Laser additive manufacturing is widely recognized to be an effective method to form complicated and custom metallic components. The existing research on metal additive manufacturing utilizes traditional alloy grades, which are designed based on the assumption that solidification occurs at equilibrium; thus, these materials are not well suited to the nonequilibrium metallurgical dynamics that are present in additive manufacturing techniques. Common issues, such as high crack susceptibility, low toughness, and low fatigue capability, as well as anisotropy, frequently occur during the fabrication of additively manufactured metallic parts. It is therefore necessary to conduct research on the design of new materials designed specifically for laser additive manufacturing in order to fully realize the potential advantages and value of the ultrafast solidification conditions. In this article, the technical bottlenecks, material design methods, and the development of new materials that are applicable to laser additively manufactured metal materials are reviewed; these materials include aluminum alloys, titanium alloys, iron-based alloys, and magnesium alloys. Finally, the potential future direction of research related to laser metal additive manufacturing is discussed.

Traditional high-strength nickel-based superalloys have a wide solidification temperature range and high proportion of low melting point eutectic phases, which are prone to cracking during rapid nonequilibrium solidification. The residual stress release and rapid nucleation of γ' precipitate during the post-heat treatment process result in crack formation for high-strength nickel-based superalloys, which limits their application and promotion in the field of additive manufacturing. In this review, the research progress in crack formation mechanism and cracking-free design (printing parameter optimization, post-treatment regulation, and alloying design) of high-strength nickel-based superalloys fabricated via additive manufacturing is presented. Additionally, research prospects related to crack control of additively manufactured high-strength nickel-based superalloys are proposed.

Selective laser melting (SLM) additive manufacturing technology holds the broad prospect for the preparation of high-performance complex metal components owing to its high processing accuracy, short manufacturing cycle, and high material usage. Magnesium (Mg) alloys are the lightest metal structural material and provide the benefits of low density, substantial specific strength and specific stiffness, good damping and shock absorption performance, and good biodegradability. Thus, it is worthwhile to employ SLM to manufacture Mg alloys, which is predicted to widen the application scope of Mg alloys. In this study, a comprehensive review on SLM of Mg alloys focusing on the preparation of Mg alloy powders, SLM process parameters, metallurgical defects, microstructure and mechanical properties of the as-built state, post-processing, and special equipment developed for SLM of Mg alloys is given. Finally, the future development trends of the SLM of Mg alloys are explored.

The postprocessing/machining of NiTi shape memory alloys (SMAs) is extremely challenging and difficult due to their low thermal conductivity and the high reactivity of ready-made NiTi parts. As a typical metal additive manufacturing technology, selective laser melting (SLM) offers significant advantages and can directly fabricate complex metallic parts, effectively address the problems of cold workability and machinability for NiTi parts. By establishing the relationship between processing parameters, microstructure, functional properties, and revealing the underlying mechanisms for altered phase transformation behavior and functional properties of SLM NiTi SMAs, it can serve as a theoretical foundation for expanding the applications of SLM NiTi SMAs. As a result, this paper comprehensively evaluates the formability, phase transformation behavior, microstructure, mechanical properties, and thermomechanical properties of SLM NiTi SMAs. Additionally, the design of SLM porous NiTi SMAs, as well as their biocompatibility, are discussed. Eventually, the future development trend and critical problems in studying SLM NiTi SMAs are investigated.

As a new manufacturing technology, additive manufacturing has brought about revolutionary changes in the aerospace, transportation, and biomedicine fields. However, since the metal materials used in additive manufacturing are still mainly traditional alloys, some of them are unsuitable for high-energy beam processing, indicating room for performance improvements. Besides, the development of additive manufacturing materials still follows the traditional trial-and-error model, seriously restricting the development of high-performance materials. Therefore, this paper discusses this situation and the existing additive manufacturing technology problems of steel, titanium alloys, and aluminum alloys, after which the application of high-throughput preparation and characterization technologies in material development and design were expounded. Combined with the principle and characteristics of high-throughput additive manufacturing preparations, the prospects and challenges of the high-throughput preparation and characterization technology of additive manufacturing in material development were expounded. Then, futuristic developmental directions of key materials for additive manufacturing development and composition optimization were proposed.

Additive manufacturing technology has greatly increased opportunities in the production of high-strength aluminum alloy complex parts. However, current additive manufactured aluminum alloy systems are still limited to castable and weldable Al-Si alloys. This impedes the development of high-performance additive manufactured aluminum alloys. Recently, various computational techniques at different scales have been gradually used to promote the development of high-performance additive manufactured aluminum alloys. This paper summarizes the research achievements in the field of computationally-assisted design of additive manufactured aluminum alloys and their preparation from domestic and foreign scholars and presents representative cases from atomic, mesoscopic, and macroscopic scales and machine learning. The different calculation methods used to assist alloy designs are analyzed and their shortcomings are presented. Finally, the prospect on how to improve the application of multi-scale computation techniques in the development of high-performance additive manufactured aluminum alloys is presented, and some specific development directions are also clarified.

This paper summarizes the research progress of friction stir additive manufacturing (FSAM) technology at home and abroad. FSAM is fast-forming, has high additive efficiency, and provides environmental protection. In addition, as a solid-phase additive technology, it effectively avoids shrinkage, porosity, and other defects caused by other melt-additive methods during molding. Currently, reported FSAM methods can be roughly divided into four categories: axial additive manufacturing, radial additive manufacturing, consumable friction-stir tool additive manufacturing, and superimposed plate additive manufacturing. The microstructures and properties of friction stir, laser, and arc additive samples are listed in detail. The advantages and disadvantages of the different additive methods and their application fields are expounded. The companies of friction stir additive equipment, the preliminary applications, and the development direction of friction stir additive equipment designed in the future are introduced. It lays a foundation for further application of friction stir additive technology.

To meet the requirements of the long fatigue life and high reliability of the key components of the aeroengine as well as solve the challenges of “structure control” and “performance control” based on the fact that plastic deformation can effectively eliminate internal stress and close metallurgical defects generated by the thermal effect, a laser integrated additive manufacturing technology with alternately thermal/mechanical effects is developed. In this study, Ti6Al4V alloy was chosen as the research object. The distributions of residual stress and metallurgical defects and the microstructural evolution of the formed components were systematically studied. The effects of surface laser shock peening (LSP) and interlayer LSP without coating (LSPwC) treatments on mechanical properties were investigated using a tensile test. The results showed that after LSP, tensile residual stress was transformed into compressive residual stress. Additionally, laser shock waves could effectively improve the metallurgical defects in selective laser melting (SLM)-formed components. Moreover, high-density dislocation structures and numerous twins in two directions were produced in coarse α' martensite by laser shock waves, which jointly promoted the grain refinement of α' martensite. The ultimate tensile strength and elongation of Ti6Al4V fabricated by the laser integrated additive manufacturing technology with alternately thermal/mechanical effects reached 1543 MPa and 15.53%, which are 46.5% and 91.5% higher than those of the SLM-formed components, respectively, yielding a good combination of strength and ductility.

Laser melting deposition (LMD) combines the laser cladding and rapid prototyping manufacturing technologies, and can be used for swift prototyping of complex parts with excellent comprehensive properties. However, due to its unique metallurgical conditions, it is easy to develop penetrating columnar crystals and coarse primary grains along the building direction. This remarkably reduces the mechanical properties of the alloy. The root cause of this issue can be traced back to the thermodynamic and dynamic metallurgical processes. Thus, this study proposes an oscillating laser melting deposition (OLMD) based on laser oscillating welding technology, and aims to elucidate the metallurgical structure and defects of laser melt deposition. OLMD modifies the motion trajectory of the molten pool using a laser in situ oscillation, and directly impacts the temperature gradient and solidification rate, thus improving the microstructure of titanium alloy by LMD. Furthermore, the microstructure evolution and mechanical properties of TC4 titanium alloy produced using OLMD were studied using OM, SEM, EBSD, and a Vickers hardness tester. The results indicate that the optimum process parameters of laser melting deposition without oscillation are as follows: the laser power is 1000 W, scanning rate is 8 mm/s, and powder feeding rate is 6.92 g/min. The optimum technological parameters of linear oscillation are as follows: the frequency is 200 Hz and the oscillation amplitude is 1.5 mm. Addition of linear laser oscillation considerably improved the morphology of the molten pool, and defects such as porosity and cracks were not observed. The overall number and size of columnar crystals reduced, and the grains were equiaxed. When compared to the sample without oscillation, the average grain size of Ti-6Al-4V alloy with linear oscillation decreased from 5.20 μm to 4.37 μm, while hardness increased from 418.00 HV to 428.75 HV.

Selective laser melting (SLM) is a widely used high-precision additive manufacturing technology that can achieve arbitrarily complex structures. The powder size used by SLM is generally 15-53 μm, which is suitable for manufacturing parts with a forming accuracy within tens of microns. However, the reason why smaller or larger particle size powders are not suitable is not yet clear. The effect of particle size on SLM processability was studied by simulation and experimentation. Three powder particle sizes of AlSi10Mg were used to study the behavior of powder spreading and melting/solidification during SLM by discrete element and computational fluid dynamics methods, respectively. The macroscopic forming quality of the formed samples was tested. The results show that the powders with a particle size below 20 μm agglomerate vigorously to form many cavities, and the powders with a particle size above 53 μm tend to form few large cavities. The relative density of the powder bed with the medium particle size is 7.69% and 3.17% higher than those of the fine and large particle sizes, respectively. The melt channels of the fine and coarse particle sizes are irregular due to the uneven quality of powder laying when the powder bed is melted. However, after multilayer melting, defects in the melt channel of the fine particle size are partially alleviated. With the increase in particle size, the melt channel surface flatness decreases, the fine particle size powder samples have more porosity, and the coarse particle size powder has a few unfused defects. The processability of the medium particle size for SLM is the best among them. The relative density of the sample with the medium particle size reach 99.8%, which is 1.4% and 0.4% higher than those of samples with fine and coarse particle sizes, respectively.

2Cr13 martensite stainless steel has been widely used for the manufacturing of surgical tools and turbine blades. Contrary to the conventional fabrication technologies, there are several remarkable advantages in the fabrication of 2Cr13 parts by adopting wire arc additive manufacturing (WAAM) technologies, such as excellent metallurgical bonding, high production efficiency, near-net-shape production, and limited environmental contamination. In this work, the effect of interlayer dwelling temperature (110-550^oC) on microstructural and mechanical properties has been revealed, providing a new approach for the active control of the performances of 2Cr13 buildups produced by wire-arc additive manufacturing. The part with a dwelling temperature of 550^oC was featured by elongated acicular martensite features, with a slightly enhanced fiber-like texture, along with minor fine irregular-reverse austenite structures, dispersed among martensite gaps. This special martensitic distribution was mainly caused by the grain-broken effect under the intensive thermal shock from liquid melting pool. Consequently, the enhanced tensile strength and microhardness were obtained due to grain refinement, although exhibiting an obvious anisotropy in tensile properties. The parts with dwelling temperatures of 110-180^oC were characterized by relatively coarsened martensite laths, with a random texture type, within block-shaped ferrite matrix. The average martensite size was gradually refined due to the increased cooling rate by lowering interlayer temperature. The isotropic mechanical properties of all three parts (110-180^oC) were similar because of the similar martensite laths.

Laser powder bed fusion (L-PBF) is an innovative additive manufacturing method with great potential for fabricating complex geometrical components with integrated functionalities. In the aerospace industry, the Al-Cu-Mg (2024Al) alloy is widely used because of its excellent mechanical properties and low density; however, its disadvantages include low printability and high crack susceptibility. This work investigates the effects of in situ TiB₂ particles on the microstructure and tensile properties of the solution-treated (510^oC treat 1 h and then cooling by water) and T6-treated (i.e., solution and aging treatments) L-PBF fabricated 2024Al alloy at room temperature. Equiaxed grains with an average size of approximately 5.8 μm dominate in the printed 2024Al-2%TiB₂ alloy because of the high cooling rate during the L-PBF process and the heterogeneous nucleation effect of the TiB₂ particles. After the T6 heat treatment, many uniformly distributed, fine, and long precipitation strips formed in both the 2024Al and 2024Al-2%TiB₂ alloys. The 2024Al-2%TiB₂ alloy has ultimate tensile and yield strengths of (458.2 ± 6.5) and (398.4 ± 2.7) MPa, respectively; further, it has a maximum elongation of (3.4 ± 0.4)%. These parameters indicate a substantial improvement in the strength and elongation of the 2024Al-2%TiB₂ alloy compared to those of the 2024Al alloy. Furthermore, the mechanical properties of the T6-treated 2024Al-2%TiB₂ alloy are comparable to those of the wrought T6-treated 2024Al-T6 alloy. The main strengthening mechanisms of the 2024Al-2%TiB₂ alloy include solid solution strengthening, dislocation strengthening, grain boundary strengthening, precipitation strengthening, Orowan strengthening, and load-bearing strengthening induced by TiB₂ particles. In conclusion, 2024Al-2%TiB₂ alloy manufactured using the L-PBF method provides excellent printability and room-temperature tensile properties.

The NiTi alloy is a key material in aerospace and biomedical fields owing to its excellent superelasticity and high shape memory effect. Laser directed energy deposition (LDED), as an advanced additive manufacturing technology, made the preparation of NiTi alloys with high shape memory effect possible. In this study, the NiTi alloy was fabricated via LDED using Ni and Ti powder feedstock. The microstructure, phase content, and phase transformation of the alloy were analyzed by XRD, phase fitting, SEM, EDS, and DSC. Next, the shape memory effect was tested using compressed cylindrical samples. When the laser energy density was low, several Ni₄Ti₃ phases were produced in the NiTi alloy. The Ni₄Ti₃ phase disappeared with an increase in the laser energy density. When the laser energy density was 20.0 J/mm², the NiTi alloy showed a high compressive breaking strength of 2878 MPa and a compression failure strain of 34.9%, and the sample also showed a shape recovery rate of 88.2% after 20 cyc of compression.