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Study on the impact characteristic of spot welds base on high speed tensile method
Yongqiang Zhang, Jianbin Ju, Can Fu, Xiaopei Wang, Rigui Yi
( Shougang Research Institute of Technical, Beijing 100043, China)
Abstract: Spot welding is the most widely used technology in the
modern car body. The impact characteristic of spot welds is very
important in vehicle crash safety. In this paper, the effects of loading
speed on impact characteristic of spot welds have been studied by the
method of high speed tensile experiments. It shows that the RSW joint
strength increases with the increase of tensile speed. With the increase
of base material strength, base material thickness and tensile speed,
the trend of interface failure is increasing. The strength of RSW joint
decreases sharply, once the interface failure occurs. The results of this
study may be useful and guiding for the correct material selection and
reasonable welding process in body design.
Key words: spot welding; high speed tensile; impact characteristic
DOI: 10.7512/ j.issn.1001-2303.2017.13.15
0 IntroductionWith the rapid development of the national economy and the
increase of national income and living standards, the demand for
automobiles has increased year by year. At the same time, the society
also has a higher request for automobile quality, especially vehicle
crash safety. At present, the research on the vehicle crash safety is
more concerned with the material itself, but there is less study on the
impact characteristic of the joints between materials. However, during
the actual impact, the joints often break down first, so it is necessary
to study the impact characteristic of the connection joints in the body.
In the white body, the resistance spot welding is the most widely used
method in body welding due to its lower cost, simple control and
automatic process. The working principle of resistance spot welding
is to compress the welding workpieces between two electrodes and
make them heated to a molten or plastic state to form a metallurgical
bond using resistive thermal effect produced by the current flowing
through the contact surface of the workpieces and the adjacent area.
Resistance spot welding is an important connection method in the
automobile manufacturing industry. According to statistics, the number
of body welds for each car is 4000 to 5000[1--2]. In this paper, the impact
Yongqiang ZhangEmail: [email protected]
characteristic of spot welds under different tensile velocity was studied
base on high speed tensile method.
1 Experiment conditionsThe experiment materials were DP780 of 1.2mm and 1.6mm,
340LA of 1.2mm. The spot welding experiments were carried out on
medium frequency DC spot welding equipment. To improve the stability
of the welding results, the spot welding samples were held by the robot.
Fig.1 Spot welding equipments
Yongqiang Zhang , born in 1978, Native place of Hebei Province, Senior Engineer, Ph.D in Materials Science and Engineering. Currently working as an engineer at Shougang Research Institute of Technology. The main areas of research are welding technology of various materials, sensing and process control. Focus on the connection technology of automotive steel, the joining technology of dissimilar materials for automobile steel and lightweight materials, and the study of joint characteristics, including mechanical characteristics, impact characteristics and fatigue characteristics. Nearly 10 patents have been authorized and more than 30 papers have been published.
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increase of the tensile speed, the peak load of the spot weld increased
from 16.0kA under the static load to 22.3kN under the tensile speed
of 5m/s and then gradually become stable. When the welding current
was 9000A, the nugget diameter was 7.6mm.With the increase of the
tensile speed, the peak load of spot weld increased from 17.3kA under
the static load to 34.6kN under the tensile speed of 5m/s. When the
tensile speed increased to 10m/s, the peak load decreased sharply to
21kN. Figure 4 shows the damage process of spot welds with button
mode failure which was photographed by high speed photography,
the material was 1.2mm DP780, welding current was 9000A and the
tensile speed was 5m/s.
Through the observation of broken specimens, it can be found that
the interface failure occur at the condition of 9000A current and 10m/s
tensile speed. In other cases, 1.2mm DP780 had button mode failure.
The tensile tests of the 1.2mm 340LA spot welds were carried out
under five different tensile speed: 0.01m/s, 0.1m/s, 1m/s, 5m/s, 10m/s and
the results were compared to the 1.2mm DP780, as shown in figure 5.
When the welding current was 9 000A, the welding nugget diameter
of 1.2mm 340LA was 6.7mm.With the increase of the tensile speed,
the peak load of the spot weld increased from 9.6 kA under the static
load to 18.9kN under the tensile speed of 5m/s and then gradually
become stable. When the tensile speed was from 0.01m/s to 10m/s,
the spot welds were all button mode failure. Under the tensile speed of
5m/s, the peak load of the DP780 spot weld was twice that of 340LA
spot weld. When the tensile speed reached 10m/s, the peak load was
decreased because of the interface failure of the DP780 spot weld
Fig.2 High speed tensile test device
The experiment equipments are shown in Figure 1.
A spot welding electrode of 6mm diameter was selected and the
electrode material is CrZrCu. The welding parameter, as shown in table 1,
identifies the current range of three different specification materials.
When the diameter of the nugget reaches 4√ ( t is the thickness of
the material), the current is defined as the lower limit current. When the
welding spatter occurs, the current is defined as the upper limit current.
The welding current range of three kinds of different specification
materials are determined by the welding tests, as shown in table 2.
It is generally believed that the quality of the spot welds in the current
range can meet the requirements of the welding quality standard of the
automobile factory. The effect of different tensile speed on mechanical
properties of spot welds was studied by welding 1.2mm DP780 steel
at the current of 9kA. The effect of welding process parameters on
mechanical properties of spot welds was studied by welding 1.2mm
DP780 steel at the current of 8kA and 9kA. The effect of material
thickness on mechanical properties of spot welds was studied by
welding DP780 steel 1.2mm and 1.6mm at the current of 9kA. Under
the above experimental conditions, the nugget diameter is shown in
Table 3。
Table 3 Welding current and welding nugget size
Specificationsand grades
Welding current/A
Nugget size/mm
Static load force/kN
1.2mm DP780 9 000 7.6 17.3
1.2mm DP780 8 000 6.4 16.0
1.6mm DP780 9 000 7.1 25.3
1.2mm 340LA 9 000 6.7 9.6
The tensile tests were carried out on the high speed tensile testing
machine. For convenience of observation, the speckle was sprayed on
the spot before the tests. High speed photography was used to observe
the damage of spot welds in the tensile process. High speed tensile test
device is shown in Figure 2。
2 Experiment resultsThe tensile tests of the 1.2mm DP780 spot welds were carried out
under five different tensile speed: 0.01m/s, 0.1m/s, 1m/s, 5m/s, 10m/
s and the experiment results are shown in figure 3. When the welding
current was 8 000 A, the nugget diameter was 6.4 mm.With the
t
Table 1 Welding parameters
Specificationsand grades
Electrode pressure/ kN
Welding time/ms
Holding time/ms
1.2mm DP780 3.1 240 200
1.6mm DP780 4.2 300 200
1.2mm 340LA 3.1 240 200
Specificationsand grades
The lower limit current/ A
The upper limit current/A
1.2mm DP780 7 200 9600
1.6mm DP780 7 600 10800
1.2mm 340LA 7 600 10700
Table 2 Welding current range
Fig.3 Effect of tensile speed on peak load of 1.2mm DP780 spot welds
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which was only slightly larger than the peak load of 340LA spot weld
under the same tensile speed.
The tensile tests of the 1.6mm DP780 spot welds were carried out
under five different tensile speed: 0.01m/s, 0.1m/s, 1m/s, 5m/s, 10m/s and
the results were compared to the 1.2mm DP780, as shown in figure 6.
When the welding current was 9000A, the welding nugget diameter
of 1.6mm DP780 was 7.1mm. With the increase of the tensile speed,
the spot welds were all in the form of interface failure. The fluctuation
of the peak load was very large and the stability of the data was poor,
but the overall trend was increasing. In the case of tensile speed of 5m/s,
the peak load was 24.5kN which was much less than the peak load of
1.2mm DP780 spot weld due to the interfacial failure of 1.6mm DP780
spot weld.
The results of the high speed tensile test indicate that the increase
of the base material strength, the material thickness and the welding
nugget diameter have all increased the tendency of the interface
failure. The button mode failure is the damage of the base material or
the heat affected zone around the spot weld. When the button mode
failure occurs, the strength of the spot weld interface is greater than
the strength of the material around the spot weld. With the increase
of the material strength, welding nugget diameter and the material
thickness, it usually takes a greater force to destroy the material around
the spot weld. When the force is greater than the interface strength,
the interface mode failure occurs. On the other hand, in the high speed
impact process, the effect of internal defects (such as crack, as shown
in figure 7) on the quality of the weld nugget is significantly higher than
that in the static loading and the tendency of the interface failure is
increased. So it is very important to control the quality of spot weld and
Fig.4 The process of button mode failure under high speed tensile
(1.2mm DP780,welding current of 9kA,tensile speed of 5m/s)
reduce the internal defects to improve the RSW joint strength.
3 Conclusions(1) Under the condition of button mode failure, the RSW joint
strength is increasing as the tensile speed increases. When it reaches
certain speed, the RSW joint strength is stable.
(2) With the increase of base material strength, base material
thickness and tensile speed, the trend of interface failure is increasing.
(3) In the process of high speed tensile, once the interface failure
occurs, the RSW joint strength will decrease sharply.
References:
Fig.6 Effect of base material thickness on the peak load of solder joints
Zhang Yongqiang, etc. Study on the Effect of Mechanical
Properties on Weld Lobe of Zinc Coated Steel Sheet for Auto,
Proceedings of 2nd international symposium on automobile steel,
2013.5
Liu Zhaowei, etc. Advancement and trend of monitoring control
in resistance spot welding, Electric Welding Machine, 2006.9
[1]
[2]
Fig.5 Effect of base materials on peak load of spot welds
Fig.7 The crack at the interface of the nugget