A COMPARATIVE STUDY ON LASER WELDED JOINTS: MECHANICAL
AND METALLURGICAL ASPECTS
E. Bayraktar*1, D. Kaplan2, and B. S. Yilbas*3
Supmeca -Paris (EA2336), School of Mechanical and Manufacturing Engineering, France
2
ARCELOR Group, Paris-France
3
King Fahd University of Petroleum & Minerals, Box 1913, Dhahran 31261, S. Arabia
*
Corresponding authors: [email protected] [email protected]
1
ABSTRACT
In order to satisfy the requirements of productivity and quality, in recent years, LASER welding processes of certain
grades of steels mainly thin sheets assemblies or Tailored Welded Blanks (TWB) have given many successful advantages
in manufacturing engineering. This welding process of in particular makes it possible very easily to joint the steel parts in
thickness and grade identical or different in many industrial applications (car industry, pipe lines, tubes…). Additionally,
Interstitial Free (IF) steels with very low C and N contents have been successfully developed in order to perform specific or
complex deep drawing operations in the automotive industry. Major developments of TWB by means of LASER welding in
this area have also occurred. Certain grades of IF steel are particularly suitable for deep drawing operations of TWB due
to high “r” values (Lankford ratio) [1-4]. However, because of the narrowness of the weld bead and important hardness
gradient, determination of “toughness” (definition of a criterion of the « ductile/brittle » transition during the impact test) of
the welded connection (weld bead and Heat Affected Zone) confirm very delicate in LASER welding. The main objective of
this paper was to make a comparative study for microstructural and mechanical-toughness characterisation of the steel
assemblies, base metal or welded by LASER. For that a procedure of test specifically adapted was used for welded joints
(t<4mm). The first part of this study consider different steel grades such as a grade of the FE360BFN steel (t = 4mm) and
a grade of the Interstitial Free Steel (IF-Ti, t = 0.8mm) and HR 60 (t=2.5mm) welded by LASER that largely used in the
automotive industry.
Introduction
Laser welding process is very well used in modern technology, for example, in automotive industry besides resisting spot
welding, regarding to other welding process. Even its certain disadvantages, this process gives many facilities in
manufacturing engineering such as deep penetration with precise narrow welds, very small heat affected zone, HAZ, low
heat input, very short times welding processing, high productivity, small distortion level, no secondary processing, etc.
Today, using of tailored blanks by means of LASER welds of thin sheets such as IFS or other application for joining of
parts is very well improvement in popular manufacturing of environmental car [1-8].
In fact, mechanical and metallurgical properties of LASER welded joints result in different toughness behaviour due to
thermal and strain cycles (thermo-mechanical effect) in the welded structures. This difference in fracture behaviour
influences the performance of welded joints during their usage. Thus, they can present some brittleness after forming. The
desired behaviour can be obtained through an optimisation of the design of the part, and through the intrinsic quality of the
base materials and their welds. However, from the conventional bending or tensile tests, it is difficult to identify the
evolution of the welding conditions or the composition towards an optimal solution. Widely held of the brittle fracture tests
were developed in order to characterize relatively thick specimens of which thickness higher than 10 mm. In order to study
the mechanical and metallurgical behaviours of the welded sheets/plates, it is quasi impossible to carry out the notched
deflection tests in these specimens because of the heavily plastic deformation of the specimens. These simultaneous
evolution of materials and of the joining techniques need adapted tests for assessing the dynamic behaviour of these
structures.
Certain grades of steels studied here such as IF, HR60, Fe360, are particularly suitable for using in the automotive
industry operations. Joining of these grades by means of LASER welding for example, major developments of tailored
blanks of IFS and the general application of LASER welding in this area resulted in a considerable research interest to
satisfy the quality requirements of this process.
The first stage of this study aims to carry out a comparative study for microstructural and mechanical-toughness
characterisation of the steel assemblies, base metal or welded joints by LASER considering different steel grades such as
a grade of the FE360BFN steel (t = 4mm) and a grade of the Interstitial Free Steel (IF-Ti, t = 0.8mm) and HR 60
(t=2.5mm).
For that a procedure of test specifically adapted was used for base metals and also welded joints (t<4mm) [1, 3].
Experimental
Different grades of steels were used in this study. They are cold rolled steel grades used principally in car industry. The
carbon and manganese contents vary from 1.4×10−3 to 5×10−3 wt% and from 200×10−3 to 1440×10−3 wt%, respectively
(Table 1). Thickness of steels varies from 0.7 to 4mm. Welding power energy varied from 1 to 5.5 KW with the welding
speeds going from 2, 2.5 to 3 m/min. The morphology of the welded zone made it possible to evaluate the values of the
thermal yielding-efficiency of LASER welding process going from 9% to 13%. Structural examination allowed to estimate
the speed of solidification about 105 – 106 (C/s). The temperature of ductile-brittle transition from the base metal and
LASER molten metal were measured by impact tensile test (ITT) developed recently [1-6]. The principle of the impact
tensile testing had been explained formerly [1, 2, 4]. All of details on this new methodology have been published in [1-4].
Briefly, this test is based on the use of a specially designed two-body tensile specimen, including a smooth part and a
notched section. This specimen is mounted in a special device called crash simulation device and the whole setting is
brought to the desired testing temperature by means of a cooling system (in liquid nitrogen).
Steel grade
FE360 - a (4 mm)
IF Ti (0.8 mm)
HR60 (2.5mm)
C
164
1.4
82
Mn
397
205
1441
Si
18
4
46
Type
FE360 - a (4mm)
IF-Ti (0.8mm)
HR60 (2.5mm)
σy (MPa)
282
175
558
S
8
10
0.9
P
12
13
15
a)
Al
35
38
33
LD
UTS (MPa) A (%)
412
37
339
41
634
18
b)
Ni
Cr
Cu
Nb
V
Ti
33
17
20
22
13
12
2
35
3
1
107
1
σy (MPa)
TD
UTS (MPa)
A (%)
186
555
346
630
41
16
N
3.3
3.5
3.6
Table 1 a) Chemical compositions (10-3 wt %) and b) tensile properties of the steel grades
Evolution of the structure of solidification depending on the welding conditions:
The (primary and tertiary) structures of solidification were examined in the LASER molten metal by using a suitable
reagent. This type of examination makes it possible to evaluate the speed of solidification. A metallographic etching makes
it possible to reveal the structure of solidification. In the plane of cut of the molten metal of steel grades, a vertical central
zone is observed, usually called central chimney zone and a formed side or dendrite zone of which growth was stopped by
this central chimney. Figure 1 shows the microstructures of the steels studied in this paper. Additionally, taking into
account their composition, solidification occurs in the field of δ followed by a transformation into an austenite phase.
In the case of the microstructure of steel grade of Fe360, initial structure is ferrite-pearlite (dα = ~10 µm) and a
decomposition of pearlite islands are observed on a short distance (Heat affected zone, HAZ). A considerably high cooling
rate has caused to a transformation of martensite in weld metal zone and also HAZ.
In the case of the steel grade of IF-Ti, of which initial structure is ferrite with a grain size of (dα = 11 µm), a very large grain
growth zone (called abnormal/excessive grain growth phenomenon) in the HAZ with a grain size of d ≅ 80 µm that the
causes have been discussed in detail in the former papers [9].
In the case of HR60, it is noted that the initial structure is composed from polygonal and acicular ferrite. However,
considering the high cooling rate of LASER welding process, the presence of the structure of martensite can be found in
weld metal zone. In all the case, lower welding speed has made wider the weld molten zone.
Fe360
IF-Ti
HR60
Fig. 1. Microstructures of the steel grades studied in this paper (Etched by Bechet Beaujard & Marschal)
As will be seen in the next section, for all grade of steels studied here, on can find a considerable difference in stressstrain behaviour of base metal and molten weld zone. This situation can cause naturally a fracture deviation from weld
metal to base metal in certain type of tests whatever the welding speed.
From these evaluation questioned just above, surface of the molten weld zones can be measured to explain the relation
between the surface of the welded zone and linear energy of the welding process as indicated in the Figure 2 for different
applications.
Surface (mm²)
12
SF
10
8
Q (kW)
6
1kW
1.35kW
4
2kW
2.7kW
2
5.2kW
5.5kW
0
0
0,5
1
1,5
2
2,5
3
3,5
4
E (KJ/cm)
Fig. 2. Evolution of surface of weld metal zone as a function of the linear energy of welding process
Within the framework of this study, to estimate speeds of cooling rate during the solidification starting from interdendritically spacing, the relations proposed in the literature were used for measurement inter-dendritically spacing
(primary or secondary) in order to evaluate the cooling rate during the solidification [10, 11].
The welding parameters considered for this study were used in the theoretical relation proposed by Grong [10]:
λp = α(q/v3/4)
(1)
where λp is measured primary inter-dendritically spacing (µm), α is proportionality factor, q is energy (W) and v is welding
speed (mm/s).
For the steel grade of FE360, one could obtain a value of α = 0.62, for HR is a value of α = 0.22 and also for steel grade
of IF-Ti, a value of α = 0.23. Evidently, in literature, there are also semi-empirical relations making it possible to determine
the cooling rate (VR) starting from measurement of primary or secondary inter-dendritically spacing. (As usually, cooling
rate (VR) is a product of solidification rate, R and thermal gradient, G).
Concerning the primary inter-dendritically spacing, λp, one could define the following relation for general C-Mn steels
(here, for the steel grade of Fe360 and also HR60) proposed by Frederickson [11].
λp = 137(VR)-0.2
(2)
where VR is cooling rate (°C/min) and λp is measured i primary inter-dendritically spacing (µm),
Additionally, cooling rate can be also determined by using the values of the measured secondary inter-dendritically
spacing (µm) that is more suitable for the very low carbon steels (here for the steel grade of IF-Ti) [12].
This relation is given in the following form:
λs = 688(VR)-0.36
(3)
where VR is cooling rate (°C/min) and λp is measured secondary inter-dendritically spacing (µm)
Figure 3 indicates only the evolution of primary inter-dendritically spacing depending on the cooling rate for different steel
grades studied here. Thus, it is interesting to compare the whole of these results so as to specify certain effects on the
speed of cooling under different conditions (thickness, composition). It is worthwhile to point out them here in order to
highlight the conclusions obtained within the framework of this study.
IF-Ti (t = 0.8mm) VR ~ 106 (°C/s) from the measured primary inter-dendritically spacing
5
VR ~ 10 (°C/s) from the measured secondary inter-dendritically spacing
HR60 (t = 2.5mm) VR ~ 105 (°C/s) from the measured primary inter-dendritically spacing
5
VR ~ 10 (°C/s) from the measured secondary inter-dendritically spacing
FE360 (t = 4mm) VR ~ 104 (°C/s) from the measured primary inter-dendritically spacing
4
VR ~ 10 (°C/s) from the measured secondary inter-dendritically spacing
It will be noted that the cooling rates vary in a range going from 104 to 106 °C/S in the experimental field of this study.
These results are well comparable with other researches on LASER welding in the literature, where it was noted that the
5
8
cooling rate varied in a range going from 10 to 10 K/s [11].
Fig. 3. Relation between the measured primary inter-dendritically spacing and cooling rate (Fe360, HR60, IF-Ti)
Toughness characterisation of LASER welds
Toughness characterisation of LASER welded joint (very high energy density) is an important point for steel plates used in
car industry. Generally, the toughness evaluation of the LASER welds is carried out by classic “Charpy V-notch” testing.
However, many previous researches have shown that this type evaluation results in some inconvenience such as facture
path deviation from weld zone to base metal due to different mechanical behaviour and very narrow weld bead, etc.
Moreover, the increase in cooling time, i.e. the increase in welding heat input weakens the fracture toughness. Evidently
abrupt changes of mechanical behaviour create a mechanical heterogeneity that makes very complex to evaluate the
toughness test results applied on the welded joints. The reason why, another type of toughness evaluation method based
on impact tensile –simulated crash test has been developed [1,3]. General details of this test have been given previously.
Here, only a recall will be given. Then, the results obtained on various steel grades, discussed in this paper, will be
presented.
In accordance with the principle of this test, the specimens are submitted to impact tensile testing (ITT) at different
temperatures. According to testing temperature, fracture mode varies:
At low temperatures, brittle fracture occurs. Due to stress concentration, fracture always occurs in the notched section.
Failure criterion is the attainment of the cleavage stress by the maximal local stress. This event happens before the
smooth section has reached plastic yield. In other words, notched section fails by brittle fracture while smooth section
stress remains purely in the elastic domain. Fracture energy obtained is naturally low. In fact, stress applied in the notch
area in ITT is limited by the load capacity of the smooth section. Thus, this test should avoid the situation where ductile
initiation occurs at the notch tip [2].
At high temperatures, the specimen fails by ductile fracture. At the first time, ductile failure will always occur in the smooth
specimen. This can be derived from the plastic consolidation in the notched specimen, and because of the fact that ductile
failure criterion at a given applied tensile load is satisfied primarily in the section of the smooth specimen. In other words,
the consolidation in the notched section leads to the fact that the ratio [2-4]:
resistance to the tensile load of a notched specimen
)
(
(4)
resistance to the tensile load of a smooth specimen
becomes greater than 1. Therefore, in a mixed tensile specimen including a smooth part and a notched part, it can be
expected that a transition occurs with temperature between the two fractures modes (Figure 4a). This transition of the
fracture mode contains the basic principle of the impact tensile test. Experience shows that transition between the two
fracture modes in ITT occurs within a very narrow temperature range. Thus, transition temperatures may be defined with
very great precision, better than 5-10°C. Only a few testing specimens are needed for transition temperature
determination.
Figure 4 displays impact tensile test results for different steel grades studied here by using a simple impact tensile
geometry. One could observe the effect of welding speed (process parameter) and also the effect of the composition
(materials parameter) on the ductile/brittle transition behaviour. In order to simplify the toughness evaluation of base and
weld parts, the transition temperature can also be determined in an equivalent manner according to the fracture mode, by
stating if fracture occurs in the notched or in the smooth section and/or of the fracture energy level by using this type of
representation i.e depending on the welding speed (Figure 5). For this ranking of materials and evaluation of toughness
values of the different parts (base and weld molten zone), one could identify a matching parameter, α that can displays a
competition between the smooth part and notched (welded) part of the specimen [2].
Experimental results have shown that an increase in the welding speed and/or a reduction of the specimen thickness lead
to a fall in the temperature of transition (increase in toughness values). In conclusion, the development of these
experimental results, in other words mechanical and metallurgical evaluation carried out here opens the way of the detail
studies to evaluate the influence of the welding conditions and/or the influence of the chemical compositions on the
toughness behaviours of the welded joints by LASER process [5-9, 13, 14].
a)
0
Tc (°C)
Timpact-trans (°C)
-100
LASER WELDING
Fe360b(4mm)
-125
-25
Base
Metal
Fe360b (4mm)
-50
IF-Ti (2mm)
-75
-150
Fe360a (4mm)
-100
Fe360a(4mm)
IF-Ti
-125
-175
Fe360b (4mm)
IF-Ti (2mm)
HR (2.5mm)
IF-Ti(0.8mm)
Weld
IF-Ti
-150
HR60 (2.5mm)
HR (3mm)
-175
-200
Fe360a (4mm)
FePo6G(1.5mm)
FePo6G (1.5mm)
-200
-225
0
25
50
75
100
125
150
V (mm/s)
-225
0
50
100
b)
150
-3
C (10 wt%)
200
c)
Fig. 4. The simple geometry for ITT a) and effect of welding speed b) and composition c) on the Impact ductile/brittle
transition values
Fracture
Mode
Fracture
Mode
IF-Ti (t=0.8mm), LASER, Q= 5,2 kW
v=2.5m/min
t=0.8mm
α=0.9 (BM)
In the
smooth
part
HR60, Welded joint
LASER, Q=5.5kW, α =1.3
t=2.5mm
In the
smooth
part v=3m/min
α=1.23 (HAZ)
v = 3 m/min
α=2.3 (WM)
In the
notched
part
In the
notched
part
v =2.5 m/min
321
-200
-150
-100
-50
0
50
100
-220
-200
-180
-160
-140
-120
-100
-80
-60
Temperature
a) IF-Ti
-40
-20
0
TEMPERATURE (0C)
b) HR60
Fig. 5. Ductile/brittle transition behaviour of base and weld parts according to the fracture mode
Fracture surface analysis of the broken specimens has been carried out after ITT test for each grade of steel.
In the case of Fe360, the fracture surfaces tested at low temperature present fracture topography par cleavage in the
notched section. The facet sizes are of the same order as that of the ferrite grains. At high temperatures, the fracture
occurs always in the smooth part of the specimen and fracture appeared in ductile mode. IF-Ti shows the similar results
and many secondary cracks were detected at the base metal.
One can find the geometrical particles of (Ti(CN)) with a size up to 5-6 μm and also small size of (CN) or (MnS) at the
quasi cleavage zone. Fracture surfaces of the steel of HR60 at low temperature in the notched section showed quasi
cleavage but fracture mode at high temperature is always ductile mode. Fracture surfaces regularly contain small
geometrical particles of (CN) and also (MnS) with small sizes of 3-5 µm. All of these observations were presented in
Figure 6.
Fig. 6a Microstructure of IF-Ti, left and its fracture surface (intergranular) after ITT test, right
Fig. 6b. Fracture surfaces of Fe360, left and HR60 right after ITT test
In order to give more detail on the fracture phenomena of the broken specimens, circle shape grids have been applied on
ITT specimens by means of photo deposited prior to tests. Figures 7a and 7b illustrate the variation of strain along the
specimens, when testing carried out at different temperatures. This type of presentation gives very clear idea on the local
strain rate and the local deformation zones
In these examples, fracture occurs in the smooth part in the case of Fe360 at room temperature. At the fracture side,
strain level can attain 60%. In the case of HR60, fracture occurs in the notched area at the temperatures of -170, -180 and
-196°C. From strain distribution, it may be seen that strain is localised in the notched area, in as much concentrated as
testing temperature is low. Deformation does not take place at the smooth part, which means that this region remains in
the elastic range during fracture. For the ITT performed at −170 °C, strain levels can reach about 25%. As typical fracture
−1
time is in the range of 0.1–0.5 ms, this means that local strain rate may be as high as 100–200 s that is well approach to
the real crash test conditions, i.e. this is a representative of the strain rates experienced in automotive collisions.
This type of tests and the methods of analysis are well adapted for the characterisation of welded parts for ranking the
materials or the manufacturing conditions with respect to their resistance to fracture in the presence of a defect.
1
1
Ductile fracture
in base metal
FE360BFN
ε1
ε 1, ε 2
ε 1, ε 2
ε2
20°C
13J
0,6
HR, Q=5,5kW, v=2m/min, t=2.5mm,
ε1
0,6
ε1
ε1
ε2
0,8
0,4
0,2
-170°C (5.6J)
0,2
-180°C (5,2J)
-196°C(3.2J)
0
ε2
-0,2
ε2
ε1
C-L
ε2
-0,2
BM
HAZ
Weld-M
-0,4
-0,6
-0,6
BM (α=1)
-0,8
-1
-1
-15
-10
-5
0
5
10
Distance from the fracture side (mm)
15
-5
a)
-4
-3
-2
-1
0
Distance from fracture side (mm)
b)
Fig. 7 Variation of longitudinal local strain with the distance from the fractured surface; a) Fe360 ductile fracture at the
smooth side and b) HR60 brittle fracture at the notched area
Conclusion
Results presented in this paper indicate that it is advantageous to evaluate the resistance of welded plates to fracture in
dynamic loading conditions by these practical methods.
Mechanical and metallurgical evaluation carried out here can facilitate the way of the detail studies to estimate the
influence of the welding conditions and/or the influence of the chemical compositions on the toughness behaviours of the
welded joints by LASER process. It is very useful and practical tool in manufacturing engineering for evaluation of a defect
during the production.
Acknowledgements
This project is supported by ARCELOR CED Auto Applications. The authors are indebted to the research managers of
ARCELOR Auto for their valuable help for this project which is going on.
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