Conference Paper

Evaluating the performance of selected constitutive laws in the modeling of friction stir processing of Mg Alloy AZ31b -- Toward a more sustainable process

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... In cases of steep state variables' gradients, as the case with FSP, it was proven difficult for most of these constitutive laws to predict the values of state variables over the entire range of variation. In recent work [6], Ammouri et al. evaluated two of the more popular material models in friction stir processes: Sellars-Tegart [7] and Johnson-Cook [8] constitutive laws where each model showed advantages over the other at different regimes of interest. The reason for adopting these two models was due to their ease of implementation in numerical solvers and the availability of their material constants in the literature for many metals of interest. ...
... The FEM model is comprised of a tool, a workpiece, and a backing plate underneath the workpiece. See detailed description of the FEM model in the work by Ammouri et al. [6]. For each of the constitutive relations, the reference test-case simulation was run at what was found [12] to be optimum process parameters for the model's tool geometry: 1000 RPM and 90 mm/min. ...
... The maximum reported flow stress values by the ST and ZA models were 205 MPa and 210 MPa respectively. Results found here are similar to those found previously[6] by the authors when comparing the stress outcomes of the JC and ST material models. Values of 630 MPa that exceeds the material's ultimate stress were noted in the stir zone of the model using JC. ...
Conference Paper
Utilizing a proper material model for describing the mechanical behavior of any material is key for a successful simulation of friction stir processing (FSP) where temperature, strain, and strain rate gradients vary abruptly within, and when moving away, from the stirring zone. This work presents a comparison of how faithfully do three different constitutive equations reproduce the state variables of strain, strain rate, and temperature in an FEM simulation of a test-case FSP (1000 rpm spindle speed, and 90 mm/min feed). The three material models considered in this comparison are namely: Johnson-Cook (JC), Sellars-Tegart (ST), and Zerilli-Armstrong (ZA). Constants for these constitutive equations are obtained by fitting these equations to experimental mechanical behavior data collected under a range of strain rates and temperatures of twin-rolled cast wrought AZ31B sheets. It is widely recognized that JC-based models over predicts stress values in the stir zone whereas STbased models are incapable of capturing work hardening outside of the stir zone. Therefore, a ZA model, being a physical based-HCP specific model, is hereby investigated for its suitability as a material model that would overcome such drawbacks of JC- and ST-based models. The equations from the constitutive models under consideration are fed into an FEM model built using DEFORM 3D to simulate the traverse phases of a friction stir process. Amongst these three material models, comparison results suggest that the HCP-specific ZA model yield better predictions of the state variables: strain, strain rate, and temperature, and, consequently, the estimated values for flow stresses.
... In cases of steep state variables' gradients, as the case with FSP, it was proven difficult for most of these constitutive laws to predict the values of state variables over the entire range of variation. In recent work [6], Ammouri et al. evaluated two of the more popular material models in friction stir processes: Sellars-Tegart [7] and Johnson-Cook [8] constitutive laws where each model showed advantages over the other at different regimes of interest. The reason for adopting these two models was due to their ease of implementation in numerical solvers and the availability of their material constants in the ...
... The FEM model is comprised of a tool, a workpiece, and a backing plate underneath the workpiece. See detailed description of the FEM model in the work by Ammouri et al. [6]. For each of the constitutive relations, the reference test-case simulation was run at what was found [12] to be optimum process parameters for the model's tool geometry: 1000 RPM and 90 mm/min. ...
... The maximum reported flow stress values by the ST and ZA models were 205 MPa and 210 MPa respectively. Results found here are similar to those found previously[6] by the authors when comparing the stress outcomes of the JC and ST material models. Values of 630 MPa that exceeds the material's ultimate stress were noted in the stir zone of the model using JC. ...
... In cases of steep state variables' gradients, as the case with FSP, it was proven difficult for most of these constitutive laws to predict the values of state variables over the entire range of variation. In recent work [6], Ammouri et al. evaluated two of the more popular material models in friction stir processes: Sellars-Tegart [7] and Johnson-Cook [8] constitutive laws where each model showed advantages over the other at different regimes of interest. The reason for adopting these two models was due to their ease of implementation in numerical solvers and the availability of their material constants in the ...
... The FEM model is comprised of a tool, a workpiece, and a backing plate underneath the workpiece. See detailed description of the FEM model in the work by Ammouri et al. [6]. For each of the constitutive relations, the reference test-case simulation was run at what was found [12] to be optimum process parameters for the model's tool geometry: 1000 RPM and 90 mm/min. ...
... The maximum reported flow stress values by the ST and ZA models were 205 MPa and 210 MPa respectively. Results found here are similar to those found previously[6] by the authors when comparing the stress outcomes of the JC and ST material models. Values of 630 MPa that exceeds the material's ultimate stress were noted in the stir zone of the model using JC. ...
Article
An experimentally verified finite element model was used to estimate the strain rate and the temperature values which were, consequently, used in calculating the Zener–Hollomon parameter, Z-parameter, of twin-roll-cast (TRC) AZ31B after being refined by FSP (using range of spindle speeds of 600–2000 rpm and tool feed rates ranging from 75 to 900 mm/min). In the finite element model, an HCP specific Zerilli–Armstrong constitutive relation was used to describe the mechanical behavior of AZ31B. The resulting grain size values were experimentally measured for the observed microstructure of all processed samples. Dynamic recrystallization was identified to be the main mechanism involved in the grain refinement. A linear relation of the form ln d = a × ln Z + b was determined relating the average grain size (d) to the Z-parameter with a and b being equal to −0.23 and 8.79, respectively. These coefficients differed from values reported by others for AZ31 magnesium alloy with this difference being attributed to different material processing techniques used in the as-received condition. The resulting equation can be used in controlling the grain size during friction stir processing by varying the process parameters (spindle speed and tool feed) that would in turn affect the instantaneous value of the Zener–Hollomon parameter.
... In cases of steep state variables' gradients, as the case with FSP, it was proven difficult for most of these constitutive laws to predict the values of state variables over the entire range of variation. In recent work [6], Ammouri et al. evaluated two of the more popular material models in friction stir processes: Sellars-Tegart [7] and Johnson-Cook [8] constitutive laws where each model showed advantages over the other at different regimes of interest. The reason for adopting these two models was due to their ease of implementation in numerical solvers and the availability of their material constants in the literature for many metals of interest. ...
... A non-linear regression analysis was used to fit published [11] [6]. For each of the constitutive relations, the reference test-case simulation was run at what was found [12] to be optimum process parameters for the model's tool geometry: 1000 RPM and 90 mm/min. ...
... This relation is constructed from an experimentally validated 3D FE model that was constructed using the commercial DEFORM 3D software. The FE model utilized the physical based Zerilli-Armstrong material model for HCP material which is capable of predicting accurate state variables[7]. The Zener-Hollomon parameter was used to calculate the predicted grain size from the resulting state variables of the FEM simulations. ...
Article
Microstructural modification via dynamic recrystallization (DRX) is the main mechanism involved in grain refinement associated with friction stir processing. Introduced in this work is a methodology for controlling grain size through the manipulation of process parameters in friction stir processes, FSP. The methodology was demonstrated on FSP using a simple shoulder/pin tool on work material comprised twin-roll cast (TRC) magnesium-aluminum-copper, AZ31B. A robust 3D FE model was used to simulate a test matrix the resulting state variables of which were used to relate the desired output (here, average grain size) to the controlled input process parameters. The Zener-Hollomon parameter (temperature compensated strain rate or Z-parameter) was used to calculate the predicted grain size from the state variables estimated from FEM simulations. Then, a power equation that relates average grain diameter, d, to the input process parameters, spindle speed (N) and tool feed (f). The proposed methodology was validated by comparing experimental test cases whose measured average grain size diameter was found to compare favorably with that predicted by the power equation. Evidence of grain size refinement and homogeneity was observed in the friction stir processed samples of Magnesium alloy AZ31B in agreement with the proposed equation over a wide range of these two parameters. Copyright © (2014) by the Society of Manufacturing Engineers All rights reserved.
... Selecting a proper material model for describing the mechanical behavior of any material is key for a successful simulation of friction stir processing where temperature, strain, and strain rate gradients vary abruptly within, and when moving away, from the stirring zone. The Zerilli-Armstrong (ZA) is a physical based constitutive relation that holds advantages over the empirical constitutive relations that are usually used as "de-facto" in the simulation of friction stir processes [11][12][13]. The ZA model is such a model that is based on thermally activated dislocation mechanics. ...
Article
Full-text available
Introduced in this work are correlations that capture the behavior of thrust force and torque vs. input process parameters in friction stir processing (FSP) of twin-roll cast (TRC) AZ3 IB. The correlations are based on the findings of an experimentally validated robust 3D FE model that was used to simulate the FSP process at different values of tool rotational and traverse speeds. The findings are fitted into simple power equations relating thrust force and torque to input parameters of spindle speed and feed. An experimental test matrix was used to validate the proposed correlations. The correlation equations were found to be able to predict the experimentally measured forces during friction stir processing with good statistical significance with average estimate errors of 6.2% and 5.4% for the thrust force and torque, respectively. The thrust force and torque exhibited opposite trends with increasing tool rotational speed. The thrust force increased while the torque decreased as the tool rotational speed increased. Copyright © (2014) by the Society of Manufacturing Engineers All rights reserved.
... The traditional experimental approaches being time and money consuming, urged the need for numerically simulating the friction processing techniques. Early modeling of the process utilized thermal models which were later upgraded to coupled thermo-mechanical models using several commercial finite element (FE) software such as DEFORM [6,7] and ABAQUS [8]. These models are used to predict the state variables of the friction stir processes and when coupled with artificial intelligent techniques, such as artificial neural networks (ANN), can be powerful tools to predict the output material microstructure similar to what has been done by [9]. ...
... The traditional experimental approaches being time and money consuming, urged the need for numerically simulating the friction processing techniques. Early modeling of the process utilized thermal models which were later upgraded to coupled thermo-mechanical models using several commercial finite element (FE) software such as DEFORM [6,7] and ABAQUS [8]. These models are used to predict the state variables of the friction stir processes and when coupled with artificial intelligent techniques, such as artificial neural networks (ANN), can be powerful tools to predict the output material microstructure similar to what has been done by [9]. ...
... The traditional experimental approaches being time and money consuming, urged the need for numerically simulating the friction processing techniques. Early modeling of the process utilized thermal models which were later upgraded to coupled thermo-mechanical models using several commercial finite element (FE) software such as DEFORM [6,7] and ABAQUS [8]. These models are used to predict the state variables of the friction stir processes and when coupled with artificial intelligent techniques, such as artificial neural networks (ANN), can be powerful tools to predict the output material microstructure similar to what has been done by [9]. ...
... The traditional experimental approaches being time and money consuming, urged the need for numerically simulating the friction processing techniques. Early modeling of the process utilized thermal models which were later upgraded to coupled thermo-mechanical models using several commercial finite element (FE) software such as DEFORM [6,7] and ABAQUS [8]. These models are used to predict the state variables of the friction stir processes and when coupled with artificial intelligent techniques, such as artificial neural networks (ANN), can be powerful tools to predict the output material microstructure similar to what has been done by [9]. ...
Conference Paper
Grain size determines to a large degree the mechanical properties of the friction stir processed (FSP) material. Developed in this work is a numerical (FEM) based-model for predicting values of the Zener-Hollomon parameter (Z-parameter) as function of input process parameters during friction stir processing of AZ31B. Prediction of Z values is desirable given that direct relations exist between the Z-parameter and the average grain size in the dynamically recrystallized zone (DRX). For this purpose, utilized in this work is a robust finite element model with a suitable constitutive equation and boundary conditions the results of which have been previously validated against published experimental data. A virtual test matrix constituting of 16 cases (4 spindle speed, N, x 4 feed, f) was run. Based on resulting state variables of strain rates and temperatures at a representative point within the stir zone, a statistically-validated power equation model was developed that relates Z-parameter values to input parameters of speed and feed. The results of the numerically developed power equation were validated against experimental results. This model can be readily used in future control frameworks to FSP produce AZ31B sheets of a predefined target grain size.
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The dynamic friction properties of the extruded AZ31 magnesium alloy with the initial average grain size of 15 μm were investigated by the ring compression test at 473 and 523 K and in a strain rate range from 1.0×10−2 to 3.0 s−1. Two types of the tool, WC-Co tool (WC) and WC-Co coated with diamond like carbon tool (DLC) were used. At 523 K, few differences in terms of the friction coefficient were observed due to the difference with or without DLC. At 473 K, the friction coefficient for the sample deformed by DLC tool was smaller than that done by WC tool. The investigation of the texture near the surface of the tested work pieces with different tools reveals that the integration degree of the grains within 10 degree from 〈0001〉 direction to compressive axis in the sample deformed by the DLC tool was smaller than that done by WC tool. It was concluded that the larger friction could enhance alignment of the planes perpendicular to the compressive direction to the basal plane even if under same testing condition.
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Constitutive laws commonly used to model friction stir welding have been evaluated, both qualitatively and quantitatively, and a new application of a constitutive law which can be extended to materials commonly used in FSW is presented. Existing constitutive laws have been classified as path-dependent or path-independent. Path-independent laws have been further classified according to the physical phenomena they capture: strain hardening, strain rate hardening, and/or thermal softening. Path-dependent laws can track gradients in temperature and strain rate characteristic to friction stir welding; however, path-independent laws cannot. None of the path-independent constitutive laws evaluated has been validated over the full range of strain, strain rate, and temperature in friction stir welding. Holding all parameters other than constitutive law constant in a friction stir weld model resulted in temperature differences of up to 21%. Varying locations for maximum temperature difference indicate that the constitutive laws resulted in different temperature profiles. The Sheppard and Wright law is capable of capturing saturation but incapable of capturing strain hardening with errors as large as 57% near yield. The Johnson-Cook law is capable of capturing strain hardening; however, its inability to capture saturation causes over-predictions of stress at large strains with errors as large as 37% near saturation. The Kocks and Mecking model is capable of capturing strain hardening and saturation with errors less than 5% over the entire range of plastic strain. The Sheppard and Wright and Johnson-Cook laws are incapable of capturing transients characteristic of material behavior under interrupted temperature or strain rate. The use of a state variable in the Kocks and Mecking law allows it to predict such transients. Constants for the Kocks and Mecking model for AA 5083, AA 3004, and Inconel 600 were determined from Atlas of Formability data. Constants for AA 5083 and AA 3004 were determined with the traditional Kocks and Mecking model; however, constants for Inconel 600 could not be determined without modification to the model. The temperature and strain rate combinations for Inconel 600 fell into two hardening domains: low temperatures and high strain rates exhibited twinning while high temperatures and low strain rates exhibited slip. An additional master curve was added to the Kocks and Mecking model to account for two hardening mechanisms. The errors for the Kocks and Mecking model predictions are generally within 10% for all materials analyzed.
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Restoration models for hot working of metals and alloys are reviewed in the context of their applicability to friction stir welding (FSW) and friction stir processing (FSP). Two of these models are used to interpret microstructure and microtexture data for two aluminum alloys subjected to FSP. The need for further experiments and model extensions to accommodate the transients and steep gradients in the strain, strain rate and temperature experienced by materials during FSW and FSP are discussed.
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The dynamic friction properties of the extruded AZ31 magnesium alloy of the grain size of 20 pm were investigated by ring upsetting method test at 523, 548 and 573 K at strain rate of 1.0 x 10(-2) s(-1), where all the initial testing conditions were the climb-controlled dislocation creep. The MoS2 lubricant maintained lower dynamically friction coefficient (m value) than the oil lubricant. The difference in m values between machined surface and polished surface was unclear. The m values for WC-Co and diamond like carbon (DLC) tools were similar in MoS2 lubricant. The m values for DLC tool were lower than those for the WC-Co tool in the oil lubricant. The extruded direction influenced to the friction properties. The aspect ratio of the inner diameter on 90 degrees to extruded direction after testing was almost isotropic; on the other hand, the anisotropy occurred on 0 degrees and 45 degrees. The extent of anisotropy at 548 K was the highest, although the lower temperature, the higher the critical shear stress of non-basal plane. The condition at 523 K, where the tine grain sizes less than 3 mu m could be obtained by dynamic recrystallization during deformation, is suitable temperature to make superplasticity at the given strain rate. [doi:10.2320/matertrans.P-M2010811]
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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.
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Hot workability relates to the ability of a metal or alloy to be deformed under conditions of high temperature (> O.6Tm, where Tm is the melting temperature in degrees Kelvin) and relatively high strain rates (10-1 to 103 S-1). The two characteristics that govern hot workability are strength and ductility. In a previous review, the relationship between strength and structure under hot-working conditions was examined with particular reference to deformation to large strains leading to essentially steady-state conditions.
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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.
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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.
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Process simulation assists the forming engineer in establishing and optimizing process variables and die design. As a result, process development effort and costs are reduced. In order for forming industry to take full advantage of process simulation, simulation tools should be capable of predicting actual physical phenomena accurately, and offer streamlined simulation procedures so that the required engineering effort in using it should be acceptable. In this paper, capabilities of the FEM code deform are presented. deform is a two-dimensional FEM program capable of simulating a wide class of forming processes with various material models. The automatic mesh generation and remeshing capability of the deform system is proved to be an essential feature of a streamlined simulation procedure for industrial applications. Example solutions are also presented to demonstrate the capabilities of the deform system.
  • C Sellars
  • W Tegart
Sellars, C. and W. Tegart, 1972, "Hot Workability". International Metallurgical Reviews, 1972: p. 1-24.
Model-Based Optimization of Process Parameters in the Friction Stir Processing of AZ31b with Adaptive Cooling The 10th Global Conference on Sustainable Manufacturing FEM Analysis of the Effects of Processing Parameters and Cooling Techniques on the Microstructure of Friction Stir Welded Joints
  • A H Ammouri
  • A H Kheireddine
  • T Lu
  • I S Jawahir
  • R F Hamade
  • A H Kheireddine
  • R F Hamade
  • A H Ammouri
  • G Kridli
Ammouri, A.H., Kheireddine, A. H., Lu, T., Jawahir, I.S., Hamade, R.F., 2012, " Model-Based Optimization of Process Parameters in the Friction Stir Processing of AZ31b with Adaptive Cooling ", GCSM2012, The 10th Global Conference on Sustainable Manufacturing, Oct 31-Nov 2, 2012, Istanbul, Turkey [19]. Kheireddine, A. H., Hamade, R.F., Ammouri, A.H., G. Kridli, 2012, " FEM Analysis of the Effects of Processing Parameters and Cooling Techniques on the Microstructure of Friction Stir Welded Joints ", IMECE2012-88943, Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition, IMECE2012, November 9-15, 2012, Houston, Texas, USA [20]. Ulacia, I., et al., 2011, "Tensile characterization and