Calculating residual manufacturing stresses in braze joints using ANSYS

Abstract

The furnace braze process consists of heating the piece parts, braze forms, and tooling, followed by the subsequent flow of the braze material and cooling of the brazed assembly and tooling. A typical application in the manufacture of traveling-wave tubes (TWT's) is the construction of ceramic RF windows. A ceramic disk is brazed into a copper sleeve to form a vacuum-tight RF window. ANSYS, a large-scale general-purpose finite-element code, can be used to calculate the residual manufacturing stresses after the braze operation is completed. To solve this nonlinear problem, discrete load steps are used to account for the temperature-dependent material properties and continuous plastic deformation of the braze material and copper sleeve. If the tooling does not constrain the piece parts during the heating phase, the analytical simulation need only encompass the second phase, the cooling from the stress-free braze solidus temperature. This can be accomplished quite nicely in ANSYS by selecting a zero strain reference temperature (TREF) at the braze solidus temperature and modifying the various material thermal coefficients of expansion to yield a table of "effective" thermal coefficients of expansion. If the tooling constrains one or both of the piece parts in the RF assembly, two TREF's ∃ a TREF of room temperature for the piece parts and tooling, and a TREF at the braze solidus temperature for the braze material. A thermal structural analysis of the residual manufacturing stresses in the braze joint would require simulation of the entire furnace braze process. For example, the tooling may be used to restrain the thermal growth of the copper sleeve to control the braze gap. This introduces radial and hoop compression in the copper sleeve during the heating phase, which must be accounted for as initial conditions to the cooling phase. At present, ANSYS does not have a provision for multiple TREF's, which would facilitate a straightforward solution. The following paper will develop a method to numerically simulate two TREF's using the present ANSYS finite-element code.