| Nowadays,low-carbon economy is a social and economical development strategy for the countries all over the world. In transportation field, one of approach for security and high-speed, energy saving and emission decreasing is to reduce the weight of vehicles. Aluminium alloys were widely applied in transportation, aerospace, bridge, electronics and electricity, architecture, energy and power, mechanical fabrication fields due to their low density that is 1/3 of steel, high specific strength,good plasticity and processing properties,acceptable casting and welding properties and favorable high temperature, riveting, veneering properties.6N01 and 7N01 aluminium alloys are two kinds of wrought aluminium alloys which were developed by Japan.They are good materials for producing aluminium double-layer structural railway high speed vehicles because of their comparable high strength,good processing and welding properties.In this project, 6N01 and 7N01 aluminium alloys were being tested the tensile performance in varies ageing state provided by the factory, then they were being designed to be ageing treated in our independent ageing technics and then being analyzed the difference between designed technics properties and given samples technics properties comparatively;furthermore, they were being ageing treated in a series of ageing time and temperature, then being investigated the relationships between micro-hardness changes and the evolution of precipitates that made micro-hardness as a reference standard. Also they were being studied the microstructure characteristics in the two materials in varies ageing state using OM, SEM, EDS, DSC, TEM methods. The experimental investigation shows that:1. The best properties were achieved in T5 state for 6N01, yield strength and tensile strength were 228.7MPa and 265.4 MPa,respectively, elongation was 10.8%,the designed technics properties was close to that of T5, yield strength and tensile strength were 225.1MPa and 248.1MPa, elongation was 8.5%, both of them had high yield ratio; yield strength and tensile strength were low in T4 and H112,but with high elongation, which were 23.8% and 20.6, respectively. From the tensile fracture morphology, many dimple fractures had been seen in T5,T4 ,H112, which indicated that they were ductile fractures.2. As the ageing temperature was elevated,the time to peak hardness was shortened for 6N01, and also the peak hardness was much lower. The peak hardness time was 24 h, 12 h, 2 h, 1 h for 160℃, 180℃, 210℃, 240℃, respectively. Peak hardness was 119HV, 117HV, 103HV, 102HV, the lower ageing temperature, the slower over-ageing rate, vice versa.3. OM, SEM and EDS were used to analyze the phases constituents of 6N01 after it was being solution heat treatment and being aged. It was found that there were some indissoluble intermediates, which were mainly rod-likeα-AlFeMnSi,α-AlFeSi phases.These indissoluble intermediates could dissolve a handful of Mg, meanwhile a great number of black particles were found, EDS showed that the atoms ratio of Mg and Si was close to 2, which indicated that they might were Mg2Si. Ageing precipitation sequences under 180℃, 210℃, 240℃were investigated using DSC and TEM technology for 6N01. When 6N01 was aged at 180℃for 12 h, the main precipitates wereβ,, and a small quantity ofβ, phases, aged for 24 h, the number ofβ, phases increased,the particles size ofβ,, enlarged. While aged at 210℃and 240℃for 2 h, the main strengthen phases wereβ,, particle size was average 50nm. Aged for 8 h at 240℃, 6N01 had been completely over aged,only equilibrium phasesβwere found.4. The best properties were obtained in T5 state for 7N01, with high yield strength and tensile strength, which were 314.9MPa and 374.3MPa, respectively. elongation was 16.1%. Though tensile strength was high for H112 state, however,the yield strength was low, only 233.5 MPa, elongation was high for H112 state that was 19.1%. Fracture morphologies showed that raptures were ductile fractures in all state.5. When 7N01 aluminium alloy was aged at 120℃, there were two peaks hardness, which appeared at 48 h and 96 h, the peak hardness were 138HV and 146HV, respectively. While the ageing temperature was elevated to 180, 200, 260, 300℃,the peak hardness time was shortened sharply, furthermore, their peak hardness was much lower than that aged at 120℃.6. OM morphology showed that there was obvious deformation with fiber texture which was called extrusion effect. Due to the function of Mn and Cr, there was no evident recrystallization. SEM and EDS analysis showed that there were some undissolved intermediates, which contained high concentration of Fe, Mn. According to some literatures, the undissolved intermediates were α-AlFeMnSi. Many black fine particles were found , which might beMgZn/ Mg5Zn6 phases. DSC and TEM showed that the first peak hardness appearing at 48 h aged at 120℃were attributed to GPII zones. The second peak hardness occurring at 96 h was mainly due to the contribution of GPII andη,phases. While 7N01 was aged at 180℃and above that, the peak hardness was achieved rapidly, then decreased rapidly too. For 7N01, when it was aged at 180℃for 4 , 8 , 12 , 24 h, there were mainly long plate-likeη,phase and round platesη. Ageing time from 4 h to 24 h, it was found that the transformation ofη,toη. Till 24 h, there were only round platesηphase,ηphases precipitated in the grain boundaries.
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