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.     

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.     

Suppression of porosity in beam weaving laser welding

Abstract

The present paper describes a beam weaving laser welding technique to suppress argon or nitrogen porosity, which may appear during laser welding of low carbon steel. Bead on plate welding was performed using a 3 kW CO₂ laser. The weaving frequency was varied within 0–30 Hz and the weaving amplitude within 0–2 mm during welding. The experimental results show that under 2.4 kW laser power and 1.0 m min^-1 welding speed, the nitrogen porosity decreases remarkably with increasing frequency, and it can be eliminated for a weaving frequency of 22 Hz with 0.5 mm weaving amplitude. Under 2.4 kW laser power and 1.5 m min^-1 welding speed, beam weaving laser welding can also effectively reduce argon porosity at a weaving frequency of 22 Hz and amplitude of 1.0–1.5 mm.     

Fibre laser welding of aluminium alloy

The objectives of this research are to investigate the effects of various welding conditions on penetration and defect formation, to clarify their welding phenomena and to develop the procedure of reduction of the defect. Fibre laser bead-on-plate welding was performed on several aluminium alloys, in particular A5083, at the power of 6 or 10 kW and several power densities from 0.4 kW/mm². It was found that the weld beads were narrower and deeper with an increase in the laser power density. For example, fully penetrated weld beads in 10 mm thick plates were produced at the laser power density of 640 kW/mm² and the welding speed of 10 m/min. However, convex–concave bead surfaces were formed. Moreover, in the case of the high power density, no porosity and many pores were present at high and low welding speeds, respectively. On the other hand, in the case of the ultra-high power density, few pores were generated in high speed welding. These reasons were interpreted by observing keyhole behaviour, bubble formation and the molten pool geometry during high power fibre laser welding with a high-speed video camera and microfocused X-ray transmission in situ observation method. Moreover, the porosity in the weld bead was reduced and prevented by the utilization of nitrogen gas instead of Ar gas, or the forward inclination angle of 40° (50° from the right angle) in Ar shielding gas.     

Effect of reduced pressure atmosphere on weld geometry in partial penetration laser welding of stainless steel and aluminium alloy with high power and high brightness laser

This study investigated the effect of reduced pressure atmosphere (from ambient pressure of 101–0·1 kPa) on partial penetration welding. A 16 kW disc laser with a 1030 nm wavelength was employed to weld SUS304 stainless steel and A5052 aluminium alloy at a welding speed of 17 mm s^?1. Penetration depths in the stainless steel and the aluminium alloy reached 26 and 23 mm respectively at a reduced pressure of 10 kPa. These depths are >1·6 times deeper than those obtained at 101 kPa. The high speed video imagery showed that in SUS304 welding, the keyhole inlet size decreased and the welding process was stabilised at lower pressures. In A5052 welding, in contrast, the keyhole inlet size increased by a factor of >4 at pressures of 1 and 0·1 kPa compared with that at 101 kPa, resulting in instable welding process and shallow penetration.     

High power laser welding of Nb microalloyed steel plates

Abstract

Laser welding studies on 12 mm thick Nb microalloyed steels were done using a 25 kW CO₂ laser at welding speeds of up to 3 m min^?1 with the aim of identifying the influences of Nb and carbon on laser weldability and weld properties. Welds were examined for solidification flaws, penetration characteristics, microstructure and mechanical properties. Solidification cracking did not appear to be influenced by carbon or Nb in the ranges studied when welding at power levels between 22 and 25 kW. The area fraction of martensite was shown to increase with decreasing energy input and increasing carbon content. Weld metal toughness was improved by reducing carbon content in the range 0·08–0·05%C but was little influenced by Nb. Weld metal and heat affected zone hardness levels were reduced, as expected, by reducing carbon content. The results show that increasing Nb content leads to higher strength material without significant loss of toughness, while lowering carbon content can improve toughness without loss of strength. Thus low carbon Nb microalloyed steels with about 0·05%C can achieve a good combination of parent plate and laser weld properties and appear to be suitable for high power laser welding situations.     

Effects of pulsed and continuous Nd–YAG laser beam waves on welding of Inconel alloy

Abstract

In the present work a 2·5 kW high power Nd–YAG laser is used in the bead on plate (BOP) and butt welding of Inconel 690 plates of thickness 3 mm. Welding is performed using a rectangular laser pulse, for which the peak to base power ratio W_r is reduced from an initial value of 10 to a value of 1, maintaining an identical mean power of 1·7 kW. Therefore, the welding mode changes from a pulsed wave to a continuous wave. The BOP results indicate that the depth of the weld penetration increases at a lower travel speed and/or a higher value of W_r. In the butt welding process, as W_r is increased from 1 to 10, the cellular microstructure of the weld remains relatively unchanged, but the macroporosity formation ratio decreases from 7·1% to 0·6%. At low values of W_r, macroporosity is identified primarily in the root region. However, as W_r increases, the associated periodic high power increases the agitation of the molten pool and probably causes bubbles to float upwards. Consequently, at higher values of W_r, the regions of macroporosity are distributed randomly throughout the weld. Although microcracks are not apparent within any of the welds, each weld exhibits slight microporosity. This microporosity decreases as W_r increases. The present results confirm that a pulsed laser beam with an appropriate peak power can be used to achieve a compromise between the mechanical properties and surface roughness of the weld for Inconel 690 in Nd–YAG laser welding.     

Laser stabilisation of arc cathode spots in titanium welding

Abstract

Cathode spot formation is very pronounced during arc welding of titanium and titanium alloys. The dynamic behaviour of these spots was observed to interfere with metal transfer during welding, this interference being a fundamental cause of poor weld quality in these alloys. In the present work, stabilisation of the arc cathode spot with a focused Nd–YAG laser beam during pulsed gas metal arc welding of titanium was investigated. The laser beam was focused near the leading edge of the weld pool and the laser power and focus spot size were varied to determine the values required to confine the cathode spot to the laser focus position. The results showed that, for fixed welding conditions, the laser power required to prevent cathode spot motion varied as a function of focus spot size. The required laser power was minimised at 200 W for a spot size of 0.6 mm. The laser stabilised arcs had lower voltage but approximately the same current density as stabilised arcs. Increased welding speeds required marginally higher laser powers to stabilise the spot, but the minimum power was still attained with a 0.6 mm focus spot diameter. The laser power density required for stabilisation decreased as spot size was increased, varying from almost 10⁶ W cm^?2 at the smallest spot size to approximately 10⁴ W cm^?2 at the largest. Cathode spot stabilisation improved weld quality by reducing spatter generation and weld bead irregularity.     

Correlation between weld formation and processing parameters in CO2 laser welding of steels

Abstract

Laser welding, which has undergone rapid development in the past few decades, is one of the most important applications in laser materials processing. Although some general data are available, precise welding parameters are equipment specific. In the present study, a series of autogenous laser welds on mild and stainless steels has been investigated, using a Trumpf 3·0 kW CO² laser system, to establish welding parameter windows. The correlation between laser power, welding speed, and weld bead profile for bead on plate welding has been obtained. For a constant laser power, penetration depth reaches a stable value as welding speed exceeds 11 000–13 000 mm min^-1. This value is defined as the penetration threshold. Lower welding speed produces deeper penetration. However, under such conditions, the unstable keyhole and weld pool could result in undercut and porosity. The maximum penetration achievable for sound welds on both mild steel and stainless steel was investigated. The correlation between penetration threshold and power level was also established. The parameter windows established for autogenous welds can be adopted effectively on butt jointsif welding speed is reduced by 25%.     

PA position full penetration high power laser beam welding of up to 30 mm thick AlMg3 plates using electromagnetic weld pool support

Abstract

Full penetration 15 kW Yb fibre laser butt welding of thick AlMg3 (AW 5754) plates was performed in PA position. A contactless inductive electromagnetic weld pool support system was used to prevent gravity dropout of the melt. The welding speed needed to achieve 20 mm penetration was ～0·5 m min^?1. An ac power supply of ～244 W at 460 Hz was necessary to completely suppress gravity dropout of the melt and eliminate sagging of the weld pool root side surface. The oscillating magnetic field can suppress the Marangoni convection in the lower part of the weld pool. The system was also successfully used in the full penetration welding of 30 mm thick AlMg3 plates.     

CO2 and Nd–YAG laser beam welding of 5754–O aluminium alloy for automotive applications

Abstract

Federal regulations have recently been enacted to reduce significantly the atmospheric pollution caused by motor vehicles. This has compelled automotive manufacturers to improve the fuel efficiency of cars and light trucks by using lightweight materials such as aluminium. The focus of the present work is to develop welding procedures for autogenous CO₂ (continuous mode) and Nd–YAG (continuous mode) laser beam welding of 5754–O aluminium alloy. The mechanical and microstructural characteristics of the welded joints were evaluated using tensile tests, microhardness tests, optical microscopy, and chemical analysis. Results indicate that this alloy can be autogenously laser welded with full penetration, minimum surface discontinuities, and little if any loss of magnesium through vaporisation from the fusion zone. The total elongation (all weld metal) in the longitudinal direction for 5754–O laser welds produced using 5 kW CO₂ and 3 kW continuous wave (CW) Nd–YAG shows a slight decrease with increasing travel speed. Studies indicate that the decreasing tendency is probably due to the orientation of the grains with respect to the loading direction. The welds produced using the 5 kW CO₂ laser at travel speeds between 127 and 212 mm s^-1 displayed a total longitudinal elongation of 19.13–15.12% and those produced using the 3 kW CW Nd–YAG laser at travel speeds between 85 and 148 mm s^-1 displayed a total longitudinal elongation of 22.6–18.15%, compared with the base metal value of 28.1%. An observation of great interest was that the weld surface condition did not have any effect on the ductility of the 5754–O aluminium alloy studied in the present investigation.     

Special features of the formation of welded joints in vertical laser-arc welding of low-alloy steels

ABSTRACT

Investigations were carried out into the special features of the technology of vertical laser-arc welding of large structures made of high-strength and low-alloy steels with a variable welding gap and a large root face.

The parameters of the conditions of vertical laser-arc welding of root passes in the low-alloy steel 30–40 mm thick with a root face of 10 mm (welding speed, the amplitude and frequency of transverse oscillations, the intensity of welding current, arc voltage, laser radiation power) resulting in the formation of high-quality welded joints are determined. The experimental results show that the laser radiation power, required for producing continuous penetration of the root face, depends on the thickness of the parent metal: for the metal thickness of 14–30 mm the sufficient laser radiation power is 3.8 kW, for a thickness of 40 mm it is no less than 4.6 kW to penetrate a depth of 5 mm.

The welded joints have a fine-dispersed acicular structure of the martensitic – bainitic type. The experimental results were used to develop a technology of welding of low-alloy steels of different thickness using robotic equipment.     

Laser pressure welding of Al alloy and low C steel

Abstract

This study was performed to obtain fundamental knowledge concerning the development of laser pressure welding technology for the joining of dissimilar metals. Laser pressure welding of Al alloy A6061 and low C steel SPCC sheets was carried out to investigate the effects of the roller pressure, laser beam scanning speed and irradiation position on the tensile shear and peel strength of welded joints. The interfaces of the joints were observed and analysed by SEM and EDX, and the formation phases on the peeled surfaces were identified with XRD. It was revealed that prevention and suppression of oxidation during welding was extremely important to the production of a sound joint with good mechanical properties. The highest tensile strength and the highest peel strength of joints were obtained at a laser power of 1·8 kW, laser scanning speed of 30 Hz, laser irradiation position at the centreline, roller pressure of more than 245 MPa and welding speed of 0·5 m min^?1 in an Ar atmosphere. The fracture occurred not in the welded zone but in the A6061 base alloy specimen.     

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.     

Welding of high thicknesses using a fibre optic laser up to 30 kW

The industrial applications of high-power fibre optic lasers include welding, 2D and 3D cutting, remote cutting and welding, brazing and surface treatments. The availability of fibre optic lasers with power outputs in excess of 10 kW might allow the development of novel fields of application in the welding of high thickness pieces: shipbuilding and offshore industries, pipe and cable manufacture and other heavy industry sectors. Carrying the beam by fibre optics allows high flexibility, even for the production of very large pieces, such as in the shipbuilding sector. This study describes the laser welding of high thickness pieces using a 30 kW laser and a 200 μm diameter fibre. On the one hand, such lasers allow a weld penetration depth of over 30 mm in a single pass, and very high process speeds for thinner materials on the other. Combining lasers with conventional arc welding techniques (hybrid welding) allows further optimization of weld quality and makes it possible to weld butt joints with a ‘gap’ of up to 1 mm. This paper presents the most recent results from very high-power fibre optic laser welding along with new applications in the manufacturing sector.     

超高功率激光-电弧复合焊接特性分析

为了对超高功率激光-电弧复合焊接过程特性有深入的理解. 借助高速摄像,以焊接过程中羽辉和飞溅为主要研究对象,对比分析了激光功率从5 kW增加到30 kW时,激光热源与不同电弧热源复合,以及是否填丝对焊接特性的影响规律. 结果表明,激光功率增加,羽辉和飞溅面积的均值都呈增加趋势,两者的波动程度也呈上升趋势;冷丝的添加在降低焊缝熔深的同时使激光-MAG复合焊接过程中的稳定性变差;激光-TIG复合填丝焊接过程的稳定性明显优于另外两种焊接形式;高功率激光复合焊接时,高温羽辉对激光的散射和吸收作用会使熔深增加趋势放缓.     

Laser welding of copper–nickel alloys: a numerical and experimental analysis

Abstract

Melt run trials were carried out on Cu–Ni bars using a CO₂ laser source in order to analyse the effects of welding parameters (i.e. laser power, welding speed) on geometrical characteristics and on the microstructure of the bead. Experimental results were then used to determine the source parameters to be employed in a finite element model (FEM) of the welding process, with particular attention paid to the thermal field induced by the laser beam. A specific procedure, named 'automatic remeshing technique', was used in order to minimise the computation time. The aim was to create a reliable numerical model, suitable for the optimisation, in practical cases, of welding processes of these kinds of materials. A good correlation, in terms of predicted cooling rates, with the values calculated from SDAS measurements, was observed.     

Power attenuation and focus shift of solid state laser beam caused by laser-induced plume

Currently, remote laser welding using solid-sate lasers is widespread in industry. Meanwhile, it is well known that the laser-induced plume blown up from the processing point affects penetration in laser welding, through the attenuation and the refraction of the laser beam. These phenomena in carbon dioxide laser welding have been investigated well and it is widely recognized that using the shielding gas flow to blow away the laser-induced plume is very important. However, in remote laser welding it is not easy to maintain the shielding gas flow to the processing point. By the way, these phenomena depend on the wavelength of the laser. So, quantitative knowledge of the attenuation and refraction of the solid state laser beam are necessary in achieving stable penetration in remote laser welding with this laser. This study was made to determine the attenuation coefficient and the amount of the effective focus shift caused by refraction of the laser beam in the plume, through melt run experiments with a YAG laser. The attenuation coefficient of the laser beam was estimated to be 0.00090 mm^?1 from the dependence of the cross-sectional area of weld metal on the laser power and the plume length. This value is about one twentieth of the attenuation coefficient of a carbon dioxide laser beam at welding found in the literature. The amount of focus shift was estimated to be 0.67 mm per 100 mm plume length, from the dependency of penetration depth on the defocusing distance and the plume length. Comparing the 3 mm of plume length, this value is centesimal of the estimated value by Beck et al. [The effect of plasma formation on beam focusing in deep penetration welding with CO₂ lasers. J. Phys. D: Appl. Phys. 1995;28:2430–2442] in CO₂ laser welding. Therefore, a solid-state laser such as a YAG laser is considered to be a suitable laser source for remote laser welding.     

Nd:YAG激光+CMT复合热源电弧形态和熔池形貌

以304不锈钢为研究对象,借助高速摄像记录了YAG激光+CMT复合热源焊接的电弧形态和熔池形貌,研究了不同激光功率和光丝间距下的YAG激光+CMT复合热源焊接的电弧形态和熔池彤貌,并与CMT焊接进行了比较,分析了YAG激光+CMT复合热源焊接的电弧形态和熔池形貌发生变化的机理.结果表明,在Nd:YAG激光+CMT复合热...     

Effect of plate thickness on very low‐cycle fatigue properties of butt welded joints containing weld defects. Study of acceptable defect size in girth welds of gas pipelines (Report 2)

Summary

To develop a joining technology for thick ceramics, the fundamental characteristics of high‐power CO₂ laser welding of 87% A1₂O₃ ceramics have been investigated. The results suggest that a penetration depth of 20 mm is possible at a welding speed of 6.5 mm/sec and laser power of 10 kW. The porosity ratio is lower at lower input powers, also tending to decrease with a decreasing welding speed at the same input power. The bending strength decreases with an increasing porosity ratio. Porosities, especially large sink mark porosities at the bead centre, strongly affect the bending strength of welded joints in alumina ceramics. The root bending strength of 4 mm thick butt‐welded plate has the same value as the base material.