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.     

Spectroscopic monitoring of penetration depth in CO2 Nd:YAG and fiber laser welding processes

A method for on-line monitoring of the joint penetration depth is reported, based on the spectroscopic analysis of the optical emission collected from laser-metal interaction zone. This technique has been applied to quite diverse welding procedures (c.w. CO₂ laser, cw fiber laser and pulsed Nd:YAG laser) of stainless steel plates. Although the acquired spectra reveal different characteristics according to the employed laser source, a discrete contribution of several iron lines can be highlighted in both types of investigated welding processes. Starting from the measurement of the intensities of those lines it is possible to calculate the plasma electron temperature, which was found to decrease as far as the laser power is enhanced together with the penetration depth, in static as well as dynamic conditions. Such a behavior does not correspond to a real decrease of the plasma plume temperature but has to be ascribed to the position of the optical collimator collecting light only from the top of the keyhole. Indeed, for deeper penetrations the hottest core of the plume moves down into the vaporized capillary so that the plasma temperature measured on top of the keyhole appears lower at higher incident powers. The electron temperature signal could be useful for the development of a feedback loop system of the laser power to control the weld penetration depth.     

低碳钢激光预熔活性焊接法

以8 mm厚的低碳钢为研究对象,在氧气的保护下,用小功率光纤激光在待焊焊件表面进行预熔处理,使表面熔化生成一层氧化层,然后用TIG焊或激光电弧复合焊覆盖氧化层,研究工艺参数对焊缝成形的影响.结果表明,激光预熔后进行覆盖焊接,电弧没有收缩,熔深增加到1.5倍左右,表面成形良好.随着电流的增加,熔深增加,但激光预熔后的焊道增加更快.随着焊接速度的增加,熔深减小.随着激光预熔功率的增加,熔深增加.随着复合焊接中激光功率的增加,熔深增加.焊缝含氧量的增加,使得表面张力温度系数由负变正,是低碳钢激光预熔活性焊接熔深增加的主要原因.     

Comparative study on laser welding characteristics of aluminium alloy under atmospheric and subatmospheric pressures

Laser bead on plate welding of 10 mm thick aluminium alloy under atmospheric and subatmospheric pressures were comparatively investigated. With the decrease of ambient pressure, the penetration depth increased sharply at first and then gradually levelled off. The largest penetration depth could reach 8·7 mm when welded under the pressures of 10¹ Pa, while only 4·9 mm under atmospheric pressure. Weld bead without any porosity was produced under ambient pressures of 10^?1 Pa. The average tensile strength of joints welded under the pressure of 10¹ Pa was 300·2 MPa. The tensile strength remained constant as the ambient pressure decreased further. The shielding effect of plasma plume on laser beam was suppressed as the ambient pressure decreased. Therefore, the laser power deposition inside the keyhole was enhanced effectively. Under subatmospheric pressure, the porosity defects were eliminated effectively due to the keyhole stability and the change of liquid flow, i.e. moving upward along the rear wall of keyhole.     

Evaluation of molten pool geometry with induced plasma plume absorption in laser-material interaction zone

The presence of plasma affects laser material processing technology because, according to process parameters, a large portion of the energy emitted by the laser source is absorbed by the plasma plume without hitting the workpiece. The only way to avoid a significant reduction in process efficiency, due to plasma absorption, is thus to decrease the plasma formation by controlling the working parameters.An original analytical system for the prediction of the actual energy transmitted to the workpiece was developed by modelling the plasma plume physical state related to the process parameters. In this way, by determining the laser beam energy lost in the plasma plume and the conduction energy transmitted to the workpiece, an evaluation of the laser material interaction could be carried out.The developed model allows to evaluate the geometry of the molten pool by means of the computation of the interface between the solid and the remelted material. The effect of the plasma plume presence, by comparison with a modelisation without plasma implemented in similar way by the authors, was to reduce the molten pool and in particular the penetration depth and it permits to have close simulation results to experimental data.For the model validation several experiments were performed on an austenitic stainless steel with a CW CO₂ laser source. The experimental activity was developed by varying process speed and power level up to 1200 W when in the range of conduction welding.     

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.     

Full penetration welding of thick high tensile strength steel plate with high-power disk laser in low vacuum

This study was undertaken in order to investigate the effect of reduced ambient pressure from an atmospheric pressure (101 kPa) to 0.1 kPa on one-pass full penetration welding of thick high-tensile strength steel plate of 23 mm thickness. A 16 kW disk laser of 1030 nm in wavelength was employed to weld HT980 grade plates at the speed of 5–25 mm/s. In partial penetration welding, it was revealed that humping phenomena occurred easily. Full penetration welding of the high-tensile strength steel plates could not be achieved at 101 kPa. On the other hand, full penetration welding was obtained at the welding speed of less than 20 mm/s at the pressure of less than 10 kPa. Especially, at 0.1 kPa, and 17 and 20 mm/s, sound weld joints without defects were obtained. According to the observation results of a keyhole inlet and a surface molten pool during welding with a high-speed video camera, the melt in front of a keyhole was smaller and the behaviour of a keyhole and a plume was much more stable at 0.1 kPa than at 101 kPa. Moreover, in the full penetration welding, spattering was suppressed under the proper conditions. Such phenomena became more stable in fast welding. It was revealed in laser welding of thick high-tensile strength steel plates that the formation of narrow I-shaped weld beads by achieving full-penetration welding in low vacuum was essential for the production of sound welds without defect.     

镁合金的低功率激光活性焊

试验研究几种常见活性剂对镁合金激光焊接过程的影响.结果表明,在低功率激光条件下,氧化物和氯化物活性剂能够增加镁合金激光焊接的熔深和深宽比.试验中,SiO2能够增加焊接熔深220%左右,证明通过使用活性剂来增加熔深,降低镁合金薄板激光焊接的成本是可行的;焊接热输入是影响活性激光焊熔深的重要参数;熔深增加的主要原因是活性剂微细粉末在激光作用初期增加对激光能量的吸收率.     

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.     

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.     

Nd:YAG激光+CMT电弧复合热源横焊工艺参数对焊缝成形的影响

以304不锈钢为对象,借助焊缝成形参数来评价YAG激光+CMT电弧复合热源横焊焊缝的成形特征,研究了Nd:YAG激光+CMT复合热源横焊过程中焊接工艺参数对焊缝成形的影响.结果表明,在CMT电弧焊接中加入激光可以改善横焊焊缝成形;在激光能量和焊接电流一定时,光丝间距存在一个最佳匹配,使得Nd:YAG激光+CMT复合热源横焊焊缝成形良好;与其它复合热源焊接相对比激光功率对熔深影响较大,对横焊焊缝成形的影响程度与焊接电流有关;焊接速度对横焊焊缝成形影响显著;离焦量对横焊焊缝成形影响较小;电弧功率对横焊焊缝的偏离度影响显著.     

Coaxial hybrid process of hollow cathode TIG and YAG laser welding

To make progress in laser-arc hybrid welding, especially when applied to thick steel plate, it is essential to get the remarkable synergic effect of deep penetration. It is suggested that the effective process is combining hollow cathode TIG with high beam quality laser welding. Penetration of hollow TIG/YAG hybrid welding was investigated by numerical analysis. A keyhole surface model taking into account the laser beam parameter product (BPP) was developed. The calculated keyhole surface profiles of coaxial hybrid welding are wider and deeper than that of YAG laser welding alone. As a result, single-pass butt welding of 20 mm thickness steel plate was achieved under the conditions of BPP = 0.9 mm mrad at laser power 1.8 kW. To confirm numerical results, coaxial hybrid welding tests with a hollow cathode TIG was conducted. The welding penetration was about two times as deep as a YAG laser alone. It was found that coaxial hybrid welding with hollow TIG/YAG had a large synergic effect.     

MB8镁合金CO2激光焊接工艺及接头性能

采用高功率CO2激光器对MB8镁合金进行一系列激光焊接工艺试验,并对焊缝成形的形成规律、接头的微观组织和力学性能进行研究.结果表明:焊缝熔深基本和激光功率呈正比关系,激光功率每增加1 kW,熔深约增加2 mm.焊缝熔合区组织主要由粗大的等轴晶组成,晶粒内有连续析出的条纹状β-Mn,晶粒尺寸约为60μm;热影响区组织为相对细小但大小不一的等轴晶,晶粒尺寸为15~40μm.在消除焊缝表面缺陷后,焊接接头抗拉强度能达到基材的90%左右.此外,熔合区内的粗大晶粒、晶粒内连续分布的第二相及焊缝内气孔缺陷是造成接头拉伸性能低于基材的主要原因.     

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.     

Experimental study of phenomena and multiple reflections during inclined laser irradiating

The phenomena during inclined laser irradiating on the metal surface were investigated to imitate the behaviour on the front keyhole wall in deep penetration laser welding. The results revealed that the velocity of molten layer increased when the laser power density increased. The molten layer could be torn off to form droplets owing to the high evaporating pressure induced by the high power density laser. The heating efficiency of laser induced vapour plume was lower than that of the molten metal. In reflection experiments, only when the incident angle was 88°, the absorption of reflected laser was greater than that of the first incident laser. The concaved surface of molten layer had a great effect to the reflected laser. During multiple reflections, the considerable absorption of laser energy mainly happened in the first two times. The calculated data were closer to the actual conditions when the influence of polarisation variation was avoided.     

Laser welding of Ti6Al4V titanium alloys

The high strength to weight ratio and excellent corrosion resistance of titanium alloys allow diverse application in various fields including the medical and aerospace industry. Several techniques have been considered to achieve reliable welds with minimum distortion for the fabrication of components in these industries. Of these techniques, laser welding can provide a significant benefit for the welding of titanium alloys because of its precision and rapid processing capability. For pulse mode Nd:YAG laser welding, pulse shape, energy, duration, repetition rate and peak power are the most important parameters that influence directly or synergistically the quality of pulsed seam welds. In this study, experimental work involved examination of the welding parameters for joining a 3-mm thick titanium alloy using the Lumonics JK760TR Nd:YAG pulsed laser. It has been determined that the ratio between the pulse energy and pulse duration is the most important parameter in defining the penetration depth. Also it has been observed the variation of pulse duration at constant peak power has no influence on the penetration depth. Consequently, to increase the penetration depth during welding, the role of the laser parameters such as pulse energy and duration and peak power have been investigated to join 3 mm thick Ti6Al4V.     

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.     

Analysis of the effect of deviations in surface preparation technology on the size of internal defects in pulsed-arc welding

Investigations were carried out into the effect of the parameters of pulsed-arc welding on the formation of oxide inclusions when the quality of surface preparation (missing individual areas in grinding) is impaired. The length of the oxide inclusions was estimated on the basis of the ratio of the total length of the oxide films to the length of the welded joint (Σl_f/L_w) and the dispersion was estimated by the deviation of the sum of the lengths of the oxide films to their number (Σl_f/n). The results show that the largest length of the oxide inclusions was obtained at a gap of 1 mm, but the average length of the oxide films in the weld metal is maximum in the gap size range 1.7–2 mm. The increase of the welding speed reduces the total length of the oxide inclusions. The lowest value of the total length of the oxide inclusions in the weld metal and the smallest average dimensions of the films were recorded in pulsed-arc welding in the conditions with ‘hardness’ of the conditions of 0.7 (τ_pulse = 0.3 s, τ_off = 0.21 s).     

Effects of output waveforms on penetration for Nd: YAG laser welding

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薄板钛合金激光焊熔透稳定性临界条件的计算

钛合金薄板熔透激光焊研究发现,在一定焊接参数条件下,由于金属蒸气和光致等离子体的作用,即使焊接过程的工艺参数稳定不变,也存在全熔透不稳定过程,其特征是焊缝表面成形均匀不变,而焊缝背面出现熔透与未熔透之间交替跳跃的成形特征,这种不稳定焊接过程属于激光深熔焊过程的本征特性,主要取决于焊接过程穿孔形成的稳定性。基于小孔形成机理和孔壁能量平衡的分析,提出了小孔穿孔速度与焊接速度相匹配的熔透稳定性物理模型,将穿孔速度与激光功率密度、焊接速度、材料物理性能、板厚联系起来,并建立了熔透焊所需最小激光功率密度计算关系式,理论计算结果与试验结果基本一致。所建立的关系式可用于判断焊接工艺参数深熔焊熔透稳定性。