Welding the Same Part Twice: Does the Second Weld Open the First?
As soon as a part has several joints, or an internal assembly is welded before the housing, this question appears: does vibration from the second weld crack or open the first? It usually does not fail on the spot; strength quietly drops and the failure appears after assembly.
Why an existing joint is affected
A finished weld is a rigid connection and vibration travels through it. Three situations cause trouble.
Resonance. If the structure containing the first joint has a natural frequency near the frequency of the second weld, energy is amplified there and the joint sees alternating stress far above expectation.
Reflection and standing waves. Vibration reflects at interfaces within a structure and amplitude adds at certain locations. An existing weld is itself an acoustic impedance interface and readily becomes a point where energy concentrates.
A second thermal exposure. Where two joints sit close together, heat from the second reaches the first. A solidified joint reheated towards its softening range while also being vibrated can be destroyed.
Principles for sequencing
Inside out. Weld the interior first and the exterior after, so that at every step the finished joints are downstream of the load path rather than upstream. Welding the housing first puts the housing joint in the path of every later internal weld.
Hardest first. Put the most demanding joint early, so the effect of later operations can be assessed in validation. A critical joint left until last is made on a part whose state has already been changed by earlier steps, and its parameters are harder to stabilise.
Separate them. Weld together whatever can be welded together in one shot, and where separate operations are unavoidable, put as much distance between them as possible.
Practical measures
Support the existing joint in the fixture. The most effective single measure. During the second weld the fixture should provide rigid support on both sides of the first joint to suppress relative motion there. No relative motion, no alternating stress.
Use different frequencies. Welding the two joints at different frequencies avoids resonance — for example internal components at 35kHz or 40kHz and the housing at 20kHz or 15kHz. That also satisfies the separate rule of high frequency for small parts and low frequency for large ones. Our range covers 15kHz to 40kHz, so a multi-stage product can be specified station by station.
Reduce the energy of the second weld. Less energy means less reaching the first joint, which requires an efficient joint design on the second.
Shorten the time. Conduction needs time, so a short, high-amplitude combination has less thermal effect on a neighbouring joint.
How to validate
The essential point is to test the first joint after all operations are complete, not when it has just been welded. This is routinely missed: process validation at station one happens at station one, the part is released, and nobody measures what later stations did to it.
The correct approach is to take finished assemblies, destructively examine the strength and fusion of the first joint, and compare against samples that received only the first weld to see how far it fell. The process stands only if the drop is within an acceptable range.
Sealed parts need one more item: leak test after all operations, not only after the first weld.
Fix the sequence at design
Welding sequence is not something the production line arranges; it belongs in the design. Structure, joint positions and how internal components are held all determine which sequences are feasible. Putting multi-stage interaction into the design review costs far less than discovering it during commissioning.
Joint design and multi-stage sequencing are on the downloads page. Machines from 15kHz to 40kHz, energy mode and per-part monitoring are on the ultrasonic welding machine pages.
Send the structure and the intended sequence for a multi-joint part to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.