Papers by Keyword: Dendrite Growth

Abstract: The dependency of morphology development and dendrite growth on welding conditions (laser power, welding speed and welding configuration) is numerically analyzed to decrease nucleation and growth of stray grain formation during laser processing aerospace component surface of ternary Ni-Cr-Al single-crystal superalloy. Proper (001)/[100] welding configuration crystallographically initiates three axisymmetrical distributions of microstructure development, i.e. stray grain formation, morphology development and dendrite trunk spacing, alongside the advancing solid/liquid interface of molten pool, whereby metallurgical properties are increased. Unpromising (001)/[110] welding configuration tends to crystallographically possesses unaxisymmetrical microstructure development to favor substantial crack-vulnerable dendrite size and morphology. Epitaxial [001] columnar dendrite growth region is favored for single-crystal dendrite growth, while vulnerable [100] equiaxed dendrite growth region is more susceptible to solidification cracking. The lower heat input is used, the smaller stray grain formation, negligible columnar/equiaxed transition (CET) and finer dendrite trunk spacing are consistently promoted by narrower constitutional undercooling ahead of solid/liquid interface to improve crack-resistant microstructure development and weld integrity. When comparing between [100] dendrite growth region on the right side and [010] dendrite growth region on the left side, (001)/[110] welding configuration spontaneously engenders severer stray grain formation, insidious columnar/equiaxed transition and coarser dendrite trunk spacing on the right side to deteriorate microstructure development with restriction of the same heat input on both sides of weld pool. The mechanism of asymmetrical solidification cracking as result of crystallography-induced microstructure degradation is therefore proposed. The theoretical predictions of asymmetrical solidification cracking susceptibility are comparable with experiments.

Abstract: Correlation between welding conditions, such as laser power, welding speed and welding configuration, microstructure anomalies, including columnar/equiaxed transition (CET) and stray grain formation, and metallurgical factors, such as aluminum redistribution and supersaturation near dendrite tip, is elucidated by thorough numerical analysis to explain prevalent phenomenon of insidious microstructure development and face the challenge of microstructure amelioration as well as solidification cracking resistance improvement during course of ternary single-crystal Nickel-Chromium-Aluminum superalloy melt-pool solidification for crack-free laser surface modification. Metallurgical factors of γ gamma phase microstructure development strongly depend on welding configuration, and heat input is not as important as welding configuration. Although melted-pool center region is susceptible to microstructure anomalies, (001)/[100] welding configuration possesses auspicious growth crystallography under which the bimodal profiles of solid aluminum concentration and liquid aluminum supersaturation ahead of dendrite tip are symmetrically distributed to potentially decrease solute redistribution and increase resistibility to solidification cracking. Dissimilarly, (001)/[110] welding configuration possesses insidious growth crystallography under which the profiles of solid aluminum concentration and liquid aluminum supersaturation ahead of dendrite tip are asymmetrically distributed to preferably facilitate alloying plentiful enrichment and differentiate problematical microstructure instability on half side of molten pool. In the bottom of molten pool, beneficial Al-barren [001] dendrite is kinetically driven by homologous single-crystal epitaxial growth without columnar/equiaxed transition. On the right side of molten pool, detrimental Al-rich [100] dendrite is spontaneously susceptible to stray grain formation with equiaxed morphology. Symmetrical microstructure development is more enrichment-resistant than asymmetrical microstructure development, which is crystallographically ascribable to favorable thermo-metallurgical factors, i.e. aluminum redistribution mitigation and supersaturation relief, and substantially reduce metallurgical degradation and microstructure failure. The faster welding speed and the lower laser power are used, the smaller aluminum concentration and supersaturation ahead of dendrite tip are kinetically incurred to suppress columnar/equiaxed transition and stray grain formation with a number of thermometallurgical factors contribution to considerable microstructure amelioration and increasingly improve solidification cracking resistance and vice versa. Shallow molten-pool shape with symmetrical growth crystallography efficiently advances anomalies-resistant microstructure development with diminution of partition driving forces for solute redistribution and supersaturation during nonequilibrium solidification instead of deep molten-pool shape with asymmetrical growth crystallography. The latter importantly aggravates stray grain formation. Useful combination of optimum solidification conditions and favorable growth crystallography predominantly minimizes diffusion-controlled solute deviation and microstructure anomalies for excellent single-crystal superalloy laser processing without cracking, and possesses tenable relationship between welding conditions, solute redistribution and microstructure anomalies. The mechanism of crystallography-aided concentration fluctuation and copious supersaturation behind prominent phenomenon of microstructure anomalies, where nucleation and growth of stray grain formation near dendrite tip are activated, is consequently proposed. The comparison between numerical analyses and experiment results are valid and satisfactory. Microstructure anomalies are predictable for proper understanding of multicomponent microstructure development in the interior of fusion zone. The theoretical methodology is reproducible as consequence of availability of thermodynamic and kinetic properties of Nickel-based or Iron-based single-crystal superalloys.

Abstract: Mould casting and drop-tube techniques were used to solidify a AlCoCrFeNi_2.1 eutectic high-entropy alloy under conditions of high cooling rate. The samples obtained from two different methods present the same phase constituent, FCC and B2 phases. During mould casting experiments the alloy almost solidified into the eutectic structure consisting of lamellar and anomalous morphology, with a tiny fraction of cellular and dendrite morphology being observed at certain sites of the sample surface due to the corresponding high cooling rate. Instead, during drop-tube experiments a typical, coarse dendrite structure of FCC single phase was formed across the entire 106-150 μm particle. The cellular structure can also be formed directly from the melt. The rest region solidified into the general eutectic morphology as was observed in the casting rods. The results clearly indicate the transition from coupled eutectic growth to single-phase dendrite growth with increasing departures from equilibrium for the multi-component AlCoCrFeNi_2.1 eutectic high-entropy alloy.

Abstract: The thermal-metallurgical modeling of microstructure development was further advanced during single-crystal superalloy weld pool solidification by coupling of heat transfer model, columnar/equiaxed transition (CET) model and multicomponent dendrite growth model on the basis criteria of minimum dendrite velocity, constitutional undercooling and marginal stability of planar front. It is clearly indicated that heat input (laser power and welding speed) and welding configuration simultaneously influence the stray grain formation, columnar/equiaxed transition and dendrite growth. For beneficial (001) and [100] welding configuration, the microstructure development along the solid/liquid interface is symmetrically distributed about the weld pool centerline throughout the weld pool. Finer columnar in [001] epitaxial dendrite growth region is kinetically favored at the bottom of the weld pool. For detrimental (001) and [110] welding configuration, the microstructure development along the solid/liquid interface is asymmetrically distributed. The dendrite trunk spacing along the solid/liquid interface from the beginning to end of solidification morphologically increases on the left side of the weld pool, while it spontaneously decreases on the right side. The vulnerable location of solidification cracking is confined in the [100] dendrite growth region on the right side of the weld pool because of increasing metallurgical contributing factors of severe stray grain formation, centerline grain boundary formation and coarse dendrite size. The mechanism of crystallography-dependent asymmetrical solidification cracking due to microstructure anomalies is proposed. It is crystallographically favorable for predominant morphology instability to deteriorate weldability. Active [100] dendrite growth region is diminished in the shallow elliptical weld pool by optimum low heat input (low laser power and high welding speed) with (001) and [100] welding configuration to essentially facilitate single-crystal solidification conditions and provide enough resistant to solidification cracking. Moreover, the theoretical predictions agree well with the experiment results. The reliable weldability maps are therefore established to determine the prerequisite for successful crack-free laser welding or cladding. The useful model is also applicable for other single-crystal superalloys with similar metallurgical properties.

Abstract: The effects of melt flow on dendrite growth during solidification are studied by the quantitative phase field model coupling the Navier-Stokes equations. Through analyzing the relationship between flow velocity and dendrite growth rate in simulations, a flow Péclet number involving with characteristic flow velocity, characteristic length of the zone affected by flow and thermal (solute) diffusion coefficient, is suggested for dendrite growth under convections. The growth rate increment due to flow follows a power-law relationship with the Péclet number. As the Péclet number is much higher than one, the influence of convection on dendrite growth is apparent, whereas as it is below one, the flow effects can be neglected.

Abstract: Numerical simulations based on a new regularized phase field model were presented, simulating the solidification of magnesium alloy. The effects of weak and strong interfacial energy anisotropy on the dendrite growth are studied. The results indicate that with weak interfacial energy anisotropy, the entire dendrite displays six-fold symmetry and no secondary branch appeared. Under strong interfacial energy anisotropy conditions, corners form on both the main stem and the tips of the side branches of the dendrites, the entire facet dendrite displays six-fold symmetry. As the solidification time increases, the tip temperature and velocity of the dendrite and facet dendrite finally tend to stable values. The stable velocity of the facet dendrite is 0.4 at ε₆ is 0.05 and this velocity is twice that observed (0.2) at ε₆ is 0.005.

Abstract: In order to study the growth process and morphology of dendrite directly, a phase field model of binary alloy was established. In this model the order parameter equation was coupled with the temperature field and the solute field. The growing processes and morphology of dendrite were simulated by using this phase field model. Through analyzing the results, we discussed the effects of anisotropic strength and temperature gradient on dendrite morphology. The results shows that with the increasing of anisotropic strength, the dendrite growth rate of the dendrite will increase and the secondary branches appear more clearly. Besides, the temperature gradient has influence on the appearance of secondary arms during the dendrite growing. With the increase of temperature gradient, the size of secondary dendrite arms increase.

Abstract: The phase field coupling with the temperature distribution field is a good method to simulate the dendrite growth during isothermal solidification. The effects of super-cooling degree on the dendrite growth are studied in the process of Nickel isothermal solidification. The results indicate that super-cooling degree has an important effect on the dendrite growth. When the super-cooling degree is small, the rate of crystal growth all is small and the shape of Crystal nucleus is an approximative ball. As super-cooling degree increases, the rate of crystal growth become large fast and the main branch Grow up quickly and its shap become long and thin, and secondary branch appear in the root between every two main branch. Ever, as the super-cooling degree become very large, the more arm dendrite appear and the rate of the main branch is smothered.

Abstract: The phase field coupling with the temperature distribution field is used to simulate the dendrite growth during isothermal solidification. The effects of coupling coefficient on the dendrite growth are studied. The results indicate that grain shape is hypertrophy dendritic crystal When the coupling coefficient is low. As coupling coefficient increases, the main branch of dendritic crystal become long and thin, and has the stable front interface. When the coupling coefficient is large, the dendrite growth becomes very complicated and the front interface of it is very unstable. Meanwhile, the temperature distribution field of main branch tip increases slowly first, then decreases; in addition to the main branch ,the temperature field increases monotonously.

Abstract: Coupling the force flow field with the phase field model for the isothermal growth of dendrite, Sola algorithm is used to calculate the flow speed and pressure of liquid metal, Using double grid numerical method to reduce the calculation amount of computer simulation, The space factor and time factor are introduced to improve the accuracy of double grid numerical calculation, Taking Al-2%-Cu alloy as an example, the dendrite growth process of the binary alloy under forced convection environment is simulated; The simulation results can capture the real dendrite growth and interactions of the liquid metal flow in the process of dendrite growth under forced convection environment: In the incident flow regions, the dendrite morphology is complex, the secondary dendrite is lush and the growth speed is fast due to the influence of liquid metal flow. In the back flow regions, the growth of dendrite changes the flowing pressure among the liquid metals, it causes the regional complex flow patterns and there are two opposite eddy current; the grow speed of the main branch which grows perpendicular to the initial flow direction is the fastest and presents tilt growth phenomenon. When the space coefficient value is appropriate, the dual mesh method can save calculation time effectively.