CDS Method for Casting Aluminium-Based Wrought Alloy Compositions: Theoretical Framework

Controlled diffusion solidification (CDS) is a novel and simple process that enables the formation of non-dendritic microstructure of primary Al phase through mixing two liquid alloys of different composition and temperature together. A quaternary alloy (Al-5.0Cu-0.35Mn-0.25Ti, wt.%), having a similar chemical component with ZL205A, was fabricated using controlled diffusion solidification (CDS) method with different mixing temperature. The mixing temperature of two liquids mostly affects the cast structure especially the primary Al phase. Results show that CDS can reduce the element segregation degree inside the grains. Microstructure evolution during solidification initiates from a primary nucleus firstly and then changed to a non-dendritic grain structure. The thermal analysis confirms the thermodynamic conditions for the formation of non-dendritic grain structure evolution.

To check solidification of aluminum alloy (AL-LM13) and nano zirconium oxide (ZrO2) composites by copper chills in water as a cooling medium has been tried with this experimental study. The key objective of this investigation focuses on analyzing the effect of copper chills on microstructural characteristics, mechanical properties, and solidification kinetics of the AL-LM13 alloy incorporating varying proportions of ZrO2 nanoparticles. The goal of adding ZrO2 nanoparticles was to improve the AL-LM13 alloy’s wear resistance and mechanical qualities. This alloy was selected because to its favorable casting properties. The exceptional thermal conductivity demonstrated by copper chills was utilized as the cooling medium throughout the course of the experimental procedure. A sand casting methodology was employed to produce the cast specimens. The experimental framework included various configurations of chill placements along with distinct ZrO2 concentrations (0%, 1%, 2%, and 3% by weight). The procedures for solidification were carefully examined, and the resultant microstructures were characterized employing optical and scanning electron microscopy methods. The specimens’ hardness, impact resistance, and tensile strength were assessed using predetermined criteria. The findings indicated that the application of copper chills significantly enhances cooling rates, leading to the development of refined microstructures with superior grain morphology. An addition of ZrO2 nanoparticles improved the 240mechanical properties further with the maximum value observed at 2 weight%. Research shows copper chills and ZrO2 help cooling and improve mechanical properties of Al-LM13 alloy composites (aluminium and silicon) making them suitable for using in engineering purposes. Future work will focus on the long-term stability and performance of these composites under operational conditions.

The cast specimens for this study were prepared with a final composition of Al-5.8Zn-2.2Mg-2.5Cu alloy by using a relatively new casting process that is also known as controlled diffusion solidification (CDS). In an earlier publication, evaluation of the uniaxial tensile properties of these castings was reported and discussed. In the present study, the effect of the CDS process was examined on the formation of secondary and intermetallic phases (IMPs) evolving during solidification. A quantitative image analysis was carried out using the light optical microscopy (LOM) and scanning electron microscopy (SEM) techniques. The quantitative metallography showed that the total phase fraction of the eutectic phases is greater than that predicted by the thermodynamic calculations, assuming the Scheil–Gulliver (S–G) (nonequilibrium) paradigm, which strongly suggests a significant shift from conventional wisdom regarding solute redistribution during solidification. Furthermore, the fractography results confirmed an intergranular ductile fracture mode in the castings under tension load. The microcracks/voids were initiated from shrinkage cavities or fractured IMPs such as Mg2Si and Cu2FeAl7; these phases solidified at the grain boundary (GB) areas, alongside the eutectic phases, such as sigma-Mg(Zn,Cu,Al)2 and S-(CuMgAl2), during the final stage of solidification. It was shown that the GB eutectic phases would further provide an easy pathway for the growth and propagation of initial microcracks/voids.

Taking pure Al (938.15 K) and Al-12Si (858.15 K) melts as two precursors and the Al-8Si alloy as the target alloy, the effect of the standing time after mixing on the microstructure of a mixture during controlled diffusion solidification with simultaneous mixing was investigated via a simulation and experiment. The simulation results indicate that the entrapped air will promptly form bubbles and cause the mixture to overflow within a short time of 1.2 s. An effective blending action still occurs during the initial stage (i.e., 0–0.5 s) of the standing process, resulting in the significant homogenization of the temperature field due to the thermal conductivity being much higher than the solute diffusivity. This is due to the large difference between the two thermophysical parameters, causing nuclei to rapidly form in the pure Al melt close to the interface of the pure Al/Al-12Si melts during mixing. Some of the nuclei will remelt, and others will only grow towards the pure Al melt side in a stable solid/liquid interface during standing, resulting in nondendritic Al grains and an increase in their size but a decrease in their number. These changing tendencies of grain morphology, size and number due to the standing time are consistent with those from the experiment, implying the employed simulation and calculation, as well as the achieved results, are reasonable and reliable.

Thermodynamic modeling of G-phase and assessment of phase stabilities in reactor pressure vessel steels and cast duplex stainless steels

The purpose of the present work is to compare the segregation behaviour of Ti solute in low solute alloys, such as nominally pure Al, and high solute alloys typical of Al–Si casting alloys. Microprobe measurements of Ti segregation within grains show that, under the solidification conditions employed, the measured partition coefficient of Ti in pure Al is 6.7 compared with the phase diagram prediction of ∼7.5. Similar measurements in an Al–7Si–0.3Mg alloy resulted in a partition coefficient of 3.2. Based on the measured partition coefficients, this translates into a reduction in the growth restriction factor (a measure of the segregating power of solute elements) from 8.75 K in pure Al to 3.36 K in Al–7Si–0.3Mg with an addition of 0.05 wt-%Ti. This may explain why Ti in solution is a less effective grain refiner in casting alloys than in low solute content wrought alloys. In addition, the measured Ti segregation profiles were compared with predicted profiles based on the Scheil or similar solidification relationships. It was found that the predictions of the Scheil equation produced a good fit with the measured Ti segregation profiles once it was adapted for the geometrical nature of dendritic solidification.

Threshold hydrogen for pore formation during the solidification of aluminum alloys

These Ecole polytechnique federale de Lausanne EPFL, n° 2642 (2002)Section des materiauxFaculte des sciences et techniques de l'ingenieurInstitut des materiauxLaboratoire de simulation des materiauxJury: Herve Combeau, Benoit Commet, Parviz Navi, Karen Scrivener, Philippe Thevoz Public defense: 2002-10-25 Reference doi:10.5075/epfl-thesis-2642Print copy in library catalog Record created on 2005-03-16, modified on 2017-05-10

Hydrogen porosity in directionally solidified aluminium–copper alloys: a mathematical model

In many industrial casting processes, knowledge of the solid fraction evolution during the solidification process is a key factor in determining the process parameters such as cooling rate, stirring intensity and in estimating the total solidification time. In the present work, a new method of estimating solid fraction is presented, which is based on calorimetric principles. In this method, the cooling curve data at each point in the melt, along with the thermal boundary conditions, are used to perform energy balance in the mould, from which solid fraction generation during any time interval can be estimated. This method is applied to the case of a rheocasting process, in which Al-Si alloy (A356 alloy) is solidified by stirring in a cylindrical mould placed in the annulus of a linear electromagnetic stirrer. The metal in the mould is simultaneously cooled and stirred to produce a cylindrical billet with non-dendritic globular microstructure. Temperature is measured at key locations in the mould to assess the various heat exchange processes prevalent in the mould and to monitor the solidification rate. The results obtained by energy balance method are compared with those by the conventional procedure of calculating solid fraction using the Schiel equation.

Controlled diffusion solidification (CDS) is a casting process that depends on mixing two precursor alloys to make a required resultant alloy. The CDS process can form a non-dendritic microstructure happening from copious nucleation, resulting in minimizing the tendency toward hot tearing. Pure aluminum was mixed into Al-33wt%Cu through funnels that have 9 and 6 mm diameters to make Al-4.7wt%Cu. Ansys software was employed to simulate the temperature, density, and velocity redistribution in the mixture during the mixing step. The results show that the thermal curves give better indications to predict the periods of nucleation and the heat transferring to the environment. Furthermore, the copious nucleation that occurs in the CDS process can be improved by controlling the agitation during the mixing step by decreasing the velocity of the mixture near the crucible wall and increasing the velocity in the middle of the mixture. The velocity around 1.7 ms−1 and 0.2 ms−1 happening at the middle and near crucible wall, respectively, gives better non-dendritic microstructure.

The effects of temperature difference between two precursor melts and angle between two gates for pouring the two melts on the microstructure of controlled-diffusion-solidified Al–8Si alloy were investigated by simulation and experiment. The simulation results showed that only the temperature difference was higher than 50 K, the nucleation rate during mixing could reach a critical value (corresponding to ~30% of solidified mesh number in simulations), and thus, a microstructure with small and spheroidal primary grains was obtained. Increasing the gate angle was beneficial for achieving a mixed melt with smaller and more alloy 1 pockets, and thereby was helpful for increasing nucleation rate and obtaining an ideal nondendritic microstructure. The subsequent experiment results confirmed these simulation results. When the temperature difference and gate angle were at 80 K and 140° respectively, an Al–8Si casting with grain size of 52.0 μm and a shape factor of 1.40 was achieved.

A large number of real-world problems nowadays require that decisions be made in the presence of uncertainties. Traditionally, the task of separation systems design neglects the influence of uncertainties. This work, however, explores the optimization strategy of a frequently encountered equilibrium-based distillation system design problem in the face of uncertainties. In particular, uncertainties arising from thermodynamic model parameters, which represent the vapor-liquid equilibrium (VLE) behavior of non-ideal mixtures involved in the system and are germane to the rational design of the separation system, are considered in the work. Joint confidence regions for the key thermodynamic model parameters estimated by the nonlinear least squares method have been incorporated in the analysis of the uncertain regions. We propose a novel two-stage stochastic programming solution scheme utilizing the optimum searching capacity of the simulated annealing algorithm in the first (design) stage and ASPEN Plus flowsheet simulator in the second (operating) stage in which Hammersley sequence sampling scheme has been implemented. A case study that investigates the optimal design of a two-column distillation system in an isopropanol (IPA) process will be demonstrated. Overall, this study provides us with a useful means to scrutinize the uncertainties and to optimize our design by quantifying the impact associated with them.

A set of self-consistent, thermodynamic model parameters is presented to describe the phase equilibria of Palladium-Lead (Pd-Pb) and Palladium-Tin (Pd-Sn) systems. A sublattice model is used for thermodynamic modeling of the intermediate phases. The experimental data agree very well with the values calculated using the optimized model parameters. Several isothermal and vertical sections of the ternary system, relevant to microelectronic soldering applications, are calculated. Several experiments are proposed, the data from which will be extremely helpful in refining the thermodynamic model parameters.

2024-T3 aeronautical aluminum alloy is a high-strength hard aluminum alloy, belonging to the Al-Cu-Mg series. It is mainly used for manufacturing various high-load mechanical parts such as aircraft skeletons. 2024-T3 aluminum alloy is difficult to be welded with traditional fusion welding, especially welded with another material such as titanium alloy or copper alloy. The welding quality of 2024-T3 aluminum alloy can be improved effectively by friction stir welding, which is favored in the aerospace and shipbuilding fields. Accurately grasping the welding transient evolution mechanism is the key to improving welding quality. It plays an important role in promoting the application of friction stir welding in industry. In this paper, the material physical parameters that change with the temperature were calculated by using the CALPHAD method, and the numerical model for friction stir welding was established. The transient changes in the temperature and stress field of the workpiece and the load of the stirring head were obtained. The temperature, stress, and strain of the advancing side and retreating side were compared and analyzed. The temperature of the workpiece surface and the stress around the keyhole were revealed. Zeiss Sigma 500 field emission scanning electron microscope was used to perform micro-characterization experiments, and the accuracy of the model was verified by comparing the simulation with the experiment. This research is of considerable significance for accurately revealing the mechanism of friction stir welding.