Mechanical properties evolution during post-welding-heat treatments of double-lap Friction Stir Welded joints

The present study focuses on double-lap Friction Stir Welded (FSW) joints in 2024T3 and 7075T6 aluminium alloys subjected to several post-welding-heat treatments at warm (typical aging) and high temperature (solution range) followed by room temperature deformation (tensile tests). The effect of post-welding-heat treatments on the microstructure and mechanical properties of double lap FSW joints were investigated. Polarized Optical Microscopy (POM) and Scanning Electron Microscopy (SEM) analysis reveal a progressive change in grain size and morphology in high temperature post-welding-heat treated joints, leading to Abnormal Grain Growth in the stir zone. Stress–strain curves are rather flat for 200° and 300 °C post-welded heat treated joints while, for the other set of samples, stress increases with strain to reach maximum stress of 140–160 MPa. Micro-hardness profiles measured on transversal sections of post-welded heat treated joints reveal conditions (temperature and time) of hardness homogeneity at top, bottom and central nugget zone and/or along the whole measured profile. When homogeneity is reached, fracture occurs in the nugget. A relationship between hardness and tensile properties has been applied in the nugget.     

Effect of aging parameters on the mechanical properties of naturally aged Al–Mg–Si alloy

Among the extruded products within the Al–Mg–Si system, AA6082 alloy is regarded as high strength alloy which is used for automotive structural applications. Room temperature storage in between the solution heat treatment and the paint bake cycle is unavoidable problem and undergoes natural aging. Hence, natural aging time in the conventional T4 condition must be modified after the solution heat treatment in order to avoid any hardening during storage before forming processes. The present work was investigated to improve paint bake response of extruded 6082 profiles by employing pre‐aging in between the solution heat treatment and the paint bake in order to obtain sufficient strength for the required in‐service dent resistance. Pre‐aging treatment was performed for 5 min at 180 °C, 200 °C and 225 °C to improve bake hardening response of extruded 6082 profiles. Tensile tests and micro hardness measurements are performed to determine the natural aging effect on mechanical properties. In order to understand the precipitation hardening of this aluminum profile with a different natural aging time, differential scanning calorimetry measurements are performed.     

Modeling the hardness and yield strength evolutions of aluminum alloy with rod/needle-shaped precipitates

For heat-treatable aluminum alloy containing rod/needle-shaped precipitates, a strengthening model for aging hardening behavior has been developed to describe the variations of dimension and volume fraction of the precipitates, and the yield strength and hardness with aging time. The model incorporates both the strengthening mechanisms of dislocation cutting the precipitates at underaged stage and bypassing the precipitates at overaged stage. It has been shown that the model predictions broadly agree with the experimental data for Al–Mg–Si alloys at varied aging time and aging temperatures. Thus, the model can be used to predict the variation of yield strength and hardness with aging time and aging temperature.     

Residual stress distribution and microstructure in the friction stir weld of 7075 aluminum alloy

In order to study the relationship between residual stress (RS) and the microstructure of friction stir weld (FSW), RS profiles through thickness in the un-welded aluminum alloy 7075 plate and in middle layer of its FSW joint were determined nondestructively by the short-wavelength X-ray diffraction (SWXRD) and neutron diffraction. Microstructure and mechanical properties of the FSW joint were also studied by optical microscopic analysis, and microhardness and tensile strength measurements. RS profiles measured by the two methods had the same distribution trend. The maximum tensile RS tested by SWXRD and neutron diffraction in transverse and longitudinal direction occurred in the weld nugget. Microhardness in the direction perpendicular to the weld line showed a “W” shape distribution. Position of the local maximal extremum of RS in thermo-mechanically affected zone corresponded to that of minimal microhardness. The grain-refined strengthening caused by the recrystallization in the weld nugget kept the joint from fracturing at this region notwithstanding the maximum tensile RS. And the tensile fracture occurred near the boundary of welding zone and thermo-mechanically affected zone where minimum of hardness and maximum of RS appear at the same position.     

焊后时效对7046铝合金搅拌摩擦焊接头力学性能的影响

对厚度为3.5mm的7046铝合金挤压板材进行搅拌摩擦焊接并对焊接接头进行人工时效,研究了焊后时效对接头力学性能的影响.结果 表明,焊接接头时效前的硬度分布大致呈"W"形,抗拉强度为406.5 MPa,焊接系数为0.8,拉伸时在后退侧热影响区与热机影响区的过渡位置出现断裂,此处的硬度值最低,断裂面上有大量的韧窝;进行1...     

Evolution of microstructures and mechanical properties in similar and dissimilar friction stir welding of AA5086 and AA6061

In this work, thermo-mechanical behavior and microstructural evolution in similar and dissimilar friction stir welding of AA6061-T6 and AA5086-O have been investigated. Firstly, the thermo-mechanical behaviors of materials during similar and dissimilar FSW operations have been predicted using three-dimensional finite element software, ABAQUS, then, the mechanical properties and the developed microstructures within the welded samples have been studied with the aid of experimental observations and model predictions. It is found that different strengthening mechanisms in AA5086 and AA6061 result in complex behaviors in hardness of the welded cross section where the hardness variation in similar AA5086-O joints mainly depends on recrystallization and generation of fine grains in weld nugget, however, the hardness variations in the weld zone of AA6061/AA6061 and AA6061/AA5086 joints are affected by subsequent aging phenomenon. Also, both experimental and predicted data illustrate that the peak temperature in FSW of AA6061/AA6061 is the highest compared to the other joints employing the same welding parameters.     

Joint properties of dissimilar Al6061-T6 aluminum alloy/Ti–6%Al–4%V titanium alloy by gas tungsten arc welding assisted hybrid friction stir welding

Hybrid friction stir butt welding of Al6061-T6 aluminum alloy plate to Ti–6%Al–4%V titanium alloy plate with satisfactory acceptable joint strength was successfully achieved using preceding gas tungsten arc welding (GTAW) preheating heat source of the Ti alloy plate surface. Hybrid friction stir welding (HFSW) joints were welded completely without any unwelded zone resulting from smooth material flow by equally distributed temperature both in Al alloy side and Ti alloy side using GTAW assistance for preheating the Ti alloy plate unlike friction stir welding (FSW) joints. The ultimate tensile strength was approximately 91% in HFSW welds by that of the Al alloy base metal, which was 24% higher than that of FSW welds without GTAW under same welding condition. Notably, it was found that elongation in HFSW welds increased significantly compared with that of FSW welds, which resulted in improved joint strength. The ductile fracture was the main fracture mode in tensile test of HFSW welds.     

人工时效对7003/7046铝合金搅拌摩擦焊接头组织和力学性能的影响

采用差示扫描量热分析、硬度、室温拉伸实验、背散射电子衍射和透射电子显微镜等手段研究了焊后人工时效对7003-7046异种铝合金搅拌摩擦焊接(FSW)接头的微观组织和力学性能的影响。结果表明：FSW接头后退侧(7046铝合金侧)的硬度明显高于前进侧(7003铝合金侧),两侧平均硬度的差约为30HV;人工时效后接头的硬度提高,两侧平均硬度的差增大到约50HV,接头的屈服强度提高,抗拉强度略有提高,伸长率几乎不变。根据时效前后FSW接头不同区域微观组织的特征,分析了接头力学性能变化的原因。     

Cu-3.0Ni-0.75Si合金时效析出动力学分析

为研究Cu-3.0Ni-0.75Si合金时效过程中沉淀相的析出与长大规律,及其对合金硬度的影响,采用涡流电导仪和布氏硬度计分别测量合金的电导率和硬度,根据导电率与新相析出量之间的关系分析合金的时效析出动力学过程.结果表明,在350℃下时效,合金硬度随时效时间的延长,先升高后趋于平缓;在450℃、550℃下时效,合金硬度随时效时间的增加快速上升,到达峰值后缓慢下降;时效温度越高,合金硬度峰值越低,但硬度达到峰值所需的时间越短.温度一定,随时效时间的增加,合金电导率在时效初期快速升高,至峰值后趋于平缓.根据Cu-3.0Ni-0.75Si合金在450℃时效过程中电导率的变化,通过Avrami方程推导出相应的相变动力学方程及电导率方程分别为f=1-exp(-0.052 2t0.717 61)和σ=15.2+16.3[1-exp(-0.052 2t0.717 61)],采用相关系数检验法及F检验法对电导率方程的可信性进行检验,结果说明时效析出动力学方程和电导率方程具有一定的可靠性.对比由电导率经验方程得出的电导率理论值与测量得出的实验值,该理论值与实验值有良好的吻合度.     

Hastelloy C-276中碳化物析出及晶界贫Mo规律研究

研究了Hastelloy C-276在550,750,800℃等温时效不同时间后的相析出及晶界附近Mo的质量分数变化规律,结果表明:M6C型碳化物在较短时间内会沿晶界析出,且随碳化物的不断析出晶界附近Mo的质量分数会不断降低直到某一最小值(约为6%),然后开始升高.由于镍基耐蚀合金中Mo元素含量对其耐蚀性能有重大影响,...     

Process model for strength of age hardenable aluminium alloy welds

Abstract

An existing process model for hardness prediction in age hardenable aluminium alloy welds is presented and analysed. One of the key criticisms of this model is that its derivation assumes softening is due to precipitate dissolution alone. The influence of precipitate coarsening has been determined by developing an equivalent model for softening owing to coarsening. It is shown that the experimentally derived master curves that form the basis of the model are capable of representing softening by a mixture of precipitate coarsening and dissolution. Methods to predict post-weld natural aging are discussed, and a new method is presented based on direct prediction of the Guinier–Preston zone fraction. The model has been applied to friction stir welding. Model predictions agree well with measured hardness profiles, and the sensitivity of the predictions to temperature is discussed.     

Influence of temperature and aging time on hardness and magnetic properties of the maraging steel grade 300

The magnetic properties and hardness of a Ni–Co–Mo–Ti maraging steel 300 grade were measured as function of aging temperature. The austenite and martensite phase quantifications in the different heat treatment conditions were carried out by X-ray diffraction using direct comparison method. The behavior of the hardening, magnetization saturation and coercive force against aging temperature and time were explained taking into account the variation of austenite volume fraction with aging time and temperature.     

Fatigue experimental analysis and numerical simulation of FSW joints for 2219 Al–Cu alloy

Fatigue properties of friction stir welding (FSW) butt joints for Al–Cu alloy 2219‐T6 were investigated by experimental analysis and numerical simulation. Microstructure characteristics of FSW butt joints for 2219 aluminium alloy were studied during different fatigue stages. Micro hardness values and grain sizes across the FSW joint at different cycles were measured to study the fatigue properties of the joint. Local mechanical performances of the FSW butt joints were investigated based on the micro tensile tests. Fatigue parameters of different regions in the FSW joints were obtained from the four‐point‐correlation method. The local stress and strain response of the FSW joints were obtained based on mechanical performances of the micro tension specimens. The comparison results between simulation and tests analysis show that the built finite element model is effective for estimating the weak areas for FSW joints.     

Effect of heat treatment on tensile properties of friction stir welded joints of 2219-T6 aluminium alloy

Abstract

The effect of post-weld heat treatment (PWHT) on the tensile properties of friction stir welded (FSW) joints of 2219-T6 aluminium alloy was investigated. The PWHT was carried out at aging temperature of 165°C for 18 h. The mechanical properties of the joints were evaluated using tensile tests. The experimental results indicate that the PWHT significantly influences the tensile properties of the FSW joints. After the heat treatment, the tensile strength of the joints increases and the elongation at fracture of the joints decreases. The maximum tensile strength of the joints is equivalent to 89% of that of the base material. The fracture location characteristics of the heat treated joints are similar to those of the as welded joints. The defect free joints fracture in the heat affected zone on the retreating side and the joints with a void defect fracture in the weld zone on the advancing side. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.     

Investigation of distribution of lithium atoms in the boundary layer of the flow of combustion products

The results are given of measurements of the profiles of temperature and concentration of lithium in the cross section of the boundary layer formed under conditions of flow of combustion products of propaneair flame past the surface of a flat metal plate. A model of chemically nonequilibrium laminar boundary layer is used to perform numerical simulation of various components of the combustion products. A comparison of the measured profiles of lithium concentration and profiles of hydrogen atoms obtained from the experimental data with the predicted results on a cooled and uncooled plate demonstrates that the suggested model of chemical kinetics describes the measurement results with good accuracy.     

Experimental evaluation and prediction of forming limit of FSW blanks made of AA 6061 T6 sheets at different weld orientations and weld locations

The main objective of the present work is to predict the forming limit of friction stir welded (FSW) sheets made of AA 6061T6, having different weld orientations, weld locations, and made at two different welding speeds. The predicted forming limit curves (FLCs) are validated with experimental FLCs. The thickness gradient based necking criterion (TGNC) and major strain‐rate ratio based necking criterion (MSRC) are used to predict the forming limit. The significance of single zone model and double zone model in FLC prediction is discussed. A decrease in hardness is witnessed in the weld zone as compared to base material. With increase in shoulder diameter and decrease in rotational speed, hardness has improved in the weld zone. The forming limit predictions of un‐welded sheets and FSW sheets coincide well with experimental results. The predicted FLCs of FSW sheets from TGNC and MSRC are equally accurate as compared to experimental FLCs in all the weld locations. Both TGNC and MSRC predict almost the same forming limit in 90° weld orientation, while TGNC showed better prediction in 45° weld orientation. FSW sheets with double zone models show better prediction accuracy than single zone models in most of the cases, except in the case of weld at centre location and at longitudinal orientation. There is only slight deviation between single zone and double zone model predictions. The failure location and failure pattern predictions are also agreeing well with the experimental FLCs.     

Material stirring during FSW of Al–Cu: Effect of pin profile

Friction stir welding (FSW) joins the material in solid state, and it gets evolved as a new and effective technique to join dissimilar materials such as aluminum and copper. FSW tool design and configuration critically affect the joint quality. This study has evaluated the effect of different pin profiles used during FSW of AA5754 Al alloy and commercially pure copper in a butt configuration on the microstructure, material movement, and microhardness for the different joints. The joining is performed through the different pin profiles of cylindrical, taper, cylindrical cam, taper cam, and square shape at the rotational and welding speed of 900?rpm and 40?mm/min respectively. Among all joints, the square pin profile provides good joining and microhardness. Square tool pin profile facilitates good amount of mixing at nugget zone, which consequently increases the hardness. The material movement in square tool pin profile joint is also studied on the longitudinal plane to understand the effect of pulsating and stirring action on the material mixing pattern in dissimilar FSW. It is evident that the softer material in the stir zone gets more stirring, and the flow lines are clearly visible for the stirred material.     

Microstructure and mechanical properties of double-side friction stir welded 6082Al ultra-thick plates

In the present work, 80 mm thick 6082Al alloy plates were successfully double-side welded by friction stir welding(FSW). The relationship between the microstructures and mechanical properties was built for the double-side FSW butt joint with more attention paid to the local characteristic zones. It was shown that a phenomenon of microstructural inhomogeneity existed in the nugget zone(NZ) through the thickness direction. The grain size presented an obvious gradient distribution from the top to the bottom for each single-pass weld, and the microhardness values decreased from both surfaces to the middle of the NZ.The lowest hardness zone(LHZ) exhibited a "hyperbolical"-shaped distribution extending to the middle of the NZ. Similar tensile properties were obtained in the three sliced specimens of the FSW joint, and the joint coefficient reached about 70% which achieved the same level as the conventional FSW Al alloy joints. Finite element modeling proved that the "hyperbolical"-shaped heat affected zone(HAZ) was beneficial to resisting the strain concentration in the middle layer specimen which helped to increase the tensile strength. Based on the analysis of the hardness contour map, tensile property and microstructural evolution of the joints, an Isothermal Softening Layer(ISL) model was proposed and established, which may have a helpful guidance for the optimization on the FSW of ultra-thick Al alloy plates.     

热循环、腐蚀和自然时效对 2024 铝合金搅拌摩擦焊接头力学性能的影响

研究了焊接过程中热循环、焊后腐蚀和自然时效对2024铝合金搅拌摩擦焊(FSW)接头力学性能的影响。结果表明,同一焊缝不同位置的力学性能存在一定的差异,靠近插入点位置的屈服强度和抗拉强度明显低于其他位置,之后不断升高,趋于稳定,屈服强度和抗拉强度的最大值分别约为310 MPa和415 MPa;随着自然时效时间的增加,接头屈服强度和抗拉强度在前2个月增长较快,增长率最大分别为2%和1.5%,之后趋于稳定,而伸长率的增长率先较快,增长率最大为6%,之后变缓;在不同的自然时效时间下,腐蚀均明显降低2024铝合金FSW接头的屈服强度、抗拉强度和伸长率,减小率的最大值分别为6.5%,6.1%和19.8%;随着自然时效时间的增加,腐蚀后接头的力学性能下降更严重。     

Microstructure simulation and ballistic behaviour of weld zones in friction stir welds in high strength aluminium 7xxx plate

The effect of friction stir welding (FSW) on the knockdown in properties and failure mechanisms of high strength aluminium plate (AA7010-T7651) under ballistic impact has been investigated by modelling and ballistic testing, simulated, and actual weld zone microstructures, in order to systematically study the link to weld microstructural variation. FSW of thick plate leads to an extremely wide heat affected zone (HAZ). The microstructures within the weld zones were successfully modelled and simulated. However, the maximum loss of hardness in the HAZ resulted in only a 20% reduction in the ballistic limit (V₅₀). The maximum V₅₀ was associated with the highest level of hardness reached prior to a change in failure mechanism, from ductile hole enlargement, using an armour piercing (AP) projectile, or plugging with a fragment simulation projectile (FSP) to excessive spalling and associated adiabatic shear banding at too high hardness levels. The behaviour of the nugget zone has also been examined. The trends in the ballistic data are discussed in the context of the weld zone microstructures and fracture mechanisms.