Superplasticity and Diffusion Bonding in a Micro-Duplex Stainless Steel (Avesta 2304)

J. Pilling, Z.C.Wang, R.I.Todd and N.Ridley

(Michigan Tech and Manchester University)

Introduction

Diffusion bonding (DB) is often combined with superplastic forming (SPF) in the manufacture of complex cellular structures from microduplex a/b titanium alloys. The work described in this paper relates to the strongly emerging interest in the application of DB/SPF in the manufacture of heat exchangers from duplex stainless steels. Previous work has shown that duplex stainless steels can, in fact, be processed to develop the necessary superplastic properties, but little is known of their suitability for diffusion bonding given the highly protective chromium oxide normally present on their surfaces. Alloys such as 3RE60 and Zeron 100 are both superplastic and can be diffusion bonded although the process cycle for diffusion bonding has been difficult to predict. The present work involves the characterisation of the superplastic behaviour of Avesta 2304, and a/g stainless steel together with a study of the process of diffusion bonding.

Experimental

Tensile specimens of a microduplex stainless steelwere cut from a 1.5mm thick sheet of Avesta 2304, composition Fe-23Cr-4Ni-0.1N by weight, with a gauge length of 10mm, parallel to the rolling direction. The mechanical properties were determined using a repeated strain rate jump test to evaluate both the strain rate dependence of the flow stress and the strain rate sensitivity at temperatures from 880°C to 1000°C. In addition, constant strain rate tensile tests were carried out over the same temperature range at strain rates of 1x10-3 and 1x10-4 /s to determine both the elongation to failure and the extent of strain hardening. The microstructure was characterised in terms of the mean a/g grain size, while the effect of temperature on the volume fraction of the a and g phases was determined from samples that had been annealed and quenched from the test temperatures.

Sheet samples measuring 25 x 40 x 0.5 mm were prepared by grinding to a P60 finish and the surface roughness measured using Talysurf. The surface profiles were then digitised and a Fourier transform applied to the resulting waveform to determine the spectral distribution of roughness wavelengths. Pairs of sheets were electron beam welded around their edges prior to diffusion bonding, which was carried out using a hot isostatic press (ABB Q1H-9 Mini Hipper) at a temperature of 940°C and a pressure of 3MPa. Samples were bonded for periods of 1 to 60 minutes and the extent of bonding determined from metallographic examination of the linear percentage of porosity along the bond line.

Results

Superplasticity: The variation of flow stress with strain rate is shown in Fig. 1, from which it is evident that Avesta 2304 exhibits typical superplastic behaviour with a maximum strain rate sensitivity, m, occuring in the strain rate range 10-4 to 10-3/s, with m-values between 0.5 and 1. The activation energy for deformation in both regions II and III was determined to be ~210 kJ/mol. The temperature compensated strain rate is plotted as a function of stress in Fig. 2, from which the stress dependance of the strain rate can be seen to fall into two distinct regions: At low stresses, Region II, the stress exponent was found to be n=1.2 (m = 0.8) indicative of superplastic behaviour, while at the higher stresses, Region III, the stress exponent was n=3.2 (m=0.3) which was more consistent with conventional dislocation glide/climb controlled creep, i.e.,



where stress is expressed in MPa and in s-1.




Constant strain rate tensile tests showed that the flow stress increased with increasing strain at both 10-3 and 10-4 /s at each of the test temperatures studied, Fig. 3



Diffusion Bonding

The surfaces to be joined were found to have a roughness which consisted of two distinct waveforms, a short wavelength roughness with an amplitude of ~1µm having a wavelength in the range 1 to 4µm which was superimposed on a longer range waviness of ~50 to 55µm having an amplitude between 2 and 3µm. The variation of the area fraction bonded with time, determined experimentally, is shown in Fig. 4, together with that predicted by the model of Isostatic Diffusion Bonding by Pilling.



During the initial stages of bonding, the measured rate of void removal was faster than predicted and would correspond to the removal of the small voids created by contact between the 1-4µm wavelength roughness at the peaks of the 50-55µm waviness. However, once those small interfacial voids were removed, the bonding rate would slow considerably, as observed, as the removal of the larger voids created by the longer range waviness began to dominate the bonding process. During this second stage of bonding, any new voids created as the short wavelength (1-4µm) asperities impinged on the two surfaces would be removed relatively quickly in comparison to the longer wavelength (50-55µm) voids.

Acknowledgements

This work was supported by a grant from the Engineering and Physical Sciences Research Council, UK.

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