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Hinchy, Eoin
University of Limerick
in Cooperation with on an Cooperation-Score of 37%
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document
Microscopy analysis of nickel superalloy joint microstructures and high--- temperature braze flowability using different braze compositions
Abstract
Nickel superalloy turbine components are extensively used in the hot section of aero and land based gas turbine engines. Non---uniform cooling of turbine components causes thermal stresses in the alloy, which result in thermal fatigue cracking during service life. Cracks greater than 250µm in width are healed using a two---part braze mixture. A high---melt superalloy powder is mixed with a low--- melt braze powder. The low---melt material contains boron which, acts as a melting point depressant (MPD) – lowering the melting point of the braze below that of the substrate. The brazed component is heated in a vacuum furnace above 1200˚C where the presence of the MPD causes the braze to melt. High temperature diffusion of boron MPD from the braze into the substrate causes the melting temperature of the liquid braze to increase above the bonding temperature – resulting in isothermal solidification. Excessive boron within the joint however can form detrimental brittle boride secondary phases, drastically weakening the joint. It is imperative that the braze completely fills cracks, while also producing a joint which is free from brittle boride phases. This study uses a novel technique to study the effects of high---melt to low---melt ratio on high temperature braze flowability and the formation of brittle boride eutectic phases. Three separate ratios of high---melt superalloy to low---melt braze material were examined using scanning electron microscopy and backscattered electron detection. Backscattered electron imaging has the ability to clearly highlight boride phases within the nickel matrix. The ratios of high---melt to low---melt used were 80:20, 50:50 and 20:80. The excessive high--- melt sample showed very poor melting and joint filling, with the least number of detrimental secondary phases. The 50:50 sample showed good filling with the formation of more secondary phases within the braze material. The excessive low---melt sample showed excellent filling, with the disadvantage of a very high concentration of boride phases in the braze material and diffusion zone.