In Situ Observation of High-Temperature Microstructure Evolution and Phase Transformation of 2195 Al-Li Alloy
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INTRODUCTION
THE 2195 Al-Li alloy is an excellent candidate for the low temperature propellant tanks due to its outstanding properties such as low density (2.47 to 2.50 g/cm3), high specific strength and rigidity, excellent corrosion resistance and good welding suitability.[1–4] The high strength of this alloy results from the addition of Li, which is acknowledged from a long time ago. Peters et al.[5] have summarized that every weight percent Li added to aluminum alloys reduces the weight by 3 pct and raises the elastic modulus by 4 pct. NASA’s weight reduction program for the space shuttle’s external tank primarily involves the replacement of the present 2219 or 2A14 aluminum alloy with 2195 Al-Li alloy.[6] In the present decade, Rioja and Liu[7] did a very thorough review of the physical, mechanical and metallurgical variables of Al-Li alloy over the passing decades and expected the Al-Li alloy as the next promising generation of air and space craft material.
JIANYU YUAN, JINCHENG PANG, GUOJUN XIE, YING WANG, and LU HAN are with the Aerospace Research Institute of Materials & Processing Technology, Beijing, 100076, P.R. China. Contact e-mail: [email protected] Manuscript submitted June 21, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The fabrication of space craft with 2195 Al-Li alloy includes the welding process, which presents a main challenge since the welding parameters have to be properly controlled in order to eliminate defects both in the weld joint and heat affected zone (HAZ). Many efforts have been taken to investigate this problem. Chen and Chaturvedi[8] studied the effects of weld heat-affected zone simulation and assessed post weld heat treatment on the microstructure and mechanical performance of 2195 Al-Li alloy. Qin et al.[9] studied the precipitation and microstructure in friction stir welded 2195 Al-Li alloy, which shows the main strengthening phase of the matrix and its dissolving behavior during welding. Zhang et al.[10] studied the microstructure and mechanical properties of laser beam welded Al-Li alloy and assessed the effects of dendritic solidification structure with the LiAlSi and CuAl2 phases and a small quantity of Mg2Si phase at GBs. Moreover, the microstructure evolution of 2195 Al-Li alloy under exterior stress has also been investigated by different researchers.[11,12] The variation of grain size and orientation have been recognized at elevated temperatures. Similar efforts to study the effects of temperature on the microstructures and mechanical properties of the melted zone in the welded joints for other kinds of aluminum alloys, such as 2219 alloy, have also been tried by Li et al.[13] and Wang et al.[14] The evolution of a phase and h¢ (Al2Cu) phase in the partially melted zone and the effects of aging treatment on the mechanical
Table I. Chemical Composition of 2195 Alloy (Weight Percent) Alloying Elements
Other Elements
Material
Cu
Li
Mg
Ag
Zr
Al
Fe
Si
Ti
Mn
Actual Nominal
3.8 3.7~4.3
1.09 0.8~1.2
0.50 0.25~0.8
0.26 0.25~0.6
0.12 0.08~0.16
bal. bal.
0
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