A method for double-sided friction stir spot welding

A rotating anvil similar to a pinless friction stir welding (FSW) tool can be applied to friction stir spot welding (FSSW) of thin metal plates. FSSW is a solid-state joining process that is currently being used by automotive manufacturers as an alternative to rivets and traditional resistance spot welding. The principal detractor of this process is the keyhole left by pin extraction, which can be detrimental to the weld strength. A pinless tool can be used to eliminate the keyhole. However, this approach is limited to joining thin sheet (≤1 mm). Using a rotating anvil with the FSSW process permits the joining of thicker cross sections, improves the mechanical strength of the spot weld and reduces the reaction forces on the spot welding frame. A numerical model of the process, tensile shear tests and macrosection analysis are used to evaluate the spot welds.Macrosection and numerical analysis reveals that the material flow between the pinless tool and rotating anvil is complex and unique to this process. It has been found that the use of a rotating anvil for FSSW is a viable means to create quality spot welds in thicker weldments.     

Simultaneous measurement of tool torque,traverse force and axial force in friction stir welding

Simultaneous measurement of the tool torque, traverse force and axial force during friction stir welding process is of great significance to the understanding of the underlying process mechanism and the optimizing of the process parameters. Different from the traditional measurement methods using load cell or rotating component dynamometer, an indirect but economical methodology is used in this study for the simultaneous measurement of the traverse force, axial force and tool torque by monitoring the output torques of the servo motors and main spindle three-phase AC induction motor inside the FSW machine. The values of the traverse force, axial force and tool torque are determined under different welding conditions, and the influencing factors are examined. The measured results in friction stir welding of AA2024-T4 aluminum alloys at different combinations of tool rotation speed and welding speed lay foundation for process optimization.     

Comparison of mechanical properties of pure copper welded using friction stir welding and tungsten inert gas welding

The objective of this research is to investigate the mechanical properties including bonding, tensile strength, and impact resistance of pure copper welded using friction stir welding (FSW) method and compare them with that of tungsten inert gas (TIG) welding. Micro-hardness tests are performed on pure copper, TIG welded copper and FSW welded copper to determine the effect of heat on the hardness of welded coppers. Tensile strength tests and notch tensile strength tests are performed to determine the mechanical properties of different weld process.In this experiment, it is found that the notch tensile strength and the notch strength ratio for FSW (212 MPa, 1.10) are significantly higher than those (190 MPa, 1.02) of TIG welding. For the impact tests, the weld zone and heat-affected zone energy absorption values for FSW (2.87 J, 2.25 J) are higher than those (1.32 J, 0 J) of TIG welding. XRD tests are performed to determine components of copper before and after welding process for TIG and FSW.     

Plasma arc welding: Process,sensing, control and modeling

This article introduces the basic principles of plasma arc welding (PAW) and provides a survey of the latest research and applications in the field. The PAW process is compared to gas tungsten arc welding, its process characteristics are listed, the classification is made, and two modes of operation in PAW, i.e., melt-in and keyhole, are explained. The keyhole mechanism and its influencing factors are introduced. The sensing and control methodologies of the PAW process are reviewed. The coupled behaviors of weld pool and keyhole, the heat transfer and fluid flow as well as three-dimensional modeling and simulation in PAW are discussed. Finally, a novel PAW process variant, the controlled pulse keyholing process and the corresponding experimental system are introduced.     

Numerical simulation of residual stresses for friction stir welds in copper canisters

In an attempt to map the residual stress distributions after friction stir welding of copper canisters, a three-dimensional thermo-mechanical model has been formulated by coupling heat transfer and elasto-plasticity analyses. The transient temperature field around the tool is simulated by a moving heat source. The simulation shows that the residual stress distribution in a thick-wall copper canister is sensitive to the circumferential angle and asymmetrical to the weld line. Both tensile and compressive stresses emerge along the weld line and its vicinity. The maximum tensile stress appears in the circumferential direction on the outer surface. The maximum tensile stress, whether it is predicted by the finite element method or measured by the hole-drilling technique and the X-ray diffraction method, does not exceed 50 MPa in general.     

Experimental investigation on flexural behavior of friction stir welded high density polyethylene sheets

Flexural strength is one of the main criteria in evaluation of the mechanical properties of polymeric joints. The flexural strength of thermoplastics, such as high density polyethylene (HDPE) sheets, is influenced by friction stir welding parameters. The determination of the welding parameters plays an important role in the weld strength. In the present study, the response surface method (RSM) was used as a statistical design of experiment technique to set the optimal welding parameters. The designed tool was consisted of a rotating pin, a stationary shoulder (shoe) and a heating system inside shoe. Rotational speed of the pin, tool traverse speed and shoe temperature were considered as varying parameters. Obtained results show a significant relationship between considered properties and processing parameters through an analysis of variance (ANOVA) study and the response surface method. It was found that welding at a high level of rotational speed and a lower level of tool travel speed increases weld flexural strength by reducing size of defects.     

Double-electrode arc welding process: Principle,variants, control and developments

Double-electrode gas metal arc welding (DE-GMAW) is a novel welding process in which a second electrode, non-consumable or consumable, is added to bypass part of the wire current. The bypass current reduces the heat input in non-consumable DE-GMAW or increases the deposition rate in consumable DE-GMAW. The fixed correlation of the heat input with the deposition in conventional GMAW and its variants is thus changed and becomes controllable. At the University of Kentucky, DE-GMAW has been tested/developed by adding a plasma arc welding torch, a GTAW (gas tungsten arc welding) torch, a pair of GTAW torches, and a GMAW torch. Steels and aluminum alloys are welded and the system is powered by one or multiple power supplies with appropriate control methods. The metal transfer has been studied at the University of Kentucky and Shandong University resulting in the desirable spray transfer be obtained with less than 100 A base current for 1.2 mm diameter steel wire. At Lanzhou University of Technology, pulsed DE-GMAW has been successfully developed to join aluminum/magnesium to steel. At the Adaptive Intelligent Systems LLC, DE-GMAW principle has been applied to the submerged arc welding (SAW) and the embedded control systems needed for industrial applications have been developed. The DE-SAW resulted in 1/3 reduction in heat input for a shipbuilding application and the weld penetration depth was successfully feedback controlled. In addition, the bypass concept is extended to the GTAW resulting in the arcing-wire GTAW which adds a second arc established between the tungsten and filler to the existing gas tungsten arc. The DE-GMAW is extended to double-electrode arc welding (DE-AW) where the main electrode may not necessarily to be consumable. Recently, the Beijing University of Technology systematically studied the metal transfer in the arcing-wire GTAW and found that the desired metal transfer modes may always be obtained from the given wire feed speed by adjusting the wire current and wire position/orientation appropriately. A variety of DE-AW processes are thus available to suit for different applications, using existing arc welding equipment.     

Modeling and simulation of arc: Laser and hybrid welding process

Welding is a fabrication process to join two different materials. Among the many welding processes, the arc and laser welding processes are the most widely used. Great effort is required to understand the physical phenomena of arc and laser welding due to the complex behaviors which include liquid phase, solid phase and, gas phase. So it is necessary to conduct numerical simulation to understand the detailed procedures of welding. This paper will present the various numerical simulation methods of the arc welding processes such as arc plasma, gas tungsten arc welding, gas metal arc welding, laser welding, and laser–arc hybrid welding. These simulations are conducted by the finite element method, finite differential method and volume of fluid method to describe and analyze the various welding processes.     

A tutorial on learning human welder's behavior: Sensing,modeling, and control

Human welder's experiences and skills are critical for producing quality welds in manual GTAW process. Learning human welder's behavior can help develop next generation intelligent welding machines and train welders faster. In this tutorial paper, various aspects of mechanizing the welder's intelligence are surveyed, including sensing of the weld pool, modeling of the welder's adjustments and this model-based control approach. Specifically, different sensing methods of the weld pool are reviewed and a novel 3D vision-based sensing system developed at University of Kentucky is introduced. Characterization of the weld pool is performed and human intelligent model is constructed, including an extensive survey on modeling human dynamics and neuro-fuzzy techniques. Closed-loop control experiment results are presented to illustrate the robustness of the model-based intelligent controller despite welding speed disturbance. A foundation is thus established to explore the mechanism and transformation of human welder's intelligence into robotic welding system. Finally future research directions in this field are presented.     

Magnetic pulse welding: Interface of Al/Cu joint and investigation of intermetallic formation effect on the weld features

This paper attempts to highlight emerging issues when joining dissimilar metal pairs by magnetic pulse welding. For this purpose, joint of Al/Cu and Al/Al welded with the same parameters are compared. Al/Cu combination involves a formation of an interfacial intermetallic phase whereas Al/Al joint is bonded by metal continuity at the interface. The intermetallic is found to be amorphous, nanoporous, and damaged by a multi-directional cracks. The formation of such features is respectively explained by a melting confined at the interface followed by high rate cooling, cavitation phenomenon and solidification shrinkage. Mechanical characterizations of the Al/Cu joint show brittle fracture at the interface with fragmentation of the [0] intermetallics (glass type rupture). The intermetallic defects such as pores and cracks strongly reduce the breaking load level when compared to the Al/Al case. On the other hand, the Al/Al joint shows ductile fracture with plastic deformation of the interface.     

Volatile organic compounds concentration profiles and control strategy in container manufacturing industry: Case studies in China

Volatile organic compounds (VOCs), important precursors of ozone (O₃) and fine particulate matter (PM_2.5), are the key to curb the momentum of O₃ growth and further reducing PM_2.5 in China. Container manufacturing industry is one of the major VOC emitters, and more than 96% containers of the world are produced in China, with the annual usage of coatings of over 200,000 tons in recent years. This is the first research on the emission characteristics of VOCs in Chinese container manufacturing industry, including concentration and ozone formation potential (OFP) of each species. The result shows that the largest amounts of VOCs are emitted during the pretreatment process, followed by the paint mixing process and primer painting process, and finally other sprays process. The average VOC concentrations in the workshops, the exhausts before treatment and the exhausts after treatment are ranging from 82.67–797.46 , 170–1,812.65 , 66.20–349.63 mg/m³, respectively. Benzenes, alcohols and ethers are main species, which contribute more than 90% OFP together. Based on the emission characteristics of VOCs and the technical feasibility, it is recommended to set the emission limit in standard of benzene to 1.0 mg/m³, toluene to 10 mg/m³, xylene to 20 mg/m³, benzenes to 40 mg/m³, alcohols and ethers to 50 mg/m³, and VOCs to 100 mg/m³. The study reports the industry emission characteristics and discusses the standard limits, which is a powerful support to promote VOCs emission reduction, and to promote the coordinated control of PM_2.5 and O₃ pollution.     

催化湿式氧化技术原理与应用

催化湿式氧化法是目前国际上处理毒害高浓度有机废水的最佳选择之一,文章就催化湿式氧化法的原理、工艺和应用等进行了介绍.     

中国酸沉降:来源、影响与控制

总结我国酸沉降观测与模拟、来源与影响以及控制与成效,不仅能为未来我国大气污染治理提供科学依据,也能为国际酸沉降控制提供参考.本文回顾了40年来我国酸沉降研究的发展历程,对有关酸雨时空分布特征、致酸气体排放、土壤和地表水酸化、酸化缓冲机制、临界负荷区划以及酸雨控制对策等研究进展进行综述.我国酸雨污染经历了快速发展、污染缓...     

紫外活化过硫酸盐降解二苯甲酮-4的动力学影响及降解机理与风险评价

活化PS（过硫酸盐）氧化工艺对于降解水中新兴微污染物具有潜在应用价值.为研究活化PS体系对BPs（二苯甲酮类）有机防晒剂的降解性能,以BP4（二苯甲酮-4）为研究对象,采用UV/PS（紫外活化过硫酸盐）工艺降解BP4,比较单一UV、单一PS和UV/PS 3种工艺对BP4的去除效果,考察各因素对UV/PS工艺去除BP4动力学的影响,同时探究BP4降解机理并进行风险评价.结果表明:BP4降解过程符合准一级反应动力学模型;最佳PS投加量为1.0 mmol/L,反应30 min后BP4去除率可达94%,增加PS投加量或降低初始c（BP4）均可促进BP4降解,无机阴离子（HCO₃-和Cl-）对BP4降解均有抑制作用,酸性条件有利于BP4降解;基于HPLC-MS/MS鉴定出8种中间产物,并提出降解路径,费氏弧菌毒性试验和ECOSAR v1.10软件预测表明,UV/PS工艺降解BP4过程中生成的中间产物比母物质毒性更高.研究显示,UV/PS工艺可有效去除BP4,但其中间产物可能会造成潜在的生态风险,后续需进一步深入研究.      

环境污染加剧石质文物风化:机理、过程及防护措施

石质文物的风化问题由来已久,在影响石质文物风化的各类因素中,环境恶化对石质文物的风化影响日益严重,但现有研究大多忽略了环境污染加剧的影响。在此背景下,本文从酸性气体排放、温度效应和水体污染三个方面阐述了环境污染加剧对石质文物风化的影响和作用机理,并对现有石质文物的防护措施进行综合比较分析,提出石质文物的保护建议和措施,为新形势下石质文物的保护研究提供新的思路。     

2015年12月北京市空气重污染过程分析及污染源排放变化

2015年12月,北京市及周边地区连续多次出现重污染天气.在此期间,北京市空气重污染应急指挥部两次发布红色预警.为厘清该月重污染的发生过程、生消变化,测算了应急措施下的污染源排放变化情况,并采用数值模拟和地面观测相结合的分析方法,对重污染的形成原因进行初步分析,同时对应急措施的环境效果进行评估.结果表明:1虽然2015年12月北京市主要大气污染物排放量较去年同期有所下降,但排放强度仍然较大,是重污染过程的内因;气象扩散条件不利是重要的外因,地面风速弱,大气稳定度高,相对湿度高,边界层高度降低,源排放及气象因素共同导致了此轮重污染过程.2红色预警应急措施可实现污染物日排放强度减少36%左右,PM_2.5浓度下降11%~21%,预警的应急措施不能扭转重污染的态势,但对于缓解PM_2.5污染加重趋势有明显的效果.3在重污染天气下,污染物仍在大气中累积,应急措施最明显的效果发生在实施后的48~72 h后,因此建议在PM_2.5浓度快速上升前36~48 h实施减排措施,从而对空气质量预报准确性提出更高的要求.     

Characteristics and recent trends of sulfur dioxide at urban, rural, and background sites in North China: Effectiveness of control measures

SO₂ measurements made in recent years at sites in Beijing and its surrounding areas are performed to study the variations and trends of surface SO₂ at different types of sites in Northern China. The overall average concentrations of SO₂ are (16.8 ± 13.1) ppb, (14.8 ± 9.4) ppb, and (7.5 ± 4.0) ppb at China Meteorological Administration (CMA, Beijing urban area), Gucheng (GCH, relatively polluted rural area, 110 km to the southwest of Beijing urban area), and Shangdianzi (SDZ, clean background area, 100 km to the northeast of Beijing urban area), respectively. The SO₂ levels in winter (heating season) are 4-6 folds higher than those in summer. There are highly significant correlations among the daily means of SO₂ at different sites, indicating regional characteristics of SO₂ pollution. Diurnal patterns of surface SO₂ at all sites have a common feature with a daytime peak, which is probably caused by the downward mixing and/or the advection transport of SO₂-richer air over the North China Plain. The concentrations of SO₂ at CMA and GCH show highly significant downward trends (-4.4 ppb/yr for CMA and -2.4 ppb/yr for GCH), while a less significant trend (-0.3 ppb/yr) is identified in the data from SDZ, reflecting the character of SDZ as a regional atmospheric background site in North China. The SO₂ concentrations of all three sites show a significant decrease from period before to after the control measures for the 2008 Olympic Games, suggesting that the SO₂ pollution control has long-term effectiveness and benefits. In the post-Olympics period, the mean concentrations of SO₂ at CMA, GCH, and SDZ are (14.3 ± 11.0) ppb, (12.1 ± 7.7) ppb, and (7.5 ± 4.0) ppb, respectively, with reductions of 26%, 36%, and 13%, respectively, compared to the levels before. Detailed analysis shows that the differences of temperature, relative humidity, wind speed, and wind direction were not the dominant factors for the significant differences of SO₂ between the pre-Olympics and post-Olympics periods. By extracting the data being more representative of local or regional characteristics, a reduction of up to 40% for SO₂ in polluted areas and a reduction of 20% for regional SO₂ are obtained for the effect of control measures implemented for the Olympic Games.     

中国大气污染治理:进展·挑战·路径

党的十八大以来,生态文明建设不断推进,其中大气污染治理是生态环境三大攻坚战之一.自2013年《大气污染防治行动计划》发布以来,我国开展了针对PM_2.5（细颗粒物）污染治理的一系列举措,在燃煤污染和移动源污染控制等领域取得了显著成绩,全国空气质量明显好转.相对于2013年,2017年京津冀、长三角、珠三角三大区域ρ（PM_2.5）年均值分别下降了40%、34%和28%.然而,目前我国不少区域和城市仍然面临着解决PM_2.5污染的急迫需求,并且O₃（臭氧）污染的重要性逐渐凸显,因此我国空气质量改善工作仍面临巨大挑战.今后在建设生态文明和"美丽中国"的进程中,围绕《打赢蓝天保卫战三年行动计划》目标,应重视对非电行业、柴油货车等重点源的控制,加强控制氮氧化物（NO_x）和挥发性有机物（VOCs）排放,持续推进能源和结构转型,协同推动我国积极应对气候变化和持续改善空气质量.      

Development of systems for detection, early warning, and control of pipeline leakage in drinking water distribution: A case study

The integrated system for the detection, early warning, and control of pipeline leakage has been successfully developed to manage the pipeline networks of Beijing.     

Comparison of mesophilic and thermophilic anaerobic digestions of thermal hydrolysis pretreated swine manure: Process performance, microbial communities and energy balance

Anaerobic digestion (AD) of swine manure (SM) commonly shows low biogas output and unsatisfactory economic performance. In this study, thermophilic AD (TAD, 50 ± 1 °C) was combined with thermal hydrolysis pretreatment (THP, 170 °C/10 bar), to investigate its potential for maximizing biogas yield, securing successful digestion and microbial diversity, as well as improving energy balance. Four lab-scale continuously stirred tank reactors were operated for 300 days and compared with each other, i.e., reactor 1 (raw SM fed in mesophilic AD: RSM-MAD), reactor 2 (THP-treated SM fed in MAD: TSM-MAD), reactor 3 (RSM-TAD), and reactor 4 (TSM-TAD). The results showed that THP was efficient to increase methane production of SM, TSM-TAD mode led to the highest methane yield (129.8 ± 40.5 mL-CH₄/g-VS/day) among the tests (p < 0.05). Although TAD was more likely to induce free ammonia (> 700 mg/L) or volatile fatty acids (> 6000 mg/L) accumulation compared with MAD in start-up phase, TSM-TAD treatment mode behaved a sustainable digestion process in a long-term operation. For TSM-TAD scenario, higher Shannon–Weaver (3.873) and lower Simpson index (0.061) indicated this mode ensured and enlarged the diversity of bacteria communities. Phylum Bathyarchaeota was dominant (59.3%−90.0%) in archaea community, followed by Euryarchaeota in the four reactors. RSM-MAD treatment mode achieved the highest energy output (4.65 GJ/day), TSM-TAD was less effective (−17.38 GJ/day) due to increased energy demands. Thus improving the energetic efficiency of THP units is recommended for the development of TSM-TAD treatment mode.