Defect formation is a critical challenge for powder-based metal additive manufacturing (AM). Current understanding on the three important issues including formation mechanism, influence and control method of metal AM defects should be updated. In this review paper, multi-scale defects in AMed metals and alloys are identified and for the first time classified into three categories, including geometry related, surface integrity related and microstructural defects. In particular, the microstructural defects are further divided into internal cracks and pores, textured columnar grains, compositional defects and dislocation cells. The root causes of the multi-scale defects are discussed. The key factors that affect the defect formation are identified and analyzed. The detection methods and modeling of the multi-scale defects are briefly introduced. The effects of the multi-scale defects on the mechanical properties especially for tensile properties and fatigue performance of AMed metallic components are reviewed. Various control and mitigation methods for the corresponding defects, include process parameter control, post processing, alloy design and hybrid AM techniques, are summarized and discussed. From research aspect, current research gaps and future prospects from three important aspects of the multi-scale AM defects are identified and delineated. 

Multi-scale defects in powder-based additively manufactured metals and alloys

Additive manufacturing of 18% nickel maraging steels: defect, structure and mechanical properties: A review

In this work we studied the influence of incline angle on the microstructure, tensile and corrosion response of Inconel 718 (IN 718) samples fabricated via selective laser melting (SLM) with build directions ranging from 0° to 45°. Results reveal that the fraction of Laves phase increases with increasing incline angle and the -direction of crystals align along the vertical direction in the samples fabricated with different incline angles. Tensile tests of the printed samples were performed at room temperature and the results showed that the yield strength and ultimate tensile strength increase with increasing incline angle. The highest strength was obtained for the sample with an incline angle of 45°. However, the ductility remains relatively independent of the incline angle. The dependence of microstructure and tensile behavior on the incline angle was rationalized in terms of the temperature gradient during deposition and the ensuing anisotropy in the crystal orientation. The corrosion resistance of SLMed IN 718 in 3.5% NaCl solution increased slightly with increasing incline angle, and this behavior was attributed to the changes in crystallization texture and grain boundary density.

Influence of build orientation on microstructure, mechanical and corrosion behavior of Inconel 718 processed by selective laser melting

Control of grain structure, phases, and defects in additive manufacturing of high-performance metallic components

The adaptation of additive manufacturing (AM) for Ni-based superalloys has gained significance in aerospace and power-generation industries due to the ability to fabricate complex, near-net-shape components on-demand and with minimal material waste. Besides its advantages, challenges remain in metal AM, especially for printing complex alloys such as superalloys. These challenges are often linked to heterogeneity in the as-fabricated parts and continue to limit the practical applications of AM products. A thorough understanding of the relationship between the complex AM process and the resulting microstructure heterogeneity needs to be established before mitigation strategies can be developed. The ability to fabricate more homogeneous Ni-based superalloy parts is expected to unlock not only better mechanical properties but also additional fields of applications. This review aims to summarize the current understanding of heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys. Microstructure heterogeneities discussed include heterogeneity in the chemical composition, phase constitution, porosity, grain and dendrite morphology, and solid-state precipitates. Related heterogeneities in hardness, tensile, creep, fatigue, and residual stress are discussed to represent mechanical properties, and mitigation strategies are summarized. The origins of heterogeneity in the as-fabricated parts are linked to the variations in AM thermal conditions caused by the complex thermal histories. 

Electron and laser-based additive manufacturing of Ni-based superalloys: a review of heterogeneities in microstructure and mechanical properties

The element segregation, Laves phase formation, and mechanical properties of the laser metal deposited IN718 superalloy by the flat top laser beam (FTLB) and gaussian distribution laser beam (GDLB) are studied. It is found that the Laves phase formation in the gaussian distribution laser deposited IN718 (GDLD-IN718) is substantially higher than that in the flat top laser deposited IN718 (FTLD-IN718). The higher production of the Laves phase in the GDLD-IN718 contributes to the higher microhardness and lower tensile strength (about 20% reduction) of the as-deposited IN718 than that of the FTLD-IN718. The element redistribution behavior in the laser rapid solidification under both of the lasers are also studied through the finite element simulation. The results show that the severe laser energy concentration in the center of the GDLB produces higher molten pool temperature, lower horizontal thermal gradient to vertical thermal gradient ratio (GX/GZ) of the solid-liquid interface. These typical thermal characteristic of the GDLB generated molten pool eventually results in a lower redistribution coefficient of the alloying elements and as a result, the interdendritic element segregation and Laves phase formation are dramatically improved in the GDLD-IN718. The present comparative study proves that the FTLB is more superior for the laser additive manufacturing than that of the GDLB.

Study on the element segregation and Laves phase formation in the laser metal deposited IN718 superalloy by flat top laser and gaussian distribution laser

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Study on the element segregation and Laves phase formation in the laser metal deposited IN718 superalloy by flat top laser and gaussian distribution laser