Detection of the real joint interface between ultrasonically welded aluminium strands (Al-strands) and copper terminals (Cu-terminals) connections using non-destructive testing methods

In the rapidly electrifying world of modern cars, the electrical system is the nervous system, carrying power from the battery to the motor and every control unit in between. As vehicles shift from 400-volt systems to more powerful 800-volt architectures to enable faster charging, the demand on these electrical connections has grown immense. To handle the heat and current, engineers often need to join two different metals: aluminum, which is lightweight and cheaper, and copper, which is an excellent conductor but more expensive. Joining these dissimilar metals is tricky because they behave differently under stress and heat. The industry standard for making these connections is ultrasonic welding, a process that uses high-frequency vibrations to press the metals together until they fuse without melting. However, because the welding happens so quickly and inside the material, it is difficult to see if the bond is truly solid or if hidden gaps remain. If a connection fails, the car could lose power or overheat, making it vital to find a way to inspect these joints without breaking them apart.

A team of researchers at RWTH Aachen University in Germany set out to solve this problem by testing various non-destructive methods to see the actual bonded area inside these aluminum-to-copper connections. They focused on a specific type of joint where a bundle of aluminum wires is welded to a copper terminal, a common setup in electric vehicle wiring. The researchers knew that the strength of the joint depends on how much of the aluminum actually fuses with the copper, but they needed a way to measure this invisible interface without destroying the part. To do this, they created two groups of test samples: one group welded with perfect settings to create strong, reliable joints, and another group welded with poor settings to create weak, defective ones. They then subjected these samples to a battery of different inspection techniques, ranging from thermal imaging to sound waves, to see which method could clearly distinguish a good bond from a bad one.

The first group of methods they tried relied on heat. The idea was that a well-bonded area would conduct heat differently than a poorly bonded one, creating a visible temperature pattern on the surface. The researchers used lasers and halogen lamps to heat the samples and infrared cameras to watch how the heat spread. However, they found that this approach failed completely for these specific materials. Because copper is such an excellent conductor of heat, any warmth introduced at one spot spread so quickly across the entire terminal that it smoothed out all the local differences. The cameras saw a uniform glow rather than the distinct patterns needed to spot a defect. The heat simply moved too fast through the metal to reveal the hidden gaps between the wires and the terminal.

Next, the team turned to sound, specifically using ultrasonic waves, which are high-frequency vibrations similar to those used in medical imaging. They tested two variations: one that used a laser to generate the sound waves and another that used a traditional probe with a water coupling. The laser method showed promise in theory, but it ran into a practical wall. The welding process leaves small, textured marks on the back of the copper terminal where the machine holds it down. These marks scrambled the laser-generated sound waves, making the images too noisy to interpret. The researchers found that they would have to mechanically grind these marks off the part before they could get a clear image, a step that is impossible in a real factory line where parts must be inspected as they come off the machine.

The method that finally worked was scanning acoustic microscopy, a technique that uses a focused probe to send sound waves into the joint and listen for the echoes. This approach proved to be the most effective tool for the job. When the researchers scanned the strong, well-made joints, the images showed a large, continuous area where the aluminum wires had successfully fused with the copper terminal. In contrast, the scans of the weak, defective joints revealed a very different picture: only small, scattered islands of bonding, with large gaps where the metals were merely touching but not fused. The images clearly showed that the poor welds lacked the extensive contact needed for a strong connection.

To confirm that these visual findings actually mattered, the researchers compared the size of the bonded area seen in the scans with the physical strength of the joints. They pulled the samples apart in a machine that measured how much force was required to break them. The results were clear and consistent. The samples that showed large, continuous bonded areas in the acoustic scans withstood significantly higher forces, ranging from 4,250 to 5,000 newtons. The samples with small, patchy bonded areas failed at much lower forces, between 900 and 1,600 newtons. This confirmed that the size of the invisible bond directly predicts how strong the joint will be. The study also looked at computed tomography, a type of 3D X-ray imaging, which could show where the wires were packed tightly but could not definitively prove if they were chemically fused or just pressed together. The acoustic method remained superior because it could specifically identify the metallurgical bond.

The study concludes that while many high-tech inspection methods exist, scanning acoustic microscopy is the most reliable way to see the true quality of these ultrasonic welds without damaging the part. It successfully bridges the gap between the invisible internal structure and the measurable strength of the joint. Although the equipment is currently too slow and complex for checking every single part on a fast-moving assembly line, the findings provide a powerful tool for engineers. They can now use this method to fine-tune welding machines, validate new designs, and ensure that the critical connections in electric vehicles are safe and robust. By understanding exactly how the bond forms and how to see it, manufacturers can move closer to a future where every electric car has a wiring system that is as reliable as the technology it powers.