• Journal of Korea Foundry Society
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• v.19 no.6
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• pp.472-483
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• 1999

The effects of rapid solidification and Cr addition to the microstructure variations of the rapid solidified Ti-48Al-xCr(X = 2,4,6) alloys have been investigated using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The segregated coarse ${\gamma}$ phase was eliminated and the microstructure was refined by rapid solidification of the cooling rate of $10^4-10^6\;^{\circ}C/sec$. The lattice parameters and tetragonality of ${\gamma}$ phase decrease with the increase Cr content and by the rapid solidification. Non-equilibrium phase ${\alpha}$ remains at room temperature condition, which would be resulted from the restriction of phase transformation ${\alpha}$ to ${\alpha}_2+{\gamma}$.

• Journal of Powder Materials
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• v.16 no.6
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• pp.438-441
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• 2009

Mg-Cu composites were successively fabricated using a combination of rapid solidification and extrusion processes. In addition, the microstructural variation of the composite was investigated by performing the extrusion repeatedly. It resulted that the composite formed an uniform and dense structure by the extrusion, and the microstructure became fine as the extrusion conducted repeatedly. The microstructural variation was known to be dependent on the number of extrusion and the area reduction ratio. The tensile strength was also measured as a function of the microstructural variation.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.193-198
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• 2002

In 1his paper, applicability of laser welding to joining process of single crystal nickel base superalloy turbine blades was investigated. Because heat input of laser welding is more precisely controlled 1han TIG welding, it is possible to optimize solidification microstructure of the welds. Since in single crystal nickel base superalloy the crystal orientation have a significant effect on the strength, it is important to control the solidification microstructure in the fusion zone. A single crystal nickel base supera1loy, CMSX-4, plates were bead-on welded and butt welded using a $CO_2$ laser. The effects of microstructure and crystal orientation on properties of the weld joints were investigated. In bead-on weldling, welding directions were deviated from the base metal [100] direction by 0, 5, 15 and 30 degrees. The welds with deviation angles of 15 and 30 degrees showed fusion zone transverse cracks. As the deviation angles became larger, the fusion zone had more cracking. In the cross section microstructure, the fusion zone grains in 0 and 5 degrees welds grew epitaxially from the base metal spins except for the bead neck regions. The grains in the bead neck regions contained stray crystals. As deviation angles increased, number of the stray crystals increased. In butt welding, the declinations of the crystal orientation of the two base metals varied 0, 5 and 10 degrees. All beads had no cracks. In the 5 degrees bead, the cross section and surface microstructures showed that the fusion zone grains grew epitaxially from the base metal grains. However, the 10 degrees bead, the bead cross section and surface contained the stray crystals in the center of the welds. Orientations of the stray crystals accorded with the heat flow directions in the weld pool. When the welding direction was deviated from the base metal [100] direction, cracks appeared in the area including the stray crystals. The cracks developed along the grain boundaries of the stray crystals with high angles in the final solidification regions at the center of the welds. The fracture surfaces were covered with liquid film. The cracks, therefore, found to be solidification cracks due to the presence of low melting eutectic. As the results, in both bead-on welding and butt welding the deviation angles should be control within 5 degrees for preventing the fusion zone cracks. To investigate the mechanical properties of the weld joints, high temperature tensile tests for bead-on welds with deviation angles of 0 and 5 degrees and the butt welds with dec1ination angles of 0, 5 and 10 degrees were conducted at 1123K. The the tensile strength of all weld joints were more 1han 800MPa that is almost 80% of the tensile strength of the base metal. The strength of the laser weld joints were more than twice that of tue TIG weld joints with a filler metal of Inconel 625. The results reveals 1hat laser welding is more effective joining process for single crystal nickelbase superalloy turbine blades 1han TIG welding.

• Journal of Welding and Joining
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• v.16 no.6
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• pp.59-68
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• 1998

In this study, effects of solidification modes (primary $\delta$-ferrite, primary ${\gamma}$-austenite) on the pit initiation and propagation in the 304L and 316L austenitic stainless steel weld metals were investigated. The solidification mode of weld metal was controlled by the addition of nitrogen to Ar shielding gas. Through the electrochemical experiments (potentiodynamic anodic polarization and potentiostatic time-current transient test) and metallographic examination (microstructure and elemental distribution), the following results were obtained. The more the volume content of nitrogen in the shielding gas were, the lower critical current density for passivity was observed. In comparison with weldments solidified through the primary $\delta$-ferrite solidification mode and the primary ${\gamma}$-solidification mode, the former showed higher critical pitting potential and a longer incubation time for stable pit initiation than the latter. However, in the pit propagation stage the former exhibited a faster dissolution rate than the latter. These results were believed to ee related to the distribution of alloying elements such as Cr, Mo, Ni and S.

• Journal of Korea Foundry Society
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• v.32 no.5
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• pp.219-224
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• 2012

The effects of Mg and Si contents on the microstructure and mechanical properties in Al-Mg alloy (ALDC6) were investigated. The results showed that phase fraction and size of $Mg_2Si$ and $Al_{15}(Fe,Mn)_3Si_2$ phase in the microstructure of Al-Mg alloy were increased as the Mg and Si contents were raised from 2.5 to 3.5 wt%. With Si content of 1.5 wt%, freezing range of the alloy was significantly reduced and solidification became more complex during the final stage of solidification. While there was no significant influence of Mg contents on mechanical properties, Si contents up to 1.5 wt%, strongly affected the mechanical properties. Especially elongation was reduced by about a half with more than 1.0 wt%Si in the alloy. The bending and impact strength were decreased with increased amount of Si in the alloy, as well. The lowered mechanical properties are because of the growth of particle shaped coarse $Mg_2Si$ phase and precipitation of the needle like $\beta$-AlFeSi in the microstructure at the last region to solidify due to presence of excess amount of Si in the alloy.

• Korean Journal of Metals and Materials
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• v.50 no.12
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• pp.897-905
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• 2012

Directional solidification experiments were carried out in a hypoeutectic Al-11.3Si-3.5Cu system to investigate the microstructural evolution with the solidification rate. At a fixed temperature gradient, a dendritic microstructure was observed at a constant speed of more than $25{\mu}ms^{-1}$, a cellular interface developed at $5{\mu}ms^{-1}$ and the growth rate of $0.5{\mu}ms^{-1}$ led to the stability of the planar interface. The results revealed that primary silicon phases formed among cells, even though the studied Al-Si alloy system formed the composition within a hypoeutectic silicon composition. This suggests that the liquid concentration among cells during solidification reached a higher concentration, i.e., the eutectic concentration. It is, however, interesting that primary silicon phases did not form during a dendritic growth of more than $25{\mu}ms^{-1}$. These experimental observations are explained using the theoretical models on the interface temperatures.

• Korean Journal of Materials Research
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• v.31 no.7
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• pp.409-419
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• 2021

The effect of solidification rate on micro-segregation in investment casting of IN738LC superalloy was studied. In Ni-based superalloys, the micro-segregation of solute atoms is formed due to limited diffusion during cast and solidification. The microstructure of cast Ni-based superalloys is largely divided into dendrite core of initial solidification and interdendrite of final solidification. In particular, mosaic shaped eutectic γ/γ' and carbides are formed in the interdendrite of the final solidification region in some cases. The micro-segregation phenomena formed in regions of dendrite core and interdendrite including eutectic γ/γ' and carbides were analyzed using OM, SEM/EDS and micro Vickers hardness. As a result of analysis, the lack of (Cr, W) and the accumulation of Ti were measured in the eutectic γ/γ', and the accumulation of (Cr, Mo) and the lack of Ti were measured in the interdendrite between dendrite and eutectic. Carbides formed in interdendritic region were composed of (Ti, W, Mo, C). The segregation applied to each microstructure is mainly due to the formation of γ' with Ni₃(Al,Ti) composition. The Ni accumulation accompanied by Cr depletion, and the Ti accumulated in the eutectic region as a γ' forming elements. The Mo tends to diffuse out from the dendrite core to the interdendrite, and the W diffuse out from the interdendrite to the dendrite core. Therefore, the accumulation of Mo in the interdendrite and the deficiency of W occur in the eutectic region located in the interdendrite. Heat treatment makes the degree of the micro-segregation decrease due to the diffusion during solid solution. This study could be applied to the heat treatment technology for the micro-segregation control in cast Ni-based superalloys.

• Journal of Korea Foundry Society
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• v.37 no.4
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• pp.108-114
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• 2017

In this study, we investigated the effect of the Si content on the secondary dendrite arm spacing (SDAS) of hypoeutectic Al-Si binary alloys in the range of 4~10 wt% Si. Cooling curves were measured during the solidification of the alloy cast in a step-wise mold. We compared two kinds of solidification time: the first is the total solidification time for both dendritic and eutectic growth, and the second is the solidification time for only dendritic growth. The proportional constant in the relationship between SDAS and cooling rate was estimated, as this constant represents the stability of the cast microstructure. The proportional constant decreased with the Si contents from 4 wt% to 8 wt%, and it remains relatively uniform with up to 10 wt% of Si.

• Corrosion Science and Technology
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• v.23 no.2
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• pp.104-112
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• 2024

Al-Zn-Mg-Si alloy coatings have been developed to inhibit corrosion of cold rolled steel sheets, and an understanding of the alloy system helps prevent coating defects. We used a Bridgman furnace to characterise the nature and formation mechanisms of the phases present in the quaternary system with 0.4 wt% Fe. In the directional solidification experiments we imposed steep temperature gradients and varied the pull rate. After the samples were quenched in the furnace, detailed characterization of the samples was carried out by electron microscopy (SEM/EDS). From the dT/dt vs T plots of the cooling curves of the alloys, the solidification path was determined to be $Liquid{\longrightarrow[80]^{544-558}}{\alpha}-Al{\longrightarrow[80]^{453-459}}Al/Mg_2Si{\longrightarrow[80]^{371-374}}Al/Zn{\longrightarrow[80]^{331-333}}Zn/mgZn_2$. The formation mechanisms of the Mg and Zn containing phases and their morphology was discussed together with the effects of the cooling rate. Key findings include the lengthening of the mushy zone in directionally solidified samples remelted against a positive temperature gradient, as well as an enrichening of the α-Al phase by Zn through remelting. Mg₂Si and other Si based phases were observed to adopt a much finer faceted microstructure in favour of a script-like microstructure when exposed to the higher cooling rate of coolant quenching.

• Proceedings of the Materials Research Society of Korea Conference
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• 1999.11a
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• pp.41-41
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• 1999