Semi-solid Isothermal Treatment Research Articles

Strength-ductility synergy of Mg-7.5 wt.% Sn binary alloys promoted by semisolid isothermal treatment coupled with hot extrusion

In this paper, we propose to adopt a new process for as-extruded Mg-7.5wt.%Sn binary alloy, i.e., semisolid isothermal heat treatment coupled with hot extrusion. Semi-solid isothermal treatment achieved nanosizing of micron-layer lamellar eutectic in the Mg-7.5wt.% Sn binary alloy, while subsequent hot extrusion crushed this nanoeutectic to nanoparticles, resulting in high strength (303 ± 2.8MPa for UTS and 246 ± 0.7MPa for YS,) and high elongation (11.5 ± 0.1% for EL) of the Mg-7.5wt.% Sn binary alloy. Grain refinement, multi-scale particles (broken nanoscale and submicron particles, and dynamically precipitated nanoscale phases), and high-density dislocations are the main contributors to the high strength. Meanwhile, the good ductility of the alloy is promoted by the softening effect due to dynamic recrystallisation as well as the weakening and random distribution of the texture.

Differences in microstructure and properties of Ti-based amorphous composites between recrystallization and partial remelting and semi-solid isothermal treatment

The as-cast specimens of Ti48Zr18V12Cu5Be17 amorphous composites were prepared by copper mold suction casting. Next, the as-cast specimens were treated using semi-solid isothermal treatment (SSIT) and recrystallization and partial remelting (RAP). The effects of SSIT and RAP on the microstructure and plasticity were analyzed. The results showed that the microstructure changed from fine crystals in the as-cast specimens to coarse bar crystals and near-spherical crystals in the SSIT and RAP specimens, respectively. The crystals of RAP specimens were finer and rounder than those of SSIT specimens due to recrystallization. In addition, the RAP specimens had high plasticity (20.93%), which is 428.5% and 45.2% higher than the as-cast and SSIT specimens, respectively. By observing the shear bands of the fractured specimens, it was found that the expansion of shear bands could not be impeded by the fine β-Ti crystals in the as-cast specimens, leading to an infinite extension that induces brittle fracture in the specimens. The essential cause of the poor plasticity of the as-cast specimens was revealed. In addition, the coarse β-Ti crystals effectively blocked the shear band expansion in the SSIT specimens, and a large number of shear bands were generated in these crystals. In contrast, the crystals of the RAP specimens had a greater number and density of shear bands compared to those of the SSIT specimens, and these shear bands intersected with each other in different directions. This revealed the mechanism by which the SSIT and RAP methods enhance the plasticity of amorphous composites.

Achieving strength-ductility synergy in Al-4.5 wt%Cu binary alloy through semisolid isothermal treatment coupled with ECAP

Bulk nanostructured (NS) aluminum alloys typically have high strength but with extremely limited ductility. An advanced two-step thermomechanical processing, i.e. semisolid isothermal treatment coupled with equal-channel angular pressing (ECAP), was adopted to achieve high strength (413±2.5 MPa) and high elongation (12.0±0.6 %) in bulk NS Al-4.5 wt%Cu binary alloy. The broken nanoscale Al2Cu particles (∼50–80 nm) from lamellar eutectic, dynamic precipitations Al2Cu (∼30–60 nm), refined grains, and high-density dislocations are believed to be key contributors for the high strength. The bimodal grain structure, which caused a well-balanced work hardening and a good deformation compatibility, is critical for the enhanced strength-ductility synergy. This work demonstrates a new approach for improving strength-ductility synergy in interdendritic-eutectic type aluminum alloys.

Effect of cross-sectional reduction ratio on microstructure evolution of semi-solid 7075 aluminum alloy prepared by RFSIMA process

Semi-solid billets of 7075 aluminum alloy were fabricated by radial forging strain induced melting activation (RFSIMA) process. Effect of the cross-sectional reduction ratio on microstructure of fabricated 7075 aluminum alloys was analyzed. The results indicate that the average grain size of α-Al decreased from 109μm to 70μm as the cross-sectional reduction ratio increased from 0% to 73% during the 20-minute isothermal treatment at 620°C.The spheroidization trend of the grains became more pronounced; During the semi-solid isothermal treatment, the segregation of Cu at grain boundaries increased slowly with the increase in the cross-sectional reduction ratio. Zn and Mg exhibited uniform distribution without significant segregation at low cross-sectional reduction ratios (43% and below). When the cross-sectional reduction ratio reached 62%, slight segregation of Zn and Mg was observed at the grain boundaries. At a cross-sectional reduction ratio of 73%, the segregation of Zn and Mg at the grain boundaries reached a level similar to that of Cu. The optimal cross-sectional reduction ratio for the RFSIMA process was found to be 62%; the microstructural evolution mechanism of the semi-solid 7075 aluminum alloy prepared by the RFSIMA process was investigated.

A new process for efficient purification of 6061 aluminum alloy scrap under semi-solid and super-gravity conditions

A new method for efficient and sustainable recovery of pure Al from 6061 aluminum alloy scrap based on semi-solid processing and supergravity separation was proposed in this study. Firstly, the microstructural variations of the alloy scraps after semi-solid isothermal treatment (SSIT) were investigated, and the liquid phase accumulated with a large amount of impurity elements would gradually aggregate at the grain boundaries and eventually form a dendritic network, while the spherical Al particles within the grain had high purity. Subsequently, the inter-dendritic liquid was successfully extruded from the intergranular by super-gravity separation technology and the pure Al was efficiently recovered. In this process, the effects of separation time (i.e., isothermal treatment time, t), separation temperature (i.e., isothermal treatment temperature, T) and gravity coefficient (G) on the purification effect were evaluated. The recovery rate of pure aluminum (Al ≈ 99.3 wt%) was 81.98% at the optimum conditions of t = 15 min, T = 640 °C, G = 500. In addition, it was found that increasing the separation time and temperature was beneficial to the purification effect of the alloy scrap but had a negative impact on the recovery rate. Therefore, this study has designed a new process for super-gravity purification and recovery, which realized cyclic recycling and significantly improves the recovery efficiency of aluminum scrap.

The microstructure evolution of Al-5Ce alloy prepared by strain-induced melt activation during semi-solid isothermal treatment process

The coarse grains and rough eutectic microstructure of Al-Ce alloy seriously damage its performance. In this study, Al-5Ce alloys with fine eutectic fibers and bimodal grain distribution were fabricated by strain-induced melt activation (SIMA) process. The results demonstrated that with increasing isothermal temperature and soaking time, the Al11Ce3 phase changes from coarsening to remelting and the α-Al grains size decreases due to decomposition by liquid droplets during the semisolid isothermal treatment. And during the subsequent quenching process, the high cooling rate leads to the solidification of fine α-Al grains and the refinement of eutectic fibers and fiber spacing when the Al concentration exceeds the upper limit of eutectic composition, which forms bimodal grains with fine eutectic fibers.

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.

Microstructural evolution and anisotropic tensile properties of a bimodal 6A02 Al semi-solid billet

The effects of isothermal temperature and duration on the microstructures and anisotropic mechanical properties of bimodal 6A02 Al semi-solid billets were studied. These semi-solid billets were fabricated by semi-solid isothermal treatment of hot extruded aluminum alloy (SSITHEAA) method. Elongated and equiaxed grains owning different dislocation densities coexisted at 610–650 °C for 5–30 min. With the increase of temperature, the intensities of micro-textures and intragranular dislocation decreased. The average Schmid factors in transverse directions were still larger than those along extruding directions, preliminarily predicting anisotropic features from the perspective of grain orientation. The anisotropic features of bimodal semi-solid microstructures were verified firstly by uniaxial tensile tests in 0°, 45°, and 90° directions, among which 0 ° specimen at 610 °C/ 20 min exhibited the highest tensile strength and ductility (yield strength σy of 103.8 MPa, maximum tensile strength σmax of 286.9 MPa, and elongation δ of 22.0%). Different from microstructures at solid temperatures, recrystallization degraded mechanical strength of semisolid microstructures to some degree once the grain boundaries were soaked with liquid phase. The dislocation strengthening inside the deformed grains and solid solution strengthening were two dominant strengthening mechanisms. Besides, the elongated grains improved the mechanical properties through dislocation pileup inside the grains, “bridging” effect, “necking” effect and decreasing the content of both flaws and liquid phase. The in-plane anisotropy (IPA) factor indicated that the elongation bore the most serious anisotropy. On the whole, the isotropy of 650 °C/20 min billet was the best, and the corresponding IPA values of σy, σmax and δ were 21.9%, 26.2%, and 47.0%, respectively. For 0° specimens, ductile fracture was dominant, but brittle characteristic was increasingly significant for 45° and 90° specimens.

Effects of temperature and time on three-dimensional microstructural evolution of semi-solid 2A14 aluminum alloy during short process preparation of semi-solid billets

To shorten the preparation process of semi-solid billets, semi-solid billets of 2A14 aluminum alloy were prepared by wrought aluminum directly semi-solid isothermal treatment (WADSSIT) process. Three-dimension (3D) combined microstructure evolution, namely transverse direction (TD) surface, rolling direction (RD) surface, and normal direction (ND) surface, was studied. Effects of temperature and holding time on average grain size and average shape factor were investigated. The results showed that the optimum conditions for preparation of 2A14 semi-solid billets by this process were 615 °C and 20 min (average grain size of 124 μm and shape factor of 0.81). Electron backscatter diffraction (EBSD) observations indicated that the microstructure was completely recrystallized when it was heated to 600 °C. Grain size was increased with the increase of temperature and grew up slowly with the holding time prolonging. Roundness was increased with increase of holding time but was not sensitive to temperature.

The Effect of CRITSIMA Process Parameters on the Microstructure Evolution and Element Segregation of Semi-Solid CuSn10P1 Alloy Billet

In this paper, based on the as-cast CuSn10P1 alloy. Semi-solid CuSn10P1 alloy billet was prepared by cold-rolled isothermal treatment strain-induced melting activation (CRITSIMA). The effects of cold-rolling reduction, isothermal temperature, and isothermal time on the microstructure of semi-solid copper alloy billet were studied by metallographic microscope and Image-Pro Plus software. The changes of primary elements in as-cast and semi-solid microstructure were analyzed briefly by a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The results show that with the increase of cold rolling reduction, the average grain diameter of semi-solid microstructure decreases gradually, the average grain roundness increases first and then decreases, and the liquid fraction of the microstructure remains unchanged. During semi-solid isothermal treatment, with the increase of isothermal temperature and the extension of isothermal time, the average grain diameter increases gradually, the average grain roundness increases first and then decreases, and the liquid fraction increases gradually. When cold rolling reduction is 30%, isothermal temperature is 900 ℃, and isothermal time is 20 min, a better microstructure can be obtained. The average grain diameter, average grain roundness, and liquid fraction of semi-solid alloy billet are 66.45 μm, 0.71, and 12.78%, respectively. Sn and P diffuse from the intergranular liquid to the grain inside during the isothermal treatment from as-cast to semi-solid.

Short-Term Oxidation Behavior, Microstructure Evolution and Compression Behavior of Nickel-Based Superalloy GH4037 in Solid and Semi-Solid States

Semi-solid processing combines the advantages of traditional forging and casting methods, so it has received much attention recently. However, the research on semi-solid behaviors of Nickel-based superalloys has been rarely reported. In order to investigate the behaviors of Nickel-based superalloy at solid and semi-solid states, oxidation experiments, isothermal treatment experiments and deformation experiments of GH4037 alloy were studied. Short-term oxidation experiments of GH4037 alloy were carried out at a solid temperature (1200 °C) and a semi-solid temperature (1360 °C). The results indicated that the oxides formed at 1200 °C were mainly composed of TiO2, Cr2O3 and a small amount of spinels NiCr2O4, while the oxides formed at 1360 °C consisted of the spinels of NiCr2O4, NiWO4 and NiMoO4 besides TiO2 and Cr2O3. Microstructure evolution of GH4037 alloy after semi-solid isothermal treatment at 1370 °C and 1380 °C was studied. The results indicated that semi-solid microstructures consisted of equiaxed solid grains and liquid phases. The average grains size and shape factor of solid grains were affected by melting mechanism and grain growth mechanism. Compression behaviors of GH4037 alloy after compressed at 1200 °C and 1360 °C were investigated. The results indicated that the flow stress of 1360 °C decreased significantly compared to that of 1200 °C. The deformation zones in the specimens were divided into three parts: the difficult deformation zone, the large deformation zone, and the free deformation zone. At 1200 °C, the deformation mechanism was plastic deformation mechanism. At 1360 °C, sliding between solid particles (SS), liquid flow (LF), flow of liquid incorporating solid particles (FLS), plastic deformation of solid particles (PDS) coexisted in the compression specimen.

Achieving nanoscale Al2Cu dispersoids in wrought Al-4.5wt.%Cu alloy by semi-solid isothermal treatment and post room-temperature ECAP

Al2Cu phase is the main strengthening phase of high-strength wrought Al-Cu based alloys. In this paper, a novel composite process coupling the semi-solid isothermal treatment and post room-temperature (RT) equal channel angular pressing (ECAP) is proposed to refine Al2Cu particles in a conventionally extruded Al-4.5wt.%Cu alloy. The results show that, after the optimized semi-solid treatment (610℃/10 min + 20℃ water quenching) the initial microscale Al2Cu particles transformed into lamellar eutectic at grain boundaries with a thickness of 70 ± 9 nm. The following ECAP broke the lamellar eutectic into nanoscale Al2Cu dispersoids, which significantly enhanced the tensile strength. As a result, the alloy subjected to the composite process exhibited the highest RT ultimate tensile strength(UTS) of 460 MPa and yield strength(YS) of 447 MPa compared with the initial extruded and direct ECAPed alloys.

Effects of Isothermal Temperature and Soaking Time on Water Quenched Microstructure of Nickel-Based Superalloy GH3536 Semi-Solid Billets.

Semi-solid billets of GH3536 alloy were prepared by semi-solid isothermal treatment of wrought superalloy method. GH3536 samples were soaked at several semi-solid temperatures (1350 °C, 1360 °C, 1364 °C, and 1367 °C) for 5–120 min. The effects of temperature and soaking time on the microstructure of GH3536 billets were studied. The results indicated that the microstructure was affected by coalescence mechanism, Ostwald ripening mechanism, and breaking up mechanism. Semi-solid microstructure of GH3536 alloy was composed of spherical solid particles and liquid phases, and the liquid phases affected the microstructure greatly. At 1350 °C, the coalescence mechanism was dominant at the early stage of isothermal treatment, then the Ostwald ripening mechanism played a major role for the longer soaking times. At higher temperatures, the breaking up mechanism occurred to form large irregular grains and small spherical grains. As the heating continued, the Ostwald ripening mechanism was dominant. However, at 1364 °C and 1367 °C, the solid grains had irregular shapes and large sizes when the isothermal time was 120 min. The optimum parameters for the preparation of GH3536 semi-solid billets were: temperature of 1364–1367 °C and soaking time of 60–90 min.

Spheroidizing Process of 2A12 Aluminum Alloy Grains during Heating up and Semisolid Isothermal Treatment Stages

Spheroidizing Process of 2A12 Aluminum Alloy Grains during Heating up and Semisolid Isothermal Treatment Stages

Deformation Characteristics and Constitutive Equations for the Semi-Solid Isothermal Compression of Cold Radial Forged 6063 Aluminium Alloy.

Al-Mg-Si based alloys are popular alloys used in the automotive industry. However, limited studies have been performed to investigate the microstructure, deformation characteristics, and deformation mechanism for the semi-solid 6063 alloys. In this study, the cold radial forging method and semi-solid isothermal treatment (SSIT) are proposed in the semi-solid isothermal compression (SSIC) process to fabricate high-quality semi-solid 6063 billets. The effects of deformation temperature, strain rate, and strain on the microstructure, deformation characteristics, and deformation mechanism of the SSIC of cold radial forged 6063 alloys were investigated experimentally. Constitutive equations were established based on the measured data in experiments to predict the flow stress. Results show that an average grain size in the range from 59.22 to 73.02 μm and an average shape factor in the range from 071 to 078 can be obtained in the microstructure after the cold radial forged 6063 alloys were treated with SSIT process. Four stages (i.e., sharp increase, decrease, steady state, and slow increase) were observed in the true stress- true strain curve. The correlation coefficient of the constitutive equation was obtained as 0.9796 while the average relative error was 5.01%. The deformation mechanism for SSIC of cold radial forged aluminum alloy 6063 mainly included four modes: The liquid phase flow, grain slide or grain rotation along with the liquid film, slide among solid grains, and the plastic deformation of solid grains.

Shear bands of as-cast and semi-solid Ti48Zr27Cu6Nb5Be14 bulk metallic glass matrix composites

The as-cast Ti48Zr27Cu6Nb5Be14 bulk metallic glass matrix composites (BMGMCs) were fabricated using a copper mold suction casting method. Then, the semi-solid BMGMC samples were obtained following an isothermal treatment (heating at 900 °C for 10 min, then cooling with water). The microstructure and compression property were investigated by scanning electronic microscopy (SEM) and universal mechanical tester. As a result of the isothermal treatment, the crystal shapes change from fine, granular, and dendritic to spherical or vermicular, and the average crystal size of the as-cast and semi-solid samples is 2.2 µm and 18.1 µm, respectively. The plasticity increases from 5.31% in the as-cast to 10.23% in the semi-solid samples, with an increase of 92.66%. The shear bands from different areas of the side surfaces of as-cast and semisolid compression fracture samples were observed. The characteristic changes of multiplicity, bend, branch and intersection of shear bands in different areas indicate that the deformation of as-cast and semi-solid samples is non-uniform during compression. It is found that poor plasticity of the as-cast samples or good plasticity of the semi-solid samples are reflected by characteristics of the shear bands. The semi-solid isothermal treatment improves the plasticity by forming large crystals which can block the expansion of shear bands and promote the multiplicity of shear bands.

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.

The Microstructural Evolution of a Cold Rolled AlSi9Mg Aluminum Alloy after Semi-Solid Isothermal Treatment Process

The key factor in semi-solid metal forming process is the preparation of semi-solid billet with fine and spherical microstructure. The recrystallization and partial melting (RAP) process consists of cold deformation and semi-solid isothermal treatment (SSIT) is a promising method, because that many alloys are always supplied in the deformed state. In this work, a cold rolled AlSi9Mg aluminum alloy was used directly to fabricate semi-solid billets. During the RAP process, samples cut from the cold rolled AlSi9Mg aluminum alloy were treated with different isothermal holding temperatures and times, and the effects of isothermal treatment parameters on the microstructures of AlSi9Mg alloy semi-solid billets were investigated. The results show that, with increasing the isothemal holding temperature and time, the average grain size of solid grains is gradually increased, the spheroidization degrees of solid grains are sharply increased and then follows by a slight improvement. The high-quality semi-solid billets can be prepared with SSIT process at 570 °C for 5~10 min.

Investigation of the Microstructure and Mechanical Properties of AZ31/Graphene Composite Fabricated by Semi-solid Isothermal Treatment and Hot Extrusion

Magnesium matrix composites reinforced with 0.3 wt.% graphene were fabricated by semi-solid isothermal treatment using various reheating conditions and hot extrusion. The microstructures of the hot-extruded AZ31/graphene composites were clearly refined, with the finest grain size (7.05 µm) achieved after reheating at 610°C for 30 min. Energy-dispersive x-ray spectroscopy analysis revealed that increasing the reheating temperature promotes diffusion of the solute elements and affects the mechanical properties of the composite. The optimal mechanical properties were achieved after reheating at 620°C for 30 min, with a yield strength of 214.82 MPa and an ultimate tensile strength of 310.79 MPa. The significant improvement in the mechanical properties of the composite was mainly attributed to the refined grain size, uniformly redistributed solute elements, addition of graphene, and close interfacial bonding.

Influence of reheating temperature on the microstructures and tensile properties of a short-carbon-fiber-reinforced magnesium matrix composite

The present work investigates the effect of the reheating temperature on the microstructure evolution and tensile properties of a magnesium composite reinforced by short carbon fibers. This composite was fabricated by semi-solid isothermal treatment and hot extrusion. The results indicate that the dynamic recrystallization grain size of the composite decreases with increasing reheating temperature, and there is strong interfacial bonding between the short carbon fibers and magnesium matrix. After reheating at 610 °C for 30 min, the strength of the composite is significantly improved, particularly the yield strength. The refined grains and good interfacial bonding are responsible for the excellent strength of this composite.