Solidification Of Molten Pool 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.

Research on gas pore formation and inhibition mechanism of high nitrogen steel during laser direct metal deposition

Nitrogen pore formation is an inherently tricky problem that limits the application of high nitrogen steel (HNS) in various fields. In the present study, a CA-LBM coupling model was proposed to understand the nitrogen pore formation mechanism during laser direct metal deposition (DMD) of HNS. Based on the proposed model and theoretical analysis, the effects of pulse frequency and shielding gas nitrogen content on the porosity were clarified systematically. Results showed that after the onset of solidification, the ferrite phase with extremely low nitrogen solubility precipitated first, inducing the continuous enrichment of nitrogen in the residual liquid phase, which in turn provided the prerequisite for nitrogen pore nucleation. The application of quasi-continuous-wave (QCW) laser mode accelerated the molten pool solidification, and shortened the nitrogen supersaturated region, allowing more nitrogen to be solid dissolved in the matrix. On this basis, the porosity was inhibited to a certain extent. Compared with the high-frequency pulse, the low-frequency pulse showed better porosity inhibition. In addition, following the classic nucleation theory, nitrogen segregation and nitrogen solubility synergistically dominated nitrogen pore formation. The nitrogen partial pressure, determined by the shielding gas nitrogen content, played a dual role in porosity defects. Elevated nitrogen partial pressure served to increase nitrogen solubility while promoting the nitrogen absorption process to intensify nitrogen segregation. However, due to the short duration of the molten pool, the nitrogen absorption process could not be fully processed. Nitrogen solubility enhancement was the key factor in inhibiting porosity during the DMD process.

Cold-wire assisted arc direct-energy deposition of VC-strengthening 5A06 aluminum alloy: Microstructures and mechanical properties

In this study, vanadium carbide (VC) was initially introduced into the molten pool using the cold wire method to refine the deposit grains and enhance the mechanical properties of arc direct energy deposition (ADED) aluminum alloy. Next, the grain structure's evolutionary behaviors and the second phase in the VC-strengthening ER5A06 aluminum alloy deposits were investigated, and the grain refinement mechanism was determined. The findings demonstrated that the cubic Al10V phase was produced after introducing VC, and the grain structure in the middle stable region of the building wall was transformed from coarse columnar grains (with an average size of 42.3 μm) into equiaxed fine grains (with an average size of 16.4 μm). Both Al10V and VC were coherent to α-Al in terms of orientation: (020)α-Al//(444)Al10V, [101―]α-Al//[112―]Al10V, and (111)α-Al//(111)VC and [011]α-Al//[011]VC. Their interatomic and interplanar spacing misfits were lower than 5 %, facilitating the heterogeneous nucleation of α-Al during molten pool solidification. Moreover, under multiple thermal cycles of ADED, the pinning effect of VC and Al10V effectively restricted the grain growth, thus enabling the building wall to steadily maintain a uniform, equiaxed, and fine-grained structure. Due to grain refinement and homogenization, the horizontal and vertical ultimate tensile strengths of the straight wall were improved by 16.8 % and 50.2 % (351.6 and 348.1 MPa), respectively, thereby eliminating the mechanical anisotropy. This work can provide a theoretical foundation and a novel method for the fine microstructure regulation and reinforcement of ADED aluminum alloy components.

Unveiling arc deflection instability in narrow gap laser-arc hybrid welding of thick Ti-6Al-4V plate

Unveiling the mechanism behind arc deflection under narrow gap laser arc hybrid welding (NGLAHW) is of paramount significance in mitigating laser-arc coupling failure and welding porosity for thick Ti-6Al-4V plate structure. This paper utilizes high-speed camera technology, data statistics and model prediction to comprehensively study the influence of constrained groove space and droplet transfer mode on arc deflection. The results demonstrate that narrowing groove gap exerts a substantial compressive effect on welding arc behavior, particularly rendering it more susceptible to arc deflection when the groove gap is smaller than the arc length. The small droplet transfer mode is characterized by stable arc behavior, whereas the large droplet transfer mode tends to be susceptible to arc instability due to the droplet deviation induced by turbulent metal vapor. Particularly noteworthy is the emergence of substantial arc deflection when the droplet deviation exceeds the critical threshold of 0.51 mm, which diminishes the driving force for droplet transfer and subsequently leading to potential short-circuiting explosions and unstable welding process. Moreover, the lateral force induced by arc deflection drives a deviation in arc heat source with accelerated solidification of molten pool, consequently resulting in an elevation in welding porosity. This paper has provided a comprehensive insight into the mechanism driving arc deflection in NGLAHW, which holds paramount importance in enabling effective strategies to regulate the arc behavior.

Improving weld forming quality and process adaptability by using a copper liner in laser-MAG hybrid overhead welding

During overhead welding, gravity inevitably drives the molten pool downward, resulting in poor adaptability and concave defect. To address this issue, this paper proposed a novel laser-MAG hybrid overhead welding technique utilising a copper liner (COL). Compared to single-arc welding, laser-MAG hybrid welding can reduce the volume of the molten pool, while the laser increases its support force. The COL can accelerate the molten pool's solidification rate. Additionally, the adhesion force between the molten pool and the COL can partially counterbalance gravity. As a result, good weld formation still was achieved even when the misalignment was 0–1 mm and the butt gap was 0–2 mm. Moreover, the back reinforcement height and width were not less than 0.71 and 3.66 mm, respectively.

Numerical investigations of arc plasma characteristic parameters evolution and metal properties in GMAW-based WAAM of Al alloy with an integrated model

Understanding the arc plasma characteristic parameters evolution and the metal thermodynamics is of great significance for guiding the improvement of the forming accuracy and performance of GMAW-based WAAM in engineering. In this study, we have developed a three-dimensional arc plasma-droplet-molten pool integrated model of GMAW-based WAAM of Al alloy, which simultaneously includes the heating of arc plasma, the vaporization of Al metal, the transfer of droplet, the melting and solidification of molten pool, and the relative motion between wire and substrate. Based on this sophisticated model, the effect of Al vapor on the arc plasma characteristics, the dynamic evolution of arc plasma characteristics during the metal transfer process, and the heat transfer and flow behaviors of liquid metal are comprehensively investigated. The simulation results show that the Al vapor is principally generated from the surface of droplet, which leads to the decrease of arc plasma temperature and arc pressure, and the expansion of current path. In a droplet transfer cycle, the average temperature of plasma decreases first and then increases, affected by the distribution of current density. Most of heat for the formation of molten pool comes from the energy transfer of droplet, and the droplet impact effect on the molten pool flow is significantly greater than Marangoni effect. Finally, the simulation results, including droplet transfer process and deposited layer profile, are validated by experimentally measured results, which are in good agreement with each other.

Effect of laser remelting on copper‑nickel alloy coating prepared by extreme high-speed laser cladding

Copper‑nickel alloy is an important corrosion resistant material for marine environment. Advanced extreme high-speed laser cladding (EHLA) technology can realize a metallurgical bonding of copper‑nickel alloy and obtain better surface and structural characteristics. However, because of the high heat conductivity and high reflectivity of copper, copper-alloy coatings prepared by EHLA have defects such as segregation, blowhole, and cracks. In this study, EHLA copper‑nickel alloy coating was improved by in-situ laser remelting process. A single pass finite-element-model of the remelting process was built to analyze the effect of remelting speed on the solidification of molten pool. The surface morphology, microstructure, element distribution, and phase composites of coatings at different remelting speeds were characterized, and the anti-corrosion of the coatings was further discussed by constant potential polarization and EIS. The results show that heat input increases the convection strength of liquid in molten pool, promotes the uniformity of the coating microstructure and reduces the defects. Although the columnar crystals to equiaxed crystals transformation of the coatings did not obviously changed at different remelting speed, the high heat input during remelting speed of 5 m/min promoted the alloying of copper‑nickel coating and eliminated the defects, obtaining the best corrosion resistance. The research will provide some technical guidance and theoretical support for the practical application of copper‑nickel alloy coating with EHLA-LR process in marine equipment.

Effects of laser oscillation on metal mixing, microstructure, and mechanical property of Aluminum–Copper welds

Laser welding of dissimilar metals is important in many industrial applications. However, as dissimilar metals get mixed during the melting process, intermetallic compounds are often formed in the welds which can significantly undermine the electrical and mechanical properties of the welds. This poses a critical challenge to the widespread utilization of this welding technique. Compared with conventional line-scan laser welding, oscillating laser welding offers additional processing parameters to control the welding process. Although some work has been reported on oscillating laser welding of dissimilar metals, a mechanistic understanding of this process was still missing. The research objective of this work was to reveal the physical mechanisms and evaluate their relative significance to the fluid flow, metal mixing, and microstructure evolution in the molten pool in oscillating laser welding of dissimilar metals. A combination of experiments and simulations was leveraged to achieve the objective. Four fluid flows have been found to determine the metal mixing in the molten pool, and their dependences on the laser oscillating parameters were discussed. In addition, the thermo-solutal conditions of the molten pool solidification were quantified as functions of the laser oscillating parameters, and the effects of the thermo-solutal conditions on the final weld microstructures were analyzed.

Improvement of the welded joints properties of P460NH steel with AC-assisted GTAW arc

Aiming at the problems of small grain size and difficult welding of P460 NH steel, an efficient and high-quality welding method of AC assisted arc welding is proposed. In this AC-assisted arc welding process, the alternating current was added to the filling wire to form an additional electromagnetic field. The alternating electromagnetic force generated by the interaction of the main arc and other electromagnetic fields can stir the molten pool during the welding process. The feasibility of improving the grain size and mechanical properties of microstructures with AC power in 0-30A was confirmed. The experiment proved that the arc oscillation caused by the alternating electromagnetic force can alleviate the root stay of the traditional GTAW process. When the AC strength doesn't exceed 20A, the average grain size decreases while the content of acicular ferrite and granular bainite increases. The average grain size is 3.258 μm refined to 2.960 μm. Moreover, the tensile strength and hardness of welded joints first increase with the addition of current and then decrease with the rise of current to 30A. Finally, the mechanism of AC on microstructure and solidification of molten pool was discussed.

Research on the evolution mechanism of solidified structure during laser cladding IN718 alloy

• An improved CA-FE model is used to explore the evolution of the microstructure. • A CET model is established to quantitatively study the solidification parameters. • The relationship between solidification parameters and microstructure is explored. • The mechanism of grain growth during the solidification is revealed. • Increasing the laser power and scanning speed will promote the CET process. In the direct energy deposition (DED) process of IN718, the microstructure significantly affects the properties of the workpieces, but the evolution mechanism of the microstructure has not been systematically studied, so it is valuable to understand the solidification process in the molten pool. In this study, a simplified temperature field model is established and the cellular automata (CA) model is used to explore the evolution of the microstructure during DED, which can effectively simulate the microstructures of IN718 with different process parameters. Then a columnar to equiaxed crystal transition (CET) model is established to quantitatively reveal the relationship between solidification parameters and microstructures throughout the solidification process, and the mechanism of grain growth during the solidification of molten pool is further explored, finding that increasing the laser power and scanning speed will promote the CET process with the equiaxed crystal content increasing from 6.18% to 33.8%. This study can be used to simulate the solidification process of IN718 during DED and obtain the required microstructures by optimizing the processing parameters.

Study on properties and microstructure of laser beam butt welded joints of Al-Si coated USIBOR® 1500 steel

The aim of the work is to analyze the laser welding quality of USIBOR®1500-AS, without the coating removing. Through metallographic analysis, the martensitic phase was found both in the Fused Zone and in the Heat Affected Zone. Within the Fused Zone was, also, found a certain amount of Al-Si coating, which essentially depended on the welding speed. That coating modified the molten pool solidification mechanism, but did not affect the welds characteristics, in terms of defects and hardness. The welding speed did not affect the hardness values, while strongly influenced the Fused Zone and Heat Affected Zone widths.

Laser-arc hybrid additive manufactured Al[sbnd]Cu alloy for T-shaped structure: Microstructure evaluation and molten pool behavior

T-shaped structure is widely used in the aeronautic and aerospace industries for the development of lightweight applications. However, poor metallurgical bonding and thermal defects by remelting at the intersection is prone to occur in the additive manufacturing of the structure. In this study, T-shaped structure of AlCu alloy was fabricated by laser-arc hybrid additive manufacturing (LAHAM), and the molten pool behavior of the process was analyzed. According to the remelting of intersection, it was divided into the top layer and the remaining layers. The remelting zone of intersection (RZI) was obvious in the top layer. During the solidification of molten pool from the boundary to the center, the ratio of temperature gradient G to solidification rate R (G/R) at the S/L interface decreased from 2.57 × 108 (K·s)/m2 to 9.27 × 107 (K·s)/m2, and promoted the transformation of equiaxed dendrites. In the remaining layers, the recoil pressure, much smaller G/R and larger G∙R made the grains as fine equiaxed along the z direction in the laser zone (LZ), while coarse columnar were observed in the arc zone (AZ). Due to the microstructure changes, the deposited T-shaped structure exhibited a yield strength of 146.5 ± 2.2 MPa and an ultimate tensile strength of 290.3 ± 5.9 MPa.

Laser powder bed fusion of Zr-modified Al–Cu–Mg alloy: Crack-inhibiting, grain refinement, and mechanical properties

The application of laser powder bed fusion (L-PBF) in conventional high-strength wrought aluminum alloys is a significant challenge due to its high cracking sensitivity. In this study, 2024Al and 1.3-wt% Zr-modified 2024Al alloy deposits were prepared by L-PBF using pre-alloyed powder. The solidification cracking susceptibility of the 1.3-wt% Zr-modified 2024Al alloy was reduced to ∼0.4 of that of the 2024Al alloy. Compared to the complete columnar α-Al grain microstructure in the 2024Al deposit, the crack-free Zr-modified 2024Al deposit exhibited a heterogeneous grain structure having columnar α-Al grains in the inner region of the molten pool and ultrafine α-Al equiaxed grains at the fusion boundary. Based on the time-dependent nucleation theory and constitutional undercooling analysis ahead of solidification interface, the precipitation behavior of primary Al3Zr nanoparticles is analyzed, and it is deduced that the high intensity of effective nucleation sites by nanosized primary Al3Zr at the fusion boundary of molten pool leads to the formation of equiaxed grains at the beginning of the solidification of molten pool. The as-deposited Zr-modified 2024Al exhibited a yield strength (YS) of ∼376 ± 7 MPa and an ultimate tensile strength (UTS) of 441 ± 7 MPa with an elongation of 14.1 ± 1.6%. The YS and UTS increased to 402 ± 9 MPa and 483 ± 37 MPa after T6 heat treatment, respectively. Both the tensile properties of the as-deposited and T6-treated Zr-modified 2024Al were comparable to the wrought 2024-T651 alloy and higher than most of the previous L-PBFed Al–Cu–Mg alloys.

The Investigation of microstructure evolution in electron beam additive manufacturing by phase field method

The mechanical performance of the components is highly related to the microstructure characteristics in the electron beam additive manufacturing (EBAM). The solidification process with complicated behaviors during the EBAM plays a major role in the microstructure formation. This paper is focused on the dendrite evolution in the solidification process of the EBAM of Ti-6Al-4V. A phase field model coupled with macroscopic solidification of molten pool which takes the heat transfer of molten metal and transient temperature field into consideration is proposed. The microstructure evolution and solute field distribution about columnar crystal are analyzed quantitatively. The simulated results are compared with that from the analytical model and good agreement between them has been found. It is demonstrated that the evolution of microstructure is mainly determined by the temperature gradient and solidification velocity of the molten pool. The solute enrichment behavior and severe micro-segregation of columnar crystal have great effect on the morphology and growth velocity of microstructure during the EBAM. The proposed phase field model can be used to reduce the internal structure defects and select the optimal process for the desired components of the EBAM with excellent microstructure and performance.

Effect of rescanning with reduced interlayer times on microstructure and properties of titanium alloy via selective laser melting

In the process of selective laser melting, laser rescanning technology is often used to optimize the residual stress and other properties of the formed parts. In order to improve the performance of parts and reduce the manufacturing time concurrently, this paper proposed a combined rescanning strategy. Based on finite element analysis, molten pool solidification behavior was simulated and studied. Ti6Al4V alloy samples were fabricated and analyzed by changing the rescanning strategies and process parameters. The microstructure, relative density, size of pore defects and residual stress were investigated under different rescanning strategies. It can be seen that the average cooling rate ranked in a descending order of SLM > re-SLM+IL1 > re-SLM, and samples formed by rescanning without layer interval had the best relative density and residual stress optimization effect, while the microstructure of each scanning strategy was all acicular α′ phase. When the number of rescanning interlayers and laser power was “one-layer” and 140 W respectively, the residual stress went down from 353 to 294 MPa. Finally, a simplified model was proposed to calculate the time cost for fabrication of rescanning with reduced interlayer times.

Numerical Analysis of Microstructure Anomalies during Laser Welding Nickel-Based Single-Crystal Superalloy Part III: Amelioration of Solidification Behavior

The contribution of crystallography-dependent metallurgical factors, such as supersaturation of liquid aluminum and minimum dendrite tip undercooling, to solidification behavior and microstructure development is numerically analyzed during Ni-Cr-Al ternary single-crystal superalloy molten pool solidification to better understand thermodynamic and kinetic driving forces behind solidification cracking resistance. The variation of supersaturation of liquid aluminum and minimum dendrite tip undercooling with location of solid/liquid interface is symmetrically consistent in (001)/[100] welding configuration. By comparison, the variation is asymmetrically consistent in (001)/[110] welding configuration. The different distribution is attributed to growth crystallography and dendrite selection. Significant increase of supersaturation of liquid aluminum and dendrite tip undercooling from [010] dendrite growth region to [100] dendrite growth region preferentially aggravates microstructure development as result of nucleation and growth of stray grain formation with the same heat input on each half of the weld pool in (001)/[110] welding configuration. High heat input (both increasing laser power and decreasing welding speed) exacerbates supersaturation of liquid aluminum and dendrite tip undercooling by faster diffusion to incur stray grain formation with severity of contributing thermometallurgical factors for susceptibility to solidification cracking, while low heat input (both decreasing laser power and increasing welding speed) ameliorates microstructure development and increases resistance to solidification cracking. Weld microstructure of optimum welding conditions, such as combination of low heat input and (001)/[100] welding configuration, is less susceptible to solidification cracking to suppress asymmetrical microstructure development and improve weld integrity potential rather than insidious welding conditions, such as combination of high heat input and (001)/[110] welding configuration. Severer supersaturation of liquid aluminum and wider dendrite tip undercooling occur in the [100] dendrite region as consequence of alloying enrichment, while smaller supersaturation of liquid aluminum and narrower dendrite tip undercooling occur in the [001] dendrite region as consequence of alloying depletion to spontaneously facilitate epitaxial growth of single-crystal essential. Symmetrical (001)/[100] welding configuration decreases growth kinetics of dendrite tip with smaller overall supersaturation of liquid aluminum and dendrite tip undercooling than that of asymmetrical (001)/[110] welding configuration regardless of combination of laser power and welding speed. Mitigation of supersaturation of liquid aluminum and dendrite tip undercooling simultaneously alleviate crack-susceptible microstructure development and solidification cracking. Additionally, the appropriate mechanism of solidification cracking resistance improvement through modification of crystallography-dependent supersaturation and undercooling of dendrite tip is proposed. Calculation analyses are sufficiently explained by experiment results in a reasonable way. The additional purpose of this theoretical analysis is to evaluate solidification cracking susceptibility of similar nickel-based or iron-based single-crystal superalloys.

Influence of laser pulse on solidification of molten pool during laser welding of dissimilar Ti-based amorphous alloys

Dissimilar Ti-based amorphous alloy sheets were butt welded by a pulsed laser and metallurgical joints were obtained. The β-Ti crystal grains in nanometre scale unevenly distribute on the amorphous background in the fusion zone. With the increase of laser pulse energy and pulse duration, the crystalline β-Ti grains grow up from spherical to dendritical profile, but the crystalline size reaches its maximum of about 1 µm, which is much smaller than the grains in base metal. The refinement of the grains in fusion zone results from the fast cooling speed during welding. The microhardness in the welded joints ranges from 470 to 525 HV. Average tensile strength of the welded joint is about 1510 MPa (86% of pure Ti-based amorphous alloy BM and 91% of β-Ti dendrite reinforced Ti-based amorphous alloy BM).

Effect of spreading of the melt pool on the deposition characteristics in laser directed energy deposition

Laser direct energy deposition (DED) is an innovative additive manufacturing technology with tremendous potential for remanufacturing and restoration of dies, molds, and components for aerospace. Accurate prediction of the deposition geometry is important to control the repair quality. In laser DED, the powder particles are fused onto the melt pool formed on the substrate surface. To estimate the deposition geometry estimating the spreading and solidification of molten pool is of utmost importance. Therefore, a computation fluid dynamics (CFD) model has been developed to estimate the melt pool size on the substrate and the spreading of the melt pool over the substrate before solidification. The model can be also used to estimate the catchment efficiency and deposition geometry. The experimental observations of solidified deposition geometry on a substrate corroborate the proposed model. The CFD model is utilized to develop data-driven models correlating the process parameters with the deposition geometry. This study, observes that larger the width of the deposition, important is the inclusion of the spreading of melt pool in the estimation of the deposition geometry.

Microstructural Evolution and Mechanical Properties of Electron Beam–Welded Ti70/TA5 Dissimilar Joint

AbstractThe dissimilar titanium alloys Ti70/TA5 are welded by using electron beam welding. The microstructure and mechanical properties of the welded joints are systematically investigated, and the welding parameters are optimized. Results show that the fusion zone (FZ) is mainly α’ martensite, and the heat-affected zone (HAZ) in the Ti70 side consists of fine α’ martensite, residual α phase, and original β phase, while the HAZ in the TA5 side is composed of coarser α phase, serrated and acicular α phase. Transmission electron microscope (TEM) analysis demonstrates that the martensite in the FZ presents the lath-like morphology. There are high-density dislocations within martensite, which has a certain orientation relationship with the β phase. Under the appropriate welding procedure, the tensile strength of the dissimilar joint is close to that of the TA5 base metal. The joint fracture dominantly presents the characteristic of ductile fracture. During welding, electron beam scanning is beneficial to improving the solidification of molten pool and grain refinement; thus, the mechanical property of the welded joint is increased to a certain extent.

Numerical analyses of molten pool evolution in laser polishing Ti6Al4V

Abstract As one advanced manufacturing process, laser polishing has been widely used in precision machining of metal or nonmetal materials. However, the evolution mechanism of molten pool formed during polishing process has not been thoroughly studied. This paper studies the polishing process of Ti6Al4V with a continuous wave (CW) fiber laser under the top-hat distributed heat source. A two-dimensional (2D) axisymmetric numerical transient model is established to simulate the formation process of the surface of Ti6Al4V after laser heating to form a molten pool and then cooling. It reveals that the evolution mechanism of physical processes involved in heat transfer, heat radiation, heat convection, melting and solidification during the polishing process. Particularly, the capillary and thermocapillary regimes (Marangoni flow) play a key role in smoothing the free-form surface of molten pool. The freedom surface can be smoothed effectively before the solidification of molten pool. In order to verify the correctness of the simulation model and explore the influence of laser irradiation duration on the material, the surface topography of the polished sample was investigated. It is found that the errors of the depth and width of the remelting zone are respectively less than 8 % and 22 % by the comparison of simulation and experimental results. Likewise, the depth and width of molten pool expand with the laser radiation duration increasing. Meanwhile, the polished surface topography becomes smoother.