Supercooled Zone Research Articles

Laser additive manufacturing of Ti and Ce co-modified 2195 difficult-to-process aluminum alloy: Grain refinement, cracking suppression and enhanced mechanical properties

High cracking susceptibility of Al-Li alloys with Ti/CeB6 addition is thoroughly suppressed in laser powder bed fusion (LPBF) processing of Ti/Ce co-modified 2195 alloys at relatively high scan speeds, while the cracking suppression mechanism and phase formation in these composites are not clarified. In this work, microstructure evolution and mechanical performance of the LPBF-fabricated Ti/Ce co-modified 2195 are investigated to reveal their cracking suppression and strengthening mechanisms. The results show that apparent grain refinement of the composites is ascribed to high supercooling from rapid formation of constitutional supercooling zone in front of solid–liquid interfaces by high-Q-value Ti solute, and heterogeneous nucleation of in situ formed Al3Ti and Al11Ce3 precipitates. Their synergistic interactions promote formation of fine equiaxed grains and thus inhibit crack initiation. The composites exhibit high microhardness of 100 ± 5 HV0.2, nano-hardness of 1.6 ± 0.1 GPa and elastic modulus of 97 ± 3 GPa, where the elastic modulus increases by ∼27% and ∼31% compared to those of LPBF-processed and conventionally manufactured 2195 alloys, respectively. A tensile strength of ∼336 MPa and an elongation of ∼3% are obtained from in-situ synchrotron X-ray diffraction measurement. The improved properties are derived from grain refinement and Orowan strengthening. Based on the optimal processing parameter and composition, a bracket component filled with lattice structures is designed and manufactured with good manufacturing quality and processing accuracy.

Grain refinement of flux-cored wire arc additively manufactured duplex stainless steel through in-situ alloying of Ti

One of the major problems in the wire arc additive manufacturing (WAAM) of duplex stainless steel (DSS) is the epitaxial growth of coarsen columnar ferrite grains crossing the deposition layers, which leads to the mechanical property anisotropy and increases the risk of hot tearing. In this investigation, Ti was in-situ alloyed to the WAAM DSS to promote grain refinement of ferrite. The results revealed that the ferrite transformed from columnar to equiaxed morphology with the increase of Ti content. In the sample with 0.5%Ti addition, some fine equiaxed ferrite grains with an average grain size of 39 μm formed at the fusion line, which inhibited the epitaxial growth of columnar grains. With further increase of Ti addition to 1.0%, the ferrite grains in the as-deposited WAAM DSS completely transformed from columnar to equiaxed structure, with an average grain size of 12 μm at the fusion line and 19 μm at the interior of deposition layer. Ti2N-Al2O3 particles with an average diameter of 400 nm were found in the equiaxed grains at room temperature, which existed in the form of TiN-Al2O3 in the molten pool and promoted the heterogeneous nucleation of ferrite. The analyzation based on Interdependence Theory demonstrated that Ti refined the ferrite in WAAM DSS by promoting the formation rate of the constitutional supercooling zone ahead of the solid/liquid interface and by introducing effective nucleat particles into it.

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.

In Situ Wire + Powder Synchronous Arc Additive Manufacturing of Ti-Cu Alloys.

In this study, a new wire + powder synchronous arc additive manufacturing technique was used to manufacture Ti-Cu alloys. The microstructure and properties of the as-fabricated alloys were studied. The results showed that the prepared Ti-Cu alloys have good properties. The Cu with high growth restriction factor can increase the constitutional supercooling zone in the Ti-Cu alloys, which can override the negative effect of a high thermal gradient in the manufacturing process. Through the observation of the microstructure, the as-printed Ti-Cu alloy specimens have equiaxed fine-grained microstructure. Through corrosion performance analysis, the Cu can also make the passivation film of the alloy more compact and make the alloy more corrosion resistant.

Effect of liquid film constitutional supercooling at the end of solidification on the hot cracking behavior of Al-4.0 wt.%Cu alloy

• Molecular dynamics method was used to calculate the solute diffusion coefficient. • Effect of liquid film constitutional supercooling on hot cracking was studied. • The influence of dendrite growth on hot cracking behavior was excluded. In this study, the molecular dynamics (MD) calculation method was used to calculate the diffusion coefficient of solute atoms in the liquid film (LF) at the end of solidification, and the effect of constitutional supercooling in LF on the hot cracking tendency of Al-4.0 wt% Cu alloy was investigated combined with experiments. The MD calculation result indicated that the diffusion coefficient of Cu elements ( D L ) increased as the atomic percentage of that in the LF decreased at a specific temperature within the solidification range (620 °C was chosen in this study). Supercooling degree produced by constitutional supercooling ( ΔT c ) decreased while the constitutional supercooling zone width ( Δx ) increased with D L , which facilitated extending the LF complementary shrinkage zone during solidification, and cellular grains easily formed after solidification, thus reducing the hot cracking tendency of Al-4.0 wt.%Cu alloy. The experimental results concluded that the hot cracking tendency of Al-4.0 wt.%Cu alloy decreased as the atomic percentage of Cu elements in the LF decreased, which verified the accuracy of the MD calculation results.

Experimental study on the grain evolution induced by thermal characteristics during oscillation laser welding of IN718

• Grain morphology was significantly improved during oscillation laser welding. • Two-color pyrometer was used to measure the cooling rate and temperature gradient. • Effect of thermal behaviors on grain distribution was investigated. • Tensile strength and ductility rate of IN718 joint were simultaneously strengthened. An oscillation laser beam was conducted to refine the grain size of IN718 welded joint. The thermal characteristic differences between non-oscillating and oscillating laser welding and their influence on the microstructure and mechanical properties were investigated using a two-color pyrometer. The experiment results showed that the grain size reduced from 291.03 μm(0 Hz) to 171.27 μm(150 Hz) and the crystalline orientation weaken. The derivation of temperature cycle curve indicated the cooling rate in the center and edge of molten pool(150 Hz) increased by 64.4% and 111.9%, and the temperature gradient decreased from 218.5 0 C/mm(0 Hz) to 73 0 C/mm(150 Hz), which illustrated an increase in constitutional supercooling zone width and the nucleation sites to refine grain size. The tensile load and ductility rate of oscillation laser-welded IN718 joint were simultaneously improved.

Research on plasma arc additive manufacturing of Inconel 625 Ni–Cu functionally graded materials

In this work, a plasma arc additive manufacturing technique with a double-wire feedback mechanism was used to manufacture Inconel 625 Ni–Cu functionally graded materials. The microstructure, mechanical properties and corrosion properties of the deposited alloy were evaluated. Microstructure observation and tensile testing of parts with different element contents indicated that functionally graded materials were obtained through the manufacturing process. The results have shown that due to the very high growth restriction factor Q of Cu, its addition can increase the constitutional supercooling zone, resulting in a decrease in columnar and dendrites and an increase in equiaxed crystals in the Inconel 625 Ni–Cu functionally graded material, as well as changing the Schmid factor (SF) distribution. The tensile strength and ductility of the Inconel 625 Ni–Cu functionally graded materials increased with increasing Cu content, while the corrosion resistance of the alloy decreased with increasing Cu content during electrochemical corrosion tests.

In-situ alloying of a fine grained fully equiaxed Ti-based alloy via electron beam powder bed fusion additive manufacturing process

Alloy design using Additive Manufacturing (AM) methods is an interesting research area attracting the attention of researchers across the world. Formation of strong texture, columnar grains, and chemical inhomogeneity in AM-designed alloys are among the serious challenges dealt with in various research works. This paper addresses these challenges for the first time for a Ti6Al4V–7Cu alloy produced via Electron Beam Powder Bed Fusion (EB-PBF) method following an in-situ alloying approach. In this work, a mixture of Ti6Al4V and elemental Cu powders was used as the feedstock. The results showed a relatively homogenous Cu distribution in the product due to the large melt pool created during the EB-PBF process, a development indicating the method's applicability for in-situ alloying and alloy design. The prior β columnar grains were effectively converted into equiaxed grains due to a constitutionally supercooled zone created by the rejection of Cu solutes in front of the solidification front and a relatively lower thermal gradient during EB-PBF compared with other AM processes. The addition of Cu enhanced the microhardness of Ti6Al4V, which could be considered an outcome of equiaxed grain formation, solid solution strengthening mechanism, and Ti2Cu intermetallic precipitation.

Effect of Nucleant Particle Agglomeration on Grain Size

Solute accumulation/depletion in the liquid around a growing solid particle during the solidification of metallic melts creates a constitutionally supercooled (CS) zone that has a significant effect on the final solidified grain structure. In this paper, we introduce two mechanisms related to the CS zone that affect grain size: one is the grain initiation free zone (GIFZ) that describes the inability of nucleant particles located in the CS zone for grain initiation and the other is re-melting (RM) of solid particles due to overlap of CS zones. Based on these two mechanisms, we have systematically analysed the effect of nucleant particle agglomeration on grain size. We found that nucleant particle agglomeration has a significant effect on grain size and is responsible for the discrepancy between theoretically predicted grain size and the experimental data. In addition, our numerical analysis suggests that under normal solidification conditions relevant to industrial practice solid particle re-melting has little effect on grain size and thus may be ignored during theoretical analysis. A practical implication from this work is that significant grain refinement can be achieved by dispersing the nucleant particles in the melt prior to solidification.

Effect of Ce on solute redistribution in liquid ahead of solid–liquid interface during solidification of Fe–4 wt.%Si alloy

The high efficiency of Ce addition in grain refinement of δ-ferrite in a cast Fe–4 wt.%Si alloy was verified. In order to further understand the solute effect of Ce on the grain refinement of δ-ferrite, the conventional directional solidification technique, which enabled to freeze the solid–liquid interface to room temperature, was used to investigate the interfacial morphology and solute redistribution in the liquid at the front of the interface, together with thermodynamic calculation of the equilibrium partition coefficients of Ce and Si in Fe–4 wt.%Si–Ce system using the Equilib module and the FsStel database in FactSage software system. Metallographic examination using a laser scanning confocal microscope showed a transition of the solid–liquid interface from planar to cellular in the Fe–4 wt.%Si alloy after adding 0.0260 wt.% Ce during the directional solidification experiment. Further, electron probe microanalysis revealed an enhanced segregation of Si solute in the liquid at the front of the solid–liquid interface due to the Ce addition. This solute segregation is considered as the cause of planar to cellular interface transition, which resulted from the creation of constitutional supercooling zone. Thermodynamic calculation indicated that Ce also segregated at the solid–liquid interface and the Ce addition had negligible effect on the equilibrium partition coefficient of Si. It is reasonable to consider that the contribution of Ce to the grain refinement of δ-ferrite in the cast Fe–4 wt.%Si alloy as a solute was marginal.

Effect of Fe Content on the As-Cast Microstructures of Ti–6Al–4V–xFe Alloys

In this work, the evolution of the solidification microstructures of Ti–6Al–4V–xFe (x = 0.1, 0.3, 0.5, 0.7, 0.9) alloys fabricated by levitation melting was studied by combined simulative and experimental methods. The growth of grains as well as the composition distribution mechanisms during the solidification process of the alloy are discussed. The segregation of the Fe element at the grain boundaries promotes the formation of a local composition supercooling zone, thus inhibiting the mobility of the solid–liquid interface and making it easier for the grains to grow into dendrites. With the increase in Fe content, the grain size of the alloy decreased gradually, while the overall decreasing trend was mitigated. The segregation of Fe was more obvious than that of Al and V, and the increase in Fe content had less effect on the segregation of Al and V.

Tailored microstructures and strengthening mechanisms in an additively manufactured dual-phase high-entropy alloy via selective laser melting

Dual-phase high-entropy alloys (DP-HEAs) with excellent strength-ductility combinations have attracted scientific interests. In the present study, the microstructures of AlCrCuFeNi3.0 DP-HEA fabricated via selective laser melting (SLM) are rationally adjusted and controlled. The mechanisms engendering the hierarchical microstructures are revealed. It is found that the AlCrCuFeNi3.0 fabricated by SLM at the scanning speed of 400 mm s−1 falls into the eutectic coupled zone, and increasing the scanning speed will make this composition deviate away from the eutectic coupled zone due to the increased cooling rate. The enrichment of Cr and Fe solutes with large growth restriction values ahead of the solid/liquid interface can develop a constitutional supercooling zone, thus facilitating the heterogeneous nucleation and near-equiaxed grain formation. The synergy of the near-eutectic DP nano-structures and near-equiaxed grains instead of columnar ones effectively suppresses cracking for the as-built DP-HEA. During the tensile deformation, the intergranular back stress hardening similar to the grain-boundary strengthening is discovered. Meanwhile, the near-eutectic microstructures comprised of soft face-centered cubic and hard ordered body-centered cubic (B2) DP nano-structures lead to plastic strain incompatibility within grains, thus producing the intragranular back stress. The Cr-rich nano-precipitates inside the B2 phase are found to be sheared by dislocation gliding and can complement the back stress. Additionally, multiple strengthening mechanisms are physically evaluated, and the back stress strengthening contributes obviously to the high performances of the as-built DP-HEA.

Synthesis of Bulk Amorphous Alloy from Fe-Base Powders by Explosive Consolidation

A Fe61Cr2Nb3Si12B22 amorphous alloy rod sample of 8.8 mm diameter has been successfully prepared through explosive consolidation. The structure and thermal stability of the as-synthesized sample have been analyzed through X-ray diffraction (XRD) and differential scanning calorimeter (DSC) analysis. The results demonstrate that the sample still retains an amorphous structure, and the glass transition temperature (Tg), the crystallization onset temperature (Tx), the supercooled liquid zone (ΔTx) (Tx − Tg) and the reduced glass transition temperatures (Trg) (Tg/Tm) are 784 K, 812 K, 28 K, and 0.556, respectively. Its microstructure has been investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The average microhardness of the alumina compact is about 1069 HV.

The effect of the melt thermal gradient on the size of the constitutionally supercooled zone

Recent verification of the analytical Interdependence model by a numerical solidification model (µMatIC) confirmed the critical role of constitutional supercooling (CS) in achieving sufficient undercooling to trigger successful nucleation events. The location of the maximum amount of CS (ΔTCSmax) is some distance from the interface of the previously growing grain and this distance contributes to the final as-cast grain size. The effect of the thermal gradient, G, on the size of the CS zone (CSZ) was neglected in that work. However, G is expected to affect the size of the CSZ (i.e. the length of the CSZ, x’CSZ, and the location of ΔTCSmax, x’CSmax). This investigation assesses the effect of G on x’csz and x'CSmax. A range of G values is introduced into both the analytical and the numerical models to obtain a correlation between the value of G and the dimensions of the CSZ. The result of a test case from the analytical model shows that x’CSmax initially decreases rapidly and then decreases gradually approaching zero at very high values of G. Independent of the analytical model, the results from the numerical model replicate the trend obtained from the analytical model.

The Influence of the Effect of Solute on the Thermodynamic Driving Force on Grain Refinement of Al Alloys

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants.

Numerical analysis of constitutional supercooling during directional solidification of alloys

Some limitations of Tiller’s morphological stability criterion are discussed in the present study. This criterion assumes a purely diffusive regime in the melt as well as a planar solid–liquid interface and a constant solidification rate. But experimental works in agreement with previous numerical modeling have shown a significant decrease of the growth rate and a variable interface curvature during the concentrated semiconductor alloys solidification. The mathematical expression of the morphological stability criterion was derived by using Tiller’s equation, predicting the solute distribution in the liquid. The numerical computations performed in this study show a significant disagreement between the numerical results and Tiller’s formula. Numerical modeling conducted in conditions when the supercooling should occur, show that the Tiller’s stability criterion cannot predict the moment of interface destabilization. The interface destabilization is numerically observed when some fluctuations appear in the liquid solutal profiles and cause the appearance of a supercooled zone inside the liquid at small distance from the interface. The present numerical results are not in contradiction with the basic elements of the classical constitutional supercooling theory, providing only that the stability criterion cannot predict the moment of the interface destabilization.

Effect of boron addition on microstructure and mechanical properties of TiC/Ti6Al4V composites

TiC/Ti6Al4V composites with different boron additions were successfully fabricated, and the effects of the boron content on the microstructure, mechanical properties of composites were investigated. The results show that the dendritic TiC in the composites is remarkably refined when the boron content is less than 0.06wt.%. The average primary axial length of TiC dendrite is decreased from 150μm to 50μm according to the increasing of the boron content from 0.01wt.% to 0.06wt.%. And TiC dendrite gradually changes from the coarse dendrite to the fine dendrite or granular, but the abrasive property is obviously reduced with the increase of boron content. On the other hand, when the boron content further increases from 0.1wt.% to 0.6wt.%, the dendritic TiC can not be refined obviously, the fiber-shape TiB begins to appear and the abrasive property of composites is improved. The compressive strength and hardness (HRC) of composites are obviously increased in the whole addition range. The refinement mechanism of boron is attributed to the combined effects of the increase in nucleation rate at the constitutionally supercooled zone in front of the solid–liquid interface and the reduction in growth rate of TiC. The improvement of its abrasive property is mainly attributed to the existence of large size TiC and fiber-shape TiB during the abrasion.

Peculiarities of the initial transient process of binary melt crystallization

The time dependence of the crystallization rate V(t) in the transient process that occurs during crystal pulling into a cold zone at a constant rate W has been investigated within the one-dimensional time-dependent model of binary melt solidification using numerical and analytical approaches. It is shown that the character of the dependence V(t) in this process is mainly determined by the parameter B w = V c /W, where V c is the crystallization rate at which the constitutional supercooling zone is formed in the melt. At B w ≥ 1, the function V(t) monotonically approaches its steady-state value W, while at B w < 1 V(t) is generally nonmonotonic. The nonmonotonic character of V(t) can manifest itself both as aperiodic damping and in the form of damping oscillations. At B w ∼ 0.1, the vibrational amplitude of the crystallization rate in the initial transient process may reach several tens of a percent of W.

On the dependence of the critical crystallization rate on the initial impurity concentration in melt

The dependence of the critical crystallization rate on the initial impurity concentration in melt is derived by determining the condition at which a nonplanar solution to the stationary diffusion problem arises. It is suggested that the conditions at which defects arise at the interface differ from those obtained when crystallization becomes stationary. The initial transient process of binary melt solidification under crystal pulling at a constant rate has been studied numerically within a 1D model. It is shown that the character of the time dependence of the crystallization rate is determined by the ratio of the crystal pulling rate to solidification rate Vc, when a constitutional supercooling zone is formed in the melt.

EFFECT OF ZIRCONIUM ADDITION ON MICROSTRUCTURE AND MECHANICAL PROPERTY OF TiC/Ti6A14V COMPOSITES

TiC/Ti6A14V composites with different Zr additions were prepared successfully in a consumable vacuum arc furnace equipped with a water-cooled copper crucible and the effect of the Zr content on the microstructure and mechanical property of 15 vol.% TiC/Ti6A14V composites was investigated by XRD, SEM and hardness testing. The results show that when the level of Zr addition is less than 4 wt.%, the morphology of the primary TiC in the composites is dendrite, and the petal-shape, piece-shape or palpus-shape eutectic TiC separates out around the primary TiC . The average size of the primary TiC decreases and the amount of eutectic TiC increases gradually with increasing Zr content. The effects of Zr on morphology of the primary TiC weaken with further addition of Zr . And the hardness (HRC) of composites was obviously increased in the whole range of Zr addition. The refinement mechanism of Zr was attributed to the combined effects of increase in nucleation rate at the constitutionally supercooled zone ahead of the solidification front and reduction in growth rate.