Laser Powder Bed Fusion Parts Research Articles

Surface Finish Analysis of Gradient Voltage Electrochemical Polishing of 316L Stainless Steel Parts Forming by Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) of complex-structure 316L stainless steel (316L ss) parts has a wide application prospects in aerospace, biomedical, and defense industry fields. However, the surface roughness (Ra) of the LPBF sample is unsatisfactory due to the process characteristics of layer-by-layer selective melting and cumulative forming, which limits its applications in the engineering field. Herein, a gradient voltage electrochemical polishing strategy is proposed based on the characteristics of electrochemical polishing technology, which can polish complex structures. The mechanisms of polishing process parameters and polishing strategy on the surface finish of LPBF parts are investigated. The gradient voltage polishing strategy is extended to complex structures, and the Ra of the inner surfaces of square and round tubes are successfully reduced to about 1 μm. The gradient electrochemical polishing process for surface finish post-treatment of LPBF parts can broaden the engineering applications of complex-structure metal parts.

Facile manipulation of mechanical properties of Ti-6Al-4V through composition tailoring in laser powder bed fusion

Ti-6Al-4 V alloy when processed by laser powder bed fusion (LPBF) is a useful material which can be used for the manufacture of complex 3D components for aerospace and medical applications. LPBF fabricated Ti-6Al-4 V typically shows high tensile strength (>1200 MPa) but poor ductility (<10%), explained by the characteristic microstructures that form under high cooling rates and multiple thermal cycles. Here, the composition of the feedstock Ti-6Al-4 V powder was modified via the addition of commercially pure Ti (CP Ti) to achieve improved ductility of LPBF parts through an in-situ modified composition, while maintaining good strength. The LPBF printed Ti alloy (with nominal composition Ti-3Al-2 V) showed a tensile strength of ∼1000 MPa, with improved ductility (∼13%), comparable to wrought Ti-6Al-4 V. These properties are thought to be explained by the decreased c/a ratio (where c and a are the lattice parameters of the titanium hexagonal close-packed phase) resulting in enhanced dislocation slip. Good relative density was also achieved using the modified composition under the same processing parameters. Some non-uniform regions were seen in the developed alloys, as well as evidence of increased martensite α′ thickness. It is proposed that with optimized parameters, further enhancements may be achieved. This work has shown that microstructural and mechanical properties can be facilely manipulated with proper CP Ti addition.

Melt pool signatures of TiN nanoparticle dry-coated Co25Cr25Fe25Ni25 metal powder in laser-powder-bed-fusion

Metal powders in laser-powder-bed-fusion (L-PBF) often exhibit cohesive flow resulting from interparticle adhesion. Nanoparticle dry-coating can improve powder flowability and promote powder layer densification. A Co25Cr25Fe25Ni25 metal powder (20–90 µm) is dry-coated with TiN particles with a diameter of 16 nm at low concentrations of up to 69 ppm. The dynamic angle of repose decreased and bulk powder density increased compared to the uncoated state from 49 ° and 4.67 g/cm3 to 29 ° and 4.81 g/cm3 with dry-coating of TiN, respectively. UV/Vis spectroscopy showed negligible alterations by TiN additions on the powder light absorption. The powder modifications strongly affected their corresponding processability in L-PBF and reduced the melt pool signatures of the in situ detected confocal single-color pyrometer signal as well as ex situ measured melt pool depth and width. With increasing flowability, a significant decrease in thermal emission and melt pool size was observed. The results demonstrate the impact of powder flowability and bulk powder density on the quality of L-PBF parts when particle interactions are actively modified.

Numerical methods in predicting residual stresses in laser powder bed fusion developed parts – a scoping review

The study reviews the numerical methods for predicting residual stresses in parts manufactured with additive manufacturing (AM) technique, such as laser powder bed fusion (L-PBF). L-PBF is a fast-growing technology with enormous potential for creating complex geometries with improved properties as compared to conventional processes. However, parts produced with L-PBF are susceptible to higher magnitudes of residual stresses, particularly tensile stresses as compared to compressive stresses, leading to geometrical distortions. While newly developed materials offer excellent properties and benefits to the industrial sector, the residual stresses that develop when parts are produced using the L-PBF process remain unexplored. The study evaluates three numerical simulation methods, such as thermomechanical modelling (TMM), inherent strain method (ISM), and multi-scale modelling (MSM) used to predict residual stresses in L-PBF parts. Each method has its advantages and limitations, and the best method depends on the specific application and available resources. The review highlights the potential benefits of numerical simulation methods for predicting residual stresses in L-PBF parts and compared to experimental results. Thus, the study recommends that future research should focus on improving the accuracy of numerical simulation methods for predicting residual stresses in L-PBF parts through refinement processes and exploring the impact of residual stresses on the L-PBF parts through detailed characterization.

Fatigue prediction and life assessment method for metal laser powder bed fusion parts

In this paper, an industry-accepted fatigue model based on generalized stress-life (S–N) curves is adapted for metal parts fabricated by laser powder bed fusion (LPBF). Initial defects inherent to the fabrication process, such as part porosity, are related to fatigue life performance. Hereto, additively manufactured test specimens are fatigue tested and used to formulate a function to predict fatigue life based on the size of initial defects. The predictions correlate well with experimental results and provide a quality measure to expel outliers. The method can be used to predict the life expectancy of LPBF parts based on a priori detected defect sizes.

Process Optimization for Up-Facing Surface Finish of AlSi10Mg Alloy Produced by Laser Powder Bed Fusion

This work investigates the effects of various processing parameters (laser power, scanning speed, hatch distance and beam offset) on the resultant inclined up-facing surface roughness of AlSi10Mg alloys produced by laser powder bed fusion (LPBF). A two-step approach, orthogonal test followed by the Doehlert matrix design (DMD) test is used to efficiently optimize the up-facing surface and contour parameters. The former method aims to determine the significance of variables while the latter one facilitates a rapid optimization. The results show that the interaction and interdependency among the parameters are of great significance to the obtainable surface roughness. Using a rational design of experiments, the optimized up-facing surface roughness of Ra of 5.4 μm is achieved. This is attributed to the elimination of the laser partition track and the reduction in irregularities at the edges of the parts. This work demonstrates an effective approach of experimental processing parameter optimization to improve the surface finish of LPBF parts.

Influence of gaseous environment on the properties of Inconel 625 L-PBF parts

The effect of gaseous atmosphere during Laser Powder Bed Fusion (L-PBF) was analysed on an Inconel 625 alloy. The bead dimensions, the surface finish, the porosity rate and the resulting microstructure were analysed for three different gases (Argon, Nitrogen and Helium) in an industrial L-PBF machine, using rather high energy density promoting keyhole welding regime. No clear effect of gas nature on energy coupling was shown when considering fusion beads on L-PBF parts. However, this work shows that gas, and especially helium mostly modifies surface finish, due to a modification of heat dissipation at the interface between the built part and the powder bed. This effect is clearly beneficial for vertical builds but is balanced by a possible and detrimental reduction of surface tensions for inclined and overhanging surfaces. The data presented here indicates a moderate benefit of He and N2 gases on porosity rates at high laser energy, but the as-built microstructures do not exhibit detectable differences, at the scale considered for the analyses (SEM + EBSD). Some physical aspects of the liquid/gas interface are discussed at the end of the paper to explain the observed results.

Failure modes in dual layer thickness Laser Powder Bed Fusion components using a novel post-mortem reconstruction technique

To exploit the design freedoms of Powder Bed Fusion, parameters can be varied within sub-volumes of components to achieve the optimal part for both service conditions and manufacturing productivity. This involves prioritising mechanical strength in areas of structural significance and high volumetric build rates in areas of low structural significance. In theory, a component with similar mechanical behaviour to that seen in standard Laser Powder Bed Fusion parts can be built in significantly less time and at a reduced cost. In practice however, the boundary between such regions is yet to be understood and discretising components into sub-volumes can induce interfacial defects. In this study, an in-depth analysis of interfaces between disparate layer thickness volumes in single components has been explored, to gain information vital to solving interface quality issues so that LPBF design freedoms can be fully exploited. A novel 3D reconstruction technique has been demonstrated to characterise transient plastic behaviour of interfacial pores post-fracture. This technique enables post-mortem evaluation of additively manufactured parts and tracking of pore deformation during subsequent mechanical testing. X-ray Computed Tomography (XCT) identified interfacial pores up to 170 µm Feret diameter, with a voxel resolution of 6 µm. Micro tensile testing with in-situ microscopy exhibited a real-time mechanical response, observing evidence that these interfacial defects lead to fracture at interface locations. The 3D reconstruction technique found that pores constricted 10.0 – 14.1% in the x direction and 10.3 – 14.6% in the y direction after fracture – normal to the loading direction. These findings contribute towards improving Additively Manufactured biomedical implants and airframe components with reduced time and cost. • LPBF components are discretised into sub-volumes with different layer thicknesses. • XCT revealed pores are introduced at the interface between sub-regions. • Interface plane orientation has a great effect on interfacial porosity. • Micro tensile failure modes are observed at the interfaces with in-situ microscopy. • A novel 3D pore reconstruction technique is demonstrated.

The Effect of Sandblasting and Bead Blasting on the Surface Finish of Dry Electrolyte Polishing of Laser Powder Bed Fusion Parts

The Effect of Sandblasting and Bead Blasting on the Surface Finish of Dry Electrolyte Polishing of Laser Powder Bed Fusion Parts

Microstructural Development of Ti-6Al-4V Alloy via Powder Metallurgy and Laser Powder Bed Fusion

A detailed study was carried out to gain a better understanding of the microstructural differences between Ti-6Al-4V parts fabricated via the conventional powder metallurgy (PM) and the laser powder bed fusion (L-PBF) 3D printing routes. The parts were compared in terms of the constituent phases in the microstructure and their effects on the micro- and nano-hardness. In L-PBF parts, the microstructure has a single phase of martensitic α′ with hcp crystal structure and acicular laths morphology, transformed from prior parent phase β formed upon solidification of the melt pool. However, for the sintered parts via powder metallurgy, two phases of α and β are noticeable and the microstructure is composed of α grains and α + β Lamellae. The microhardness of L-PBF processed Ti-6Al-4V samples is remarkably higher than that of the PM samples but, surprisingly, the nano-hardness of the bulk martensitic phase α′ (6.3 GPa) is almost the same as α (i.e., 6.2 GPa) in PM samples. This confirms the rapid cooling of the β phase does not have any effect on the hardening of the bulk martensitic hcp α′. The high microhardness of L-PBF parts is due to the fine lath morphology of α′, with a large concentration of low angle boundaries of α′. Furthermore, it is revealed that for the α phase in PM samples, a higher level of vanadium concentration lowers the nano-hardness of the α phase. In addition, as expected, the compacting pressure and sintering temperature during the PM process led to variations in the porosity level as well as the microstructural morphology of the fabricated specimens, which will in turn have a significant effect on the mechanical properties.

Analysis of the interdependent relationship between porosity, deformation, and crack growth during compression loading of LPBF AlSi10Mg

For parts loaded in tension and shear, it is clear that voids created during manufacturing generally decrease part strength and ductility. This study examines how these manufacturing defects affect the mechanical performance of laser powder bed fusion (LPBF) AlSi10Mg specimens loaded in compression. Cylindrical samples of LPBF AlSi10Mg with varying levels of porosity and different microstructures were loaded in-situ in compression and characterized using X-ray computed tomography (XCT). Bulk porosity influenced the mechanical behavior of LPBF AlSi10Mg parts loaded in compression, with increasing porosity leading to decreasing yield strength, ultimate strength, and part ductility. However, bulk porosity did not influence the failure mode of the material during compression. Instead, microstructural changes caused by different heat treatments were the determining factor as to whether a part would fail via a critical shear crack or slowly collapse during loading. Data suggests that as bulk porosity increased, the strain required to propagate a crack from nucleation to catastrophic part failure also increased. Based on observations from XCT data, we hypothesize that large pores in the microstructure blunted the growing crack. These results suggest that, by increasing the resilience of LPBF AlSi10Mg parts loaded in compression, porosity may enhance certain desirable properties of LPBF parts during compressive loading.

Laser Powder Bed Fusion of Powder Material: A Review.

Laser powder bed fusion (L-PBF) has drawn increasing interest from enterprises because of its great advantages in additive manufacturing of metal, ceramic, and polymer materials. Although people pay more attention to L-PBF, many problems such as poor density need to be solved urgently. In this article, the commonly used materials of L-PBF are systematically reviewed, and the latest development of the application of L-PBF powder materials is analyzed. The background, application, and molding mechanism of a new type of green material for L-PBF are comprehensively introduced, which shows its great application potential. According to the characteristics of different materials, the methods to improve the quality of L-PBF parts are discussed. The existing problems and prospects of L-PBF are summarized.

High cycle fatigue performance evaluation of a laser powder bed fusion manufactured Ti-6Al-4V bracket for aero-engine applications

Due to the stringent certification requirements and complex engine operating environment, the design of AM components for aero-engine application remains a challenging task. The high cycle fatigue performance of a weight-optimised bracket made using the Laser Powder Bed Fusion (LPBF) process was studied through shaker table testing. The results are discussed together with the observed mechanical and fatigue strengths in conjunction with material characterisation of microstructure, surface roughness, defects, micro-hardness and fracture surfaces. The debits in mechanical and fatigue strengths are expected due to process dependent surface roughness and the density of defects present. From the shaker table test results, it was concluded that the proposed weight-optimised LPBF bracket is capable of meeting the performance targets. The fundamental vibration mode of the bracket assembly was 84 Hz, which was much higher than the 1st Low Pressure shaft speed (48 Hz) of the target engine and thus avoids potential resonance. The bracket achieved its target inertial g load capability of 20 g and it was demonstrated that the bracket had enough redundancy in its load transfer paths should a strut fail during service of the engine. Lack of fusion voids and micro-cracks present on or near the surface were the prime sites for crack initiation. It has been shown that the as-built surface can cause a significant reduction (up to 40%) in the fatigue strength when compared to machined Ti-6Al-4V. A safe life regime for LPBF component design has been presented based on the Kitagawa-Takahasi diagram, modified using the Chapetti curve, which effectively links the material fatigue properties and performance of LPBF parts with intrinsic defects.

Additive manufacturing of Zn-Mg alloy porous scaffolds with enhanced osseointegration: In vitro and in vivo studies

Biodegradable metals (BM) and additive manufacturing (AM) are regarded revolutionary biomaterials and biofabrication technologies for bone repairing metal implants, the combination of both, namely AM of BM, is thus expected to solve the dual technical difficulties including "conventional medical metals are biologically inert and exist in the human body permanently" and "conventional manufacturing processes are inadequate to fabricate personalized implants of complicated structure”. This work additively manufactured biodegradable Zn-Mg alloy porous scaffolds by laser powder bed fusion (L-PBF). By using the pre-alloyed Zn-xMg (x = 1, 2 and 5 wt.%) powder and the optimized processing conditions, high fusion quality with the relative density greater than 99.5% was confirmed for the L-PBF parts. The influence of Mg content on microstructure, mechanical properties, in vitro corrosion, cytocompatibility, in vivo degradation, biocompatibility and osteogenic effect was investigated. Fine α-Zn grains and precipitation phases including Mg2Zn11 and MgZn2 were observed in the Zn-xMg L-PBF parts. The hardness increased, and the strength increases firstly and then decreased with increasing the Mg content. The compressive strength and elastic modulus of Zn-1Mg porous scaffolds reached the highest as 40.9 ± 0.4 MPa and 1.17 ± 0.11 GPa, respectively, equivalent to those of cancellous bone. The corrosion rate and cell viability slightly rose with increasing the Mg content. Histological analysis after 6-week and 12-week implantation in rabbit femurs showed enhanced bone formation around the Zn-1Mg porous scaffolds compared with pure Zn counterparts. In summary, Zn-1Mg porous scaffolds produced by L-PBF presented promising results to fulfill customized requirements of biodegradable bone implants. Statement of significanceAdditive manufacturing of biodegradable metal porous scaffolds is expected to solve the dual challenges from customized structures and bioactive function required for bone implants. It was the first to present a systematic in vitro and in vivo investigation into the compositions, microstructure, mechanical properties, biodegradation, biocompatibility and osteogenic effect of additively manufactured Zn-Mg alloy porous scaffolds. Reliable formation quality and performance evaluation was achieved by using the pre-alloyed Zn-xMg (x = 1, 2 and 5 wt.%) powder and the optimized laser powder bed fusion process. Although the Zn-1Mg scaffolds exhibited promising mechanical strength, biocompatibility, and osteogenic effect, their degradation rate needs to be further accelerated compared with the term of bone reconstruction.

On the efficiency of machine learning for fatigue assessment of post-processed additively manufactured AlSi10Mg

Laser powder bed fusion (LPBF) is receiving widespread attention for its capability to build components with complex geometries. Post-processing can address the adverse effects of various imperfections exhibited in LPBF parts in their as-built state, including inhomogeneous microstructure, tensile residual stresses and poor surface quality. In a recent experimental study, we investigated the influences of different post-processing techniques including heat treatment and shot peening as well as their combination on rotating bending fatigue behavior of V-notched LPBF AlSi10Mg samples. Herein, we further examined those samples regarding the specific parameters that directly influence fatigue performance with the aim to develop a deep learning based approach by means of artificial neural network. The effect of yield stress, ultimate tensile strength, elongation, porosity, microhardness, compressive residual stresses, and surface roughness and morphology were assessed and implemented in the model. Fatigue behavior of the samples was predicted and analyzed using sensitivity and parametric analyses. The obtained results reveal the high potential of deeply learned neural network for unlocking the role of post-processing on fatigue performance of LPBF AlSi10Mg samples.

CT-based method to measure metal powder characteristics and to study their influence on the quality of additively manufactured parts

Laser powder bed fusion (LPBF) is increasingly used to produce metal industrial components for high value-added sectors, such as aerospace, automotive and biomedical. However, mechanical and structural properties of LPBF parts are often hindered by the large quality variability, poor geometrical and dimensional characteristics, complex surface texture and low density. The quality of the feedstock material is an important aspect to be taken into account, as it significantly influences such possible issues. In particular, metal powder used in LPBF should have shape and size distribution designed to facilitate good flowability, spreading and packing behaviour, so that the final fabricated parts have acceptable density, surface finish and mechanical properties. This work focuses on the accuracy of simultaneous measurement of powder size and shape from three-dimensional reconstructions obtained by X-ray computed tomography (CT). Results of CT measurements are compared with results from other methods based on laser diffraction and scanning electron microscopy. Different materials and powder morphologies were investigated. In addition, the CT-measured powder characteristics can be used to improve other CT analyses of LPBF parts.

The effect of shot peening on corrosion performance of anodized laser powder bed fusion manufactured AlSi10Mg

Anodizing is commonly used on aluminium to improve the surface properties of the product for better corrosion resistance and wear resistance. In this research, the anodizing of laser powder bed fusion (LPBF) manufactured AlSi10Mg parts which were subjected to prior shot peening (SP) were investigated. Anodized specimens were analysed using SEM imaging and the corrosion properties were examined considering the effect of residual stress relief heat treatment and the SP process. The results showed that the SP deteriorates the corrosion performance of the material as such, but together with the subsequent anodizing it resulted to the lowest corrosion current of all investigated structures. As the SP can be used to lower the surface roughness of the LPBF parts and to increase fatigue life as well, the results of this work further encourage the use of SP with anodizing.

Hybrid thermal, mechanical and chemical surface post-treatments for improved fatigue behavior of laser powder bed fusion AlSi10Mg notched samples

Complex geometries can be produced by laser powder bed fusion (LPBF) techniques in a layer-by-layer manner. These parts exhibit inhomogeneous microstructure and poor surface quality in their as-built state. Performing post-treatments to modify these imperfections can play a substantial role in enhancing the performance of LPBF parts. However, the effects of post treatments on local geometrical irregularities are not still well documented. In this study, four different post-treatments including heat treatment, mechanical and chemical surface treatments as well as their combination were considered. Their effect was studied on microstructure, surface, and mechanical properties of LPBF V-notched AlSi10Mg samples. The as-built samples were subjected to two different shot peening processes (using different Almen intensity, shot diameter, and shot hardness), chemical polishing and electro-chemical polishing, in individual and combined configurations. Comprehensive microstructural characterization was carried out and the surface state of the samples was studied in detail in terms of surface morphology and roughness. In addition, mechanical properties including microhardness and residual stresses were measured and finally the fatigue behaviors of the samples were analyzed and compared at a constant stress level. All post treatments led to improved fatigue life. The combination of the aforementioned post-treatments led to a remarkable fatigue life improvement up to 414 times higher compared to the as-built state.

Structuring by laser remelting as a method for waviness reduction on additive manufactured parts

The surface roughness of parts manufactured by laser powder bed fusion (LPBF) is often not sufficient for the intended application. Therefore, surface finishing is necessary. Conventional finishing techniques have many disadvantages like accessibility of complex geometries, high time and cost consumption and a negative environmental impact due to the use of abrasives. As a solution laser polishing is investigated. In this study it was found that surface roughness of LPBF parts up to a spatial wavelength of 0.5 mm can be reduced by ∼96% with laser polishing, while waviness can only be reduced by ∼55%. To further reduce waviness, structuring by laser remelting (WaveShape) is investigated. The basic idea is to structure the inverse of the initial structure and thus to “de-structure” the waviness. A structured profile of a laser polished LPBF surface is “de-structured” with a variation of process parameters. The results show the potential of a waviness reduction of 85%.

Microstructure of aluminum alloys manufactured via laser powder bed fusion: A review

Laser powder bed fusion (LPBF) enables layer-by-layer manufacturing and provides distinguished microstructure compared with conventional processing methods. Microstructure controlling of LPBF parts requires deep insights into the solidification process. The aim of this article is to review the solidification fundamentals and microstructure characteristics during the LPBF process, and to serve as a guide on how to tailor microstructure for a given part. To clarify the distinguished microstructural characteristics of LPBF parts, casting is briefly discussed for comparison. The article begins with the introduction of solidification theory and microstructure features in LPBF and casting. Thereafter, the attempts to tailor microstructure in LPBF and casting are addressed and compared. The final part of this paper provides an outlook on the remaining challenges and potential research topics of microstructure controlling in LPBF with Al alloys.