Selective electron beam melting (SEBM) is a type of additive manufacturing (AM) that involves multiple physical processes. Because of its unique process conditions compared to other AM processes, a ...detailed investigation into the molten pool behavior and dominant physics of SEBM is required. Fluid convection involves mass and heat transfer; therefore, fluid flow can have a profound effect on solidification conditions. In this study, computational thermal-fluid dynamics simulations with multi-physical modeling and proof-of-concept experiments were used to analyze the molten pool behavior and resultant thermal conditions related to solidification. The Marangoni effect of molten metal primarily determines fluid behavior and is a critical factor affecting the molten pool instability in SEBM of the Co–Cr–Mo alloy. The solidification parameters calculated from simulated data, especially the solidification rate, are sensitive to the local fluid flow at the solidification front. Combined with experimental analysis, the results presented herein indicate that active fluid convection at the solidification front increase the probability of new grain formation, which suppresses the epitaxial growth of columnar grains.
Abstract
Solidification characteristics in the meltpool drive the process-microstructure relationship which helps dictate the material properties of as-built parts in additive manufacturing; ...therefore, being able to accurately and quickly simulate the size, shape, and solidification characteristics in the melt pool is of great interest to the field. This study investigates various important physical phenomena (dynamic material properties, fluid-flow, radiation and vaporization) which can either be included or neglected in a continuum finite volume model (FVM) and their effect on the solidification conditions. Additionally, since the simplest form of such a model (conduction only) has an analytic solution which is much faster, its viability is also considered. Since the inclusion of some of these physical phenomena will inherently change the net energy input as well as the amount of energy needed to achieve melting of a control volume, each set of included phenomena had an effective absorption efficiency which was calibrated to closely match the dimensions of the melt pool to that of the ground truth data. The ground truth data for this study was defined to be the output of the FVM which included all the physical phenomena (OF). This study then goes on to compare the effects on solidification conditions each of these calibrated models has. It was found that most of the change in solidification conditions comes from the inclusion of latent heat. A posterior correlation factor (PCF) is then introduced to enable an analytic model to predict similar solidification conditions to OF model.
•The heat and mass transfer process in weld pool for various conditions was studied.•The solidification conditions along the trailing pool boundary were analyzed.•The morphological evolution of ...solidification structures was simulated by CA model.•We discussed the formation mechanisms for different weld grain structures.
The heat and mass transfer and the morphological evolution of the various weld solidification structures for the entire weld were simulated through the macro-micro modeling which combined the welding process and the solidification structures in the weld pool. The weld profiles were calculated for different welding conditions, and the effects of the welding parameters on the solidification conditions were analyzed along the trailing pool boundary. The formation mechanism of the axial columnar structure and the equiaxed grains in the weld were studied by the developed model. The results indicate that the temperature gradient G, solidification rate Rw, and the ratio G/Rw along the trailing pool boundary are related with the location in the weld and the welding parameters. If there does not exist heterogeneous nuclei in the weld pool, the axial columnar structure forms in the central region of the weld at low welding speeds owing to the anisotropic kinetics of the dendritic structure. When the heterogeneous nucleation occurs in the weld pool, the weld structure is directly determined by the competitive growth between the columnar and new nucleated grains. At low welding speeds, the preferential orientations of the survival grains get closer to the welding direction with the decrease of the distance to the weld centerline. With the increase of the welding speed and welding current, more grains form in the weld pool, and the new nucleated grains also become more competitive. The columnar grains from the fusion line are blocked by the equiaxed grains. The continuous equiaxed grain zone forms in the weld, and the columnar to equiaxed transition (CET) occurs.
The formation of low-angle grain boundaries (LABs) in the rejoined platforms of a Ni-based single crystal superalloy under different directional solidification rates was investigated by the ...experimental investigation and the ProCAST simulation. The results showed that the growth morphology and orientation evolution of dendrites in the platforms were different under the withdrawal rates in the range of 60–100 μm/s and then resulted in different types of LABs. At lower withdrawal rates, the longitudinal LABs were common in the rejoined platforms. Both the sliver defects and the orientation deviation of original primary dendrites from two independent growth paths could cause the longitudinal LABs in the platforms. At higher withdrawal rates, the dendrite growth patterns were more complex and the secondary branches with lateral growth tended to deviate from their original orientation, eventually leading to the formation of some transverse LABs. Finally, some suggestions to prevent the formation of different LABs are provided.
Powder Bed Fusion with Laser Beam of Metals (PBF-LB/M) is one of the fastest growing technology branches. More and more metallic alloys are being qualified, but processing of aluminum wrought alloys ...without cracks and defects is still challenging. It has already been shown that small parts with low residual porosity can be produced. However, suffering from microscopic hot cracks, the fracture behavior has been rather brittle. In this paper different combinations of temperature gradients and solidification rates are used to achieve specific solidification conditions in order to influence the resulting microstructure, as well as internal stresses. By this approach it could be shown that EN AW-2024, an aluminum-copper wrought alloy, is processable via PBF-LB/M fully dense and crack-free with outstanding material properties, exceeding those reported for commonly manufactured EN AW-2024 after T4 heat treatment.
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•Developed free surface based model for laser spot welding.•Interface evolution dynamics is considered.•In-depth comparison with the conventional flat surface based model.•Significant ...difference in temperature, phase interface, thermal cycle and flow field.•Local solidification conditions in the melt pool are quantified and compared.•Indicative as-solidified weld grain structure estimated based on GR and G/R.
In this article, a free surface based thermo-fluidic model for conduction mode laser spot welding has been developed. This approach is different from the conventional numerical modelling of conduction mode laser spot welding which assumes flat top surface. The development of metal-gas interface in this model is captured by integrating a free surface capturing scheme with the thermo-fluidic model. The free surface is tracked by using Volume of Fluid (VOF) method. This numerical model solves mass, momentum and energy conservation equations coupled with phase change (melting and solidification) and VOF. The present model accounts for the interfacial forces, such as surface tension and Marangoni force responsible for curvilinear evolution of the interface. This additional physics of interfacial forces is completely ignored in the conventional flat surface based models. The results from the free surface model is validated with the experimental results reported in literature. For evaluation purpose the predictions made by free surface model are compared in detail with those from the conventional flat surface model. The evolution of melt pool characteristic, temperature distribution, phase boundary, velocity field and thermal cycle are compared for both models. Also, the local solidification condition as well as non-dimensional thermo-fluidic parameters are described and compared. The free surface model showed larger melt pool dimensions, higher peak temperature and velocity, along with delay of overall solidification in the computational domain. The calculated local solidification conditions in the melt pool are used to estimate the indicative as-solidified weld grain structure based on GR and G/R values.
Cast aluminum matrix composites are promising materials for the production of responsible and particularly critical parts used in the high-technology fields of industry such as aerospace, automotive, ...electronics, etc. In this work, hybrid aluminum matrix composite reinforced with in-situ formed Mg2Si and Al3Ti particles were synthesized successfully in gravity casting conditions, and the influence of thermal conditions of solidification on the structural parameters of endogenous reinforcing phases was investigated. The results show that an increase in the cooling rate during solidification due to the application of a copper mold instead steel mold leads to reduction in the average size of the Mg2Si primary crystals, improvement of the distribution uniformity and significant increasing of their total quantity; at the same time, the average size of Al3Ti particles decreases substantially but their quantity is approximately the same for both types of molds.
As weld pool geometry and thermomechanical strains are known to affect formation of solidification cracks, the influence of a superimposed beam oscillation on these characteristics is investigated ...for this paper. In this context the effects of the oscillation frequency and focal diameter on the weld pool and temperature field are determined by means of infrared thermography. As a result an increase of the weld pool size for larger focal diameters and a more even shape of the seam edges for higher frequencies can be identified.
In the present article, some important trends have been shown regarding the relationship between mechanical properties, microstructure and solidification variables of Zn–Al alloys castings. Some ...theoretical dendritic growth models, expressing secondary spacings as function of tip growth rate or local solidification time, have been tested against experimental data obtained during unsteady-state solidification. Based on these dendritic models, on analytical expressions describing the position of solidus and liquidus isotherms in the unidirectional solidification of binary alloys and on experimental results concerning tensile testing of casting samples, expressions have been developed permitting a correlation between ultimate and yield strength, dendrite secondary spacings and solidification processing variables.
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