Emerging metallic systems for additive manufacturing : in-situ alloying and multi-metal processing in laser powder bed fusion

Creating metal parts by Fused Deposition Modeling and Sintering

ABSTRACT Laser powder bed fusion (LPBF) additive manufacturing has been advancing in the fabrication of metallic multi-material structures with intricate structures and refined material layouts. Herein, a comprehensive review of the recent achievements of multi-material structures via LPBF is provided in terms of interface characteristics and strengthening methods, critical technical issues and potential applications. It begins with the introduction of multi-material structures and the scope of the review. The interface characteristics (including representative multi-material types printed by LPBF, interfacial microstructure, defects, etc.) and strengthening methods of multi-material structures are then presented. Thereafter, the critical technical issues in LPBF for multi-material structures are discussed with regard to equipment development, data preparation, thermodynamic calculation and process simulation, and powder cross-contamination and recycling. Moreover, the potential applications (particularly in biomedical, electronic, aerospace) are illustrated and discussed. Finally, the outlook is outlined to provide guidance for future research.

Recent progress on additive manufacturing of multi-material structures with laser powder bed fusion

The lattice structure with graded composition and microstructure in function of position, allowing for smooth transition and combination of multiple desirable features, has great prospects in various applications. Laser powder bed fusion (LPBF) is an effective approach to preparing the gradient-material lattice structure by altering the proportion of raw powder materials. In this study, in-situ TiB/Ti6Al4V gradient-material bulk samples and Schoen Gyroid scaffolds with various relative densities were manufactured by LPBF for the first time. Microstructure evolution, manufacturing quality, and mechanical responses were analyzed in detail. The results show that the graded layers exhibit good metallurgical bonding with irregular band-shaped transition zones at the interface. The in-situ synthesized TiB phase particles improve the strength and grain refinement, but decrease the dimensional accuracy and induce more defects. The elastic modulus and compressive yield strength of gradient-material lattice structures increase with the rise of the relative density. The TiB/Ti6Al4V gradient-material lattice structures have higher stiffness and strength compared with homogeneous Ti6Al4V counterparts. A combined model for predicting and designing the mechanical properties of lattice structures with graded layers is used to offer a new methodology to design complex components with multiple materials combined with superior performances.

Microstructure evolution and mechanical properties of TiB/Ti6Al4V gradient-material lattice structure fabricated by laser powder bed fusion

This research illustrates the rationale of adopting a preferred printing sequence by examining crack generation predominated by resultant interfaces and microstructural inhomogeneity, through underlying governing mechanisms in directed energy deposition of 316L stainless steel/Inconel 625 (SS316L/IN625) bimetals. For this purpose, microstructural and crystallographic characterizations augmented by numerical simulations were employed on additively manufactured two distinct interfaces, i.e. Type-I (IN625 deposition on SS316L) and Type-II (SS316L deposition on IN625). Changing the printing sequence generated these two types of interfaces with unique morphologies, which was found attributable to the compositional variations and mismatch in thermal properties. Type-I interface was typified by gradual-change composition in the transition zone, causing the IN625 grains to grow epitaxially on the grains of SS316L. Type-II interface was characterized as a compositional sudden-change zone (CSCZ) adjacent to SS316L, leading to merging bidirectional nucleation and grain growth from the bottom IN625 and upper CSCZ, and lack of epitaxial growth. Additionally, high cracking susceptibility occurred near the Type-II interface rather than the Type-I interface, which was related to solidification and liquidation cracking, and further promoted ductility dip cracking. This research will provide a guideline for the additive manufacturing of bimetals with the consideration of printing sequence to control interface formation for a crack-free structure.

Microstructural characteristics and crack formation in additively manufactured bimetal material of 316L stainless steel and Inconel 625

The multi-material selective laser melting (SLM) technology has been widely studied in recent years. In this paper, 316L stainless steel (316SS) and Inconel 718 (IN718) multi-material parts were manufactured by SLM and their interfacial characteristics and mechanical properties were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, electronic universal testing machine and microhardness tester. The interface metallograph and higher comprehensive mechanical performance with elasticity modulus (103 ± 3 MPa), elongation (28.1 ± 2%) and ultimate tensile strength (596 ± 10 MPa) proved good metallurgical bonding at the interfaces (∼100 μm) between 316SS and IN718. Nevertheless, some cracks and holes, which were found at or near the interfaces, suggested special SLM processing parameters (such as laser power, laser scanning speed, layer thickness, hatch spacing, remelting, fabrication sequence and layers of interface) should be applied at the interfaces.

Wang Xiangyu

Papers

Interfacial characterization and mechanical properties of 316L stainless steel/inconel 718 manufactured by selective laser melting