The choice of AC or DC equipment can be driven by a number of unrelated variables. In general, two major areas drive the choice between AC or inverter DC: first are electrical considerations, second are weld quality considerations. The electrical considerations include the following:

It was determined that the most commonly used variable for weld quality is weld button size. This parameter is measured in production on a periodic basis to ensure the weld process is under control. The intent of this article is to examine how several key variables interact when welding DP-600 material with both AC and inverter DC.

The intercycle cooling effect of AC affects the heat input into the weld by changing the total time that current flows during weld time. Joule's law states Q=I²rt. With phase-shifted AC current, there can be a substantial amount of off time between half cycles. During this off time, the electrodes cool the weld metal, thus reducing the total heat over a given weld time.

Experimental Procedure
The material used to conduct this experiment is detailed as follows:

The equipment used to perform the test was as follows:

Common Equipment

Meters and Gauges

DC-Specific Welding Equipment

AC Welding Specific Equipment:

The experimental procedure for this test used the exact same welding equipment (with the exception of the transformer and weld control), welding the same material to investigate the effects of AC compared to inverter DC. The same weld button size was maintained for AC and inverter DC. By maintaining the same size weld button, weld quality issues such as tensile strength and cross tension could accurately be compared. Maintaining the same weld button size required that the AC to DC current be adjusted due to the intercycle cooling effect that was described previously. With the exception of the weld current, all other parameters were identical. Thirty-five samples of 1-mm G DP-600 were run for both AC and inverter DC. This experiment was duplicated on 1.50-mm G DP-600. The following weld parameters were used:

Table 1 shows the secondary current and the primary power.

Tests were started with fresh caps that had 50 conditioning welds before current settings were determined. Conditioning welds were done approximately 500­1000 A under the operating current. Table 2 shows the data that were collected for this experiment.

Tension and cross-tension testing procedures were performed as follows:

Samples were prepared with the following parameters as shown in Figs. 1 and 2. Figure 1 depicts a tensile shear coupon while Fig. 2 depicts a cross-tension coupon. The sample coupons followed industry convention.

Discussion
When comparing the effects of AC to inverter DC on DP-600 material, it is not a simple matter of one being good or bad. Certain unrelated variables will ultimately drive the decision of single-phase AC or inverter DC. The electrical evaluation of AC compared to DC shows differences in two major areas. When comparing the single-phase AC load to the three-phase inverter load, the balance load of the three phases becomes an improvement when considering electrical distribution. Along with a balanced three-phase load, the power factor of the inverter load is approximately 0.99 or unity. This also is an improvement when compared to typical single-phase AC with power factors ranging from 0.3 to 0.8 based on the design of the welding circuit.

Tensile shear and cross-tension testing helps determine the ultimate strength and the fracture mode of welded specimens. Unfortunately, the tensile shear test is a combination of tensile and shear and not pure shear. This combination of tensile and shear on the welded specimens will only affect the base material or the heat-affected zone. This is not a good indication of weld quality, but it is a good method of evaluating the consistency of the welding process used.

Fig. 3 — 1-mm G DP-600 welded with AC current.	Fig. 4 — 1-mm G DP-600 welded with inverter DC.
Fig. 5 — 1.5-mm G DP-600 welded with AC current.	Fig. 6 — 1.5-mm G DP-600 welded with inverter DC.

Cross tension applies stresses to the weld in a direction normal to the surface of the weld. This test is a better indication of the quality of the weld than the tensile shear test. But, again, one must understand that this test does have some peeling action occurring during it that does not apply all of the loading on the weld itself.

A metallograph is the best indication of the quality of the weld. It can be used to determine nugget size, penetration depth of the weld nugget, and porosity within the weld nugget.

For this study, the performed peel tests resulted in a consistent measured button size for the AC and inverter DC welds. Further investigation should be considered to better understand the difference between the measured peeled button and the measured nugget on the metallograph.

Conclusion
When looking at the decision of whether to use AC or inverter DC to weld DP-600 material, the following conclusions can be drawn:

Electrical

Weld Quality
Based on the data in Table 2, the application of AC or inverter DC welding had no major effects on the overall quality of the weld.

In summary, field experience along with this type of experiment will ultimately drive the final decision on which type of welding is best suited for the application. Current trends in automotive design indicate an increased application of thinner-gauge materials. Continued evaluation will be required to understand the advantages of one RSW method vs. the other as the industry moves toward thinner gauges of high-strength materials.

Based on a paper presented at the AWS Detroit Section's Sheet Metal Welding Conference XI, May 13­15, 2004.