Laser welding with activating flux

Abstract

The effect on laser welding of activating flux, originally developed to improve penetration depth for tungsten inert gas welding, was investigated. Both mild and stainless steels were tested. Results show that weld penetration capability relates closely to the laser power level and welding speed of the process. Significant improvement in penetration was only observed when laser welding was performed in the conduction mode (i.e. weld aspect ratio <1). For the range of parameters studied, the best penetration capability improvements achievable for mild steel, type 304 stainless steel, and duplex stainless steel were found to be 41, 53, and 63% respectively. These data are of use in the selection of parameters for laser welding with activating flux.     

Weld pool shape prediction in plasma augmented laser welded steel

Abstract

A welding process that combined plasma arc welding with laser welding was used to make autogenous bead on plate welds on a sheet stock of a carbon steel. A wide range of welding parameters (arc current, laser power, weld speed) was employed. The experimental weld pool shapes were analysed and the data were used to train a neural network to predict weld pool shape as a function of process conditions. The predictions of the neural network model showed excellent agreement with the experimental results, indicating that a neural network model is a viable means for predicting weld pool shape. Using the model, a parametric study was carried out to examine the influence of process conditions on the final weld pool profile.     

High quality welding of stainless steel with 10 kW high power fibre laser

Abstract

The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in bead on plate welding of type 304 austenitic stainless steel plates with a 10 kW fibre laser beam. The penetration depth reached 18 mm at the maximum at 5 mm s^?1. At 50 mm s^?1 or lower welding speeds, however, porosity was generated at any fibre laser spot diameter. On the other hand, at 100 mm s^?1 or higher welding speeds, underfilling and humping weld beads were formed under the conventionally and tightly focused conditions respectively. The generation of spatters was influenced mainly by a strong shear force of a laser induced plume and was greatly reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was dependent upon several dynamic or static factors, such as melt volume above the surface, strong melt flow to the rear molten pool on the top surface, solidification rate and narrow molten pool width and corresponding high surface tension. Its suppression was effective by producing a wider weld bead width under the defocused laser beam conditions or reduction of melt volume out of keyhole inlet under the full penetration welding conditions. Concerning porosity, X-ray transmission in situ observation images demonstrated that pores were formed not only from the tip of the keyhole but also at the middle part because of high power density. The keyhole behaviour was stabilised using a nitrogen shielding gas, resulting in porosity prevention. Consequently, to produce high quality welds in 10 kW high power fibre laser welding, the reduction procedures of welding defects were required on the basis of understanding their formation mechanism, and 10 kW fibre laser power could produce sound deeply penetrated welds of 18 mm depth in a nitrogen shielding gas.     

Evaluation of TIG flux welding on the characteristics of stainless steel

Abstract

The aim of the present study was to investigate the effect of specific oxide fluxes on the surface appearance, weld morphology, retained δ ferrite content, hot cracking susceptibility, angular distortion and mechanical properties obtained with the tungsten inert gas (TIG) process applied to the welding of 5 mm thick austenitic stainless steel plates. An autogenous gas tungsten arc welding process was applied to stainless steels through a thin layer of activating flux to produce a bead on plate welded joint. The MnO₂ and ZnO fluxes used were packed in powdered form. The experimental results indicated that the 80% MnO₂–20% ZnO mixture can give full penetration and also a satisfactory surface appearance for type 304 stainless steel TIG flux welds. TIG welding with MnO₂ and/or ZnO can increase the measured ferrite number in welds, and tends to reduce hot cracking susceptibility in as welded structures. It was also found that TIG flux welding can significantly reduce the angular distortion of stainless steel weldments.     

Characterisation of plasma induced during high power fibre laser welding of stainless steel

Abstract

The objective of this research is to obtain a fundamental knowledge of generation behaviour and ionised state of a plume or plasma induced during bead on plate welding of a 20 mm thick type 304 stainless steel plate with a 10 kW fibre laser beam of 0˙9 MW mm^–2 power density, on the basis of 10 000 to 40 000 flames s^–1 high speed video observation and spectroscopic analysis. The high power fibre laser produced a partial penetration weld of 12 mm in depth at 50 mm s^–1 welding speed. According to the high speed observation pictures, the laser induced plume was repeatedly generated from a keyhole at the interval of about 0&dot5 ms period to reach 12 mm in maximum height. The spectroscopy indicated the line spectra of neutral atoms of alloying elements of type 304 such as iron (Fe), chromium (Cr) and manganese (Mn). However, ionised spectra of alloying elements and line spectra of argon (Ar) neutral atom were not apparently detected under these welding conditions. Furthermore, the temperature and the ionisation degree of the laser induced plume were calculated to be approximately 6000 K and 0&dot02 respectively, by the Bolzman plots and Saha's equation. Consequently, the plume induced with the 10 kW fibre laser beam of the ultra high power density was judged to be weakly ionised plasma from these experimental results.     

Effect of weakly ionised plasma on penetration of stainless steel weld produced with ultra high power density fibre laser

Abstract

A weakly ionised plasma can be generated in stainless steel welding with a 10 kW fibre laser beam at the ultra high power density of ～1 MW mm^–2 in Ar shielding gas. The objectives of this study are to obtain a fundamental knowledge of optical interaction between a fibre laser beam and the weakly ionised plasma, and to evaluate effects of the plasma on weld penetration. The optical interaction was investigated by the high speed video observation or the power meter measurement of another probe fibre laser beam, which passed horizontally through the weakly ionised plasma induced during bead on plate welding of a 20 mm thick type 304 plate with a 10 kW fibre laser beam of 0˙9 MW mm^–2 in power density. The probe laser observed was refracted at 0˙6 mrad angle in average, which was much lower than the 90 mrad divergence of the focused fibre laser beam. The attenuation of the probe laser was measured to be ～4%, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering. Moreover, a stable laser welding process could be produced at such ultra high power density that 11˙5 mm deep penetration was obtained even if the laser peak power was modulated 1 ms periodically from 10 to 8˙5 kW. It was consequently considered that the optical interaction between the 10 kW fibre laser beam and the weakly ionised plasma was too small to exert the reduction in weld penetration.     

Study of the Performance of Stainless Steel A-TIG Welds

The purpose of the present work was to investigate the effect of oxide fluxes on weld morphology, arc voltage, mechanical properties, angular distortion and hot cracking susceptibility obtained with TIG welding, which applied to the welding of 5 mm thick austenitic stainless steel plates. A novel variant of the autogenous TIG welding process, oxide powders (Al₂O₃, Cr₂O₃, TiO₂, SiO₂ and CaO) was applied on a type 304 stainless steel through a thin layer of the flux to produce a bead on plate welds. The experimental results indicated that the increase in the penetration is significant with the use of Cr₂O₃, TiO₂, and SiO₂. A-TIG welding can increase the weld depth to bead-width ratio, and tends to reduce the angular distortion of the weldment. It was also found that A-TIG welding can increase the retained delta-ferrite content of stainless steel 304 welds and, in consequence, the hot-cracking susceptibility of as-welded is reduced. Physically constricting the plasma column and reducing the anode spot are the possible mechanism for the effect of certain flux on A-TIG penetration.     

Influence of GMAW-P current waveforms on heat input and weld bead shape

Abstract

Manufacturers use different, proprietary pulse current programming algorithms in their synergic 'one knob control' pulsed gas metal arc welding (GMAW-P) power supplies. Since the different pulse waveforms produced by these supplies can produce somewhat different welding characteristics, inconsistent results can be obtained when transferring welding procedures between different GMAW-P power supplies. A primary objective of this work was to characterise the differences in welding heat input and weld bead shape that could be produced by the pulsed current waveforms from four different commercial supplies. To eliminate the affects of subtle differences in electrical characteristics and to ensure that the exact shape of all waveforms was fully known, the comparison was also done by simulating the waveforms generated by three of the four power supplies on the fourth supply, which was equipped with waveform programming capability. Then, the four waveforms were used to create 'bead on plate' welds over a range of wire feed speed settings, and corresponding heat inputs were calculated from current and voltage samples recorded by a computer data acquisition system during welding. Welds were also done at the same wire feed speed setting using a constant voltage supply. All welds were then cross-sectioned for penetration and dilution measurements. In general, all of the waveforms produced good metal transfer and weld quality. However, the heat input and beads shapes varied noticeably. The heat inputs for the four pulse waveforms and constant voltage welds differed by as much as 150 J mm^?1 (17% of the maximum heat input) at the highest wire feed speed of 212 mm s^?1. The weld penetration differed by 1 mm (22% of the maximum penetration) at a wire feed speed of 169 mm s^?1 and the dilution differed a maximum value of 6.5% dilution (22% of the maximum dilution) at a wire feed speed of 169 mm s^?1 .     

Effect of active flux addition on laser welding of austenitic stainless steel

Abstract

The use of active flux in tungsten inert gas (TIG) welding is known to increase its weld depth. The present paper involves study of active flux laser beam welding (ALBW) of austenitic stainless steel sheets with respect to its effect on plasma plume, microstructure and mechanical properties of the resultant weldments. ALBW performed with SiO₂ as the flux significantly modified shape of the fusion zone (FZ) to produce narrower and deeper welds. Plasma plume associated with the process was considerably smaller and of lower intensity than that produced during bead on plate laser beam welding (LBW). Flux addition during LBW produced thin and rough weld bead associated with humping. The development of such a weld bead is cause by reversal in the direction of Marangoni flow by oxygen induced inversion of surface tension gradient, widely fluctuating plasma plume and presence of oxides on the weld pool surface preventing free flow of the melt. Active flux laser weldments exhibited lower ductility than that of bead on plate laser weldments.     

Ferritic stainless steel welding with the A-TIG process

The main advantage of the TIG with flux process (A-TIG welding) is the possibility of obtaining greater penetration of the weld bead while employing the same welding parameters as conventional TIG welding. Various studies have shown the influence of active fluxes on the geometric characteristics of stainless steel austenitic welds. However, little is known about the influence of this process on the geometric and metallurgical characteristics of the weld beads in ferritic stainless steels. In this work, different types of flux are applied when welding ferritic stainless steel with the objective of verifying possible influences on the weld bead's profile, on its visual appearance, on the microstructure, on the hardness of the welded zone and on the impact resistance (Charpy test). The bead-on-plate welds were produced without using any addition metal. Six types of flux were used – one being an oxide produced in a laboratory (TiO₂) and five commercial fluxes. The results showed that use of the flux allows an increase in penetration with significant changes in the appearance of the weld bead. It was also confirmed that the microstructure and the hardness of the weld bead for the steel studied were not affected by the type of flux used, with the microstructure analysed under an optical microscope. The steel in the study showed a high degree of fragility at ambient temperature.     

Effects of magnesium content on dual beam Nd : YAG laser welding of Al – Mg alloys

Abstract

The effects of Mg content on the weldability of aluminium alloy sheet using the dual-beam Nd:YAG laser welding process have been studied by making bead-on-plate welds on 1.6 mm thick AA 1100, AA 5754 (3.2 wt-%Mg) and AA 5182 (4.6 wt-%Mg) alloy sheets. Whereas all full-penetration laser welds made in 1100 aluminium were of excellent quality,many of the welds produced in the aluminium–magnesium alloys exhibited rough, spiky underbead surfaces with drop-through and undercut. A limited range of process variables was found, however, that allowed welds with acceptable weld bead quality to be produced in the 5754 and the 5182 alloy sheet. Goodwelds were only produced in these alloys if the lead/lag laser beam power ratio was ≥1. Weld penetration and the maximum welding speed allowing full penetration keyhole-mode welding were observed to increase with Mg content. This was attributed to the effect of Mg on the vapour pressure within the keyhole and the surface tension of the Al–Mg alloys. Significant occluded vapour porosity was seen in the 5754 and 5182 alloy welds with borderline penetration; however, there was no evidence of porosity in the acceptable full-penetration welds with smooth underbead surfaces. Hardness profiles in the 5754 and 5182 welds showed a gradual increase in hardness from the base metal values through the heat affected zone (HAZ) to a peak in hardness in the weld metal adjacent the fusion boundary. It is possible that this increase in hardness may be the result of the presence of Mg₂Al₃ or metastable Mg₂Al₃′ precipitates in this region of the weld and HAZ.     

Factors controlling the magnesium weld morphology in deep penetration welding by a CO2 laser

In laser welding with power density beyond 10⁴ W · mm⁻², the formation of plasma cavities, commonly referred to as keyholes, leads to deep penetration welds with high aspect ratios. In this paper, the morphologies of keyhole welds produced with a 6 kW CW CO₂ laser on two die-cast magnesium alloys, AZ91 and AM50, are compared. It was found that the two magnesium alloys responded differently to laser welding. Though irregular weld cross-section profiles were consistently observed on each materials, bead dimensions often varied with the welding variables in contrasting ways. For both alloys, important characteristics of the weld beads such as depth, width, crown height (hump), and surface ripples were analyzed as a function of the welding parameters, most particularly the heat input. Results show that the use of heat input, a variable grouping two welding parameters into one, was often inadequate in characterizing the bead morphology. Several explanations are given, including base metal vaporization, but the process of bremsstralung absorption explains it well and rationalizes many observed characteristics of laser weld morphology.     

TIG焊活性剂对焊缝成形的影响

活性化焊接（A－TIG焊）在90年代末期受到国外的高度重视，同传统的TIG焊（钨极惰性气体保护电弧焊）相比，在相同的规范下活性化焊接能够大幅度地提高生产率、降低生产成本。中针对不锈钢和钛合金材料，研究了在A－TIG焊中单一成分的活性剂和涂敷量对焊缝成形的影响。试验结果表明，与无活性剂的焊缝相比，活性剂CaF2,SiO2,NaF,Cr2O3和TiO3都能够有效地增加不锈钢和钛合金焊缝的 熔深，随着涂敷量的增加，焊缝熔深也相应的增加，熔宽减小。但涂敷有CaF2活性剂的不锈钢焊缝成形不好，涂敷有Cr2O3的钛合金焊缝正面熔宽没有明显的变化。在不锈钢焊接中，活性剂SiO2的作用效果最好；而钛合金的焊接中CaF2的作用效果最好。电弧收缩和熔池表面张力的变化是活性剂增加熔深的主要原因。     

Microstructures and properties of laser/arc hybrid welds and autogenous laser welds in pipeline steels

Abstract

Lasers are capable of producing welds with deep penetration, low distortion and faster travel speeds, compared to arc welding. More recently, laser/arc hybrid welding processes have also been generating interest for industrial fabrication. In this paper, six carbon–manganese, mainly pipeline, steels were welded using both autogenous Nd:YAG laser welding, and Nd:YAG laser/MAG hybrid welding. The improvements in weld microstructures and weld metal toughness that are possible when using the hybrid process are described and illustrated. Laser/arc hybrid welding is shown to be a process that can generate good quality welds in commercially available pipeline steels. It also has the potential to complete girth welds in these steels with significantly fewer welding passes than are currently required for arc welded pipelines, reducing the joint completion time.     

Effect of process parameters of micro-plasma arc welding on morphology and quality in stainless steel edge joint welds

Abstract

The effects of the process parameters of micro-plasma arc welding (micro-PAW) on the morphology and quality of stainless steel edge joint welds were investigated. Micro-PAW was applied on type 304 stainless steels to produce an edge joint weld. Welding experiments were carried out for various combinations of arc current, welding speed, arc length, shielding gas, and clamp distance, with all other operating conditions held constant. The experimental results indicated that the collimated shape of the low current plasma arc was mainly responsible for the low sensitivity of the weld morphology to variations in the nozzle standoff distances. The arc voltage increased with increasing quantity of added hydrogen in the argon shielding gas. It was also found that satisfactory edge joint welds can be formed using a clamp distance of 0.35 mm, and that the edge joint penetration on a 0.1 mm thickness stainless steel is about 60% of the clamp distance.     

大功率盘形激光焊接过程等离子体图像特征分析

研究一种基于等离子体图像特征的大功率盘形激光深熔焊质量分析及检测的新方法．以大功率盘形激光焊接Type 304不锈钢板为试验对象,应用高速摄像机摄取焊接过程中的等离子体图像,通过图像处理技术提取等离子体的面积和高度特征．以熔宽作为衡量焊接过程稳定性的因素,对比焊接过程中等离子体图像和焊接试件的熔宽变化,研究相邻等离子体...     

Weldability of 1.6 mm thick aluminium alloy 5182 sheet by single and dual beam Nd: YAG laser welding

Abstract

The weldability of 1.6 mm thick 5182 Al–Mg alloy sheet by the single- and dual-beam Nd:YAG laser welding processes has been examined. Bead-on-plate welds were made using total laser powers from 2.5 to 6 kW, dual-beam lead/lag laser beam power ratios ranging from 3:2 to 2:3 and travel speeds from 4 to 15 m min^-1. The effects of focal position and shielding gas conditions on weld quality were also investigated. Whereas full penetration laser welds could be made using the 3 kW single-beam laser welder at speeds up to 15 m min^-1, the underbead surface was always very rough with undercutting and numerous projections or spikes of solidified ejected metal. This 'spikey' underbead surface geometry was attributed to the effects of the high vapour pressure Mg in the alloy on the keyhole dynamics. The undesirable 'spikey' underbead geometry was unaffected by changes in focal position, shielding gas parameters or other single-beam welding process parameters. Most full penetration dual-beam laser welds exhibited either blow-through porosity at low welding speeds (4–6 m min^-1) or unacceptable 'spikey' underbead surface quality at increased welding speeds up to 13.5 m min^-1. Radiography revealed significant occluded porosity within borderline or partial penetration welds. This was thought to be caused by significant keyhole instability that exists under these welding conditions. A limited range of dual-beam laser process conditions was found that produced sound, pore-free laser welds with good top and underbead surface quality. Acceptable welds were produced at welding speeds of 6 to 7.5 m min^-1 using total laser powers of 4.5–5 kW, but only when the lead laser beam power was greater than or equal to the lagging beam power. The improved underbead quality was attributed to the effect of the second lagging laser beam on keyhole stability, venting of the high vapour pressure Mg from the keyhole and solidification of the underbead weld metal during full penetration dual-beam laser welding.     

Experimental study of laser welding with applied electrical potential

Abstract

The objective of the present study is to develop laser welding with an applied voltage potential, to increase the weld bead root size in laser welding and thereby to improve the welding speed and the butt joint gap tolerance. The influences of the experimental conditions, namely, input laser power, applied voltage between plate and backside electrode, welding speed, plasma operating gaseous species (air, argon, or argon–helium), and the butt joint gap, on the plasma stability and the weld bead were investigated. The weld beads are evaluated from the point of view of the bead appearance, the penetration depth, the ratio of the widths of the weld bead root and weld bead face, and the smoothness of the bead surface. It is found that to stabilise the plasma, it is preferable to set the plate polarity as the cathode (electrode positive), and to use argon or helium gas as a plasma operating gas. Also, it is concluded that this novel method is effective in increasing the bead root/face width ratio, and the melting area. Although it is necessary to optimise the experimental conditions to avoid overheating and melting of the plates, the present method is applicable for higher speed and wider gap butt joint welding than in conventional laser welding.     

CO_2激光焊接600MPa DP钢接头组织与性能

采用CO2激光对抗拉强度为600MPa,厚度1.4mm的DP钢进行焊接.研究焊接速度对焊缝外观和截面成形的影响、接头的组织特点、硬度、强度和成形能力.结果表明,激光功率相同,焊接速度较低时焊缝易产生气孔,焊接速度较高时易发生飞溅;焊接速度对焊缝熔深及熔宽也有影响.焊缝区组织主要由马氏体构成,从焊缝、焊接热影响区到母材,组织中马氏体含量下降,接头的最高硬度出现在焊缝或热影响区.在平行于焊缝方向,焊接接头的抗拉强度高于母材,垂直于焊缝方向,接头的抗拉强度与母材相当.由于焊缝出现马氏体组织,接头的塑性和韧性降低,板材的冲压成形能力下降.     

焊接参数对铝合金激光——MIG电弧复合焊缝熔深的影响

以5A06(LF6)铝合金为研究对象,研究了铝合金激光-MIG电弧复合焊(HYBRID焊)时焊接参数的变化对焊缝熔深的影响规律,并将焊接参数的变化对激光-MIG电弧复合焊、MIG电弧焊、激光焊的焊缝熔深的影响进行了综合比较分析.结果表明,若以等效于激光焊熔深的MIG电弧功率为分界线,可以把MIG电弧的功率分成高、低能量两个区,当激光与低能量电弧区的电弧复合时,激光对焊缝的熔深起主导作用,HYBRID焊的熔深大于该区的MIG电弧焊;当激光与高能量电弧区的电弧复合时,电弧对焊缝的熔深起主导作用,HYBRID焊缝主要体现为MIG焊缝的特点,加入激光的优势主要体现在增加焊接速度方面,即高速焊时也可获得良好的焊缝成形.