Joining dissimilar metals—such as aluminum to steel, copper to stainless, or titanium to nickel alloys—is the “final frontier” of industrial welding. As automotive manufacturers push for lighter multi-material BIW (Body-in-White) structures and EV battery producers struggle with high-conductivity copper-to-aluminum busbar joins, the margin for error in the weld zone has shrunk to near zero.
The challenge isn’t just the difference in melting points; it’s the physics of the interface. When two metals with vastly different thermal conductivities and expansion coefficients meet, the resulting heat-affected zone (HAZ) often develops brittle intermetallic compounds (IMCs). If the heat input is too high, the IMC layer grows thick and the joint fails under stress. If it’s too low, you get lack of fusion.
Thermal imaging has emerged as the critical tool for navigating this narrow “Goldilocks zone.” By providing real-time, high-resolution temperature profiling of the asymmetric heat distribution, inline monitoring systems allow manufacturers to control the interface chemistry in ways that traditional visual or electrical monitoring cannot.
The Physics of Dissimilar Metal Joins
In a standard steel-to-steel weld, the thermal properties are symmetrical. In a dissimilar join, the thermal gradient is skewed.
Consider a Steel-to-Aluminum laser weld:
- Steel melting point: ~1500°C
- Aluminum melting point: ~660°C
- Thermal conductivity: Aluminum is roughly 4-5x more conductive than steel.
If you center the heat source on the joint line, the aluminum acts as a massive heat sink, pulling energy away, while the steel side overheats. This asymmetry leads to uneven penetration and, more dangerously, the formation of brittle Fe-Al intermetallic layers. Research shows that keeping the IMC layer thickness below 10 micrometers is essential for structural integrity—a feat that requires micro-second control of the cooling rate.
The IMC Risk: Intermetallic compounds are essentially “glass-like” layers between the metals. They are hard but extremely brittle. Thermal imaging is the only non-contact method capable of monitoring the cooling rate (t8/5) in real-time to predict and prevent excessive IMC growth.
How Thermal Imaging Monitors the Interface
Unlike a single-point pyrometer, a thermal camera captures a 2D map of the entire weld zone. For dissimilar metals, this spatial data is vital for three reasons:
1. Monitoring Asymmetric Heat Distribution
Inline systems like HeatCore AI track the temperature gradient across the joint. By analyzing the “thermal footprint,” the system can detect if the arc or laser beam has drifted too far toward the more conductive metal, which would cause a sudden drop in peak temperature and a failure to fuse the less conductive side.
2. Real-Time Cooling Rate (t8/5) Analysis
The time it takes for the weld to cool from 800°C to 500°C (t8/5) determines the final microstructure. In dissimilar welding, the cooling rate on the “fast” side (e.g., copper or aluminum) can be so rapid that it induces quenching cracks, while the “slow” side (steel) stays hot long enough to grow thick IMCs. Thermal imaging monitors both sides simultaneously, flagging deviations that fall outside the qualified ISO 15614-1:2017 procedure.
3. Detection of “Cold” Defects in High-Conductivity Metals
Copper and aluminum are notorious for “cold starts” and lack of fusion because they pull heat away from the start of the weld so quickly. A thermal camera detects the “thermal lag” at the start of the bead, allowing the system to signal the robot to adjust travel speed or power dynamically to compensate.
Applications in Key Industries
EV Battery Manufacturing: Copper to Aluminum
The most common dissimilar join in the EV world is the battery tab-to-busbar connection. This usually involves laser welding thin copper to aluminum. Because copper reflects laser energy and aluminum dissipates it, the process window is incredibly narrow. Thermal monitoring ensures that the laser energy is sufficient to penetrate the copper without vaporizing the aluminum underneath.
Aerospace: Titanium to Nickel Alloys
In jet engine components, titanium is often joined to nickel-based superalloys. These materials are prone to solid-state cracking if the thermal cycles are not strictly controlled. Thermal imaging provides the “digital twin” of the thermal history required for aerospace traceability under ISO 3834-2:2021.
Automotive: Multi-Material BIW
Modern car frames use high-strength steel (HSS) and aluminum to balance weight and safety. Resistance spot welding (RSW) of these combinations is notoriously difficult. Thermal imaging of the “expulsion” (spatter) and the cooling profile of the nugget allows for 100% inline inspection, replacing destructive “chisel tests” that only sample a fraction of production.
Standards and Compliance for Dissimilar Welds
Qualifying a dissimilar metal welding procedure requires more rigorous testing than similar metals. Key standards that govern these processes include:
- ISO 15614-1:2017: The primary standard for qualifying welding procedures for steels and nickel alloys. It includes specific requirements for dissimilar joins regarding tensile and bend testing.
- ISO 15614-12:2021: Covers the qualification of spot, seam, and projection welding, which are frequently used for multi-material automotive components.
- ISO 15614-14:2013: Specifically addresses laser-arc hybrid welding, a common process for thick-section dissimilar joins in shipbuilding and heavy industry.
Pro Tip: When using thermal imaging for procedure qualification (PQR), the thermal records can be used as “supplementary essential variables.” This means if you can prove the thermal profile of a production weld matches the qualified PQR, you have a much stronger case for reducing downstream NDT.
Integrating Thermal Monitoring into Your Process
Adding thermal monitoring to a dissimilar welding cell follows a specific workflow:
- Emissivity Calibration: Different metals emit infrared radiation differently. A “dual-wavelength” or “short-wave” (SWIR) camera is often required to handle the varying emissivity of shiny aluminum vs. dull steel.
- Baseline Mapping: Run a series of “good” and “bad” welds (intentionally inducing lack of fusion and burn-through) to map the thermal signatures.
- AI Training: Use HeatCore AI to recognize the specific thermal gradient of a successful dissimilar join.
- Inline Feedback: Connect the monitoring system to the robot controller to adjust power or travel speed in real-time based on the interface temperature.
Conclusion: The Future is Multi-Material
As industries move toward more complex, lightweight, and high-performance structures, the ability to join dissimilar metals reliably will be a major competitive advantage. Thermal imaging moves this process from “black box” trial-and-error to a data-driven science.
By monitoring the interface in real-time, manufacturers can:
- Reduce scrap rates in expensive multi-material components.
- Guarantee joint integrity in safety-critical EV and aerospace parts.
- Accelerate the qualification of new material combinations.
Ready to see the heat? Explore Therness HeatCore AI or book a technical consultation to discuss your specific dissimilar metal joining challenges.
Related reading:
- AI Weld Defect Detection: Thermal vs Vision vs Acoustic
- Heat Input, Cooling Rate, and T8/5: The Science of Weld Quality
- ISO 3834 and EN 1090: Welding Traceability with Thermography
