Semi-solid Metal Research Articles

Liquid Segregation Phenomenological Behaviors of Ti14 Alloy during Semisolid Deformation

The liquid segregation phenomenon and its effect on deformation mechanism of Ti14 alloy in semisolid metal processing were investigated by thermal simulation test. Microstructure of depth profile was determined by cross-section quantitative metallography, and liquid segregation phenomenon was described by Darcy's law. The results show that segregation phenomenon was affected by solid fraction, strain rate, and deformation rate. More liquid segregated from center to edge portion with high strain rate and/or deformation ratio as well as low solid fraction, which caused different distribution of dominating deformation mechanism. The relationship between liquid segregation and main deformation mechanism was also discussed by phenomenological model.

Mechanical properties and corrosion resistance of steel X210CrW12 after semi-solid processing and heat treatment

Semi-Solid Metal Processing (SSM) is one of the modern methods of steel forming. SSM of hot rolled X210CrW12 tool steel was conducted at 1250°C, in order to obtain about 30% of liquid phase. The microstructure consisted of globular austenitic grains surrounded by eutectic mixture (ferrite, austenite and M7C3 carbides). The average hardness of as-SSM samples was 470HV5, while the coefficient of abrasion resistance reached 7.9*10−6mm3/Nm. The compression strength of thixo-casts of X210CrW12 steel reached 4850MPa at the plastic strain of 34%, although the yield stress was only 810MPa. The X210CrW12 alloy just after thixo-casting revealed disadvantageously small wear resistance due to a large amount of metastable austenitic structure. A heat treatment consisting of 2h tempering at 555°C and additional low temperature tempering at 250°C led to the hardness increase up to 650HV5 as well as to the improvement of abrasion resistance to 1.8*10−6mm3/Nm. The compression strength and plastic strain decreased to 3140MPa and 8.9%, respectively, while the yield stress increased up to 2050MPa. Such very good mechanical properties were due to a complex microstructure consisting of globular grains containing the tempered martensite, fine perlite and M7C3 carbides surrounded by eutectic mixture. Tempering at 630°C for 2h led to the transformation of primary austenitic grains into perlite and carbides. It caused a decrease in hardness down to 495HV5 and the compression strength to 2300MPa, and the deterioration of abrasion resistance to 7.4*10−6mm3/Nm. The corrosion measurements revealed that the X210CrW12 steel directly after SSM was susceptible to the pitting corrosion in 0.1M sodium chloride solution. The corrosion resistance of thixo-casts after tempering at 555°C/2h slightly decreased compared with the as-thixoformed samples.

Evolution of primary α-Al particles during isothermal transformation of rheocast semi solid metal billets of A356 Al–Si alloy

Semi solid metal (SSM) processing is a relatively new technology for metal forming, different from the conventional metal shaping technologies that use either solid or liquid metals as the starting material. Semi solid metal processing deals with semi solid slurries, in which non-dendritic solid particles are dispersed in a liquid matrix. The solid volume fraction could vary between 0·1–0·5 with apparent viscosity close to that of liquid state. The slurry flows easily under pressure and makes complicated shapes with high degree of die filling and integrity. There are different ways to prepare SSM slurries including the application of direct or indirect mechanical stirring, chemical treatments of the melt, and manipulation of pouring temperature.One of the important parameters controlling the mechanical and flow characteristics of SSM billets is the morphology of the primary solid phase, e.g. α-Al for Al–Si alloys. The morphological evolution of primary α-Al particles of SSM processed A356 Al–Si billets is studied with variant soaking times, 0–480 s, at 595°C, i.e. 0·33 fraction solid before the applied compression force. The effect of applied compression force of 33 kgf on the evolution of dendritic morphology was also investigated at the same temperature and within the same holding time range. The microstructure was characterised quantitatively and showed that spheroidisation and coarsening are active during the course of treatment with the application of compressive forces bringing about further refinement.Le traitement de métal à l’état semi-solide (SSM) est une technologie relativement nouvelle de formage des métaux, différente des technologies conventionnelles de façonnage des métaux qui utilisent des métaux soit à l’état solide ou liquide comme matériel de départ. Le traitement SSM traite des coulis à l’état semi-solide, dans lesquels des particules solides non-dendritiques sont dispersées dans une matrice liquide. La fraction volumique du solide peut varier entre 0·1 et 0·5, avec une viscosité apparente proche de celle de l’état liquide. Le coulis s’écoule facilement sous pression en des formes compliquées avec un haut degré de remplissage du moule et d’intégrité. Il existe différentes façons de préparer les coulis de SSM, incluant l’application d’agitation mécanique directe ou indirecte, les traitements chimiques du bain et la manipulation de la température de coulage.L’un des paramètres importants contrôlant les caractéristiques mécaniques et d’écoulement des billettes de SSM est la morphologie de la phase solide primaire, par exemple Al-α dans les alliages d’Al–Si. On a étudié l’évolution morphologique des particules primaires d’Al-α de billettes traitées en SSM d’Al–Si A356, avec des temps d’incubation variant de 0 à 480 s, à 595°C – c’est-à-dire fraction solide de 0·33 avant la force appliquée en compression. On a également examiné l’effet de la force de compression appliquée de 33 kgf sur l’évolution de la morphologie dendritique, à la même température et dans la même gamme de temps de rétention. On a caractérisé quantitativement la microstructure et l’on montre que la sphéroïdisation et le grossissement sont actifs tout au long du traitement, l’application de forces compressives amenant davantage d’affinement.

Influence of casting parameters on microstructure and mechanical properties of AC2A thixocasting

Thixocasting is a semisolid metal (SSM) casting technology that combines the benefits of spheroidal non-dendritic microstructure of SSM and near net shape casting of conventional die casting, leading to extremely high integrity and exceptional mechanical properties of cast components. In this paper, the effects of casting parameters and T6 heat treatment on the microstructure and mechanical properties of fork-like thixocast components of AC2A aluminium alloy were investigated. The microstructures of specimens of both as cast and T6 cast components were observed. Uniform distribution of fine solid particles of globular primary phase was found for all casting conditions. However, low solid fraction of the primary phase was noticed at the edge of each casting, where enriched solutes were also detected. Significant increase in porosity was found in the biscuit and runner part of the castings. The effects of casting parameters on phase separation, porosity distribution and mechanical properties of the castings are discussed.

Optimization of semi-solid metal processing of A356 aluminum alloy

Semi-solid metal (SSM) processing has been recognized as an advanced process to produce high-quality and low-cost engineering components. The cooling slope method is a simple route that can develop non-dendritic slurry for various purposes with reduced equipment and processing costs. In this study, the cooling slope method was employed to produce the A356 feedstock in SSM processing. The dendritic primary phase in the conventionally cast A356 alloy has transformed into a non-dendritic one through the use of ingots cast over a cooling plate with a different pouring temperature. After pouring, the melt that becomes semi-solid at the end of the plate is consequently poured into cylindrical steel molds with different mold temperatures. Also, the process has been conducted in different cooling slopes and different cooling lengths. Then, a back-propagation neural network has been designed to correlate the process parameters to grain size. Finally, genetic algorithm (GA) has been used to optimize the process parameters. Results indicate that the pouring temperature, mold temperature, cooling slope, and cooling length have significant effects on size and morphology of α-Al phase. The GA can optimize the process as well.

The Factors to Influence the Friction Stir Welding of Aluminum Casting Semi-Solid Metal Dissimilar Joint between SSM 356 and SSM 6061

The objective of this research is to investigate the effect of parameters on the microstructure and mechanical properties of friction stir welded butt joints of dissimilar aluminum alloy sheets between Semi-Solid Metal (SSM) 356 and 6061. The base metal of SSM 356 and 6061 was located on the advancing side and on the retreating side, respectively. The cylindrical pin was used as the welding tool geometry. Friction stir welded using different tool rotation speed (710, 1,000 and 1,400 rpm) and welding speed (80,112 and 160 mm/min). From investigated, the maximum average tensile strength of 175.40 MPa was achieved for the joint produced at tool rotation speed 1,400 rpm, welding speed 112 mm/min. Metallurgy and welding structures occurred of aluminum composite material will consist of two types of definition than the base metal.

Effect of Cooling Rate on Microstructure of Semi-Solid AZ61 Slurry Produced via Ultrasonic Vibration Process

The semi-solid metal (SSM) processing, including thixoforming and rheocasting, offers the opportunity to manufacture net-shaped components with complicated shape and good mechanical properties. More and more researches have been focused on rheocasting in recent years because of its low cost and high productivity [1,. Rheocasting involves stirring the solidifying alloy to prepare non-dendritic semisolid slurry, then shaping the slurry directly. Ultrasonic vibration (USV) has the potential to be a simple and effective process to produce semisolid metal slurry. Previous investigations have revealed that when USV is applied on the solidifying melt, microstructure changes occur involving grain refinement, increased homogeneity, reduced micro-segregation as well as degassing [3-.

Effect of Welding Speed and Tool Pin Profile on Weld Quality of Friction Stir Welded Semi-Solid Aluminium Alloy 356

The effect of welding speeds and tool pin profiles on microstructure and mechanical properties of semi-solid metal 356 joints produced by friction stir welding was investigated. In this work, the joints were made by using a fixed rotating speed of 1750 rpm with varying welding speed of 80, 120 and 160 mm/min. In addition, two different types of tool pins, cylindrical and square pin, were applied. The Scanning Electron Microscope (SEM) reveals fine microstructure and uniform dispersion of Si (Silicon) particles obtained from cylindrical pin than that of square pin. Transverse and longitudinal tensile strengths obtained from cylindrical pin are greater than square pin. Furthermore, the joint made from 1750 rpm, 160 mm/min with cylindrical pin shows highest strength.

Temperature distribution and its influence on microstructure of alloy AZ31 during semisolid rheo-rolling process

In the backward slip zone, the buckling direction of the isothermal lines near the surface of the alloy was toward the exit of the roll gap, the buckling direction of the isothermal lines at the centre was toward the entrance of the roll gap and semisolid metal deformation happened. In the forward slip zone, the buckling direction of the isothermal lines was toward the exit of the gap, the bending degree increased from the neutral flow plane to the exit of the roll gap and solid metal deformation took place. The microstructure of the strip became more refined with the decrease in casting temperature. When the casting temperature was between 670 and 690°C and the rolling speed was 0·167 m s−1, the alloy AZ31 strip with cross-section size of 4×160 mm was prepared. The mechanical properties of the product were higher than that of conventional roll casting.

An Enhanced Herschel–Bulkley Model for Thixotropic Flow Behavior of Semisolid Steel Alloys

An enhanced model based on Herschel–Bulkley model has been proposed. Modifications are based on the attempts to overcome the limitations of the previously used models. The proposed model predicts thixotropic time-dependent flow behavior at medium shear rates, while reaching toward Newtonian viscosity at higher shear rates, which is a typical behavior of semisolid metal slurries. Rheology tests on M2 high-speed steel were then conducted using a self-developed high-temperature Searle-type rheometer for three different solid fractions. All of the tests were performed in an electric furnace with an argon-controlled atmosphere. Two series of experiments were performed to evaluate the rheological behavior of the material: steady-state flow stress experiments to determine the equilibrium flow curves, and step-change of shear-rate experiments to determine the time-dependent characteristics of the material. The model parameters were then derived using the experimental results and calibrated by curve fitting of experimental data. The model was then linked to a commercial CFD software, and simulation of the process was conducted to evaluate the model. The results show that the model fits well with the experimental data and is capable of simulating a wide range of shear rates compared with typical Herschel–Bulkley model.

Investigation of Fusion Weldments of Semi-Solid Aluminium A356 Alloy: Pool Geometry and Microstructure

A fusion welding technique to join a semi-solid processed A356 cast plate is explored using Gas Tungsten Arc Welding (GTAW). Semi-solid metal (SSM) billets of non-dendritic microstructure produced by rheocasting in a mould placed inside a linear electromagnetic stirrer were used for this study. GTAW experiments were conducted to simulate different thermal gradients near the fusion zone. The geometries of the weld pool as well as the temperature gradient in the fusion boundary were measured to understand the microstructure evolution. Simulation of the welding process was performed to aid in the analysis. Quantitative metallography provided the shape factor as a measure of globularity of the primary a-Al phase. Based on the studies, a model has been proposed to explain the observation of globular microstructure in the fusion zone of the welds. Conclusions show a positive correlation of thermal gradient with globular microstructure formation in this class of alloys.

Process Optimization and Microstructure Control for Twin Screw Rheo-Extrusion of an AZ91D Magnesium Alloy

Twin screw rheo extrusion (TSRE) is a continuous semisolid forming process. The process takes advantage of the thixotropic properties of semisolid metal slurry and the high shear, controllable forward driving and self-sweeping capabilities of a counter rotating twin screw mechanism, offering a one-step melt-to-product forming technology with significant savings on energy consumption and manufacturing cost. The present work was carried out to investigate the feasibility of the process for processing light alloys using an AZ91D magnesium alloy and to obtain optimized operation conditions for microstructure control. A range of processing parameters was tested for slurry making, feeding and extrusion and their effect on microstructure development was examined. Experimental results showed that slurry with non-dendritic primary phase particles of various volume fractions, typical of semisolid microstructure, was obtained prior to extrusion. The particles were further refined during extrusion due to dynamic shearing applied by the twin screw mechanism under controlled thermal schemes and the final microstructure obtained after extrusion was dominated by uniformly distributed equiaxed, non-dendritic spherical grains and the average grain size was in the range of 35-80 mm, depending on processing parameters. This paper presents the performance of the TSRE process, focussing on process optimization and microstructure control. Mechanisms of microstructural refinement and particle morphology evolution are discussed.

Yield Stress in Semi-Solid Alloys – The Dependency on Time and Deformation History

An important aspect of the rheological characterization of semi-solid metals is the determination of the materials ability to exhibit yield stress. The yield stress is the stress level at which the material cease from purely elastic to plastic deformation. In semi-solid metal suspensions the yield stress is not a constant value, but depends on the transient state of the material. The investigation of yield stress in semi-solid metal alloys has been initiated in previous works, however, a profound understanding of the influence of the material history on the yield stress is so far lacking. In semi-solid metal suspensions, the physical cause of the yield stress is generally described by the formation of a particle "skeleton" structure which can hold a certain stress level without being subject to irreversible deformation. The yield stress in alloy suspensions can be described as a function of structural properties i.e. the solid fraction and the particle size, form and spatial distribution. It has been observed in previous works that the yield stress of semi-solid metals depends on the duration of resting time. It is generally assumed that this is due that as the material is resting, the particle network build up over time leading to an increase in the yield stress. In addition, we assume that the yield stress is also a function of the shear history as this affects the particle spatial distribution. The goal of the proposed work was to investigate the yield stress in semi-solid alloys including its dependency on the materials shear history and resting time. This was investigated both from a structural and a rheological point of view. The yield stress of the semi-solid alloys was measured with shear stress ramp and oscillation tests. Structural imaging measurements of quenched samples were performed to correlate structural parameters with the value of the yield stress. However, in semi-solid alloys, the experiments performed at a long time scale are influenced by aging of the material (Ostwald ripening). Therefore, rheological experiments were also performed with synthetic suspensions to investigate the long term yield stress dependencies without errors caused by the aging effects.

Application of X-Ray Microtomography to Quantify the Liquid Fraction of M2 Steel for Semi-Solid Forming Process

Thixoforging, one variant of semi-solid metal processing in which the metallic alloys are processed at low liquid fraction (0.1< Fl < 0.3), is used to produce complex parts with high mechanical properties. Steel thixoforging faces more challenges as compared to that of low melting point materials due to high processing temperature and lack of understanding of the thermomechanical behavior of materials in the given conditions. It is crucial to study the microstructure at the semi-solid state to improve the understanding of the thixoforging process since the material behavior strongly depends on main parameters: the liquid fraction, its distribution as well as the coherence of the solid skeleton. The microstructure has a great influence on the viscosity of the material, on the flows and finally on the final shape and mechanical properties of the thixoforged parts. Here, the characterization of the volume percentage and distribution of liquid fraction at the semi-solid state with high energy 3D X-ray microtomography was investigated on M2 steel grade as a ‘model’ alloy. The obtained results have been compared to 2D observations using EDS technique in SEM on heated and quenched specimens. They showed a good correlation making both approaches very efficient for the study of the liquid zones at the semi-solid state.

Thixoforging of Ultrasound Treated 6060 Aluminum Alloy

In the last years researches on thixotropic materials have been developed in order to introduce this new technology in manufacturing processes. For instance, when considering high pressure die-casting, several applications are present in literature mainly related to low melting point alloys (Al and Mg) because of the limited die life experienced when casting higher melting materials. In this case, semi-solid metal forming allows to work at lower temperature with subsequent increase in die life and reduction in production costs, combined with lower porosity level in the casting. On the other hand, in the case of conventional forging, semi-solid processing needs higher performance materials and/or coatings for the mould because of the working temperatures; however, the advantages of obtaining near net shape part in a single step, with reduced machining and finishing costs, make the semi-solid technology competitive. The present paper deals with the thixoforging of aluminum 6061 alloy, whose semi-solid feedstock material was obtained by ultrasound treatment. The application of ultrasonic waves to liquid or solidifying alloys has been already demonstrated to be an effective technique for the obtainment of globular microstructure. Along with a refining effect, ultrasound can also produce a series of beneficial effects, such as hydrogen degassing or oxide and non-metallic inclusion removal, which all improve mechanical properties of the component. The aim of this research was to investigate the influence of process parameters on final forged part quality. The solid fraction percentage as a function of temperature was measured by differential scanning calorimetric analysis. The geometry of the die was properly designed and optimized by FEM simulation in order to be suitable for forging semi-solid material, allowing a comparison with conventional forging process. 14 K-type thermocouples were used for monitoring the temperature of top and bottom dies; an instrumented 100 ton press was also equipped with load cells to acquire the forging force. A deep metallurgical analysis of the forged parts was performed in order to evaluate their mechanical properties and quality.

Microstructural Evolution of Solid-solution-treated Zn–22Al in the Semisolid State

The effect of solid-solution-treatment on the semisolid microstructure of Zn–22Al with developed dendrites was investigated. Forming Zn–22Al products by semisolid metal processing offers significant advantages, such as reductions in macro-segregation, porosity and forming costs. Thermal and microstructural analyses of the formed Zn–22Al alloy were performed by differential scanning calorimetry, scanning electron microscopy and optical microscopy. The changes in the microstructures and phase transformation in response to various solid-solution-treatments were analysed. In this study, as-cast samples were held isothermally at 330 °C for 0.5–5 h and then partially remelted at a semisolid temperature of 438 °C for 1 h to produce a solid-globular grain structure in a liquid matrix. A non-dendritic semisolid microstructure could not be obtained when the traditionally cast Zn–22Al alloy with developed dendrites was subjected directly to partial remelting. After solid-solution-treatment at 330 °C, the black interdendritic eutectics were dissolved, and the dendritic structures gradually transformed into uniform β structures when the treatment time was increased. The coarsened and merged dendrites were separated as a result of penetration by the liquid phase and melting of the residual eutectic at sites along the former grain boundaries. The microstructure of the solid-solution-treated sample transformed into a small globular structure; the best shape factor of 0.9, corresponding to a particle size of 40 ± 16 μm, is achieved when the sample was treated for 3 h followed by direct partial remelting into its semisolid zone.

Microstructural Evolution of Electromagnetically Stirred Feedstock SSM Billets During Reheating Process

Semi-solid metal (SSM) processing is an effective alternative to the classical manufacturing processes of casting and forging. The key issue in SSM is the production of suitable feedstock with guaranteed continuous supply. To produce good quality billets, the current work studied the thixocastability of electromagnetically stirred (EMS) Al–7Si alloy billets and compared them with conventionally cast billets to establish the effect of EMS in improving the quality of feedstock. In all cases, the intention was to study the microstructure of the billets just before feeding it into the high pressure die casting machines and therefore the billets were reheated to thixocasting temperature and isothermally held for 10 min before being quenched in water. The outcome of EMS application was the formation of refined and more globular SSM billet structures with less entrapped eutectic. The implication of pursuing such work is to guarantee continuous supply of feedstock to enable the establishment of manufacturing facilities, similar to mini mills for steel industry, where the feed stock is shipped to casting site to be shaped in high pressure die casting machines.

Microstructure and mechanical properties evolution of 6061 aluminum alloy formed by forward thixoextrusion process

Thixoforming process is a semisolid metal processing route, which involves forming of alloys in the semisolid state to near net shaped products. To investigate the effects of thixoextrusion parameters on microstructure and mechanical properties of commercial 6061 aluminum alloy, forward extrusion experiments were carried out in the semisolid state. The results showed that fine and globular microstructure can be achieved using semisolid forming. SEM micrographs and mechanical tests results showed that the tensile behavior of thixoformed specimens at billet temperatures below 620°C is relatively elastic and brittle. With increasing die temperature up to 620°C more elongation value and ductility were obtained in the thixoformed specimens.

An Overview of Semisolid Processing of Aluminium Alloys

Semisolid metal processing (SSM) or thixoforming is a new technology that offers several advantages over liquid processing and solid processing. This process utilizes semisolid behavior as well as reduces macrosegregation, porosity, and forming forces during shaping process. A lot of research work has been carried out by various researchers in order to exploit the potential of this process to produce different products especially for automotive industry. This paper will summarise the rheological behavior of aluminium alloys in semisolid slurries, thixoformability of modified aluminium alloys, the effect of feedstock production method on mechanical properties, and the importance of developing low-cost raw materials for semisolid processing.

New frontiers for thixoforming

The continuous increase in demand for light alloys in different applications involves manufacturing high integrity and superior performance components using cost-effective and safety processes. In this direction, the research communities together with manufacturing industries are focusing their attention to develop new products using innovative procedures, i.e., Semi-Solid Metal (SSM) processing routes. The central issue is the technological transfer to industry for affordable mass production. The purpose of this paper is to present an overview of alternative methods of production of enhanced performance light alloys components for critical areas. The origin, development and current evolution of the SSM processing are illustrated with characteristics of real-life components for automotive industry. Different techniques are presented to provide a general idea prevalently on the commercially available SSM processing techniques. A comparison with traditional methods, in particular High- and Low-Pressure Die Casting technologies, in terms of process parameters and properties is illustrated and discussed.