STUDY ON MICRO-DAMAGE OF AL ALLOY WELDED JOINT IN
AEROSPACE THERMAL CYCLING CONDITION
Cheng JINa, Jitai NIUb, Shiyu HE and Long GAN
School of Materials Science and Engineering, Harbin Institute of Technology
Harbin 150001, P.R.China
a
[email protected], [email protected]
ABSTRACT
Mechanical properties deterioration and damage mechanism of 5A06 aluminum alloy welded joint under thermal cycling
condition were investigated based on simulated aerospace thermal cycling condition. The mechanical performance was
measured before and after thermal cycling process. The microstructure and fracture surfaces were observed by optical
microscope and scanning electron microscopy. Results show that thermal cycling condition can induce damage in the welded
joint. Lots of micro voids formation can be observed during the thermal cycling process. Micro voids nucleation and evolution
of the around the particles cause the mechanical properties deterioration of the welded joint. The particles formed in welding
heat-affected zone are the source of local damage in thermal cycling condition. The thermal mismatch stress assist the
external stress can cause debonding between the particles and base alloy. The contribution of the thermal mismatch stress to
void nucleation has been studied and introduced to Gurson-Tvergaard-Needleman void nucleation model. The process of
thermal stress assist void nucleation was also simulated by FEM.
Introduction
Aluminum alloy is one of the most important construction materials, due to its high specific strength, good corrosion resistance
property and good weldability. It has been used in almost every branch of transportation industry [1]. Especially, in aerospace
industries its light weight, good corrosion resistant properties, and no low temperature point for brittleness transformation make
it be widely used in weight sensitive and cryogenic construction [2].
Because of the different microstructure and mechanical performance between the welded joint and base materials, the welded
joints often become the weakest part in the whole welded structures. Their performance evolution and failure behavior
determine the serve life of the whole welded structures.
The aerospace industry currently uses an amount of tungsten inert gas (TIG) welding technology to join aluminum alloy
structures. When the space vehicles travelling in earth orbit during the period of serving in aerospace, some construction
material will subject to the space cyclic thermal load as well as the working load [3,4]. That will cause the performances
deterioration and material microstructure damage [5,6]. Many works had been done with respect to the damage of composites
induced by thermal cycling. They found thermal cycling could cause the micro-damage and change the failure modes of metal
matrix composites (MMC). The different coefficients of thermal expansion (CTE) between the reinforcement and the matrix
play a key role in provoking a high stress distribution at the interfaces [7]. Over a long time thermal cycling, the formation of
porosity or micro-voids at the matrix/reinforcement interface can be observed [8]. As a result, the evolution of the voids
decreases its mechanical performance.
However, few literatures have been found to investigate the metal alloy welded joint performance in aerospace thermal cycling
condition. Hongbin GENG [9] has found the thermal cycling could also cause the microstructure change and performance
deterioration in Al-Li alloy welded joint.
In order to evaluate the long-life reliability and guarantee the safety of the welded structure under services, it is essential to
determine its damage mechanisms and the law of its performance deterioration. This paper focuses on the mechanical
performance deterioration and the damage mechanisms of a 5000 serial aluminum alloy butt-welded joint under simulated
aerospace thermal cycling condition.
Damage in ductile metals
It has been widely recognized that voids nucleation and growth around the inclusions and second-phase particles is the key
mechanism of damage and rupture for ductile metals at meso scale [10]. The presence and evolution of voids causes a
progressive damage of the material due to the load-carrying capability. Taking the voids volume fraction as the damage metric,
Gurson [11] firstly proposed a yield function and a void evolution law for a porous material. This function was derived by
studying a single void in elastic-plastic matrix. Tvergaard and Needleman [12] extended the Gurson model to take into account
the acceleration failure process induced by voids coalescence. That is the so-called GTN model and it reads:
σ e2
3q σ
+ 2 f *q1 cosh( 2 h ) − 1 − q3 ( f * ) 2 = 0
2
σM
2σ M
Φ=
⎧f
f* =⎨
⎩ fc + K ( f − fc )
where
(1)
if f ≤ f c
if f > f c
(2)
σ e = von Mises equivalent stress
σ M = flow stress of the matrix material
σ h = macroscopic hydrostatic stress
f = void volume fraction
q1;q2;q3 = parameters of the GTN model
fc = critical void volume fraction
K = damage accelerate factor
During damage process, the void volume fraction increases. The rate of void volume fraction is consisted of a void growth and
a void nucleation part, and it is written in the form
f = fgrowth + fnucleation
(3)
The void growth part is proportional to the plastic volume dilatation rate and given by,
In general, the void nucleation rate is described by,
fgrowth = (1 − f ) E kkP
(4)
fnucleation = Aσ M + Bσ h
(5)
where A and B are void nucleation factor in GTN model, which depends on the void nucleation model used.
Experimental
A typical composition of the parent aluminum alloy 5A06 sheet used in this study is presented in Table 1. The sheets with 3mm
thickness were butt-welded by TIG welding using the same filler wire material. The welding parameters are shown in Table 2.
Table 1. Compositions of 5A06 aluminum alloy
Mg
Mn
Si
Fe
Zn
Cu
Ti
Al
Other
5.8-6.8
0.5-0.8
≤0.4
≤0.4
≤0.2
≤0.1
0.02-0.1
Bal.
≤0.1
Table 2. TIG welding conditions
Filler wire
material
Filler wire
diameter
Tungsten electrode
diameter
Welding current
(AC)
5A06
3mm
2.5mm
120～130A
Wire feed
rate
60～70
Argon flow
7L/min
mm/min
Welded joint specimens were cut across the butt-join welded sheets, as shown in Figure 1.
Figure 1. Dimension of the welded joint specimen
Carefully X-ray non-destructive detections were carried out on these welded specimens. The specimens with any detectable
defects were eliminated to ensure the results consistency in the following tests.
Thermal cycling tests were conducted using a homemade machine. The test parameters were pre-programmed and put into
the supervisory control computer. Several welded joint specimens as well as base alloy specimens were put into the thermal
isolation cavity of the machine. During the test, an external 70MPa tensile load was applied on the specimens. The specimens
were heated using current heat effect and cooled by spraying liquid nitrogen into the test cavity. By means of a thermocouple
fixed on the specimen, the temperature is controlled. The cycling temperature range is between –100~100℃ and the cyclic
period is 90 minutes.
Monotonic tension test of the specimens were conducted both before and after the thermal cycling test, in order to determine
the mechanical performance decrease induced by thermal cycling. The fracture surface was observed by scanning electron
microscopy (SEM) and EDS to analyze the damage and fracture mechanism.
The microstructures of the welded joint specimens were examined before and after thermal cycling test. The polished
specimens, but without etching, were observed to reveal the micro-voids evolution.
Results
Figure 2 shows the variation of the specimens’ mechanics properties vs. thermal cycles.
Figure 2. Welded joint specimens strength vs. thermal cycles
From Figure 2 we can see, the thermal cycling process can cause the decrease of tensile properties. Many micro-voids could
also be seen in the microstructure (without etching) photo, as shown in Figure 3.
Figure 3. Micro-voids distribution in the HAZ
In tensile test, all the welded specimens were failed in HAZ. The SEM fractographs of the failed joints specimens before and
after thermal cycling are compared as shown in Figure 4. Several tiny dimples can be found in Fig. 4(a). In contrast, more
larger and deeper dimples are observed in Fig. 4(b). In the larger dimples we can see particles in them. Meanwhile, larger
voids formed around the particles, which indicated they are the source of failure.
Figure 4. Fractographs of the welded joints (a) before and (b) after 200 hours thermal cycling
By EDS analysis, the impurities were determined as Al-Mn-Fe impurities, Mg-Si compound, Mg2Si and some Al8Mg5 and MgO
particles. During the welding process these particles accumulate in the HAZ near the welding fusion line.
The microstructure observation and fractographic analysis reveal that the void evolution around the particles is the main
damage mechanism of 5A06 aluminum alloy welded joint under thermal cycling condition. And the thermal cycling can assist
the void nucleation, which accelerate the damage process in thermal cycling condition.
Analysis of the Results
For the purpose of understanding the thermal cycling assist voiding, it is convenient to consider a spherical volume unit
containing a particle of spherical shape, as shown in Figure 5.
Figure 5. Spherical volume unit containing particle
Each constituent phase is assumed to be elastic-plastic perfect and isotropic solid, but the yield strength of the second phase
particle is much higher than that of the Al alloy matrix. And the interface of the matrix and the second phase particle is
assumed to be well bonded.
From generalized Hooke’s law, the strain during temperatures varying can be expressed as,
1
(σ r − 2νσ θ ) + α∆T
E
(1 −ν )σ θ −νσ r
+ α ∆T
εθ =
E
εr =
(6)
Where E is Young’s modulus, νis the Poisson ratio, T is the temperature and αis the coefficient of thermal expansion.
ε r ,σ r
and
εθ , σ θ
are the stain, stress at radial direction and tangential direction respectively. M. OLSSON et al. [13]
calculated the interface pressure P during temperature varying in the elasto-plastic framework with no other external load.
Using subscript 1 refers to the matrix material and subscript 2 refers to second phase particle, the interface pressure induced
by temperature varying can be expressed by,
2 E1
(α 2 − α1 )(1 − f )∆T
3(1 −ν 1 )
P=
2 M el
[1 −
(1 − f )]
3
(7)
where Mel is so called elastic mismatch parameter and given by,
M el =
E1 1 − 2ν 1 1 − 2ν 2
(
−
)
1 −ν 1
E1
E2
(8)
According to Equation (7) and (8), we can see, the interface pressure P can be positive or negative. Which indicate the
interface can be in traction during temperature varying even if there is no external stress applied. In general, α1 > α 2 and
−∞ < M el < 3 2 , so when ΔT>0, P<0, the interface is in the status of traction. That means the thermal mismatch stress P
can assist the void nucleation.
Above analysis is no external load applied. If the material is applied an external stress during thermal cycling process, the
thermal mismatch stress P can associate the external load to control the void nucleation process. Considering the stress P is
induced by thermal and mechanical mismatch, and recalling that the object is assumed to be isotropic, the stress P has no
effect on Mises stress σ e . It just has the contribution to the macroscopic hydrostatic stress σ h . After reorder the stress
direction, the void nucleation Equation (5) in GTN model can be express by,
fnucleation = Aσ M + B(σ h − P )
(9)
The process of thermal mismatch stress assist void nucleation can be analyzed by FEM in a more complex condition, including
non- spherical unit, materials hardening behavior, temperature dependence performance and so on. The results are shown in
Figure 6.
Figure 6. FEM simulation void nucleation in thermal cycling condition
From Figure 6 we can see, the stress concentrate at the interface between the particle and matrix alloy. And debonding can
occur if the applied external stress plus the thermal mismatch stress is high enough.
Conclusions
The micro damage mechanics of 5A06 aluminum alloy welded joint under thermal cycling condition was analyzed at meso
scale. The thermal cycling can cause the performance deterioration of the joint. Micro-voids formation around the particles in
HAZ is the main reason of strength decrease. The difference CTE between the particles in HAZ and the base alloy induce
thermal mismatch stress, which can cause the interface debonding. The thermal mismatch stress can assist to void nucleation.
Micro-voids around the particles undergo nucleation, growth and coalesce. It is the main reason, which induces performance
deterioration of 5A06 aluminum alloy welded joint.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 90205035).
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