Cracking the Challenge of Steel–Copper Fusion

Steel and copper are widely used materials in manufacturing, often side-by-side in electrical, thermal, and structural applications. But joining them by fusion-based techniques like laser welding or additive manufacturing remains a persistent challenge. Their vastly different thermal and metallurgical properties — especially their melting temperatures, thermal conductivities, and mutual solubilities — cause severe stresses and interfacial instabilities during processing. The result? Cracks.

These cracks don’t just appear randomly — they reflect complex interactions between melting, solidification, and local chemistry. Prior studies have proposed various explanations for cracking in the steel-copper system — from thermal stress and liquation to solidification-related issues and chemical embrittlement — yet definitive experimental evidence has been lacking. This ambiguity stems from the difficulty of observing crack formation as it occurs.

In this study, researchers used operando synchrotron X-ray radiography and diffraction to follow crack formation as it happened during laser processing of stainless steel–copper joints. Unlike conventional methods, which look at the aftermath, this approach provided a real-time window into the evolving melt pool, microstructure, and strain state.

This time-resolved characterization allowed the team to disentangle the different mechanisms responsible for failure — showing that cracking is not driven by a single cause, but by a combination of metallurgical and mechanical factors.

By closely analyzing the sequence of events during fusion, the study identified three major cracking modes:

1. Solidification Cracking: As the copper-rich melt solidifies, poor feeding of the mushy zone and thermal shrinkage cause gaps to open between dendrites. If the liquid can’t flow fast enough to fill them, cracks appear along grain boundaries.
2. Liquation Cracking: Localized remelting at the steel side weakens grain boundaries. When the surrounding material contracts or strains slightly during cooling, these semi-molten boundaries can fail, leading to interfacial cracks.
3. Metal-Induced Embrittlement (MIE): Perhaps the most insidious mechanism, this occurs when molten copper penetrates along steel grain boundaries, reducing their cohesion without full melting. This leads to brittle intergranular fracture, driven not by heat, but by chemistry.

Crucially, the study also shows that cracking is not unavoidable. By adjusting processing conditions — particularly the energy input and melt pool geometry — it was possible to avoid excessive copper melting and reduce the stress concentrations that lead to failure. In some configurations, crack-free joints were achieved even between these highly incompatible metals.

This work provides a mechanistic roadmap for designing more reliable dissimilar metal joints, with implications for applications in power electronics, heat exchangers, and multi-material additive manufacturing.