Minimizing Residual Stress in Brazed Joints by Optimizing the Brazing Thermal Profile

Ceramic to metal brazing is a common bonding process used in many systems such as automotive engines, aircraft engines, and electronics. Typically, a braze joint is heated to the solidus temperature of the braze and then cooled back to room temperature. In most cases the materials in a braze joint have mismatched stiffnesses and thermal expansions that lead to residual stress during the thermal cool down. These residual stresses reduce the strength of the joint or lead to a potential catastrophic failure. In this study we use viscoplastic and thermo-elastic material models to find an optimum thermal profile for a Kovar® washer bonded to an Alumina button that is typical of a tension pull test. A number of active braze filler materials are included in this work. Cooling rates, annealing times, aging, and thermal profile shapes are related to specific material behaviors. The viscoplastic material models are used to represent the creep and plasticity behavior in the Kovar and braze materials while the thermos-elastic material model is used on the Alumina. 

Material fits for each material are presented with data.  The Kovar® is particularly interesting because it has a curie point at 400°C that creates a unique thermal strain profile and temperature dependent stiffness. This complex behavior incentivizes the optimizer to maximize the stress above the curie point with a fast cooling rate and then favors slow cooling rates below the curie point to anneal the material. It is assumed that, if failure occurs in these joints, it will occur in the ceramic material. Consequently, the maximum principle stress of the ceramic is minimized in the objective function. Specific details of the stress state are considered and discussed.

Dakota, an optimization tool developed at Sandia National Laboratories, is used to predict optimal brazing temperature profiles that result in minimizing residual stresses in the ceramic. Finite element analysis (FEA) is used to predict the residual stresses due to the applied temperature profiles and the resulting strength of the brazed joint. Heat transfer of the system is neglected.  It is assumed that the entire system changes temperature simultaneously. Additionally, sensitivity analyses are performed to predict the variability in predicted stresses due to manufacturing perturbations such as the braze material thickness and meniscus shape. Where possible, the strength predictions are compared to experimental results from tension test data.  A comparison between the optimum profile for multiple braze materials is presented in detail.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

SAND2020-3730 A