Diffusion bonding of superplastic 7075 aluminium alloy

doi:10.1016/S0921-5093(98)00958-7

Materials Science and Engineering: A

Volume 266, Issues 1–2, 30 June 1999, Pages 295-302

https://doi.org/10.1016/S0921-5093(98)00958-7 Get rights and content

Abstract

Diffusion bonding (DB) of superplastic 7075 Al alloy has been investigated for various temperatures, pressures and times, using a Gleeble 1500 test machine. After the removal of surface oxide, an organic solution was used to protect the surfaces prior to bonding. The strengths achieved after bonding were dependent on interface grain boundary migration and on grain growth during the bonding process. Under optimum conditions, bonds having parent metal shear strength and microstructure were obtained. The optimum temperatures for diffusion bonding, 510–520°C, corresponded with those at which the material exhibited optimum superplastic behavior. The characteristics and mechanisms of bonding are discussed.

Introduction

There is considerable interest in extending the diffusion bonding (DB) and superplastic forming (SPF) technology which has been developed for Ti alloys to other superplastic materials, and, in particular, to Al alloys. While several high strength Al alloys have either been designed and/or specifically processed to give excellent superplastic properties, diffusion bonding of these materials is difficult because of the tenacious surface oxide which inhibits metal to metal contact. Hence, to achieve a sound bond it is necessary to remove the surface oxide at least partially, or to disrupt its continuity.

Previous procedures used to produce diffusion bonds in superplastic Al alloys have been outlined by Partridge [1], Pilling [2], and Ridley and Livesey [3]. Methods have included the solid state bonding of uncoated surfaces using static compression [4], [5], [6], [7]; or gas pressure [5], [8], [9]; the use of a range of interlayers [7], [10]; the deposition of protective coatings on, or implantation in, argon sputter-cleaned surfaces[11], [12], [13]; the application by vapour deposition, or roll cladding, of transient liquid interlayers to the sufaces to be bonded [3], [5], [6], [7], [14], [15]. Despite these studies a commercially acceptable DB or DB/SPF process has not yet been developed for Al alloys.

In the present work diffusion bonding of superplastic 7075 Al alloy has been investigated by adopting a new approach to overcome the problem of re-oxidation of cleaned surfaces prior to bonding. This involved the application of an organic solution to form a protective layer on an oxide-free surface obtained by mechanical cleaning. The liquid then volatilized in vacuum during the bonding procedure leaving no residue.

Section snippets

Material

The starting material was Al alloy 7075 of wt% composition: Al–1.73Cu–2.41Mg–5.94Zn–0.38Mn–0.22Cr which was received in the form of 12 mm thick hot-rolled plate. The alloy was thermomechanically processed to develop a superplastic microstructure using a Rockwell type treatment [16]. The procedure involved: (1) solution treatment at 500°C for 5 h followed by water quenching; (2) overageing initially at 140°C for 3 h and then at 180°C for 3 h, water quenching; (3) warm rolling at 200°C to a

Mechanical testing

The absolute values of bond shear strength were not used to evaluate the extent of bonding because specimens having different bonding conditions (i.e. different thermal histories) could show variations in mechanical strength. Further, the effect of specimen constraint and bending stresses during testing could influence strength. Hence, to exclude these effects, the specific strength, μ, was used to assess mechanical properties and is defined as: $μ=τ_{db} /τ_{b}$ where τ_db is the bond strength, and τ_b is

Conclusions

1.
Diffusion bonds with parent metal shear strength and microstructure have been produced in a superplastic 7075 Al alloy using a procedure which involved coating the cleaned surfaces with an organic solution.
2.
The main characteristics of the organic solution, which consisted of acetone, styrene, and polystyrene, were that it could be readily coated onto the cleaned surfaces to be joined so protecting them from oxidation, and that it volatilized completely on heating in vacuum to the bonding

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¹: Present address: Department of Metal Forming, Northeastern University, Shenyang, People’s Republic of China.

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Diffusion bonding of superplastic 7075 aluminium alloy

Abstract

Introduction

Section snippets

Material

Mechanical testing

Conclusions

Scripta Metall. Mater.

Mater. Sci. Eng.

Mater. Sci. Eng.

Acta Metall.

Superplasticity (AGARD-LS-168)

J. Mater. Sci. Technol.

J. Mater. Sci.