Capillary brazing under microgravity (TEXUS-II) and 1g conditions

1120. Adv. Space1981. Res. Printed Vol. 1, inpp.1l © COSPAR, Great Britain.

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CAPILLARY BRAZING UNDER MICROGRAVITY (TEXUS-.II) AND 1G CONDITIONS K. Frieler,’ N. Phlippovich,’ R. Stickler’ and W. Bathke2 ‘University of Vienna, Austria 2BAM, Berlin, FRG ABSTRACT Experiments of vacuum brazing under both microgravity and 1-g conditions show the effect of hydrostatic pressure on gap-filling. The absence of buoyancy forces under microgravity affects the microstructure of the solidified braze in the joint. INTRODUCTION Brazing under vacuum is a frequently applied method for joining metals. However, investigations of brazing reactions and mechanisms remain largely empirical because of the multitude of base metal/filler metal multicomponent systems and the multiplicity of the interactions of numerous essential parameters. In order to study details of the brazing processes and the effects of gravity on gap—filling mechanisms a well defined base metal/filler metal system was selected. Brazing was carried out under 1—g in a laboratory furnace, microgravity (~-g) conditions were obtained during the TEXUS-Il experiment. The TEXUS (Technologische Experimente unter Schwerelosigkeit) program is a national space research project of the VederaT Republic of Germany, the experiments are carried out by means of sounding rockets flights. DESIGN OF THE TEXUS-lI BRAZING SPECIMEN The specimen geometry selected for the experiments consists of an assembly of thinwalled tubes forming annular gaps The tubes were machined from commercial grade pure Ni(purity 99.6 wt.%). The filler metal consisted of a near-eutectic alloy of 71.7 wt.% Ag, 28 wt.% Cu, and 0.3 wt.% Li, melting at 1033 K. No fluxing agent was required since Li promotes the spreading of the melt by reaction with the superficial oxide film. The geometry and dimensions of the TEXUS-Il specimen are shown in Fig.1. The specimen consists of two parts fitted one inside the other. The outer part forms a concentric gap of 200 1.1m secured by cams protruding from the inner cylinder A segment of the braze contained in a ring-shaped groove was neutron activated to produce s-and y-active ilOmAg, in order to study mixing phenonema in the liquid phase during gap-filling The inner part of the specimen contains a sickle-shaped gap of variable clearance from 0 to 2000 urn The insert is locked by four set screws This set up provides a suitable test configuration for evaluating the gap—filling capacity The whole assembly was placed into a stainless steel~artridgeand sealed after evacuation to an internal pressure of less than 10 Pa.

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THERMAL CYCLE The time—temperature profile of the TEXUS-Il experiment is shown in Figs. 2 and 3. For the braze metal to fill the gap, a minimum temperature of 1103 K is required Since the thermocouples were attached to the outer wall of the cartridge, an indication of at least 1290 K had to be obtained in order to account for the thermal gradient between the cartridge wall and the specimen. Unfortunately, as the readings from the thermocouples revealed, the desired peak temperature was not reached in all locations, neither in the 1—g nor in the micro—g experiment. 1290 K f~i—i~ 1001

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Fig. 2: Time-temperature profile of the Fig. 3: Time-temperature profile of the micro-g experiment 1-g experiment MI CROSTRUCTURE Effect of gap width. Although the ternary system AgCuNi is not described completely in literature {1,2}, the appropriate binary systems are well established. While Ag and Ni are immiscible in the liquid state, addition of Cu promotes the solubility. Cu forms with Ni a continuous series of solid solutions with melting ranges considerably above the brazing temperature ~3}.During the brazing cycle transport phenonerna may give raise to (i) adsorption of Cu at the base metal/melt interface and (ii) partial dissolution of Ni in the braze alloy. Due to these reactions changes in the melt composition occur which result in a modified eutectic or dendritic microstructure upon solidification In the 1—g and p-g specimens the following types of microstructure could be distinguished with increasing gap width Ci) A continuous Ag-rich phase with a layer of Cu adsorbed on the base metal surface, (Fig.4), (ii) Appearance of a discontinuous Cu-rich phase near the center of the joint, (Fig.5), (iii) Formation of a coarse eutectic with Ag-rich dendrites, and (iv) Appearance of large Cu-rich dendrites with a Ni—containing core, dispersed in a fine acicular eutectic (Fig.6). Effect of gravity In principle the same types of microstructure could be observed in both the 1—g and i-g specimens ~4 5} However the distribution of the various phases showed marked differences between these two specimens. These differences can be classified as follows:

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Fig.4: Microstructure in a minimum width Fig.5: Microstructure in a narrow joint joint ~ (i) In small gaps, reduced mass trans_____________________________ port under u—g conditions promotes ___________________________________ precipitation of primary co—phase, (ii) In wide joints, increased porosity was observed in the u-g specimen, __________________________________ probably due to outgassing reactions and the lack of buoyancy forces to _______ remove the gas bubbles. ____________________________ (iii) At 1100 K. the density difference _____________________________________ between the melt and the Cu-rich dendrites is approximately 0.95 g’a~. Thus, appreciable sedimentation of these dendrites may be expected to Fig.6: Microstructure in a wide joint occur under 1—g conditions. Actually, in very wide joints, say between 1000 and 2000 urn, dendrites were found to concentrate in the upper portion, Fig.7. No sedimentation could be observed in the u-g specimen. Effect of the time-temperature profile. The equilibrium composition of the filler metal in the joint is determined by the reaction described by the equation: Ni(5) + (Ag,Cu)(1) (Ni~Cu)(5)+ Ag(5) _______________________ _______________________

In a given time span of approximately one minute for the filler metal to remain in the liquid state, the reaction cannot reach equilibrium except in the narrowest joints, say smaller than 50 pm. However, if the time for the braze metal to remain liquid is extended, more Cu can react with the Ni. As a consequence,marked changes in the composition of the braze metal can occur even in wide joints. The excess of Ag is precipitated as co—phase dendrites. The Cu-rich dendrites formed earlier sometimes act as nuclei for this process, Fig.8. Fig.7: Joint brazed

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Fig.8: co—phase dendrite core formed afterwith 10 h Cu-Ni at 1100 K

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FLOW OF MOLTEN BRAZE Both the 1-g and p—g specimens did not reach the desired temperatures and thermal equilibrium. Therefore, the 200 pm gaps of both specimen were filled only partially in the shorter portion, the bulk of the braze remaining in the depot region. Auto— radiographic examination, Figs. 9 and 10, revealed that circumferential flow of the braze did occur inside the depot prior to gap-filling. In addition, radioactivity could be detected in every part of the filled gap. Since both effects could be observed in the 1-g and in the u-g specimens it can be assumed that the transport mechanisms responsible for the mixing are independent of gravity. -

Fig.9: Autoradiography of specimen Fig.1O: Autoradiogrphy of specimen brazed under p-g brazed under 1—g CONCLUSIONS The following conclusions can be drawn from the TEXUS-Il brazing experiment: (i) Vacuum brazing in a resistance heated furnace can be applied for joining metals under u-g conditions, (ii) Under p-g consitions gaps with a width of up to 2000 pm can be filled due to the action of capillary forces, (iii) Gravitation influences the distribution of phases in the solidified braze, (iv) Transport phenonema occur during the liquid state inside the filler depot apparently independent of gravity conditions. Verification of these findings is expected from the FSLP experiments. ACKNOWLEDGEMENTS The research work at the University of Vienna was supported by a grant from the Austrian Ministry pf Science and Research. The -nvestigations at the BAM were supported by a grant from the German Ministry of Research and Technology (Ref. No. 01QV158—Z/SN—SLN 7785) administered by the DFVLR-Cologne. REFERENCES {1} 0.J. Larson, Metallurgical Analysis of Skylab M552 and M557 Samples, Final Report, Grumman Research Dept., Report RE-565, (1978) {2} A.T. Sievert and R.W. Heine, Met.Trans.A, 8A, 515, (1976) {3} Gmelins I-tandb~ich der Anorganischen Chemie, 8th Edition, Springer, Berlin (1972) {4} W. Bathke, N. Philippovich, R. Stickler and K. Frieler, Shuttle/Spacela~ Utilization, Final Report Project TEXUS-II, DFVLR, 1978, p.62 ~5}N. Philippovich, W. Bathke, R. Stickler, K. Frieler, 3rd European Symposium “Material Science in Space” Grenoble 1979, ESA SP—142 (1979), p.95