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A Relation between Grain Size and Process Parameters in Friction Stir Processing of AZ31B
Abstract
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.
In order to select the optimum parameters for friction stir welding of twin-roll cast (TRC) AZ31B Mg-Al-Cu alloy, mechanical and microstructural characterizations are performed on friction stir welded 3 mm thick sheets under different processing parameters of rotational spindle speed and feed rate. Used was a tool made from high resistance Sverker 21 steel alloy with a 19 mm diameter cylindrical shoulder and a 6.4 mm diameter pin that extrudes 2.7 mm from the bottom of the shoulder. A large number of welded joints were prepared according to a test matrix of various combinations of rotational speed and feed rate. Tensile tests at different temperatures and strain rates were performed on specimens cut with their major axis perpendicular to the welded joints. Static strength tests of these specimens revealed that joints welded at a speed of 1200 rpm and a feed of 150 mm/min resulted in strengths as high as 94% of that of the as received TRC AZ31B metal. For specimens welded at spindle speeds larger than 1400 rpm, microstructural observations revealed the presence of micro cracks and voids located in the zone between the nugget and the thermally affected zone. In order to monitor the effect of lengthy testing times and the observed grain growth associated with slow strain tests, annealing tests under different temperatures and time durations were performed while monitoring grain diameter. Very limited grain growth have been obsereved after annealing and thus the mechanical performnace of the joints was solely due to the effect of the friction stir welding.
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.
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.
Nanograined structures with an average grain size of similar to 85 nm have been achieved in solution hardened AZ31 magnesium alloy by two-pass friction stir processing under rapid heat sink in which the second pass has a lower heat input. The mean hardness of the nanograined region reaches similar to 1.5 GPa (or 150 H(v)), about three times that of the matrix. The evolution of the nanograined structure is also investigated. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Ultrafine-grained (UFG) microstructures with an average grain size of 100–300 nm are achieved in solution-hardened AZ31 Mg–Al–Zn alloy prepared by friction stir processing equipped with a rapid heat sink. The mean hardness of the UFG region reaches ∼120Hv, which is more than twice as high as that of the AZ31 matrix. The grain refinement kinetics are analyzed and the results are self-consistent.
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.
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 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.
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.