Microstructural evolution of multilayered AISI 316L-440C steel composites manufactured by laser powder bed fusion

Abstract

This study presents a systematic study of multilayered steel-steel composites prepared via laser powder bed fusion (L-PBF) in an open-architecture additive manufacturing system equipped with a multiple powder delivery module. Stainless steel AISI 316 L powder and high‑carbon stainless steel 440C powder were chosen as the model material system. The printed samples were characterized using optical microscopy, X-ray diffraction, electron backscattered diffraction, energy-dispersive X-ray spectroscopy, X-ray tomography, and micro-hardness to understand the complex manufacturing process and its influence on the microstructure and properties. The evolution of the multilayered structure was assessed, revealing that significantly more pronounced remelting occurs when the 440C powder is deposited on the 316 L band, generating a thick 440C/316 L transition region and a thin 440C band. Cracks were observed primarily at the transition regions, and were attributed to a “hot cracking” mechanism. The cracks were associated with significant chemical segregation along the grain boundaries at these regions, which exhibit intrinsically higher hot cracking susceptibility than the single-material bands. This was particularly prevalent for mixtures with <50 wt% SS440C (balance AISI 316 L), as corroborated by thermodynamic calculations and observed crack locations. The processing-microstructure relationship was discussed in terms of the formation of defects at the transition regions. Challenges associated with crack removal in multi-material systems were discussed and guidelines for manufacturing multi-material, multilayered composites via L-PBF were provided.