Grain structure of laser remelted 7075 aluminium alloy in presence of Al2O3 particles

Abstract

The effect of inert particles on grain structure development from alloy melt during laser rapid solidification has been investigated. It is found that the presence of Al₂O₃ particles may disrupt the usual epitaxial grain structure evolution of an aluminium 7075 alloy when processed by laser surface remelting. This result, in addition to observations of crystal termination at the particles and grain refinement in particle dense regions, indicates that a mechanism of particle restricted grain growth operates.     

Fatigue behavior of AA7075-T6 aluminum alloy coated with ZrN by PVD

The present investigation has been conducted in order to study the effect of the deposition of a ZrN coating, of 3 μm in thickness, on the static mechanical properties and fatigue behavior of a 7075-T6 aluminum alloy substrate. It has been determined that the coating deposition process gives rise to a significant decrease in such properties, which is not fully compensated for the presence of the film. When fatigue tests were carried out in a 3 wt.% NaCl solution at low alternating stresses, the ZrN film partially compensated for the decrease in fatigue properties of the coated substrate. Extensive delamination of the coating from the substrate was observed under the action of cyclic stresses greater than approximately 220 MPa. Below this stress and in the presence of NaCl, the behavior of the coated material approached that of the uncoated alloy, which highlighted the good corrosion resistance of the ZrN coating and its ability to protect the substrate when it remained adhered to the latter.     

Equivalent crack size model for pre-corrosion fatigue life prediction of aluminum alloy 7075-T6

Corrosion damage can significantly reduce the service life of aluminum alloy structures and endanger the structural integrity of aircraft. Here, aiming at center-hole sheet specimens of aluminum alloy 7075-T6, uniaxial fatigue tests and post-fracture analysis are performed to investigate the effect of corrosion pits on the pre-corrosion fatigue behavior. Then the best correlated parameters between corrosion pits and equivalent cracks are identified through Pearson correlation analysis. It is found that for single-crack initiations ar_ECS (equivalent crack depth 1 aspect ratio) vs. ar_cri (critical pit depth 1 aspect ratio) are best correlated with correlation coefficient of 0.9, while the best correlated parameters for multi-crack initiations are ar_ECS (equivalent crack depth 1 aspect ratio) vs. r_cri (aspect ratio) with correlation coefficient of 0.69. Equivalent crack size (ECS) models are correspondingly developed with these best correlated parameters for single- and multi-crack initiations, respectively. The pre-corrosion fatigue lives predicted with our models agree well with the experimental results and the maximum error factor is about 1.6.     

Part-through fracture toughness (KIe) and crack-tip opening displacement (CTOD) of welded joints of AA2014-T6 at low temperatures

The part-through fracture toughness (K_Ie) and crack-tip opening displacement (CTOD, δ_m) of welded joints of aluminum alloy (AA) 20l4-T6, including the weld metal, the fusion zone (FZ), the heat-affected zone (HAZ), and the base material, were investigated at both liquid nitrogen temperature and liquid helium temperature with surface-crack tension (SCT) specimen and single-edge-notched bend (SENB) specimen respectively. Results indicate a conventional fusion welding process leads to formation of second-phase precipitations and inclusions, which cause significant reduction of fracture resistance at the weld metal and the FZ by fractographic analyses of fractured surface.     

Fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P deposit

An investigation has been carried out in order to study the fatigue and corrosion–fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P (EN) deposit, in the as-plated condition, of approximately 38–40 μm in thickness and a high P content, of approximately 18 wt%. The results obtained, show that the EN coating can give rise to a significant improvement in the fatigue and corrosion–fatigue performance of the substrate, depending on the testing conditions. When the coated system is tested in air, it is observed that the increase in fatigue properties decreases as the alternating stress applied to the material increases. At stresses of the order of 0.4 σ_0.2% the increase in fatigue life is more than about 100%. However, as the stress increases to values in the range of 0.7 σ_0.2%, no improvement in the fatigue performance of the system is observed and the behavior is similar to that of the uncoated substrate. Under corrosion–fatigue conditions, the fatigue life is observed to increase between approximately 60% and 70%, depending on the stress applied. It is shown that fatigue cracks are associated with nodular-like defects present on the surface of the coated samples. The deleterious effect of such defects seems to be more pronounced as the alternating stress applied to the material increases. A crude estimate of the yield strength of the EN coating from tensile measurements indicates that such a parameter is in the range of 3.8 GPa, in agreement with the computation of the absolute hardness of the deposit, of about 4 GPa, by means of Meyer’s law. It is also shown that the EN deposit has a very good adhesion to the substrate even when the system is subjected to tensile stresses greater than the yield strength. Such characteristics as well as the higher mechanical properties of the EN coating in comparison with the aluminum alloy substrate and the preservation of its integrity during fatigue testing contribute to the better fatigue performance of the coated system.     

Effect of friction stir welding on the microstructure and mechanical properties of AA 6063-T6 aluminum alloy

In this study, AA 6063-T6 alloy plates were joined via friction stir welding using three different pin geometries (i. e., helical threaded, pentagonal and triangular) under various process parameters of tool rotational speed and welding speed. The microstructures and mechanical properties of the various welded joints were investigated. Macro-structural observations revealed that kissing bonds occurred in the welded joints due to fractured oxide layers. X-ray diffraction analysis indicated that the stir zones of the welded joints exhibited phases of Al₈Fe₂Si, Al₅FeSi, and Mg₂Si. In the welded joints, processed using a helical threaded pin, no tunnel-type defect was detected to occur; specimens were fractured outside of the joint region during tensile tests, indicating that the kissing bonds formed in the stir zones did not cause any deterioration in tensile strength or ductility. The welded joints processed using a helical threaded, pentagonal and triangular pin at 500 min⁻¹ tool rotational speed and 80 mm min⁻¹ welding speed exhibited a ductile deformation behavior along with a tensile strength in the range of 153 MPa to 155 MPa.     

Microstructure-based multistage fatigue modeling of aluminum alloy 7075-T651

The multistage fatigue model for high cycle fatigue of a cast aluminum alloy developed by McDowell et al. is modified to consider the structure-property relations for cyclic damage and fatigue life of a high strength aluminum alloy 7075-T651 for aircraft structural applications. The multistage model was developed as a physically-based framework to evaluate sensitivity of fatigue response to various microstructural features to support materials process design and component-specific tailoring of fatigue resistant materials. In this work, the model is first generalized to evaluate both the high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes for multiaxial loading conditions, with appropriate modifications introduced for wrought materials. The particular microstructural features of relevance to fatigue in aluminum alloy 7075-T651 include micron-scale Fe-rich intermetallic particles and rolling textures. The model specifically addresses the role of local constrained cyclic microplasticity at fractured inclusions in fatigue crack incubation and microstructurally small crack growth, including the effect of crystallographic orientation on crack tip displacement as the driving force. The model is able to predict lower and upper bounds of the fatigue life based on measured inclusion sizes.     

基于热加工图的7075铝合金热塑性变形工艺参数优化识别

在Gleeble-1500热模拟试验机上进行多组热压缩试验,得到7075铝合金在成形温度573～723K,应变速率0.01～10s-1下的真应力-应变数据,用此数据作为计算应变速率敏感指数（m值）、功率耗散因子（η值）、失稳判据（ξ（ε.）值）三重判据的基本模型。通过三重判据构建包含应变在内的7075铝合金热加工图,并观察试样变形后的微观组织来验证热加工图,最终判断该合金在试验范围内的最佳变形参数。结果表明7075铝合金热加工的安全区集中在高温低应变速率区,并随着应变的增加,η值逐渐增加;通过金相观察,稳定变形区,材料由于变形发生动态再结晶而使晶粒细化;不稳定变形区,裂纹伴随着流动位错带的产生而被发现,因此可以通过包含应变的热加工图所确定的最佳工艺参数来保证无缺陷的7075铝合金锻件。     

Evaluation of low strain rate constitutive equation of 7075 aluminium alloy at high temperature

Abstract

The 7075 aluminium alloy is one of the most important engineering alloys utilised extensively in aircraft and transportation industries due to its high specific strength. In the present research, the flow behaviour of this alloy has been investigated using hot compression test at strain rates of 0·001, 0·01, 0·1 and 1 s^?1 and temperatures of 350, 400 and 450°C. The results reveal that dynamic softening occurred in these temperatures and strain rates. The activation energy, strain rate sensitivity and two constitutive equations (hyperbolic sine law and the power law) are derived from the results. It is shown that the hyperbolic sine law has a better agreement with the experimental results.     

Effect of Temperature on Material Transfer Behavior at Different Stages of Friction Stir Welded 7075-T6 Aluminum Alloy

In this work, the morphologies of weld of 7075-T6 aluminum alloy via friction stir welding （FSW） were analyzed by optical microscopy, the temperature field was attained by numerical simulation, and the effect of temperature on material transfer behavior in the thermal-mechanical affected zone （TMAZ） at different stages was mainly investigated. The FSW process consists of three stages. It is very interesting to find that the maximum transfer displacement of material appears at the final stage of welding process, then at the stable stage and at the initial stage, which results from the difference of peak temperatures at different stages. At any stage, the material in TMAZ near the surface of weld transfers downwards, the material in the middle of weld moves upwards and the material near the bottom of weld hardly moves. In any cross section of weld, the largest transfer displacement of material appears in the middle of weld. The increase of rotational velocity and the decrease of welding speed are both beneficial to the transfer displacement of material in the middle of weld.     

A study of fatigue crack growth of 7075-T651 aluminum alloy

Both standard and non-standard compact specimens were employed to experimentally study the crack growth behavior of 7075-T651 aluminum alloy in ambient air. The effects of the stress ratio (R), overloading, underloading, and high–low sequence loading on fatigue crack growth rate were investigated. Significant R-ratio effect was identified. At the same R-ratio, the influence of specimen geometry on the relationship between crack growth rate and stress intensity factor range was insignificant. A single overload retarded the crack growth rate significantly. A slight acceleration of crack growth rate was identified after a single underload. The crack growth rate resumed after the crack propagated out of the influencing plastic zone created by the overload or underload. A parameter combining the stress intensity factor range and the maximum stress intensity factor can correlate the crack growth at different stress ratios well when the R-ratio ranged from −2 to 0.5. The parameter multiplied by a correction factor can be used to predict the crack growth with the influence of the R-ratio, overloading, underloading, and high–low sequence loading. Wheeler’s model cannot describe the variation of fatigue crack growth with the crack length being in the overload influencing zone. A modified Wheeler’s model based on the evolution of the remaining affected plastic zone was found to predict well the influence of the overload and sequence loading on the crack growth.     

Prediction of cutting force based on machining parameters on AL7075-T6 aluminum alloy by response surface methodology in end milling

Cutting forces modeling is the basic to understand the cutting process, which should be kept in minimum to reduce tool deflection, vibration, tool wear and optimize the process parameters in order to obtain a high quality product within minimum machining time. In this paper a statistical model has been developed to predict cutting force in terms of geometrical parameters such as rake angle, nose radius of cutting tool and machining parameters such as cutting speed, cutting feed and axial depth of cut. Response surface methodology experimental design was employed for conducting experiments. The work piece material is Aluminum (Al 7075-T6) and the tool used is high speed steel end mill cutter with different tool geometry. The cutting forces are measured using three axis milling tool dynamometer. The second order mathematical model in terms of machining parameters is developed for predicting cutting forces. The adequacy of the model is checked by employing ANOVA. The direct effect of the process parameter with cutting forces are analyzed, which helps to select process parameter in order to keep cutting forces minimum, which ensures the stability of end milling process. The study observed that feed rate has the highest statistical and physical influence on cutting force.     

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082-T6 aluminium alloy

The present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082-T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD_0.2) decreases in the range of ∼18 to 36% whereas the value of the non-linear fracture mechanics parameter  J _0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.     

高周疲劳载荷下6061-T6铝合金的温度演化

为了研究金属材料在疲劳载荷下的温度变化,采用红外热像系统对高周疲劳载荷下6061-T6铝合金的温度演化进行分析,用热像图对疲劳裂纹尖端的塑性区进行测量.结果显示,疲劳加载作用下,循环次数达到107次时6061-T6铝合金试样表面温度的变化分为四个阶段:初始温升阶段、温度缓降阶段、温度二次缓慢上升阶段和温度快速上升阶段.结合热弹性理论、铝合金塑性变形的微观机制分析并预测疲劳载荷下温度的演化和宏观裂纹扩展时裂纹尖端塑性区域大小.宏观裂纹开始扩展时,裂纹尖端的塑性区域可达3.6 mm2,红外热像仪测得结果为3.46 mm2,测试结果与理论结果吻合.     

焊后热处理对AA7204-T4铝合金搅拌摩擦焊接头组织与力学性能的影响

采用搅拌摩擦焊对AA7204-T4铝合金板材进行焊接,研究焊后热处理对FSW接头组织与力学性能的影响。结果表明:焊后(AW)态焊核区平均晶粒尺寸和再结晶分数分别为4.7μm和81.9%,焊后直接人工时效(AA)后二者分别为4.8μm和82.4%,焊后固溶+人工时效(SAA)后分别为5.9μm和86.5%,AA态并未对焊核区的晶粒结构产生明显的影响,而SAA态则使其分别提高了25.5%和5.6%。AW态,AA态和SAA态接头抗拉强度分别为296.6,318.2 MPa和357.4 MPa,AA态对接头力学性能提升有限,而SAA态则有效提升接头力学性能,焊接系数达92.0%。焊后固溶淬火导致接头连接界面"S"线处产生微裂纹,接头断裂于此,且伸长率严重降低。     

Numerical simulation and experimental investigation on friction stir welding of 6061-T6 aluminum alloy

A modified three-dimensional model was established to simulate the friction stir welding of the 6061-T6 aluminum alloy. A detailed calculating method of the heat generation was proposed by taking account of the contact conditions between the tool and the work-piece. The results show that the heat mainly generated within the region close to the shoulder, the high temperature exists within the upper portion of the weld and decreases along the thickness direction. The strong material flow mainly occurs within the region around the tool and the material ahead of the tool sweeps toward the RS and finally deposits behind the tool. During this procedure the material is extruded to experience different shear orientations, and a defect-prone region exists in the region where material flow is weak. The temperature field and material flow behaviors predicted by the simulation method are in good agreement with the results obtained by the experiments.     

6061-T6铝合金中厚板-节点套多层多道焊数值模拟

为研究铝合金中厚板-节点套接头在多层多道焊后的残余应力和变形分布,本文基于ABAQUS软件建立了该接头三维有限元模型,采用双椭球热源、生死单元法以及顺序耦合法,对6061-T6铝合金中厚板-节点套多层多道焊进行数值模拟,并分析了接头的温度场,以及在夹具约束下的焊接残余应力及变形的分布情况。研究结果表明:数值模拟与实际接头的熔池形状吻合度较高;摆动焊接过程中温度曲线呈多峰结构;焊件的升温速率明显大于冷却速率,且冷却速率随时间逐渐减小;焊接残余应力主要集中在焊缝及夹具区域,且小于6061-T6铝合金在室温下的屈服强度;接头的最大横向残余应力为129.9 MPa,中厚板上的横向残余应力大于节点套上的横向残余应力;接头的最大纵向残余应力为132.9 MPa,沿焊接方向,焊缝处的纵向残余应力呈山峰状分布;该接头在Y轴方向上的变形最大,为1.494 mm,该接头的最终变形结果为上凸变形。     

Flow behavior and processing map for hot deformation of ATI425 titanium alloy

Flow behavior and processing map play important roles in the hot deformation process of titanium alloys. In this research, compression Gleeble tests have been carried out to investigate the stress–strain relationship at temperatures ranging from 700 to 1000℃ and strain rates ranging from 0.001 to 1 s~(-1) for ATI 425 titanium alloy. Arrhenius type constitutive equation was obtained to describe the compressive flow behavior with modification of additional deformation dead zone, friction model, temperature model and strain rate. The introduction of novel calculation method for value in Arrhenius equation gives more accurate fitting than traditional one. Processing maps were drawn based on the distribution of dissipator co-content, and optimized deformation temperature and strain rate range obtained. It is proven to be accurate and effective through the experimental results. The microstructure analysis shows that more dynamic recrystallization can be achieved in the area with larger value on the processing map.     

Growing and critical ovalization for sharp-notched 6061-T6 aluminum alloy tubes under cyclic bending

In this paper, we present results from experiments dealing ovalization in 6061-T6 aluminum alloy tubes which have been notched and subjected to cyclic bending are presented. It was discovered that the tubes continuously ovalized to a critical quantity when they buckled. Tubes with a smooth surface and five different notch depths were considered. The ovalization–curvature curve exhibited a symmetrical and ratcheting increase with the number of bending cycles. Deeper notch depths caused greater ovalizations. In addition, the trend of the ovalization at negative extreme curvature and number of bending cycles relationship was distinguished into three stages. Finally, the empirical form proposed by Lee, Hung, and Pan in 2010 was employed for describing the above-mentioned correlation in the first two stages. It was found that the experimental and simulated data agreed quite well.