Semi-solid Isothermal Heat Treatment 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.

High-ductility fine-grained Mg-1.92Zn-0.34Y alloy fabricated by semisolid and then hot extrusion

The combination of semisolid and hot extrusion processing was applied to refine the icosahedral quasicrystalline phase (I-phase) in an extruded Mg-1.92Zn-0.34Y(wt.%) alloy for the first time. The semisolid isothermal heat treatment transformed the micron-sized I-phase particles into nano lamellar eutectic (α-Mg + I-phase) with a lamellar spacing of ~86 nm. After subsequent hot extrusion at 250 °C, the nano lamellar eutectic phases were broken into uniformly dispersed nanoscale I-phase particles. What's more, the matrix microstructure was significantly refined with an equiaxed average grain size of 2.59 ± 0.81 µm, and an unusual texture component (most of the grains’ c-axis is parallel to the extrusion direction) was observed. The processed alloy exhibited a high tensile elongation to failure (EL) of 44 ± 2.6% with an ultimate tensile strength (UTS) of 258 ± 2.0 MPa and a tensile yield strength (TYS) of 176 ± 1.6 MPa at room temperature. The high ductility from the combined effects of the grain refinement, dispersion of nanoscale I-phase particles, and the unusual texture. The uniform dispersion of nanoscale I-phase particles could promote grain refinement by particle stimulated nucleation mechanism, and thus bring the unusual texture (where the c-axis is aligned parallel to the extrusion direction during dynamic recrystallization, which contributed to ductility).

Microstructure Evolution of 2A12 Aluminum Alloy under Isothermal Heat Treatment Direct Writing Process

A metal-melting direct writing process, using semi-solid isothermal heat treatment to form high-quality semi-solid components, realized the integrated innovation of semi-solid formation and additive manufacturing. An experimental study was carried out on semi-solid isothermal heat treatment for metal-melting direct-writing technology, using 2A12 aluminum alloy as raw material. The semi-solid isothermal heat treatment was carried out over different temperature ranges, and four-stages evolution mechanism of the semi-solid microstructure in the semi-solid melting direct writing process was investigated. The effects of holding temperature and time on the microstructure of the semi-solid isothermal heat treatment of the alloy were put forward. According to the analysis of the results of the semi-solid-melting direct-writing test, the corresponding relationship between semi-solid microstructure and extrusion formability was found. The results show that when the holding temperature is 640–650 °C and the holding time is 20–25 min, the liquid phase rate can reach about 40%, and the direct-writing forming technology can be carried out stably.

Effect of La addition on semi-solid microstructure evolution of Mg-7Zn magnesium alloy

Semi-solid billets of Mg-7Zn and Mg-7Zn-0.3La alloys were prepared by semi-solid isothermal heat treatment. The effects of the La element on the as-cast and semi-solid microstructures of Mg-7Zn alloy were investigated. Meanwhile, the effects of isothermal temperature and holding time on the evolution of the semi-solid microstructure of Mg-7Zn-0.3La alloy were also studied. Results indicate that the addition of a small amount of La can significantly refine the as-cast and semi-solid microstructure. During the semi-solid thermal transformation, the size and shape factor of solid particles decrease at first and then increase with the increase of isothermal temperature and holding time. The semi-solid microstructure of Mg-7Zn-0.3La alloy obtained by holding at 605 °C for 30 min is the optimal. The average size of solid particles, shape factor, and solid fraction are 42 µm, 1.45 and 61.8%, respectively. At the same time, a comparative study on the coarsening process of particles in the semi-solid billets of Mg-7Zn and Mg-7Zn-0.3La alloys reveals that the addition of La effectively decreases the coarsening rate of solid particles and restricts the growth of solid particles.

Direct preparation of semi-solid billets by the semi-solid isothermal heat treatment for commercial cold-rolled ZL104 aluminum alloy

Direct preparation of semi-solid billets by the semi-solid isothermal heat treatment for commercial cold-rolled ZL104 aluminum alloy

Effect of ECAP process on liquid distribution of AZ80M alloy during semi-solid isothermal heat treatment

Two kinds of semi-solid samples of AZ80−0.2Y−0.15Ca (wt.%) (AZ80M) magnesium alloy were prepared by semi-solid isothermal heat treatment of materials with and without equal channel angular pressing (ECAP) process. The microstructures of initial and semi-solid treated samples were compared and analyzed. The results showed a significant difference in the liquid phase distribution between three-pass ECAP processed (3P) and as-received samples during the isothermal heating process. The semi-solid 3P sample showed a more uniform liquid distribution due to its smaller dihedral angle. Besides, the coarsening processes of solid grains of as-received and 3P samples were dominated by the coalescence and Ostwald ripening mechanism, respectively. The difference of coarsening processes was mainly related to the proportion of the high-angle grain boundaries in materials, which further affected the evolution behavior of the liquid pools.

Microstructure and Gd-rich phase evolution of as-cast AZ31-xGd magnesium alloys during semi-solid isothermal heat treatment

The microstructure and Gd-rich phase evolution of as-cast AZ31-xGd (x=0, 1.5 wt.%, 2.0 wt.% and 2.5 wt.%) magnesium alloys during semi-solid isothermal heat treatment were investigated deeply in the present work. Results showed that the lamellar (Mg,Al)3Gd phases transformed into the particle-like Al2Gd phases in AZ31 magnesium alloys with Gd addition during semi-solid isothermal heat treatment, leading to yielding more spherical α-Mg grains. When Gd content is 2.0 wt. %, the size of semi-solid spherical grains reaches the minimum. The main mechanism of grain refinement lies in the remelting of dendritic branches as well as the auxiliary effect of a small number of Al2Gd particles as grain refining inoculants. Meanwhile, Al2Gd particles enriched at the solid-liquid interfaces can remarkably retard the growth rate of α-Mg grains. A reduction of deformation resistance has been successfully achieved in AZ31-2.0Gd magnesium alloy after semi-solid isothermal heat treatment, which shows a moderate compressive deformation resistance (230 MPa), comparing to the as-cast AZ31 magnesium alloy (280 MPa) and semi-solid AZ31 magnesium alloy (209 MPa).

Effect of Microstructures and Mechanical Properties of Large Deformation High Entropy Alloy CoCrCuFeNi in Semi-solid Isothermal Heat Treatment

The semi-solid slurries of the CoCrCuFeNi high entropy alloy (HEA) were fabricated through the recrystallization and partial melting (RAP) process by cold-rolling and partial remelting. The temperature range of the semi-solid region and the relationship between the liquid fraction and the temperature were determined by the differential scanning calorimetry (DSC) curve. The effect of isothermal temperature and holding time on the evolution of the microstructure and mechanical properties of the rolled samples was analyzed. The results show that the microstructure was significantly deformed, and the tensile strength has been increased by 107% after 63% rolling deformation of the CoCrCuFeNi high entropy alloy (HEA). The high-entropy alloy after cold rolling was maintained at 1150 and 1300 ° C for 20, 30, 60, and 120 minutes respectively, the plasticity has been improved compared with the rolled high entropy alloy. The optimal plasticity was reached 13.7% and 7.9% at 1150 ℃ and 1300℃ for 30 minutes, respectively. After semi-solid isothermal heat treatment, the grain morphology changed from dendritic of as-cast or rolled to spherulite and the grain size increased significantly with time and the holding temperature increased.

The Influence of Holding Time on the Microstructure Evolution of Mg–10Zn–6.8Gd–4Y Alloy during Semi-Solid Isothermal Heat Treatment

A semi-solid microstructure of Mg–10Zn–6.8Gd–4Y alloys is acquired via an isothermal heat treatment process, and the effects of the holding time on the microstructure evolution of Mg–10Zn–6.8Gd–4Y alloys are investigated. The results show that the microstructure of the cast alloy is composed of primary α-Mg dendritic grains with a eutectic structure (W-phase and eutectic Mg) distributed at the grain boundaries. The primary α-Mg dendritic grains grow in size with increasing holding time, and they tend to grow into more globular structures in the initial stage; they then become a bit more dendritic, as small branches grow from the grain boundaries after holding the sample at 580 °C for 10 min. Meanwhile, the interdiffusion of magnesium atoms within the eutectic region, and between the primary α-Mg and eutectic structure, leads to the formation of fine and relatively globular eutectic Mg grains in the eutectic structure after holding for 10 min. The eutectic Mg grains begin to grow, coarsen, coalesce, or be swallowed by the surrounding primary grains, causing fluctuations of the general grain size. Over the whole isothermal heat treatment process, two mechanisms—coalescence and Ostwald ripening—dominate the grain coarsening.

Effects of trace Cr on as-cast microstructure and microstructural evolution of semi-solid isothermal heat treatment ZC61 magnesium alloy

The content and kind of trace elements in magnesium alloys have important effects on their ascast and semi-solid microstructures. In this research work, effects of trace Cr on as-cast and semi-solid microstructures of ZC61 magnesium alloy were investigated by metal mold casting and semi-solid isothermal heat treatment. The results show that the addition of Cr can refine the α-Mg phase without generating a new phase, noticeably change the eutectic phase, and decrease the average size of solid particles at the same isothermal heat treatment conditions. Non-dendritic microstructures of all alloys are constituted of α1-Mg phases, α2-Mg phases and eutectic phases after water quenching. With isothermal temperature increased or holding time prolonged, the eutectic microstructure (α-Mg+MgZn2+CuMgZn) at the grain boundaries in as-cast alloy is melted preferentially and then turned into semi-solid non-dendritic microstructure by processes of initial coarsening, microstructure separation, spheroidizing and final coarsening. Especially when the ZC61-0.1Cr alloy was treated at 585 °C for 30 min, the ideal non-dendritic microstructure can be obtained, and the corresponding solid particle size and shape factor were 37.5 μm and 1.33, respectively. The coarsening process of solid α-Mg phase at higher temperature or longer time, which is affected by both combining growth and Ostwald ripening mechanism, is refrained when Cr is added to the ZC61 alloy.

Effect of Semi-solid Isothermal Heat Treatment on Microstructure of 7075 Alloy

Effect of Semi-solid Isothermal Heat Treatment on Microstructure of 7075 Alloy

Evolution and distribution of Al 2 Sm phase in as-extruded AZ61– x Sm magnesium alloys during semi-solid isothermal heat-treatment

Abstract The evolution and distribution of Al2Sm phase in as-extruded AZ61–xSm (x=0, 1.5, 2.0 and 2.5, mass fraction, %) magnesium alloys during semi-solid isothermal heat treatment were investigated. The results showed that when as-extruded AZ61 magnesium alloys were modified with Sm, the smaller and rounder grains were obtained during semi-solid isothermal heat treatment. When the Sm content is 2.0% (mass fraction), the average size of the globular grains reached the smallest value of 90 μm. Although a few Al2Sm particles existed in the α-Mg grains, most of Al2Sm particles solidified at the edge of the globular grains with the width of ~20 μm. These phenomena are mainly attributed to the forces acting on Al2Sm particles in front of the solid–liquid interface, leading to Al2Sm particles accumulating at the solid–liquid interface and then solidifying at the edge of the globular grains in the quenching process.

Effects of Semi-solid Isothermal Heat Treatment on Microstructures and Damping Capacities of Fly Ash Cenosphere/AZ91D Composites

The fly ash cenosphere/AZ91D composites were successfully prepared and isothermally heat-treated at different temperatures for different time. The effects of semi-solid isothermal heat treatment on the microstructures and damping capacities of fly ash cenosphere/AZ91D composites were investigated. With the increase in isothermal temperature or holding time, the small liquid droplets within grains increased in size but decreased in quantity. The average size and shape factor of Mg2Si particles increased with the rise of isothermal temperature. The damping capacities of the composites were improved by isothermal heat treatment. At room temperature, the composites after heat treatment at 520 and 550 °C had a higher damping capacity due to interface damping when the strain amplitude was lower than about 8.8 × 10−5, and the composite after heat treatment at 580 °C had a better damping capacity because of the dislocation damping under the condition of high strain amplitude. The damping capacities of the composites increased with the rise of the test temperature, and the damping mechanisms varied depending on different test temperatures. The interface damping played an important role when the test temperature was below about 100 °C, and the dislocation damping and grain boundary damping took effect with the rise of test temperature.

Effects of Sm addition on microstructural evolution of Mg-6Zn-0.4Zr alloy during semi-solid isothermal heat treatment

The application of segmental semi-solid thixoforming of magnesium alloys is confined due to the dimensional distinction existing in solid particles of the alloy billet from edge to center zones. In the present study, the effects of Sm addition on the microstructural evolution of Mg-6Zn-0.4Zr and Mg-6Zn-4Sm-0.4Zr alloys by semi-solid isothermal heat treatment were investigated, to obtain optimum semi-solid microstructures for the subsequently thixoforming. The results indicate that the grains of the Sm-bearing alloy are evidently refined and gradually evolve from dendritic to globular and elliptic particles. In addition, the distinctly dimensional effect of the Mg-6Zn-0.4Zr alloy is eliminated with 4% Sm addition; the particle sizes in all zones from center to the edge of the billet are almost identical. With the increment of isothermal heat treatment temperature, the dendritic microstructures completely disappear, and meanwhile, the irregular and globular particles gradually form. The size, morphology and the distribution of solid particles mainly depend on the formation and permeation of the liquid phase in the process of isothermal heat treatment. As the isothermal temperature increases from 570 °C to 610 °C, the average size and shape factor of solid particles of both the alloys with and without Sm addition gradually decrease while the liquid fraction gradually increases.

Microstructure Evolution of Semisolid Mg-2Zn-0.5Y Alloy during Isothermal Heat Treatment

Microstructure evolution of extruded Mg-2Zn-0.5Y (at%) alloy in semisolid isothermal heat treatment process was investigated. The results show that in the alloy there are α -Mg, Mg3-Zn6-Y1 (I-Phase) and Mg3-Zn3-Y2 (W-phase), and the average grain size of α -Mg is 7 μm. During the isothermal heat treatment at 793 K, the grains continue growing and no liquid phase appears until isothermal holding time reaches 4.0 min. With isothermal temperature rising and holding time increasing, solid α -Mg grains in the alloy gradually grow and are separated by liquid phases. Meanwhile, the liquid phase locating both at the grain boundaries and in the grain interior significantly increases. It is also found that at higher solid fractions, particle coarsening mechanism and solid particle remelting mechanism both play major roles. While at lower solid fractions, Ostwald ripening is the major mechanism and coalescence of adjacent particles could be still observed.

Porosity Defects Induced by Semi-Solid Isothermal Heat Treatment in Cast Hypereutectic Al-18% Si Alloy

The Present study was undertaken to investigate the porosity defects induced by semi-solid isothermal heat treatment in cast hypereutectic Al-18% Si alloy for different holding time. All specimens were heated up to 585 °C then hold for 10, 20, 30 and 40 min. in an electrically heated resistance furnace with heating rate of 10 °C.min-1. After the semi-solid heat treatment, the samples were taken out immediately for water quenching. Bulk density, total porosity and pore sizes at center and edge of each treated sample were investigated. It was found that the porosity in final treated samples increases by increasing the holding time. The pore size increases and the density decreases steeply with increasing the semi-solid isothermal holding time. Above 20 min., holding time the pore size at the edge shows higher value compared with that measured at the center of sample. It might be recommended that shorter holding time during semi-solid isothermal heat treatment should be selected in order to obtain minimum porosity percentage and higher density Al-18% Si alloy castings.

SEMI-SOLID HEAT TREATMENT OF HYPEREUTECTIC AL-18% SI ALLOY CONTAINING IRON-RICH INTERMETALLICS

In this research the effect of semi-solid heat treatment on matrix structure and iron-rich intermetallics of hyper-eutectic Al-18% Si containing 0.95%Fe is studied. Semi-solid heat treatment at heating temperature of 585 C shows a significant refinement of ironrich intermetallic phase. In as cast samples ,needle-like iron-rich intermetallics phases was observed, otherwise, plate-like and fine plate-like iron-rich intermetallics phases was observed for all samples heat treated in semi-solid state. Hardness values for semi-solid heat treated Al-18% Si Alloy for all heating time range (10-40 min) are relatively higher compared with the as cast one. Hardness value of 72.5 HRB is achieved by heating Al-18% Si alloy at heating temperature of 585 C for 20 min. At the early stages of heating time (up to 20 min), a significant decreases of weight loss is observed in Al-18% Si alloy when semi-solid isothermal heat treatment was applied at 585 C heating temperature. However, semi-solid isothermal heat treatment performed above heating time of 25 min might result in increases of weight. The optimum semi-solid heating treatment condition was achieved at the temperature of 565C for the range of 15 to 25 mins.

Influence of Semi-Solid Isothermal Heat Treatment on Microstructure and Mechanical Properties of Ductile Cast Iron

In this research, the effect of semi-solid isothermal heat treatment on microstructure, hardness and impact toughness of ductile iron (DI) is studied. Heat-treated air-cooled ductile iron shows spheroid graphite, cementite and fine pearlite matrix structure. At the early stages of heating time (up to 20 min), the DIs show significant decrease in amounts of graphite and significant increase in amounts of cementite. By increasing heating time, above 20 min, the DIs show slightly decrease in amounts of graphite and slightly increase in amounts of cementite. Hardness values increase by increasing heating time. Meanwhile, the impact toughness decrease with increasing heating time. The optimum heating treatment condition for reasonable structure and mechanical properties could be achieved at the temperature of 1165℃ for the heating time range of 10 to 15 min.

Effect of Holding Time on the Microstructure of Mg-3Si-1.70Sn Alloy during Semisolid Isothermal Heat Treatment

The microstructural evolution behavior of Mg-3Si-1.70Sn alloys during semisolid isothermal heat treatment was investigated by optical microscopy observations in the present study. With the holding time increasing, the grains evolve to be more spheroidal and 15min is adequate for the alloy to be spheroidized, after this coarsening begins to happen. The amount of the liquid islands inside α-Mg grains decreases and that of the liquid phase at the grain boundaries increases with the prolonged holding time. Moreover, the morphology of eutectic Mg2Si phases changes from the Chinese script and/or long-fiber shape to short fiber or spherical shapes during semisolid isothermal heat treatment, which can be attributed to the Rayleigh shape instability.

Influence of Semi-Solid Isothermal Heat Treatment on Microstructure of Gray Cast Iron

Materials serve as an enabling technology contributing to solutions in problems of concern to society. In this research, the effect of semi-solid isothermal heat treatment on graphite morphologies and matrix structure for gray cast iron were studied. Microstructure observations and measurements of flaky graphite morphology are reported as a function of isothermal heating time range of 5 to 25 minutes after isothermal heating temperature at 1163℃. The effect heating time, on the semisolid microstructures during partial re-melting was investigated. Flack graphite morphology was changed significantly by isothermal heating of gray cast iron at 1163℃ for heating time above 15 min resulting fine graphite morphology in matrix structure. Hardness increases with increasing heating time due to the amount of cementite and fine pearlite matrix for air cooled gray cast iron. The optimum heating treatment condition was achieved at the temperature of 1163℃ for the range of 15 to 20 min.