Effect of intermetallic compounds on the fracture behavior of dissimilar friction stir welding joints of Mg and Al alloys

Joining Mg to Al is challenging because of the deterioration of mechanical properties caused by the formation of intermetallic compounds(IMCs) at the Mg/Al interface. This study aims to improve the mechanical properties of welded samples by preventing the fracture location at the Mg/Al interface. Friction stir welding was performed to join Mg to Al at different rotational and travel speeds. The microstructure of the welded samples showed the IMCs layers containing Al12Mg17(γ) and Al3Mg2(β) at the welding zone with a thickness(< 3.5 μm). Mechanical properties were mainly affected by the thickness of the IMCs, which was governed by welding parameters. The highest tensile strength was obtained at 600 r/min and 40 mm/min with a welding efficiency of 80%. The specimens could fracture along the boundary at the thermo-mechanically affected zone in the Mg side of the welded joint.

Changing Rule of Carbon-Enriched Zone and Diffusion Behavior of Carbon in Aging 0Cr6Mn13Ni10MoTi/1Cr5Mo Dissimilar Welded Joints

The microstructures, the changing rule of carbon-enriched zone, the diffusion behaviors of elements C and Cr, and thecarbide type of 0Cr6Mn13Ni10MoTi/1Cr5Mo dissimilar welded joints after aging at 500℃ for various times and afterlong-term service in technical practice were investigated by using the optical microscopy electron probe microanalysis,scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that in aging0Cr6Mn13Ni10MoTi/1Cr5Mo dissimilar welded joints, the main carbides are M_3C and a few carbides are M_7C_3 andM_(23)C_6. The M_3C carbide decomposition and dissolution with increasing aging time or aging temperature and theanti-diffusion of C and Cr cause the decrease and disappearance of the carbon-enriched zone. The results are differentfrom those of the A302/1Cr5Mo dissimilar welded joints in previous studies.

EFFECT OF CARBON MIGRATION ON CREEP PROPERTIES OF Cr5Mo DISSIMILAR WELDED JOINTS WITH Ni-BASED AND AUSTENITIC WELD METAL

In this paper, the effect of carbon migration on creep properties of Cr5Mo dissimilar welded joints with Ni-based (Inconel 182) and Cr23Nil3 (A302) austenitic weld metal was investigated. Carbon migration near the weld metal/ferritic steel interface of Cr5Mo dissimilar welded joints was analyzed by aging method. Local creep deformations of the dissimilar welded joints were measured by a long-term local creep deformation measuring technique. The creep rupture testing was performed for Cr5Mo dissimilar welded joints with Inconel 182 and A302 weld metal. The research results show that the maximum creep strain rate occurs in the decarburized zone located on heat affect zone (HAZ) of Cr5Mo ferritic steel. The creep rupture life of Cr5Mo dissimilar welded joints with A 302 weld metal decreases due to carbon migration and is about 50% of that welded with Inconel 182 weld metal.

Research on the creep damage and interfacial failure of dissimilar metal welded joint between 10Cr9Mo1VNbN and 12Cr1MoV steel

The mechanical properties, creep damage, creep rupture strength and features of interfacial failures of welded joints between martensite (SA213T91) and pearlite steel (12Cr1MoV) have been investigated by means of argon tungsten pulsed arc welding, high temperature accelerated simulation, creep rupture, mechanical property tests and scanning electronic microscope (SEM). The research results indicate that the mechanical properties of overmatched and medium matched joint deteriorate obviously, and they are susceptible to creep damage and failure after accelerated simulation operation 500 h, in the condition of preheat 250℃, and post welding heat treatment 750℃×1 h. However, the mechanical properties of undermatched joint are the best, the interfacial failure tendency of undermatched welded joint is less than those of medium and overmatched welded joint. Therefore, it is reasonable that low alloy material TR31 is used as the filler metal of weld between SA213T91and 12Cr1MoV steel.

Effect of tool pin profile on microstructure and tensile properties of friction stir welded dissimilar AA 6061e AA 5086 aluminium alloy joints

Joints between two different grades of aluminium alloys are need of the hour in many light weight military structures.In this investigation,an attempt has been made to join the heat treatable(AA 6061) and non-heat treatable(AA 5086) aluminium alloys by friction stir welding(FSW)process using three different tool pin profiles like straight cylindrical,taper cylindrical and threaded cylindrical.The microstructures of various regions were observed and analyzed by means of optical and scanning electron microscope.The tensile properties and microhardness were evaluated for the welded joint.From this investigation it is founded that the use of threaded pin profile of tool contributes to better flow of materials between two alloys and the generation of defect free stir zone.It also resulted in higher hardness values of 83 HV in the stir zone and higher tensile strength of 169 MPa compared to other two profiles.The increase in hardness is attributed to the formation of fine grains and intermetallics in the stir zone,and in addition,the reduced size of weaker regions,such as TMAZ and HAZ regions,results in higher tensile properties.

Influence of Creep Strength of Weld on Interfacial Creep Damage of Dissimilar Welded Joint between Martensitic and Bainitic Heat-Resistant Steel

The mechanical properties, creep rupture strength, creep damage and failure characteristics of dissimilar metal welded joint （DMWJ） between martensitic （SA213T91） and bainitic heat-resistant steel （12Cr2MoWVTiB（G102）） have been investigated by means of pulsed argon arc welding, high temperature accelerated simulation, mechanical and creep rupture test, and scanning electronic microscope （SEM）. The results show that there is a marked drop of mechanical properties of undermatching joint, and low ductility cracking along weld/G102 interface is induced due to creep damage. Creep rupture strength of overmatching joint is the least. The mechanical properties of medium matching joint are superior to those of overmatching and undermatching joint, and creep damage and failure tendency along the interface of weld/G102 are lower than those of overmatching and undermatching joint after accelerated simulation for 500 h, 1 000 h, 1 500 h, and the creep rupture strength of medium matching joint is the same as that of undermatching joint. Therefore, it is reasonable that the medium matching material is used for dissimilar welded joint between martensitic and bainitic steel.

Numerical Investigation on Fracture Initiation Properties of Interface Crack in Dissimilar Steel Welded Joints

Fracture toughness property is of significant importance when evaluating structural safety.The current research of fracture toughness mainly focused on crack in homogeneous material and experimental results.When the crack is located in a welded joint with high-gradient microstructure and mechanical property distribution,it becomes difficult to evaluate the fracture toughness behavior since the stress distribution may be affected by various factors.In recent years,numerical method has become an ideal approach to reveal the essence and mechanism of fracture toughness behavior.This study focuses on the crack initiation behavior and driving force at different interfaces in dissimilar steel welded joints.The stress and strain fields around the crack tip lying at the interfaces of ductile-ductile,ductile-brittle and brittle-brittle materials are analyzed by the numerical simulation.For the interface of ductile-ductile materials,the strain concentration on the softer material side is responsible for ductile fracture initiation.For the ductile-brittle interface,the shielding effect of the ductile material plays an important role in decreasing the fracture driving force on the brittle material side.In the case of brittle-brittle interface,a careful matching is required,because the strength mismatch decreases the fracture driving force in one side,whereas the driving force in another side is increased.The results are deemed to offer support for the safety assessment of welded structures.

Interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints during aging

The interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints with a nickel-based filler metal during aging was studied in detail to elucidate the mechanism of premature failures of this kind of joints.The results showed that not only a band of granular Cr_(23)C_(6)carbides were formed along the fusion boundary in the ferritic steel during aging,but also a large number of granular or plate-like Cr_(23)C_(6)carbides,which have a cube-cube orientation relationship with the matrix,were also precipitated on the weld metal side of the fu-sion boundary,making this zone be etched more easily than the other zone and become a dark etched band.Stacking faults were found in some Cr_(23)C_(6)carbides.In the as-welded state,deformation twins were observed in the weld metal with a fully austenitic structure.The peak micro-hardness was shifted from the ferritic steel side to the weld metal side of the fusion boundary after aging and the peak value increased signific-antly.Based on the experimental results,a mechanism of premature failures of the joints was proposed.

Numerical Simulation on Interfacial Creep Failure of Dissimilar Metal Welded Joint between HR3C and T91 Heat-Resistant Steel

The maximum principal stress, von Mises equivalent stress, equivalent creep strain, stress triaxiality in dissimilar metal welded joints between austenitic（HR3C） and martensitic heat-resistant steel（T91） are simulated by FEM at 873 K and under inner pressure of 42.26 MPa. The results show that the maximum principal stress and von Mises equivalent stress are quite high in the vicinity of weld/T91 interface, creep cavities are easy to form and expand in the weld/T91 interface. There are two peaks of equivalent creep strains in welded joint, and the maximum equivalent creep strain is in the place 27-32 mm away from the weld/T91 interface, and there exists creep constrain region in the vicinity of weld/T91 interface. The high stress triaxiality peak is located exactly at the weld/T91 interface. Accordingly, the weld/T91 interface is the weakest site of welded joint. Therefore, using stress triaxiality to describe creep cavity nucleation and expansion and crack development is reasonable for the dissimilar metal welded joint between austenitic and martensitic steel.

Effect of heat treatment temperature on dissimilar welded joint

The dissimilar metals 1Crl8Ni9 and 16MnR are welded by shielded metal arc welding process using electrode A312. The corrosion experiments are carried out on welded joint samples, which is as-welded and post-weld heat treatment at 650 ℃, 750 ℃ and 850 ℃, for 2 h in 70% sodium hydroxide solution. EDS and X-ray diffraction analysis are carried out on the samples after corrosion. Average corrosion rate calculation and microhardness measurement are conducted on both as- welded and post-weld heat treatment samples. The results indicate that average corrosion rate of as-welded joint metal is smaller than that of post-weld heat treatment joint metal. Compared with that of post-weld heat treatment at 750 ℃ and 850 ℃ for 2 h, the average corrosion rate of welded joint after post-weld heat treatment at 650 ℃ for 2 h increases greatly.

Study of fractal character of the fusion line in dissimilar welded joint

The fractal dimension of the fusion line in different dissimilar welded joints is measured with Box Dimension Method.The non scale region of the fusion line with fractal character is calculated. The fusion line in the dissimilar welded joint is proved to be a fractal structure. The change and influence factors of the fractal dimension of the fusion line are studied.

Creep Rupture of Mismatched Welded Joints of Steels with Dissimil ar Creep Strengths

The finite element analysis of mismatched welded jo ints with a 30°groove angle was performed to study the mechanical behavior of D MWJs (dissimilar metal welded joints). It is concluded that the distribution of stress triaxiality in the DMWJs is uneven, especially near the fusion lines. The degree of creep strength mismatch has remarkable effect on the distribution. Th e higher the level of mismatch is, the more uneven the distribution is and the e asier for premature failure to occur in the joint.

钛/铝异种金属扩散连接及其界面调控的研究进展

钛/铝复合构件具有轻量化、高强度、耐腐蚀等优势,使其广泛应用于新能源汽车、航空航天等领域。但是钛/铝之间的物理性能差异较大,且冶金反应容易生成如TiAl_(3),TiAl等脆性的金属间化合物,导致钛/铝异种金属接头性能下降。为了解决上述问题,基于扩散连接技术在异种金属连接方面变形小、无污染、冶金结合致密等优势,文中详细讨论了扩散连接技术在钛/铝异种金属连接上的应用,以及连接温度、保温时间、压力等焊接工艺参数、焊后热处理工艺、中间层对接头的影响。重点总结了工艺调控和添加中间层调控钛/铝接头的组织和性能。结果表明,工艺调控和添加中间层调控相结合可以最大限度的调控脆性金属间化合物的类型和降低金属间化合物的厚度。在此基础上,讨论了钛/铝异种金属扩散连接的发展趋势。

激光功率对铝/铜激光熔钎焊接头组织及性能的影响

采用激光熔钎焊方法对5052铝合金和T2紫铜进行对接试验,研究了激光功率(2000,2200,2400,2600,2800 W)对接头显微组织和力学性能的影响。结果表明:不同激光功率下接头铜侧钎焊区组织均以柱状晶为主,随着激光功率的增加,焊缝区树枝晶由细针状转变为雪花状;铜侧钎焊区由界面层和金属间化合物层组成,2000,2600 W激光功率下的界面层主要为AlCu相,金属间化合物层主要为Al_(4)Cu_(9)相,2400 W激光功率下的界面层主要由AlCu相和CuZn相组成,金属间化合物层主要为CuZn_(5)相;焊缝区主要由Al_(4.2)Cu_(3.2) Zn_(0.7)和Al_(0.71)Zn_(0.29)相组成。随着激光功率的增加,金属间化合物层厚度增加。激光功率对铜侧钎焊区的显微硬度影响较大,各接头的最高显微硬度均出现在铜侧钎焊区。随着激光功率的增加,接头最高硬度增加,抗拉强度先增大后减小;当激光功率为2400 W时,接头的抗拉强度最大,达到铝合金母材的92%,此时接头在靠近铝合金母材处断裂,而其他激光功率下制备的接头均在铜侧钎焊区和焊缝区处断裂。

Impact wear properties of dissimilar joints between bainitic frogs and pearlite rails

The impact wear behavior and damage mechanism of dissimilar welded joints between U26Mn frog and U75 V rail before and after normalizing treatment were studied by cyclic impact tests.The experiment indicated that the impact wear volume of the joints increased with the increasing number of impact cycles.The main wear mechanisms include pitting wear,mild fatigue wear,delamination wear,and fatigue wear,and plastic deformation was the primary impact wear mechanism.Among them,fatigue wear had the greatest influence on wear volume,while other wear mechanisms had limited effect.The impact wear resistance of the base material was better than that of the heat-affected zone.Normalizing treatment was beneficial to improving the impact wear resistance of welded joints owing to its effect to promote pearlite recovery,grain refinement,and uniform distribution of grains.The martensite generated in the rail welded joints aggravated the impact wear damage to the materials,which should be avoided.

超临界电站锅炉马氏体/奥氏体异种钢焊接接头失效模式分析

某350 MW超临界循环流化床机组T91+TP347HFG异种钢焊接接头发生开裂泄漏。本文采用宏观检查、化学成分分析、力学性能测试、硬度测定、金相组织形貌观察等方法对焊接接头试样进行研究,分析其开裂原因。试验结果表明:该异种钢焊接接头泄漏的原因是焊接质量不过关,且未按照要求进行焊后热处理工作。

焊后热处理对SA-182F1和SA-106C钢接头显微组织和力学性能的影响

采用钨极氩弧焊和焊条电弧焊焊接SA-182F1和SA-106C钢.对焊接接头进行了615℃保温8h的消除应力处理.检测了焊接接头热处理前、后的显微组织和力学性能.结果表明:热处理前、后焊接接头的拉伸和冲击性能均满足要求,消除应力处理使接头拉伸性能降低、冲击韧度提高;热处理前后焊接接头的显微组织没有差异.

反应堆压力容器接管安全端焊接残余应力模拟及其焊接参数优化研究

目的针对反应堆压力容器接管安全端焊接残余应力较大易导致应力腐蚀开裂的问题,探究焊接工艺参数对焊接残余应力的影响,并寻找最佳工艺参数。方法利用有限元参数化建模方法建立反应堆压力容器接管安全端的三维热-力耦合模型,模拟其焊接过程,研究焊接残余应力的变化情况。采用正交试验设计法分析了焊接电流、焊接速度及层间冷却时间对焊后最大等效应力的影响,建立了焊后最大等效应力与焊接电流、焊接速度及层间冷却时间的二次回归模型,基于该模型利用遗传算法寻优焊接参数。结果焊接残余应力峰值靠近熔合区位置,残余应力较高,超过了材料的屈服应力;各参数按对焊后等效残余应力峰值的影响由大到小的顺序依次为焊接速度、焊接电流、层间冷却时间。正交试验所得最佳工艺参数如下:焊接电流为610 A,焊接速度为20 mm/s,层间冷却时间为400 s,经遗传算法进一步优化后所得的最佳参数如下:焊接电流为610 A,焊接速度为23 mm/s,层间冷却时间为427 s。通过仿真验证遗传算法优化结果,得到焊接残余应力的峰值为373 MPa,比未优化前减小了44 MPa。结论优化后的工艺参数有效降低了焊接残余应力,提高了反应堆压力容器接管安全端服役过程中的安全性,该研究方法可为实际焊接生产提供一定的参考。

T91+TP347H异种钢焊接接头失效分析

某电厂超临界锅炉运行中发现高温再热器出口T91/TP347H异种钢焊接接头发生断裂致使锅炉泄漏。对失效焊接接头进行宏观形貌观察、化学成分分析、金相检验、硬度测试等一系列检验,结合该锅炉的运行情况和历史故障情况,综合分析该锅炉异种钢焊接接头的失效原因。结果显示,焊缝成形差、焊接质量不合格是导致出现接头开裂的直接原因;异种钢接头本身化学成分差异大,性能不均匀,在长期的高温和应力作用下发生元素迁移,造成交界面性能下降,这是焊接接头出现开裂的根本原因;机组的深度调峰、频繁启停调峰则进一步加速了裂纹的产生。经过失效原因分析,并借鉴异种钢焊接接头的管理标准及实践经验,建议加强焊接质量控制和优化金属监督策略,提高无损检测的抽样率,并注意控制机组调峰速率。可为同类型电厂异种钢焊接接头管理提供参考。

桥式起重机箱形梁异种钢焊接接头疲劳损伤的声发射表征

文中通过声发射技术对桥式起重机箱形梁Q345B/Q235B异种钢焊接接头疲劳损伤过程进行表征,发现了声发射特征参数变化取决于疲劳裂纹扩展不同阶段的特点,得以利用声发射动态监测桥式起重机循环载荷下的疲劳损伤状态,从而实现及早预警的目的。另外,通过对异种钢焊接接头焊缝和热影响区2种不同缺陷位置疲劳实验各阶段典型声发射信号的对比分析,获知热影响区比焊缝位置更易发生疲劳断裂,故箱形主梁在服役过程中应重点关注异种钢焊接接头热影响区位置并及早发现危险。