· ·158

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: zhangyongqiang@shougang.com.cn

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.

· ·159

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

· ·160

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