【作者】 何立子；

【导师】 崔建忠； 张晓博；

【作者基本信息】 东北大学 ， 材料成形与控制工程， 2001， 博士

【摘要】 Al-Mg-Si 系合金密度低、耐腐蚀、焊合性好,具有良好的综合机械性能。本文在Al-Mg-Si 系铝合金基础之上,调整合金元素和过渡族元素,改进其组织和性能,开发出一种适合舰载飞机的中强、耐蚀铝合金。本文研究的主要结果如下: 正交实验表明,Cu 和Si 对Al-Mg-Si 系合金的强化效果最显著,Mg 次之,而稀土La 对强化不起作用。采用电子探针、EDS 分析和TEM 组织观察,得出含微量元素的Al-Mg-Si-Cu 系合金-T6 的时效析出相有:β(Mg2Si)相、θ(CuAl2)相和S(CuMgAl2)相,弥散相和夹杂相有α(AlFeSi)、β(AlFeSi)、AlMnFeSi、AlCrFeSi 和ZrAl3等。系统地研究了Cu、Si、Mg 和Mn 含量变化对Al-Mg-Si 系合金组织性能的影响。结果表明:Cu 含量增加可明显提高合金的强度和硬度,但塑性下降。Cu 含量增加,合金中的主要时效强化相θ”的数量相对增加;因含CuAl2相的共晶组织熔点较低,合金的过烧敏感倾向增加;合金的沿晶断裂倾向增加。Si 加快合金的硬化速度,合金中的针状β”相的析出密度随其含量增加而增大,合金显著强化,但含量过高时合金中析出较多的单晶Si,合金的强度和塑性明显下降。Mn 含量增加,增加了粗大夹杂相α(AlMnFeSi)的数量,合金的强度降低。同时,系统研究一些过渡族元素对Al-Mg-Si 系合金组织性能的影响。结果表明:Cr 和Ti 减少了铸态组织中粗大含Fe 杂质相β(Al9Fe2Si2)的数量,促进了尺寸较小、形状较利于变形的α(Al12(Fe,Cr)3Si)相的形成,增加了含Fe 弥散相的体积分数。Mn 和Zr同时添加对再结晶的抑制效果最显著,提高合金的再结晶温度,再结晶晶粒细化。Cr和Ti 对合金的时效特性基本没有影响,添加0.3%Mn 和0.08%Zr 的硬化效果最显著。添加Ag 和Zn 后合金的显微组织发生了明显不同的变化,使合金同时具有高密度细小的晶内析出相(T6 组织的特征)和较大、不连续分布的晶界析出相(T7 组织的特征)。微量的Cr、Ti、Mn 和Zr 提高合金的塑性,Cr 和Ti 对强度影响不大,但适当含量的Mn 和Zr 可提高合金强度。均匀化研究表明:少量的Mn可明显加快均匀化过程中片状的β-AlFeSi相向较小的、粒状α-AlFeSi 相的转变,转变过程中涉及到Fe/Si 比的增加和第二相颗粒的细化。实验合金在570℃均匀化空冷过程中析出粗大的β’相,在急冷中没有观察到。变形行为研究表明:实验合金在峰时效状态(170℃×7h)和过时效状态(170℃×32h)状态下的应变硬化指数n 值较低,合金的塑性较低。添加微量元素后,合金的断口由细小且深的韧窝组成,断裂方式由沿晶断裂转变为穿晶断裂。提出了断裂应变εf与第二相更多还原

【Abstract】 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更多还原