Comparison of copper filler infiltration behavior in ultra-thin stainless steels 304, 304L, and 316L during brazing

This study systematically examines the infiltration of a 12-μm-thick copper filler during the brazing of 50-μm-thick 304, 304L, and 316L stainless steel plates at 1090–1120 °C. Penetration depths varied from zero to complete through-thickness infiltration (50 μm), depending on alloy composition and temperature. Notably, 316L steel exhibited up to 87 % less penetration than 304 steel due to its higher nickel content. Complete infiltration led to a joint thickness reduction of up to 70 %. Quantitative analysis correlated penetration depth with measured dihedral angles (0–150°). The underlying mechanism is controlled by grain boundary groove dynamics, transitioning from groove-dominated to wetting-dominated above a wetting transition temperature, estimated at 1090–1100 °C for 304/304L and 1110–1115 °C for 316L. This transition to a wetting-dominant regime causes an exponential increase in penetration depth. These findings provide fundamental insights into grain boundary infiltration and offer critical guidance on material selection and process optimization for brazing ultra-thin stainless steels.