Alloys In Semi-solid State Research Articles

Anisotropic microstructure and thixo-compression deformation behavior of extruded 7075 aluminum alloy in semi-solid state

In order to examine the micro-mechanism behind the thixotropic behavior of semi-solid alloys, microstructural effects on the compression deformation of semi-solid 7075 aluminum alloy prepared by recrystallization and partial melting (RAP) method were investigated. The fibrous and deformed microstructure of as-extruded 7075 aluminum alloy can transform into elongated and globular grains via reheating to semi-solid states, and exhibiting dominated {100}<001> recrystallization texture and preferred crystal growth in extrusion direction. The anisotropic microstructure of semi-solid alloys depends on the remelting temperatures to a great degree. Correspondingly, the initial peak stress of compression stress-strain curves also demonstrated as anisotropic behavior. It was analyzed that the crystallographic texture caused orientation dependent Young's modulus and a corresponding elastic response, and the disagglometation of the saturated solid skeleton led to an increase of viscoplastic stress, which relates to the amount of bonds between solid grains.

Measurement of viscous properties on Al-5mass%Mg alloy in semi-solid state by tensile test with high-frequency induction heating

Measurement of viscous properties on Al-5mass%Mg alloy in semi-solid state by tensile test with high-frequency induction heating

The Effect of Equal Channel Angular Pressing on Coarsening Kinetics of AZ80–0.2Y–0.15Ca Alloy in Semisolid State

Semisolid samples of the AZ80–0.2Y–0.15Ca wt% (AZ80M) magnesium alloy are fabricated using equal channel angular pressing (ECAP) followed by semisolid isothermal heating treatment. The effect of the number of ECAP passes on the coarsening dynamics of the alloy at different semisolid temperatures is investigated. The results show that as the number of ECAP passes increases, the coarsening rate constant (K) shows a similar trend downward and then upward at different temperatures. And the K value of the sample after three passes of ECAP processing (3P) is the lowest, which is primarily attributed to the maximum proportion of high‐angle grain boundaries (HAGBs) and the most uniform grain size in the 3P sample. At high solid fractions, a high proportion of HAGBs hinders the coalescence of the adjacent solid grains. At low solid fractions, uniform and fine grains result in insufficient driving force for the Ostwald ripening process. These lead to the lowest K value in the 3P sample.

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.

A Study on Bar Drawing Process of A356 Alloy in Semisolid State

A bar drawing process of an aluminium alloy in semisolid state is presented in the work. The drawing process depends on various parameters such as temperature, die-angle, shear rate etc., accordingly a study is considered. The work involves development of a model to investigate the drawing process of A356 alloy in semisolid range. The rheology of the alloy in semisolid state shows a distinct behaviour and reduces energy requirement during the drawing process. In the context, a model suitably represents the rheology of the alloy is considered to perform a study of the process in details. An analytical and a numerical solutions are combined together to solve the governing equations. Finally, in the work, the distribution of velocity, viscosity variation and drawing power of the semisolid alloy under shear are predicted in the domain. It is found that the energy requirement is reasonably less in case of semisolid bar drawing process compare to a conventional bar drawing process. Finally, the drawing power required to deform a conventional solid A356 alloy is compared with that of the semisolid A356 alloy.

Rheological Investigation of Semisolid AlSi7 Alloy by Means of Oscillation Experiments

Dynamic Mechanical Analysis (DMA) of semisolid aluminum alloys was performed in a rheometer of Searle type. DMA was applied on a binary AlSi7 alloy to demonstrate the advantages of the method for the investigation of the behavior of alloys in semi-solid state and it was compared to classical shear experiments. Frequency sweeps, amplitude sweeps as well as constant condition experiments (CCE) were performed. It became obvious that elastic properties are getting more dominant with increasing resting time without shearing. The shift from a more viscous to more elastic nature of the material can be quantified. Interestingly, it was found that the semi-empirically based Cox-Merz rule, usually applied for polymers, holds for the semisolid material as well. This allows investigating the shear viscosity under different relevant conditions - important to improve material models for die-filling simulation of semisolid alloys.

Preparing a Solid Filament for Use in Additive Manufacturing of Metals

Additive manufacturing (AM) or three-dimensional (3D) printing has now become one of the significant green manufacturing methods for use in many sectors of industry. Among numerous additive manufacturing methods that can produce geometrically complex components by adding material in consecutive layers, the most prevalent 3D printing method uses polymeric filaments as feedstock. Meanwhile, metal additive manufacturing requires complex technologies such as selective laser melting of the powder, being the most common metal 3D printing today. This study deals with the development of a metallic filament for use in a novel metal AM process, and the advantages of applying metal wires as a 3D printer input material. The aim is to describe how to achieve a readily printable metal filament that could be used in desktop metal printers. An investigation was conducted on the raw material preparation, design considerations, and microstructural evolution. However, it is very difficult to control such deposition of molten metal in a layer-wise manner. Extrusion of metal alloys in semisolid state is a good candidate process that could provide good deposition characteristics for metal wires. The results of this work show that semisolid deposition of a metal wire represents an affordable additive manufacturing technique. This study illustrates that, by designing an effective thermomechanical procedure, metallic filaments with the desired extrudability could be produced. The effect of process control on the rheological properties of the filament was also investigated. Finally, the desired filament microstructure was characterized to ensure a successful deposition process. The study of the mechanical properties of the filament and deposited beads yielded the desirable values, in turn confirming the success of the process to build a fully dense metallic part.

Microstructure Analysis and Rheological Behavior of Magnesium Alloys at Semi-solid Temperature Range

Microstructure and rheological properties of AZ91 and Elektron 21 magnesium alloys in semi-solid state were studied. Differential scanning calorimetry revealed significant differences in melting range in analyzed materials. Isothermal annealing for 5, 30 and 80 min, in semi-solid range, corresponding to 50 and 70% of solid fraction, showed slower microstructure coarsening kinetics in Elektron 21 due to the presence of Zr- and RE-enriched precipitations. Ostwald ripening and coalescence resulted in decrease in grains density with increasing annealing time and temperature. Hardness after annealing showed overall tendency to diminish with soaking time probably due to the growth of average grain size. Rheological analysis showed tendency to lower the value of dynamic viscosity coefficient as shear rate rise, which confirmed non-Newtonian flow behavior. The highest values of dynamic viscosity coefficient were obtained for alloy AZ91.

Microstructural Evolution during Partial Melting and Semisolid Forming Behaviors of Two Hot-Extruded Magnesium-Rare Earth Alloys

Reheating experiments and semisolid compression tests were conducted on two hot-extruded magnesium-rare earth (Mg-RE) alloys, Mg-8.20Gd-4.48Y-0.36Zr-3.34Zn and Mg-3.75Gd-5.15Y-0.75Zr-3.05Zn by using a multistage hot compression test machine. Dissolution of eutectic compounds, growth of grains, and partial melting took place during the reheating of these Mg-RE alloys and resulted in spherical semisolid slurries at certain temperatures (580 °C for Mg–8.20Gd–4.48Y–3.34Zn–0.36Zr, 560 °C for Mg-3.75Gd-5.15Y-0.75Zr-3.05Zn). Owing to the different alloying element contents of these two Mg-RE alloys, eutectic compounds with different morphologies were found inside them after reheating and rapid cooling processes. The forming characteristics of these two Mg-RE alloys in semisolid state were discussed based on the results of compression tests.

Unexpected dynamic recrystallization behavior of Ti-7Cu alloy in semi-solid state

Titanium alloy obtained from semi-solid processing has received increasing attentions recently for its excellent formability and mechanical properties. This work intends to unveil the puzzling deformation process of the alloy during semi-solid processing. Through the compressive tests of semi-solid Ti-7Cu under various conditions (temperatures or strain rates), the deformation is found to be dominated by the plastic deformation of solid particles. Strikingly, the dynamic recrystallization (DRX), as commonly occur during hot deformation, is also observed. Based on the Poliak's theory and Avrami equation, the critical strain that triggers the DRX and the DRX kinetics are analyzed. It is found that the kinetics of DRX during semi-solid deformation is highly sensitive to temperature and strain rate. More specifically, higher percentages of DRX grains present at lower temperatures or higher strain rates, which is in contrary with that in solid state. Such phenomenon is considered as originated from the lubricating effect caused by the flow of the liquid phase during semi-solid deformation. Further, a modified DRX kinetic model is developed based on the Zener–Hollomon parameter Z, the predicted grain size agrees well with the experimental measurements.

Rheology of StelliteTM 21 Alloy in Semi-Solid State

Abstract The main objective of this study was to conduct an analysis of the rheological properties of StelliteTM 21 alloy in the semi-solid state, as the results could be used for identifying the appropriate temperature range for thixoforming of this alloy, and a secondary objective of the experimental work was the development of mathematical model of the alloy’s apparent viscosity. Such viscosity models are necessary for numerical simulations of the thixoforming processes. The StelliteTM 21 alloy exhibits high hardness and thus shaping in the semi-solid state is promising route of production of parts from this alloy. Within the confines of experimental work the measurement methods of the rheological properties at high temperatures was developed. They are based on the use of specially designed viscometer equipped with high temperature furnace.

Behaviour of Semisolid Slurry Flows through a Channel

In case of metal sheet forming of alloys in semisolid state, modelling of the process is very essential to predict flow behaviour, temperature distribution of the alloy etc. towards improvement of the product quality and to reduce manufacturing costs. Accordingly, the present work develops a model to predict the behaviour during metal sheet forming of an Al-alloy (A356) in semisolid state. The semisolid alloy passes through a rectangular channel having small depth and larger width. The alloy in semisolid state is cooled from the top at a controlled rate. In the model, the respective flow field is represented by the momentum conservation equation. The non-Newtonian behaviour of the semisolid slurry is incorporated considering the Herschel–Bulkley model. The agglomeration and de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter. The temperature field is predicted considering the transient energy conservation equation, and hence the fraction of solid is continuously updated. The solution considers an apparent viscosity of the semisolid alloy as a function of structural parameter, shear stress and shear rate. The governing equations are finally solved by finite difference method. The work predicts velocity, temperature and liquid fraction distribution of the semisolid slurry.

Microstructure Evolution and Coarsening Mechanism of 7075 Semi-Solid Aluminum Alloy Pre-Deformed by ECAP Method

To investigate the influence of refined grains on the microstructure of 7075 aluminum alloy in semi-solid state, a new strain induced melting activation (SIMA) method was put forward containing two main stages: pre-deformation with equal channel angular pressing (ECAP) method and isothermally holding in the semi-solid temperature range. The breaking up and growth mechanisms of the grains and kinetics of equiaxed grains coarsening during the semi-solid holding were investigated. The results showed that the average grain size after ECAP extrusion decreased significantly, e.g., microstructure with average globular diameter less than 5μm was achieved after four-pass ECAP extrusion. Obvious grain coarsening had been found during isothermal holding in the semi-solid state and the roundness of the grains increased with the increasing holding time. The proper microstructure of 66.8μm in diameter and 1.22 in shape factor was obtained under proper soaking condition (at 590°C for 15 min). Two coarsening mechanisms, namely, coalescence in lower liquid fraction and Ostwald ripening in higher liquid fraction contributed to the grain growth process.

Transgranular liquation cracking of grains in the semi-solid state.

Grain refinement via semi-solid deformation is desired to obtain superior mechanical properties of cast components. Using quantitative in situ synchrotron X-ray tomographic microscopy, we show an additional mechanism for the reduction of grain size, via liquation assisted transgranular cracking of semi-solid globular microstructures. Here we perform localized indentation of Al-15wt.%Cu globular microstructures, with an average grain size of ∼480 μm, at 555 °C (74% solid fraction). Although transgranular fracture has been observed in brittle materials, our results show transgranular fracture can also occur in metallic alloys in semi-solid state. This transgranular liquation cracking (TLC) occurs at very low contact stresses (between 1.1 and 38 MPa). With increasing strain, TLC continues to refine the size of the microstructure until the grain distribution reaches log-normal packing. The results demonstrate that this refinement, previously attributed to fragmentation of secondary arms by melt-shearing, is also controlled by an additional TLC mechanism.

An Investigation on the Effects of Pre-deformation and Temperature on Thixotropic Microstructure of AA2014 Aluminum Alloy

With copper as the main alloying addition, alloy from the 2XXX family offers high strength at low specific weight and are widely employed for aircraft, wheels and major structural components. However, they suffer from low forming speeds and require high forming pressures. Hence, it is very attractive to form these alloys in semisolid state for technical and economical reasons. In this study, one of the most important wrought alloys for aerospace applications, AA2014 was used. In order to form the best semi-solid forming products, thixotropic character must be known. For this purpose, AA2014 alloy as-cast and warm extruded samples were heated to 600, 610 and 620 °C in semi-solid region and held for 0, 5 and 10 min in the induction coil. All samples were cooled with water at room temperature. Microstructure analysis was conducted with metallographic methods and shape factor values were calculated. With increasing predeformation rate and heating temperature, the shape factor increases. Experimental results show that spherical shaped thixotropic microstructures which is required for semi-solid forming can be created in AA2014 alloy.

Characterization of semisolid deformation behavior and constitutive analysis of Ti230 alloy

The semisolid compressive deformation behavior of Ti230 titanium alloy at the temperature range of 1273 K to 1573 K and strain rate range of 0.005 to 5 s−1 has been investigated by hot compressive testing. The relationship between flow stress and deformation parameters and microstructure were discussed. The results show that the alloy expresses obvious yield point in semisolid state. The maximum decreased with decreasing solid phase fraction and/or decreasing strain rate, and a transition in stress occurred at a solid phase fraction close to 0.95, which is considered to be associated with the decrease in amount of solid bridges between grains and the decomposing effects of the liquid on α-Ti matrix. A deformation constitutive equation for Ti230 alloy in semisolid state was established considering high-temperature deformation activation energy Q. Furthermore, microstructural analysis confirmed that increasing temperature could reduce the deformation-resistant stress and lead to the transformation of softening mechanism from plastic deformation of solid particles (PDS) to sliding between the solid particles (SS).

Deformation characteristic of semi-solid ZCuSn10 copper alloy during isothermal compression

The compression tests were carried out by Gleeble-1500 thermo-mechanical simulator with samples of semi-solid ZCuSn10 alloy prepared by strain-induced melt activation (SIMA) process. The original microstructure and the deformation temperature of semi-solid ZCuSn10 alloy are different. The strain is 0.2, and the strain rate is 1 s−1 for the compression test. The results show that when the semi-solid ZCuSn10 alloy was prepared by SIMA process, the liquid fraction of semi-solid microstructure increases, and the solid grain is smaller, more uniform and more inclined to be round as the rolling pre-deformation increasing. The results also indicate that the deformation resistance of ZCuSn10 alloy in semi-solid state decreases with the deformation temperature increasing or the solid fraction of original microstructure decreasing. The stress–strain curves of the isothermal compression can be divided into quasi-elastic deformation stage and plastic deformation stage, and there are three deformation zones in the samples after isothermal compression, namely the difficult deformation zone, the large deformation zone and the free deformation zone. In the three deformation zones, the main deformation mechanism is flow of liquid incorporating solid particles (FLS) mechanism, plastic deformation of solid particles (PDS) mechanism and liquid flow (LF) combining with FLS mechanism, respectively.

Modelling of Extrusion Process for Aluminium A356 Alloy

In the present work, a model is developed to study extrusion process of A356 alloy in semi-solid state. The distinct rheology of the semisolid alloy reduces energy necessity during extrusion process. Accordingly, a proper rheological model of the alloy is considered in the model towards a detailed study of the process. A combination of analytical and numerical solutions is considered for solving the governing equations. The work finally predicts distribution of velocity and shear stress of the alloy under shear in the considered domain. It also predicts the energy requirement during the extrusion process. It is demonstrated that for semisolid extrusion, reasonably less energy is required as compared to a conventional extrusion process Keywords: Extrusion, semi-solid alloy, apparent viscosity, extrusion power

Effect of Semi-Solid Processing on Iron-Bearing Intermetallic Compounds in A380 Aluminum Alloy

Al-Si-Fe alloys are a general purpose of die casting alloys, widely used to manufacture automotive parts. Forming this alloy in semi-solid state can eliminate important problems in die casting process. In the present work, low superheat melt was employed to produce modified microstructure and non-dendritic A380 alloy feed stocks. The melt was cast on a cooling slope plate at 615 °C to obtain ingots. Then, ingots are thixoformed using a hydraulic press after heating them at 570 °C for 15 minutes, yielding a microstructure with predominantly α–Al globules, Si particles, and modified β-phase intermetallic compounds. The effect of semi-solid processing on the morphology, size, and distribution of iron-bearing intermetallics was studied. The results of image analysis of the samples showed that by using semi-solid method, the shape factor of α-Al phase improved, and iron containing intermetallics were modified in size and distribution. These changes in the aspect ratio and redistribution of the intermetallics improved the mechanical properties such as hardness of the products.

Effects of Pulsed Magnetic Field on the Microstructure and Mechanical Properties of Mg&lt;sub&gt;97&lt;/sub&gt;Y&lt;sub&gt;2&lt;/sub&gt;Zn&lt;sub&gt;1&lt;/sub&gt; Alloy

Effect of pulsed magnetic field treatment on the microstructure and mechanical properties of Mg97Y2Zn1 alloy has been investigated. When the pulsed magnetic field is applied on the alloy in semi-solid state, the α-Mg was modified from developed dendrite to fine rosette, resulting in a refined solidification microstructure with the grain size decreased from 4 mm to 0.5 mm. The volume fraction of the second phase ( X phase) increased by about 10 %. The yield strength, fracture strength and plasticity were improved by 21 MPa, 38 MPa and 2.4 %, respectively. The improvement of mechanical properties was attributed to the refined grain size and increased volume fraction of X phase.