chapter-11

CHAPTER-11 INTRODUCTION 1.1 GENERAL BACKGROUND A. series. of. aluminium alloys. commercialised position. over. amongst. have been developed and. the past years which occupy a. the various alloys used. unique. in engineering. applications, the aerospace industries in particular. These alloys. are characterised by their high strength. ratio,. good. formability, resistance to corrosion and high. thermal conductivity. applications their and. to weight. The main wrought alloys for aerospace. are Al-Cu and Al-^Zn-Mg-Cu alloys which derive. desired properties from thermo-mechanical. the cast alloys like Al-Cu,. treatments. Al-Mg alloys which .derive. their properties from thermal treatments. In course of the past work,. various alloying elements have. been alloyed with aluminium so as to produce alloys of high strength and low density and the attempt to develop an Al-Li e alloy is no exception to such an endeavour. ‘since the most effective. way of reducing the overall weight of an aircraft. is by using considered applications. an alloy of. to have. low density,. Al-Li. a very high potential. for. points. a. reference. to the Al-Li phase diagram. possibility of 1 hardenable in nature . The. out. concept. aerospace. as lithium (density - 0.53 gm/cc) addition can. bring down the density of the alloy considerably. more,. alloy was. the. producing. of introducing Li in A1 alloys. 1. Further (Fig-1.1). an alloy,. age. in structural. LITHIUM PERCENT ATOMIC. Do 3 a n i V a 3 d W 3 i. application. perhaps. dates. back to 19202. The first such. commercial Al^Li alloy, named. Sceleron^ was. developed in. Germany in 1922 but was phased out due to the developments in Duralumin alloys.. Since then attempts have been made by. many 4«14 |-0 develop and commercialise the Al^Li alloys. Unfortunately, the past attempts did not yield the desired results due to the dificulties associated with melting and casting10'11 toughness. 1^. poor. of these. ductility12 alloys.. and. low fracture. Further, the. incomplete. understanding of the strengthening mechanism of this alloy was the main cause of failure of developing this into a potential material for aircraft industries. «. In the recent past, the hike in fuel prices has resulted in the extensive search for new aluminium alloys of high specific strength.. Consequently the AlxLi alloys have been. investigated more intensively in the last decade. Perhaps * the major research activities on Al^Li alloys started only after 1970 and the development of this alloy has taken a concrete shape in the '80's. ALCAN has claimed1^ to have developed medium strength low density commercially viable Al«Li alloys.. The development of Al«Li alloys has proceeded. in two directions.. Ingot metallurgy combined withthermo*-. mechanical processing and rapidly solidified powder metallurgy approach1®.. In the recent years, a number of. review articles have appeared covering different aspects of Al^Li alloy systems.. The existing literatures on these. alloys as are currently available are mostly confined to the 3. studies on thermo^mechanical treatments and structure property correlation.. It appears that the extensive. research as above has been able to establish clearly the hardening mechanism of the alloy17. However, no literature seems to have appeared reporting on the use of Al^Li alloys in the form of castings.. Further, no publication seems to. have considered it necessary to deliberate upon the melting technology of the alloys.. A bare mention is found in most. of the literatures regarding the difficulties in melting O these alloys. Balmuth et al in his review paper covering 61 publications concluded that melting and ingot casting of Al alloys containing Li are complicated by the reactivity of Li making these alloys systems undesirable from producers view point.. the. Averill et al-*-® and Divecha et a l ^. had reported about the problem related to melting and casting Al^Li alloys.. It thus appears that not much work. has been reported on the melting and casting characteristics of the Al«Li alloys.. Perhaps the work done so far are. considered as proprietary in nature.. Furthermore, no. literature seans to have reported about the use of light Al« Li alloy castings either in as cast condition or after thermal treatment of the same.. It may also be mentioned. here that the past work on Al<-Li alloy systems were mostly carried out with ultra pure (99.7%) Al and 99.9% Li metals. Keeping these in view, the present work has been undertaken uti 1 ising commercially pure Al (99.7%) and Li (99.8%) with the objectives as envisaged in 1.2 below.. 4.