Effect of Solution Treatment on Fatigue Crack
Propagation Behaviour of Magnesium Alloy
M. A. M. Daud
1, Z. Sajuri
2, J. Syarif
3, M. Z. Omar
4Abstract - An investigation on the effect of solution treatment on fatigue crack propagation (FCP) behaviour of AZ61 magnesium alloy was carried out. A centre cracked plate tension (CCT) specimen was prepared from an extruded cylindrical AZ61 magnesium alloy rod. The solution treatment was performed at 400oC for one hour to get homogeneous solid solution before quench
in water. The FCP test was conducted in a laboratory air environment under a constant amplitude sinusoidal loading with a stress ratio of 0.1 and a frequency of 10 Hz. The FCP curve for solution treated samples was then compared to that of the extruded AZ61 magnesium alloy. Results showed that solution treatment shifted the FCP curve to the left and demonstrated a lower fatigue crack propagation resistance at the high stress intensity region. The threshold value was recorded at 0.91 MPa√m.
Keywords:
Fatigue crack propagation, solution treatment, threshold value, magnesium alloy
σN nominal stress
σys yield stress
σuts ultimate tensile strength
Pmax maximum load
B specimen thickness
W width ofgauge position
a crack length Hv Vickers hardness
R stress ratio
oC degree Celsius K stress intensity factor
ΔK stress intensity factor range
ΔKth threshold stress intensity factor range
F(α) geometrical factor
da/dN fatigue crack propagation rate
C constant
m slope of the curve
I. Introduction
Magnesium alloys are being widely utilized especially, in transportation and aerospace industry due to their lightweight and with high specific strength [1]-[3]. For many years, magnesium alloys have been attractive to engineer due to their low density (1.74 g/cm3) compared to counterparts such as aluminium and
titanium. Nowadays, magnesium alloys are mostly used for static parts such as cases, housings, brackets, panels, etc., but these materials also indicate that they have a potential to be used as load-bearing components, e.g. wheels, which will be subjected to fatigue loading [4], [5]. In order to use magnesium alloys as a high strength structural component, especially in automotive,
aerospace and other transportations industries, it is very important to make their fatigue characteristics clear and understand the fatigue crack propagation (FCP) mechanism. Wrought magnesium alloy such as the extruded AZ61 have good mechanical properties and widely utilized especially for chassis in car and other strategic applications. Until now, there has been growing interest in studies on fatigue behaviour of magnesium alloy. These include crack growth behaviour in die cast AZ91D [6], low cycle fatigue behaviour of die cast AZ91E-T6 [7], fatigue crack growth of rolled plate AZ31 [8], fatigue of calibre rolled AZ91D [9], fatigue of extruded AZ91D [10], fatigue behaviour of die cast AZ91, AM60B and AZ91E-T4 in very high cycle regime [11]-[13]. Hilpert and Wagner examined the fatigue performance of extruded AZ80 in ambient air and NaCl [14]. It was found that there was no pronounced effect of NaCl solution on fatigue life at higher stress but fatigue life was considerably reduced at stress below 125 MPa. Shih et al. studied the fatigue life of AZ61A and reported that cracks initiate from subsurface or surface inclusions [15]. There is no clear information or finding as to whether heat treatment (aging) accelerates the FCP rate of magnesium alloy or does not. Bag and Zhou pointed out that the aging treatment reduces the FCG rate of as-cast AZ91 alloys mostly, due to the larger deviation and branching of the crack from the plane of maximum stress caused by the inhomogeneous microstructure [16]. However, in contrast, Kobayashi et al. proposed that the precipitates have a negative effect on the FCP rate of AZ91 [17].
the influence of solution treatment on the FCP rate of AZ61.
II. Experimental Procedure
The material used in this study was AZ61 magnesium alloy. The chemical composition of the material used is shown in Table 1. AZ61 magnesium alloy has about 6% of aluminium and 1% of zinc as its major alloying elements.
Prior to the investigation on effect of solution treatment on fatigue crack propagation behaviour of AZ61 magnesium alloy, series of heat treatment processes were performed to identify the effect of each process on the mechanical property, i.e. hardness of the alloy after treatment.
Solution treatment was performed at 400oC for 60
minutes followed by quenching in water. To investigate the effect of aging temperature, samples were heated to 100, 150, 200, 250oC for 30 minutes before quenched
in water. The hardness of these samples were then measured and compared to the hardness of solution treated sample. It was found that the aging treated samples showed significantly lower hardness compared to that of solution treated sample, which exhibited hardness value of Hv 71 as shown in Fig.1 (a). The increase in aging temperature improved the hardness but it was then saturated at the temperature above 200oC at Hv 60. Based on these results, further detail
aging treatment was performed to investigate the effect of aging time on the hardness of AZ61 magnesium alloy. Aging treatment was applied on solution heat-treated samples at different time intervals of 30, 60, 90, 120 and 150 minutes at aging temperature of 200oC.
Figure 1(b) shows the result of effect of aging time on hardness. From the result, it shows that AZ61 magnesium reached the peak aging after only 60 minute of aging time. The hardness at the peak aged was Hv 61.
From the above initial test results, solution treated sample demonstrated the best mechanical property in hardness compared to that of aged samples. Based on these results, it is worth to investigate the effect of solution heat treatment on fatigue behaviour of AZ61 magnesium alloy before proceeding detail investigation on the effect of aging treatment of the investigated
TABLE 1
THE CHEMICAL COMPOSITIONS OF AZ61 MAGNESIUM ALLOY (WT.%)
Al Zn Fe Si Mn Mg
6.53 0.96 0.002 0.024 0.164 Bal.
alloy. Moreover, similar study on the effect of aging treatment on fatigue behaviour in extruded AZ61 and
AZ80 magnesium alloys have been performed by Uematsu et al. [18].
To investigate the effect of heat treatment on fatigue crack propagation rate of the AZ61 magnesium alloy; all samples were firstly heated in furnace to 400oC and
held for one hour before quenching them in water. Time and temperature range used in this study were based on the ASTM Standard [19]. Figure 2 shows the microstructure of the material used. The average grain size was about 20 μm.
(a) Effect of aging temperature at aging time of 30 minutes
(b) Effect of aging time at 200oC Fig. 1. Vickers hardness as a function of aging treatment
(a) As-extruded (b) Solution treated
The specimen used for fatigue crack propagation rate test was centre cracked-plate tension (CCT) specimen. Figure 3 shows the geometry of the specimen according to ASTM E647-08 standard [20]. The dimension of the specimen was determined by following equation according to the test standard:
ys
Pmax is the maximum load, B is the specimen thickness,
W is the width of gauge position and a is the crack length. A screw type fixture was used in the CCT specimen. To avoid the excessive lateral deflection or buckling of the CCT specimen during the test, the gauge length and thickness of gage position was limited to 12 mm and 2 mm, respectively. The gage position was then polished with 500 to 1500 grit emery papers to obtain a smooth surface.
The fatigue crack propagation rate test was conducted by using a pneumatic fatigue testing machine (14 kN maximum capacity) and to investigate the effect of heat treatment on fatigue crack propagation behaviour. The tests were performed at frequency of 10 Hz by using sinusoidal loading form. A stress ratio R = 0.1 was applied in the tests. The loading direction was in the extrusion direction of the material and the testing was carried out at room temperature.
The crack propagation curve (crack propagation rate da/dN versus stress intensity factor range ΔK) was obtained by using K-decreasing and K-increasing test procedures. The decreasing and increasing load steps are 5 - 7% of the previous loading value. The stress intensity factor value for the CCT specimen was calculated using the following equation:
Fig. 3. Centre cracked-plate tension (CCT) specimen used in the FCP tests
a F
K (2)
Here, F(α) is a boundary correction factor which depends on the ratio of the crack length a to the width of the specimen W. For the CCT test specimen used in this study, the boundary correction factor is given as [19], microscope. The threshold stress intensity factor ΔKth
was determined when a crack growth is not observed for 106 cycles. A hole with a 1 mm diameter was drilled
in the centre of the specimen before introducing a 1.35 mm notch by EDM (electrical-discharge machining) to facilitate fatigue pre-cracking.
The procedure for introducing a pre-crack was followed the ASTM standard [20]. The specimen was aligned so that the load distribution is symmetrical. The load ratio R during pre-cracking is the same as the load ratio used in the fatigue crack propagation test. The pre-cracking was interrupted after a pre-crack length equal to 0.1 of specimen thickness was attained at maintained pre-cracking propagation rates of about 10-8
m/cycle.
III. Result and Discussion
The comparison of fatigue strength of solution treated and extruded AZ61 magnesium alloy is shown in Figure 4. The figure shows that fatigue strength of the solution treated samples increase as that compared to the fatigue strength of the as-extruded AZ61 samples. The fatigue limit for solution treated and as-extruded AZ61 were 180 MPa and 150 MPa, respectively. The higher fatigue strength observed for
103 104 105 106 107 108
Number of cycles to failure, Nf (cycles)
M
Fig. 4. Fatigue strengths of solution treated and as-extruded samples
(a)
(a)
TABLE 2
MECHANICAL PROPERTIES OF AZ61 MAGNESIUM ALLOY
Material
the solution treated sample is believed due to higher tensile strength and also higher hardness properties compared to that of the as-extruded sample as shown in Table 2.
After the solution treatment the increment in hardness from Hv 67 to Hv 71 is believed due to the solid solution strengthening. In the heat treatment process, the solution treated samples were heated into the solid solution zone where atoms of alloying elements dissolved into the matrix. In this condition, the samples were quenched in water, which limit the time for precipitation to takes place.
Optical micrographs revealed that there is no precipitation of second phase observed in the solution treated sample. Further, the aging processes performed
Fatigue crack
at different aging times and temperatures were unable to achieve higher hardness compared to that of solution treated sample due to limitation of second phase precipitation. This result was in aligned with the results obtained by Uematsu et al. who reported that precipitation of Mg17Al12 in AZ61 magnesium alloy is
very limited due to low percentage of Al content as compared to other magnesium alloy with higher Al higher stress concentration sites such as inclusions or foreign particles at sub-surface. For the as-extruded samples, the extrusion and intrusion play a prominent role in initiating fatigue crack. Therefore, the increased in hardness and yield strength in solution treated samples delay the fatigue crack initiation and result in a higher fatigue life. The initiation and propagation mechanisms of fatigue crack in both samples are illustrated in Fig. 5.
Detailed fracture surface observations showed that foreign particle was found at the fatigue fracture origin of the solution treated samples especially for the samples which exhibited higher fatigue life more than 105 cycles as shown in Fig. 6(a). The foreign particle
size observed was about 20 to 30µm. Pile-up of slips deformation at near the foreign particle during fatigue cycles contributed to high stress concentration at around the foreign particle and resulted in fatigue crack initiation. In contrast, SEM observation results on fracture surface of as-extruded samples showed that there was no evidence of foreign particle at the fatigue fracture origin. The fatigue crack initiation site was relatively flat as shown in Fig. 6(b).
The FCP curve of solution treated AZ61 magnesium alloy as a function of stressintensityfactor range (ΔK) at room temperature is shown in Fig. 7. The FCP curve for as-extruded AZ61 magnesium alloy is also plotted in the same figure for comparison [8]. From the figure, it can be noted that there is not much difference in the FCP resistance for both curves at a low ΔKregion. However the difference of fatigue crack propagation resistance can be seen at a higher ΔK
region above 2.0 MPa√m. Fatigue crack propagation resistance for solution treated samples is found lower as compared to the as-extruded samples. It can be considered that the crack in as-extruded AZ61 has more frequent chances of encounter with grain boundaries due to the smaller grain size, resulting in a slower propagation rate. This argument is similar to the FCP behaviour of AZ31B-L as mentioned by Uematsu et al. [22].
The arrows in the figure indicate that the threshold value of stress intensity factor range at ΔKth. From the
figure, the threshold value for solution treated AZ61 magnesium alloy is at 0.91 MPa√m. This value is almost at par to that of the extruded magnesium alloy where the threshold value is at 0.92 MPa√m. From the result, it can be concluded that heat treatment does not affect the threshold value of AZ61 magnesium alloy. However, a slight difference in the fatigue crack propagation resistance is been demonstrated at higher ΔK region. Similar FCP behaviour of the AZ91D magnesium alloy was also reported by Kobayashi et al. [17].
The da/dN-ΔK curves obtained at the Paris regime can be expressed as: calculated using the least square method and the results are shown in Table 3.
Fig. 6. Observations were done using SEM equipped EDX. (a) Foreign particle (b) Flat surface
Stress intensity factor range, Δ K (MPa.m1/2)
Fa
Fig. 7. Fatigue crack propagation behavior of solution treated and as-extruded AZ61 magnesium alloy
a b
TABLE 3
CRACK PROPAGATION PARAMETER AT THE PARIS REGIME FOR AZ61 MAGNESIUM ALLOY
(a) Overview
(b) Micrograph of fracture surface at ‘a’ in (a) at low ΔK region
(c) Micrograph of fracture surface at ‘b’ in (a) at high ΔK region
Fig. 8. Fracture surface observations of FCP test for solution treated specimen.
IV. Observation of Fracture Surface
Figure 8 shows the fracture surface of fatigue crack propagation test specimens tested at R = 0.1 for solution treated AZ61 magnesium alloy. Figures 8(a), (b) and (c) show the macroscopic and microscopic fractograph at the low ΔK and at high ΔK regions, respectively. The fracture surface appearance of the entire fracture surface was predominantly a featureless
slip fracture. The directions of the slips patterns are almost the same as the crack propagation directions. Some faceted surface of cleavage fracture was appears together with the featureless slip fracture as shown in Fig. 8 (a). The striations-like slip markings can be seen from the fracture surface as shown in Fig. 8 (b).
V. Conclusion
Experimental investigation on the effect of solution treatment to the fatigue crack propagation behaviour of AZ61 magnesium alloy was carried out. Based on the results the following conclusions are made:
1. Solution treated sample demonstrated superior mechanical properties and higher fatigue limit as compared to as-extruded AZ61 magnesium alloy.
2. Foreign particle in sub-surface served as the fatigue fracture origin for solution treated samples especially at high fatigue life region.
3. The solution treatment shifted the FCP curve at high
ΔK region to the left and demonstrated lower fatigue crack propagation resistance as compared to as-extruded sample which has more frequent chances of encounter with grain boundaries due to the smaller grain size, resulting in a slower propagation rate.
4. The threshold stress intensity factor range ΔKth for
solution treated and as-extruded are almost identical at the value of 0.91 MPa√m.
Acknowledgement
The author would like to thank Prof. Dr. Y. Mutoh of Nagaoka University of Technology, Japan for providing the extruded AZ61 magnesium alloy and the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for sponsoring this project under the Science Fund Research Grant (03-01-02-SF0048).
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Authors’ information
1,2,3,4Dept. of Mechanical and Materials Engineering, Faculty of
Engineering & Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia.
1Mohd Ahadlin Mohd Daud is a PhD student in Department of
Mechanical & Materials Engineering, Universiti Kebangsaan Malaysia. He received his Master Degree (Mechanical
Department of Mechanical &
Materials Engineering, Universiti
Kebangsaan Malaysia. He received his
PhD (Material Science) from Nagaoka
University, Japan in 2005. He has
taught several courses throughout the
undergraduate and postgraduate
curriculum. His research
interest includes fatigue and fracture
mechanics, advanced materials and powder metallurgy
3Dr Syarif Junaidi is a senior lecturer at the Department of
Mechanical & Materials Engineering, Universiti Kebangsaan Malaysia. He received his PhD (Material Science) from Kyushu University, Japan in 2003. He has taught several courses throughout the
undergraduate and postgraduate
curriculum. His research interest includes materials processing, advanced materials and powder metallurgy.
4Dr Zaidi Omar is an Associate Professor
at the Department of Mechanical & Materials Engineering, Universiti Kebangsaan Malaysia. He received his semisolid processing, advanced materials and metal matrix composite.
