Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes Part Ⅱ: Apparent Viscosity

This study investigated the rheological properties of semi-solid metal. An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheological processes such as slurry preparing,delivering and model filling. The rheological properties of SSM (semi-solid metal) slurry was described by an analytical model in terms of microstructural parameters, which consist of effective solid fraction, particle size and shape, and flow parameters such as mean velocity, fluctuant velocity and relative velocity between liquid and solid phase. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer. And the maximum relative error between the theoretical value and measured one is less than 10%.The results of experiment and theoretical calculation also indicate that the microstructural parameters and flow parameters are two major factors that affect the apparent viscosity of semi-solid alloys, and fluctuant velocity and relative velocity between liquid and solid phase are the key factors to distinguish between steady and transient rheological properties.     

Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes Part Ⅱ： Apparent Viscosity

This study investigated the rheological properties of semi-solid metal. An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheological processes such as slurry preparing, delivering and model filling. The rheological properties of SSM （semi-solid metal） slurry was described by an analytical model in terms of microstructural parameters, which consist of effective solid fraction, particle size and shape, and flow parameters such as mean velocity, fluctuant velocity and relative velocity between liquid and solid phase. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer. And the maximum relative error between the theoretical value and measured one is less than 20%. The results of experiment and theoretical calculation also indicate that the microstructural parameters and flow parameters are two major factors that affect the apparent viscosity of semi-solid alloys, and fluctuant velocity and relative velocity between liquid and solid phase are the key factors to distinguish between steady and transient rheological properties.     

Time-Dependent Rheological Properties of Semi-Solid Metal Alloys

Semi-solid metal alloys, as used in thixoforming, have a special microstructure of globular grains suspended in a liquid metal matrix. The complex rheological properties are strongly influenced by the local solid fraction, particle shape, particle size and state of agglomeration. There is a high demand for models and software tools allowing the simulation of semi-solid casting processes. The material under investigation is a tin-lead alloy (Sn-15%Pb) which exhibits a similar microstructure to aluminium alloys. The experiments were performed with a concentric cylinder rheometer of the Searle type. Initially, the liquid alloy is cooled down to the semi-solid range under constant shearing and then kept under isothermal conditions for further experimentation. Based on the experimental data, a single-phase model has been derived where the semi-solid alloy is regarded as a homogeneous material with thixotropic properties and the microstructure is characterised by a structural parameter. The model consists of two parts: the equation of state, including a finite yield stress, and a rate equation for the structural parameter. The model equations are employed in numerical software and used for the simulation of characteristic filling cases and the comparison with the conventional filling.     

Structure and properties of AIMgSi1 alloy tailored for semi-solid forming

Thixoforming requires semi-solid slurries, which contain a high volume fraction of non-dendritic solid phase with special grain morphology. Volume fraction, geometry and connectivity of the solid -phase has to be kept within narrow limits. The paper illustrates the development of a new aluminium based wrought alloy AIMgSi1 alloy, adapted with barium, characterised by a microstructure which exhibits a low sensitivity towards fluctuations in process parameters resulting in improved mechanical properties of the alloy. The rheological behaviour of the new alloy by means of backward extrusion experiments is described, and an overview of the static and dynamic strength properties is given that can be achieved when the alloy is processed by means of the New Rheocasting process.     

The influences of the microstructure morphology of A356 alloy on its rheological behavior in the semi-solid state

In this paper, the rheological behavior of semi-solid A356 alloy with different solid morphology was studied with an improved static shear test method. The results indicated that the rheological behavior of the alloy was significantly influenced by the structural morphology of the alloy. The alloy had quite different rheological properties even though the same fraction of solid existed in the semi-solid state. The rheological behavior of the alloy fitted a five-element model (H₁–[N₁|H₂]–[N₂|S]) for the as-cast microstructure with developed primary (α–Al dendrites, whereas it fitted a six-element model ([H₁|S₁]–[N₁|H₂]–[N₂|S]) for degenerated dendritic or spheroidal primary α–Al, which had been obtained by electromagnetic stirring and spray deposition, respectively. Computation results showed that the deforming capability and shear rate of the semi-solid alloy increased remarkably with the change of primary α–Al from developed dendrites to degenerated dendrites, and then to spheroidal structures. On the other hand, the temperature dependence of the rheological properties of the semi-solid alloy with spheroidal or degenerated dendritic primary α–Al was much less than that with developed primary α–Al dendrites.     

The influences of the microstructure morphology of A356 alloy on its rheological behavior in the semi-solid state

In this paper, the rheological behavior of semi-solid A356 alloy with different solid morphology was studied with an improved static shear test method. The results indicated that the rheological behavior of the alloy was significantly influenced by the structural morphology of the alloy. The alloy had quite different rheological properties even though the same fraction of solid existed in the semi-solid state. The rheological behavior of the alloy fitted a five-element model (H₁–[N₁|H₂]–[N₂|S]) for the as-cast microstructure with developed primary (α-Al dendrites, whereas it fitted a six-element model ([H₁|S₁]–[N₁|H₂]–[N₂|S]) for degenerated dendritic or spheroidal primary α-Al, which had been obtained by electromagnetic stirring and spray deposition, respectively. Computation results showed that the deforming capability and shear rate of the semi-solid alloy increased remarkably with the change of primary α-Al from developed dendrites to degenerated dendrites, and then to spheroidal structures. On the other hand, the temperature dependence of the rheological properties of the semi-solid alloy with spheroidal or degenerated dendritic primary α-Al was much less than that with developed primary α-Al dendrites.     

Analysis of on-deposition solidification during the Osprey spray-deposition process

The on-deposition process during spray deposition was studied. The theory of forced convection in the case of laminar flow is applied to evaluate the heat-transfer coefficient between the atomizing gas and the semi-solid/semi-liquid thin film on the surface of the deposit. The cooling rate of the on-deposition process was estimated, based on the heat-flow analysis. The results agree well with experimental observations of the microstructure of spray-deposited aluminium alloy, as well as other similar rapid solidification processes, such as gas atomization of metal powders.     

Evolution of microstructure and mechanical properties for 9Cr18 stainless steel during thixoforming

Hot compression tests were carried out in the semi-solid state of 9Cr18 stainless steel on Gleeble-1500 thermal simulation testing machine to investigate the effects of thixoforming parameters on its microstructure and mechanical properties. In this paper, microstructure was observed by scanning electron microscopy (SEM) and analyzed using energy dispersive spectrometer (EDS), and true stress–stain curves of the specimens with different initial microstructures after thixoforming were obtained to study the deformation mechanism. The results showed that thixoforming parameters such as reheating temperature and the strain rate had a significant influence on microstructure and mechanical properties evolution of 9Cr18 semi-solid billet. With increasing reheating temperature or decreasing strain rate, average size of carbides decreased from 2 μm to 0.5 μm, and the phenomenon of liquid extrusion during thixoforming became more obvious. During thixoforming, carbon atoms diffused to molten metal from austenite in the centre of specimens. When thixoforming temperature reached 1300 °C, martensitic transformation occurred after rapid cooling. Flow stress of semi-solid billet was lower than traditional ingot casting and hot rolled state steel, when reheated to the semi-solid range, due to their different original microstructure.     

Rheology of semi-solid fresh cement pastes and mortars in orifice extrusion

Short fiber-reinforced semi-solid fresh cement pastes and mortars, tailored for extrusion, have much lower water-to-binder ratio and higher viscosity than normal cement pastes or mortars. The rheology of these pastes or mortars cannot be characterized by traditional rheology test methods suitable for normal fresh cement pastes or mortars with much greater water-to-binder ratio and lower viscosity. In this paper, orifice extrusion is employed to calibrate rheology of the semi-solid fresh cement mortar. An analytical model is developed for orifice extrusion of semi-solid pastes and mortars obeying a rigid-viscoplastic constitutive relationship, von-Mises yield criterion and the associated flow rule. Orifice extrusion results are interpreted using the analytical model and the established experiment data interpretation method, and the associated rheological parameters are derived for the semi-solid fresh cement mortar. This study provides a simple analytical model, together with experiment and data interpretation methods, for characterizing the complex intrinsic rheological behavior of semi-solid fresh cement pastes or mortars.     

基于SIMA法的铝合金复杂构件触变锻造成形技术

采用半固态触变锻造技术成形铝合金构件,容易实现轻量化、低成本、短流程制造,因此在汽车、航空航天等领域应用广泛,但是对于高强铝合金复杂形状构件触变锻造存在半固态坯料制坯工序复杂、制件固液偏析严重和力学性能较弱等问题。基于SIMA制坯方法,提出了分级热处理、快速感应重熔和梯度等温处理等重熔工艺以及触变-塑变复合成形等新成形技术,优化了变形铝合金二次重熔半固态组织调控和触变锻造技术,获得了良好的半固态球晶组织并成形出合格的制件,最后提出了铝合金触变锻造成形中仍需解决的问题和发展方向。     

铜合金半固态流变成形及制备技术研究进展

金属半固态成形已有近50年的发展历史,因其加工优势和市场价值备受关注,并形成了很多加工方法,其中半固态流变成形工艺效率高、成本低,具有较好的发展前景。由于铜合金熔点普遍较高,目前针对其进行的半固态工艺研究较少。简要综述了针对铜合金的半固态流变成形及制备技术的研究新进展,包括各新工艺的加工过程、最优工艺参数选择及工艺过程的机理等,分析了各工艺目前尚存在的问题和原因。阐述了半固态流变制备工艺模型及机理的新进展,并对其发展趋势进行了分析和展望,认为半固态流变成形技术未来的研究方向有半固态浆料制备新方法、新型半固态合金和半固态组织演变机制及其控制方法等。     

Melt-state shear flow and elasticity of a thermoplastic fluorosulphonated—PTFE copolymer

Some rheological properties of a perfluorosulphonated PTFE copolymer have been measured in order to characterise the melt-state viscous and elastic behaviour of a thermoplastic precursor of Nafion®, an ion-selective membrane used in the electrochemical industry. Steady-state shear viscosity measurements show conventional pseudoplastic flow behaviour over a wide temperature range, under high shear conditions. These data have been modelled to a high level of accuracy using polynomial simulations to obtain Carreau model coefficients and flow activation energies. Using an orifice die in a capillary rheometer, calculated extensional viscosity data are shown to decrease with stress and are more temperature-sensitive than shear viscosity. Although die swell increases with shear rate in a conventional manner, unusual and complex die swell data (at a fixed shear stress) have been obtained in response to an increase in process temperature. This behaviour is attributed to the breakdown of a small-scale network of domains and ordered crystalline material in the amorphous matrix of the precursor, as exemplified by a very broad melting endotherm. Different modes of deformation have been proposed to explain the rheological data observed across the process temperature range. The observed changes to the flow mechanism and elastic character of the melt carry practical implications for the extrusion processes and developed microstructure of film products manufactured from this precursor copolymer.     

金属半固态浆料非枝晶微观组织形成机制与表征

半固态技术自20世纪70年代由麻省理工学院开发以来,因其独特的优势而在全球范围内得到了广泛的研究。非枝晶组织的形成机理是半固态成形技术的基石,它决定了在特定条件下应采用哪些工艺来获得优质的半固态组织,对开发新工艺具有重要的指导作用。经过近半个世纪的发展,半固态浆料制备过程中非枝晶结构的形成机理已经发展出许多不同的观点。由此衍生了丰富的半固态浆料制备技术,大大促进了半固态领域的发展。为了进一步理解凝固过程中球晶的形成机制,对各种非枝晶组织的形成机制进行了梳理,基于制备原理将半固态制浆技术分为搅拌制备技术类和低过热度制备技术类,并对常见的几种技术的原理和应用进行了总结。半固态产品的性能在很大程度上取决于其微观组织结构,但半固态形态复杂,传统的二维表征手段对材料内部空间结构的理解可能存在误差。对半固态浆料的二维微观组织和三维显微组织表征技术进行总结分析,为理解金属半固态浆料的微观结构演变和微观组织与性能之间的关系提供了理论基础。     

Three-dimensional injection molding simulation of AZ91D semi-solid magnesium alloy

Magnesium alloys are increasingly used in automotive, aeronautic and electronic applications to produce high performance, light weight parts. In the thixomolding process a semi-solid slurry is injected into a mold at controlled temperature such that the melt has specific flow behavior. This allows the fabrication of near net shape components with controlled microstructure and good mechanical properties. The numerical modeling of such applications present unusual challenges for both the physical modelling and the solution algorithm. This paper presents a 3D numerical solution algorithm for the simulation of the injection molding of semi-solid AZ91 magnesium alloys. The methodology deals with the shear thinning, temperature dependent viscosity behavior and is able to accurately solve the high velocity flows encountered during semi-solid magnesium molding. A segregated algorithm is used to solve the Navier–Stokes, energy and front tracking equations. The position of the flow front in the mold cavity is computed using a level set approach. Equations are integrated in time using an implicit Euler scheme and solved by stabilized finite element methods. The approach is applied to the injection of a tensile bar and the results compared with experimental data. The methodology presents the robustness and cost effectiveness needed to tackle complex industrial applications.     

Microstructure,mechanical and tribological properties of high velocity oxy fuel thermal spray coating: A review

Iron alloy based amorphous coating materials have enormous potential in wide range of applications such as petrochemical, aerospace, ocean, and electronic communications due to their better mechanical properties, chemical properties, magnetic properties and tribological properties. The industrial applications of coating are increasing rapidly due to many advancements in the material development and coating deposition techniques. The present paper reviewed the recent progresses in deposition technologies, development of new high order alloys and composite based coating materials. In this regard, change in microstructure, elemental composition, mechanical and tribological properties on performance of iron alloy based coating properties were presented. It can be concluded that the tribological properties of coating is dependent on pre-coating and post-coating factors. Pre-coating factors include coating deposition techniques, coating layer thickness and coating parameters such as spray distance, oxygen flow rate etc. Post-coating factors include microstructure, hardness, fracture toughness and adhesion strength. Therefore, multi-criteria decision making techniques can be the best approach to find the optimum formulation of coating materials to achieve desired set of objectives under the conflicting criteria.     

Simulation of liquid infiltration and semi-solid extrusion for composite tubes by quasi-coupling thermal-mechanical finite element method

As a new metal forming technology, the liquid infiltration and semi-solid extrusion process can produce various composite parts, such as tubes, bars, and shaped products, in a single process. In this paper, the liquid infiltration and semi-solid extrusion process for forming a composite tube is simulated by means of thermal rigid-plastic quasi-coupling FEM method. The key technologies such as the handling of liquid phase zone, the transition between liquid and solid phase zones, the grid re-meshing method and the establishment of the boundary condition have also been studied. Based on the FEM simulation software developed by the authors and the grid re-meshing technology, the distribution of stress field, strain field and deformation force in the liquid infiltration and semi-solid extrusion process for forming composite tubes are obtained. The deformation force simulation results accord with the experimental data, indicating the reliability of the system. Therefore, the present research is theoretically valuable in the product quality control and the process parameter choice.     

Influence of rheological behaviour on extrusion parameters during non-continuous extrusion of multi-base propellants

During the production of multi-base propellants, in order to identify both the optimal and safety conditions to be used at processing these rubber-like compounds, its rheological behaviour is considered as a key factor. Thus, an analysis of their rheological moduli at the end of the mixing stage and prior to the non-continuous extrusion (ram extrusion) is required so as to predict the optimum parameters to be assumed for both the mixing and extrusion processes. The rheological analysis of this multi-base propellant is carried out by means of a DHR-1 rotational rheometer of TA Instruments, and the complex modulus is proposed as a generalized indicator of the rheological response of these special materials. The complex interrelations among the main parameters of the extrusion process (steady extrusion pressure and extrusion ram velocity) together with the rheological properties of these multi-base propellants, will serve to deduce useful technical guidelines for improving the manufacturing system productivity, safety processing conditions and final product quality.     

Biaxial deformation behavior and mechanical properties of a polypropylene/clay nanocomposite

Polymer nanocomposites offer the potential of enhanced properties such as increased modulus and barrier properties to the end user. Much work has been carried out on the effects of extrusion conditions on melt processed nanocomposites but very little research has been conducted on the use of polymer nanocomposites in semi-solid forming processes such as thermoforming and injection blow molding. These processes are used to make much of today’s packaging, and any improvements in performance such as possible lightweighting due to increased modulus would bring significant benefits both economically and environmentally. The work described here looks at the biaxial deformation of polypropylene–clay nanocomposites under industrial forming conditions in order to determine if the presence of clay affects processability, structure and mechanical properties of the stretched material. Melt compounded polypropylene/clay composites in sheet form were biaxially stretched at a variety of processing conditions to examine the effect of high temperature, high strain and high strain rate processing on sheet structure and properties.     

Mechanical properties of bio compatible functional prototypes for joining applications in clinical dentistry

In the presented research, work investigations have been made for mechanical properties of the functional prototypes prepared from biocompatible filament of fused deposition modelling (FDM), comprising of hydroxyapatite (HAp), polypropylene (PP) and polyvinyl chloride (PVC). The functional prototypes will be used in clinical dentistry (mainly for joining application for job-type production activities). The filament has been prepared in house using twin screw extrusion process. For evaluation purpose, standard tensile specimens as per ASTM D-638 have been prepared on FDM. This study highlights the effect of three parameters of FDM (namely: infill percentage, layer thickness and speed of extrusion head) on the mechanical properties (namely: load at peak and load at break). The results of the study suggest that infill density has majorly contributed, 92% on load at peak and 89% for load at break, and deposition speed has very less contribution i.e., 1% towards the mechanical strength of the specimen. Further, the results are supported with thermal analysis using differential scanning calorimeter (DSC), which ensures that the specimen prepared are thermally stable and can be put in for joining applications for job-type production activities in clinical dentistry.     

Pressure-driven mold filling model of aluminum alloy melt/semi-solid slurry based on rheological behavior

The pressure-driven mold filling ability of aluminum alloy melt/semi-solid slurry is of great significance in pressure casting processes, and the rheological behavior of the alloy has a crucial effect on the mold filling ability according to fluid dynamics. In this work, a pressure-driven mold filling model is first proposed based on the rheological behavior of the alloys. A356 alloy is employed as an example to clarify the rheological behavior of aluminum alloys, which obeys the power law model and is affected by temperature. The rheological behavior of the alloy in semi-solid state is modelled with the coupling of shear rate and temperature. The stop of mold filling attributes to the pressure loss which is caused by the viscosity during the flow of the melt/semi-solid slurry. Pressure loss caused by viscous flow and heat transfer between the alloy and the mold are calculated and coupled during the mold filling of the melt/semi-solid slurry. A pressure-driven mold filling model of aluminum alloy melt/semi-solid slurry is established based on steady-state rheological behavior. The model successfully predicts the filling length of melt/semi-solid slurry in pressure casting processes. Compared with the experimental results,the model can provide a quantitative approach to characterize the pressure-driven mold filling ability of aluminum alloy melt. The model is capable of describing the stop filling behavior of other aluminum alloys in pressure casting processes with corresponding rheological parameters and heat transfer coefficient.