Microstructural coarsening of semi-solid aluminium alloys Atkinson, H.V.; Liu, D.
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
11/2008, Letnik:
496, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Semi-solid metal processing involves processing metallic alloys between the solidus and the liquidus. The microstructure must be non-dendritic and consist of spheroids of solid in a liquid matrix. ...Two potential routes to suitable starting material are recrystallisation and partial melting (RAP) and cooling slope (CS). Here the microstructural coarsening of such materials in the semi-solid state is compared with rates in the literature.
A slightly lower coarsening rate was observed for RAP route 2014 alloy with 37% fraction liquid than for CS route 2014 alloy with 17% fraction liquid despite the higher fraction liquid for the former. For the CS route, an increase in fraction liquid gave a higher coarsening rate. A relatively high coarsening rate was observed in a modified 2014 alloy with the Fe, Mn and Zn stripped out of the composition and a relatively low Si content. The coarsening rate was also relatively high for alloy 201 prepared by the CS route compared with alloy 2014 either by RAP or CS. It is likely that relatively low coarsening rates are associated with the presence of particles which are inhibiting the migration of liquid film grain boundaries, either through a pinning mechanism or through impeding diffusion through the liquid film at the boundary.
Semisolid metal (SSM) processing is a forming process which occurs within solidus and liquidus temperatures, and it requires feedstock billets with spheroidal microstructures. In this paper, a novel ...gas-assisted direct thermal method (DTM) is proposed as a method to create feedstock billets with fine spheroidal microstructures. Effect of different combination parameters between pouring temperature and holding time during the gas-assisted DTM technique on the microstructure of aluminium alloy 7075 feedstock billets was investigated. Pouring of molten aluminium alloy 7075 was conducted at temperatures between 645 and 685 °C, while holding time was set between 20 and 60 s. The melt was cooled down within the semisolid temperature range in a cylindrical copper mould with the help of carbon dioxide (CO
2
) gas before quenching in room temperature water. Results revealed that the smallest primary phase grain size formed at the combination parameters of 645 °C pouring temperature and 20-s holding time. Furthermore, the same combination parameters also produced the highest circularity value. The addition of external CO
2
gas surrounded the copper mould as a rapid cooling agent was found to have a significant improvement of 36.4% to the formation of smaller primary grain and spheroidal structure. It is concluded that the lowest pouring temperature and shortest holding time coupled with rapid cooling condition from the addition of external CO
2
during DTM produced the finest and most globular structure of primary phase size.
Simple rheocasting processes Haga, Toshio; Kapranos, P
Journal of materials processing technology,
12/2002, Letnik:
130
Journal Article
Recenzirano
This work describes two recently invented simple rheocasting processes. One process used a cooling slope and the other low superheat casting in order to generate semisolid slurries with spheroidal ...microstructures that are amenable to thixoforming. In the former process, A356 aluminum alloy was poured into the lower part of a die and immediately an upper die, containing an internal cavity, was inserted in the lower die half. The A356 alloy was in a semisolid slurry form when it flowed into the lower die via a cooling slope. In the latter process, the A356 changed from fully liquid to semisolid slurry condition by cooling in the lower die half after being poured into it as low superheat casting. The primary crystals of the cast metal in the both processes became spheroidal. There was no major observable difference between conventional thixocasting microstructures and those of the two processes used in the present study. The mechanical properties obtained in the present study were: tensile strengths of 310
MPa and 18% elongation for the process using the cooling slope, and tensile strengths of 290
MPa and 12% elongation for the process using the low superheat casting.
The effects of isothermal holding process on the microstructure evolution of semi-solid 7075 strength aluminum alloy produced by the recrystallisation and partial remelting(RAP) process were ...investigated. Tensile mechanical properties of as-received and thixoformed alloys at room temperature were examined. The results show that the microstructure of as-received alloy exhibits remarkable orientation along the deformation direction.With the increase of isothermal holding time, the solid particle grain size continuously increases and the degree of spheroidization also improves. Coalescence mechanism is dominant when reheated at 595 °C and Ostwald ripening mechanism is dominant when reheated at 615 °C in the semi-solid state. A lower coarsening rate is observed for 615 °C(coarsening rate K = 391 μm3 s-1) compared with the coarsening rate for 595 °C(coarsening rate K = 501 μm3 s-1). The RAP 7075 aluminum alloy can be successfully thixoformed and the filling of components is good after thixoforming. The thixoformed components exhibit favorite mechanical properties.
Two kinds of billetless thixoforming processes were devised in the present study. One process employed a cooling slope, and the other exploited low superheat casting. In the former process, molten ...metal was poured down an inclined cooling slope in order to generate the semisolid slurry. The semisolid slurry flowed and solidified into a die located at the bottom end of the slope. In the latter process utilizing low superheat casting, the melt was poured directly into the die and solidified. The need to produce an ingot for further processing by thixoforming can be eliminated since the molten metal can be shaped directly into the semisolid state by synchronizing the pouring into the die with the thixoforming operation. The size of ingot can be easily varied accordingly by appropriate choice of ladle and die. The cooling slope is a very simple, compact and cheap piece of equipment with low associated running costs. The primary crystals of test pieces generated in the course of this present study were spheroidal, and the general morphology of the ingot microstructures, when heated up to semisolid condition, was very similar to the microstructures obtained by conventional semisolid casting routes. The mechanical properties of test pieces produced from ingots cast in this present study were very similar to properties of products made by conventional thixoforming. Tensile strengths of 293
MPa and 15% elongation were obtained after T6 heat treatment.
This paper presents the effects of different cooling rates on thermal profiles and microstructures of aluminum 7075. The 7075 alloy was heated in a graphite crucible to 750°C. In the experimental ...work two thermocouples were used to record the temperatures at the center and 30mm from the center of the graphite crucible. A slow cooling rate condition was achieved by placing the crucible into a chamber with Kaowool insulation. A higher cooling rate was achieved by placing the crucible in open atmosphere with controlled air flow over the crucible. The slow and high cooling rates were 0.03°C/s and 0.4°C/s respectively. The Data Acquisition (DAQ) system implemented using LabVIEW software was used to record the temperature-time profiles. The enthalpy of phase change at each temperature was estimated from the cooling curves. The changes of cooling rate were directly related to phase transformation including at liquidus, eutectic and solidus temperatures. The dendritic coherency point (DCP) was determined from analysis of the temperature difference between two thermocouples. The formation of DCP was found to be delayed with use of the slow cooling rate. DCP occurred at 615.2°C (0.75 fraction solid) for the slow cooling rate and at 633.1°C (0.3 fraction solid) for the higher cooling rate. The microstructure features were also found to alter significantly with the different cooling rates used. The microstructure was more spheroidal for the slow cooling rate compared with the higher cooling rate.
The microstructure evolution during semisolid equal channel angular pressing and semisolid isothermal heating of a cast A356 aluminum alloy is reported. It is demonstrated that partially melted ...dendritic cast structure undergoes substantial plastic deformation during semisolid equal channel angular pressing. Coarsening of the fragmented dendritic microstructure during subsequent semisolid isothermal heating gives rise to spheroidal solid phase surrounded by liquid metal.
For semi-solid metal forming, it is essential to fabricate the semi-solid materials with spheroidal microstructure. Among several fabrication techniques of the semi-solid materials, (strain-induced ...melt activation (SIMA) is an ideal candidate with the advantages of simplicity and low equipment costs. In this paper, the microstructure evolution of LY11 alloy (approximately corresponding to ASTM 2017) was investigated in the SIMA process, which had two essential stages: deformation and isothermal heat treatment. The deformation stage was conducted using a CSS-1100C material testing machine and the isothermal heat treatment stage was performed in a resistance furnace. Different levels of deformation temperatures, ram velocities, isothermal temperatures, and holding times were used in this investigation. The microstructure of LY11 alloy was observed by a NEOPHOT-1 optical microscope. The results indicated that the processing parameters must be selected properly to obtain the fine, uniform and spheroidal microstructure by SIMA. The deformation-recrystallization mechanism for microstructure evolution in SIMA process was supported by experimental evidence.PUBLICATION ABSTRACT
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