Materials Science Forum Vols. 490-491 (2005) pp 436-441 online at http://www.scientific.net © (2005) Trans Tech Publications, Switzerland Online available since 2005/Jul/15
Residual stress relaxation and cyclic deformation behavior of deep rolled AlMg4.5Mn (AA5083) at elevated temperatures P. Juijerm, I. Altenberger, U.Noster, B. Scholtes University of Kassel, Institute of Materials Engineering, Moenchebergstrasse 3, 34125 Kassel, Germany
[email protected] Keywords: deep rolling, residual stress relaxation, high temperature fatigue
Abstract The cyclic deformation behavior of deep rolled and polished aluminium wrought alloy AlMg4,5Mn in the temperature range 20-300°C has been investigated. Results of quasistatic tension and compression tests of untreated specimens in the temperature range 20-300°C are presented. To characterize the fatigue behavior for stress-controlled tests as a function of test temperature, s-n curves, cyclic deformations curves and mean strains as a function of number of cycles are given. The residual stress- and work hardening states near the surface of deep rolled aluminium alloy AlMg4.5Mn before and after fatigue tests were investigated by X-ray diffraction methods. The investigated AlMn4.5Mn aluminium alloy shows cyclic hardening until fracture at all stress amplitudes in stress-controlled fatigue tests at 25-150°C. With increasing temperature the deformation behavior shifts from cyclic hardening to cyclic softening. Below a certain stress amplitude at a given temperature deep rolling led to a reduction of the plastic strain amplitude as compared to the untreated state through cyclically stable near-surface work hardening as indicated by stable FWHM-values. This reduction in plastic strain amplitude is associated with enhanced fatigue lives. The effectiveness of deep rolling is governed by the cyclic and thermal stability of nearsurface work hardening rather than macroscopic compressive residual stresses. Since nearsurface work hardening is known to retard crack initiation, deep rolling is also effective in temperature- and stress ranges where macroscopic compressive residual stresses have relaxed almost completely, but where near-surface work hardening prevails. Above certain stress amplitudes and temperatures, deep rolling has no beneficial effect on the fatigue behavior of AlMg4.5Mn. This is a consequence of instable near-surface microstructures, especially instable near-surface work hardening. 1. Introduction The aluminum-magnesium or 5xxx alloys in sheet-form are widely used in welded components and have ben considered for some time to be the most appropriate choice for vehicle structures. Recent applications of Al-Mg alloys in automotive structures include the use of higher-magnesium containing alloys such as AA5454 and AA5083 for applications such as wheels, chassis and sub-frames where emphasis is put on welded materials strength [1] Fatigue failure is still considered to be one of the most costly technical problems in industry. Hence, the optimized use of aluminum-alloys with acceptable fatigue properties is gaining importance. Very simple and comparably cheap methods to enhance fatigue properties are mechanical surface treatments such as shot peening, deep rolling or laser-shock peening. The main beneficial effects of mechanical surface treatments are near-surface work hardening and compressive residual stresses, and in some cases also a smoothening of the surfacetopography, as induced by all deep rolling or roller-burnishing treatments. Whereas the near-
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surface cold work primarily serves to retard or inhibit crack initiation, the compressive residual stresses decelerate or even stop fatigue crack growth [2,3]. However, these effects are only beneficial if near-surface properties remain stable under mechanical and thermal loading. Although a number of studies already exist dealing with the stability of compressive residual stresses and work hardening during fatigue loading of different steels [4,5], magnesium- or titanium-alloys [6,7,8] at elevated temperatures, none of these investigations cover high temperature fatigue of mechanically surface treated aluminum alloys. Therefore, this paper discusses s-n-behavior, and cyclic deformation curves of non- and mechanically surface treated (deep rolled) AlMg4.5Mn at room temperature and elevated temperature. Moreover, the near-surface residual stress and work hardening states of deep rolled specimens were investigated before and after fatigue testing at ambient and elevated temperature. 2. Material and experimental details The AA5083 raw material was delivered as warm rolled sheet with a thickness of 15 mm. The chemical composition of this alloy is given in table 1. Cylindrical fatigue specimens with a diameter of 7 mm and a gauge length of 15 mm were machined from the sheets. The rolling direction during the fatigue tests corresponds to the rolling direction of the sheet. The samples were investigated without any further heat treatment. The non-mechanically surface treated samples were chemically milled in the gauge length leading to a material removal of 100 µm. The deep rolling treatment was carried out with a hydrostatically seated rolling device with a 6.6 mm ball and 100 bar rolling pressure. Quasistatic tension- and compression tests were carried out using a mechanical testing machine at a strain rate dε/dt = 10-3s-1. Tensioncompression fatigue tests were performed using a servohyraulical testing device under stress conrol without mean stress (R = -1) and with a test frequency of 5 Hz. Stress-strain-hysteresis measurements were carried out with a capacitative extensometer. The elevated temperature tests were performed with two controlled resistance heating elements. Residual stress depth profiles were determined by successive electrolytical material removal applying the classical sin²ψ-method using CrKα-radiation at the {222}-planes and ½ s2 = 18.56x10-5 mm2/N as elastic constant. Near surface work hardening was characterized by the full width at half maximum (FWHM) values of the X-ray interference lines. 3. Results and discussion Quasistatic tensile and compression tests revealed a continuous decrease of the 0.2%-proof stress with rising temperature above 150oC [9]. Therefore, also a signficant impact of temperature on fatigue strength was expected. Fig. 1 exhibits non-statistically evaluated s-ncurves of polished (non-surface treated) specimens. As expected, the fatigue strength and fatigue lives declined strongly with increasing temperature, especially in the temperature range 150-300oC. For instance, the fatigue life of the polished condition at room temperature was roughly 110,000 cycles for a stress amplitude of 175 MPa, but only 2500 cycles for the same stress amplitude at 250oC. Fig. 2 shows cyclic deformation curves of polished specimens at elevated temperatures and a stress amplitudes of 175 MPa. With increasing stress amplitude and temperature the plastic strain amplitude increases and fatigue lifetimes (number of cycles to fracture) decrease. Whereas at room temperature AlMg4.5Mn exhibits pronounced cyclic hardening behavior until failure, at temperatures above 200oC the material initially cyclically hardens and then softens until fracture (Fig. 2). Cyclic softening becomes especially pronounced at temperatures above 200oC and stress amplitudes higher than approx. 145 MPa. Furthermore, elevated temperatures also significantly promote cyclic creep of this alloy [9]. The near-surface stress state state of deep rolled AlMg4.5Mn before and after fatigue, as characterized by X-ray diffraction, is presented in Fig. 3. After deep rolling, compressive
438
Residual Stresses VII, ICRS7
residual stresses as well as increased FWHM (half-width)-values were measured at the surface and in near-surface regions up to a distance from the surface of roughly 500 µm. At a depth of 30 µm, maximum compressive stresses of about –240 MPa were detected. After fatigue at stress amplitudes below 205 MPa and temperatures below 150oC, plastic strain Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
wt.%
0.40
0.40
0.10
0.40-1.0
4.5
0.05-0.25
0.25
0.15
Bal.
Table 1: Chemical composition of AlMg4.5Mn
260 240
20° C
stress amplitude σ a (MPa)
150°C 220
200°C
200 250°C
180 160
300°C
140 120 100 1,0E +02
1,0E +03
1,0E +04
1,0E +05
1,0E +06
num ber of cycles to failure N f
Fig. 1: Non-statistically evaluated s/n-curves of polished specimens in the temperature range 20-300°C
plastic strain amplitude (o/oo)
5 σa = 175 MPa, polished
4 3 300°C
2 250°C
1 200°C 150°C RT
0 1
10
100
1000
10000
100000 1000000
number of cycles
Fig. 2: Cyclic deformation curves of polished AlMg4.5Mn for different test temperatures and a stress amplitude of σa = 175 MPa amplitudes are diminished and fatigue lifetimes are increases as compared to the untreated surface condition. At room temperature, the cyclic deformation behaviour of deep rolled AlMg4.5Mn is characterized by cyclic hardening for all stress amplitudes, similar as in the untreated condition. As an example, this is demonstrated in Fig. 4 for a stress amplitude of
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205 MPa for the untreated and deep rolled surface condition. However, at stress amplitudes above 205 MPa and a test temperature of 150oC, the deep rolled state exhibits initially lower plastic strain amplitudes than the untreated specimens only during the first 20 % of the fatigue life; then this behavior is reversed and the plastic strain amplitudes of the deep rolled states exceed the ones of the untreated state (Fig. 5). Due to the high cyclic plastic strains and high temperatures the fatigue process is associated with pronounced residual stress relaxation. Already at room temperature, fatigue loading at a stress amplitude of 205 MPa causes a reduction of surface compressive residual stresses by 60 % (Fig. 3). At 150oC, compressive residual stresses relaxed almost completely after half the number of cycles to failure. Consequently, the influence of macroscopic residual stresses under these conditions is negligible, as already observed in other cases, where surface residual stresses relaxed already in the first fatigue cycles [10]. Nevertheless, in contrast to nearsurface compressive residual stresses, near-surface work hardening remains stable during room temperature fatigue at this stress amplitude, thus leading to a marked fatigue life improvement (Fig. 6). Diminished near-surface FWHM-values at 150oC or higher indicate that near-surface work hardening starts to anneal out (Fig. 3). Through the “relaxation” of the near-surface work hardening the beneficial effect of deep rolling on the fatigue behavior disappears above a certain stress amplitude-temperature-combination, as can be seen in the s/n plots in Fig. 6. If stress amplitude-temperature-combinations are chosen in such a way that the resulting fatigue lives are lower than 6000-12000 cycles to failure, no lifetime improvement by deep rolling occurs. The effectiveness of deep rolling as a means to enhance fatigue life for given stress amplitudes and test temperatures can be summarized in Fig. 7. From the s-n-curves a boundary line in a stress amplitude-temperature-plot can be derived, below which deep rolling leads to fatigue life improvement and above which the thermomechanical instability of nearsurface microstructures renders the deep rolling treatment ineffective.
0 2,00 1,90 -100
unloaded
-150
after Nf/2 cycles, stress amplitude 205MPa, at 20°C after Nf/2 cycles, stress amplitude 205MPa, at 150°C after Nf/2 cycles, stress amplitude 240MPa, at 20°C
-200 -250 -300 0,00
FWHM [°]
Residual stress [MPa]
-50
1,80 1,70 1,60 1,50 1,40
0,10
0,20
0,30
0,40
distance from surface (mm)
0,50
0,00
0,10
0,20
0,30
0,40
0,50
distance from surface [mm]
Fig. 3: Residual stress- and FWHM-depth-profiles of deep rolled AlMg4.5Mn after isothermal fatigue to half the number of cycles to failure (stress amplitude σa = 205 MPa)
440
Residual Stresses VII, ICRS7
plastic strain amplitude (°/°°)
4 polished deep rolled
3,5 3
σa = 205 MPa
2,5 2 1,5 1 0,5 0 1
10
100
1000
10000
100000
number of cycles
Fig. 4: Cyclic deformation curves of polished and deep rolled specimens (T = 20°C, σa = 205 MPa)
plastic strain amplitude (o/oo)
4
σa = 220 MPa T = 150°C
3
2 deep rolled 1 polished 0 1
10
100
1000
10000
100000
number of cycles
Fig. 5: Cyclic deformation curves of polished and deep rolled specimens (T = 150°C, σa = 220 MPa) 260
stress amplitude σ a (MPa)
240 220 200 180 160
polished specimens, 20°C polished specimens, 150°C deep rolled specimens, 20°C deep rolled specimens, 150°C
140 120 100 1,0E+03
1,0E+04
1,0E+05
1,0E+06
number of cycles to failure Nf
Fig. 6: Non-statistically evaluated s/n-curves of polished and deep rolled specimens for test temperatures of 20°C and 150°C
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stress amplitude (MPa)
250
deep rolling ineffective
235 220 205
fatigue life enhancement through deep rolling
190 175 160 145
20
60
100
140
180
220
260
300
temperature (°C)
Fig. 7: Mechanical and thermal conditions under which deep rolling improves fatigue life 4. Conclusions The effectiveness of deep rolling for fatigue life enhancement is influenced by the cyclic and thermal stability of near-surface work hardening rather than macroscopic compressive residual stresses. Since near-surface work hardening inhibits or retards crack initiation, deep rolling enhances also the fatigue behavior in temperature- and stress-regimes where macroscopic compressive residual stresses have vanished almost completely, but where nearsurface work hardening remains stable. At low and medium stress amplitudes and a given temperature deep rolling led to a reduction of the plastic strain amplitude as compared to the untreated state. This reduction of the plastic strain amplitude is caused by cyclically stable near-surface work hardening and is associated with enhanced fatigue lives according to the Coffin-Manson-law. At high stress amplitudes and temperatures, deep rolling does not improve the fatigue behavior of AlMg4.5Mn. It is assumed that this is a consequence of instable near-surface microstructures, especially instable near-surface work hardening. References [1] J.A. van der Hoeven, L. Zhuang, B. Ijmuiden, B. Schepers, P. de Smet and J.P. Baekelandt, Aluminium, Vol. 78, Giesel Verlag GmbH (2002). [2] Advances in Surface Treatments (ed. A. Niku-Lari), Pergamon Press, Oxford, 1987. [3] B. Scholtes, In: Structural and Residual Stress Analysis by Nondestructive Methods (ed. V. Hauk), Elsevier, Amsterdam, 1997, p. 590 [4] H. Holzapfel, V. Schulze, O. Voehringer, Mater. Sci. Eng. A Vol. 248 (1998), p. 9 [5] I. Altenberger, E.A. Stach, G. Liu, R.K. Nalla, R.O. Ritchie, Scripta Mater. Vol. 48 (2003), p. 1593 [6] U. Noster, I. Altenberger, B. Scholtes, In: Magnesium Alloys and their Applications (Ed. K. U. Kainer), Wiley-VCH, Weinheim (2000), p. 274 [7] R.K. Nalla, I. Altenberger, U. Noster, G.Y. Liu, B. Scholtes, R.O. Ritchie, Mater. Sci. Eng. A Vol. 355 (2003), p. 216 [8] I. Altenberger, R.K. Nalla, U. Noster, G. Liu, R.O. Ritchie, B. Scholtes, Mat. wiss. u. Werkstofftech. Vol. 34 (2003), p. 529 [9] P. Juijerm et al., Mater. Sci. Eng., 2004, in print. [10] U. Martin. I. Altenberger, B. Scholtes, K. Kremmer, H. Oettel, Mater. Sci. Eng. A Vol. 246 (1998), p. 69 [11] I. Altenberger, B. Scholtes, Mater. Sci. Forum Vol. 337-349 (2000), p. 382
