| Al-Mg-Si alloys are widely used in industry because of it's favorable properties such as low density, high corrosion resistance and excellent weldability and so on. On the base of Al-Mg-Si alloys, the alloying elements were adjusted and transitional metals were added to improve the microstructures and mechanical properties, a new type of alloy with medium strength and high corrosion resistance was developed, which is suitable for the need of carrier-based aircrafts. Some main research results are presented as the followings: The orthogonal experiments show that Cu and Si are the most important, Mg is less,whereas La is no useful to strengthening Al-Mg-Si alloys. The main precipitates and constituent phases of the Al-Mg-Si-Cu alloys with transitional Metals-T6 are researched through EPMA,EDS and TEM. The ageing precipitates are β(Mg2Si), θ(CuAl2) and S(CuMgAl2), the constituent phases are α(AlFeSi), β(AlFeSi), AlMnFeSi , AlCrFeSi and ZrAl3. The influences of the variations of Cu,Si,Mg and Mn contents were studied on the microstructures and performances of Al-Mg-Si alloys systematically. With increasing Cu content, strength and hardness of Al-Mg-Si alloys are greatly improved, but ductility dropped because that the number of the main strengthening phases (θ") increased, that the low melting temperature of the eutectic structures including CuAl2 leads to the trend of the overheating sensibility of Al-Mg-Si alloys and that the trend of intergranular fracture increases. Si increases the hardening rate and strength of the alloys because of increasing density of β"phases. Whereas the primary Si appears when Si content is higher than the stoichiometric value of Mg/Si for Mg2Si, which is harmful to mechanical properties. The strength dropped also due to increasing the coarse α(AlMnFeSi) with higher Mn content. The effects of some transitional metals on the microstructures and properties were investigated. The transitional metals promote the formation of more dispersed phases (α(Al12(Fe,Cr)3Si)) which have smaller and more favorable shape. Adding both Mn and Zr is the most effective on recrystallization resistance, which increases recrystallized temperature and makes recrystallized grain finer of Al-Mg-Si alloys. Adding Cr and Ti has no effect on ageing characteristics, whereas addition of 0.3%Mn ,0.08%Zr leads to a much stronger effect on ageing characteristics. An apparent change takes place in the microstructures by adding Ag and Zn, fine precipitates with high density distribute homogeneously within grains and GBPs(grain boundary precipitates) become coarser and discontinuous. Trace Cr,Ti,Mn and Zr are favorable for ductility, Cr and Ti have a little effect on strength, however, suitable addition of Mn and Zr also can elevate the strength. Mn can promote the speed of transition from flaky β(AlFeSi) to spherical α(AlFeSi) in homogenization process, which increase the ratio of Fe/Si and refine secondary phase particles. Homogenized at 570℃, coarse β'is undiscovered after water quenching, it is found after air cooling. The strain hardening exponent(n) is low under the peak-aging(170 ℃×7h) and over-aging(170℃×32h) conditions. Added transitional metals, the fracture mode turns from intergranular to transgranular fracture and the fractograph is made of deep and fine dimples, The relations between the fracture stain (εf) and the volume fraction of secondary phases, or the width of PFZs(precipitate free zones) and the size and the number of GBPs are presented as the following: It is the first time to widely study the influences of the ageing conditions on the microstructures of Al-Mg-Si alloys. Through DSC, the ageing precipitation sequence of Al-Mg-Si-Cu alloys is: GP zones β"β'β. The orthogonal experiments show that the θ"optimum ageing procedure is 140℃×22h+170℃×5h. With increasing the ageing temperature, the softening rate of the alloy increases, which leads to the reduction of hardness. With increasing the ageing time, the precipitates within grains becomes coarser, PFZs become wider, and GBPs become discontinuous and coarse also. When pre-ageing temperature is 20℃, "negative effect on the strength"appears, on the contrary, "positive effect on the strength"appears if the pre-ageing temperature increase above 80℃. According to TEM observation, the needle-like precipitates are coarser when pre-aged at 20℃. But "positive effect"appears also when the pre-ageing time(20℃) is as long as 3 weeks. For a short aging time(5min), the hardness is lower than pre-ageing, because of the remelting of GP zones. The optimum retrogressive temperature is 200℃. It is found through TEM observation that there are fine and dense needle-like precipitates ,coarse secondary phases within grains, and two type of (one is big, the other is small) discontinuous precipitates at grain boundaries. The hardness of the alloys obtained by thermomechanical treatment is higher than that in single aging, with the deformation ratio increasing, the hardness of the alloys increases and the time to maximum hardness becomes shorter, and the aging hardenability εf = k???? 1?f f????orεf ?k"?Dw3N...
|
PDF Full Text Request