1,721,014 research outputs found
Determination of biaxial stress–strain curves for superplastic materials by means of bulge forming tests at constant stress
No full textAs the characterization of superplastic materials requires elevated temperatures and strain rate control, standard bulge testing procedure with optical measuring systems is not feasible for the determination of biaxial stress–strain curves. Standard superplastic bulge forming tests performed at constant pressure can be used for the identification of material constants or formability evaluation, but they are not applicable for accurate characterization of deformation behavior providing stress–strain curves at constant strain rates. The present paper is aimed to provide a technique for the direct characterization of stress–strain behavior of superplastic materials in conditions of biaxial tension. This technique is based on bulge forming testing with a closed-loop pressure control procedure allowing one to maintain the value of the effective stress at the dome pole at the predefined constant level. Thus, the variation of strain rate is reduced compared to the constant pressure testing. The results are evaluated using a double-step numerical procedure which provides the way to calculate the stress–strain curves, corresponding to constant referenced strain rates. The developed technique was used to characterize superplastic aluminum alloy Alnovi-U in conditions of biaxial tension at 500 °C
Characterization of stress-strain behavior of superplastic titanium alloy by free bulging tests with pressure jumps
No full textThe work is dedicated to determination of stress-strain behavior of Ti6Al4V alloy deformed in conditions of biaxial tension provided by free bulging testing. The dome height during each test was continuously measured and recorded using a magnetostrictive position transducer. All the tests were performed using stepped pressure regime with jump pressure changing between two values at evenly spaced time moments. This experimental technique provides the possibility to study strain rate sensitivity index variation during the test and subsequently construct strain and strain rate dependent material model. The output data of each test include the evolution of dome height, subsequent pressure regime and final thickness of the specimen at the dome pole. In the framework of this study the processing of such data in order to evaluate the material behavior is discussed. Inverse analysis with different material models was implemented as well as special direct technique allowing one to construct stress-strain curves based on the results of free bulging tests with pressure jumps. The obtained material model was verified by finite element simulation
Wire laser additively reinforced blanks: Effect of the laser power on the bending strength of a single layer reinforcement
Get PDFThis study investigates the application of Wire Laser Metal Deposition (w-LMD), a form of Directed Energy Deposition (DED) additive manufacturing, to enhance the production process of automotive components, specifically through the development of patchwork blanks with localized reinforcements. The research focuses on reinforcing 22MnB5 steel sheets with beads of 316L steel using a laser beam at various power levels, aiming to achieve maximum strength with minimal use of material. The resulting components, referred to as wire-Laser Additively Reinforced Blanks (w-LARB), demonstrated a substantial increase in strength, up to 87%, as verified by bending tests. Notably, the study reveals that a relatively low laser power can still yield significant mechanical improvements, underlining the efficiency of the process in terms of material usage and energy consumption. Furthermore, the high repeatability of the w-LMD process confirms its potential for widespread industrial adoption in automotive manufacturing
Superplasticity and Superplastic Forming
Get PDFIn both academic and industrial research endeavours, driving forces are essential to keep the interest alive for a specific topic [...
GA-based optimization to control the thickness distribution in components manufactured via superplastic forming
No full textSuperplastic forming (SPF) is a net-thinning sheet metal forming process that allows to achieve very complex geometries; nevertheless, due to its deformation mechanics, it is difficult to achieve a uniform thickness distri-bution in the final component since the most stretched regions of the blank are contemporarily characterized by the most severe thinning. With the aim of proposing a robust approach, a new methodology able to optimize the thickness distribution in components produced by SPF is proposed. Although two different titanium-based (Ti-6Al-4 V ELI) axisymmetric components - i.e., a hemisphere and a frustum of cone - were chosen as case studies, the methodology can be extended to any material and to any geometry, even considering more than one objective function. The superplastic forming was numerically simulated, and the Finite Element (FE) model was embedded into an automatic procedure managed by the multi-objective genetic algorithm MOGA-II. The blank's initial thickness distribution was modelled using a second-order polynomial function and its coefficients were determined to get, at the end of the SPF process, a uniform thickness in the formed part irrespective of the specific geometry. Titanium blanks were machined on a 5-axis milling machine to obtain the calculated optimal initial thickness distribution; subsequently, such blanks were superplastically formed and inspected in order to measure the thickness distribution. Tests showed a very good fitting with the numerical prediction thus con-firming the robustness and the effectiveness of the proposed methodology
A bulge-test based viscoplastic model for superplastic deformation behaviour of a magnesium alloy
No full textIn this work, a new approach is proposed for modelling the superplastic deformation of AZ31B magnesium alloy sheets. Gas bulge tests were performed at 450 °C under three constant gas pressure levels of 0.4, 0.7, and 1.0 MPa. The dome height evolutions according to time were recorded in experiments and then translated into stress–strain curves using a new analytical approach. In order to predict the superplastic behaviour of the material, the Variable m-value Viscoplastic (VmV) model was considered in this study, whose constants were directly assessed from the bulge test results. The bulge forming of the magnesium alloy sheet was simulated with ABAQUS/Standard using the VmV model. The predicted dome height-time profiles and thickness distributions along the specimens were compared with the experimental results. Finally, a real case study (a resorbable cheekbone prosthesis) was simulated implementing the VmV model for validation purposes. The results from the simulation of both the bulge tests and the case study manufacturing revealed that the proposed model is able to effectively reproduce the superplastic behaviour of the AZ31B magnesium alloy
Numerical/experimental investigation of the effect of the laser treatment on the thickness distribution of a magnesium superplastically formed part
No full textThe growing need for high-performance components in terms of shape and mechanical properties encourages the adoption of integrated technological solutions. In the present work, a novel methodology for affecting the superplastic behaviour and, in turn, the thickness distribution of magnesium alloy components is proposed. Through heat treatments using a CO2 laser, the grain size was locally changed, thus modifying the superplastic behaviour in a predefined area of the blank. Both the grain coarsening produced by the laser heat treatment and the superplastic forming of the heat treated blank were simulated using a finite element model, which allowed to set the related process parameters for the manufacturing of the investigated case study (a truncated cone). The thermal finite element model of the laser heat treatment, calibrated using the experimental temperature evolutions acquired in specific areas during the heat treatment, was used to evaluate the influence of process parameters on the grain size evolution. The laser heat treatment was able to significantly promote the grain growth, increasing the mean grain size from about 8 μm to twice (about 17 μm). The resulting grain size distributions were implemented in the mechanical finite element model of the superplastic forming process and the combination of laser parameters which allowed to obtain the most uniform thickness distribution on the final component was finally experimentally reproduced and measured for validation purposes. Even in the case of the laboratory scale application, characterised by quite small dimensions, the proposed approach revealed to be effective, to improving the thinning factor (tMIN/tAVG) of the formed part from 0.85 to 0.89, and providing an increase in the thickness uniformity of about 4.7%
A new experimental approach for modelling the constitutive behaviour of sheet metals at elevated temperature through interrupted bulge tests
No full textModelling the deformation behaviour of materials plays a fundamental role in the process design of formed components. In this work, an original methodology based on interrupted hot bulge tests has been proposed for evaluating the effective stress and strain values in a wide range of strain rates. The strain rate value was instantaneously calculated in each test by a new approach based on continuous acquisition of the dome height. The authors conducted bulge tests on the AZ31B magnesium alloy at elevated temperature (450°C) and interrupted the tests at different levels of the strain at the dome apex. The corresponding dome height at which the test had to be stopped was calculated by a predictive model. The strain rate and the stress values evaluated through the analysis of the samples from interrupted bulge tests were correlated using two different constitutive models. The constitutive models calibrated using the proposed approach were finally implemented in the numerical simulations of the bulge tests in order to compare the results with the experimental data. Both the constitutive models revealed to be accurate, showing a good agreement between numerical and experimental dome height versus time curves, especially when using the phenomenological constitutive model, which allowed to keep the discrepancy below 12% in a very large pressure range. Thus, also the effectiveness of the proposed methodology was demonstrated
Mechanical characterization of CO2 laser beam butt welds of AA5083
No full textLaser beam welding experiments have been carried out on 3-mm thick aluminium–magnesium alloy 5083 specimens in butt-joint configuration.
The mechanical properties of the joints have been evaluated by performing tensile tests, hardness profiles across the weld sections, porosity
measurements and EDX analyses. A design of experiment technique has been used to study the effects of the welding speed (v) and the incident laser power (P) on the aforementioned response variables that are considered to be representative for the weld quality. By comparing the welds obtained by operating at constant linear energy input released onto the material (P/v ratio), the best results have been found for higher laser powers and welding speeds. A clear correlation was found between the incidence of porosity, the tensile strength and the hardness of the fused zone. Welding reliability was enhanced for selected sets of process parameters capable of producing butt-joints showing mechanical properties very competitive if compared with the performances obtained, on similar aluminium alloys, using alternative joining technologies like friction stir welding or gas tungsten arc welding
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