The Al-Si coating of ultra-high strength steel has been applied to hot stamping more and more widely, owing to solving the problem of oxidation and decarburization. However, the evolution of Al-Si coating during the heating process was rarely studied in the previous study. The tests about the influence of heating parameters, such as heating temperature, heating rates and dwell time, on properties of the Al-Si coating were carried out on the Gleeble-3500 thermal simulator. The properties of the Al-Si coating, for instance, volume fraction of FeAl intermetallics, α-Fe layer as well as porosity and 3D surface topography, were explored in the study. Results showed that more and more Kirkendall voids and cracks appeared in the Al-Si coating when the heating temperature exceeded 600°C. The heating rates almost had no influence on properties of the Al-Si coating when the temperature was equal to or lower than 500°C. The volume fraction of FeAl intermetallics in the coating with dwell time from 3 s to 8 min at 930°C was0, 6.19%, 17.03% and 20.65%, separately. The volume fraction of the α-Fe layer in the coating changed from zero to 31.52%with the prolonged dwell time. The porosity of the coating ranged from 0.51% to 4.98% with the extension of dwell time. The unsmooth degree of the surface of the coating rose gradually with the increasing of heating rates and the extension of dwell time.The 3D surface topography of the coating was determined by the comprehensive effect of atoms diffusion, new formed phases,surface tension and the degree of oxidation of the coating surface. Experiments indicated that rapid heating was not suitable for the coating when the temperature exceeded 500°C. Experiments also demonstrated that enough dwell time was essential to obtain the superior properties of the coating. 相似文献
Force closure and stability problems exist in heavy gripping devices. Based on a typical gripping device for forging operations, the workpiece and the gripping device are innovatively considered as a single mechanism in this paper, and its global topological configuration is obtained. The changing element method is used to establish the optimized topology geometry of the gripping device by decreasing the degrees of freedom of the gripping mechanism and improving the force closure performance. Therefore, a new idea for the innovative design of heavy-duty forging gripping devices is provided. As the effect of revolute joint friction on loading capacity of the gripping device cannot be ignored, a unified mechanical equation including friction in terms of the topological adjacency matrix is established to analyze the force closure performance and calculate the loading capacity. The conclusions are provided by experiments.
In this study,austenitizing heat treatment before hot stamping of Al-10%Si coated boron steel is first investigated through environment scanning electron microscopy(ESEM)equipped with energy dispersive x-ray analysis(EDAX).The cracking behavior of the coating was evaluated using Gleeble 3500,a thermo-mechanical simulator under uniaxial plastic deformation at elevated temperatures.The extent and number of cracks developed in the coating were carefully assessed through an optical microscope.The coating layer under hot-dipped condition consists of an Al-Si eutectic matrix,Fe2Al7Si,Fe3Al2Si3 and Fe2Al5,from the coating surface to the steel substrate.The coating layer remains dense,continuous and smooth.During austenitization,the Al-rich Fe-Al intermetallics in the coating transform to more Fe-rich intermetallics,promoted by the Fe diffusion process.The coating finally shows the coexistence of two types of Fe-Al intermetallics,namely,FeAl2 and FeAl.Microcracks and Kirkendall voids occur in the coating layer and diffusion zone,respectively.The coating is heavily cracked and broken into segments during the hot tensile tests.Bare steel exposed between the separate segments of the coating is oxidized and covered with a thin FeOx layer.The appearance of the oxide decreases the adhesion of the Al-Si coating.It is found that the ductile FeAl is preferred as a coating microstructure instead of the brittle FeAl2.Therefore,the ductility of the Al-Si coating on hot stamping boron steel could be enhanced by controlling the ductile Fe-rich intermetallic phase transformations within it during austenitization.Experiments indicate that a higher austenitizing temperature or longer dwell time facilitate the Fe-rich intermetallics transformation,increasing the volume fraction of FeAl.This phase transformation also contributes to reducing the crack density and depth. 相似文献
In this study, a thermo-plastic-martensite transformation coupled model based on the von Mises yield criterion and the associated plastic flow rule is developed to further improve the accuracy of numerical simulation during hot stamping. The constitutive model is implemented into the finite element program ABAQUS using user subroutine VUMAT. The martensite transformation, transformation-induced plasticity and volume expansion during the austenite-to-martensite transformation are included in the constitutive model. For this purpose, isothermal tensile tests are performed to obtain the flow stress, and non-isothermal tensile tests were carried out to validate the constitutive model. The non-isothermal tensile numerical simulation demonstrates that the thermo-plastic-martensite transformation coupled constitutive model provides a reasonable prediction of force-displacement curves upon loading, which is expected to be applied for modeling and simulation of hot stamping.
