Journal of Materials Engineering and Performance最新文献

Residual stress is a key factor affecting the properties of ceramic/metal brazed joints. In this work, the residual stress of sapphire/Ti6Al4V alloy joints brazed with CuTi+B composite filler after cooling and reheated to 400 °C was calculated by finite element numerical simulation method. The effect of the B content in the filler and the filler thickness on the residual stress distribution of brazed joints after cooling was studied. The results show that the residual stress is mainly concentrated in the area near the filler layer. The maximum von Mises stress is located in the filler layer and a large Z-axial (the direction perpendicular to the brazed surface) tensile stress is on the sapphire near the filler layer. The B content and the filler thickness have no significant effect on the distribution of residual stress, but they have a certain effect on the value of residual stress of the sapphire near the filler layer. The von Mises stress and the Z-axial stress of this area increase with the increase of B content, and they decrease first and then increases with the increase of filler thickness. When the sapphire/Ti6Al4V joint is reheated to 400 °C, its residual stress significantly reduces and the residual stress on the periphery of the filler layer changes from compressive stress to tensile stress. As the B content is 1 and 3 wt%, the sapphire/Ti6Al4V joint with a smaller residual stress can be obtained. The microstructure observation and shear tests of the joints show that cracks appear in the high stress concentration area and the fracture starts from the area with maximum axial tensile stress, which verifies the accuracy of the simulation results.

In this article, the yield strength, tensile strength, and the microstructure of the wrought aluminium alloy 7449 rolled thick plate have been studied through thickness under different temper conditions. For all heat treatments, it has been proven that the yield strength and tensile strength values increase from the surface to the centre. The largest difference between the centre and the surface, in both properties, occurs in the case of a sample aged at room temperature for 120 h (TTA temper). The sample artificially aged at 120 °C for 24 h (TTB temper) shows the best strength-gradient relationship of the tensile properties through the thickness. Metallographic characterisation carried out by optical and scanning electron microscopy shows much finer elongated grains in the region near the surface of the plate than in the centre, with incipient recrystallisation in the area near the surface. In addition, electron backscattered diffraction technique, used for micro-texture analysis, has proven the presence of a gradient of crystallography texture in the plate. This explains the yield strength gradient, since the rate of change of the Taylor factor through thickness correlates with the rate of the change of yield strength in the longitudinal direction for the samples studied.

AISI 52100 is a high carbon alloy steel typically used in bearings. One hardening heat treatment method for AISI 52100 is austempering, in which the steel is heated to above austenitizing temperature, cooled to just above martensite starting (Ms) temperature in quench media (typically molten salt), held at that temperature until the transformation to bainite is completed and then cooled further to room temperature. Different austempering temperatures and holding times will develop different bainite percentages in the steel and result in a variation in mechanical properties. In the present work, the bainitic transformation kinetics of AISI 52100 were investigated through experiments and simulation. Molten salt austempering trials of AISI 52100 were conducted at selected austempering temperatures and holding times. The microstructure of austempered samples were characterized with optical microscope and x-ray diffraction. The bainitic transformation kinetics were analyzed by Avrami equations using measured hardness data. The CHTE quench probe was used to measure the cooling curves in the molten salt from austenitizing temperature to the selected austempering temperatures. The heat transfer coefficient (HTC) was calculated with the measured cooling rates and used to calculate the bainitic transformation kinetics via DANTE software. The experimental results were compared with the calculated results and displayed good agreement.

The hole-drilling method is one of the most widespread techniques to measure residual stresses. Since the introduction of the Integral Method to evaluate non-uniform stress distributions, there has been a considerable improvement in the instrumentation technology, as step increments of about 10 microns are now achievable. However, that spatial resolution makes the ill-posedness of the problem stand out among other sources of uncertainty. As the solution becomes totally dominated by noise, an additional regularization of the problem is needed to obtain meaningful results. Tikhonov regularization is the most common option, as it is also prescribed by the hole-drilling ASTM E837 standard, but it has only been studied in the reference case of a hole with no eccentricity with respect to the strain rosette. A recent work by Schajer addresses the eccentricity problem by defining a correction strategy that transforms strain measurements, allowing one to obtain the solution with the usual decoupled equations. In this work, Tikhonov regularization is applied to the eccentric hole case through the influence functions approach, in order to avoid the introduction of new error-compensating functions and bias-prone interpolations. Some useful general considerations for a practical implementation of the procedure and an experimental test case on an aluminum specimen are presented.

This study employs neutron diffraction to investigate the relationship between residual stress and coating thickness in cold sprayed 304L austenitic stainless steel. Results show that shot peening predominantly impacts the residual stress profile, leading to substantial in-plane compressive force. The impact of laser heating, a widely used method to alter cold spray's microstructural properties, on the coating's residual stress is also analyzed. The findings indicate that the maximum compressive residual stress in the in-plane component is mainly independent of coating thickness, which suggests that the material properties determine the maximum residual stress. The cold sprayed deposits possessed compressive, nearly biaxial strain and stresses. After laser heating, these stresses were replaced by tensile residual stresses. Two analytical models, the Tsui and Clyne and the Boruah models, for predicting residual stresses are also evaluated, and both models provide reasonable fits to the experimental data. At this point, the deviations between the experimental results and the models are principally caused by the inability of the current models to address plastic deformation and relaxation, and the residual stresses generated by thermal gradients.

The influence of Y₂O₃ addition on densification, physical, mechanical, thermal, and oxidation properties of ZrB₂-20 vol.%SiC- (0-15 vol.%Y₂O₃) composites was investigated in the present study. Powders of ZrB₂-SiC-Y₂O₃ were cold compacted uniaxially, and green compacts were densified by pressure-less sintering. Results indicate that Y₂O₃ addition improves the sinterability and mechanical properties, whereas it diminishes the electrical and thermal conductivities of the investigated composites. Removal of surface oxides by the additives and segregation of Y₂O₃ particles at the triple junction of the ZrB₂ grains enhances densification. Reduction in porosity (9.5-4.2%) through Y₂O₃ addition (0-15 vol.%) improves hardness (up to 52%), relative elastic modulus (up to 9%), and fracture toughness (up to 26%) of the investigated composites. The electrical conductivity has been observed to vary in the range of 2.67-1.92 10⁶ S/m, and thermal diffusivity values decrease with an increase in Y₂O₃ content and temperature. Oxidation studies indicate that the ZrB₂-SiC composite shows better oxidation resistance than other investigated composites. Characterization of oxidized scales confirms the formation of a thicker oxide layer over the samples containing Y₂O₃.

Residual stresses in sheet metal parts are internal stresses that remain after the release of elastic strains at the end of metal forming operations. When the formed sheet metal part is removed from the forming tools, it not only springs back but also acquires a through-thickness residual stress distribution. Residual stresses for monolithic aluminum sheets and aluminum/polypropylene/aluminum sandwich laminates after springback of a U-channel formed by draw bending are presented in this paper. The forming stresses at the end of punch travel and residual stresses at the end of punch withdrawal are numerically determined using LS-DYNA, a well-established nonlinear finite element software. The through-thickness forming stress distribution is determined using an explicit forming simulation, following which the through-thickness residual stress distribution is determined using an implicit springback simulation. Stress distributions are studied at the die corner, punch corner and along the wall of the U-channel. Both forming and residual stresses in the sandwich laminate are found to be lower than those in monolithic aluminum of equivalent thickness. In sandwich laminates with the same skin thickness, higher residual stresses are observed in the skin layers as the core thickness is increased. The residual stresses at the punch corners of the formed U-channels are more influenced by changes in the die and punch corner radii than at the die corner.