Obtaining residual stress along weld beads by means of analytical and experimental methods

In CO₂ arc welding of solid wire, metal transfer phenomena and spatter generation are investigated with rectangular pulse current, and a low spatter CO₂ arc welding process with high frequency pulse current is developed. The optimal conditions of high frequency pulse CO₂ arc welding are in the range of peak current: 450–550 A and pulse frequency: 450–750 Hz. These high frequency pulse currents have the effect of droplet oscillation due to resonance between applied pulse frequency and the natural frequency of the droplet. The droplet is transferred consistently every 9–11 pulses and the average interval of metal transfer is about 16 ms which is reduced to half that of conventional CO₂ arc welding. This average droplet weight is 34 mg, showing a large reduction in comparison with that of the conventional method. As a result, total spatter weight is reduced by 70% in comparison with the conventional method, and especially, large spatters more than 0.5 mm in diameter are reduced from 1.5 to 0.2 g/min.     

Effect of REM addition of wire on CO2 gas shielded arc phenomenon

Carbon dioxide (CO₂) gas shielded arc welding is the main arc welding method, but it generates a large amount of spatter during welding. The root cause of spatter lies in the fact that the droplet undergoes repeated irregular shaking. To solve this problem, spatter generation modes were clarified and the effects of polarity and rare earth metal (REM) addition of the wire on CO₂ gas shielded arc welding were investigated. As a result, when welding is performed with an electrode negative (DCEN) polarity using REM added wire, it was found that a conical arc plasma is formed, and the droplet which is transferred from the wire tip to the molten pool is fine and continuous, in what is termed ‘spray transfer’. Thus, spatter generation was reduced to 10% of amount of the conventional CO₂ gas shielded arc welding (from 0.058 to 0.005g/s).     

An analysis of butt joints in a railway viaduct

Summary

Undercut and/or humping are generated when the arc length in high-speed pulsed MAG welding is too high, and spatter is generated if it is too low. Refinement of the droplets from the wire and simultaneous maintenance of one droplet transfer per pulse avoid short-circuiting in a short arc and enable spatter and highspeed/high-efficiency welding to be rendered compatible with each other. The requirements are to maintain a stable droplet shape during one droplet transfer per pulse, to maximise the welding speed limit without spatter generation, and to develop a technology for arc length shortening at the limit of short-circuiting by wire droplet refinement as objectives basically centred on pulsed MAG welding. This paper describes an investigation of the factors controlling droplet transfer through a division being made into the driving force imparted to the droplets and the deformability of the weld metal receiving this force as well as a method of droplet refinement based on resolution of these aspects. Through the simple rectangular wave pulse of a high peak current being set for a short time using an inverter-type pulsed power source at a constant wire diameter to increase the electromagnetic pinch force, one droplet transfer per pulse is possible on condition that the droplet volume is reduced by around 30% as compared with that obtained using a conventional power source with a chopping transistor on the secondary side. To increase the droplet deformability, the wire composition can be changed to achieve one droplet transfer per pulse on condition that the droplet volume is reduced by 20%. These droplets also show the same surface tension as conventional droplets as well as a lower viscosity coefficient. Through a combination of a pulsed current waveform and improved wire properties, the droplet volume can be reduced to around one half that of conventional droplets. The critical speed of spatter- and defect-free welding is then increased as an improvement immediately applicable on an actual automotive mass production line.     

Study on metal transfer modes and welding spatter characteristics of self-shielded flux cored wire

Abstract

The metal transfer behaviour of self-shielded flux cored wire and the resultant welding spatter were investigated by using a high speed camera. Three modes of metal transfer were found, i.e. bridging transfer without arc interruption in explosive form or by surface tension, globular repelled transfer and droplet transfer, while the former two modes played a key role in the weld metal transfer. Correspondingly, the bridge explosion, discontinuous globular repelled process and the misalignment of droplets transfer were the main factors causing the welding spatter.     

Effect of power source characteristic on CO2 short circuiting arc welding

Abstract

Some problems are reported concerning the observation of the weld pool and effects of the power source characteristic on arc stability in CO₂ short circuiting arc welding. First, the effect of a power source with a constant voltage characteristic on CO₂ short circuiting arc welding is investigated by analysing the behaviour of the voltage and the current. From the results of this analysis, the cause of spatter generation is identified. A new power source characteristic is then proposed to improve the stability and the self-regulation of the arc. By adjusting the power source characteristic, the metal transfer can be stabilised in the CO₂ short circuiting arc welding process, i.e. the present authors have developed a power source having a non-linear characteristic. Its performance is verified by carrying out experiments.     

金属粉芯型药芯焊丝熔滴过渡及飞溅观察分析

采用高速摄影技术对金属粉芯型药芯焊丝的熔滴过渡及飞溅进行观察分析,总结了金属粉芯型药芯焊丝在试验参数下的熔滴过渡类型和特征以及飞溅类型和特征,阐述了熔滴过渡特征以及飞溅特征产生的原因．结果表明,采用100％CO2气体保护时,焊接过程中电弧电压波动较大,熔滴过渡不稳．以排斥过渡为主,少量细颗粒过渡和爆炸过渡,焊接飞溅大;采用5％CO2＋95％Ar保护时,熔滴过渡为单一射滴过渡,熔滴过渡平稳,电弧稳定,焊接飞溅小;金属粉芯型药芯焊丝飞溅形式主要包括：气泡放出型飞溅、缩颈飞溅、熔滴爆炸飞溅以及电弧力引起的飞溅．     

高频交变磁场对大电流GMAW熔滴过渡和飞溅率的影响

在熔化极气体保护焊过程中,采用大送丝速度,增大焊接电流和焊丝伸出长度是提高焊接熔敷率的直接途径.但当熔滴过渡转变为旋转射流过渡时,电弧不稳,飞溅增大,焊缝成形变差.施加不同频率的纵向交变磁场,对焊缝成形进行控制.采用高速摄像技术,拍摄焊接过程中的电弧形态和熔滴过渡,研究不同频率的磁场对熔滴过渡和焊接飞溅率的影响规律.结果表明,熔滴过渡形式不同,产生飞溅的机理不同;外加频率为1 000 Hz纵向交变磁场时,电弧的旋转半径减小,电弧的挺度增大,旋转射流过渡时电弧更稳定,焊接飞溅率降低,焊缝成形改善.     

Metal transfer instability in gas metal arc welding

Abstract

This work presents a simplified model of metal transfer in gas metal arc welding. The model incorporates key features of metal transfer including the change in droplet diameters as welding moves from the globular into the spray metal transfer region, and the increase in welding voltage that is observed to occur as the pendant droplet grows. The model predicts that an instability arises in the globular metal transfer region, which leads to deterministic chaos and complex limit cycles with many droplet sizes. The instability also causes deterministic chaos with a characteristic gap in droplet diameters at the transition to spray mode metal transfer. The model explains observed features of metal transfer in some detail, including the existence and location of preferred bands of droplet sizes. Whether the instability is present or not defines the boundary between chaotic globular metal transfer and the stable drop spray transfer mode. The identification of deterministic chaos in gas metal arc welding metal transfer opens the way for new approaches to welding control.     

Fume formation from spatter oxidation during arc welding

Abstract

Spatter and fume formation rates during arc welding both increase and decrease in a similar manner as welding parameters change. Previously, this fume–spatter relationship has been attributed to evaporation of the spatter caused by oxidation. In this work, a simulated spatter oxidation test did not detect significant fume formation, but high speed videography showed fume trails behind large spatter droplets. Heat balance calculations show that only spatter droplets larger than a few millimeters evaporate and produce fume in significant amounts. Since most spatter particles are smaller than 2 mm, it is not likely that evaporation from spatter contributes significantly to fume. It is proposed that the correlation between spatter and welding fume is instead related to how the temperature of the welding surface affects formation rates of both spatter and fume.     

波控CO2焊短路过渡过程的计算机仿真及试验

提高CO2焊工艺性能、减少飞溅的有效方法之一就是通过改进焊接电源设备，对电弧及熔滴过渡行为加以控制。本在确定了短路过渡的合理的瞬时电流特征的基础上，提出了CO2焊短过渡过程的AWP(Adapting welding physics process）波形控制思想，利用MATLAB的仿真工具建立了数字仿真模型，并得出仿真波形。建立了试验系统，将仿真结果与试验结果相比较，两的一致性很好，证明屯所建模型的合理性和实用性。     

Effects of welding parameters on melting energy of CO2 laser–GMA hybrid welding

Abstract

A series of CO₂ laser–gas metal arc (GMA) hybrid welding experiments were carried out on the mild steel workpiece to investigate the effects of the welding parameters, such as laser power, arc current and the distance between laser and arc D _LA, on the melting energy. A dimensionless parameter psi was introduced to indicate the change in the melting energy of hybrid welding. The results showed that with different welding parameters, the melting energy of hybrid welding was changed by the two heat sources (laser and arc) interaction. With an optimal combination welding parameters, psi can be increased up to 23%. Finally, the role of the two different mechanisms in the heat sources interaction was quantitatively discussed in terms of psi. It can be concluded that when D _LA<4 mm, the interaction between the laser induced plasma and the arc plasma dominates the heat sources interaction, therefore the changes of melting energy, whereas the heat sources interaction is only dominated by the preheating mechanism when D _LA≥4 mm.     

高铬镍奥氏体焊丝Ar/He/CO2保护脉冲GMAW电弧形态与熔滴过渡

为解决高强度Cr-Ni奥氏体焊丝脉冲GMAW电弧挺度不足,熔滴过渡不稳定的问题,文中采用高速摄像手段对Ar/He/CO₂不同组合气体保护下的脉冲GMAW电弧形态与熔滴过渡进行了对比研究,以期优化混合气体成分.结果表明,氩气弧熔滴过渡容易,但电弧漂移、挺度差;氦气和CO₂气体的加入可提高电弧挺度、增大电弧能量、熔滴过渡变为1脉多滴,先一个大滴,接着几个小滴;氦气的比例越大,第一个熔滴的尺寸越大;CO₂气体可克服阴极斑点漂移,但比例不能超过5%;40% Ar+58% He+2% CO₂三元组合的电弧挺度大,熔滴过渡均匀平稳,是奥氏体焊丝脉冲GMAW厚板焊接较理想的混合气体组分.     

电弧熔丝脉冲GTAW熔滴过渡行为分析

提出了电弧熔丝脉冲GTAW焊接工艺,在非熔化极与工件之间的GTA电弧加入脉冲电流,不仅具有传热与传质分离和无飞溅等优点外,同时通过脉冲GTAW控制熔池熔深与形状,减小热影响区,减少合金元素的烧损.结果表明,随着焊丝距离工件高度的减小,熔滴过渡形式从大滴过渡转为无飞溅搭桥过渡,并且距离越近,熔滴过渡频率越快.当焊丝距离工件一定高度,焊接过程一脉一滴时,存在一个最优的频率可以实现最小的熔滴尺寸.     

Determination of the melting rate of coated electrodes in manual arc welding with modulated current

Summary

This study deals with shielded metal arc (manual metal arc, MMA) welding and CO₂ gas shielded arc welding, measuring the force required to remove adhering spatter from the surface of base metal by using different filler metals and by changing the conditions of the surface of the base metal. Rolled steel for general structural use (SS400) was used as base metal.

A high titanium oxide type electrode and a low hydrogen type electrode were used for shielded metal arc welding, a solid wire and a flux‐cored wire were used as filler metal for CO₂ gas shielded arc welding respectively. In order to examine the relationship between the condition of the surface of the base metal and the force required to remove spatter, a base metal whose surface was ground by an electric grinder, one which was not ground by an electric grinder, and another which was coated with an anti‐spatter compound were used for the experiments.

Whichever filler metal was used, the spatter which adhered to the surface of the base metal was located within 100 millimetres from the weld line. In those cases, the force required to remove the adhering spatter can be measured as mostly below 98 N. The spatter needing more than 98 N to remove was located mostly within 20 mm of the weld line.

No matter whether the scale was on the base metal or not, the difference of the force required to remove the spatter was small. When we used the base metal coated with anti‐spatter compound, in some cases we found spatter on the base metal, and in other cases we did not. When we found spatter, it was located within 40 millimetres of the weld line and the force required to remove it was below 20 N.     

Application of optical sensor for arc welding

Summary

In CO₂ gas-shielded arc welding, spatter is scattered and adheres to the base metal surface. The main factors affecting any difference in the bonding force remain obscure. This paper examines the bonding force of spatter adhering to the surface of SS400 base metal (rolled steel sheets) under different temperature conditions in CO₂ gas-shielded arc welding using 1.2 mm dia. solid wire. The following four types of base metal surface condition were adopted:

Type 1: As-received surface covered with an oxide film;

Type 2: Ground surface machined with a plain grinder (R_max = 0.6 μm);

Type 3: Surface with fume adhering to the ground surface;

Type 4: Free-ground surface machined with a disc grinder (R_max = 7–16 μm).

The base metal was heated by a 14 mm dia. x 600 mm ceramic heater arranged on the back of the base metal. The bonding force was measured as the shearing force of the spatter. Some 400 spatter particles on each surface were examined. The particles mostly have diameters ranging between 0.4–1.4 mm.

At a base metal temperature below around 450 K, the spatter on the scaled surface is insufficiently hot to melt the base metal and cannot adhere by melting the base metal surface. The shear stress is mostly below around 40 MPa. Despite a rising base metal temperature, however, spatter removal is still possible at less than 300 MPa.

Fume on the surface or the surface roughness do not affect the bonding force as directly as scale on the surface. The temperature, however, affects the bonding force. At a base metal temperature below around 450 K, the bonding force is slightly smaller.

When spatter adheres to the surface, heat causes the microstructure to change with corresponding hardening. The spatter and HAZ of the base metal here show a slightly higher Vickers hardness value (some spatter particles having HV470) than the base metal hardness (HV156–165), although this depends on the surface conditions and temperature.     

环保焊枪吸烟对小电流CO2气体保护电弧焊的影响

采用环保焊枪进行了小电流二氧化碳气体保护电弧焊平焊位置焊接,通过分析焊接过程中电弧形貌、电流电压和熔滴过渡方式变化以及焊缝成形、焊缝金属拉伸力学性能和X射线探伤结果,评估了吸烟功率变化时的吸烟效果以及吸烟行为对于焊接过程和焊缝质量的影响规律. 结果表明,使用环保焊枪可以显著降低小电流二氧化碳气体保护电弧焊时飘散在周围空间中的焊接烟尘. 吸烟过程虽然使得短路过渡熔滴频率略有增加,悬挂熔滴和电弧的稳定性略为变差,但对焊缝成形和焊接缺陷都无影响,焊缝金属屈服强度略有减小,抗拉强度略有增加.     

Spatter reduction in gas metal arc welding of stainless steel sheets using controlled bridge transfer process

Abstract

In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by controlling the short circuit current to a low level just before the re-arcing. The controlled bridge transfer (CBT) process, which optimises the accuracy of predicting the re-arcing in real time in response to the metal transfer, realises stable, low spatter level. In this research, the methods for controlling short circuit transfers to minimise spatter and realise stable arcs in GMAW of stainless sheet using argon rich shielded gases are investigated. The new CBT process has been developed by applying the specific arc length estimation method that is not affected by abnormal rise in arc voltage. This process can suppress the spatter generation caused by a fluctuation in the vibratory motion of the weld pool or inaccurate prediction of the re-arcing in the succeeding short circuit/re-arcing cycle, and thereby spatter free GMAW in the short circuit transfer mode can be carried out even on stainless steels.     

Thermal and metal transfer behaviours in pulsed current gas metal arc weld deposition of Al–Mg alloy

Abstract

The control of pulsed current gas metal arc (GMA) welding is highly critical owing to the simultaneous influence of the pulse parameters on thermal and metal transfer behaviours of the process. An analytical model has been developed to provide a theoretical understanding of the influence of pulse parameters on the behaviour of metal transfer and thermal characteristics in pulsed current GMA welding using Al–Mg filler wire. The variations in thermal and metal transfer behaviours with changes in pulse parameters have been satisfactorily analysed considering a summarised influence of pulse parameters defined by a dimensionless factor &phis; = (I _b/I _p)ft _b, proposed previously. A large number of process parameters have been considered, as a result of using four different GMA welding power sources. The hypothesis has been verified using some previously reported experimental results. The theoretical model may be useful in the control of pulse parameters to achieve desired behaviours of thermal and metal transfer under different conditions of weld fabrication, thereby facilitating more universal application of GMA welding.     

Prediction of weld quality in pulsed current GMAW process using artificial neural network

Abstract

Weld joint dimensions and weld metal mechanical properties are important quality characteristics of any welded joint. The success of building these characteristics in any welding situation depends on proper selection-cum-optimisation of welding process parameters. Such optimisation is critical in the pulsed current gas metal arc welding process (GMAW-P), as the heat input here is closely dictated by a host of additional pulse parameters in comparison to the conventional gas metal arc welding process. Neural network based models are excellent alternatives in such situations where a large number of input conditions govern certain outputs in a manner that is often difficult to adjudge a priori. Six individual prediction models developed using neural network methodology are presented here to estimate ultimate tensile strength, elongation, impact toughness, weld bead width, weld reinforcement height and penetration of the final weld joint as a function of four pulse parameters, e.g. peak current, base current, pulse on time and pulse frequency. The experimental data employed here are for GMAW-P welding of extruded sections of high strength Al–Zn–Mg alloy (7005). In each case, a committee of different possible network architectures is used, including the final optimum network, to assess the uncertainty in estimation. The neural network models developed here could estimate all the outputs except penetration fairly accurately.     

Influence of welding parameters on distribution of wire feeding elements in CO2 laser GMA hybrid welding

Abstract

The effect of welding parameters on the distribution of wire feeding elements has been investigated during CO₂ laser and pulsed gas metal arc hybrid welding process. The molten metal flow on the pool surface and inside of the samples was observed by a high speed video camera and an in situ X-ray transmission imaging system respectively. The results indicate that the fluid flow towards the inside of keyhole, namely inward flow, improves the homogeneity of weld metal. The distribution of alloying elements is more homogeneous in leading laser compared with leading arc, since both of the drag force of the plasma jet and momentum of droplet promote the inward flow in leading laser. Almost homogeneous distribution of alloying elements can be attained if the oxygen content in the shielding gas is more than 2%, since the Marangoni flow direction changes from outward to inward with increasing the oxygen content.