查看更多>>摘要：The shape of welding arc is closely related to the quality of welding. For micro plasma arc welding (MPAW), however, the change of arc shape is not obvious enough to evaluate the quality of welding, due to the small size of welding arc. Thus, it needs to extract the specific features of arc shape through image processing algorithm to judge its change quantitatively. Furthermore, for workpiece with bright surface, it is difficult to extract the arc shape directly from the arc images because of the strong arc reflection on the workpiece surface. In this paper, a new method based on vertical Sobel operator was proposed to overcome the above-mentioned challenges, and to extract the clear arc shapes from the arc images of pulse micro plasma arc welding (P-MPAW). The arc length, arc width and the height difference between two sides of arc were extracted as characteristic parameters to reveal the influence of different welding defects on the arc shape. The changes of arc characteristic parameters have been studied in depth, when there are different defects such as burn through, misalignment, undercut, concave deformation, excessive gap and overlap. The results show that, excessive welding gap and misalignment, caused by the assembly and machining accuracy, have obvious characteristics in the instantaneous arc shape. The de-fects caused by welding thermal process like burn through, concave deformation, overlap and undercut have their own distinctive characteristics in the changing process of the arc shape.
查看更多>>摘要：Due to chip-flow interference and coordination, the two types of chips formed in the processes of double-edged cutting make a remarkable difference to cutting force components, tool service life, and machined parts quality. However, the chip formation mechanisms of both types are poorly understood, and the existing constitutive models generally fail to capture the deformation characteristics of the workpiece material during double-edged cutting. To this end, a new combined constitutive model composed of plasticity and damage parts is presented and applied to the finite element (FE) model for double-edged cutting of AISI 1045. The plasticity part considers the impacts of strain, strain rate, and temperature on the flow stress of workpiece material. The damage part considers the effects of stress states on the equivalent plastic strain at damage initiation. Double-edged cutting simulations were carried out employing the combined constitutive model and the Johnson-Cook (JC) model, respectively. To validate the presented constitutive model, experiments for double-edged cutting of AISI 1045 were completed, and the predicted results were compared with experimental results. It is concluded that the presented constitutive model substantially enhances the predicting accuracy for chip morphologies and cutting force components compared to the JC model. In addition, the two types chip formation mechanisms in doubleedged cutting are revealed through analyzing the chip generation in simulations and the chips collected from the experiments. The new combined constitutive model contributes to the simulation-based optimization of the tool/ process parameters in double-edged cutting to reduce the cutting forces or produce the desired chip morphology.
查看更多>>摘要：To join dissimilar metals with significant differences in characteristics, such as molybdenum (Mo) and stainless steels (SS), is essential to take full advantage of their combined performances. In this work, Mo and SS304 have been jointed with Ni interlayer using a diffusion bonding technique at different bonding temperatures. The results show that a MoNi brittle intermetallic compound is easy to form in the interface of the Mo/SS304 joint with increasing the bonding temperature, which will reduce the shear strength of the joint because a crack is easy to form between Mo and the intermetallic compounds. Meanwhile, Mo/SS304 joints have also been prepared by using NixCu1-x (x = 0.45, 0.24, or 0) as an interlayer. The result shows that reducing Ni content can avoid the formation of intermetallic compounds, and the fracture occurs between the interlayer and Mo diffusion zone far away from the Mo side. This work shows that the fracture properties of the Mo/SS304 joint can be modified by changing the composition of the interlayer. The best shear strength of the Mo/SS304 joint is 140 MPa. This work is essential for effectively joining Mo and SS304 using a low-temperature diffusion bonding method.
查看更多>>摘要：Due to the economic efficiency of shear cutting, mechanical cutting is used for almost every sheet metal component processed in large numbers. The process parameters recommended both in the literature and in factory regulations generally apply to low- strength, single-phase steel alloys with low ECS. This does not necessarily apply to current types of high-strength alloys, such as those used in body and chassis components for passenger cars or in frame structures of commercial vehicles. As a result, metal working industries and teaching institutes have developed a range of non-standard testing methods to identify the ECS of metals. The ability to prevent edge cracking by the choice of appropriate cutting parameters eliminate the need for costly and timeconsuming testing, which in turn reduces machine, tool, material and labor costs. This paper investigates various edge crack testing methods based on current single and multi-phase steel alloys and objectively evaluates them based on multiple criteria. The findings were then linked to the state of knowledge. A particular aspect of this document as compared to a conventional review paper is that the materials used can be regarded as constant across all test methods. This enables an objective comparison and evaluation of the methods used. A simple discussion or comparison of the recorded parameters of the state of knowledge is not expedient due to the large number of metallic alloy systems per material group and the fact that the alloy composition is generally not explicitly documented.
查看更多>>摘要：The production of parts involving the combination of different materials has gained a lot of interest in recent years as a strategy for achieving weight reduction. Particular attention has been paid to the joining process of mechanical components obtained with different production processes. This paper aims at studying the feasibility of laser welding to join rolled 6082-T6 thin sheet to A357 sheet produced by Laser-based Powder Bed Fusion (LPBF). The role of welding parameters, LPBF scan strategy and post heat treatment on the weld bead formation has been studied by means of microstructural and mechanical characterization. The results showed that a higher welding speed reduces the porosity in the weld bead obtaining values of about 5% in terms of percentage of fused area, while the post heat treatment of the LPBF sheet has the greatest influence on the final properties of the joint compared to the scanning strategy used. In particular, a complete T6 heat treatment of the additive sheets leads to an ultimate tensile strength of about 200 MPa closes to the reference condition 6082-6082. Hardness measurements showed that a drop in the values occurred at the center of the bead regardless of the treatment conditions while Digital Image Correlation (DIC) strain analysis confirmed that higher deformation occurred in the Fused Zone and in the Heat Affected Zone. SEM-EDS analysis revealed the presence of intergranular precipitates which contain Fe, Si, Mg and Mn in the fused zone.
查看更多>>摘要：A comprehensive systematic comparative study on high-strain-rate tests (Split Hopkinson Pressure Bar), with and without in-situ heating of the specimens and their respective influence on the quality of empirical material models is presented. The determination of material constitutive model parameters is one of the most challenging aspects of the modelling and simulation of machining processes. Chip formation and process forces show a significant dependence on the actual constitutive model and its parameters as well as on the testing method. Typically, the influences of strain, strain rate, and temperature are investigated in separate experiments of quasistatic compression tests and tests, because the most widespread phenomenological constitutive material models (e.g. Johnson-Cook model) neglect interactions between temperature and strain rate. In contrast, the presented work demonstrates, that a coupled experimental approach of strain rate and temperature in the same test increases the quality of such uncoupled material models as well. The authors compared both approaches (separated and in situ temperature-dependent experiments) by determining the constitutive model parameters for AISI 5115 steel samples taken from a single material batch. The parameters are calculated based on a covariance matrix adaptation evolution strategy and applied in identical two-dimensional orthogonal FEM cutting simulations. Process forces and chip thickness values were used for comparison with the machining experiments. The work therefore gives new aspects to decide for a suitable experimental approach when calibrating a constitutive equation.
查看更多>>摘要：Machining oxygen-free electronic (OFE) copper could be challenging but is not widely studied because few industrial or critical components requires to master the machined sub-surface characteristics. CERN radio frequency cavities are one of the applications, especially because the turned surface is not the functional one of the final products. The niobium coating post process, which gives superconductive properties to the cavity, largely depends on the machined surface characteristics. The present study relies on an experimental approach of the cutting process, through thermal and mechanical probing of high precision, pollution free, turning. Cutting forces and thermal load on the tool are detailed for finish turning. The critical uncut chip thickness, defined at macroscale as the limit between cutting and ploughing behavior, is also a frontier at microscale. Consequently, surface integrity is evaluated by advance microstructural analysis (EBSD and FIB), imposed by the thinness of the affected layer. Grain recrystallization appears in the first 0.6 micrometers below the surface and deformed grains are observed up to 4 micrometers for cutting regime, while the thickness of the layers is three time larger in case of ploughing regime. Hence surface integrity of OFE copper finish turning is characterized and optimal cutting conditions are defined. The research shows that simple cutting tests can quickly narrow down to optimal cutting condition, which are then confirmed through metallurgical analysis, even in the edge case of pure OFE copper, hence relevant to other material.
查看更多>>摘要：Laser-assisted grinding (LAG), as a potential machining method, is expected to achieve high-efficiency machining without any surface damage or sub-surface damage. However, grinding force tends to exert serious impact on the surface damage during LAG process. In this paper, a grinding force predictive model for the LAG process was established, which has taken the combined effects of temperature-dependent mechanical properties of the material, statuses of grit-material micro interaction, and stochastic shapes and random distributions of abrasive grits into consideration. This model also reveals the mechanism for the reduction of grinding force during LAG. In the meantime, the simulative grinding force distributions of workpiece surface with different laser powers were obtained. LAG experiments of zirconia ceramic were carried out to validate this model. It is found that the modelled forces are in good agreement with the measured forces and the error rates can be confined within 12 %. In addition, the effect of grinding parameters on grinding force has been investigated. It is demonstrated that the grinding force can be reduced by a certain percentage ranging from 29.4%-60.1% using the optimal machining parameters. Within a certain threshold, higher laser power can improve the surface integrity and decrease the depth of damage. This work is expected to provide significant guidance for promoting the development of the laser-assisted machining technologies.
查看更多>>摘要：Local thermal history can significantly vary in parts during metal Additive Manufacturing (AM), leading to local defects. However, the sequential layer-by-layer nature of AM facilitates in-situ part voxelmetric observations that can be used to detect and correct these defects for part qualification and quality control. The challenge is to relate this local radiometric data with local defect information to estimate process error likelihood in future builds. This paper uses a Short-Wave Infrared (SWIR) camera to record the temperature history for parts manufactured with Laser Powder Bed Fusion (LPBF) processes. The porosity from a cylindrical specimen is measured by ex-situ micro-computed tomography (mu CT). Specimen data from the SWIR camera, combined with the mu CT data, are used to generate thermal feature-based porosity probability maps. The porosity predictions made by various SWIR thermal feature-porosity probability maps of a specimen with a complex geometry are scored against the true porosity obtained via mu CT. The receiver operating characteristic curves constructed from the predictions for the complex sample demonstrate the porosity probability mapping methodology's potential for in-situ based porosity detection.
查看更多>>摘要：Further development of the car lightweight in automotive industry significantly depends on the reliability of the Al/steel dissimilar joint, since more and more Al alloy parts are used in the car manufacturing. It has been proved that the Al/steel dissimilar joint strength can be effectively increased when the intermetallic compound (IMC) growth, especially the Fe2Al5 layer thickness, is controlled at certain level. Because the Fe2Al5 is the dominant phase of the entire IMC layer in the Al/steel dissimilar joint, and cracks usually initiate at this location when damage is conducted. Therefore, for the automotive industry assembly line, the prediction model monitoring the Fe2Al5 layer thickness of each Al/steel weld seam is desired to ensure the dissimilar joint quality and reliability. In the present research, a numerical dynamic growth model based on element diffusion laws is established to calculate the IMC layer thickness of Fe2Al5 phase during the variable polarity cold metal transfer (VP-CMT) welding of aluminum to steel in the function of welding heat input and location. To build up effective connection between the welding heat input and the actual temperature history of Al/steel interface, a finite element analysis (FEA) model is customized for the present VP-CMT process. The Fe2Al5 layer thickness calculation results are further validated using electron back-scattered diffraction (EBSD) characterization under scanning electron microscopy (SEM), confirming a 2.3 % deviation between the model prediction and actual layer thickness.
NSTL
Elsevier