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 400
o
C for 60 minutes followed by quenching in water. To investigate
the effect of aging temperature, samples were heated to 100, 150, 200, 250
o
C 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
200
o
C 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 200
o
C. Figure 1b 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 400
o
C 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 200
o
C Fig. 1. Vickers hardness as a function of aging treatment
a As-extruded b Solution treated
Fig. 2. The optical micrograph of a as-extruded and b solution treated samples
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 W
a BW
P N
2
1
max
Here, σ
N
is the nominal stress, σ
ys
is the yield stress, P
max
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 dadN 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
F a
K
2 Here, Fα 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],
2 4
2
sec 06
. 025
. 1
F
3 where
W a
2
4
The crack length was measured using travelling microscope. The threshold stress intensity factor ΔK
th
was determined when a crack growth is not observed for 10
6
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
mcycle.
III. Result and Discussion
