Effect of quenching cooling rate on residual stress and microstructure evolution of 6061 aluminum alloy

In this study,the cooling rate was manipulated by quenching with water of different temperatures(30,60 and 100℃).Surface and internal residual stresses in the quenched 6061 aluminum alloy samples were measured using hole-drilling and crack compliance methods,respectively.Then,the processability of the quenched samples was evaluated at cryogenic temperatures.The mechanical properties of the as-aged samples were assessed,and microstructure evolution was analyzed.The surface residual stresses of samples W30℃,W60℃and W100℃is−178.7,−161.7 and−117.2 MPa,respectively along x-direction,respectively;and−191.2,−172.1 and−126.2 MPa,respectively along y-direction.The sample quenched in boiling water displaying the lowest residual stress(~34%and~60%reduction in the surface and core).The generation and distribution of quenching residual stress could be attributed to the lattice distortion gradient.Desirable plasticity was also exhibited in the samples with relatively low quenching cooling rates at cryogenic temperatures.The strengthes of the as-aged samples are 291.2 to 270.1 MPa as the quenching water temperature increase from 30℃to 100℃.Fine and homogeneous β"phases were observed in the as-aged sample quenched with boiling water due to the clusters and Guinier-Preston zones(GP zones)premature precipitated during quenching process.

热输入对钛铝异种合金激光自熔钎焊接头组织与力学性能的影响

使用IPG 6000光纤激光器对TC4钛合金与6061铝合金进行对接焊,通过激光束偏移到铝侧,获得钛铝异种合金的激光自熔钎焊接头,通过金相显微镜、扫描电镜以及拉伸测试等分析了热输入对TC4/6061钛铝异种接头组织以及力学性能的影响。结果表明,当焊接热输入超过133 J/mm,较多的脆性金属间化合物在界面生成,金属间化合物主要为TiAl3以及TiAl2;当热输入小于133 J/mm,金属间化合物层很薄且在1μm左右,金属间化合物主要为TiAl3。当焊接热输入为107 J/mm时,TC4/6061钛铝异种接头抗拉强度最大,达到212 MPa,约为6061铝合金的63%,接头断裂方式为韧脆混合型断裂。

铝合金表面处理增强Al/CFRP胶黏接头剪切强度研究

随着碳纤维增强树脂基复合材料(CFRP)、铝合金等轻质结构材料在航空航天、轨道交通以及武器装备等领域的应用大幅度增加,CFRP/铝合金叠层结构对承载能力与安全性的要求明显提高,合适的铝合金、CFRP表面处理工艺可有效改善CFRP/铝合金叠层结构的界面状态,增强综合力学性能。本研究通过优化6061铝合金磷酸阳极氧化与硅烷复合处理工艺,研究了铝合金与CFRP表面状态对CFRP/铝合金叠层结构剪切强度与失效形式的影响规律。研究表明:铝合金经磷酸阳极氧化可使接头剪切强度达9.4MPa,是未阳极氧化处理接头剪切强度的1.76倍;进一步与硅烷浸渍复合处理后可将剪切强度提高至14.9MPa,可广泛应用于航空航天、汽车制造、武器装备等关键零部件的结构轻量化领域。

Influences of electric-hydraulic chattering on backward extrusion process of 6061 aluminum alloy

The possibility of the electric-hydraulic chattering technology and its application in the cold extrusion were presented.The conventional and electric-hydraulic chattering assisted backward extrusion processes were performed on 6061 aluminum alloy billets at room temperature.The experimental results showed that 5.65% reduction in the extrusion load was attained if the die and ejector were vibrated at a frequency of 100 Hz and amplitude of 0.013 mm in the longitudinal direction.The friction coefficient at the billet and tool system interface determined from the finite element analysis（FEA） decreased from 0.2 without chattering to 0.1 with application of electric-hydraulic chattering.The higher values of instantaneous velocity and direction change of material flow were achieved during the chattering assisted backward extrusion process.The strain distribution of the chattering assisted backward extrusion billet revealed lower maximum strain and smoother strain distribution in comparison with that produced by the conventional extrusion method.

Effect of joining temperature on microstructure and properties of diffusion bonded Mg/Al joints

The joining of AZ31B Mg alloy to 6061 Al alloy was investigated at different joining temperatures by vacuum diffusion bonding method. The microstructures of Mg/Al dissimilar joints were studied by means of optical microscopy （OM）, scanning electron microscopy （SEM） and energy dispersive X-ray （EDX）. The results show that the thickness of each layer in the diffusion zone increases with the increase of joining temperature, and the microstructure changes obviously. At joining temperature of 440 °C, the diffusion zone is composed of Mg2Al3 layer and Mg17Al12 layer. At joining temperatures of 460 and 480 °C, the diffusion zone is composed of Mg2Al3 layer, Mg17Al12 layer, eutectic layer of Mg17Al12 and Mg-based solid solution. The width of high-hardness zone in the joint increases with increasing joining temperature, and the micro-hardnesses at different locations in the diffusion zone are significantly different. The joining temperature of 440 °C offers the highest tensile strength of 37 MPa, and the corresponding joint exhibits brittle fracture at the intermetallic compound layer of Mg17Al12.

Simulation of temperature and stress in 6061 aluminum alloy during online quenching process

The cooling curves of 6061 aluminum alloy were acquired through water quenching experiment. The heat transfer coefficient was accurately calculated based on the cooling curves and the law of cooling. The online quenching process of complex cross-section profile was dynamically simulated by the ABAQUS software. The results suggest that the heat transfer coefficient changes during online quenching process. Different parts of the profile have different cooling velocity, and it was verified by water quenching experiment. The maximum residual stress of the profile was predicted using FEM simulation based on ABAQUS software The relations between the temperature and stress were presented by analyzing the data of key points.

微弧氧化-溶胶凝胶复合表面技术改善铝合金的耐腐蚀性能

通过微弧氧化-溶胶凝胶复合表面处理技术来提高铝合金的耐腐蚀性能,分析了多层凝胶层对6061铝合金耐腐蚀性能的影响。采用扫描电镜(SEM)、X射线衍射仪(XRD)和电化学分析等方法对膜层的表面形貌、化学组成、结构以及耐腐蚀性能进行了表征。研究表明:TiO2溶胶渗入微弧氧化陶瓷膜的微孔以及裂纹中,能有效阻挡腐蚀介质的扩散和渗透;复合处理后的试样较仅微弧氧化处理更平滑、致密;膜层除了γ-Al2O3相外,经高温退火处理后出现TiO2锐钛矿,并形成较好的晶相结构;在3.5%NaCl溶液中的室温电化学行为中,复合处理较微弧氧化处理后的试样自腐蚀电位上升最高约400 mV,自腐蚀电流密度最高减小约3个数量级,极化电阻明显增大;随着凝胶层厚度的增加耐腐蚀性能逐渐增强,但当凝胶层数大于4时,膜层龟裂现象严重,并导致耐腐蚀性能开始下滑。

停放时间对6061铝合金性能及微观组织的影响

采用宏观检测与显微分析相结合的手段,研究不同停放时间下6061铝合金的力学性能与微观组织的变化规律。结果表明:停放时间对6061铝合金的晶粒尺寸有较大影响,晶粒尺寸呈先增大后减小的趋势;停放时间对6061铝合金弹塑性变形过渡阶段有较大影响,但对弹性及塑性阶段变形的影响很小。6061铝合金在停放为0~2 h时,力学性能无明显变化,变形后试样表面光滑,变形协调性较好;停放时间为12 h时,材料的抗拉强度和屈服强度降至最低,但伸长率提高,试样表面呈橘皮形貌,变形均匀性较差;停放时间为24 h^15 d时,合金的强度回升并逐渐趋于稳定。结果表明:随着停放时间的增长,6061铝合金断口的韧窝直径和深度不断增加,第二相粒子的尺寸也不断增大,形状由球状、带状逐渐转变为板状、块状。通过研究得到6061铝合金满足汽车结构强韧化需求,综合性能最优需求的停放时间为12~24 h。此研究结果能够为6061铝合金加工工艺优化提供理论指导。

挤压工艺对喷射成形6061铝合金组织与性能的影响

在2000kN四柱液压机上对铸态、喷射态6061铝合金进行挤压处理,研究了挤压温度、挤压比对喷射态合金微观组织及力学性能的影响,分析了挤压态合金的断裂机理,并对比了铸态、喷射态合金挤压后的力学性能。结果表明,当挤压比为6.25时,随着挤压温度的升高,合金试样发生再结晶的晶粒数量增加,到250℃时形成均匀细小的等轴晶组织;合金的硬度和抗拉强度随挤压温度的升高而降低;但伸长率却随挤压温度的升高而升高。当挤压温度为250℃时,合金晶粒尺寸随挤压比的增大而减小;伸长率和抗拉强度随挤压比的增大而升高;而硬度受挤压比变化的影响则不大。挤压态合金的断裂机理为微孔聚集断裂。"喷射+挤压"态合金的抗拉强度和伸长率都比"铸造+挤压"态合金的高。

6061铝合金触变压缩数值模拟

基于连续多孔介质的材料模型方法,将半固态材料假设为连续可压缩多孔介质骨架,利用DEFORM有限元软件进行单轴压缩模拟,分析触变成形过程中应变速率对6061铝合金成形性能的影响,得出初始固相体积分数为0.82的半固态6061铝合金的固相率分布及应变应力场分布。模拟结果表明,随着变形程度的增大,材料中心区域固相率分布较边缘均匀,随着变形速度的增加,材料的等效应变场分布与固相率分布呈现出相同的变化趋势。模拟曲线与实验曲线吻合,证明了数值模拟的可行性。

6061铝合金ECAP变形前后的空泡腐蚀试验研究

利用20 kHz超声振荡空泡腐蚀设备,研究6061铝合金170℃等径角挤压(Equal-channel angular pressing,ECAP)四道次变形前后在蒸馏水中的空泡腐蚀行为。测量单位面积累积失重量,利用SEM和TEM观察6061铝合金ECAP变形前后空泡腐蚀的表面形貌,分析腐蚀层微观组织。结果表明,ECAP可使位错增殖,晶粒细化,变形后的试样单位面积累积失重量显著降低。位错增殖是空泡冲击腐蚀层的主要微观机制,增殖形式多为位错圈和晶界增殖。空泡腐蚀10 h,6061铝合金腐蚀层晶粒尺寸均发生了不同程度地细化,位错增殖形成高密度位错区,导致显微区域循环硬化;疲劳变形带引起低密度位错区,导致显微区域循环软化。6061铝合金经过ECAP前后的抗空泡腐蚀能力受到显微区域循环硬化和软化两种相互竞争因素共同作用。

复合锻造提高6061铝合金汽车转向节耐磨和耐蚀性能

分别采用常规锻造和复合锻造制备了6061铝合金汽车转向节用制品试样,并对试样进行了室温和300℃高温条件下耐磨损性能以及中性盐雾条件下耐腐蚀性能的测试与分析。结果表明,复合锻造可以显著提高试样的耐磨损性能和耐腐蚀性能。与常规锻造相比,复合锻造试样在室温和300℃高温条件下的磨损体积分别减小32%、56%,96 h盐雾腐蚀试验后的质量损失率下降了1.59%。

热处理对6061铝合金铸锭淬火应力及力学性能的影响

文章研究了不同热处理制度对6061铝合金铸锭淬火应力及力学性能（抗拉、屈服强度、延伸率及硬度）的影响,并通过金相、SEM和EDS方法初步探讨了其微观组织变化。结果表明,合金在淬火敏感区（230-440℃）采用60-80℃的水淬火,其淬火应力明显小于室温水淬火,但是其抗拉、屈服强度、延伸率及硬度与用室温水淬火几乎相当。

Effects of processing parameters on microstructure and mechanical properties of powder-thixoforged 6061 aluminum alloy

A new processing technology,powder thixoforming,for preparation of particle reinforced metal matrix composites was proposed and 6061 aluminum alloy was prepared by powder thixoforging.6061 ingots were first prepared by cold-pressing the atomized 6061 alloy powders,and then the ingots were partially remelted followed by thixoforging.The effects of reheating time,mould temperature and reheating temperature on microstructure and mechanical properties of the thixoforged alloys were investigated.The results indicate that all of the three parameters have large effects on the microstructure and mechanical properties.Owing to the microstructure changes,the fracture regime varies with the processing parameters.Furthermore,cracks always initiate from shrinkage porosities and inclusions,and then propagate either along the secondarily solidified structures or primary particles.The ultimate tensile strength,elongation and hardness of the resulting alloy are up to 196 MPa,11.0%and HV 55.7 respectively.

Effect of ball milling on microstructural evolution during partial remelting of6061 aluminum alloy prepared by cold-pressing of alloy powders

The effect of ball milling on the microstructural evolution was investigated during partial remelting of 6061 aluminum alloy prepared by cold-pressing of atomized alloy powders.The results indicate that the microstructural evolution of 6061 aluminum alloy can be divided into three stages,the dissolution of eutectic phases and the coarsening and growth behavior of the resulting grains,structural separation and spheroidization of primary particles,and the final coarsening behavior of the particles.Compared with the alloy without ball milling,ball milling accelerates the first stage of microstructural evolution due to the energy stored in the powders,but the latter two stages are slowed down because of the formation of large-sized powders.Moreover,the finer the as-cold-pressed microstructure is,the smaller and more spherical the primary particles in the final semisolid microstructure are.Furthermore,properly elevating the heating temperature is beneficial for obtaining small and spheroidal particles.

Effect of post-weld heat treatment on microstructure and mechanical properties of welded joints of 6061-T6 aluminum alloy

6061 aluminum alloy T-joints were welded by double-pulsed MIG welding process. Then, the post-weld heat treatment was performed on the welded T-joints. The weld microstructure under different aging temperature and time was investigated by transmission electron microscopy and scanning electron microscopy. The mechanical properties were examined by hardness test and tensile test. The results showed that the micro-hardness was sensitive to heat treatment temperature and time. Increasing temperature was beneficial to the shortening of peak aging time. There were a large number of dislocations and few precipitates in the welded joints. With the increase of post-weld heat treatment temperature and time, the density of dislocation decreased. Meanwhile, the strengthening phase precipitated and grew up gradually. When the post-weld heat treatment temperature increased up to 200℃, large Q' phases were observed. And they were responsible for the peak value of the micro-hardness in the welded joints.

Influence of sequence of cold working and aging treatment on mechanical behaviour of 6061 aluminum alloy

The influence of combination of different designated precipitation hardening and cold working on the tensile properties of 6061 aluminum alloy was investigated. The results indicate that applying single aging at 180 ℃ for 4 h in different thermal-mechanical treatments improves both the strength and elongation. However, double aging does not improve the mechanical properties. In addition, pre-aging shows a negative effect on the subsequent precipitation hardening of material. The changes in mechanical properties were discussed by explanation of microstructural evolution due to the competition of precipitation hardening, strain hardening and work softening processes.

Optimization of ECAP-RAP process for preparing semisolid billet of 6061 aluminum alloy

6061 aluminum alloy semisolid billet was prepared by the equal-channel angular processing(ECAP)-recrystallization and partial(RAP)process(a combination of equal-channel angular processing and recrystallization and partial remelting).The effects of different process parameters on the alloy microstructure were studied and the quantitative relationship between the process parameters and microstructure was established by response surface methodology(RSM)to optimize the process parameters.According to the orthogonal test,the holding temperature and holding time of the four ECAP-RAP process parameters were found to have the greatest impact on the microstructural characteristics,including average grain size and average shape factor.Through RSM,it was also found that when the average grain size or the average shape factor is optimized separately,another will be degraded.When the two indexes were simultaneously considered,the optimal process parameters were found to be a holding temperature of 623°C and holding time of 13 min,and the corresponding average grain size and average shape factor were 35.97μm and 0.8535,respectively.Moreover,comparing the experimental and predicted values,the reliability of the established response surface model was verified.

Solidification behavior of 6061 wrought aluminum alloy during rheo-diecasting process with self-inoculation method

The semisolid slurry of the 6061 wrought aluminum alloy was prepared by the self-inoculation method（SIM）. The effects of the isothermal holding parameters on microstructures of rheo-diecastings were investigated, and the solidification behavior of 6061 wrought aluminum alloy during the rheo-diecasting process was analyzed using OM, SEM, EDS and EBSD. The results indicate that the isothermal holding process during slurry preparation has great effect on primary α（Al） particles（α1）, but has little effect on the microstructure of secondary solidification in the process of thin-walled rheo-diecasting. Nucleation is expected to take place in the entire remaining liquid when the remaining liquid fills the die cavity, and the secondary solidification particles（α2） are formed after the process of stable growth, unstable growth and merging. The solute concentration of remaining liquid is higher than that of the original alloy due to the existence of α1 particles, hence the contents of Mg and Si in α2 particles are higher than those in α1 particles.

Evolution of Residual Stresses in Micro-arc Oxidation Ceramic Coatings on 6061 Al Alloy

Most researches on micro-arc oxidation mainly focus on the application rather than discovering the evolution of residual stresses. However, residual stresses in the surface coatings of structural components have adverse effects on their properties, such as fatigue life, dimensional stability and corrosion resistance, etc. The micro-arc oxidation ceramic coatings are produced on the surfaces of 6061 aluminum alloy by a homemade asymmetric AC type of micro-arc oxidation equipment of 20 kW. A constant current density of 4.4___0.1 A/dm2 and a self-regulated composite electrolyte are used. The micro-arc oxidation treatment period ranges from 10 min to 40 min, and the thickness of the ceramic coatings is more than 20 Bin. Residual stresses attributed to 7-A1203 constituent in the coatings at different micro-arc oxidation periods are analyzed by an X-ray diffractometer using the sin2~u method. The analysis results show that the residual stress in the ceramic coatings is compressive in nature, and it increases first and then decreases with micro-arc oxidation time increase. The maximum stress value is 1 667_＋20 MPa for period of 20 min. Through analyzing the coating thickness, surface morphology and phase composition, it is found that the residual stress in the ceramic coatings is linked closely with the coating growth, the phase composition and the micro cracks formed. It is also found that both the heat treatment and the ultrasonic action release remarkably the residual compressive stress. The heat treatment makes the residual compressive stress value decrease 1 378 MPa. The ultrasonic action even alters the nature of the residual stress, making the residual compressive stress change into a residual tensile stress.