Research On Microstructures And Properties Control Of Al-Zn-Mg Alloy Profiles For High-speed Trains

As the key materials applied in body structures of high-speed trains,Al-Zn-Mg alloy profiles are serviced in harsh service conditions,such as alternating and high loads,acidic corrosion environment,it is badly needed to coordinate their different kinds of performances,for instance,strength,toughness,corrosion resistance and fatigue resistance.For this reason,in this paper,starting from the control of grain structures and multi-scale precipitates,conducted the basic research of the effects of microalloying elements,extrusion processes and aging treatments on the strength,toughness,corrosion resistance and fatigue crack growth resistance,and explored key technologies for the preparation of alloys with high comprehensive performance used in high-speed trains.The main conclusions are as follows:（1）The effects of microalloying on grain structure and properties of Al-Zn-Mg alloy profiles were investigated.By adding an appropriate amount of Cr and Zr,nano-scale dispersed precipitates are formed during homogenization annealing,which can significantly pin grain boundaries,sub-grain boundaries and dislocations,and inhibit the formation and growth of recrystallized grains,thus the grains are refined,and a grain structure dominated by low angle grain boundaries（LAGBs）and fiber-like deformed structures can be obtained,which greatly improves the strength,toughness,corrosion resistance,and reduces the fatigue crack growth rate（CGR）.The recrystallization fraction of the surface and center layers of Al-Zn-Mg profiles without Cr and Zr are higher than 85%,and the grains are coarse.After aging processes,the yield strength and tensile strength are significantly lower than that of profiles containing Cr and Zr,the fracture toughness and stress corrosion resistance are serious deterioration.（2）The hot deformation behaviors and recrystallization of Al-Zn-Mg alloys used in high-speed trains was investigated by hot compression test,the Arrhenius hyperbolic sine constitutive equation and processing maps were established,revealing the dynamic softening mechanism and microstructure evolution law of the alloy under different deformation conditions.Considering comprehensively,the optimized deformation conditions are 470-520℃/0.004-0.05 s^-1.Under these conditions,the alloy has good forming performance and can obtain uniform deformed structures.（3）The influence of extrusion methods on grain structures and properties of Al-Zn-Mg alloy profiles was investigated,and established uniform deformed grains and substructures as the grain structure mode for high-performance Al-Zn-Mg alloy profiles.Both forward double-acting piercing extrusion using annular billets and backward extrusion using solid bar billets can obtain this kind of grain structures.Since perforation is not required in extrusion using annular billets,the deformation temperature rise was smaller,uniform deformation could be achieved,and a finer grain structure was obtained.In peak aging and over-aging,the profiles’tensile strength reached 396.5 MPa and366.6 MPa,respectively.The conditional fracture toughness K_Q under over-aged tempers in T-L and L-T direction were 46.6 MPa·m^1/2 and 50.3 MPa·m^1/2,respectively.The tear strength and UIE（Unit Initiation Energy）in T-L were597.6 MPa and 194.4 N/mm,respectively,the exfoliation corrosion（EXCO）was PB level,and the stress corrosion sensitivity index I_SSRT was 2.38%.In the four-point bending test,the time of the crack occurring and fracture was 580 h and 1736 h,respectively.Meanwhile,it obtained excellent resistance to fatigue crack growth,whenΔK=20 MPa·m^1/2,the CGR in T-L and L-T direction was8.91×10^-4 mm/cycle and 8.40×10^-4 mm/cycle,respectively.（4）A new variable-rate non-isothermal aging process（variable-rate NIA）that applies different heating/cooling rates in different temperature ranges was proposed,the mechanical properties and corrosion resistance of the Al-Zn-Mg alloys were improved simultaneously,and its mechanism was investigated.The alloy produced by the variable-rate NIA obtained a 5-7%higher strength than that of T6 and a 14-16%higher strength that of T73,the tensile strength and yield strength reached 389.9 MPa and 345.2 MPa,respectively,the elongation after fracture was 16.1%.Meanwhile,the alloy has a higher resistance to intergranular corrosion（IGC）and EXCO than that of T73 temper.A lower heating/cooling rate（5℃/h）applied in the low temperatures range（100°C→145°C,145°C→120°C）and a faster heating/cooling rate（5℃/h）applied in the high temperatures range（145°C→175°C→145°C）result in a sufficient precipitation in the grains but avoiding severe coarsening,furthermore,a sufficient“secondary precipitation”happened during the cooling process,so that a higher volume fraction of not coarsened precipitates was obtained.Meanwhile,due to the proper exposure time at higher temperatures,discontinuous grain boundaries precipitates and moderate widths of PFZ are obtained on the grain boundaries,resulting in a simultaneously obtaining of high strength and high corrosion resistance.（5）Variable-rate NIA significantly improved the fracture toughness and resistance of fatigue crack propagation.The conditional fracture toughness K_Qwas increased by about 35%-40%compared with T73,the tear strength obtained in the Kahn tear test was higher than that of T6 and T73 by 67.3 MPa and 87.5 MPa,respectively,and the UIE reached 206.9 N/mm,which is higher than that of T73 by 7.8 N/mm.WhenΔK=20 MPa·m^1/2,the CGR of the variable-rate NIA was 7.41×10^-4 mm/cycle,which was 43.43%lower than that of the T6 peak-aged and 21.33%lower than that of T73 temper.Compared with T73 temper,the variable-rate NIA process has a larger number of new GP zones and smallη′particles formed during the cooling process,which increases the reversibility of dislocation slip,and leads to the reduced fatigue crack growth rate.Figures:119;Tables:33;References:233.