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Model-Based Optimization of Process Parameters in the Friction Stir Processing of AZ31b with Adaptive Cooling
... The friction coefficient vs. temperature behavior that was arrived at by tuning in this work is shown in Figure 3 co-plotted on which are the experimental values from [17]. The performance of the utilized FE model along with the assumed friction coefficient was previously validated by the authors [18][19]. Thermal conductivity [10] 96 N/(s K) ...
... The simulation process parameters for the traverse phase were 1000 RPM for the tool rotational speed and 90 mm/min for the tool traverse speed. These parameters were selected according to optimum processing conditions for AZ31b which were determined by the authors [18]. For each constitutive model, simulations were run and the state variables were determined and logged for comparison. ...
... The heat transfer coefficient between the tool-workpiece and backing plate-workpiece interfaces was set to 11 kW/(m 2 ºC) [7]. The friction coefficient used at the tool-workpiece interface was set as function of temperature with an initial value of 0.28 at room temperature that increases to 0.32 at 908K and drops to 0.02 as temperature exceeds 910K [8]. ...
Introduced in this work is a relation that captures the behavior of grain size with the varying process parameters in friction stir processing of AZ31B. The relation was based on the results of a 3D FE model that was used to run simulations of the process at different tool rotational and traverse speeds. The model was validated by comparing its state variable outputs to experimental results found in the literature. The coefficients of the proposed relation were determined for magnesium alloy AZ31B. This proposed relation will aid in controlling the output grain size in computerized friction stir processes.
... The heat transfer coefficient between the tool-workpiece and backing plate-workpiece interfaces was set to 11 kW/(m 2 ºC) [7]. The friction coefficient used at the tool-workpiece interface was set as function of temperature with an initial value of 0.28 at room temperature that increases to 0.32 at 908K and drops to 0.02 as temperature exceeds 910K[8]. The model was validated with experimental data available in the literature by tracking the temperature history of an observation point on the traverse line at a distance of 8.5 mm below the surface for two different test cases. ...
Introduced in this work is a relation that captures the behavior of grain size with the varying process parameters in friction stir processing of AZ31B. The relation was based on the results of a 3D FE model that was used to run simulations of the process at different tool rotational and traverse speeds. The model was validated by comparing its state variable outputs to experimental results found in the literature. The coefficients of the proposed relation were determined for magnesium alloy AZ31B. This proposed relation will aid in controlling the output grain size in computerized friction stir processes.
Proper numerical modeling of the Friction Stir Processes (FSPs) requires the identification of a suitable constitutive equation which accurately describes the stress-strain material behavior under an applicable range of strains, strain rates, and temperatures. While some such equations may be perfectly suitable to simulate processes characterized by low (or high) strains and temperatures, FSPs are widely recognized for their relatively moderate ranges of such state variables. In this work, a number of constitutive equations for describing flow stress in metals were screened for their suitability for modeling Friction Stir Processes of twin roll cast (TRC) wrought magnesium Mg–AL–Zn (AZ31B) alloy. Considered were 4 different reported variations of the popular Johnson–Cook equation and one Sellars–Tegart equation along with their literature–reported coefficients for fitting AZ31B stress–strain behavior. In addition, 6 variations of the (rarely used in FSPs simulations) Zerilli–Armstrong equation were also considered along with their literature–reported coefficients. The screening assessment was based on how well the considered constitutive equations fit experimental tensile stress–strain data of twin roll cast wrought AZ31B. Goodness of fit and residual sum of squares were the two statistical criteria utilized in the quantitative assessment whereas a ‘visual ’ measure was used as a qualitative measure. Initial screening resulted in the selection of one best fitting constitutive equation representing one of each of the Johnson–Cook, Sellars–Tegart, and Zerilli–Armstrong equations. An HCP–specific Zerilli–Armstrong constitutive equation (dubbed here as ZA6 ) was found to have the best quantitative and qualitative fit results with an R2 value of 0.967 compared to values of 0.934 and 0.826 for the Johnson–Cook and Sellars–Tegart constitutive equations, respectively. Additionally, a 3D thermo–mechanically coupled FEM model was built in DEFORM 3D to simulate the experimental tensile test from which the experimental load–deflection data was obtained. The three ‘finalist ’ equations were fed into the FEM simulations and were compared based on the 1) simulations’ running times and 2) goodness of fit of the simulation results to the experimental load–deflection data. It was found that the ZA6 constitutive equation exhibited favorable run times even when contrasted against the simpler mathematical form of the Sellars–Tegart equation. On average, the ZA6 equation showed improvements in solution time by 5.4% as compared with the Johnson–Cook equation and almost identical solution time (0.9% increase) with that of the ST equation. This result indicates that the proposed equation is not numerically expensive and can be safely adopted in such FEM simulations. Based on the favorable running times and goodness of fit, it was concluded that the HCP–specific Zerilli–Armstrong constitutive equation ZA6 holds an advantage over all other considered equations and was, therefore, selected as most suitable for the numerical modeling of FSP of twin roll cast AZ31B.
The ram speed and the billet temperature are the primary process variables that determine the quality of the extruded magnesium profile and the productivity of the extrusion operation. The optimization of the extrusion process concerns the interplay between these two variables in relation to the extrudate temperature and the peak extrusion pressure. The 3D computer simulations were performed to determine the effects of the ram speed and the billet temperature on the extrudate temperature and the peak extrusion pressure, thereby providing guidelines for the process optimization and minimizing the number of trial extrusion runs needed for the process optimization. A case study on the extrusion of an AZ31 X-shaped profile was conducted. The correlations between the process variables and the response from the deformed material, extrudate temperature and peak extrusion pressure, were established from the 3D FEM simulations and verified by the experiment. The research opens up a way to rational selection of the process variables for ensured quality and maximum productivity of the magnesium extrusion.
Purpose
– The friction stir welding (FSW) process comprises several highly coupled (and non‐linear) physical phenomena: large plastic deformation, material flow transportation, mechanical stirring of the tool, tool‐workpiece surface interaction, dynamic structural evolution, heat generation from friction and plastic deformation. This paper aims to present an advanced finite element (FE) model encapsulating this complex behaviour and various aspects associated with the FE model such as contact modelling, material model and meshing techniques are to be discussed in detail.
Design/methodology/approach
– The numerical model is continuum solid mechanics‐based, fully thermo‐mechanically coupled and has successfully simulated the FSW process including plunging, dwelling and welding stages.
Findings
– The development of several field variables are quantified by the model: temperature, stress, strain. Material movement is visualized by defining tracer particles at the locations of interest. The numerically computed material flow patterns are in very good agreement with the general findings from experiments.
Originality/value
– The model is, to the best of the authors' knowledge, the most advanced simulation of FSW published in the literature.
This study is intended to present a straightforward and computationally efficient methodology for optimizing the process parameters of friction stir welding (FSW) of 6061 aluminum alloy. In particular, it is shown how to minimize the heat affected zone (HAZ) distance to the weld line in the joined parts using a Taguchi opti-mization method and a temperature-field finite element model. The peak temperature during the process has also been minimized. Since the method is used for the first time in relation to the HAZ objective function, an auxiliary full factorial search is conducted to ensure Taguchi's orthogonal design assumption for the FSW problems. Results confirm that the method can be successfully used for minimizing both the HAZ distance to the weld line and the peak temperature, with a minimal number of simulation runs via orthogonal arrays. In addition, a new ANOVA analysis on the L 9 orthogonal array with three factors is performed and results in-dicate that among the parameters considered (i.e., the tool rotational speed, transverse speed, and the axial force), the most significant parameter on the weld quality is the rotational speed, followed by the axial force and transverse speed.
In conventional sheet forming processes, such as stamping or drawing, significant contact phenomena take place between workpiece
and die surfaces. Especially, relative motion and normal loads generate friction which influences some aspects of processes
such as material flow, tools wear and life and total force needed to complete the process. In the current paper an experimental
test campaign has been carried out using a large scale pin-on-disk device designed and realized by the Authors to investigate
the influence of pressure, sliding velocity and temperature. The purpose is to test the developed device and to find which
and how these parameters mostly affect friction. The pin-on-disk test consists of two specimens, a pin and a plate representing
respectively die and workpiece, which are compressed by means of a known force and then moved one over the other. Compression
and friction forces are sampled during the tests and the friction coefficient is estimated as the ratio of these two forces.
The tested materials are H13 die steel on FeP04 and AZ31 sheets.
The relationship between the resulting grain size and the applied working strain rate and temperature for the friction stir processing in AZ31 Mg is systemically examined. The Zener–Holloman parameter is utilized in rationalizing the relationship. The grain orientation distribution is also studied using the X-ray diffraction.
The hot ductility and strength, as well as constitutive equations were determined for Mg-3Al-1Zn (AZ31), Mg-5,5Al-3Zn (AZ63) Mg-8Al-1Zn (AZ91) and Mg-6Zn-0.6Zr (ZK60) by torsion testing across the range 180-450°C, 0.01 to 1.0 s-1. Optical observations show that twinning occurs extensively at low strain to reorient grains without suitable slip planes. At 180°, slip is normally limited to the basal system except when stress concentrations at grain or twin boundaries enhance other systems. However as T rises above 300°C, thermally activated pyramidal or prismatic slip causes noticeable dynamic recovery near the twin and grain boundaries, as observed in TEM. The development of misoriented regions leads to formation of dynamic recrystallization grains. These restoration mechanisms markedly raises the ductility above 300°C. The constitutive equations were employed in extrusion modeling.
An FE model has been used to study the effect of localised dynamic cooling on the residual stresses developed during friction stir welding. The main aim of the work was to see if the cooling power and source positions required, to achieve significant residual stress reductions in friction stir welds, were compatible with the FSW process and recent developments in CO 2 cooling systems. Comparisons were made between welds produced with a single cold spot placed over the weld line, either ahead or behind the tool, or with cold spots both leading and trailing the tool. Simulations revealed that a large reduction in the residual stresses can be obtained, particularly at the weld line, but the benefits are very dependent on the size and positioning of the cooling sinks. Cold spots placed behind the heat source have the greatest effect in reducing the build up of trailing tensile stresses, by counteracting the compressive plastic misfit of the hot soft weld metal. Overall, it is shown that the application of local dynamic cooling has potential as a practical solution for controlling residual stresses in industry.
Microstructure and tensile behaviors of AZ31 magnesium alloy prepared by friction stir processing (FSP) were investigated. The results show that microstructure of the AZ31 hot-rolled plate with an average grain size of 92.0 μm is refined to 11.4 μm after FSP. The FSP AZ31 alloy exhibits excellent plasticity at elevated temperature, with an elongation to failure of 1050% at 723 K and a strain rate of 5×10−4 s−1. The elongation of the FSP material is 268% at 723 K and 1×10−2 s−1, indicating that high strain rate superplasticity could be achieved. On the other hand, the hot-rolled base material, which has a coarse grain structure, possesses no superplasticity under the experimental conditions.
Friction stir processing (FSP) was used to fabricate AZ31/Al2O3 nanocomposites for surface applications. The effects of probe profile, rotational speed and the number of FSP passes on nanoparticle distribution and matrix microstructure were studied. The grain refinement of matrix and improved distribution of nanoparticles were obtained after each FSP pass. By increasing the rotational speed, as a result of greater heat input, grain size of the base alloy increased and simultaneously more shattering effect of rotation, cause a better nanoparticle distribution. The average grain size of matrix of the composites was in the range of 1–5μm and the microhardness of them was 85–92Hv.
Friction stir processing is a new thermo-mechanical processing technique that leads to a microstructure amenable for high strain rate superplasticity in commercial aluminum alloys. Friction stirring produces a combination of very fine grain size and high grain boundary misorientation angles. Preliminary results on a 7075 Al demonstrate high strain rate superplasticity in the temperature range of 430-510 °C. For example, an elongation of >1000 % was observed at 490 °C and 1×10-2 s-1. This demonstrates a new possibility to economically obtain a superplastic microstructure in commercial aluminum alloys. Based on these results, a three-step manufacturing process to fabricate complex shaped components can be envisaged: cast sheet or hot-pressed powder metallurgy sheet + friction stir processing + superplastic forging or forming.
This paper proposes a model based approach for the optimisation of friction stir welding processes. The proposed approach starts with building a model of the process. For this study, the thermal model developed by Chao and his associate for friction stir welding of AA 2195-T8 is replicated using Fluent. Once developed, the thermal model is then used to simulate the process. Two surrogate models, one linear and one non-linear, are constructed to relate three process parameters with maximum temperature at a selected location, using the simulation data generated by the thermal model. A constrained optimisation model is next formulated, which is eventually solved by five population based metaheuristrics to find the optimal solutions for the studied friction stir welding process. The optimal solutions are primarily constrained by the lower bound of the temperature. The lower this lower bound temperature is the higher travel speed can go. The linear surrogate model results in a slightly better optimal solution than the non-linear model when the temperature constraint is loose and the converse is true when the temperature constraint is tight. Comparing all five metaheuristics, differential evolution has the best performance, followed by particle swarm optimisation, ant colony optimisation, genetic algorithm, and lastly harmony search.
Cryogenic cooling with CO2 was applied during friction stir welding of AA2024-T351 in order to reduce the temperature increase during welding, and thus improve the corrosion resistance of the weld. The effect of cryogenic cooling on corrosion susceptibility was investigated with gel visualisation, immersion tests and local electrochemical measurements. The most susceptible area for both uncooled and cooled welds was in the heat-affected zone (HAZ) region, which showed intergranular attack. Cryogenic cooling had no detectable influence on the degree of anodic reactivity in the weld region. However, it did decrease the width of the reactive HAZ.
The ability to generate nano-sized grains is one of the advantages of friction stir processing (FSP). However, the high temperatures generated during the stirring process within the processing zone stimulate the grains to grow after recrystallization. Therefore, maintaining the small grains becomes a critical issue when using FSP. In the present reports, coolants are applied to the fixture and/or processed material in order to reduce the temperature and hence, grain growth. Most of the reported data in the literature concerning cooling techniques are experimental. We have seen no reports that attempt to predict these quantities when using coolants while the material is undergoing FSP. Therefore, there is need to develop a model that predicts the resulting grain size when using coolants, which is an important step toward designing the material microstructure. In this study, two three-dimensional computational fluid dynamics (CFD) models are reported which simulate FSP with and without coolant application while using the STAR CCM+ CFD commercial software. In the model with the coolant application, the fixture (backing plate) is modeled while is not in the other model. User-defined subroutines were incorporated in the software and implemented to investigate the effects of changing process parameters on temperature, strain rate and material velocity fields in, and around, the processed nugget. In addition, a correlation between these parameters and the Zener-Holloman parameter used in material science was developed to predict the grain size distribution. Different stirring conditions were incorporated in this study to investigate their effects on material flow and microstructural modification. A comparison of the results obtained by using each of the models on the processed microstructure is also presented for the case of Mg AZ31B-O alloy. The predicted results are also compared with the available experimental data and generally show good agreement.
Friction stir welding (FSW) is an energy efficient and environmentally “friendly” (no fumes, noise, or sparks) welding process, during which the workpiece are welded together in a solid-state joining process at a temperature below the melting point of the workpiece material under a combination of extruding and forging. Significant microstructural evolution takes place during FSW: in particular continuous dynamic recrystallization (CDRX) phenomena result in a highly refined grain structure in the weld nugget and strongly affect the final joint resistance. In the paper two different analytical models aimed to the determination of the average grain size due to continuous dynamic recrystallization phenomena in FSW processes of AA7075-T6 aluminum alloys have been implemented in a 3D FEM model and numerical analyses of the welding processes have been performed to verify their effectiveness.
Recently friction stir processing (FSP) has emerged as an effective tool for enhancing sheet metal properties through microstructure modification. Significant grain refinement and homogenization can be achieved in a single FSP pass leading to improved formability, especially at elevated temperatures. FSP is a solid-state process where the material within the processed zone undergoes intense plastic deformation resulting in dynamically recrystallized grain structure. Most of the research conducted on FSP focuses on aluminum alloys. Despite the potential weight reduction that can be achieved using magnesium alloys, very little is reported on FSP of magnesium alloys. In this work, we examine the possibility of using FSP to modify the microstructure and properties of commercial AZ31B-H24 magnesium alloy sheets. The effect of various process parameters on thermal histories, resulting microstructure and properties are investigated. Preliminary results are promising and it is shown that FSP leads to finer and more homogenized grain structure.