热轧对7055铝合金淬透性的影响

为研究热轧对7055铝合金淬透性的影响,采用末端淬火实验、硬度测试、光学显微镜和透射电镜研究了不同冷却速率处合金的组织和性能.实验结果表明:合金热轧后淬透性大大降低,淬透深度由约100 mm降至约45 mm,这主要与慢速淬火时析出的η平衡相的数量和尺寸的改变有关;热轧增加了η平衡相的形核位置,并引入大量的位错,增加了η平衡相的长大速率.因此,热轧合金固溶后慢速淬火时,析出的η平衡相的数量和尺寸都大大增加,时效后形成的η’沉淀强化相数量减少,硬度随冷却速率减小而下降的程度增加,合金的淬透性降低.     

微量Ge对7056铝合金组织和淬火敏感性的影响

通过力学拉伸、末端喷淋淬火实验,结合金相观察、硬度测试、电导率测试、扫描电镜(SEM)和透射电镜(TEM)等测试分析方法,研究微量Ge元素对7056铝合金组织和淬火敏感性的影响。结果表明:添加微量Ge细化了合金晶粒,晶粒尺寸由137μm减小到114μm;添加微量Ge在晶界上形成了粗大的Mg_2Ge相,并抑制了T相的形成;与无Ge合金相比,添加Ge后无沉淀析出带(PFZ)宽度变窄,从190nm减小到90nm,同时晶界析出相尺寸减小;添加微量Ge显著降低了合金的淬火敏感性,合金淬透深度提高了约25%,这与慢速淬火时析出的η平衡相的数量和尺寸有关;Ge降低了合金中的空位浓度,η平衡相在缺陷处的非均匀形核及长大被抑制,时效后η′沉淀强化相数量增多,慢速冷却时合金强度下降程度减小,合金淬透性提高。     

淬火速率对Al-Zn-Mg-Cu-Cr合金挤压棒材组织及硬度的影响

通过硬度测试、扫描电镜、透射电镜研究淬火速率对Al-Zn-Mg-Cu-Cr合金挤压棒材组织及硬度的影响。结果表明:淬火速率低于100℃/s时,硬度开始明显下降;2℃/s淬火时,硬度下降了43%。淬火速率低于100℃/s时,随着淬火速率降低,冷却过程中(亚)晶界及晶内弥散粒子处非均匀形核析出η平衡相的数量和尺寸明显增加,时效强化效果明显降低。相同淬火速率时,晶内η平衡相尺寸大于晶界η平衡相尺寸。在所研究的淬火速率范围内建立起硬度值与η平衡相面积分数间的定量关系。     

7085铝合金的淬火敏感性

采用分级淬火法测定了7085铝合金的TTT和TTP曲线,并结合透射电镜对其淬火敏感性进行了研究。结果表明,在等温保温过程中7085铝合金淬火态电导率升高,时效态硬度下降,在中间温度段电导率及硬度变化较快;TTT和TTP曲线的鼻尖温度约为320℃,淬火敏感区间为250~370℃;等温保温过程中,过饱和固溶体分解析出平衡相η,在320℃附近析出速率达到最大,随着等温保温时间的延长,η相聚集并长大,时效后出现大量无沉淀析出区域,导致性能显著下降;在淬火敏感区间,较高的相变驱动力和较大的扩散速率是η相快速析出长大的主要原因。     

7085铝合金剥落腐蚀的淬火敏感性

采用末端淬火实验、剥落腐蚀实验结合扫描和透射电镜分析,研究了7085铝合金剥落腐蚀的淬火敏感性。结果表明:7085铝合金的耐剥落腐蚀性能随淬火速率的降低而下降,腐蚀等级由EA级变为EB级,最大腐蚀深度从135μm增加至315μm,腐蚀深度与淬火速率的对数呈线性关系。淬火速率降低导致晶界析出相尺寸增加、无沉淀析出带宽化和在亚晶处也有η相析出,是造成剥落腐蚀淬火敏感性的主要原因。     

Zn/Mg比及时效温度对Al-Zn-Mg-Cu系合金析出行为的影响

采用热力学模拟软件中的铝合金模块研究了Al-Zn-Mg-Cu系合金中不同Zn/Mg比及时效温度对析出相GP区及η'相的析出规律的影响。结果表明,GP区含量随Zn/Mg比的增加而降低,低温时效能够促进GP区的形成;而η'相含量随Zn/Mg比的增加略微降低,时效温度对η'相含量影响不大;与120℃相比,合金在160℃时效时η'相含量达到6.5%的时间缩短16 h;在相同温度下时效时,随Zn/Mg比的增加,η'相含量达到6.5%的时间先缓慢增加,当Zn/Mg比大于2.91时则快速增加。合金的淬火敏感性随Zn/Mg比的增加有所降低;随Zn/Mg比从2.46提高到3.20,GP区和η'相的鼻尖温度分别从150℃和330℃降低到140℃和320℃;临界冷却速率随Zn/Mg比的增加而逐渐降低。     

非等温回归再时效对7050铝合金组织与力学性能的影响

采用扫描电镜(SEM)、透射电镜(TEM)、硬度测试和拉伸测试研究了非等温回归再时效对7050合金组织和力学性能的影响。非等温回归处理的加热速率为5℃/min,回归终了温度包括160℃、190℃、220℃、260℃和300℃。结果表明:经120℃/24 h+RT■190℃+120℃/24 h的非等温回归再时效处理后,合金的力学性能最佳,其抗拉强度约为588 MPa,屈服强度约为558 MPa,延伸率约为23%,硬度值约为199HV。预时效态合金晶界平衡相η呈不连续分布,其尺寸约为49～70 nm,晶内析出相η'的平均尺寸约为5～6 nm;在190℃的非等温回归处理过程中,预时效态合金中的析出相发生了部分回溶,相的平均尺寸变小,其晶界相和晶内相的尺寸分别约为5～10 nm和3～6 nm,回归态合金晶界呈多列平行且无沉淀析出带(PFZ)不明显;再时效态合金的晶界相尺寸最大,约为91～108 nm,晶内相弥散,尺寸约为4～10 nm,无沉淀析出带变得明显,其宽度约为41 nm。     

6063铝合金挤压型材的TTP曲线测定及其应用研究

通过中断淬火技术测定了6063铝合金挤压型材的时间-温度-性能(TTP)曲线,并通过透射电子显微镜观察其微观组织,采用淬火因子分析法预测不同淬火速率对合金硬度的影响.研究表明,6063挤压型材的TTP曲线鼻尖温度约为360℃,淬火敏感区间为280~410℃.等温保温时,过饱和固溶体分解析出无强化效果的β平衡相,会削弱时效强化效果,在360℃附近的相变速率最快,随着保温时间的延长,粗大β相数量和尺寸增加.为了在较好的时效强化效果和较小的残余应力之间求得平衡,6063挤压型材在线淬火时,在淬火敏感区间的冷却速度最好略大于15℃/s,高于410℃和低于280℃时可适当降低冷却速率.     

LD7铝合金淬火敏感性及相变动力学规律

通过分级淬火的方法,结合所测LD7铝合金淬火态电导率和时效态硬度拟合出合金的时间-温度-转化率(TTT)曲线和时间-温度-性能(TTP)曲线。利用透射电镜(TEM)和Johnson-Mehl-Avrami(JMA)方程研究了铝合金等温淬火过程中的组织变化和相变动力学规律。结果表明:LD7铝合金TTT和TTP曲线的鼻尖温度均为350℃,孕育期约为2s,淬火敏感区间为290~410℃;等温保温过程中过饱和固溶体析出第二相粒子,鼻尖温度附近析出速率最快;随保温时间的延长,第二相富集长大且粗化,无沉淀析出区域变宽,晶界第二相连续化,导致合金强化效果下降;根据相变动力学方程拟合出的S曲线,解释了合金在350℃附近淬火敏感性最大以及淬火敏感区间合金第二相主要以针状形态析出的规律。     

回归再时效中预时效温度对7050铝合金应力腐蚀性能的影响

采用硬度测试、电导率测试、慢应变速率拉伸、透射电镜和扫描电镜等方法,研究了回归再时效热处理工艺中预时效温度对7050铝合金微观组织和应力腐蚀性能的影响。结果表明:随着预时效温度升高,回归再时效后7050铝合金晶内析出相从以GP区为主转变为以η′相为主,晶界析出相逐渐粗化,晶界变得不连续分布,合金应力腐蚀敏感性降低;但晶界无沉淀析出带宽度增加,120℃时达到140nm,易导致应力集中和阳极溶解,合金抗应力腐蚀性能下降。预时效温度为80℃,即稍微欠时效时,7050铝合金抗应力腐蚀性能较好,在缓慢应变速率(10-6s-1)和3.5%NaCl溶液腐蚀介质下,合金抗拉强度为473.5MPa,伸长率为10.67%,应力腐蚀指数为0.05824。     

Effect of processing parameters on quench sensitivity of an AA7050 sheet

Secondary precipitation takes place in alloy 7050 during slow quenching after solution and results in a significantly decreased content of Mg, Zn elements. Optical microscopy, transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) study showed the distribution of Al₃Zr particles plays a vital role in quenching process. The equilibrium η phases mainly precipitate at Al₃Zr particles within the recrystallized grains and other high energy areas (primarily grain boundaries). The influence of the processing (homogenization, hot rolling and solution) parameter on the quench sensitivity is also investigated by mechanical property examination (T6 temper). A ramping heat homogenization, controlled hot rolling (67% rolling reduction and 3-5 s⁻¹ deformation rate) and two-stage solution treatment result in lesser recrystallization and fewer high angle grain boundaries, and lower the boundary angles within sub-structures. The decreasing number of heterogeneous precipitation sites endows the study alloy with good quenching sensitivity and better mechanical properties.     

7075-H18铝合金板材的淬火敏感性与等温转变行为EI北大核心CSCD

以2 mm厚的7075-H18高强度铝合金板材为研究对象,采用微观组织分析、显微硬度测试、动力学建模与计算相结合的方法,对板材的淬火敏感性、等温相变行为及转变动力学进行系统研究,构建"时间-温度-性能"关系图。结果表明:7075-H18板材具有较高的淬火敏感性。"时间-温度-性能"关系图的鼻尖温度约为350℃,孕育期仅为0.23 s,淬火敏感温度区间为271~404℃,转变量为0.5%的曲线对应的临界线性冷却速率为969.7℃/s,超过了冷模淬火所能达到的冷却速率。板材在等温淬火过程中主要形成粗大的η平衡相,等温时间越长,晶内与晶界的η相尺寸越大,晶界η相越趋于连续分布,且无沉淀析出带变宽。基于实验数据构建的等温转变动力学模型,对7075-H18板材的等温相变过程进行准确预测,板材在350℃等温过程中沉淀相的析出速率最大,等温相变类型为"形核-长大"型相变,相变过程为层片状沉淀相的长大、增厚及互相吞并。理论计算结果与透射电子显微镜下观测到的η相特征以及"时间-温度-性能"关系图相一致。     

多级时效热处理对7056铝合金析出组织与耐蚀性的影响

利用透射电子显微镜(TEM)、扫描电子显微镜(SEM)、力学性能测试、电导率测试以及剥落腐蚀、慢应变速率拉伸应力腐蚀(SSRT)、Tafel循环极化曲线等手段研究多级时效热处理对7056铝合金析出组织及耐蚀性的影响。结果表明:过时效再时效热处理后溶质原子二次析出,晶内析出相体积分数增大,晶界析出相粗化断开,无沉淀析出带宽化。与120℃/24h相比,采用120℃/6h预时效工艺有利于晶内细小析出相回溶和粗大相长大。再时效热处理可提高过时效合金的强度和电导率,与峰时效和回归再时效相比,合金的抗拉强度损失不大,电导率明显提升。过时效再时效热处理后,合金晶界处连续阳极溶解被有效避免,抗剥落腐蚀和抗应力腐蚀性能增强。     

含Zr、Sc的Al-Zn-Mg-Cu合金的低周疲劳行为

利用透射电子显微镜和低周疲劳试验机研究了单级时效状态及回归再时效状态两种含Zr、Sc的Al-Zn-Mg-Cu合金的微观组织和低周疲劳性能。结果表明:单级时效基体析出相以η′相为主,晶界析出连续分布平衡相,并伴有晶间无析出带;回归再时效基体析出相略有长大,晶界析出相长大明显,无析出带变宽。低周疲劳加载条件下,合金在0.4%~0.7%外加总应变幅范围内表现出循环稳定性;在0.8%的应变幅下,呈现先软化后硬化。在0.4%~0.6%较低的外加总应变幅范围内,回归再时效合金表现出较高的低周疲劳寿命。两种状态合金的塑性应变幅和弹性应变幅与载荷反向周次之间均成直线关系,并可分别用Coffin-Manson公式和Basquin公式来描述。两种状态的合金的疲劳裂纹均萌生于试样表面,并以穿晶方式扩展。     

Microstructure evolution and stress rupture properties of K4750 alloys with various B contents during long-term aging

The relationship among B content,microstructure evolution and stress rupture properties of K4750 alloy during long-term aging were investigated.After aging at 800 ℃ for 1000 h,the decomposition degree of MC carbides of K4750 alloys with 0B,0.007 wt.％ B and 0.010 wt.％ B were basically identical,which indicated that B has no inhibition on MC carbide decomposition during long-term aging.The MC carbide decomposition was accompanied by the formation of M23C6 carbides and a small number of η phases,which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from the γ matrix.In addition,M23C6 carbides in boron-free alloy were in continuous chain and needle-like η phases were precipitated near them,while M23C6 carbides in boron-containing alloys remained in granular distribution and no η phases precipitation around them.Adding B could delay the agglomeration and coarsening of M23C6 carbides during long-term aging,which was because the segregation of B at grain boundary retarded the diffusion of alloy elements,thus weakened the local fluctuation of chemical composition near grain boundary.The stress rupture samples of K4750 alloys with various B contents after aging at 800 ℃ for 1000 h were tested at 750 ℃/380 MPa.The results indicated that the stress rupture properties of boron-containing alloys were significantly better than that of boron-free alloy,which could be attributed to the increase of grain boundary cohesion strength and the optimization of M23C6 carbide distribution due to the addition of B.     

Correlation between precipitate microstructure and mechanical properties in AA7075‐T6 aluminum alloy friction stir welded joints

Microstructural evolution and mechanical properties of friction stir welded AA7075‐T6 aluminum alloy were examined. Grain structure and precipitate evolution in the stir zone and heat‐affected zone were evaluated using optical microscope and differential scanning calorimetry. A significant grain refinement and dissolution of η′ precipitates in the stir zone were found, but chromium‐bearing dispersoids remained nearly unchanged. The main particles in the stir zone and heat‐affected zone were η precipitates as well as Guinier‐Preston zones formed during post‐weld natural aging. The small recrystallized grains were observed in the thermo‐mechanically affected zone next to the stir zone. A W‐shaped hardness distribution where soft region was produced in the heat‐affected zone at a short distance from the stir zone were obtained. Hardness profiles of the welds were explained by precipitate distributions. Friction stir welding resulted in the reversion and coarsening of η′ precipitates. The formation of Guinier‐Preston zones in the stir zone and some parts of the heat‐affected zone during post‐weld natural aging increased the hardness. In transverse tensile specimens, fracture occurred in a location with the minimum hardness at either advancing or retreating side randomly. Further, influences of welding parameters on mechanical properties were investigated.     

挤压温度对AZT802镁合金组织和性能的影响

将铸态AZT802合金分别在350℃、380℃和400℃下挤压,随后进行T5时效处理,研究不同挤压温度对AZT802镁合金挤压态和时效态组织和性能的影响。结果表明,当挤压温度为350℃时,晶粒尺寸分布不均匀,同时观察到大块的条状第二相沿挤压方向析出。当挤压温度高于350℃时,挤压态合金获得均匀等轴晶粒,第二相以颗粒状形貌沿晶界均匀分布。经T5时效处理后,颗粒状Mg_2Sn相均匀分布于基体中,Mg_(17)Al_(12)相以连续相和非连续相析出,非连续析出相随时效前挤压温度的升高而逐渐增多。力学性能测试结果表明,AZT802合金在380℃下挤压,并进行175℃(3h)T5时效处理后,获得最佳综合力学性能。