Comparative experimental study on the modification of microstructural and mechanical properties of friction stir welded and gas metal arc welded EN AW‐6061 O aluminum alloys by post‐weld heat treatment

In this paper, the effects of post‐weld heat treatment on modification of microstructures and mechanical properties of friction stir welded and gas metal arc welded AA6061‐O plates were compared with each other. Gas metal arc welding and friction stir welding were used as the applicable welding processes for AA6061‐O alloys. The applied post‐weld heat treatment consisted of solution heat treatment, followed by water quenching and finally artificial aging. The samples were classified as post‐weld heat treated and as‐welded joints. The microstructural evolution, tensile properties, hardness features and fracture surfaces of both as‐welded and post‐weld heat treated samples were reported. The results clearly showed that friction stir welding process demonstrated better and more consistent mechanical properties by comparison with the gas metal arc welding process. The weld region of as‐welded samples exhibited a higher hardness value of 80 HV0.1 compared to the base material. In addition, the feasibility of post‐weld heat treatment in order to enhance the mechanical properties and to obtain more homogeneous microstructure of 6061‐O aluminum alloys was evaluated.     

Microstructure and residual stress distributions in friction stir welding of dissimilar aluminium alloys

The aim of this investigation was to study the effect of welding heat input and postweld natural aging on residual stress, microstructure, and precipitation distribution in different zones of dissimilar friction stir welding of 8 mm thick plates of AA6082-T6 and AA7075-T6. It was found that atomic diffusion occurs at the interface of the materials in the stir zone of the joints. Transmission electron microscopic investigations showed that reprecipitation of fine Guinier–Preston zone, β′, and η′ precipitates resulted in increased micro-hardness in the SZ after natural aging. An increase in welding heat input resulted in decreased maximum tensile residual stress and increased size of the tensile residual stress region. Natural aging within the SZ and thermo-mechanical affected zone resulted in 15–20 MPa reduction of the residual stress in these zones.     

Microstructure and mechanical properties of friction stir welded 7136–T76 aluminium alloy

Abstract

The microstructure of the weld was examined by light and electron microscopy (scanning and transmission). The various regions, i.e. thermomechanically affected zone, heat affected zone and unaffected base material, were studied in detail to better understand the microstructural evolution during friction stir welding and its impact on basic mechanical properties. The change in morphology of the strengthening phases reflected the relative temperature profile and the amount of deformation across the welded joint during the stir welding process. The centre of the weld was composed of fine grains and coarse particles identified mainly as MgZn₂. In the thermomechanically and heat affected zones, the grain size was not uniform, and the strengthening phases filled the grain interiors, while grain boundaries were surrounded by precipitation free zones. The size of the strengthening phase decreased towards the base material. The hardness profile of the friction stir weld displayed the lowest hardness on the retreating side. Tensile properties of the weld itself were superior to those for material containing weld.     

Effects of welding speed on the microstructure and hardness in friction stir welding joints of 6005A-T6 aluminum alloy

The 6005A-T6 aluminum alloy was friction stir welded at different welding speeds. The peak temperature, microstructure and mechanical properties were examined for these joints. A special attention was devoted to the relationship between the precipitates evolution within different zones and the local hardness. In the nugget zone (NZ) experiencing the highest peak temperature, the β″ precipitates dissolved into α-Al matrix during welding, and the hardness of NZ depended on the level of natural aging (NA) at different welding speeds. The thermo-mechanically affected zone (TMAZ) is characterized by elongated grains with a high density of dislocations. The welding speed had not a significant effect on hardness in this zone. The heat-affected zone (HAZ) contains the transformation of β″–β′, the precipitation of Q′ and the coarsening of precipitates. The HAZ close to the joint center line exhibited the minimum hardness due to the coarsening of β′ and Q′ precipitates while the HAZ far from it having a high hardness level was mainly related to coherent β″ precipitates. The HAZ hardness and joint strength have an increased tendency with increasing the welding speed. It can be explained by increasing the density of Q′ or β″ precipitates.     

Effect of post-weld natural aging on mechanical and microstructural properties of friction stir welded 6063-T4 aluminium alloy

Influence of natural aging on mechanical and microstructural properties of friction stir welded 6063-T4 aluminium alloy plates was investigated through mechanical testing, X-ray diffraction studies, and transmission electron microscopy, for aging times up to 8640 h. Mg–Si co-clusters formed during the natural aging process resulted in an increase in strength, decrease in ductility, and occurrence of serrated plastic flow. Hardness increase from aging was fastest in welds obtained at higher tool rotational speeds due to greater amount of “quenched-in” vacancies from higher peak stir zone temperatures. Peak broadening analyses and classical Williamson–Hall plots were used to investigate the effect of friction stir welding and post weld natural aging on microstrain in different weld regions. Higher microstrain was found in stir zone as well as heat affected zone as compared to that for base metal, albeit for different reasons.     

Metallurgical characterization of pulsed current gas tungsten arc,friction stir and laser beam welded AZ31B magnesium alloy joints

This paper reports the influences of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on mechanical and metallurgical properties of AZ31B magnesium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy and X-Ray diffraction technique were used to evaluate the metallurgical characteristics of welded joints. LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints due to the formation of finer grains in weld region, higher fusion zone hardness, the absence of heat affected zone, presence of uniformly distributed finer precipitates in weld region.     

Evaluation of the microstructure and mechanical properties of friction stir welded 6005 aluminum alloy

Abstract

The microstructural change related with the hardness profile has been evaluated for friction stir welded, age hardenable 6005 Al alloy. Frictional heat and plastic flow during friction stir welding created fine and equiaxed grains in the stir zone (SZ), and elongated and recovered grains in the thermomechanically affected zone (TMAZ). The heat affected zone (HAZ), identified only by the hardness result because there is no difference in grain structure compared to the base metal, was formed beside the weld zone. A softened region was formed near the weld zone during the friction stir welding process. The softened region was characterised by the dissolution and coarsening of the strengthening precipitate during friction stir welding. Sound joints in 6005 Al alloys were successfully formed under a wide range of friction stir welding conditions. The maximum tensile strength, obtained at 507 mm min^-1 welding speed and 1600 rev min^-1 tool rotation speed, was 220 MPa, which was 85% of the strength of the base metal.     

Microstructure and mechanical properties of friction stir welded 6082 AA in as welded and post weld heat treated conditions

Friction stir welding of 6082 AA-T651 was performed using three different combinations of feed rates (90, 140 and 224 mm/min) and tool rotational speeds (850, 1070 and 1350 rpm). Mechanical properties of the weldments were evaluated by hardness measurements on the transverse section and tensile testing, while microstructure evaluation was done by optical microscopy and electron back scattered diffraction (EBSD). Irrespective to welding parameters the dynamically recrystallized grains in the stir zone were measured to be in the range of 2–3 μm for different feeds rates and rotational speeds. A considerable loss in hardness in the stir zone and more severely in the thermo-mechanically affected zone was noted due to dissolution of β′ and β″ second phase particles. A post weld heat treatment (PWHT) of 175 °C for 5 and 12 h was given to the weldments for all welding conditions and the mechanical properties and microstructure were re-evaluated. The hardness and strength were partially recovered and this was attributed to the possible re-precipitation of the β″ precipitates. The grain size barely exhibited a change, whereas the texture displayed a significant diminish in the Goss orientation after PWHT.     

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...     

Effects of the Reversely Rotating Assisted Shoulder on Microstructures During the Reverse Dual-rotation Friction Stir Welding

The reverse dual-rotation friction stir welding(RDR-FSW) has the capability to adjust the heat generation because of the separately designed tool shoulder and tool pin.The welding torque exerted on the workpiece by the reversely rotating shoulder is opposite to that exerted by the rotating tool pin,so the total welding torque is reduced,which is beneficial to reducing the clamping requirement of workpieces.In the present paper,a RDR-FSW joint was welded in a condition similar to the optimal welding condition of conventional FSW,and microstructures in various zones were investigated by comparison,aiming to highlight effects of the reversely rotating assisted shoulder.Due to the heat conduction of the middle cylinder and the bottom end cover on which the assisted shoulder was machined,the thermal effect of RDR-FSW was smaller than that of the conventional FSW.Moreover,the effect of assisted shoulder on the plastic flow or deformation of material or was constrained in a thin layer near the weld top surface,and thus the flow of material especially along the thickness direction was clearly decreased in the RDR-FSW.In the heat-affected zone(HAZ),the precipitate coarsening was the main evolution and was completed through the dissolution of small precipitates and the continuous growth of large precipitates.By contrast,the dissolution degree of precipitates increased significantly in the thermomechanically affected zone(TMAZ),and a small amount of original meta-stable precipitates transformed to block-shaped stable precipitates.Precipitate evolutions in the shoulder affected zone(SAZ)and the weld nugget zone were similar,i.e.the majority of original meta-stable precipitates dissolved into the matrix and the remainder transformed to stable precipitates,though the dissolution degree was greater in the SAZ.Compared with the conventional FSW joint,the coarsening degrees of precipitates in the HAZ and TMAZ of RDR-FSW joint were much smaller,as well as the dissolution degrees of precipitates in all four specified zones.     

Single-stage aging behaviour and precipitate evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy

Property evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy treated at 120°C, with the microstructure subjected to under-aging (UA), peak-aging (PA) and over-aging (OA), is investigated. Precipitate size distributions and distances between neighbouring precipitates are determined. Results indicate that a certain time is required to reach peak hardness and yield strength, and that peak values can be sustained for a relatively long time. As the aging time increases, the conductivities increase persistently. As the alloy temper varies from UA to PA to OA, the main matrix precipitates change from ‘GPI zone and GPII zone and η′ phase’ to ‘GPII zone and η′ phase’ and then to η′ phase. Meanwhile, the precipitate size distribution becomes broader, and the average precipitate size increases.     

Joint properties of friction stir welded AZ31B– H24 magnesium alloy

Abstract

The weldability of friction stir welded hot rolled AZ31B-H24 magnesium alloy sheet, 4 mm in thickness, was evaluated, varying welding parameters such as tool rotation speed and travel welding speed. Sound welding conditions depended mainly on sufficient heat input during the welding process. Insufficient heat input, which was generated in the case of higher travel speed and lower rotation speed, caused an inner void or lack of bonding in the stir zone. The microstructure of the weld zone was composed of five regions: base metal, heat affected zone, thermomechanically affected zone, stir zone I and stir zone II. Unlike the general feature of friction stir welded aluminium alloys, the grain size of the weld zone was larger than that of the base metal. Stir zones I and II were characterised by partial dynamic recrystallisation and full dynamic recrystallisation, respectively. The hardness of the weld zone was lower than that of the base metal owing to grain growth. A wider range of defect free welding conditions was acquired at higher tool rotation speed and lower welding speed. The maximum tensile strengh was 240 MPa, which was ~85% of the base metal value of 293 MPa. The fracture location was close to the stir zone.     

800 MPa级超高强度铝合金的时效析出行为

采用硬度、电导率、室温拉伸测试方法,研究110~140℃范围内时效不同时间后新型铝合金性能的变化。利用透射显微镜(TEM)观察合金的组织形貌特征。结果表明:该新型铝合金最佳的时效工艺为110℃保温24 h,此条件下合金的抗拉强度,屈服强度和伸长率分别为808,785 MPa与6.9%。时效温度是影响合金析出相种类、密度和尺寸的主要因素。在110℃时效时,合金主要的析出相是GPⅠ区、GPⅡ区和亚稳η′相。110℃时长时间(直至96 h)时效后,GPⅠ区和GPⅡ区仍能稳定存在。与110℃时效相比,在140℃时效时,析出过程加速。当140℃时效4 h后,未观察到GP区的存在,主要的析出相为η′相;140℃时效24 h后,主要的析出相为η′相和η相。     

Influences of Friction Stir Welding and Post‐Weld Heat Treatment on Al–Cu–Li Alloy

In this paper, the influences of friction stir welding (FSW) and post‐weld heat treatment (PWHT) on the microstructures and tensile properties of Al–Cu–Li alloy are investigated. After FSW, strengthen loss occurred in the welding area. Remarkable softening occurs in the thermo‐mechanically affected zone (TMAZ) resulting from dissolution of Al₃Li (δ′) phases. Recrystallization and precipitation of ultra‐fine δ′ phases take place in the nugget zone (NZ) that lightens the softening degree of this zone. A noteworthy enhancement in the hardness and tensile strength of the joint is achieved after T8 re­aging treatment (3% ? pre‐deformation, 30 h at 152 °C). However, re‐solution treatment coupled with re‐aging treatment leads to ductility deterioration in the joint because coplanar slip of coarse Al₃Li phases induces severe stress concentration during plastic deformation.

     

Thermische Nachbehandlung der laserstrahlgeschweißten Al‐Legierungen AlSi1MgMn und AlCu4Mg1

Post heat treatment of the laser beam welded aluminium alloys AlSi1MgMn and AlCu4Mg1 Laser beam welded age hardenable aluminium alloys often exhibit a loss in strength in the fusion and the heat affected zones, compared to the uninfluenced base material. A material‐compatible combination among a base material, a welding filler material, as well as welding parameters and a suitable post heat treatment of the welded joint allows to improve the weld seam properties. The base material AlSi1MgMn (6082) was welded in the aging condition T4 using AlSi12 and AlSi7Mg ‐ filler materials and the welded joint was completely aged at different temperatures and times, in order to adjust an almost constant hardness profile over the base material, heat affected zone and fusion zone. The base material AlCu4Mg1 (2024) was welded in the aging condition T351 using a AlCu6Mn ‐ filler material and the welded joint was naturally aged. The aging behaviour, the residual stress, the static and dynamic properties of welded joints were examined. The properties can be clearly improved by the post heat treatment.     

The microstructure and mechanical properties of friction stir welded Al–Zn–Mg alloy in as welded and heat treated conditions

High strength aluminium alloys generally present low weldability because of the poor solidification microstructure, porosity in the fusion zone and loss in mechanical properties when welded by fusion welding processes which otherwise can be welded successfully by comparatively newly developed process called friction stir welding (FSW). This paper presents the effect of post weld heat treatment (T₆) on the microstructure and mechanical properties of friction stir welded 7039 aluminium alloy. It was observed that the thermo-mechanically affected zone (TMAZ) showed coarser grains than that of nugget zone but lower than that of heat affected zone (HAZ). The decrease in yield strength of welds is more serious than decrease in ultimate tensile strength. As welded joint has highest joint efficiency (92.1%). Post weld heat treatment lowers yield strength, ultimate tensile strength but improves percentage elongation.     

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.     

Precipitate evolution in friction stir welding of 2219-T6 aluminum alloys

Precipitate evolution in friction stir welding of 2219-T6 aluminum alloys was characterized by transmission electron microscopy. In the weld nugget zone and the thermo-mechanically affected zone some metastable precipitates overaged to equilibrium phase while others solutionized into the aluminum solid solution. In the heat-affected zone the precipitates coarsened.     

Effect of weld morphology on mechanical response and failure of friction stir welds in a naturally aged aluminium alloy

Friction stir butt welds in 6063-T4 aluminium alloy were obtained using square and two tapered tool pin profiles. Tensile tests at 0°, 45°, and 90° to the weld line, hardness contours in the weld cross-section, temperatures in the heat affected zones, cross-sectional macrographs, transmission electron micrographs, and X-ray diffraction studies were used to characterize the welds. In transverse weld specimen, tunnel defects appearing at higher weld speeds for tapered pin profiles, were found to result in mechanical instabilities, i.e. sharp drops in load–displacement curves, much before macroscopic necking occured. Further, in comparison to the base metal, a marked reduction in ductility was observed even in transverse specimen with defect free welds. Hardness contours in the weld cross-section suggest that loss in ductility is due to significant softening in heat affected zone on the retreating side. Transmission electron microscopy images demonstrate that while recovery and overaging are responsible for softening in the heat affected zone, grain size refinement from dynamic recrystallization is responsible for strengthening of the weld nugget zone. X-ray diffraction studies in the three weld zones: weld nugget zone, heat affected zone, and the base metal corroborate these findings. A weld zone model, for use in forming simulations on friction stir welded plates of naturally aged aluminium alloys, was proposed based on mechanical characterization tests. The model was validated using finite element analysis.     

Microstructure,microhardness and corrosion susceptibility of friction stir welded AlMgSiCu alloy

Microstructure, microhardness and corrosion susceptibility of friction stir welded joint in an AlMgSiCu alloy were investigated. It was found that the joint exhibits different corrosion susceptibility among the microstructural zones. The base material is the most susceptible to intergranular corrosion because of the presence of continuous cathodic precipitates (Si and Q phases) at grain boundaries and the precipitate free zone along the grain boundaries. The coarsening of intergranular precipitates and the precipitation of Q′ phases in the grain bodies reduce intergranular corrosion susceptibility but introduce pitting corrosion in the heat-affected zone. The significant elimination of intergranular corrosion both in nugget zone and thermo-mechanically affected zone is related to the low volume fraction of intergranular precipitate. Microhardness variations depend on the evolution of intragranular precipitates. The dissolution and/or coarsening of the strengthening precipitates result in the softening within the welded zone.