Low-Temperature Interface Reaction Between Titanium and the Eutectic Silver-Copper Brazing Alloy
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Section I: Basic and Applied Research
Low-Temperature Interface Reaction Between Titanium and the Eutectic Silver-Copper Brazing Alloy J. Andrieux, O. Dezellus, F. Bosselet, and J.C. Viala
(Submitted June 13, 2008; Accepted: 21 July 2008) Reaction zones formed at 790 °C between solid titanium and liquid Ag-Cu eutectic alloys (pure and Ti-saturated) have been characterized. When pure Ag-Cu eutectic alloy with 40 at.% Cu is used, the interface reaction layer sequence is: aTi/Ti2Cu/TiCu/Ti3Cu4/TiCu4/L. Because of the fast dissolution rate of Ti in the alloy, the reaction zone remains very thin (3-6 lm) whatever the reaction time. When the Ag-Cu eutectic alloy is saturated in titanium, dissolution no longer proceeds and a thicker reaction zone with a more complex layer sequence grows as the reaction time increases. Four elementary chemical interaction processes have been identified in addition to Ti dissolution in the liquid alloy. These are growth of reaction layers on Ti by solid state diffusion, nucleation and growth from the liquid of TiCu4, isothermal solidification of silver and, finally, chemical conversion of the Cu-Ti compounds by reaction-diffusion in the solid state. A mechanism combining these processes is proposed to account for the constitution of Ti/Ag-Cu/Ti joints brazed at 780-800 °C.
Keywords
chemical potential gradients, diffusion paths, intermetallics, metals, ternary system
1. Introduction The Ag-Cu eutectic alloy with 40 at.% Cu (28 wt.% Cu) has already shown good wetting and bonding characteristics with regard to conventional brazing of titanium parts at temperatures higher than 850 C.[1] This behavior is explained by the formation of binary Ti-Cu compounds that promote reactive wetting and spreading at the liquid/ solid interface.[1-4] Surprisingly, data are very scarce about brazing of titanium parts at temperatures lower than 850 C, although promising observations have been made.[3] In fact, operating at temperatures closer to the melting point of the alloy (which is of 780 C) should be favorable to the preservation of the microstructure and composition of the titanium base metal. Moreover, formation of copper-rich Ti-Cu solid compounds should render possible isothermal solidification of the Ag-Cu brazing alloy. In other words, the development of a process for brazing titanium or titanium alloys by transient liquid phase bonding at low temperature should be very attractive from an applied perspective.[5,6] For these reasons, it was decided to further investigate the chemical interaction between titanium and the eutectic Ag-Cu alloy at temperatures down to 780 C, the temperature of the eutectic transformation.
J. Andrieux, O. Dezellus, F. Bosselet, and J.C. Viala, Laboratoire des Multimate´riaux et Interfaces, LMI, UMR CNRS 5615, Universite Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, France. Contact e-mail: [email protected].
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In the first approach at investigating the kinetics of compound formation in the Ag-Cu-Ti system, cold-pressed mixtures of Ag, Cu, and Ti powders were heate
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