Influence of welding current shape on expulsion and weld strength of ...

Influence of welding current shape on expulsion and weld strength of resistance spot welds P. Podrzˇaj*, I. Polajnar, J. Diaci and Z. Karizˇ The present paper presents the influence of welding current shape on weld strength of resistance spot welds of zinc coated mild steel sheets. The influence is analysed at different levels of the electrode wear. Welding currents with different peak values and different RMS (root mean square) values were used in the experiment. The results show that welding current with high peak values implies higher weld strength. Keywords: Resistance spot welding, Cold weld, Expulsion, Weld strength, Welding current shape

Introduction Resistance spot welding (RSW) is a material joining process that produces coalescence of faying surfaces of the workpieces with the heat obtained from resistance to welding current through the workpieces held together under pressure by electrodes.1 The amount of generated heat is the most influential variable for resistance weld quality. The heat generation is the most intense at the sheet/sheet interface owing to the highest resistance to welding current flow. As a result, a volume of molten material (welding nugget) is formed at this interface. The size of welding nugget depends primarily on heat generation and is extremely important for weld strength. For mild steels, larger welding nugget generally implies higher weld strength. RSW is a commonly used material joining procedure in mass production. There are for example few thousands of spot welds on an automobile body2 and improving their quality is a continuing process in RSW research.3–11 Current research in the area of weldability, electrode wear and spot weld quality is stimulated by increased use of zinc coated steels in robotised mass production in automotive industry.12–15 There are two types of errors if the amount of generated heat is not appropriate: (i) if the amount of generated heat is to low, the welding nugget is undersized or it exhibits brittleness during tearing. Both cases are considered unacceptable.16 The resulting weld is called cold weld (ii) if the amount of generated heat is to high, the welds experience expulsion on welding are, therefore, considered unacceptable.16 Namely, welding nugget volume cannot exceed certain limits. Faculty of Mechanical Engineering, University of Ljubljana, Asˇkercˇeva 6, 1000 Ljubljana, Slovenia *Corresponding author, email [email protected]

ß 2006 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 15 December 2005; accepted 21 January 2006 DOI 10.1179/174329306X101391

Otherwise the solid material around the welding nugget is not able to withstand the pressure of the electrodes anymore. As a result the collapse of the welding area (expulsion) takes place. The amount of generated heat should therefore be between cold weld and expulsion. The basic parameters that affect the amount of generated heat are: (i) the shape and diameter of the electrode tips. They are important for contact resistances. Larger electrode tip diameters generally imply lower contact resistances (ii) welding force. Welding force is important because it determines the contact resistances. Higher welding force implies lower contact resistance (iii) welding current. Higher welding current implies a greater rate of heat generation (iv) welding time. Longer welding times imply a larger amount of generated heat assumed quasiconstant rate of heat generation. The amount of generated heat alone is not enough to determine the size of the welding nugget. The shape and diameter of electrode tip have important influence on the process of heat generation and therefore on weld nugget growth, particularly during welding of zinc coated steel. There is therefore in general no single combination of appropriate parameters, but a set of them as shown in Fig. 1. The range of acceptable welding parameters depends on the state of electrode tips. As the tips gradually degrade during successive numbers of produced weld spots, the range of acceptable parameters moves towards higher currents.17,18 Electrode tip degradation is particularly severe in welding of zinc coated steel. The range of acceptable welding parameters changes much more rapidly in this case. The appropriate parameter selection is usually obtained with experiments and is then documented in the form of welding schedules,19 which usually apply welding current in its RMS value. The present paper discusses the difference between different welding current curves with the same RMS value at different stages of electrode life.

Science and Technology of Welding and Joining

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Influence of welding current shape on resistance spot welds

1 Range of acceptable welding currents and times depends on number n of successive weld spots produced by pair of electrode tips: all other welding parameters are assumed constant

Experimental Single phase resistance welding machines use thyristors as the welding current source. These semiconducting elements are capable of controlling high currents (several kA). The thyristor is fired with a pulse on the gate. Two thyristors are needed in single phase ac welding machine as shown in Fig. 2. Each of the thyristors is used to conduct the welding current iw one way as shown in Fig. 3. The welding current conduction is initiated with the pulse on the gate of the thyristor (iG1 or iG2) and stops at the end of the half period of the net frequency. The pulses are generated by the controller whose inputs are welding parameter settings and a switch which is used to start the welding process. AC welding machines employ a welding transformer (Fig. 2) to transform voltages to lower values. The welding transformer setting can therefore be used to adjust the welding current iw (the current in secondary circuit). The RMS welding current iRMS is defined by the following equation 0 t 11=2 ð0 1 iRMS ~@ i2 (t)dtA (1) t0 w where iw is the welding current (time dependant) and t0 is the period of current oscillations. RMS welding current

a high transformer setting T – high current amplitude and small firing angle a; b low transformer setting T – low current amplitude and large firing angle a; c current pulses which control thyristors to obtain current waveform in b 3 Welding current shapes exhibiting same RMS values: t, time; imax, maximal welding current; imax,t, theoretical maximal welding current; tc, welding current conduction time; iG1, gate current (first thyristor); iG2, gate current (second thyristor)

2 Welding source with thyristors

can therefore be controlled by a transformer setting T and a firing angle of the thyristor a. The same RMS value of the welding current can be obtained by different values of the firing angle a (thyristor setting) and the maximum value of the welding current imax (transformer setting) as shown in Fig. 3. It is already known from the comparison between the ac and dc resistance welding that high current peaks are more likely to cause an expulsion.20–22 The authors, however, also wanted to study the influence of the welding current shape on the weld strength with welding currents ranging from the cold weld to the expulsion and throughout the electrode life. Stationary welding machine Elektroda TA 60-S was used in the experiment. There are nine possible firing angle levels and five possible transformer settings which

0


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4 Weldpieces used in experiment: DIN 50124

result in the nominal RMS current range from 1 to 15 kA as shown in Table 1. The welding force was 5 kN and welding time 300 ms. Approximately 5 spots/ min were made during the experiment. The weldpieces were made of 1 mm thick zinc coated steel sheets as shown in Fig. 4. The electrode caps used were type G Nr.66.0k (D513 mm, d55 mm). The welds were made at different nominal RMS welding current settings ranging from the cold weld to the expulsion area at different levels of electrode wear: (i) new electrode (0 weld) (ii) electrode after 100 welds (iii) electrode after 250 welds (iv) electrode after 400 welds (v) electrode after 600 welds (vi) electrode after 900 welds (vii) electrode after 1300 welds (viii) electrode after 1700 welds. The nominal RMS welding currents were constant during welding of a certain spot weld (no current slope was used). The measurements were repeated three times at each level of the electrode wearout. All the specimens obtained were tested for tensile strength on a ZWICK Z050 machine with ZWICK 8406 50 kN grips and a MultiSens Type 6336.107 extensiometer. The test speed was 10 mm min21. The test X-pert software was used to conduct the measurements.

Results Figure 5 shows the expulsion occurrence at different nominal RMS welding currents and firing angle settings for a new electrode pair. If parameter a setting is equal to four (ordinate) and transformer setting T is changed from 1 to 5, nominal RMS welding currents will change from 4.75 (T51) to 6.47 kA (T55) on abscissa. All welds corresponding to these parameters are normal ones without expulsion. If parameter a setting is increased to 5, it can be seen that expulsion welds occur as the transformer setting is increased to T54 or 5. However, if parameter a setting is increased to 6, expulsion welds are made even at the lowest transformer setting (T51). Similarly, if constant transformer setting T51 is taken for example and parameter a setting is

5 Expulsion occurrence as function of nominal RMS welding current and firing angle: new electrode

increased from 4 to 7, it can be seen that parameter a setting 5 corresponds to normal welds and parameter a setting 6 to expulsion ones. This means that if any of the parameters (a or T) is increased, we move towards the expulsion area. However, Fig. 5 suggests that nominal RMS welding current alone can be used to separate expulsion points from normal ones. Parameter combinations with nominal RMS welding current below ,6.5 kA correspond to normal welds and parameter combinations above ,7 kA correspond to expulsion area. If however the results in Fig. 6 are observed when 900 welds have already been made, it can be seen that this is not the case. There is namely a parameter combination (a59, T51) at nominal RMS welding current of 10.7 kA with normal welds and a parameter combination (a57, T54) at a lower nominal welding current of 10.55 kA with expulsion welds. This fact suggests that nominal RMS welding current alone is not enough for expulsion prediction. The shape of the welding current curve is important as well. High peak values of welding current (high T) are much more likely to cause the expulsion occurrence than welding currents of the same RMS value but lower peak values (lower T). Figure 7 shows the results after 1700 welds, when the electrode is slowly

Table 1 Nominal RMS welding currents iN (kA) for different firing angle levels a and transformer settings T TQ aR 1 1 2 3 4 5

1.19 1.30 1.43 1.51 1.62

2

3

4

5

6

7

8

9

2.60 2.86 3.01 3.23

3.56 3.89 4.29 4.52 4.85

4.75 5.19 5.72 6.03 6.47

5.94 6.49 7.15 7.54 8.09

7.13 7.79 8.58 9.04 9.70

8.32 9.09 10.01 10.55 11.32

9.50 10.38 11.44 12.06 12.94

10.70 11.68 12.87 13.56 14.55

6 Expulsion occurrence as function of nominal RMS welding current and firing angle: after 900 welds


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7 Expulsion occurrence as function of nominal RMS welding current and firing angleafter 1700 welds

9 Weld strength v. nominal RMS welding current at different welding current shapes after 100 welds

being worn out. It can be seen in this case that welds with and without expulsion are made at the same settings. It is thought that uneven mushrooming of the electrode face increases randomness of the process outcome such as expulsion/no expulsion weld. But higher peak values are in general still more likely to cause expulsion. Also, Figs. 5–7 also show that the expulsion occurs at higher welding currents as the electrode wears out. This is due to the larger contact area which implies lower welding current densities. Figure 8 shows a plot of weld strength versus the number of welds made with an electrode pair at a constant parameter setting. As expected and already known, the weld strength decreases with the number of welds made owing to larger contact area between electrodes and weldpieces and consequently lower current density. It can also be observed that the weld strength does not dissipate much among the repeated measurements. Figures 9 and 10 show the plots of the weld strength versus the nominal RMS welding current at different welding current shapes. The parameters that define the welding current shape are the transformer setting T and the firing angle a. In theory the same nominal RMS welding current can be achieved with different combinations of parameters T and a (low T and high a and vice

versa, as already indicated in Fig. 3). Because the welding machine used for the present experiment is of only limited sets of parameter settings, as shown in Table 1, similar nominal RMS welding currents can be achieved with different parameter settings. For example, nominal RMS welding current of 10.55 kA can be achieved with a57 and T54 (similar current shape as the one in Fig. 3a) and nominal RMS welding current of 10.70 kA can be achieved with a59 and T51 (similar current shape as the one in Fig. 3b). If welds made with these two nominal RMS welding currents are observed in Fig. 10, it can be seen that nominal RMS welding current alone cannot be used to distinct appropriate weld region from expulsion region as shown in Fig. 1. All three welds made with lower nominal RMS welding current (10.55 kA) are namely expulsion ones, whereas welds made with higher nominal RMS welding current (10.70 kA) are all without expulsion. Cold weld region was estimated based on the weld strength and visual inspection of welds as a region with a weld strength below 1500–2000 N in both cases. It can be seen in both figures that the welds closer to the cold weld region increase in weld strength if parameter a is increased. But as the expulsion area is closer, an even

8 Weld strength v. number of welds made: iN57130 A, a56

10 Weld strength v. nominal RMS welding current at different welding current shapes after 1300 welds


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greater impact of parameter a can be observed (Fig. 10). In this case, however, the weld strength tends to decrease with increasing values of parameter a. This fact is extremely important because the best quality welds are made in this region (high weld strength and no expulsion).

Conclusions The results of the experiment have shown that despite the fact that welding schedules generally give recommendations about the RMS welding current, it is not enough to avoid expulsion. The shape of the welding current curve is of great importance as well. High peak values of the welding current are much more likely to cause expulsion than welding currents of the same RMS value but lower peak values. Also, the shape of the welding current is important to the weld strength as well. The welds close to the expulsion region made with high peak values of welding current have higher welding strength than those made with the welding current of the same RMS value but lower peak values.

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