Laser-arc hybrid welding of wrought to selective laser molten stainless steel

Selective laser melting (SLM) is a successful tool-free powder additive technology. The success of this manufacturing process results from the possibility to create complex shape parts, with intrinsic engineered features and good mechanical properties. Joining SLM steel to similar or dissimilar steel can overcome some limitations of the product design like small dimension, undercut profile, and residual stress concentration. In this way, the range of applications of the SLM process can be broadened. In this paper, the hybrid laser welding of selective laser molten stainless steel was investigated. A high-power fiber laser was coupled to an electric arc and austenitic stainless steel wrought and SLM parts were welded together. The power and speed parameters were investigated. The joints were analyzed in terms of weld bead profile, microstructure, microhardness, and tensile test. The efficiency of the welding process was evaluated through the line energy input versus the weld molten area.     

Numerical modeling on the formation process of keyhole-induced porosity for laser welding steel with T-joint

A mathematical model of laser welding steel with a T-joint was developed in this paper to investigate the formation process of keyhole-induced porosity, helping to understand the mechanism of porosity formation in laser welding steels with heavy section. Solidification model and adiabatic bubble model were coupled in this model, which could more approximately reflect the formation process of bubble and its evolution into porosity. The volume-of-fluid (VOF) method was taken to track free surfaces of keyhole and porosity. The numerical results showed that the unstable keyhole during the laser welding process induced the collapse of keyhole and then resulted in bubbles in the molten pool. These bubbles moved following with the fluid flow in the molten pool, where some bubbles could escape out of molten pool under the competition of flow and solidification speed. But some bubbles captured by a solidified wall during the migration process in the molten pool would evolve into porosities. It was also found that some bubbles formed adjacent to a fusion line were easier to be captured by a solidification surface, which could give explanation for some porosities occurring close to the fusion line. A good agreement of simulation and experimental results proved the reliability of this mathematical model, while the mechanism of porosity formation was better clarified with this model.     

铝合金MIG+激光复合焊接工艺研究

研究轻量化轿车用3A21铝合金M IG 激光复合焊接工艺,探讨工艺参数对焊缝成型的影响规律及激光与电弧的复合作用。试验结果表明,采用M IG 激光复合焊接工艺可以显著提高熔深和焊速,达到采用小功率激光焊机实现铝合金的激光焊接。在比较宽的工艺参数范围内M IG YAG激光复合焊接铝合金具有焊缝成型美观等优点,熔深和焊速均显著提高,大大提高生产率。     

Investigation on weldability of magnesium alloy thin sheet T-joints: arc welding, laser welding, and laser-arc hybrid welding

Four welding methods, including laser welding, gas tungsten arc (GTA) welding, laser–GTA hybrid welding, and laser–GTA hybrid welding with cold welding wire, are used to investigate the weldability of T-joints of magnesium alloy thin sheet. Stake welding process is presented in this paper in order to overcome the defects, such as stress concentration and deformation, and improve the accessibility of T-joints in fillet welding process. The effect of heat source type on weldability of T-joints is analyzed. The microstructures and mechanical properties are investigated. Experimental results indicate that comparing with the other three welding methods, laser–GTA hybrid welding with cold welding wire is the most effective process for T-joints of magnesium alloy thin sheet. In this process, T-joints are full penetration and the toes are smooth and round, and besides, reinforcement forms on the upside of weld bead by the filled wire. The mechanical properties of T-joints made with laser–GTA hybrid welding with cold welding wire achieve 90 % of that of base metal and are superior to that without welding wire.     

Adaptive volumetric heat source models for laser beam and laser + pulsed GMAW hybrid welding processes

Laser + pulsed gas metal arc welding (GMAW) hybrid welding process is an attractive joining technology in industry due to its synergy of the two processes. It is of great significance to conduct fundamental investigations involving mathematical modeling and understanding of the hybrid welding process. In this study, an adaptive heat source model is first developed for laser beam welding. Through combining the ray-tracing method with the keyhole profile determination technique based on the local energy balance, the keyhole shape and size are calculated and correlated to the distribution parameters of the volumetric heat source model. Then, thermal action characteristics in laser + pulsed GMAW hybrid welding are considered from viewpoint of macro-heat transfer, and a combined volumetric heat source model for hybrid welding is developed to take consideration of heat input from laser, pulsed gas metal arc, and overheated droplets. Numerical analysis of thermal conduction in hybrid welding is conducted. The shape and size of fusion zone and weld dimension in the quasi-steady state are calculated for various hybrid welding conditions, which have a fair agreement with the experimental results.     

Process modeling and parameter optimization using neural network and genetic algorithms for aluminum laser welding automation

In the automotive industry, applying aluminum alloys to car chassis have become an important concern in order to reduce car weight. In aluminum laser welding, the strength of weld is typically reduced by porosity, underfill, and magnesium loss. In order to overcome these problems, laser welded with filler wire was suggested. In this study, experiments on the laser welding AA5182 of aluminum alloy with AA5356 filler wire were performed with respect to laser power, welding speed, and wire feed rate. The experiments showed that the tensile strength of the weld was higher than that of the base material under certain conditions. Using the experimental results, a neural network model was proposed to predict the tensile strength. To optimize the process parameters, a fitness function was formulated, taking into account weldability and productivity. A genetic algorithm was used to optimize the laser power, welding speed, and wire feed rate. The optimal value of these parameters was considered to be the proper process conditions in terms of weldability and productivity.     

Interaction between laser-induced plasma/vapor and arc plasma during fiber laser-MIG hybrid welding

Hybrid plasma is an important physical phenomenon in fiber laser-MIG hybrid welding. It greatly affects the stability of the process, the quality of the weld, and the efficiency of energy coupling. In this paper, clear and direct proofs of these characteristics are presented through high-speed video images. Spectroscopic analysis is used to describe the characterization of hybrid plasma. The hybrid plasma forms a curved channel between the welding wire and the keyhole during the fiber laser-MIG hybrid welding process. The curved channel is composed of two parts. The laser-induced plasma/vapor expands due to the combined effect of the laser and the MIG arc, forming an ionization duct, which is one part of the curved channel. The resistance of the duct is smaller than that of other locations because of the rise in electrical conductivity. Consequently, the electrical arc is guided through the duct to the surface of the material, which is the other part of the curved channel. The spectral intensities of metal elements in laser-MIG hybrid welding are much stronger than those in MIG-only welding, whereas the spectral intensities of shielding gas element in laser-MIG hybrid welding are much weaker.     

Application of DFB diode laser sensor to reacting flow (I)- estimation and application to laminar flames -

Diode laser sensor for measuring gas temperature and species concentration in combustion chamber was developed using 2.0 μm distributed teed back lasers. To evaluate the measurement sensitivity of diode laser sensor system, CO₂ survey spectra near 2.0 μm were measured and compared with the calculated one. This diode laser absorption sensor was applied to measure gas temperatures in a premixed flat flame of CH₄-air mixture. Experimental results were in good agreement with the values by an R-type thermocouple within 6.12%. In addition, successful demonstration of measurement of gas temperature and species concentration in a soot flame showed the promising possibility of diode laser absorption sensors for practical combustion system with non-intrusive method.     

Effect of gap on plasma and molten pool dynamics during laser lap welding for T-joints

The aim of the present research is to discuss the effect of gap on plasma plume, keyhole, and molten pool dynamics during laser lap welding for T-joints. The authors observe plasma plume, keyhole opening, and molten pool images by high-speed camera in different gaps during CO₂ laser overlap welding of T-joints. The results show that gap causes beam energy fluctuations in the keyhole and leads to the instability of welding process. In laser spot welding, zero-gap and small gap greatly affect the stability of plasma and keyhole, which causes the formation of cavities in the weld metal, while a proper gap can help prevent porosity formation. In laser continuous welding, the disruption and closure of front keyhole wall at the gap periodically changes with the gap, which causes the formation of plenty of porosities at the gap. The instability of keyhole is closely related to dynamics of plume and molten pool, which gives an insight into the mechanism of porosity formation during laser overlap welding.     

Neural network modeling and analysis of the material removal process during laser machining

To manufacture parts with nano- or micro-scale geometry using laser machining, it is essential to have a thorough understanding of the material removal process in order to control the system behaviour. At present, the operator must use trial-and-error methods to set the process control parameters related to the laser beam, motion system, and work piece material. In addition, dynamic characteristics of the process that cannot be controlled by the operator such as power density fluctuations, intensity distribution within the laser beam, and thermal effects can significantly influence the machining process and the quality of part geometry. This paper describes how a multi-layered neural network can be used to model the nonlinear laser micro-machining process in an effort to predict the level of pulse energy needed to create a dent or crater with the desired depth and diameter. Laser pulses of different energy levels are impinged on the surface of several test materials in order to investigate the effect of pulse energy on the resulting crater geometry and the volume of material removed. The experimentally acquired data is used to train and test the neural network's performance. The key system inputs for the process model are mean depth and mean diameter of the crater, and the system outputs are pulse energy, variance of depth and variance of diameter. This study demonstrates that the proposed neural network approach can predict the behaviour of the material removal process during laser machining to a high degree of accuracy.     

车身覆盖件点焊结构虚拟节点建模及应用

为了研究大量焊点快速建模及保证焊点的位置精度问题,提出了虚拟节点法以模拟点焊结构.首先在实际焊点位置处建立参考点,并把该参考点沿焊接面的法线方向向两焊接面投影,得到两个虚拟节点;焊接单元由该虚拟节点对构成,并在两虚拟节点之间引入一短梁单元来模拟焊接关系.其次,在每个虚拟节点和其周围网格节点之间建立分布耦合约束,通过耦合关系实现虚拟节点和其周围网格节点运动上的协调.经试验与比较表明,该方法仿真结果与试验结果一致性较好,比传统方法更有效地模拟实际焊点的力学行为,证实了虚拟节点法应用于模拟点焊结构的可行性.     

脉冲激光焊接Hastelloy C-276合金的熔池流动传热特性分析

基于流体动力学方程和传热方程建立了三维瞬态模型,用于研究脉冲激光焊接0.5 mm厚Hastelloy薄板时熔池的流动行为及传热特性.应用Fluent软件,采用有限容积法(FVM)求解控制方程,用SIMPLE算法处理速度与压力的耦合.引入Pe来衡量焊接熔池中对流传热与传导传热的相对强弱,并以此分析焊接熔池的传热特性.结果表明:沿焊接方向,焊接熔池的流动速度随着离熔池中心距离的增加先增加后减小;在给定试验条件下,熔池流动速度在离熔池中心0.2 mm左右时出现最大值,且沿焊接方向前方稍大于后方,而后迅速减小为零;焊接熔池中对流的存在使得焊接熔池熔深较小而熔宽较大;最终的焊接形貌由对流传热与传导传热相互作用而成.对焊缝形貌的数值模拟结果与实验结果进行了比较,计算结果与实验结果吻合较好.此模型可为脉冲激光焊接Hastelloy C-276薄板时熔池流体流动行为的分析提供理论依据.     

基于XeF(C-A)放大器的混合(固态/气态)超高功率飞秒激光系统

提出了基于前端发生器和终端放大器,使用光泵浦XeF(C-A)激活介质的太瓦级混合激光器(THIA00)系统.前端发生器由长532 nm的连续激光泵浦的钛宝石飞秒脉冲振荡器,脉冲展宽机构,532 nm的脉冲激光泵浦的再生多通道放大器,衍射光栅压缩器和二次谐波发生器(KDP)组成.其光束输出参数为:脉冲持续时间50 fs,...     

A study on the development of photoelastic experimental hybrid method for colour isochromatics (I)

Isochromatics obtained from photoelastic experiment shows the stress distributions of the full field of a structure under load. Therefore, stress distributions of the structure can be read at a glance through isochromatics. Many experimental data can be obtained from isochromatics which are then used in various photoelastic experimental hybrid methods for stress analysis. Monochromatic light has however, until now been used in the photoelastic experimental hybrid method to produce black and white isochromatics. The use of black and white isochromatics in photoelastic experimental hybrid method for black and white isochromatics requires high fringe orders in order to obtain sufficient experimental data for photoelastic hybrid techniques. Accordingly, this paper develops the photoelastic experimental hybrid method for color isochromatics in which a fringe order of 1 is enough to gather the experimental data of the photoelastic experimental hybrid method. The method was applied to validate stress concentration problems. Experimental results from this study indicated that the photoelastic experimental hybrid method for color isochromatics is more precise than the photoelastic experimental hybrid method for black and white isochromatics. The use of few fringe orders in photoelastic experimental hybrid method for color isochromatics can offer significant advantages in stress analysis of real components using reflective-type photoelastic experimental method.     

Numerical modeling of dynamic recrystallization during nonisothermal hot compression by cellular automata and finite element analysis

In this study, dynamic recrystallization during nonisothermal hot compression was numerically simulated by cellular automata and finite element analysis. A modified cellular automata model was developed by introducing a new parameter for considering solute drag effect. The isothermal hot compression tests of pure copper were carried out to verify the modified cellular automata model by comparing material behavior and average grain size. The effect of solute drag was numerically considered and compared to the experimental data and the numerical data obtained by conventional cellular automata without solute drag effect. Then, the modified cellular automata model was applied to a nonisothermal hot compression by combining with a finite element analysis. The finite element analysis was conducted to acquire local parameters such as strain, strain rate, and temperature. These values were provided to the cellular automata model as input. The local changes of microstructure and average grain size were simulated by cellular automata and compared with nonisothermal hot compression results. The simulation results were in reasonably good agreement with experimentally determined microstructures by electron backscattering diffraction. The developed model was further applied to simulate a hot gear blank forging process to check its applicability. With the current approach, local microstructures can be determined for better understanding microstructural changes during the nonisothermal process.     

Experimental investigation on deformation and wear of WC tool during friction stir welding (FSW) of stainless steel

In this paper, the effect of friction stir welding (FSW) parameters on wear and deformation behavior of tungsten carbide (WC) tool employed in the welding of AISI 304 austenitic stainless steel (SS) is reported. In addition, the wear and deformation of the tool are also characterized. Three FSW parameters, namely shoulder diameter, tool rpm, and traverse speed each at three levels were considered. Experiments were performed as per Taguchi’s L₉ orthogonal array to investigate the effect of these parameters on wear and plastic deformation of the tool. Wear at the pin root and bottom face of the pin attributed to diffusion and attrition mechanisms, respectively, were observed. Significant deformation of the tool was also observed during welding which caused bulging of the shoulder with an increased cone angle of the pin.