Columnar-to-equiaxed Transition Research Articles

Effect of TiB2 on the microstructure and mechanical properties of TiAl alloy produced by laser direct energy deposition

A Ti-48Al-2Cr-2Nb alloy was prepared by laser directed energy deposition (DED). The microstructure of the DED-produced Ti-48Al-2Cr-2Nb alloy exhibited a coarse columnar structure along the building direction. Through a combination of experiment and numerical simulation, the temperature gradient and solidification rate of the single-track sample were calculated theoretically. The columnar to equiaxed transition (CET) behavior of molten pool solidification was quantitatively analyzed, and the microstructure transition diagram was established. The effect of TiB2 on the microstructure and mechanical properties of the TiAl alloy was investigated. The columnar changed to equiaxed grains in the vertical cross section by adding nanometer-sized TiB2 particles and the microstructure was significantly refined. The grain size was reduced from 48.5 μm to 4.9 μm. Some equiaxed γ grains were distributed around the banded TiB2. The compressive properties were improved from 628.4 MPa to 698.8 MPa by adding nanometer-sized TiB2 particles. Finally, the solidification behavior and the microstructure refinement mechanism caused by the addition of TiB2 particles were discussed.

Effect of ultrasonic parameters on microstructure and mechanical properties of non-contact ultrasonic-assisted laser metal deposition of Fe-based powder

In order to study the effects of ultrasonic parameters on the microstructure and mechanical properties of non-contact ultrasonic assisted laser deposition, Fe-based powder was deposited on 42CrMoA steels with different ultrasonic amplitudes (UA) and excitation distances (d). The results show that the ultrasonic-assisted laser metal deposition (LMD) of Fe-based powder increases depth and width of the deposition layers. Due to the cavitation effect of ultrasonic, it effectively refines the grains and promotes the columnar to equiaxed transition (CET) of grains at the interface. When d = 50 mm, the grain size greater than 8 μm accounts for only 3.1 %, but the ultrasonic vibrations haven’t changed the phase type of the structures. With ultrasonic-assisted, the cracks generated in the deposition layer can be effectively inhibited and the mechanical properties can be improved. When d = 80 mm and UA = 30 μm, the average microhardness of the deposition layer reaches 772.9 HV, which is 95.8 HV higher than that without ultrasonic-assisted deposition. When UA = 40 μm, the tensile strength is 1237.9 MPa, which is increased by 26.8 % compared to without ultrasonic-assisted deposition. Compared to without ultrasonic-assisted, the friction coefficient of deposition layers is significantly decreased by 25.5 % with the application of UA = 30 μm.

Solidification and tribological behaviors of CoCrFeNiSi0.5-(Al,Ti) non-equiatomic high entropy alloy coatings fabricated by direct laser deposition

In order to evaluate the difference in the effects of Al and Ti elements on the microstructure and tribological behaviors of CoCrFeNi-based high entropy alloy (HEA) coatings, CoCrFeNiSi0.5-(Al,Ti) non-equiatomic HEAs coatings were prepared using direct laser deposition (DLD). The phase constitution, solidification microstructure and tribological behaviors of the HEA coatings were investigated. The results indicated that CoCrFeNiSi0.5 HEA coating was composed of face centered cubic (FCC) phase with microstructure of columnar and cellular grains. The addition of Al and Ti led to a columnar-to-equiaxed transition (CET) in the HEA coatings. When only Al was added (CoCrFeNiSi0.5Al0.5), the primary phase transformed into a body centered cubic (BCC) phase (94 vol%). Additionally, Cr3Si formed in the interdendrite (ID) region, and a spinodal decomposition structure emerged in the dendrite (DR) region. Conversely, with the inclusion of Ti alone (CoCrFeNiSi0.5Ti0.5), the matrix maintained its FCC phase, but Ni16Ti6Si7 (G phase, 20.9 vol%) and Cr15Co9Si6 (15.1 vol%) were generated in the ID and DR regions, respectively. Incorporating both Al and Ti (CoCrFeNiSi0.5Al0.5Ti0.5) significantly reduced the G phase content in the ID region to 7.9 vol% and led to the formation of dispersed Fe3Al nanoparticles with an average size of 99 nm in the DR region. The CoCrFeNiSi0.5 HEA coating exhibited the lowest hardness (249 HV0.1), the highest friction coefficient (0.778), and the poorest wear resistance. For the HEA coatings containing Al and Ti, the microhardness significantly increased to over 700 HV0.1, and the friction coefficient decreased to around 0.6. However, the wear resistance of the CoCrFeNiSi0.5Al0.5 HEA coating was almost equivalent to that of the CoCrFeNiSi0.5 with a wear loss of 5.1 mg. By contrast, the HEA coatings containing Ti demonstrated notably improved wear resistance, with wear losses of only 2.2 mg and 2.4 mg for the CoCrFeNiSi0.5Ti0.5 and CoCrFeNiSi0.5Al0.5Ti0.5 HEA coatings, respectively.

Exceptional grain refinement induced by dispersed MgO particles in TIG-welded AZ31 alloy

Due to the low content of alloying elements and the lack of effective nucleation sites, the fusion zone (FZ) of tungsten inert gas (TIG) welded AZ31 alloy typically exhibits undesirable coarse columnar grains, which can result in solidification defects and reduced mechanical properties. In this work, a novel welding wire containing MgO particles has been developed to promote columnar-to-equiaxed transition (CET) in the FZ of TIG-welded AZ31 alloy. The results show the achievement of a fully equiaxed grain structure in the FZ, with a significant 71.9% reduction in grain size to 41 μm from the original coarse columnar dendrites. Furthermore, the combination of using MgO-containing welding wire and pulse current can further refine the grain size to 25.6 μm. Microstructural analyses reveal the homogeneous distribution of MgO particles in the FZ. The application of pulse current results in an increase in the number density of MgO (1-2 μm) from 5.16 × 104 m−3 to 6.18 × 104 m−3. The good crystallographic matching relationship between MgO and α-Mg matrix, characterized by the orientation relationship of [112‾0]α−Mg//[01‾1]MgO and (0002)α−Mg//(111)MgO, indicates that the MgO particles can act as effective nucleation sites for α-Mg to reduce nucleation undercooling. According to the Hunt criteria, the critical temperature gradient for CET is greatly enhanced due to the significantly increased number density of MgO nucleation sites. In addition, the correlation with the thermal simulation results reveals a transition in the solidification conditions within the welding pool from the columnar grain zone to the equiaxed grain zone in the CET map, leading to the realization of CET. The exceptional grain refinement has contributed to a simultaneous improvement in the strength and plasticity of welded joints. This study presents a novel strategy for controlling equiaxed microstructure and optimizing mechanical properties in fusion welding or wire and arc additive manufacturing of Mg alloy components.

Improving the processability and grain structures of additively manufactured Al-Fe-Cu-xZr alloy: Experiment and high-fidelity simulation

Additive manufacturing (AM) offers significant advantages over conventional manufacturing techniques in processing Al alloys, however, high-strength Al alloys produced by AM commonly encounter a few challenges including poor processability and undesirable microstructures with coarse columnar grains and even micro-cracks, leading to poor mechanical properties of the resultant Al alloys. Here, we demonstrate that these issues can be largely resolved by micro-alloying of Zr element in the Al-based alloys. In this study, a series of novel Al-1Fe-0.6Cu-xZr alloys with different content of Zr (x=0.3, 0.6, 0.8, 1.3 at%) are designed and then subjected to laser powder bed fusion (LPBF). It is found that, with increasing Zr content, the processability of Al alloys is significantly improved, as evidenced by the gradual reduction of melt splashing and defects (including pores, balling, necking and discontinuity) during LPBF, which eventually leads to decrease of surface roughness and improvement of processability. High-fidelity simulation reveals that the processability is closely related to the melt velocity gradient (MVG) in melt pools. Increasing Zr content results in a more uniform distribution of MVG and decreased Marangoni convection, which favors the stability of melt pool and good processability. On the other hand, Zr content also plays an important role in microstructure. Although the four LPBF Al alloys all exhibit notably heterogeneous structures with alternate distribution of coarse grain zones (CGZs) and fine grain zones (FGZs) in the whole samples, with increasing Zr content, the coarse columnar grain structure in CGZs is suppressed and transforms to the fine equiaxial grain structure. The occurrence of columnar-to-equiaxed transition (CET) is attributed to the decreased ratio between temperature gradient (G) and solidification rate (R) at the solid-liquid interface with increasing Zr content. In addition, the increase in Zr content also leads to a gradual refinement of the grain size in both CGZs and FGZs due to the increased cooling rate. Beneficial from the good processability and fully refined equiaxial gain structure, nearly full-dense and high-strength Al alloys with 0.8 and 1.3 at% Zr are successfully fabricated, which show a good combination of high strength and ductility. The findings in this work provide a guild for design of novel Al alloys with good processability and advanced mechanical properties for additive manufacturing.

Effects of Rare Earths on Microstructure and Wear Resistance in Metal Additive Manufacturing: A Review

Rare earth elements (REEs) doping technology can effectively control the microstructure and improve the quality and performance of materials. This paper summarizes the research progress of REEs in metal additive manufacturing (MAM) in recent years and briefly introduces the effects of REEs on the molten pool fluidity, purified structure, and interfacial bonding between the molten cladding layer and substrate. It focuses on the mechanism of the role of REEs in the refinement and homogenization of microstructures, including grain growth, columnar to equiaxed transition (CET), and elemental segregation. The reasons for the influence of REEs on the homogenization of the structure and elemental segregation are analyzed. The effects of REE type, content, and dimension on hardness and wear resistance are investigated. Finally, tribological applications of REEs in biological and high-temperature environments are summarized, and the impact of REEs-modified alloys is summarized and prospected.

Insights into the microstructural design of high-performance Ti alloys for laser powder bed fusion by tailoring columnar prior-β grains and α-Ti morphology

A high-performance Ti-Ni-B alloy with good tensile properties and reduced mechanical anisotropy was developed by promoting the columnar to equiaxed transition (CET) of prior-β grains and modifying α-laths to equiaxed grains. Both Ni and B contributed to the refinement of columnar prior-β grains during the L→β phase transformation by generating constitutional undercooling. Compared with Ni, B had a superior capability of generating constitutional undercooling, which not only replaced a significant amount of Ni with a minor addition to reduce the formation of brittle eutectoid, but also reacted with Ti to form TiB to promote heterogeneous nucleation of α-Ti grains during the β→α phase transformation. Together with the restricted growth of α-laths induced by the refinement of prior-β grains, a fully equiaxed α-Ti structure was obtained. The competition between the negative effect of brittle eutectoid and the positive role of α-lath to equiaxed grain transition on the ductility of as-printed Ti-Ni-B alloys was fundamentally governed by the morphology of eutectoid and technically dependent on the Ni-B content. When the addition was 1.2Ni-0.06B (wt.%) or less, the positive effect of α-lath on equiaxed grain transition can effectively mitigate the issue of reduced ductility caused by brittle eutectoid. In contrast, at 1.8Ni-0.09B or greater, the negative effect of eutectoid dominated. New insights into microstructural design obtained through the aforementioned approach were presented and discussed.

Synergistic control of microstructures and properties in eutectic high-entropy alloys via directional solidification and strong magnetic field

The AlCoCrFeNi2.1 eutectic high-entropy alloy (Ni2.1 EHEA), as an exemplary representative of the high-entropy alloy family, has garnered significant research attention owing to its exceptional comprehensive properties. In this study, we investigated the influence of various growth velocities on the microstructure, lamellar spacing, and mechanical properties of the Ni2.1 EHEA. We observed that at lower growth velocities, the structure consisted of an alternating face-centered-cubic (FCC) phase and B2 phase lamellae aligned in a single direction, with the lamellae orientation parallel to the direction of the heat flow. The yield strength increased with the growth rate, while the ultimate tensile strength decreased with increasing the growth velocity. Ductility remained relatively consistent, and a double yield phenomenon was observed in the elastic-plastic deformation region. Under conditions of high growth velocities, the microstructure transitioned from a single-directional full lamellar structure to a multi-stage lamellar arrangement. The most favorable comprehensive mechanical properties were achieved at a growth rate of 200 μm/s, resulting in a yield strength of 450 MPa, an ultimate tensile strength of 1092 MPa, and a remarkable ductility of ∼32% in the directionally solidified samples—double that of the arc-melted sample. The evolution law of directional solidification structures under the coupling effect of different magnetic fields and different growth rates was studied. The interaction of the thermoelectric-magnetic force and thermoelectric-magnetic convection and the potential mechanism of microstructure evolution under the effect of magnetic field were deeply analyzed. The results reveal that at a growth rate of 2 μm/s, the spacing between eutectic layers decreases as the magnetic induction intensity increases, leading to the transformation of some regular layers into spherical layers. Similarly, at a growth velocity of 10 μm/s, the eutectic structure exhibited a Columnar-to-equiaxed transition (CET). However, as the growth rate further increases, the limited exposure time to the magnetic field prevented significant structural changes.

Effect of laser power on microstructure and mechanical properties of pure Zn fabricated via laser powder bed fusion

The preparation of biodegradable pure Zn porous scaffolds using additive manufacturing technology has great potential for application. However, the mechanical properties of pure Zn are not sufficient for the clinical application of stents. An object of this paper was to investigate the effect of laser power on microstructural evolution and tensile properties of pure Zn fabricated via laser powder bed fusion (LPBF). The results demonstrated that the temperature gradient and heat storage were the main factors determining the microstructure. The formation of columnar grains was due to the high temperature gradient in the early printing stage, and the equiaxed grains in the middle and top region were attributed to the heat storage induced recrystallization and columnar-to-equiaxed transition (CET), respectively. In addition, as the laser power increased from 40 W to 60 W, the recrystallization degree increased, and the Kernel average misorientation value and Taylor factor decreased gradually, resulting in a significant increase in the ductility. The mechanical properties were maximized at the laser power of 60 W and the scanning rate of 150 mm/s (yield strength, tensile strength and elongation of samples can reach 82.02 MPa, 116.20 MPa and 37.16 %, respectively). This work provides theoretical support for optimizing the mechanical properties of LPBF Zn-based materials by microstructural regulation.

Grain refinement of NiTi alloys during ultrasound-assisted wire-arc directed energy deposition

ABSTRACTAchieving grain refinement in the wire-arc directed energy deposition (DED) process of NiTi alloy is highly challenging. We have successfully obtained NiTi alloy with a fine microstructure and observed an improvement in superelastic recoverability by introducing an ultrasound-assisted field during the deposition process. The results show that the average grain size of the NiTi alloy samples decreased from 147.3 to 72.2 μm. Additionally, the recoverable strain of the samples increased significantly from 3.7% to 4.4%. This is attributed to the ultrasonic cavitation and acoustic streaming phenomena during the ultrasound-assisted process, which promotes the columnar-to-equiaxed transition (CET) during the solidification process. Fundamentally, the key to the enhancement of recoverable strain after ultrasound-assisted is that the formation of equiaxial grains provides more strain compatibility to the grain boundaries, which reduces the formation of dislocations during deformation.

Multi-physics multi-scale simulation of microstructure evolution in entire welds of 2219-T6 aluminum alloy: An MPF-LBM study

The microstructure of fusion welds significantly affects its mechanical properties. Existing studies on the dynamic evolution of welding molten pools have exposed great limitations in considering the flow field and the solid motion of free dendrites. In this paper, a multi-physics multi-scale coupling simulation of thermal-mass-flow was conducted to study the solidification behavior during the welding of 2219-T6 aluminum alloy. The developed multi-phase field lattice Boltzmann model realized the direct full coupling of phase, temperature, solute, solid motion, and flow fields. It is also combined with a continuous nucleation model to simulate heterogeneous nucleation and columnar-to-equiaxed transition (CET), and a transient growth-condition model to simulate the movement and solidification of the entire molten pool. The model and simulation results were verified by welding experiments, electron back-scattered diffraction, and scanning electron microscopy tests. Good agreements are achieved in terms of grain structure, dendrite morphology, and grain size. The undercooling field, fluid flow, solid motion, dendrite morphology, solute distribution, and quantitative parameters during the whole solidification process were analyzed in detail to reveal the true evolution of thousands of dendrites in fusion welds. The results show that solute convection caused by fluid flow significantly affects the dendrite morphology. The solid motion of equiaxed dendrites squeezes the solute enrichment layer and forms a solute hysteresis transport effect. A band of equiaxed dendrites gathers in front of the columnar dendrites and blocks their growth, forming a mechanical blockade mechanism of the CET process. The equiaxed dendrites dominated the subsequent solidification after CET is complete, and the final microstructure consists of brief-growth columnar dendrites and dense equiaxed dendrites. The top tangential flow causes a band of equiaxed dendrites with large gaps in the upper weld, which was presumed to be the source of surface cracks in welds with high residual stress.

The columnar to equiaxed transition of CoCrNi medium-entropy alloy fabricated by laser directed energy deposition

Understanding and revealing the evolution mechanism of the columnar to equiaxed transition (CET) that occurred in the medium-entropy alloys (MEAs) fabricated by laser directed energy deposition (LDED) is crucial for achieving the tunable microstructure and mechanical properties. In the present work, a CET map was established to explain the effect of the laser energy density (LED) on the CET of the LDED fabricated single tracks of the CoCrNi MEA. The results show that the CET observed in single tracks is related to the morphological transformation of cellular substructure, which is governed by constitutional undercooling. With the increase of LED, the maximum value of the constitutional undercooling exceeds the critical nucleation undercooling earlier, which facilitates the heterogeneous nucleation and inhibits the epitaxial growth of rod-like cells, and finally causes a promoted CET. The correlation among the average cell size, geometrically necessary dislocation density, and average hardness of single tracks was also discussed. The present work not only uncovers the mechanism of the CET and grain growth in the LDED-fabricated CoCrNi MEA but also provides theoretical guidance for fabricating other MEAs with ideal microstructure and performance.

A fully kinetic phase diagram-coupled multicomponent columnar-to-equiaxed grain transition model with an application to additive manufacturing

The columnar-to-equiaxed transition (CET) is known to impact crack formation during the additive manufacturing of metallic alloys. While previous experiments have shown that CET is tunable via its alloying elements, a rigorous multicomponent model to demonstrate the impact of multi-alloying components on CET is still lacking. In this study, we developed a multicomponent model by fully coupling the phase diagram of the kinetic interface condition. Building upon the binary model reported by Gaumann et al. our model replaces the restrictive approach of calculating the non-equilibrium partition coefficient and liquidus slopes with kinetic phase diagram calculation. The extended multicomponent model was validated by comparing it with the Al–Cu results reported by Gaumann et al. CET transition curves were computed for two Al–Cu–Mg–Si–Zn alloys manufactured using laser powder bed fusion. The results are in qualitative agreement with our own and previously reported experimental results. These findings suggest that the proposed multicomponent CET model is a valuable tool for designing AM alloys and optimising processing parameters.

Elucidating the promoting mechanism of nitrogen in the columnar-to-equiaxed transition of steel ingot

Based on the behavior of heat and mass transfer ahead of the advancing columnar front, combined with the nucleation and growth of dendrites, as well as the blocking mechanism of columnar-to-equiaxed transition (CET), the promoting mechanism of nitrogen in the CET of steel ingot was elucidated. It was found that the CET moves from the center to the edge of the ingot as the nitrogen content increases, resulting in a decrease in the columnar dendrite zone and an increase in the equiaxed dendrite zone. The occurrence of this phenomenon can be attributed to two factors. On the one hand, an increase in nitrogen content leads to an increase in critical nucleation supercooling and incubation time of dendrites, causing a decrease in the number of dendrites nucleation. Additionally, the higher nitrogen content reduces the restriction effect of solute on the growth of columnar dendrites, leading to an increase in dendrite size. On the other hand, within the constitutional supercooling (CS) zone ahead of the advancing columnar front, both the CS and temperature gradient decrease with increasing nitrogen content, while the CS increases and the temperature gradient decreases with the distance from the advancing columnar front. Consequently, the nitrogen solute primarily influences the nucleation and growth of columnar and equiaxed dendrites, as well as the length, temperature gradient, and growth restriction factor Qc of the CS zone, resulting in changes in the heat and mass transfer behavior ahead of the advancing columnar front, thereby promoting CET.

Effects of pulsed frequency on the microstructure characteristics and properties of Ti6Al4Vlaser melting deposition additive manufacturing parts

Laser melting deposition–produced Ti6Al4V additive manufacturingparts tend to manifest as coarse columnar grains. This paper optimises the laser pulse frequency to manage the microstructuralcharacteristics of Ti6Al4V parts and enhance their tensile properties and corrosion resistance. A fiber laser processing equipment with a laser power of 1200 W and a spot diameter of 2.0 mm was employed for all experiments, which provides pulsed lasers at 4, 40, 400, and 4000 Hz at a scanning speed of 8 mm/s and a duty cycle of 95 % for comparative research.The findings demonstrated that increasing the pulse frequency improved the deposition efficiency.An appropriate pulse frequency could promote the refinement and columnar-to-equiaxed transition (CET) of primary β grains.At 400 Hz, the Ti6Al4V-deposited parts yielded refined equiaxed grains.The average grain size in the middle of the sample was about 206.8 μm,and the microstructure mainly comprised fine α-laths in a lamellar structure, which yielded the best overall performance of the parts, with the ultimate tensile strength and elongation reaching 1083 MPa and 4.09 %, respectively.Therefore, improvingthe structural properties of partsby regulating the deposition process, which is conducive to the online regulation of the forming quality of laser deposition parts, is an effective and feasible approach.

A multiphase modeling for investigating temperature history, flow field and solidification microstructure evolution of FeCoNiCrTi coating by laser cladding

In this work, a multiphase numerical model was established to investigate the non-isothermal flow and microstructure evolution during laser cladding. The model considered the beam-powder interaction, temperature dependent material properties, the effect of buoyancy and surface tension on convection. The accuracy of the model was verified comparing the simulated coating geometric morphology with the experimental results. Simulation indicated that the temperature and flow of the molten pool were interactive and mutually causal. On the one hand, the temperature field affected the tension gradient and the liquid flow direction. On the other hand, the velocity field significantly affected the heat transfer in the molten pool. Specifically, the maximum fluid velocity was 9.58 mm/s and the Peclet number (PeT) value was less than 5 at the initial stage, indicating that conduction dominated in heat transfer. Furthermore, the PeT value was greater than 200 when the maximum velocity reached 344 mm/s. The heat transfer in the molten pool was mainly thermal convection. In addition, the distribution diagram of solidification parameters (G and R) was calculated to quantitatively establish the internal correlation between solidification parameters and microstructure combing with experimental observation, and revealed the columnar-to-equiaxed transition (CET). Eventually, the microhardness results showed that HEA coating could improve the Ti6Al4V surface microhardness. Simultaneously, the CET behavior significantly affected the microhardness of the coating.

The effect of thickness on the defects and anisotropy of thin-wall Hastelloy X fabricated via laser powder-bed fusion

Laser powder-bed fusion (LPBF) possesses unique advantages in manufacturing complex structures. The mechanical properties of thin-wall structures are crucial for the application. This study investigates the influence of wall thickness on defects and mechanical properties of LPBF Hastelloy X (HX) superalloy. As the wall thickness decreases from 1.5 mm to 0.25 mm, the diameter of sub-grains inside the molten pool increases, exhibiting a progressively enhanced <100>//transverse direction (TD) texture in thin wall structure. Meanwhile, decreasing wall thickness leads to increased porosity and cracks due to unstable molten pool and higher residual stress. The crack initiates at columnar to equiaxed transition (CET) position and propagates along the building direction (BD). The tensile strength of the LPBF HX specimens in the horizontal and vertical directions is 773.5 MPa and 642.1 MPa with thickness of 1.5 mm, respectively, while the elongation of both specimens is close to 35%. When the wall thickness decreases to 0.25 mm, the elongation significantly decreases due to the evolution of microstructure anisotropy and the cracks along the BD, reaching 10.1% and 24.3% for horizontal and vertical specimens. This work illustrates the defects, microstructural evolution and the mechanical properties anisotropy in thin-wall structures of LPBF HX, providing a foundation for adjusting and optimizing the performance of subsequent thin-wall printing processes.

Zn additions modifying microstructure, thermal parameters and cytotoxicity of Sn-0.7Cu eutectic solder alloys

Alloys from the SnCu systems have superior properties as compared to the SnPb alloys, such as mechanical and corrosion resistance. However, SnCu alloys still exhibit low oxidation resistance, poor wettability and low reliability. One way to optimize such properties is by adding alloying elements, such as zinc. Zn is generally used in Pb-free solder alloys to minimize intermetallic compound growth in soldered joints, refine the microstructure and increase mechanical strength, in addition to reducing the final cost. The addition of Zn is motivated by the effects it promotes such as microstructural refining, suppression of intermetallics in the substrate/alloy reaction layer, increase in fluidity and mechanical properties. Therefore, this study aims to understand the effects of Zn additions (0.2 and 0.5 in wt%) on solidification thermal parameters (growth rate-V and cooling rate-Ṫ), macrostructure, microstructure, phase transformation, macrosegregation and cytotoxicity in directionally solidified eutectic Sn-0.7wt.%Cu alloy under transient heat flow regime of solidification in copper sheet. Thus, the samples have been characterized by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD). The CALPHAD (CALculation of PHAse Diagrams) method by Thermo-calc software was used to generate property diagrams and isopleths. Cytotoxicity analysis was performed based on cell viability with incubation periods of 15 and 30 days for the alloys, with subsequent exposures of the extracts for 24 and 48 h. The Zn additions caused slight changes in the phase transformation temperatures of the Sn-Cu-Zn alloys. Macrostructures exhibiting columnar-to-equiaxed transition (CET) were observed for the Sn-Cu-Zn alloys. The microstructures of the Sn-Cu-Zn alloys were predominantly dendritic with a matrix rich in tin (β-Sn) surrounded by a eutectic mixture of the β-Sn + Cu6Sn5 + CuZn phases. After CET on Sn-Cu-Zn castings, low velocity eutectic cells have been observed. Moreover, Zn additions refined and did not change the dendritic arrangement of the Sn-Cu-Zn alloys, when compared to the Sn-0.7wt.%Cu alloy. However, increasing the Zn content did not affect the microstructural scale in the ternary alloys when compared to each other. The variations in the microstructural scale did not influence the toxicity of the examined alloys, but factors such as incubation time and chemical composition did. In general, Zn improved the cell viability of the Sn-Cu-Zn alloys, but still with moderate cytotoxic levels.

A novel approach to regulate the microstructure of laser-clad FeCrNiMnAl high entropy alloy via CeO2 nanoparticles

In this work, the mechanical mixing process was successfully used to inoculate FeCrNiMnAl powder with CeO2 nanoparticles, which contributed to the heterogeneous nucleation of fine equiaxed grains. In this way, the nano-CeO2/FeCrNiMnAl HEA composite coating with low dilution, cracks-free was successfully fabricated. The effects of different addition of nano-CeO2 (0 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%) on the microstructure and properties of HEA composite coatings were mainly explored. The thermodynamic calculations on HEA composite coatings with different contents of CeO2 were conducted to predicted their phase composition. With the modification of nano-CeO2, the microstructure exhibited homogenous, fine-grained and equiaxed. To further reveal the effect of CeO2 on the microstructure, this work studied the columnar-to-equiaxed transition (CET) behavior of HEA coating during solidification process. As a comparison, the heterogeneous nucleation behavior of fine equiaxed grains induced by nano-CeO2 in HEA composite coatings was studies. The results showed that CeO2 nanoparticles were enriched at the grain boundaries of HEA composite coating, and it not only had a drag effect, but also hindered the growth and development of grains. However, excessive nano-CeO2 (2.0 wt% CeO2) could affect the forming quality of HEA coating and have a negative impact on the mechanical properties. After regulating the grain structure, the mechanical properties of HEA composite coating have been greatly improved. While 1.0 wt% nano-CeO2 was added, the microhardness of HEA composite coating reached to the maximum (670.4 HV0.2). Compared with substrate, the average microhardness has increased almost 91.5 %. The friction performance of 1.0 wt% CeO2/FeCrNiMnAl HEA composite coating was the optimal as well and the wear rate reached 3.12 × 10−6 mm3/N m. The tribo-oxide layer was produced on the worn surface of HEA composite coating.

Revealing the evolution of microstructure and mechanical properties with energy density to achieve high-strength Ti-6wt%Cu alloy by laser metal deposition

In this work, Ti-6wt%Cu alloys were fabricated by laser metal deposition (LMD) technology to understand how the energy densities impact the microstructure and consequent mechanical properties. The results show that a bimodal microstructure of ultrafine eutectoid lamellae and Widmanstätten-α can be obtained at low energy density (118 J/mm3), while higher energy density produces coarse colony-α structure. The multi-scale microstructural refinement is achieved due to the fast cooling rate and low heat accumulation in low energy density deposits, resulting in a sound combination of property with tensile strength of 1063 MPa and elongation of 10%. Based on the solidification theories and accurate solidification parameters in the LMD process, the columnar to equiaxed transition (CET) was proposed to elucidate the refinement mechanism of near-equiaxed prior-β grains. Furthermore, detailed microstructural characterization identified the effect of Cu segregation on the formation of eutectoid lamellae. The Cu aggregation at low energy density plays a crucial role in phase transition and microstructural refinement, as well as account for orientation heterogeneity of lamellae. In addition, the strengthening mechanisms were discussed by considering microstructural evolutions due to different energy densities.