Abstract Laser powder bed fusion (LPBF) is an emerging additive manufacturing technique that is currently adopted by a number of industries for its ability to directly fabricate complex near-net-shaped components with minimal material wastage. Two major limitations of LPBF, however, are that the process inherently produces components containing some amount of porosity and that fabricated components tend to suffer from poor repeatability. While recent advances have allowed the porosity level to be reduced to a minimum, consistent porosity-free fabrication remains elusive. Therefore, it is important to understand how porosity affects mechanical properties in alloys fabricated this way in order to inform the safe design and application of components. To this aim, this article will review recent literature on the effects of porosity on tensile properties, fatigue life, impact and fracture toughness, creep response, and wear behavior. As the number of alloys that can be fabricated by this technology continues to grow, this overview will mainly focus on four alloys that are commonly fabricated by LPBF—Ti-6Al-4 V, Inconel 718, AISI 316L, and AlSi10Mg. 

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A critical review on the effects of process-induced porosity on the mechanical properties of alloys fabricated by laser powder bed fusion

Despite the recent progress in additive manufacturing (AM) process and technology, challenges in the repeatability and reproducibility of AM parts still hinders the adoption of this technique in many industries. This is particularly difficult when a part is qualified on a particular part on a certain machine using optimised parameters. If a manufacturer wishes to expand production to multiple machines, the ability to translate these optimised parameters to different machines much be understood. In this study, four different metal L-PBF printers were used to produce 316L tensile testing samples using the same processing parameters and metal powder supplied from a single batch from the same supplier. In addition to the analysis of the correlation between the input parameters and the output measures, this study reports that despite the same set process parameters, there is significant variations were found in the mechanical performance and properties of the AM samples produced on the different L-PBF metal additive manufacturing machines. For the range of the input processing parameters and the resulting input volumetric energy density applied of 21–37 J/mm3, values of (4–42)%, (200–716) MPa, and (52–214) GPa were obtained for the elongation, ultimate tensile strength and elastic modulus on additively manufactured 316L samples respectively.

Comparison of the porosity and mechanical performance of 316L stainless steel manufactured on different laser powder bed fusion metal additive manufacturing machines

Heat Treatment for Metal Additive Manufacturing

Additive manufacturing (AM) is one of the recently studied research areas, due to its ability to eliminate different subtractive manufacturing limitations, such as difficultly in fabricating complex parts, material wastage, and numbers of sequential operations. Laser-powder bed fusion (L-PBF) AM for SS316L is known for complex part production due to layer-by-layer deposition and is extensively used in the aerospace, automobile, and medical sectors. The process parameter selection is crucial for deciding the overall quality of the SS316L build component with L-PBF AM. This review critically elaborates the effect of various input parameters, i.e., laser power, scanning speed, hatch spacing, and layer thickness, on various mechanical properties of AM SS316L, such as tensile strength, hardness, and the effect of porosity, along with the microstructure evolution. The effect of other AM parameters, such as the build orientation, pre-heating temperature, and particle size, on the build properties is also discussed. The scope of this review also concerns the challenges in practical applications of AM SS316L. Hence, the residual stress formation, their influence on the mechanical properties and corrosion behavior of the AM build part for bio implant application is also considered. This review involves a detailed comparison of properties achievable with different AM techniques and various post-processing techniques, such as heat treatment and grain refinement effects on properties. This review would help in selecting suitable process parameters for various human body implants and many different applications. This study would also help to better understand the effect of each process parameter of PBF-AM on the SS316L build part quality.

A Critical Review on Effect of Process Parameters on Mechanical and Microstructural Properties of Powder-Bed Fusion Additive Manufacturing of SS316L.

The effects of laser nitriding on the microstructural evolution and wear and corrosion behaviors of the NAK80 mold steel (AISI P21) were investigated. A high-power diode laser with a laser energy density of 1125 J/mm2 was employed for laser nitriding of NAK80. To examine the microstructure of laser-nitrided NAK80, diverse characterization methods such as scanning electron microscopy with electron backscatter diffractometry, high-resolution transmission electron microscopy with electron energy loss spectrometry, and atom probe tomography were utilized. These analyses revealed that the laser-nitrided region consisted of rod-like aluminum nitride (AlN) precipitates with various nitrides and carbides in the martensite matrix, resulting in the surface hardening of NAK80 from 409 to 536 HV. As a result, the laser-nitrided NAK80 exhibited an enhancement in the wear resistance in comparison to that of the base metal in the fretting test. The wear loss of laser-nitrided NAK80 was reduced by 33.4% compared to that of the base metal after 40,000 fretting cycles. However, a deterioration in the corrosion resistance was observed for the laser-nitrided NAK80 compared to the base metal. It may be associated with the elemental segregation in the vicinity of the AlN precipitates, which was observed in the atom-probe tomography.

Microstructural characterizations and wear and corrosion behaviors of laser-nitrided NAK80 mold steel

The influence of heat treatment on the microstructure, mechanical properties, and wear behaviors of stainless steel 316L (SS316L) produced via selective laser melting (SLM) was investigated. The fabricated SLM samples were subjected to two different heat treatments: a typical furnace-type heat treatment conducted at 1100 °C for 0.5 h and hot isostatic pressing performed at 1100 °C and 100 MPa for 1.5 h. High-density SLM samples with low porosities were obtained by increasing the laser power and decreasing the scan speed. The heat treatments of the fabricated SLM samples induced the removal of porosity, cellular microstructure, and dense dislocation structures with a slight increase in grain size. In terms of mechanical properties, the fabricated SLM samples exhibited similar hardness and tensile strength properties to those of the conventional SS316L, while a significantly lower elongation was evident. The heat treatments of the fabricated SLM samples improved elongation, while the surface hardness and tensile strength decreased owing to microstructural evolution. During the pin-on-disk test, the conventional SS316L and fabricated SLM sample exhibited similar wear resistance values, which decreased after the heat treatments of the fabricated SLM samples owing to the heat treatment-induced surface softening.

Heat treatment effect on the microstructure, mechanical properties, and wear behaviors of stainless steel 316L prepared via selective laser melting

Laser heat-treatment and laser nitriding were conducted on an AISI P21 mold steel using a high-power diode laser with laser energy densities of 90 and 1125 J/mm2, respectively. No change in surface hardness was observed after laser heat-treatment. In contrast, a relatively larger surface hardness was measured after laser nitriding (i.e., 536 HV) compared with that of the base metal (i.e., 409 HV). The TEM and electron energy loss spectroscopy (EELS) analyses revealed that laser nitriding induced to develop AlN precipitates up to a depth of 15 μm from the surface, resulting in surface hardening. The laser-nitrided P21 exhibited a superior wear resistance compared with that of the base metal and laser heat-treated P21 in the pin-on-disk tribotests. After 100 m of a sliding distance of the pin-on-disk test, the total wear loss of the base metal was measured to be 0.74 mm3, and it decreased to 0.60 mm3 for the laser-nitrided P21. The base metal and laser heat-treated P21 showed similar wear behaviors. The larger wear resistance of the laser-nitrided P21 was attributed to the AlN precipitate-induced surface hardening.

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Won Sang Shin

Papers

Heat treatment effect on the microstructure, mechanical properties, and wear behaviors of stainless steel 316L prepared via selective laser melting

Effect of laser heat-treatment and laser nitriding on the microstructural evolutions and wear behaviors of AISI P21 mold steel