Published Article / Numerical modelling has successfully been used as an efficient tool to convert a gravity cast brake calliper to a thixocasting process. The thixo-module of Procast has been used for the modelling process to obtain optimum processing parameters. Results from interrupted shot castings show excellent correlation with the fluid dynamics and flow pattern of the model. The level and location of porosity revealed by non-destructive X-rays and microscopic analyses showed good correlation with the model prediction.

"Semi-solid metal (SSM) processing has emerged as a preferred manufacturing method due to the superior quality associated with semi-solid castings. In recent years, the driving force to reduce process cost has led to the development of a few rheocasting (also termed slurry-on-demand) processes. These include UBE’s New Rheocasting (NRC) process [1], Idra Prince’s Semi-Solid Rheocasting (SSR) process [2], and THT’s Sub-Liquidus Casting (SLC®) process [3]. A novel slurry-making SSM process developed at ACRC/MPI, termed the “Continuous Rheoconversion Process” (CRP), is a passive liquid mixing technique in which the nucleation and growth of the primary phase are controlled using a specially designed “reactor”. The reactor provides heat extraction, copious nucleation and forced convection during the initial stage of solidification, leading to the formation of thixotropic structures. In these studies, the critical issues/challenges to optimize the CRP for industrial applications have been addressed through validation experiments and pre-industrial trials."

"New, economical semi-solid metal (SSM) processes rely on forced convection during solidification to influence non-dendritic growth. The fundamental mechanisms that produce SSM microstructures in the presence of forced convection (due to fluid flow) are not fully understood. The objective of this work is to elucidate these mechanisms through the use of a new semi-solid slurry-making technique. Employing an apparatus developed at WPI, two alloy melts are mixed within a static reactor that induces convection and rapid cooling. Experiments carried out with this apparatus, named the “Continuous Rheoconversion Process” (CRP), result in globular semi-solid microstructures throughout a wide range of processing conditions. These conditions include the superheat in the melts before mixing, cooling rate of the slurry through the SSM range, and the presence or absence of inoculants in the melts. The results comprise repeatable sets of semi-solid microstructures having fine particle size and shape factors approaching unity. Even in the absence of melt inoculants, uniform distributions of α-Al particle sizes of about 60µm are attainable. Entrapped liquid is not present in the majority of the samples obtained with the CRP, and irregular particles that form in the process are of a limited distribution. Variation of slurry analysis methods indicates that these structures can be obtained consistently for both thixocasting and rheocasting applications. The design of the mixing reactor leads to turbulent fluid flow just as solidification commences. The results suggest that the following factors must be considered in identifying the mechanisms operating under the above conditions: copious nucleation of the primary phase; dispersion of nuclei throughout the bulk liquid; and inhibited remelting of nuclei due to temperature uniformity. In the CRP, these factors consistently lead to suppression of dendritic growth, significant grain refinement, and globular slurries. The exact fundamental mechanism leading to this effect is yet to be uncovered; however it is clear that temperature gradients ahead of the liquid are such that a cellular, non-dendritic morphology is the most stable growth form. Through further exploration of the process and identification of the operating mechanisms, future development of economical, continuous rheocasting methods will be facilitated."

Se revisan los antecedentes y el estado del arte en la conformación de aleaciones de aluminio en estado semisólido. Se describen los diferentes procesos, sus ventajas e inconvenientes y su interés industrial de acuerdo con la información disponible. Se plantean los aspectos teóricos de la conformación en estado semisólido relacionados con: las propiedades tixotrópicas y reológicas de los materiales; los mecanismos de formación de lodos; la estructura de los lingotes; y los efectos del recalentamiento y de los tratamientos térmicos.Mediante técnicas de Thixocasting se han fabricado, con aleaciones A357 y A356, diferentes piezas: lingote, semicomponente y componentes para automóvil y motocicleta. En estos materiales se estudia el efecto de los tratamientos térmicos en la microestructura, en las propiedades mecánicas y en los mecanismos de fractura.La experimentación realizada permite determinar el efecto de la temperatura en: la velocidad y mecanismo de crecimiento del silicio; la evolución de la fase  y los compuestos intermetálicos; y en la distribución más o menos heterogénea del magnesio. Así mismo, la producción de semicomponentes en diferentes condiciones, mediante un exhaustivo control de las condiciones de fabricación, ha permitido optimizar los parámetros de producción mediante procedimientos estadísticos.Se estudian los mecanismos de corrosión en soluciones salinas en los diferentes materiales, mediante ensayos en cámara de niebla salina, medidas de impedancia y técnicas de polarización.Finalmente se realiza un estudio comparativo de los resultados obtenidos con componentes fabricados por Thixocasting, por otros procesos de conformación en estado semisólido (New Rheocasting y Sub Liquidus Casting) y por los métodos convencionales, poniendo de manifiesto las ventajas e inconvenientes de las nuevas tecnologías.

In several applications of casting, dendritic microstructure is not desirable as it results in poor mechanical properties. Enhancing the fluid flow in the mushy zone by stirring is one of the means to suppress this dendritic growth. The strong fluid flow detaches the dendrites from the solid-liquid interface and carries them into the mold to form slurry. The detached dendrites coarsen in the slurry and form into rosette or globular particles based on processing conditions. This slurry offers less resistance to flow even at a high solid fraction and easily flow into the die-cavity. The above principle is the basis of a new manufacturing technology called “semi-sold forming” (SSF), in which metal alloys are cast in the semi-solid state. This technique has several advantages over other existing commercial casting processes, such as reduction of macrosegregation, reduction of porosity and low forming efforts. A major challenge existing in semisolid manufacturing is the production of metallic slurry in a consistent manner. The main difficulty arises because of the presence of a wide range of process parameters affecting the quality of the final product. An established method of producing slurry is by stirring the alloy using an electromagnetic stirrer. From an elaborate review of literature, it is apparent that solidification in presence of electromagnetic stirring involves a wide range of shear and cooling rates variation. However, the CFD models found in the literature are generally not based on accurate rheological properties, which are known to be functions of the relevant process parameters. Hence, there is a clear need for a comprehensive numerical model for such a solidification process, involving accurate rheological data for the semisolid slurry subjected to a range of processing conditions. The objective of the present work is to develop a numerical model for studying the transport phenomena during solidification with linear electromagnetic stirring. The study is presented in the context of a billet making process in a cylindrical mould using linear electromagnetic stirring. The mould consists of two parts: the upper part of the mould is surrounded by a linear electromagnetic stirrer forming the zone of active stirring, and the lower part of the mould is used to cool the liquid metal. The material chosen for the study is Al-7.32%Si (A356) alloy, commonly used for die casting applications. A complete numerical model will therefore have two major components: one dealing with rheological behavior of the semisolid slurry, and the other involving macroscopic modeling of the process using computational fluid dynamics (CFD) techniques. For the latter part of the model, determination of rheological behavior of the slurry is a pre-requisite. The rheological characteristics of the stirred slurry, as a function of shear rate and cooling rate, is determined experimentally using a concentric cylinder viscometer. Two different series of experiments are performed. In the first series, the liquid metal is cooled at a constant cooling rate and sheared with different shear rates to get the effect of shear rate on viscosity. In the second series of experiments, the liquid metal is cooled at different cooling rates and sheared at a constant shear rate to obtain the effect of cooling rate on viscosity. During all these experiments, the shear rate is calculated from the measured angular velocity of spindle using inductive position sensor; viscosity of the slurry is calculated based on the torque applied to the slurry and angular velocity of the spindle; and the solid fraction is calculated from measured temperature of the slurry based on Schiel equation. From these data, a constitutive relation for variable viscosity is established, which is subsequently used in a numerical model for simulating the transport phenomena associated with the solidification process. The numerical model uses a set of single-phase governing equations of mass, momentum, energy and species conservation. The set of governing equations is solved using a pressure based finite volume technique, along with an enthalpy based phase change algorithm. The numerical simulation of this process also involves modeling of Lorentz force field. The numerical study involves prediction of temperature, velocity, species and solid fraction distribution. First, studies are performed for a base case with a moderate stirring intensity of 250A primary current and 50 Hz frequency. It is found that the electromagnetic forces have maximum values near the mould periphery, which results in an ascending movement of the slurry near the mould periphery. Because of continuity, this slurry comes down along the axis of the mould. Stirring produces a strong fluid flow which results good mixing in the melt. Correspondingly, a homogenized temperature distribution is found in the domain. Because of strong stirring, the solid fraction in the slurry is found to be distributed almost uniformly. It is also found that fragmentation of dendrites increases solid fraction in the slurry with processing time. During processing, the continuous rejection of solute makes the liquid progressively solute enriched. It is predicted from the present study that the remaining liquid surrounding the primary solid phase finally solidifies with a near-eutectic composition, which is desirable from the point of view of semisolid casting. Correspondingly, a set of experiments are performed to validate the numerically predicted results. The numerical predictions of temperature variations are in good agreement with experiments, and the predicted flow field evolution correlate well with the microstructures obtained through experiments at various locations, as observed in the numerical results. Subsequently the study is extended to predict the effect of process parameters such as stirring intensity and cooling rate on the distributions of solid fraction and solute in the domain. It is found, from the simulation, that the solidification process is significantly affected by stirring intensity. At increasing primary excitation current, the magnitude of Lorentz force increases and results in increase of slurry velocity. Correspondingly, the fragmentation of dendrites from the solid/liquid is more during solidification at higher stirring intensity, which increases the fraction of solid in the slurry to a high value. It is also found that the solute and fraction of solid in the liquid mixes well under stirring action. Thus, a near uniform distribution of solute and solid fraction is found in the domain. It is found that stirring at high currents produces high solid fraction in the liquid. Also, at very low cooling rate, the solid fraction in the liquid increases. The present study focuses on the model development and experimental validation for solidification with linear electromagnetic stirring for producing a rheocast billet. Further studies highlighting the effects of various process parameters on the thermal history and microstructure formation are also presented.

The challenges of weight reduction and good strength in automotive industry have drawn considerable interest in thixocasting technologies. Joining of such components with conventional fusion welding creates voids, hot cracking, distortion in shape, and more importantly evolution of dendritic microstructure that ultimately would lead to inferior mechanical properties of the weld region. Thus, the purpose of making thixocast component is lost. The friction welding which is a solid state joining process can avoid defects associated with melting and solidification in a typical fusion weld and can be a promising alternative. This process produces a weld under compressive force at the contact of workpieces rotating or moving relative to one another to produce heat and plastically displacing material from the faying surfaces. Research on semisolid processing has its origin in the early 1970s. However, from the literature survey on semisolid processing it is clear that, till date, not much work has been done in field of joining of semisolid processed components. In the area of solid state welding, in particular, it is not at all explored. In view of this, the present work is focused on exploration of joining of Thixocast A356 Aluminium alloy component by friction welding and comparison of its performance with friction weld of conventionally cast sample of the same alloy. The study is carried out experimentally as well as numerically. Moreover, the material behaviour of thixocast component at elevated temperature in solid state is also described with the help of processing maps and constitutive modelling. The hot workability of thixocast and conventionally cast A356 alloy is evaluated with the help of processing maps developed on the basis of the dynamic materials model approach using the flow stress data obtained from the isothermal compression test in wide range of temperature (300-500℃) and strain rates (0.001s-1-10s-1). The domains of the processing map are interpreted in terms of the associated microstructural mechanism. On comparing the flow stress at elevated temperature of thixocast and conventionally cast A356 alloy samples, it is observed that the flow stress of the latter showed higher value at different strain level, temperature and strain rates. This indicates that the flow property of the thixocast alloy sample is better than that of the conventionally cast one (i.e. response to plastic flow is better for the former); while at room temperature thixocast sample has higher strength. Moreover to investigate the general nature of the influence of strain, strain rate and temperature on the compressive deformation characteristics of thixocast A356 and conventionally cast A356 aluminium alloy, a comprehensive model describing the relationship of the flow stress, strain rate and temperature of the alloys at elevated temperatures is proposed by hyperbolic-sine Arrhenius-type equation and Johnson-Cook model. The validity of descriptive results based on the proposed constitutive equation is also investigated and a comparison between two constitutive models is also made. In order to numerically model the friction welding process of a thixocast A356 aluminium alloy and conventionally cast alloy of same material using a finite element method (FEM), temperature dependent physical properties, mechanical properties as well as viscoplastic constitutive equations were used in the model. A two- dimensional axisymmetric finite element model has been developed. The modelling is based on a coupled thermomechanical approach. First, a nonlinear, transient two-dimensional heat transfer model is developed to determine the temperature fields. Later, the temperature fields are used as input for a nonlinear, two-dimensional structural model in order to predict the distortions and von Mises stress. The finite element models are parametrically built using APDL (ANSYS Parametric Design Language) provided by ANSYS. The validation of the model is carried out by comparing with the experiment. Once validated, the thermomechanical model was used to perform parametric studies in order to investigate effects of various process parameters on temperature and stress distribution in the workpieces. This helps in deciding the range of parameters for friction welding experiments in order to get good weld. Both thixocast and conventionally cast samples exhibited similar temperature distribution during the friction welding process, because of identical thermophysical properties. The magnitude of von Mises stress distribution during friction welding of thixocast A356 sample is found to be lower than that of the conventionally cast sample. It is because of their different constitutive behaviour at elevated temperature. Moreover, the developed FEM model can be successfully used to predict the residual stress at various locations for different set of parameters and geometry for friction welding of thixocast and conventionally cast A356 alloy. This helps in reducing time consuming and expensive experiments on residual stress measurement. The chosen experiments based on Taguchi L27 orthogonal array were conducted on the friction welding machine which works on the principles of continuous drive-mechanism. The experimental specimens were machined from thixocast A356 aluminium alloy connecting rods as well as conventionally cast A356 aluminium alloy ingot in the form of cylindrical bars of dimensions 85mm length and 20mm diameter. The parameters used for welding were friction pressure, rpm, forge pressure, burn-off, and upset pressure. The effects of welding parameters on performance characteristics (i.e. tensile strength and weld efficiency) were evaluated. Taguchi method was applied to investigate the influence of each parameter on strength of joints and evaluate the combination of parameters that leads to the highest weld strength. Accordingly optimum process parameters was identified which helps in achieving the tensile strength of more than parent material. The optimized process parameters for friction welding of thixocast A356 aluminium alloy are rpm = 500, friction pressure = 60, upset time = 5, upset pressure = 100 and burn off = 5. The empirical relationships were also developed to predict the tensile strength. The developed relationship can be effectively used to predict the tensile strength of welded joint with a correlation coefficient of 0.86, which indicates the strong positive relationship between predicted and experimental data. Friction welding of thixocast A356 aluminium alloy helps to achieve very fine eutectic silicon particles of the order of 0.4 at the interface due to severe plastic deformation taking place during welding. Obtaining such fine eutectic silicon particles is difficult to be achieved within few seconds of processing by any other method. The hardness variation of friction welded thixocast alloy shows higher value as compared to that of a conventionally cast sample in the heat affected zone, which indicates better weld strength of the former. This was also confirmed by the tensile strength studied and fatigue test. This indicates that weldability of cast alloys will get improved if the microstructure is modified to globular type.