氧-天然气切割和焊接技术研究

研究了氧-天然气金属切割和焊接技术.结果表明:通过设计出优质快速切割喷嘴和双层焊接焊炬,采用合适的切割和焊接工艺规范和措施,氧-天然气切割和焊接技术能获得良好的切割质量和焊接质量,且能满足一般效率要求;与氧-乙炔气体相比,氧-天然气切割和焊接技术成本最低,具有显著的推广应用价值.     

激光能量和辅助气体对切割能力影响的数值模拟

采用旋转高斯激光体热源和氧气流量控制的氧铁燃烧反应放热复合热源,基于计算流体力学建立了能够反映激光切割中激光能量、辅助气体和切缝之间相互作用的多相流模型.利用该模型对以氧气和氮气为辅助气体的激光切割过程进行了数值模拟,通过改变激光功率和辅助气体压力,研究了热输入和辅助气体流场对激光切割能力的影响,并对两种辅助气体的切割结果进行了比较和分析.结果表明,所采用的计算模型较好地模拟出激光功率和辅助气体对激光切割能力的影响,并对切缝形状进行预测.     

The influence of nozzle design on HVOF exit gas velocity and coating microstructure

A simple device was constructed for determining a value for the average combustion gas velocity at the exit plane of a high-velocity oxyfuel gun. This device was used to measure the velocities of a standard factory-made barrel nozzle and a specially designed de Laval nozzle as a function of the fuel/oxygen ratio and the total mass flow rate. The Mach number of the de Laval nozzle was 1.42. The maximum combustion gas exit velocities determined for the standard and the de Laval nozzles were 1100 and 1550 m/s, respectively. The maximum velocity depends on the fuel/oxygen ratio but is independent of the total flow rate. The effect of increased combustion gas velocity on coating quality is demonstrated.     

Cutting under active and inert gas shields: A contribution to the mechanics of chip flow

This paper is focused on the interaction between cutting medium and freshly formed surfaces with the aim at providing a new level of understanding on the mechanics of chip flow in metal cutting.The methodology draws from specially designed orthogonal metal cutting experiments performed in dry conditions under active and inert gas shields for observation of chip flow and measurement of friction, chip-compression factor and forces acting on the cutting tools.The presentation is a step towards clarification of common belief among researchers and practitioners that active gases influence metal cutting by showing that oxygen acting on the freshly cut surfaces of lead may change friction, chip compression factor, chip curling and forces to a level that goes significantly beyond what has been said and written in the context of metal cutting fundamentals.The overall content of the paper is original and discloses new experimental results and explanations about the influence of the surrounding medium on tribological conditions at the tool–chip contact interface and about the correlation between surrounding medium, surface roughness, freshly cut surfaces and chip curling.     

焊炬氧气胶管回火爆炸事故分析

在使用氧-乙炔焰气焊、气割和预热金属过程中,回火现象时有发生,但一般情况下都是向乙炔气体通道里回火.介绍了一起在使用大型焊炬预热金属过程中专向助燃气体通道里猛烈回火爆炸的案例,并展开一系列的实验分析,阐述事故发生、发展和演变的过程,描绘氧气工作压力与发生氧气带回火现象的大致关系曲线,给出导致这起严重生产事故的根本原因,...     

Effect of cutting process on the stress corrosion susceptibility of AISI 304L stainless steel

During manufacturing of a component, cutting, turning, grinding, and milling operations are inevitable and these operations induce surface residual stresses. In this study, it is shown that, depending on the process employed for cutting, residual stresses generated at the cut surfaces can vary widely and they can, in turn, make the cut surfaces of austenitic stainless steel (SS) prone to stress corrosion cracking (SCC). An austenitic SS 304L plate was cut using three different procesess: bandsaw cutting, cutting using the cut-off wheel, and shearing. Surface residual stress measurement using the X-ray diffraction (XRD) technique is carried out close to the cutting edges and on the cross-section. SCC susceptibility studies were carried out as per ASTM G36 in 45% boiling magnesium chloride solution. Optical microscopic examination showed the presence of cracks, and confocal microscopy was used to measure the depth of cracks. The study confirmed that high tensile residual stresses present in the cut surfaces produced by cut-off wheel and shear cutting make the surfaces susceptible to SCC while the surfaces produced by bandsaw cutting are resistant to SCC. Hence, it is shown that there is a definite risk of SCC for product forms of austenitic SS with cut surfaces produced using cutting processes that generate high tensile residual stresses stored for a long period of time in a susceptible environment.     

Analysis of heat effects in laser cutting of steels

Efficient control of laser cutting processes is closely related to knowledge of heat effects in the cutting front and its surroundings. Similar to other machining processes using high power densities, in laser cutting processes it is very important to monitor the heating phenomena in the workpiece material due to heat input. In laser cutting processes with oxygen as an auxiliary gas, cutting energy is a combination of laser beam energy and the energy of the exothermic reactions occurring in the cutting front. The presence of oxygen in the process increases cutting efficiency, but also causes additional physical processes in the cutting front that render a more detailed analysis of the cutting phenomena difficult. The aim of this article is to analyze the emission of infrared rays from the cutting front with a photodiode, statistically analyze the temperature signals, and optimize the laser cutting process based on a critical cutting speed. The measured infrared radiation temperature signal was, on the basis of calibration, converted into a temperature that was related to the formation of macro- and microstructures and to the change in microhardness in the surface layer of the cut. On the basis of experimental results, it was proved that heat effects in the cutting front decisively influenced the quality of cut. Finally, factor analysis was used to establish statistical relations among variables of the laser system, variables of the cutting process, and geometrical characteristics of the cut.     

Influence of the stress,strain, and temperature on the surface roughness of an AlSl 52100 steel due to an orthogonal cut

In recent years, the finite element method (FEM) has become the main tool for simulating the metal cutting process because research based on trial and error is time consuming and requires high investment. Early studies were done by different investigators. In this research AISI 52100, hardened steel (62 HRC) was selected for an orthogonal machining process as well as metal cutting simulation using the software DEFORM-2D. This software is based on a forging process and has been adapted to an orthogonal machining process. The results of simulated cutting forces were compared with experimental cutting force data to validate the orthogonal cut simulation. Also, the surface roughness was measured, and the influence of the stress, strain, and temperature on the surface roughness was studied.     

激光切割过程辅助气体动力学性能的数值模拟

激光切割过程中辅助气体的动力学性能对切缝形成过程具有重要影响.利用VOF算法和深度自适应激光热源,建立了能反映切割过程中辅助气体和切缝之间相互作用的多相流模型,结合切割试验对模型的有效性进行了验证.采用多相流模型分析了从打孔到稳定切割过程中切缝形貌、辅助气体动力学性能和温度场分布.计算结果表明,在未切透阶段,受切割前沿形状和切割深度等的影响,辅助气体动力学性能不断发生变化;当切割过程稳定后,辅助气体流场几乎不发生变化,切缝形状和温度场也不再改变;所建模型能够有效反映辅助气体动力学性能对切割深度和切缝宽度的影响.     

Laser welding of 3 mm thick laser-cut AISI 304 stainless steel sheet

The objective of the present work was to study the laser weldability of laser-cut 3 mm thick AISI 304 austenitic stainless steel sheet (using oxygen as an assist gas). For minimizing heat input during laser cutting, which is an important factor influencing the thickness of the oxide layer on the cut surface, laser cutting was performed in pulsed mode. The results of the study demonstrated that although the laser welding of laser-cut specimens did not result in the formation of weld defects, the resultant laser weldments exhibited reduced ductility with respect to base metal and bead-on-plate laser weldments. Laser-cut and laser-welded specimens also displayed higher notch sensitivity than the base metal. However, laser-cut and laser-welded specimens still possessed enough ductility to pass guided bend tests.     

Generation of ferromagnetic micro- and nanoparticles by laser and mechanical milling methods

The generation of micro- and nanoparticles using laser cutting of steel strips of the thickness of 0.2–0.6 mm and the separation of particles according to their size were studied. Under optimal cutting conditions (laser power, gas pressure before the nozzle), the cutting quality is good when a small amount of fully oxidized erosion products at the cut edge of a slit is obtained. The study showed that the cascade filter system can be used to separate the particles up to nanometer dimensions. The simulation of the gas flow can be used to create optimal conditions for collecting generated particles of the required dimensions. The modelling data confirmed that small particles from the liquid metal state in the cut slit are generated most effectively using nozzles forming ring flows. The larger particles modification with aluminium and zinc by milling was performed.     

燃气燃烧特性对火焰切割性能的影响

根据燃气物理化学性质的分析,比较燃气燃烧速度,火焰质量分布,温度的高低,阐述了乙炔,丙烷,等燃气燃烧特性对火焰切割性能的影响。最后对燃气的选用进行了讨论。     

Comparative study of WC-cermet coatings sprayed via the HVOF and the HVAF Process

The high velocity air fuel (HVAF) system is a high-velocity combustion process that uses compressed air and kerosene for combustion. Two WC-cermet powders were sprayed by the HVAF and the high-velocity oxyfuel (HVOF) processes, using an AeroSpray gun (Browning Thermal Systems Inc., Enfield, New Hampshire) and a CDS-100 gun (Sulzer Plasma Technik, Wohlen, Switzerland) respectively. Several techniques, including x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, were used to characterize the microstructures and phase distribution of the powders and coatings. In addition, mechanical properties such as hardness and wear resistance (pin-on-disk) were investigated. A substantial amount of W₂C was found in the HVOF coatings, as well as a high concentration of tungsten in the binder phase, indicating that oxidation and dissolution processes change the composition and microstructure from powder to coating during spraying. This was in contrast to the HVAF coatings in which composition and microstructure were unchanged from that of the powder. Additionally, the wear resistance of the HVAF coatings was superior to that of the HVOF coatings.     

Oscillation overlaid shearing of sheet

One of the most frequently used sheet metal processing methods is shearing. While using the shearing method a cut edge is usually created which often clearly fails to meet the desired quality. The optimum cut edge quality is as rectangular and as burr-free as possible. Trials made at the IFUM showed, that the quality of cut edges can be improved by overlaying the movement of the punch with an oscillation. Through different test rows the mechanism and impact of oscillation on the shearing process are investigated. This paper reports about tensile and friction tests made on sheet metal with oscillation overlay, high speed cutting tests and test series with a new designed tool which allows stamping sheet metal also with an oscillation overlay. The test series show, that oscillation can have an effect on the tensile strength, friction and work hardening of sheet metal and as a result on the cutting force and the cut edge quality.     

Obtaining a submerged arc welding flux of the MnO–SiO2–CaO–Al2O3 – CaF2 system by fusion

Plate temperature and heat input in an oxyfuel gas cutting process with H₂/LP gas and LPG flame are calculated by three-dimensional FE heat conduction analyses. FE analyses are performed by using moving coordinates, and cutting groove temperature is determined by iterative calculation. The two-dimensional groove temperature distribution determined by Matsuyama's theory is chosen as the initial values in this iterative calculation. The heat transfer properties of the preheating flame are determined by using the genetic algorithm-based heat transfer estimation technique proposed in the previous report. The validity of the proposed numerical procedure and the accuracy of the determined groove temperature are examined by comparing the calculated and measured plate temperature and heat-affected zone sizes. Heat input due to preheating, q_G, and that due to self-burning of steel, q_B, are estimated in these analyses, and they are compared with the heat inputs estimated by Wells' and modified Wells' equations. The relation between the heat transfer characteristics of the preheating gas flame and plate temperature distribution is examined, and the cutting performance improvement mechanisms of hydrogen preheating are discussed. As a result, the followings are found: (1) the three-dimensional groove temperature distribution can be calculated by performing the iterative analyses procedure proposed in this study; (2) the critical cutting speed can be estimated once the gas heat transfer parameters are known; (3) it is not appropriate to evaluate the magnitude of cutting thermal deformation only from the preheating gas's total calorific value; (4) under the conditions chosen, the heat generated by self-burning is inadequate to maintain the cutting process, and it is essential to supplement heat by preheating; (5) the faster cutting speed and smaller total heat input of H₂/LP gas are results of the larger local heat transfer coefficient below the gas ejection hole. It is supposed that the improvement in oxyfuel gas cutting performance can be achieved by modifying the heating apparatus so that the local heat transfer coefficient becomes larger.     

气割工艺分析

对切割氧气压力、切割速度及倾角、割嘴与割件距离等对切口表面的质量的影响因素进行了分析,并提出改进质量的措施。     

New attachment for controlling gas flow in the HVOF process

During the decade, the high-velocity oxyfuel (HVOF) process proved to be a technological alternative to the many conventional thermal spray processes. It would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow deposition of ceramic coatings. Based on a numerical analysis, a new attachment to a standard HVOF torch was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the attachment was investigated by spraying several coating materials including metal and ceramic powders. Particle conditions and spatial distribution, as well as gas phase composition, corresponding to the new attachment and the standard HVOF gun, were compared. The attachment provides better particle spatial distribution, combined with higher particle velocity and temperature. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Mathematical modeling of the gas and powder flow in HVOF systems

A mathematical model was developed to describe the gas dynamics and heat-transfer mechanism in the gas/particle flow of high- velocity oxyfuel (HVOF) systems. A numerical solution was carried out using a PC- based computer program. One- dimensional predictions of the temperature and velocity profiles of gas and particles along the axis of flow were obtained to conduct cost- effective parametric studies and quality optimization of thermal spray coatings produced by HVOF systems. The numerical computer model allows for the variation of the HVOF system parameters, such as air/fuel ratio and flow rates, cooling water inlet temperature and flow rate, barrel length, standoff distance, particle size, and gun geometry. Because of the negligible volume of the powder relative to the gas, the gaseous phase was modeled as continuous nonadiabatic, and friction flow with variable specific heats and changing cross- sectional areas of flow. The generalized continuity, momentum, and energy equations with the influence parameters were used to model the gaseous flow regime and predict its thermodynamic properties. Empirical formulas for the mean axial decay of both velocity and temperature in the supersonic jet plume region were generated from published measurements of these parameters using laser Doppler velocimeter and Ray leigh scattering techniques, respectively. The particle drag and heat- transfer coefficients were calculated by empirical formulas in terms of Reynolds, Nusselt, and Prandtl numbers to evaluate both the momentum and heat transferred between the combustion gases and the powder particles. The model predictions showed good agreement with the particle and gas temperature and velocity measurements that are available in the literature.     

Influence of assist gas nature on the surfaces obtained by laser cutting of Al-Cu alloys

This paper focuses on the influence of assist gas nature on the surface finishing and chemistry of cut edges obtained during CO₂-laser cutting of an aluminium-copper alloy by means of a non-conventional cutting head. Argon, oxygen, nitrogen and compressed air were studied as assist gases. Surface characterization was accomplished by means of X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) analyses of cut edges, showing the formation of oxides and nitrides. Argon was revealed as the more efficient assist gas to obtain best quality results. This makes argon the gas of choice when laser cutting Al-Cu alloys, excluding its higher cost.     

Grey relational analysis coupled with principal component analysis for optimization design of the cutting parameters in high-speed end milling

This paper investigates optimization design of the cutting parameters for rough cutting processes in high-speed end milling on SKD61 tool steel. The major characteristics indexes for performance selected to evaluate the processes are tool life and metal removal rate, and the corresponding cutting parameters are milling type, spindle speed, feed per tooth, radial depth of cut, and axial depth of cut. In this study, the process is intrinsically with multiple performance indexes so that grey relational analysis that uses grey relational grade as performance index is specially adopted to determine the optimal combination of cutting parameters. Moreover, the principal component analysis is applied to evaluate the weighting values corresponding to various performance characteristics so that their relative importance can be properly and objectively described. The results of confirmation experiments reveal that grey relational analysis coupled with principal component analysis can effectively acquire the optimal combination of cutting parameters. Hence, this confirms that the proposed approach in this study can be an useful tool to improve the cutting performance of rough cutting processes in high-speed end milling process.