Correlation spectroscopy as a tool for detecting losses of ligand elements in laser welding of aluminium alloys

The plasma plume induced during laser welding of metals is a mixture of metal vapour, coming from the vaporised weld pool surface and shielding gas. The influence of the shielding gas on the welded joints quality is not yet well understood and very few investigations, to the best of our knowledge, were addressed to study its role in case of welding of aluminium–magnesium alloys. In this paper we present a study of the dynamics of plasma plume produced in laser welding of 5xxx aluminium alloys by means of correlation spectroscopy. By our results we can correlate the influence of the welding speed, in case of ineffective gas shielding, to the loss of alloying elements. Finally, the results obtained are consistent with the EDX analysis performed in post-processing on the welded joints.     

A study on shielding gas contamination in laser welding of non-ferrous alloys

Laser welding of non-ferrous alloys is a high-productivity and cost-effective joining technology, which gained an undoubted interest especially in aerospace, chemical and medical industry, where high strength and corrosion resistant mechanical parts are required. Unfortunately some of the most used non-ferrous alloys are highly reactive with respect to the components of the environmental atmosphere: oxygen, nitrogen, hydrogen and humidity. This reactivity leads to the formation of porosities and to oxides and nitrides inclusion, which are responsible for a decrease of ductility and strength in welded joints.According to this a good shielding technique of the weld pool is of primary importance in order to obtain sound beads and reliable manufacturings. This paper deals with the opportunity of simulating the shielding gas behavior by means of Computational Fluid Dynamics software in order to understand the relationship among the outlet position, the shielding gas type and its flow rate.A simulation activity was carried out in order to evaluate the behavior of shielding gas concentration surrounding the weld pool. The simulated welding environment was simplified without considering the presence and the effect of the plasma plume. The main results concern the shielding gas contamination prediction with respect to the distance from the beam-material interaction zone.     

Dissimilar autogenous full penetration welding of superalloy K418 and 42CrMo steel by a high power CW Nd:YAG laser

Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of γ-FeCr_0.29Ni_0.16C_0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni₃Al γ′ phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility.     

盘型激光焊金属蒸汽特征色调-色饱和度-亮度分析法研究

对于大功率盘型激光焊,金属焊件表面在激光束辐射下强烈汽化并形成等离子体状的金属蒸汽羽状物. 该金属蒸汽羽状物可逆向激光束传输,对激光有明显的屏蔽作用, 降低激光辐射至焊件的能量密度,影响焊接效率和质量. 因此研究金属蒸汽特征变化规律及其与焊接质量之间的关联 ,可实现由金属蒸汽特征实时监测激光焊接状态. 以10 kW大功率盘型连续激光焊接304不锈钢钢板为试验对象, 应用高速摄像机摄取金属蒸汽动态图像,将其转换至色调-色饱和度-亮度空间, 提取金属蒸汽面积、激光束受影响路径长度等相关特征量, 以焊缝熔宽的变化作为衡量焊接状态稳定性的参数. 通过金属蒸汽特征值的均值统计和方差分析,试验证明根据金属蒸汽面积和激光束受影响路径长度等金属蒸汽特征可有效地反映熔宽质量, 从而对焊接状况做出动态评估.     

Optimization of laser butt welding parameters with multiple performance characteristics

This paper presents a study carried out on 3.5 kW cooled slab laser welding of 904 L super austenitic stainless steel. The joints have butts welded with different shielding gases, namely argon, helium and nitrogen, at a constant flow rate. Super austenitic stainless steel (SASS) normally contains high amount of Mo, Cr, Ni, N and Mn. The mechanical properties are controlled to obtain good welded joints. The quality of the joint is evaluated by studying the features of weld bead geometry, such as bead width (BW) and depth of penetration (DOP). In this paper, the tensile strength and bead profiles (BW and DOP) of laser welded butt joints made of AISI 904 L SASS are investigated. The Taguchi approach is used as a statistical design of experiment (DOE) technique for optimizing the selected welding parameters. Grey relational analysis and the desirability approach are applied to optimize the input parameters by considering multiple output variables simultaneously. Confirmation experiments have also been conducted for both of the analyses to validate the optimized parameters.     

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.     

利用环形激光视觉传感的焊缝三维恢复

为了实现复杂条件下的焊缝检测与跟踪,通过激光扫描技术,开发了环形激光视觉传感器,与机器人手爪一起构成焊接机器人"手-眼"系统,从而获得焊缝三维信息。改善了传统的点状或线条状激光视觉传感器信息量少、解释模糊以及跟踪方向单一等问题。研究了用于焊缝定位与跟踪的环形激光视觉传感器系统的内外参量标定,提出了实现焊缝三维计算的模型,最后,以对接接头、搭接接头、角接接头为对象进行了试验验证,并对标定精度进行了分析。结果表明:环形激光在不同的焊缝表现了不同的形态,结合标定试验结果,能够计算被检测焊缝的三维坐标,计算精度能够满足试验要求。     

Gas protection optimization during Nd:YAG laser welding

Many laser processes, such as welding or surface treatments are associated with an undesired phenomenon, which is oxidation. The solution commonly employed to solve this problem approaches the shielding gas and/or the shielding gas device. What we propose in this paper is a methodology with the goal to optimize the protection gas device design as well as the gas flow in the case of laser welding and surface treatments. The pressure created by the gas flow on the sample surface is recorded and analysed together with the operating parameters influence in order to reach the objectives. The nozzle system designed and presented below assures the protection against material oxidation using minimal gas flow rates and increases the welding penetration in the case of high-power Nd:YAG laser welding.     

Relationship between weld quality and optical emissions in underwater Nd: YAG laser welding

In underwater laser welding (LBW), the weld quality is very dependent on the shielding conditions of the local dry cavity when other welding parameters are fixed. Thus, diagnosing the stability of the local dry cavity is the key to monitoring of underwater LBW. In this work, a sensing system containing two optical sensors was set up to detect the infrared and ultraviolet waveband of the optical emissions induced in the welding. The effect of water on the laser welding was studied first by conducting direct underwater welding, which was performed without any method to exclude the water from the welding zone. A kind of plasma that has strong ultraviolet emission formed if the water depth was more than several millimeters, which considerably reduced the penetration depth. Then a gas-shielding nozzle was used to form a local dry cavity for underwater LBW. The relationship between the optical signals and the weld quality with various shielding conditions were investigated. The results show that the detected signal well reflects the shielding condition variations of the local dry cavity. The optimal shielding condition could be determined by signal stability.     

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.     

Spectral diagnostics of a vapor-plasma plume produced during welding with a high-power ytterbium fiber laser

We have conducted spectroscopic studies of the welding plasma formed in the process of welding with an ytterbium fiber laser delivering output power of up to 20 kW. The influence of shielding gases (Ar, He) on different parts of the welding plume is investigated. The absorption coefficient of the laser radiation by the welding-plume plasma is estimated. Scattering of 532-nm probe radiation from particles of the condensed metal vapor within the caustic of a high-power fiber laser beam is measured. Based on the obtained results, conclusions are made on the influence of the plasma formation and metal vapor condensation on the radiation of the high-power fiber laser and the stability of the welding process.     

Analysis of the correlation between plasma plume and keyhole behavior in laser metal welding for the modeling of the keyhole geometry

The process of laser metal welding is widely used in industry. Nevertheless, there is still a lack of complete process understanding and control. For analyzing the process we used two high-speed cameras. Therefore, we could image the plasma plume (which is directly accessible by a camera) and the keyhole (where most of the process instabilities occur) during laser welding isochronously. Applying different image processing steps we were able to find a correlation between those two process characteristics. Additionally we imaged the plasma plume from two directions and were able to calculate a volume with respect to the vaporized material the plasma plume carries. Due to these correlations we are able to conclude the keyhole stability from imaging the plasma plume and vice versa. We used the found correlation between the keyhole behavior and the plasma plume to explain the effect of changing laser power and feed rate on the keyhole geometry. Furthermore, we tried to outline the phenomena which have the biggest effect on the keyhole geometry during changes of feed rate and laser power.     

Optimal design for laser beam butt welding process parameter using artificial neural networks and genetic algorithm for super austenitic stainless steel

Laser welding input parameters play a very significant role in determining the quality of a weld joint. The joint quality can be defined in terms of properties such as weld bead geometry, mechanical properties and distortion. Therefore, mechanical properties should be controlled to obtain good welded joints. In this study, the weld bead geometry such as depth of penetration (DP), bead width (BW) and tensile strength (TS) of the laser welded butt joints made of AISI 904L super austenitic stainless steel were investigated. Full factorial design was used to carry out the experimental design. Artificial Neural networks (ANN) program was developed in MatLab software to establish the relationships between the laser welding input parameters like beam power, travel speed and focal position and the three responses DP, BW and TS in three different shielding gases (Argon, Helium and Nitrogen). The established models were used for optimizing the process parameters using Genetic Algorithm (GA). Optimum solutions for the three different gases and their respective responses were obtained. Confirmation experiment has also been conducted to validate the optimized parameters obtained from GA.     

Comparative analysis of the structure of metal in the arc and laser welding of cryogenic steel

A comparative study of the structure of joints produced by arc and laser welding with austenitic nitrogen-containing cryogenic Cr–Ni–Mn steel is performed. The dimensions of seams, the dispersity of the structure, and the microhardness in different zones of the welded joints are studied. It is shown that laser welding results in better structure-sensitive characteristics of welded joints than arc welding.     

保护条件变化的激光焊接等离子体光谱分析

保护气体在激光焊接过程中起重要的作用,保护条件的改变对焊接质量会有显著地影响。研究Nd∶YAG激光焊过程中保护条件的变化对激光等离子体的电子温度和电子密度等特征参数的影响,通过设计分步减小保护气流量的激光焊试验进行规律性研究,通过模拟实际可能发生的保护不良的激光焊试验进行验证性研究。在试验研究过程中,利用光谱仪采集激光焊接过程中产生的光致等离子体的光谱信息,通过相对光强法计算不同保护条件下等离子体的电子温度,通过斯塔克展宽机制计算不同保护条件下等离子体的电子密度。研究结果表明,保护条件的改变对Nd∶YAG激光焊接过程中产生的光致等离子体的电子温度和电子密度有重要影响,随着保护条件的变化,光致等离子体的电子温度和电子密度的平均值会发生变化,其波动幅度也会发生变化。在保护良好的条件下,等离子体的电子温度和电子密度均较小,且波动幅度也较小;在保护不良的条件下,等离子体的电子温度和电子密度都比较大,且波动幅度也比较大,这种变化的特征有助于对激光焊接过程进行质量监控。     

Effect of carbon nanotube amount on polyethylene welding process induced by laser source

This paper reports a study of the laser welding of polymeric joints composed by two polyethylene sheets, one pristine and the other filled by carbon nanotubes. In order to know the material modifications occurring during the laser exposure and the phenomena that seal the polymeric sheets, the physical, chemical and mechanical features of the welded area were analysed. The mechanical resistance of the welded joint and the structural changes of the polymer were checked by mechanical shear tests, absorption coefficient measurements, calorimetric analyses, mass quadrupole spectrometry and electron microscopy. The welding effectiveness was investigated as a function of the filler concentration.     

Shielding gas influences on laser weldability of tailored blanks of advanced automotive steels

The effects of shielding gas types and flow rates on CO₂ laser weldability of DP600/TRIP700 steel sheets were studied in this work. The evaluated shielding gases were helium (He), argon (Ar) and different mixtures of He and Ar. Weld penetration, tensile strength and formability (Erichsen test) of laser welds were found to be strongly dependent upon the shielding gas types. The ability of shielding gas in removing plasma plume and thus increasing weld penetration is believed to be closely related to ionization potential and atomic weight which determine the period of plasma formation and disappearance. It was found that the higher helium shielding gas flow rate, the deeper weld penetration and the lower weld width.     

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.     

Characteristics of deep penetration laser welding of dissimilar metal Ni-based cast superalloy K418 and alloy steel 42CrMo

Experimental trials of autogenous deep penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 5.0 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser output power, welding velocity and defocusing distance on the morphology, welding depth and width as well as quality of the welded seam were investigated. Results show that full keyhole welding is not formed on both K418 and 42CrMo side, simultaneously, due to the relatively low output power. Partial fusion is observed on the welded seam near 42CrMo side because of the large disparity of thermal–physical and high-temperature mechanical properties of these two materials. The microhardness of the laser-welded joint was also examined and analyzed. It is suggested that applying negative defocusing in the range of Raylei length can increase the welding depth and improve the coupling efficiency of the laser materials interaction.     

An ultrasonic testing method for gas pressure welded joints of reinforcing steel bars

Gas pressure welding for jointing of reinforcing steel bars is most commonly used in Japan and the Far Eastern regions, where there are frequent earthquakes. An ultrasonic testing method has been developed for gas pressure welded joints. An intrinsic feature of the ultrasonic testing method is that there is a correlation between the ultrasonic testing results and the mechanical property (tensile strength) of joints. This feature enables non-destructive detection of gas pressure welded joints with tensile strength below a reference value. The theoretical justification for the correlation between the ultrasonic testing results and the tensile strength of the joints is discussed.