Primary Al Grains Research Articles

Isothermal coarsening of cooling slope processed semi-solid A380 Al alloy slurry

ABSTRACT Present study depicts the effect of isothermal holding on microstructural morphology of the cooling slope processed slurry of A380 Al alloy. Along with the primary and eutectic phase morphology, significant effect of isothermal slurry coarsening on Iron (Fe)-rich intermetallic formation is evidenced here. The study also reports the effect of adding refining (0.15 wt% Al-5Ti-1B master alloy) and modifying (0.1 wt% Al-10Sr master alloy) agents within the melt on formation sequence and percentage of Fe- and Mn-rich intermetallics within the slurry matrix. Furthermore, modified Lifshitz-Slyozov-Wagner equation is proposed here to quantify the relative influence of Ostwald ripening and coalescence, towards determining slurry microstructure, during isothermal slurry holding stage. Optimum slurry morphology, i.e. minimum size and maximum sphericity of constituent primary Al grains as well as uniform distribution of Fe, Manganese (Mn) and Copper (Cu)-rich intermetallics, is achieved for the refiner+modifier added slurry, after 7 minutes of isothermal holding.

Optimisation of melt pouring temperature and low superheat casting of Al-15Mg2Si-4.5Si composite

ABSTRACT In this study, a computational fluid dynamics (CFD) model is employed to optimise the melt pouring temperature during Low superheat casting (LSC) of the Al-15 Mg2Si-4.5Si composite. The die cavity considered to perform the die filling simulations corresponds to the as-cast tensile specimens, as per ASTM B557 guidelines. The findings of the study include melt temperature distribution, solid fraction distribution during filling as well as during solidification, velocity distribution of the melt and surface defect concentration. Experimentation is performed to develop low superheat cast composite based on the numerically estimated optimum melt pouring temperature of 650°C. Mixture of irregular dendritic, polygonal and equiaxed shaped primary Mg2Si grains are observed within the cast parts, whereas primary Al grains are found to be of dendritic morphology with occasional presence of spheroids. Marked improvements in microstructure and mechanical properties have been evidenced in the LSC composite compared to its conventional cast counterpart.

Effect of process variables and melt treatment conditions during cooling slope semi solid slurry generation process of Al-7Si-0.3 Mg alloy

Simulation studies on semi solid slurry preparation of A356 alloy employing cooling slope technique, with the help of an existing multiphase and multiscale model, are presented here. The constituent phases considered in the multiphase model are the liquid melt of A356 alloy, near spherical primary Al grains, and air. The numerical study features prediction of slurry morphology, for a desired set of process conditions of cooling slope slurry formation technique. In particular, the effects of melt pouring temperature into the slope, slope angle variation and addition of grain refiner and modifier within the melt on the thermophysical and morphological properties of the generated slurry are studied. Attempts are made to gain insight into the transport phenomena of cooling slope slurry generation process, with respect to volume fraction of constituent phases, morphological evolution of solidifying primary Al grains, viscosity distribution, temperature field, velocity field, macro and micro segregation, and isothermal coarsening kinetics of α-Al grains. The efficacy of the multiphase model is validated by performing relevant experiments.

Control of kink-band formation in mille-feuille structured Al/Al2Cu eutectic alloys

Kink bands have recently received significant attention owing to their ability to increase the strength and ductility of some Mg alloys. In this study, we first demonstrated that it is also expected even in Al alloys, by controlling the morphology of the introduced kink bands. Several directionally solidified Al–Cu alloys, wherein an Al/Al2Cu eutectic lamellar microstructure developed, were focused, and the variations in deformation behavior with microstructure were examined. The alloys can be deformed at room temperature, and the yield stress exhibits strong anisotropy. A high yield stress appears when stress is applied parallel to the lamellar interface, accompanied by kink-band formation. The microstructure was found to play an important role in controlling kink-band formation and the resultant mechanical properties. Only a few large kink bands form in a eutectic alloy specimen with a full lamellar microstructure, which is accompanied by a lower yield stress. Thus, the kink band cannot be used positively in it. In hypoeutectic alloys in which primary Al grains coexist with the lamellar microstructure, however, significantly high yield stresses appear, which are accompanied by the homogeneous formation of small kink bands. The results demonstrate that microstructural control can vary the role of the deformation kink band from the fracture (buckling) mode to the deformation mode via the change in its morphology. This enables a large increase in yield stress while maintaining the ductility of the Al/Al2Cu eutectic alloy. The high yield stress is maintained at temperatures up to ~300 °C owing to the high thermal stability of the lamellar microstructure. These findings provide new ways to develop novel high-temperature high-strength Al alloys.

Globularization of Primary Phase of Al–7Si–0.3Mg Alloy During Cooling Slope Processing and Isothermal Holding

This study is aimed towards process design for semi-solid slurry generation of Al–7Si–0.3Mg alloy using cooling slope as a slurry maker, and subsequent isothermal holding to promote globularization of slurry microstructure. The study primarily reports globularization mechanism of cooling slope processed semi-solid slurry of the alloy, and efforts are directed to establish process control by investigating the effect of coarsening during isothermal holding on the shape, size and distribution of primary Al grains. As an outcome of this study, we propose a modified Lifshitz–Slyozov Wagner equation to quantify the effect of coarsening mechanisms prevailing during isothermal holding of the said alloy slurry. The evolution of primary phase morphology due to addition of grain refiner has also been studied and ideal processing condition is identified for the end goal of component development through rheo pressure die casting route.

Effects of Fe, Mn, chemical grain refinement and cooling rate on the evolution of Fe intermetallics in a model 6082 Al-alloy

The solidification microstructures formed in a model 6082 alloy with 0.2–1.0 wt % Fe were examined under different cooling rates and the effects of 0.5 wt % Mn and Al–5Ti–1B grain refiner addition investigated. The results were compared against Thermo-Calc, differential scanning calorimetry (DSC) and cooling curve analysis. Fe promotes primary-Al grain refinement from growth restriction and constitutional undercooling effects but increases detrimental β–(Al9Fe2Si2) intermetallic. Mn contributes to primary-Al grain refinement from growth restriction and promotes the formation of α–Al15(Fe,Mn)3Si2 at the expense of increased intermetallic content. Al–5Ti–1B inoculation not only produces the strongest refinement of primary-Al grains but also refines the Fe-intermetallics by enhancing their nucleation and restricting their growth volume in the interdendritic liquid pockets.

Three-dimensional phase field simulation of spheroidal grain formation during semi solid processing of Al-7Si-0.3 Mg alloy

The present study reports development and application of a three dimensional phase field (PF) model, to investigate the microstructure evolution mechanism responsible for generation of spheroidal primary Al grains, associated with cooling slope processed semi solid slurry of A356 aluminium alloy. The cooling slope rheoprocessing technique involves solidification of flowing melt on an inclined slope surface, where gravity assisted fluid flow and shearing action between the flowing melt and the slope surface causes formation of near spherical primary solid fragments, and subsequent isothermal globularisation of the evolving primary solid. The present PF model implements a seed density based nucleation model. The seed density requirements to simulate microstructure evolution for different simulation/melt treatment conditions of cooling slope processing have been estimated based on initial experimentation. The simulated micrograph of slope exit state i.e, at the end of cooling slope processing has been used to estimate the number density and average size of constituent α-Al grains of the generated slurry, at slope exit state. The values obtained are fed subsequently as input parameters to simulate post slurry generation isothermal globularisation process. The model predictions are validated experimentally, thus establishing its capability to predict the characteristics of semisolid slurry at different stages of cooling slope rheoprocessing, in terms of solid content (volume fraction), diameter, density, and shape factor (sphericity) of nucleated primary Al grains.

Phase field modelling of microstructure evolution and ripening driven grain growth during cooling slope processing of A356 Al alloy

A phase field model is developed for simulation of microstructure evolution during semi solid slurry generation process of A356 aluminium alloy using a cooling slope. First, experiments are performed to evaluate the number of seeds required within the simulation domain to simulate near spherical microstructure formation, occurring during cooling slope processing of the melt. Subsequently, microstructure evolution is studied employing a phase field method. Simulations are performed to understand the effect of cooling rate and melt treatment in the form of grain refiner and modifier addition on the slurry microstructure. The results obtained from mesoscopic phase field simulations are grain size, grain density, degree of sphericity of the evolving primary Al phase and the amount of solid fraction present within the slurry at different time frames. The simulation is extended to predict the microstructure evolution during the post slurry generation isothermal holding stage, by feeding in the average particle size and the number of primary Al grains within the simulation domain, which are estimated from the simulated micrograph obtained at the end of slurry generation simulation. Results obtained from the simulation studies are validated against corresponding experimental observations. Insight into the cooling slope slurry generation technique is obtained from the numerical findings, which are found to be useful for process control during component development using a Rheo pressure die casting system.

Study of Microstructure Evolution during Semi-Solid Processing of an In-Situ Al Alloy Composite

In the present work, effect of pouring temperature (650°C, 655°C, and 660°C) on semi-solid microstructure evolution of in-situ magnesium silicide (Mg2Si) reinforced aluminum (Al) alloy composite has been studied. The shear force exerted by the cooling slope during gravity driven flow of the melt facilitates the formation of near spherical primary Mg2Si and primary Al grains. Shear driven melt flow along the cooling slope and grain fragmentation have been identified as the responsible mechanisms for refinement of primary Mg2Si and Al grains with improved sphericity. Results show that, while flowing down the cooling slope, morphology of primary Mg2Si and primary Al transformed gradually from coarse dendritic to mixture of near spherical particles, rosettes, and degenerated dendrites. In terms of minimum grain size and maximum sphericity, 650°C has been identified as the ideal pouring temperature for the cooling slope semi-solid processing of present Al alloy composite. Formation of spheroidal grains with homogeneous distribution of reinforcing phase (Mg2Si) improves the isotropic property of the said composite, which is desirable in most of the engineering applications.

Hydrogen generation using the corrosion of Al-Sn and Al-Si alloys in an alkaline solution

We investigated the effects of adding Sn and Si to Al alloys on the corrosion of the alloys and the generation of hydrogen from an alkaline solution using the alloys. With increasing Sn content of up to 20 wt% in the Al-Sn alloy, the volume fraction of the Sn phase as a cathodic site at grain boundaries increased, and consequently, the hydrogen generation rate from an alkaline solution by the alloy also increased. In addition, the quenched Al-Sn alloys had smaller grain sizes compared to the furnace-cooled alloys, and accordingly, exhibited a slightly higher hydrogen generation rate. A galvanic cell was formed between the Al grain and the Sn phase of the grain boundary, and accordingly, intergranular type corrosion was observed on the Al-Sn alloys. Compared with the Al-Sn alloys, a more uniform type corrosion was observed on the Al-Si alloys because the nobler Si was uniformly distributed in the eutectic region formed between the primary Al grains. The hydrogen generation rate increased with an increasing Si content up to 10 wt% and was greater for the furnace-cooled samples than that for the quenched samples due to the more clearly formed eutectic structure.

Fe Bearing Intermetallic Phase Formation in a Wrought Al–Mg–Si Alloy

This paper investigates the two dimensional (2D) and three dimensional (3D) morphologies of the Fe bearing intermetallics that forms during the direct chill casting of an AA6063 Al alloy. An intermetallic phase extraction technique was used to facilitate 3D interconnectivity, morphology and fraction of intermetallics. Metallographic 2D analyses suggest the presence of Chinese-script-type and needle-type morphology Fe bearing intermetallics typically at the primary Al grain boundaries, whereas 3D analyse of the extracted intermetallics suggests those particles have dendrite-type and platelet-type morphologies. In-addition, globular shaped intermetallics which were observed in 2D within the primary Al grains where observed to have sphere shaped rosette-type morphology in 3D. αc-AlFeSi and β-AlFeSi were the two dominant intermetallic phases observed in the as-cast billet. Clusters of Ti rich particles were observed at the point from which growth appears to have started suggesting a possible nucleating site for the Fe intermetallics to form during solidification.

Microstructure evolution of Al-Si semi-solid slurry by gas bubble stirring method

A novel technique of introducing gas bubble stirring during solidification was studied to prepare Al-Si semi-solid slurry. The microstructure evolution of the slurry during slow cooling process after stirring was investigated. The effects of the solidification rate on the microstructure of the semi-solid slurry were investigated under three different solidification conditions. The results show that fine non-dendritic slurry can be obtained using the gas bubble stirring method. Ripening and coarsening of primary Al grains are observed during the slow cooling process, and at last coarsened eutectic Si appears. Primary Al grains with different sizes and eutectic Si are obtained, corresponding to three different solidification rates.

Electro-pulse modification and reheating process for production of thixotropic microstructure in AlSi 7Mg alloy

Electro-pulse modification (EPM) was used to change the dendritic structure of AlSi 7Mg alloy to globular one. The effects of the modified temperature, electro-pulse frequency and time on the solidified structure were examined. The results show that these parameters play an important role in the solidified microstructures. That is to say, under the same modified temperature, the solidified microstructure will be improved greatly with the increase of electro-pulse frequency and time, but when they exceed to limit values, the solidified microstructure will become worse, resulting from the decrease of the ratio of nucleation. The experimental results indicate that the suitable modified temperature of AlSi 7Mg alloy is 720 °C, and the appropriate electro-pulse frequency and time are 5 Hz and 40 s, respectively. Then the microstructures produced by suitable EPM process were reheated at temperatures between liquidus and solidus, the primary-Al grains ripen further and become more spherical, which is favorable to the semi-solid forming of AlSi 7Mg alloy.