https://doi.org/10.17576/jkukm-2020-32(3)-18
The Evolvement of Mechanical Properties and Microstructure of Commercial Aluminum Alloy 6061 via High-Pressure Torsion
Cheeranan Krutsuwan Nuphairode*, Intan Fadhlina Mohamed, Fauziana Lamin, Wan Fathul Hakim Wan Zamri, Mohd Zaidi Omar
Mechanical Engineering Programme,
Center for Engineering Materials and Smart Manufacturing (MERCU),
Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, Malaysia Zenji Horita
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka, 819-0395, Japan.
*Corresponding author: cheeranankrutsuwan@gmail.com Received 10 October 2019, Received in revised form 14 February 2020
Accepted 01 May 2020, Available online 30 August 2020
ABSTRACT
A precipitation-hardened AA6061 alloy is processed by high-pressure torsion (HPT) technique which has demonstrated its success in refining grain structures up to the nanometer-scale. With the aim of upscaling the strengthen lightweight materials for larger applications, this study is essential to analyse the related strengthening mechanisms for further strengthening potential and microstructure alteration on sample with extra diameter size. This severe plastic deformation (SPD) technique simultaneously contributes to an intensification of dislocation density which caused by the grain refinement. This study presents properties enhancement of 30mm diameter of AA6061 alloy after being compressed at 4GPa pressure and subjected to a 0.2 rpm torsional force for 1 and 3 turn(s) at room temperature. For the result testing, Vickers microhardness, tensile test, X-ray diffraction (XRD) and Transmission electron microscopy (TEM) were used as to analyse the mechanical properties as well as the microstructure changes. As a result, grain structures were refined to 200nm, and its mechanical properties, such as tensile strength and hardness is enhanced to ±600MPa and ±172HV, respectively. The homogeneity of hardness is increased with the number of turns or the torsional strain applied. For this particular aluminium alloy, the grain refinement and grain boundary hardening were identified as the main hardening mechanisms by the process of HPT.
Keywords: Commercial AA 6061; dislocation hardening; grain boundary strengthening; high-pressure torsion; severe plastic deformation
INTRODUCTION
Grain refinement of bulk materials has been actively conducted along several decades due to the great opportunity of attaining enhanced properties by severe plastic deformation (SPD) procedure, as formulated in the classic Hall-Petch relationship (Hall 1951) and had been proven in many other reports (Horita et al. 2001; Mohamed, Yonenaga, et al. 2015; Tejedor et al. 2019; Valiev et al. 2010).
This procedure is more powerful in processing porosity- free samples than other conventional techniques such as nanopowder compaction or gas condensation. (Edalati
& Horita, 2016; Azushima et al. 2008). SPD processing has shown its ability in producing ultrafine-grained (UFG) microstructures for numerous range of materials from pure metals to its alloys, including the intermetallic form (Valiev et al. 2000). SPD also have the ability to enhance the corrosion resistance of materials. (M. A. Gebril et al. 2018;
Mohamed Abdelgawad Gebril et al. 2019). Successful grain refinement was also evidenced in non-metallic materials such as composite, amorphous and ceramics (Edalati &
Horita 2016; Starink et al. 2013).
Nowadays, several SPD techniques are available.
Besides HPT, other techniques including equal channel angular pressing (ECAP), large strain extrusion machining (LSEM), accumulative roll-bonding (ARB), asymmetric rolling (ASR) and friction stir processing (FSP) are also gaining special interest among the SPD communities.
However, in recent years, HPT beats others due to its capability in evolving a more significant grain refinement (Naghdy et al. 2016). In addition, the method is also applicable to generate consolidated materials from metal powders. Interestingly, all of these capabilities can be conducted at room temperature, as a premature failure due to the large straining is minimised by the existence of the high hydrostatic pressure (Yoo et al. 2012). Meanwhile, the
torsion provides large shear straining, severely deforms the processed samples and ultimately contributes to improved strength.
In particular, several mechanisms involved during the strengthening process. Besides the grain refinement that analytically formulated by the Hall-Petch relationship, (Fleischer 1963; Labusch 1970), (Hirsch 1959) and (Mohamed, Yonenaga, et al. 2015) revealed other mechanisms that contribute to strengthening in HPT, including solid solution, dislocation accumulation and precipitation hardening. These mechanisms were analytically formulated by the Fleischer and Labusch relationship, the Bailey-Hirsch and the Orowan relationship, respectively. (Chandrasekaran 2003) reported that metal alloy that is generally constructed with the primary element plus alloying elements in a certain percentage might experience the solid solution strengthening.
The combination of a different atomic dimension of these alloying elements could distort the strain field. Even though the hardening percentage by the solid solution mechanism is comparatively lower, the mechanical properties are possible to be further increased due to additional grain refinement by the solute atoms (Edalati et al. 2014). This phenomenon explains the importance of understanding the related hardening mechanisms in HPT for a particular material.
Despite the discussed HPT promising potential, studies on the wider and bigger sample are still lacking. Several studies (Hawas 2013; Lamin et al. 2018; Aïcha Loucif et al. 2010; Mohamed et al. 2013; Mohamed, Lee, et al. 2015) reported a positive effect of giant straining by HPT to the hardness as well as the strength of AA6061, but smaller sample size was studied; typically 9-10mm. Hence, this work aims to assess the properties enhancement, including the microstructure and the mechanical properties of a wider diameter of 30mm thin disc commercial age-hardenable AA6061. Accordingly, verification of related strengthening
mechanism is also conducted to maximise the strengthening possibility, beyond the saturation levels.
METHODOLOGY
This study utilizes a 1 mm thickness commercial AA 6061 cold-rolled sheet. With aluminium as the primary element, other alloying content was summarized in Table 1.
30 mm diameter disc samples were cut from the rolled- sheet using a wire-cutting electric discharge machine (EDM). Then, a solution treatment was conducted at 540°C for four hours in an oven, and the treated sample was then immediately quenched into ice water.(Mattos et al. 2017) The prepared samples were subjected to HPT under a 4 GPa pressure for 1 or 3 turn(s) with a rate of 0.2 rpm at room temperature.(Ito & Horita 2009). The disc was thinning to a range between 0.7 and 0.8mm by the HPT procedure.
The range was kept constant for the simplification of the equivalent strain calculation.
Sample grinding was performed using a series of sandpapers grit, i.e. 400, 800, 1000 and 1200, and continued with polishing to produce a smooth mirror-like surface- finishing. Vickers micro hardness test was conducted using a Mitutoyo HM-102 tester by applying a 200 g indentation in 5 seconds. A thorough hardness measurement was considered at six radial directions to assess the sample homogeneity. As illustrated in Figure 1, each radial measurement consists of 14 indentation points with a 1 mm equal distance.
As for the tensile specimen preparation, the disc sample was further polished to a range between ~0.6 and 0.7 mm thickness. Here again, EDM was utilized to cut six tensile specimens from a disc sample. As shown in Figure 2, the dumb-bell shaped specimens were positioned at 4 mm, 8 mm and 12 mm from the disc centre. The tensile specimen
TABLE 1. Element content of the AA6061 in weight percentage
Mg Si Cu Fe Cr Zn Ti Al
0.96 0.59 0.29 0.29 0.02 0.01 0.01 Bal.
Source: Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University
TEM and XRD specimens
Hardness measurements
FIGURE 1. Schematic diagram of hardness measurement and specimen preparation for XRD and TEM microstructural observation
has a gauge length and width of 1.5 mm and 0.7 mm, respectively. The tensile test was conducted by pulling the specimen at an initial strain rate of 2.0 x 10-3s-1 utilising a mini tensile machine.
Whereas for the microscopic specimen preparation, the thickness of the processed sample was decreased to 0.4 mm by grinding. It was punched to a smaller 3mm diameters disc, as shown in Figure 1. Two specimens were prepped for each disc, for transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. Then, the grinding process was continued until an approximately 0.12 mm thickness obtained. Perforated thinning using a twin-jet electro polisher was used in a percentage ratio of 30:70 HNO₃ and CH₃OH solution at -13°C and 10V. Microstructure evolution was analysed using TEM Hitachi H-8100 in a conventional transmission mode with a 200kV accelerating voltage of a parallel beam. Meanwhile, the Cu Kα was utilised for the XRD test with an accelerating voltage 40kV and a current of 40 mA.
RESULTS AND DISCUSSION
A. MECHANICAL PROPERTIES
Evolution of Vickers microhardness along the disc radius for the sample processed at 1 turn in shown in Figure 3.
The hardness ranges between 110 HV and 172 HV from the centre towards the disc edge. The increasing hardness distribution seems saturated at approximately 170 HV. The hardness evolution pattern was also evidenced in other HPT- processed aluminium alloys, as modelled by Ariffin et al.
(2019). The main strengthening mechanism attributes to the total strengthening was discussed in the microstructure evolution, following this subsection.
The hardness distribution was re-plotted opposing to the developed equivalent strain. The equivalent strain is defined as in Eq. (1).
(
1)
0 32( )
= −s
∫
N t Nπr dNε (1)
The abbreviation of all equations was listed in Table 2.
Since all the HPT samples retain an almost similar thickness, a constant thickness and non-slippage condition were considered. Therefore, the equivalent strain is expressed as in Eq. (2)
2
= 3rN t
ε π (2)
It was revealed that the hardness increases as the torsional straining increased (Figure 4). A comparable positive hardness-straining evolution behaviour was reported in various metallic materials (Edalati et al. 2011; Edalati,
Yokoyama, et al. 2010; Kawasaki et al. 2010, 2011; Aicha Loucif et al. 2012). For this particular sample with N=1 and N=3, the hardness evolution becomes saturated at 170 HV as the equivalent strain reaches approximately 25. The saturation starting point for both turns are almost consistent.
This saturation behaviour was contributed by two contrast mechanisms, which are hardening and softening. These mechanisms were occurred as a result of lattice defects with dislocation annihilation, led by dynamic recovery and recrystallization (Edalati et al. 2008; Starink et al. 2013;
Vorhauer & Pippan 2004).
Figure 5 outlines the tensile testing curves of the post-HPT tensile specimens (N = 1, T = R.T) for the three tested positions, i.e. 4 mm, 8 mm and 12 mm from the disc centre. As a result, it can be noted that the strength fundamentally increases from 150 MPa to a certain level, ranges between 620 MPa to 650 MPa, as listed in Table 3.
It ought to be noticed that the increment of tensile strength is directly proportional to the distance from the disc centre.
It is understood that the equivalent strain experienced at a localised position is directly proportional to the disc radius.
Therefore, the higher strain imposed at the outer radius could explain this strength development pattern.
B. MICROSTRUCTURE EVOLUTION
The grain size strengthening, ∆σGB, is linked by the equalization of Hall-Petch as follows.
∆ GB= y− 0 = ky
σ σ σ d (3)
The description of abbreviations used in this equation can be referred to in Table 2. After HPT-processing with N = 1, the grain was successfully refined to ~200 nm. This significant grain refinement leads to the material hardening by ~111 HV.
The optical micrograph of the solution treated sample is shown in Figure 6(a). It was noticed that the specimen has an equate grain size of ~80μm whereas the rest of the figure displays TEM images of samples after HPT, which are located at both edge and centre of the discs (N=1, T
= R.T). TEM micrograph evidenced an impressive grain refinement near to the disc centre. With one turn (𝑁=1), in which the centre was strained about ɛ=~18, the grain size is refined to an average size of ~800 nm (Figure 6(b)).
Interestingly, the grain was further refined to ~200 nm at the edge area (Figure 6(c)). By and by, as discussed earlier, the lower imposed equivalent strain contributes to the larger grain structures at the inner disc radius as compared to the edge. The refined grain size was accompanied by a higher dislocation density as revealed in the calculation from the XRD analysis later.
Analysis of the XRD peaks broadening depicts the evolvement of crystallite size D and dislocation density 𝜌 of the processed samples. Calculation of these
FIGURE 2. Three different position of the tensile specimens i.e. 4 mm, 8 mm and 12 mm and its dimension.
FIGURE 3. Vickers microhardness variation along the 15 mm disc radius
TABLE 2. Description of abbreviation in all the equations
Symbol Description
r Distance of the hardness measurement point with respect to the centre
s Slippage fraction
N Number of HPT turn
t(N) Disc thickness after N
ε Lattice strain
σy Hall-Petch stress/strength
d The average grain size,
ky Hall-Petch slope (0.166 MNm−3/2).
β FWHM in radian
λ Wavelength of X-ray beam (0.1542 nm for Cu Kα),
D Crystallite size
ρ Dislocation density
ϴ Bragg angle
b Burgers vector (0.286 nm for Al)
FIGURE 4. The evolvement of Vickers microhardness with an
increase in equivalent strain for 1 and 3 turns. FIGURE 5. Plotting of nominal stress vs nominal strain for post- HPT sample (N=1)
TABLE 3. Tensile strength of samples after HPT-processing at 3 different positions from the centre Condition/ Position from the centre (mm) Tensile strength (MPa)
As-solution treated ~150
12 ~650
8 ~630
4 ~620
parameters has been done by utilising the Williamson Hall method (Williamson and Hall 1953; Warren &
Averbach 1952):
0.9 2
= +
cos sin
D
β θ ε θ
λ λ (4)
2
14.4 2
= b
ρ ε (5)
Again, Table 2 is referred for the description of abbreviations used in these equations.
XRD spectrums of pre-HPT sample and post-HPT- samples are illustrated in Figure 7. The peak broadening of both samples was then determined by utilising the software called Diffrac.suite Eva. Referring to the Eq. (4), 𝛽 represents the range of peak broadening, in radians, of an individual XRD spectrum. Later, Scherrer’s equation (Scherrer 1918) was incorporated to calculate the crystallite size, as expressed in Eq. (6).
= 0.9 D cos
λ
β θ (6)
Based on Figure 7, through the image of the 5 peaks appeared, it can be noticed that the peaks of the post-HPT specimens, the red and black spectrums at the bottom, are broader than that of the pre-HPT specimen, the upper blue
spectrum. This shows that the HPT produces samples with finer crystallites. This phenomenon is contributed by the factor of crystallite domain structured repetition (Bragg 1969; Révész et al. 2010; Wilson 1921). In view of this, crystallites that formed in the post-HPT samples poses high disorientations, inconsistently organised, and thus turn out peak broadening. It is also can be spotted that from other phases, there are zero extra peaks arising. This can be confirmed that no phase transformation or hydrides formation developed during the HPT-processing. This result implies that the transformation of phase and precipitation formation are not evidenced during the strengthening process. A similar phenomenon was also reported in other materials like Ti, Zr and Hf (Edalati et al. 2009; Edalati Horita et al. 2010; Zrnik et al. 2008)
Table 4 tabulates the average crystallite size and dislocation density of the observed specimens. Crystallites were refined, a minimum by 55% from 63.3 nm to 28.3 nm, at the disc centre. The comparison of the average crystallite size at the disc edge and disc centre are not too significant as the homogeneity of disc surface is almost achieved as the N or the torsional strain increased. Compared to TEM result, the grain size obtained is about six to 7 times larger than that of crystallite size obtained by XRD. This is on the grounds that XRD estimates the coherent diffraction domains which incorporate grains and subgrains as well as the dislocation cells which are isolated from one another by little contrasts in direction, ordinarily 1°−2°. Be that as it may, TEM perception regularly gives the spans of grains with generally high point grain limits. Aside from which,
(a)
(b)
(c)
FIGURE 6. Comparison of microscopic observation between (a) specimen before HPT using an optical microscope, and TEM image of specimens sampled at two different locations: (b) near to the disc centre while (c) near to the disc edge. Both HPT-processed specimens
were extracted from a sample that subjected to one turn (N=1).
20 40 60 80
(a)
(c) (220) (222)
(311) (200)
Int ens ity
Bragg Angle 2θ AA6061
HPT: P=4GPa, N=3, ω=0.2 rpm, T=R.T
(111)
(b) Al
FIGURE 7. Comparison of XRD spectrums of different condition between (a) before HPT;and after HPT, at distinctive position: (b) the disc centre and (c) the disc edge
getting the dislocation density from XRD profiles is a lot simpler than performing quantitative dislocation density investigation under TEM. Subsequently, XRD profile analysis is generally combined with TEM procedures to investigate the auxiliary properties of plastically deformed materials. TEM gives direct pictures of grain size and thick dislocations yet different parameters like microstrain is constantly estimated by XRD (Gubicza et al. 2004; Ungar 2001; Ungár et al. 2005)
CONCLUSION
The purpose of the current study was to assess the evolution of commercial AA6061 properties as a result of HPT processing for 1 and 3 turns. Before HPT, the pre-HPT sample poses a microhardness of 65 HV. Significant enhancement of microhardness was evidenced as the equivalent strain increased and it reaches a saturation level at an approximately 172 HV while shear straining further increased with an increase in the HPT turn number. In conjunction, the tensile strength of the processed samples also improved, up to 650 MPa. Even though elongation reduced, in particular near the edge, this strengthening process still preserves some percentage of ductility, especially towards the disc centre.
Correspondingly, TEM microstructural observation at the saturation level revealed a significant grain refinement of
~200 nm. The results of this investigation show that the primary hardening mechanism of this 30 mm AA6061 is grain hardening and dislocation hardening. An implication of this is the possibility for further strengthening potential on the upscaled AA6061 samples by any possible alteration of the existing process.
DECLARATION OF COMPETING INTEREST
None.
ACKNOWLEDGEMENTS
This work was financed by the Fundamental Research Grant Scheme (FRGS/1/2016/TK03/ UKM/02/4) by the Ministry of Higher Education Malaysia. Thank you to the Department of Materials Science and Engineering, Faculty of Engineering at Kyushu University, Fukuoka, Japan for providing us with the HPT facility and other lab equipment.
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