Resistance Spot Welding of Complex Stack-Ups
– Application of MFDC Electric Servo-Gun Technology
Jerry E. Gould, PhD | Technology Leader, Resistance and Solid State Welding
New automotive designs are continuously employing new grades of steels (e.g., AHSS), as well as complex designs to meet requirements of structural rigidity, safety, and weight reduction. These materials and designs often result in combinations for resistance spot welding referred to as complex stack-ups. Complex stack-ups are those of greater than two sheets, in which the thicknesses individual materials vary. Of concern for resistance welding are those complex stack-ups in which a thin sheet is attached to a relatively thick combination of other materials. Under such conditions, there are concerns about achieving sufficient heat balance for resistance welding. Insufficient heat balance when resistance spot welding such stack-ups can result in, at worst, poor attachment of the outside sheet, or at best, reduced process robustness for the application. Historically, variations in electrode configurations have been used to improve heat balance in such applications. However, the manufacturing strategies in automotive plants have been to simplify electrode configurations, both in types of materials and allowable sizes. As a result, an electrode configuration based solution for complex stack-ups is not viable.
Recently, medium-frequency direct current (MFD) electric servo guns have become available for use in automotive production. These systems combine MFDC power with force application through electric servo motors and ball screws. The resulting systems have a much broader process capability than previous weld guns. Specific capabilities include the ability to apply forge forces and the ability to sequence currents with these forge forces.
In this program, the use of MFDC servo guns for resistance spot welding a specific complex stack-up has been investigated. The stack-up of interest here is of a thin-thick-thick variety. Weldability has been investigated using design-of-experiment (DOE) techniques. This DOE has been organized to characterize how a range of MFDC servo gun processing conditions affects weldability of this stack-up. In addition, the DOE has been designed to include some electrode variations. The results of this study suggest that electrode variables (geometry, conductivity) are still the most effective way of achieving heat balance in complex stack-up welds. However, it is understood that electrode variations cannot be considered a solution for welding complex stack-ups in high volume vehicle applications. Using similar geometry and material electrodes, nugget penetrations into the thin attached sheet on the order of 40% could be achieved by using a combination of short weld times, high forces, and an early application of forge force. It was also noted that these high penetrations were accompanied by a relatively high indentation (~0.5-mm) on the thin sheet side of the stack-up. The results suggest benefits of using the features offered by MFDC electric servo-gun systems for such complex stack-up applications.
For more information, please contact Jerry Gould at 614.688.5121 or jgould@ewi.org. |
