Effect of residual stress on the strength of an alumina–steel joint by partial transient liquid phase (PTLP) brazing

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Abstract

The effect of residual stress on the strength of joints made between alumina and stainless using partial transient liquid phase (PTLP) brazing with the Ni of an 80Ni20Cr core interlayer was studied. It was found that the residual stress caused by the thermal expansion mismatch between alumina (Al2O3) and stainless steel-304 (SS304) deteriorated the Al2O3–SS304 joints severely compared to Al2O3–Al2O3 joints with the same brazing parameters. The high residual stress zones obtained through finite element method (FEM) simulation corresponded well to the fractographic observation of the Al2O3–SS304 joints. FEM simulation also demonstrated that Ni and Ni–Cr filler metals are compliant layers that are suitable to relieve thermal residual stress in the Al2O3–SS304 joints.

Introduction

Ceramics exhibit an excellent resistance to wear and corrosion at high temperatures. Compared to metals they are usually stronger and have lower coefficients of thermal conductivity and of thermal expansion. However, ceramics remain brittle and suffer from a wide scatter in strength, preventing their introduction as monolithic parts in engineering structures. In order to improve their reliability and to make up for their low workability, ceramics must be combined with metals.

Various ceramic-to-metal joining techniques have been developed and improved over the past 50 years, among which solid state diffusion bonding and reactive metal brazing have been explored most extensively for this purpose. Although the joints by these two techniques can meet the requirements in some cases, these methods, however, have limitations, in particular the latter route when used to produce ceramic–metal joints that are required to withstand high operation temperatures. A novel and promising method proposed for creating high temperature resistant interfaces between dissimilar materials is the partial transient liquid phase (PTLP) method [1], [2], [3], [4]. In this technique, multi-layer metals composed of a lower melting and thin outer layer on a thick core of refractory metal are used. A film of liquid can be formed at the joining temperature either because the melting point of the outer layer has been exceeded or because reaction between the core and the outer layer has produced a phase that is molten at that temperature. The liquid wets the solid surfaces and interdiffuses with the parent materials and core interlayer to produce a more refractory solid material. Homogenization of the outer layer during bonding or subsequent thermal treatment makes it possible to produce joints with higher operation temperatures. This method is still in the initial research stage and there exist many problems that need to be investigated, e.g. residual stress induced by thermal expansion mismatch, the microstructure and mechanical properties of the interface and the strength of the ceramics itself. The stress relief of the joints during and after joining is considered to be the most difficult problem [5]. In most cases, there is the need to investigate the residual stress distribution and endeavor to find some methods to minimize such residual stresses. The aim of this research is to investigate the influence of thermal expansion mismatch between ceramics and metals on the joint bond strength, such as alumina (Al2O3) and stainless steel-304 (SS304) joints made by PTLP brazing using a Ni or Ni–Cr core interlayer, and use the finite element method to simulate the stress distribution along the joint in order to improve the bond strength.

Section snippets

Materials

The ceramic used in this work is alumina with a relative density of 94%. The metal is SS304, which is widely used in structural components. Both base material blocks were cut into small pieces with the dimension of 20mm×12mm×4mm for brazing and for four-point bending tests.

The metal foils used as filler materials were commercially available and the pure metals or the foils were of high purity. The Ni foil as a core layer was 125 μm thick and the 80Ni20Cr foil as another core layer was 89 μm thick

Bond strength of ceramic–ceramic joints and ceramic–metal joints

The influences of the bonding parameters on the room temperature strengths of the joints are shown in Fig. 2. The results demonstrate that the bond strength of the Al2O3–Al2O3 joint is much higher than that of the Al2O3–SS304 joint under the same brazing conditions irrespective of the Ni or Ni–Cr core interlayer. Accordingly, it can be concluded that this difference is mainly caused by the thermal property mismatch between these two base materials. Fig. 3 illustrates that the CTE of SS304 is

Conclusions

The Al2O3–Al2O3 and Al2O3–SS304 joints brazed by PTLP bonding method have been investigated and the critical problem of residual stresses resulting from thermal expansion mismatch has been examined. From the results obtained, the following conclusions can be drawn:

  • 1.

    The Al2O3–Al2O3 joints have much higher bond strength than the Al2O3–SS304 joints even with the same brazing parameters. This demonstrated that the joint property can be deteriorated severely by the residual stress caused by the

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