Columnar Dendritic Microstructure Research Articles

Effect of Composition and Pouring Temperature of Cu-Sn on Fluidity and Mechanical Properties of Investment Casting

The composition and pouring temperature are important parameters in metal casting. Many cast product failures are caused by ignorance of the influence of both. This research aims to determine the effect of adding tin composition and pouring temperature on fluidity, microstructure and mechanical properties including tensile strength and hardness of tin bronze (Cu-Sn). The Cu-Sn is widely used as employed in the research is Cu (20, 22 and 24) wt.%Sn alloy using the investment casting method. Variations in pouring temperature treatment TS1 = 1000°C and TS2 = 1100°C. The mold for the fluidity test is made with a wax pattern then coated in clay. The mold dimensions are 400 mm long with mold cavity variations of 1.5, 2, 3, 4, 5 mm. Several parameters: increasing the pouring temperature, adding tin composition, decreasing the temperature gradient between the molten metal and the mold walls result in a decrease in the solidification rate which can increase fluidity. The α + δ phase transition to β and γ intermetallic phases decreases fluidity at >22wt.%Sn. The columnar dendrite microstructure increases with the addition of tin composition and pouring temperature. The mechanical properties of tensile strength decrease, hardness increases and the alloy becomes more brittle with increasing tin composition.

High temperature oxidation behaviour of NiCrAlY clad developed on SS-304 through microwave irradiation

The current study investigates the underlying causes of the microstructural features, microhardness and high-temperature oxidation behaviour of the NiCrAlY powder cladded stainless steel (SS-304) substrate. The cladding was performed by utilizing an industrial multimode microwave apparatus, operating at 1.1 kW and 2.45 GHz with a 900 s processing time. The fabricated clad layer unveiled perfect metallurgical integration with the substrate and was noticed to be free from obvious flaws, including cracks or improper bonding at the clad-base material interface. The deposited clad layer revealed an equiaxed dendritic microstructure at the top while a typical columnar dendritic microstructure at the centre. The X-ray diffraction (XRD) examination of the clad sample revealed the existence of intermetallic compounds like γ′-Ni3Al, and Al5Y3O12 in addition to a solid solution of Iron (Fe) and Nickel (Ni) and chromium (Cr) enriched matrix. The microhardness of the clad layer was noticed to be higher than that of the uncladded SS-304 specimen. High-temperature oxidation investigation performed at 900 °C in air revealed weight gain by all the samples (bare and clad) in a parabolic fashion, initially for around 10 cycles of oxidation, beyond which the weight gain is noticed to saturate. The maximum oxidation resistance exhibited by the clad sample was credited with having intensified α-Al2O3 in the surface layer.

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.

Influence of growth velocity on fragmentation during directional solidification of Al – 14 wt.% Sn alloy studied by in-situ synchrotron X-radiography

Directional solidification of Al – 14 wt.% Sn alloy was investigated by means of synchrotron X-radiography at the European Synchrotron Radiation Facility (ESRF). The dynamics of the solidification microstructure formation was studied in-situ and in real-time. In the present manuscript, a special attention is given to the influence of increasing growth velocity on the fragmentation phenomenon of dendrite arms, which has an important role and impact on the final grain structure. Fragmentations of both secondary dendrite arms and more strikingly part of primary trunks have been observed. The experiments show that the number of fragments increases concomitantly with the solidification growth rate until it reaches a limit value after which fewer or no more fragments are observed. The possible origins of the fragmentation are discussed in detail and the likelihood of each of these potential mechanisms is estimated to understand the reason of both primary and secondary arm detachment. It is shown that the fragmentation in a transient regime is correlated to the variations of the solute accumulation, latent heat rejection and the curvature of the attachment neck during solidification. Moreover, during these experiments, Columnar-to-Equiaxed Transition (CET) induced by the fragmentation phenomenon is observed. The velocity of the dendrite fragments due to the buoyancy force is a major parameter to control the CET and our results show that a reverse Equiaxed-to- Columnar Transition can even be observed. Once the growth velocity is of the same order or higher than the fragment flotation velocity, the fragments are trapped within the columnar dendritic microstructure, and CET cannot take place or the equiaxed regime cannot be sustained.

Effect of laser pulse power on solidification cracking susceptibility in surface processing of a high carbon high chromium tool steel

High carbon, high chromium tool steels are deep hardening steels widely used for cold forming dies. A 400 W Nd:YAG laser was employed to melt the surface of the AISI D2 tool steel with 5 and 6 ms single pulses of laser at 4–10 J energy levels. The microstructural study showed that complete dissolution of large primary carbides occurred during melting, but the cooling rate was too fast to form any primary carbide. Increasing the laser energy (peak power) led to a coarser dendritic microstructure and a reduction in solidification cracking intensity. The analyses indicate that the coarser dendritic structure has promoted the backfilling effect. However, despite having a coarser dendritic structure, the material laser processed with a preheat of 300 °C showed the most severe crack intensity due to the provision of sufficient time in cooling for the formation of a network of brittle carbides in the inter-dendritic regions. The HAZ was found to be sound although reaching a hardness of 750 HV. • A fine columnar dendritic microstructure was formed by laser remelting, which became coarser with increased laser peak power. • Facilitation of the backfilling effect in higher laser peak powers reduced the intensity of solidification cracks.. • The formation of the inter-dendritic carbide phase in the preheated sample increased the solidification cracking intensity.

Metallurgical, mechanical and corrosion characteristics of vibration assisted gas metal arc AA6061-T6 welded joints

Microstructure, mechanical properties and corrosion performance of gas metal arc AA6061-T6 aluminum alloy welded joints under the effect of mechanical vibration during welding have been investigated. The vibration frequencies, f used in this study were in the range of 0-500 Hz. Subsequently, several experiments were performed including microstructural observation, microhardness measurements, tensile tests, fatigue crack growth tests combined with fractography study and electrochemical polarization tests in 3.5% NaCl solution. Results indicated that the application of mechanical vibration increased tensile strength of the welded joints up to 83.3 % higher than the weld without vibration achieved at a frequency of 300 Hz. It was found that the strengthening mechanisms in the vibrated weld joints occurred mainly by the grain refinement of columnar dendritic and equiaxed dendritic microstructures in the weld metal region. Such microstructures could contribute to fatigue crack growth inhibition in the weld metal. Based on electrochemical corrosion tests, the vibration assisted weld joints were found to have better corrosion resistance than as-welded weld joint. The improved corrosion performance was likely attributable to suppression of intermetallic compound formation such as Mg2Si by mechanical vibration during weld solidification.

Effects of shear flows on columnar dendritic microstructure during rapid solidification of IN718 alloy: A cellular automaton-lattice Boltzmann modeling study

The growth of columnar dendrites during rapid directional solidification of IN718 alloy in the presence of shear flows is simulated by using a coupled cellular automaton-lattice Boltzmann (CA-LB) model. The developed CA-LB model is benchmarked by the mesh independency of the steady-state dendrite growth velocity and the primary dendrite arm spacing (PDAS) under different cooling rates. The CA-LB model is then applied to simulate the columnar dendrite growth under different shear flows and cooling rates in the conditions of rapid solidification. The values of PDAS under different conditions are determined from the simulation results, and are correlated with the shear flow rate (Ush) and cooling rate (CR). A unified fitting formula is developed to represent the effects of Ush and CR on the evolution of PDAS. The PDAS is found to decrease with CR increasing, but increase with Ush increasing. Moreover, it is revealed that the influence of Ush on PDAS becomes stronger as CR increases.

Microstructural administered mechanical properties and corrosion behaviour of wire plus arc additive manufactured SS 321 plate

Wire plus arc additive manufacturing (WAAM) technology with higher deposition rate and efficient material utilization was employed to fabricate a stainless steel 321 (SS 321) wall for the first time. In this work, the microstructural characteristics, mechanical properties and corrosion performance of as-built SS 321 were evaluated. The micrographs confirmed the presence of columnar and equiaxed dendrites along the building direction, and recrystallization of grains was noticed due to the re-melting of former layers. The microstructure was dominantly austenite with a small fraction of ferrite within the austenitic matrix. Better tensile properties were noticed for as-printed SS 321 WAAM samples in-comparison to wrought counterpart. This is corroborated to the equiaxed and columnar dendritic microstructure with small fraction of ferrite (FN). The hardness decreased from bottom (247 HV) to top (196 HV) region in SS 321 WAAM plate and is attributed to the microstructural difference with varying amount of ferrite (6.3 to 3.7 FN). The impact strength of samples in the horizontal and vertical direction was 116 ± 2 J and 114 ± 2.5 J respectively, and is comparable with the wrought AISI 321 (123 ± 1.5 J). The reduction in impact toughness is attributed to the ferrite (<6.3 FN) fraction. Polarization curves and Nyquist plots elucidate the excellent pitting resistance of SS 321 WAAM specimens, and the corrosion rate was less than 1 mils per year (mpy). Corrosion cracks were absent, and the passive film formation in the WAAM specimens were compact and highly stable for corrosive environments.

Multiphase polymorphic nanoparticles reinforced titanium matrix composite produced by selective electron beam melting of a prealloyed composite powder

A Ti−6Al−3Sn−4Zr−0.9Mo−0.4Si−0.4Y−0.5B (wt.%) titanium alloy matrix composite (TMC) with multiphase polymorphic TiB whiskers and Y2O3 and (Ti, Zr)5Si3 nanoparticles was successfully fabricated by selective electron beam melting (EBM) of a prealloyed composite powder. The rapid hypoeutectic solidification during EBM results in a columnar dendritic microstructure with three dimensional fine networks of TiB whiskers and Y2O3 nanoparticles uniformly distributed in the interdendritic regions of the parent β-Ti phase. This EBM-built TMC exhibits a remarkably enhanced tensile strength and hardness compared with its matrix alloy at room temperature and elevated temperatures up to 700°C. The ultimate tensile strength of the EBM-built TMC reaches 1185 MPa with an elongation to fracture of 3.2% at room temperature, and still retains a high value of 750 MPa at 600°C. This study opens up a new way to manufacture high-performance components of TMCs with multiphase polymorphic reinforcements and complex shapes.

An integrated Eulerian-Lagrangian-Eulerian investigation of coaxial gas-powder flow and intensified particle-melt interaction in directed energy deposition process

In this work, a comprehensive framework consisting of coaxial gas-powder flow and melt-flow dynamics models is developed for the laser-assisted directed energy deposition process. Firstly, by using the coupled Eulerian-Lagrangian models, the dynamics of the carrier-shielding gases, the particle-stream trajectory, and the in-flight temperature rise of the powder-particles are predicted. This is done by solving the momentum transfer equations, the k-ε turbulence equations, the discrete phase equation and the energy equation. Thereafter, using the particle-stream characteristics above the melt-pool free-surface, an interface-tracking melt thermo-hydrodynamics model is developed in an open-source environment to track the incident powder-particles in the melt-pool. Using the developed framework, the spatial and temporal variation of the melt-pool morphology due to the powder-particle impingement is analyzed. The computational results of particles-melt interaction show an erratic flow pattern with an oscillatory behavior of the melt-pool free-surface. The incident powder-particle creates a crater on the free-surface of the melt-pool and generates a radially outward strong ripple-wave. The velocity field of the melt-pool shows a highly non-linear behavior which is extremely sensitive to the particle impact. It was found that the thermo-capillary stresses dominate only at the instances when there is no particle insertion and the flow is stabilized. Using the solidification parameters obtained from the melt-pool dynamics model, the influence of the fluctuating thermo-fluidic field on the resultant columnar dendritic microstructure is studied. The comparison of the numerically predicted deposition dimensions and the dendritic arm spacings shows a good correlation with the corresponding experimentally measured values from the in-house controlled experiments.

Effect of the Volume Energy Density and Heat Treatment on the Defect, Microstructure, and Hardness of L-PBF Inconel 625

In this article, we investigated the effect of the process parameters and heat treatment on the defect formation, microstructure, and hardness of Inconel 625 manufactured by the laser powder-bed fusion (L-PBF) process. Specimens were fabricated with various scan speeds, laser powers, and hatch spacing to give a range of volume energy density (VED) between 108 and 156 J/mm3. SEM analysis was conducted to identify the effects of the VED on the various defects. A columnar-dendritic microstructure was found in all specimens. The effect of heat treatment on the microstructure, including grain growth, carbide precipitations, and phase transformation, was also studied. The results of this study show that VED has an influential effect on the formation of defects, such as lack of fusion porosity, keyhole, gas pores, and micro-cracks. The microstructure and hardness dependency of the L-PBF Inconel 625 material on the VED and heat treatment emphasizes the importance of the optimum selection of VED and post-processing in the L-PBF manufacturing procedure. Finally, the hardness of the heat-treated and as-built specimens was measured, and their variations with VED were specified.

Electron-beam welding of high-entropy alloy and stainless steel: microstructure and mechanical properties

ABSTRACT An autogenously dissimilar Al0.1CoCrFeNi-high entropy alloy/stainless steel (i.e., AISI 304) weld joint was produced by an electron-beam welding technique. The weld metal followed mode A of solidification and resulted in a fully austenitic columnar dendritic microstructure due to rapid cooling and lower chromium equivalent to nickel equivalent (Creq/Nieq) ratio (1.12–1.14). Tensile test sample fractures at the base metal (Al0.1CoCrFeNi-high entropy alloy) and illustrates higher strength (the yield and ultimate strength of 310 ± 10 MPa and 560 ± 15 MPa, respectively) than that of Al0.1CoCrFeNi-high entropy alloy, ensuring the suitability of the electron-beam welding for the Al0.1CoCrFeNi-high entropy alloy/AISI 304 stainless steel joint structures designed with respect to Al0.1CoCrFeNi-HEA properties. The influence of the manufacturing process (electron beam welding) is highlighted in terms of microstructure and mechanical properties.

Revealing the mechanisms by which magneto-hydrodynamics disrupts solidification microstructures

A key technique for controlling solidification microstructures is magneto-hydrodynamics (MHD), resulting from imposing a magnetic field to solidifying metals and alloys. Applications range from bulk stirring to flow control and turbulence damping via the induced Lorentz force. Over the past two decades the Lorentz force caused by the interaction of thermoelectric currents and a magnetic field, a MHD phenomenon known as Thermoelectric Magnetohydrodynamics (TEMHD), was also shown to drive inter-dendritic flow altering microstructural evolution. In this contribution, high-speed synchrotron X-ray tomography and high-performance computational simulation are coupled to reveal the evolution, dynamics and mechanisms of solidification within a magnetic field, resolving the complex interplay and competing flow effects arising from Lorentz forces of different origins. The study enabled us to reveal the mechanisms disrupting the traditional columnar dendritic solidification microstructure, ranging from an Archimedes screw-like structure, to one with a highly refined dendritic primary array. We also demonstrate that alloy composition can be tailored to increase or decrease the influence of MHD depending on the Seebeck coefficient and relative density of the primary phase and interdendritic liquid. This work paves the way towards novel computational and experimental methods of exploiting and optimising the application of MHD in solidification processes, together with the calculated design of novel alloys that utilise these forces.

Microstructure and high temperature mechanical properties of wire arc additively deposited Stellite 6 alloy

In this work, wire arc additive deposition of Stellite 6 alloy was carried out using cold metal transfer (CMT) process and the microstructural features, microhardness, and high temperature mechanical properties were studied. The as-deposited multi-layer wall was free of any defects. High temperature compression test samples were extracted parallel to the build direction, and the tensile samples were prepared along parallel and perpendicular orientation to the build direction. The hot compression test data was used to predict the flow behavior of the additively deposited alloy using an Arrhenius type equation at a strain of 0.6. The flow behavior of the additively deposited Stellite 6 alloy was similar to the conventional Co-based alloys in spite of the layer-by-layer macrostructure and microstructural variations. Microstructure, hardness, and phase analysis studies were carried out for the deformed samples after hot compression. The high temperature tensile properties were found to be anisotropic and location-specific, which were correlated with the columnar dendritic microstructure, alignment of the layer interfaces and variation in the secondary dendrite arm spacing due to repeated thermal cycles and difference in the cooling rate.

Microstructure and High Temperature Mechanical Properties of Wire Arc Additively Deposited Stellite 6 Alloy

In this work, wire arc additive deposition of Stellite 6 alloy was carried out using the cold metal transfer (CMT) process and the microstructural features, microhardness, and high temperature mechanical properties were studied. The as-deposited multi-layer wall was free of any defects. High temperature compression test samples were extracted parallel to the build direction, and the tensile samples were prepared along parallel and perpendicular orientation to the build direction. The hot compression test data was used to predict the flow behavior of the additively deposited alloy using an Arrhenius type equation at a strain of 0.6. The flow behavior of the additively deposited Stellite 6 alloy was similar to the conventional Co-based alloys in spite of the layer-by-layer macrostructure and microstructural variations. Microstructure, hardness, and phase analysis studies were carried out for the deformed samples after hot compression. The high temperature tensile properties were found to be anisotropic and location-specific, which were correlated with the columnar dendritic microstructure, alignment of the layer interfaces and variation in the secondary dendrite arm spacing due to repeated thermal cycles and difference in the cooling rate.

Microstructure and mechanical properties of rene 41 alloy manufactured by laser powder bed fusion

Precipitation hardenable Ni-based superalloy Rene 41, considered to have moderate weldability, was produced by laser powder bed fusion (LPBF). Crack-free and dense parts were obtained directly from processing. Parts were characterized in both as-built condition and after aging heat treatment. In the as-deposited condition, scanning electron microscopy and electron back scattered diffraction analysis revealed a fine columnar dendritic microstructure with a preferred orientation along <100>. Transmission electron microscopy analysis revealed the absence of γ′ precipitates in the as fabricated state. After heat treatment, the columnar grain morphology was maintained and formation of distinct carbide particles at the grain and cell boundaries were observed along with the precipitation of fine and homogeneously distributed γ’. The size of these γ′ precipitates were measured as 8–30 nm which is smaller compared to the reported values in the literature. Significant increase in the microhardness value from 347 HV to 548 HV is observed after the heat treatment. Increased room temperature yield and ultimate tensile strength values reaching 857 and 1165 MPa were measured for the samples built parallel to the base plate in the as built state. These values increased to 1263 and 1580 MPa after the heat treatment, ascribed to the presence of fine γ’ precipitates. Fractography analysis revealed that failure occurred in a ductile manner for both as-fabricated and heat-treated samples.

Novel welding of Al0.5CoCrFeNi high-entropy alloy: Corrosion behavior

Laser welding of Al0.5CoCrFeNi high-entropy alloy has been studied in detail. Base metal (BM) shows equiaxed grains with three different regions; (a) Cr–Fe rich, (b) Al–Ni rich phases and (c) Al-rich particle. Laser welding has resulted in evolution of columnar dendritic microstructure with dissolution of Al rich particles in the weld metal (WM), where dendrites are Cr–Fe rich and interdendrites are Al–Ni rich regions. The corrosion experiment shows higher corrosion current density (2.83 × 10−5 mA/cm2) in WM than the BM (8.63 × 10−6 mA/cm2) implies higher corrosion rate. However, the WM exhibits higher (nobler) corrosion potential as compared to BM. Due to this nobler potential, WM acts as the cathode and the BM act as an anode when weldment (BM + WM) is analyzed. Hence, the weldment (BM + WM) exhibits lower corrosion current density (4.11 × 10−6 mA/cm2) like BM.

Fabrication and Characterization of AlxCrCuFeNi2 High-Entropy Alloys Coatings by Laser Metal Deposition

High-entropy alloys (HEAs) are becoming new hot spots in the metallic materials community, which are defined to contain equiatomic or close-to-equiatomic compositions. HEAs can possess many interesting mechanical properties, and in particular, they have the great potential to be used as coating materials requiring high hardness and wear resistance. In this study, the feasibility of fabrication AlxCrCuFeNi2 (x=0,0.75) HEAs was investigated via laser metal deposition from elemental powders. The microstructure, phase structure, and hardness were studied by an optical microscope, scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), electron backscatter diffraction (EBSD) and Vickers hardness tester. The bonding between the AlxCrCuFeNi2 (x = 0,0.75) HEAs and AISI 304 stainless steel were good combinations. The Al0.75CrCrFeNi2 alloy consisted of columnar dendritic microstructure with Al/Ni enrichment in the dendritic regions. The phase structure of the AlxCrCuFeNi2 (x = 0,0.75) HEAs were face center cubic structure as identified by EBSD. Vickers hardness results indicate that the average hardness of CrCuFeNi2 HEA was 175 HV. With the addition of aluminium, the Vickers hardness of Al0.75CrCuFeNi2 HEA increased to 285 HV.

Effects of Selective Laser Melting additive manufacturing parameters of Inconel 718 on porosity, microstructure and mechanical properties

The effect of SLM parameters on porosity, microstructure and mechanical properties is studied. To this purpose, the Selective Laser Melting (SLM) technology is applied to manufacture Inconel 718 specimens. The material, the manufacturing process, the Hot Isostatic Pressure (HIP), heat treatment, observation procedures and characterisation of mechanical properties are presented. A columnar-dendritic microstructure was observed on all the SLM specimens and a Volumetric Energy Density (VED) effect on the latter was also noted. The rate of porosity varies in relation to the VED and is considerably reduced after HIP. The heat treatment erases the dendritic microstructure, significantly enhances microhardness and confers on the alloy tensile mechanical properties comparable to forged Inconel 718.

Fabrication of Crack-Free Nickel-Based Superalloy Considered Non-Weldable during Laser Powder Bed Fusion.

Crack-free Hastelloy X fabricated through laser powder bed fusion (LPBF) from powder with a standard chemical composition is reported. Electron backscatter diffraction (EBSD) analysis evidenced columnar grains parallel to the building direction. The typical LPBF columnar dendrite microstructure was found to be finer than reported elsewhere. Mo-enriched carbides (~50 nm), presumed to play an important role in the cracking behavior of the alloy, were confirmed along interdendritic regions. Crack-free condition was maintained after heat treatment at 1177 °C for 1 h followed by water quenching, and the resulting microstructure was analyzed.