Solidification Morphology Research Articles

Mechanism for the induction and enhancement of inclusions on crack source and simulation analysis for hot tearing tendency of aluminum alloy

Inclusions are typical discontinuous defects, those defects significantly reduce the local mechanical properties of castings during solidification, and play a strong role in inducing and enhancing hot tearing in alloys. Based on the coupling characterization of micro solidification morphology and macro inclusions, this paper proposed a construction method of heterogeneous model containing multi-scale information, and carried out dynamic analysis on the evolution process of stress field at the end of solidification, so as to investigate the influence mechanism of inclusions on the initiation and propagation of hot tearing. The results showed that inclusions and their local content significantly changed the stress field at the end of solidification, which easily resulted in local stress concentration and induced crack source in defects area. During the crack propagation, nearby inclusions greatly reduced the propagation resistance and promoted the formation of hot tearing, which had significant impact on the propagation path of hot tearing. Introducing inclusions defect into stress analysis and comprehensively considering the induction and enhancement of inclusions on hot tearing can better improves the prediction accuracy of hot tearing, which has engineering guiding significance for casting process optimization and product quality control.

Investigations on the effect of standing ultrasonic waves on the microstructure and hardness of laser beam welded butt joints of stainless steel and nickel base alloy

Joining dissimilar metals with superior quality is important to provide tailored, lightweight, and cost-efficient components. Expensive and durable materials are exceptionally used where the cheaper material would not withstand the requirements. With laser beam welding, dissimilar metals can already be joined with high precision, low heat input, and a customizable mixing degree. Introducing ultrasonic excitation into the weld pool is a promising approach for further improvements like customizing the solidification morphology and avoiding weld defects. The experiments are carried out with round bars of 30 mm diameter made of 1.4301 steel alloy and 2.4856 nickel base alloy. Ultrasonic-assisted laser beam butt welding is conducted on rotating specimens with a laser beam power of 7.75 kW and a welding speed of 0.95 m/min. The specimens are evaluated by metallographic cross sections, hardness measurements, and energy-dispersive x-ray spectroscopy (EDX). The ultrasound is used to excite an eigenmode of the sample and the weld position is varied at stress- and displacement-nodes. Two different mechanisms of acoustic grain refinement are revealed. Heterogeneous nucleation is fostered in weld seams that are positioned in stress-nodes, and the fragmentation of dendrites is fostered in displacement-nodes. The welds' chemical compositions correspond to the change of solidification morphology.

Role of Nb on the phase stability and morphology of Ti–Ni–Cu–Nb alloys

Ti52-xNi38Cu10Nbx alloys with Nb additions of x = 4, 6, and 10 at.% were produced through arc melting. The phase constitution and morphology were evaluated through X-ray diffraction, microstructural analyses (scanning electron microscopy/energy-dispersive X-ray spectroscopy), and thermodynamic calculations (ThermoCalc). The as-cast alloys exhibited a TiNi matrix, β-Nb/TiNi eutectic, and Ti2Ni precipitates. Under equilibrium conditions, TiNi and a small fraction of β-Nb were expected. Increasing the Nb content resulted in a higher β-Nb volume fraction, development of different morphologies for Ti2Ni precipitates, and formation of Ti particles. The Scheil calculator and virtual composition of the liquid were used to simulate the effect of chemical segregation during solidification. We demonstrated that the metastable phases formed during solidification and the Nb content played a vital role in the solidification sequence, phase distribution, and morphology of the alloy.

MEMS-Based in situ electron-microscopy investigation of rapid solidification and heat treatment on eutectic Al-Cu

The solidification behavior of a eutectic AlCu specimen is investigated via in situ scanning transmission electron microscope (STEM) experiments. Solidification conditions are varied by imposing various cooling conditions via a micro-electro-mechanical system (MEMS) based membrane. The methodology allows the use of material processed by a melting and casting route close to industrial metallurgically fabricated material for in situ STEM solidification studies. Different rapid solidification morphologies could be obtained solely on a single specimen by the demonstrated strategy. Additional post-solidification heat treatments are investigated in terms of observation of spheroidization of lamellas during annealing at elevated temperatures.

Processability, microstructure and precipitation of a Zr-modified 2618 aluminium alloy fabricated by laser powder bed fusion

Additive manufacturing offers the opportunity to produce complex geometries from novel alloys with improved properties. Adapting conventional alloys to the process-specific properties can facilitate rapid implementation of these materials in industrial practice. Nevertheless, the processing of conventional alloys by laser powder bed fusion is challenging, particularly in cases of pronounced susceptibility to hot cracking. Previous studies have demonstrated the positive influence of zirconium on the susceptibility to hot cracking of high-strength aluminum alloys, although its influence on precipitation formation, which is immensely important for 2xxx alloys, remains largely unexplored. This work investigates the optimum process parameters and precipitation formation of 2618 modified by zirconium in laser powder bed fusion. The addition of zirconium results in the production of a crack-free, high-density (~99.9%) material. The microstructure is characterized by a trimodal grain size distribution. Very fine (~0.5 µm) equiaxed grains, nucleated by L1 2 -Al 3 Zr precipitation at the melt pool boundary, followed by columnar-dendritic grains (5–15 µm long, 1–3 µm wide) and coarser equiaxed grains (1–3 µm) form during solidification. The grain boundaries are populated predominantly by (Al,Cu) 9 FeNi, but also by Mg 2 Si, Al 2 CuMg, and AlCu, which presumably impede grain growth and promote a very fine-grained, low-textured microstructure, even in regions where L1 2 -Al 3 Zr are absent. The as-built microhardness of 1360 ± 74 MPa exceeds that of the known high-strength Al alloys tailored to additive manufacturing, Addalloy™ and Scalmalloy®. The results provide a better understanding of precipitate formation in Zr-modified 2xxx series alloys and pave the way for the commercialization of further 2xxx alloys adapted to additive manufacturing. • 2618 is known to show pronounced solidification cracking issues when processed with LPBF. • Zr-modified 2618 exhibits high-density, crack-free microstructure with trimodal solidification morphology. • Far-from-equilibrium solidification leads to GB segregation of (Al,Cu) 9 FeNi, Mg 2 Si, Al 2 CuMg and AlCu. • Hardness exceeds that of conventionally produced 2xxx alloys (e.g. 2024, 2618) as well as Addalloy™ and Scalmalloy®.

Gravity effects on temperature distribution and material flow in the keyhole pool of VPPA Al welding

This study investigated heat and mass transfer as well as the effect of gravity on keyhole formation in the molten pool of keyhole plasma arc welding. The flow trajectory of the molten pool was obtained via high-speed camera images illuminated by a laser light system, and the temperature distribution was obtained using an infrared thermal camera. The experimental results showed that the molten metal flowed rearward and downward at the top surface of the weld pool and thereafter to the rear side of the keyhole. There are two types of reverse flow of molten metal on the rear side of the keyhole. The different welding positions had minimal effect on the flow pattern; however, the flow velocities were different. Further, for the temperature distribution, although the keyhole front side yielded a similar high-temperature area under different welding positions, the area of the high temperature on the keyhole rear side in the vertical position welding was much larger than that in the flat position. Consequently, the influence of the force on the keyhole molten pool was evaluated, and the flow pattern of the molten pool was found to be dominated by the sheer force. Moreover, the flow velocity and solidification morphology of the molten pool were affected by the gravity at different welding positions. In addition, the correlation mechanism of the temperature distribution, flow, and solidification morphology of the keyhole molten pool was analyzed using the combined action of shear force and gravity, which further revealed the mechanism of the influence of gravity on the keyhole welding process.

Effect of Y2O3 contents on microstructural, mechanical, and antioxidative characteristics of Al2O3-ZrO2-Y2O3 coatings

The Al2O3-ZrO2-Y2O3 ternary ceramic coatings (AZY-TCC) with different Y2O3 contents were successfully prepared by thermal explosion spraying assisted rapid solidification. The microstructural, mechanical, and antioxidative characteristics of different solidification morphologies for AZY-TCC were tested and analyzed. The results show that, along the solidification direction, AZY-TCC mainly contained nanocrystalline, nano-eutectic, submicro-eutectic and micro-eutectic regions. According to the growth rate of these areas, the solidification morphology mapping of AZY-TCC with different Y2O3 contents was drawn. As the Y2O3 contents increased from 1.5 mol% to 16 mol%, ZrO2 gradually changed from m-phase to t-phase, and finally stabilized to c-phase. When the Y2O3 content was up to 12 mol%, the excess Y2O3 and Al2O3 formed a small amount of Y3Al5O12 (YAG) phase, and a typical Al2O3-ZrO2-YAG ternary eutectic structure appeared with 16 mol% Y2O3. The hardness and fracture toughness of AZY-TCC with different Y2O3 contents reached up to 23.57 ± 0.66 GPa and 5.35 ± 0.38 MPa·m1/2, which were much higher than those of other ceramic coatings with the same composition, and AZY-TCC with 3 mol% Y2O3 had excellent friction and wear resistance and oxidation resistance. Consequently, the high-performance AZY-TCC provides theoretical guidance for its microstructural evolution under rapid solidification.

Phase field assisted analysis of a solidification based metal refinement process

Ultra pure metals have various applications, e. g. as electrical conductors. Crystallization from the melt, e. g. via zone melting, using the segregation of impurities at the solidification front is the basic mechanism behind different technical processes for the refining of metals and semi-metals. In this paper, we focus on a crystallization methodology with a gas cooled tube (“cooled finger”) dipped into a metallic melt in a rotating crucible. The necessary requirement for purification in a solidification process is a morphologically stable solidification front. This is the only way to enable macroscopic separation of the impurities, e. g. by convection. For cellular or dendritic solidification morphologies, the segregated impurities are trapped into the interdendritic melt and remain as microsegregations in the solidified metal. Morphological stability depends on the temperature gradient G at the solidification front, the solidification front velocity V front and thermodynamic alloy properties like the segregation coefficients of the impurity elements. To quantify the impact of these parameters on the morphological evolution, especially on the planar/cellular transition and thus on microsegregation profiles, phase field simulations coupled to a thermodynamic database are performed for an aluminium melt with three impurities, Si, Mn and Fe. In particular, we have investigated the morphology evolution from the start of solidification at the cooled finger towards a stationary growth regime, because in the technical process a significant fraction of the melt solidifies along the initial transient. To solve the transient long range temperature evolution on an experimental length scale, the temperature field has been calculated using the homoenthalpic approach together with a 1D temperature field approximation. The simulations provide the process window for an energy efficient purification process, i. e. low thermal gradients, and elucidate the benefit of melt convection.

Analyzing effects of temperature gradient and scan rate on metal additive manufacturing microstructure by using phase field-finite element method

Predicting the evolutionary behavior of microstructures with the help of numerical simulation techniques has become an essential tool for studying the solidification process of metal additive manufacturing. As a mesoscopic model based on the diffusion interface theory, phase field method (PFM) can be used to predict the evolution of solidification microstructure. The open-source PFM framework PRISMS-PF can not only efficiently solve systems of equations with billions of degrees of freedom, but also provide a simple adaptive mesh control module. In this paper, based on the open-source PFM framework PRISMS-PF, a phase field-finite element method (PFM-FEM) simulation flow for the solidification process of A356 aluminum alloy additive manufacturing in the two-dimensional case was established. The effects of temperature gradient, scan rate and initial solid-phase morphology on solute concentration, dendrite spacing and dendrite morphology were analyzed and compared with experimental results for verification. Analyzing the results for different temperature gradients and scan rates cases, it was found that the increase of temperature gradient or scan rate made the primary dendrite arm space decrease; as the ratio of temperature gradient to scan rate decreased, the solidification morphology gradually changed from flat crystal to cellular crystal, columnar crystal, and even dendritic structure. Analyzing the results for different initial solid-phase morphology cases, it was found that the influence of initial solid-phase morphology on dendrite growth increased as the ratio of temperature gradient to scan rate decreased. The above influence rules were mainly related to the composition overcooling zone under different conditions. This paper is expected to provide a theoretical support for the effective regulation of solidification microstructure in metal additive manufacturing.

In-situ X-ray investigation of growth behavior of primary silicon particles and aluminum dendrites during solidification of a hypereutectic Al-Si-Cu alloy

The solidification behavior of hypereutectic Al-18 wt%Si-10 wt%Cu alloy was investigated using in-situ X-ray radiography. Sheet-like samples were solidified both under isothermal cooling conditions and in a moderate temperature gradient with different process parameters. The real-time observation method allows for the investigation of the growth of faceted primary Si particles and non-faceted Al dendrites. Cooling down below the liquidus temperature, firstly primary silicon particles nucleate and grow. At low cooling rates just a few Si particles nucleate and grow continuously to large sizes, whereas at higher cooling rates a larger number of smaller Si particles appear. As cooling continues below the nucleation temperature of aluminium, simultaneous growth of both columnar and equiaxed Al dendrites in the melt surrounding the primary Si particles was observed. In-situ observation as well as post-mortem analysis of the solidification morphology demonstrated that nucleation and growth of Al dendrites in the melt was nearly independent of existing Si particles and no indication was found that the Al phase starts growing from the surface of Si particles. The growth of the equiaxed Al dendrites can be explained by the thermal undercooling controlled by the application of a constant and continuous cooling, but the reason for the fast growing columnar Al dendrites in an isothermal melt is still not clear.

Investigation of temperature distribution and solidification morphology in multilayered directed energy deposition of Al-0.5Sc-0.5Si alloy

The temperature distribution, geometry and size of the melt pool, and solidification parameters were computed using the heat transfer and material flow model for the directed energy deposition process. The thermal cycle and melt pool size were computed for different process parameters such as laser power, deposition speed, and laser beam radius across the multiple layers for deposition of Al-0.5Sc-0.5Si alloy. The computed thermal cycles at various locations from the centerline and melt pool size showed fair agreement with the experimentally measured result. The computed thermal gradient and solidification rate were mapped over the solidification map, which was in agreement with the experimentally observed microstructure. The transition in solidification morphology with varying melt pool depth and process parameters were fairly observed inside the melt pool. The fraction of equiaxed solidification morphology increases with the reduction in a thermal gradient.

Two-Dimensional Transient Heat Transfer Model for High-Temperature Laser-Scanning Confocal Microscopy

Abstract High-temperature laser-scanning confocal microscopy (HT-LSCM) has proven to be an excellent experimental technique through in situ observations of high temperature phase transformation to study kinetics and morphology using thin disk steel specimens. A 1.0 kW halogen lamp within the elliptical cavity of the HT-LSCM furnace radiates heat and imposes a nonlinear temperature profile across the radius of the steel sample. When exposed at the solid/liquid interface, this local temperature profile determines the kinetics of solidification and phase transformation morphology. A two-dimensional numerical heat transfer model for both isothermal and transient conditions is developed for a concentrically solidifying sample. The model can accommodate solid/liquid interface velocity as an input parameter under concentric solidification with cooling rates up to 100 K/min. The model is validated against a commercial finite element analysis software package, strand7, and optimized with experimental data obtained under near-to equilibrium conditions. The validated model can then be used to define the temperature landscape under transient heat transfer conditions.

Comparison of the Porosity of Aluminum Alloys Castings Produced by Squeeze Casting

The results of researches on porosity and structure of castings from AlMg9, AlSi7Mg and AlCu4Ti alloys produced by squeeze casting and for comparison by gravity die casting are presented. The tests were carried out on 200x100x25mm plates squeeze casted under 90MPa pressure. Prior to the commencement of experimental studies, numerical simulations of solidification were made for the selected alloy (AlSi7Mg) in order to determine the potential locations of shrinkage porosity. As part of the study, the porosity distribution in the plate cast was assessed by taking samples for measurements from the center and edge of the casting. It was found that the area particularly vulnerable to the presence of shrinkage porosity is the central part of the casting and the zone extending from its center to the upper surface. Due to the wide temperature range of solidification of the examined alloys, diffused porosity occurs in castings, and the shape of the pores formed is conditioned on the solidification morphology. The average porosity of squeeze castings is two times smaller than gravity die castings and is at the level of 1.0-1.5% depending on the type of alloy. In addition, as a result of pressing, the shrinkage porosity in the central part of the plate is reduced and its distribution becomes uniform throughout the volume. High pressure acting on solidifying castings ensures a significant increase of grains density in microstructure and decrease of SDAS.

Ultra-short pulsed laser powder bed fusion of Al-Si alloys: Impact of pulse duration and energy in comparison to continuous wave excitation

Laser assisted powder bed fusion of Al-40 wt%Si using ultra-short laser pulses is presented. The effects of the pulse duration as well as the pulse energy on the melt pool size, solidification morphology and material properties have been investigated. The experiments have been carried out with a mode-locked fiber laser system delivering pulses from 500 fs up to 800 ps at a wavelength of 1030 nm. Comparative investigations have been performed using a continuous wave Yb-fiber laser operating at 1070 nm. The results show that the melt pool width is reduced at shorter pulse durations and higher repetition rates while maintaining the same average power. Additionally, keyhole melting is achieved in pulsed operation after exceeding the threshold fluence for single pulse ablation. In comparison to continuous wave radiation, powder bed fusion using ultra-short laser pulses leads to a more uniform melt pool shape with refined primary Si and eutectic structure that is accompanied with an improvement of the mechanical properties.

A model for predicting the effects of buoyancy driven convection on solidification of binary alloy with nanoparticles

Convection during the solidification of a binary alloy with nanoparticles governs the nanoparticle, species and temperature distribution in the domain and thus the solidification rate and morphology. Hence, a numerical model to predict the effect of convection on solidification with nanoparticles is essential. In this work, a volume averaged enthalpy method-based model is developed to predict the nanoparticle, solute and temperature distribution during micro-scale solidification of binary alloy with nanoparticles. In addition to nanoparticle transport due to Brownian motion based diffusion, the effects of thermal buoyancy, solutal buoyancy and nanoparticle driven convection are considered in the model. The effect of nanoparticles on the solidification process is also incorporated into the model. From single dendrite simulations, it is observed that convection plays a significant role in dendrite growth and nanoparticle distribution. Also, it is observed that the effect of nanoparticle driven convection is insignificant. Microstructure simulations with large number of dendrites show that the effect of convection on the dendrite growth is diminished due to the reduction in space available for flow in the interdendritic region. It is also found that the presence of convection increases the homogeneity of nanoparticle distribution in the final microstructure.

Role of laser powder bed fusion process parameters in crystallographic texture of additive manufactured Nb–48Ti alloy

Additive manufacturing, known as “3D printing”, is a set of manufacturing technologies that can build parts with complex geometries in a process by adding layers of powder material. The laser powder bed fusion (L-PBF) process is characterized by selectively melting layers of particulate material at a micrometer scale in repetitive patterns. This technology is expected to improve Young's modulus of metallic biomaterials, by controlling the microstructure and crystallographic texture– influenced by the laser power and scanning speed parameters which promote an oriented heat extraction. Implant materials should have low Young's modulus, to avoid a large mismatch with that of the bones. This work investigates the role of the laser power and scanning speed on microstructure and crystallographic texture formation of the Nb–48Ti alloy, fabricated by laser powder bed fusion process, using pre-alloyed plasma atomized powder. The microstructure was characterized by optical microscopy (OM), scanning electron microscopy (SEM), backscattered electron diffraction technique (EBSD) for crystallographic texture, and Young's modulus was obtained indirectly via EBSD data. The microstructure showed a cellular dendritic solidification morphology formed by epitaxy at the edge of the melt pools. Texture results indicated that higher values of power and scanning speed favored the increasing of a near-cube-on-face texture and a reduction in Young's modulus.

Self-Organization, Entropy Generation Rate, and Boundary Defects: A Control Volume Approach.

Self-organization that leads to the discontinuous emergence of optimized new patterns is related to entropy generation and the export of entropy. Compared to the original pattern that the new, self-organized pattern replaces, the new features could involve an abrupt change in the pattern-volume. There is no clear principle of pathway selection for self-organization that is known for triggering a particular new self-organization pattern. The new pattern displays different types of boundary-defects necessary for stabilizing the new order. Boundary-defects can contain high entropy regions of concentrated chemical species. On the other hand, the reorganization (or refinement) of an established pattern is a more kinetically tractable process, where the entropy generation rate varies continuously with the imposed variables that enable and sustain the pattern features. The maximum entropy production rate (MEPR) principle is one possibility that may have predictive capability for self-organization. The scale of shapes that form or evolve during self-organization and reorganization are influenced by the export of specific defects from the control volume of study. The control volume (CV) approach must include the texture patterns to be located inside the CV for the MEPR analysis to be applicable. These hypotheses were examined for patterns that are well-characterized for solidification and wear processes. We tested the governing equations for bifurcations (the onset of new patterns) and for reorganization (the fine tuning of existing patterns) with published experimental data, across the range of solidification morphologies and nonequilibrium phases, for metallic glass and featureless crystalline solids. The self-assembling features of surface-texture patterns for friction and wear conditions were also modeled with the entropy generation (MEPR) principle, including defect production (wear debris). We found that surface texture and entropy generation in the control volume could be predictive for self-organization. The main results of this study provide support to the hypothesis that self-organized patterns are a consequence of the maximum entropy production rate per volume principle. Patterns at any scale optimize a certain outcome and have utility. We discuss some similarities between the self-organization behavior of both inanimate and living systems, with ideas regarding the optimizing features of self-organized pattern features that impact functionality, beauty, and consciousness.

The Development of Plate and Lath Morphology in Ni5Ge3

Drop-tube processing was used to solidify rapidly a congruently melting, single phase intermeallic Ni5Ge3. This process results in the production of powders with diameters that are between 850–53 μm. After etching that occurs at low cooling rates (850–150 μm diameter particles, 700–7800 K s–1), an isolated plate and lath microstructure in what is an otherwise featureless matrix constitutes the dominant solidification morphology. By contrast, when the cooling rates are higher (150–53 μm diameter particles, 7800–42 000 K s–1), it is isolated hexagonal crystallites within a featureless matrix, which constitute the dominant solidification morphology. The TEM analysis of selected area diffraction shows that plate and lath microstructures are variants of e-Ni5Ge3, which are partially ordered. By contrast, the isolated hexagonal crystallites are revealed to be disordered. However, the featureless matrix of both microstructures are the fully ordered variant of the same compound. The plate and lath has a very different EBSD and GOS signatures to the hexagonal crystallites structure. Histogram of the correlated grain orientation angle distribution across grain orientation in plate and lath microstructure sample from the 300–212 μm fraction showing predominance of low angle grain boundaries. However, grain orientation in isolated hexagonal crystallites from 150–106 μm revealing the distribution typical of random grain orientation.

Effect of Cooling Rates on the Local—Overall Morphology Characteristics of Solidification Structure at Different Stages for High Carbon Steel

The solidification characteristics of 70 steel at the stage of the superheat elimination and the liquid–solid phase transformation were analyzed at cooling rates from 10 to 150 °C/min based on a high-temperature confocal scanning laser microscope (HT-CSLM). Secondary dendrite arm spacing (SDAS) and fractal dimension (D) were used to quantitatively describe the local compactness and overall self-similar complexity of the solidification morphology. It was found that the cooling rate had a very important influence on the local and overall morphology characteristics of solidification structures. At the superheat elimination stage, the cooling rate affected the morphology of the microstructure through the dynamic structural fluctuation between the generation and disappearance of atomic clusters in the molten steel. At the liquid–solid phase transformation stage, the cooling rate affected the local morphology of the microstructure by affecting the solute diffusion rate between dendrite arms, while it affected the overall morphology by changing the concentration undercooling at the front of all solidified interfaces. The presented results show that adjusting the cooling system at the superheat elimination stage can also be an important way to control the solidified morphology of different alloys.

Presence of έ and ε crystal structures in rapidly solidified intermetallic compound Ni5Ge3

The congruently melting, single phase intermetallic Ni5Ge3 has been subject to rapid solidification via drop-tube processing wherein powders, with diameters between> 850 and 150 µm, are produced. At low cooling rates (> 850 µm diameter particles,< 700 K s−1) the dominant solidification morphology, revealed after etching, is that of partial plate and lath microstructure in an otherwise featureless matrix. At slightly high cooling rates (850–150 µm diameter particles, 700–7800 K s−1) complete plate & lath microstructure observed, again imbedded within a featureless matrix. Selected area diffraction analysis in the TEM reveals that partial plate & lath are the disordered form of έ-Ni5Ge3 in a matrix of the ordered (double-superlattice) form. Partial plate & lath has a very different EBSD to the plate & lath structure. Histogram of the correlated grain orientation angle distribution across grain orientation in partial plate & lath microstructure sample from the> 850 µm fraction showing random grain orientation. However, grain orientation in plate & lath structure from 300 to 212 µm fraction displaying the distribution predominance of low angle grain boundaries.