• Common defects and anomalies in powder bed fusion metal additive manufacturing. • Formation mechanism and practical mitigation strategies are discussed. • Defects/anomalies are classified as powder-related, processing-related, and post-processing related issues. • Properties such as mechanical behavior and corrosion resistance of defective parts are discussed. • Current challenges, gaps, and future trends are discussed. Metal additive manufacturing is a disruptive technology that is revolutionizing the manufacturing industry. Despite its unrivaled capability for directly fabricating metal parts with complex geometries, the wide realization of the technology is currently limited by microstructural defects and anomalies, which could significantly degrade the structural integrity and service performance of the product. Accurate detection, characterization, and prediction of these defects and anomalies have an important and immediate impact in manufacturing fully-dense and defect-free builds. This review seeks to elucidate common defects/anomalies and their formation mechanisms in powder bed fusion additive manufacturing processes. They could arise from raw materials, processing conditions, and post-processing. While defects/anomalies in laser welding have been studied extensively, their formation and evolution remain unclear. Additionally, the existence of powder in powder bed fusion techniques may generate new types of defects, e.g., porosity transferring from powder to builds. Practical strategies to mitigate defects are also addressed through fundamental understanding of their formation. Such explorations enable the validation and calibration of models and ease the process qualification without costly trial-and-error experimentation. 

Defects and anomalies in powder bed fusion metal additive manufacturing

Effects of porosity on the mechanical properties of additively manufactured components: a critical review

The microstructure, temperature-dependent mechanical properties and deformation behaviors of equiatomic CoCrFeMnNi high-entropy alloy (HEA) additively manufactured by selective laser melting (SLM) were investigated. SLM-built HEA had a face-centered cubic (FCC) single-phase random solid solution. In addition, SLM-built HEA was composed of epitaxial growth grains, dislocation network and nano-sized oxides. Room- and high-temperature compression tests confirmed that SLM-built HEA has outstanding mechanical properties in all temperature ranges compared to equiatomic CoCrFeMnNi HEAs reported up to the present. The excellent mechanical properties of SLM-built HEA were achieved with fine grains, high dislocation density and fine precipitates at low temperatures (25 °C to 600 °C), and by high dislocation density and fine precipitates at high temperatures (700 °C or higher). On the other hand, the deformation microstructure showed that slip and deformation twins are the main deformation mechanisms from 25 °C to 600 °C, and slip and partial recrystallization are the main deformation mechanisms above 700 °C. Based on the above findings, this study also discusses correlations among the microstructure, superior mechanical properties and deformation mechanisms of SLM-built equiatomic CoCrFeMnNi HEA.

Superior Temperature-Dependent Mechanical Properties and Deformation Behavior of Equiatomic CoCrFeMnNi High-Entropy Alloy Additively Manufactured by Selective Laser Melting

Effect of heat treatment on the microstructure and high temperature oxidation behavior of TiC/Inconel 625 nanocomposites fabricated by selective laser melting

Although additive manufacturing (AM) has developed significantly due to its numerous process advantages, some adverse effects include large columnar grains and detrimental tensile residual stress, which can weaken the performance of the final part. In this study, a new post-treatment method combining heat treatment and laser shock peening was employed to alter the microstructure and mechanical properties of AM-processed Inconel 625 alloy (IN625). The microstructural evolution of IN625 after different treatments was studied. Results showed that many γ′' precipitates were formed after heat treatment. After heat treatment followed by laser shock peening treatment, gradient microstructure along depth direction was generated. High-density dislocation structures and refined grains were produced in the subsurface layer after heat treatment followed by laser shock peening treatment. Moreover, the precipitates interacted with dislocations. High levels of compressive residual stress could be generated after heat treatment followed by laser shock peening treatment, resulting in high ultimate tensile strength and large elongation. This combined heat treatment and laser shock peening method is helpful in enhancing the mechanical performance of AM parts through changing their microstructure and residual stress distribution.

Microstructural evolution and mechanical properties of selective laser melted a nickel-based superalloy after post treatment

Effect of post-treatment on the microstructure and high-temperature oxidation behaviour of additively manufactured inconel 718 alloy

This study investigated and compared the tensile and high-cycle fatigue properties of AISI 1045 steel and Cr electroplated AISI 1045 steel. First, tensile and fatigue specimens were machined from AISI 1045 steel (substrate), and two kinds of Cr electroplating layers with different layer thickness were applied. The substrate, AISI 1045 steel consisted of an a-Fe matrix and pearlite, and the two Cr electroplating layers were measured to have thicknesses of 13.1 μm and 53.9 μm, respectively. The Cr layer did not show signs of peeling from the substrate, but initial micro cracks were present within the Cr electroplating layer. Tensile test results confirmed that the Cr electroplated specimens had similar yield strengths and tensile strengths to those of the substrate AISI 1045 steel, but elongation decreased significantly. High cycle fatigue results confirmed that the fatigue limit (~10 cycles to fatigue failure) of the substrate steel was 600 MPa, and the fatigue limit decreased significantly to 500 MPa (13.1 μm thickness) and 325 MPa (53.9 μm thickness) as the Cr electroplating layers were formed. This study also observed fracture surfaces of tensile and high cycle fatigue fractured specimens, and the deformation mechanisms of the Cr electroplated steel were suggested in connection with microstructures. (Received January 4, 2019; Accepted January 28, 2019)

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Yeon-Ji Kang

Papers

Effect of post-treatment on the microstructure and high-temperature oxidation behaviour of additively manufactured inconel 718 alloy

Effect of Cr Electroplating Layer Thickness on the Tensile and High Cycle Fatigue Properties of AISI 1045 Steel

Microstructural and Wear Properties of WC-based and Cr 3 C 2 -based Cermet Coating Materials Manufactured with High Velocity Oxygen Fuel Process