Reduction of the stresses introduced during the diffusion bonding of dissimilar materials

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Abstract

Residual stresses in joints of dissimilar materials resulting from thermal effects are a common cause of joint failure during processing or of deterioration in operational characteristics. Therefore, it is crucial that methods of controlling such stresses are investigated, especially in the case of brittle materials. In this paper the dependence of the stresses on some design and technological parameters are studied. New approaches for controlling these stresses are proposed. Data relating to these and other already-established methods are verified and expanded.

Introduction

The stresses introduced by processing which result from the cooling of joints of dissimilar materials in general causes deterioration of their strength and operational characteristics. This is why defining methods for controlling these stresses is of interest in the practice of diffusion bonding (DB), brazing and similar heating-based joining processes 1, 2, 3, 4, 5, 6.

The aim of the current study was to verify and extend data on established approaches for controlling the thermal stresses induced in joints, as well as to present and study new methods. The investigations concentrate on how such stresses are affected by design and technological parameters which have not been investigated, or for which incomplete or contradictory results have been published in the literature 1, 2, 3, 4, 5, 6.

In this paper, the problems considered are the effect of the joint design (symmetrical or asymmetrical), the number of material couples, the interlayer and its thickness and the application of pressure, during cooling, on the stresses.

Section snippets

Methods of investigation

Appropriate `strength of materials' analysis methods were selected for calculation: the finite-element method (FEM) and the layer separation method (LSM) [7]. The former of these (FEM) is a widely-used method, whilst the latter (LSM) is a more approximate approach, but is considered suitable for stress investigations in thin plates and has the advantage of being simpler in application.

Sandwich-like joints of one or more sets of similar couples of consecutively arranged thin plates were

Results

The present results on the effects of the joint design (symmetrical or asymmetrical) on the residual stresses (Fig. 2) confirm the suggestions of other authors (7, 8), that in symmetrical design joints the stresses are lower. In addition, it was established here that the stresses are lower when the axis of symmetry lies on the metal, independently of the ratio n of the thickness of the ceramic investigated, hce, to that of the metal, hme (range of n investigated, n=hce/hme=0.25–8).

In more

Conclusions

(1) It was confirmed that the replacement by symmetrical joints is an opportunity for reducing the undesirable tensile stresses in the ceramic component on a ceramic–metal joint, with a positive effect on joint strength. Symmetry with an axis lying on the metal is recommended.

(2) Increasing the number of sets of similar ceramic–metal joints in the Al2O3 ceramic and alloy compound, is recommended as a measure for reducing the above-mentioned tensile stresses.

(3) Variations in inter-layer thickness

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