Volume 85 International Scientific Journal Issue 2 published monthly by the June 2017 World Academy of Materials Pages 80-85 and Manufacturing Engineering
The effect of deformation degree on the microstructure of the 6060 aluminium alloy
M. Koralnik a, *, B. Adamczyk-Cieślak a, M. Kulczyk b, J. Mizera a a Faculty of Materials Science and Engineering, Warsaw University of Technology, ul. Woloska 141, 02-507 Warszawa, Poland b Institute of High Pressure Physics of the Polish Academy of Sciences UNIPRESS, ul. Sokołowska 29, 01-142 Warszawa, Poland * Corresponding e-mail address: [email protected]
ABSTRACT
Purpose: All results obtained in the present study allowed to analyse the changes in the microstructure and texture of the commercial 6060 aluminium alloy, after deformation process by severe plastic deformation. There were compare two deformation degree samples received by cumulative hydrostatic extrusion. Design/methodology/approach: The samples of the 6060 alloy were subjected to a one- pass and three-passes extrusion process and next the age hardening. The microstructure changes were investigated by using transmission and scanning electron microscopy. To study the texture evolution the X-ray diffraction were made. Findings: The microscopic observations results presented the refinement of microstructure as a result of deformation process. The evolution of fibrous character of texture was observed. There were noted the disappearance of fibrous component <100> during subsequent deformation processes and generation the fibrous component <111> after high deformation degree. In addition, for each state, the presence of cubic texture component was recorded. Research limitations/implications: For the future research are planned to analyse changes in mechanical properties after hydrostatic extrusion combinate with age hardening of investigated materials. Originality/value: The paper focuses on the investigation of microstructure and texture evolution after modern method of plastic deformation. Keywords: Metallic alloys; Hydrostatic extrusion; Microstructure and texture changes Reference to this paper should be given in the following way: M. Koralnik, B. Adamczyk-Cieślak, M. Kulczyk, J. Mizera, The effect of deformation degree on the microstructure of the 6060 aluminium alloy, Archives of Materials Science and Engineering 85/2 (2017) 80-85.
MATERIALS
80 80 © Copyright by International OCSCO World Press. All rights reserved. 2017 1. Introduction Introduction reductions (R) and cumulated true strain ( cum ), which are given by the equations:
The 6xxx series (Al-Mg-Si) aluminium alloy are one of 2 (1) = D0 the most popular materials for lightweight constructions at R 2 DF industrial applications. The Al-Mg-Si alloys belong to a group of alloys able to strengthened by age hardening and cum =lnR (2) are characterized by good corrosion resistance, weldability, The reduction R (equation 1) is the ratio of the cross superior combination of strength to density and high section surface area of the initial material before and after formability [1]. Thanks to the good ability to strengthen by extrusion, which corresponds to the cumulated true strain deformation the aluminium alloys are subjected to severe (equation 2) with the true strain being defined as the plastic deformation (SPD) processes. The studies of the cum natural logarithm of R. effect of the deformation degree on the microstructure and To evaluate the effect of deformation degree on the properties of age hardening aluminium alloys, after SPD microstructure and texture, the materials were subjected to processes have been described mainly for samples obtained a one-pass and three-passes extrusion process. Single by equal channel angular pressing [2-4]. deformation was performed with reduction diameter from The present research is concerned with the influence of 50 mm to 20 mm, corresponding to a low deformation the deformation degree obtained by SPD realized by high sample LDD (Low Deformation Degree sample). To achieve pressure processing by hydrostatic extrusion (HE) process more deformed sample HDD (High Deformation Degree), on the microstructure and texture of the industrial extrusions were performed according to the sequence: aluminium alloy. initial diameter 50 mm was reduced to 20 mm, then
reduced to 10 mm and in the next step to 5 mm. The all
HE processes were carried out at room temperature 2. Materials Materials andand methodsmethods conducted in a cumulative manner. The transmission electron microscopy (TEM) was used 2.1. Materials Materials for for research research to study the microstructural evolution. Observations were carried out on sections which were perpendicular to the The material used in the present study was the extrusion direction of materials. The samples were cut out commercially available Al-Mg-Si alloy EN AW-6060, in the from cross sections of the materials as a 3 mm discs and form of hot extruded rod with 50 mm diameter. In the initial prepared by applying electrolytic polishing. state the rod was solution heat treated at 530°C for 1 h and To obtain a complete characterization of the micro- subsequently water-quenched to room temperature. The structure, there were used the stereological methods solution heat treated sample was subjected to ageing at supported by computer image analysis [7]. Main measured 150°C for 24 h. The sample after heat treatment having the parameter describing the structure is d 2 – equivalent average grain size about 50 m and irregular shape. In the diameter corresponding to the grains size, which is defined initial state (before deformation) alloy exhibited a fibrous as the diameter of a circle which has the surface area equal character of texture the <100> fibre was predominant and to the surface area of a given grain. The d 2 values was amounts to 25% of volume fractions. Also, the presence of a determined for more than 300 randomly selected grains, cubic texture component with 30% fractions was estimated. to measure average grain size with the well statistic. To analyse the influence of plastic deformation on the In addition, a descriptions of the grains shape were made. microstructure and texture, the hydrostatic extrusion was The ratio of the grain’s longest chord and its perimeter made. The samples were solution annealed, then subjected to the equivalent diameter, allow to designate shape to severe plastic deformation and after that aged. The heat factors: and , which are sensitive to grain elongation and treatment parameters were the same like, at initial state of the expansion of grain boundary, respectively. the research material. The microstructure studies by TEM were supplemented by observations using the scanning electron microscope 2.2. Work Work methodologymethodology (SEM). Investigations were carried out on sections perpendicular to the extrusion direction at the polished The aluminium rods were subjected to severe plastic surface. The primary precipitates were examined by deformation by cumulative hydrostatic extrusion [5,6]. The scanning electron microscope at electron backscattering basic coefficients describing the process are extrusion mode combined with energy dispersive X-ray spectroscopy
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(EDS). To verify the chemical composition of the particles Table 1. the spot and map-scanning analysis were made. The Basic parameters of the hydrostatic extrusion examples of precipitate analysis were presented. The Number D , D , R, , Sample 0 F cum secondary phases formed as a result of heat treatment, were of passes mm mm - - small in size what made impossible to observe and LDD 1 50.11 19.97 6.30 1.84 analysed them using SEM. HDD 3 50.11 5.14 95.04 4.55 The samples texture was analysed by the X-ray diffraction method (XRD), at sections perpendicular to the 3.2. The The analysis analysis of the ofmicrostructure the microstructure extrusion direction, applying filtered Co K radiation. observations Based on the measured pole figures (111), (200), (220) and observations (311) there were determined the orientation distribution functions (ODF) were computed for each sample. From In the Figure 2 the TEM images of ultrafine grained obtained results the quantitative analysis of major texture 6060 alloy after hydrostatic extrusion process were components was made. The quantitative results of texture presented. The microstructure is highly deformed with show components with more than 5% of volume fraction. many dislocations and heterogeneity of grain size.
3. Results Results and and discussion discussion
3.1. The The plastic plastic deformation deformation by hydrostatic by hydrostatic extrusionextrusion
The three consecutive passes of a hydrostatic extrusion of a 6060 aluminium alloy rods, are presented in the Figure 1. The initial peaks, which follows the compression period, represent the pressure necessary to begin the extrusion process. Then the deformation proceeds under steady pressure, i.e. with a constant extrusion velocity.
Fig. 1. Pressure characteristics during hydrostatic extrusion process
All curves show a linear course of the extrusion. The pressure during one-pass process and at the third pass of cumulative hydrostatic extrusion, ranged respectively Fig. 2. Microstructure of a) LDD and b) HDD samples 370 MPa and 440 MPa. The higher pressure observed obtained in the HE processes during the next passes indicates strengthening of the material. The parameters of the hydrostatic extrusion are The stereological analysis (Table 2) showed the very strong given at the Table 1. structural refinement as a result of deformation process.
82 82 Archives of Materials Science and Engineering The effect of deformation degree on the microstructure of the 6060 aluminium alloy
The equivalent diameter was reduced from 50 µm at initial At the next step, the quantitative analysis of those elements state to 0.44 µm in the sample after low deformation degree. (Table 3) were made. Based on the analysis of the EDS The next two deformation processes were less affected by results and the literature review, the phase has been the fragmentation of the structure. The average grain size identified as Al 5FeSi particles [8,9]. The observed remains (0.31 µm) is 30% smaller than that observed in the first step amount of magnesium in the results for the particle is of extrusion. The value of elongation factor and the related to the presence of this element in the matrix. expansion of grain boundary are similar for both samples.
Table 2. Stereological parameters of the microstructure after HE processes
Sample d 2, m CV(d 2) , - , - LDD 0.44 0.48 1.52 1.31 HDD 0.31 0.46 1.46 1.28
The Figure 3 shows TEM images of the microstructure near to primary rod-like particles (indicated by arrows). Observations revealed that the deformation decreased the grain size in their neighbourhood. This effect is related to the accumulation of dislocations and energy during the deformation process, resulting as a larger amount of grain boundaries. Fig. 4. The EDS spot analysis at the matrix and particle
Fig. 3. Microstructure area with visible primary precipitates
In order to verify the chemical composition of the particles, EDS analysis was used. Because of the chemical composition was not changed at the individual samples, the exemplary EDS measurements were presented in the paper. At the Figure 4 the spot-scanning analysis of elements were presented. Based on the measurement, it was possible to identify the elements occurring at the participate. The EDS spot analysis were supplemented by mapping scanning. Figure 5 shows the distribution of the elements aluminium (Al), magnesium (Mg), silicon (Si) and iron (Fe), respectively. Fig. 5 Elements distribution from EDS mapping
Volume 85 Issue 2 June 2017 83 M. Koralnik, B. Adamczyk-Cieślak, M. Kulczyk, J. Mizera
Table 3. The quantitative analysis of the elements Analysed Element, weight % area Al Mg Si Fe Matrix 99.40±0.42 0.26±0.03 0.34±0.09 - Participate 73.22±0.39 0.13±0.03 7.27±0.16 19.38±0.32
As a result of the precipitation hardening during the aging process, the secondary particles were created (Fig. 6). According to the literature, the phase was identified as metastable precipitate Mg-Si ’’ [10,11]. The secondary phases were present in the samples as a needle-like shape and were coherently bound or semicoherent with the matrix. It can be observed the precipitation free zone about 50 nm near the grain boundaries, the same width for both samples. The width of these areas indicates the distance of solutes diffusion [12]. Fig. 7. Measured pole figures (111), (200), (220) and (311) for LDD sample
Fig. 6. Secondary precipitates at a) LDD and b) HDD samples
3.3. The The texture texture analysis analysis
Based on the XRD studies, the pole figures (111), (200), (220) and (311) are shown in Figures 7 and 8. The texture results revealed the fibrous texture for samples, which is characteristic for hydrostatic extrusion of fcc materials [13,14]. The volumes percentage of the individual texture components are given in Table 4. Fig. 8. Measured pole figures (111), (200), (220) and (311) for HDD sample Table 4.
The results of XRD quantitative analysis The results received in the present work have allowed Volume fraction of texture Component to determine the evolution of the fibrous texture. component, % designation Comparing results for non-deformed material and LDD {hkl}
84 84 Archives of Materials Science and Engineering The effect of deformation degree on the microstructure of the 6060 aluminium alloy
component (~7%) decreased after HE processes from 30% [5] J.J. Lewandowski, A. Awadallah, Hydrostatic Extrusion at the initial state of the alloy. Whereas, in the HDD sample of Metals and Alloys, ASM Handbook, Vol. 14A: appears the additional rotated cubic component {0 0 1} Metalworking: Bulk Forming, 2005, 440-447. <1 1 0>. This effect may be related to the texture evolution [6] M. Lewandowska, K.J. Kurzydłowski, Recent during the heating of the material at the cumulative development in grain refinement by hydrostatic deformation process [15]. In the HDD sample, the extrusion, Journal of Material Science 43 (2008) maximum level values (above 61), indicate a sharp fibrous 7299-7306. texture, while in case of LDD sample, the level values [7] T. Wejrzanowski, W.L. Spychalski, K. Rozniatowski, slightly exceeded 14. The low background level indicates K.J. Kurzydłowski, Image based analysis of complex strong texture in the tested materials. microstructures of engineering materials, International Journal of Applied Mathematics and Computer Science 18 (2008) 33-41. 4. Conclusions Conclusions [8] L. Sweet, S.M. Zhu, S.X. Gao, J.A. Taylor, M.A. Easton, The effect of iron content on the iron- In the present paper, the effects of deformation degree containing intermetallic phases in a cast 6060 on the microstructure and texture of industrial aluminium- aluminum alloy, Metallurgical and Materials magnesium-silicon alloy were presented. The results Transactions A 42 (2011) 1737-1749. obtained in the present study lead to the following [9] P. Widlicki, Influence of hydrostatic extrusion on conclusions: mechanical properties and microstructure in aluminum • The hydrostatic extrusion process allows to obtain alloys, Doctoral thesis, Warsaw University of Techno- heterogeneous ultrafine grained microstructure. logy, 2009. • The TEM observations expose strong defected structure [10] S.J. Andersen, H.W. Zandbergen, J. Jansen, C. with many dislocations. Traeholt, U. Tundal, O. Reiso, The crystal structure of • As a result of deformation the fibre <111> were the ’’ phase in Al-Mg-Si alloys, Acta Materialalia 46 generate, the component become more intense with (1998) 3283-3298 increase degree of material deformation. [11] D. Maisonnette, M. Suery, D. Nelias, P. Chaudet, • The cubic texture in all investigated samples was T. Epicier, Effects of heat treatments on the retained after the hydrostatic extrusion process. microstructure and mechanical properties of a 6061 aluminium alloy, Materials Science and Engineering A 528 (2011) 2718-2724. References [12] W. Chromi ski, S. Wenner, C.D. Marioara, R. Holmestad, M. Lewandowska, Strengthening [1] I.J. Polmear, Light alloys: from traditional alloys to mechanisms in ultrafine grained Al-Mg-Si alloy nano crystals, 4 th Edition, Butterworth-Heinemann, processed by hydrostatic extrusion – influence of Burlington, 2006. ageing temperature, Materials Science and [2] M. Karo , A. Kopy ş , M. Adamiak, J. Konieczny, Engineering A 669 (2016) 447-458. Microstructure and mechanical properties of the [13] L. Styczynski, W. Pachla, S. Wojciechowski, annealed 6060 aluminium alloy processed by ECAP Thermal-softening processes in polycrystalline method, Archives of Materials Science and aluminium during hydrostatic extrusion, Metal Engineering 80/1 (2016) 31-36. Science 16 (1982) 525-528. [3] M. Ilieva, R. Radev, Effect of severe plastic [14] A. Bois-Brochu, C. Blais, F.A. Goma, D. Larouche, deformation by ECAP on corrosion behavior of J. Boselli, M. Brochu, Characterization of Al–Li 2099 aluminium alloy AA 7075, Archives of Materials extrusions and the influence of fiber texture on the Science and Engineering 81/2 (2016) 55-61. anisotropy of static mechanical properties, Materials [4] P. Snopi ski, T. Ta ski, O. Hilšer, A. Lubos, Effect of Science and Engineering A 597 (2014) 62-69. ECAP process on structure and hardness of AlMg3 [15] J. Hirisch, Texture and anisotropy in industrial aluminium alloy, Archives of Materials Science and applications of aluminium alloys, Archives of Engineering 84/2 (2017) 79-85. Metallurgy and Materials 50 (2005) 21-34.
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