Hisao Esaka

Liquid-Phase Separation in the Interdendritic Region After Growth of Primary β-Sn in Undercooled Sn-2.8Ag-0.3Cu Melt

An unusual microstructure consisting of both Sn-Ag3Sn and Sn-Cu6Sn5 binary eutectic structures is observed in actual solder balls. In this study, the solidification process of the Sn-Ag3Sn binary eutectic structure after the growth of primary β-Sn in an undercooled Sn-2.8Ag-0.3Cu alloy was investigated by using thermal analysis and interruption tests to understand the formation of the unusual microstructure. First, fine Ag-enriched liquid zones formed around β-Sn after the growth of primary β-Sn. The Ag-enriched zones then gradually enlarged with the accumulation of Ag from the remnant liquid with a decrease in temperature. This indicated that the liquid-phase separation occurred in the remnant liquid after the nucleation of β-Sn. Eventually, when the temperature of the specimen decreased to approximately the binary eutectic temperature, eutectic Ag3Sn nucleated in the Ag-enriched zones. From interruption tests, we determined the precursor of the Sn-Ag3Sn binary eutectic structure before the beginning of Sn-Ag3Sn binary eutectic solidification. This finding corresponds to the precursor of the Sn-Cu6Sn5 binary eutectic structure observed in the Sn-1.0Ag-0.5Cu alloy.

Influence of Supercooling on the Formation of Primary Phase during Solidification

It is reported actual volume fraction of the primary phase in alloys is larger than the equilibrium value. Larger volume fraction of the primary phase may cause shrinkage cavities and surface or internal cracks. Although control of the solidified structure is important for the quality of cast products, this problem has not been fully elucidated. Taking these results into account, this study has been carried out in order to comprehend the phenomenon of larger volume fraction of primary phase. The Sn-Pb alloy has been used as a test alloy to examine the relation between supercooling for nucleation and the volume fraction of primary phase. Actually, the volume fraction of the primary phase in Sn-Pb alloy is larger than that of the lever rule. It was also observed that the volume fraction of β-Sn decreases with decreasing the supercooling in the early stage of solidification. In the final stage of solidification, however, the effect of supercooling on volume fraction of primary phase is small. Futhermore, when the supercooling was low, the volume fraction of primary phase slowly increased.

In-Situ Observation of Horizontal Centrifugal Casting using a High-Speed Camera

In order to understand the solidification process of horizontal centrifugal casting, experimental equipment for in-situ observation using transparent organic substance has been constructed. Succinonitrile-1 mass% water alloy was filled in the round glass cell and the glass cell was completely sealed. To observe the movement of equiaxed grains more clearly and to understand the effect of movement of free surface, a high-speed camera has been installed on the equipment. The most advantageous point of this equipment is that the camera rotates with mold, so that one can observe the same location of the glass cell. Because the recording rate could be increased up to 250 frames per second, the quality of movie was dramatically modified and this made easier and more precise to pursue the certain equiaxed grain. The amplitude of oscillation of equiaxed grain ( = At) decreased as the solidification proceeded.

Formation of Primary Intermetallic Compounds in Sn-Ag-Cu Alloys

Sn-Ag-Cu alloys are considered one of the most favorable lead-free solder systems. In slowly-cooled eutectic Sn-Ag-Cu alloys, sometimes large primary Ag3Sn or Cu6Sn5 intermetallic compounds (IMCs) form. These IMCs may affect the mechanical properties of solders. However, explanations for the formation of these IMCs are still not clear. This study deals with interrupted tests in order to clarify the nucleation of IMCs in the liquid phase. In this study, Sn-4.41Ag-0.63Cu and Sn-3.30Ag-1.47Cu alloys were prepared. According to the thermodynamic calculation, Pandat, the equilibrium solidification paths are described as follows: Sn-4.41Ag-0.63Cu :L → primary Ag3Sn → binary eutectic (Ag3Sn +Sn) → ternary eutectic; Sn-3.30Ag-1.47Cu :L → primary Cu6Sn5 → binary eutectic (Cu6Sn5 + Sn)→ ternary eutectic. The actual solidification process was different from the estimation from the equilibrium phase diagram. In the case of Sn-4.41Ag-0.63Cu, only Ag3Sn grew as a primary phase in the liquid, while in the case of Sn-3.30Ag-1.47Cu, not only primary Cu6Sn5 but also pseudo-primary Ag3Sn grew in the liquid. Ag3Sn may nucleate easily in the liquid phase, but Cu6Sn5 would not nucleate in the liquid.

3-D analysis of grain selection process

It is known that the grain selection plays an important role in the manufacturing process for turbine blades. There are some analytical or numerical models to treat the grain selection. However, the detailed mechanism of grain selection in 3-D is still uncertain. Therefore, an experimental research work using Al-Cu alloy has been carried out in order to understand the grain selection in 3-D.A mold made by Al2O3 was heated to 600 °C ( = liquids temperature of the alloy) and was set on a water-colded copper chill plate. Molten Al-20 wt%Cu alloy was cast into the mold and unidirectional solidified ingot was prepared. The size of ingot was approximately 25×65H mm. To obtain the thermal history, 4 thermocouples were placed in the mold. It is confirmed that the alloy solidified unidirectionally from bottom to top. Solidified structure on a longitudinal cross section was observed and unidirectional solidification up to 40 mm was ensured. EBSD analysis has been performed on horizontal cross section at an interval of ca.200 μm. These observations were carried out 7-5 mm from the bottom surface. Crystallographic orientation of primary Al phase and size of solidified grains were characterized. A large solidified grain, the crystallographic orientation of which is approximately along heat flow direction, is observed near the lowest cross section. The area of grain decreased as solidification proceeded. On the other hand, it is found that the area of grain increased.

Observation of peritectic reaction in Ag-Sn alloys

In general, peritectic reaction is known that the secondary phase grows as it surrounds the primary phase. However, relationship between primary and secondary phase is still uncertain. Therefore, to clarify this, an experimental work has been preformed using Ag-35 mass%Sn alloy. The alloy was melted in an alumina crucible and then cooled at a constant cooling rate. The specimen temperature was measured with a thermocouple. Thereafter, the specimen was dropped into a water bath at a various temperature to quench the solid/liquid interfacial morphology and solute distribution. Then the specimen was polished and was etched, if necessary. The result of the thermal history, supercooling and recalescence was observed near the peritectic temperature. The observation by scanning electron microscope (SEM) unveiled many fine protrusions ( = e-phase) on a primary ζ-phase, which is quenched at early state of peritectic reaction. In addition, concentration profile was measured by energy dispersive X-ray spectrometry (EDX) and ζ, e and liquid phases were identified. As a result, it was confirmed that e-phase grew along the surface of ζ-phase and it did not grow into the ζ-phase. Further, result of electron backscatter diffraction (EBSD) analysis, e-phase was found to be single crystal within the range of this study.

In Situ Observation of Solidification in Horizontal Centrifugal Casting Process

Horizontal centrifugal casting is widely used for production of water pipes, rolls and so on. Advantage of this process is to press molten metal against the permanent mold due to centrifugal force. Thanks to this, one can obtain fine structure and sound castings1). On the other hand, macroscopic segregation on tangential direction sometimes forms. The formation mechanism of macroscopic segregation has not well understood. This is because the solidification process itself is still uncertain since it is very complex2-4). Thus, in order to understand the solidification process in horizontal centrifugal casting an in-situ observation has been made in this study. Transparent organic substance has been used to simulate the metal solidification.

Determination of Crystallographic Orientation near a Chill Zone Using Ghost Lines

Many crystals nucleate on the mold surface when the molten alloy is poured in a mold cavity. Because the crystallographic orientations of these crystals are random, the solidified structure near the mold surface is very complex. The ghost lines, which are sometimes thick and the angle between them is not 90 degrees, are often observed in this region. However, if the crystallographic structure of this alloy is cubic, such as bcc or fcc, the ghost lines are very regular. In order to understand the geometry of ghost lines, Al-20 mass%Cu alloys were unidirectionally solidified with constant growth velocity. The solidified structures on the obliquely crossed section were observed. The ghost lines were quite regular and parallel to each other in a solidification grain. The angles and the ratio of the width of ghost lines were measured and crystallographic orientations were estimated using these parameters, based on the solid analytical geometry. EBSD analysis were also performed on the area, where the ghost lines were characterized, and the precise crystallographic orientations were decided. The comparison between both analytical values indicated that the differences between them are within 10 degrees and it can be safely concluded that the estimation for crystallographic orientation using ghost lines agreed well with the EBSD analysis.

Crystallographic Investigation of the Initial Solidification Grain Structure in Al-Si Alloy

As-solidified structure of an ingot is composed of the chill, columnar and equiaxed zones. The whole solidified structure is strongly affected by the chill crystals. Some initial solidification grains have been observed on the ingot surface and thought to be traces of the nucleation point. The aim of this study is, therefore, to develop the experiment technique to make one ‘grain’ and to crystallographically investigate the initial solidification grain using EBSD analysis. In order to start solidification at a very specified position, a small metallic protrusion was installed on an insulating plate. Al-6 wt%Si alloy was melted at 800 °C and was poured on the metallic protrusion. In this study, the amount of protrusion was varied to investigate the growth mechanism of the initial solidification grain. The longitudinal cross section of the specimen was observed by an optical microscope, a scanning electron microscope. The starting position of solidification was the area that was on the metallic protrusion. In this initial solidification grain, it was difficult to observe the dendritic structure. The shape of this grain was about hemispherical. The grain area seemed to increase with increasing the amount of protrusion. The results of EBSD analysis showed that almost all initial solidification grains were composed by several crystals. The reason of this is that the nucleation frequency may increase with the amount of protrusion. The dendrite grew radially from the initial solidification grain continuously. The crystallographic structure was also continuous on the boundary of the initial solidification grain.

Growth of Solidification Grain of Aluminum Alloy Near a Mold

Surface quality as well as internal quality of cast products of aluminum alloys are strongly affected by the process of initial solidification. Control of solidified structure in this region is therefore quite important. In order to understand the growth of solidified grain, crystallographic characterization has been performed using EBSD (Electron Backscattered Diffraction) in this study. Al-6 mass%Si alloy was cast at 750°C on the chill plate. Longitudinal cross section of solidified shell was analyzed. In the region of initial solidification, many small crystals nucleated on the mold surface. The crystallographic orientations of these grains were random. It is normally found that an unfavorable grain was eliminated by a favorable grain. However, occasionally, we have found that an unfavorable grain enlarged its size. In this case, dendrite, the growth direction of which was far from the heat flow direction, gradually changed its crystallographic orientation from unfavorable one to favorable one. The grain enlarged its size by multiplication of dendrite arms. Crystallographic orientation of dendrite changed little by little when it branched. This kind of phenomena may take place in unsteady condition, such as initial solidification region.