Residual stresses in friction stir welded parts of complex geometry

Residual stresses play a key role on the mechanics underlying the fatigue crack growth propagation of welded joints. Indeed, compressive residual stresses may induce a beneficial enhancement of the fatigue life under loading condition whereas tensile residual stresses may act to increase the stress distribution at crack tip, resulting in a life-threatening condition of the welded structure. In-process distortion and final geometry of welded joints are also affected by residual stresses. In this paper, the longitudinal residual stress distributions in friction stir welding (FSW) joints were investigated for butt and skin–stringer geometries, including lap and T configurations. To measure residual stresses, the cut-compliance and the inverse weight-function methodologies were adapted for skin–stringer FSW geometries via finite element analysis. AA2024-T4 and AA7075-T6 aluminum alloys were used to weld dissimilar skin–stringer joints whereas butt joints were made of AA2024. The effect of most relevant process parameters as well as the cooling during welding process was also investigated for a better understanding of welding residual stresses. Our findings suggest that FSW of complex skin–stringer geometries produces higher residual stresses than those of butt joints, and that the cooling water flux further reduces residual stresses. Changes of process parameters did not affect markedly residual stress distribution.     

Accurate measurement of residual stresses of 2219-T87 aluminum alloy friction stir welding joints based on properties of joints

In order to avoid the deviation caused by calculating the residual stresses of welding joints with the release coefficients of the same parent metal, a new method has been proposed based on the properties of weld joints. Since the mechanical property of 2195-F Al alloy is close to that of 2219-T87 Al alloy Friction stir welding (FSW) joint, 2195-F Al alloy is selected as the substitute material of 2219-T87 Al alloy FSW joint in the calibration test. Release coefficients of 2195-F Al alloy and proper correction coefficient are used to calculate the residual stresses of 2219-T87 Al alloy FSW joints. Compared to the results calculated with release coefficients of its parent metal, it has been proved that the proposed correction method is more precise in residual stresses measurement of weld joints. The mathematical analysis and the results of verification tests have shown that this new method is reliable, and it will provide guidance in academic research and engineering application of the spherical indentation strain-gauge method.     

低碳钢焊缝金属强度组配对焊接残余应力分布的影响规律

采用热弹塑性有限元方法,对平板对接接头中焊缝金属强度组配对焊接残余应力分布的影响进行了研究。结果表明,对于高组配接头,纵向残余应力的峰值接近焊缝金属的屈服极限,出现在焊缝中心。对于低组配接头,纵向残余应力的峰值接近母材的屈服极限,并发生在焊缝附近。随着对含有残余应力的接头施加外载,焊缝金属强度组配对纵向应力分布的影响依然存在,但残余应力的影响在减小。对承受外载的焊接接头,逐步卸除外载后,焊缝横截面     

固溶处理对7B04-O铝合金搅拌摩擦焊接接头微观组织与力学性能的影响

对航空用3 mm厚的带有包铝层的7B04-O铝合金板材进行搅拌摩擦焊接(Friction stir welding,FSW),研究固溶处理对搅拌摩擦焊接接头的微观组织和力学性能的影响。结果表明,当转速为800 r/min、焊接速度为200 mm/min、焊接工具轴肩直径为12 mm时,可得到表面美观、致密无缺陷的搅拌摩擦焊接接头。焊核区发生动态再结晶,形成细小的等轴晶。经固溶处理后,焊核的上部及底部晶粒都发生了异常长大,而中部区域晶粒仍然为细小的等轴晶组织。焊态接头的拉伸试样断裂在母材位置,抗拉强度达到199 MPa,与退火态母材抗拉强度相当,断后伸长率达到12%。在新淬火状态下,接头的抗拉强度为310 MPa,为相同热处理母材的91.4%,断后伸长率为11.2%,试样断裂在焊核区,呈不完全的韧性断裂。     

反应堆压力容器内壁环形锻件焊接残余应力三维有限元数值模拟

某新型反应堆压力容器内壁设计了环形锻件与筒体内壁焊接的环形焊接结构。该种结构形式的焊缝首次在反应堆压力容器中出现,无成熟经验可以借鉴。为了了解该种复杂结构形式及大厚度焊缝的焊接残余应力幅值及分布规律,基于ANSYS有限元分析软件,建立了反应堆压力容器内壁环形锻件多层多道焊接三维有限元模型。在此基础上,以带状移动温度热源作为焊接热源模型计算出多层多道焊接的瞬态温度场结果,采用热-力间接耦合法,得到了焊接应力场计算结果。模拟结果表明,焊缝区域环向应力从上表面到下表面分布趋势为拉应力-压应力-拉应力,呈现自平衡的分布形式。根部焊道区域的环向应力为拉应力。焊缝上轴向应力最大为300 MPa左右;焊缝上下表面径向应力较大,达到400～500 MPa左右;峰值等效应力出现在焊缝根部区域,幅值最大约700 MPa。     

Mechanical properties of friction stir welding and metal inert gas welding of Al-Zn aluminum alloy joints

In this work, an Al-Zn aluminum alloy was welded using friction stir welding (FSW) and metal inert gas (MIG) welding methods. The comparison between microstructure and mechanical properties of the two different joints was mainly discussed. Results showed that defect-free joints can be obtained using both the two welding methods. Due to much less heat input, grains of the stir zone (SZ) of the FSW joint are finer than those of the MIG joint, resulting into higher hardness. Tensile strength of the MIG and FSW joints respectively reaches 68.6 and 73.2% of the BM. The FSW joints own much better fatigue properties than the MIG joints and reach its infinite life at using 90 MPa. The FSW joint owns two cracks during the fatigue test.     

铝合金搅拌摩擦焊技术研究

论述了搅拌摩擦焊的焊接方法、工艺过程和基本原理；通过大量的工艺参数优化，解决了焊缝中的孔洞问题，使ＬＦ６板材搅拌摩擦焊的焊缝达到与母材等强，ＬＹ１２焊后未经热处理，连接强度接近母材的８０（。由金相分析可以看出，搅拌摩擦焊属于固相连接，焊缝晶粒细小，无气孔、夹杂、裂纹等焊接缺陷。初步应力测试发现，搅拌摩擦焊焊缝比熔焊焊缝的残余应力低。试验研究结果表明，搅拌摩擦焊焊接过程稳定可靠，焊接接头性能良好，具有广泛的应用前景。     

球罐环焊缝接头残余应力的三维非线性有限元分析

建立了球罐环焊缝焊接温度场和焊接应力应变场三维移动热源有限元分析模型,考虑了材料的热物理性能和力学性能随温度而变化,应用单元生死技术模拟焊接填充过程,模拟计算出移动热源作用下的温度场,以及以温度场为基础的环焊缝接头焊接应力应变场的分布规律:温度场结果表明,由于焊接的热输入和速度不同,以及热源加载体积不相等,每道焊接的最高温度均不相等。应力场的分析结果表明,在球罐内表面的焊缝及近缝区,呈现双向残余拉应力(经向和周向),而在外表面的对应区域,经向残余应力是压应力,周向残余应力为拉应力。     

A study on mechanical characteristics of the friction stir welded A6005-T5 extrusion

In this paper, A6005-T5 extruded aluminum alloy sheets which are used for floor, roof or wall panels of railroad vehicles were welded by the friction stir welding (FSW) and gas metal arc welding (GMAW) techniques. The mechanical characteristics including the tensile strength, micro-hardness and fatigue strength of the FSW joint were compared to those of the base metal and GMAW joints. In order to determine the relationship between the welding variables of FSW and the mechanical characteristics of the joint, the response function was derived using the least square method and the sensitivity analysis was performed. The rotational speed, welding speed and tilting angle of the welding tool were chosen as design variables. On the basis of the Plackett-Burman design table, eight different FSW experiments were done, and then the effects of design variables on the mechanical characteristics of the FSW joint were analyzed. The result showed that the welding speed has a most significant effect on the tensile and fatigue strength. In the case of the micro-hardness, the effect of the tilting angle was the biggest.     

应用X射线衍射法测定轴类工件中的残余应力

根据测试距车轴表面2mm深度处残余应力的要求,推导了剥层测残余应力的修正公式,利用生死单元法模拟了车轴车削的过程,验证了该修正公式的正确性,并采用X射线衍射法测定了距某车轴表面2mm深度处的轴向和周向残余应力。结果表明:X射线衍射法和修正公式能有效测出距车轴表面2mm深度处的残余应力;轴向残余应力最大值(绝对值)和周向残余应力最大值(绝对值)分别出现在0°测点和90°测点,其值分别为-139.15MPa和-105.21MPa,轴向残余应力和周向残余应力各点的差值均小于EN13261标准规定的40MPa。     

陶瓷/金属钎焊接头残余应力的测量及数值计算

用微区X射线衍射仪测量了陶瓷／金属接头陶瓷表面离边缘0．2mm处的残余应力的大小和分布，用热弹塑性有限元方法，在考虑了材料性能参数随温度变化的情况下，对接头残余应力的大小和分布进行了计算。结果表明，计算值和测量值的趋势基本一致，都是在陶瓷侧的外表面靠近界面的地方存在着最大的轴向拉应力。X射线衍射测量结果还表明，Si3N4／40Cr接头的钎料层较厚时，接头的残余应力值较高。     

Effect of shoulder geometry on residual stress and fatigue properties of AA6082 fsw joints

The tool geometry is a crucial characteristic of the friction stir welding (FSW) process; its design is the key to the successful FSW application for a wide range of materials and thicknesses improving the weld strength and fatigue life. The present study investigates the influence of three shoulder geometries on the FSW joint performance, in terms of residual stresses state, microhardness profile and mechanical properties of 1.5 mm thick AA 6082-T6 FSW joints in the butt-joint configuration. The three tool geometries are characterized by three different shoulders: a shoulder with scroll, a shoulder with a shallow cavity, and a flat shoulder. Transverse and longitudinal tensile tests at room temperature were performed in order to evaluate the mechanical properties, respectively, of the joints and of the stirred zone, while the fatigue tests were performed transversally to the joint line. This paper was recommended for publication in revised form by Associate Editor Jooho Choi Marta De Giorgi received academic degree and Ph.D. degree in Materials Engineering from Lecce University in 1998 and 2002 respectively. She is currently a Post-Doc researcher at Salento University in Lecce, Italy. Dr. De Giorgi’s research interests are in the area of fatigue, residual stress, welded joints and advanced materials. She studies these subjects by experimental and numerical means. Agostino Scialpi graduated with honour in Mechanical Engineering in 2002 from Technical University of Bari, Italy. He attended a Master on Rapid Prototyping Techniques, is qualified as International Welding Inspector (Basic Level, IWI-B) and is Professional Member of The Welding Institute (grade: Graduate). He obtained a PhD in Environmental Engineering in 2006 from Technical University of Bari. Francesco Panella received full mechanical engineering degree in 1996 and Ph.D. degree in Materials Engineering from Lecce University in 1999. Actual position: Senior Researcher at University of Lecce-Italy and assistant Professor in: Machine Design. Main Research fields: Fatigue, welding, thermo-elasticity and Image correlation techniques for stress analysis, advanced Materials characterisation Luigi De Filippis graduated in Mechanics Engineering at Polytechnic of Bari, Italy in 1998, he started his career with the Ph.D. Degree (2001) in Advanced Production Systems at Polytechnic of Bari. His scientific activities concerne: numerical simulation and experimental investigation of laser welding, forming and heat treatment; rapid prototyping and reverse engineering.     

A study on natural aging behavior and mechanical properties of friction stir-welded AA6061-T6 plates

Mechanical properties, microstructural events, residual stresses, and aging behavior of friction stir-welded AA6061-T6 were investigated in this work. Microstructural and mechanical characterizations of the friction stir-welded joints in as-welded and post-welded conditions were made by means of optical metallography, transmission electron microscopy, X-ray diffraction for determination of residual stresses, tensile testing, and hardness measurements. It was found that weld strength and hardness variations after welding are mainly dependent on the imposed heat input per unit length. Besides, the kinetics of natural aging in the welded samples was found to be noticeable within the first 14 days, and its effect decreases considerably in longer aging durations. The residual stress measurements show that subsequent natural aging leads to considerable relaxation of residual stress of about 22 MPa, while this effect is particularly significant in the stir zone and the thermomechanically affected zone.     

Effects of welding speed on microstructures and mechanical properties of AA2219-T6 welded by the reverse dual-rotation friction stir welding

Reverse dual-rotation friction stir welding (RDR-FSW) is a novel FSW technology in which the tool pin and the assisted shoulder rotates reversely, thus it has the capability to obtain appropriate welding conditions through adjusting the rotating tool pin and surrounding assisted shoulder independently. In the present study, a RDR-FSW tool system was designed and successfully applied to weld high strength aluminum alloy 2219-T6, and the effects of welding speed on microstructures and mechanical properties were investigated in detail. At a constant rotation speed of 800 rpm for both the rotating tool pin and the reversely rotating assisted shoulder, defect-free joints were obtained at welding speeds ranging from 50 to 150 mm/min, while a cavity defect appeared at the three-phase confluction on the advancing side when the welding speed increased to 200 mm/min. With increasing of the welding speed, the width of the softened region decreased, but the minimum microhardness value increased gradually. When compared with the joints welded by the conventional FSW, there is only a minor variation of the Vickers hardness across the stirring zone in the joint welded by the RDR-FSW. The maximum tensile strength 328 MPa (73.7 % of the base material) was obtained at the welding speed of 150 mm/min, while the elongation reached its maximum 7.0 % (60.9 % of the base material) at the welding speed of 100 mm/min. All defect-free joints were fractured at the weakest region with the minimum Vickers hardness, while for the joint with cavity defects the fracture occurred at the defect location. The tensile fracture was in the ductile fracture mode.     

B4C/6061Al复合材料焊接接头组织与力学性能研究*

基于碳化硼中¹⁰B同位素优良的热中子吸收能力,铝基碳化硼复合材作为中子吸收材料越来越多的应用于核电站中。但碳化硼颗粒的加入使该材料的可焊性变差,因此研究其焊接行为变得十分必要。采用钨极氩弧焊(Tungsten inert gas,TIG)和搅拌摩擦焊(Friction stir welding,FSW)对体积分数为30%的B₄C/6061Al复合材料进行焊接,研究不同焊接方法、焊缝填充材料对复合材料对接接头微观组织及力学性能的影响。B₄C/6061Al复合材料焊接接头拉伸性能如下：FSW焊>TIG焊(Al-Si焊丝)>TIG焊(6061Al焊丝)>TIG焊(6061Al-Mg焊丝)>TIG焊(无填充)。TIG焊缝区容易产生气孔、B₄C颗粒分布不均匀及有害生成相是导致其力学性能不佳的主要原因。FSW可以有效避免基体金属与增强相的高温化学反应,使得焊缝区的晶粒细化,增强相颗粒的分布比TIG焊均匀,为30%B₄C/6061Al复合材料最佳焊接方法,其接头的室温拉伸强度达247 MPa,为母材强度的85%。     

Design of tool system for the external nonrotational shoulder assisted friction stir welding and its experimental validations on 2219-T6 aluminum alloy

The nonrotational shoulder assisted friction stir welding (NRSA-FSW) is still in the feasibility study stage. To reveal details in the tool system designing and highlight advantages of this novel technology, the tool system for the NRSA-FSW was designed and utilized to weld high-strength aluminum alloy 2219-T6 for validations. Compared with the joints welded by the friction stir welding (FSW) without assistance of the nonrotational shoulder (NRS), the effect of the NRS on the weld formation and mechanical properties was illustrated. At a constant welding speed 100 mm/min, defect-free joints can only be obtained at the tool rotation speed 800 rpm by the FSW without assistance of the NRS, but the NRSA-FSW can produce defect-free joints in a wider range of tool rotation speeds 600–900 rpm. The NRS prevented all plasticized materials from escaping from the stirring zone, thus the weld nugget zone transformed from the basin-type formation to the spherical formation with increasing of the stirring effect when the tool rotation speed increased gradually. For joints welded by these two FSW processes, both the tensile strength and the elongation showed nearly the same trend with the tool rotation speed, but the NRSA-FSW can produce joints with the maximum tensile strength in a wider range. Compared with the maximum joint efficiency 71.2 % of the FSW without assistance of the NRS, the maximum tensile strength obtained by the NRSA-FSW also reached 69.0 % of the base material. All tensile specimens machined from defect-free joints fractured at the weakest region with minimum Vicker’s hardness; while for those joints with cavity defects, the fracture occurred at the defect location.     

搅拌摩擦焊和钨极氩弧焊焊接接头的残余应力

利用X射线衍射法测量了铝合金搅拌摩擦焊和钨极氩弧焊(TIG)焊接接头表面的残余应力分布.结果表明:两种接头在焊缝区及其周围的残余应力存在着明显的变化,在平行于焊缝方向,应力呈"W"型的分布,焊缝外残余应力值迅速下降;钨极氩弧焊焊接接头残余应力的最大值位于热影响区;在热影响区,搅拌摩擦焊接接头残余应力平均值比TIG焊接接头的约低15%～25%.     

Process responses and resultant joint properties of friction stir welding of dissimilar 5083 and 6061aluminum alloys

The effects of combinations of dissimilar aluminum alloys during Friction stir welding (FSW) on the process response and resultant joint properties are experimentally investigated using two dissimilar automotive structural aluminum alloys. Depending on the materials on the advancing and retreating sides of the tool travel direction during FSW, four different material combinations are considered. FSW joints without macroscopic defects are successfully fabricated for the four different material combinations. The optical microscopy results show that the macroscopic material mixing behaviors of the two dissimilar material combinations during FSW are somewhat different from each other, even though the process responses during joining are not much different. The results of the quasi-static tensile tests and EBSD analysis demonstrate that the mechanical behaviors and orientation changes of the joint during tensile deformation are affected by the material locations with respect to the tool travel direction during FSW.

     

钛合金大厚度试件电子束焊接残余应力有限元分析

基于热弹塑性有限元理论,建立了钛合金大厚度试件电子束移动热源的焊接残余应力三维数值分析模型,分析研究了厚度为75 mm的TC4电子束焊接试件残余应力分布规律。计算结果表明,大厚度电子束焊接试件在焊缝及其附近宽度约40 mm的范围内存在极其复杂残余应力,试件表面的焊缝及其附近20 mm左右的区域存在着对结构强度有利的横向残余压应力,但试件内部残余应力水平要高于表面的残余应力,尤其是在距起始和收尾端10mm及约1/4厚度处的区域,存在着三向的残余拉应力,且数值较高,对钛合金平板电子束焊接接头力学性能将具有重要影响,应引起足够的重视。钛合金平板电子束焊接残余应力的计算结果与实测结果基本一致。     

Effective predictions of ultimate tensile strength,peak temperature and grain size of friction stir welded AA2024 alloy joints

A series of welds were made by friction stir welding (FSW) under different welding and rotation speeds. A 2D ultimate tensile strength (UTS) map was developed based on various experimental data to predict the UTS of friction stir welded AA2024 alloy joints. The accuracy of the UTS map was evaluated by comparing the estimated UTS with the corresponding experimental results from the FSW of the same material available in the open literature. Analytical models were developed to estimate the peak temperature and grain size in the nugget zone. The predicted optimal peak temperature and welding and rotation speeds for AA2024 were within the windows of 400–465 °C, 175–350 mm/min and 800–1,200 rpm, respectively, under which the joint tensile strength could be higher than 458 MPa (about 94.6 % of the base metal) and the estimated average grain sizes in the nugget zone were about 2–3.9 μm.