Semi-solid Range Research Articles

Processing of Hypereutectic AlSiCuFe Alloys in the Semi-Solid State

Hypereutectic Al-Si based alloys are expected to be more wear resistant than the AlSi8Cu3Fe alloy, currently used in compressor connecting rods in die cast grades. Die casting of such alloys, however, is not straightforward. Semi-solid processing was thus considered to overcome the problems encountered in casting hypereutectic Al-Si alloys. Two experimental hypereutectic AlSiCuFe alloys were obtained by adding 8 to 12 wt% Si to the AlSi8Cu3Fe alloy. The ingots of these alloys were melted and cooled to very near their liquidus points, 625°C and 675°C respectively, before they were poured into a permanent mould in order to produce nondendritic feedstock for thixoforming. This low superheat casting process largely replaced α-Al dendrites with relatively smaller α-Al rosettes. The slugs machined from the ingots thus obtained were thixoformed after they were heated in situ in the semi-solid range for 5 minutes in a laboratory press. Several thixoformed parts were heat treated to the T6 temper with an accompanying increase in hardness from 91-96 HB to 130-131 HB.

Semisolid processing of near-eutectic and hypereutectic Al–Si–Cu alloys

Semisolid processing of near-eutectic and hypereutectic versions of alloy 380 offers to overcome the problems encountered in casting hypereutectic Al–Si alloys and was thus explored in the present work. Experimental near-eutectic and hypereutectic Al–Si–Cu alloys obtained by adding elemental silicon to the 380 alloy were melted and were cooled to within 5 to 15 °C of their liquidus points before they were poured into a permanent mould in order to produce non-dendritic feedstock for thixoforming. This low superheat casting (LSC) process largely replaced α-Al dendrites with relatively smaller α-Al rosettes in all alloys. The slugs machined from the LSC ingots thus obtained were thixoformed after they were heated in situ in the semisolid range, between 568 and 573 °C, for 5 min in a laboratory press. Semisolid soaking sufficed to produce the required globular structure even when some dendritic features were retained in the starting feedstock. The hardness of the thixoformed parts which ranged between 84 and 96 HB have increased to 121–131 HB after the T6 heat treatment, implying a considerable age-hardening potential. The T6 treatment also improved the morphology of the eutectic silicon with potential benefits regarding the ductility of the thixoformed part.

Wear properties of thixoformed AlSiCuFe alloys

High-performance compressors which use low-viscosity lubricants face heavier wear problems and thus require a material more resistant to wear than the current high pressure die cast AlSi8Cu3Fe alloy in connecting rod applications. While hypereutectic versions of this alloy are expected to offer superior wear resistance, their processing via die casting is not straightforward. Semisolid processing of hypereutectic AlSiCuFe alloys offers to overcome the problems encountered in casting hypereutectic Al-Si alloys. A series of experimental AlSiCuFe alloys obtained by adding elemental silicon to the parent AlSi8Cu3Fe alloy were melted and were cooled to within 5 to 15°C of their liquidus points before they were poured into a permanent mould in order to produce non-dendritic feedstock for thixoforming in the present work. The slugs machined from the ingots thus obtained were thixoformed after they were heated in situ in the semisolid range for 5 minutes in a laboratory press. The hardness of the thixoformed and heat treated parts were 84–96 HB and 121–131 HB, respectively. A modified Falex Block on Ring equipment was employed to investigate the wear properties of these alloys under service conditions which prevail in connecting rod applications in compressors. The wear resistance of the thixoformed near-eutectic AlSiCuFe alloy was found to be the highest. In spite of a notable increase in the hardness of the wear test samples, the T6 heat treatment did not appear to have an impact on the wear resistance.

DETERMINATION OF THE THIXOCASTING TEMPERATURES OF AZ91D AND OTHER Mg ALLOYS USING A QUENCHING METHOD

The thixocasting injection temperature is a crucial processing parameter, frequently unavailable in the literature, especially for newly developed alloys. In this work, a simple and rapid method derived from the quenching method was developed to obtain an acceptable thixocasting injection temperature range for various magnesium alloys: AZ91D, AJ62x, AXJ530, MRI153 and ZAEX10440. Small specimens of each alloy were heated to appropriate temperatures within the semisolid range and quenched. Their microstructure was observed optically and the proportion of phase was measured using quantitative image analysis. The microsegregation was characterized and two different heating cycles prior to quenching were studied. The apparent volume fraction of the primary solid was determined as a function of quenching temperature between liquidus and solidus. These results were found useful to obtain a practical injection temperature range for each alloy and to obtain good quality thixocast parts.La température d’injection pour la thixocoulée est un paramètre crucial de procédé qui n’est pas toujours disponible dans la littérature et ce, surtout dans le cas de nouveaux alliages développés en laboratoire. Dans cet article, une méthode simple et rapide, dérivée de la méthode de trempe, est proposée pour estimer un intervalle de température d’injection d’alliages de magnésium tels que AZ91D, AJ62x, AXJ530, MRI153 et ZAEX10440. Des spécimens de chaque alliage ont été chauffés à différentes températures dans l’intervalle semisolide et trempés. Leur microstructure a été observée en microscopie optique et analysée quantitativement par analyse d’image afin de mesurer la proportion de la phase solide primaire. La microségrégation a également été mesurée dans les phases après trempe et deux cycles thermiques différents ont été comparés. La fraction volumique apparente de la phase solide primaire a été déterminée en fonction de la température de trempe entre le liquidus et le solidus. À partir de ces résultats, un intervalle de température a été estimé pour chaque alliage afin d’obtenir par thixocoulée des pièces d’excellente qualité.

Thixo-formability of the AA6063 alloy: Conventional production processes versus electromagnetic stirring

Abstract Cold-formed AA6063 alloy has an excellent mechanical behaviour, and is used in a wide range of structural applications. In Brazil this alloy is produced in large amounts, due to wide application at low cost. The present work aims to present a new use for this material, allowing the subsequent manufacture of metal parts by thixo-forming. The objectives of this work are (a) to establish a suitable condition for the production of thixo-formable structures using electromagnetic stirring during semi-continuous casting, with and without grain refiner, and (b) to compare this to the same alloy produced by the conventional process. The alloy was supplied by Companhia Brasileira do Aluminio (CBA), and processed in a semi-continuous casting equipment at the Thixo-Forming Laboratory of Unicamp, which has a controllable electromagnetic field from 600 to 2000 W/kg of material, induced into the alloy during solidification. The final alloy structure was characterized in terms of grain size, primary particle size and shape and rheological behaviour for 45% solid fraction. Furthermore, results were compared to the chemical refining (Al–1 wt%Ti–0.5 wt%B), as well as the combination of both production methods. According to the thixo-formability evaluation protocol, the commercially produced AA6063 alloy has an excellent degree of thixo-formability, presenting a very low-apparent viscosity (105 Pa s), despite the difficulty of its processing, i.e., a very narrow semi-solid range (only 23 °C). In fact, the most important feature for thixo-formability is to have raw material presenting a very refined structure, as well as, the most globular structure. In this case, the commercial alloy presented grain size of 86 μm and rheocast quality index 0.60. For commercial processing better equipment and a better temperature control will be indispensable in the commercial development of thixo-forming operations for this type of material.

Melt infiltration of SiCp reinforced Al matrix composite by newly designed pressure infiltration technique

Newly designed pressure infiltration technique was developed to clarify the molten metal flow through solid network, aiming to obtain property materials and, simultaneously, the purified matrix as a filtrated metal. An experimental apparatus was constructed to control the microstructure morphology for varied fraction of solid and applied pressure. A sample, Al-7 wt.% Si-20 vol.% SiCp was squeezed on the double layers of semi-solid aluminum alloy cake and alumina porous medium by using argon gas pressure. Permeability values were estimated by Darcy's law in order to know the molten metal flow characteristics through solid network in the semi-solid range. It was found that the permeability suddenly decreased with increasing fraction of solid, and filtration mechanism was changed between depth and cake filtration near fraction of solid 0.25 due to much difference of the flow resistance between the two. Increasing applied pressure decreased the permeability due to the consolidation of solid grains. For pressurized sample on the filter and filtrated metal, SiCp content became to as high as 38 and less than 0.4 in volume percentage depending on fraction of solid, respectively. The pressurized samples with high SiCp content showed low wear rate and high hardness, when compared with original composite sample.

Correlation between ignition and oxidation behaviours of AZ91 magnesium alloy

The ignition resistance of AZ91 alloy is investigated during continuous heating experiments and when held at constant temperature in the semisolid range. It is found that during continuous heating, ignition takes place close to the liquidus temperature when a significant amount of liquid is present. Similarly, ignition occurs during an isothermal hold in the semisolid region, but after an incubation time that depends on the temperature. Resistance to burning is closely related to high temperature oxidation, which is particularly complex in the case of this alloy. The non-protective nature of the oxides together with the liquid oxidation and the possible role of Mg vapour seem to be the reasons for the AZ91 ignition.

고효율 전동기용 Cu Rotor의 반응고 성형과 공정변수 제어

Rotor in small-medium induction motor has been usually manufactured by aluminum diecasting. In order to improve the efficiency of induction motors, newly developed Cu-Ca alloys have been investigated. The electrical conductivity in the Cu alloys containing Ca less than <TEX>$1.0wt\%$</TEX> was higher than <TEX>$80\%$</TEX> IACS. Cu-Ca alloy is desirable for the thixoforming process because it has wide semi-solid range over <TEX>$150^{\circ}C$</TEX>. In this study, Cu-rotor with thixoforming process was developed to replace the conventional aluminum diecasting rotor. Analysis was performed for the microstructure of thixoforming rotor. Effect of incomplete filling on the efficiency of induction motor was discussed.

Forming Experiments and Die Development for Processing Steels in the Semi-Solid Range

Forming Experiments and Die Development for Processing Steels in the Semi-Solid Range

The effect of isothermal mechanical stirring on an Al–Si alloy in the semisolid condition

Abstract Isothermal rheocasting studies were carried out on Al–5.2Si alloy under different processing conditions. The parameters varied are the shear rate, time of stirring, addition of grain refiner and the cooling rate. The addition of grain refiner prior to stirring has been found to influence the size and morphology of the primary particles. The shear rate–time combination has found to have tremendous effect in deciding the primary particle morphology. By proper selection of the processing parameters, globular primary particles can be obtained by mechanical stirring with a radial flow impeller in the semisolid range. The observed particle growth with increasing isothermal shearing time could be understood with the available theories of particle growth.

Experimental Investigations and Computer Simulation of the Liquid Fraction Evolution during the Solidification of Alloys Suitable for Semi-Solid Processing

One important parameter for the processing of materials by semi-solid forming is the actual distribution of the solid and liquid phases in the semi-solid range. This parameter defines the process stability for the forming step. Therefore it is necessary to obtain information about the materials behaviour in the semi-solid state for different materials grades. This kind of information can be obtained by experimental studies in the interesting temperature range or by calculations with simulation programs using thermodynamic data validated by experiments. This work shows the results of experimental studies and thermodynamic calculations of the solidification and heat treatment behaviour of the aluminium alloy A319 and the steel X210CrW12. The experimental studies of solidification and heat treatment of these alloys were carried out using a differential thermal analysis system (DTA). The theoretical fraction of liquid content was calculated from the DTA signal by using a software module called Corrdsc. The experimental data obtained were used to validate the thermodynamic simulations of the solidification of semi-solid alloys. The simulations of the solidification process were carried out for equilibrium conditions, with the Scheil-Gulliver model as well as with diffusion calculations. The equilibrium and Scheil-Gulliver calculations were performed by the program Thermo-Calc, and the diffusion by the program DICTRA. The required thermodynamic and mobility data for multicomponent systems were taken from the data bases COST 507 light alloys, TCFE2000 Steel/Alloys and MOB2 mobility and from newly added data. The comparison of calculated phase transformations and fractions of liquid content with experimental data revealed a good agreement.

Rheological modelling on semi-solid metal alloys and simulation of thixocasting processes

Semi-solid metal alloys, as used in thixoforming, have a special microstructure of globular grains suspended in a liquid metal matrix. The material under investigation is a tin–lead alloy (Sn–15% Pb) which exhibits a similar microstructure as aluminum alloys. The experiments were performed with concentric cylinder rheometers. Initially, the liquid alloy is cooled down to the semi-solid range under constant shearing and then kept under isothermal conditions for further experimentation. The microstructure is characterized in dependence of the shearing time. The rheological techniques consisted of step change of shear rate and shear stress ramp experiments for different solid fractions (40–50%). 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 characterized 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 into numerical software and used for the simulation of a characteristic thixocasting process. The results are compared to real experiments.

Microstructural and mechanical characterisation of an Al-21.8 wt.% Ge brazing alloy with a globular morphology of the primary Al-rich phase

This paper is concerned with the microstructural and mechanical characterisation of a hypoeutectic Al-Ge alloy designed to join the plasma facing components of thermonuclear fusion reactors to permanently cooled substrates. The alloy was prepared from pure aluminium and germanium by casting, hot rolling and heat treatment above the eutectic temperature. This treatment leads to a globular morphology of the primary Al-rich phase provided that the thickness reduction during rolling is sufficient, about 20%. Increasing the rolling reduction, however, leads to liquid entrapment in the globules, the amount of which depends on the temperature in the semisolid range. The mechanical behaviour of the material was determined in tensile tests at room and elevated temperature after various heat treatments in the semisolid state and compared with the mechanical properties after hot rolling. In addition, compression tests were also carried out in the semisolid state. These tests show that the globular materials deform under very low stresses without segregation of the liquid. A comparison is made with non-globular materials obtained simply by partial remelting of the as-cast alloy.

溶湯撹はん法によるナノサイズＳｉＣ粒子のアルミニウム合金への複合化

The fabrication of metal matrix composite (MMCp) by vortex process is the general method due to the advantage of its convenience. However in the case of fine particles the surface property of the particle appears dominantly, especially in nano-size particles the agglomeration occurs and the fabrication of MMCp is reduced to be difficult. It is analyzed that the strength of the MMCp is dominated by the ascent of the dislocation density. Further, as the dispersion strengthening mechanism can be anticipated, dispersions of more fine particles into the matrix are required. In this study, the influences on various primary factors for solving an agglomeration problem are examined using SiC particles of 0.3 μm (the minimum diameter in the market) into 6061 Al alloy by the vortex method. From this study, the following results are obtained, (1) It is found that MMCp with no agglomeration can be fabricated by conducting the following procedure; pre-treatment, disagglomeration of powder, Ca addition and stirring in the semi-solid range. (2) It is deduced that the main factors of forming the agglomeration in the matrix of MMCp are the yield of SiO2 on the surface of SiC and the reduction of the void ratio of SiC powder.

Materials Selection for Semisolid Processing

The mechanical behavior of alloys in the semisolid state is very sensitive to the volume fraction of solid. In this paper, a simple criterion for processability in the semisolid range is introduced, based on the sensitivity of the volume fraction of solid with respect to minor temperature variations. In addition it is proposed that the processability in the semisolid state can be enhanced using microsegregated material.

Mechanical behavior of alloys with equiaxed microstructure in the semisolid state at high solid content

The mechanical response in compression of A2014 and Al–4 wt% Cu alloys with equiaxed microstructure in the semisolid state at high volume fraction of solid (>0.6) is studied. When the amount of liquid is low, deformation in compression is highly non-uniform due to strain localization. In the range of conditions examined no phase segregation was detected. The stress–strain curve exhibits a peak at low strain, around 5–10%, and then decreases to a plateau value. Secondary phase particles in A2014 that remain solid in the semisolid range of the alloy are responsible for the lower peak and higher plateau stress compared to Al–4 wt% Cu alloy. The effect of the grain size for the alloys tested was small. Strain rate jump experiments show that the instantaneous strain rate sensitivity in the semisolid state is similar to that of the solid phase. As the volume fraction of solid decreases, the plateau stress becomes rate insensitive to the strain rate jump. The results allow the relative importance to be assessed of various mechanisms responsible for resistance to flow of semisolid alloys with equiaxed microstructure in compression.

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.

Effect of solidification conditions on deformation behaviour of semi-solid Sn-Pb alloys

The compressive behaviour of Sn-Pb alloys is studied with materials of conventionally dendritic structure and non-dendritic (rheocast) structure obtained through mechanical stirring during solidification. The alloys are found to deform similarly in the fully solid state but their behaviour becomes very different in the semi-solid range depending on the solidification mode. The holding time in the semi-solid state before compression also affects the mechanical properties: the influence of these two parameters is discussed in terms of the initial structure of the alloy and its evolution. Advantages of using semi-solid materials in metal forming processes are also presented.