The 3D printing method, alternatively known as additive manufacturing (AM), is promising for rapid tooling and layered micromanufacturing. However, significant fundamental research and applied study in the 3D printing area are still necessary to develop new manufacturing mechanisms for combining multi-materials for multiscale and multi-functionality behaviors. Among those materials, particles with unique mechanical, thermal, electrical, optical, and other functional properties can find broad applications in structural composites, thermal packaging, electrical devices, optoelectronics, biomedical implants, energy storage, filtration, and purification. This review will first briefly cover the 3D printing basics before presenting the critical factors in polymer/particle-based printing. We will then introduce a spectrum of different printing mechanisms, i.e., vat polymerization-based, jetting-based, material extrusion-based, powder bed fusion-based, and a few other less utilized 3D printing methods, with a summary of the processing parameters, advantages, disadvantages, and future challenges of each printing technique. During this discussion of 3D printing, we will also present generally used polymers and particles, namely, liquid monomers, viscous inks, compliant gels, stiff filaments, and loosely packed pellets containing micro and nanoscale particles. The emphasis of this review is on the general printing mechanisms applicable in particle- and polymer-relevant processing. To end, this review identifies provides future perspectives regarding some new application examples. Identifying challenges in materials science and manufacturing processes will give direction to the fabrication of multifunctional systems for diverse applications, especially when using multi-materials (e.g., polymers and particles) at multiple scales (e.g., nanoscale morphologies and macroscale structures) for multifunctional systems.

3D printing for polymer/particle-based processing: A review

Heat transfer—A review of 2005 literature

The design of a novel neutron shielding B4C/Al composite containing Gd

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

Inconel 718 powders have been hot-isostatic-pressed (HIPed) at representative temperatures to investigate the variations in microstructure, tensile properties and tensile fracture mode of the powder compact. Microstructure of the powder compacts were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and so on. The results showed that the interdendritic precipitates inherited from the powders were partially retained in the powder compacts when the powders were HIPed at or below 1210 °C but were eliminated when HIPed at and above 1260 °C. The grain size uniformity of the powder compacts first increases and then decreases with increasing HIPing temperature. Prior particle boundaries (PPBs) were observed in the powder compacts HIPed at and below 1260 °C but was eliminated when HIPed at 1275 °C. The PPBs were decorated with carbide particles, the amount of the carbide particles at the PPBs decreases with increasing HIPing temperature. Most of the PPBs were pinned by the carbide particles in the compacts HIPed at 1140 °C. When the HIPing temperature was increased to 1210 °C and 1260 °C, a large number of PPBs de-pinned and moved beyond the pinning carbide particles, leading to grain growth and leaving carbide particles at the site of the original PPBs within the new grains. With increasing HIPing temperature, the 0.2% yield strength of the powder compacts at 650 °C decreases, the tensile elongation increases, and the tensile fracture mode changed from inter-particle dominant fracture to fully dimple ductile fracture.

Influences of hot-isostatic-pressing temperature on microstructure, tensile properties and tensile fracture mode of Inconel 718 powder compact

In-situ TiB reinforced titanium matrix composites (TMCs) were fabricated by selective laser melting (SLM) of ball-milled Ti6Al4V–TiB2 powders. Optimized SLM processing and stress relief annealing were applied to obtain crack-free and fully dense composites. TiB reinforcement is mainly present in the form of whisker clusters and exhibits a quasi-continuous distribution in TMC1 (2 vol%TiB) while a full-continuous distribution in TMC2 (5 vol%TiB). The distribution of TiB whisker clusters in primary β-Ti grain is not consistent with the complete dissolution mechanism proposed previously. As a result, a dual mechanism of direct reaction and precipitation has been put forward to describe the formation of TiB phase. The microhardness, compressive strength and tensile strength of TMC1 are improved by 14%, 36%, 25% respectively, compared with those of Ti6Al4V alloy. These enhancements can be mainly attributed to Hall-Petch strengthening and load-bearing transformation strengthening. The fracture surface of TMC1 after tensile testing shows a mixture of regions of cleavage facets with regions of small dimples.

Microstructure evolution and mechanical properties of in-situ Ti6Al4V–TiB composites manufactured by selective laser melting

AA2024 aluminum alloy easily forms coarse columnar grains and cracks during selective laser melting (SLM) Herein, a novel grain refiner composed of TiC nanoparticles and TiH2 powders was used to develop the microstructure of selective laser melted AA2024 aluminum alloy Due to the heterogeneous nucleation of α-Al promoted by the cooperation of TiH2 and TiC, the microstructure of the TiC–TiH2/AA2024 composite converted from columnar grains with an average area of 7290 μm2 into equiaxed grains with an average area of 34 μm2, and the morphology of the second phase changed from streamline to network-shaped Moreover, cracks in the TiC–TiH2/AA2024 composite were intrinsically eliminated because of grain refinement Benefitting from elimination of columnar grains and inhibition of cracks as well as the combined effect of refinement strengthening and Orowan strengthening, the TiC–TiH2/AA2024 composite exhibited excellent mechanical properties, the tensile strength and elongation of as-built samples rose from 240 ± 10 MPa and 03 ± 02% in AA2024 alloy to 390 ± 15 MPa and 120 ± 05% in TiC–TiH2/AA2024 composite After T6 heat treatment, TiC–TiH2/AA2024 composite exhibited a tensile strength of 490 ± 20 MPa and elongation of 160 ± 1%, which were comparable to those of wrought AA2024 alloys The results show that aluminum alloy with excellent mechanical properties can be prepared by SLM with effective microstructure control

Grain refinement and crack inhibition of selective laser melted AA2024 aluminum alloy via inoculation with TiC–TiH2

Microstructure evolution during semi-solid powder rolling and post-treatment of 7050 aluminum alloy strips

Grain refinement is critical to surpassing the bottlenecks of inherent hot tearing of high-strength aluminum alloys fabricated by additive manufacturing (AM). In this study, a synergistic grain-refining strategy including heterogeneous nucleation, solute-driven growth restriction and nanoparticle-induced growth restriction was introduced to control the microstructure of Al-Zn-Mg-Cu alloys during the laser powder bed fusion (LPBF) process. Crack-free Al-Zn-Mg-Cu alloys with significantly refined grains were safely fabricated via LPBF by coincorporation of TiC and TiH2 particles. In-situ L12-Al3Ti particles were produced to promote the heterogeneous nucleation. The grain growth was restricted by adding Ti solute, while introduced TiC nanoparticles (NPs) improved the density of heterogeneous nucleation sites and blocked grain growth physically. The resultant elimination of columnar grains and hot cracks in the (1 wt.%) TiC- and (0.8 wt.%) TiH2-modified Al-Zn-Mg-Cu alloy resulted in excellent ultimate tensile strength (UTS) of 593 ± 24 MPa, yield strength (YS) of 485 ± 41 MPa and elongation (EL) of 10.0% ± 2.5% under the T6 condition. This study provides new insights into improving the grain microstructure and mechanical properties of high-strength aluminum alloys during LPBF. 

Enhanced strength and ductility in Al-Zn-Mg-Cu alloys fabricated by laser powder bed fusion using a synergistic grain-refining strategy

Semi-solid powder rolling (SSPR) is a novel method to prepare metal strips, which combines the advantages of powder rolling and semi-solid forming. Besides, it is a near-net shape forming with high efficiency and energy saving. 10% B4C–AA2024 composite strips with the relative density of 0.75–0.95 were prepared by SSPR. The effects of temperatures on the microstructures, brinell-hardness and tensile strength of B4C–AA2024 composite strips were analyzed. The laws of microstructure evolution and densification were discussed. The results show that the best liquid fraction to prepare a 10% B4C–AA2024 composite strip ranges from 38% to 60%. The liquid phases in the powders exist in the form of isolated melt pools when liquid fraction is lower than 20%. When liquid fraction is higher than 20%, liquid phases forms a fluid network, lapping mutually or filling in the pores among the powders. Liquid phases play an important role in the formation of microstructures. B4C particles can restrain the generation of micro-cracks during the solidification process.

Yunzhong Liu

Papers

Microstructure evolution and mechanical properties of in-situ Ti6Al4V–TiB composites manufactured by selective laser melting

Grain refinement and crack inhibition of selective laser melted AA2024 aluminum alloy via inoculation with TiC–TiH2

Microstructure evolution during semi-solid powder rolling and post-treatment of 7050 aluminum alloy strips

Enhanced strength and ductility in Al-Zn-Mg-Cu alloys fabricated by laser powder bed fusion using a synergistic grain-refining strategy

The effects of temperatures on microstructure evolution and mechanical properties of B4C–AA2024 composite strips prepared by semi-solid powder rolling