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.     

Influences of rotation speed on microstructures and mechanical properties of 6061-T6 aluminum alloy joints fabricated by self-reacting friction stir welding tool

A 6061-T6 aluminum alloy was self-reacting friction stir welded by using the specially designed tool with unequal shoulder diameters at a constant welding speed of 150 mm/min to investigate the effect of rotation speed on microstructure and mechanical properties of the joints. Excessive flash on the bottom surface of the joint and groove defects on both surfaces of the joint were formed when the lower shoulder diameter was much smaller. The suitable shoulder sizes were determined as 16 and 18 mm in lower shoulder diameter and upper shoulder diameter, respectively. The grain size and the dislocation density in the weld nugget zone (WNZ) increased with increasing rotation speed. The tensile strength of joints first increased with increasing rotating speed and then decreased remarkably as a result of the formation of void defect. The joints welded at lower rotation speeds were fractured in the thermal mechanically affected zone (TMAZ). However, the fracture locations of the defect-free joints were changed to the heat affected zone (HAZ) at higher rotation speeds.     

Analyzing dissimilar friction stir welding of AA5754/AA7075

AA5754/AA7075 was butt-welded by friction stir welding, and the joint of each weld case was identified by ultimate tensile strength, percentage of elongation, and hardness. Moreover, the significance of each parameter was investigated, and a mathematical relation was constructed by regression analysis. A defect-free joint was achieved in the case of a weld produced with 1000 rpm of tool rotational speed, 80 mm/min of welding speed, and an 22-mm tool shoulder diameter. Most of the failures are located at the bottom of the pin and side of AA7075. The ultimate tensile strength (UTS) decreases with increasing welding speed (WS) or increasing tool rotational speed (TRS). Hardness distribution in the weld zones varied dependent on the nugget zone formation affected by TRS and WS. The present study also investigated the significance and contribution of each parameter on the UTS by analysis of variance (ANOVA). From the results of ANOVA, the conclusion reached is that the all the parameters have a great influence on UTS. The contributions are 41.41 % for WS, 17.58 % for diameter, and 13.28 % for TRS. Moreover, a full quadratic model was constructed between the parameters and the UTS value. The results show that the variation from the predicted values was between 0.41 % and 10.36 %. The strength of the model was analyzed by R-Sq. The achieved R-Sq is 0.892, which means that there is a strong relation between predicted and actual values.     

2A14厚板铝合金SSFSW角焊缝接头组织及性能

采用自主研制搅拌针长度为8.5mm的静止轴肩搅拌工具和2A14-T4厚板铝合金进行150°角焊缝接头静止轴肩搅拌摩擦焊工艺试验,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:在500~700r/min主轴转速与40~100mm/min焊接速度范围内均可获得表面光滑无内部缺陷的角焊缝接头,其外观尺寸可精确控制基本无残余焊接角变形。焊缝区主要由焊核(Stir zone,SZ)组成,SZ形状类似搅拌针圆锥台状或椭圆状、其宽度沿厚度方向分布比较均匀;热力影响区(Thermal mechanical affected zone,TMAZ)及热影响区(Heat affected zone,HAZ)宽度明显较小。焊缝区硬度分布具有明显不均匀特征,最薄弱区位于TMAZ与HAZ的交界处。主轴转速变化对焊缝区平均硬度影响较小,但随着焊接速度增加其平均硬度明显增大。角焊缝前进侧等效拉伸强度大于后退侧,等效拉伸强度随转速增加而减小,焊速的增大而增大。在500r/min-100mm/min焊接工艺下所得到的接头等效拉伸强度最高,可达到母材的79.24%。在拉-剪复合承载模式下,角焊缝拉伸试样宏观塑性变形很小呈现脆性断裂特征。     

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.     

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.     

Influence of energy induced from processing parameters on the mechanical properties of friction stir welded lap joint of aluminum to coated steel sheet

Friction stir welding has been attempted to evaluate joint strength of lap joint between aluminum sheet (AA6063) and zinc-coated steel (HIF-GA) sheet under different combination of rotational speed and traverse speed. The shear strength decreases significantly when rotational speed increases from 700 to 1,500 rpm at a traverse speed of 30 mm/min. At traverse speed of 50 mm/min, increasing rotational speed from 700 to 1,500 rpm, shear strength remains more or less the same. However, at a traverse speed of 100 mm/min, the shear strength increases significantly with increasing rotational speed from 700 to 1,500 rpm. Essentially, higher fracture load of the lap joint is obtained within a certain range of energy. The results have been correlated with the microstructural characteristics at the bond interface using energy dispersive X-ray spectroscopy, electron probe micro analyzer, and X-ray diffraction. The results show that characteristics of intermetallic compound formed at the interface derived from energy input takes predominating role towards lap joint of Al and coated steel. Furthermore, force and torque responses influenced by the processing parameters can be utilized as weld quality check.     

Friction stir welding characteristics of 2219-T6 aluminum alloy assisted by external non-rotational shoulder

Friction stir welding (FSW) of 2219-T6 aluminum alloy assisted by external non-rotational shoulder was carried out, and effects of the welding speed on microstructures and mechanical properties were investigated in detail. Defect-free joints were obtained in a wide range of welding speeds from 50 to 300 mm/min. The microstructural deformation and weld formation were dominated by the rotating tool pin and subsize concave shoulder but the non-rotational shoulder exerted very little effects for all joints. Compared with the weld obtained by conventional FSW, less intense stirring effects in FSW assisted by external non-rotational shoulder can only generate a narrower thermomechanically affected zone, whose width decreased with increasing of the welding speed. Microstructures and Vickers hardness distributions showed that this new welding process is beneficial to improving the asymmetry and inhomogeneity, especially in the weld nugget zone. The maximum tensile strength was up to 69 % of the base material.     

Multi-response optimization using the Taguchi-based grey relational analysis: a case study for dissimilar friction stir butt welding of AA6082-T6/AA5754-H111

This paper presents a multi-response optimization process for dissimilar friction stir welding of AA6082/AA5754 aluminum alloys. An L₉ orthogonal array was constituted for the experiments. Three welding parameters—tool shoulder diameter-to-pin diameter (D/d) ratio, tool rotational speed (TRS), and welding speed (WS)—were associated with tensile strength and elongation. An optimization process was started to determine the signal-to-noise (S/N) ratio. Grey relational analyses were performed utilizing the S/N ratio. According to the results of a series of analyses, the optimal welding condition was determined as 4 for D/d, 1,000 rpm for TRS, and 100 mm/min for WS. The analysis of variance results showed that all the welding parameters are statistically significant at 95 % confidence level. Additionally, the joint efficiency of welding fabricated at the optimal condition was compared for both AA6082 and AA5754. This revealed that the joint efficiency is 66 % for AA6082 and 92 % for AA5754.     

Study on microstructure and mechanical properties of dissimilar steel joint developed using friction stir welding

In the present study, microstructure and mechanical properties of dissimilar weld of structural steel and ferritic stainless steel (FSS) plates of thickness 3 mm were investigated. The plates were butt welded by friction stir welding and defect-free welds were produced at a traverse speed of 20 mm/min and rotational speed of 508 rpm using a tungsten carbide tool. The weld joint consisted of alternate bands of both steels resembling an onion ring pattern. In the weld joint, six distinct regions were found including both the base metals. The stir zone of structural steel revealed refined grain structure of ferrite, pearlite, and martensite whereas in ferritic stainless side, highly refined ferritic grains with grain boundary martensite was observed and also confirmed by x-ray diffraction (XRD). The hardness of the weld joint varies from 186 to 572 HV. This scatter of hardness in stir zone is due to the presence of metal from both sides. The ultimate tensile and yield strengths of the transverse weld specimens was higher than the structural steel base metal whereas lower than the ferritic stainless steel, having fracture from structural steel side.     

The effect of austenitic interlayer on microstructure and mechanical behaviors in keyhole plasma transfer arc welding of ferritic stainless steel couple

In this study, AISI 430 ferritic stainless steel couple of 10 mm thickness was welded by keyhole plasma transferred arc welding (KPTAW) process with or without filler wire addition using AISI 316L austenitic stainless steel interlayer of 2 mm thickness. Welded joints were manufactured with constant traverse speeds (0.01 m/min) under two different welding currents (110 and 130 A) at two different plasma gas flow rates (1.1 and 1.2 l/min), nozzle diameter (2.4 mm), and a shielding gas flow rate (25 l/min). In order to determine the microstructural changes that occurred, the interface regions of the welded samples were examined by scanning electron microscopy (SEM), optic microscopy, X-ray diffraction, and energy dispersive spectrometry after KPTAW. Microhardness and V-notch impact tests were conducted to determine the mechanical properties of the welded samples. In addition, fracture surface was examined by SEM after impact test.     

Investigating the formation of intermetallic compounds during friction stir welding of magnesium alloy to aluminum alloy in air and under liquid nitrogen

This research demonstrates the use of submerged friction stir welding under liquid nitrogen as an alternative and improved method for creating fine-grained welds, and hence, to alleviate formation of intermetallic phases. Magnesium alloy and aluminum alloy were joined by friction stir welding in two environments, namely air and liquid nitrogen, with 400 rpm rotation and 50 mm/min travel speed. The temperature profile, microstructure, scanning electron microscope energy dispersive X-ray spectroscopy analysis and hardness were evaluated. In the stir zone of air-welded specimen, formation of brittle intermetallic compounds causes the weld to crack. These phases were formed because of constitutional liquation. The stir zone of under liquid nitrogen-welded specimen showed that formation of intermetallic compounds is suppressed significantly because of lower heat input.     

Predictions of the optimized friction stir welding process parameters for joining AA7075-T6 aluminum alloy using preheating system

The scope of this investigation is to evaluate the effect of welding parameters on the mechanical properties and microstructural features of 3-mm-thick AA7075-T6 aluminum alloy subjected to gas heating system as a preheating source during friction stir welding. Toward this end, a gas heating system was designed to heat up the weld seam just ahead of rotating tool to soften the material before being stirred. Three welding parameters, five levels, and a central composite design (CCD) have been used to minimize the number of experimental conditions. The joining parameters such as tool rotational speed, welding speed, and shoulder diameter have a significant influence on determining the mechanical properties of the welded joints. It was found that using preheating system mostly can result in higher total heat input into the weld joint and effectively reduces the formation of defects when unsuitable process parameters were used. Also, an attempt has been made to establish the mathematical model to predict the tensile strength and microhardness of the joints. The optimal welding conditions to maximize the final responses were investigated and reported. The results show that the joint fabricated at a rotational speed of 1,050 rpm, welding speed of 100 mm/min, and shoulder diameter of 14 mm exhibited higher mechanical properties compared to other joints.     

无针搅拌摩擦点焊材料流动行为及其与接头宏观形貌、晶粒特征的相关性

搅拌摩擦焊接头材料流动行为是优化焊接工艺的根本所在,目前关于无针搅拌摩擦点焊流动行为尚未形成统一的认识。以0.02 mm镍箔为示踪材料,采用轴肩端面具有渐开线凹槽的无针搅拌头,改变旋转速度和焊接时间进行1.8 mm厚2198-T8铝锂合金搭接搅拌摩擦点焊试验,借助微焦点锥束三维CT设备、扫描电镜等测试手段,研究材料流动行为及其对接头宏观形貌、晶粒特征的影响。结果表明,轴肩下方的金属在轴肩挤压和摩擦热作用下先软化,以螺旋形向下向内流动形成搅拌区;随着焊接时间的延长,搅拌区金属向上和向外流动增强,致使搅拌区外缘界面翘曲,形成Hook缺陷。随旋转速度或焊接时间增大,搅拌区金属向下和向上向外流动加剧,焊核的深度和直径增大、晶粒更细小;下板金属软化程度加强,搅拌区外缘下板更多的塑化金属向上向外流动,致使Hook更翘曲。研究结果为深入了解无针搅拌摩擦点焊材料流动行为和优化焊接工艺提供了理论基础。     

Investigations of friction stir welding process using finite element method

The aim of this study is to investigate the process of friction stir welding (FSW) by using finite element method (FEM). Currently, the materials that are difficult to be joined with conventional fusion methods can now be easily joined with the method of friction stir welding. In this paper, the welding capability of many different materials with this method has been investigated by using analytical and numeric methods. In this study, a finite element (FE) model was developed for welding process with friction stir welding of AZ31 magnesium alloy. This model was performed by the software of DEFORM 3D finite element in 960, 1,964, and 2,880 rpm rotational speeds and in 10 and 20 mm?min^?1 transverse speeds. The temperature values taken from experiments and the temperature values with FEM are compared, and according to these results, it can be stated that the FE model gives reasonable results with experimental results based on temperatures values. Hence, the FE model can be used to predict other parameters of FSW process in future studies.     

Low-cycle fatigue of a friction stir welded 2219-T62 aluminum alloy at different welding parameters and cooling conditions

Strain-controlled low-cycle fatigue tests and microstructural evaluation were performed on a friction stir welded 2219-T62 aluminum alloy with varying welding parameters and cooling conditions. Cyclic hardening of friction stir welded joints was appreciably stronger than that of the base material. The cyclic stress amplitude increased, and plastic strain amplitude and fatigue lifetime slightly decreased with increasing welding speed from 60 to 200 mm/min but were only weakly dependent of the rotational rate between 300 and 1,000 rpm with air cooling. Friction stir welded joints with water cooling had higher stress amplitude and fatigue life than that with air cooling. Fatigue failure of the joint occurred in the HAZ where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.     

静轴肩摩擦搅拌焊温度场仿真分析与参数优化

为了解决非刚性支撑条件下传统搅拌头易陷入被焊接板材而导致焊接失败的问题,设计研发了静轴肩焊接结构。通过建立有限元仿真计算模型,并使用热红外成像仪对焊接表面温度进行实时监测,分析不同工艺参数下静轴肩摩擦搅拌焊焊接过程中的温度场变化情况。使用设计研发的静轴肩摩擦搅拌焊进行现场试验并对完成焊接表面无缺陷的焊缝与母材进行拉伸试验对比,检测其焊缝机械强度,并对断口进行微观组织分析。结果表明：在使用静轴肩搅拌头焊接过程中,产热量主要来源于搅拌针轴肩的摩擦生热和搅拌针端部的摩擦生热,搅拌针的侧面摩擦生热和静轴肩的摩擦生热占比较小;对产热量影响较大的是主轴压力和主轴转速,C轴转速对产热量影响不大;在主轴压力为2940～3430 N,主轴转速为1000 r/min,C轴转速为0.05 r/min的工艺参数下,完成焊接的焊缝表面光滑无飞边,内部无沟槽隧道缺陷,焊缝抗拉性能达到母材的71.5%左右;焊缝断口存在分层现象,靠近焊接表面的上层呈脆性断裂特性,下层呈延性断裂特性,与母材相比,焊缝试样的延伸率和抗拉强度均有所降低。     

AZ31B镁合金搅拌摩擦焊焊接工艺研究

采用搅拌摩擦焊对AZ31B镁合金板材进行了焊接试验,研究了搅拌头旋转速度、焊接速度和搅拌头轴肩下压量对焊接接头成形质量的影响。结果表明,搅拌头转速过快或焊接速度过慢时,焊缝会出现局部过热甚至熔化现象;反之,当搅拌头转速不够或焊接速度过快时,材料不能充分流动,会形成隧道型缺陷或表面沟槽。当搅拌头轴肩下压量过小时,焊缝内部组织疏松或出现孔洞、隧道型缺陷,焊缝表面出现沟槽,甚至使焊缝金属液外溢;搅拌头轴肩下压量过大,会造成摩擦力及搅拌头前移阻力增大、焊缝凹陷及出现飞边。当搅拌头转速为1200～1500r／min、焊速为30～60mm／min,搅拌头轴肩下压量为1．5～2．0mm时,可得表面成形良好、内部无孔洞和隧道的焊缝。     

DESIGN AND EVALUATION OF AN ULTRAHIGH SPEED MICRO-MACHINING SPINDLE

A new ultrahigh speed micro-spindle has been developed for micro-milling that can be used at rotational speeds approaching 500,000 rpm. Since conventional ball bearings or fluid lubricated journal bearings cannot be used at such speeds, the new micro-spindle uses a set of journal and thrust foil bearings. Prior to fabrication of the micro-spindle, rotordynamic analysis of the rotor with an attached cutting tool confirmed that the rotor would be stable at the desired speeds. The cutting tool was then attached to the rotor using a shrink-fit approach. The micro-spindle was integrated with a 3-axis micro-milling machine. Cutting experiments were performed on an aluminum alloy at speeds greater than 300,000 rpm using 125 and 300 μm end-mills. Vibration spectra for free rotation and during cutting confirmed the dynamic stability of the micro-spindle. The vibration spectrum was dominated by the rotational frequency and was free of deleterious vibrations. The increase in rotational speed to 450,000 rpm in micro-milling of aluminum alloy allowed an increase in feed rate to nearly 750 mm/min, thus increasing the material removal rate by more than two orders of magnitude. The dimensional accuracy of several straight cuts made at different feed rates was measured.     

2219铝合金双轴肩搅拌摩擦焊工艺及接头组织性能研究

通过对8mm厚2219铝合金进行双轴肩搅拌摩擦焊试验,研究了不同焊接速度对接头成形、组织演变及其对力学性能的影响规律。工艺试验结果表明:在固定转速(200r/min)下,不同焊接速度下的接头均成形良好,未出现微裂纹、隧道以及疏松等焊缝表面缺陷。随着焊接速度的增加,接头区域晶粒尺寸减小;接头显微硬度受到晶粒尺寸与沉淀相分布的制约,硬度分布曲线呈“W”形,热影响区硬度最低。并且随着焊接速度的增加,接头最低硬度和抗拉强度逐渐提高,断裂位置发生在热影响区与热影响区交界处。在焊接速度为350mm/min时,接头抗拉强度达到最大值335MPa,约为母材的72.8%。