Microstructural analysis of laser micro-welds between copper and aluminum

Currently used ultrasonic welded joints for assembly and packaging of Li-Ion batteries have reliability concerns for automotive applications, as the battery is subjected to vibration and other mechanical loads. The sealing of the battery can is very critical for safety. Due to battery weld failures in recent years, the postal service has put ban on shipping Li-ion batteries via regular mail. A laser based alternative joining technology has the potential to offer robust, fast and cost-effective processing of Li-Ion batteries. Before the technology can be fully implemented, it is imperative to understand the effects of various process parameters on the robustness of the weld. In the present analysis, a preliminary study is performed to understand the effect of laser scanning speed on the micro-structural and physical characteristics of the materials in the weld area that ultimately affect the bond quality. Samples are created by welding aluminum and copper in lap shear configuration using a continuous wave fiber laser. Two sets of samples are created using a laser power of 225 W; however, the scanning speeds are 300 and 400 mm/s. Scanning electron microscopy and energy dispersive spectroscopy are performed in the weld area to understand the microstructural and physical characteristics of the joint that may have been affected by the processing parameters.     

Optimization of laser butt welding parameters based on the orthogonal array with fuzzy logic and desirability approach

This paper presents the study carried out on 3.5 kW cooled slab laser welding of 904 L super austenitic stainless steel. The joints had butts welded with different shielding gases like 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. Taguchi approach is used as statistical design of experiment (DOE) technique for optimizing the selected welding parameters. Fuzzy logic and desirability approach are applied to optimize the input parameters considering multiple output variables simultaneously. Confirmation experiment has also been conducted for both the analyses to validate the optimized parameters.     

Characterization of localized laser assisted eutectic bonds

An innovative bonding process for silicon and single crystal quartz has been developed and investigated using various material science characterization methods, such as TOF-SIMS, SEM, EDX and XRD. The bonding process combines the principles of laser transmission welding, eutectic bonding and bonding by localized heating. A focused laser beam (low power, max. 0.83 W) is transmitted through a quartz medium to intermediate layers of chromium, gold and tin at the silicon–quartz interface to provide localized heating and bonding. This bonding process is particularly suitable for bonding wafers containing temperature sensitive devices as it confines the temperature increase to a small area. Bond strength of over 15 MPa is comparable to most localized bonding schemes. This process provides a simple yet robust bonding solution with rapid processing time, selectivity of bonded area and corrosion resistant joints.     

Laser welding of light alloys and superalloys

The chemico-physical characteristics of aluminum cause considerable difficulty in the welding of this material. The high level of specific power and the restricted area involved put laser technology in the position of being most promising for resolving these difficulties.This paper discusses the welding of aluminum using laser beam technology and presents the most significant experimental results.     

Using a neural network for predicting the average grain size in friction stir welding processes

In the paper the microstructural phenomena in terms of average grain size occurring in friction stir welding (FSW) processes are focused. A neural network was linked to a finite element model (FEM) of the process to predict the average grain size values. The utilized net was trained starting from experimental data and numerical results of butt joints and then tested on further butt, lap and T-joints. The obtained results show the capability of the AI technique in conjunction with the FE tool to predict the final microstructure in the FSW joints.     

Finite element modeling of multi-pass welding and shaped metal deposition processes

This paper describes the formulation adopted for the numerical simulation of the shaped metal deposition process (SMD) and the experimental work carried out at ITP Industry to calibrate and validate the proposed model. The SMD process is a novel manufacturing technology, similar to the multi-pass welding used for building features such as lugs and flanges on fabricated components (see Fig. 1a and b). A fully coupled thermo-mechanical solution is adopted including phase-change phenomena defined in terms of both latent heat release and shrinkage effects. Temperature evolution as well as residual stresses and distortions, due to the successive welding layers deposited, are accurately simulated coupling the heat transfer and the mechanical analysis. The material behavior is characterized by a thermo-elasto-viscoplastic constitutive model coupled with a metallurgical model. Nickel super-alloy 718 is the target material of this work. Both heat convection and heat radiation models are introduced to dissipate heat through the boundaries of the component. An in-house coupled FE software is used to deal with the numerical simulation and an ad-hoc activation methodology is formulated to simulate the deposition of the different layers of filler material. Difficulties and simplifying hypotheses are discussed. Thermo-mechanical results are presented in terms of both temperature evolution and distortions, and compared with the experimental data obtained at the SMD laboratory of ITP.     

Process parameter optimization of lap joint fillet weld based on FEM–RSM–GA integration technique

This study introduces a welding process design tool to determine optimal arc welding process parameters based on Finite Element Method (FEM), Response Surface Method (RSM) and Genetic Algorithms (GA). Here, a sequentially integrated FEM–RSM–GA framework has been developed and implemented to reduce the weld induced distortion in the final welded structure. It efficiently incorporates finite element based numerical welding simulations to investigate the desired responses and the effect of design variables without expensive trial experiments. To demonstrate the effectiveness of the proposed methodology, a lap joint fillet weld specimen has been used in this paper. Four process parameters namely arc voltage, input current, welding speed and welding direction have been optimized to minimize the distortion of the structure. The optimization results revealed the effectiveness of the methodology for welding process design with reduced cost and time.     

基于DSP的数字弧焊逆变电源系统的研究

根据铝导杆焊接的工艺要求和被焊母材的特性,设计了以数字信号处理器TMS320LF2407A为控制系统核心,采用IGBT逆变技术的PMIG焊接电源。焊接控制系统由硬件和软件两部分组成,根据PMIG焊接特性采用了变参数数字PI（比例、积分）调节器的设计,通过程序实现反馈信号的采样处理及数字PI调节,对焊接电源进行准确的控制。由仿真结果可以看出,该设备具有控制精度高、性能稳定等特点,适合于铝导杆的焊接。     

Gas permeability of adhesives and their application for hermetic packaging of microcomponents

 The use of adhesives with low gas permeability in packaging microcomponents provides a simple, low-cost alternative to the standard techniques for hermetic packaging. The permeability to helium of various epoxy resin adhesives was determined by means of a mass spectrometer in reference specimens of defined dimensions. The adhesive with the lowest permeability to helium was chosen to bond an evacuated LIGA gyrometer package (flatpack, volume=1.4 cm³). For comparison, a few packages were closed by laser welding. Subsequently, the pressure rise in the sealed packages due to gas permeation and desorption from the walls was measured by the integrated microsensor, and the permeation rate was determined. The permeation rates of as low as 10⁻¹⁷ m³/s achieved in bonded packages are comparable to those measured in laser welded packages. Received: 21 December 1999/Accepted: 9 February 2000     

Electrostatic sprayed SnO2 and Cu-doped SnO2 films for H2S detection

This paper presents the ability of electrostatic sprayed tin oxide (SnO₂) and tin oxide doped with copper oxide (1, 2, and 4 at.% Cu) films to detect different pollutant gases, i.e., H₂S, SO₂, and NO₂. The influence of a copper oxide dopant on the SnO₂ morphology is studied using scanning electron microscopy (SEM) technique, which reveals a small decrease in the porosity and particle size when the amount of dopant is increased. The sensing properties of the SnO₂ films are greatly improved by doping, i.e., the Cu-doped SnO₂ films have large response to low concentration (10 ppm) of H₂S at low operating temperature (100 °C). Furthermore, no cross-sensitivity to 1 ppm NO₂ and 20 ppm SO₂ is observed. Among the studied films, the 1 at.% Cu-doped SnO₂ layer is the most sensitive in the detection of all the studied gases.     

Mechanical and electrical properties of electroplated copper for MR-imaging coils

Mechanical and electrical properties of electroplated copper films were investigated to be used in an active tracking system for the patient-safe localisation of a catheter in intravascular interventions using magnetic resonance imaging. For this system spiral micro coils with high aspect ratio are fabricated on a planar polyimide foil to be wrapped around a catheter tip. Due to this special assembly method the coils and the copper, respectively, have to withstand high mechanical forces. The influence of the electroplating parameters as current density and shape, temperature and sample motion on the mechanical and electrical properties is investigated. These samples were characterised using nanoindentation, tensile tests, laser profiler and four-point prober. Results show that a low constant current density at a temperature of 40°C and sample motion generate flexible and high-Q copper coils.     

Experiment study of laser thermal enhanced electrochemical deposition

The experiment of laser thermal enhanced electrochemical deposition was investigated in this paper and the deposition layer of copper was obtained when laser irradiated the stainless steel substrate through the electrolyte. Then the scanning electron microscope was utilized to observe the surface morphology and cross-sectional morphology of the deposition layer. According to morphology observed, the laser thermal effect was discussed and the mechanical of laser enhanced electrochemical deposition was analyzed. The results show that the thermal effect of laser could increase the electrode potential and speed up the reduction reaction rate of electrochemical deposition. Compared with the ordinary electrodeposition, the grain size of electrodeposition has also been greatly reduced. When the laser energy was 0.2 mJ (20 kHz), the deposition rate was better and when the laser energy was between 0.2 mJ (20 kHz) and 0.4 mJ (20 kHz), the deposition layer with high tensile strength was gained. The results are useful to the development of the later electrochemical machining technology.

     

A facile strategy to integrate robust porous aluminum foil into microfluidic chip for sorting particles

High efficiency integration of functional microdevices into microchips is crucial for broad microfluidic applications. Here, a device-insertion and pressure sealing method was proposed to integrate robust porous aluminum foil into a microchannel for microchip functionalization which demonstrate the advantage of high efficient foil microfabrication and facile integration into the microfluidic chip. The porous aluminum foil with large area (10 × 10 mm²) was realized by one-step femtosecond laser perforating technique within few minutes and its pores size could be precisely controlled from 3 μm to millimeter scale by adjusting the laser pulse energy and pulse number. To verify the versatility and flexibility of this method, two kinds of different microchips were designed and fabricated. The vertical-sieve 3D microfluidic chip can separate silicon dioxide (SiO₂) microspheres of two different sizes (20 and 5 μm), whereas the complex stacking multilayered structures (sandwich-like) microfluidic chip can be used to sort three different kinds of SiO₂ particles (20, 10 and 5 μm) with ultrahigh separation efficiency of more than 92%. Furthermore, these robust filters can be reused via cleaning by backflow (mild clogging) or disassembling (heavy clogging).     

Study and design of resistive switching behaviors in PMMA-based conducting-bridge random-access memory (CBRAM) devices

The fabrication and the performance of PMMA resistive switching device have been studied by using FR-4 (copper), PMMA (poly methyl methacrylate) and aluminum as the active anode, the solid electrolyte and the inert cathode respectively. By etching the copper surface with the acid solution [4HNO₃ + 11H₃PO₄ (98 %) + 5CH₃COOH] at 60 °C for 2 min, a good performance of Cu/PMMA/Al device, which can switch until 2300 cycles, has been realized. The spin rate for forming the PMMA coating plays a decisive role in the performance of Cu/PMMA/Al device. The best performance of the Cu/PMMA/Al device was obtained only when the spin rate of deposition of PMMA reached 4000 rpm (low thickness).

     

Defect identification in friction stir welding using discrete wavelet analysis

This article discusses on the detection of fault occurred during friction stir welding using discrete wavelet transform on force and torque signals. The work pieces used were AA1100 aluminum alloys of thickness 2.5 mm. The plates were 200 mm in length and 80 mm in width. Presence of defect in welding causes sudden change in force signals (Z-load), thus it is easier to detect such abrupt changes in a signal using discrete wavelet transform. Statistical features like variance and square of errors of detail coefficients are implemented to localize the defective zone properly as it shows better variations (in defective area) than the detail coefficient itself.     

Laminated fabrication of micro-stepped gear mold based on femtosecond laser cutting of stainless steel foil

This paper proposes that a micro-stepped structure is fabricated based on the characteristics of femtosecond laser (high-precision and cold machining) and the laminated manufacturing principle in rapid prototyping. This process is composed of two parts: the single-layer graphics cutting and the layer laminating. The cutting experiment results show that when the femtosecond laser power is 230 mW and the cutting speed is 50 μm/s, the cutting effect of the 20 μm thick 0Cr18Ni9 stainless steel foil is guaranteed and the over-ablation of the lower layer steel foil caused by excessive power is avoided. During the experiment, the principles and error of focus adjustment are analyzed and a compensation plan is proposed. When the laminated procedure is finished, the overall intensive spot welding is performed on the area surrounding the mold cavity to reinforce the mold cavity and reduce the gap between layers. For a six-layer micro-mold, the nugget thermal field simulations and the metallographic analyses showed that when the welding time is 10 ms, the pressure is 20 N and the voltage reaches 0.22 V, the overall welding effect is satisfactory. Eventually, three kinds of micro-stepped gear molds are prepared and a micro-mold is bulged and replicated by Cu foils.     

Multi-objective optimal design of small scale resistance spot welding process with principal component analysis and response surface methodology

This paper investigates the effects of welding parameters on the welding quality and optimizes them in the small scale resistance spot welding (SSRSW) process. Experiments are carried out on the basis of response surface methodology technique with different levels of welding parameters of spot welded titanium alloy sheets. Multiple quality characteristics, namely signal-to-noise (S/N) ratios of weld nugget diameter, penetration rate, tensile shear load and the failure energy, are converted into an independent quality index using principal component analysis. The mathematical model correlating process parameters and their interactions with the welding quality is established and discussed. And then this model is used to select the optimum process parameters to obtain the desired welding quality. The verification test results demonstrate that the method presented in this paper to optimize the welding parameters and enhance the welding performance is effective and feasible in the SSRSW process.     

Laser beam welded structures for a regional aircraft: weight,cost and carbon footprint savings

Laser beam welded structures offer great opportunities for the lightweight design of fuselage structures in order to reduce structural weight for increased fuel efficiency. Our main objective is to validate and demonstrate that laser beam welding (LBW) technology provides the best opportunities in terms of weight reduction, production time and energy consumption for manufacturing aircraft components. To this end, a comparison in terms of energy, process time, cost and carbon footprint is assessed against the ‘conventional’ manufacturing process of riveting, to prove that LBW is actually an environmental friendly process. Manufacturing of a four-stringer stiffened flat subscale component was the case of the present work that was called in the Clean Sky Eco-Design Airframe (EDA) project as the B1 demonstrator (742 mm × 384 mm). The LBW process has been broken down into several sub-processes and activities according to the Activity Based Costing (ABC) methodology and the weight reduction, production time and energy consumption results were compared against the respective of the riveting process. It was proved that for the specific subscale LBW component, it consumes half the energy and can be processed in less than half the time needed (in serial processing of the component) with riveting. Manufacturing of the component with the LBW process (door to door approach) is more environmentally friendly, since it produces 53% less CO_2e emissions than the respective riveted process. This is a clear advantage to this manufacturing process in order to assure a sustainable life cycle of the final product.     

激光视觉搭接焊缝的图像识别

研究了激光视觉搭接焊缝的图像识别方法。针对原始焊接坡口激光图像中的噪声,比较了均值滤波和中值滤波去噪方法,提出了一种改进的滤波方法,采用自适应阈值调整的最大方差法计算滤波图像的二值化阈值,从而实现对图像的分割。针对搭接坡口激光图像提出了三种坡口中心位置的识别和提取方法,并通过图像处理实验比较了三种识别方法的坐标误差、识别正确率。试验证明,最大方差法的图像处理过程和快速Hough变换识别法有效,能满足焊缝实时跟踪的要求。     

Multi-objective process parameters optimization of hot-wire laser welding using ensemble of metamodels and NSGA-II

Hot-wire laser welding (HLW) can reduce the dispersion of laser energy and improve deposition efficiency by preheating filler wire during laser welding process. In order to obtain sound welding results, it is crucial to select appropriate process parameters. In this study, an optimization methodology based on ensemble metamodels (EM) which take the advantages of the prediction ability of stand-alone metamodels (Kriging, RBF and SVR), and Non-dominated sorting genetic algorithm (NSGA-II) are presented to obtain optimum process parameters during stainless steel 316L hot-wire laser welding. Firstly, EMs are developed through minimizing the generalized mean square Leave-one-out (LOO) errors to find the optimum weight factors of the used stand-alone Kriging, RBF and SVR metamodels. And then the EMs are applied to establish the relationships between process parameters (i,e., laser power (LP), welding speed (WS) and hot-wire current (I)) and welding results (i,e., welding depth-to-width ratio (DW), welding reinforcement (BR) and tensile strength (TS)). During optimization process, NSGA-II is employed to search for multi-objective Pareto optimal solutions based on EMs. In addition, the main effects of multiple process parameters on welding results are analyzed. The verification tests indicate that the obtained optimal process parameters are effective and reliable for producing expected welding results (maximized DW, maximized TS and desired BR value). In general, the proposed optimization method can provide a reliable guidance for HLW in engineering practice.