Microstructural and mechanical characterisation of an Al-21.8 wt.% Ge brazing alloy with a globular morphology of the primary Al-rich phase

Abstract

This paper is concerned with the microstructural and mechanical characterisation of a hypoeutectic Al-Ge alloy designed to join the plasma facing components of thermonuclear fusion reactors to permanently cooled substrates. The alloy was prepared from pure aluminium and germanium by casting, hot rolling and heat treatment above the eutectic temperature. This treatment leads to a globular morphology of the primary Al-rich phase provided that the thickness reduction during rolling is sufficient, about 20%. Increasing the rolling reduction, however, leads to liquid entrapment in the globules, the amount of which depends on the temperature in the semisolid range. The mechanical behaviour of the material was determined in tensile tests at room and elevated temperature after various heat treatments in the semisolid state and compared with the mechanical properties after hot rolling. In addition, compression tests were also carried out in the semisolid state. These tests show that the globular materials deform under very low stresses without segregation of the liquid. A comparison is made with non-globular materials obtained simply by partial remelting of the as-cast alloy.

Introduction

Brazing is a classical joining technique [1] where a metal placed between the two parts to be joined is heated up to its melting point. This leads to the formation of a metallurgical bond between the brazing alloy and the respective parts. The brazing is normally carried out at 30–50°C above the liquidus temperature of the filler metals. The brazing alloys are classically of eutectic composition. Sometimes other elements are added. For instance Ti or Zr is added to the Ag-Cu eutectic for reactive or wetting purpose. Similarly Mg is used in aluminium brazing alloys for reducing the surface oxides. The brazing of aluminium alloys is usually performed with filler metal based on the Al-Si eutectic with a eutectic temperature of 577°C: the brazing temperature is then ∼600°C.

For several years, brazing in the semisolid state has been considered for joining plasma facing components (PFCs) to permanently cooled, high conductivity copper substrates in fusion reactors [2]. One of the advantages of this type of brazing is the possible easy in-situ replacement of damaged components by heating just above the solidus temperature, and this could be done whatever be the shape of the components. However, the alloy must exhibit a low viscosity to allow this reversible brazing so a globular morphology of the solid phase is desired. For this application the joint must in addition constitute a ‘thermal bond layer’ (TBL) to provide a good thermal contact between the PFCs and the substrate [3]. It must be also compliant to reduce the interfacial thermal stresses, metallurgically compatible with both the PFCs and the substrate materials and stable in service conditions. Finally, to allow easy replacement the solidus temperature of the alloy must not be too high; nevertheless it should be high enough to avoid melting in operating conditions.

One system which can fulfil these conditions is the Al-Ge system which exhibits a eutectic temperature at 424°C [4]. Changing the composition of the alloy below the eutectic (51.6 wt.% Ge) allows the liquid volume fraction at a given temperature to be varied and also the semisolid range to be changed. Experiments have been carried out with such alloys for several years and some of the results have already been reported on various compositions [5], [6], [7], [8]. One of the most promising compositions is Al-21.8 wt.% Ge since it leads to a relatively large volume fraction of liquid (37%) at the eutectic temperature.

The aim of this paper is to detail all the results obtained with this alloy. The fabrication procedure of sheets will be first briefly recalled. The microstructure of the material will then be characterised as a function of the fabrication parameters. The mechanical properties of the as-produced sheets will thereafter be presented at room temperature as well as at elevated temperature. Finally, the rheological properties in the semisolid state will be detailed and compared with those of non-globular microstructures.