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.     

A study of the shielding gas influence on the laser beam welding of AA5083 aluminium alloys by in-process spectroscopic investigation

In laser welding, the shielding gas is commonly used to stabilize the welding process, to improve welded joints features and to protect the welded seam against oxidization. Besides the type of shielding gas used, the nozzle parameters play an essential role. In fact, the chemical composition of the shielding gas and the flow geometry are key factors limiting the size of the plasma plume and its contamination by the surrounding atmosphere, and affecting the final quality of the welded joints.In this work, we present an experimental study on the complex physical phenomena occurring during the interaction between the plasma plume, the laser beam and the shielding gas by using an in-process spectroscopic investigation of the plasma plume characteristics under different operating conditions. A correlation was found between the spectral features and the formation of oxide layers on the surface of the welding seam, caused by defective gas shielding and by the vaporization of alloying elements. Experimental results have given useful indications for the development of innovative welding nozzle for application in laser welding of aluminium alloys.     

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.     

Effect of welding wires on microstructure and mechanical properties of 2A12 aluminum alloy in CO2 laser-MIG hybrid welding

This paper represented the effect of welding wires on microstructure and mechanical properties of 2A12 aluminum alloy in CO₂ laser-metal inter gas (MIG) hybrid welding. Plates of 2A12 aluminum alloy were welded by ER4043 and ER2319 welding wires, respectively. Full penetration joints without any defects were produced. The X-ray diffraction was used to analyze the phase composition, while the scanning electron microscopy (SEM) was conducted to study the microstructure, segregation behaviors of major alloying elements and the eutectics formed at dendrite boundaries in the joints. The results showed that silicon and copper were concentrated at the dendrite boundaries and α-Al + Si + Al₂Cu + Mg₂Si eutectic was formed if the ER4043 welding wire was used. However, only copper was concentrated at the dendrite boundaries and α-Al + θ eutectic was formed by ER2319 welding wire. Finally, the tensile tests were performed and the fracture surfaces were analyzed. The results showed that the joint efficiency by ER2319 and ER4043 welding wires reached up to 78% and 69%, respectively. Coarse dimples and voids had been observed in the fractographs. The joints showed a transgranular type failure.     

Pulsed Nd:YAG laser welding of AISI 304 to AISI 420 stainless steels

The technique to weld AISI 304 stainless steel to AISI 420 stainless steel with a pulsed Nd:YAG laser has been investigated. The main objective of this study was to determine the influence of the laser beam position, with respect to the joint, on weld characteristics. Specimens were welded with the laser beam incident on the joint and moved 0.1 and 0.2 mm on either side of the joint. The joints were examined in an optical microscope for cracks, pores and to determine the weld geometry. The microstructure of the weld and the heat affected zones were observed in a scanning electron microscope. An energy dispersive spectrometer, coupled to the scanning electron microscope, was used to determine variations in (weight %) the main chemical elements across the fillet weld. Vickers microhardness testing and tensile testing were carried out to determine the mechanical properties of the weld. The results of the various tests and examinations enabled definition of the best position for the incident laser beam with respect to the joint, for welding together the two stainless steels.     

Microstructure characteristics of laser-MIG hybrid welded mild steel

To deepen the understanding of laser-arc hybrid welding, the weld shape and microstructure characteristics of laser-metal inert gas hybrid welded mild steel were analyzed. The results showed typical hybrid weld could be classified as two parts: the wide upper zone and the narrow nether zone, which were defined as arc zone and laser zone, respectively. In the hybrid weld, the microstructure, alloy element distribution and microhardness all have evident difference between laser zone and arc zone. The microstructure of arc zone consists of coarse columnar dendrite and fine acicular dendrite between the columnar dendrites, but that of laser zone is composed of fine equiaxed dendrite in weld center and columnar dendrite around the equiaxed dendrite. Compared to arc zone, laser zone has finer grain size, higher microhardness, smaller alloy element content in the fusion zone and narrower heat affected zone. The discussions demonstrated that the observed difference was caused by the difference of temperature gradient, crystallizing and the effects of arc pressure on the molten pool between laser zone and arc zone.     

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.     

The influence of laser pulse waveform on laser-TIG hybrid welding of AZ31B magnesium alloy

By dividing laser pulse duration into two parts, three kinds of laser waveforms are designed, including a high power density pulse (HPDP) laser in a short duration set at the beginning of the laser waveform. This paper aims to find out the laser pulse waveform and idiographic critical values of HPDP, which can affect the magnesium penetration in laser-tungsten inert gas (TIG) hybrid welding. Results show that when the laser pulse duration of HPDP is not more than 0.4 ms, the welding penetration values of lasers with HPDP are larger than otherwise. Also, the welding penetration values of laser with HPDP have increased by up to 26.1%. It has been found that with HPDP, the laser can form the keyhole more easily because the interaction between laser and the plate is changed, when the TIG arc preheats the plate. Besides, the laser with high power density and short duration strikes on the plates so heavily that the corresponding background power can penetrate into the bottom of the keyhole and maintain the keyhole open, which facilitates the final welding penetration.     

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.     

A study of the radiation of a Nd:YAG laser-MIG hybrid plasma

To study the coupling of a Nd:YAG laser beam and a MIG arc with the spectrum of the plasma, a new hollow probe method is used to collect radiation from specific points within the arc plasma. With the probe, the spatial distributions of the MIG arc radiation with and without the Nd:YAG laser hybrid plasma are collected and analyzed by a fiber spectrometer. Through analysis of the spectrum, radiation in the ultraviolet (Fe II dominated), visible (Fe I dominated) and infra (Ar I dominated) spectral zones are acquired for comparison of the difference between the MIG and Laser-MIG hybrid welding processes. The electronic temperature is calculated using the Boltzmann plot method and the electronic density is calculated using the Stark broadening method. The results show that the Laser-MIG hybrid welding processes cause the plasma energy to focus on the center of the welding arc and approach the welding pool. An ionizing duct abundant with Fe ions will be formed near the position of the impinging laser beam, which makes the welding arc stable. The electronic temperature of the Laser-MIG hybrid welding arc is a little higher than that of the MIG. The electronic density of Laser-MIG hybrid welding is higher than that of the MIG.     

Concurrent laser welding and annealing exploiting robotically manipulated optical fibers

Present investigation reports on the effects of incorporating pre- and post-heating on the mechanical properties of laser-welded joints, in normal air condition. Two common types of steels, i.e. mild steel, and stainless steel were welded with Lumonic's MS 830 Nd³⁺:YAG laser machine, with an output capacity of 400 W. Due to the low integrated energy input required for laser welded joints, the welded region are often cooled too rapidly via conduction to the surrounding material and atmosphere, which leads to hardness discontinuities in the fusion and heat affected zone. The effects of in-process laser annealing on the mechanical properties and microstructure of laser-welded joints are important in any manufacturing operation. To improve the poor weld characteristics, this work investigates the use of automated dual-beam delivery system to implement a pre- or post-heating technique, simultaneously with the welding process. The results show that proper selection of the control parameters for the pre- or post-heating can reduce the hardness of the weld significantly and improve the welded joints mechanical properties, such as higher tensile strength and better durability.     

Dual beam method for laser welding of galvanized steel: Experimentation and prospects

Laser welding of zinc-coated steel sheets in lap configuration poses a challenging problem, because of the zinc vapours spoiling the quality of the weld. In continuation to the earlier work, the novel solution of dual laser beam method for lap welding of galvanized steel sheets is discussed here in view of the recently obtained observations and ensuing concerns. In this method the precursor beam cuts a slot, thus making an exit path for the zinc vapours, while the second beam performs the needed welding. The metallurgical analysis of the welds is encouraging showing absence of zinc in the welded area. In the current work on this technique, new experimental results have been obtained verifying the earlier observations. Along with this, the possibility of using a transversely split-up beam for the welding purposes with this approach is discussed and analyzed in this paper. This new technique is expected to be very useful in prospective industrial applications requiring higher welding throughput along with the needed quality.     

Effects of relative positioning of energy sources on weld integrity for hybrid laser arc welding

This study is concerned with the effects of laser and arc arrangement on weld integrity for the hybrid laser arc welding processes. Experiments were conducted for a high-strength steel using a 4 kW Nd: YAG laser and a metal active gas (MAG) welding facility under two configurations of arc–laser hybrid welding (ALHW) and laser–arc hybrid welding (LAHW). Metallographic analysis and mechanical testing were performed to evaluate the weld integrity in terms of weld bead geometry, microstructure and mechanical properties. The morphology of the weld bead cross-section was studied and the typical macrostructure of the weld beads appeared to be cone-shaped and cocktail cup-shaped under ALHW and LAHW configurations, respectively. The weld integrity attributes of microstructure, phase constituents and microhardness were analyzed for different weld regions. The tensile and impact tests were performed and fracture surface morphology was analyzed by scanning electron microscope. The study showed that ALHW produced joints with a better weld shape and a more uniform microstructure of lath martensite, while LAHW weld had a heterogeneous structure of lath martensite and austenite.     

Influence of shielding gas pressure on welding characteristics in CO2 laser-MIG hybrid welding process

The droplet transfer behavior and weld characteristics have been investigated under different pressures of shielding gas in CO2 laser and metal inert/active gas (laser-MIG) hybrid welding process. The experimental results indicate that the inherent droplet transfer frequency and stable welding range of conventional MIG arc are changed due to the interaction between CO2 laser beam and MIG arc in laser-MIG hybrid welding process, and the shielding gas pressure has a crucial effect on welding characteristics. When the pressure of shielding gas is low in comparison with MIG welding, the frequency of droplet transfer decreases, and thedroplet transfer becomes unstable in laser-MIG hybrid welding. So the penetration depth decreases, which shows the characteristic of unstable hybrid welding. However, when the pressure of shielding gas increases to a critical value, the hybrid welding characteristic is changed from unstable hybrid welding to stablehybrid welding, and the frequency of droplet transfer and the penetration depth increase significantly.     

Research on laser welding of vehicle body

Based on many experiments of CO₂ laser welding of vehicle body, joint microstructure and stress–strain curve of specimen are analyzed. The deep punching performance acquired by adopting Ar as protective gas is better than that of the one acquired by adopting N₂ as protective gas. Meanwhile the percentage of zinc in welding seam can be effectively controlled by means of blowing side protective gas. In this paper, welding penetration and width are shown to vary with laser power and speed of welding. The results indicate that some flaws such as gas hole, crack and softening of HAZ do not appear in laser welding seam in sheet steel of automobile bodies if technology parameters optimizes. The deep punching performance of tailor-welding sheet is fine.     

Influence of microsupersonic gas jets on nanosecond laser percussion drilling

This paper reports on etching rates and hole quality for nanosecond laser percussion drilling of 200-μm thick 316L stainless steel performed with micro supersonic gas jets. The assist-gas jets were produced using nozzles of 200, 300 and 500 μm nominal throat diameters. Air and oxygen were used separately for the process gas in the drilling trials and the drilling performance was compared to drilling in ambient conditions. The highest etch rate of 1.2 μm per pulse was obtained in the ambient atmosphere condition, but this was reduced by about 50% with assist-air jets from the 200 μm nozzle. Increasing the jet diameter and/or using oxygen assist gas also decreased the etching rate and increased the hole diameter. The 200 μm nozzle using air-assist jets produced the least amount of recast and gave the best compromise for etching rate. A combination of plasma shielding and different gas dynamic conditions inside the holes and at the surface are correlated to the observations of different drilling rates and hole characteristics.     

Energy law preserving continuous finite element schemes for a gas metal arc welding system

In this paper a modifed continuous energy law was explored to investigate transport behavior in a gas metal arc welding(GMAW)system.The energy law equality at a discrete level for the GMAW system was derived by using the finite element scheme.The mass conservation and current density continuous equation with the penalty scheme was applied 10 improve the stability.According to the phase-field model coupled with the energy law preserving method,the GMAW model was discretized and a metal transfer process with a pulse current was simulated.It was found that the numerical solution agrees well with the data of the metal transfer process obtained by high-speed photography.Compared with the numerical solution of the volume of fuid model,which was widely studied in the GMAW system based on the finite element method Euler scheme,the energy law preserving method can provide better accuracy in predicting the shape evolution of the droplet and with a greater computing efficiency.     

Modelling the spot shape influence on high-speed transmission lap welding of thermoplastics films

Modelling high-speed laser lap welding of thermoplastic films has been accomplished and the influence of laser beam spot shape, dimensions, and position relative to sample displacement was analysed.Engineering parameters predicted by the model were applied to lap weld of high- and low-density polyethylene transparent samples with thickness between 10 and 100 μm, and experimentally validated. Experimental set-up allowed reaching welding constant linear velocities up to 10 m/s.Theoretical and experimental data show coincidence. Weld strength increases for larger beam spot diameters, and elliptical beam spots increase weld efficiency, allowing higher processing speeds or decreasing required laser power. An angular deviation of elliptical beam spot with regard to the sample's movement direction causes an increase of weld strength and a decrease of welding speed.     

侧吹辅助气流对激光立焊5A90铝锂合金气孔影响

基于试验设计软件Design Expert V8设计试验,开展激光焊接5A90铝锂合金工艺试验,研究侧吹保护气流参数对焊缝气孔的影响规律,拟合侧吹流量、侧吹角度和等效气孔点数的函数图像,建立数学模型,优化保护气流参数,预测Ⅰ级焊缝保护气流参数范围。实验结果表明：侧吹气流的加入能明显抑制焊缝气孔,侧吹流量是显著因子,对气孔影响较大,随着流量的增加,气孔点数先增后减,在侧吹流量为5~7 L/min时有最小气孔点数;侧吹角度和交互项是不显著因子,在侧吹角度为20~30范围内,有一个较优值。气孔点数的最优响应模型为三阶模型,该模型拟合良好,经验证试验检验,模型误差范围小于10%,Ⅰ级焊缝易在侧吹流量为3~9 L/min,侧吹角度为15~52范围内获得。     

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4 mm厚5083 H116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150 A,且热源间距不大于4 mm,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318 MPa,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。