Mg和Cu元素调控的7xxx系铝合金凝固特性及强韧化机理研究

发布时间：2018-02-22 09:33

本文关键词： 高强度铝合金成分微观组织力学性能热力学　出处：《北京科技大学》2016年博士论文　论文类型：学位论文

【摘要】：我国目前在航空用高强Al-Zn-Mg-Cu系合金(7xxx系铝合金)的生产、研究方面取得了不少成果,但在成分设计、组织控制和强韧化等原理、机理认识方面仍存不足。成分设计对该系合金强度、韧性和耐蚀性的作用规律及机制属领域核心内容,相关资料国外鲜有公开报道。系统研究该系合金成分—组织—性能间的关系对于理解、消化国外先进Al-Zn-Mg-Cu系合金设计理念及开发具有自主知识产权的新合金具有重要理论和工程应用价值。本文基于对该系合金高温相组成影响更为显著的Mg、Cu元素含量的调控,设计了系列高Zn含量(约8.5 wt%)的Al-Zn-Mg-Cu合金,研究Mg、Cu元素在制备流程(铸造、均匀化、轧制、固溶和时效)中对组织和主要性能的作用规律及机制。此外,采用计算热力学、动力学技术对一些重要实验现象进行了模拟分析,实验与模拟所得规律基本一致。主要研究结论如下：设计合金的实际凝固路径介于Scheil model和Level rule两种凝固模式之间,且更接近前者。铸锭中存在的大块σ相和(σ+θ)团块的含量主要由Mg含量决定,Cu元素的作用次之,具体为：Mg含量越高,形成的大块。相越多,(σ+0)团块越少；Mg含量相近时,合金Cu含量越高,形成的大块σ相越多,一般地(σ+θ)团块越多。高Mg高Cu含量合金难以形成(σ+0)团块。设计合金经460℃/168 h单级长时和(460℃/24h+475℃/24h)双级均匀化处理后的相组成与热力学计算所得Al-Zn-Mg-Cu (Zn=8.5 wt%)四元相图等温截面基本相符。研究显示,460℃平衡相组成受Mg含量影响显著：低Mg合金更易进入Al单相区,即使其含有高的Cu含量。475℃较460℃能更充分溶解第二相,此时平衡相组成主要由(Mg+Cu)含量决定：(Mg+Cu) 4.35 wt%时合金可获得A1单相固溶体,否则难以获得Al单相固溶体。低Mg含量或高Zn:Mg比有助于加速合金时效析出进程,即合金能更快达到峰时效状态并具有更大的过时效速率。这是由于在第一级时效120℃/6 h过程中,低Mg含量或高Zn:Mg比能促进GPII区的形成,而高Mg含量或低Zn:Mg比会促进GPI区的形成。由于GPI热稳定性较GPII差,高Mg含量或低Zn:Mg比合金在120℃至160℃升温过程中较少的GP区能够保存下来成为第二级160℃时效阶段η相的形核点。Cu并不明显影响时效进程。相同时效状态下,设计合金的电导率、硬度、强度和韧性主要由Mg含量决定：提高Mg含量,合金硬度、强度提高,而电导率、韧性会相应降低。提高Cu含量也能获得类似效果,但效果较弱。实验及热力学模拟研究表明,提高Mg含量会显著增加欠时效及过时效态合金中析出相粒子的体积分数,不明显影响欠时效态合金粒子尺寸,但会一定程度上减小过时效态合金粒子尺寸,进而提高欠时效/过时效态合金的强度和硬度。提高合金Cu含量可在一定程度上增大析出相粒子体积分数,使合金强度和硬度获得提高,但其影响明显弱于Mg。随Mg含量增加,T76态合金晶界析出相粒子面积分数增大,晶内与晶界无析出区屈服应力差也会提高,这将促进沿晶断裂,导致合金韧性降低。Cu对合金韧性的影响较Mg弱。对于低/中Mg含量(如1.5/2.0 wt%)合金,提高Cu含量会增大晶内与晶界无析出区屈服应力差,并提高合金再结晶分数,这将在一定程度上促进沿晶断裂,导致合金韧性降低。对于高Mg含量(如2.5 wt%)合金,提高Cu含量还会增加固溶处理后粗大未溶金属间化合物粒子的数量,进而增大合金断裂倾向,使韧性进一步降低。
[Abstract]:China's current use of high-strength Al-Zn-Mg-Cu alloy in aviation (7xxx Aluminum Alloy) production, the research made a lot of achievements, but in the composition design, principle of microstructure control and toughening mechanism, understanding is not enough. The strength of alloy composition design, function mechanism and regularity of toughness and corrosion resistance of the genus the core content areas, foreign related data rarely publicly reported. Study on the alloys component, microstructure and properties of the relationship between the understanding of the new alloy Al-Zn-Mg-Cu alloy digesting foreign advanced design concepts and development with independent intellectual property rights has important theoretical and practical value. The high temperature of the alloy phase composition effect is more remarkable. Based on Mg, Cu control elements, designed a series of high Zn content (about 8.5 wt%) of Mg, Al-Zn-Mg-Cu alloy, Cu element in the preparation process of preparation (casting, homogenization, rolling, solid Effect of solution and aging) law and mechanism of organization and the main performance. In addition, the calculation of thermodynamics, dynamics techniques were simulated and analyzed some important experimental phenomena, experimental and numerical simulation results are basically consistent. The main conclusions are as follows: between the actual design of alloy solidification path between Scheil model and Level rule two solidification mode and, closer to the former. In the presence of large ingot and sigma (sigma + 0) mass content is mainly decided by the content of Mg, effect of Cu elements, in particular: the content of Mg is higher, the formation of bulk phase. More (+0) mass less; similar content of Mg, content of alloy the higher the Cu, the more large the formation of sigma phase, general (+ Sigma Theta) mass. More high Mg alloy with high Cu content (+0) to form clumps. Design of alloy is 460 DEG C /168 h single and prefect (460 DEG /24h+475 DEG /24h) double stage after homogenization treatment phase group With the thermodynamic calculation of income Al-Zn-Mg-Cu (Zn=8.5 wt%) four yuan isothermal section of phase diagram basically. Research shows that 460 C balance phase composition was significantly affected by Mg content: low Mg alloy is more susceptible to Al single-phase regions, even if its containing high content of Cu.475 is 460 DEG C to more fully dissolved in the second phase, the phase equilibrium mainly composed of (Mg+Cu) content: (Mg+Cu) 4.35 wt% alloy can obtain A1 single-phase solid solution, otherwise it is difficult to obtain Al single phase solid solution. The low Mg content or high Zn:Mg ratio helps accelerate the aging process of alloy, the alloy can be quickly reached the peak aging state and has a greater rate of aging. This is because in the first stage of aging at 120 for /6 h, low Mg content or high Zn:Mg ratio can promote the formation of GPII, and high Mg content or low Zn:Mg ratio will promote the formation of GPI area. Due to the thermal stability of GPI was lower than GPII, high or low Zn:Mg ratio and Mg content The gold in the 120 to 160 DEG C during the heating process less GP area to survive to become second 160 ageing stage phase nucleation of.Cu does not significantly affect the aging process. The same aging conditions, conductivity, design of alloy hardness, strength and toughness is mainly determined by the content of Mg, increase the Mg content, the hardness of the alloy that strength is improved, but the electrical conductivity and toughness will decrease. The increase of Cu content can get a similar effect, but the effect is weak. Experimental study and thermodynamic simulation show that increasing the content of Mg significantly increased the volume fraction of precipitates under particle aging and overaging alloy, the influence is not obvious under aged alloy particle size. But to some extent reduce the over aged alloy particle size, thus improving the underaged / overaging alloy strength and hardness of the alloy. Cu can improve the content of a certain increase of precipitates particle volume fraction, the The strength and hardness of alloy is improved, but the effect was weaker than Mg. with the increase of Mg content, the particle phase T76 alloy precipitation area fraction increases, grains and precipitate free zone yield stress difference will increase, which will promote the intergranular fracture toughness of the alloy, lead to reduce the influence of.Cu on the toughness of the alloy is Mg weak. For the low content of Mg (1.5/2.0 / wt%) alloy, increase the content of Cu increases in grain and precipitate free zone yield stress, and improve the recrystallization fraction, this will to some extent promote the intergranular fracture toughness of the alloy, lead to reduced. For the high Mg content (such as 2.5 wt%) alloy, increase the content of Cu will increase the number of solid solution treatment after undissolved intermetallic particles, thereby increasing the alloy fracture tendency, the toughness is further reduced.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.21

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