Fatigue behaviour of commercially pure aluminium processed by rotary swaging
Mustafa A. Abdulstaar, Mansour Mhaede, Manfred Wollmann & Lothar Wagner Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN 0022-2461 Volume 49 Number 3 J Mater Sci (2014) 49:1138-1143 DOI 10.1007/s10853-013-7792-9
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Author's personal copy J Mater Sci (2014) 49:1138–1143 DOI 10.1007/s10853-013-7792-9
Fatigue behaviour of commercially pure aluminium processed by rotary swaging Mustafa A. Abdulstaar • Mansour Mhaede Manfred Wollmann • Lothar Wagner
•
Received: 14 July 2013 / Accepted: 1 October 2013 / Published online: 12 October 2013 Ó Springer Science+Business Media New York 2013
Abstract Fatigue strength of ultrafine-grained commercially pure aluminium (Al 1050) produced by severe plastic deformation (rotary swaging) was investigated. Fatigue tests were carried out on smooth and notched specimens. Results show improved static and fatigue strength of the rotary swaging processed material. However, the processed material was highly notch sensitive due to low work hardening capability, low ductility as well as low uniform strain. It was found that post-deformation annealing above recrystallization temperature can additionally enhance the work hardening capability and the ductility of the swaged material, which led to marked reduction in fatigue notch sensitivity. At the same time, this reduction is accompanied with pronounced loss in strength. Fatigue notch sensitivity of commercially pure aluminium can be good correlated to the work hardening capability and ductility. This behaviour was discussed in details based on microstructure and mechanical properties study.
Introduction The microstructure of metals can be significantly changed by subjecting the material to severe plastic deformation (SPD) processes such as rotary swaging (RS) [1, 2], equal channel angular pressing (ECAP) [3–5], high pressure M. A. Abdulstaar (&) M. Mhaede M. Wollmann L. Wagner Institute of Material Science and Engineering, Clausthal University of Technology, Agricolastr. 6, 38678 Clausthal Zellerfeld, Germany e-mail:
[email protected] M. Mhaede Faculty of Engineering, Zagazig University, Zagazig, Egypt
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torsion (HPT) [6, 7] and friction stir processing [8–10], or accumulative roll-bonding (ARB) [11, 12]. These processes belong to the top down approach and are capable of leading to substantial grain refinement so that the grains can be reduced to the sub-micrometre or even the nanometre range. UFG Metals and alloys are expected to have high strength as a result of structural refinement, according to the Hall–Petch relationship, where the yield stress varies with the reciprocal of the square root of the grain size. On the other hand, UFG materials have limited ductility due to susceptibility to deformation localization. One approach to increase the ductility and at the same time preserve high strength of UFG materials is the formation of a structure with bimodal grain distribution in which nanocrystalline grains provide strength and larger grains facilitate a high ductility during deformation [13], another approach is based on the precipitation of a second-phase in the metallic matrix during deformation or annealing treatment which modifies the slip band distribution during deformation and thereby increase ductility [14]. It is relevant to have information also on the fatigue notch sensitivity, because the fatigue performance of notched bodies might be very poor even if the fatigue performance of smooth bodies is good. The stress concentration at a notch is usually described by the geometrical notch factor Kt = rmax/rnom which mainly depends on the notch root radius, the notch depth and the mode of loading. Experiments have shown that the HCF strength of a notched specimen not only depends on Kt, but also on materials properties such as microstructure and mechanical properties. Therefore, a fatigue notch factor Kf was introduced which relates the smooth HCF strength for a given number of cycles (e.g. 107) to the notched one: (Kf = ra107smooth/ra107notched). Accordingly, the fatigue notch sensitivity q can be expressed as [15]:
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q ¼ ðKf 1Þ = ðKt 1Þ
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ð1Þ
Two limiting cases can occur: q = 0 and q = 1. The first case is given if ra107smooth = ra107notched, the latter if Kf = Kt. Taking the difference between tensile strength and yield strength (UTS-YS) as a coarse measure for work-hardening capability, it was reported [16] that this difference affects the fatigue notch sensitivity significantly. The higher the difference between tensile strength and yield strength (work-hardening capability), the lower is the fatigue notch sensitivity and vice versa. It was found that AISI steel 304 was low notch sensitive (q = 0.32), while Beta C Titanium was almost fully notch sensitive (q = 0.9). The workhardening capability (UTS-YS) of AISI 304 and Ti-Beta C was 390 and 10 MPa, whereas the tensile elongation was 0.8 and 0.2, respectively. Ultrafine-grained materials proceed by SPD usually have higher fatigue notch sensitivity than their coarse-grained counterparts, this was reported for UFG copper [17] and UFG Ti [18]. Pervious study on the fatigue and fatigue notch sensitivity of UFG Ti [19] showed that post deformation annealing could enhance the materials ductility and lower fatigue notch sensitivity. Moreover, the notch sensitivity of the UFG Ti was greater than that of coarse-grained CP-Ti. It is generally accepted that high strength materials with low ductility are more notch sensitive than low strength materials with high ductility. In the present study, rotating bending fatigue of ultrafine-grained aluminium 1050 processed by RS was conducted on smooth and notched samples. Furthermore, annealing above recrystallization temperature of RS processed materials was applied to generate fine and coarsegrained materials. Fatigue performance of various grain sizes is compared and contrasted.
Table 1 Chemical composition of Al 1050 (wt%) Fe
Si
Mn
Mg
Ti
Zn
V
Al
0.135
0.095
0.055
0.025
0.015
0.013
0.010
Bal.
Table 2 Details of thermo-mechanical treatments and grain sizes of Al 1050 Condition
Thermo-mechanical treatment
Grain size (lm)
Ultra fine grain (UFG)
Swaging
0.75
Fine grain (FG)
Swaging ? 500 °C for 2 h
35
Coarse grain (CG)
Swaging ? 600 °C for 2 h
450
and diameter of 25 and 5 mm, respectively. Optical microscopy (OM) was used to document the microstructure after electrolytical etching by Barker’s reagent. Fatigue tests were performed in rotating bending (R = -1). Tests were performed on hour-glass-shaped (smooth) or circumferentially notched (2.0 B Kt B 2.5) specimens. This range of Kt values related to the difference in the samples minimum diameter and notch depths obtained after machining. These differences become more significant after electropolishing of the samples, as the removed surface layers are not almost constant in all samples. Test frequency was about 50 Hz. The surface of fatigue specimens was ground and electrolytically polished. Three conditions were investigated in this study, thermo-mechanical treatments and grain sizes are summarized in Table 2.
Results and discussion Microstructure
Experimental procedure The investigation was performed on commercially pure (CP) Al 1050. The material was received as hot extruded bar with chemical composition as given in Table 1. Cylindrical samples were machined out from the extruded bar parallel to the extrusion direction. The samples were then swaged using RS machine to obtain ultrafine-grained microstructure. After RS a microstructure with a grain size of 0.75 lm was obtained, as reported in [2]. The swaged material was further annealed for 2 h at 500 and 600 °C to generate a microstructure with an average grain size of 35 and 450 lm, respectively. In addition to Vickers hardness measurements, tensile tests (initial strain rates 6.7 9 10-4 s-1) were performed using threaded cylindrical specimens having gage lengths
The grain morphology of ultrafine-grained aluminium observed by means of EBSD is shown in Fig. 1. The deformation degree (true strain calculated from cross sectional area reduction) is of about 3. Taking the critical misorientation as 2°, the average grain size was found to be 0.75 lm. The structure is characterized with cell structure of low angle grain boundaries (LAGBs) within bigger grains. In the plane parallel to swaging direction, the grains exhibit elongated shape; more details were reported in a previous work [2]. It has been reported that the formation of non-equilibrium grain boundaries with high angle misorientations facilitate intergranular sliding during plastic deformation which would enhance the material’s ductility [13, 20]. The microstructure of fine and coarse-grained conditions is shown in Fig. 2a, b, respectively. The
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Fig. 1 Color coded orientation map for the plane normal to the swaging direction in (a) and parallel to the swaging direction in (b)
Fig. 2 Microstructure of the fine-grained in (a) and coarsegrained condition in (b)
Table 3 Tensile and hardness properties of Al 1050 in various conditions Condition
YS (MPa)
UTS (MPa)
Ultra fine grain (UFG)
158
163
Fine grain (FG)
35
Coarse grain (CG)
20
UTS - YS (MPa)
Elongation (%)
HV 10
5
11.6
49
68
33
37.3
21.6
72
52
43.1
20
Mechanical properties
Fig. 3 Engineering stress–strain curve of different conditions
microstructures are generated by annealing the swaged materials above the recrystallization temperature to generate different grain sizes. There is a great difference between the two conditions in terms of grain size; in terms of mechanical properties they are much comparable.
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Figure 3 shows the engineering stress–strain response of the investigated materials under tensile loading. It is obvious that UFG material has superior strength in comparison to the fine grained (FG) and coarse-grained (CG) materials, at the same time, the ductility (tensile elongation percent to fracture) and uniform strain of UFG are much less. As seen, the work-hardening capability of FG and CG materials is markedly higher than that of UFG material.
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Fig. 4 S–N curves of smooth samples in rotating beam loading (R = -1)
The tensile and hardness values of the three conditions are listed in Table 3. It is clearly seen that FG and CG conditions have comparable hardness values. High cycle fatigue (HCF) The S–N curves of UFG, FG and CG materials in both smooth and notched conditions are depicted in Figs. 4 and 5. Figure 4 shows the fatigue performance of the smooth samples of FG, CG and UFG materials. It is clear that, the
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fatigue life of UFG material is significantly higher than that of FG and CG materials. The fatigue limit of UFG material is about 125 and 80 % higher than that of CG and FG materials; this may be attributed to high dislocation density accumulated during cold working. It was reported that the strength enhancement of UFG material is dependent mainly upon the dislocation density that built up inside the material during deformation [2]. Figure 5 represents the fatigue performance of CG, FG and UFG materials in both smooth and notched conditions. While the HCF performance of CG and FG materials is only slightly deteriorated by the notch (q = 9 and 28 %, respectively), UFG is much more affected (q = 83 %). The fatigue results of all tested materials together with the calculated Kf and q values are listed in Table 4. Figure 6 shows that the higher workhardening capability and ductility, the lower is the fatigue notch sensitivity. Pervious study on pure titanium [19] showed that the ductility can considerably influenced the fatigue notch sensitivity of ultrafine-grained material. The increase of the static strength can enhance the material resistance to crack nucleation, and at the same time, the fatigue life of samples is defined by their crack propagation resistance and can depend on the material ductility [21]. For CG and FG materials, the ratio of the fatigue strength to the yield strength (ra 107/YS) is C1 for both smooth and notched conditions (Table 4), while for UFG
Fig. 5 S–N curves of smooth and notched samples in rotating beam loading (R = -1) for UFG in (a), FG in (b) and CG condition in (c)
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Table 4 Fatigue performance of the various conditions ra 107 Smooth (MPa)
ra 107 Notched (MPa)
ra 107/YS Smooth
Notched
UFG
90
50
0.57
0.32
1.96
1.8
83
FG
50
35
1.43
1.0
2.52
1.42
28
CG
40
35
2.0
1.75
2.56
1.14
9
Condition
Kt
Kf
q (%)
Fatigue notch sensitivity, (%)
100 80 60 40 20 0 0
10
20
30
40
50
60
UTS-YS
(a)
(b)
Fig. 6 Fatigue notch sensitivity (q) against (UTS-YS) difference in (a) and tensile elongation in (b)
material it approximately equals 0.57 and 0.32 for smooth and notched samples, respectively. This low ratio of UFG material is attributed to its limited ductility in cyclic deformation, which promotes early crack initiation, and greater availability of grain boundaries in orientations favourable for crack propagation in a material with a finer grain structure. The fatigue life in HCF regime is controlled by crack nucleation in a smooth body, where crack propagation is often dominating in the LCF regime [22, 23]. Moreover, in HCF regime, the cyclic loading of UFG materials yield coarse grains at lower stresses, thereby providing a natural pathway for early strain localisation and premature failure either due to cracking at the interfaces between the coarse grains and the surrounding finegrain matrix or due to transgranular surface crack initiation in a coarse shear band [24]. The occurrence of large-scale shear bands associated with UFG processing is a crucial factor in the fatigue life. Fatigue cracks most often initiate at and propagate along these shear bands, which can extend over a large number of ultrafine grains or even propagate across a whole specimen [23]. It was reported that the crack growth rate in the nearthreshold region is higher in the UFG state than in the ordinary CG materials; however, the picture might be opposite for relatively high stress intensities. Compared to CG material, stable fatigue cracks in the UFG specimens propagate with much less out-of-plane deflections, providing faster fatigue crack growth rates [25]. It was found that the fatigue threshold (DKth) corresponding to the lowend limit of the curve decreased after ECAP of a non-heat
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treatable 5056 Al alloy [26] and 6061 Al alloy [27]. Additionally, it was reported that UFG AA 6063 produced by ECAP, show significantly lower fatigue threshold values (DKth) and higher crack growth rates (da/dN) than their coarse grained counterparts, limited work-hardening capability and ductility after SPD have unfavourable effects on fatigue crack growth properties [28]. In the current investigation, it has shown that enhancement of UFG work-hardening capability and ductility by annealing can considerably reduce the fatigue notch sensitivity. However, this reduction is accompanied by loss in fatigue strength. The necessity arises for other SPD methods that ensure suitable ductility for the material beside high strength, especially under stress concentrator condition. One method is the production of UFG material by friction stir processing, previous study [9] on pure Al 1050, showed that friction stir processed material has excellent strength and ductility, in addition the equiaxed grains in friction stir processed zone had very low dislocation density. Another study [29] on 6063-T6 aluminium alloy showed that friction stir processed material has low fatigue notch sensitivity than the base material. Therefore, work-hardening capability and ductility are essentials factors for designing components, where notches are encountered in service.
Conclusion The effects of RS as a SPD process and post-deformation heat treatments on the microstructure, mechanical properties
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and fatigue performance of Al 1050 has been investigated, the following conclusions could be drawn out: 1.
2.
3.
4.
UFG material of average grain size of 0.75 lm could be produced by RS; FG and CG with average grain size of material 35 and 450 lm, respectively, are produced by post-deformation annealing. UFG material shows enhanced static and fatigue strength, at the same time, the processed material has low tensile ductility, work-hardening capability and uniform strain. Work-hardening capability and ductility enhancement is achieved by post-deformation annealing, namely in FG and CG materials. However, annealing heat treatment results in drop in static and fatigue strength of UFG material. The UFG material was highly fatigue notch sensitive (q = 83 %). The calculated notch sensitivity factor of FG and CG materials was 28 and 9 %, respectively. Fatigue notch sensitivity in pure Al 1050 can be good correlated to the work-hardening capability and ductility, the higher work-hardening capability and ductility the lower fatigue notch sensitivity. This suggests that work-hardening-capability and ductility have beneficial effects on fatigue crack growth rate.
Acknowledgements The first author would like to thank the German academic exchange service (DAAD) for supporting his stay at TU Clausthal.
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