Equiaxed Crystals Research Articles

Microstructure and mechanical properties of Mg-Nd-Zn-Zr alloy fabricated by TIG-based wire – Arc directed energy deposition with pulsed current

In this study, the Mg-Nd-Zn-Zr alloy thin-wall specimens were prepared by tungsten inert gas (TIG) based wire-arc directed energy deposition (WA-DED) with different pulsed currents. The arc characteristics of different pulsed current frequents were observed. The microstructures, and mechanical properties of as-deposited (AD), 5 Hz, 10 Hz, 15 Hz, and 20 Hz specimens were systematically analyzed and evaluated. The uniformly equiaxed crystals with random grain orientations and intergranular network Mg-Nd-Zn eutectics were found in the WA-DED fabricated Mg-Nd-Zn-Zr alloy thin-wall specimens. No significant voids were found. The microstructures were regulated, and the mechanical properties were improved by adjusting the pulsed current frequency. The 10 Hz specimen had the optimal microstructure with an average grain size of 10.35 μm. Concurrently, the 10 Hz specimen exhibits excellent strength-ductility synergy and isotropic, benefiting from the finely equiaxed crystals. The average microhardness of the 10 Hz specimen was 68.51 HV0.2, and the ultimate tensile strengths in the building and traveling directions were 223 MPa and 229.7 MPa, respectively, and the yield strengths in the building and traveling directions were 138.3 MPa and 145.3 MPa, respectively. Notably, the elongations in the building and traveling directions of the 10 Hz specimen were 16.8 % and 17.4 %, respectively. The local strain evolution and fracture surfaces of AD and 10 Hz specimens in the building and traveling directions were observed. The mechanisms of grain refinement and mechanical properties improvement were revealed.

Investigation of corrosion resistance offered by the Fe-based clad layer under salt spray and electrochemical workstations

The present work aims to evaluate the electrochemical characteristics of the Fe–based alloy coating formed on the 27SiMn steel substrate under neutral salt spray and 3.5 wt% NaCl environments. By adopting the high-speed laser cladding technique, the Fe-based clad layer was fabricated to perform microstructural and electrochemical characterizations. The microstructure and phase of the coating were analyzed using a scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD), and its corrosion performance was also discussed using the salt spray corrosion chamber and the electrochemical workstation. The results show that the microstructure of the coating surface contains equiaxed crystals and long dendritic crystals which arise due to the higher solidification rate after the cladding process. The Fe–based coating was rich in FeCr phase throughout its microstructure without the formation of the intermetallic compounds. Compared to the substrate, the corroded surface of the coating tends to be compact containing a few corrosion pits after different corrosion times, which is attributed to the formation of Cr oxide on it. The electrochemical tests on the potentiodynamic polarization curve (PPC) and electrochemical impedance spectrum (EIS) indicate that the corrosion resistance of the coating is superior to the substrate, presenting a higher corrosion potential (−0.298 V), lower passive current density (1.36 × 10−6 A•cm2), and higher charge transfer resistance (8.23 × 106 Ω·cm2). Additionally, the corrosion mechanism of coating in salt spray and electrochemical tests is the local pitting corrosion due to the autocatalytic effect on the localized region, which facilitates Cl− ions penetrating the coating and leads to the dissolution of Fe and Cr oxides.

Enhanced high temperature mechanical and oxidation behavior of direct energy deposited TiC/Inconel 718 gradient coatings

While Inconel 718 is commonly employed in high-temperature settings, its performance at elevated temperatures is notably diminished in comparison to its performance under ambient conditions. In this study, Inconel 718 (IN718), Inconel 718 with 5 layer of Inconel 718–5 wt% TiC gradient coatings (CG-5), and Inconel 718 with 5 layer of Inconel 718–10 wt% TiC gradient coatings (CG-10) were fabricated using direct energy deposition (DED) to investigate the influence of TiC content on the mechanical and oxidation properties at elevated temperatures through the regulation of microstructural changes. Following the incorporation of TiC, the microstructures underwent a transformation into refined equiaxed crystals, carbides, and columnar crystals, replacing the coarsened columnar crystals and Laves phase present in IN718. Compared to IN718, the high-temperature tensile strength of CG-5 and CG-10 has increased by 80% and 180% at 900 ℃. Meanwhile, the mass gain due to oxidation per unit area of CG-5 and CG-10 has decreased by 23% and 11%, respectively. The results indicate that CG-10 exhibits a combination of highest high-temperature tensile strength and the enhanced resistance to high-temperature oxidation when compared to IN718 and CG-5.

Effect of interlayer ultrasonic impact on the microstructure, mechanical and corrosion properties of wire arc additive manufacturing AZ31 Mg alloy thin wall

AZ31 Mg alloy thin walls are prepared using wire arc additive manufacturing (WAAM) and interlayer ultrasonic impact (UI) techniques with cold metal transition (CMT) serving as the heat source. The microstructure, mechanical and corrosion properties of thin walls prepared by WAAM and WAAM + UI are studied. Results showed that the recrystallization area fraction along traveling direction (TD) increased by 68.6% after UI treatment, and many fine equiaxed crystals were formed, resulting in grain refinement, anisotropy reduction, mechanical and corrosion properties improvement. The average grain size along TD decreased from 66.6 ± 3.5 μm to 32.7 ± 1.6 μm. Through UI treatment, the ultimate tensile strength (UTS) and elongation (EL) along TD increased from 205 MPa to 230 MPa and 13.5%–17%, respectively. The anisotropic percentage of UTS and EL were decreased from 10.8% to 4.5%, and 42.1%–9.7%, respectively. Electrochemical experimental results showed that the average corrosion rate along TD decreased from 1.93 mm year−1 to 1.53 mm year−1. Grain refinement, dislocation density variation and texture strength reduction were the main reasons for these results.

Cellular automata-lattice Boltzmann model for polycrystalline solidification with motion of numerous dendrites

A GPU-accelerated cellular automata-lattice Boltzmann combinatorial model is developed for calculating the preferred growth, movement, and collision behavior of equiaxed crystals in supercooled melts of binary alloys. For moving dendrites, the growth is computed in a dynamic grid that grows with the body, and continuous movement is achieved by moving the dynamic grid. The impulse-based method is used for the collision of dendrites to calculate the post-collision velocity. Each module of the model was rigorously benchmarked, proving that the model has good computational accuracy and efficiency. The model was used for modeling the solidification of an Al-3 wt% Cu alloy, simulating the growth of abundant kinematic equiaxed crystals in a rotating flow and the falling and stacking of dendrites in droves and subsequent grain growth during the columnar to equiaxed transition, respectively.

Microstructure, corrosion resistance and high temperature oxidation properties of AlCrFeNiCu high-entropy alloy coating by high-speed laser cladding

AlCrFeNiCu high-entropy alloy (HEA) coating was prepared on Zirconium (Zr) rod by high-speed laser cladding (HSLC) technique. The effect of different laser power and scanning speed on the forming quality of the coating was studied. When the specific energy density was in the range of 3.0–3.2 J/mm2, the AlCrFeNiCu HEA coating with good adhesion to the substrate, low dilution rate (small heat affected zone) and no cracks were obtained. The phase structure, microstructure, microhardness, corrosion resistance and high temperature oxidation resistance of the coating were tested and analyzed. The results show that the AlCrFeNiCu HEA coating is composed of 76.9 % BCC phase and 23.1 % FCC phase, in which the BCC phase is mainly composed of disordered BCC phase rich in Cr and Fe elements and ordered B2 phase rich in Al and Ni elements, and the FCC phase is rich in Cu elements. Due to the rapid cooling of HSLC, the coating structure gradually changes from columnar crystal to equiaxial crystal from bottom to top. The hardness of the coating is as high as 805 HV, which may be caused by the solution strengthening effect, lattice distortion effect and slow diffusion effect of HEA. In 3.5 % NaCl solution and 0.1 mol/L KOH solution, the self-corrosion current density of the coating is 4.209 × 10−8 A/cm2 and 4.331 × 10−8 A/cm2, respectively. The passivation film on the surface of the coating is composed of a variety of oxides, and the oxygen vacancy density of the passivation film in the two solutions is 2.552 × 1020 cm−3 and 5.794 × 1020 cm−3, with fewer pitting pits on the surface. The structural integrity of the coating can be maintained after 60 min oxidation at 1200 °C. The oxidation process follows the growth kinetics curve, and the surface oxides mainly comprise FeAl2O4, Al2O3 and Cr2O3. AlCrFeNiCu HEA coating improves the corrosion resistance and high temperature oxidation resistance of the Zr-4 alloy.

Microstructure and flame-retardant properties of the TF550 titanium alloy by ultrasonic vibration assisted laser solid forming

More and more attention has been paid to the application of flame retardancy and surface strengthening of titanium alloys in the aerospace field. In this paper, ultrasonic vibration assisted the laser solid formed (LSFed) TF550 flame-retardant titanium alloy was studied, and the influence of ultrasonic vibration on the microstructure and flame-retardant properties of the LSFed TF550 alloy was analyzed. The results showed that the LSFed TF550 alloy after ultrasonic vibration treatment effectively reduced the generation of pores. When the ultrasonic amplitude was 60 μm, the porosity was the lowest. The porosity of the sample was reduced from 3.84 % to 0.53 %, with the reduction of 86 %. The ultrasonic vibration results in the CET transformation of LSFed TF550 alloy specimen, and the average grain length-width ratio decreases from 5.3 to 1.2, and the microstructure of the specimen changes from the average width of 214 μm to the average grain size of 144±12 μm. Especially when the ultrasonic amplitude is 90 μm, the equiaxial grain size and the grain length-width ratio are the lowest. The effect of ultrasonic vibration at different positions was different. The higher the deposition height, the more columnar crystals in the microstructure and the less equiaxed crystals. When the deposition height was 43 mm, equiaxed crystals and columnar crystals began to coexist. Due to the grain refinement effect of ultrasonic vibration, the number of grain boundaries was increased, and the flame-retardant properties of the LSFed TF550 alloy was improved.

Synergistic solid–liquid composite and rapid solidification preparation of aluminum-clad steel wires

Solid-liquid composites and rapid solidification were synergistically employed to improve the interfacial-bonding and mechanical properties of prepared aluminum-clad steel wires. The effects of the process parameters on the performance of the wires were studied. The experimental results showed that good metallurgical interfaces can be prepared because of the synergistic strengthening effects between the solid–liquid composite and rapid solidification. The interfacial diffusion zone composed of FeAl3 and Fe2Al5 increased the bonding strength of the wires by 13 % compared with that of wires prepared by using the rolling process. The equiaxed fine crystal tissue morphology of the cladding aluminum and twin crystal organization resulted in good mechanical properties. Compared with LB35 and LB40, the conductivity and tensile strength were increased by 14 % and 22 %, respectively.

Process optimization and microstructure of Ti3Zr1.5NbVAl0.25 high entropy alloy produced by directed energy deposition

As an emerging materials preparation method, directed energy deposition (DED) has its unique advantages in the preparation of high entropy alloys (HEAs). However, improper process parameters can lead to numerous defects, affecting material properties. In this study, a Ti3Zr1.5NbVAl0.25 HEA was produced by DED. The effects of process parameters on single track cladding layer (STCL) were studied by response surface methodology (RSM), and the parameters were optimized accordingly. It is indicated that the microstructure of the STCL is composed of columnar and equiaxed crystals and the grain growth is parallel to the heat flux direction. 〈001〉//BD (building direction), 〈111〉//SD (scanning direction) and {100}〈001〉 textures were observed in molten pool. The microhardness of the molten pool measures 299.99 HV0.2, while the estimated Young's modulus and yield stress are 87.66 GPa and 945 MPa, respectively. This work provides unique insights into optimizing the DED process parameters of HEAs, further obtaining the solidification microstructure and mechanical properties of STCL.

Microstructure and properties of SiC ceramic-modified aluminium alloy fabricated using wire arc additive manufacturing

A method for improving the properties of the deposited metal in wire arc additive manufacturing of Al–Si alloy is proposed here, which introduces SiC ceramic particles into the side-axis wire feed. The effects of SiC ceramic particles on the macroscopic morphology, microstructure and mechanical properties, corrosion resistance, and wear resistance of deposited metals were studied. The results show that the introduction of SiC particles will not affect the surface-forming quality of the deposited metal; this is likely due to the stable droplet transfer and no splashing. The transverse and longitudinal tensile strengths are increased by 10.1 % and 11.3 %, respectively, while the corresponding elongations are increased by 33.1 % and 47.2 %, respectively. Further, the corrosion resistance and wear resistance of Al–Si alloy deposited metals containing SiC are improved. The (100) base surface has strong texture characteristics along the deposition direction X0, and the maximum polar density decreases to 13.63, which indicates that SiC promotes the transformation of columnar crystals to equiaxial crystals. The interface between SiC and Al is non-coherent, and the performance improvement lies in the enhanced dispersion distribution and load transfer of SiC. A ring structure was prepared using double-wire arc additive manufacturing technology, and the feasibility of side-axis wire feeding technology was verified.

Melt pool control-assisted additive manufacturing of thin-walled parts

Thin-walled aerospace components manufactured by laser directed energy deposition are susceptible to thermal accumulation effects owing to their deposition path and efficiency, further leading to the edge collapse. The melt pool width, as the carrier of thermal input, is an important geometric factor reflecting the state of the melt pool. Therefore, an in-situ melt pool control technique was proposed. The melt pool width as the control target was monitored using a coaxial camera, and the laser power was adjusted in real time based on the proportional-integral-derivative algorithm. A thermal accumulation model in conventional mode and a thermal suppression model in control mode were derived. The surface roughness, porosity, microstructure, and tensile properties of AISI 316 L stainless steel were compared between two modes. The results demonstrated that the roughness and porosity of thin-walled parts were respectively reduced by 45.3% and 82.7% after control. The periodic evolution of interlayer columnar and equiaxed crystals, and the increase in deformation twins contributed to a simultaneous increase in tensile strength and toughness, with a maximum increase of 35.7% in elongation.

Microstructure evolution and performance of TiMoCrWxTay refractory high-entropy alloy coatings prepared using laser cladding

Refractory high entropy alloy (RHEA) coatings containing W and Ta elements at the same time usually have excellent performance at high temperature, which are promising candidate materials for high temperature protective coating materials. In this work, the effects of W and Ta on the microstructure and performance evolution of TiMoCrWxTay RHEA coatings have been investigated. Experimentally, TiMoCrWxTay coatings are mainly composed of BCC phase, Laves phase and Ti-rich phase, with W and Ta atoms preferably dissolved in BCC phase. The W atoms promote the formation of dendrites and the growth of rod-like and equiaxed crystals and the Ta atoms promote the formation of cellular crystals and produce the effect of fine crystal strengthening. W and Ta atoms have significant influence on the tribological properties of TiMoCrWxTay coatings. The tribological properties of TiMoCrWxTay coatings at 800 °C are better than those at room temperature. TiMoCrWxTay RHEA coatings follow near-linear oxidation kinetics during the oxidation of 50 h at 800 °C.The oxidation resistance of the coatings first increases and then decreases with the increase of Ta content. Ta2O5, TiO2 and other oxides may have a shielding effect on the oxidation of W. In particular, The W0.3Ta0.5 shows the best tribological properties and oxidation resistance, and the mass gain of W0.3Ta0.5 is only 13.4 mg/cm2 after the oxidation test for 50 h. It indicates that TiMoCrWxTay coatings can obtain good high temperature performance by optimizing the content of W and Ta in RHEA coatings. This work can provide guidance for the research and application of TiMoCrWxTay RHEA coatings.

Influence of Pre-strain on the Microstructure and Mechanical Properties of GH4169 Superalloy

Abstract In this study, GH4169 superalloy was subjected to pre-straining after aging, with pre-strain deformations ranging from 0.2% to 10%. Subsequently, the tensile performance of the alloy was tested at various deformation levels. The microstructure of several pre-tensile deformed alloys was also examined. The results show that when the pre-strain deformation increases, the strength of the alloy is improved, the elongation is greatly reduced, and the orientation of equiaxed crystals increases. The pre-strain of GH4169 superalloy increases LAGB and dislocation density while decreasing HAGB and annealing twins.

New insights into oxidation mechanism and kinetics of 9Cr–1Mo ferritic-martensitic steel in oxygen-saturated liquid lead-bismuth eutectic

The evolution of oxide scales on a 9Cr–1Mo ferritic-martensitic steel (T91) in oxygen-saturated liquid Lead-Bismuth Eutectic (LBE) at 500 °C is investigated. While taken out at around 500 °C, the T91 steel, after corrosion, shows clean surface with little LBE adhesion due to the poor wettability induced by surface oxides. The results reveal the evolution of surface oxides from irregular strip shapes to regular crystalline grains, and stack-packed equiaxed crystals in the outer magnetite layer. Thermally-induced cracks are captured in the oxide scale on T91 steel after 1000 h exposure to LBE. A new oxidation mechanism is proposed, with consideration of the formation of the outer layer by metal dissolution/oxide precipitation, and a thin film of chromium oxide at the inner/outer layers interface. The dependency of temperature and oxygen concentration on the oxidation kinetics of T91 steel in LBE is discussed.

Properties and microstructure regulation of electrodeposited ultra-thin copper foil in a simple additive system

Hydroxyethyl cellulose (HEC) has been commonly used in a variety of complex formulations for acid copper plating. However, the roles of HEC acting in acid copper plating still lacks of systematic investigation. To explore the efficacy of HEC in the deposition of the ultra-thin electrodeposited copper foil (ED-Cu), we designed a simple formulation system, in which HEC was used as the single organic additive. Using electron backscatter diffraction (EBSD), microstructures of the prepared ED-Cu was comprehensively investigated. The results showed that the ED-Cu was characterized by a mixed distribution of columnar and equiaxed crystals. Grain morphology, dislocation density and crystal orientation of the ED-Cu could be regulated by HEC concentration. According to the cyclic voltammetry (CV) and chronoamperometry (CA) results, the introduction of HEC between 0–200 ppm led to a polarizing effect, which marginally increased with the HEC concentration. Meanwhile, the increase of HEC concentration enhanced the nucleation rates of copper and reduced the grain size during instantaneous nucleation. The introduction of the HEC also altered the preferred orientation of the ED-Cu foil. Mechanical results showed that the optimum concentration of HEC addition was 125 mg l−1.

Effect of B4C content and particle sizes on the laser cladded B4C/Inconel 625 composite coatings: Process, microstructure and corrosion property

B4C/Inconel 625 composite coatings were successfully prepared on 20 pipeline steel by laser cladding, in which B4C ceramic was selected as reinforcement phase to improve the microstructure and corrosion property of Inconel 625 coating. The laser cladding parameters were optimized and the effect of B4C content and B4C particle sizes on the microstructure and properties of B4C/Inconel 625 composite coatings were studied in detail. With increasing liner energy density and decreasing powder feeding speed, the crack ratio of the composite coatings reduces. NiB phase forms due to the in-situ reaction of B4C with Ni element in Inconel 625, and a thin layer of planar crystal also forms at the coating/substrate interface. As the addition of B4C content ranged 5 wt.%∼10 wt.% and the particle sizes ranged 10 μm–60 μm, the quantities of equiaxed crystals increase obviously and the coarse columnar crystals are also refined, which contributes to the improved microhardness and corrosion resistance of the B4C/Inconel 625 composite coatings. When the B4C content is 10 wt.% and particle size is 10 μm, the maximum microhardness of the composite coating is about 567HV0.2, which is 241.2% to that of Inconel 625 coating. The optimal corrosion resistance of B4C/Inconel 625 composite coatings is obtained when the B4C content is 5 wt.% and particle size is 60 μm.

Formation mechanism and mechanical behavior of gradient nanograin structure in directional solidified Ti3Al alloy: Atomic-scale study

The formation mechanism of Ti3Al alloy during a directional solidification process was systemically investigated by means of molecular dynamics (MD) simulations, and its mechanical behavior was explored by comparing with its nanograined (NG), coarse-grained (CG) and gradient nanograined (GNG) counterparts. It is found that the solidified front forms equiaxed crystals first, then they transform into columnar crystals, and the GNG structure is formed finally. Noticeably, the grains will grow preferentially in the direction parallel to the solidification direction. Besides, it is also found that the directional solidified alloy with the GNG structure has higher tensile strength and better ductility than its NG and CG counterparts. The GNG structure not only suppresses strain localization and grain growth in its small grain regions, but also promotes more cross dislocations in the large grain regions, resulting in a better mechanical performance.

Tribological and electrochemical behaviors of FeCoNiCrMox HEA coatings prepared by internal laser cladding on 316L steel tube

In order to improve the performance of the inner wall of the 316L hydraulic cylinder, FeCoNiCrMox (x = 0.2, 0.4, 0.6) high entropy alloy coatings were prepared by laser cladding. The effects of the Mo element on the phase, microstructure, hardness, and tribological as well as electrochemical behaviors of the coating were investigated. The results show that the coatings consist of the FCC phase. The FCC phase increases, and the grain orientation becomes more complex with increased Mo element content. The coatings are mainly composed of columnar and equiaxial crystals. Mo0.4 coating has the finest grain structure, resulting in the coating with a maximum hardness of 421.99 HV, 2.11 times that of the substrate. The nanoindentation test shows that the elastic modulus of the HEA coating increases with the increase of Mo content. The dry sliding wear test shows that adding the Mo element improves the wear performance of the coating. The wear volume loss and wear rate of Mo0.4 coating were the lowest, 12.242 mg and 2.88 × 10−4 mm3•N−1•m−1, respectively. In the 3.5 wt% NaCl solution, the polarization curves and EIS results indicated that adding the Mo element improved the corrosion resistance of the coating. The FeCoNiCrMox coatings significantly improved the hardness, tribological, and electrochemical behaviors of the 316L substrate.

Effect of ultrasonic shot peening on microstructure and mechanical properties of GH3230 superalloy during selective laser melt solution heat treatment

Additive manufacturing (AM) is widely used because of its well-known advantages, but there are some disadvantages including microstructure inconsistencies and harmful residual stresses, which can lead to premature part failure. In this study, the microstructure, and mechanical properties of GH3230 alloy processed by AM are changed by a new post-treatment method, which combines thermal solution treatment (ST) and ultrasonic shot peening (USP). The microstructure evolution of GH3230 after ST and USP after ST is investigated experimentally. The experimental results show that equiaxed crystals and M23C6 carbides appear after solution treatment. After ST, USP is performed, and the gradient microstructure is formed perpendicular to the shot peening surface. Dislocation entanglement and grain refinement occur in the plastically deformed layer. In addition, the carbide interacts with the dislocation. After ST, USP can produce a high level of residual compressive stress in the plastic deformation layer. This combination of ST and USP helps to improve the mechanical properties of additive manufactured parts by changing their microstructure and residual stress distribution.

Microstructure and Mechanical Properties Ultrasonic Assistance Laser Welded Joints of Beta Titanium Alloy with Multiple Vibrators

Aiming at the problem of deterioration of the properties of beta titanium alloy welded joints due to many porosity defects and coarse grains, multi-vibrator ultrasonic-assisted laser welding (M—ULW) technology was used to improve the structure and properties of beta titanium alloy welded joints. The microstructure evolution, tensile strength, elongation, and fracture behavior of the weld joint were studied through scanning electron microscopy, electron back-scatter diffraction, and a universal testing machine. The results show that ultrasonic vibration has no effect on the phase composition of titanium alloy welds during ultrasonic-assisted laser welding. However, it caused all grains in the weld to be transformed into equiaxed grains, and the higher the amplitude, the finer and more uniformly distributed were the equiaxed grains. When the ultrasonic amplitude reached 20 μm, the fine equiaxed crystals were uniformly distributed throughout the weld, and the average grain size of the weld was 56.15 um, which is only one-third of that of the unultrasonicated laser welded joint. Ultrasonic refinement makes the joint grain size decrease, weakens the beta titanium alloy {200} direction weaving, increases the dislocation density within the weld; and increases the tensile strength of the welded joint. The tensile strength of the welded joints exceeded that of the base material by 907 MPa, and the elongation was significantly increased by a factor of 1.8 compared with that of the un-ultrasonicated laser welded joints, resulting in a shift of the fracture location from the center of the weld to the heat-affected zone.