Materials Science Forum ISSN: 1662-9752, Vol. 889, pp 143-147 doi:10.4028/www.scientific.net/MSF.889.143 © 2017 Trans Tech Publications, Switzerland Submitted: 2016-10-31 Accepted: 2016-11-03 Online: 2017-03-20 Investigation on Mode I Propagation Behavior of Fatigue Crack in Precipitation-Hardened Aluminum Alloy with Different Mg Content S.F. Anis1,2, a, M. Koyama2,b and H. Noguchi2,c * 1 Department of Mechanical Engineering, University of Technology, Malaysia, Jalan Sultan Yahya Petra, 54100 Kuala Lumpur, Malaysia 2 Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan a firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Keywords: Al6061-T6 alloy, Rotating bending fatigue test, Fatigue striation, Mode I crack, Dynamic strain aging. Abstract. The influence of excess Mg on the Mode I propagation of fatigue crack was examined in newly developed precipitation-hardened Al alloy containing Zr and excess Mg. The aim of this study was to evaluate the underlying factor affecting fatigue crack growth rate in the stage II region. For this purpose, the rotating bending fatigue tests were performed in constant amplitude loading, and replication technique with an optical microscope was used to measure the crack growth in the Al alloys. Through analyses of the crack propagation on the specimen surface and striation formation of the fracture surface, the effects of excess Mg in the Al alloys were clarified to promote the occurrence of mode I fatigue crack, and decelerate the fatigue crack propagation. These facts suggest that the dynamic strain aging of Mg induces the formation of fatigue striation and reduce the driving force of the crack propagation. The findings were supported by the fractographic observations in the fatigue crack propagation region. Introduction Aluminum alloy 6061 is commonly used in engineering application such as automotive parts and aircraft structure because of their excellent corrosion resistance, light weight, and good weldability. However, there are still undesired characteristics of this alloy such as no fatigue limit, less fatigue resistance and a large scatter in fatigue life  compared to steel . Therefore, fatigue behavior of this alloy has been subjected to numerous studies to improve fatigue material properties  . For instance, several researchers   have developed a new 6061-based alloy with an additional content of Mg, and they have successfully confirmed the ability to generate fatigue limit in facecentered cubic (FCC) metals via fatigue test and coaxing effect test. However, the effect of excess Mg on fatigue crack propagation behavior in the Al6061-T6 alloy has not been investigated sufficiently. Generally, the fatigue crack growth can be divided into two stages; stage I and stage II . The cracks begin to grow across several grains in the stage I are strongly affected by slip characteristics and material surface condition such as microstructure and surface roughness. Meanwhile, in stage II, a crack tends to propagate perpendicular to the loading direction, and the fatigue crack growth rate (FCGR) depends on the crack growth resistance of the material. An important characteristic of this stage is the presence of fatigue striation formation that represents Mode I fatigue crack growth . The aim of this study was to investigate the influence of excess Mg on fatigue crack propagation behavior in the stage II region. The investigation has not limited to the observation of fatigue crack growth on the specimen surface, but also included the fractographic examination of the fracture surface. The implications of the results for the fatigue crack propagation and the striation formation were then discussed. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.scientific.net. (#103369317, University of Auckland, Auckland, New Zealand-12/11/17,10:33:01) 144 Engineering and Innovative Materials V Material characterization The present study deals with two types of newly developed precipitation-hardened Al6061-T6 alloys with an additional element of Zirconium (Zr) in both Al alloys and the presence of excess Magnesium (Mg) in the latter, in which we denote as Al6061-Zr and Al6061-Zr-Mg. The addition of Zr to Al alloys has exhibited positive effects on microstructure via grain refinement and improvement in the mechanical properties . On the other side, the influence of Mg in Al alloys has been investigated to promote dynamic strain aging (DSA)   and to assist nonpropagation of a crack in the fatigue limit regime  . Details about the chemical composition for both Al alloys are listed in Table I. The manufacturing processes for both Al alloys were as follows. The billets of these alloys with a diameter about 155 mm were obtained by a semi-continuous casting method and subjected to a homogenization treatment at 823 K for 14,400 s. The billets were extruded to form the round bars with a diameter of 23 mm at 773 K. The extruded bars were then solid solution treated at 813 K for 3600 s in an air furnace and immediately water-quenched. Subsequently, T6 aging was carried out at 463 K for 14,400 s. Table 1. Chemical composition of the precipitation-hardened Al alloys (wt%) Element Si Fe Cu Mn Mg Cr Ti Al Zr Excess Mg Al6061-Zr alloy 0.55 0.21 0.24 0.09 0.9 0.26 0.02 bal. 0.16 Al6061-Zr-Mg alloy 0.55 0.2 0.23 0.09 1.39 0.26 0.02 bal. 0.14 0.49 Experimental Procedure Smooth specimens of the alloys were used for rotating bending fatigue test with the geometry shown in Fig. 1(a). All specimens were mechanically polished using fine emery papers with the grit number up to 3000 at the central of the specimens to remove scratches on the surface, and subsequently electro-polished at 30 V and 323 K in a solution of 40 g of gelatin, 40 g of oxalic acid dehydrate and 2000 ml of phosphoric acid with a concentration of 85% to eliminate the work hardened layer. Fatigue tests were carried out by using Ono-type rotating bending fatigue test machine in room temperature at the stress ratio R was equal to -1 and frequency f of 60 Hz (sinusoidal waveform). The replica technique with a microscope was used to observe the crack growth on the specimen surface. A schematic illustration of crack length measurement is depicted in Fig. 1(b). The fractographic observation of the fracture surfaces was performed by a scanning electron microscope (SEM). To investigate the behavior of the Mode I crack, the area of fatigue striation was measured at seven different regions, located at about 500, 1000, 1500, 2000, 2500, 3000 and 3500 µm from the crack initiation site, as shown in Fig. 2(a). Fig. 2(b) shows the size of the examination area in each region. The striation area was calculated as striation ratio of the striation area to the observation area. (a) (b) Fig. 1. Schematic illustration: (a) rotating bending fatigue specimen, and (b) fatigue crack measurement. Materials Science Forum Vol. 889 145 Fig. 2. SEM images of the fatigue fracture surfaces: (a) seven regions of fatigue striation evaluation, and (b) size of observation area for each region. Results and discussion S-N curves. Fig. 3 compares the S-N diagrams of the two Al alloys. The fatigue tests were performed at four different stress amplitudes. It was found that the cracks were easily coalesced each other at high stress amplitude of 250 MPa due to the easiness of crack initiation. At finite life regime, even with additional content of Zr, the fatigue strength of Al6061-Zr-Mg alloy demonstrates similar result from the previous studies   with a distinct fatigue limit, which is absent in the Al6061-Zr alloy. This phenomenon was believed to be due to the addition of a strain aging capability that can be strengthening the material in the vicinity in front of the crack tip. This finding implies that the ability of Al alloy with additional Mg content resists against small crack growth. In contrast, for Al alloy without excess Mg, the fatigue failure occurred even in the range of fatigue limit (107 ~ 108). To minimize the tendency of occurrence crack coalescence, the investigation on Mode I fatigue crack growth behavior is focused on the stress amplitude of 200 MPa. Fatigue crack growth behavior. In both Al alloys, the fatigue crack was initiated at defect on the specimen surface due to high local stress concentration, and Fig. 4 compares the fatigue crack growth curves of the main cracks at stress amplitude of 200 MPa. Generally, fatigue crack in the Al6061-Zr-Mg alloy was initiated earlier than that in the Al6061-Zr alloy at the beginning of initiation life. The easiness of the crack initiation in the Al6061-Zr-Mg alloy is attributed to the existence of DSA in this alloy. It is known that strain localization associated with DSA can assist fatigue crack initiation . However, the fatigue crack in Al6061-Zr-Mg alloy propagates more slowly with increasing crack length than that in the Al6061-Zr alloy. It can be seen clearly after 2 105 cycles. This implies the different behavior of fatigue crack propagation in both Al alloys, and might be influenced by the Mode I crack. To investigate these behaviors in more detail, we observed the fracture surface by using SEM. Influence of excess Mg on mode I propagation behavior. By fractographic observations, the Mode I fatigue crack propagation could be examined based on the formation of fatigue striation on the fracture surface. To examine the different behavior of Mode I crack, the striation ratios were analyzed to determine the influence of excess Mg in generating Mode I crack in the stage II region. It found that Al6061-Zr-Mg alloy is easier to generate Mode I crack with a larger fatigue striation area compared to Al6061-Zr alloy, as shown in Table II. The retardation of the FCGR in Al6061Zr-Mg alloy might be due to the high tendency in generating striation formation. This phenomenon possibly facilitates crack closure mechanism , which induces contact between mating fracture surfaces during the loading cycle, resulting in the reduction of the local driving force for the crack growth and deceleration of the FCGR. 146 Engineering and Innovative Materials V Fig. 3. S-N fatigue data of precipitationhardened Al alloys. Fig. 4. Crack length versus number of cycles at the stress amplitude of 200 MPa. Table 2. Effect of excess Mg on striation formation in Al alloys. Distance from crack Striation ratio, AS/AO (%) initiation site (µm) Al6061-Zr alloy Al6061-Zr-Mg alloy 500 0.35 0.48 1000 0.66 1.32 1500 1.03 8.03 2000 6.30 17.03 2500 14.85 36.20 3000 34.40 60.95 3500 59.90 81.10 Conclusions In this study, the fatigue crack propagation in stage II region was examined experimentally by using smooth specimen. The influence of excess Mg on Mode I fatigue crack propagation has been highlighted. Based on experimental results, the main conclusions obtained in this study can be drawn as follows: (1) The fatigue crack in Al6061-Zr-Mg alloy was initiated earlier and propagate slower in stage II region than that in the Al6061-Zr alloy. (2) The fatigue striation examination confirmed the enhancement of the Mode I crack propagation in the Al alloy with additional Mg content, and it may be attributed to the DSA effect in this alloy. (3) The retardation of FCGR in Al6061-Zr-Mg alloy was believed to be due to the high tendency in generating striation formation, which possibly facilitates crack closure mechanism. In addition, it is speculated that the DSA of Mg has a significant influence on FCG behavior in Al6061-T6 alloy through restricting the motion of the dislocation, which requires more force to cause the dislocations to facilitate slip along the crystal planes. Further research would be continued to examine in detail the effect of DSA on Mode I fatigue crack growth. An understanding of the crack growth behavior of Al6061-T6 alloys with excess Mg is necessary because the scatter of FCGR might be influenced by the DSA phenomenon. These studies will be addressed in the phases of future research. 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