摘要
本文对两种可用普通铸锭法生产的新型高强度铝合金进行了较为系统的研究。根据两种合金的目前发展情况,对Al-Zn-Mg-Cu系合金重点研究了微合金化元素Zr的影响机理与合适添加量,而对Al-Cu-Li系合金进行了比较全面的研究,目的是为发展我国的新型高强铝锂合金奠定理论基础和试验基础。
本文对微量Zr元素在合金凝固与退火过程中行为的研究表明,当合金中的Zr含量超过0.06%时,Zr元素可以对铸态晶粒带来约30%的细化效果,但是,Zr元素含量稍高,约0.16%时,就有可能析出一次初生Al_3Zr,这限制了Zr元素的有效添加量。均匀化退火是亚稳Al_3Zr相析出的主要工艺过程,对Al-Zn-Mg-Cu-(Zr)合金的研究发现,亚稳Al_3Zr在该超高强铝合金中的析出较在Al-Zr二元合金中提前。随Zr含量的增加,亚稳Al_3Zr相的数量增加,合金中析出的η平衡相的数量也增加而尺寸减小。
含Zr量不同的合金在锻压后的组织有所不同,这主要表现在亚晶粒的大小方面,Zr含量越高,合金的亚晶粒越细小。固溶处理后,合金的组织差别会更明显,不含Zr或Zr含量低于0.06%的合金都表现出了以再结晶晶粒为主的组织,而Zr含量高于0.06%的合金都呈现出了以未再结晶晶粒为主的组织。理论分析表明,要想获得稳定的未再结晶组织,保证W(Zr)≥0.10%是必须的。
Zr元素会对超高强铝合金的时效动力学产生明显的影响,主要表现在抑制GP(Ⅰ)区的形成和加速η′相的析出两个方面。究其原因,前者主要是因为固溶Zr与淬火空位易于结合,减缓了合金在时效过程中Zn、Mg等溶质原子的扩散所致,而后者与Al_3Zr相促进η′相的析出有关。虽然Zr元素可以改变Al-Zn-Mg-Cu合金的时效进程,但是,Zr元素对Al-Zn-Mg-Cu合金力学性能的影响主要受其对合金晶粒组织的改变,而因析出相变化所引起的性能改变要小一些。
综合考虑熔铸、成型、热处理以及力学性能等方面的因素,本文认为,Zr作为一种有效的组织、性能调整元素,必须精确地控制其含量,过低则起不到应有的作用,过高则有可能带来类似Fe、Si等杂质元素的不利影响。本研究中超高强铝合金的合适Zr元素添加量应在0.10%~0.14%之间。
本文对Al-Cu-Li三元合金的研究表明,Cu、Li含量是影响合金强塑性的重要因素,提高Cu含量往往可以提高合金的强度,而增加Li含量却会降低合金的强度。合金的强度、延伸率受合金饱和程度的影响也很大,并且在Li含量较高时更敏感。发展新型高强铝锂合金的一个较合适的基础成分范围是Cu:3.0%~5.0%,Li:1.0%~1.5%。
本文对铝锂合金的各向异性问题进行了比较系统的研究,结果发现,合金的平面内各向异性主要受材料的变形特点和织构控制,而与材料的晶粒结构关系不大。添加微合金化元素改变合金中的析出相,进而改变合金的变形行为,是降低材料各向异性的一条重要途径。此外,通过添加Mn元素改变铝锂合金的再结晶行为,控制合金中的织构组份和强度,也是减小材料各向异性的重要手段。
对Al-Cu-Li系合金的微合金化研究发现,Mg、Ag、Zn是提高铝锂合金强度的重要的微合金化元素,但是,单独添加这些元素的效果往往不明显,当以Mg+Ag、Mg+Zn或Mg+Ag+Zn的组合方式加入时,合金强度会得到大幅度提高。Mn元素的添加不仅对控制合金的各向异性具有重要意义,而且可以带来一定的强化效果,这与合金中的其它微合金化元素存在与否无关。但是,Mn元素的存在往往会引起Al-Cu-Li合金的淬火敏感性增加,这一点与Mn在7000系铝合金中的影响相似。
综合对Al-Cu-Li合金的研究,提出了发展新型高强铝锂合金的基本思路是:采用高Cu低Li的基础合金成分;添加微量的Mn元素改善合金的变形特点,控制合金的晶粒组织和织构,以降低铝锂合金的各向异性;采取多元微合金化的方法进一步提高合金的强韧性。提出了一个有发展前途的含有Mg、Zn、Mn等微合金化元素的新型超高强铝锂合金的合金原型,其屈服强度达635MPa,延伸率接近5%,而不含Ag元素,因此该合金在密度、成本等方面更具优势。
In the present thesis, two advanced high-strength aluminum alloys produced by ingot-metallurgy method were investigated. The strengthening mechanisms and the effect of trace element zirconium in an Al-Zn-Mg-Cu alloy were studied. And the alloying effects and strengthening behaviors of a high-strength and low-density aluminum alloy, Al-Cu-Li system, were also investigated.
The influences of element Zr in the solidification and homogenization processes were investigated. It is found that the grain sizes could be significantly refined with the addition of Zr. When the alloy contains more than 0.06%Zr, its grain sizes are smaller about thirty percent than that without Zr element. The primary Al_3Zr compounds (about 50um) are also observed in this study, when the content of Zr reaches 0.16%. The morphology of the Al_3Zr phases similar to that of the compounds containing Fe or Si element should be avoided in the solidification microstructures. After the ingots were homogenized, another metastable type Al_3Zr particles, which precipitate as coherency phases withα(Al) matrix are found in the Zr-bearing alloys and can exist for a long period of time. It is also found that the precipitation speed of the metastable Al_3Zr in Al-Zn-Mg-Cu alloys is much faster than that in Al-Zr binary alloys. The intensity of the Al_3Zr dispersoids and the equilibriumηphases also increase with the volume of Zr increasing.
After the alloys forged, the sizes of subgrains are different with the variation of the Zr content. When the alloys were soluted for two hours at 470℃, the difference becomes more significant owing to the extent of recrystallization processing in each alloy. When the content of element Zr reaches 0.06%, the grain sizes could be smaller than 10um. According to the model proposed by Nes and Wert, it is necessary to keep the content of Zr higher than 0.1% in order to retain the unrecrystallized microstructures.
In ageing process, Zr atoms in solution suppress the formation of GP( I) regions and the metastable Al_3Zr particles promote the precipitation ofη' phase. Thus, the age-hardening curves of the high strength aluminum alloys with different Zr content can be identified as three types. The mechanical property results show that the strength of the alloys increases with the content of zirconium, but the elongation reaches a peak at 0.06%Zr and then decreases with the content of Zr increasing. This is mainly due to the addition of element Zr can significantly retard the recrystallization and refine the microstructure of the alloys.
Based on the study mentioned above, it is suggested that the content of Zr should be in range of 0.10%-0.14% in the Al-Zn-Mg-Cu alloy.
The effects of the content of Cu and Li on the mechanical properties of Al-Cu-Li alloys were investigated according to the metastable phase diagram. It is found that the strength of the alloys increases with the Cu content increasing. However, the effect of element of Li is in the reverse direction. Furthermore, the mechanical properties of the Al-Cu-Li alloys can also be affected by the degree of the supersaturation. The higher the Li content is, the more significant the effect of the degree of supersaturation will be. Appropriate contents of Cu and Li are proposed that the Cu element should be in the range of 3.0%-5.0% and the Li element in the range of 1.0%-1.5%.
The mechanical anisotropies of Al-Li alloys were also studied in this paper. The results show that the mechanical anisotropies are mainly affected by the deformation character and the crystalline oriental distribution of the materials. However, the grain microstructures have little effect on the anisotropy. The in-plane anisotropy of Mn-bearing alloy is lower than that without Mn because the microstructure and the intensity and component of the crystallize texture were significantly changed by the addition of element Mn.
The effects of microalloying elements such as Mg, Ag, and Zn on the mechanical properties of Al-Cu-Li alloys were also studied extensively. The strengthening effects of Mg+Ag, Mg+Zn and Mg+Ag+Zn additions are much higher than the individual addition of Mg, Ag or Zn. The element Mn can also bring some extent of strengthening effects on the alloys whether the other microalloying elements present or not. But the quenching sensitivity becomes more seriously when the alloys contain element Mn. A new high-strength Al-Li alloy containing Mg, Zn and Mn was developed for aircraft applications in this study. Its promising mechanical properties were found to be:σ_b= 657 MPa,σ_(0.2)= 635 MPa andδ= 4.9%.
Finally, the principles of developing high strength Al-Li alloys were proposed that:①the baseline alloy should contain more than 3.0%Cu and less than 1.5%Li;②the element Mn could be selected to improve the deformation performance and control the grain microstructure and crystalline texture;③addition of microalloying elements would be necessary to obtain a super-high strength alloy with yield strength higher than 600MPa.
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