Assessment of self-reacting bobbin tool friction stir welding for joining AZ31 magnesium alloy at inert gas environment

Research work presented in this study has the primary target of exploring joint attributes of AZ31 magnesium alloys using friction stir welding process with a modified tool referred as bobbin tool.Effects of inert and open atmosphere on mechanical properties are evaluated over a wide range of welding speed and tool rotation speed.Comparison of the research findings from the inert atmosphere bobbin tool were made with the traditional process of friction stir welding.The results depicted improved joint properties for inert atmosphere welding.Low and intermediate range of tool rotational speed is found to be favorable for bobbin tool friction stir welding without and with an inert medium,respectively.Controlled atmosphere due to inert medium leads to less oxidation of the AZ31 Mg alloy leading to superior joint properties.Microstructural investigations are also made with the aim of evaluating the impact of bobbin tool and inert medium on joint properties.In each aspect for joining of AZ31 Mg alloy,bobbin tool with inert medium is found to be an effective solution for joining with improved mechanical properties compared to without inert bobbin tool as well as conventional tool friction stir welding.

Effects of postweld aging on the microstructure and properties of bobbin tool friction stir-welded 6082-T6 aluminum alloy

Samples of 6082-T6 aluminum alloy were subjected to bobbin tool friction stir welding (BT-FSW), and the joints were treated by postweld natural aging (PWNA) and postweld artificial aging (PWAA). The microstructure, microhardness, and tensile properties of the aged and as-welded specimens were investigated. Transmission electron microscopy (TEM) observations revealed that a large number of Guinier–Preston (GP) zones precipitated in the form of a network on the stir zone (SZ) after PWNA for 60 d, and a large number of β'' phases precipitated in the matrix for after PWAA for 6 h. As the aging time increased, the microhardness of the SZ and the thermomechanically affected zone (TMAZ) increased significantly, and the hardness of the SZ after PWAA for 6 h was close to that of the base metal (BM). With increasing PWNA time, the strength and strain increased slightly. When the PWAA time increased, the strength clearly increased, with a maximum value of 279.9 MPa after 6 h, while the strain decreased.

Enhanced Fatigue Properties of 2219 Al Alloy Joints via Bobbin Tool Friction Stir Welding

In the present study,2219-T87 Al alloy plates,4 mm in thickness,were subjected to bobbin tool friction stir welding(BTFSW)under relatively high welding speeds of 200 and 400 mm/min,with the aim to analyze the effect of welding speeds on fatigue properties of the joints.The results showed that the tension–tension high-cycle fatigue performance of the BT-FSW joints at room temperature was significantly enhanced compared to that of other joints of 2xxx series Al alloys counterparts.Particularly at a high welding speed of 400 mm/min,the fatigue strength of the joint reached 78%of the base material together with a high tensile strength of 311 MPa.It was found that the joint line remnants had no effects on the fatigue properties of the BT-FSW joints due to the elimination of root flaws under the action of the lower shoulder.Most of the samples with the welding speed of 200 mm/min failed at the thermo-mechanical zone(TMAZ)during fatigue tests,attributable to the coarsened grains and precipitates,but all of the samples with high welding speed of 400 mm/min randomly failed at the nugget zone due to the improved hardness value in the TMAZ.

Effects of traverse speed on weld formation,microstructure and mechanical properties of ZK60 Mg alloy joint by bobbin tool friction stir welding

ZK60B(Mg-6%Zn-0.6%Zr)alloy joints fabricated by bobbin tool friction stir welding(BTFSW)with various traverse speeds were investigated.The sound joint fabricated by the BTFSW was possible under the appropriate welding parameters.The severe plastic deformation during BTFSW resulted in dispersion and segregation of the Zr-rich particles within the stirred zone(SZ)followed by evolution of a bimodal grain structure with distributed bands of 0.8-1.7μm ultrafine grains and 4.1-7.1μm equiaxed grains.Micro-hardness of SZ is substantially reduced in contrast to that of parent metal(PM)in spite of the finer grain size owing to dissolution of Mg-Zn based precipitates having hardening effects on alpha-Mg matrix.With the decrease in traverse speed,randomization degree of the plasticized metal flow increases,which is evidenced by the randomized arc line pattern at the low traverse speed.Among all defect-free joints,the 200 mm/min joint exhibits the weakest isotropy of texture within SZ and the best tensile properties,which has reduced ultimate tensile strength and yield strength by 5.4% and by 22.2%,respectively,as compared to the PM.The randomized texture hinders the joint fracturing within SZ at low elongation.Therefore,a relatively high elongation of 10.8% was achieved,which corresponded to 72% of the PM value.

Improving weld formability by a novel dual-rotation bobbin tool friction stir welding

A novel dual-rotation bobbin tool friction stir welding （DBT-FSW） was developed, in which the upper shoulder （US） and lower shoulder （LS） have different rotational speeds. This process was tried to weld 3.2 mm thick alunlinum-lithium alloy sheets. The metallographic analysis and torque measurement were carried out to characterize the weld formabiliW. Experimental results show that compared to conven- tional bobbin tool friction stir welding, the DBT-FSW has an excellent process stability, and can produce the defect-free joints in a wider range of welding parameters. These can be attributed to the significant improvement of material flow caused by the formation of a staggered layer structure and the unbalanced force between the US and LS during the DBT-FSW process.

Assessing the Bonding Interface Characteristics and Mechanical Properties of Bobbin Tool Friction Stir Welded Dissimilar Aluminum Alloy Joints

This study focuses on the bonding interface characteristics and mechanical properties of the bobbin tool friction stir welded dissimilar AA6056 and AA2219 aluminum alloy joints using diff erent welding speeds.Voids arise solely in the stir zone at the AA2219 side.A distinct boundary with limited material mixing develops at the middle section of the bonding interface,while excellent material mixing with an irregularly jagged pattern forms at the top and bottom sections of the bonding interface.Increasing the welding speed,the material mixing is rarely changed at the middle section in comparison with the bottom section.Furthermore,a small diff erence between Guinier–Preston dissolution and Q phase precipitation leads to rare change of hardness in the heat aff ected zone(HAZ)at the AA6056 side.The increased hardness of the HAZ at the AA2219 side is attributed to avoidance of the dissolution ofθ’’phase precipitates.A maximum tensile strength of 181 MPa is obtained at 300 mm min-1.Fractures occur at the AA6056 side near the top and bottom surfaces and at the bonding interface in the middle section of the joints.The regions close to the top and bottom surfaces of the joints show a better ductility.

Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate

7085-T7452 plates with a thickness of 12 mm were welded by conventional single side and bobbin tool friction stir welding （SS-FSW and BB-FSW, respectively） at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation （DIC） was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone （HAZ）, the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone （WNZ） before yield behavior for all BB-FSW specimens. The ultimate tensile strength （UTS） of SS-FSW joints presented the highest value of 410 MPa, 82% of the base material, at a rotational speed of 300 rpm and welding speed of 60 mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process, The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic （SEM） analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.

Research Progress of Bobbin Tool Friction Stir Welding of Aluminum Alloys:A Review

Bobbin tool friction stir welding(BT-FSW)is a variant of conventional friction stir welding(FSW).It can be used to weld complex curvature structures and closed sections by adding an extra shoulder instead of a rigid backing anvil,which expands the potential application of FSW in aerospace,railway,automotive and marine industries.BT-FSW has some signifi cant advantages over conventional FSW such as no root fl aws,full weld penetration,low stiff ness requirements for machines and fi xtures,balanced heat input,lower distortion and thus has broad prospects for development.At present,there have been numerous research reports on BT-FSW,but its widespread use is still restricted due to various factors such as tool life,process stability,control complexity and implementation cost.In this paper,the domestic and foreign research progress of BT-FSW is reviewed from four aspects of bobbin tool design and classifi cation,temperature fi eld and fl ow fi eld during welding,microstructure and mechanical properties of welded joints as well as industrial application,and then the possible research hotspots of BT-FSW in the future are pointed out.This paper mainly aims to help researchers have a comprehensive and in-depth understanding of BT-FSW.

5083铝合金搅拌摩擦焊缝多区域力学性能研究

对12 mm厚的5083铝合金双轴肩搅拌摩擦焊接头不同区域进行金相组织观察、拉伸试验、硬度测试和疲劳试验,对疲劳试验后的试样断口进行SEM观察。结果表明:焊核区晶粒明显细化,热影响区晶粒粗化,热机影响区晶粒发生严重变形并带有方向性。接头整体硬度近似呈非对称的“W”形分布。从母材到焊核区硬度先减小后增大。母材硬度值最高,焊核区次之。母材、焊核区、热影响区的疲劳寿命差别较大,母材疲劳性能最好,焊核区次之,热影响区最弱。各区的疲劳裂纹源位于疲劳试样的表面,扩展区中可以看到疲劳条带,瞬断区断裂形式均为韧性断裂。

AA6056双轴肩搅拌摩擦焊接头非均匀性分析

采用双轴肩搅拌摩擦焊对4 mm厚6056铝合金进行了焊接,分析了接头不同特征区的显微组织和力学性能的非均匀特性.结果表明,接头横截面等效应变呈哑铃状,与焊缝横截面宏观形貌一致;接头横截面沿焊缝中心左右两侧塑性应变呈非对称分布状态,其中接头前进侧的塑性应变值高于后退侧;搅拌区靠近前进侧和中间位置的大角度晶界比例高于后退侧,前进侧和中间位置主要为B型{112}<110>织构,后退侧为A型{111}<110>织构;焊缝的带状区局部发生不完全动态再结晶,变形晶粒比例高达58.9%,带状区为B型{112}<110>织构,织构强度为10.6,高于搅拌区其它位置;接头热影响区的应变超过18%,高于整体接头应变3.6%,热力影响区的应变与整体接头相近,而搅拌区的应变小于整体接头.

中厚板2219铝合金双轴肩搅拌摩擦焊焊接残余应力及微观组织演变

为了研究双轴肩搅拌摩擦焊缝内部焊接残余应力的大小及分布情况,本研究采用短波X射线衍射进行焊接件内部残余应力的无损检测分析;采用光学显微分析、显微硬度和电子背散射衍射(Electron Back-scattering Patterns,EBSD)对焊缝的前进侧和后退侧的母材、热影响区、热机械影响区和焊核区的组织结构演变进行了分析。金相观察结果显示双轴肩搅拌摩擦焊的接头组织在厚度方向上近似于对称分布,呈“腰鼓形”,焊核区与热机影响区的界面为近似双曲线,前进侧热机影响区的分界线更明显。EBSD扫描结果显示热影响区、热机械影响区均存在较强的形变组织;焊核区在剪切变形和焊接循环热的双重影响下发生了动态再结晶,主要为弱取向组织,小角度晶界含量较大。短波X射线衍射结果表明,双轴肩FSW焊接板内部板厚中心层,纵向方向残余应力均大于横向方向;沿着焊缝,拉应力较大区间位于距焊缝起始端150~250 mm的范围内,最大拉应力为244 MPa。

6082-T6铝合金双轴肩搅拌摩擦焊接头组织与性能研究

采用双轴肩搅拌摩擦焊方法对4 mm厚6082-T6铝合金板进行对接试验,对接头的组织和力学性能进行了研究。结果表明,接头焊核区发生了动态再结晶,呈现出典型的等轴细晶组织。在下压量为0.2 mm,搅拌头转速为700 r/min,焊接速度为300 mm/min的参数下,接头的抗拉强度为222.42 MPa,约达到母材的70.1%。拉伸接头在热影响区发生断裂,呈现出典型的韧性断裂模式;焊接接头横截面的硬度呈“W”形分布,硬度值在接头两侧的热影响区位置最低。接头的正弯、背弯强度接近,均在24 MPa左右。在搅拌针表面螺纹向下挤压作用下,在近下轴肩区域上下塑性流动材料交汇产生弱连接现象,导致接头的背弯试样断裂程度更大。

6082-T6铝合金双轴肩搅拌摩擦焊接头成形及力学性能

采用分体式双轴肩搅拌摩擦焊(Bobbin-tool friction stir welding,BT-FSW)实现了10 mm厚6082-T6铝合金板材的焊接,搅拌针表面的加工螺纹特征有助于搅拌针诱发周围材料的单一剪切变形,在增加搅拌针周围材料变形复杂程度的同时提高应变速率,从而有助于弱化材料在BT-FSW接头区域组织。结果表明,当转速为300 r/min,焊接速度为400 mm/min,且压入量为0.5 mm时,获得表面及内部宏观成形良好的BT-FSW接头,接头宏观形貌呈典型的“哑铃形”;在较高程度的塑性变形影响下,BT-FSW接头内的晶粒发生动态再结晶从而生成细小的等轴晶;由于接头不同区域的摩擦热输入分布不同,接头的硬度分布呈典型的W形分布;接头抗拉强度约为母材强度的77%,断后伸长率约为母材的69%。

铝锂合金的搅拌摩擦焊及其改型工艺研究进展

铝锂合金拥有较高的比强度和比刚度以及高损伤容限,且耐蚀性好、低温疲劳性能优异,是航空航天、轨道列车等领域实现构件轻量化的理想结构材料之一。搅拌摩擦焊是高强铝锂合金焊接成形的优选方法之一。首先探讨了铝锂合金熔化焊接的特点及难点,然后分析了铝锂合金的搅拌摩擦焊接的研究现状及挑战,随后讨论了国内外学者对铝锂合金搅拌摩擦焊接改型工艺的相关研究进展,最后对铝锂合金搅拌摩擦焊接的未来研究方向进行了展望。

2A14-T6铝合金双轴肩搅拌摩擦焊特征及接头组织性能分析

成功实现了2A14-T6高强铝合金的双轴肩搅拌摩擦焊,获得了表面成形良好,无内部缺陷的优质接头.试验发现,在焊缝的焊核内存在一个由上、下轴肩和搅拌针所驱动的材料塑性流动交汇区,该交汇区靠近焊缝下表面.微观分析表明,焊核上部的晶粒尺寸要小于其中部和下部的晶粒尺寸.焊缝各区的块状第二相在焊接中发生了溶解和粗化,导致接头内出现了宽度近乎轴肩直径二倍的软化区;焊缝各层硬度分布接近,没有出现常规搅拌摩擦焊中常见的接头各层异性现象.经拉伸测试证实,双轴肩焊接接头的强度系数达到了71%,拉伸时断在了焊核内的材料流动交汇区处.

2219-T4铝合金双轴肩FSW与常规FSW接头性能对比研究

对常规搅拌摩擦焊和双轴肩搅拌摩擦焊技术进行了比较,试验材料为6mm厚2219-T4铝合金。对两种焊缝的微观结构、显微硬度和力学性能进行了测试。结果表明双轴肩焊缝的宏观形貌与常规搅拌摩擦焊焊缝的不同,呈哑铃型。且由于双轴肩焊接的热输入量较高,其接头的显微硬度和力学性能普遍低于常规搅拌摩擦焊接头。双轴肩搅拌摩擦焊接头的抗拉强度可达到常规搅拌摩擦焊接头的90%。

6082铝合金双轴肩搅拌摩擦焊试板温度场研究

采用热电偶测温技术系统测定了6mm6082铝合金双轴肩搅拌摩擦焊试板各特征点的温度变化曲线,分析了双轴肩搅拌摩擦焊过程中焊接试板不同区域的温度场分布特征。双轴肩搅拌摩擦焊搅拌头上、下轴肩同时产热,比传统搅拌摩擦焊产热量大,且热输入方式及试板接触散热条件也有很大不同,因此,其稳定焊接速度较大,从而导致双轴肩搅拌摩擦焊试板温度场分布特征与传统搅拌摩擦焊明显不同。双轴肩搅拌摩擦焊过程分为加速焊接和稳定焊接两个阶段,起始阶段,随着焊接速度的增加,靠近起始端测温点的温度逐渐升高,而远离起始端各测温点的温度升温则非常缓慢,当焊接速度达到较高的稳定焊接速度,搅拌头接近后续各测温点时,其温度值瞬间急剧升高,然后随着搅拌头的远离,温度值逐渐下降。不同区域测温点温度测试结果显示,靠近下轴肩试板测温点的温度高于靠近上轴肩试板、后退侧的温度明显高于前进侧;与单轴肩搅拌摩擦焊接试板相同,距离焊缝越近的位置温度上升和下降的越剧烈,峰值温度越高;焊接速度提高,各测温点的峰值温度依次降低,随着测温点远离焊缝中心,焊接速度对其温度分布的影响作用逐渐减弱。

2A14-T6铝合金双轴肩搅拌摩擦焊接温度场研究

搅拌摩擦焊接过程的温度场分布对焊接接头质量有非常重要的影响。本文应用ANSYS有限元分析软件对2A14-T6铝合金双轴肩搅拌摩擦焊接过程的温度场进行数值模拟,研究了焊接过程试板表面和厚度方向温度分布特征。运用热电偶测温技术对试板特征点进行温度测量,分析各特征点温度变化规律,同时验证了模拟的正确性。

双轴肩搅拌摩擦焊接实验系统研究

为了适应现代焊接实验教学的需要,研究了双轴肩搅拌摩擦焊接的数字化控制过程,介绍了双轴肩搅拌摩擦焊机、双轴肩搅拌摩擦头的基本设计原理,并对焊接过程中搅拌头的轴向力、平面受力及前导区温度进行了检测。基于搅拌头轴向力信号、平面力信号及前导区温度检测等环节,利用分体式双向可调节的双轴肩搅拌摩擦头,配以合理的焊接工艺参数,实现不同厚度试板的焊接与参数监控;最后基于Smith预估算法构建了搅拌头前导区温度控制系统。该套实验系统功能齐全,具备开放性和灵活性,可根据教学要求开展多项实验,有利于培养学生的科研素养、动手能力。

双轴肩搅拌摩擦焊技术在铝合金车体制造中的应用发展

针对双轴肩搅拌摩擦焊技术特点进行介绍,对双轴肩搅拌摩擦焊在铝合金车体制造中的应用现状进行了分析,并提出了由传统搅拌摩擦焊技术衍生出的新型搅拌摩擦焊技术在轨道车辆制造行业的未来发展趋势和应用对象。采用双轴肩搅拌摩擦焊工艺成为国内外的又一个研究热点,加快搅拌摩擦焊技术在铝合金车体应用对提高国内轨道车辆制造水平至关重要。