Columnar Dendritic Structure Research Articles

Optimizing Selective Laser Melting of Inconel 625 Superalloy Through Statistical Analysis of Surface and Volumetric Defects

This article delves into optimizing and modeling the input parameters for the selective laser melting (SLM) process on Inconel 625. The primary aim is to investigate the microstructure within the interlayer regions post-process optimization. For this study, 100 layers with a thickness of 40 µm each were produced. Utilizing the design of experiments (DOE) methodology and employing the Response Surface Method (RSM), the SLM process was optimized. Input parameters such as laser power (LP) and hatch distance (HD) were considered, while changes in microhardness and roughness, Ra, were taken as the responses. Sample microstructure and surface alterations were assessed via scanning electron microscopy (SEM) analysis to ascertain how many defects and properties of Inconel 625 can be controlled using DOE. Porosity and lack of fusion, which were due to rapid post-powder melting solidification, prompted detailed analysis of the flaws both on the surfaces of and in terms of the internal aspects of the samples. An understanding of the formation of these imperfections can help refine the process for enhanced integrity and performance of Inconel 625 printed material. Even slight directional changes in the columnar dendrite structures are discernible within the layers. The microstructural characteristics observed in these samples are directly related to the parameters of the SLM process. In this study, the bulk samples achieved a microhardness of 452 HV, with the minimum surface roughness recorded at 9.9 µm. The objective of this research was to use the Response Surface Method (RSM) to optimize the parameters to result in the minimum surface roughness and maximum microhardness of the samples.

Structures in grain-refined directionally solidified hypoeutectic Al-Cu alloys: Benchmark experiments under microgravity on-board the International Space Station

Benchmark solidification experiments were successfully performed under microgravity conditions on-board the International Space Station (ISS) within the ESAprogramme CETSOL (Columnar-to-Equiaxed Transition in SOLidification Processing). Cylindrical samples of grain-refined Al-4wt.%Cu, Al-10wt.%Cu and Al-20wt.%Cu alloys were directionally solidified in a gradient furnace to investigate columnar and equiaxed dendritic growth structures as well as the columnar to equiaxed transition under diffusive conditions. The determination of temperature gradients; interface velocities; and cooling rates at liquidus, solidus, and eutectic front positions provides well-defined thermal experimental characterization. The evaluation of the flight samples demonstrates that no significant macrosegregation along the sample axis occurred and no radial effects were observed. Therefore, purely diffusive solidification behaviour without any residual melt convection can be assumed for these microgravity experiments. The analyses of the microstructure in longitudinal cross-sections show dendritic structures without any pore formation and the averaged eutectic fraction is largely constant along the sample. The samples of refined Al-4wt.%Cu alloy show a sharp CET from columnar dendrites to a fine equiaxed steady-state grain structure whereas in the samples of refined Al-10wt.%Cu and Al-20wt.%Cu alloy, only equiaxed dendritic grain growth is observed. A quantitative analysis of the equiaxed grain morphology shows, that the shapes of the equiaxed dendrites depend on the applied temperature gradient, but the grain sizes in radial and longitudinal directions are identical. Therefore, a fully equiaxed dendritic growth structure without dendrite elongation was obtained. Compared to experiments in microgravity with non-refined Al-Cu alloys the average equiaxed grain size is about three times smaller.

Microstructure and mechanical properties of Al-Zn-Mg-Cu alloy fabricated by multi-wire arc-based directed energy deposition

ER5356, ER2319 and Zn wires were synchronously fed using a multi-wire gas metal arc directed energy deposition (DED) system. This allowed the successfully fabrication of Al-6.0Zn-2.5Mg-1.5Cu alloy. The microstructure and mechanical properties of the fabricated parts were studied. It is shown that the main precipitated phases are η and nanoscale η'. The upper and lower parts of the deposited parts are composed by coarse equiaxed grains and columnar dendrite structures, respectively. The average microhardness of the fabricated alloy was 118.5 HV. The ultimate tensile strength and elongations in the perpendicular to the building direction (BD) and parallel to the BD were 267 MPa and 2.7 %, 238 MPa and 2.3 %, respectively. The average tensile strength is higher than that of 7075-O alloy. The fracture mode of the samples composed brittle and ductile features, although the former dominated. This multi-wire arc DED approach provides a new choice for the in-situ synthesis of Al-Zn-Mg-Cu alloys.

Effect of Mn addition on microstructure and corrosion behavior of AlCoCrFeNi high-entropy alloy

In this study, the effects of Mn content on the microstructure and corrosion resistance of AlCoCrFeNiMnx (x = 0, 0.25, 0.5, 0.75, and 1) high-entropy alloys were investigated. The results indicate that with increasing Mn content, the HEAs consistently comprise nanophase BCC geometry with different shapes embedded in the ordered B2-phase matrix, and the morphology of the microstructure changes from a coarse dendritic structure to a fine dendritic structure, then to a completely columnar dendrite structure. The microhardness of the HEAs increases gradually to 620 HV0.2 when x = 1, owing to the solid-solution and fine-grain strengthening. The corrosion resistance of the HEAs in 3.5 wt% NaCl solution first increases and then decreases, reaching a maximum with the lowest self-corrosion current density of 2.869 × 10−8 A∙cm−2 when x = 0.5. Improvement in corrosion resistance is related to a reduction in degree of composition segregation in the microstructure of HEAs, which leads to the formation of a dense and complete passivation film on the surface of the alloys. However, when the Mn content is in the range of 0–0.5, the pitting potential of the HEAs gradually decreases, although the passivation current density also gradually decreases. Higher Mn content (x > 0.5) leads to a transition of the HEAs from passivation to active dissolution. This indicates that the stability of the passivation film on the HEAs decreases with higher Mn content. XPS analysis revealed a gradual decline in the relative content of Cr2O3 and a gradual increase in the presence of unstable oxides such as MnO and Mn2O3 in the passivation film, leading to this phenomenon. The present work reveals that the addition of Mn in the AlCoCrFeNi HEA affects not only the microstructure and composition segregation but also the composition and stability of the passivation film, leading to a non-monotonic variation in the corrosion performance of the present HEAs with respect to the Mn content.

Microstructure evolution and mechanical properties of friction welded IN713LC and AISI 4140

The microstructure and mechanical behavior of rotary friction-welded IN713LC nickel-base superalloy with AISI 4140 steel have been investigated. The microstructure of IN713LC changed from a columnar dendritic structure to equiaxed grains due to dynamic recrystallization with considerable change in in-grain misorientation, morphology, and volume fraction of gamma prime (γ′) precipitates. The observations indicate a counteracting effect of heat input and degree of deformation which resulted in achieving minimum grain size at an intermediate friction pressure. Phase transformation was observed in the weld interface/thermomechanically affected zone region of AISI 4140 steel. These changes in microstructure contribute to the variation in the microhardness across the weld joint. Furthermore, a maximum weld joint tensile strength of 922 MPa was observed at an intermediate friction pressure of 110 MPa.

Strengthening effect of laser peening on fatigue performance and life extension of laser cladded IN718 nickel-based alloy parts

Strengthening effects of laser peening (LP) on fatigue life extension of IN718 nickel-based alloy parts repaired by laser cladding (LC) were investigated. Special attention was paid on the microhardness, residual stress, microstructures and fatigue performance. The results show that LP increases the microhardness and compressive residual stress of LCed samples and the induced severe plastic deformation closes the metallurgical defect. In addition, LP changes the microstructure of LCed parts, transforms a large number of columnar dendritic structures into cellular dendritic structures. The grains in the subsurface were refined and a high-density dislocation structure is produced as well. As a result, while compared with the matrix sample, the fatigue life of LPed sample with laser power density of 11.30 GW/cm2 is increased by 54.9 %. Finally, synergistic strengthening mechanism of LP on LCed IN718 nickel-based alloy parts through defective pore closure effect, high-amplitude residual stress, and microstructure evolution was revealed.

Investigation of mechanical properties and microstructure characterisation of MMA-MIG-TIG-SAW welded EN8 steel joints via shot blasting method

The industries of energy, transportation, and other engineering components all benefit greatly from the use of medium carbon (C) steel EN8. For these applications, welding is typically used to join the material. Conventionally welded joints, however, struggle to meet the exacting standards of industrial applications. In this comprehensive study, the tensile strength, microhardness, and microstructure of joints made of medium C EN8 steel using manual metal arc (MMA), metal inert gas (MIG), tungsten inert gas (TIG), and submerged arc (SAW) welding were investigated. These weld joints were then subjected to the shot blasting method to determine the effect on the mechanical properties of these weldments, Different response characteristics have been studied in relation to the shot blasting with various time periods. A detailed characterisation of the samples has been carried out optical microscopy, surface electron microscopy (SEM), energy dispersive spectroscopy (EDS), optical emission spectroscopy (OES), microhardness, and tensile testing with fractographical analysis. The maximum percentage increase in the microhardness in fusion zone (FZ) occurred in case of TIG welded specimens, i.e. 29.63% after 25 min of abrasive blasting period. However, the maximum change in the tensile strength, i.e. 10.53% has been observed for MMA welded specimens after 15 min of abrasive blasting period. Microstructural analysis revealed TIG has a columnar dendritic structure, MIG and SAW have an equiaxed dendritic structure. MMA has a cellular dendritic structure. The outcomes revealed that abrasive blasting improves the mechanical characteristics of industrial MMA, MIG, TIG, and SAW welded specimens.

CHARACTERIZATION OF CORROSION BEHAVIOR OF INCONEL 617 WELDED JOINTS USING CMT-GMAW MULTI-CONTROL WELDING

This work addressed the effect of CMT-GMAW multi-control welding of Inconel 617 alloy with different wire feed rates (8.9, 9.4, and 9.9 m/min) on the microstructure, hardness, and electrochemical properties. The weld joints are composed of columnar dendritic structure with cellular crystals. Electron backscattered diffraction (EBSD) study showed equiaxed dendrite at the center of weld metal with growth direction perpendicular to the fusion boundary. The weldments showed diffraction peaks at 43.53[Formula: see text], 50.12[Formula: see text], and 74.82[Formula: see text], and these peaks mainly represent gamma ([Formula: see text]) and gamma prime ([Formula: see text]) phases along with the carbide peaks of Ti (C, N), M[Formula: see text]C6 and M6C. The Base metal (BM) had a lower hardness (232 ± 10 HV[Formula: see text]) and lower corrosion rate (0.212 mpy) than the weld joints. The increase in wire feed rate (WFR) results in the decrease of microhardness (267 ± 5 − 251 ± 6 HV[Formula: see text]) and increase in corrosion rate (1.833-28.140 mpy). The base metal exhibited higher potential ([Formula: see text]) and lower current density ([Formula: see text]) than the weld joints. As wire feed rate (WFR) increases, heat input increases; solidification time increases, grain boundaries coarsen, resulting in a lower grain boundary (GB) density, and hence increased carbide precipitation and segregation in weld zone leading to higher stable anodic current density, which caused corrosion resistance to deteriorating. The BM was more corrosion resistant than the weld joints. The metallurgical and physical changes caused by the welding process affect the corrosion resistance of the weld joints. This leads to the weld metal corroding faster than the base metal.

Effect of Current Waveforms during Directed Energy Deposition of 4043 Aluminum Alloy on Microstructure, Hardness, and Wear of Alloy.

Wire arc additive manufacturing (WAAM) was employed to fabricate 4043 aluminum alloy walls. To investigate the effects of sinusoidal, triangular, and rectangular waveforms of alternating current (AC) and their transients on the wall geometry, microstructure evolution, hardness, and wear properties were evaluated. The root mean square (RMS) current value was maximum for the rectangular and minimum for the triangular waveform. The section produced by the triangular waveform had the highest height-to-width ratio, indicating that this waveform can be a favorable choice for creating components using WAAM. The optical micrographs of the transverse cross-section of the printed sections revealed the grain structure produced with this waveform to be heterogeneous, having a columnar dendritic structure at the bottom and equiaxed at the top portion. The waveforms also had an impact on the hardness and wear characteristics of all the walls, which were attributed to their cooling rate.

Microstructural analysis of microwave processed EWAC + Cr3C2 cladding on SS-304 substrate

The surface layer of SS-304 substrate was modified by a composite cladding of EWAC + Cr3C2 (20%, 40%, & 60% by weight) powder performed by utilizing the concept of microwave hybrid heating technique. The experimental testing was carried out in a home microwave applicator with an Al2O3 shield. Claddings of around 1 mm thickness were produced by exposing materials to microwave irradiation at a frequency of 2.45 GHz and 900 W of power. The entire system was exposed to microwave environment for about 360 s. The microstructural findings showed that the clads were produced via efficient metallurgical bonding, which entailed melting of clad powder along with diffusion of the substrate surface layer. The clads showed hard carbides in a columnar dendritic structure surrounded by a softer nickel-iron based matrix. The XRD spectrum shows that the clad also contains solid solution of Fe/Ni, chromium carbide (Cr23C6 and Cr3C2) and various intermetallic phases like NiSi, Ni3Si, and FeNi3. The average Vickers microhardness of the microwave processed clads was observed to be 700 ± 31 HV for EWAC + 60% Cr3C2, 580 ± 28 HV for EWAC + 40% Cr3C2 and 460 ± 23 HV for EWAC + 20% Cr3C2 respectively. The observed microhardness for all cases was higher than the substrate microhardness (260 ± 5 HV), which indicates the usefulness of the cladding for wear resistance applications.

Dependence of Eutectic Fraction on Inclination Angle of Columnar Dendrite Structures in Al-3 mass% Cu Alloy Analyzed by Phase-field Simulation

Dependence of Eutectic Fraction on Inclination Angle of Columnar Dendrite Structures in Al-3 mass% Cu Alloy Analyzed by Phase-field Simulation

Microstructure, strain hardening behavior, segregation and corrosion resistance of an electron beam welded thick high-Mn TWIP steel plate

In the present work, the microstructure, strain hardening behavior, segregation and corrosion resistance of a high-Mn TWIP steel joint fabricated by electron beam welding (EBW) method were investigated. It was observed that in the fusion zone (FZ), columnar dendritic structure and cellular structure were formed and the texture component of ⟨100⟩∥X-axis was developed. Annealing twinning events hardly occurred during the solidification of weld metal since few Ʃ3 and Ʃ9 grain boundaries existed in the FZ. The 0.2% yield strength (YS) of the joint is comparable to that of the base metal (BM). Deformation twinning played an important part in increasing the strain hardening rate. C, Mn and Si elements were found to segregate in the interdendritic regions, nevertheless, Al element was discovered to concentrate in the centres of the dendrites. The microsegregation of alloying elements and the formation of (Fe,Mn)3C cementites increased the corrosion sensitivity of interdendritic regions, which led to the occurrence of preferential corrosion in these regions and then reduced the corrosion resistance of the joint.

Experimental investigations on the multi-layered SS316L wall fabricated by CMT-based WAAM: Mechanical and microstructural studies

In this research work, the cold metal transfer (CMT)-based wire-arc additive manufacturing (WAAM) process was used to fabricate a multi-layered wall using SS316L wire electrode. The wall was built with optimised CMT-WAAM parameters derived from a multi-variable regression analysis. The mechanical and microstructural characteristics of the multilayered wall were studied at the bottom, middle, and top regions. The macrostructure in all three regions of the wall indicates that the layers have completely fused together. The optical micrograph displays fine equi-axed dendrites in the bottom region, columnar grains with residual δ-ferrite in the middle region, and columnar dendritic structure in the top region. The tensile and impact test values of the bottom, middle, and top regions of the wall show that the values are comparable to their wrought counterparts. It was observed that the hardness value decreased with an increase in the build height. Scanning electron microscopy (SEM) analysis of the fractured surfaces of the tensile and impact test specimens showed dimples, indicating high ductility of the as-built wall and successful fabrication.

Studies on the Effect of Substrate Preheating, Interlayer Dwell, and Heat Treatment on Microstructure, Residual Stress, and Mechanical Properties of IN625 Superalloy Built by Direct Metal Deposition

Abstract In this investigation, IN625 alloy samples were processed by the directed energy deposition (DED) approach under various metal deposition strategies such as substrate preheating, interlayer dwell and with combined substrate preheating, interlayer dwell, as well as postheat treatment. The processed sample’s microstructural characteristics, residual stress, microhardness, and tensile properties are assessed in comparison to the manufacturing strategies. Rapid heat dissipation caused finer microstructure near the substrate. There is a growth of columnar grain structure epitaxially in the build direction. The progressive microstructure change seen in the build direction across the cross section was due to the gradual rise of heat accumulation between subsequent layers. The interdendritic zones contained Laves phases. Laves phases have a high Nb, Mo, as well as Si content, according to the energy-dispersive spectroscope (EDS) spectrum. The field emission scanning microscopy (FESEM) microstructural morphology of the deposited samples after their postheat treatment has shown a new microstructure with the combination of equiaxed (recrystallized) and columnar dendritic structure with the reconstruction of columnar dendritic solidification microstructure into equiaxed grains. Heat treatment caused the Laves phases to dissolve in the matrix of IN625 alloy, which led to the precipitation of nanometric γ″ phases. The deposition strategies with substrate preheating significantly decreased the residual stress with moderately improved mechanical properties. The combination of substrate preheating, interlayer dwell, and postheat treatment has shown an outstanding reduction of residual stress along with a remarkable improvement in tensile strength with the retainment of an equivalent ductility compared with other strategies.

Investigating the effects of double-pass TIG welding on the mechanical and microstructural properties of AISI 1008 steel

ABSTRACT Most premature failures of double-pass weldments are associated with limited knowledge of the mechanical and microstructural evolution following the welding process. Thus, this study aims to investigate the effect of double-pass tungsten inert gas welding on the mechanical and microstructural properties of type AISI 1008 mild steel joints. Double-pass TIG welding in butt joint configuration was adopted to weld the plates maintaining a 2.5 mm root gap in between plates. The double-pass tungsten inert gas welded AISI 1008 joints showed improved ultimate tensile strengths in contrast to the unwelded counterpart, with the least joint efficiency of 82.67 % (weld W3). An increase in the heat input led to a corresponding increase in the weld metal hardness, whereas the hardness of the heat-affected zones increased first and then decreased. The internal geometry of the welds reveals an increase in the bead width, height of reinforcement, width of the heat-affected zone, and weld metal area as the heat input increases. Lastly, the microstructural evolutions of the weld metal zones were primarily dominated by widmanstatten ferrite, acicular ferrite, and pearlite structures, accounting for the enhanced tensile strength and hardness of the joints. Meanwhile, the heat-affected zones were characterised by coarse columnar dendrite structures resulting in lower hardness values. This study gives an insight into the mechanical and microstructural properties of the industry’s most frequently used steel grade, which has not been given much attention in past studies. The research finding enriches the welding theory of A1SI 1008 steel for industrial and academic benefits.

Evolution of microstructure and wear mechanism in laser clad Al0.5CoCrFeNiSi0.5Tix coatings in response to the Ti addition

In this work, the microstructural evolution and its influence on the wear mechanism with the increasing Ti content are investigated in double-layer Al0.5CoCrFeNiSi0.5Tix coatings. The results indicate that FCC solid solution as the matrix phase of first layer changes from cellular structure to columnar dendritic structure due to the addition of Ti. By contrast, BCC solid solution consisting of disordered BCC and ordered B2 are found to be the matrix phase of second layer. Ti promotes the transition of ordered B2 phase from speckle-shape to stripe-shape, and then to square-shape. Besides, Ti facilitates the in-situ synthesis of TiN and TiSi2 in the Ti-containing coatings. Hardness of the coatings is dramatically improved due to the strengthening effect caused by Ti. The acting wear mechanism changes from abrasive wear of coating Ti0 to oxidational wear and mechanical mixing of Ti-containing coatings. The wear weight loss of the coatings decreases drastically with the increasing Ti content, indicating that the microstructural evolution caused by Ti addition has a positive influence on the wear resistance.

Investigations of Microstructure and Mechanical Properties in Wire + Arc Additively Manufactured Niobium–Zirconium Alloy

Herein, the feasibility of the gas tungsten arc welding‐based wire + arc additive manufacturing process for fabricating thin wall structures of niobium‐1 wt% zirconium (NbZr1) alloy is investigated. Three different heat input conditions (low, medium, and high) have been selected for fabricating it. The microstructure is characterized by using optical microscopy, scanning electron microscopy, X‐ray diffraction, energy‐dispersive spectroscopy, and electron backscattered diffraction (EBSD). The microstructure shows the columnar dendritic structure elongated in the build direction. No cracks or porosity are observed in the structure. Average Vickers hardness for low, medium, and high heat input conditions are 146.6, 162.1, and 163.5 HV, respectively. There is an increasing trend of microhardness value along the deposition height, which can be attributed to the difference in secondary dendritic arm spacing and the formation of precipitates. The tensile strength of the specimen is comparable to the conventional and additively manufactured structures. EBSD analysis confirms that possible subgrains are responsible for good mechanical properties at room temperature. In the majority of the tensile samples, the failure mechanism has been identified as a ductile fracture. The mechanical characteristics fluctuate with locations in each of the thin walls, suggesting anisotropy in the deposits.

Prediction of the primary dendritic arm spacing in the laser metal deposition of Inconel 718 superalloy using the numerical and experimental techniques

In order to obtain the knowledge about the formation of the oriented structure in the additive manufacturing process, it is required to gain an accurate understanding about the formation and growth of grains. Hence, in this paper, the simulation of laser metal deposition was performed by the volume of fluid technique to predict the primary dendritic arm spacing, fluid flows, and geometry of the deposited layer. Moreover, the laser metal deposition of the Inconel 718 superalloy was performed experimentally to verify the results of simulation. From the results of simulation, a maximum error of about 8% was observed in the prediction of the geometry of the deposited layer. In addition, the difference between the value of primary dendritic arm spacing in the experimental and simulation results was about 14%. The results have also shown that a cellular dendritic structure was formed when the ratio of the temperature gradient to the cooling rate was more than 9 × 108, while when this ratio was less than 9 × 108, a columnar dendritic structure was formed. The results of simulations and experiments showed that an increase in the laser power from 350 to 450 W led to an improvement of about 9.5% in the size of primary dendrite arms due to the reduction in the temperature gradient, while the increase in the scanning speed from 3 to 5 mm/s resulted in a reduction of about 21% in the size of primary dendrite arms due to the faster cooling rate.

Experimental investigations and parametric effects on depositions of super duplex stainless steel in wire arc additive manufacturing

Identification of suitable processing conditions is essential for successful fabrication of engineering components of any material by wire arc additive manufacturing (WAAM). Objectives of this article are to determine optimal process parameters for deposition of super duplex stainless steel (SDSS) ER2594 in gas metal arc welding (GMAW)-based WAAM using response surface methodology (RSM) and characterisation of the deposited materials. The developed RSM-based models of deposition geometry in terms of bead width and bead height, and deposition rate based on experimental data showed higher values of width, height and deposition rate for increasing voltage and wire feed speed due to higher heat input onto the tip of wire and decreasing torch travel speed and gas flow rate. Results of tensile tests of the deposited SDSS exhibited increase of mechanical properties like yield strength, ultimate tensile strength and elongation by 28.8%, 14.3% and 84%, respectively, compared to that of the cast SDSS material. Microstructural characterisations of the deposited SDSS wall using SEM exhibited a typically higher ratio of austenite to ferrite phases (77:23) with oriented columnar dendritic structure compared to that of the base material (50:50). Pre-heating of the substrate could enhance the deposited bead size with higher width due to higher energy available for melting and spreading of the beads.

Enhancing mechanical properties and degradation performance of Mg−0.8wt.%Ca alloy by directional solidification

The mechanical properties and degradation performance in Hank's solution of the directionally solidified (DSed) Mg−0.8Ca (wt.%) alloys were investigated and compared with those of the as-cast alloy. The microstructure was studied by OM, SEM, TEM, and EBSD. It is found that the columnar grains in DSed alloys have a consistent growth orientation of <1120>. The mechanical strength and ductility of Mg−0.8Ca alloy are significantly improved after directional solidification. Besides, the corrosion resistance of Mg−0.8Ca alloy in Hank's solution is dramatically enhanced after directional solidification. The Mg−0.8Ca alloys in Hank's solution mainly undergo microgalvanic corrosion, and the corrosion products consist of Mg(OH)2, (Ca, Mg)3(PO4)2, and hydroxyapatite (HA). The superior properties of the DSed Mg−Ca alloy are beneficial from the columnar microstructure and the redistribution of the eutectics. The DSed Mg−0.8Ca alloy with columnar dendritic structure is expected to be used as biodegradable materials.