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Thixoforging of Wrought Aluminum Thin Plates with Microchannels
Bolouri, A.,Kang, C. G. Springer Science + Business Media 2014 Metallurgical and Materials Transactions A - Physi Vol.45 No.5
<P>A direct die-filling thixoforging method is designed to fabricate aluminum thin plates with a pattern of microchannels in a single forming operation. Extruded AA2024 and AA7075 wrought aluminum billets are used. A recrystallization and partial remelting process is used to prepare the semisolid slurries required for the forming process. Under a thixoforging pressure of 70 MPa, AA7075 thin plates are successfully thixoforged in a temperature range of 883 K to 893 K (610 A degrees C to 620 A degrees C), corresponding to liquid fractions of similar to 30 to 50 pct in the semisolid slurry. AA2024 thin plate requires a thixoforging temperature range of 888 K to 898 K (615 A degrees C to 625 A degrees C), corresponding to the liquid fractions of similar to 45 to 60 pct. Final microstructures of the thin plates comprise primary alpha-Al equiaxed globular grains in a matrix of a solidified liquid phase. With increasing thixoforging temperature, the yield strength values continuously decrease. The ultimate tensile strength (UTS) values of the thin plates initially decrease with increasing thixoforging temperature from 883 K to 888 K (610 A degrees C to 615 A degrees C) and from 888 K to 893 K (615 A degrees C to 620 A degrees C) for the AA7075 and AA2024 thin plates, respectively. The UTS values stabilize and slightly enhance when the thixoforging temperatures are further increased to 893 K and 898 K (620 A degrees C and 625 A degrees C) for the AA7075 and AA2024 thin plates, respectively. Very brittle behavior (elongation value of similar to 1 pct) is observed for the AA7075 thin plates thixoforged at 883 K and 888 K (610 A degrees C and 615 A degrees C). The elongation value increases to 3 pct with increasing the thixoforging temperature to 893 K (620 A degrees C). In contrast, larger elongation values (between 4 and 6 pct) are achieved for the AA2024 thin plates. Increasing the thixoforging pressures from 70 to 100 MPa and then to 150 MPa improves the tensile properties of the thin plates. The tensile properties of the thixoforged thin plates are linked to their microstructural characteristics and processing conditions and are discussed here in detail.</P>
Tensile properties and microstructural characteristics of indirect rheoformed A356 aluminum alloy
Bolouri, A.,Bae, J.W.,Kang, C.G. Elsevier Sequoia 2013 Materials science & engineering. properties, micro Vol.562 No.-
The effects of rheoforming parameters such as the rheoforming temperature and pressure on the tensile properties of H-shaped components were examined. An H-shaped component is useful for practical investigation into the influence of different wall thicknesses on the microstructural and mechanical properties. A simulation study on the effect of forming pressure on the die filling was conducted using the DEFORM-3D package, and the results were compared with that obtained using the actual rheoforming. The semisolid slurries were prepared by employing the electromagnetic stirring (EMS) technique. The components were rheoformed at different forming conditions and their tensile properties in the center and side positions were evaluated and linked to the corresponding microstructure. The tensile properties remained approximately constant when the rheoforming temperature was increased from 580<SUP>o</SUP>C to 590<SUP>o</SUP>C, but was followed by a substantial reduction at 600<SUP>o</SUP>C. Moreover, the tensile properties in the center and side positions were distinct. Increasing the rheoforming pressure from 200MPa to 250MPa had a harmful effect on the elongation values, whereas the effect on the ultimate tensile strength values was limited. This trend could be linked to the increase in prevalence of internal defects such as the liquid segregation.
Bolouri, A.,Kang, C.G. Pergamon Press 2014 RENEWABLE ENERGY Vol.71 No.-
Metallic bipolar plates for polymer exchange membrane (PEM) fuel cells are currently manufactured by stamping of thin sheets. However, there are dimensional and shape errors of microchannels because of forming limitation such as spring back of thin sheets after stamping. On the other hand, stamping process is limited to commercially available sheet alloys, which restricts the development of a high corrosion resistant substrate aluminum alloy. Here, thixoforming (a commercial semisolid route) that is applicable to a wide range of aluminum alloys is proposed for net-shape micromanufacturing of aluminum bipolar plates with high dimensional stability. High corrosion resistance cast A356 (Cu-free) and wrought AA7075 (~2% Cu) aluminum billets are used for this study. Initial billets are heated at different semisolid temperatures. Subsequently, the semisolid slurries are injected into the die cavity. A356 and AA7075 aluminum bipolar plates are successfully fabricated by thixoforming with very small deviation of 0.7% and 1.5% from the nominal value of 0.300 mm in the microchannel depth, respectively. A multilayer coating of TiN/CrN is deposited on the surface of thixoformed bipolar plates through a commercially available magnetron sputtering technique. Electrochemical corrosion tests show that coated-thixoformed A356 (Cu-free) bipolar plates have significantly lower corrosion current densities than coated-thixoformed AA7075 (~2% Cu) bipolar plates. This seems to be due to the deleterious effect of Cu alloying element on the corrosion resistance of aluminum alloys that clearly confirms the importance of substrate material development for corrosive PEM fuel cell environment. It is suggested that specific high corrosion resistance aluminum alloy for PEM fuel cell application can be simply designed and then thixoforming can be efficiently and cost effectively employed to fabricate net-shape aluminum bipolar plates.
Thixoforming of AI7075 Alloy with High Solid Fraction
Amir Bolouri(아미르 볼루리),Chang Hyun Jang(장창현),Mohammad Shahmirl(모하메드 샤미리),Chung Gil Kang(강충길) 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.10
Aluminum 7075 alloy has been used as the feedstock for thixoforming in order to investigate the thixoformability of the alloy at high solid fratcions. Strain induced melting activation (SIMA) route has been employed to prepare the alloy to behave as the thixotropic material when reheated into the semisolid state. The results of reheating experiments at 590℃, 600℃ and 610℃ corresponding to high slid fractions (above 0.75) are presented. The reheating experiments were conducted for various holding times: 10min, 15min and 20min. According to the microstructure was achieved. Thixforming experimental at 590℃ failed. At 610℃, the optimum forming condition obtained for 300kN and 400kN at 0.75m/s ram speed.
Fabrication of AA7075 Thin Plates for PEM Fuel Cells by Thixoforging
Amir Bolouri(아미르),Chang Hyun Jang(장창현),Chung Gil Kang(강충길) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.5
Aluminum 7075 alloy extrusion billets have been used as the feedstock for thixoforging of thin plates. Recrystallisation and remelting (RAP) route has been employed to prepare the semisolid slurries. A direct die-filling system was designed for thixoforging of thin plate. Thin plates were thixoforged at the temperatures of 610℃, 615℃ and 620℃ with application of 70 MPa forging pressure. The quality of thin plates was examined by X-ray radiography. Hot tearing defects were detected when thixoforging was conducted at higher temperatures. The elongated grains in the initial extrusion billets were transferred to the globular grains within the semisolid slurry. However, some interconnected grains were observed. The microstructure of thixoforged thin plates indicated that a disagglomeration effect was active during thixoforging.
Manufacturing of Metgallic Aluminum Bipolar Plates by Thixoforming Process
Amir Bolouri,장창현(Chang-hyun Jang),강충길(Chung-gil Kang) 한국소성가공학회 2012 한국소성가공학회 학술대회 논문집 Vol.2012 No.5
Thixoforming investigations have been developed, mainly for manufacturing of bulk components. However, the knowledge is very poor regarding the fabrication of thin cross section of alloys. In this research work, an experimental approach has been employed to study the effectiveness of thixoforming process for the fabrication of microchannel for bipolar plates of polymer electrolyte membrane (PEM) fuel cells. A356 alloy in the continuous casting state has been used for this study. Recrystallisation and remelting (RAP) route has been employed to prepare feedstock for thixoforming process. The feedstock with different liquid fractions of 75%-50% were prepared and used for the thixoforming process. The bipolar plates with the thickness of 1.2 mm were successfully fabricated. The presence of any internal defect such as localized porosity was examined using X-ray radiography. The effects of liquid content of feedstock on the formability and die-filling phenomena are thoroughly investigated and discussed. Results showed that at all the applied liquid contents fabrication of bipolar plate is achievable. However, reproduction of bipolar plates is very high at high liquid fraction of 75%.