In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in th...In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.展开更多
The influence of Sr addition on pore formation in directionally solidified Al-12.3wt.% alloy was investigated using X-ray detection, optical microscope, and SEM-EDX. Results indicate that addition of Sr significantly ...The influence of Sr addition on pore formation in directionally solidified Al-12.3wt.% alloy was investigated using X-ray detection, optical microscope, and SEM-EDX. Results indicate that addition of Sr significantly increases the number density and volume fraction of porosity. The considerable rise in volume fraction of porosity is attributed to the remarkable increase in the numbers of pores formed. It is found that Sr solute in liquid Al-Si al oy can diffuse into the oxide inclusions to form loose oxide aggregations which have more activity as the nucleation sites for porosity. Adding more Sr considerably increases the numbers of active nucleation sites. There is an obvious fluctuation of pore number density during steady state solidification, which is believed to be related to a fluctuation of local hydrogen supersaturation induced by the competition of pore nucleation and growth for hydrogen solute supplement.展开更多
Effects of Ce addition on microstructure, castability （fluidity and hot tearing sensitivity）, mechanical properties and electric conductivity of commercial purity aluminum （CP-AI） were investigated through microst...Effects of Ce addition on microstructure, castability （fluidity and hot tearing sensitivity）, mechanical properties and electric conductivity of commercial purity aluminum （CP-AI） were investigated through microstructure observation and performance tests. Results show that adding Ce in a CP-AI can considerably refine the grains, and has an important influence on the amount, crystallographic forms, and distribution of secondary phases. Addition of Ce also has a large impact on the fluidity and hot tearing sensitivity （HTS） of the CP-AI. With the addition of Ce from 0.1wt.% to 0.5wt.%, the fluidity of CP-AI is first increased remarkably and then decreased, and the HTS has an opposite response. The best castability of the studied alloys appears to be at 0.2wt.%-0.3wt.% Ce addition. The remarkable improvement in castability is attributed to the considerable refinement of grain structure. Ce addition can also lead to a significant rise in electric conductivity. The maximum conductivity of the as-cast CP-Al is 59.7% IACS with an addition of 0.2wt.%Ce. The T7 heat treatment can further improves the conductivity to 60.7% IACS. The Ce-induced evolution of the secondary phases is believed to be the mechanism for it.展开更多
Al-Si eutectic growth mechanism was investigated in a directionally-solidified AI-1 3 wt% Si alloy with different strontium （Sr） and magnesium （Mg） additions, growth velocities and temperature gradients. Macro- an...Al-Si eutectic growth mechanism was investigated in a directionally-solidified AI-1 3 wt% Si alloy with different strontium （Sr） and magnesium （Mg） additions, growth velocities and temperature gradients. Macro- and micro- scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying AI-Si eutectic front and the final morphology of eutectic grains in the solidified A1-13 wt% Si alloys. By varying （tailoring） these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition （CET） of eutectic structure, which can be interpreted on the basis of Hunt＇s Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.展开更多
基金funded by the Natural Science Foundation of China under grant No:50771031GM Research Funding under contract No:GM-RP-07-211
文摘In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.
基金financially supported by National Natural Science Foundation of China (grant No.51244008)GM Research Foundation under contract No.GM-RP-07-211
文摘The influence of Sr addition on pore formation in directionally solidified Al-12.3wt.% alloy was investigated using X-ray detection, optical microscope, and SEM-EDX. Results indicate that addition of Sr significantly increases the number density and volume fraction of porosity. The considerable rise in volume fraction of porosity is attributed to the remarkable increase in the numbers of pores formed. It is found that Sr solute in liquid Al-Si al oy can diffuse into the oxide inclusions to form loose oxide aggregations which have more activity as the nucleation sites for porosity. Adding more Sr considerably increases the numbers of active nucleation sites. There is an obvious fluctuation of pore number density during steady state solidification, which is believed to be related to a fluctuation of local hydrogen supersaturation induced by the competition of pore nucleation and growth for hydrogen solute supplement.
基金supported by GM Research Foundation under contract No.GB1279-NV
文摘Effects of Ce addition on microstructure, castability （fluidity and hot tearing sensitivity）, mechanical properties and electric conductivity of commercial purity aluminum （CP-AI） were investigated through microstructure observation and performance tests. Results show that adding Ce in a CP-AI can considerably refine the grains, and has an important influence on the amount, crystallographic forms, and distribution of secondary phases. Addition of Ce also has a large impact on the fluidity and hot tearing sensitivity （HTS） of the CP-AI. With the addition of Ce from 0.1wt.% to 0.5wt.%, the fluidity of CP-AI is first increased remarkably and then decreased, and the HTS has an opposite response. The best castability of the studied alloys appears to be at 0.2wt.%-0.3wt.% Ce addition. The remarkable improvement in castability is attributed to the considerable refinement of grain structure. Ce addition can also lead to a significant rise in electric conductivity. The maximum conductivity of the as-cast CP-Al is 59.7% IACS with an addition of 0.2wt.%Ce. The T7 heat treatment can further improves the conductivity to 60.7% IACS. The Ce-induced evolution of the secondary phases is believed to be the mechanism for it.
基金financially supported by the National Natural Science Foundation of China(No.50771031)GM Research Foundation(No.GM-RP-07-211)
文摘Al-Si eutectic growth mechanism was investigated in a directionally-solidified AI-1 3 wt% Si alloy with different strontium （Sr） and magnesium （Mg） additions, growth velocities and temperature gradients. Macro- and micro- scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying AI-Si eutectic front and the final morphology of eutectic grains in the solidified A1-13 wt% Si alloys. By varying （tailoring） these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition （CET） of eutectic structure, which can be interpreted on the basis of Hunt＇s Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.
