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
4
Abstract
- 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 400
o
C 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
σ
N
nominal stress σ
ys
yield stress σ
uts
ultimate tensile strength P
max
maximum load B
specimen thickness W
width of gauge position
a crack length
Hv Vickers hardness
R stress ratio
o
C degree Celsius
K stress intensity factor
ΔK stress intensity factor range
ΔK
th
threshold stress intensity factor range Fα
geometrical factor dadN
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 gcm
3
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].
This study aims to investigate the influence of solution treatment on the FCP rate of extruded AZ61
magnesium alloy, and the emphasis is on understanding
the influence of solution treatment on the FCP rate of AZ61.
II. Experimental Procedure
