The effect of interrupted aging on the yield strength and uniform elongation of precipitation-hardened Al alloys

The yield strength and elongation are both important properties of engineering alloys. In precipitation-hardenable alloys, these properties are directly related to the interaction between the precipitates and mobile dislocations and an inverse correlation is usually observed. It was recently reported that bimodal microstructures containing both shearable and shear-resistant precipitates, generated by interrupted aging schedules, can simultaneously increase both the yield strength and the uniform elongation in selected Al alloys. This is an effect of considerable technological significance but the physical origin is not understood. An explanation for the interrupted aging effect is offered in this contribution. The mechanical response of a model Al–Cu alloy after interrupted aging has been characterized using uniaxial tensile tests and tension–compression Bauschinger tests. The undeformed and deformed microstructures have been characterized using transmission electron microscopy, scanning transmission electron microscopy and differential scanning calorimetry. It is shown that dissolution of the shearable Guinier–Preston (GP) zones occurs during uniaxial deformation, and that the repartition of solute from the GP zones to the matrix can provide a positive contribution to strain hardening that is quantitatively capable of explaining the observations of enhanced elongation. The result depends on the effect of the repartitioned solute on the plasticity of the Al matrix and, using a series of Al–X (X = Cu, Mg, Si, Zn) binary solid solutions, the effect of solute in solution on the mechanical response is quantified. This information is incorporated into models for the yield strength and strain hardening of precipitate-containing microstructures. It is suggested that simultaneous increases in both strength and elongation due to interrupted aging may be expected in 2xxx series alloys but are less likely to be significant in the 6xxx series alloys and are unlikely in most 7xxx series.     

Determining the effect of microstructure and heat treatment on the mechanical strengthening behavior of an aluminum alloy containing lithium precipitation hardened with the δ′ Al3Li intermetallic phase

The effect of the thermal treatment and composition on microstructure and subsequent mechanical behavior of an Al-2.6 wt.% Li-0.09 wt.% Zr alloy that was solution heat treated (SHT) and artificially aged for a series of aging times and temperatures was studied. The underaged, peakaged, and overaged thermal heat treatments were studied to determine the effect of the microstructure and processing on the mechanical properties. The precipitates in the microstructure, which impede dislocation motion and control the precipitation strengthening response as a function of aging practice, were analyzed as the basis for controlling the strengthening depending on their size distribution, average size, and interparticle spacing. The average particle size, spacing, and size distribution were determined from the microstructure as a function of the thermal processing and composition. For the demonstration alloy, the primary strengthening was a direct consequence of ordered coherent Al₃Li (δ′) intermetallic precipitates, which are uniformly distributed throughout the microstructure and restrict the glide motion of dislocations during plastic deformation. The Al₃Li average particle size, distribution, spacing, and volume fraction are closely related to the overall mechanical behavior and are a result of the heat treating practice and composition. Consequently, a micromechanical model was developed for predicting the precipitation hardening response in terms of the variation in polycrystalline strength with aging time, aging temperature, and composition. The overall micromechanical model, which was determined from the particle coarsening kinetics, dislocation mechanics, thermodynamics, resolved shear stress, as well as the dislocation particle shearing and bypassing mechanisms, accurately predicted the mechanical strength in the underaged, peak-aged, and overaged tempers of the demonstration alloy.     

Validation of a systematic approach to modeling spray quenching of aluminum alloy extrusions, composites, and continuous castings

Optimal cooling of aluminum alloys following the high- temperature extrusion process suppresses precipitation of intermetallic compounds and results in a part capable of possessing maximum strength and hardness after the subsequent age- hardening process. Rapid quenching suppresses precipitation but can lead to large spatial temperature gradients in complex- shaped parts, causing distortion, cracking, high residual stress, and/or nonuniform mechanical properties. Conversely, slow cooling significantly reduces or eliminates these undesirable conditions but allows considerable precipitation, resulting in low strength, soft spots, and/or low corrosion resistance. This study presents a systematic method of locating and operating multiple spray nozzles for any shaped extrusion such that uniform, rapid cooling and superior mechanical and metallurgical properties are achieved. A spray nozzle data base was compiled by measuring the distribution of spray hydrodynamic parameters (volumetric spray flux, mean drop diameter, and mean drop velocity) throughout the spray field of various industrial nozzles. Spray heat transfer correlations, which link the local spray hydrodynamic parameters to the heat transfer rate in each of the boiling regimes experienced by the surface, defined the spatially nonuniform boundary conditions in a numerical model of the quenching process that also accounted for interference between adjacent spray fields. New correlations, offering increased accuracy and less computational time, were formulated for the high- temperature boiling regimes which have a critical influence on final mechanical properties. The quench factor technique related predicted thermal history to metallurgical transformations occurring within the extrusion to predict hardness distribution. The validity of this unique approach was demonstrated by comparing model predictions to the temperature response (and hardness after artificial aging) of an L- shaped Al 2024- T6 extrusion to quenches with multiple, overlapping water sprays. The validation study reported herein concludes by exploring the possibility of applying quenching technology to improving the properties of extruded metal- matrix composites such as SiC_p/Al 6061 and cast alloys.     

Definition of quality in cast aluminum alloys and its characterization with appropriate indices

In this paper, the issue of “quality” of cast aluminum alloys from various viewpoints is interpreted. Many methods to characterize the quality of materials are available; the methods used currently for the quality evaluation of cast aluminum alloys include nondestructive testing, characterization of the microstructure, and mechanical testing. With regard to mechanical testing, a number of quality indices have been devised to evaluate and characterize the quality of cast aluminum alloys. As these quality indices use different mechanical properties for the quality evaluation, they are expected to lead to different results. In this work, the application of proposed quality indices and their suitability is discussed for a number of situations, including minor variations in chemical composition, different solidification rate, solid solution and artificial aging heat treatments.     

预时效对车身用6016铝合金烘烤硬化性能的影响

采用硬度测试、拉伸试验和透射电镜等手段研究了不同预时效处理对6016铝合金烘烤前后微观组织和力学性能的影响。结果表明:6016铝合金具有较强的自然时效硬化能力,自然时效24 h的6016铝硬度比固溶态合金硬度增加了45.6%。自然时效超过24 h以后,合金硬度值变化不大。通过预时效处理可以显著提高6016铝合金的烘烤硬化效果。经550 ℃×30 min固溶+160 ℃×10 min预时效处理后,6016铝合金规定塑性延伸强度为131.4 MPa,伸长率为24.7%。再经175 ℃×30 min烘烤后合金规定塑性延伸强度达到199.5 MPa,烘烤硬化值(BH)为68.1 MPa,此工艺为6016铝合金车身板最佳的热处理工艺。     

8090Al—Li合金的低周疲劳行为

研究了不同热处理状态８０９０Ａｌ－Ｌｉ合金的循环疲劳与断裂行为。结果表明，相同时效状态合金的循环应力随应变幅的增加而提高，低应变幅下时效状态影响较小；随应变幅的提高，时效状态的影响增强，合金出表现出的循环硬化，软化以及低周疲劳性能与位错组态，沉淀相的尺寸，晶界ＰＦＺ的形成和断口形貌密切相关。     

通过双级固溶热处理制备新型高强Al−10.5Si−3.4Cu−0.2Mg合金

通过析出硬化提高Al?Si?Cu合金的力学性能.这些合金对时效硬化的反应非常缓慢.为了解决这一问题,在Al?10.5Si?3.4Cu合金中分别加入0.2％、0.4％和0.7％(质量分数)的镁.该新型合金在固溶处理阶段经过两种不同的析出硬化过程.结果表明,添加不同含量的镁可加速该合金对时效处理的响应,提高其硬度和强度.双...     

Dry and clean age hardening of aluminum alloys by high-pressure gas quenching

When precipitation-hardenable aluminum parts are water quenched, distortion occurs due to thermal stresses. Thereby, a costly reworking is necessary, and for this reason polymer quenchants are often used to reduce distortion, with the disadvantage that the quenched parts have to be cleaned after quenching. In opposition to liquid quenchants, gas quenching may decrease distortion due to the better temperature uniformity during quenching. Furthermore, cleaning of the quenched parts can be avoided because it is a dry process. For this purpose, a heat-treating process was evaluated that included a high-pressure gasquenching step. Gas quenching was applied to different aluminum alloys (i.e., 2024, 6013, 7075, and A357.0), and tensile tests have been carried out to determine the mechanical properties after solution annealing, gas quenching, and aging. Besides high-pressure gas quenching, alloy 2024 was quenched at ambient pressure in a gas nozzle field. The high velocity at the gas outlet leads to an accelerated cooling of the aluminum alloy in this case. Aluminum castings and forgings can be classified as an interesting field of application of these quenching methods due to their near-net shape before the heat treatment. Cost savings would be possible due to the reduced distortion, and therefore, less reworking after the precipitation hardening.     

冷加工和时效处理顺序对6061铝合金力学性能的影响(英文)

研究不同的析出硬化和冷加工组合对6061铝合金拉伸性能的影响。结果表明,在不同的热处理过程中,在180℃单时效4h能提高合金的强度和伸长率。然而,双时效处理不能改善其力学性能。另外,预时效对随后的析出硬化有负面影响。合金力学性能的变化归因于析出硬化、应变硬化和加工软化的竞争而引起的显微组织演变。     

铸造铝合金微观组织对应变硬化指数预测的影响

研究了铸造铝合金的微观组织参数包括二次枝晶臂间距(SDAS)、Si颗粒的形貌率和体积分数,与拉伸过程中内应力的关系,根据Hollomon和内应力公式建立了应变硬化指数n的定量预测关系式。结果表明:n值反映了材料在较大塑性变形下(在发生塑性松弛以后)的硬化能力.定义了微结构硬化能力参数,并用常见的几种铸造铝合金材料进行了验证,所推导的理论关系式及线性拟合关系式能较好地预测n值,对同一牌号的铸造铝合金材料,n值对颗粒相形貌率和SDAS的依赖性强,而对颗粒相体积分数的依赖性不明显,颗粒相形貌率和SDAS值越大,n值越小,对A319和A356/357铝合金,最佳修正系数分别是0.17和0.11,预测平均误差在10%左右。     

Effect of aging time and temperature on exfoliation corrosion of aluminum alloys 2024‐T3 and 7075‐T6

Two types of aluminum alloys, 2024‐T3 and 7075‐T6, have been selected in this study to investigate the effect of metallurgical aspects on exfoliation corrosion. To determine and evaluate the metallurgical effects of heat treatments on corrosion behaviour of these alloys, G34 ASTM test was selected to investigate the exfoliation corrosion behaviour. The results showed that with increasing the aging time for the aluminum alloy type 2024‐T3 the susceptibility to exfoliation corrosion increases, while for type 7075‐T6 decreased. These results refer to precipitation of the intermetallic compound phases such as CuAl₂, and MgZn₂, in 2024‐T3 and 7075‐T6 respectively. The amount of these phases increases with increasing the aging time for both alloys. The investigations showed the phases that initiate in 2024‐T3 act as anode sites while in 7075‐T6 they act as cathode sites.     

On certain aspects of strain rate sensitivity of sheet metals

The formability of a material depends upon the strain hardening and strain rate hardening of the material. In this study, constitutive parameters using the power law constitutive equation are determined for six different strength steels and two aluminum alloys over different strain ranges, including approximations of the postuniform elongation range. Constitutive parameters are found to be different at different strain ranges. The strain hardening of steels increases with strain at low strain levels (less than 5%) and decreases at high strain levels (greater than 10%). Strain rate hardening decreases with strain for all steels and aluminum alloys. Uniform elongation depends only on strain hardening, and postuniform elongation depends only on strain rate hardening. However, the total elongation depends on both strain hardening and strain rate hardening.     

Effect of chemical composition variation on microstructure and mechanical properties of a 6060 aluminum alloy

The 6XXX series aluminum alloys (Al-Mg-Si) are widely used in many different engineering and architectural applications. These alloys usually undergo a thermal treatment, which consists of a heat treatment solution and artificial aging, since the desirable mechanical properties depend on the microstructural state of the material. The recycling of materials has been increasing recently for economic and ecologic reasons. By using scrap was raw material, important reductions in energy and total costs can be achieved, and, at the same time, negative environmental impacts can be greatly reduced. In the present work, the possibility of using a larger amount of scrap as raw material in the production of an AA 6060 alloy is evaluated by analyzing the difference in microstructure and mechanical properties between a commercial 6060 alloy and a variation with higher Fe and lower Si contents that was specially produced for this study. Both materials were placed into a heat treatment solution at 560 °C for 1 h, and then underwent water quenching followed by artificial aging at 180 °C for different periods of time. Hardness and tension tests were used to evaluate the mechanical properties. Light and transmission electron microscopy have been used to determine important features such as grain size before and after being placed into the heat treatment solution, and the characteristics of the second-phase particles in the two materials. This study leads to the conclusion that a higher amount of scrap material can be used in the production of 6060 Al alloy without significant changes in mechanical properties compared with the more usual compositions.     

多轴载荷下ZL101铝合金的低周疲劳行为

研究不同等效应变幅下ZL101铝合金在多轴比例和非比例载荷下的低周疲劳行为,并用透射电镜观察合金的疲劳行为中的位错结构。结果表明：合金在两种加载方式下均表现为循环硬化;在非比例载荷下合金表现出附加强化,但程度不明显;合金的疲劳寿命随等效应变幅的增加而降低,合金在非比例加载下的疲劳寿命低于比例加载时的疲劳寿命。对位错结构的观察表明,随等效应变幅度的提高,合金的低周疲劳位错结构从交叉位错带转化为位错胞,合金在非比例加载下更易形成位错胞结构。     

Identification of heat treatments for better formability in an aluminum-lithium alloy sheet

Research in the weight of an automobile is a continuous process among auto manufacturers. The “body in white” (BIW, i.e., the body of the car) deserves attention, being a major contributor to the weight of the vehicle. By virtue of a high strength to weight ratio (density smaller than aluminum) and a higher Young’s modulus than aluminum, aluminum-lithium alloy sheet appears to hold promise as an autobody material. Because auto components are required in large numbers and are formed at room temperature, formability under these conditions becomes significant. Aluminum-lithium alloys acquire, because of aging over a short period of time, a good amount of strength and hence dent resistance. In principle, they can be given, through suitable heat treatments, a high formability as well as dent resistance, i.e., an ideal combination of properties. To this end, tensile properties have been determined for a number of heat treatments comprising three different solutionizing temperatures and for three aging times at each of the three aging temperatures. Considerable influence of heat treatment was observed on the mechanical properties (which in turn characterize both formability and dent resistance), such as the strain hardening exponent, average normal anisotropy, yield stress, ultimate tensile stress, and percentage elongation to failure. For each property, the best three heat treatments leading to a high formability were identified. Consequently, heat treatments that imparted the greatest formability for processes such as deep drawing and stretch forming have been identified. The investigations show that the best heat treatment for one property may not be the best for another property, calling for a compromise to obtain the most practicable heat treatment schedule. Results shed light on not only the biaxial formability but also springback behavior that is important in the BIW components. Further, the properties obtained from the heat treatment giving good formability in deep drawing were used to simulate car body fender and the S-rail using sheet metal forming simulation software PAMSTAMP2G. A comparison of simulation of aluminum-lithium alloy fender and S-rail with those made from steel demonstrates advantages using aluminum-lithium alloys in terms of weight reduction. Finally, based on the current oil prices and the projected demand for oil in the next decade, aluminum-lithium alloys seem to have an edge despite the difficulties in manufacturing, assembly, and joining of the aluminum-lithium components.     

铸造铝合金在多轴非比例载荷下的低周疲劳行为研究

主要研究了356铸造铝合金在多轴非比例载荷下的低周疲劳性能。结果表明，合金在循环变形过程中表现为循环硬化特征，循环硬化的程度表现出对非比例加载路径的依赖性；在相同等效应变幅值下，合金的非比例加载低周疲劳寿命小于比例加载，且非比例加载低周疲劳寿命也表现出对非比例加载路径的依赖性；在多轴疲劳寿命预测模型中考虑非比例加载路径对疲劳寿命的影响可以大大提高寿命模型预测的精确度。     

耐磨蚀合金设计原则的探讨

对耐磨蚀合金的成分与组织设计思想作了较全面的讨论．根据系统研究的结果，对形变强化，第二相强化，热处理制度以及钝化膜作用等在发展耐磨耐蚀合金中可能起的作用进行了深入和细致的分析．     

A356合金的低周疲劳行为及塑性应变能

采用电解低钛铝合金、工业纯铝与Al-10Ti中间合金,制备了具有不同钛含量的电解加钛A356合金(EA356合金)和熔配加钛A356合金(MA356合金),研究了加钛方式和钛含量对A356合金的应变能密度和低周疲劳性能的影响。结果表明:4种合金均表现为明显的循环硬化行为;具有较高钛含量的E14、M14合金的循环硬化能力高于低钛含量的E10和M10合金;合金的塑性应变能密度受应变幅的影响且具有循环相关性;高应变幅时,塑性应变能较高但随循环周次变化较小;当应变幅较低时,合金的塑性应变能较小但变化较大,特别是塑性较好的E10和M10合金;无论是电解加钛还是熔配加钛,钛含量为0.1%的E10和M10合金的的塑性应变能密度和疲劳寿命均优于钛含量为0.14%的E14和M14合金;合金的疲劳寿命对加钛方式不敏感,在相同钛含量下,两种加钛方式的合金具有相近的低周疲劳寿命。     

Al—Yb合金的析出强化行为

研究了稀二元Al一Yb合金在不同时效温度下的析出强化行为。结果表明,在时效过程中析出了细小、弥散的L12结构Al3Yb相,时效析出所能达到的峰值硬度为400～416MPa,各温度的峰值硬度比较接近。L12结构的Al3Yb析出相的粗化动力学遵循LSW理论,表明粗化由溶质原子扩散控制。当Al3Yb析出相的半径小于11nm时其与基体保持共格关系,大于11nm时开始出现半共格现象。当析出相的半径小于2nm时,时效强化的机理是位错切割机制,大于2nm时则主要为位错绕过的Orowan机制。     

A mathematical model coupled to CALPHAD to predict precipitation kinetics for multicomponent aluminum alloys

A mathematical model was developed to simulate the precipitation kinetics during heat treatment of multicomponent aluminum alloys. The model is based on the general numerical framework proposed by Kampmann and Wagner, and features a full coupling with CALPHAD software for the evaluation of the Gibbs–Thomson effect. It also does not rely on the assumption that precipitate phase composition is stoichiometric or uniform, and is therefore applicable for predicting complex precipitation kinetics encountered in industrial practices. Applications of the model to various aging treatments of binary Al–Sc alloys and a ternary Al–Sc–Zr alloy were conducted. It was found that the model predictions for extended time coarsening kinetics are in good agreement with the analytical Lifshitz–Slyozov–Wagner coarsening theory. Its ability to reproduce the complex precipitation pathways in multicomponent alloys was demonstrated by simulation of the precipitation kinetics for an Al–0.09 at.% Sc–0.03 at.% Zr alloy. Comparison of the simulation results with experimental measurement has also highlighted research directions that require further effort.