Reversing DC and the Benefits of Joining Lightweight Materials
November 28, 2016
With the need to produce lighter weight vehicles, the switch from conventional steel applications to aluminum is beginning to take hold in the automotive industry. The aluminum applications have been primarily centered in closures. However, the auto industry is looking to expand the usage of aluminum into the Body in White (BIW). There will be a predicted increase from 4% of BIW as complete aluminum to 18% by 2025.
Need for High Production Processes
While Resistance Spot Welding (RSW) of aluminum is nothing new to the auto industry, until now it has been used in lower production rate vehicles or batch build closure applications. To meet the demand of CAFÉ standards, there is need to use it on high production lines. This need will, right or wrong, amplify the areas of concern that have plagued aluminum RSW in past applications.
And while the ability to reverse polarity with special AC systems (frequency conversion) is used in many aerospace aluminum spot welding applications today, the type of system used in aerospace applications is too large for robotic and automated type equipment and therefore, not conducive to high production processes.
The major inhibitor for the widespread use of aluminum RSW applications has been cap life. In general, DC systems may have lower electrode life, and a greater tendency than comparable AC systems of electrode-to-sheet sticking due to the increased erosion of the positive (anode) electrode. With the decrease in electrode life, manual interaction with tooling increases (electrode changes), this in turn decreases uptime of the machine and overall jobs per hour (JPH).
With the increase in applications using aluminum, different series of aluminum plus applications of 3 thickness (3T) welding are possible design options. However, with different series aluminum, polarity sensitivity can be present, especially when welding 3T applications. Therefore, change in the resistance and heating characteristics can make 3T welding very difficult in production applications.
It has been proven through prior studies that having the ability to change polarity when using DC to weld aluminum is helpful in increasing electrode life. It is also helpful in obtaining proper weld nugget growth when challenged with dissimilar alloys, and multiple 3T and even 4T thicknesses.
However, with robotic welding applications in high rate production, it is unrealistic to expect that robot programs/tooling can be modified to ensure proper polarity for each spot weld to resolve the RSW aluminum difficulties—robotic weld gun geometry and the assembly process present a big challenge.
The solution to this problem is provided when the welding transformer switches polarities by a simple signal from the weld controller. The signal system functions within the transformer and can be easily incorporated into existing welding controllers, for high rate production.
Increased Electrode Life & Control Nugget Migration
Past studies have shown an increase of 30%-60% electrode life when selecting the proper polarity to stack-up orientation. Now, a test study conducted by RoMan confirms the increase in electrode life by being able to control polarity within the weld schedule by means of a RoMan Reversing DC power supply (RDC). The test study also shows the effects of alternating the Anode and Cathode, and proves the ability to control nugget migration in dissimilar series aluminum welds.
To download the complete Reversing DC Test Study quantifying the Benefits of Joining Lightweight Materials with a RoMan RDC transformer, click the button below