Evolution of secondary phases and properties of 7B04 aluminum alloy during DC homogenization

The effects of the direct current （DC） on the evolutions of hardness and morphology of the secondary phases in 7B04 aluminum alloy homogenized at 380？465 ℃ for 2 h were investigated in detail by electric conductivity measurement, hardness test, X-ray diffraction analysis, field emission scanning electron microscopy and energy dispersive spectrometry. The results show that with increasing temperature from 380 to 465 ℃, the electric conductivity of normal homogenized sample decreases from 34.9%IACS to 28.7%IACS, the hardness increases from HV 96 to HV 146, and the area fraction of secondary phase reduces from 4.5% to 1.89%. While, DC homogenized sample has a higher hardness, a lower electric conductivity and a smaller area fraction of secondary phases at the same temperature. The DC enhances the homogenization process by promoting the diffusibility of the solute atoms and the mobility of vacancy.

Effect of connection processes on mechanical properties of 7B04 aluminum alloy structures

The static and fatigue properties of 7B04 aluminum alloy structures connected by riveting and refill friction stir spot welding(refill FSSW)were compared and analyzed.Results show that the static compression load of the typical structure connected by riveting and refill FSSW fluctuated in the range of 117-124 kN,and the shear load was in the range of 89-95 kN.Welds spacing had a small influence on the static load of the structures joined by refill FSSW.However,the fatigue life of riveted structures was lower than that joined by refill FSSW.For the welded structure,the heterogeneous microstructures of the welded joint led to the uneven microhardness,and the hook at the lap interface bent upwards at the same time.These factors made the welded structures during the fatigue test failure along the path of sleeve moving.

Initial corrosion behavior and mechanism of 7B04 aluminum alloy under acid immersion and salt spray environments

The initial corrosion behavior and mechanism of 7B04 aluminum alloy under acid immersion and salt spray environments(pH=3.5)are studied by Scanning Electron Microscope(SEM),optical microscope,Fourier Transform Infrared Spectroscopy(FT-IR),X-Ray Diffraction(XRD),potentiodynamic polarization,Electrochemical Impedance Spectroscopy(EIS),and Scanning Kelvin Probe(SKP).The results show that pitting corrosion occurs at the initial corrosion stage,and the potential difference between the second phase particles is the main cause of pitting.Pitting pits on different locations gradually expand and coalesce with the proceeding of corrosion.The main components of corrosion products are Al_(2)O_(3),Al(OH)_(3),and AlCl_(3),and the generation rate of the corrosion product layer under the salt spray environment is larger than that under acid immersion environment.Under both environments,the Volta potential distribution first disperses and then concentrates,while the charge transfer resistance first decreases and then increases with the corrosion time.The Volta potential gradually shifts in a positive direction,indicating that corrosion products have an inhibitory effect on corrosion.After the same corrosion time,the corrosion product layer resistance and the expectation of the Volta potential of the salt spray sample are higher than those of the immersion sample.Comparatively,the corrosion current density for the salt spray sample is significantly lower than the immersion sample,which indicates that the thicker the corrosion product layer,the stronger the inhibition of corrosion reaction.

基于扫描Kelvin探针和FT-IR的7B04-T74铝合金/涂层体系丝状腐蚀研究

对7B04-T74铝合金进行硫酸或铬酸阳极氧化后再喷涂底漆或底漆+面漆,从而制备4类试样,而后开展标准丝状腐蚀试验,利用扫描Kelvin探针(SKP)和傅里叶变换红外光谱(FT-IR)技术对7B04-T74铝合金/涂层体系的腐蚀行为和机理进行研究。光学显微镜观察发现4类试样上腐蚀丝的数量较少,且长度不超过3 mm,表明4类试样均具有较好的耐蚀性。SKP测试结果表明,4类试样跨划痕区域的伏打电位随试验时间的变化规律相似,划痕处与周围的涂层-金属界面之间的电位差引起丝状腐蚀的发生,腐蚀机理为阳极破坏;单个腐蚀丝的伏打电位值朝着头部降低,朝着尾部升高,头部和尾部之间的电位差促进腐蚀丝的传播。FT-IR分析显示丝状腐蚀产物的成分主要为Al(OH)3、Al2O_(3)和AlCl3及其部分水解产物,发生的电化学反应类似于7B04-T74铝合金在氯离子作用下的点蚀过程。对比分析结果表明,2种阳极氧化方式对丝状腐蚀的影响差异性较小。