Late Stage Of Solidification Research Articles

An integrated microporosity model of 3D X-ray micro-tomography and directional solidification simulations for Ni-based single crystal superalloys

The shrinkage microporosity formed during the directional solidification of Ni-based single crystal superalloys can significantly reduce the fatigue life of turbine blades, which may lead to a catastrophic flight accident due to the failure of aeroengines. We developed an integrated mesoscale model, which combined the shrinkage pressure drop from dendritic growth using cellular automaton method and X-ray microtomography characterization for Ni-based single crystal superalloys. The critical shrinkage pressure drop was derived from the simulation and compared to the empirical functions and experimental measurements. It was found that the pressure drop was so critical for predicting the microporosity accurately, and its relationship with the pore size and fraction was established via direct comparison to X-ray tomography. Using this integrated model, the pore size, percentage, and distribution as a function of directional solidification conditions can be successfully simulated. The model can predict small pores at the roots of the secondary dendrite arms in the early stage of solidification, and the large ones in the interdendritic liquid at the late stage of solidification. Therefore, this proposed approach has demonstrated to predict not only the influence of the processing conditions such as withdrawal rate on pore volume fraction, but also the root cause of pore size distributions.

Effect of pulse shaping on solidification process and crack in 5083 aluminum alloy by pulsed laser welding

Solidification crack was a common welding defect in welding of aluminum alloy.Whensolidification shrinkage and heat shrinkage of thevulnerable zoneand its surrounding solids were hindered, it willcause grain boundary cracking owing to insufficient supply of liquid metal.Therefore, cracks generally appear in the middle part of aluminum alloy welding spot at the late stage of solidification. Compared with continuous laser, pulsed laser could meet more precise heat input control and was widely used in aluminum alloy sheet welding.In this research, by designing different pulse shaping to improvethe solidification condition of the5083aluminum alloy pulsed laser welding spot, the numerical simulation was used for analyzing the solidification rate and temperature gradient of molten pool.Application of ramp-down pulse decreased crack cross-sectional area from 420475 µm2 to 0, and step-down pulse could also effectively control the crack radius. The results showed that effect of suppressing solidification crack on the horizontal direction of the welding spot was obvious to the vertical direction. Using ramp-down or step-down pulse shaping also reduced the volume of the liquid molten pool at the end of the laser process, which decreased the shrinkage stress at the late stage of solidification.

Multiscale solidification simulation of Sr-modified Al-Si-Mg alloy in die casting

Thermomechanical casting simulations incorporating solidification models are widely applied to improve the dimensional accuracy of casting components. The solidification path is commonly described based on Scheil-Gulliver assumptions which however fail to describe the effect of Strontium in Sr-modified Al-Si-Mg casting alloys. Strontium, even in small amounts, strongly affects the solidification morphology of the Al-Si eutectic together with its growth undercooling and hence the fraction solid–temperature relation during later stages of solidification. In order to address this problem, a dedicated micro-macro simulation approach is proposed here. The macroscopic thermomechanical casting simulation is linked with a spatially resolved multi-phase field simulation of the microstructure. Due to the fine fibrous morphology of the Si eutectic, a two-level homogenization scheme was applied to derive the effective mechanical properties of the Al alloy during its solidification. The proposed multiscale simulation and homogenization approach was applied to a permanent mould casting using an axisymmetric bowl cast from A356 for experimental validation.

Mechanical behavior and microstructure of hypereutectic high chromium cast iron: the combined effects of tungsten, manganese and molybdenum additions

The combined effects of W, Mn and Mo additions on the microstructure and properties of a hypereutectic high chromium cast iron (HCCI) were studied. The results indicated that W was distributed uniformly in the matrix and carbides. The W and Mo addition promoted the formation of M2C and M6C carbides. When 13wt.% Mn was added to the samples, the solubility of Mn in the matrix increased the stability of the austenite phase and improved the toughness of the materials. Moreover, Mn and Cr form the M7C3 carbides in hypereutectic HCCI. A large number of (Fe, Mn)3C carbides were produced in the matrix during the later stage of solidification, which significantly improved the wear resistance of the matrix. When the amount of W, Mn and Mo added was 6wt.%, 15wt.% and 4.2wt.%, respectively, the HCCI exhibited the best performance herein. The addition of 6wt.% W led to an increase in the average hardness of the samples and the microhardness of carbides of up to 57.5 HRC and 1680 HV, respectively. Similarly, it is worth mentioning that the impact toughness of the 6% W sample reached 6.35J/cm2, and the mass loss of the Cr16 and 0% W samples was 1.43 and 1.23 times that of the 6% W sample, respectively. The wear resistance of the 6% W sample showed no significant change before and after heat treatment. The combination of W, Mn and Mo simultaneously improved the wear resistance and impact toughness of the hypereutectic HCCI.

Creation of the Composition of a Complex Expanding Additive and its Influence on the Strength of Concrete

We study the influence of admixtures on the linear expansion of cement stone. By the method of mathematical planning of the experiments, we determined the optimal relationship between the expanding components in the composition of cement. The importance of simultaneous introduction of several components is explained not only by the expanding action of each of these components but also by their mutual influence. Gypsum is used not only as an inhibitor of the hydration of lime but also as a sulfate component used for the formation of calcium hexasulfoaluminate in the later stages of solidification. This enables us to affect both the formation of crystal structure and the strength of the cement stone in general. We also performed mathematical processing of the results of experimental investigations.

Complex Precipitates of TiN-MCx in GCr15 Bearing Steel

Nitride and carbide are the second phases which play an important role in the performance of bearing steel, and their precipitation behavior is complicated. In this study, TiN-MCxprecipitations in GCr15 bearing steels were obtained by non-aqueous electrolysis, and their precipitation mechanisms were studied. TiN is the effective heterogeneous nucleation site for Fe7C3 and Fe3C; therefore, MCxcan precipitate on the surface of TiN easily. The chemistry component of MCx consists of M3C and M7C3 (M = Fe, Cr, Mn) and Cr3C2. TiN-MCx with high TiN volume fraction, TiN forms in early stage of solidification, and MCx precipitates on TiN surface after TiN engulfed by the solidification advancing front. TiN-MCx with low TiN volume fraction, TiN and MCx form in late stage of solidification, TiN can not grow sufficiently and is covered by numerous precipitated MCxparticles.

Quality index and hot tearing susceptibility of Al–7Si–0.35Mg– x Cu alloys

Abstract The effects of Cu addition (0.5%, 1%, 1.5%, 2%, and 3%, mass fraction) on the quality index (Qi) and hot tearing susceptibility (HTS) of A356 alloy were investigated. According to the results, Cu addition up to 1.5% increases the Qi by almost 10%, which seems to be due to its solid solution strengthening and dispersion hardening effect of Cu-rich Al2Cu and AlMgCuSi compounds. However, further addition of Cu (up to 3%) decreases the Qi by almost 12%, which is likely due to the reduction of tensile strength and elongation caused by increased volume fraction of brittle Cu-rich intermetallics and microporosities in the microstructure. It is also found that Cu increases the HTS of A356 alloy measured by constrained rod casting method. According to the thermal analysis results, Cu widens the solidification range of the alloy, which in turn, decreases its fluidity and increases the time period during which the mushy-state alloy is exposed to the hot tearing susceptible zone. SEM examination of the hot tear surfaces in high-Cu alloys also demonstrates their rough nature and the occurrence of interdendritic/intergranular microcracks as convincing evidences for the initiation of hot tears in the late stages of solidification in which there is not enough time for crack healing.

Development of a new class of high strength copper alloy using immiscibility route in Cu-Fe-Si system: Evolution of hierarchical multi-scale microstructure

In this paper, a new high strength copper based alloy (Cu-20 at%Fe-5 at% Si) in Cu-Fe-Si system is reported that is designed by accessing submerged liquid miscibility gap of the Cu-Fe system and ternary addition of Si for a hierarchical design of microstructure. The alloy was synthesized by suction casting of the melt into a copper mould. This paper further elucidates the sequence of evolution of metastable phases in the microstructure due to non-equilibrium solidification through detailed characterizations of the microstructure of the alloy using transmission electron microscopy. The microstructure containing particles with globular morphology of the Fe-rich DO3 ordered phase suggests its origin in the liquid state consistent with the expectation of a metastable liquid immiscibility. The solute rejection process during the later stage of solidification and the solid state cooling results in the evolution of Fe rich nano sized fully coherent ordered particles in the Cu-rich matrix. This particular alloy has achieved an excellent mechanical strength both at room temperature and at elevated temperature. The correlations between microstructure and mechanical behavior have been discussed in details.

Influence of macrosegregation on solidification cracking in laser clad ultra-high strength steels

Alloy selection is critical for the performance of aerospace components repaired through laser cladding, as unsuitable combinations of clad and substrate materials can lead to defects during deposition. Different combinations of laser clad ultra-high strength steels have been studied to examine the effect of material composition on clad quality. Clad/substrate combinations of Aermet®100/300M, Aermet®100/4340, and 4340/300M were trialled using a range of processing parameters. While no defects occurred in the 4340/300M samples, solidification cracking was observed in the Aermet®100 multi-track clad samples, especially on 300M substrates. The cracking originates from macrosegregation trails caused by differences in melting temperature between clad and substrate. These trails interfere with liquid feeding beneath them when the substrate has a higher liquidus temperature, with entrapped liquid leading to short solidification cracks. A second larger form of solidification cracking was found in Aermet®100/300M due to Aermet®100 solidifying faster in the late stages of solidification, as this can entrap liquid in the inter-dendritic regions leading to cracking. Both forms of cracking can be avoided by increasing the laser interaction time during cladding, as this slows the solidification process to allow for more complete mixing and liquid feeding.

Hot-Tearing Assessment of Multicomponent Nongrain-Refined Al-Cu Alloys for Permanent Mold Castings Based on Load Measurements in a Constrained Mold

The hot-tearing resistance of multicomponent Al-Cu alloys during permanent mold casting was investigated using a constrained permanent mold in which the load and temperature were measured. The nominal Cu composition was varied from 5 to 8 wt pct. Casting experiments were conducted without adding any grain-refining inoculants. The following variables, which were obtained from the measured load data during casting, were considered to assess the hot-tearing resistance of the Al-Cu multicomponent alloys: “V”-like signature in the load rate variation, load at solidus point, and load rate average over the freezing range. In addition, a hot-tearing criterion based on the variation of the fraction of solid in the late stages of solidification was used. It was found that all criteria considered can accurately predict the alloys with the lowest and highest hot-tear resistance, respectively. It was found that the rate of measured load during casting could be used to indicate substantial hot tearing. However, the load rate variation could not be used to detect when small hot tears were present. Among all the criteria considered, the load at the solidus point shows an excellent agreement with experimentally observed hot-tearing resistance for all but one alloy. The poorly resistant hot-tearing alloys exhibited mainly coarse columnar grains while the most hot-tearing resistant alloys exhibited a much more refined grain microstructure. This is the first study in which good hot-tear resistance is demonstrated for multicomponent Al-Cu alloys with nominal Cu content greater than 7 wt pct.

Investigating gas-phase defect formation in late-stage solidification using a novel phase-field crystal alloy model

We study late-stage solidification and the associated formation of defects in alloy materials using a novel model based on the phase-field-crystal technique. It is shown that our model successfully captures several important physical phenomena that occur in the late stages of solidification, including solidification shrinkage, liquid cavitation and microsegregation, all in a single framework. By examining the interplay of solidification shrinkage and solute segregation, this model reveals that the formation of gas pore defects at the late stage of solidification can lead to nucleation of second phase solid particles due to solute enrichment in the eutectic liquid driven by gas-phase nucleation and growth. We also predict a modification of the Gulliver-Scheil equation in the presence of gas pockets in confined liquid pools.

Effect of mould temperature, grain refinement and modification on hot tearing test in Al-7Si-3Cu alloy

Abstract In the past decades, a great deal of work had been done on improving the mechanical properties and castability of Al-Si-Cu alloy in foundry industries through grain refinement and modification techniques. Among these alloys, Al-7Si-3Cu alloys are popularly used in automotive applications to produce lighter and more fuel efficient vehicles. In spite of these advantages, when this alloy is cast via permanent mould casting process, it shows a high susceptibility to hot tearing. Inoculation and hot tearing are important factors affecting the quality of the final casting products. Effect of mould temperatures on hot tear formation has been studied at room temperature, 90 °C, 160 °C, 230 °C and 300 °C. Higher mould temperature helps in reducing hot tear susceptibility of the alloy by refilling the material in later stage of solidification. For hot tear test, a permanent mould having test bar of size 260 mm long and 20 mm in height with constraint at the end has been used. Al-5Ti-1B grain refiner has been added in order to Ti content in order to from 0.040 wt% Ti to 0.050 wt% Ti to minimize the hot tear susceptibility of the alloy through alterations in grain size and morphology. The Al-7Si-3Cu alloy has a columnar grain structure with a large grain size, while with the addition of grain refiner it shows a finer equiaxed structure. The effect of processing of Al-5Ti-1B grain refiner and hot rolled or cast in combination with Al-10Sr modifier have also been studied to observe their effect on minimizing hot tear formation. The fracture surfaces of the specimen have been investigated by SEM. The closed packed plans and presence of intermetallic particle on the fracture surface is identified using TEM.

Microstructural Evolution in AlMgSi Alloys during Solidification under Electromagnetic Stirring

Equiaxed solidification of AlMgSi alloys with Fe and Mn was studied by electromagnetic stirring to understand the effect of forced flow. The specimens solidified with a low cooling rate, low temperature gradient, and forced convection. Stirring induced by a coil system around the specimens caused a transformation from equiaxed dendritic to rosette morphology with minor dendrites and, occasionally, spheroids. This evolution was quantitatively observed with specific surface Sv. The precipitation sequence of the phases was calculated using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) technique. Melt flow decreased secondary dendrite arm spacing λ2 in the AlSi5Fe1.0 alloy, while λ2 increased slightly in Mg-containing alloys. The length of detrimental β-Al5FeSi phases decreased only in AlSi5Fe1.0 alloy under stirring, whereas in Mg-containing alloys, changes to the β-Al5FeSi phase were negligible; however, in all specimens, the number density increased. The modification of Mn-rich phases, spacing of eutectics, and Mg2Si phases was analyzed. It was found that the occurrence of Mg2Si phase regions reduced fluid flow in the late stages of solidification and, consequentially, reduced shortening of β-Al5FeSi, diminished secondary arm-ripening caused by forced convection, and supported diffusive ripening. However, the Mg2Si phase was found to have not disturbed stirring in the early stage of solidification, and transformation from dendrites to rosettes was unaffected.

Solidification modelling for coupling prediction of porosity and segregation

Porosity is a common casting defect induced by solidification shrinkage as well as gas segregation, nevertheless, it may exert a beneficial effect on reducing alloying element segregation. Unfortunately, the influence of porosity formation on alloying elements segregation in casting alloys have not been synthetically investigated by neither experiment nor theoretical analysis. In this regards, here a theoretical model, based on analyses of the redistribution behaviours of both gas element and the alloying elements, is proposed to simultaneously predict the porosity and segregation in as-solidified Al-based alloys. First, Al-4.5 wt pct Cu alloy with a columnar dendritic interface is selected to mathematically predict the porosity forming process. The modelling results show the same tendency with Poirier’s prediction, but the volume percentage of porosity calculated by the present model is lower than that of Poirier’s. Besides, the present model predicts a decrease in porosity fraction at late stage of solidification, and more than the previous studies, the gas escaped from eutectic liquid is also evaluated. Then, the impact of porosity formation on the velocity of interdendritic feeding flow is estimated, the porosity in the mushy zone apparently slow down the suction of interdendritic liquid. Finally, a mathematical model for element segregation is derived with consideration of porosity formation. Numerical results show that the solute enrichment in interdendritic liquid gets relieved slightly with porosity formation. Consequently, porosity will reduce the solute segregation.

Tensile Deformation Behavior of Al-Cu 206 Cast Alloys near the Solidus Temperature

To study the micromechanics of semisolid deformation, a modified experimental set-up is employed in Gleeble 3800 thermomechamical testing unit to achieve a uniform temperature distribution in partially remelted aluminum samples. The temperature variation was markedly reduced to one degree for a length of 4-5 mm in the middle of tensile samples. High temperature semisolid tensile tests of Al-Cu 206 cast alloys were performed at different temperatures near solidus with a strain rate of 10-3 s-1, corresponding to the solid fractions (fs) between 1 and 0.95. The stress-displacement curves with different fs were measured and analyzed. The microstructure and fracture surface of samples were examined by optical and scanning electron microscopes. The relation between the microstructural characteristics, tensile properties and fracture behavior of semisolid 206 samples at high fs were explored. Mush deformation mechanisms were discussed in term of defect nucleation and propagation at the late stage of solidification.

Microstructure evolution and mechanical properties of quasicrystal-reinforced Mg-Zn-Y alloy subjected to ultrasonic vibration

Although the icosahedral quasicrystal phase found in Mg-Zn-Y alloys has some outstanding characteristics, the coarse α-Mg dendrites and the agglomeration of the secondary phases in as-cast microstructure restrict the improvements in the mechanical properties. In this study, the semisolid slurry of Mg-6Zn-1.4Y alloy was obtained with ultrasonic vibration (UV) treatment and then formed by rheo-squeeze casting (RSC) process. The effects of UV on the microstructure evolution and mechanical properties were systematically investigated. With UV, primary α-Mg grains and the agglomerated Mg-Zn-Y compounds were significantly refined. Notably, a large mass of fine and granular quasicrystal I-phase particles precipitate in the grains at the later stage of solidification. The RSC alloy subjected to 6W/mL UV exhibited the optimal mechanical properties, with the yield strength of 129MPa, the ultimate tensile strength of 231MPa and the elongation of 18.5%. Compared with the samples without UV, they are increased by 18.3%, 14.9% and 55.5%, respectively.

Analysis and control of central cracks in the bloom continuous casting of microalloy 49MnVS3 steel

In order to control central cracks in continuous casting of microalloy 49MnVS3 steel, the formation mechanism of central cracks has been studied by analysing the element segregation, crack morphology, hot ductility, precipitates and centre macrosegregation. It was found that the centre macrosegregation of carbon and sulphur and the precipitation of (Mn, Fe)S in grain boundary at the later stage of solidification could decrease the solidus temperature and enlarge the high temperature brittle zone, meanwhile, the precipitations of MnS and Ti(C, N) in grain boundary could make cracks easier to propagate, both of which lead to the formation of central cracks. Based on the above analysis, the centre macrosegregation of carbon and sulphur and the inclusions including MnS and the total in the centre of bloom were decreased by using final electromagnetic stirring and machine soft reduction together, thus, the central cracks of bloom were controlled successfully.

Analysis and simulation of non-metallic inclusions in spheroidal graphite iron

Non-metallic inclusions in spheroidal cast iron (SGI) reduce fatigue strength and yield strength. This type of inclusion usually accumulates at grain boundaries. Papers addressing this topic show the overall impact of both the fraction of so-called white (carbides) and black (non-metallic) inclusions on mechanical properties. In the present work we focus on the origin and the formation conditions of black Mg-bearing inclusions, further distinguishing between Si-bearing and non-Si-bearing Mg inclusions. The formation was simulated applying thermodynamic approaches. Moreover, appropriate experiments have been carried out and a large number of particles have been studied applying innovative feature analysis with regard to shape, size, and composition. Magnesium silicates are predicted at elevated oxygen concentrations, whereas at low levels of oxygen sulphides and carbides appear at a late stage of solidification. Experiments with three consecutive flow obstacles show that the amount of magnesium silicates decrease after each of the three obstacles, whereas the fraction of non-Si-bearing inclusions remains approximately constant. The size of inclusions divides in halves over the flow path and the number of particles increases accordingly. We point out that based on feature analysis Mg-O-C bearing inclusion show disadvantageous form factors for which reason this kind of inclusions may be extremely harmful in terms of crack initiation. All results obtained indicate that magnesium silicates are entrapped on mould filling, whereas Mg-(O, C, S, P, N) bearing particles are precipitates at late stages of solidification. Consequently, the only avoidance strategy is setting up optimum retained magnesium content.

Heatline Visualization During Solidification of a Eutectic Solution in a Rectangular Cavity

The present work is focused on the visualization of heatline during solidification. The investigation considers a rectangular cavity filled with eutectic aqueous ammonium chloride solution. In order to develop Rayleigh-Bennard convection in the cavity, the temperatures of the top and bottom walls are maintained below and above eutectic temperature of the solution, respectively, whereas the vertical walls are assumed to be adiabatic. The solidification process is modelled by a set of mass, momentum and energy conservation equations coupled with boundary conditions. The governing differential equations are solved using semi-implicit finite volume method according to SIMPLER algorithm with the help of line-by-line TDMA solver. The evolution of heatline pattern during solidification is presented. From the evolution of solid–liquid interface, it is found that the solidified layer undergoes remelting at the solid–liquid interface during later stages of solidification as a consequence of thermal superheat. In the present work, the cause of the remelting phenomenon during solidification is investigated in details using energy flux vectors. It is observed that convection loop pattern i.e. energy flow below the remelted interface changes at the time of remelting.

Remelting and interface dynamics during solidification of a eutectic solution in a top-cooled rectangular cavity: A numerical study

The present work is a numerical investigation on the occurrence of remelting phenomenon during solidification of a eutectic aqueous ammonium chloride solution in a top-cooled rectangular cavity. The analysis is performed following a fixed-grid single-domain approach. Accordingly, the solidification process is represented by a set of mass, momentum and energy conservation equations, where the enthalpy update scheme is used to trace the solid–liquid interface during solidification. The set of governing differential equations is solved using semi-implicit finite volume method, based on SIMPLER algorithm, applying line-by-line TDMA solver. The present simulation predicts the distribution of temperature and streamline along with fraction of solid in the computational domain, and evolution of the solidified layer during solidification. The simulation shows remelting of solidified layer at the solid–liquid interface during later stages of solidification due to irregular superheat of the bulk solution convected towards the solid–liquid interface. Hence, the occurrence of the remelting phenomenon during solidification is studied by evaluating the interface growth history as a function of the process parameters, namely, cavity height and thermal gradient imposed across the cavity. It is found that there is a small increase in the solidification growth rate with increase in cavity height, where the possibility of remelting is high. With the increase in imposed thermal gradient, the solidification growth rate increases significantly and the occurrence of remelting disappears.