Study On Heat Treatment Process Of The Cast Al-Li-Cu-Mg-Zr Alloy

As deforming Al-Li alloy, cast Al-Li alloy is one of the most promising light metal alloy for production of aircraft nowadays for its low specific density, high specific strength, low growth rate of fatigue crack, high resisting force of stress corrosion, wide range of Li content etc., so has great potentials in thin wall products of complex aviation structure. At the present time, deforming Al-Li alloy has been extensively used in modern aerospace plane, but it is initial step in cast Al-Li alloy study. Based on detail reviews of the progress of Al-Li alloys, the heat treatment process of the cast Al-Li-Cu-Mg-Zr alloys (Al-3.02Li-4.24Mg-1.26Cu-0.14Zr-0.27Y &Al-2.94Li-1.19Mg-1.24Cu-0.16Zr-0.20Y) was studied. Vacuum melting method and mathematically orthogonal heat treatment method, were carried out together. The effects of the heat treatment process and heat treatment processing parameters on the microstructure and mechanical properties of the testing cast Al-Li-Cu-Mg-Zr alloys have been investigated by means of mechanical property measurement, optical microscopy, scanning electron microscopy and microhardness tester, respectively. Influence on microstructure and mechanical properties of the test Al-Li-Cu-Mg-Zr alloys caused by different heat treatment process has been studied. Influence degree of heat treatment processing parameters such as solution temperature, solution time, ageing temperature and ageing time on microstructureand performance of the testing Al-Li-Cu-Mg-Zr alloys has been analyzed. Similarly, heat treatment strengthening mechanism of cast Al-Li-Cu-Mg-Zr alloy has also been discussed.The main results are as follows:1. The results of the orthogonal heat treatment test indicated that influence degree of heat treatment processing parameters on microstructure and performance in turn is solution temperature, ageing time, solution time and ageing temperature. The best heat treatment process for cast Al-Li-Cu-Mg-Zr alloy should be:53O°C X 14h+190°CX8h.2. Effects on microhardness and tensile strength by heat treatment process are consistent. The tensile strength and microhardness maximizing treated by 53O°C X 14h+190°CX8h.3. With solution time expand (or solution temperature grows), precipitated phase has been dissolved in Al matrix step by step while solution temperature ( or solution time) is unchangeable. So the solute atoms (Mg> Li) diffuse between intracrystalline and intergranular mutually, that makes the PFZ narrow, even vanished. The size of equilibrium phase on grain boundary diminishes. However, the degree of intergranular continuity increases. At the same time, the strength of grain boundary increases and the microstructure becomes homogeneous.4. 8'(Al3Li) is the main strengthening phase in cast Al-Li-Cu-Mg-Zr alloy. It often likes a ball and distributes in the Al matrix homogeneously. It nucleates and precipitates from the Al-Li supersaturated solid solution, which strengthens the Al-Li-Cu-Mg-Zr alloy.5. p' (Al3Zr) can be the nucleating center of 8', which accelerates the ageing precipitation process of 5'.6. The main reasons of brittle fracture in cast Al-Li-Cu-Mg-Zr alloy are coplanar slipping, which caused by dislocation cutting 5', and PFZ, which generates on grain boundary.