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.     

Influence of displacement constraints in thermomechanical analysis of laser micro-spot welding process

The evolution of mechanical components into smaller size generating a need for microwelding of these components using laser which offers better control as compared to arc and plasma processing. The present article describes the numerical simulation of laser micro-spot welding using finite element method. A two dimensional Gaussian distributed surface heat flux as a function of time is used to perform a sequentially coupled thermal and mechanical analysis. The model is used for simulating laser micro-spot welding of stainless steel sheet under different power conditions and configurations of mechanical constraints. The temperature dependent physical properties of SS304 have been considered for the simulation and an isotropic strain hardening model has been used. The simulated weld bead dimensions have been compared with experimental results and temperature profiles have been calculated. The maximum deformation of 0.02 mm is obtained with maximum laser power of 75 W. The thermal stress is more inducing factor to temperature induced residual stresses and plastic strain as compared to mechanical constraints. The plastic strain changes significantly by displacement constraints as compared to residual stress.     

Problems and issues in laser beam welding of aluminum–lithium alloys

Aluminum–lithium (Al–Li) alloys are very attractive for potential aerospace applications due to their lower density, higher specific strength and rigidity, better corrosion and fatigue crack growth resistance properties, compared to conventional aluminum alloys. Laser beam welding (LBW), being an advanced joining method, has already been approved for applications involving aluminum alloys. However, there are still a number of problems and issues to be answered and solved in LBW of Al–Li alloys. In this review, the properties of Al–Li alloys and the characteristics of LBW are introduced, the formation and prevention of the main weld defects such as porosity and hot cracking are discussed, and then the weld microstructure and the joint mechanical properties are described in highlight. At the end, an outlook on future trends is presented.     

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.     

油库设备焊割动火的安全防护

总结了油库设备焊割动火作业中油蒸气的清除、隔离方法及其组织实施要点,有利于油库整修改造过程中焊割动火作业的安全管理。     

激光法与传统方法比较测定土壤粒度分布的研究

激光法测定土壤粒度是近年来应用的新方法,它的特点是测定速度快、效率高,比传统的经典测定法省时省力省试剂消耗,实验室对环境的二次污染也小,特别是在处理和测定大量样品时,它的优势更为明显。本文作者选择内蒙古地区有代表性的暗棕壤、草甸土、黑土、黑钙土、栗钙土、棕钙土和风沙土7种土壤类型共42个土壤样本为例,采用激光法与传统比重计法对土壤粒度分布测定进行了方法比对研究,设计了激光法三个参数三个水平的正交实验,结果表明粘粒含量激光法测定低于传统方法,粉粒含量则相反,砂粒含量两种方法测定结果最接近;得出了两种方法之间的相关系数和回归方程,P〈0．05时,最大相关系数是：粘粒含量r=0．5822,粉粒含量r=0．9094,砂粒含量r=0．9152;本研究结果还表明激光法与传统比重计法两种方法确定的质地大致相同,认为虽然土壤的粘粒部分就目前的测定结果来看不适合用激光法测定,但粉粒部分和砂粒部分可以用激光法测定后通过两种方法之间的线性回归方程换算,从而提高测定效率。这一研究结果明确了激光法与传统比重计法测定土壤粒度分布的差异,对使用两种不同方法研究土壤颗粒组成具有借鉴作用,并且在土壤批量样品粒度的分析测定中具有非常实用的推广意义。