Numerical Optimization by Finite Element Method of Stainless Steel/Glass-Epoxy Composite Bolted Joint under Tension and Compression

The aim of this study was to optimize the geometry and the design of metallic/composite single bolted joints subjected to tension-compression loading. For this purpose, it was necessary to evaluate the stress state in each component of the bolted join. The multi-material assembly was based on the principle of double lap bolted joint. It was composed of a symmetrical balanced woven glass-epoxy composite material plate fastened to two stainless sheets using a stainless pre-stressed bolt. In order to optimize the design and the geometry of the assembly, ten configurations were proposed and studied: a classical simple bolted joint, two joints with an insert (a BigHeadR insert and a stair one) embedded in the composite, two “waved” solutions, three symmetrical configurations composed of a succession of metallic and composites layers, without a sleeve, with one and with two sleeves, and two non-symmetrical constituted of metallic and composites layers associated with a stair-insert (one with a sleeve and one without). A tridimensional Finite Element Method (FEM) was used to model each configuration mentioned above. The FE models taked into account the different materials, the effects of contact between the different sheets of the assembly and the pre-stress in the bolt. The stress state was analyzed in the composite part. The concept of stress concentration factor was used in order to evaluate the stress increase in the highly stressed regions and to compare the ten configurations studied. For this purpose, three stress concentration factors were defined: one for a monotonic loading in tension, another for a monotonic loading in compression, and the third for a tension-compression cyclic loading. The results of the FEM computations showed that the use of alternative metallic and composite layers associated with two sleeves gived low values of stress concentration factors, smaller than 1.4. In this case, there was no contact between the bolt and the composite part and the most stressed region was not the vicinity of the hole but the end of the longest layers of the metallic inserts.

Microstructure,Corrosion and Mechanical Properties of Medium-Thick 6061-T6 Alloy/T2 Pure Cu Dissimilar Joints Produced by Double Side Friction Stir Z Shape Lap-Butt Welding

A novel double side friction stir Z shape lap-butt welding(DS-FSZW)process was proposed to achieve excellent mechanical properties of Al/Cu medium-thick dissimilar joints.The influence of welding parameters on weld microstructure and properties of DS-FSZW joint were systematically investigated.It indicated that defect-free medium-thick Al/Cu DS-FSZW joint could be achieved under an optimal welding parameter.DS-FSZW joint was prone to form void defects in the bottom of the second-pass weld.The recrystallization mechanisms at the top and middle of the weld nugget zone(WNZ)were continuous dynamic recrystallization(CDRX)and geometric dynamic recrystallization(GDRX).While the major recrystallization mechanism at the bottom of the WNZ was GDRX.DS-FSZW joint of the optimal welding condition with 850 r/min-400 mm/min was produced with a continuous thin and crack-free IMCs layer at the Al/Cu interface,and the maximum tensile strength of this joint is 160.57 MPa,which is equivalent to 65.54%of pure Cu base material.Moreover,the corrosion resistance of Al/Cu DS-FSZW joints also achieved its maximum value at the optimal welding parameter of 850 r/min-400 mm/min.It demonstrates that the DS-FSZW process can simultaneously produce medium-thick Al/Cu joints with excellent mechanical performance and corrosion resistance.

Experimental study on the shear performance of quasi-NPR steel bolted rock joints

Quasi-NPR(negative Poisson’s ratio)steel is a new type of super bolt material with high strength,high ductility,and a micro-negative Poisson’s effect.This material overcomes the contrasting characteristics of the high strength and high ductility of steel and it has significant energy-absorbing characteristics,which is of high value in deep rock and soil support engineering.However,research on the shear resistance of quasi-NPR steel has not been carried out.To study the shear performance of quasi-NPR steel bolted rock joints,indoor shear tests of bolted rock joints under different normal stress conditions were carried out.Q235 steel and#45 steel,two representative ordinary bolt steels,were set up as a control group for comparative tests to compare and analyze the shear strength,deformation and instability mode,shear energy absorption characteristics,and bolting contribution of different types of bolts.The results show that the jointed rock masses without bolt reinforcement undergo brittle failure under shear load,while the bolted jointed rock masses show obvious ductile failure characteristics.The shear deformation ca-pacity of quasi-NPR steel is more than 3.5 times that of Q235 steel and#45 steel.No fracture occurs in the quasi-NPR steel during large shear deformation and it can provide stable shear resistance.However,the other two types of control bolts become fractured under the same conditions.Quasi-NPR steel has significant energy-absorbing characteristics under shear load and has obvious advantages in terms of absorbing the energy released by shear deformation of jointed rock masses as compared with ordinary steel.In particular,the shear force plays a major role in resisting the shear deformation of Q235 steel and#45 steel,therefore,fracture failure occurs under small bolt deformation.However,the axial force of quasi-NPR steel can be fully exerted when resisting joint shear deformation;the steel itself does not break when large shear deformation occurs,and the supporting effect of the jointed rock mass is effectively guaranteed.

Study on Key Joining Technology and Test Method of Steel/Al Hybrid Structure Body-in-White

Green and low carbon promote the application and development of light-weight materials in body-in-white. Large-scale die-casting Al alloy (DCAA) and high-strength thermo-formed steel sheet (TFSS) have put forward higher requirements for the application of joining technology of high-strength steel/Al dissimilar materials. Taking the new die-casting Al alloy body as an example, this paper systematically studies the progress of the latest joining methods of steel/Al dissimilar material with combination of two-layer plate and three-layer plate. By analyzing the joining technologies such as FSPR, RES, FDS and SPR, the technology and process characteristics of steel/Al dissimilar material joining are studied, and the joining technical feasibility and realization means of different material combination of the body are analyzed. The conditions of material combination, material thickness, material strength, flange height, preformed holes and joint spacing for achieving high-quality joining are given. The FSPR joining technology is developed and tested in order to meet with the joining of parts with DCAA and TFSS, especially for the joining of three-layer plates with them. It finds the method and technical basis for the realization of high quality joining of dissimilar materials, provides the early conditions for the application of large DCAA and TFSS parts in body-in-white, and meets the design requirements of new energy body. .

Weld Joint Efficiency of the Kazakhstani Constructional Steel

In the Republic of Kazakhstan, the regulatory framework in construction based on Eurocodes has been in force since 2015. However, Kazakhstani produced steel has not been studied for compliance with the requirements of Eurocode 1993. This has resulted in limited use of Kazakhstani structural steel in construction. The feasibility of using structural steel in welded joints has been experimentally investigated. To verify the application of such joints in construction, including earthquake engineering, experimental studies of welded joints made of structural steel produced by Arcelor-Mittal in Temirtau have been carried out. In total, 7 types of structural steel of various thicknesses were selected. Five specimens have been used in each series of tests. The Brinell hardness values of the weld joint, yield strength of steel and tensile strength, relative rupture strain were determined. It was found that for all types of structural steel, the quality of weld joints complied with the requirements of Eurocode 1993—a sample rupture appeared along the plates (main body of the metal), not along the weld joints. It has been established that structural steel produced in the Republic of Kazakhstan fully complies with the requirements of Eurocode 1993. The studies on the dependence of Brinell hardness values of weld joint steel on the yield strength, tensile strength and relative rupture strain have been carried out. The correlation dependences between the values of yield strength of steel and tensile strength, relative rupture strain and BH Brinell hardness were studied. The results of work will allow for significantly increasing the use of Kazakhstani structural steel in seismic and conventional areas of the Republic of Kazakhstan.

Mn25Al7钢焊接接头的疲劳性能试验研究

Mn25Al7钢是一种新型的轻质高强船用钢,目前针对Mn25Al7钢的疲劳性能尚未开展相关试验研究。本文对Mn25Al7钢的母材、对接焊接接头及T型焊接接头等典型节点进行疲劳试验研究,分别基于名义应力方法和热点应力方法拟合得到典型节点疲劳等级曲线,并与现有规范的疲劳等级曲线进行对比分析。对三种试样的断口形貌进行观测,分析其裂纹扩展规律和断裂机理。试验结果表明:Mn25Al7钢母材疲劳寿命高于按规范设计的普通钢材;规范低估了母材和磨平焊趾的对接焊缝接头疲劳寿命,但能较好地评估T型焊缝接头的疲劳寿命。通过对比分析三种试件的损伤速率差别,进一步对断口形貌进行了系统分析,发现初始裂纹源和焊接残留物会造成疲劳强度的降低。本研究可为船用高强钢的疲劳寿命预测提供理论依据和试验支撑。

不锈钢表面Al/Cu涂层制备与第一性原理计算

为了研究Al/Cu涂层中金属间化合物的生成顺序和形成机理,在304不锈钢基体表面制备了Al/Cu涂层。对试样进行热处理使得Al/Cu涂层原位反应生成金属间化合物,随后测试了涂层的高温抗氧化性能。采用第一性原理计算Al-Cu金属间化合物的焓、熵、吉布斯自由能和热容等数据。将热力学和扩散动力学相结合,提出有效生成自由能模型来预测化合物在Al/Cu界面的生成顺序。结果表明:Al/Cu涂层中首先形成Al_(2)Cu相,在Al/Cu界面处出现了AlCu相,加热时间增加至20 h后出现了富Cu的Al_(4)Cu_(9)相。由第一性原理计算得出,Al-Cu金属间化合物的形成顺序为Al_(2)Cu→AlCu→Al_(4)Cu_(9),与实验结果一致。此外,氧化实验结果表明,涂层具有良好高温抗氧化性能。

铝脱氧钢中Al_(2)O_(3)夹杂物与Al_(2)O_(3)-C质水口耐材的粘附机理研究

铝脱氧钢连铸过程中的水口堵塞一直是困扰生产的难题,弄清水口堵塞的形成机理对解决该问题非常重要。研究首先在实验室制备了含有单一、量多的Al_(2)O_(3)夹杂物的钢液,然后与Al_(2)O_(3)-C质耐材棒反应不同时间,研究了钢中Al_(2)O_(3)夹杂物与耐材棒的粘附行为,在此基础上探讨了铝脱氧钢连铸过程水口堵塞的形成机理。研究发现,耐材棒与钢液反应一段时间后,在耐材棒表面由内向外逐渐形成两层Al_(2)O_(3),第1层致密平滑,厚度较薄;第2层松散粗糙,厚度较厚。水口堵塞的形成机理为,耐材插入钢液一段时间内,因温度升高,耐材表面区域的SiO_(2)、Al_(2)O_(3)与C反应生成的SiO、Al_(2)O气体向钢液扩散,在耐材表面分别与钢液中的[Al]、耐材中的SiO_(2)反应生成Al_(2)O_(3);耐材棒表面的SiO_(2)也会与钢液中的[Al]反应生成Al_(2)O_(3);三者共同作用形成第1层致密的Al_(2)O_(3)。第1层Al_(2)O_(3)层形成后,钢液中的Al_(2)O_(3)夹杂物易于在其上面附着,形成第2层松散的Al_(2)O_(3)。随着时间的延长,Al_(2)O_(3)层变厚,水口堵塞逐渐形成。第1层Al_(2)O_(3)形成机理以化学反应为主,增长缓慢;第2层以钢中夹杂物粘附为主,增长较快。

Hot deformation behavior of Fe–27.34Mn–8.63Al–1.03C lightweight steel

Hot compression tests were performed to investigate the hot deformation behavior of Fe–27.34Mn–8.63Al–1.03C lightweight steel and optimize the hot workability parameters. The temperature range was 900–1150℃ and the strain rate range was 0.01–5 s^(-1)on a Gleeble-3800 thermal simulator machine. The results showed that the flow stress increased with decreasing deformation temperature and increasing strain rate. According to the constitutive equation, the activation energy of hot deformation was 422.88 kJ·mol^(-1). The relationship between the critical stress and peak stress of the tested steel was established, and a dynamic recrystallization kinetic model was thus obtained. Based on this model, the effects of strain rate and deformation temperature on the volume fraction of dynamically recrystallized grains were explored. The microstructural examination and processing map results revealed that the tested steel exhibited a good hot workability at deformation temperatures of 1010–1100℃ and strain rate of 0.01 s^(-1).

An innovative joint interface design for reducing intermetallic compounds and improving joint strength of thick plate friction stir welded Al/Mg joints

Friction stir welding of dissimilar Al/Mg thick plates still faces severe challenges, such as poor formability, formation of thick intermetallic compounds, and low joint strength. In this work, two joint configurations, namely inclined butt(conventional butt) and serrated interlocking(innovative butt), are proposed for improving weld formation and joint quality. The results show that a continuous and straight intermetallic compound layer appears at the Mg side interface in conventional butt joint, and the maximum average thickness reaches about 60.1 μm.Additionally, the Mg side interface also partially melts, forming a eutectic structure composed of Mg solid solution and Al_(12)Mg_(17) phase.For the innovative butt joint, the Mg side interface presents the curved interlocking feature, and intermetallic compounds can be reduced to less than 10 μm. The joint strength of innovative butt joint is more than three times that of conventional butt joint. This is due to the interlocking effect and thin intermetallic compounds in the innovative joint.

Evaluation of microstructure and mechanical properties of squeeze overcast Al7075-Cu composite joints

Al7075-Cu composite joints were prepared by the squeeze overcast process.The effects of melt temperature,die temperature,and squeeze pressure on hardness and ultimate tensile strength(UTS)of squeeze overcast Al7075-Cu composite joints were studied.The experimental results depict that squeeze pressure is the most significant process parameter affecting the hardness and UTS.The optimal values of UTS(48 MPa)and hardness(76 HRB)are achieved at a melt temperature of 800℃,a die temperature of 250℃,and a squeeze pressure of 90 MPa.Scanning electron microscopy(SEM)shows that fractured surfaces show flatfaced morphology at the optimal experimental condition.Energy-dispersive spectroscopy(EDS)analysis depicts that the atomic weight percentage of Zn decreases with an increase in melt temperature and squeeze pressure.The optimal mechanical properties of the Al7075-Cu overcast joint were achieved at the Al2Cu eutectic phase due to the large number of copper atoms that dispersed into the aluminum melt during the solidification process and the formation of strong intermetallic bonds.Gray relational analysis integrated with the Taguchi method was used to develop an optimal set of control variables for multi-response parametric optimization.Confirmatory tests were performed to validate the effectiveness of the employed technique.The manufacturing of squeeze overcast Al7075-Cu composite joints at optimal process parameters delivers a great indication to acknowledge a new method for foundry practitioners to manufacture materials with superior mechanical properties.

Electrochemical impedance spectroscopy study on the corrosion of the weld zone of 3Cr steel welded joints in CO_2 environments

The welded joints of 3Cr pipeline steel were fabricated with commercial welding wire using the gas tungsten arc welding （GTAW） technique. Potentiodynamic polarization curves, linear polarization resistance （LPR）, electrochemical impedance spectroscopy （EIS）, scan- ning electron microscopy （SEM）, and energy-dispersive spectrometry （EDS） were used to investigate the corrosion resistance and the growth of a corrosion film on the weld zone （WZ）. The changes in electrochemical characteristics of the film were obtained through fitting of the EIS data. The results showed that the average corrosion rate of the WZ in CO2 environments first increased, then fluctuated, and finally de- creased gradually. The formation of the film on the WZ was divided into three stages： dynamic adsorption, incomplete-coverage layer forma- tion, and integral layer formation.

Multiaxial Fatigue Analyses of Stress Joints for Deepwater Steel Catenary Risers

In the present study, the dynamic and fatigue characteristics of two types of stress joints are investigated under ocean environmental condition. Connected with the riser and the platform, stress joint at the vessel hang-off position should be one of the main critical design challenges for a steel catenary riser （SCR） in deepwater. When the riser is under a high pressure and deepwater working condition, the stress state for the joint is more complex, and the fatigue damage is easy to occur at this position. Stress joint discussed in this paper includes two types： Tapered Stress Joint （TSJ） and Sleeved Stress Joint （SSJ）, and multiaxial fatigue analysis results are given for comparison. Global dynamic analysis for an SCR is performed first, and then the local boundary conditions obtained from the previous analysis are applied to the stress joint FE model for the later dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than the yield limit of material and the damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs： for TSJ, the place where the stress joint connects with the riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of the inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. In consideration of various factors, designers should choose the most suitable type and also geometric parameters.

Interfacial characterization of resistance spot welded joint of steel and aluminum alloy

The dissimilar material resistance spot welding of galvanized high strength steel and aluminum alloy had been conducted. The welded joint exhibited a thin reaction layer composed of Fe2Al5 and Fe4Al13 phases at steel/aluminum interface. The welded joint presented a tensile shear load of 3.3 kN with an aluminum alloy nugget diameter of 5.7 mm. The interfacial failure mode was observed for the tensile shear specimen and fracture occurred at reaction layer and aluminum alloy fusion zone beside the interface. The reaction layer with compounds was the main reason for reduction of the welded joint mechanical property.

Dissimilar welding of high nitrogen stainless steel and low alloy high strength steel under different shielding gas composition:Process,microstructure and mechanical properties

Ar-N_(2)-O_(2)ternary shielding gas is employed in dissimilar welding between high nitrogen steel and low alloy steel.The effect of O_(2)and N_(2)is investigated based on the systematical analysis of the metal transfer,nitrogen escape phenomenon,weld appearance,nondestructive detection,nitrogen content distribution,microstructure and mechanical properties.There are two nitrogen sources of the nitrogen in the weld:high nitrogen base material and shielding gas.The effect of shielding gas is mainly reflected in these two aspects.The change of the droplet transfer mode affects the fusion ratio,N2in the shielding gas can increase nitrogen content and promote the nitrogen uniform distribution.The addition of 2%O_(2)to Ar matrix can change the metal transfer from globular transfer to spray transfer,high nitrogen base material is thereby dissolved more to the molten pool,making nitrogen content increase,ferrite decrease and the mechanical properties improve.When applying N2-containing shielding gas,arc stability becomes poor and short-circuiting transfer frequency increases due to the nitrogen escape from droplets and the molten pool.Performance of the joints is improved with N_(2)increasing,but internal gas pores are easier to appear because of the poor capacity of low alloy steel to dissolve nitrogen,The generation of pores will greatly reduce the impact resistance.4-8%N2content in shielding gas is recommended in this study considering the integrated properties of the dissimilar welded joint.

Microstructure and mechanical property of resistance spot welded joint of aluminum alloy to high strength steel with especial electrodes

Dissimilar material joining of 6008 aluminum alloy to H220 YD galvanized high strength steel was performed by resistance spot welding with especial electrodes that were a flat tip electrode against the steel surface and a domed tip electrode upon the aluminum alloy surface. An intermetallic compound layer composed of Fe2Al5 and FeAl3 was formed at the steel/ aluminum interface in the welded joint. The thickness of the intermetallic compound layer increased with increasing welding current and welding time, and the maximum thickness being 7. 0 μm was obtained at 25 kA and 300 ms. The weld nugget diameter and tensile shear load of the welded joint had increased tendencies first with increasing welding current （ 18 -22 kA） and welding time （ 50 - 300 ms）, then changed little with further increasing welding current （ 22 - 25 kA） and welding time （300 -400 ms）. The maximum tensile shear load reached 5.4 kN at 22 kA and 300 ms. The welded joint fractured through brittle intermetallic compound layer and aluminum alloy nugget.

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.

Experimental Study on Seismic Performance of SRUHSC⁃RC Frame Exterior Joints with Cyclic Loads

SRUHSC⁃RC(Steel Reinforced Ultra High Strength Concrete⁃Reinforced Concrete)structures are used extensively in super high⁃rise buildings.However,it has obvious brittleness.Wrapping structural steel inside is a good way to alleviate the brittleness problem.The purpose of this study is to investigate seismic behavior of SRUHSC⁃RC exterior joints on the basis of experimental results of eleven specimens under the reversed cyclic loading.The relationship among the force⁃displacement curve,ductility,energy dissipation property,strength degradation,stiffness degradation,the strains of steel bone and stirrup at the core area of the joint were analyzed based on the test results.It is shown that axial compression ratio and stirrup volumetric ratio have significant effects on the shear behavior of SRUHSC⁃RC frame exterior joints.The general requirement on the axial⁃load level and the amount of confinement stirrup for the joints in the design is suggested.

Al-Si镀层22MnB5激光焊的熔池流动特性及元素分布

熔池流动行为对镀层元素在焊缝中的分布起决定性作用.采用建立激光焊熔池三维瞬态数值模型的方法,分析AlSi镀层22MnB5高强度钢激光焊的熔池流动行为及其对焊缝中Al元素分布的影响规律.在焊接试验中,根据能谱仪对焊接接头的扫描数据验证数值模型的可靠性.结合模拟结果与试验结果对激光焊熔池流动行为及焊缝Al元素分布规律进行研究.结果表明,根据激光焊热输入与功率密度阈值的不同,熔池形貌呈现无匙孔、未完全贯穿匙孔和完全贯穿匙孔3种特征,并存有不同的温度场、流场、流速和稳定性.无匙孔熔池由于流速分布均匀,涡流产生较少,稳定性最好.镀层Al元素在焊接接头上下焊趾区域易产生偏聚,较之上表面镀层,下表面镀层进入熔池后Al元素偏聚现象更明显.因此,上部Al元素均质化分布显著优于下部.

Al含量对含铝奥氏体不锈钢在高温超临界二氧化碳中均匀腐蚀性能的影响研究

为进一步提升奥氏体不锈钢作为超临界二氧化碳核反应堆候选包壳材料的耐腐蚀性能,对比研究了3种不同Al含量的含铝奥氏体不锈钢及不含Al基材在650℃/20 MPa的超临界二氧化碳环境中的均匀腐蚀行为。结果表明,材料的腐蚀增重随Al含量增加而降低,不同Al含量材料的腐蚀增重均近似服从抛物线生长规律。Al含量低于1.5wt%时,材料表面生成双层富Fe氧化膜,保护性差,渗碳层厚度可达约12μm;Al含量高于2.5wt%时,材料表面生成保护性氧化膜,外层富Cr、内层富Al,氧化膜及基体中仍存在渗碳行为,渗碳层厚度减小至约6μm。造成差异的原因是较高Al含量能有效促进保护性富Al氧化膜的形成,抑制Fe的向外扩散和C的向内扩散,进而提升材料的耐氧化和渗碳属性。