Diffusion Bonding of Advanced Aerospace Metallics
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DIFFUSION BONDING OF ADVANCED AEROSPACE METALLICS DAVID V. DUNFORD AND ANDREW WISBEY Materials and Structures Dept., Defence Research Agency, Farnborough, Hants, UK ABSTRACT Advanced aluminium and titanium alloys, metal matrix composites (MMC's) and intermetallic compounds are of considerable interest to the aerospace industry. These materials offer significant mechanical improvements over many conventional materials. Appropriate joining technologies are being developed to utilise the advantages these materials offer in aerospace applications. Diffusion bonding offers considerable potential as a joining process. This keynote paper will review diffusion bonding with reference to these advanced metallic systems. INTRODUCTION Considerable advances in metallic systems pertinent to the aerospace community have been made in recent decades. The goals of lighter and stiffer airframe structures have led to potential applications for lower density aluminium - lithium alloys and the rapid development of both continuous fibre and particulate reinforced metal matrix composite systems. Higher temperature and lower density metallic systems are being developed for gas turbine applications to improve thrust to weight ratios and engine efficiency. The development of improved manufacturing technologies, such as superplastic forming, has enabled complex structures to be fabricated from expensive materials cost effectively by maximising material utilisation. To preserve these improvements and allow successful incorporation into aerospace structures, advanced joining techniques are required. Conventional fusion welding may cause unacceptable microstructural damage and poor mechanical properties. Solid state joining can offer significant advantages for some advanced materials. Several solid state processes depend on high pressures and large scale deformation; for example pressure welding, roll bonding and explosive bonding for example. These processes are used to produce semi-finished products such as clad sheet or plate but are generally unsuitable for MMC's. In friction and ultrasonic welding, intense deformation confined to the bond interface region raises the temperature and disrupts and disperses the oxide films before welding. These processes are discussed in detail in reference 1. In diffusion bonding (DB), through thickness deformation is usually small (5% or less), low pressures (much less than the macroscopic yield stress) and high temperatures (> 0.5 Tm, where Tm is the absolute melting point) are typical. Hence the deformation is confined primarily to surface asperities [2]. DB by hot isostatic pressing (HIP) under inert gas pressure has the unique advantages of high pressure (up to 300 MPa) and temperatures (up to 2000'C) with minimum total deformation giving greater flexibility in component shape. This process is used extensively for consolidating powders. Compared with other joining processes, diffusion bonding offers [3]: i) ii) iii) iv) v) vi)
joint strengths approach or equal parent material strength low deformation an
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