The influence of acceleration forces on dendritic growth and grain structure
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INTRODUCTION
EARLIER studies ~'2 have proposed fluid flow as a determinant in the final structure of castings. The more recent discussions3 allude to the control of convection as crucial to the regulation of grain size and material homogeneity. The presence of convection with its thermal pulses and fluid movement induces remelting,4 fragmentation, and generally disrupts the dendrite solidification process. In addition, interdendritic flow contributes to macrosegregation in castings? '6 Experiments related to Materials Processing in Space have led to investigations of such phenomena through analysis of the convection driving force: gravity or acceleration. Metal model casting simulation experiments on earth and in low gravity7'8 indicated that in the absence of gravity induced acceleration, the resulting structure would be large grained and columnar. Conjunctively, the dendrite secondary ann spacings are more uniform and larger than their earth-based counterparts. The purpose of the present research was to extend the metal-model experiments to metal systems and to study the effect of acceleration force from the low gravity (10 -4 g)* *The symbol g is used in the discussion to represent the gravitational constant 980 cm/s 2.
available during sounding rocket flights, up to 5 g in a centrifuge apparatus, thus magnifying the influence of the gravity force on the structure. This paper shows grain size distribution and shape, constituent segregation, and dendrite arm spacings in the Sn-Pb system solidified at three selected cooling rates under the range of accelerations. A rationale will be proposed for the increase in secondary ann spacings at low gravity, based on previous dendrite coarsening theories.4'9'1~ II.
EXPERIMENTAL PROCEDURE
A centrifuge furnace apparatus was designed and built to solidify alloys under various accelerations. The furnace and a counterweight were attached to the rotating arm of the centrifuge by a pivot rod, which ensured that the acceleration force was directed along the crucible axis. (Earlier studies showed that this configuration produced a gravity M. H. JOHNSTON, Materials Engineer, and R. A. PARR, Metallurgical Engineer, are with the Marshall Space Flight Center, AL 35812. Manuscript submitted January 7, 1980. METALLURGICALTRANSACTIONS B
gradient along the axis of 0.1g/cm at 3.5 g.) Centrifuge instrumentation consisted of a direct current magnetic torque motor, a low-noise electrical slip ring, a gas quench system, temperature recorders, and a programmable temperature controller. The electronic control system and the furnace canister were prototype units for a Space Processing Applications Rocket (SPAR) flight furnace. The Sn-15 wt pct Pb alloy (212.7 ~ melting point) was prepared from elements of 99.9 pct nominal purity by melting in air and casting into a cold graphite crucible. The samples were removed and placed into a second graphite crucible for processing. Each sample was 76.2 mm wide by 101.6 mm long by 12.7 mm deep. The acceleration force was applied parallel to the longe
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