Effects of Welding Processes and Post-Weld Aging Treatment on Fatigue Behavior of AA2219 Aluminium Alloy Joints

AA2219 aluminium alloy square butt joints without filler metal addition were fabricated using gas tungsten arc welding (GTAW), electron beam welding (EBW), and friction stir welding (FSW) processes. The fabricated joints were post-weld aged at 175 °C for 12 h. The effect of three welding processes and post-weld aging (PWA) treatment on the fatigue properties is reported. Transverse tensile properties of the welded joints were evaluated. Microstructure analysis was also carried out using optical and electron microscopes. It was found that the post-weld aged FSW joints showed superior fatigue performance compared to EBW and GTAW joints. This was mainly due to the formation of very fine, dynamically recrystallized grains and uniform distribution of fine precipitates in the weld region.     

热处理条件对搅拌摩擦镁合金接头各区域抗弹行为的影响

采用搅拌摩擦焊(FSW)工艺制备AZ31B镁合金焊接接头,并在不同条件下进行热处理。研究AZ31B镁合金焊后热处理(PWHT)不同区域的抗弹行为,使用7.62 mm×39 mm穿甲弹,冲击速度为(430±20)m/s。分析热处理前后搅拌摩擦焊接头的显微硬度。结果表明,PWHT工艺(250°C,1 h)能提高热处理后搅拌摩擦焊接头的显微硬度。扫描电镜(SEM)结果显示,热处理使α-Mg晶粒细化,形成细小的析出相,使β-Mg17Al12相溶解到Mg基体中。通过穿深(DOP)试验评估PWHT不同区域的抗弹行为。热处理后接头母材区(BMZ)的DOP值较小。抗弹试验后对弹坑周边3个区域的横截面进行SEM表征,观察到绝热剪切带(ASB)的形成。     

Bimodal nanocrystallization of NiTi shape memory alloy by laser shock peening and post-deformation annealing

In this paper, surface nanocrystallization of NiTi intermetallic alloy by a novel method is reported. The NiTi alloy is processed by laser shock peening (LSP) and controlled annealing. The microstructure of the NiTi alloy after processing is characterized by transmission electron microscopy. At the top surface of the material, a nanostructure with bimodal grains is obtained. The mechanism of the formation of the bimodal microstructure is discussed. At the material subsurface, deformation twins are generated by LSP and retained after controlled annealing. Tensile test results showed that both strength and ductility are significantly improved through LSP and controlled annealing.     

A Successful Synthesis of the CoCrFeNiAl0.3 Single-Crystal, High-Entropy Alloy by Bridgman Solidification

For the first time, a face-centered-cubic, single-crystal CoCrFeNiAl_0.3 (designated as Al0.3), high-entropy alloy (HEA) was successfully synthesized by the Bridgman solidification (BS) method, at an extremely low withdrawal velocity through a constant temperature gradient, for which it underwent two BS steps. Specially, at the first BS step, the alloy sample underwent several morphological transitions accompanying the crystal growth from the melt. This microstructure evolves from as-cast dendrites, to equiaxed grains, and then to columnar crystals, and last to the single crystal. In particular, at the equiaxed-grain region, some visible annealing twins were observed, which indicates a low stacking fault energy of the Al0.3 alloy. Although a body-centered-cubic CoCrFeNiAl (Al1) HEA was also prepared under the same conditions, only a single columnar-crystal structure with instinctively preferential crystallographic orientations was obtained by the same procedure. A similar morphological transition from dendrites to equiaxed grains occurred at the equiaxed-grain region in Al1 alloy, but the annealing twins were not observed probably because a higher Al addition leads to a higher stacking fault energy for this alloy.     

挤压镁合金AZ31的压缩过程中的组织及织构演化

对常规挤压态镁合金AZ31压缩过程的组织及织构演化进行了扫描电镜-电子背散射衍射(SEM-EBSD)原位观察。结果表明材料的初始组织为等轴晶,晶粒的平均尺寸为76微米,晶粒内部未发现形变孪晶。材料的初始织构类型为典型的{11-20}丝织构,即大多数晶粒的<11-20>晶向平行于棒材的挤压方向(ED)。在压缩过程中,多数晶粒内部开始出现拉伸孪晶,随着压缩应变的增加,孪晶片层不断增厚,导致晶内的孪晶合并成大的孪晶并占据晶粒内部的大部分区域进而使孪晶的体积分数不断增加。随着压缩压缩应变的增加初始丝织构不断减弱并有新的基面织构形成。实验表明压缩过程中的{10-12}<10-11>孪生而非滑移是引起压缩过程中织构演化的主要原因。     

Al-Mg-Mn-Sc-Zr合金板材搅拌摩擦焊和氩弧焊焊接接头的对比

研究Al-Mg-Mn-Sc-Zr合金的焊接工艺和焊接接头组织和性能。采用搅拌摩擦焊（FSW）和氩弧焊（TIG）2种焊接工艺对该合金的热轧和冷轧-退火2种使用态板材进行焊接。采用比较研究的方法测定和研究焊接接头的力学性能和显微组织,利用光学显微镜和透射电子显微镜研究焊缝的显微组织和力学性能之间的关系。结果表明,与基材相比,Al-Mg-Mn-Sc-Zr合金的热轧和冷轧-退火板材的FSW和TIG焊接接头的强度均下降,但FSW焊接系数高于TIG焊接系数。这是因为FSW焊接接头焊核区亚结构强化的丧失和Al3（Sc,Zr）的析出强化作用的极少量丧失,而TIG焊焊接接头的软化主要原因是其形变强化的完全丧失和Al3（Sc,Zr）的析出强化作用的大部分丧失,且搅拌摩擦焊焊核区晶粒比TIG焊的焊缝区晶粒更细小。     

AZ31镁合金摩擦搅拌焊在慢拉伸应力腐蚀过程中的组织演化和氢致延迟开裂

研究AZ31镁合金摩擦搅拌焊组织的演化,包括织构和断口变化。利用中子衍射仪测量织构。利用光学显微镜和扫描电子显微镜观察应力腐蚀开裂（SCC）样品的金相及断口形貌。利用X射线衍射仪研究SCC样品的断口表面结构。结果表明,在母材表面形成了明显的基面织构。然而,搅拌区晶粒发生了基面旋转,大多数晶粒的基面沿着焊接方向倾斜25°。在慢拉伸应力作用下,在空气和侵蚀性溶液中分别形成了羽毛状孪晶和氢化物。在溶液中,穿晶裂纹扩展,最终断裂在回转侧。在断口表面存在的氢化物表明镁合金应力腐蚀可能存在氢致延迟开裂机制。     

An investigation on metallurgical characteristics of tungsten based tool materials used in friction stir welding of naval grade high strength low alloy steels

A non-consumable tool is a vital requirement for friction stir welding (FSW) of high melting point alloys such as steel and titanium. In this investigation, an attempt was made to understand the pre-weld and post-weld microstructural characteristics of three tungsten based alloy FSW tools viz. 90%W, 95%W and 99%W. A naval grade high strength low alloy (HSLA) steel plates of 5 mm thickness were welded using the above tools with a tool rotational speed of 600 rpm and welding speed of 30 mm/min. Microstructural characteristics of the FSW tools, before and after welding, were analyzed using optical microscopy (OM) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). From this investigation, it is found that the tool made of 99% W doped with 1% La₂O₃ exhibited microstructural stability at elevated temperatures during FSW process.     

Tungsten Inert Gas and Friction Stir Welding Characteristics of 4-mm-Thick 2219-T87 Plates at Room Temperature and −196 °C

2219-T87 aluminum alloy is widely used for fabricating liquid rocket propellant storage tank, due to its admirable cryogenic property. Welding is the dominant joining method in the manufacturing process of aerospace components. In this study, the tungsten inert gas welding and friction stir welding (FSW) characteristics of 4-mm-thick 2219-T87 alloy plate at room temperature (25 °C) and deep cryogenic temperature (?196 °C) were investigated by property measurements and microscopy methods. The studied 2219 base alloy exhibits a low strength plane anisotropy and excellent room temperature and cryogenic mechanical properties. The ultimate tensile strength values of TIG and FSW welding joints can reach 265 and 353 MPa at room temperature, and 342 and 438 MPa at ?196 °C, respectively. The base metal consists of elongated deformed grains and many nano-scaled θ (Al₂Cu) aging precipitates. Fusion zone and heat-affected zone (HAZ) of the TIG joint are characterized by coarsening dendritic grains and equiaxed recrystallized grains, respectively. The FSW-welded joint consists of the weld nugget zone, thermo-mechanically affected zone (TMAZ), and HAZ. In the weld nugget zone, a micro-scaled sub-grain structure is the main microstructure characteristic. The TMAZ and HAZ are both characterized by coarsened aging precipitates and elongated deformed grains. The excellent FSW welding properties are attributed to the preservation of the working structures and homogenous chemical compositions.     

Deformation Characterization of Friction-Stir-Welded Tubes by Hydraulic Bulge Testing

In this article, the large-diameter thin-walled aluminum alloy tubes were produced using a hybrid process combining friction-stir welding (FSW) and spinning. For this novel process, rolled aluminum alloy sheets with a thickness about 2–3 times the wall thickness of target tube, were FSW to form cylinders, and then the cylinders were subjected to spinning to get thin-walled aluminum alloy tubes. Both experimental and simulation study were conducted to investigate the deformation characterization of the FSW tube during hydraulic bulge testing, and the stress and strain states and thickness distribution of the FSW tube were investigated. It was found that the common defects of FSW tube can be significantly improved by specific welding devices. The ductility of the tube is considerably improved with nearly two times higher bulge ratio than as-spun tube after annealing treatment at 300°C. But the annealed tube still shows a high nonuniform wall thickness distribution due to the inhomogeneous deformation characteristics. With increasing deformation of the tube, the gap between the hoop and axial stress for the weld and base metal (BM) decreases. However, the hoop and axial stress of the weld are always greater than those of the BM at the same pressure.     

Heterogeneous structure-induced strength and ductility synergy of α-brass subjected to rapid cooling friction stir welding

The 2 mm-thick α-brass plates were successfully joined using conventional friction stir welding (CFSW) with air cooling and rapid cooling friction stir welding (RCFSW) with liquid CO₂ cooling. The microstructure and mechanical properties of the two welds were carefully investigated by electron back-scattered diffraction and transmission electron microscopy. The stir zone of CFSW exhibited homogeneous equiaxed grains, while the stir zone of RCFSW showed a heterogeneous grain structure, i.e. ultrafine grains containing massive dislocations and nano twins. Compared with the CFSW, yield strength and ultimate tensile strength of RCFSW were increased by 31% and 24%, respectively. The enhanced yield strength and improved strain hardening capacity were attributed to grain boundary strengthening and dislocation strengthening. Furthermore, good ductility was achieved due to the released stress concentration of the nano twins caused by the plastic deformation.     

AZ91D薄板搅拌摩擦焊

实现了厚度为2.2 mm铸造镁合金AZ91D薄板的搅拌摩擦焊和钨极氩弧焊,分析了搅拌摩擦焊工艺参数对焊接接头成形的影响和接头组织变化,考察了搅拌摩擦焊接头的力学性能.在搅拌头旋转速度为1 380 r/min时得到了比较理想的焊接接头,而1 960 r/min的转速过大.接头不同区域所受的机械力和热量不同,显微组织明显不同.搅拌区晶粒细小,显微硬度和强度都有所提高.搅拌摩擦焊接头力学性能与热输入有关;与氩弧焊接头相比,搅拌摩擦焊接接头的性能更好.     

2219-T87铝合金搅拌摩擦焊接头组织与力学性能

采用搅拌摩擦焊方法对8mm厚2219-T87铝合金进行了焊接.对接头的宏观形貌、微观组织、显微硬度及断口形貌进行了分析.结果表明,焊核区为细小的等轴晶粒,晶粒尺寸远小于母材;热机影响区发生了弯曲变形;热影响区组织出现了明显粗化.前进边热机影响区和焊核区形成明显分界线,后退边相对模糊.搅拌摩擦焊对接头各区域沉淀相分布形态有重要影响.接头室温拉伸强度可以达到母材的70%以上.沿焊缝横截面的显微硬度的分布显示,硬度最低点位于后退侧热影响区区域,断裂位置位于后退侧热影响区处,接头的断裂形式为韧性断裂.     

Precipitation twinning

The paper describes the heterogeneous nucleation of mechanical twins in a TiAl alloy containing Ti₃AlC perovskite precipitates. Examination of the deformation structure by high-resolution electron microscopy has revealed that overlapping planar faults were generated at the perovskite particles. These faults can apparently easily rearrange into embryonic twins and subsequently grow into large twins. The nucleation mechanism was found to be closely associated with the misfit character and stress state of the precipitates. The structural data serves as input for the modelling of the precipitation-induced twinning process. Based on these calculations, the thickness and length of the embryonic twins can be predicted, which agree well with the experimental observations.     

焊接参数对7075铝合金搅拌摩擦焊接头组织及腐蚀行为的影响

不同参数下对1．6mm厚7075铝合金板进行了搅拌摩擦焊接,在分析了焊缝表面组织的基础上,采用浸泡试验对焊缝表面的腐蚀行为进行研究,讨论了硬度分布与腐蚀发生的关系。结果表明,焊后轴肩作用区晶粒细化明显。随转速的增加,焊缝上表面热机械影响区范围加宽、轴肩作用区硬度增大。焊缝上表面腐蚀最严重的区域为轴肩作用区,在EXCO溶...     

Friction stir welding of 304 stainless steel using Ir based alloy tool

Abstract

In order to investigate the durability of an Ir based alloy tool, friction stir welding (FSW) of 304 stainless steel plates was performed under various welding conditions, and the mechanical properties of the joints were measured. Defect free joints are possible under certain conditions, and the mechanical properties of the joints are the same as the base material. When the rotation pitch is ～0·3, the degree of wear of the Ir based alloy tool with 1·4 mm probe height, which was used for the FSW of a 304 stainless steel plate with 2 mm thickness, was less than or equal to that of the polycrystalline cubic boron nitride tool with 2 mm probe height, which was used for the FSW of a 304 stainless steel plate with 9 mm thickness. The result of the tool life test in this study showed that the Ir based alloy tool enabled the FSW of 304 stainless steel over a 75 m length.     

镁合金薄板的搅拌摩擦焊工艺

搅拌摩擦焊是一种新型的塑化连接工艺,对于用熔化焊方法难于焊接的有色金属,有着极大的应用潜力。本文针对我国航空航天工业中常用的MB8镁合金,试验研究了镁合金薄板的搅拌摩擦焊工艺,对焊缝的成形特点、接头组织特征及力学性能进行了分析探讨。结果表明,MB8镁合金塑化连接的接头外观成形良好,内部无气孔、裂纹,焊后焊件几乎无变形,接头性能可达到母材抗拉强度的76％。塑化连接接头由焊核和热影响区组成,焊核区是经历了动态再结晶的细小晶粒,热影响区的晶粒较粗大。     

Microstructure evolution of TA15 titanium alloy subjected to equal channel angular pressing and subsequent annealing at various temperatures

TA15 titanium alloy was successfully processed for the first time by equal channel angular pressing (ECAP) in the temperature range of 900-1000 °C and annealed in a wide temperature interval from 650 to 800 °C. The investigation was achieved by light microscope (LM), scanning electron microscope (SEM) and transmission electron microscope (TEM) on the microstructure evolution of TA15 alloy subjected to ECAP and subsequent annealing after ECAP. In the present work, equal channel angular pressing (ECAP) was taken as the effective method to acquire severe plastic deformation (SPD). The studies we have performed show that grains have been obviously refined and well globularized after ECAP. When TA15 alloy was pressed at the temperatures of α + β phase region equiaxed microstructure was created. There was an increase in the equilibrium grain size with increasing pressing temperature, while a decrease in the volume fraction of equiaxed α phase. TEM microstructural images illustrate that an amount of deformation twins emerged while pressing TA15 below α-β transformation temperature (T_β), which led to the continued plastic deformation through the restarting of many slip bands. Severe coarsening took place in β grains during ECAP at the temperature above T_β. A larger number of well globularized and more homogeneous equiaxed α phase of TA15 alloy annealed after ECAP has been attained. Furthermore, with annealing at the optimum temperature, grains have not grown significantly.     

Effects on the Surface Texture,Superplastic Forming,and Fatigue Performance of Titanium 6AL-4V Friction Stir Welds

The speed and feed effects of the friction stir welding (FSW) process on the surface texture along the top of a butt welded nugget were studied. The tests were conducted using fine grain (0.8-2 μm) titanium alloy 6Al-4V with a nominal thickness of 2.5 mm. It was shown that the pin tool marks along the top surface of the weld can be highly detrimental to both the superplastic forming (SPF) characteristics and the fatigue performance of welded panels. Removing the marks by machining the top surface after FSW was found to eliminate the predominant tearing of the weld during SPF and most of the fatigue life of across the weld was also restored. Through additional development of the FSW process parameters, the butt welded nugget was made to have equivalent SPF characteristics as the parent sheet material. By using a water-cooled pin tool and other cooling techniques, it is believed that the weld zone can be kept below the beta transus temperature during FSW, which enables the formation of a grain structure that is uniquely conducive to superplastic behavior, when compared to conventional fusion welding processes.     

高镁铝合金搅拌摩擦交叉焊接头微观组织与力学性能

为满足大型铝合金船舶壁板的制造需求,对新一代高镁铝合金进行了搅拌摩擦交叉焊接试验. 结果表明,交叉焊接头成形良好,搅拌区晶粒尺寸最小,热力影响区晶粒形态没有明显方向性,与单道搅拌摩擦焊相比,交叉焊接头搅拌区晶粒组织更细. 显微硬度测试结果表明,交叉焊接头显微硬度变化范围较小,前进侧接头软化明显;拉伸试验测试结果表明,交叉焊接头抗拉强度为340 MPa,为母材强度的87%,对比搅拌摩擦焊接头抗拉强度358 MPa略微降低,在热影响区断裂,断裂方式为45°韧性断裂;疲劳裂纹萌生于焊缝底部,在最大应力150 MPa下循环超2 × 106次未断裂,疲劳性能良好,瞬断区断裂方式为韧性断裂.