Critical influence of FeAl and ɑ-Fe phases in coating layers on resistance spot weldability of Al–Si coated press-hardening steels

Al–Si coated press-hardening steels (PHS) are extensively employed in body-in-white structures due to their exceptional strength, yet their resistance spot (RS) weldability is critically influenced by coating alloying layers. This study systematically investigates the effect of individual phases in the alloying layers to identify the dominant phase and propose an optimal coating microstructure through the hot stamping process. Weldability with diverse coating structures under varying heating durations was evaluated. Additionally, specimens with exclusively α-Fe phase retention via coating-removal treatments were used as a comparative group. Key results indicate that increasing the fraction of either α-Fe or FeAl phases narrows the suitable current range (SCR) and deteriorates weld mechanical performance. Quasi-static and dynamic resistance measurements reveal that the FeAl phase significantly elevates contact resistance and dynamic peak resistance, enhancing heat input efficiency and accelerating heat accumulation, thereby promoting premature expulsion. Coating analysis at the nugget nucleation interface further indicates that α-Fe cannot be expelled prior to nugget formation, whereas outer alloying compound layers exhibit expulsion capability. Moreover, Al released from α-Fe destabilizes the molten pool, triggering expulsion even under low heat input. Thus, premature expulsion is attributed to two mechanisms: the FeAl phase enhances heating efficiency whereas the α-Fe phase reduces the expulsion threshold. Consequently, a coating structure minimizing both α-Fe and FeAl phases is proposed for hot-stamping processes to improve RS weldability of PHS. To address high-content α-Fe and FeAl scenarios, a dual-pulse welding process was developed, enabling stable nugget growth and extending the SCR from 0.4 to 1.0 kA.