Materials Science Forum Vols. 575-578

Abstract: Aiming at the radiator with tube-to-plate structure applied usually in aeroplane, a two-dimensional model for finite element analysis was established in this work. By ANSYS software, the temperature field and stress field of electron beam brazing (EBB) 1Cr18Ni9Ti stainless steel radiator by two kinds of process were numerically simulated. The calculated results of temperature field show, by the stage-by-stage heating process, the uniform temperature distribution of radiator faying face was obtained. The temperature of most regions is between 1042~1051°C, which is in the range of brazing temperature. The calculation results of stress field indicate, for radial residual stress, the obvious stress concentration region was found in faying face by direct-heating process; while there was no stress concentration in faying face by stage-by-stage heating process. For circumferential residual stress, compared the stage-by-stage heating process with direct-heating process, the peak value of tensile stress reduces by 11.2%. Compared circumferential residual stress with radial residual stress by two kinds of brazing process, the peak value of circumferential tensile stress is higher than radial tensile stress. So the dangerous position of faying face is along circle direction, namely, the heating direction of scanning electron beam. Consequently, the temperature difference between different positions in faying face must be controlled well during heating. The reduction of temperature difference can fall the peak value of tensile stress and improve the distribution of residual stress.

Abstract: During the research the following problem was raised: to define the pulse laser radiation influence value on different powder materials, as well as to define experimentally the thickness and width relations of a sintered powder layer from technological laser radiation parameters.

Abstract: Magnesium alloys are being increasingly used in automotive and aerospace structures. In this study, welding of AZ61 magnesium alloy with 10 mm thickness was carried out using vacuum electron beam welding (EBW). By using the finite element model and the 3D moving double ellipsoid heat source model, numerical simulation method was employed to study the influence of the electron beam current on the temperature field of welding process and weld penetration. The microstructure and microhardness of weld joint obtained by the optimized vacuum EBW process had been investigated in detail. The results show that the numerical simulation result basically matches the experimental result. A favorable joint had been obtained by EBW for AZ61 magnesium alloy, in which heat affected zone was not evident, the fusion zone (FZ) consisted of fine-equiaxed grain. The weld hardness was greater than that of the base metal.

Abstract: The glass-to-metal seals are widely used in the solar thermal power. When a glass-to-metal seal is cooled in the process of diffusion welding, the residual stresses are generated due to different thermal contraction between the two materials. The residual stresses built up along the interface near the end of the seal can induce welded joints to crack and decrease the fatigue intensity of the welded joints and thus are of technical importance. In order to obtain the residual stresses existed in the diffusion welded joints, the glass-to-metal vacuum diffusion sealing process were simulated by using finite element software ABAQUS. Furthermore, the influences of temperature, time, vacuum, and seal pressure on the strength of the glass-to-metal diffusion welding were analyzed. The optimization of the diffusion welding process parameter based on the simulation of the residual stress and analysis of the micro-structure and the macro-mechanical performance of the diffusion welded seals was carried out. The distribution of residual stress on the surface of the glass-to-metal joint caused by welding is measured by X-ray diffraction method, and compared with the result of the numerical simulation to prove the validation of the finite element model.

Abstract: High residual tensile stress is an important factor contributing to stress corrosion cracking (SCC). Shot peening can impose compressive stresses on the surface of welded joints that negate the tensile stresses to enhance the SCC resistance of welded joints. In the present work, the distribution of residual stress caused by welding is measured by X-ray diffraction method. The maximum stress in the weld is close to the yield strength of AISI 304 stainless steel, and the stresses are negative at both ends of the weld and far from the weld. The X-ray method is also used to measure stress caused by shot-peening. The results show that the higher the peening coverage, the higher the residual compressive stresses in the surface of weldments. While under the same condition, the residual compressive stresses induced by glass beads shot-peening are larger than those by cast steel shots. Temperature and stress fields of welding are simulated by using ABAQUS codes. The 3-D solid elements are used in FEM. Temperature depending on material properties as well as the convection and radiation as boundary conditions are considered. The 3-D linear reduced-integration elements are used to simulate the shot peening process. The results of simulation have a good agreement with experimental data. All unpeened and peened weldments are immersed in boiling 42% magnesium chloride solution during SCC test. Unpeened specimens crack after immersion for 6 hours. The steel-peened specimens with 50% coverage crack after 310 hours, while the steel-peened specimens with 100% coverage crack for 3500 hours. However, steel-peened specimens with 200% coverage and glass-peened specimens with 50%, 100% and 200% coverage are tested for a total of 3500 hours without visible stress corrosion cracks in the peened surfaces. The experiment results indicate that shot peening is an effective method for protecting weldments against SCC and weldments peened by glass beads resist SCC better than those peened by steel shots.

Abstract: In order to predict the geometry character of laser surface micro-texturing, the finite element analyzing software ANSYS is used to simulate temperature field and crater on the laser ablation. The influence and change regulation of laser intensity, laser pulse number and pulse duration in laser surface texturing are analyzed in detail. The simulation results conclude the best laser intensity in laser-pulse and materials interactions on certain conditions, and the best pulse duration in nanosecond laser micromachining. This research establishes the foundation for laser machining regular non-smooth surface in a rapid and effective way.

Abstract: Metal transfer mode of the low hydrogen type structural steel covered electrode is the one that coexists as the globular short-circuiting transfer and the fine droplet transfer in the flux-bridge. Component of the metal transfer modes has a direct effect on usability of the covered electrode. The oscillograms of the arc voltage and the welding current gained using the traditional photoelectric oscillograph can only qualitatively describe the general characteristics of metal transfer, but can’t make a quantitative analysis. Using the ANALYSATOR HANNOVER, the welding electrical parameters are measured and analyzed. Four characteristic information values correlated with usability of the covered electrode, such as the frequency of the globular short-circuiting transfer, the globular short-circuiting time, the mean value of the globular short-circuiting time and the mean value of the weighted arcing time, are extracted. The method of principal component analysis is applied to determining the evaluation index for usability of the covered electrode. Thereby quantitative evaluation for usability of the covered electrode is realized. A new method is offered for scientifically evaluating usability of the low hydrogen type structural steel covered electrode.

Abstract: Thermal simulating technology was used to simulate weld CGHAZ of microalloyed steel with different thermal cycle for the purpose of investigation on morphology of M-A constituent and its influence on toughness. The experimental results showed that in comparison with base metal specimens after thermal cycle have poorer toughness for its larger size, elongated and sharp massive shape and non-uniform distribution. Toughness of specimen value has maximum value with maximum area fraction for cooling time of 7s, and then it will drop whether cooling time is longer or shorter. Moreover, its mean chords of all specimens are all smaller than 0.5μm. Only those with the length larger than 2μm or the length-width ratio exceeding 4, cleavage fracture can occur. So it is concluded that M-A constituent is not the main influencing factor of impairing toughness for steel with different thermal cycle for its smaller mean chord and area fraction.

Abstract: Laser bending is a flexible forming process which forms sheet metal by means of stresses induced by external heat instead of external forces. In this paper, a three-dimensional coupled thermal-mechanical model for numerical simulation is established with finite element code ABAQUS. Some key problems about the simulation of laser bending are investigated in detail, and the reasonable solutions are presented. Taking AISI-1008 steel as an example, numerical simulations are carried out for the complex contour forming of sheet by using Sequentially Coupled Thermal-Stress Analysis technique. Then the corresponding experiments are performed to validate the simulation results. Good correlation between the numerical simulation and the experimental results was demonstrated.

Abstract: The numerical simulation and experimental measurement of temperature distribution in electrical field activated sintering of titanium powders were carried out. The simulated and experimental results were in good agreement. It was shown that the sintering temperature gradually decreased from the center of sample to the outer. To improve the performance of sintered material, the sintering temperature gradient had to minimize. A method, electric field-activated sintering coupled with axial alternating magnetic field, was proposed to homogenize sintering temperature field. The simulation of sintering temperature field was also conducted under different magnetic field intensity. It was proved that the maximum radial sintering temperature difference in sample was reduced by about three fourths, owing to the skin effect of induced current caused by alternating magnetic field.