inconel 718 is studied. • The borided layer is consisting of borides of Ni, Cr and Fe together with γ (Ni-Cr-Fe) matrix phase. • Formation of boride phases leads to the significant improvement in hardness (∼5 times) and wear resistance (∼8 times). • In the borided layer, microstructural morphology is changed from cellular to equiaxed dendrites. • The phase stability of borided layer is discussed based on phase equilibria information and thermo-kinetic theory. Effect of laser power density (LPD) on microstructure and tribological properties of laser borided Inconel 718 is studied. Boriding is carried out using laser powder bed fusion based additive manufacturing process at three LPDs of 17.0, 22.6 and 28.3 kW cm −2 . Laser power density played significant role on surface roughness, homogeneity and depth of the borided layer. Higher laser power density led to lower surface roughness, improved homogeneity and higher depth of borided layer. Structural, chemical and microstructural characterization of the borided layer confirmed the formation of Nickel boride (Ni 2 B/Ni 3 B), Chromium boride (CrB/Cr 2 B) and Iron boride (FeB/Fe 2 B) along with the matrix γ (Ni-Cr-Fe) phase in borided layer. The content of γ phase in borided layer is found to be increased with increase in LPD due to dilution effect. The presence of different boride phases in the borided layer caused significant improvement in hardness (∼5 times) and wear resistance (∼8 times) than the substrate Inconel 718 for LPD of 17.0 kW cm −2 . At higher LPD, due to increased content of soft γ phase, both hardness and wear resistance are reduced. Across the borided layer, microstructural morphology is changed from cellular to equiaxed dendrite with a clear columnar to equiaxed transition. The formation of different boride phases and microstructural evolution in the borided layer is discussed in view of available phase equilibria information of Ni-Cr-Fe-B system and thermo-kinetic theory. The findings of present study will be helpful in developing the Ni baser super alloys with improved tribological properties for advanced technological applications under extreme service conditions.