Semi-solid Slurry Research Articles

Microstructural evolution mechanism of semi-solid slurry: a study using Phase-Field-Lattice-Boltzmann scheme

Abstract The formation mechanism study of a spherical crystal is the theoretical basis for preparing high-quality semi-solid slurries with homogeneous spherical microstructure. The interaction between thermal-solute diffusion during solidification and solid-liquid interface migration determines the morphology evolution of microstructure according to the representation theory. Comprehensive research about the effects of local variates on the microstructural evolution of semi-solid slurry in a Representative Elementary Volume (REV) by 2D cases was conducted. A modified Phase-Field-Lattice-Boltzmann scheme using techniques of parallel computing and adaptive mesh refinement (Para-AMR) was adopted. Actual processing parameters were coupled with the numerical scheme. Mesoscopic results were upscaled by averaging them over the simulation domain. Five aspects, growth space, initial undercooling degree, cooling rate, natural convection, and forced convection, were investigated. It is found that the formation of spherical grain requires a specific initial undercooling degree, cooling rate, and nuclei density. Study results indicate that melt convection has little effect on the morphological evolution of grain in the case of rapid isotropic growth, which was verified by experiments using the SEED process. Besides, forced convection has a significant effect on the morphology evolution of grain controlled by solute diffusion, which provides a possibility of an extension of the process window about pouring temperature and alloys with narrow solidification range for preparing semi-solid slurries.

Evaluation of Microstructural Characteristics of High-Si Al Alloy Cylindrical Samples Produced by Rheological Squeeze Casting with Bottom-Up Filling and Flow State Analysis of the Alloy Semi-Solid Slurries

The microstructures of high-Si Al alloy cylindrical simples produced by rheological squeeze-casting with bottom-up filling were investigated. The effect of process parameters on the distribution of hard phases (primary Si and Fe-rich phase) was evaluated, and flow characteristics of semi-solid slurry of high-Si Al alloy were investigated. Differences in volume fractions (VFs) of hard phases between the middle and upper layers of cylindrical samples decreased as the squeezing speed increased. Moreover, the flow state of the semi-solid alloy slurry transitioned from laminar to turbulent flow as the squeezing speed was increased. Movement of gases throughout the semi-solid alloy slurry filling process was analysed under different process parameters. The results provide insights for improving rheological casting of high-Si Al alloy and other Al matrix composites.

Microstructure and Mechanical Properties of ZA27 Alloy Rheo-Diecasting Process Fabricated by Self-Inoculation Method

Abstract Semisolid slurry of ZA27 alloy was prepared by Self-inoculation Method (SIM), the effects of isothermal holding parameters on microstructures of rheo-diecasting were researched, and the mechanical properties of ZA27 rheo-diecastings were tested. The results indicate that the dendritic microstructure of ZA27 alloy formed by permanent mold casting can be significantly modified by RDC with SIM, and obtain fine spherical microstructures. After comprehensive consideration, the suitable melt treatment temperature for the ZA27 alloy semisolid processing is 550~560°C. The isothermal holding process of ZA27 alloy slurry has great effect on primary α-Al particles (α1), while has little effect on the microstructure of secondary solidification in the process of thin-walled rheo-diecasting, and the suitable isothermal holding time of semisolid slurry for rheo-diecasting is 3 min. Compared with HPDC, the RDC process can increase the tensile strength of ZA27 alloy by more than 6.9%, with a maximum increase of about 12.5%. The best performance die casting can be obtained when the slurry holding time is 3-5 min.

Formation of non-dendritic microstructures in preparation of semi-solid Mg-RE alloys slurries: Roles of RE content and cooling rate

Abstract The proper alloy composition ranges for rheo-forming of Mg-RE alloy was obtained through thermodynamic calculations. The low frequency electromagnetic stirring (LFEMS) process was used to fabricate semisolid slurries of Mg-Gd-Zn series alloys, and the role of solute Gd content and the cooling rate in the LFEMS process were systematically investigated. Both the Gd content and cooling rate show effective influences on the microstructure of the Mg-Gd-Zn alloy semisolid slurries. Fine and spherical primary Mg phase can only be obtained by decreasing the cooling rate to less than 1.4 C°/min and increasing the solute Gd content to no less than 9 wt.%. The formation mechanism of non-dendritic morphology of primary Mg phase in Mg-Gd-Zn alloy was proposed to be the dendrite arm root re-melting induced by curvature undercooling and ripening induced by low cooling rate.

Multiphase modelling of the transient flow for Sn-15Pb and 357.0 alloys in semi-solid die casting process

Abstract The transient flow behavior of the semi-solid slurries for Sn-15Pb and 357.0 alloys during die casting process is investigated by experimental and numerical methods. A model for the transient-state three-dimensional multiphase flow based on the kinetic theory of granular flow is developed. This model is applied to investigate the distribution of solid concentration, flow state, pressure variation of the semi-solid slurry in the die casting process. The three important aspects of the multiphase flow, flow regime, particle-particle interaction and particle-liquid interaction, are interpreted according to the characteristic of the system. The model is verified by partial filling and in-situ observation experiments. Then it is applied for the phase segregation study of an elbow cavity encountered in practice. Results indicate that the 3D multiphase model can depict the flow characteristics of the semi-solid slurry very well. The inner arc side of the elbow cavity is easier to have liquid phase segregation than the outer arc side for the lower velocity gradients of the solid and liquid phases.

A Comparison of the Use of Ultrasonic Melt Treatment and Mechanical Vibration in the Manufacture of Al5Si5Zn Alloy Feedstock for Thixoforming

The use of physical agents when casting aluminum alloys has proven to be an effective route for grain refinement and avoids the inconvenience of residual impurities left in the material when chemical agents are used. The application of ultrasonic waves to the molten metal before casting generates acoustic cavitation, which promotes extensive heterogeneous nucleation and contributes to degassing of the metal. In addition, the application of mechanical vibration during solidification has been proven to promote dendrite fragmentation, and therefore, grain refinement. The aim of this work is to evaluate microstructural refinement due to cavitation produced by ultrasonic melt treatment (UST) of Al5Si5Zn alloy (Al-5wt pctSi-5wt pctZn) and to compare the resulting microstructure with that achieved with and without simple mechanical vibration (MV) during casting so that the best manufacturing procedure for refining aluminum silicon feedstock for subsequent thixoforming can be identified. After casting, the alloy produced under each condition was partially melted to a 0.45 solid fraction to obtain a primary phase with a spheroidized microstructure. The rheological behavior of each semisolid slurry was also evaluated. Microstructural characterization was performed using optical and scanning electron microscopy. Mechanical performance was evaluated by means of tensile tests and hardness measurements. The use of ultrasonic stirring for 30 seconds resulted in slightly better mechanical performance than the other casting conditions. However, because of the short life expectancy of the sonotrodes, mechanical vibration can be considered a simpler, superior solution for feedstock production.

Optimization of Pouring Temperatures and Stirrer Speed Parameters on a Semi-Solid Slurry of ADC12 Al Alloy Prepared by Mechanical Stirring

This paper presents an experimental design approach for the optimization process parameters for the aluminum solid alloy casting of ADC12 alloys. To achieve this goal, the pouring temperatures and stirring speed are selected and two levels of this parameter are considered. Design of expert (DOE) of tests was used for experimental design and analysis of results. The aluminum ADC12 slurry is stirred by a mechanical stirrer (round rods stirrer models) with a variation of speed 250, 300, 350 rpm for 20 seconds. Furthermore, the aluminum slurry of ADC12 is poured on a metal mold with a temperature of 620 °C, 600 °C, and 580 °C. The microstructure characteristics were examined by direct observation using optical microscopy, secondary α-Al phase dendrite arm spacing and shape factor were identified. The mechanical properties were investigated by the hardness test and tensile test. The mechanical properties and microstructure of aluminum alloys ADC12 made with semi-solid rheocasting casting technology using several parameters pouring temperature and stirring speed have been studied, and the results obtained can be synergized as follows. ADC12 aluminum alloy slurry preparation using a round rod mechanical stirrer on semi-solid casting can improve mechanical properties. Low pouring temperature and high stirring speed are reliable techniques for producing the right mechanical and microstructure properties for a semi-solid casting process (Rheo casting). The pouring temperature and stirring speed are recommended to produce optimal mechanical properties and the round microstructure is 580 °C and 350 rpm.

Effect of Temperature Gradient on the Grain Size Homogeneity of SEED Produced Semi-Solid Slurries by Phase-Field Simulation.

The distribution homogeneity of grain size affects the fluidity of the semi-solid slurry, which in turn affects the properties of the casting. One key factor affecting grain size uniformity resides in the nucleation number, which has been studied thoroughly, while the other factor is temperature gradient which has not been investigated yet. In this study, the microstructure evolutions under certain temperature gradients are investigated by experiment and simulation using a two-dimensional quantitative phase-field (PF) model. A parallel and adaptive mesh refinement algorithm is adopted to solve the nonlinear phase-field equations. The results indicate that temperature gradient can affect the size distribution of microstructure in the semi-solid slurry prepared by the SEED process. A higher temperature gradient (in the range of 0.230~0.657 °C/mm) along the radial direction is beneficial to the homogeneity of the grain size in a slurry.

Effect of Pouring Temperature on Microstructure and Properties of A356 Alloy Strip by a Novel Semisolid Micro Fused-Casting for Metal

A novel semisolid micro fused-casting (MFC) for preparing A356 alloy strips is proposed, and the effects of process parameters of pouring temperature on the microstructure and properties of A356 alloy strips are investigated. MFC means that the semisolid metal slurry was pressed out from the outlet of bottom of crucible to the movable plate, and directly solidified and formed layer by layer. The microstructure and properties of A356 semisolid alloy slurry were influenced by the cooling conditions. Results show that the aluminu alloy A356 strip samples fabricated by micro fused-casting had good performances and uniform structures with the pouring temperature at 595 °C and the substrate movement speed at 18 mm/s. The fine grains of the primary α-Al phase with average grain size of 53 μm and shape factor up to 0.72 was obtained, the ultimate tensile strength of the aluminum alloy A356 strip reaches 243.79±3.91 MPa, while the average vickers hardness is 82.65±1.86 HV.

Cooling Behavior and Microstructure of Semisolid A201 Aluminum Alloy Prepared by the SEED Process

The biggest challenge in semisolid processing of high-performance aluminum alloys is the narrow temperature processing windows of these alloys, and as a result, the preparation of qualified semisolid slurries is very important. High solid fraction slurries of high-strength A201 alloy were prepared by the Swirled Enthalpy Equilibration Device (SEED) process. The cooling behavior and microstructures of the A201 slurries produced by the standard, as well as a modified, SEED process were investigated. The results show that qualified A201 slurry can be produced by decreasing the pouring temperature and controlling the processing time in the SEED process. The modified SEED process significantly reduced the radial temperature gradient of the melt, due to the slow cooling rates involved, with the resulting slurries being more uniform, with more spherical microstructures, as compared to those produced by the standard SEED process. The formation of the nondendritic grain structure in the SEED process is attributed to the uniformly distributed large number of nuclei within the melt and the slow cooling of the melt in the containing crucible.

Microstructure and Simulation of Semisolid Aluminum Alloy Castings in the Process of Stirring Integrated Transfer-Heat (SIT) with Water Cooling

The continuing quest for aluminum castings with fewer defects has aroused the interest in rheological high-pressure die casting (RHPDC). A new machine, stirring integrated transfer-heat (SIT) device, based on the mechanical stirring and enhanced cooling for semisolid slurry preparation was proposed to produce the thin-walled communication shell part by RHPDC process. The influence of SIT process characteristics on the microstructure was studied. The microstructure of SIT-RHPDC shell part in different locations was investigated. Combined with simulation, the flow characteristics and the temperature field change in the melt under the stirring-only process and different stirring with internal water-cooling conditions were studied to get a better understanding of the nucleation, grain growth and solidification behavior. Under the dual action of agitation and heat dissipation in SIT device, the fluid convection has changed the temperature field of the melt significantly. The temperature field of the melt becomes relatively uniform from a great temperature gradient and the continuous cooling rate is increased to 5 °C/s. In the same time, the slurry has a greater supercooling degree, and flows more evenly in the filling process. The experimental results indicate that compared with microstructure of an HPDC component, the microstructure at different locations of SIT-RHPDC part is composed of refined rosiness and nearly spherical particles.

Critical Assessment 32: Controling the melting behaviour of cast structures through solid-state deformation

ABSTRACTThe role of solid-state deformation of cast structures in controlling the morphological transformations, accompanying their subsequent melting is critically assessed. When dendritic morphologies, present after casting, are subjected to solid-state deformation with a strain level sufficient to trigger recrystallisation, they are replaced by equiaxed grain structures. Heating of such structures to temperatures of the solidus-liquidus range initiates melting at triple junctions and along grain boundaries, causing disappearance of dendrites, which are replaced by globular features. The semisolid slurry formed this way exhibits thixotropic properties, positively affecting its flow behaviour and properties after solidification. Although a variety of conventional and exotic deformation techniques generate very fine grain structure after recrystallisation, it does not lead to substantial differences in morphological characteristics of the semisolid slurry.

Tensile properties and microstructure of rheo-diecast 7075 alloy prepared by serpentine channel process

A semisolid slurry of 7075 aluminum alloy was prepared by means of a serpentine channel process (SCP) and rheo-diecasting. The influences of pouring temperature during slurry preparation, the injection pressure and die preheat temperature on the mechanical properties and microstructure of a rheo-diecast 7075 aluminum alloy were investigated. The results show that the as-cast strength and elongation of the tensile samples were 210–260 MPa and 0.2%–1.7%, respectively, with an injection pressure of 130 MPa, a pouring temperature of 700–720 °C, a preheat temperature of 280–350 °C, and a plunger speed of 0.5 ms−1. The results also show that the aged strength and elongation of the tensile samples were 420–453 MPa and 1.0%–1.4%, respectively, when the alloy solution was treated at 470 °C for 12 h and aged at 120 °C for 24 h. The substantial shrinkage porosity in the 7075 aluminum alloy tensile samples was the main cause of low elongation.

Effects of Si Content and Forging Pressure on the Microstructural and Mechanical Characteristics in Semi-solid Forging of Al-Si-Mg Alloys

In this study, semi-solid forging (SSF) using electromagnetic stirring of Al-Si-Mg alloys was carried out. To evaluate the acceptability for producing a semi-solid slurry of an alloy with various Si contents, the temperature sensitivity of the solid fraction and solidification range were calculated by Thermo-Calc. The effects of the Si content and forging pressure on the microstructural and mechanical properties, such as tensile strength and elongation, were ascertained. Specimens were taken from two typical positions on the semi-solid-forged products, where one position corresponds to a region of directly applied forging pressure and the other region corresponds to the position of slurry being squeezed and extruded. Microstructural characteristics such as average grain size, volume fraction, and form factor of the α-Al were evaluated as the Si content was changed from 5 to 7 wt pct at 0.5 wt pct intervals. As the Si content increased, the particle size of α-Al and tensile strength increased, while the volume fraction of α-Al and elongation decreased. At lower Si contents, solidification shrinkage was observed, resulting in a decrease in the elongation. At 5 and 6 wt pct Si, α-Al particles were agglomerated, and shrinkage was observed. Uniform, fine, and globular microstructures were obtained at 6 wt pct Si. At 6 and 7 wt pct Si, α-Al particles were coarsened. The forging pressure was changed from 100 to 250 MPa, and at a forging pressure of 250 MPa, shrinkage was observed. As the forging pressure was increased from 100 to 200 MPa, the tensile strength and elongation increased, and the particle size of α-Al decreased. At a forging pressure of 250 MPa, α-Al particles were deformed and agglomerated, which decreased the strength.

Studies on die filling of A356 Al alloy and development of a steering knuckle component using rheo pressure die casting system

In this study, a computational fluid dynamics (CFD) model is developed to investigate die filling of semi solid slurry as part of rheo pressure die casting (RPDC) system. The die filling cavity corresponds to that of an automobile steering knuckle, and the slurry is made of A356 aluminium alloy. The rheological model used in the CFD simulation is determined experimentally. The results obtained from present numerical model includes flow field of the slurry within the die cavity, viscosity evolution, solid fraction distribution, temperature and pressure distribution during solidification within cavity during die filling stage. The main objective of the study is to determine the gating arrangement, pouring temperature, and injection conditions for desirable microstructure and mechanical properties of the developed component. To study the effect of injection conditions on die filling capability of the said alloy slurry, five injection profiles are studied, with a variation in final injection velocity between 2–3.2 m/s. In order to corroborate the findings of the present study, microstructural morphology and structure-property correlation have been studied, primarily in the form of optical microscopy and macro hardness measurements, by obtaining samples from different locations of the solidified component.

Influence of Fine Al2O3 and Aluminium Nano-Particles on the 6061 Aluminium Alloy near the Grain Boundary of the Semi-Solid Cast Microstructure

Dual mixed slurry for AA6061/Al2O3-Al powder was formed under argon gas protection and used to fabricate composite due to the effect of free vortex flow in semi-solid stage. Initially, Al2O3 reinforcement was mixed with aluminium powder via ball milling process and then converted into reinforcement slurry. Simultaneously, primary slurry for AA6061 alloy was prepared in closed coiled sprue. Semi-solid thick slurry was prepared for a consolidated effect of liquid reinforcement addition and solid-liquid dispersion on the cast microstructure. Finally, this mixed slurry was free to flow in open mould in free vortex flow and 668oC melt temperature condition. Observations from the microstructure, EDX and XRD reveal that Al2O3 particles and aluminium particles are segregated near the grain boundary and potentially provide an effective joining between the matrix and reinforced particles. Mechanical characteristics for tensile and impact strength for Al2O3 addition confirms a significant improvement on these properties of the AA6061 alloy.

The effects of annular electromagnetic stirring parameters on microstructure evolution of rheocast AZ91 magnesium alloy

In the present study, an annular electromagnetic stirrer was used to fabricate the semi-solid slurry of AZ91 magnesium alloy under various conditions of electromagnetic intensities and times. The fabricated semi-solid slurry was injected into a step-die cavity, using a hydraulic jack with ram speed of 1m/s. The effects of electromagnetic intensity, stirring time and step thicknesses on the microstructure evolution, shape factor, globules equivalent diameter, solid fraction and microhardness of samples were investigated. The experimental results showed that obtained optimal parameters of electromagnetic stirring were 55 (A) stirring current and 30 (s) stirring time. In this conditions, the value of shape factor, was ∼0.85. By increasing the thickness of the steps, the shape factor and micro-hardness value was decreased, whereas, the equivalent diameter of globules and solid fraction were increased.

Metallurgical Structure of A356 Alloy Solidified by Mechanical Stirring

The study investigated the effects of mechanical stirring before solidification on the metallurgical structure of hypoeutectic aluminum-silicon A356. A series of stirring trials were conducted in the present study. Emphasis were placed on the morphological changes of the primary phase, which was subjected to different levels of stirring at various values of the rod material and its diameter, insertion temperature and rotation speed. It was found that when the rod was made of the same material as the molten metal, it acted as a nucleation site to generate numerous nucleated primary crystals, which separated from the rod surface continuously into the molten metal with the rotation of the stirring rod, resulting in the refinement and spheroidization of the primary crystals. The ideal semisolid slurry with homogeneous spherical and fine primary crystals could be obtained by optimizing rod insertion temperature, rotation speed and its diameter, which is a key factor in semi-solid forming.

Thixoforming of Semisolid Slurry with High Fraction Solid Fabricated by Partial Melting of Commerical Wrought Aluminum Alloys

Semisolid slurries of four wrought alloys were fabricated via partial melting of commerical wrought aluminum alloy. Thixoforming experiments of four typical parts were performed. The results showed that a large amount of equiaxed grains before soaking in semisolid state were created due to recrystallization occurred in the continuous heating from room temperature to a given temperature above recrystallization temperature. It provides a desirable microstructure to form spheroidal grains during the next soaking process in semisolid state. The microstructure of the 2A12,7A04 and 7075 semisolid slurry consisted of fine and spheroidal grains. The elongation of the thixoformed parts were higher those of the hot-rolled plate. The UTS of the thixoformed parts were close or ever higher than those of the hot-rolled plate. Although the grain size and roundness of the 5A06 semisolid slurry are not very desirable, the mechanical properties of the thixoformed part are close or ever than those of the hot-rolled plate. The high mechanical properties of the thixoformed parts further confirmed the feasibility of short-process thixoforming route

Numerical Simulation of Slurry Making Process of 7075 Aluminum Alloy under Electromagnetic Field in Rheocasting Process

Rheocasting processes have several technological advantages, such as shorter processing and lower cost than thixocasting processed, while better product quality than conventional die casting. At present, the common alloys used in rheocasting processes are mainly Al-Si alloys which have limited strength. There is a strong demand for higher strength aluminum alloys in industries, for instance the 7075 aluminum alloy. However, the solid fraction of the 7075 aluminum alloy varies significantly with the temperature change in the medium solid fraction range from 0.4 to 0.6, which poses a great challenge to the slurry preparation. The enthalpy compensation process, in which the core was electromagnetic heating in the outside of the metallic crucible containing aluminum alloy melt, has been proposed to reduce the temperature difference within semi-solid slurry in this work. This study used simulation method to analyze the effects of key process parameters on the temperature difference within the 7075 aluminum alloy slurry. The simulation results indicated that semi-solid slurry with medium solid fraction may be obtained and temperature difference may be reduced to less than 4°C.