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Special Section: Bioprinting of 3D Functional Tissue Constructs

Regular Section


Editors’ foreword to the inaugural issue of Materials Science in Additive Manufacturing
by Chee Kai Chua, Swee Leong Sing
Editors’ foreword to the inaugural issue of Materials Science in Additive Manufacturing

Original research article

by M. Hossein Sehhat, Austin T. Sutton, Chia-Hung Hung, Ben Brown, Ronald J. O’Malley, Jonghyun Park, Ming C. Leu

Particles of AISI 304L stainless steel powder were spheroidized by the induction plasma spheroidization process (TekSphero-15 spheroidization system) to assess the effects of the spheroidization process on powder and part properties. The morphology of both as-received and spheroidized powders was characterized by measuring particle size and shape distribution. The chemistry of powders was studied using inductively coupled plasma optical emission spectroscopy for evaluation of composing elements, and the powder’s microstructure was assessed by X-ray diffraction for phase identification and by electron backscattered diffraction patterns for crystallography characterization. The Revolution Powder Analyzer was used to quantify powder flowability. The mechanical properties of parts fabricated with as-received and spheroidized powders using laser powder bed fusion process were measured and compared. Our experimental results showed that the fabricated parts with plasma spheroidized powder have lower tensile strength but higher ductility. Considerable changes in powder chemistry and microstructure were observed due to the change in solidification mode after the spheroidization process. The spheroidized powder solidified in the austenite-to-ferrite solidification mode due to the loss of carbon, nitrogen, and oxygen. In contrast, the as-received powder solidified in the ferrite-to-austenite solidification mode. This change in solidification mode impacted the components made with spheroidized powder to have lower tensile strength but higher ductility.

Original research article

by Audelia Gumarus Dharmawan, Gim Song Soh

Depositing on inclined cylindrical surfaces has recently gained interest due to its potential for directly employing feedstock that forms part of the printed structure. In this paper, we present our approach to perform cylindrical path planning through converting a planar slicing data structure into a universal 3D polar data structure. This has the advantage of using off-the-shelf slicing software and adapting it for cylindrical path planning. Our approach is capable of generating cylindrical print paths of various patterns such as linear raster, circular raster, hybrid contour, and zigzag path. We demonstrate the capability of the approach to planning cylindrical print paths for two different revolved components employing these three different printing patterns. Actual printing experiments and tensile tests of the cylindrical part using wire-arc additive manufacturing were conducted and reported. It was found that they yield an average tensile strength that matches the strength of the 4340 feedstock.

Original research article

by Wenhui Yu, Zhen Xiao, Xuhui Zhang, Yetao Sun, Peng Xue, Shuai Tan, Yongling Wu, Hongyu Zheng

High-performance engineering alloys, such as 7000 series aluminum alloys, suffer poor printability in laser powder bed fusion (LPBF) additive manufacturing. An enormous challenge lies in the suppression of solidification cracks caused by solidification shrinkage and thermal stresses. Porosity formation, as one of the main concerns for LPBF application, should also be avoided at the same time. In this study, aluminum alloy (AA) 7075 with and without Zr modification was additively manufactured by LPBF. Processing parameters of laser power and scanning speed, resulting in various volumetric energy density (VED), were experimentally determined to produce crack-free components with tailored microstructure. Optical microscopy was used to reveal how the crack density and porosity vary with VED. Scanning electron microscopy and transmission electron microscopy uncovered the detailed microstructure in the molten pool and the evolution of the elemental Zr addition. The results indicate that 1 w.t.% addition of elemental Zr in AA7075 led to lower crack density compared with 0.3 w.t.% addition. In 1 w.t.% Zr-modified AA7075, crack-free components were obtained under high VED. Fine equiaxed grains, instead of large columnar grains, were formed at the bottom of the molten pool boundary due to the existence of Al3Zr compound, which favored the nucleation of aluminum grains and elimination of cracks. The phenomenon of silicon segregation near cracks remained in Zr modified alloys, although its effects on cracking were suppressed. Spherical pores in the Zr-modified AA7075 increased due to the deterioration of fluidity by unmelted particles, which distracted the Marangoni flow as well. Sufficient laser energy input can increase the viscosity and ease the pores escaping. By optimizing parameters, crack-free AA7075 parts with low porosity can be manufactured through LPBF with Zr addition.

Original research article

by Zainab N. Khan, Hamed I. Albalawi, Alexander U. Valle-Pérez, Ali Aldoukhi, Noofa Hammad, Elena Herrera-Ponce de León, Sherin Abdelrahman, Charlotte A. E. Hauser

Three-dimensional (3D) bioprinting methods vary in difficulty and complexity depending on the application desired and biomaterials used. 3D biofabrication is gaining increased traction with enhanced additive manufacturing technologies. Yet, high print resolution and efficiency for the fabrication of complex constructs still prove to be challenging. An intricate balance between biomaterial composition, machine maneuverability, and extrusion mechanism is required. While soft bioinks are highly desirable when used as a biodegradable scaffold material for tissue and organ fabrication, mechanical stiffness and shape fidelity are often compromised. Alternately, post-printing ultraviolet and chemical crosslinking processes improve fidelity but threaten cell viability. Herein, we propose a hybrid fabrication approach to facilitate 3D bioprinting using soft bioinks with instantaneous gelation properties while maintaining shape fidelity for tissue and organ structures of complex geometries. The approach entails a multi-step “3D Printed Molds to Scaffolds” method, which uses additive manufacturing to create accurate negative support structures for the desired construct. A tissue or organ model is first designed in computer-aided design (CAD) modeling software to create a negative mold structure of the desired tissue or organ. Using a Formlabs® SLA 3D printer, the negative mold is fabricated at desired scale using a biocompatible elastic resin. Then, a robotic 3D bioprinting system is loaded with a sliced g-code of the CAD model. The robot start position is aligned with the placement of the fabricated mold on the printbed. Microfluidic pumps deliver three solutions through a customized nozzle to extrude peptide bioink, which gels instantaneously. The initial layers of the structure are formed within the mold to create a solid foundation of the construct. The hybrid approach was found to enhance fidelity considerably and enabled the printing of a complex human ear structure. It is promising for tissue and organ fabrication as it offers a cost-effective support structure without increasing printing time. It could also be used as a rapid prototyping approach for researchers who do not have access to 3D bioprinting systems. Biofabrication, from printed molds to bioprinted scaffolds, will potentially enhance the printing experience with soft bioinks while preserving cell durability and viability. 

Original research article

by Yifan Fei, Jie Xu, Donggang Yao, Richard Chiou, Jack Zhou

Compared with current powder-based 3D metal printing, thixotropic metal 3D printing has great potentials and advantages in equipment cost, product quality, and process efficiency. In this paper, detailed problem statement, technique challenge, and development method regarding thixotropic metal 3D printing are discussed. A shear mixing and extruding prototype machine for thixotropic alloy fabrication was designed. We developed a direct thixotropic metal 3D printing machine and conducted a modeling and simulation process for the system. The printability of this direct metal 3D printing machine was studied. At the end, conclusions and future directions are also presented.

Original research article

by Xi Gong, Dongrui Zeng, Willem Groeneveld-Meijer, Guha Manogharan

Prior studies in metal additive manufacturing (AM) of parts have shown that various AM methods and post-AM heat treatment result in distinctly different microstructure and machining behavior when compared with conventionally manufactured parts. There is a crucial knowledge gap in understanding this process-structure-property (PSP) linkage and its relationship to material behavior. In this study, the machinability of metallic Ti-6Al-4V AM parts was investigated to better understand this unique PSP linkage through a novel data science-based approach, specifically by developing and validating a new machine learning (ML) model for material characterization and material property, that is, machining behavior. Heterogeneous material structures of Ti-6Al-4V AM samples fabricated through laser powder bed fusion and electron beam powder bed fusion in two different build orientations and post-AM heat treatments were quantitatively characterized using scanning electron microscopy, electron backscattered diffraction, and residual stress measured through X-ray diffraction. The reduced dimensional representation of material characterization data through chord length distribution (CLD) functions, 2-point correlation functions, and principal component analysis was found to be accurate in quantifying the complexities of Ti-6Al-4V AM structures. Specific cutting energy was the response variable for the Taguchi-based experimentation using force dynamometer. A low-dimensional S-P linkage model was established to correlate material structures of metallic AM and machining properties through this novel ML model. It was found that the prediction accuracy of this new PSP linkage is extremely high (>99%, statistically significant at 95% confidence interval). Findings from this study can be seamlessly integrated with P-S models to identify AM processing conditions that will lead to desired material behaviors, such as machining behavior (this study), fatigue behavior, and corrosion resistance.