Optimisation of Vacuum Brazing Conditions for Joining Stainless Steel 321 Sheets by Nickel Based Braze Foil for Regenerative Rocket Nozzle Applications by Taguchi Method

Abstract

This paper outlines the optimisation of vacuum brazing conditions for joining titanium stabilised Stainless Steel SS321 sheets by Ni-based braze alloy (Ni-Mn-Cr-Co-Si) as brazing filler metal (BFM) for two shelled rocket nozzle applications. This is a specialized brazing technique in which multiple parameters such as temperature, time, argon pressure in the furnace and vacuum level in the channels between the shells are to be precisely controlled in a complex furnace. Brazing temperature, holding time, surface pressure in terms of dead weight and vacuum levels were identified as parameters having significant effect on the bonding strength of a braze joint. Taguchi method was used as a statistical technique for design of experiments to optimise brazing conditions in terms of shear strength. L9(34 ) orthogonal array was designed for conducting the experiments with four parameters at three levels ie temperature (1190oC, 1230oC,1270oC), holding time (10min, 20min, 30min), surface pressure (100kPa, 150kPa, 200kPa) and vacuum level (0.133Pa, 1.33Pa, 13.3Pa). The optimised set of parameters ie temperature: 1230oC, time: 20min, pressure: 100kPa, vac. level: 1.33Pa has been arrived at by Taguchi method via statistical analysis through analysis of means (AVOM) and analysis of variances (ANOVA). Confirmatory test for optimised conditions agreed well with the Taguchi analysis. Also, extensive optical metallography and energy dispersion X-ray analysis across the joint were carried out to study the metallographic morphology across the joint. The fractured surfaces during lap shear tensile testing have also been studied under SEM.