Local melting and cutting of iron bulks by a synergic microwave-DC thermal skin effect

Microwaves are widely utilized in heating processes, in domestic, industrial, scientific, and medical applications. However, metal bulks (unlike metal powders or thin sheets) are commonly considered as hardly susceptible to microwave heating (due to their micrometric skin-depth, which prevents the microwave penetration into the bulk). Here, we show that adding a relatively small direct current (DC) may catalyze a localized microwave-heating (LMH) effect in the iron bulk, up to its local melting (and even further to ablation and dusty-plasma ejection). The combined DC-LMH effect is demonstrated here by cutting 8-mmØ iron rebars (made of carbon steel, with no susceptors added), which is not feasible by sole microwave or DC in these conditions. The synergic microwave and DC effect is attributed here to a combined thermal skin evolution, which jointly forms a hotspot in a mutually intensified thermal-runaway instability and deepens the microwave penetration into the iron bulk. This interpretation of the experimental findings is supported here by a simplified theoretical model of the combined microwave-DC interaction in iron, which demonstrates the thermal skin layer evolution and confirms the feasibility of a hotspot formation. The synergic DC-LMH effect may advance potential applications of microwaves in thermal processing of metals, such as melting, cutting, joining, sintering, casting, and 3D printing.