Alloys In Semi-solid State Research Articles

Simulation studies on deformation behaviour of AA2017 alloy in semi-solid state using FEA

Thixo-forming is one of the near net shaped manufacturing processes in which the final product is made between the liquidus and solidus temperatures. Forming the product at this temperature range provides large benefits compared to conventional forming methods. In the present work, deformation mechanism of AA2017 alloy in the semi-solid state has been studied. As the mechanical behaviour and deformation mechanism of semi-solid metal is completely different from that of the solid state forming, it is necessary to investigate its nature at the semi solid range. To analyse the metal flow, the stress behaviour and the corresponding strain induced, extensive finite element analysis (FEA) based simulation studies have been performed. The commercial software, DEFORM 2D was used for the simulation. The flow behaviour was modelled by formulating an Arrhenius type constitutive relation.

Investigation on solidification conditions in functionally Si-gradient Al alloys using simulation and cooling curve analysis methods

During recent years, functionally graded alloys have been widely produced by melt processing routes to achieve advanced material properties. In this paper, the new cast-decant-cast (CDC) method has been used to produce the gradient in concentration of Si particles in cross section of final Al–Si castings. For this purpose, the hypereutectic LM28 and hypoeutectic LM25 alloys were selected for each step casting into the steel mold. Cooling curve thermal analysis and simulation methods were applied to investigate the cooling behavior of first poured LM28 alloy and improve the accuracy of CDC process by determining the curves of solid fraction and temperature profiles. The final products were studied through optical microscopy, image analysis, and Brinell hardness measurement. The results showed that the silicon concentration decreased along transition zone between two alloys by increasing the decanting time in the order of 25, 40, and 50 s. This can be due to the lower temperature of exterior LM28 alloy in semi-solid state and shorter solidification time of interior LM25 alloy. This can lead a to reduction of the diffusion rate of elemental silicon along the transition zone. The microscopic scale of transition zone between two alloys developed the maximum thickness of 438 μm and hardness value of 83 HB comparing with the hardness of 88 and 62 HB for external and internal alloys, respectively. The microscopic observations and hardness evaluations confirmed the creation of functionally Si-gradient products.

The compressive deformation behavior and deformation mechanism of Ti14 alloy in semi-solid state

The semi-solid deformation behavior of Ti14 was investigated using compression tests at deformation temperatures between 1 273 and 1 423 K with strain rate of 5×10−2 s−1. Moreover, the fraction solid at different temperatures was also measured by image analysis. The results showed that the deformation temperature had strong effects on the flow stress, and the stress increased with the decrease of deformation temperature. The maximum stress depended greatly on the fraction solid, and a sharp decrease in stress occurred at a solid fraction between 0.94 and 0.98 (temperature from 1 323 to 1 373 K). This decrease was related to the decrease in the amount of solid bridges between grains. Because of the partial solid/liquid segregation during deformation, the experimental strain rates were much lower than those calculated by the flow of liquid incorporating solid particles, which suggested that the main deformation mechanism between 1 323 and 1 373 K was still plastic deformation of solid particles.

Rheology of A356 Alloy During Solidification Under Stirring

This work presents an experimental investigation on the rheology of A356 alloy in semisolid state using a high temperature Couette type viscometer. The molten liquid, resides in the annular space between the cylinders, is stirred and cooled continuously during experiments. The stirring results in fragmentation of dendrites which are transported into bulk liquid and form a semisolid slurry. The viscosity of the slurry is distinct in nature, which depends on microstructure of the suspended dendrites after coarsening. Hence, in the work, the variation of viscosity and microstructure is captured during cooling under different process parameters such as shear rate and cooling rate. Angular velocity of the inner cylinder and torque applied to stir the slurry are recorded to determine the apparent viscosity of the slurry. Temperature of the slurry is recorded to calculate the fraction of solids present in the slurry. For micrograph analysis, a vacuum quartz tube is used to remove the slurry-samples during experiments and they are quenched them in water.

Coarsening kinetics of multiaxial forging (MAF) formed AZ91D magnesium alloy in semisolid state

Multiaxial forging (MAF) was used as a strain induced melt activated (SIMA) process. The coarsening kinetics of the MAF formed AZ91D during partial remelting has been investigated. With increasing temperature, liquid is formed initially by the eutectic with lower melting point at boundaries and penetrates the recrystallised boundaries. Prolonged holding time resulted in grain coarsening. Grain coarsening in the semisolid state can be described by classical LSW (Lifshiz, Slyozov and Wanger) equation in Ostwald ripening. The coarsening rate does decreases with increasing temperature, for temperature lower than 560°C, and then increases again with further increasing temperature from 560 to 570°C.

Study on Thixotropic Property of A356 Alloy in Semi-Solid State

In the present work, the thixotropic property of a semisolid aluminium alloy (A356) under deformation is investigated numerically where the Couette flow between two parallel plates is considered. The flow field is represented by momentum conservation equations where the non-Newtonian behavior of the semisolid material is represented by the Herschel-Bulkley model. The agglomeration and the de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter influenced by the rate of strain and shear stress. The simulation predicts the flow field, rate of strain and apparent viscosity of the semisolid materials under transient and steady state conditions. It is found that the apparent viscosity shows a transient nature during sudden change in the shear rate, and its value decreases with increasing shear rate and vice-versa. It is also found that the present prediction shows a good agreement with prior work.

Rheological behaviour of AZ91D magnesium alloy in semisolid state

The rheological characterisation of semisolid AZ91D magnesium alloy has been successfully studied using a Couette rheometer. Different applied regimes by varying the temperature and the shearing in semisolid state revealed the rheological behaviour of the alloy and were confirmed by microstructural evolution. The apparent viscosity was the critical parameter that was studied during the applied regimes. The change in the morphology of the solid phase during the thermomechanical treatment was the main cause of the evolution of the apparent viscosity. The evolution of the morphology of the solid phase, from dendritic to globular, due to the shearing, had a significant impact on the rheological behaviour of the slurry which was correlated to the operating conditions by an analytical model.L’étude de la caractérisation rhéologique de l’alliage de magnésium de type AZ91D à l’état semi-solide a révélé une étroite dépendance entre les conditions opératoires et les changements structuraux. Les régimes thermo-mécaniques choisis ont montré que la vitesse de refroidissement ainsi que le cisaillement imposé avaient un effet direct sur la viscosité apparente de l’alliage. Le passage d’une structure conventionnelle dendritique à une structure globulaire a réduit de manière significative la viscosité, tributaire de l’intensité ainsi que du temps de cisaillement selon un modèle analytique.

Influência do tempo e deformação na microestrutura de ligas de alumínio reciclado condicionadas para tixoconformação

Um dos pré-requisitos para o processo de tixoconformação de ligas, é que estas apresentem microestruturas globulares. Esse trabalho utilizou o método da deformação seguido por recristalização com o objetivo de determinar a deformação e o tempo mínimos necessários para completa globularização de duas novas ligas de Al no estado semi-sólido (ESS). Buscou-se, também, encontrar a deformação necessária para um máximo refinamento microestrutural. As ligas foram fundidas a partir de sucatas de latas de alumínio, acrescentando-se Si, até atingir 3,5% (Al3,5Si), e Cu para a outra liga, até atingir-se 4% (Al4Cu). As novas ligas foram laminadas com deformações variando de 5 a 30% e levadas ao ESS com tempo de encharque variando de 1 a 60 minutos. As microestruturas foram caracterizadas por microscopia ótica. Os resultados mostram que a deformação mínima, para promover total globularização, está entre 5 e 10%. Quanto maior a deformação mais refinada é a microestrutura, porém, a partir de aproximadamente 20%, é necessário melhorar as condições de conformação para impedir a formação de trincas.

Extrusion of EN AW-2014 alloy in semisolid state

The potential of forming EN AW-2014 alloy in semisolid state was investigated. The EN AW-2014 slugs were partially melted at 610°C, to a liquid fraction of ∼15%, before they were extruded into a solid bar with a diameter of 16 mm. The ram speed used in this process was much lower than that employed in thixoforging of the same alloy to maximise heat removal from the slug and to fully solidify the liquid fraction by the time the extruded bar exits the die. The high solid fraction employed was also of help in this regard. Forming during extrusion under these conditions took place largely via rotation of the α-Al globules over one another, producing a remarkable microstructure of predominantly uniform globular α-Al grains. The forming load was at least an order of magnitude smaller with respect to that measured during hot extrusion of the same alloy, underlining the benefits of extruding the hard to form 2XXX alloys in semisolid state. Having been largely drained of its liquid, the final part of the preheated slug, however, failed to enjoy extrusion under such favourable conditions. The large extrusion deformation and temperatures well above the liquidus point have led to dynamic recrystallisation, which is responsible for the equiaxed, instead of fibrous, grain structure in this region.

Behavior of A356 Alloy in Semi-Solid State Produced by Mechanical Stirring

This investigation studied semi-solid (SSM) aluminum-silicon alloy produced by mechanical stirring. Aluminum alloys produced by this method, are widely used instead of conventional die casting and forging processes. In this research by using a mechanical stirrer slurry maker, the effects of stirring speed, the solid fraction percent and mold temperature are investigated on micro-structure and hardness of A356 aluminum alloy in semi-solid casting. By optimizing the forming parameters, dendrite microstructure changes to globular and mechanical properties improves. This is because of breaking and globularizing dendrites of primary α-AL phase. It is determined that stirring temperature of 608°C and stirring speed of 300 RPM leads to the uniform grains distribution and therefore arrives to better hardness for the produced billets.

Extrusion of Rheocast 7075 Aluminum Alloys in Semi-Solid State

Extrusion of Rheocast 7075 Aluminum Alloys in Semi-Solid State

Deformation behaviors of Al–4.5 mass% alloy in semi solid state

Deformation behaviors of Al–4.5mass%Cu alloys in semi solid state were investigated with tensile tests and their in situ observation. Deformation behaviors were classified into six stages (from stage I to stage VI, stage I was the lowest temperature range of semi solid state) by mechanical properties, morphologies of fracture surfaces and in situ observations during deformation. In the stages occured hearing (stage V and stage VI), the deformation was caused by flow of liquid phase. Therefore, the elongation could not be evaluated as the ductility of material in these stages. The range of temperature appearing the hot tearing was restricted to narrow temperature range in semi solid state, and was classified to stage II to stage IV. The tensile test method to research deformation behaviors of aluminum alloys in semi solid state with high accuracy was developed.

Rheological behavior, microstructure and hardness of A356 aluminum alloy in semisolid state using backward extrusion process

The influence of temperature on the flow behavior and rheological characteristics of an A356 alloy in the semi-solid state was investigated using backward extrusion process. Experiments were performed at 5 temperatures and 4 different wall thicknesses. Viscosities were determined using the force-displacement graphs obtained form back extrusion tests. As observed experimentally, at a constant temperature, the increase of shear rate results in the decrease of alloy viscosity exponentially. Raising the temperature increases the liquid fraction hence reduces the semi-solid alloy viscosity. Metallographic and image analyses show that, because of low forming speed, liquid has time to escape from solid phase forward the sample wall. This condition is the main reason for the segregation phenomenon seen in the base and walls. Vickers hardness test on samples reveals that the hardness increases with the decrease of temperature and wall thickness.

Compression Deformation Behavior of Al-Fe Alloy in Semi-solid State

Al-Fe alloy has good heat-resisting property,but has poor mechanical properties.Semi-solid formation can improve the alloy mechanical properties and make the alloy have application value.Since the semi-solid formation technology is closely related to the deformation behavior of the alloy in semi-solid state,systematically studying on the compression deformation behavior can provide a basis for laying down semi-solid formation technology.The effects of deformation degree,rate and temperature on the changes of true stress and true strain of electromagnetic stirred Al-Fe alloy in semi-solid state are investigated through compression deformation test by using an INSTRON-5500R electronic universal testing machine.The results show that the deformation resistance increases with the increase of deformation rate,and with the increase of deformation degree,the total strain increases,the grain size of the specimen center part tends to reduce and the proportion of liquid phase decreases obviously,while the edge of the specimen has no obvious change.The peak value of the true stress increases sharply with the decrease of deformation temperature,while the steady-state stress changes slightly,and both the grain size of the center part and the proportion of liquid phase decrease.

Compressive deformation and forging behavior of Ti14 alloy in semi-solid state

Deformation behavior and formability of Ti14, a new typical α + Ti 2Cu alloy in the semi-solid state, were investigated by compressive tests in temperature range between 1223 and 1473 K and by forging tests between 1223 and 1373 K. Tensile tests of the forged alloys were also performed to study the effect of forging on mechanical properties. The results show that the maximum compressive stress was greatly dependent on the fraction solid, and that a transition in stress occurred at a solid fraction between 0.94 and 0.98 (corresponding to the temperature between 1323 and 1373 K). This transition was related to the decrease in amount of solid bridges between grains. Excellent formability and grain refinement due to dynamic recrystallization were achieved during the semi-solid forging. Furthermore, tensile tests showed a high ultimate tensile strength and yield strength after semi-solid forging at more than 1273 K, which was attributed to the grain refinement caused by the semi-solid forging. Ductility was also improved by changing the forging ratio. These results indicate that both good formability and tensile properties can be simultaneously attained by the compressive processing in the semi-solid state.

Effects and on-line prediction of electromagnetic stirring on microstructure refinement of the 319 Al–Si hypoeutectic alloy

In this paper are presented the results of the electromagnetic treatment used to refine 319 aluminum alloy in semi-solid state. The severity of the treatment is controlled by varying the intensity of the electromagnetic field (using different AC current), which results in the different levels of microstructure modification. In this research is found that the main microstructure characteristics that are sensitive to electromagnetic stirring modification are grain size, dendrite size and a Cu phase. The electromagnetically stirred material is transformed from a dendritic structure into quasi-cellular one with a grain size refinement of up to 600%. Conversely, the silicon present in the Al–Si eutectic did not present significant modification. The results of image and thermal analysis are in good agreement allowing to propose a mathematical approach that can be used for on-line prediction of the level of microstructure modification of the ES treated melt.

Development of Non-Dendritic Microstructure of Aluminum Alloy in Semi-Solid State under Ultrasonic Vibration

The effect of processing parameters on the semi-solid microstructure has been investigated in the course of semi-solid slurry preparation of A356 Al alloy by ultrasonic vibration method. The A356 melt in temperature of 630660°C was poured into a special metal cup, and exposed to ultrasonic vibration at 20 kHz or isothermal holding for a certain time after vibration. The results show that the semi-solid slurry with primary α-Al crystals smaller than 90 μm and an average shape coefficient F S (SF was defined as SF4 A/L2P) above 0.5 could be prepared by ultrasonic vibration for a time less than 144 s. In the isothermal holding period after a short time of ultrasonic vibration, the average particle diameter increased with the increase of holding time, but F S decreased at first, and then increased a little. It was discovered that ultrasonic vibration is a good method to prepare semi-solid slurry with fine and relatively round primary crystals due to cavitation and acoustic streaming effects.

Numerical Simulation of Semi-Solid Roll Strip-Casting Process for AZ91D Magnesium Alloy

High quality magnesium alloy sheets with non-dendritic microstructure can be manufactured by the technique which incorporates semi-solid processing with continuous roll stripcasting. In particular, the process is believed to be cost-effective in mass production of magnesium sheet products. The processing parameters, such as roll speed and temperature, have significant influence on roll strip-casting process. In this paper, the commercial software, DEFORM-3D, for deformation simulation of bulk materials was used to carry out the simulation of roll strip-casting process for AZ91D magnesium alloy in semi-solid state, and the distribution of equivalent stress, equivalent strain and temperature was obtained. Based on the results, the optimization of processing parameters was discussed.

Effect of Disturbing Conditions in Semi-Solid State on Microstructure Evolution of Magnesium Alloy

In this paper the microstructure evolution of AZ91HP magnesium alloy casts produced under different disturbing treatment conditions such as non-disturbing, argon blowing and mechanical stirring in semi-solid state after holding a short time was studied. The results show that the grains of AZ91HP alloy casts refined, the secondary dendritic arms grown and the dendrites tend to rosette shape with the decrease of holding temperature. External disturbance can accelerate the evolution process, and the spherical primary α phase formed easily under mechanical stirring treatment condition. The formation of non-dendritic structure is owing to ripening of the dendritic arms, refinement of the grains and movement of the primary formed solid phases. Disturbing treatments in semi-solid state induce more equilibrium solidification and decrease the amount of brittle β-Mg17Al12 phase. Blowing argon into the refined and modified magnesium alloy in semi-solid state can obtain homogeneous non-dendritic structure and the net shaped β phase distributed on α-Mg phase boundaries become fine and thin, this may be have a good effect on the mechanical properties of the magnesium alloy casts. Due to the low disturbing strength, argon blowing can maintain the metallurgy quality of the semi-solid slurry well, and also have high efficiency to make it. This technology need not new complex equipments and can be practiced in conventional casting conditions, so it may be used in industrialize manufacture.

Microstructural Evolution and Deformation Behavior of Stainless Steel in Semi-Solid State

Thixoforming or Semi-Solid Metal Forming offers many advantages in comparison with casting and conventional forging. The purpose of the present study is to provide the basic microstructure and deformation data for austenitic and ferritic stainless steel under mushy state. As well known, the stainless steels solidify in different modes according to the different chemical compositions. In this paper, microstructural evolution of austenitic stainless steel type 304 which solidifies in FA mode ( L → L +δ → L +δ +γ →δ +γ →γ ),austenitic stainless steel type 310S which solidifies in A mode ( L → L +γ →γ ), and ferritic stainless steel type 430 which solidifies in F mode ( L → L +δ →δ )are investigated during partial remelting by way of SIMA (Strain Induced Melted Activation). The results show that A and F mode of stainless steels melt directly at the grain boundary without phase transformation during reheating. A banded structure, originating from the primary dendritic segregation of the original ingots, is observed in type 310S steel during further heating. On the other hand, a perfect globular and insegregative two-phase semi-solid structure L +δ can be obtained while heated beyond the banded three-phase L +δ +γ semi-solid state in FA mode austenitic stainless steel type 304. This spheroidization can be attributed to the peritectic reaction occurred in the L +δ +γ semi-solid state. In addition, simple compression tests of these alloys in semi-solid state for varied combination of deformation rate and deformation temperature are conducted to examine the deformation behavior of stainless steel. Flow stress curves exhibit abrupt change in various alloys, even though in the same alloy such as type 304, various flow stresses are observed according to the difference in inner microstructure or morphology. Stress of type 310S steel shows the most reduction as the deformation temperature increasing at the same strain rate condition. The Liquid is centralized to periphery by the compression force in all deformed test pieces. Fracture, observed in all alloys except type 304 steel in globular L +δ semi-solid state, should be resulted from the lack of liquid in L +δ +γ state of type 304 steel and solidification crack in type 310S and type 430 steel. Deformation of solid particles occurs only in L +δ +γ state of type 304 steel. Last in this paper, various deformation mechanisms are proposed for various microstructures.