Papers by Keyword: Thermal Contraction

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Authors: Aage Stangeland, Asbjørn Mo, Dmitry G. Eskin
Abstract: A constitutive equation for thermal strain in the mushy zone has recently been established [1]. The parameters in this constitutive relation are in the present study determined for the commercial alloys A356, AA2024, AA6061 and AA7075 in addition to an Al-4 wt% Cu alloy by combining experimentally measured contraction of a cast sample with thermomechanical simulations. The constitutive equation for thermal strain in the mushy zone reflects that there is no thermal strain in the solid part of the mushy zone at low solid fractions and that the thermal strain in the mushy zone approaches thermal contraction in fully solid as the solid fraction increases towards one. Experiments were performed at cooling rates in the range from 2 to 5.5 °C/s. The solid fractions when the tested alloys start to contract, gsth , are in the range from 0.63 to 0.94. Grain refinement increases gsth for all the tested alloys.
Authors: S. Nishijima, Yoshihide Honda, Shunkichi Ueno, Seiichi Tagawa, Tatsuya Okada
Authors: Nikolay A. Belov, V.V. Cheverikin, Dmitry G. Eskin, A.N. Turchin
Abstract: The formation of eutectics in Al–Zn–Mg–Ni and Al–Zn–Mg–Si systems is studied by means of metallography, DSC, EPMA, X-ray spectroscopy and thermodynamical calculations. Polythermal sections of the corresponding phase diagrams are constructed. The concentrations and temperatures of binary eutectic reactions L → (Al) + Al3Ni and L → (Al) + Mg2Si in quaternary alloys are determined. Nonequilibrium solidification in Al–7% Zn–3% Mg-based alloys ceases at approximately 480 °C. The alloys close by composition to binary eutectics have considerably improved casting properties as compared to the base Al–7% Zn–3% Mg composition. In particular, hot tearing susceptibility is much less in alloys with Al3Ni or Mg2Si. These results are corroborated by measurements of thermal contraction during solidification. The alloys containing binary eutectics exhibit much lower temperatures of contraction onset and less thermal strain is accumulated in the solidification range. Fine eutectic morphology enables fragmentation and spheroidization of intermetallic particles during annealing. The presence of Al3Ni and Mg2Si particles does not decrease the precipitation hardening effect associated with precipitation of the T′ (AlMgZn) phase. Improved casting properties and good mechanical properties of castings allow the application of Al–Zn–Mg alloys with binary eutectics formed by Al3Ni or Mg2Si as foundry alloys.
Authors: Dmitry G. Eskin, Laurens Katgerman
Abstract: Hot tearing is a significant problem upon direct-chill casting of high-strength aluminum alloys. The occurrence of hot cracks is related to the thermal contraction of the solid phase and to the lack of feeding by the liquid phase during solidification. It has been identified that structure features such as grain size and amount of nonequilibrium eutectics influence both phenomena involved in hot tearing. Experimental and computer-simulation results are presented for a range of model and commercial aluminum alloys. The results are obtained both during special small-scale experiments and during industrial-scale direct-chill casting. It is shown that grain refinement reduces hot tearing susceptibility of aluminum alloys through the related decrease of the temperature of thermal contraction onset and increased permeability of the mushy zone. The effects of process parameters on hot tearing are also discussed.
Authors: M. Hussain, Atsushi Nakahira, S. Nishijima, Koichi Niihara
Authors: Qing Ling Bai, Jun Cheng Liu, Yue Li, Hong Xiang Li, Qiang Du, Ji Shan Zhang, Lin Zhong Zhuang
Abstract: The hot tearing susceptibilities (HTS) of some AA7×××alloys, AA7050, AA7055, AA7085 and AA7022 were evaluated with constrained rod casting (CRC). Thermal contraction behaviors during solidification were measured as well in a T-shaped setup. The results showed that alloys with HTS from high to low were AA7055, AA7085, AA7050 and AA7022. Zn content in 7××× aluminum alloys seemed to play a major role with respect to the HTS index. Remarkable differences could be seen on thermal contraction behaviors within solidification range for each alloy. The rate and amount of thermal contraction for AA7055 was most prominent, followed by AA7085 and AA7050, while contraction curve of AA7022 was very flat together with least amount of thermal contraction. There was a well consistency between the amount of thermal contraction and HTS. Despite complex interactions of many variables in the formation of hot tear, thermal contraction behaviors within solidification range could give a quick evaluation of hot tearing susceptibility.
Authors: Kotaro Hanada, Yusuke Hashiura, Ryutaro Maeda, Toshio Sano, Hideaki Negishi
Authors: Sheng Ching Wang, Hsi Hsun Tsai
Abstract: A stabilized laser is essential for optical fiber communication network. One of the passive technique for stabilization of central wavelength of laser is based on the application of fiber Bragg gratings. Due to the positive coefficient of thermal expansion of optical fiber, the Bragg gratings within the fiber written by excimer laser gives about 0.01nm/oC shift on the central wavelength respect to the ambient temperature which leads serious problem in the communication network. Since both the temperature and tension force are linearly proportional to the central wavelength of fiber Bragg gratings. A feasible approach to derive the wavelength stabilization is to decrease the tension force of fiber Bragg gratings respect to the increase of ambient temperature. In this paper, a Kovar substrate with negative coefficient of thermal expansion is used to decrease the tension force while the environmental temperature increases. The experimental results show that the coefficient of thermal expansion of the Kovar substrate is negative and linearly proportional to the temperature. Thus, this Kovar substrate differing from the constant negative coefficient of thermal expansion ceramic substrate induces about 0.0085nm/C on the fiber Bragg gratings, which shows the well application of this Kovar for athermalization of the fiber Bragg gratings in optical communication system.
Authors: Dmitry G. Eskin, Laurens Katgerman
Abstract: Aluminium alloys during solidification change their density. This process can be conditionally divided into two stages: solidification shrinkage due to the density difference between liquid and solid phases and thermal contraction due to the temperature dependence of the solid density. Solidification shrinkage is the main cause of porosity in castings and also plays an essential role in the development of macrosegregation, whereas thermal contraction is important for the development of hot and cold cracks and is responsible for shape distortions during casting. An experimental technique has been developed and applied to binary Al–Cu alloys in order to quantify the thermal contraction in the solidification range and at subsolidus temperatures. It is shown that thermal contraction of aluminium alloys starts at rather high fractions of solid, between 80 and 95%. The experimentally determined temperature of contraction onset agrees well with the temperature at which the mushy material acquires the ability to transfer stresses. The magnitude of contraction accumulated in the solidification range corresponds well to hot tearing susceptibility of the alloy. Factors that decrease the temperature of contraction onset and the magnitude of contraction, e.g. grain refinement, are also known to decrease hot tearing. The data on the temperature at which the thermal contraction starts, on the magnitude of the contraction, and on the thermal contraction coefficient are used to model hot tearing and shape distortions during casting.
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