Abstract
Al-Li alloys have been extensively used in aerospace vehicle structure since the presence of lithium increases the modulus and reduce the density of the alloy. Especially the third generation Al-Cu-Li alloy shows enhanced fracture toughness at cryogenic temperatures so that the alloy has been used on the fuel tank of space launchers, like Super Lightweight External Tank of the Space Shuttle. Since the commercial size of the plate cannot accommodate the large tank size of the launcher, joining several pieces is required. However, lithium is highly reactive and its compounds can decompose with heat from conventional fusion welding and form different types of gases which result in formation of defects. In this study, the microstructure change is investigated after solid state welding process to join the Al-Cu-Li sheets with optical and transmission electron microscopic analysis of precipitates.
Highlights
Aluminum lithium alloy has been used for aerospace structure because its higher modulus and lower density comparing to conventional aerospace aluminum alloy [1]
The details were examined by transmission electron diffraction microscopy (TEM; Titan G2 Double Cs corrected TEM, Philips/FEI) with an acceleration voltage of 200~300 kV, and composition was determined by energy-dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS) and scanning TEM-high angle annular dark field (a)
As T phase is grown from GP zone, T’’ and T’ phases, the size of the T series is varied from 10 nm to 5 Pm
Summary
Aluminum lithium alloy has been used for aerospace structure because its higher modulus and lower density comparing to conventional aerospace aluminum alloy [1]. Solid state welding generally involves heating the metal with a suitable amount of pressure so that homogeneous and complete microstructural joining is possible with no melting of the parts to be joined. Solid state diffusion welding is an attractive manufacturing method for aerospace applications where mechanical properties in the bond area and a sound metallurgical bond are important [7] Diffusion welding is such a process in which two matched surfaces are held together at a temperature range between 0.5 of the absolute melting temperature of the materials and the room temperature under a low pressure without causing a macroscopic plastic deformation in the materials. A solid phase bond is created between the pieces of metal that have been stirred and mixed together This process requires low energy input and joins materials. Samples for the TEM observation were prepared on Molybdenum-grid by a focused ion beam (FIB)-SEM hybrid system equipped with a Ga+ focused ion beam (FIB) column (FIB; SMI3050SE, SII Nanotechnology Inc.)
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