Containerless Rapid Solidification Research Articles

A comparison of acoustic levitation with microgravity processing for containerless solidification of ternary Al–Cu–Sn alloy

The containerless rapid solidification of liquid ternary Al–5 %Cu–65 %Sn immiscible alloy was accomplished at both ultrasonic levitation and free fall conditions. A maximum undercooling of 185 K (0.22 TL) was obtained for the ultrasonically levitated alloy melt at a cooling rate of about 122 K s−1. Meanwhile, the cooling rate of alloy droplets in drop tube varied from 102 to 104 K s−1. The macrosegregation was effectively suppressed through the complex melt flow under ultrasonic levitation condition. In contrast, macrosegregation became conspicuous and core–shell structures with different layers were formed during free fall. The microstructure formation mechanisms during rapid solidification at containerless states were investigated in comparison with the conventional static solidification process. It was found that the liquid phase separation and structural growth kinetics may be modulated by controlling both alloy undercooling and cooling rate.

Numerical Simulation of Rapid Solidification Process for Micrometer Level Solder Ball Used in BGA Packaging

We numerically simulated the containerless rapid solidification for micrometer level solder ball used in BGA packaging, based on prior models for heterogeneous droplet nucleation and non-equilibrium solidification, to predict the solidification microstructure of solder ball. Results of simulations for Sn-5mass%Pb solder ball produced under different conditions are discussed.

Phase separation and structure evolution of ternary Al–Cu–Sn immiscible alloy under ultrasonic levitation condition

The containerless rapid solidification of liquid Al–15%Cu–55%Sn immiscible alloy is accomplished by single-axis ultrasonic levitation. A maximum undercooling of 209K (0.23TL) is obtained for the alloy melt at a cooling rate of about 100K/s. The phase separation pattern transforms from the vertical-type macrosegregation under normal condition into the lateral-type macrosegregation under ultrasonic levitation. The enhancement of both undercooling and cooling rate induces the refinement of microstructure and the remarkable extension of solute solubility in (Sn) phase. Theoretical analyses show that the morphology transition mainly depends on the density and surface tension differences between Al-rich and Sn-rich liquid phases. The centrifugal migration of Sn-rich droplets caused by sample rotation is the major dynamic mechanism responsible for the macrosegregation evolution under ultrasonic levitation condition.

Microstructural evolution during containerless rapid solidification of Co-Si alloys

The Co-12%Si hypoeutectic, Co-12.52%Si eutectic and Co-13%Si hypereutectic alloys are rapidly solidified in a containerless environment in a drop tube. Undercoolings up to 207K (0.14TE) are obtained, which play a dominant role in dendritic and eutectic growth. The coupled zone around Co-12.52%Si eutectic alloy has been calculated, which covers a composition range from 11.6 to 12.7%Si. A microstructural transition from lamellar eutectic to divorced eutectic occurs to Co-12.52%Si eutectic droplets with increasing undercooling. The lamellar eutectic structure of the Co-12.52%Si alloy consists of εCo and Co3Si phases at small undercooling. The Co3Si phase cannot decompose completely into εCo and αCo2Si phases. As undercooling becomes larger, the Co3Si phase grows very rapidly from the highly undercooled alloy melt to form a divorced eutectic. The structural morphology of the Co-12%Si alloy droplets transforms from εCo primary phase plus lamellar eutectic to anomalous eutectic, whereas the microstructure of Co-13%Si alloy droplets experiences a `dendritic to equiaxed' structural transition. No matter how large the undercooling is, the εCo solid solution is the primary nucleation phase. In the highly undercooled alloy melts, the growth of εCo and Co3Si phases is controlled by solutal diffusion.

A Transition from Eutectic Growth to Dendritic Growth Inducedby High Undercooling Conditions

Cu-8 wt.%Al eutectic alloy was undercooled by up to 187 K(0.14 TE) using a drop tube technique. The crystal growth andphase selection mechanisms were investigated during containerless rapidsolidification. It is found that the microstructural morphology ischaracterized by lamellar eutectic growth at small undercoolings.However, if the liquid alloy is undercooled by more than 25 K, eutecticgrowth will be suppressed completely and the dendritic growth of (Cu)solid solution dominates its solidification process. When theundercooling exceeds 153 K, a microstructural transition from coarsedendrite to equiaxed dendrite takes place.

Microstructure transition of Al-Cu eutectic alloy during containerless rapid solidification*

Abstract Droplets of Al-32.7% Cu eutectic alloy were rapidly solidified during containerless processing in a 3 m drop tube. The microstructural evolution of Al-Cu eutectic alloy depends strongly on droplet size. The eutectic growth morphology changes from regular lamellar eutectic to a kind of anomalous eutectic with the decrease in droplet size. This microstructural transition was analyzed within the current theories of eutectic growth. The results indicate that there exists a critical velocity for eutectic growth beyond which anomalous eutectic appears. The coupled zone of the Al-Cu eutectic alloy system has been calculated on the basis of the Trivedi-Magnin-Kurz TMK eutectic growth and the Lipton-Kurz-Trivedi LKT Boettinger-Coriell-Trivedi BCT dendrite growth models, which provides a further interpretation for the morphological transition.

Microstructure transition of al-Cu eutectic alloy during containerless rapid solidification

Microstructure transition of al-Cu eutectic alloy during containerless rapid solidification

Microstructural evolution during containerless rapid solidification of Ni–Mo eutectic alloys

Ni–45%Mo hypoeutectic, Ni–47.7%Mo eutectic and Ni–50%Mo hypereutectic alloys are rapidly solidified during containerless processing in drop tube. The microstructures of Ni–47.7%Mo eutectic alloy are composed of lamellar eutectic plus anomalous eutectic of Ni and NiMo phases. When the droplet size decreases, the volume fraction of anomalous eutectic becomes larger. The structural morphology transforms into Ni dendrite plus lamellar eutectic in very small droplets which are highly undercooled. The microstructures of Ni–45%Mo hypoeutectic alloy are characterized by primary Ni dendrite plus lamellar eutectic, whereas those of Ni–50%Mo hypereutectic alloy consist of NiMo dendrite plus lamellar eutectic. For both off-eutectic alloys, the experimental results show that the microstructure evolution depends mainly on droplet size. In the case of Ni–45%Mo hypoeutectic alloy, with the decrease of droplet size, the primary Ni phase transforms from dendrites to equiaxed grains. As for Ni–50%Mo hypereutectic alloy, when droplets become smaller and smaller the microstructural transition proceeds from primary NiMo dendrite plus lamellar eutectic to anomalous eutectic. The calculated highest undercoolings of the three alloys are 226, 182 and 135 K, respectively. By classical nucleation theory, Ni phase is the primary phase to nucleate for Ni–47.7%Mo eutectic alloy. The TMK eutectic growth and LKT/BCT dendritic growth theories are applied to analyze the rapid solidification process and investigate the microstructural transition mechanisms. The coupled zone of Ni–Mo eutectic alloy has also been calculated on the basis of TMK and LKT/BCT models, which covers a composition range from 45.7% to 57.1% Mo.

Containerless rapid solidification of highly undercooled Co-Si eutectic alloys

Droplets of Co–62.1%Si and Co–43.5%Si eutectic alloys with different sizes are rapidly solidified during containerless processing in drop tube. The microstructures of Co–62.1%Si eutectic alloy are mainly characterized by lamellar eutectic plus anomalous eutectic of CoSi2 and Si phases, whereas those of Co–43.5%Si eutectic alloy are mainly characterized by lamellar eutectic plus anomalous eutectic of CoSi and CoSi2 phases. For both alloys, the experimental results show that the microstructural evolution depends mainly on undercooling. In the case of Co–62.1%Si eutectic alloy, the smaller the droplet diameter, the larger the volume fraction of anomalous eutectic. When its diameter is very small, the droplet exhibits anomalous eutectic morphology entirely. As for Co–43.5%Si eutectic alloy, with the decrease of droplet size, the microstructural transition proceeds from lamellar eutectic to anomalous eutectic, even to dendrites. The nucleation rates of each phase have been calculated. The TMK eutectic growth and LKT/BCT dendritic growth theories are applied to analyze the rapid solidification process and investigate the microstructural transition mechanisms. The coupled zone around Co–43.5%Si eutectic alloy has also been calculated on the basis of TMK and LKT/BCT models, which covers a composition ranging from 40.8 to 43.8%Si. And calculated coupled zone of Co–62.1%Si covers a composition ranging from 60.2 to 81.6%Si.

Containerless rapid solidification of undercooled Cu-Co peritectic alloys

Droplets of Co-16%Cu and Co-71 .6%Cu peritectic alloys were solidified during containerless processing in a 3-m drop tube. The microstructures of Co-16% Cu alloy droplets were characterized by dendritic or equiaxed a-Co phase with a small amount of Cu-rich solid solution distributed on a- Co phase boundaries. Two thresholds of droplet diameter were observed for Co-16%Cu alloy at which "equiaxed-dendritic-equiaxed" morphological transitions occur to primary a-Co phase. This conspicuous refinement of primary a-Co grains results from the fragmentation of a-Co dendrites caused by recalescence effect. For Co-71.6% Cu alloy, the primary a-Co phase forms as coarse columnar dendrites in large droplets and equiaxed dendrites in small droplets. Theoretical calculations indicate that Marangoni migration contributes more to the growth of disperse Co-rich spheres by stimulating collision and coalescence than Stokes motion caused by the residual gravity in the falling Co-71.6% Cu alloy droplets.