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.

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.

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.

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.

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.

Preparation of semi-solid 6061 aluminum alloy slurry by serpentine channel pouring

The semi-solid 6061 aluminum alloy slurry was prepared by a serpentine channel pouring process. The effects of pouring temperature, bend number and bend diameter on the microstructures were investigated. Microstructural evolution mechanism of the semi-solid slurry during the pouring process was also analyzed. The results show that the grain is refined and the grain roundness is improved by controlling the pouring temperature close to the liquidus temperature, and the nucleation rate of primary α(Al) grains is effectively increased via increasing the bend number and decreasing the bend diameter. The primary grains are not only formed directly from the alloy melt via chilling nucleation and heterogeneous nucleation, but also evolved from the fractured dendrite fragments. Meanwhile, the heat exchange between the melt and the serpentine channel is increased by the “self-stirring” effect in the melt, which also promotes the refinement and spheroidization of primary α(Al) grains.

Segregation in squeeze casting 6061 aluminum alloy wheel spokes and its formation mechanism

Segregation can seriously damage the mechanical properties of the aluminum alloys.6061 aluminum alloy wheel spokes were prepared by squeeze casting.To investigate the formation mechanism of segregation,the microstructure of the alloy was observed using scanning electron microscopy,energy dispersive spectrometry,X-ray diffraction and electron microprobe analysis methods.The Gibbs energy of each phase during solidification was calculated by JMat Pro.Results show that the segregation phases in the R-joint of the wheel spokes are mainly composed of Mg_(2)Si,β-Al Fe Si and Al_(5)Cu_(2)Mg_(8)Si_(6)intermetallics.During the solidification of the 6061 aluminum alloy wheels,Mg_(2)Si andα-Al Fe Si phases precipitate in the mushy zone at first.With the decrease of temperature,α-Al Fe Si transforms intoβ-Al Fe Si,while Al_(5)Cu_(2)Mg_(8)Si_(6)precipitates from the solid-state aluminum alloy after solidification.Segregation at the R-joint of wheel spokes is mainly caused by insufficient cooling,so the cooling during alloy solidification should be enhanced to avoid segregation.

Fatigue limit assessment of a 6061 aluminum alloy based on infrared thermography and steady ratcheting effect

To quickly predict the fatigue limit of 6061 aluminum alloy,two assessment methods based on the temperature evolution and the steady ratcheting strain difference under cyclic loading,respectively,were proposed.The temperature evolutions during static and cyclic loadings were both measured by infrared thermography.Fatigue tests show that the temperature evolution was closely related to the cyclic loading,and the cyclic loading range can be divided into three sections according to the regular of temperature evolution in different section.The mechanism of temperature evolution under different cyclic loadings was also analyzed due to the thermoelastic,viscous,and thermoplastic effects.Additionally,ratcheting strain under cyclic loading was also measured,and the results show that the evolution of the ratcheting strain under cyclic loading above the fatigue limit undergone three stages:the first increasing stage,the second steady state,and the final abrupt increase stage.The fatigue limit of the 6061 aluminum alloy was quickly estimated based on transition point of linear fitting of temperature increase and the steady value of ratcheting strain difference.Besides,it is feasible and quick of the two methods by the proof of the traditional S-N curve.

Microstructure and mechanical properties of laser beam welding 6061 aluminum alloy with different states

With the diversification of manufacture methods, joining the same materials with different states becomes indispensable in practical application. In present work, 6061 aluminum alloys with different states were welded by laser beam welding （LBW）. The microstructures of welded joint, before and after heat treating, were investigated. The mechanical properties, such as the tensile properties and microhardness , were tested. And the fracture characteristic was observed by means of scanning electron microscope （SEM）. The results show that the 6061 aluminum alloys have superior weldability and the microstructures are different significantly in different states. Besides, the grain boundaries of the joint microstructures become unclear after the heat treating. The strength and the elongations of welded joints could reach to those of the base metal. The tensile fracture occurs in the fusion zone and near 6061-0 alloy. And the fracture presents ductile rupture. Therefore, the LBW is an effective method for 6061 aluminum alloy.

Laser welding of 6061 aluminum alloy sheet with surfactant

Aluminum alloy shows low absorption to laser for its essential high reflection property. In this paper, an active laser welding process is adopted to weld 6061 aluminum alloy with the halide suocactant coated on sheet surface to improve the welding quality. The sheets with surfactant are welded under a series of welding parameters of laser power and welding speed while the plasma plumes in the welding process are recorded by high-speed camera. Then a metaUographic analysis and a transverse tensile test are implemented to assess the sheet butt joint property. The experiments show that the weld depth is deepened and the plasma plume is smaller with surfactant. A well formed weld of 1 mm-thickness sheet of 6061 aluminum alloy can be obtained under laser power of 920 W, welding speed of 7 mm/s, zero defocusing-amount and an argon shielding gas flow of 30 L/min.

Unified Principal S-N Equation for Friction Stir Welding of 5083 and 6061 Aluminum Alloys

With the popularization of friction stir welding(FSW),5083-H321 and 6061-T6 aluminum alloy materials are widely used during the FSW process.In this study,the fatigue life of friction stir welding with two materials,i.e.,5083-H321 and 6061-T6 aluminum alloy,are studied.Fatigue tests were carried out on the base metal of these two materials as well as on the butt joints and overlapping FSW samples.The principle of the equivalent structural stress method is used to analyze the FSW test data of these two materials.The fatigue resistances of these two materials were com-pared and a unified principal S-N curve equation was fitted.Two key parameters of the unified principal S-N curve obtained by fitting,Cd is 4222.5,and h is 0.2693.A new method for an FSW fatigue life assessment was developed in this study and can be used to calculate the fatigue life of different welding forms with a single S-N curve.Two main fatigue tests of bending and tension were used to verify the unified principal S-N curve equation.The results show that the fatigue life calculated by the unified mean 50%master S-N curve parameters are the closest to the fatigue test results.The reliability,practicability,and generality of the master S-N curve fitting parameters were verified using the test data.The unified principal S-N curve acquired in this study can not only be used in aluminum alloy materials but can also be applied to other materials.

Stress based forming limit diagram for formability characterization of 6061 aluminum

Two numerical criteria of forming limit diagram（FLD） criterion and ductile fracture criterion（DFC） are presented for FLD prediction of 6061 aluminum. The numerical results are compared with the experimental FLD and also punch＇s load-displacement curve of experimental samples. Experimental FLD of this study is calculated using hemispherical punch test of Hecker. Experimental FLD is converted to FLSD and imported to the Abaqus software to predict necking of samples. Numerical results for FLSD prediction were compared with experimental FLSD. Results show that ductile fracture criterion has higher accuracy for FLD and FLSD prediction of 6061 aluminum. Comparison of numerical and experimental results for force-displacement curve of punch shows that numerical results have a good agreement with experiment.

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.

6061铝合金FSW接头组织与耐蚀性能的研究

对4 mm厚的6061-T6铝合金板进行了搅拌摩擦焊接,通过浸泡腐蚀试验和电化学腐蚀试验研究焊缝腐蚀性能,采用金相显微镜、透射电镜等观察接头的微观组织。结果表明:6061铝合金FSW焊核区晶粒显著细化,晶界第二相较少;焊核区的耐剥落腐蚀和晶间腐蚀性能优于热影响区的;焊核区比热影响区的腐蚀电位较高,腐蚀电流密度较小,焊核区表现出更好的耐腐蚀性能。

旋转摩擦挤压加工对不同时效态6061铝合金显微组织和力学性能的影响

采用旋转摩擦挤压(rotational friction extrusion,RFE)工艺对不同时效态6061铝合金进行加工,研究RFE对不同时效态6061铝合金显微组织和力学性能的影响。结果表明:RFE能够破碎6061铝合金中的难熔AlFeMnSi相,导致合金中的AlFeMnSi相细小且分布均匀。预析出粗大Mg2Si相在RFE加工过程中摩擦热和变形作用下,回溶后重新析出,致使预析出粗大Mg2Si相对材料动态再结晶中的粒子激发形核(particle stimulated nucleation,PSN)机制无影响,RFE加工后的晶粒尺寸变化较小。RFE使淬火态6061铝合金强度下降,但长时间时效态6061铝合金经RFE加工后强度得到提升。然而,RFE加工的材料内部存在孔洞,合金断后伸长率降低。

膜层厚度对6061铝合金黑色微弧氧化陶瓷膜层结构与性能的影响

为了明确膜层厚度对铝合金黑色微弧氧化陶瓷膜层的黑度、耐蚀性和硬度的影响规律,促进其实际应用,在硅酸盐-钒酸盐复合溶液体系中对6061铝合金进行微弧氧化处理,通过控制微弧氧化时间获得具有不同膜层厚度的铝合金黑色陶瓷膜层。利用测厚仪、粗糙度测量仪、SEM和EDS对黑色陶瓷膜层的表面结构、膜层成分和粗糙度等进行表征,并借助色差仪、维氏硬度计和电化学工作站等研究了膜层厚度对陶瓷膜层的黑度、显微硬度和耐蚀性的影响。结果表明:膜层厚度对黑色陶瓷膜层的表面形貌、粗糙度、显色特性、硬度和耐蚀性有较大影响。随着微弧氧化时间延长,膜层厚度增加,膜层表面更为粗糙,表层放电微孔数量减少,放电微孔孔径增大;当膜厚达到30.786μm时,陶瓷膜层的放电微孔显著增大,孔径达到10μm。随着膜层厚度增加,陶瓷膜层的黑度增加,显微硬度提高,但耐蚀性先增强后降低。当膜层厚度为20.781μm时,陶瓷膜层的黑度和硬度较高,耐蚀性最好,其在3.5%NaCl溶液中的自腐蚀电流密度为1.093×10^(-6) A/cm^(2)。

固溶处理对6061铝合金显微组织与力学性能的影响

采用光学显微镜(OM)、电子背散射衍射(EBSD)和维氏显微硬度计研究了固溶处理对压缩态6061铝合金显微组织与力学性能的影响。结果表明:压缩态6061铝合金经过固溶处理后有细小等轴再结晶晶粒形成,再结晶晶粒的形成弱化了其织构强度。随着固溶处理时间增加,合金中的再结晶晶粒增多,织构强度降低。6061铝合金经过固溶处理后由于固溶强化的作用导致其硬度显著增大,但随着固溶处理时间的增加,合金的晶粒尺寸增大,细晶强化作用降低,致使合金的硬度值有所减小。

6061铝合金非连续搅拌摩擦增材制造与性能研究

目的利用非连续搅拌摩擦沉积增材制造技术制备了6061铝合金并研究了z方向非连续搅拌对合金性能的影响。方法使用超景深电子显微镜分析了预热时间对沉积增材成形工艺的影响,并探究了该工艺条件下沉积层的宏/微观缺陷、材料流动及拉伸力学性能关联规律。结果采用z方向非连续搅拌摩擦沉积增材成形工艺,在预热时间为35 s的条件下,除无横向约束的前进侧和后退侧出现了孔洞、弧形飞边及弱结合等微观缺陷外,沉积层中原料棒与凸起搅拌作用区的组织表现为高致密无孔洞的良好形态,原料棒及刀具凸起的强搅拌作用造成z方向材料流动存在差异,进而获得力学性能沿z方向具有梯度分布的完整沉积层,提高了搅拌摩擦沉积增材制造6061铝合金的成形效率。结论在相应的主轴转速(420 r/min)、平移速度(72 mm/min)、进料速度(72 mm/min)、刀具凸起高度(1.5 mm)与预热时间(35 s)工艺参数下,获得了稳定的增材沉积层,有效抑制了沉积层原料棒及凸起搅拌作用区的宏观缺陷,并使材料性能沿z方向呈梯度分布;沉积层抗拉强度达到了原料棒的57.15%;同时伸长率提升了185.8%,为高效率沉积增材及制备梯度性能构件提供了工艺借鉴。