Low Gravity Solidification Structures in the tin-15wt%lead and tin-3wt%bismuth Alloys
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LOW GRAVITY SOLIDIFICATION STRUCTURES IN THE TIN-15WT %LEAD AND TIN-3WT%BISMUTH ALLOYS
MARY H. JOHNSTON AND RICHARD A. PARR Marshall Space Flight Center, Alabama 35812
ABSTRACT The tin-15wtlead and tin-3wtbismuth alloys have been solidified in the low-gravity environment provided by the Space Processing Applications Rocket (SPAR), on the KC-135 airplane, and at high "g" levels in a centrifuge furnace. In each case the resultant cast structure was significantly different from that obtained in ground based experiments. Earlier low-gravity studies with the metal-model system NH 4 Cl-H20 presaged these results. This paper presents and discusses the influence of changes in the gravity force on the grain structure of these materials.
INTRODUCTION In order to study the formation of small grains in the central zone of a casting, a series of experiments were performed in NASA-unique hardware. Since gravity is considered to play a major role in solidification phenomena, several experiments were performed to test this theory. Materials were solidified in a sounding rocket that provided up to 5 minutes of 10-4 'g' time, and a KC-135 airplane that provided 10-20 sec. of 10-2 'g' time. A centrifuge casting furnace was used to increase the gravity level up to 5 'g's. Therefore a range of gravity acceleration from 10-4 to 5 'g's was achieved in the experiments. This gives an adequate test of the dendrite remelting theory (which depends on fluid flow) versus other theories such as constitutional supercooling (which operates best in a quiescent liquid). The metal model solution of NH 4 Cl-H20 and the alloys Sn-15wt% Pb, and Sn-3 wt% Bi were selected for study because they were well characterized in the literature. NH4Cl-H20 has been used to visually study the formation of grains in castings [1]. The dendrite remelting in Sn-3 wt% Bi was studied earlier by Glicksman [2]. The authors had used the tin-lead system in studies on gravity effects [3]. PROCEDURE Three unique pieces of NASA hardware were used: A centrifuge furnace, the Space Processing Applications Rocket (SPAR), and a KC-135 airplane. The centrifuge furnace system has been discussed in detail in an earlier paper [4]. Through the use of electrical slip rings and rotating gas feed throughs, samples can be melted and quenched while undergoing gravity acceleration forces. Only the metal systems were studied using the centrifuge. Acceleration forces were increased up to 5 'g's. Photographic systems with the NH4 C1-H20 metal model, and a furnace identical to that on the centrifuge, were flown on a sounding rocket. These systems are discussed in more detail in previous papers [5, 6]. The sounding rocket provides 10- 4 'g's for
652 about 5 minutes. Details on the KC-135 airplane can be found in reference 7. It flies a parabolic maneuver which provides the hardware and the investigators on board with 10- 2 'g's for 20 to 30 seconds. The SPAR casting furnace (Figure 1) and optical equipment specifically designed to observe flow perturbations in metal-models were also flown
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