Multi-objective optimization of process parameters for ultra-narrow gap welding based on Universal Kriging and NSGA Ⅱ

The successful confinement of the arc by the flux band depends on the welding process parameters for achieving single-pass,multi-layer, and ultra-narrow gap welding. The sidewall fusion depth, the width of the heat-affected zone, and the line energy are utilized as comprehensive indications of the quality of the welded joint. In order to achieve well fusion and reduce the heat input to the base metal.Three welding process characteristics were chosen as the primary determinants, including welding voltage, welding speed, and wire feeding speed. The metamodel of the welding quality index was built by the orthogonal experiments. The metamodel and NSGA-Ⅱ(Non-dominated sorting genetic algorithm Ⅱ) were combined to develop a multi-objective optimization model of ultra-narrow gap welding process parameters. The results showed that the optimized welding process parameters can increase the sidewall fusion depth, reduce the width of the heataffected zone and the line energy, and to some extent improve the overall quality of the ultra-narrow gap welding process.

Development of ultra-narrow gap welding with constrained arc by flux band

Narrow gap welding has merits of lower residual stress and distortion, and superior mechanical properties of joints. A major problem of this process is the lack of fusion in sidewalls, hence many methods of weaving arc have been developed to increase heating effect of arc to the sidewalls. In this work, a new approach Without weaving arc is attempted to ensure the penetration of sidewall, and ultra-narrow gap welding with the gap of less than 5 mm was executed successfully. In this approach, the width of gap is decreased further, so that the sidewalls are made within range of arc heating to obtain the enough heat. In order to prevent the arc from being attracted by sidewall and going up along the sidewalls, two pieces of flux bands consisting of the specified aggregates are adhered to the sidewalls to constrain the arc. In addition, when flux band being heated by the arc, slag and gases are formed to shield the arc and the weld pool. This technique was tested on the welding experiment of pipeline steel with thickness of 20 mm. The involved welding parameters were obtained, that is, the width of gap is 4 mm, the welding current 250 A, and the heat input 0. 5 kJ/mm, the width of heat-affected zone is 1 -2 mm.

Weld formation and heating mechanism in ultra-narrow gap withconstricted arc by ultra-fine granular flux

Ultra-narrow gap welding （UNGW） process with high stabilization, reliability and without spatter can be achieved with constricted arc by molten slag wall, which is made from melted flux. The experiments are carried out by changing voltage under different currents. The results indicate voltage range being fit for UNGW is about 22 -31 V under the current range of 200 -320 A. With the increasing of voltage, weld formation of UNGW has the law of lack of fusion on sidewall, good weld and undercut in turn under a certain current. In addition, the action relationships among arc, molten slag wall and sidewalls can be improved by properly adjusting voltage and current of arc, which makes cathode spot properly distribute in ultra-narrow gap. Therefore, the effective control of weld formation of UNGW has been achieved.

Research on droplet transfer in oscillating arc narrow gap GMA welding

In this paper, the droplet transfer in oscillating arc narrow gap gas metal arc （ GMA ） welding was studied. According to the experimental results, the oscillating arc has effect on the droplet transfer mode. The droplet transfer frequency in narrow gap groove is higher than that in bead-on-plate welding. Because of the change of arc location in narrow gap groove, the droplet transfer in oscillating arc narrow gap changes regularly. The droplet transfer frequency near groove sidewall is higher than that at the middle of narrow gap groove.

Characteristics of the electrode melting phenomena in narrow gap MAG high-rotating-speed arc welding

The influence of rotating speed on metal transfer and the wire metal speed was studied in the high rotating speed arc narrow gap welding.The results indicate that the high rotating speed arc has benefit on the metal transfer,and that with the rotating speed increasing,the droplet volume decreases.It is shown that the rotating speed has little influence on the wire metal speed with DC electrode positive polarity(DCEP),but the melting speed decreases with increasing of rotating speed in DC electrode negative polarity(DCEN).

The comparison of multi-layer narrow-gap laser and arc welding of S32101 duplex stainless steel

Multi-layer narrow-gap welding of thick S32101 duplex stainless steel was conducted using laser welding with beam wobble process.The phase transition,grain size,phase proportion and crystal texture of welded joint were also studied and compared with gas metal arc welding process.The microhardness and tensile strength were measured and fracture surface was analyzed to evaluate the mechanical properties of welded joints.The results showed that beam wobble technology improved the misalignment of laser beam and filler wire in narrow groove and helped to avoid incomplete fusion defects.Compared to arc welding process,the groove size and heat input were reduced,while welding efficiency was increased.The faster cooling rate and lower temperature gradient of laser wobble welding favored grain refinement,while the austenite content in weld zone decreased.Both the beam wobble and swing arc were conducive to stir weld pool,optimizing the weld microstructure and joint formation.The microstructural variance in various weld passes was caused by the heat input and heat dissipation ability.The microhardness of laser welded joint was lower,while the tensile strength and elongation percentage were higher.The fracture surface of arc welded joint was featured with shallower dimples and cleavage steps.

Narrow gap space contributes to chemical metallurgy of self-shielded arc welding

The slag-free self-shielded flux-cored wire was fabricated to apply for the narrow gap welding.The results showed that narrow gap welding shows lower welding spatter compared with hardfacing except under voltage of 30 V and current of 260 A.The deposition efficiency keeps over 90%for both 12 mm and 8 mm narrow gap welding.For 12 mm narrow gap welding,when the voltage is 28 V and the current is 264 A(or 286 A),no pores are found in the narrow gap weld.In the continuous welding process,manganese vapor,aluminum vapor and CO continuously generate to form gasbag and occupy the narrow gap space,thus protecting the droplet and molten pool from the invasion of air.

Metal transfer characteristics of tandem narrow gap GMAW for flat welding position

A series of experiments of tandem narrow gap GMAW for flat welding position were carried out. The arc behavior and metal transfer process were observed by a high-speed photography system. The effects of the welding parameters on the metal transfer were investigated. The results show that the arc behavior and metal transfer process of tandem narrow gap GMA W are different from these of bead-on-plate tandem GMA W. The arc behavior and metal transfer process are influenced by the distance between the two wires, the peak voltage, the pulse frequency and the peak time. With the increase of the distance between the two wires, the metal transfer mode gradually transforms from one pulse-multi droplets into one pulse-one droplet, and the average welding current increases. With the increase of the peak voltage, pulse frequency or peak time, the metal transfer mode transforms from one pulse-one droplet into one pulse-multi droplets, and the arc tends to occur between the wire and the sidewall.

RECALLABLE TRACKING TECHNIQUE FOR TOP WELD BEADS OF NARROW GAP SUBMERGED ARC WELDING

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Rotating arc horizontal narrow gap welding of high strength quenched and tempered steel

Rotating arc borizontal narrow gap welding of quenched ＆ tempered （Q＆T） steel was innovatively performed for solving the bottleneck that the molten pool sagged due to the gravity. The shapely multilayer single pass horizontal joint could be obtairzed by using the rotating are welding process. The cold crack was not observed in the joint without controlling the heat input and selecting the consumables intentionally. Microstructure of the joint could be divided into three zones： base metal zone （BMZ） , heat-affected zone （ HAZ） and weld zone （WZ）. Because of the characteristic of the rotating arc horizontal welding process, the defects in the joints were slag inclztsion formed at the interlayer of lower side wall. Tbe tensile strength and hardness of HAZ and WZ were larger than those of BMZ. The impact toughness in WZ, HAZ and BM at 0 % is equal to 11.5, 212 and 236 J, respectively.

Influence of arc pressure on the forming of molten pool in tungsten inert gas arc butt welding with micro gap for tantalum sheet

Arc pressure is the key influencing factor to forming of molten pool. Countering the characteristic of tungsten inert gas arc welding with micro gap for tantalum sheet, according to the fundament of arc physics, a distribution model of arc pressure and forming mechanism of molten pool with micro butt gap are proposed, and the influences of arc pressure on forming of molten pool are discussed. Experimental researches for the dynamic formation process of weld molten pool by using high-speed vidicon camera show that when butt gap is appropriate, that is from 0. 1 to 0. 15 mm, molten metals formed on two workpiece uplift and grow up first, then are fused and form uniform molten pool finally.

Fine microstructure characterization of titanium alloy laser narrow gap welded joint

In this paper,the microstructure of narrow gap laser welded joint of 45 mm Ti6321 titanium alloy was characterized.The microstructure characteristics of Laser self-fusion and laser-multi-pass-narrow-gapfilling layer were observed by metallographic microscope,and the phase morphology was analyzed by transmission electron microscope.The results show that the joint was obtained using the laser-multipass-narrow-gap welding,and no volumetric defect is observed.The weld grain of self fusion welded bead tends to be equiaxed with lots of needleα’,and the width of weld metal needleα’was 0.4μm.The heat affected zone is columnar crystal with lamellarα,and The width of heat affected zonelamellarαwas 2–4μm with a large number of dislocation tangles.The weld grains of laser-multi-pass-narrow-gap filler bead are coarse columnar grains with staggered acicularα’,and the width of acicularα’in weld metal is less than 1μm with number of dislocations.The heat affected zone is thick lamellar microstructure with a width of 2–4μm with a large number of dislocation entanglement and packing.The microstructure evolution law is related with heating temperature and cooling rate.

A twin-electrode alternating current to direct current synergetic pulsed arc welding control method

A novel synergetic arc control method was used to control twin-electrode alternating current （ AC） to direct current （DC） pulsed arc welding and the mechanism of poor sidewall fusion in narrow gap welding was investigated. The synergetic arc control method easured that the arc voltage of DC welding source could switch in phase with the AC welding source. To test the reliability and operability of this method, a twin-electrode AC to DC pulsed arc welding system was set up and data was acquired through high-speed photography and electrical signal measurement system. The results show that the interactions between the two arcs can be controlled effectively and the control method is a sensitive and efficient control method.

Effect of welding heat input on HAZ character in ultra-fine grain steel welding

In this essay, we studied how heat input affected the microstructure, hardness, grain size and heat-affected zone(HAZ) dimension of WCX355 ultra-fine grain steel which was welded respectively by the ultra narrow-gap welding (UNGW) process and the overlaying process with CO 2 as protective atmosphere and laser welding process. The experimental results show when the heat input changed from 1.65 kJ/cm to 5.93 kJ/cm, the width of its HAZ ranged from 0.6 mm to 2.1 mm.The average grain size grew up from 2～5 μm of base metal to 20～70 μm and found no obvious soften phenomenon in overheated zone. The width of normalized zone was generally wide as 2/3 as that of the whole HAZ, and the grain size in this zone is smaller than that in base metal. Under the circumstance of equal heat input, the HAZ width of UNGW is narrower than that of the laser welding.

送进式焊剂片约束电弧超窄间隙焊接工艺参数匹配

为探究送进式焊剂片约束电弧超窄间隙焊接的工艺参数匹配,自主设计制作了一套送进式焊剂片约束电弧超窄间隙焊接装置,以提高焊接过程稳定性和自动化水平。利用瞬间中断电弧的方法保留了熄弧位置处焊缝形态和焊剂片末端熔化形貌,发现焊接电压和焊剂带送进速度都是通过改变焊剂片对电弧的作用长度来影响焊缝成形,焊剂片对电弧的作用长度可表现为焊剂片对电弧固壁约束面积。通过改变焊剂带送进速度和焊接电压进行了试验研究,结果表明,当焊剂片对电弧固壁约束面积适中时,电弧集中加热区域会下移至间隙底部和侧壁根部,获得根部熔合良好的焊缝;当焊剂片对电弧固壁约束面积较大时,电弧受约束程度过强,电弧加热区域集中在间隙底部,分布于侧壁根部电弧热量很少,焊道上部会有熔池堆积;当焊剂片对电弧固壁约束面积较小时,电弧受约束程度较弱,会沿侧壁攀升导致加热区域集中在两侧壁上,很难在坡口根部形成有效熔池,最终形成孔洞焊缝。确定了最佳的焊接电压范围为21.5~24.5 V,焊剂带送进速度2.1~2.6 mm/s,利用该参数进行超窄间隙单道多层焊接可以获得成形良好的焊缝。

窄间隙P-GMAW仰焊位置熔滴过渡影响因素及控制方法

管道窄间隙全位置焊接过程中,在接近仰焊位置容易出现焊接缺陷,制约了管道施工的质量和效率.文中针对管道自动焊仰焊位置成形差、可靠性低的问题,分析P-GMAW过程送丝速度、弧长修正、导电嘴到熔池距离等因素对熔滴过渡的影响规律.结果表明,增加弧长修正系数可以增大电弧对侧壁的热输入,能够缓解侧壁熔合不良的问题,但会使熔滴过渡路径难以控制.因此当焊枪运动到仰焊位置时,提高送丝速度有利于获得更小的熔滴、更高的过渡频率;降低导电嘴到熔池距离,可以增加电磁力、减小熔滴尺寸,从而更有利于仰焊位置熔滴向熔池的过渡,研究结果对于管道窄间隙焊接的工艺设计具有一定的指导意义.

基于电弧声信号的窄间隙脉冲熔化极气体保护焊侧壁熔合状态在线识别

为在焊接过程中实时了解焊缝内部的焊接状况,构建了电弧声信号实时采集系统。在焊枪摆动中心处于不同位置的情况下,进行了电弧声信号特征与侧壁熔合状态的相关性分析。分别从时域与频域中提取了与侧壁熔合状态相关性较强的电弧声特征。为进一步提高熔合状态预测的有效性,采用电弧声特征参量构建了支持向量回归的侧壁熔合状态识别模型。为减小不良特征对识别模型的影响,显著提高模型的识别精度,采用遗传算法进行了参数寻优。参数寻优后模型的总体识别率达93.33%,实现了窄间隙侧壁熔合状态的有效识别。

P-GMAW起弧过程特征分析及稳定性判别方法

脉冲熔化极气体保护焊(pulsed gas metal arc welding,P-GMAW)起弧过程易产生不稳定现象,会严重影响电弧传感焊缝跟踪精度.针对这一问题,对摆动电弧窄间隙P-GMAW不稳定起弧过程的成因进行了研究,发现送丝速度对起弧过程稳定性具有重要影响.通过对电弧图像与电信号特征进行对比分析,提取了表征电弧稳定性的电信号特征变量;为减小变量冗余性和过拟合,采用最大似然估计法筛选并提取了8个变量,并通过主成分分析法(principal component analysis,PCA)对变量进行融合,提取了方差贡献率最高的前两个主成分;根据因子载荷发现,相比熔滴过渡阶段和基值阶段,脉冲峰值阶段是电弧更易发生不稳定现象的阶段.结合提取的主成分变量与二分类Logistic回归模型建立了起弧过程电弧稳定性判别模型.通过受试者工作特征(receiver operating characteristic,ROC)曲线得到了模型的最佳阈值.结果表明,该模型对脉冲稳定性判别准确率达到了80%以上,表明模型具有良好的判别性能.该模型对提高窄间隙高低跟踪精度、保证焊接质量具有一定应用价值.

大壁厚NiCrMoV钢的窄间隙焊接工艺与接头性能

针对电站设备转子用大壁厚NiCrMoV钢,分析了材料焊接性,采用窄间隙埋弧焊工艺获得了完整的焊接接头,分析了焊接接头的组织、硬度、拉伸与弯曲性能。结果表明,整个焊接接头为完全冶金结合,无裂纹、气孔等缺陷;焊缝组织为回火贝氏体+少量回火马氏体,热影响区中的粗晶区和细晶区均为回火马氏体组织和沿晶界分布的少量碳化物,母材为回火贝氏体+回火马氏体的混合组织;接头热影响区硬度最高,焊缝硬度略低于母材;焊接接头的抗拉强度随着温度的升高逐渐降低,均满足设计要求;焊接接头经过180°弯曲性能测试均未发现微裂纹,表明接头的延展性良好。

市域(郊)铁路桥上无缝道岔至梁缝距离研究

市域(郊)铁路桥梁占比高,结合车站配线设计,道岔不可避免需要设置在高架站前后的道岔连续梁上。考虑市域(郊)铁路采用的道岔型号、桥梁结构及设计荷载与高速铁路明显不同,桥上无缝道岔至梁缝距离研究相对缺乏。由于市域(郊)铁路设计速度比地铁高,轮轨作用力大,轮轨之间的振动和冲击对周围环境影响大,因此,桥上道岔采用跨区间无缝线路技术。基于“道岔-桥梁”相互作用原理,建立“岔-板-梁-墩”一体化12号无砟轨道、无缝道岔有限元模型,对常用的单开道岔、4.3 m线间距渡线道岔至梁缝距离开展计算分析并研究其影响规律,以道岔钢轨强度、尖轨尖端位移值、心轨尖端位移值、转辙机处基本轨与桥梁相对位移值及断缝值为限值条件,提出市域(郊)铁路桥上无缝道岔至梁缝合理距离限值。研究结果表明:桥上无缝道岔始终端距梁缝距离越大,尖轨尖端位移、心轨尖端位移和转辙机处基本轨与桥梁相对位移明显减小,道岔钢轨伸缩力和制动力计算结果差异不大;温暖地区道岔始端、终端至梁缝距离不小于10 m,寒冷地区道岔始端、终端至梁缝距离不小于18 m,相关研究成果纳入《市域(郊)铁路设计规范》。