Laser welding of low-thickness zinc-coated and uncoated carbon steel sheets

Laser welding is a competitive joining process for low-thickness plates. The speeds achieved with laser welding using a CO₂ laser and the low heat input introduced to the base material are the main advantages of this process. The widespread use of laser welding will increase the productivity and accuracy of welding operations. Nevertheless, there are still important developments to be made.The results of laser welding of thin sheets (below 1.25 mm) of uncoated and zinc-coated carbon steels are presented. A CO₂ laser was used for the welding of sheets of different thicknesses and to make dissimilar welds (carbon steel to zinc-coated steel). The bead dimensions and the distortions of the welded sheets are analysed.     

Modelling of CO2 laser welding of magnesium alloys

Laser welding is an important joining process for magnesium alloys. These materials are being increasingly used in different applications such as in aerospace, aircraft, automotive, electronics, etc. To date, carbon dioxide (CO₂) neodymium-doped yttrium aluminum garnet (Nd:YAG) and the high power diode laser have been extensively used to investigate the weldability of magnesium alloys. The present work describes an analytical thermal model for the weldability of magnesium alloys (WE43) using an industrial (CO₂) laser source. The main target of the project is to present to the industrial community a simple and rapid tool for the determination of the penetration depth and the bead width as a function of both the incident laser power and welding speed. The proposed model is based on the Davis thermal approach, largely considered for the characterization of the average radius of the liquid zone, aiming at predicting the joint shape. Moreover, since during the welding process considered in this study, a protecting gas is used to avoid joint oxidation, both thermal convection and radiation phenomena in the welding area have been estimated and introduced in our model for a better characterization of the welding process. The obtained results have been compared to the experimental ones and a satisfactory correlation has been observed, indicating the reliability of the model developed in this study.     

Statistical modeling of laser welding of DP/TRIP steel sheets

In this research work, a statistical analysis of the CO₂ laser beam welding of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets was done using response surface methodology. The analysis considered the effect of laser power (2–2.2 kW), welding speed (40–50 mm/s) and focus position (?1 to 0 mm) on the heat input, the weld bead geometry, uniaxial tensile strength, formability limited dome height and welding operation cost. The experimental design was based on Box–Behnken design using linear and quadratic polynomial equations for predicting the mathematical models. The results indicate that the proposed models predict the responses adequately within the limits of welding parameters being used and the welding speed is the most significant parameter during the welding process.     

Research on laser welding of high-strength galvanized automobile steel sheets

The paper presented some useful results of deep-penetration laser welding of high-strength galvanized steel sheets, which had been carried out by a self-made CO₂ laser unit with maximum power output of 1.5 kW. The workpieces of high-strength galvanized automobile steels with thickness of 1.5 mm were butt-welded with argon as the shielding gas. The effects of such factors as laser power, welding speed, focal position, shielding gas and zinc vaporization on the quality of welds are investigated. With the processing parameters optimized and the proper shielding gas used in both coaxial and side-blow direction, most of the defects, such as pores, cracks and softening in HAZ, can be avoided in laser welding joints. The microstructure, the hardness distribution and the elemental distribution in the welding joints can be changed due to laser heating and recrystallization. In order to determine the mechanical properties of the welding joints, the static tensile strength was tested. Experimental results indicated that both the strength and microhardness of welding joints were higher than those of the base metal. Consequently, the welding quality is reliable for manufacturing of automobile bodies.     

Sinterability studies on K0.5Na0.5NbO3 using laser as energy source

The sinterability of K_0.5Na_0.5NbO₃ (KNN) ceramics by a laser beam has been investigated in the present research. A 100 W CO₂ laser with a beam diameter of 0.6 mm has been used to sinter the KNN specimens prepared on a uniaxial pressing machine. The relations between laser power and thickness of densified layer, crystallographic structures and phase compositions have been studied. A comparison has been made between laser and furnace sintered KNN samples according to the SEM, XRD and XRF results. The possibility of KNN used for the layer-wise laser direct sintering 3D components has been confirmed in this paper.     

A highly efficient 5 kW peak power Nd:YAG laser with time-shared fiber optic beam delivery

Here, we report on the development of an efficient, high peak power lamp pumped Nd:YAG laser with time-shared fiber optic beam delivery. A maximum average output power of 270 W with 100 J maximum pulse energy and 5 kW peak power has been achieved with an electrical to laser conversion efficiency of 5.4%, which is on higher side for typical lamp pumped solid-state lasers. We have improved efficiency by spectral conversion and water flow optimization in the pump cavity, with a resulting beam quality comparable to commercial systems of similar power level. The resonator has been designed for stable operation from single-shot to 200 Hz repetition rate. A study of pulse-to-pulse laser energy stability for different resonator configurations has also been performed. The resonator was designed to achieve a good beam quality for the whole range of operation with a maximum beam parameter product of 15 mm mrad (M²45). A simple mechanism for time-shared fiber optic port selection has also been devised. Material processing applications such as cutting of stainless steel sheets up to 14 mm thickness and welding of metals such as carbon steel with weld depths up to 2 mm using the developed laser system has also been reported.     

Effect of magnetic field on plasma control during CO2 laser welding

During high power CO₂ laser beam welding, the plasma above the keyhole has a shielding effect that it not only absorbs part of the laser energy but also defocuses the laser beam. As a result, the welding efficiency and the aspect ratio of the welds are influenced. In order to reduce the effect of plasma, helium as a plasma control gas has been used successfully and effectively. However, the cost of helium in Southeast Asia is extremely high and therefore the production cost is significantly increased when helium is used as a continuous bleeding plasma control gas. To search for an alternative plasma control technique, feasibility in using magnetic effect as a control tool is explored in this paper. The influences of the magnetic field strength, laser power, welding speed, field direction and shielding gas (e.g. helium and argon) on the penetration depth and the width of bead were also investigated. Experimental results indicated that the magnetic field can influence the shielding effect of the plasma without using plasma control gas. It was found that at a suitable magnetic field strength the penetration depth was increased by about 7%, but no significant difference on the width of bead was found. Moreover, it was shown that the plasma control effect can be achieved at low magnetic field strength and the penetration depth can be increased significantly under argon atmosphere.     

Optimization of laser welding of DP/TRIP steel sheets using statistical approach

Generally, the quality of a weld joint is directly influenced by the welding input parameter settings. Selection of proper process parameters is important to obtain the desired weld bead profile and quality. In this research work, numerical and graphical optimization techniques of the CO₂ laser beam welding of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets were carried out using response surface methodology (RSM) based on Box–Behnken design. The procedure was established to improve the weld quality, increase the productivity and minimize the total operation cost by considering the welding parameters range of laser power (2–2.2 kW), welding speed (40–50 mm/s) and focus position (?1 to 0 mm). It was found that, RSM can be considered as a powerful tool in experimental welding optimization, even when the experimenter does not have a model for the process. Strong, efficient and low cost weld joints could be achieved using the optimum welding conditions.     

5083铝合金光纤激光-TIG复合焊接工艺研究

采用IPG YLS-6000光纤激光器和Fronius MagicWave3000job数字化焊机,对4mm厚5083H116铝合金进行了复合焊接试验。研究了电源特性、电流大小和热源间距等工艺参数对光纤激光-钨极惰性气体保护焊(TIG)复合焊接焊缝成形的影响规律,并分析了焊接接头的缺陷、显微硬度及力学性能。结果表明,光纤激光-TIG复合焊接5083铝合金,能够明显改善焊缝成形,提高焊接过程稳定性,特别是与变极性TIG电弧复合效果更为显著;光纤激光与变极性TIG电弧复合焊接,采用激光在前的方式,电弧电流150A,且热源间距不大于4mm,可以得到具有明亮金属光泽和均匀鱼鳞纹的焊缝,焊缝无气孔和裂纹缺陷,其表面有少量的下凹;复合焊接接头抗拉强度为318MPa,达到母材强度的93%,延伸率为7.6%,高于单光纤激光焊接,断口分析为韧性断裂。     

Laser applications in nuclear power plants

This paper reports the state of the art of using a solid-state Nd:YAG laser for material processing applications such as cutting, welding and drilling of several components of operational nuclear reactors in radioactive environment. We have demonstrated several advantages of laser-based material processing over conventional methods, and these are discussed briefly. At NPCIL, we have used laser techniques to cut stainless steel sheets up to 14 mm thickness and stainless steel weld up to a depth of 3 mm. This remotely operable laser system has been engineered for its robustness with proper fixtures and tooling for various material processing operations on industrial scale.     

Weld-bead profile and costs optimisation of the CO2 dissimilar laser welding process of low carbon steel and austenitic steel AISI316

The dissimilar full depth laser-butt welding of low carbon steel and austenitic steel AISI 316 was investigated using CW 1.5 kW CO₂ laser. The effect of laser power (1.1–1.43 kW), welding speed (25–75 cm/min) and focal point position (?0.8 to ?0.2 mm) on the weld-bead geometry (i.e. weld-bead area, A; upper width, W_u; lower width, W_l and middle width, W_m) and on the operating cost C was investigated using response surface methodology (RSM). The experimental plan was based on Box–Behnken design; linear and quadratic polynomial equations for predicting the weld-bead widthness references were developed. The results indicate that the proposed models predict the responses adequately within the limits of welding parameters being used. The regression equations were used to find optimum welding conditions for the desired geometric criteria.     

Spectroscopic, energetic and metallographic investigations of the laser lap welding of AISI 304 using the response surface methodology

Spectroscopic signals originated by the laser-induced plasma optical emission have been simultaneously investigated together with energetic and metallographic analyses of CO₂ laser welded stainless steel lap joint, using the Response Surface Methodology. This statistical approach allowed us to study the influence of the laser beam power and the laser welding speed on the following response parameters: plasma plume electron temperature, joint penetration depth and melted area. A clear correlation has been found between all the investigated response parameters. The results have been shown to be consistent with quantitative considerations on the energy supplied to the workpiece as far as the laser power and travel speed were varied. The regression model obtained in this way could be a valuable starting point to develop a closed loop control of the weld penetration depth and the melted area in the investigated process window.     

Development of underwater laser cutting technique for steel and zircaloy for nuclear applications

In nuclear field, underwater cutting and welding technique is required for post-irradiation examination, maintenance, decommissioning and to reduce storage space of irradiated materials like used zircaloy pressure tubes etc., of nuclear power plants. We have developed underwater cutting technique for 4.2 mm thick zircaloy pressure tubes and up to 6 mm thick steel using fibre-coupled 250 W average power pulsed Nd:YAG laser. This underwater cutting technique will be highly useful in various nuclear applications as well as in dismantling/repair of ship and pipe lines in water.     

Third harmonic generation of CO2 laser radiation in AgGaSe2 crystal

Generation of third harmonic of CO₂ laser radiation has been obtained in a type-II, ϑ=57° cut 9 mm thick AgGaSe₂ crystal for the first time by sum-frequency-mixing of the fundamental with its second harmonic, the latter being obtained using another type-I, ϑ=55° cut 11 mm thick AgGaSe₂ crystal. The energy conversion efficiencies obtained for second harmonic and third harmonic generations are 6.3% and 2.4% respectively with the input fundamental pump power density of 5.9 MW/cm² only. The wavelength of the fundamental CO₂ laser radiation used for the generation of harmonics is 10.6 μm, P(20) line. A compact TEA CO₂ laser source has been built in the laboratory.     

Laser rapid manufacturing of Colmonoy-6 components

This paper introduces a new manufacturing technique for the fabrication of Colmonoy-6 components using laser rapid manufacturing (LRM). LRM is a upcoming rapid manufacturing technology, being developed at various laboratories around the world. It is similar to laser cladding at the process level with different end applications. In general, laser cladding technique is used to deposit material on the substrate either to improve the surface properties or to refurbish the worn out parts, while LRM is capable of near net shaping the components by layer-by-layer deposition of the material directly from CAD model. In the present study, a high power continuous wave (CW) CO₂ laser system, integrated with a co-axial powder-feeding system and three-axis workstation was used. The effect of processing parameters during multi-layer deposition of Colmonoy-6 has been studied and optimized to fabricate about a dozen bushes. Thus fabricated bushes were finally machined and ground to achieve the desired dimensions and surface finish. These bushes were tested for non-destructive testing (like-ultrasonic testing, Dye-penetrant testing), metallographic examinations, micro-hardness measurement, X-ray diffraction and thermal ageing. Results compared well with those fabricated by deposition of Colmonoy-6 on austenitic stainless steel rods using gas Tungsten arc welding (GTAW). Thus, the new manufacturing technique not only produced quality product, but also minimized machining of hard-faced material and brought significant saving of time and costly Colmonoy-6 material.     

Nd：YAG板条激光增益介质封装技术研究进展北大核心CSCD

为实现大尺寸、高储能的Nd:YAG板条激光增益介质模块的高可靠性工作,必须找到合适的封装工艺解决大尺寸无空洞、低热阻界面连接问题和界面低应力、低透射波前畸变问题。在充分了解板条激光增益介质和冷却单元的特性后,选择了延展性好的铟作为焊料,实验得到最佳焊料层厚度,通过改进封装工艺的钎焊技术将这两部分可靠地连接在一起。改进的封装工艺实现了钎焊面积大于40cm2,空洞率小于0.5%,最大空洞面积小于1mm2的技术指标,工艺重复性大于90%。通过对焊料层的优化实现了尺寸为150.2mm×30mm×2.5mm板条激光增益介质静态透射波前畸变小于1μm,成品率优于80%,静态透射波前畸变小于1.5μm的模块成品率接近100%的技术指标。采用改进封装工艺焊接的单模块Nd:YAG板条激光器稳定输出功率达到4000 W。     

Sheet metal welding using a pulsed Nd: YAG laser-robot

This paper presents a pulsed Nd: YAG laser-robot system for spot and seam welding of mild steel sheets. The study evaluates the laser beams behaviour for welding, and then investigates pulsed Nd: YAG laser spot and seam welding processes. High pulse power intensity is needed to initiate the key-hole welding process and a threshold pulse energy to reach full penetration. In seam welding, a weld consists of successive overlapping spots. Both high pulse energy and high average power are needed to keep the key-hole welding going. A 70% overlap is used to define overlapping spot welding as seam welding and to optimize process parameters because a high tensile strength joint compatible with the strength of the base material can be obtained when the overlap is ≥70%; at the same time a smooth seam with full penetration is obtained. In these cases, the joints in pulsed Nd: YAG laser welding are comparable in strength to those obtained with CO₂ laser welding. Robot positioning and motion accuracies can meet the demands of Nd: YAG laser sheet metal welding, but its cornering accuracy affects the welding processes. The purpose of the study is to evaluate the YAG laser-robot system for production in the automotive industry.     

Influences of laser in low power YAG laser-MAG hybrid welding process

The influences of laser defocusing amount △z, laser power P, space distance DLA between laser and arc on weld penetration, arc modality and stability are investigated in low power YAG laser and metal active gas （laser-MAG） hybrid welding process. The experimental results indicate that the effects of laser-induced attraction and contraction of MAG arc are emerged in hybrid welding process, which result in the augmentation of hybrid welding energy. When DLA ： -0.5 - 2 mm, △z = -2 - 2 mm and P ≥ 73 W, the synergic efficiency between laser and MAG arc is obvious, the cross section at the root of hybrid arc is contracted and the hybrid weld penetration is increased. The maximal ratio of hybrid/MAG weld penetration is 1.5 and the lowest YAG laser power that augments MAG arc is 73 W. The input of YAG laser makes the stabilities of arc ignition and combustion prominent in hybrid welding process.     

Development of portable laser machining system for laser writing applications

This study presents a portable laser machining system that consists of a fiber-optic diode laser source with a wavelength of 808 nm, optic/opto-mechanical components, a laser scanning module, and a laser energy control module. The laser beam quality was measured at different operation frequencies during system evaluation. The experimental results of beam profile evaluation indicate that the enlarged collimated beam was the TEM₀₀ mode with a roundness of approximately of 96%. The output laser power level increased as the pulse frequency increased during laser power evaluation. To control the rotating angle of the galvanometric scanning system, the deflective angle was adjusted using a 0.192 voltage to obtain a deflective value of 1mm and the maximum scan field of 100 × 100mm². The laser source operated at different frequencies, with pulse widths ranging from 530 to 48 μs. Finally, the proposed machine can also be used for black thick paper laser writing applications.     

EU194 in the United Kingdom

This paper describes some of the results obtained in the United Kingdom within the Eureka Eurolaser Project EU194 — Industrial Applications of High-Power CO₂ Lasers. Areas selected include laser welding of aluminium alloys, in particular with respect to the tensile and formability properties of the welds produced; a new technique, involving laser beam spinning, is described for thick section cutting of mild steel using only modest laser power; a comparison of thermal cutting processes is presented in terms of the kerf widths, speed, squareness, roughness and size of heat affected zone produced; a mathematical model for the welding of thin sheet material is described. along with the results of experimental verification of the predictive capabilities of the model; and finally, laser welding of tailored blanks for application in the automotive industry is described, with examples of two particular areas where this technology might be used to advantage.