The study of laser welding parameters influence on fusion zone shape and surface quality of SUS301L stainless steel.
Hongxiao, Wang ; Chunsheng, Wang ; Chunyuan, Shi 等
Introduction
The disk laser is a brilliant combination of a solid-state laser
and a semiconductor laser. The semiconductor laser provides a high
degree of pumping efficiency and the solid-state laser, as a disk laser,
provides the high beam quality [5]. The optimized combination of high
laser power plus excellent beam quality is kept constant across the
entire power range. YAG laser beam welding with a continuous wave is a
high energy density and low heat input process. The result of this is a
small heat-affected zone (HAZ), which cools very rapidly with very
little distortion, and a high depth-to-width ratio for the fusion zone
[11].
SUS301L is a kind of austenitic stainless steel with reliable
structure strength. For light weighting and environment protection,
SUS301L is widely applied in the stainless steel vehicle structure as
the outside panel with no painting [12]. Due to deformation caused by
the welding heat, the stainless steel is assembled with resistance spot
welding with small thermal importation. But because of the punched power
and thermal input of resistance spot welding electrodes, there will
leave the indentation with 1 cm in diameter on the outside surface of
the stainless steel vehicle body (shown in Figure1(a)). This influences
the appearance of public transport vehicles, and should not be ignored.
Now to laser weld SUS301L stainless steel to get an overlap joint
with no welded traces on the outside surface (shown in Figure1 (b)) and
higher tensile strength than that of resistance spot welding is a new
area of research and will be applied in the railway vehicle
manufacturing.
[FIGURE 1 OMITTED]
Ordinarily, laser beam welding involves many variables: laser
power, welding speed, defocusing distance and type of shielding gas, any
of which may have an important effect on heat flow and fluid flow in the
weld pool. In turn, this will affect penetration depth, shape and
micro-structure of the fusion zone. Both the shape and microstructure of
the fusion zone will considerably influence the properties of the joints
and the quality of the exterior surface.
There are many reports [3,6,8] which deal with the shape and
solidification structure of the fusion zone of laser beam welds in
relation to different laser parameters. However, the effect of the
combination of laser welding has up to now not been extensively
researched. More work is required for understanding the combined effect
of laser parameters on the shape of the fusion zone and the quality of
the exterior.
The present investigation is concerned with laser power, welding
speed and defocusing distance and their effects on the fusion zone shape
and micro- structure of SUS301L stainless steel overlap joints.
Experimental procedure
The materials used are SUS301L austenitic stainless steel of 1 and
2mm plate thickness with the dimension of 100 X 200 mm. The chemical
components and mechanical properties of SUS301L are given in Table1 [4].
A continuous wave (CW) fiber laser (Trudisk4002) in TEM00 mode was
used for overlap welding. TruDisk 4002 (as shown in Fig 2), is a laser
with 4 kW of power and a beam quality of 8 mm * mrad. The wavelength of
the laser is 1.0 6 [micro]m. the focal length 200 mm and the minimum
diameter of the focus 0.6 mm. Argon shielding gas was used at the flow
rate of 30 L min-1 through an 8 mm diameter size nozzle.
[FIGURE 2 OMITTED]
The overlap configuration for the thickness combination 1 + 2 mm
sample is shown in Figure 3. There is no gap between the two plates with
jig punched. The jig configuration and form of the work piece clamping
are shown in Figure 4.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
The laser beam welding parameters investigated are summarized in
Table 2. Combinations of laser power (P) of 1.5-2.5 kW and welding speed
(S) of 16-28 mm/s resulted in nominal heat inputs (HI) ranging from 0.04
to 0.48 kJ/mm. The defocusing distance ([D.sub.d]) was in the range of
-2 to 2 mm.
After welding, the specimens were visually inspected then, cut into
40 mm wide for shear tensile strength test, the left sectioned
transverse to the welding direction. The shape and microstructure of the
fusion zone were examined using optical microscopy and Scanning Electron
Microscope(SEM).
In accordance with the GB/T2651-1989 (sampling and testing methods
for tensile test of welding joints), shear tensile test was carried on
the WE-30 hydraulic tensile testing machine and the shear tension sample
was made into 40 mm wide. There were 2 cushion plates welded on the
upper and bottom plates to avoid torque. Figure 5 is showing the shear
tensile machine and the sample fastened mode.
[FIGURE 5 OMITTED]
Results and discussion
Influence of the process parameters
Influence of laser power
Both welding speed and defocusing distance were kept constant at 22
mm/s and zero respectively. The penetration depth increased sharply with
increasing laser power from 1.5 to 2.5 kW as shown in Fig. 6.
Fig. 7 shows an example of a cross section of overlap weld joint
using laser power of 2.0 kW. The microstructure of the cross section is
shown with inverted parabola-shaped for the intensity of the laser beam
has a Gaussian-like distribution (shown in Fig. 8(a)). The fusion zone
is symmetrical about the axis of the laser beam. No welding cracks or
porosity were found in any of the welds, this may be partly due to the
good crack resistance of the base metal and the welding conditions
provided.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
A. Kar and Mazumder [1] investigated a typical key-hole welding
process model and the cylindrical coordinate system(r, [theta], z) used
in the model which is shown in Fig. 8. According to this model, the
results indicated also that the development of the weld pool is
essentially symmetrical about the axis of the laser beam. Laser power
has a less influence on both weld profile and HAZ width in comparison
with its effect on penetration depth. This is in agreement with other
researchers work where they pointed out that changing laser power
between 3 and 5 kW [10] did not result in any significant change in the
size or shape of the weld.
When the laser power is lower than 2 kw and the penetration depth
is below 1.4 mm, there are no welded traces on the exterior, on the
contrary distinct welded traces will be shown on the surface.
[FIGURE 8 OMITTED]
Influence of welding speed
The influence of welding speed was investigated at the optimum
laser power (2 kW) and zero defocusing distance. Fig. 9 shows the
relationship between welding speed and fusion zone area. The fusion zone
area is defined as the half of product of depth and width of the fusion
zone. The fusion zone area decreased sharply from 1.59 to 2.26
[mm.sup.2] with the increase in welding speed from 16 to 28 mm/s.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
The dependence of the fusion zone area on welding speed was
confirmed at a different laser power (2.3 kW). A lower welding speed
resulted in a considerable increase in the fusion zone area and
consequently an increase in shear tensile strength of the overlap joint
leading to unacceptable surface quality as shown in fig 10. When the
weld speed is higher than 22 mm/s, there is no welded trace on the
exterior surface. The shear tensile strength of the overlap joint with
the speed of 25 mm/s is lower than that of the joint with the speed of
22 mm/s.
Influence of heat input
The influence of heat input as a function of laser power was
clarified [H.sub.I] = P/S. The above results have shown that the laser
power and welding speed should be optimized in order to minimize heat
input, then a satisfactory weld with reliable quality could be obtained.
This reflects one of the most notable features of laser welding compared
with other welding processes, that is small heat input.
[FIGURE 11 OMITTED]
Turning to the macrographs shown in Fig. 7 and 11, incomplete
penetration and no welded trace on the exterior surface with higher
shear tensile strength than that of RSW fusion zone profile could be
obtained using either P = 2 kW, S = 22 mm/s (Fig.7) or P = 2.3 kW, S =
25 mm/s (Fig. 11). However, P = 2 kW, S = 22 mm/s resulted in a smaller
welded trace on the exterior surface. And in laser welding, if the laser
power is too high, the spatter will increase to pollute the focus lens
increasing the risk of lens explosion [9]. When achieving the same
result, the lower laser power should be selected.
Effect of defocusing distance
Defocusing distance is the distance between specimen surface and
the optical focal point. In order to study its influence on both
penetration depth and surface quality, overlap joint was made with
changing defocusing distance between -2 and 2 mm. Laser power (2 kW) and
welding speed (22 mm/s) were selected to obtain incomplete penetration.
[FIGURE 12 OMITTED]
The relationship of the defocusing distance and the penetration of
the overlap joint is shown in Fig. 12. The penetration depth is
considerably decreased with changing defocusing distance from -2 mm to 2
mm. In laser welding, when focus is below the upper surface, the laser
power density of interior is higher than the surface, the energy
transfers deeper to the material, benefiting to weld thick materials
with deep penetration. For thin material, positive focal position is
usually used [2].
Microstructure of laser beam welds
In laser welding, the material was molten in a very short period
and solidified quickly resulting in the difference between the
microstructure of the laser welding and that of conventional welding
methods. Molten metal first crystallized in the solid- liquid interface
then grows up rapidly to the interior of the molten zone. The crystal
crystallized in the fusion line extends to the center of the weld seam
and finally combined there and almost no HAZ can be seen as demonstrated
in Figure 13 (a), and no cracking or porosity was observed.
A clear dividing line marked the boundary of the fusion zone can be
seen in the microstructure of the transition from the basement material
to the fusion zone (shown in Figure 13 (b)). For the great degree of
subcooling existing in the solid- liquid interface, the crystal of the
fusion line grows up into columnate dentrites with rapid crystallization
speed and parallel distributed perpendicular to the fusion line. In the
weld seam center, gradient of temperature is smaller than that of its
surroundings and the unfused suspended particle becomes the
solidification surface of non- spontaneous nucleation, and grows up
freely from the cooling conditions into equiaxes dendrites (shown in
Figure 13 (c)). The equiaxes grains of the center were fine and shaped
during the re-crystallization of the welding heat circulation.
[FIGURE 13 OMITTED]
Microstructures of type SUS301L steel weld metals made using two
different welding speeds, 19 and 22 mm/s, with same laser power, 2 kW,
are shown in Fig. 14a and 14b respectively. The noticeable feature is
the highly directional nature of the microstructure around the axis of
the laser beam. This is due to solidification of the weld metal at high
cooling rate compared to that of conventional GTA welding [7].
It can also be noticed that the higher the welding speed, the finer
the dendritic structure (Fig. 14b). This is attributed to an increase in
both solidification and cooling rates due to low heat input resulted
from high welding speed. Concerning the effect of laser power, the
higher the laser power, the coarser is the dendritic structure due to
decreasing cooling rate. However, the effect of laser power was
relatively less than that of welding speed.
[FIGURE 14 OMITTED]
Mechanical properties
Shear tensile test results of all overlap laser welds with
incomplete penetration showed that failure has taken place in the weld
metal. Vickers hardness measurements were carried on with a 50 g loading
force on the laser welding seam with different parameters. Figure 15 is
showing a cross-section of an overlap laser welding joint with diagram
of hardness. As expected, the hardness of the weld seam is higher than
that of the basement material. But the hardness of the weld seam center
is lower than that of the part between the center and the fusion line.
[FIGURE 15 OMITTED]
The cooling rate of the weld seam center is more moderate than that
of its surrounding part, and equiaxes dendrites come into being in the
center with the tissue of Polygonal Ferrite (PF)(as shown in Figure 16
(a)). In the zone between the fusion line and the weld seam center, the
cooling rate is rapid and columnate dentrites come into being there with
the tissue of Acicular Ferrite (AF)(as shown in Figure 16 (b)). Between
the two adjacent Acicular Ferrite (AF) there is martensite (M-A
constituent as the main element). The Cementite or Mmartensite and the
Acicular Ferrite (AF) have higher hardness than that of Polygonal
Ferrite (PF) explaining the lower hardness in the weld seam center.
[FIGURE 16 OMITTED]
Conclusions
The following conclusions can be drawn:
(1) The penetration depth increased with the increase in laser
power. And welding speed has a pronounced influence on size and shape of
the fusion zone. Increase in welding speed resulted in a decrease in the
fusion zone size.
(2) The fusion zone is symmetrical about the axis of the laser
beam. No welding cracks or porosity were found in any of the welds.
(3) The fusion zone is symmetrical about the axis of the laser beam
and no welding cracks or porosity were found in any of the welds. The
equiaxes dendrites are found in the weld seam center and columnate
dentrites between the center and the fusion line. The columnate
dentrites are toward the center and perpendicular to the fusion line.
(4) The Vickers hardness of the weld seam is higher than that of
basement material, and the micro harness in the center is lower than
that of the part between the center and the fusion line. The Cementite
or Martensite and the Acicular Ferrite (AF) in the part between the
center and the fusion line have higher hardness than Polygonal Ferrite
(PF) in the weld seam center.
References
[1] A. Kar, Mazumder. Mathematical modeling of key-hole laser
welding. J. Appl. Phys. 78 (ll), 1 December 1995, p6353-6360.
[2] B. N. Upadhyaya, et al. A highly efficient 5 kW peak power
Nd:YAG laser with time-shared fiber optic beam delivery[J]. Optics &
Laser Technology, 2008, (40): 337-342.
[3] J. Arata, et al., Trans. JWRI 5 (1976) 35.
[4] JIS G 4305:1999. Cold-rolled stainless steel plate and steel
belt technical standards. Japanese Standards Association. 1999.
[5] Maren Gast et al., In-Laminate Laser Soldering--A Gentle Method
to assemble and interconnect silicon solar cells to modules, 21st
European Photovoltaic Solar Energy Conference, Dresden, Germany, 2006
[6] N. Suutala, Meta. Trans.14 A (1983) 191.
[7] S. A. David, et al., Welding J. 66 (1987) 289.
[8] S. A. David and J.M. Vitek, Laser in Metallurgy, conference
proceedings of the Metallurgical Society of AIME (1981) p. 147.
[9] T. Zacharia, et al., Metall.Trans. 20 A (1989) 1125.
[10] T. Zacharia, et al., Welding J. 68 (1989) 12.
[11] Wang Hailin, et al.. Laser Welding of 8 mm Thick Stainless
Steel Plates. CHINESE JOURNAL OF LASERS. 2003, 5, 463-466.
[12] Xue Kezhong. Selecting Material for Railway Vehicles bady [J].
Reaching of railway Communication, 2003 (1):14-18.
Wang Hongxiao (a), Wang Chunsheng (b), Shi Chunyuan (a), He
Guangzhong (b), Wang Ting (b) and Xiao Jingfei (b)
(a) Institute of Materials Science and Engineering; Dalian Jiaotong
University; Dalian; 116028;
(b) Changchun Vehicle Railway Cooperation Ltd.; Changchun; 130062
Email: magie527@hotmail.com
Table 1: Chemical components and mechanical properties of SUS301L [4].
Chemical components/%
C Si Mn P S Ni Cr N
<0.0 <1. <2. <0.04 <0.0 6.00- 16.00- <0.2
30 00 00 5 30 8.00 18.00 0
Mechanical properties
[sigma]s/ [sigma]b/ [delta]/%
C MPa MPa HV
<0.0
>685 >930 >20 <218
30
Table 2: The laser welding parameters.
Laser Welding speed Heat Input [H.sub.I] Defocusing distance
power S(mm/s) (KJ/mm) [D.sub.d](mm)
P(Kw)
1.5 22 0.07 0
1.7 22 0.08 0
2.0 16,19,22,25,28 0.13,0.11,0.09,0.08,0.07 -2,-1,0,1,2
2.3 22 0.10 0
2.5 22 0.11 0
