Page 34 - MetalForming-January-2019-issue

Welding Well
By Robert K. Cohen
Adaptive Controls Optimize the Resistance
Seam-Welding Process—Part One
Wheel starts to roll up onto the part
Current starting after wheel already is rolling up onto part, resulting in undersized weld on front edge
Displacement data (top) and current data (bottom) collected with WeldView monitor
Using the resistance seam-welding (RSEW) process to make gas-tight seals involves making a series of overlapping spot welds. Each spot must be a fully formed nugget-free of expulsion. Insufficient heat produces undersized spots and can cause leaks. Excess heat produces expulsion, which also can cause leaks. In addition to con- trolling nugget formation, the process requires consistent spot spacing to ensure proper nugget overlap.
Welding-electrode selection represents a critical success factor, as well as applying the right electrode force, current, and weld time. The machine control must provide accurate delivery of the programmed current for each weld.
Among the biggest challenges represented by welding operations using conventional controls: Maintain consistent electrode force and consistent electrode contact area on the part being welded. Any increase in electrode contact area or weld force, or decrease in welding current, will reduce the size of each weld. Conversely, any decrease in electrode contact area or weld force, or increase in welding current, will increase the size of each weld.
When faced with weld-size variations due to electrode con- tact-area and applied-force fluctuations, operations using con- ventional controls that apply the same heat regardless of these variations are plagued with inconsistent welding performance. In these situations, adaptive controls that instantaneously react to the variations can reduce the occurrence of problem welds and increase the consistency of all welds produced.
RSEW machines efficiently make gas-tight seals by employ- ing electrode wheels that roll to the next location to form each successive weld. When integrated with a capable control, these machines can accurately control spot spacing without having to add any special positioning mechanisms or tooling.
Fabricators select between two basic modes for RSEW: inter- mittent (commonly referred to as “roll spot”), and continuous.
Intermittent Seam Welding
During intermittent seam welding, the electrode wheels advance to the desired position and stop to make each weld.
Robert K. Cohen is president of Troy, NY-based Weld- Computer, a company that he founded in 1987 with a mission to solve manufacturers’ resistance-welding challenges and to ensure weld quality and consisten- cy. WeldComputer engineers, manufactures, sells and supports welding control and monitoring solutions for aerospace, defense, medical, electronics, automotive, appliance, industrial and general manufacturing. Robert K. Cohen, President/Founder
WeldComputer Corp. rcohen@weldcomputer.com
Fig. 1—When applying current too late, an undersized front-edge weld results.
The process repeats over the entire length of the seam.
The dynamics of intermittent seam welding are similar to those for spot welding. The control can direct the length of time required to form good welds, as well as all other actions typically employed by an adaptive control to regulate weld quality. In addition to dynamically adjusting the pro- grammed current to compensate for instantaneous elec- trode-force and contact-area variations, such actions may include automatically correcting for part-surface contami- nation and poor part fitup. Also, in instances when expulsion occurs, the control can instantly terminate weld heat within 1 msec., and automatically generate a repair weld in place. Production throughput is limited by how quickly the elec- trode wheels can accelerate from a stationary state after each weld, move to the next weld position, and completely
stop to form the next weld.
Continuous Seam Welding
During continuous seam welding, the wheels continue rolling as the welds form. Unlike the intermittent process, continuous seam welding imposes the constraint of a fixed time window to make each weld. Since there is no opportunity to vary the weld time for each nugget, all adaptive decisions and compensating actions must occur as each weld forms.
The continuous process allows production at much higher speeds than can be achieved with intermittent seam welding. However, ensuring a repeatable process, when using a con- ventional control, requires good control of velocity. Excess wheel velocity will cause cold welds; insufficient wheel
32 MetalForming/January 2019
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