STRESS ANALYSIS OF LASER MARKED LOW CARBON STEEL
Z. Kalincsák*, L. Balogh**, L. Borbás***, J. Takács****,
*Dep. of Vehicles Manufac. and Rep., BME, H-1111 Bertalan L. 2., Budapest, Hungary
[email protected]
**Dep. of Mat. Phys., L. Eötvös Univ., H-1117, Pázmány P. sétány 1/a., Budapest, Hungary
[email protected]
*** Dep. of Vehicle Parts and Drives, BME, H-1111 Bertalan L. 2., Budapest, Hungary
[email protected]
**** Dep. of Vehicles Manufac. and Rep., BME, H-1111 Bertalan L. 2., Budapest, Hungary
[email protected]
ABSTRACT
Laser marking on steel surfaces is considered to apply to create local phase transformations and/or local stress level change
in order to produce bar codes on low carbon steel surfaces. The local phase transformations caused by the rapid heating and
cooling process combined with the stress accumulations within the heat affected zones [1]. The stress, generated by the
marking procedure (“marking stress”) has of a primary importance from the point of view of reading out. This kind of codes can
be widely applied in the production logistics. The investigated materials are cold rolled low carbon steel sheets (typical car
body sheets with 0.1 % C content). In this paper the working principle of this stress detection as well as some technical details
will be described.
Introduction
The productions of laser marking on the surfaces of rails have been described in Ref.[2,3]. This type of marking is considered
to apply for the detection of thermal induced stresses, as well as to produce bar codes as signals on low carbon steel surfaces
for production logistics. The mechanical stability of markings has of a primary importance for any application (particularly in the
case of magnetic reading out technique like (i.e. using Barkhausen effect, eddy current testing).
Samples are prepared from cold rolled low carbon steel sheets (content: C≈0.1 Wt%; Si 0.34 Wt%; Ti 0.06 Wt%; Mn 0.85
Wt%;). The sheets were covered with 0,01 mm thick phosphate layer. The Fig.1. shows schematically the local laser treating
process and modal structure of laser beam.
v
FIGURE 1. Process of laser scribing and power distribution in cross section of laser beam
Experimental
In the structure of low carbone steel grain coarsening can takes place Influence of laser heat treatment when the applied
energy density is sufficiently low, the local temperature not exceeds the 650 °C. In this case the recrystallisation of the cold
rolled sheet is the dominant effect (structural change), as it can be observed.
Marking
direction
a)
b)
FIGURE 2. a) Microraph on the cross section of the laser marked sheet, b) SEM micrograph on the surface of the laser
marked sheet (SEM, 35 kV)
Previously, the photostress analysis [6]. and the Moire-effect [7]. were applied for the detection of local stresses in the heat
affected zone. However the resolution of these methods was not able to reveal the changes in the stressed state of the
investigated specimen. The next method was the X-ray diffraction [8] peak profile analysis. It is a powerful tool for the
characterisation of microstructures in crystalline materials. The analysis of local micro-stress state in the laser scribed zone
was carried out within X-ray diffraction using CoKα1 radiation. The diffraction was measured two positions, in the base material
(1. measuring position) and in the laser treated zone (2. meas. pos.). The reflections were recorded in “image plate” used
Debye-Scherrer method. The applied measuring arrangement and the specimen can be seen in Fig. 1.
2. meas.pos.
X-ray beem
1. measuring
position
Laser treated zone
FIGURE 3. The applied X-ray arrangement and the geometry of specimen (6 mm x 5 mm).
Results
In the fig. 5. can be seen the Debye-Sherrer rings of base material and the laser treated part. From this image plate can be
seen the difference between the structure of base material and treated zone.
2*Θ (Reflection angle)
a)
2*Θ(Reflection angle)
b)
FIGURE 5. Measured Debye-Scherrer rings of a) base material and b) laser treated part
In the Table 1. can be seen the measured Fe-reflections and the Miller-indexes
Table 1. Degrees of reflections belong to Miller-indexes
Degree of
h
Miller index
k
l
52,377
1
1
0
77,235
2
0
0
99,705
2
1
1
123,929
2
2
0
161,399
3
1
0
reflection,
2Θ (°)
Fig. 6. shows intensity distribution versus the diffraction angle. This arose that the image plate was integrated along the
Debye-Scherrer rings. The typical peaks (reflection of Fe) were analysed using line profile analysis in order to obtain subgrain
size and dislocation density. From this results give infornations about the stress state of material.
35
In te n s ity b a s e m a te r ia l
In t e n s ity in la s e tr e a t e d z o n e
Intensity (-)
30
25
20
15
10
5
0
0
20
40
60
80
100
120
140
160
180
2 * te ta (d e g )
FIGURE 6. Intensity versus of diffraction angle in base material and laser treated zone
To analyze the peek profile two classical methods have evolved, the Williamson-Hall and the Warren-Averbach procedures.
The first is based on the full width at half-maximum (FWHM) values and the integral breadths, while the second is based on
the Furier coefficients of the profiles. Both methods provide, in principle, apparent size parameters of crystallites or coherently
diffracting domains and values of the mean square strain. In this case the Williamson-Hall method was used for qualitative
evaluation [4,5].
K=
2 sin Θ
λ
(1)
1
d
(2)
K=
Θ: reflection angle
λ: wavelength of applied radiation
d: lattice parameter
0 ,0 4
L a s e r tre a te d z o n e
B a s e m a t e r ia l
FW HM [1/nm ]
0 ,0 3
0 ,0 2
0 ,0 1
0 ,0 0
2
4
6
8
10
12
K [1 /n m ]
FIGURE 6. The Williamson-Hall plot of base material and laser treated zone
Fig. 7. shows the Debye-Scherrer rings of (310) reflection in the base material and the laser treated part. The intensity
distribution in base material is homogenous, hints to the material contains fine grain structure. Contrary, the intensity
distribution of laser treated part is inhomogeneous influence of roughened average grain structure.
b)
a)
FIGURE 7. Debye-Scherrer rings of (310) reflection, a) base material, b) laser treated part
Diffraction peaks broaden when crystallites are small or the material contains lattice defects. The decreasing of FWHM hints to
the decreasing dislocation density (the level of inner micro stress) and increasing subgrain size.
1,4
Intensity base material
Intensity laser treated zone
Intensity (-)
1,2
1,0
0,8
0,6
0,4
0,2
0,0
159
160
161
162
163
164
165
166
2*teta (deg)
FIGURE 8. Debye-Scherrer rings of (310) reflection, a) base material, b) laser treated part
Using the CMWP (Convolutional Multiple Whole Profile) method the median and the variance, m and sigma of the log normal
size distribution function of subgrain size, the area average mean subgrain size, <X>area, and the average dislocation density
and the dislocation character q have been determined for the non treated zone and the laser treated part.
The microstructure of non treated zone:
Parameters of lognormale grain size distribution are: m=94[nm], sigma=0.7; from this results follows that the average subgrain
size (weighted with surface): <X> = 320 +/- 40 [nm]
2
Dislocation density = 8.7e14 +/- 1e14 [1/m ],
q=2.2 +/- 0.2, which means that screw and edge dislocations can be found in the base material, but there is slightly more from
the screw type (only edge dislocation q = 1.25, only screw dislocation q = 2.6)
The microstructure of laser treated part:
The average subgrain size larger than 1000 [nm]. (Size above 1000 [nm] can't be determined by X-ray line profile analysis)
<X> > 1000 [nm], due to high temperature the material recrystallised.
2
Dislocation density = 3.7e14 +/- 0.5e14 [1/m ]
It can be seen, that this value is half of the value of non treated part, the microstress level was decreased, q = 1.7, which
means that slightly more edge dislocations than screw dislocations can be found in the laser treated zone ( only edge
dislocation q = 1.25, only screw dislocation q = 2.6)
These results are consistent with the SEM measurements (Figure 2. b) which also shows grain growth in the laser treated
area.
The size measured by SEM is different from the size measured by X-ray diffraction. The size calculated using X-ray line profile
analysis can be interpreted as the average size of the coherently scattering domains (subgrains), while size seen using SEM is
the size of the crystallites separated by high angle grain-boundaries. But these crystallites have a subgrain structure, where
subgrains are divided by low angle boundaries, dislocation walls.
Conclusions
CMWP [9,10] fitting was used to evaluate quantitatively the microstructure (grain size distribution and dislocation density,
dislocation arrangement and type. The decreasing of FWHM hints to the decreasing the density of dislocations and the level of
inner micro stress. The types of dislocations were also slightly changed. In the base material there were slightly more screw
dislocation contrary to the laser treated zone where edge dislocations were in majority.
Acknowledgments
This work has been supported by National Office for Research and Technology (NKTH) Project number: AGE-00015/2003,
TéTA-4/03
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