1. OPTIMIZATION OF RESISTANCE SPOT WELDING PARAMETERS WITH USING FACTORIAL DESIGN APPROACH
Umut İŞ1, Rukiye ERTAN2, Merve TEKİN2 1 Oyak Renault Otomobil Fabrikaları A.Ş. Bursa/Türkiye , 2 Uludağ Üniversitesi, Müh. Fak., Otomotiv Müh. Böl., Görükle/Bursa
ABSTRACT
Resistance spot welding is the most preferred welding method in the automobile industry. Therefore, many studies have been made about the optimization of welding parameters. In this study, the welding parameters of the robot and hanging welding machine used in the auto body department were optimized by preserving the available welding quality related to the weld nugget diameter. 2,7 mm thick steel sheets were used in experiments. Firstly, controllable parameters were determined and sets of experiments were created in minitab software. For each experimental sets, parameters were repeated in 3 spot welds. According to the calculated nugget diameters, factorial regression analysis was performed in Minitab software and the effects of binary and triple interactions on the nugget diameter were interpreted with the graphs obtained. At the end of the study, 31% energy was saved per spot by preserving the available welding qualities.
Key words: Resistance spot welding, welding nugget diameter, welding parameters
1. INTRODUCTION
Approximately 800 pieces of pressed sheet metals and 450 pieces of machined parts are used in the production of a car. In the assembly of these parts, various methods such as rivets, bending, bolting, soldering and adhesive bonding are used, but the most commonly preffered method is joining with welding connection [1]. Resistance spot welding (RSW) is the most commonly used method to joining sheet metal car parts. There are 4500 spot weld joints in the body of a car [2]. The reason why the resistance spot welding is preferred in automobile industry is that it has advantages such as high-speed welding, lower costs and adaptability for automation in high-volume or high-rate production [3].
RSW is a welding process consisted through the heat generated by the electrical current and the force applied by the electrodes. The welds occurred by this method are not continuous, the joining happens in the area where electrodes apply pressure. In RSW, the weld quality is related to the weld nugget diameter and the weld nugget diameter is also influenced by the welding parameters [4].
2. EXPERIMENTAL AND ANALYSE PROCEDURE
In this study to optimize of the welding parameters, steel sheets of 2.7 mm thickness were used for RSW. The chosen welding parameters and their values are presented in Table 1.
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Figure 3. (a) Binary interactions of pressure and current parameters
(b) Binary interactions of pulse and current parameters
(c) Binary interactions of welding and forging parameters
Table 1. Maximum and minimum welding parameters
In another binary interaction (Figure 3-b), a high current and pulse values appear to be a favorable effect on the nugget diameter. The increase in current and pulse provides parallel increase in nugget diameter. For the binary interaction of welding and forging (Figure 3-c), in the region where forging values are low, there is no major effect of the change in welding time for the nugget diameter. But, in the region where the forging is high, the increase in the welding time is a significant influence on the growth of the nugget diameter. It is necessary to use high forging times to achieve high core diameter. Finally, due to the equality obtained for the nugget diameter, two confirmations (Table 2)were obtained that minimized the energy consumption without changing the quality of the weld. These confirmations have been tested by experiments and it is seen that the obtained nugget diameters are close to the expected nugget diameter values from the equation.
Welding parameters
Minimum values
Maximum values
Welding current
9500
11500
Electrode force
350
500
Squeeze time 10 40
Welding time 15
Forging time 70
Holding time 1 3
Pulse 2 4
The number of spots 5
250
Based on the parameters in Table 1, 32 sets of experiments were created and each set was repeated for 3 spot welds. After welding, the nugget diameters were measured and calculated their averages. Samples used in experiments are shown in Figure 1.
According to the calculated nugget diameters, factorial regression analysis was performed in Minitab software. Figure 2 shows the Pareto diagram which shows effect of parameters on the nugget diameters. In this diagram, it is seen that the most effective three
Table 2. Confirmations for energy saving and welding quality
Parameters
Confirmation 1
Confirmation 2
Current
11500
Force
500
Squeeze time 40
Weld time 10
Forging time 5
Hold time 1
Pulse 3
The number of spots 2
Figure 1. Auto body sheets used
in experiments
effects on the nugget diameter are the triple interaction of pulse, current and current-force-pulse, respectively. Although the effect of the pressure alone on the nugget diameter is not very major, it is major in binary and triple interactions (For example AB and ABH).
As a result of the analysis, it is seen that some parameters have no effect alone on the welding nugget diameter.It was found that when they interact with other parameters they affected the nugget diameter. Besides, it has been observed that the current, pressure, welding time and forging parameters have a direct effect on the welding nugget diameter.
At the end of the study, 31% energy gain per spot was achieved. Resistance spot welding machine has the new parameter values as given in Table 3.
Table 3. Available and optimum parameters values
Factor A B C D E F G H
Name
Current
Force
Squeeze time
Weld time
Electrode
Forging
Hold time
Pulse
Welding Parameters
Before optimization
After optimization
Current (A)
12500
11500
Force (daN)
550
500
Squeeze time (cycle) 10 40
Weld time (cycle)
Forging time (cycle)
Hold time (cycle) 2 1
Pulse 4 3
The number of spots 5
Energy per spot (Wh)
33.33
23.00
Figure 2. Pareto charts of standardized effects
3. RESULTS AND DISCUSSION
Different results were obtained on the diameter at different current values between current and pressure (Figure 3-a). For example, even if the pressure is low or high in the region where the current is low (9 500A), there is no significant effect on the nugget diameter. However, if the pressure is low or high in the region where the current is high (eg A), there is a significant effect on the nugget diameter. For this reason, unlike practical information, it is more convenient to work at high pressures at high currents.
4. REFERENCES
[1] Çil, E., 2009 "Investigation Of Weld Zone Characteristics Of Stainless Steel And Copper Parts Joining With Friction Welding", M.Sc. Thesis, TU Institute Of Science And Technology
[2] Devarasiddappa. D (2014), “Automotive Applications of Welding Technology – A Study”, International OPEN ACCESS Journal Of Modern Engineering Research, 4, 9, pp. 13
[3] Jou M. (2003), “Real Time Monitoring Weld Quality Of Resistance Spot Welding For The Fabrication Of Sheet Metal Assemblies”, Journal of Materials Processing Technology, 132, 1-3, pp
[4] Pashazadeh H.,Gheisari Y., Hamedi M. (2014), “Statistical Modeling And Optimization Of Resistance Spot Welding Process Parameters Using Neural Networks And Multi-Objective Genetic Algorithm”,Journal of Intelligent Manufacturing , 27, 3, pp
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