Phase transformations in heat treatment of Al−Li alloys and optimization of operational properties of semifinished products from these alloys

Abstract

In recent years AI -Li alloys have become very popular in the aircraft industry. These alloys are characterized by a high specific strength and rigidity which increases substantially the load ratio and fuel efficiency of the parts. As a rule, Al -Li alloys are used in an underaged state, which provides a high ductility over the height of the semifinished products. However, the threshold stress (Ocr) in corrosion cracking in this state does not usually exceed 25 -50 N/ram 2, and the yield strength is not always satisfactory. The results of a study of the mechanical properties and corrosion resistance of some AI-Li alloys are described and the ways for optimizing these properties are described on the basis of the phase transformations in the alloys.

Decomposition of the solid solution in commercial semiproducts from aluminum alloys

“…This concept agrees well with the data of differential scanning calorimetry (DSC) of aluminum alloys. The scheme (succession) of the decomposition of the solid solution presented in [16] is implementable under industrial conditions, has been confirmed by the results of transmission electron microscopy, x-ray phase analysis, and differential scanning calorimetry, and agrees well with the thermokinetic diagrams [9,12,13,16,17]. The presented series of phase transformations are realized in actual heat treatment of a wide range of commercial stamped preforms from alloy 1420.…”

“…At the present time we have developed a regime TV2 for full-scale treatment that removes all the defects and provides a higher operational reliability than regime TVI [8]. This is connected predominantly with the 8' --* 8 phase transformation in aging and the reduction of the supersaturation of the solid solution with lithium [8,16]. This effect is especially well manifested in alloys with a high lithium content (2.0-2.5%).…”

“…Works [9,13,16,17] have been devoted to a complex study of phase transformations in various aluminum alloys.…”

Delayed fracture of aluminum alloys

“…of the mechanical properties, the crack resistance, and the corrosion resistance in alloy 1420 [9,10]. Further reduction of the volume fraction of the 5'-phase and growth of the content of the &phase occur after a stabilizing aging which, as has been shown in [15,23,24], considerably increases the corrosion resistance of aluminum-lithium alloys. The stabilizing aging (regime TV2 for alloy 1420) accompanied by segregation of a &phase considerably compensates for the decrease in ~cr in the repeated heating and reduces the residual stresses even more than regime TVI (by 65 -70%).…”

“…It seems that the low thermal stability of aluminum-nickel alloys can have a negative effect on the anisotropy of ~d-/c [6] and on crc~ [15], which finally increases the susceptibility of the material to delayed fracture. In this connection it is desirable to devote more attention to the study of the thermal stability.…”