Semi-solid Heat Research Articles

Microstructural Evolution of Cast Hyper-Eutectic Al-18% Si Alloy during Cyclic Semi-Solid Heat Treatment

The cyclic semi-solid heat treatment represents a promising technique for improving microstructure and mechanical properties of a wide range of metallic alloys. In the current research the influence of cyclic semi-solid heat treatment on microstructure of Al-18% Si alloy containing 0.8% Fe has been studied. All specimens were heated in an electrically heated resistance furnace with heating rate of 10˚C∙min −1 to 585˚C. For a complete one cycle heat treatment (5 min heating time), samples after 5 min holding at 585˚C were cooled to a temperature of 550˚C in still air cooling and the samples were taken out immediately for water quenching. It was found that heat treatment cycles should be limited to 3 cycles or less in order to maintain fine grain size and globular structure without agglomeration and coalescence. Cyclic semi-solid heat treatment changes morphology of iron-rich intermetallics phases to be plate-like and fine plate iron-rich intermetallics phases, in stead of needle-like iron-rich intermetallics phases that are observed in as-cast samples. Cyclic heating shows a relatively higher hardness for all heating cycles compared with as-cast one due to its finer and globular structure. Cyclic semi-solid heat treatment technique results in lower coarsening rate constant compared with isothermal heat treatment one due to coarsening discontinuous effect.

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.

Effects of dynamic recrystallisation during deep rolling of semisolid slab and heat treatment on microstructure and properties of AZ31 alloy

A novel process of manufacturing high performance AZ31 alloy strip was put forward by combining semisolid rolling, deep rolling and heat treatment, and the effects of dynamic recrystallisation during deep rolling of semisolid slab and heat treatment on microstructure and properties of AZ31 alloy were investigated. When the casting temperature was set from 650 to 690°C and the vibration frequency was controlled at 80 Hz, AZ31 alloy strip with the cross-section size of 4×160 mm and fine near round and rosette grains was manufactured by semisolid rolling. Dynamic recrystallisation happened during deep rolling process. Dynamic recrystallisation grain size decreases with the increase in deep rolling temperature in a certain scope. With the increase in deep rolling deformation amount, the microstructure evolved gradually from coarse primary grains and twins to fine dynamic recrystallisation grains and twist primary grains as well as small regular equiaxed grains. AZ31 alloy strip with very fine equiaxed grain in size of 6 μm was obtained by deep rolling. After solution treatment at 415°C for 20 h, the saturated solid solution of AZ31 alloy strip has been formed. After solution treatment at 415°C for 20 h and aging at 230°C for 16 h, the deep rolled AZ31 alloy strips exhibit excellent mechanical properties; the ultimate tensile strength, yield strength, elongation to failure and hardness could reach 330 MPa, 204 MPa, 11% and 71 HV respectively.

Microstructure and properties of A2017 alloy strips processed by a novel process by combining semisolid rolling, deep rolling, and heat treatment

A novel short process for producing A2017 alloy strips with notable features of near net shape, saving energy, low cost, and high product performance was developed by combining semisolid rolling, deep rolling, and heat treatment. The microstructure and properties of the A2017 alloy strips were investigated by metallographic microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, tensile testing, and hardness measurement. The cross-sectional microstructure of the A2017 alloy strips is mainly composed of near-spherical primary grains. Many eutectic phases CuAl2 formed along primary grain boundaries during semisolid rolling are crushed and broken into small particles. After solution treatment at 495°C for 2 h the eutectic phases at grain boundaries have almost dissolved into the matrix. When the solution treatment time exceeds 2 h, grain coarsening happens. More and more grain interior phases precipitate with the aging time prolonging to 8 h. The precipitated particles are very small and distribute homogenously, and the tensile strength reaches its peak value. When the aging time is prolonged to 12 h, there is no obvious variation in the amount of precipitated phases, but the size and spacing of precipitated phases increase. The tensile strength of the A2017 alloy strips produced by the present method can reach 362.78 MPa, which is higher than that of the strips in the national standard of China.

The Study of an Al-Fe-Si Alloy After Equal-Channel Angular Pressing (ECAP) and Subsequent Semisolid Heating

The effects of coupling equal-channel angular pressing (ECAP) and heating at semisolid temperature on the microstructure of an Al-Fe-Si alloy were investigated. The microstructure of the equal-channel, angular-pressed samples after semisolid heating was found to be globular, fine, and uniform. Both the globularity and the grain size of the semisolid samples increased by increasing the holding temperature and holding time. The kinetics of grain coarsening was studied and the apparent activation energy was calculated on the basis of Lifshitz, Slyozov, and Wagner theory. Coupling ECAP and semisolid heating resulted in an enhanced microstructure and mechanical properties in the Al-Fe-Si alloy.

Microstructure Evolution of AS21 Magnesium Alloy via the SIMA Process

AS is a relatively new series of Magnesium alloys. The microstructure of this alloy can be improved for semisolid processing. The current research is concerned with the microstructure evolution of AS21 under the strain induced melt activated (SIMA) process. For this purpose, the AS21 alloy is cast and compressed 10-40% at 200 °C. The semisolid heat treatment is completed in a carbonate salt bath at different temperatures between 600-620 °C. The microstructure studies show that there is no favourable microstructure evolution between 600-610 °C. At 615 °C fine globular grains are obtained with the most desired mean grain size and sphericity of 67 µm and 81%, respectively. At 620 °C an undesirable coarsening phenomenon occurs that damages the microstructure globularity. SEM micrographs show that in a successful SIMA processing, the Mg2Si phases are broken into fine particles distributed within the grains and grain boundaries.

Semi-Solid Microstructure Control of Wrought Al-Mg-Si Based Alloys with Fe and Mn Additions in Deformation-Semi-Solid-Forming Process

The effects of Fe, Mn and Fe/Mn-combined additions on the refinement of the spheroidized semi-solid α-Al grains in the wrought aluminum 6000 based alloy was investigated. The semi-solid microstructure control of Al-1.2%Mg-1.3%Si based alloys with Fe (1.0 mass%), Mn (0.7 mass%) and Fe/Mn-combined (1.0/0.7 mass%) additions was studied by the Deformation Semi-Solid Forming (D-SSF) process. This process consists of high deformation before semi-solid heating in order to introduce high density of dislocations and fragmentation of intermetallic compounds. Various second phase particles strongly affect the resultant α-Al grain size. The deformation process of the Fe-added alloy with a low rolling ratio produces fine α-Al grains with an average size of 76 μm, which is apparently smaller than that of the Fe-free alloy with an average α-Al grain size over 100 μm. The Mn addition required a high deformation ratio to achieve an average α-Al grain of 85 μm. The combination of Fe and Mn additions produces the finest grain size of 64 μm. The morphology of the Fe-intermetallic compound in the semi-solid forming process can be controlled by the cooling rate and Mn addition. The D-SSF process is a promising process to modify the harmful Fe impurity into the useful intermetallic compounds.

3D Microstructural Study of AlSi8Mg5 Alloy by FIB-Tomography

Abstract In the present work, a foundry AlSi8Mg5 alloy is investigated by FIB-Tomography in the as-cast condition and after semi-solid heat treatment. The different microstrutural features are characterized quantitatively using morphological shape factors. The 3D-shape of Mg2Si Chinese script as well as Si and impurity Fe containing phases are revealed in high detail. A comparative study with respect to 2D structures is presented as well.

Semisolid heat treatment of chromium cast iron

The effect of semisolid heat treatment on the microstructures of three chromium white cast irons has been studied. The primary objective was to examine the effect of semisolid heat treatment on the microstructures with specific emphasis on revealing the variations related to the carbide morphology of the selected materials. It is found that graphitization of carbide in a 1·5 wt.% chromium cast iron is enhanced during isothermal heat treatment in the semisolid state. The eutectic carbide in high chromium cast irons is fully or partially modified into discrete particles during heat treatment in the semisolid state.

Microscopic observation of cold-deformed Al–4Cu–Mg alloy samples after semi-solid heat treatments

Wrought aluminum alloys can be effectively fabricated by a strain-induced, melt-activated (SIMA) process. The SIMA method involves plastic deformation of an alloy to some critical reduction point and a semi-solid heat treatment in the solid–liquid temperature range. The semi-solid heat treatment is a key process to control the semisolid microstructures. In this paper, the microscopic morphology of a cold-deformed SIMA treated Al–4Cu–Mg alloy has been investigated, and the effects of microstructural evolution, precipitation behavior and dislocation morphology on the mechanical properties are discussed. The experimental results show that the number of CuAl 2 ( θ phase) precipitates and the dislocation density of Al–4Cu–Mg alloy decreased gradually by the semi-solid heat treatment. Moreover, unique dislocation morphologies including helical dislocations and dislocation loops appeared and evolved to reduce the stored energy. With an increase of the holding time in the semi-solid heat treatment, the ultimate strength and yield strength decreased. The reduction of these mechanical properties of the SIMA treated Al–4Cu–Mg alloy is mainly due to the decrease of refinement strengthening, solution strengthening, and dislocation strengthening in the semi-solid heat treatment.

Characterization of Al 7075 alloys after cold working and heating in the semi-solid temperature range

The effects of cold working and heating conditions on the microstructures of 7075 aluminum alloys were investigated in the SIMA process. Swaging was conducted to control the recrystallization structures in the mushy zone. Different levels of cold working, holding temperatures and holding times were used in this investigation. It was found that at least 50% cold working was necessary in order to obtain a uniform and fine microstructures during semi-solid heating. The holding time should be between 2 and 3 min in order to prevent grain growth. The optimum heat treatment conditions for the thixoforged 7075 parts were investigated. The mechanical properties of thixoforged 7075 aluminum alloys were compared with those of conventionally hot forged alloys.