Thermal data processing to characterise quasi-unidirectional solidification of cast aluminium alloy AA 354

Abstract

In this work the authors show how to build a semi-industrial scale macrothermal analysis experimental apparatus for low pressure aluminium casting AA 354 with quasi-unidirectional solidification. Several thermocouples were connected to a multichannel electronic device allowing a sampling rate up to 10 Hz; the thermocouples were installed in the mould at different locations to acquire the discontinuous cooling curves at those same locations.

With this type of experiment and appropriate mathematical procedures it was possible to build a reasonable response surface T?=?f(x, t) and the respective derivatives: ?T/?t and ?T/?x. Exponential polynomials were applied for modelling the curves and linear interpolation to relate the several cooling curves. Mathematical tools applied to the modelled curves allowed the authors to identify different solidification events and correlate them with the specific thermal gradient, cooling conditions and solidification fronts phenomena such as columnar to equiaxed transition of α aluminium grains. Growth velocity of proeutectic aluminium α and eutectic fronts were also determined.     

Investigation of the microstructure of alkali-activated cements by neutron scattering

Significant research has been devoted to understanding and modeling the neutron scattering behavior of Portland cement pastes. The present work examines the potential of neutron scattering to noninvasively evaluate the properties of alternative cementitious materials encountered in the field. To interpret the scattering of both alkali-activated and slag-cement systems, alternative models to those typically adopted for Portland cement pastes were considered. Investigation of alkali-activated and slag-cement coupons were conducted for different sample thickness (∼0.5, 1, 2, 4, 6, 8 and 12 mm) over a wide scattering vector (Q) range (0.0005<Q>0.03 nm⁻¹ and 0.04<Q>4 nm⁻¹) on both ultra (USANS) and conventional small-angle neutron scattering (SANS) spectrometers. This wide Q range allowed determination of the fractal properties of the microstructure as well as the radius of the main scattering particle in the material. An empirical correction for multiple scattering was made based on the Dexter-Beeman equation and demonstrated that for thicker sample widths the theory correlated reasonably well. Alkali-activated fly ash was found to have the largest R value while the OPC and slag mix exhibited the largest Guinier radius. This difference was attributed to variations in the extent of multiple scattering for the samples. Successful application of this model was limited to sample thickness ≥1 mm. A combined power law-Sabine expression was also utilized to successfully model the SANS data over a Q range of 0.04-4 nm⁻¹.     

The application of synchrotron-based spectroscopic techniques to the study of chromate conversion coatings

This work utilized synchrotron spectroscopic techniques such as extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and synchrotron infrared microspectroscopy (SIRMS) to understand the chemistry, formation, structure and aging of protective chromate conversion coatings (CCC). We shall discuss issues related CCC formed on aluminum-copper alloys. SIRMS combined with argon ion sputtering demonstrated spatial and depth chemical variations of the CCC formed on AA2024-T3. XANES indicated that in the first stages of CCC formation the thickness depends on the Cu content of the substrate and that thinner CCC appears to contain less Cr⁶⁺ for protection. Micro-XANES shows that even monolayer coverage of chromate can protect the alloy surface. EXAFS and XANES measurements were used to monitor the structural changes of the CCC with time. It was observed that a slight decrease in the Cr(VI)Cr(III) ratio occurred during the first 24 h following coating. The structure of the CCC is in close agreement with a Cr mixed oxide. EXAFS data appeared to indicate an increase in ordering in the CCC structure with time.     

反舰导弹突防能力研究

分析了典型目标舰艇防空武备的配置及其反舰导弹的抗击过程，计算了某型反舰导弹对该型舰艇的突防概率，并对结果进行了分析。     

7050航空铝合金结构材料激光冲击强化处理研究

用自行研制的激光冲击强化处理(LSP)装置对两种重要航空铝合金结构材料7050T7451，7050T7452冲击强化试验，进行了疲劳寿命对比试验。给出了反映疲劳应力水平与结构件寿命对应关系的σm-N曲线。结果表明．在67．3MPa的加载应力水平下，激光冲击处理后7050T7451结构材料的疲劳寿命提高到未经处理的435％，而7050T7452在81．4MPa的应力水平下提高到518%。并对试件进行了激光冲击处理机理的研究。结果显示试件表面具有较大的残余压应力和较高的位错密度。     

Formability and cavitation behavior of superplastic AA5083 aluminum alloy under biaxial tension

The superplastic forming potential of two fine-grained 5083 aluminum alloys were studied under biaxial tension using a pneumatic bulge test. Experiments were performed at temperatures ranging from 475 to 525 ℃ with three different strain paths ranging from equi-biaxial to approaching plane strain. The shape of the forming limited diagram（FLD） is found to be significantly different from FLDs commonly used in room temperature stamping. The effects of temperature on final thickness distribution, dome height and cavitation were investigated for the case of equi-biaxial stretching. Increasing temperature in free bulge forming can improve the thickness distribution of final parts but have no significant effect on dome height. The results indicate that determination of forming limits in SPF cannot be represented with a simple FLD and additional metrics such as external thinning and internal cavitation needed to determine the SPF potential of a material.     

7050铝合金蠕变时效成形本构模型研究

为研究7050T451铝合金蠕变时效本构模型,在160℃、不同应力条件下进行单轴拉伸蠕变试验,分析了蠕变应变、屈服强度和微观组织随时间的变化规律.基于高强铝合金析出强化理论,建立了能描述蠕变时效成形宏观及微观变化的本构方程,并运用遗传算法对材料常数进行拟合优化.研究表明,该模型在不同应力水平下与试验结果吻合良好,能够用来模拟分析蠕变时效成形过程.     

AA5083铝合金在海水中的腐蚀行为研究

采用电化学工作站研究了AA5083铝合金在海水中的极化曲线、点蚀电位、交流阻抗等性能,通过高分辨率数码显微镜、扫描电镜等表征手段对AA5083铝合金腐蚀前后的表面形貌进行了研究。研究结果表明:AA5083铝合金在静态海水中的腐蚀以点腐蚀为主,浸泡初期阻抗由容抗和感抗组成,腐蚀电位随着浸泡时间的延长不断负移,耐腐蚀性降低,点腐蚀坑越来越多,并且点腐蚀坑不断增大加深,在试板表面中间区域形成半球状点腐蚀坑,腐蚀达到一定程度后,出现Warburg阻抗,腐蚀电位逐渐稳定在-0.58V。     

共聚物钻井液降滤失剂的合成与性能评价

以丙烯酰胺(AM)、2-甲基-2-丙烯酰胺基丙磺酸(AMPS)、丙烯酸(AA)、甲基丙烯酰氧乙基三甲基氯化铵(DMC)为原料,采用氧化还原引发体系合成了一种四元共聚物水基钻井液降滤失剂。确定了最佳合成条件:单体配比为AM/AMPS/AA/DMC=55∶30∶10∶5(质量比),引发剂加量各为0.3%(单体浓度为1),单体总浓度为20%(质量分数),反应温度为50℃,溶液p H值为7,反应时间为4 h。对聚合物进行了红外表征,合成产品与设计结构一致。钻井液性能评价结果显示:淡水基浆中聚合物加量为0.7%时,常温中压滤失量为6.0 m L,经过180℃老化后,API滤失量为8.2 m L,高温高压滤失量为11.6 m L;聚合物加量为1.0%时的页岩相对回收率达到99.4%。说明AM/AMPS/AA/DMC聚合物降滤失能力强,且热稳定性和抑制性能好。     

颗粒增强铝基复合材料时效析出行为研究

时效是提高金属基复合材料力学性能和增强材料稳定性的重要方法,颗粒增强铝基复合材料的时效带来的强化效果有时甚至大于颗粒添加带来的弥散增强效果,深入研究时效析出行为对改进热处理工艺和复合材料的优化设计有至关重要的指导性作用.在当前开展的颗粒增强铝基复合材料时效行为研究的基础上,从时效机理出发,重点分析了基体、颗粒增强体、热处理工艺及加工工艺等对陶瓷颗粒增强铝基复合材料时效析出的影响,评述了当前主要采用的几种颗粒增强铝基复合材料时效析出行为的研究方法,展望了未来颗粒增强铝基复合材料时效行为可能的研究方向.