Effect of solidification conditions and surface pores on the microstructure and columnar-to-equiaxed transition in solidification under microgravity

Abstract

Microgravity solidification experiments were carried out in the Material Science Laboratory on board the International Space Station. The influence of grain refinement, rotating magnetic field (RMF) and surface pores on the microstructure and columnar-to-equiaxed transition (CET) were investigated in two selected Albased samples solidified under microgravity conditions. The increase of the furnace pulling velocity leads to a finer dendrite structure, a smaller eutectic percentage and a more uniform eutectic distribution in the interdendritic regions. On the one hand, grain refinement ensures the occurrence of CET, which is progressive in the studied experiment because of the high temperature gradient. On the other hand, in the nonrefined alloy a RMF applied during solidification fails to trigger the CET, because the forced liquid flow is too weak compared to the solidification front velocity to transport fragments from the mushy zone above the solidification front. The presence of the pores at the sample surface leads to a peculiarity in the eutectic percentage and weakens the decrease of the dendrite arm spacing for both samples. These effects are ascribed to a forced extra liquid flow into the mushy zone due to the pore that *Manuscript Click here to view linked References 2 promotes the growth of the dendrites along the liquid flow direction, resulting in elongated grains and postponing the CET in the refined alloy.

Modification of the microstructure by rotating magnetic field during the solidification of Al-7 wt.% Si alloy under microgravity

“…and B2F6, indicating that the influence of the grain refiner (GR) and of the rotating magnetic field (RMF) in stage II are negligible as concerns the recorded temperatures. This conclusion is in agreement with previous experiments [43]. Fig.5a) and the velocity of the liquidus isotherm vL (Fig.5b), which are calculated based on the temperature profiles with the procedure described in our previous work [43].…”

“…were conducted under low and high furnace pulling velocities using the MSL Bridgman-type Solidification and Quenching Furnace (SQF), which was described in details in our previous studies [43]. The SQF furnace, which consists of hot and cold zones separated by an adiabatic zone, and the sample cartridge assembly (SCA) are schematized in Fig.1.…”

“…The characterization methods in the present study have been described in detail in our previous study [43]. After their return to Earth, samples are cut into transverse and longitudinal sections of 30 mm in lengths and prepared for metallographic analysis.…”

“…Based on the binary images, the E% maps and the E% profiles along the main cylindrical sample axis are determined using a box-averaging method. The DAS is measured using a linear-intercept method [43]. For one data point of DAS, 1000 lines are generated in random orientation.…”

Columnar and Equiaxed Solidification of Al-7 wt.% Si Alloys in Reduced Gravity in the Framework of the CETSOL Project

“…Indeed, grain nucleation takes place and stops the columnar growth in both samples. However, due to the higher temperature gradient in B2-FM1, nucleation takes place in a shorter undercooled region, preventing copious and simultaneous nucleation necessary to obtain sharp CET [7]. …”