The authors gratefully acknowledge the help extended by Prof. I. Samajdar, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, for the EBSD analysis of the specimens.
Microstructure-Property Relationship for Friction Stir Processed Magnesium Alloy†
Article first published online: 3 SEP 2013
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Advanced Engineering Materials
Volume 16, Issue 1, pages 94–102, January 2014
How to Cite
Arora, H. S., Grewal, H. S., Singh, H., Dhindaw, B. K., McPhail, D., Shollock, B., Chater, R. and Mukherjee, S. (2014), Microstructure-Property Relationship for Friction Stir Processed Magnesium Alloy. Adv. Eng. Mater., 16: 94–102. doi: 10.1002/adem.201300205
- Issue published online: 9 JAN 2014
- Article first published online: 3 SEP 2013
- Manuscript Accepted: 15 JUL 2013
- Manuscript Received: 30 MAY 2013
Friction stir processing (FSP) of Mg based AE42 alloy was performed under single pass as well as double pass conditions. The evolution of microstructure was investigated using electron back scatter diffraction (EBSD) analysis. EBSD revealed that the grain size and texture varies within the nugget zone of friction stir processed region. The variation of mechanical properties across the nugget region was evaluated using nanoindentation. Hardness and Young's modulus was found to increase along the depth of the friction stir processed specimen. This was attributed to a finer grain structure with increasing depth. The friction stir processed specimen showed higher tendency toward strain hardening compared to as-cast alloy. Understanding microstructure–property relationship paves the way for optimization of FSP conditions and development of advanced functional Mg alloys.
Renewed interest in Mg alloys has led to a series of investigations on improving these materials for high specific-strength applications. There are many promising developments in new alloy design.[2, 3] In addition, several recent studies have focused on improving conventional alloys by various thermo-mechanical processing methods. Friction stir processing (FSP) is one such method, where the microstructure can be refined by severe plastic deformation.[5-7] Number of studies has demonstrated the utilization of FSP for grain refinement in magnesium based alloys. Feng and Ma found that FSP resulted in breaking of course β-Mg17Al12 phase precipitates along with significant grain refinement. Ni et al. reported that FSP resulted in grain size refinement from 300 µm for the base material to nearly 15 µm for the processed specimen. Similarly, Freeney and Mishra investigated FSP of magnesium–rare earth alloy (EV31A) and reported a reduction in grain size, as well as breakage, and dissolution of second-phase particles. Some investigations on the influence of conventional processing methods have also been reported. Stanford and Barnett produced fine grained AZ31 magnesium alloy using combination of severe hot rolling and annealing. Annealing of hot rolled AZ31 at 200 °C resulted in recrystallized grain size of 2.2 µm compared to 12 µm for the base material. Tensile strength increased from 275 MPa for the base material to 300 MPa for the specimen with refined grain structure. The improvement in mechanical properties was attributed to more homogeneous deformation. Letzig et al. investigated the influence of extrusion parameters including extrusion ratio, extrusion rate, temperature, and type of extrusion process on the properties of different magnesium alloys. They proposed that hydrostatic extrusion leads to more homogeneous microstructure with finer grain size and superior mechanical properties owing to lower processing temperature. Swiostek et al. studied hydrostatic extrusion of different Mg alloys and its effect on grain size and mechanical properties. The initial grain size between 300 and 500 µm for as-cast materials got significantly refined to nearly 2–8 µm after extrusion with the corresponding improvement in mechanical properties. Miyahara et al. found that extrusion and equal channel angular processing refined the initial grain size from 16 to 0.6 µm for AZ61 alloy.
AE42 is a die-cast Mg alloy that has been developed for high temperature automotive applications. This alloy consists of in situ particles that are precipitated during cooling. Ugandhar et al. reported that the AE42 alloy possesses good creep properties. However, only limited improvement in mechanical properties such as elastic modulus could be achieved. In one of the earlier investigations by the authors, it was observed that FSP of AE42 alloy supplemented by external undersurface cooling resulted in a refined microstructure comprising of uniformly distributed fine in situ second phase precipitates. However, the influence of FSP on texture and grain size distribution has not been evaluated. Furthermore, the influence of microstructure on mechanical behavior of the investigated alloy has not been reported. In this study, we analyze the texture/grain size distribution in the nugget zone of the friction stir processed AE42 alloy and report on its effect on mechanical properties.
FSP was carried out on a computer numerical control (CNC) vertical milling machine. A schematic representing the fixture used for FSP is shown in Figure 1. The fixture was connected to an external chiller unit through inlet and outlet pipes. It has a rectangular cavity for flow of the cooling fluid (methanol at −20°C) underneath the surface. Thus, the fixture was used for clamping the FSP specimen as well as for cooling it. Details about the cooling mechanism are given in an earlier publication. Rectangular specimens having dimensions of 80 mm × 40 mm × 3 mm were prepared from the alloy ingot. The FSP tool was cylindrical in shape with a 12 mm shoulder diameter, 4 mm pin diameter, and 2.7 mm pin length. The FSP parameters used comprises a tool rpm of 900, linear speed of 60 mm min−1, and plunge depth of 0.3 mm. FSP was done under single pass as well as double pass conditions. During the double pass, FSP tool was traversed two times over the same path in the same direction. FSP was done under the following conditions:
- Single pass FSP with under-surface cooling at −20 °C (SPUSC)
- Double pass FSP with under-surface cooling at −20 °C (DPUSC). Cooling was done after second pass only.
Friction stir processed (FSPed) specimens were sectioned in 10 mm × 3 mm size along the cross-section. The samples were ground down to 4000 grit followed by diamond polishing. The ground and polished samples were further polished using colloidal silica and ethanol. Further polishing was carried out on an ion-beam polishing machine (Precision etching system, Gattan make, model: 682). Optical microscope (Make: Leica; Model: DFC295) and electron back scatter diffraction (EBSD, FEI Quanta 3D FEG (Field emission gun)) were used for the determination of grain structure. For SPUSC specimen, EBSD scans were taken at three different locations: 0.5, 1.5, and 2.5 mm below the top surface at the center of the processed zone. However, for DPUSC specimen, EBSD scan at 0.5 mm location could not be indexed due to severe deformation near the shoulder region during the second FSP pass. Therefore, EBSD results for DPUSC have been reported for two locations only: 1.5 and 2.5 mm below the top surface. The scan area for FSPed specimens was 50 µm × 50 µm with a step size of 0.1 µm. The data was analyzed using TSL OIM 6.1.3 software in which the obtained Kikuchi patterns were indexed for the determination of grain size distribution and (0001) pole figures. Transmission electron microscopy (TEM) was done for detailed characterization of DPUSC.
Evaluation of mechanical properties of the FSPed specimens including hardness and modulus was done using nanoindentation (Make: Hysitron, Inc., Minneapolis USA; Model: TI-900). The test was done using a standard three-sided Berkovich indenter with a diamond tip. The test parameters comprise a peak load of 1 000 µN with loading and unloading rates of 100 µN s−1.
3 Results and Discussion
The optical images of the as-cast AE42 as well as SPUSC specimen are shown in Figure 2a and b, respectively. The average grain size for the as-cast alloy and SPUSC was found to be nearly 20 and 2.9 µm, respectively. Thus, the grain structure got considerably refined after FSP. EBSD maps as well as grain size distribution for SPUSC at 0.5, 1.5, and 2.5 mm locations are shown in Figure 3a through c. The corresponding data for DPUSC at 1.5 and 2.5 mm location s is shown in Figure 4a and b, respectively. The average grain size at 0.5, 1.5, and 2.5 mm locations for SPUSC was found to be 3.6, 3.4, and 2.3 µm, respectively. For DPUSC, average grain size at 1.5 and 2.5 mm depths was found to be 3 and 2.1 µm, respectively. Thus, the grain size varies across the depth for both the friction stir processed specimens. It can be seen that the grain size is more refined at greater depth from the top surface. This may be attributed to the temperature gradient during heating and cooling cycles in FSP. The major heat source in FSP is the frictional heating between the FSP tool shoulder and the specimen surface. Frictional heating causes higher temperature at the top surface of the specimen and it decrease toward the depth. Further, the cooling rate was also found to be higher toward the bottom surface of friction stir processed specimen. The fraction of submicron grains increases from 9% at 1.5 mm to 20% at 2.5 mm location in SPUSC. Similarly, for DPUSC, the fraction increases from 12.5% at 1.5 mm to 21% at 2.5 mm depth.
A schematic representing the normal direction (ND), processing direction (RD), and transverse direction (TD) is shown in Figure 5. The (0001) pole figure for the as-cast alloy is shown in Figure 6a. It can be observed from this figure that <0001> directions in the as-cast AE42 are oriented at nearly 50° from RD toward TD. Some crystal lattices have <0001> directions oriented at nearly 10–15° from RD toward TD. The (0001) pole figures for SPUSC at 0.5, 1.5, and 2.5 mm locations are shown in Figure 6b through d. From Figure 6b and c, it is evident that the <0001> directions are neither completely parallel to ND nor to TD. Instead, <0001> directions can be observed to be oriented nearly 40–45°clockwise (90°) from ND towards RD. A relatively weak <0001> parallel TD component can also be observed in Figure 6b and c. The pole figure at 2.5 mm location (Figure 6d) shows that <0001> directions have some orientation towards RD, that is, (0001) planes are oriented parallel to the FSP tool pin surface.
It is well documented[18, 19] that near the top surface of friction stir processed specimen, (0001) planes are mostly oriented parallel to the RD. In other words, <0001> parallel ND component is strong near the tool shoulder surface due to the compression stress. However, near the tool pin surface, the lattice planes have the tendency to orient parallel to the shear plane. With such an orientation, the <0001> parallel RD component strengthens and <0001> parallel ND component weakens. However, in the current investigation, the (0001) pole figures at 0.5 and 1.5 mm locations for SPUSC shows that <0001> directions are oriented nearly 40–45°clockwise (90°) from the ND towards the RD. A similar result was found by Yu et al. on another magnesium alloy AZ31. The possible reason for this behavior was proposed to be twinning, which has the tendency to reorient the <0001> parallel RD texture toward the ND. However, the pole figure at 2.5 mm location suggests the appearance of stronger <0001> parallel RD component. This may be due to higher shear force from the tool pin at this location. The (0001) pole figures for the DPUSC at 1.5 and 2.5 mm locations are shown in Figure 6e and f, respectively. The pole figures show that the <0001> directions are oriented at nearly 15–20° anticlockwise (135°) from the ND toward the RD with some inclination toward the TD. Further, the texture strength appears to get comparatively reduced during DPUSC.
TEM image of the nugget zone of DPUSC specimen is shown in Figure 7. The image shows the presence of submicron grains. It also depicts that the nugget zone has a sub-grain structure. It is known that magnesium has a hcp structure and limited number of slip systems. The basal slip system (0001) is dominant during deformation near the room temperature. and represent non basal slip systems in Mg. Couret and Caillard examined the temperature dependence of critical resolved shear stress (CRSS) for the basal and non-basal slip systems in Mg. They observed that the magnitude of CRSS for the non-basal slip significantly decreases as the deformation temperature rises above 600 K. In the current investigation, the temperature reached during FSP of AE42 was higher than 600 K, indicating possible activation of non-basal slip systems. Due to this and medium stacking fault energy (γ ≈ 125 mJ m−2) for Mg, dislocation movement through climb and glide are likely. Therefore, it is believed that during FSP of the AE42 alloy, the dislocations tend to form sub-grain structure through dislocation movement during the dynamic recovery (DRV) phase.
During DRV, the dislocations get accumulated at the sub-grain boundaries, whereas the interior of the sub-grains is devoid of dislocations. This leads to the existence of a strain gradient across sub-grain boundaries leading to their rotation to form large angles. The grain misorientation distribution for SPUSC and DPUSC was determined using EBSD and is shown in Figure 8. The figure shows a large fraction of high angle grain boundaries (HAGBs) for SPUSC and DPUSC specimens. The presence of a large fraction of HAGBs in the FSPed AE42 specimen suggests sub-grain rotation due to strain gradient. The evolution of a fine and equiaxed grain structure, the presence of sub-grains together with the existence of a large fraction of HAGBs suggest continuous dynamic recrystallization (CDRX) during FSP of AE42 alloy.
3.2 Nanoindentation Testing
The load displacement curves obtained from the nanoindentation testing at 1.5 and 2.5 mm locations for the as-cast, SPUSC as well as DPUSC specimens are shown in Figure 9a and b, respectively. The results of the nanoindentation testing are summarized in Table 1. Hardness from nanoindentation testing was determined using the following expression
where Pmax is the maximum load applied and Ac is the projected contact area at the maximum load. It is seen from Figure 9a that maximum indentation depth occurred for the as-cast alloy with a corresponding lower hardness among the three specimens. DPUSC demonstrated the least indentation depth with the highest hardness. Elastic modulus for all the specimens was determined using the following Equation (2).
where Er is the reduced modulus, Em the modulus for the material, Ei the modulus for the indenter, νm the Poisson's ratio for the material, and νi is the Poisson's ratio for the indenter. The modulus obtained from nanoindentation is the reduced modulus, Er, which is a combination of sample material and indenter elastic deformation. The Poisson's ratio and modulus value of the indenter material are 0.07 and 1140 GPa, respectively. The Poisson's ratio used for AE42 alloy was 0.35. It is seen from Table 1 that both SPUSC as well as DPUSC specimens showed higher modulus values than the as-cast alloy.
|Sr. No.||Specimen||Maximum penetration (hmax)||Hardness, H (HV)||Modulus, E (GPa)|
|Near 1.5 mm location|
|Near 2.5 mm location|
The difference in penetration depth for the as-cast, SPUSC, and DPUSC increased at 2.5 mm location compared to 1.5 mm location (Figure 9b). The decrease in penetration depth is an indication of higher hardness of the surface. Thus, SPUSC and DPUSC specimens showed higher hardness and elastic modulus at 2.5 mm location compared to 1.5 mm location. Higher hardness and modulus value at 2.5 mm location may be attributed to the finer grain structure, which is supported by the EBSD analysis.
3.3 Strain Hardening
In order to get quantitative information about strain hardening, the strain hardening exponent was calculated from the results of nanoindentation using the methodology given by Giannakopoulos and Suresh. The load-displacement curve for each of the investigated case was used to determine the maximum penetration depth, (hmax), as well as the residual depth, (hr). The value of strain hardening exponent was calculated within the nugget zone at 2.5 mm location and is given in Table 2. The fine grained friction stir processed specimens show a higher tendency for work hardening compared to the coarse grained as-cast AE42 alloy. Contrary to the observation in the current study, some of the previous investigations have reported that the strain hardening capability diminishes with the grain size reduction. These authors have reported that the friction stir processed specimen demonstrated reduced strain hardenability compared to the base material. They attributed this to the incapability of small grains to accommodate dislocations. However, the current study shows higher strain hardening for finer grained specimens.
|Max, Pmax (µN)||1000||1000||1000|
|Residual indentation depth, Hr (nm)||116.8||84.9||70.14|
|Maximum indentation depth, Hmax (nm)||153.7||119.8||111.5|
|Reduced elastic modulus, Ereduced (GPa)||54.35||74.8||76.53|
|Strain hardening exponent, n||0.642||0.721||0.738|
In an earlier investigation by the authors, it was observed that FSP of AE42 alloy resulted in a refined microstructure comprising a uniform distribution of fine in situ precipitates of Al-rare earth and Mg-rare earth elements. The precipitate size for SPUSC specimen was found to be in the range of 200 nm to 1µm which got further refined to nearly 50 nm for the DPUSC specimen. For comparison, the SEM images of SPUSC and DPUSC specimens are shown in Figure 10a and b, respectively. From these images, an inter-particle distance for precipitates finer than 1 µm size was evaluated using image analysis software. The average inter-particle distance for SPUSC and DPUSC specimens was found to be 1.7 and 1.1 µm, respectively. Thus, in DPUSC the particle size is more refined and has smaller inter-particle distance. The microstructure of the friction stir processed AE42 alloy is distinctly different from that reported by Yadav et al., where the microstructure was devoid of any second phase precipitates. The observed behavior of higher strain hardening ability of the friction stir processed AE42 alloy may be attributed to the presence of fine dispersion of second phase precipitates, which act as barriers to dislocation movement. Further, it is known that second phase precipitates with smaller inter-particle distance are more effective in restricting the dislocation movement. Therefore, an even higher strain hardening exponent for DPUSC specimen compared to SPUSC can be explained on the basis of further refinement of second phase precipitates during double pass FSP and smaller inter-particle distance.
In summary, AE42 alloy was friction stir processed under two different conditions. EBSD was used to analyze the variation in grain size and texture. It was observed that at greater depths from the tool shoulder, the grain size was more refined. The orientation of (0001) planes also varies across the depth of the friction stir processed specimen. These planes show increased tendency to orient towards the tool pin surface at greater depths from the FSP tool shoulder. Nanoindentation study showed higher hardness and modulus values towards the bottom part of the specimen cross-section. The double pass friction stir processed specimen showed higher hardness and modulus values compared to single pass. The friction stir processed specimens showed greater strain hardenability compared to unprocessed specimen, which is explained by the second phase precipitates acting as barriers to dislocation movement.
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