Mechanical strength and corrosion resistance of Al-additive friction stir welded AZ31B joints

Compared to other structural alloys,magnesium alloys have a relatively poor corrosion resistance and low mechanical strength,which can be further deteriorated when these alloys are subjected to joining processes using the existing joining methods.Herein,we propose for the first time an additive friction stir-welding(AFSW)using fine Al powder as an additive to improve the mechanical strength as well as corrosion resistance of AZ31B weld joints.AFSW is a solid-state welding method of forming a high-Al AZ31B joint via an in-situ reaction between pure Al powders filled in a machined groove and the AZ31B matrix.To optimize the process parameters,AFSW was performed under different rotational and transverse speeds,and number of passes,using tools with a square or screw pin.In particular,to fabricate a weld zone,where the Al was homogenously dispersed,the effects of the groove shape were investigated using three types of grooves:surface one-line groove,surface-symmetric grooves,and inserted symmetric grooves.The homogenous and defect-less AFS-welded AZ31B joint was successfully fabricated with the following optimal parameters:1400 rpm,25 mm/min,four passes,inserted symmetric grooves,and the tool with a square pin.The AFSW fully dissolved the additive Al intoα-Mg and in-situ precipitated Mg_(17)Al_(12)particles,which was confirmed via scanning electron microscopy,transmission electron microscope,and X-ray diffraction analyses.The microhardness,joint efficiency,and elongation at the fracture point of the AFS-welded AZ31B joint were 80 HV,101%,and 8.9%,respectively.These values are higher than those obtained for the FS-welded AZ31 joint in previous studies.The corrosion resistance of the AFS-welded AZ31B joint,evaluated via hydrogen evolution measurements and potentiodynamic polarization tests,was enhanced to 55%relative to the FS-welded AZ31B joint.

Fatigue life evaluation of girth butt weld within welded cast steel joints based on the extrapolation notch stress method

The fatigue life evaluation of the girth butt weld within the welded cast steel joint was studied based on the extrapolation notch stress method.Firstly,the mesh sensitivity of the finite element model of the welded cast steel joint was analyzed to determine the optimal mesh size.Based on the stress field analysis of the finite element model of the welded cast steel joint at the weld toe and weld root,the sharp model of the extrapolation notch stress method was applied to derive the effective notch stress of the rounded model belonging to the effective notch stress method,in which the key problem is to calculate the extrapolation point C,and the extrapolation point C has an exponential function relationship with the geometric parameters of the welded cast steel joint.By setting different values of geometric parameters,the corresponding value of parameter C is calculated,and then the functional relationship between the extrapolation point C and the geometric parameters can be obtained by the multiple linear regression analysis.Meanwhile,the fatigue life evaluation of the girth butt weld within welded cast steel joints based on the effective notch stress was performed according to the guideline recommended by the IIW(International Institute of Welding).The results indicate that the extrapolation notch stress method can effectively simplify the process of calculating the effective notch stress and accurately evaluate the fatigue life of the girth butt weld within welded cast steel joints.

Effects of temperature on fracture behavior of Al-based in-situ composites reinforced with Mg_2Si and Si particles fabricated by centrifugal casting

An aluminum-based in-situ composites reinforced with Mg2Si and Si particles were produced by centrifugal casting A1-20Si-5Mg alloy. The microstructure of the composites was examined, and the effects of temperature on fracture behavior of the composite were investigated. The results show that the average fraction of primary Si and Mg2Si particles in the composites is as high as 38%, and ultimate tensile strengths （UTS） of the composites first increase then decrease with the increase of test temperature. Microstructures of broken specimens show that both the particle fracture and the interface debonding affect the fracture behavior of the composites, and the interface debonding becomes the dominant fracture mechanism with increasing test temperature. Comparative results indicate that rich particles in the composites and excellent interface strength play great roles in enhancing tensile property by preventing the movement of dislocations.

Experimental study on the effect of flexible joints of a deep-buried tunnel across an active fault under high in-situ stress conditions

During dislocation,a tunnel crossing the active fault will be damaged to varying degrees due to its permanent stratum displacement.Most previous studies did not consider the influence of the tunnel’s deep burial and the high in-situ stress,so the results were not entirely practical.In this paper,the necessity of solving the anti-dislocation problem of deep-buried tunnels is systemically discussed.Through the model test of tunnels across active faults,the differences in failures between deep-buried tunnels and shallow-buried tunnels were compared,and the dislocation test of deep-buried segmental tunnels was carried out to analyze the external stress change,lining strain,and failure mode of tunnels.The results are as follows.(1)The overall deformation of deep-buried and shallow-buried tunnels is both Sshaped.The failure mode of deep-buried tunnels is primarily characterized by shear and tensile failure,resulting in significant compressive deformation and a larger damaged area.In contrast,shallow-buried tunnels mainly experience shear failure,with the tunnel being sheared apart at the fault crossing,leading to more severe damage.(2)After the segmental structure design of the deep-buried tunnel,the‘‘S”deformation pattern is transformed into a‘‘ladder”pattern,and the strain of the tunnel and the peak stress of the external rock mass are reduced;therefore,damages are significantly mitigated.(3)Through the analysis of the distribution of cracks in the tunnel lining,it is found that the tunnel without a segmental structure design has suffered from penetrating failure and that cracks affect the entire lining.The cracks in a flexible segmental tunnel affect about 66.6%of the entire length of the tunnel,and cracks in a tunnel with a short segmental tunnel only affect about 33.3%of the entire length of the tunnel.Therefore,a deep-buried tunnel with a short segmental tunnel can yield a better anti-dislocation effect.(4)By comparing the shallow-buried segmental tunnel in previous studies,it is concluded that the shallow-buried segmental tunnel will also suffer from deformation outside the fault zone,while the damages to the deep-buried segmental tunnel are concentrated in the fault zone,so the anti-dislocation protection measures of the deep-buried tunnel shall be provided mainly in the fault zone.The results of the above study can provide theoretical reference and technical support for the design and reinforcement measures of the tunnel crossing active fault under high in-situ stress conditions.

Corrosion behavior of the friction-stir-welded joints of 2A14-T6 aluminum alloy

The corrosion behavior of friction-stir-welded 2A14-T6 aluminum alloy was investigated by immersion testing in immersion exfoliation corrosion（EXCO） solution. Electrochemical measurements（open circuit potential, potentiodynamic polarization curves, and electrochemical impedance spectroscopy）, scanning electron microscopy, and energy dispersive spectroscopy were employed for analyzing the corrosion mechanism. The results show that, compared to the base material, the corrosion resistance of the friction-stir welds is greatly improved, and the weld nugget has the highest corrosion resistance. The pitting susceptibility originates from the edge of Al-Cu-Fe-Mn-Si phase particles as the cathode compared to the matrix due to their high self-corrosion potential. No corrosion activity is observed around the θ phase（Al2Cu） after 2 h of immersion in EXCO solution.

In-situ observation of porosity formation during directional solidification of Al-Si casting alloys

In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.

Heat transfer behavior of AZ80-1%Y alloy during low-frequency electromagnetic casting

Heat transfer behaviors of AZ80?1%Y alloy during low frequency electromagnetic casting (LFEC) and direct chilling casting were studied by in-situ temperature measurement. The results demonstrated that the low frequency electromagnetic field (EM) caused forced convection in the melt during LFEC. The forced convection led to uniform solidification velocity and temperature field. EM frequency, excitation current intensity and casting temperature could control the heat transfer behavior. The forced convection could improve the microstructure and degrade the difference in microstructure between the edge and center of billet. Appropriate parameters of low frequency EM for casting Mg alloy are 20 Hz of frequency and 60 A of electric current intensity.

大跨度钢桁架铸钢节点逆作施工力学性能分析与安装技术

针对北京朝阳站的异形铸钢节点及V形铸钢节点开展结构逆作施工过程中节点安全性能和力学性能研究。首先选取两个关键施工阶段,使用ABAQUS有限元软件分析铸钢节点关键区域的应力状态及变形特征,分析评价其受力性能和安全性能;其次以位移为控制条件分析两种铸钢节点的极限承载能力以及破坏模式,并进行参数分析研究壁厚和倒角半径对铸钢节点受力性能的影响;最后探讨了铸钢件和梭形斜柱逆作施工安装技术。结果表明:北京朝阳站大跨度鱼腹式钢桁架结构铸钢节点在施工阶段具有充足的强度储备;铸钢节点的极限承载能力满足规范要求;逆作施工安装技术为北京朝阳站大跨度复杂管桁架屋盖工程的顺利施工提供了保障。

援所罗门群岛2023年太平洋运动会体育场铸钢节点受力性能试验研究

援所罗门群岛2023年太平洋运动会体育场钢结构罩棚采用了树形支撑铸钢节点,为考察其在最不利设计工况下的受力性能,对铸钢节点进行了足尺试验研究和有限元分析。试验结果表明,在1.0倍受拉工况设计荷载及1.6倍受压工况设计荷载作用下,节点处于弹性阶段,卸载后没有明显残余变形。有限元分析结果与试验结果吻合良好,表明有限元分析可以准确模拟节点试件的实际受力状态。铸钢节点的极限承载力在受拉工况下为设计荷载的9.0倍,受压工况下为设计荷载的8.6倍,满足《铸钢节点应用技术规程》(CECS 235:2008)中的相关要求,该铸钢节点具有较高的安全储备。

四辊轧机工作辊辊颈端部铸接修复部位寿命评估

轧辊作为带钢生产轧机的核心部件,其消耗对生产成本有重要影响。随着钢铁企业竞争加剧导致的降本增效需求逐步提高,轧辊再制造循环利用得到逐步推广。大型轧机大尺寸轧辊工作层消耗后改造用于小轧机为其中主要方式之一;对于局部尺寸尤其是辊肩尺寸不足的轧辊通常采用铸接方式加长以满足进一步使用的需求;铸接再制造后轧辊的使用寿命成为保障安全生产的关键。针对上述问题,在轧辊铸接部位取样开展了疲劳性能测试,同时选用扭转疲劳模型在证明计算精度的基础上,结合基于现场生产数据建立的载荷谱,对工作辊修复后的辊颈进行了寿命预测,预期服役寿命满足现场对修复后轧辊的性能需求,可为现场轧辊安全使用维护制度建立提供理论支撑。

绿塘整体浇筑堆石混凝土拱坝施工期温度仿真研究

【目的】绿塘大坝是一座不分横缝、整体浇筑的堆石混凝土拱坝,旨在解决坝体分缝过多而带来的施工干扰大、速度慢以及堆石率低等问题。为深入探究这种新拱坝型式的温度场分布特征与应力变化规律,得到堆石混凝土坝体不分缝或少分缝的理论依据,【方法】基于实测资料开展有限元温度仿真计算,考虑上游防渗层及预制混凝土块模板、混凝土水化反应及弹性模量增长、徐变等,采用结构多场仿真与非线性分析软件SAPTIS模拟整体浇筑拱坝的分层拱圈浇筑全过程,并分析坝体温度场与应力分布规律。【结果】结果显示:绿塘大坝施工期的混凝土水化温升较低,约3~10℃,部分高温月份浇筑的混凝土温升后能达到38.6℃;施工期拱向拉应力普遍小于0.8 MPa,仅上游自密实混凝土防渗层的局部应力达到1.5 MPa,预制混凝土块模板起到辅助吸热和力学约束作用;横缝对拱坝上游面的应力改善较为明显。【结论】结果表明,堆石混凝土拱坝采用不分横缝、全断面整体上升浇筑的型式是可行的,但建议上游防渗层设置短缝和高温季节采取合适的温控措施,该坝型对促进快速筑坝技术发展具有重要意义。

基于ProCAST的密接式车钩U型接头铸造工艺优化

分析了U型接头铸件原工艺产生缩孔、缩松及浇不足、冷隔等铸造缺陷的原因,通过ProCAST软件对铸造工艺进行分析和优化,采取对铸件的浇注位置,分型面,生产工艺流程等工艺进行改进,使铸件浇不足、冷隔、缩孔、缩松等缺陷减少,提高铸件成品率和表面品质,提高生产效率,使铸件的总废品率降到了5%以下,取得了明显的经济效益。

模拟海水环境下G20Mn5QT/Q355异种钢对接焊接接头腐蚀疲劳性能试验研究

金属材料焊接接头的腐蚀疲劳失效往往会引发严重的工程事故。本工作对G20Mn5QT/Q355异种钢对接焊接接头在模拟海水环境下的腐蚀疲劳性能进行了研究,开展了其在3.5%NaCl溶液中的腐蚀疲劳试验,并对腐蚀疲劳断口进行了扫描电子显微镜试验,分析了其腐蚀疲劳的失效机理。揭示了在较大应力水平(>220 MPa)下,母材缺陷及焊接缺陷是导致裂纹形核的主要原因;而在较小应力水平(≤220 MPa)下,表面蚀坑是导致裂纹形核的主要原因。分别采用Stromeyer公式与基于三参数威布尔分布的模型建立了对接焊接接头的腐蚀疲劳寿命预测方法,结果表明三参数威布尔分布的腐蚀疲劳S-N曲线拟合精度更高,对实际工程疲劳设计具有指导意义。

现浇钢筋混凝土框架结构节点施工质量控制分析

随着社会经济的不断发展,我国建筑工程数量逐渐增多,框架结构作为建筑工程的重要部分,对于保障建筑质量具有重要意义。而在现浇钢筋混凝土框架结构中,节点属于施工重要位置,能够起到承受板、柱、梁的作用,节点施工质量对于整体结构的稳定性、安全性和耐久性具有重要影响。然而,由于多种因素的影响,节点施工质量控制存在一定的难度。基于此,本文主要结合具体工程案例,分析现浇钢筋混凝土框架结构在节点施工质量控制方面的不足与控制要点,以期保证梁柱节点的施工质量,促进建筑行业的长久以及稳定发展。

In-situ isothermal aging TEM analysis of a micro Cu/ENIG/Sn solder joint for flexible interconnects

Sn/ENIG has recently been used in flexible interconnects to form a more stable micron-sized metallurgical joint,due to high power capability which causes solder joints to heat up to 200℃.However,Cu_(6)Sn_(5)which is critical for a microelectronic interconnection,will go through a phase transition at temperatures between 186 and 189℃.This research conducted an in-situ TEM study of a micro Cu/ENIG/Sn solder joint under isothermal aging test and proposed a model to illustrate the mechanism of the microstructural evolution.The results showed that part of the Sn solder reacted with Cu diffused from the electrode to formη´-Cu_(6)Sn_(5)during the ultrasonic bonding process,while the rest of Sn was left and enriched in a region in the solder joint.But the enriched Sn quickly diffused to both sides when the temperature reached 100℃,reacting with the ENIG coating and Cu to form(Ni_(x)Cu_(1-x))_(3)Sn_(4),AuSn_(4),and Cu_(6)Sn_(5)IMCs.After entering the heat preservation process,the diffusion of Cu from the electrode to the joint became more intense,resulting in the formation of Cu_(3)Sn.The scallop-type Cu_(6)Sn_(5)and the seahorse-type Cu_(3)Sn constituted a typical two-layered structure in the solder joint.Most importantly,the transition betweenηandη’was captured near the phase transition temperature for Cu_(6)Sn_(5)during both the heating and cooling process,which was accompanied by a volume shifting,and the transition process was further studied.This research is expected to serve as a reference for the service of micro Cu/ENIG/Sn solder joints in the electronic industry.

分缝护栏致空心板梁桥L型裂缝预应力控制方法研究

为提高护栏分缝桥梁的耐久性和承载力,以台金高速公路杨司高架桥预制空心板梁桥为背景,研究通过施加预应力控制因护栏分缝引起L型裂缝的有效性,并针对既有桥梁加固和新建桥梁设计提出通用的有效方法。采用Abaqus软件建立既有桥梁和新建桥梁的有限元模型,研究车道荷载作用产生的应力放大效应;分析预应力筋布置高度和长度对边梁梁底应力的影响及对既有桥梁护栏分缝导致的预应力损失;在此基础上,提出L型裂缝的预应力控制方法。结果表明:边梁应力放大系数随桥梁跨径和护栏高度的增加而增大,其取值范围为[1.42,2.14];预应力筋布置高度越低,边梁跨中梁底应力越大;预应力筋布置长度越短,既有桥梁梁底应力越大,对新建桥梁应力没有影响;分缝护栏对既有桥梁施加预应力产生损失,对新建桥梁不会产生预应力损失;在边梁跨中施加相应的预应力,可有效抵消跨中的应力集中效应,防止主梁跨中产生L型裂缝。

天长全民健身中心体育场钢屋盖结构铸钢节点施工阶段力学性能分析

大跨度管桁架结构弦杆数量多、交错连接,传统的相贯节点由于截面限制、连接刚度不足、相贯线不足等原因,无法满足复杂大跨度管桁架钢结构杆件间连接要求.铸钢节点具有一次浇筑成型、形状适应性强、材质均匀等优势,被广泛用于空间结构中.以天长全民健身中心体育场钢屋盖项目为背景,对8种典型铸钢节点进行了力学分析,铸钢节点所连接杆件数量从4~12根不等,共分为支座铸钢节点与空间铸钢节点两类.分析时共考虑了合拢和卸载2个主要施工工况,卸载工况考虑了内外圈临时支撑卸载的顺序对铸钢节点的影响,对比了各个工况下铸钢节点的应力云图.分析结果表明:外圈临时支撑完全卸载和内圈临时支撑卸载一半时,最大Von-Mises应力仅为22 MPa,完全卸载时最大应力约为51 MPa.8种典型铸钢节点在5个主要施工工况下,应力分布较为均匀,节点与杆件连接处应力过渡合理,节点核心区域应力分布较小,能够保证大跨度管桁架主结构各杆件之间的有效连接和内力传递.

G18NiMoCr3-6铸钢焊接接头低温冲击性能不合格原因

某G18NiMoCr3-6铸钢焊接接头的低温(-40℃)冲击性能不合格。采用力学性能测试、宏观观察、化学成分分析、扫描电镜及能谱分析、金相检验、回火工艺试验等方法分析了焊接接头低温冲击性能不合格的原因。结果表明:焊缝金属在560℃回火时,在原奥氏体晶界处析出了Fe3C薄壳,降低了晶界结合力,使材料产生高温回火脆性,材料的韧-脆转变温度升高,最终造成材料发生低温脆断。

装配式建筑预制构件与竖向现浇带钢筋衔接施工技术

装配式预制叠合梁和预制墙作为建筑结构的重要组成部分,在与竖向现浇带钢筋衔接施工方法的研究是在建筑行业中非常重要的一项工作。通过预制构件与现浇带衔接节点钢筋的排布设计,符合其结构受力要求的情况下对构件钢筋的局部调整,满足各构件与现浇带钢筋衔接问题,以确保整个结构的安全性和稳定性。

荆门西站复杂铸钢节点设计及受力性能分析

荆门西站屋盖为马鞍形双曲屋面,采用网架+桁架结合的大跨度复杂空间结构体系,由于屋盖结构曲面造型复杂、平面尺度超长且站房东侧、西侧的大悬挑桁架和中间区域曲面网架采用拱形大桁架连接,使得钢结构屋盖受力异常复杂,并存在超多杆交汇和树状分叉的关键节点,该节点采用铸钢节点的形式。针对此类复杂铸钢节点,首先介绍了铸钢节点的设计,然后采用ABAQUS软件对铸钢节点在最不利荷载工况作用下的力学性能进行有限元分析,并分析了节点的极限承载力。结果表明:铸钢节点在设计荷载作用下完全处于线弹性状态,铸钢节点设计合理;按比例施加10倍设计荷载作用,提取节点的荷载-位移曲线,在4.2倍及5.4倍设计荷载作用下,节点达到极限承载力,综合多工况分析可判断所设计的铸钢节点具有较高安全储备,满足设计要求,可为相关工程的铸钢节点设计提供参考。