Friction-stir processed ultrafine grain high-strength Al-Mg alloy material K. N. Kalashnikov, T. A. Kalashnikova, A. V. Chumaevskii, A. N. Ivanov, S. Yu. Tarasov, V. E. Rubtsov, and E. A. Kolubaev
Citation: AIP Conference Proceedings 1909, 020075 (2017); View online: https://doi.org/10.1063/1.5013756 View Table of Contents: http://aip.scitation.org/toc/apc/1909/1 Published by the American Institute of Physics
Friction-Stir Processed Ultrafine Grain High-Strength Al-Mg Alloy Material K. N. Kalashnikov1,a), T. A. Kalashnikova2,b), A. V. Chumaevskii2,c), A. N. Ivanov2,d), S. Yu. Tarasov2,3,e), V. E. Rubtsov2,f), and E. A. Kolubaev2,3,g) 1
2
National Research Tomsk State University, Tomsk, 634050 Russia Institute of Strength Physics and Materials Science SB RAS, Tomsk, 634055 Russia 3 National Research Tomsk Polytechnic University, Tomsk, 634050 Russia a)
Corresponding author:
[email protected] b)
[email protected] c)
[email protected] d)
[email protected] e)
[email protected] f)
[email protected] g)
[email protected]
Abstract. It was shown experimentally that both friction stir processing parameters and the workpiece thickness have their effect on structure and mechanical properties of the processed samples. Even the 10% deflection from the optimal values resulted in the essential mechanical strength reduction. It was observed that FSP showed a tendency to suppress the serrated yielding during the tensile testing.
INTRODUCTION The use of friction stir processing (FSP) may be a good strategy to obtain the high-strength ultrafine grain aluminum alloys [1–4] by means of the severe plastic deformation (SPD). The advantages of this processing are its high-production yield and the feasibility of large article making. There are many literature sources devoted to this method use but only few of them disclose complex relationships between the processing process parameters and the resulting structural and mechanical characteristics with an eye to the influence of the workpiece dimensions. Even a smaller number of papers deals with studies of the complex structural evolution of sub microcrystalline grainsubgrain structures at atomic scale although there are some of them describing the results of molecular dynamics studies of simple atomic models [5, 6]. The majority of theoretical and experimental works in the field of SPD bulk nanostructured materials is devoted to describing their mechanical behavior under the loading and plastic deformation phenomena [7–9]. Only few works were done on samples obtained by FSP. The specificity of mechanical processing of FSP nanostructured materials is another problem to be resolved within the framework of works done before [13–15]. This work is focused on studying the effect of FSP process parameters and sample dimensions of the structure and mechanical characteristics of Al-Mg alloy samples and is a part of a complex program intended to study the high-strength FSP aluminum alloys.
MATERIALS AND METHODS Al-Mg alloy sheet samples of various thicknesses were subjected to FSP using an experimental FSP machine. After machining the samples were tested for tension and compression by the tensile test machine UTC 110M-100 1-U at 1 m/min loading rate.
Proceedings of the International Conference on Advanced Materials with Hierarchical Structure for New Technologies and Reliable Structures 2017 (AMHS’17) AIP Conf. Proc. 1909, 020075-1–020075-5; https://doi.org/10.1063/1.5013756 Published by AIP Publishing. 978-0-7354-1601-7/$30.00
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FIGURE 1. Microstructures of a base Al-Mg alloy: metal thickness 5 mm, stirring metal zone at plunging force 3150 kg, tool speed 105 mm/min, and rotation rate 450 RPM
The sample microstructure was examined by the optical microscope Altami Met 1C, SEM instrument SEMTRAC mini SM3000 and TEM JEOL JEM-2100. The micro hardness was measured by PMY-3M microindenter.
RESULTS AND DISCUSSION All FSP alloy samples show the grain refinement in the stirring zone with the final formation of the equiaxed grain-subgrain structure and the mean grain size of ~5 µm (Fig.1). The grain size distribution is close to that of the normal probability law and only slightly depends on the measurement direction compared to that of the initial hotrolled metal (Fig. 2). The microhardness of the stirring zone is reduced to HV = 880 MPa as compared to that of the base metal. The microhardness numbers are distributed unevenly across the sample thickness so that some peaks achieve 10% of its mean level. The results of mechanical tests show that the base metal strength before FSP was at the level of 302.1 and 376.5 MPa for tensile and compression tests respectively. The plastic deformation during compression and especially tensile tests was characterized by the jerky plastic flow presence, which was attributed to the Portevin– Le-Chatelier effect. This effect is manifested in the form of either quasi-periodic or non-periodic serrations in the load/displacement diagrams, which occur when dislocations overcome their pinning by the impurity clouds. 1 to 20 MPa height serrations were found during the tensile tests while 1 MPa height ones during compression. The FSP of 5 and 10 mm thick samples demonstrated their compression mean strength level of 425.3 MPa, i.e. 12.9% higher than that of the base metal. It is interesting that the FSP samples showed no serrated yielding.
FIGURE 2. The grain size distribution in longitudinal and transversal directions in the base metal (a) and within the stirring zone of FSP Al-Mg alloy (b)
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FIGURE 3. The microhardness profiles in 10 mm thick Al-Mg alloy FSP metal
The mean tensile strength of FSP 10 mm thick samples was at the level of 298.5 MPa, i.e. at the level of the base metal strength. However, the deformation curves show some specific features compared to that of the base metal (Fig. 4a). The friction stir processed samples demonstrated the lower stage III strain hardening coefficient, the longer stage III and practically zero stage IV hardening. The fracture stage was characterized by the gradual nucleation and development of cracks. The optimal FSP parameters were as follows: plunging force—3150 kg, tool speed—105 mm/min, and rotation rate—450 RPM. The 10% modification of the process parameters such as the tool travel velocity and rotation rate to 115 mm/min and 495 RPM, respectively, resulted in 8–9% tensile strength reduction while keeping the deformation curve behavior constant (Fig. 4a). The greater is the deflection of optimal parameters from their optimal values, the greater is the reduction in the tensile strength. The 5 mm thick FSP samples demonstrated their strain curves close to those obtained from the base metal samples (Fig. 4b) except for the specific type of serrations and their lower height. The fracture stage is rather long at practically constant load. The samples were fractured at 45 grad angle to the tensile axis without any necking. The almost flat fracture surface shows the cellular pattern with some quasi-brittle fracture zones (Fig. 5). The 10 mm thick FSP sample fracture occurs practically perpendicular to the tensile axis with the neck formation. The wave pattern fracture surface is of the viscous cellular type (Fig. 6).
FIGURE 4. Tensile test diagrams obtained on 10 mm thick Al-Mg alloy samples after the friction stir processing at the plunging force of 3150 kg, the tool speed of 105 mm/min, the rotation rate of 450 RPM (1) and the plunging force of 3150 kg, the tool speed of 115 mm/min, the rotation rate of 495 RPM (2); 5 mm thick samples processed at the plunging force of 2500 kg, the tool speed of 450 mm/min, the rotation rate of 550 RPM (3) and the plunging force of 2500 kg, the tool speed of 540 mm/min, the rotation rate of 650 RPM (4)
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FIGURE 5. Fracture surfaces of 5 mm thick Al-Mg FSP alloy
FIGURE 6. Fracture surfaces of 10 mm thick Al-Mg FSP alloy
CONCLUSION The results of mechanical tests on Al-Mg alloy samples subjected to the friction stir processing showed the differences between the base and friction stir processed metals from the viewpoint of their deformation behavior and as depended on the sample thickness. As shown the friction stir processed sample demonstrated 10% higher strength as compared to that of the base metal. The essential influence of the workpiece thickness and FSP parameters on the serrated yielding and fracture in mechanical testing was observed on FSP samples despite their equal grain size. This is related to the use of different tools and heat removal conditions so that the various structural-phase states might be created that modify the plastic deformation and fracture. Such modification should be taken into account in machining the ultra-fine grain materials.
ACKNOWLEDGMENTS This work has been implemented with RFBR financial support grant No. 16-48-700652.
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