During the additive manufacturing (AM) process, energy is transferred from the energy beam to the processed material. The high-energy input and uneven temperature distribution result in the high-temperature gradient, large thermal stress, and warping deformation. The scanning strategy, one of the representative AM processing parameters, plays an important role in the microstructures, mechanical properties, and residual stresses of 3D printed parts. It is necessary to review the current state of research about scanning strategy in additive manufacturing, and this paper seeks to address this need. This review mainly focuses on the scanning strategies in selective laser melting process. Various scanning strategies and their effects on mechanical properties, microstructures, and residual stresses of selective laser melted parts are summarized. Finally, some suggestions on the optimization of scanning strategy for better performance are provided based on the above analysis.

https://link.springer.com/content/pdf/10.1007/s00170-021-06810-3.pdf

Scanning strategy in selective laser melting (SLM): a review

Aerospace is a key market driver for the advancement of additive manufacturing (AM) due to the huge demands in high-mix low-volume production of high-value parts, integrated complex part geometries and simplified fabrication workflow. Rapid and significant progress has been made in the laser additive manufacturing (LAM) of aeroengine materials, including the advanced high-strength steels, nickel-based superalloys and titanium-based alloys. Despite the extensive investigation of these three families of materials by the research community, there is a lack of comprehensive review on LAM of high strength steels, and existing gaps in published reviews on Ti-based alloys and Ni-based superalloys. Furthermore, although emerging materials such as high/medium entropy alloys and heterostructured materials exhibit promising mechanical performance, rigorous characterization, testing, qualification, and certification are still required before actual application in engine parts. Thus, it is still important and relevant to have a deep understanding on the relationship between process parameters – microstructures – mechanical properties in these widely used aeroengine materials, to drive the development of superior high-value alloys. This review aims to provide a critical and in-depth evaluation of laser powder bed fusion (LPBF) and laser directed energy deposition (LDED) technologies of the mentioned aeroengine materials. The review will summarize the material properties, performance envelops and outlines the research gaps of these aeroengine materials. Furthermore, perspectives on research opportunities, materials development, and new R&D approaches of LAM for the aeroengine materials are also highlighted.

Progress and perspectives in laser additive manufacturing of key aeroengine materials

CoCr alloy processed by Selective Laser Melting (SLM): effect of Laser Energy Density on microstructure, surface morphology, and hardness

The effect of energy density on texture and mechanical anisotropy in selective laser melted Inconel 718

Residual stresses (RS) of great magnitude are usually present in parts produced by Laser Powder Bed Fusion (PBF-LB), mainly owing to the extreme temperature gradients and high cooling rates involved in the process. Those “hidden” stresses can be detrimental to a part’s mechanical properties and fatigue life; therefore, it is crucial to know their magnitude and orientation. The hole-drilling strain-gage method was used to determine the RS magnitude and direction-depth profiles. Cuboid specimens in the as-built state, and after standard solution annealing and ageing heat treatment conditions, were prepared to study the RS evolution throughout the heat treatment stages. Measurements were performed on the top and lateral surfaces. In the as-built specimens, tensile stresses of ~400 MPa on the top and above 600 MPa on the lateral surface were obtained. On the lateral surface, RS anisotropy was noticed, with the horizontally aligned stresses being three times lower than the vertically aligned. RS decreased markedly after the first heat treatment. On heat-treated specimens, magnitude oscillations were observed. By microstructure analysis, the presence of carbides was verified, which is a probable root for the oscillations. Furthermore, compressive stresses immediate to the surface were obtained in heat-treated specimens, which is not in agreement with the typical characteristics of parts fabricated by PBF-LB, i.e., tensile stresses at the surface and compressive stresses in the part’s core.

Laser Powder Bed Fusion of Inconel 718: Residual Stress Analysis Before and After Heat Treatment

Effect of laser energy density on the microstructure, mechanical properties, and deformation of Inconel 718 samples fabricated by selective laser melting

Selective laser melting (SLM) is a promising powder bed fusion additive manufacturing technique for metal part fabrication. In this paper, varying scanning speed in the range of 500 mm/s to 1900 mm/s, and laser power in the range of 100 W to 200 W, were realized from layer to layer in a cycle of 56 layers in a single cuboid Inconel 718 alloy specimen through SLM. Layer-wise variation of microstructure and porosity were acquired, showing the layer-wise controlling capability of microstructural soundness. The melt pool size and soundness are closely linked with the energy input. High energy density led to sound regions with larger, orderly stacked melt pools and columnar grains, while low energy density resulted in porous regions with smaller, mismatched melt pools, un-melted powder, and equiaxed grains with finer dendrites. With the increase of laser energy density, the specimen shifts from porous region to sound region within several layers.

Effect of Layer-Wise Varying Parameters on the Microstructure and Soundness of Selective Laser Melted INCONEL 718 Alloy

Microstructure and mechanical behavior of bright crescent areas in Inconel 718 sample fabricated by selective laser melting

Additive manufacturing provides a possibility to realize high throughput investigation of material preparation and processing. In order to thoroughly investigate the effect of the key parameters on the soundness, microstructure and mechanical properties of the fabricated parts, sixty three combinations of laser power (twelve levels) and scanning speed (sixteen levels) with fifty eight levels of laser energy densities were realized in a single Inconel 718 alloy bar shaped specimen using the laser powder-bed fusion (L-PBF) technique. The combination of parameters varied from bottom to top of the bar, with each combination repeating every eight layers. Its spatial porosity distribution was measured by the Computed tomography (CT) test; its microstructure was observed by optical microscope, scanning electronic microscope and EBSD; the microhardness was measured as well. A comprehensive view of the relationship of these parameters and the porosity fraction, melt pool stacking, primary grain arm spacing and microhardness was obtained. Although high laser power and slow scanning speed are helpful to achieve low porosity, but the laser power has more significant effect on the quality of the specimen than the scanning speed. Only if at high levels the volumetric laser energy density could be used as an index to correlate to the quality of the specimen. The processing chart for the guidance of L-PBF was obtained. As the laser power is higher than 140 W and the scanning speed lower than 1000 mm/s, sound specimens would be realized. This method demonstrated the great potential for high throughput investigations of the L-PBF process.

Tianjiao Wang

Papers

Effect of laser energy density on the microstructure, mechanical properties, and deformation of Inconel 718 samples fabricated by selective laser melting

Effect of Layer-Wise Varying Parameters on the Microstructure and Soundness of Selective Laser Melted INCONEL 718 Alloy

Microstructure and mechanical behavior of bright crescent areas in Inconel 718 sample fabricated by selective laser melting

High-throughput investigation of laser powder bed fabricated Inconel 718 alloy: Fabrication, microstructure and performance