Effect of weld curvature radius and tool rotation direction on joint microstructure in friction stir welding casting alloys

Curved welds were designed and the effects of the weld curvature radius and tool rotation direction on the microstructure of friction stir welded cast aluminum alloy joints were investigated. Results show that both the weld curvature radius and tool rotation direction have a significant influence on the microstructure of the curved joints during FSW. With decreasing weld curvature radius, the size of the tunnel defect is reduces and the proportion of fine Si particles in the stir zone increases. Si particles are finer and denser in the retreating side (RS) than that in the advancing side (AS) when both the welding direction and tool rotation direction are anticlockwise. However, when the welding direction is anticlockwise while the tool rotates clockwise, the proportions of fine Si particles decrease compared to the former situation. Furthermore, the tunnel defect is more likely to be present in the AS in the former situation.     

Process optimisation and mechanical properties of friction stir lap welds of 7075-T6 stringers on 2024-T3 skin

This paper focuses on the results of process optimisation and mechanical tests that were used to ascertain the feasibility of using friction stir welding (FSW) to join stringers to skin. The effects of process parameters on weld quality of 1.5-mm 7075-T6 stringers lap-joined on 2.3-mm 2024-T3 skins were investigated. Advancing and retreating side locations on the joint configuration were alternated to determine optimal design arrangements. The effects of travel and rotation speeds on weld quality and defect generation were also investigated. Weld quality was assessed by optical microscopy and bending tests. It was found that: (i) the increase of the welding speed or the decrease of the rotational speed resulted in a reduction of the hooking size and top plate thinning but did not eliminated them, (ii) double pass welds by overlapping the advancing sides improved significantly the weld quality by overriding the hooking defect, and (iii) change of the rotational direction for a counter clockwise with a left-threaded probe eliminated the top sheet thinning defect. Subsequently, FSW lap joints were produced using optimum conditions and underwent extensive mechanical testing program. Several assembly configurations including discontinuous and continuous welds as well as single and double pass welds were produced. The results obtained for cyclic fatigue performance of FSW panels are compared with riveted lap joints of identical geometry. S–N curves, bending behaviour, failure locations and defect characterisation are also discussed. It was found that: (i) the tensile strength of FSW joints approached that of the base material but with a significant reduction in the fatigue life, (ii) the probe plunge and removal locations served as the key crack nucleation sites in specimens with discontinuous welds, and (iii) double pass welds with overlapping advancing sides showed outstanding fatigue life and very good tensile properties. The present work provided some valuable insight into both the fabrication and application of FSW on stringer/skin lap joints.     

Anisotropic property of material arrangement in friction stir welding of dissimilar Mg alloys

Microstructure and mechanical properties of friction stir weld joints of dissimilar Mg alloys AZ31 and AZ80 were investigated in the present work. Several different welding parameters were adopted in the study, and the effects of rotation speed and welding speed on the joint quality were discussed comprehensively. In addition, material arrangement which means that AZ31 alloy was at advancing side or at retreating side has significant influence on the joint formation, including the joint microstructure and mechanical properties. A few kinds of defects were observed when the improper parameters were taken in the experiment, and the reasons for generating these defects were revealed in this work. Sound joints with good mechanical properties could be easily obtained when AZ31 was at retreating side, but it was difficult to obtain the sound joint with the contrary material arrangement. These results suggest that the material with inferior plastic deformability should be set at the advancing side and the material with superior one should be set at the retreating side in order to get sound FSW joint of dissimilar Mg alloys.     

Experimental and numerical investigations of bonding interface behavior in stationary shoulder friction stir lap welding

Stationary shoulder friction stir lap welding (SSFSLW) was employed to weld 2024 aluminum alloy. A coupled Eulerian-Lagrangian (CEL) model was developed to investigate the lap interface behavior during SSFSLW. Numerical results of material movement and equivalent plastic strain were in good agreement with the experimental work. With increasing welding speed, the distances from the hook tip to the top surface of the upper workpiece on the retreating side (RS) and the advancing side (AS) increase, while the distance between two wave-shaped alclads decreases. A symmetric interface bending is observed on the AS and the RS during plunging, while the interface bending on the AS is bigger than that on the RS during welding. The peak temperature of the interface on the AS is higher than that on the RS. The equivalent plastic strain gradually increases as the distance to the weld center decreases, and its peak value is obtained near the bottom of the weld.     

Effect of friction stir welding parameters on microstructural characteristics and mechanical properties of 2219-T6 aluminum alloy joints

A 2219-T6 aluminum alloy was friction stir welded in the present study. The results indicate that the recrystallized grains in the weld nugget zone (WNZ) of the joints exhibit the largest size in the middle part and the smallest size in the lower part. Furthermore, the void defect is formed in the joint when the rotation speed or welding speed is quite high. As the rotation speed or welding speed increases, the tensile strength of the joint firstly increases to a maximum value and then sharply decreases due to the occurrence of void defect. During tensile test, the defect-free joints welded at lower rotation speed are fractured in the WNZ, while those welded at relatively high rotation speed tend to be fractured in the heat affected zone (HAZ) adjacent to the thermal mechanically affected zone (TMAZ) on the retreating side.     

Galvanic and asymmetry effects on the local electrochemical behavior of the 2098-T351 alloy welded by friction stir welding

Scanning electrochemical microscopy(SECM) and scanning vibrating electrode technique(SVET) were used to investigate the electrochemical behaviour of the top surface of the 2098-T351 alloy welded by friction stir welding(FSW). The SVET technique was efficient in identifying the cathodic and anodic weld regions. The welding joint(WJ), which comprises the thermomechanically affected zone(TMAZ) and the stir zone(SZ), was cathodic relative to the heated affected zone(HAZ) and the base metal(BM). The reactivities of the welding joint at the advancing side(AS) and the retreating side(RS) were analyzed and compared using SECM technique in the competition mode by monitoring the dissolved oxygen as a redox mediator in 0.005 mol L~(-1) NaCl solution. The RS was more electrochemically active than the AS,and these results were correlated with the microstructural features of the welded alloy.     

Influence of heat input/multiple passes and post weld heat treatment on strength/electrochemical characteristics of friction stir weld joint

The V-95 and D-19 precipitation hardened Russian aluminum alloys are widely used in the Russian aircraft industry and these alloys are not weldable by conventional fusion weld techniques. This paper intends to evaluate the effect of spindle and weld speed on joint strength characteristics of a single pass (SP) and double pass (DP) friction stir lap weld through a common heat index and to analyze the effect of retrogression and re-ageing treatment (RRA) on joint strength and corrosion characteristics. The strength characteristics were analyzed by welding and shear testing of specimens and corrosion susceptibility of joint through immersion in EXCO solution as per ASTM G34. The trials revealed that the joint strength of the welded alloy is inversely proportional to the heat index and the DP weld provided significantly higher strength than an SP weld. The heat affected zone of the joint was found most sensitized to corrosion. The RRA treatment was found to improve the strength of the joints welded with higher heat input while it slightly degraded the joint strength for low heat input welds. The corrosion characteristics of the welded joint is also significantly improved by the post weld RRA treatment.     

Effect of welding speed on microstructures and mechanical properties of underwater friction stir welded 2219 aluminum alloy

Underwater friction stir welding (underwater FSW) has been demonstrated to be available for the strength improvement of normal FSW joints. In the present study, a 2219 aluminum alloy was underwater friction stir welded at a fixed rotation speed of 800 rpm and various welding speeds ranging from 50 to 200 mm/min in order to clarify the effect of welding speed on the performance of underwater friction stir welded joint. The results revealed that the precipitate deterioration in the thermal mechanically affected zone and the heat affected zone is weakened with the increase of welding speed, leading to a narrowing of softening region and an increase in lowest hardness value. Tensile strength firstly increases with the welding speed but dramatically decreases at the welding speed of 200 mm/min owing to the occurrence of groove defect. During tensile test, the joint welded at a lower welding speed is fractured in the heat affected zone on the retreating side. While at higher welding speed, the defect-free joint is fractured in the thermal mechanically affected zone on the advancing side.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

Effect of welding parameters on microstructure and mechanical properties of friction stir welded 2219Al-T6 joints

The effect of friction stir welding (FSW) parameters on the microstructure and mechanical properties of 5.6 mm thick 2219Al-T6 joints was investigated in detail. While the sound FSW joints could be obtained under lower rotation rates of 400–800 rpm and welding speeds of 100–800 mm/min; higher rotation rates of 1200–1600 rpm easily led to the tunnel and void defects. The FSW thermal cycle resulted in low hardness zones (LHZs) on both retreating side (RS) and advancing side (AS). The LHZs may be located at the interface between the nugget zone (NZ) and the thermo-mechanically affected zone (TMAZ), at the TMAZ, or at the heat affected zone under the varied welding parameters. The tensile strength of FSW 2219Al-T6 joints increased when increasing the welding speed from 100 to 800 mm/min, and was weakly dependent on the rotation rates from 400 to 1200 rpm. The FSW 2219Al-T6 joints fractured along the LHZ on the RS.     

Lap joint properties of FSWed dissimilar formed 5052 Al and 6061 Al alloys with different thickness

Lap joint friction stir welding (FSW) between dissimilar 5052-H112 (1 mm) and 6061-T6 (2 mm) Al alloys with different thickness was carried out with various tool rotation speeds and welding speeds according to the fixed location of each material on bottom or top sheet. Interface morphology was characterized by pull-up or pull-down from initial joint line. Amount of vertical material transports increased and thickness of 5052 resultantly lessened with increasing tool rotation and decreasing welding speed, which were the conditions of the weak bond. Higher stress concentration on the interface pull-up region, the penetration of unbonded region into the weld zone and the lessened thickness of 5052 Al part might be the reasons for lower fracture load. Higher fracture load was acquired at the lower tool rotation speed and higher welding speed when a thicker 6061 was fixed at retreating side on top sheet. Interface morphology was the most important factor determining the mechanical strength of lap FSW joints and can be manageable using FSW parameters.     

Defect features and mechanical properties of friction stir lap welded dissimilar AA2024–AA7075 aluminum alloy sheets

5 mm-Thick dissimilar AA2024-T3 and AA7075-T6 aluminum alloy sheets were friction stir lap welded in two joint combinations, i.e., (top) 2024/7075 (bottom) and 7075/2024. The influences of process conditions (welding speed and joint combination) on defects (hook and voids) features and mechanical properties of joints were investigated in detail. It was found that the hook deflects largely upwards into the stir zone (SZ) at lower welding speeds (50, 150 mm/min) in both combinations. The process conditions significantly affect the hook geometry which in return affects the lap shear strength. In all 2024/7075 joints, voids appear and the joints fracture from the tip of hook on AS along the SZ/TMAZ (thermomechanically affected zone) interface in lap shear test (tensile fracture mode). In 7075/2024 joints, the hook on RS horizontally extends a large distance into the bottom stir zone at higher welding speeds (225, 300 mm/min). The joints fracture in three modes: shear fracture along the lap interfaces, tensile fracture and the mix fracture of both. In both joint combinations, the lap shear strength generally increases with the increase of welding speed. 7075/2024 Joints show higher failure load than 2024/7075 joints at lower welding speeds while the opposite result appears at higher welding speeds.     

铝合金厚板搅拌摩擦焊焊缝局部金属的塑性流动特征

以0.02mm厚铜箔为标示材料,采用带三角平面圆锥形搅拌针对20mm厚7075-T6铝板进行焊接。通过测试沿焊缝厚度方向上温度场分布及观察标示材料分布状态,分析焊缝局部金属塑性流动行为特征。结果表明,沿焊缝厚度方向上自上而下的金属温度逐渐降低;焊缝上、下表面温度差约为90℃;同一厚度上相对称的两点,位于前进边金属的温度高于返回边约15℃。位于焊核区上部的铜箔呈细小颗粒状均匀分布;下部铜箔则呈层片状分布,且向前进边偏移。焊核区由多个呈纹路状、有序排列的洋葱环结构相互层叠而成,这与焊缝塑化金属沿轴向迁移方式发生变化有关。     

Flow patterns in friction stir welds of AA5083 and AA6082 alloys

The flow patterns in dissimilar friction stir welds of AA5083-O and AA6082-T6 alloys have been studied. It was observed that material flows (pushes but does not mix) more from the advancing side into the retreating side. Material flow from the retreating side to the advancing side only occurs in the tool shoulder domain, and the pull is greatest at the transition region between the tool shoulder domain and the tool pin domain. It was also observed that materials tend to extrude out only in the thermomechanically affected zone of the retreating side, which was influenced by rotation of both the tool shoulder and the tool pin. The finest grains were present in the regions closest to the tool edge in the retreating side. The volume fraction of recrystallized grains increases down into the deeper part of the nugget from the flow arm region. Microhardness measurements revealed that regions of lowest hardness values were the nugget and the heat affected zone of the AA6082-T6 alloy side. The welding speeds had no influence on the microhardness values per se, but affected the mixing proportions in the flow arm and in the nugget stem.     

Effect of welding parameters on microstructure and mechanical properties of friction stir welded joints of AA7039 aluminum alloy

A high strength Al–Zn–Mg alloy AA7039 was friction stir welded by varying welding and rotary speed of the tool in order to investigate the effect of varying welding parameters on microstructure and mechanical properties. The friction stir welding (FSW) process parameters have great influence on heat input per unit length of weld, hence on temperature profile which in turn governs the microstructure and mechanical properties of welded joints. There exits an optimum combination of welding and rotary speed to produce a sound and defect free joint with microstructure that yields maximum mechanical properties. The mechanical properties increase with decreasing welding speed/ increasing rotary speed i.e. with increasing heat input per unit length of welded joint. The high heat input joints fractured from heat affected zone (HAZ) adjacent to thermo-mechanically affected zone (TMAZ) on advancing side while low heat input joints fractured from weld nugget along zigzag line on advancing side.     

工具形状及工艺过程对搅拌摩擦增材成形及缺陷的影响

采用2mm厚的2195-T8铝锂合金作为增材板条,利用5种不同形状的搅拌工具进行搅拌摩擦增材工艺实验。利用金相观察和硬度测试的分析方法,重点探讨搅拌工具形状与工艺过程对增材成形、界面缺陷及硬度分布的影响。结果表明:圆柱状和三角平面圆台状搅拌针下增材界面上下材料无明显混合,偏心圆柱状和三凹圆弧槽状搅拌针有利于增材界面上下材料混合及减小界面钩状缺陷;增材前进侧界面形成致密无缺陷冶金连接,而后退侧界面材料混合不充分,钩状缺陷易伸入焊核区,且弱连接缺陷起源于此。四层增材中,相邻两层焊接方向相反的增材工艺使除顶层增材外其他增材两侧钩状缺陷向焊核区外侧弯曲,弱连接缺陷得到改善;顶层增材后退侧钩状缺陷伸入焊核区。增材焊核区有明显软化现象,但不同增材工艺下焊核区硬度分布均匀,表明搅拌摩擦增材制造可获得性能均匀的增材;相比于单道焊接工艺,来回双道焊接工艺使单层增材焊核区进一步软化;四层增材中,越靠近顶部的增材,其焊核区平均硬度越大。     

Tool geometry,rotation and travel speeds effects on the properties of dissimilar magnesium/aluminum friction stir welded lap joints

Lap joint friction stir welding (FSW) between dissimilar AZ31B and Al 6061 alloys sheets was conducted using various welding parameters including tool geometry, rotation and travel speeds. Tapered threaded pin and tapered pin tools were applied to fabricate FSW joints, using different rotation and travel speeds. Metallurgical investigations including X-ray diffraction pattern (XRD), optical microscopy images (OM), scanning electron microscopy equipped with an energy-dispersive X-ray spectroscopy (SEM–EDS) and electron probe microanalysis (EPMA) were used to characterize joints microstructures made with different welding parameters. Intermetallic phases were detected in the weld zone (WZ). Various microstructures were observed in the stir zone which can be attributed to using different travel and rotation speeds. Mechanical evaluation including lap shear fracture load test and microhardness measurements indicated that by simultaneously increasing the tool rotation and travel speeds, the joint tensile strength and ductility reached a maximum value. Microhardness studies and extracted results from stress–strain curves indicated that mechanical properties were affected by FSW process. Furthermore, phase analyses by XRD indicated the presence of intermetallic compounds in the weld zone. Finally, in the Al/Mg dissimilar weld, fractography studies showed that intermetallic compounds formation in the weld zone had an influence on the failure mode.     

铝合金薄板搭接高速 FSW 缺陷及断裂行为

目的 为了拓展搅拌摩擦焊技术应用,对薄板搭接结构高速搅拌摩擦焊工艺优化与工程应用提供 借鉴与指导。方法 采用圆锥无螺纹搅拌针,进行了 6061 铝合金薄板搭接高速搅拌摩擦焊接,对接头界 面缺陷及其断裂模式进行分析,探讨了转速对 6061 铝合金薄板搭接接头成形及性能的影响规律。结果 发现在无螺纹圆锥搅拌针、高转速（6000~9000 r/min）条件下,接头塑性金属在后退侧易形成飞边流出, 导致下板前进侧出现孔洞缺陷,且随转速增大,界面缺陷尺寸逐渐增大,当转速达到 10 000 r/min 时, 孔洞尺寸有所减小,此时接头拉剪强度最高,为 123 MPa。对试样拉剪断裂位置分析发现,高速搭接接 头断裂位置主要有两种,分别断裂在结合界面处或在前进侧下板,且转速在 9000 r/min 以上越趋向于在 结合界面断裂。结论 高转速搭接焊接必须协调轴肩相貌、焊接工装约束等条件,保证接头塑性金属充 分流动而不流失,才能获得成形良好无缺陷的接头。     

Effects of tool rotation speed on microstructures and mechanical properties of AA2219-T6 welded by the external non-rotational shoulder assisted friction stir welding

The external non-rotational shoulder assisted friction stir welding (NRSA-FSW) was applied to weld high strength aluminum alloy 2219-T6 successfully, and effects of the tool rotation speed on microstructures and mechanical properties were investigated in detail. Defect-free joints were obtained in a wide range of tool rotation speeds from 600 rpm to 900 rpm, but cavity defects appeared on the advancing side when the tool rotation speed increased to 1000 rpm. The microstructural deformation and heat generation were dominated by the rotating tool pin and sub-size concave shoulder, while the non-rotational shoulder helped to improve the weld formation. Microstructures and Vickers hardness distributions showed that the NRSA-FSW is beneficial to improving the asymmetry and inhomogeneity, especially in the weld nugget zone (WNZ). At the tool rotation speed of 800 rpm, both the tensile strength and the elongation reached the maximum, and the maximum tensile strength was up to 69.0% of the base material. All defect-free joints were fractured at the weakest region with minimum Vickers hardness in the WNZ, while for the joint with cavity defects the fracture occurred at the defect location.     

铝合金厚板静止轴肩搅拌摩擦焊接头组织及性能

采用8.5 mm厚度2A14-T4铝合金和自主研制搅拌工具进行静止轴肩搅拌摩擦焊（stationary shoulder friction stir welding,SSFSW）实验,探讨焊接工艺参数对接头组织和力学性能的影响规律。结果表明：只有在低转速工艺参数范围内（转速ω=400~600 r/min与焊接速率v=60~120 mm/min）可获得焊缝表面光滑、无缺陷厚板铝合金SSFSW焊接接头。SSFSW焊缝区主要由焊核区（NZ）组成,周围热力影响区（TMAZ）及热影响区（HAZ）宽度明显减小,焊核区与搅拌针形状类似且由两种不同尺寸细小等轴晶构成,前进侧NZ晶粒比后退侧NZ更为细小。接头显微硬度呈"W"状分布,NZ硬度值可达到母材硬度80%~90%,TMAZ与HAZ交界处存在软化区,硬度最低为母材硬度72%左右。在给定ω=500 r/min,v=140 mm/min焊接参数下,SSFSW接头抗拉强度可达到母材的88%,断裂位置多位于后退侧TMAZ与HAZ交界处软化区,具有韧性断裂特征。