Cutting by a high power laser at a long distance without an assist gas for dismantling

As the applications of laser processing progress, new fields of use are being investigated, including dismantling with power lasers. To fulfil our dismantling requirements we propose a new laser method that we have called the laser dismantling (LD) process. This cutting method uses a high-power laser at a long distance, without an assist gas, and with a focal length of the system of 1 m to 10 m. Precision and accuracy in the process are not the same as for laser cutting for production and assembly. The first application of the laser dismantling process, on which we demonstrate our method, is the dismantling of obsolete nuclear plants with remote controlled, or automatic, robots in irradiated environments. For our demonstrator, the beam from a Nd:YAG laser was focused by a multimode optical fibre. The objectives of this paper are: to discuss the criteria for determining the theoretical feasibility of LD; to discuss issues related to future industrial implementation by introducing the process's basic principles; and to compare LD with classical laser processing, which differs not only in the consideration of cutting quality and speed, but also in the cutting irregularities that could be accepted.     

Principle of massive support in the opposed anvil high pressure apparatus

The Bridgman anvil technique offers a simple and versatile means of generating very high pressures required in solid state studies. The opposed anvil technique is based on the principle of massive support. The practical case of a gasketted anvil is considered, and an expression for the maximum pressure generated under massive support is derived in terms of the geometric parameters, the strength of the anvil material and the gasket properties. In particular, for a given maximum pressure, it is possible to calculate the taper angle, the taper height and the gasket thickness from this expression. The anvil is assumed to be in the elastic region under load. Good agreement is obtained between the calculated and the experimental values for the massive support factor (msf) for various taper angles. By choosing the proper geometry, it is possible to achieve a pressure as high as 130 kbar in an alloy steel anvil. It has been clearly found that the straight portion, where the taper ends, does not really take any part in changing the stress pattern. Thus the minimum straight portion can serve the purpose, and will result in material saving. Anvils exhibit yielding at very high pressure. It is also pointed out that a further strengthening of the anvil can extend the ultimate pressure. Several methods of further strengthening the anvils are discussed.     

添加剂对不锈钢着色过程的影响

探讨了添加剂在不锈钢着色过程中的作用和影响，在特定条件下可使不锈钢成品获得满意的结果。     

组合式隔热陶瓷短杆高温SHPB实验技术

讨论和分析了当前高温分离式霍普金森压杆(SHPB)实验技术,为了获得材料在高温下可靠的动态力学性能,建立了一套在压杆和试件之间添加隔热陶瓷短杆的高温SHPB实验系统。相比于传统接触式高温SHPB方案,该系统可以使用在更高的冲击载荷和温度下,与机械对杆方案相比,实验装置及其控制要简便许多。结合有限元模拟,对陶瓷短杆及温度场对压杆中应力波传播的影响进行了相应的评估,并利用这套实验系统得到了800℃下HR2抗氢钢的动态压缩应力-应变曲线。     

Effect of water quenching process on the microstructure and magnetic property of cold rolled dual phase steel

The effects of water quenching process on the microstructure and magnetic property of cold rolled dual phase steel are investigated. Correlations of microstructure, magnetic properties and water quenching parameters are established. The results show that the microstructure of the dual phase steels mainly consists of the ferrite and martensite phase, the martensite volume fraction increases gradually on increasing the holding and quenching temperature. It is found that magnetic properties of dual phase steel are very sensitive to the quenching process. Based on the minor hysteresis loop results, the coercivity and hysteresis loss increase obviously with the increase of quenching temperature, while the remanent induction and the maximum permeability tend to decrease. Furthermore, the magnetic domain structure of the ferrite phase in the presented dual phase steel is observed by magnetic force microscopy. The mechanism of the magnetic property varying with the quenching process is also discussed.     

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.     

Densification behavior of gas and water atomized 316L stainless steel powder during selective laser melting

The densification during selective laser melting (SLM) process is an important factor determining the final application of SLM-part. In the present work, the densifications under different processing conditions were investigated and the densification mechanisms were elucidated. It was found that the higher laser power, lower scan speed, narrower hatch spacing and thinner layer thickness could enable a much smoother melting surface and consequently a higher densification.The gas atomized powder possessed better densification than water atomized powder, due to the lower oxygen content and higher packing density of gas atomized powder. A large number of regular-shaped pores can be generated at a wider hatch spacing, even if the scanning track is continuous and wetted very well. The densification mechanisms were addressed and the methods for building dense metal parts were also proposed as follows: inhibiting the balling phenomenon, increasing the overlap ratio of scanning tracks and reducing the micro-cracks.     

Mechanics and energy analysis on molten pool spreading during laser solid forming

This paper studies the issue that the molten pool width gradually increases under some conditions during laser solid forming (LSF), which can decrease the shape and dimension accuracy of LSFed component to a large extent. By using the statics analysis method and calculating the interfacial tensions at the solid-liquid-gas triple point of molten pool, the proposed two-dimensional (2D) cross-sectional model of single deposition layer illustrates qualitatively that the deposition width would increase with the increasing pool temperature at a certain powder feeding rate, which we called the pool spread behavior here. Meanwhile, by calculating the maximum equilibrium contact angle for keeping solid-liquid-gas triple point balance, it is found that the molten pool is solidified during non-equilibrium state. Furthermore, in order to control the pool temperature and decrease pool spread amount, the optimal match of pool energy and mass inputs is determined for obtaining an optimum balance between the energy input and deposition efficiencies.     

波形钢腹板箱梁的约束扭转剪应力分析

基于周边不变形理论,结合闭口薄壁杆件约束扭转的计算分析,研究了波形钢腹板箱梁在约束扭转时混凝土悬臂板上扭转剪应力的分布,并进行了计算.通过对悬臂板在约束扭转中剪力流计算公式的推导,进一步阐述了其自由扭转剪应力及翘曲扭转剪应力的分布,指出了相关文献在这部分计算中存在的问题.通过一个简支波形钢腹板组合箱梁算例,将该文方法计算结果与ANSYS有限元计算结果进行比较.结果表明:在波形钢腹板箱梁截面中,主要由波形钢腹板承受扭转剪应力,其次是混凝土底板,底板剪应力最大值发生在底板中心处,其数值近似等于腹板剪应力的一半,而混凝土顶板和悬臂部分的扭转剪应力很小;该文计算的扭转剪应力结果在总体上符合有限元得到的扭转剪应力分布规律,在悬臂自由端为0,随着离开悬臂自由端距离的增大,扭转剪应力逐渐增大并达到峰值.     

45钢柱壳膨胀断裂的应变率效应

采用前照明高速分幅照相技术拍摄到在爆轰加载下45钢柱壳表面裂纹生成、扩展及产物泄漏过程的清晰图像,较准确地测量了膨胀断裂的时间与应变。 45钢柱壳在炸药爆轰加载下,断裂应变随应变率的增加而增加,当应变率达到一定值,断裂应变随应变率的增加而降低,出现了动态断裂中的塑性峰现象。