Semi-solid Range Research Articles

Unraveling solid–liquid phase transition and microstructural coarsening of semi-solid Al0.8Co0.5Cr1.5CuFeNi HEA with dual globules for thixoforming application

Multi-phase HEAs typically have potential of semi-solid processing due to wide semi-solid range. For thixoforming route of semi-solid processing (SSP), the semi-solid billet preparation by semi-solid isothermal heat treatment (SSIT) is one of the most important process steps. This work focuses on the microstructure evolution during SSIT of a semi-solid Al0.8Co0.5Cr1.5CuFeNi HEA with dual globules for thixoforming application. This semi-solid HEA contains BCC(B2), FCC globules and Cu-rich liquid film. Solidified liquid follows K-S, epitaxial orientation relationship withneighboringBCC(B2) and FCC globules. Solutionizing point for B2 phase is higher than roughly 1175 °C, and complete disordering of BCC(B2) globule occurs at 1225 °C. While, FCC globule persists stable disordered solid solution. Solid-liquid phase transition of globules provides liquid film proliferation. CALPHAD simulation on the “hypothetical” alloy system corresponding to component constitution of individual globule successfully forecasted solid–liquid phase transition. Coupling coarsening inhibition effects, including sluggish diffusion in globules, Cu-rich liquid film as inhibitory barrier, heterogeneous globule as obstacle, account for extremely sluggish microstructural coarsening (coarsening rate is only 2.44–11.25 μm3/s). Finally, through forming ofexemplar component, intriguing potential for SSP application in component manufacturing was verified.

An investigation into the softening behavior of cast A356 alloy in the solid and semisolid deformation regimes

The current work deals with the hot deformation behavior of A356 aluminum alloy starting with the cast structure and an emphasize on the capability of flow softening. Toward this end, the hot compression tests were performed at temperatures of 150, 250, 350, 450, 500 and 550 °C in the solid range and 580 °C in the semisolid range under the typical strain rate of 0.01s−1. The materials represent high protentional for the occurrence of softening mechanism in solid state regime. During thermomechanical processing (above 250 °C), after reaching the peak stress, the dynamic recrystallization prevails over the deformation behavior and the stress level reaches a steady state regime. The α-dendritic arms are crushed and refined, and the silicon blades are mechanically fragmented and spread irregularly through the microstructure. Being in the semi-solid temperature range, the eutectic silicon phase is melted and causes a sharp decrease in the resistance against deformation. The penetration of the liquid phase along the developed sub-boundaries give rise to grain partitioning and resembles the occurrence of dynamic recrystallization. The presence of the liquid phase during semisolid forming, in addition to the lubricated flow mechanism, facilitates the deformation by providing proper condition for the occurrence of recrystallization. In the present case, a stable stress level as below as 1 MPa was recorded for the experiment cast structure.

Microstructure evolution and formation mechanism of CoCrCu1.2FeNi high entropy alloy during the whole process of semi‐solid billet preparation

• Coarsening kinetic equation describing solid grain growth was established. • Segregation in FCC 1 phase reduced and the lattice constant difference between the two phases decreased after isothermal heat treatment. • During heating up, FCC 1 and FCC 2 phase recrystallized successively, FCC 2 phase was spheroidized to a certain extent. • FCC 2 grains coalesced, grew up and spheroidized and the preferred orientations basically disappeared as heated to semi-solid range. Dual face-centered cubic (FCC) CoCrCu 1.2 FeNi semi-solid billets were prepared by semisolid isothermal treatment of wrought high entropy alloy (HEA) (SSITWH) method, and the microstructure evolution in the whole process of billets preparation was systematically investigated by optical microscopy, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The hot deformed feedstock was mainly composed of deformation structure with preferred orientations and a small number of dynamically recrystallized grains of FCC 1 phase. In the semi-solid stage, the effect of temperature and soaking time on semi-solid microstructure was studied in the range of 1130–1250 °C and 5–120 min. The semi-solid microstructure was evaluated quantitatively. The average grain size and average shape factor increased with the increase of soaking time and isothermal temperature. After isothermal heat treatment, the segregation of Cu in FCC 1 phase reduced to a certain extent. Semi-solid coarsening kinetics analysis showed that the alloy had low coarsening coefficients. When the temperatures were 1130 °C, 1175 °C, 1200 °C, 1225 °C and 1250 °C, the coarsening coefficients were 1.08 μm 3 /s, 5.95 μm 3 /s, 6.17 μm 3 /s, 17.58 μm 3 /s, 38.67 μm 3 /s, respectively. A coarsening kinetic equation describing solid grain growth was established. During heating up, FCC 1 and FCC 2 phase recrystallized successively. At higher temperature, FCC 2 phase was spheroidized to a certain extent. When temperature was raised to semi-solid range, the grains of FCC 2 phase coalesced, grew up and spheroidized and the preferred orientations basically disappeared. The types of semi-solid melting characteristics of HEAs were summarized in this paper. The semi-solid melting behavior of alloys is essentially affected by phase structure, phase number, phase volume content and composition.

Microstructure, Properties, and Numerical Simulation of Semi-Solid Aluminum Alloy under Planetary Stirring Process

In order to solve the problem of insufficient convective heat transfer of uniaxial stirred melt, the temperature field and shear rate of melt under planetary stirring were studied based on CFD simulation. The microstructure and properties of this technology were also experimentally studied. The results show that compared with the uniaxial stirring semi-solid technology, the convective heat transfer ability of aluminum alloy, semi-solid slurry in planetary stirring mode is stronger. In addition, its temperature field can be reduced to the semi-solid range faster and more evenly, which is conducive to a large number of nucleation and improves the nucleation rate. The temperature difference of the whole melt is small, so the preferred direction growth and uniform growth of dendrites are avoided, and the morphology is improved. Properly increasing the revolution and rotation speed of the stirring shaft can refine the grains of semi-solid aluminum alloy parts, improve the grain morphology, and improve the tensile strength. The planetary stirring semi-solid process is very suitable for rheological high-pressure casting.

Effect of a high magnetic field on the macro- and microstructures of Zn-Mg alloy during semi-solid isothermal annealing process

Zn-Mg alloy was isothermally annealed in the semi-solid range under a high magnetic field (HMF). Results showed that, regardless of the HMF, the specimens exhibit inhomogeneous structures, i.e., primary Zn-rich dendrites and granular (nearly spherical or platelike in three dimensions) particles are distributed in the upper and lower parts, respectively. Applying the HMF and decreasing the annealing temperature significantly contribute to the structural homogeneity, which are ascribed to the induced magnetic viscosity resistance force and the higher viscosity of the interdendritic liquid, respectively. Moreover, the HMF aligns the primary dendrites and platelike particles with their longer axes parallel to the HMF direction B. In addition, the HMF preferentially orients all the granular particles with their c-axis perpendicular to B, regardless of their sizes and shapes. The regular alignment and preferential orientation are related to the magnetocrystalline anisotropy of the primary Zn-rich crystals.

Analysis of the 355 Aluminium Alloy Microstructure for Application in Thixoforming

The 355 aluminium alloy is known to have excellent thermodynamic characteristics that render it suitable as a raw material for rheocasting and thixoforming. However, besides the controllable transition from solid to liquid phase, the refined microstructure required in the semisolid range is one of the key factors with a strong influence on the rheology of the material. This paper intends to analyse the in situ behaviour of the microstructure, in terms of morphological change, using high-temperature laser scanning confocal microscopy. The 355 alloy was prepared via conventional casting, and refined with a 30-s exposition via ultrasonic melt treatment (UST - 20 Hz, 2 kW). The material was reheated up to the thixoforming target temperature of 595 °C at which it was maintained for 0, 30, 60, 90, and 120 s, after which all the samples were cooled in water. The samples subjected to UST prior to the heat treatment were more refined in terms of microstructural evolution; they exhibited reduction in grain size (~107 ± 16 μm), smallest primary phase particle size (~81 ± 7 μm), and high circularity shape factor (~0.59 ± 0.19 μm). In situ observation methods were employed to analyse evolution mechanisms such as Ostwald ripening and coalescence.

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.

Microstructural Development of Aluminum A201 Alloy Semi-Solid Processing Feedstock Produced by Different Routes

Thixoforming or Semi-Solid Metal (SSM) forming is a manufacturing route that relies on a non-dendritic microstructure, a structure that is retained after processing resulting in components of fine and uniform microstructures with enhanced mechanical properties as compared to conventional cast products. In this project, an investigation is conducted on the microstructural development of Aluminium A201 alloy in the semi-solid range, through a combination of variable time and temperature conditions. Using equilibrium and non-equilibrium (Scheil) curves, the solidus and the liquidus temperatures as well as the melting behaviour of this alloy are estimated. The temperature range for the thixoforming process typically lies within the 40-60% fraction liquid and this is found to be between 620oC to 637oC for this particular alloy. The aim is to heat treat specimens of this alloy within the semi-solid range (the process window) and observe the resulting microstructures to establish if they comprise of near-spheroidal solid phase particles surrounded by a liquid phase. This microstructure gives the material its thixotropic properties; i.e. the material flows like a liquid when sheared but thickens and behaves like a solid when it is allowed to stand. Three different routes for obtaining the non-dendritic microstructure necessary for thixoforming are investigated in this work: Magneto-Hydrodynamic (MHD) stirring, Strain Induced Melt Activated (SIMA) and Semi-Solid Rheocasting via Cooling Slope (SSR+CS) route. These routes are compared and their potential for future thixoforming applications studied.

Thixoformability and Microstructural Evolution of Al-Si-Cu Alloys

Abstract Commercial aluminium casting alloys such as those from Al-Si-Cu family are widely used in the automotive industry because of their combinations of good fluidity and mechanical strength. Semisolid metal processing utilizing these alloys thus far has given products with better mechanical properties over those produced via traditional metal forming processes. However, there is still demand to widen the range of suitable alloys for the semisolid processing. One way of achieving this is by modifying the alloy compositions. Hence, modified alloys under this family were formulated and assessed. This paper reports results of the thermodynamics evaluation and microstructural evolution for one of these modified alloys, and comparing them with that of the commercially available A319 alloy. Data from Differential Scanning Calorimetry heating curves along the semi-solid range of the alloys were analysed and used as an approximation in determining their thixoformability. The work had demonstrated that the selected modified alloy is suitable for semisolid processing by having wider working window temperature, lower temperature sensitivity of liquid fraction and adequate solidification range. Both alloys, the modified and the commercial A319 alloys, were also successfully thixoformed and their microstructural features and quantitative measures obtained and compared. They both had given the shape factor and solid globule size within the acceptable values for the semisolid processing, i.e. of around 0.7-0.8 and 50-60 µm, respectively. However, in short, the modified alloy has shown better thixoformability characteristics than the commercial alloy.

Fabrication of Al7075–Al2O3np-based metal matrix composites with a high solid fraction for thixoforming

Abstract

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.

A Microstructural Evaluation of Friction Stir Welded 7075 Aluminum Rolled Plate Heat Treated to the Semi-Solid State

Two rolled plates of 7075 aluminum alloy were used as starting material. The plates were welded using a simultaneous double-sided friction stir welding (FSW) process. One way of obtaining feedstock materials for Semi-solid processing or thixoforming is via deformation routes followed by partial melting in the semi-solid state. As both the base plate materials and the friction weld area have undergone extensive deformation specimens were subjected to a post welding heat-treatment in the semi-solid range at a temperature of 628 °C, for 3 min in order to observe the induced microstructural changes. A comparison between the microstructural evolution and mechanical properties of friction stir welded plates was performed before and after the heat-treatment in the Base Metal (BM), the Heat Affected Zone (HAZ), the Thermomechanically Affected Zone (TMAZ) and the Nugget Zone (NZ) using optical microscopy, Scanning Electron microscopy (SEM) and Vickers hardness tests. The results revealed that an extremely fine-grained structure, obtained in the NZ after FSW, resulted in a rise of hardness from the BM to the NZ. Furthermore, post welding heat-treatment in the semi-solid state gave rise to a consistent morphology throughout the material which was similar to microstructures obtained by the thixoforming process. Moreover, a drop of hardness was observed after heat treatment in all regions as compared to that in the welded microstructure.

EFFECT OF RAPID COOLING FROM THE SEMISOLID OR LIQUID RANGE ON STRUCTURE AND PROPERTIES OF M2 TOOL STEEL

M2 tool steel was used as a feedstock material for casting from the the semisolid range between 1310-1350oC corresponding to 25 - 50 % of the liquid phase. The chemical composition of the investigated steel is following: 0.85%C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. A specially constructed machine allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. After heating of the feedstock using induction heating by the stamping of semi-liquid sample to a graphite mould pre-heated by resistance heating. The material after thixoforming from 1315oC shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness significantly increases up to 780 HV after thixo-forming due to formation of martensite. The other part was cast from the liquid phase at 1450o C using melt spinning method allowing high cooling rate. The microstructure of melt spun ribbon shows a cellular microstructure of austenitic and martensitic structure without the eutectic. The WC and M23C6 carbides were located at the cell’s boundariers. The microhardness near 1300 HV of the ribbon was much higher than that of semisolid formed samples.

Microstructural coarsening of 6061 aluminum alloy semi-solid billets prepared via recrystallization and partial melting

In this study, the Recrystallization and partial melting (RAP) process was performed to prepare 6061 aluminum alloy semi-solid billets. The microstructural evolution and coarsening of the materials were investigated. During the RAP process, the 6061 aluminum alloy plate supplied in cold rolled state was heated to the semi-solid range. The results showed that both the average grain size and shape factor of solid grains increased along with the isothermal holding time and temperature. The coarsening of solid grains could be attributed to the coalescence and Ostwald ripening mechanisms, with the latter playing an increasingly important role as the isothermal holding time and temperature increased. At isothermal holding temperatures of 620, 625, 630 and 635 °C, the coarsening rate constants reached 333.1, 833.8, 1034.7 and 1271.9 μm/s3, respectively. Moreover, the coarsening rate constant gradually increased along with isothermal holding temperature.

Critical assessment: opportunities in developing semi-solid processing: aluminium, magnesium, and high-temperature alloys

Semi-solid metal processing (SSM) is a modern metal forming technology offering net-shape metal components of complex geometry in a one-step operation. The process relies on the thixotropic behaviour of metals with non-dendritic microstructures in a wide semi-solid temperature range. The beneficial effects are currently exploited in aluminium and magnesium alloys. This alternative manufacturing route to casting and forging has generated high expectations regarding high melting alloys, although no conclusive results have been achieved so far. Current studies focus on a deep understanding of the fundamental basics of alloys in the semi-solid range, e.g. rheology, microstructural evolution, and the consequences of forming, e.g. relations between process parameters, microstructure, and properties. This critical assessment aims at defining the important needs for further development of SSM, and assessing current knowledge.

Mechanical properties of thixoformed hypoeutectic gray cast iron

The effects of thixoforming process variables on the final mechanical properties of a specially designed hypoeutectic cast iron were studied. An Fe-2.6wt%C-1.5wt%Si alloy was prepared using conventional sand casting molds and thixoformed in an eccentric press. Different temperatures and heat-treatment holding times were tested. After being heated to the semisolid state at 1160°C and 1180°C and held at these temperatures for 0, 30, 60 and 90s, the samples were subjected to the thixoforming process. The holding time in the semisolid range simulates the industrial heating process, which is time controlled rather than temperature controlled. The results indicate that the forces applied during thixoforming are closely related to the liquid fraction of the hypoeutectic gray cast iron. A comparative analysis of the behavior of the tensile properties, Vickers microhardness and porosity before and after thixoforming was carried out. The stress-strain curves were similar for the different liquid fractions studied: at 1160°C the average values were YE=282MPa, UTS=289MPa, E=0.21% and Vickers microhardness=274HV, while at 1180°C the corresponding figures were YE=272MPa, UTS=277MPa, E=0.2% and Vickers microhardness=257HV. The small variations in tensile properties can be attributed to the final graphite morphology of the thixoformed alloy. The results also show that after thixoforming, the porosity of the Fe-2.6wt%C-1.5wt%Si alloy decreased for all experimental conditions and had a maximum value of 5%. There was no significant difference in hardness between the thixoformed material and the as-cast alloy.

Modification of the Cast Structure in a Cu-12.6 Wt Pct Sn Alloy by Heat Treatment in the Semisolid State

The dendritic structure in a cast Cu-12.6 wt pct Sn alloy can be transformed to a globular structure if the alloy is heat treated in the semisolid range. A Cu-12.6 wt pct Sn alloy has a wide solidification range which encourages the development of a cored dendritic structure during solidification. In order to transform it into a globular structure, heat treatment was carried out separately at three temperatures in the semisolid range, namely 1183 K, 1153 K, and 1123 K (910 °C, 880 °C, and 850 °C) with corresponding liquid volume fractions of 50, 40, and 20 pct, respectively. An attempt was made to optimize the temperature and vol pct of liquid combination. It was observed that microstructural modification proceeded mainly through coarsening, fragmentation, and spheroidization of the dendrite arms. Semisolid heat treatment at 1123 K (850 °C) for 2 hours with 20 vol pct liquid yielded the best result. This microstructure was compared with the microstructure developed by solid-state heat treatment at 1023 K (750 °C) for 2 hours. Assessment of the structure–property co-relation demonstrated that hardness, yield stress, ultimate tensile stress, and toughness of the semisolid heat-treated samples were superior to those of subcritically annealed samples.

Experimental and simulation studies on thixoforming of AA 2017 alloy

Thixoforming is one of the near net shaped manufacturing processes which in the product will be formed between the solidus and the liquidus temperatures of the alloy. In the present work, a finite element based simulation study has been performed to simulate the thixoextrusion of AA 2017 alloy in the semisolid range. The alloy has been heated between 570 and 610°C and it was extruded in a die having die cone angles of 30, 45 and 60°. Simulation studies have been performed by using the constitutive relation that was developed by performing disc compression tests at different temperatures and strain rates. A good agreement was found between the experimental and the simulation results.

Investigation on microstructure of Al–25 wt-%Mg2Si composite produced by slope casting and semi-solid forming

The effect of cooling slope casting process and semi-solid forming on the microstructure of Al–Mg2Si composite were investigated. In the current work, in order to obtain a suitable microstructure for semi-solid forming process, including good sphericity of α-Al phase with the equivalent diameter of less than 100 μm, cooling slope method was used. Cooling curve of cast composite was plotted using thermal analysis method. Nucleation temperature of each phase was determined using the first derivative curve and cooling curve. Casting was performed at three different temperatures to find the best temperature for casting on the cooling slope. Approriate temperature for reheating in semi-solid range was determined using solid fraction curve. Samples obtained from cooling slope process were thixoformed. The results showed that the microstructure was improved in the thixoformed samples.

Effects of Cu on the Microstructures and Tensile Properties of Thixoformed Al-Si Alloys

In this study, the effects of copper content on the microstructures and tensile properties of thixoformed Al-5Si-xCu-0.5Fe (x =1.0, 2.0 and 3.0 wt. %) were investigated. For this study, three different alloys having various amounts of copper were prepared using cooling slope casting before thixoforming. The semi-solid liquid range for the alloys were estimated using the diffrential scanning calorimetry (DSC) analysis. The samples were thixoformed at 40% liquid fraction. Some of these samples were treated with a T6 aging process. The thixoformed and thixoformed T6 samples were then characterized by optical microscopy, scanning electron microscope (SEM) and energy dispersive X-ray (EDX) as well as tensile tests. The different phases formed in the thixoformed and thixoformed T6 samples were throughly investigated.The results indicate that as copper content increases, the tensile strength also increases, which might due to precipitation hardening. The thixoformed T6 alloys attained an ultimate tensile strength (UTS) as high as 303 MPa when Cu content is 3 wt%.