Laser Melting Research Articles

Microstructure instability of additively manufactured alloy 718 fabricated by laser powder bed fusion during thermal exposure at 600-1000 ℃

Microstructure and precipitation behavior of alloy 718 produced by laser powder bed fusion (L-PBF) and its recrystallized counterpart were evaluated after thermal exposure at a temperature range of 600-1000 ℃ for up to 1000h. The as-built 718 featured a microstructure of columnar grains, and dislocation cell structures (DCSs) with chemical segregations (rich in Nb, Ti, Mo and depleted in Cr, Fe) and precipitates (Laves phase and carbonitride). The DCSs remained thermally stable for up to 1000h at 600 ℃ or 100h at 700 ℃, but only 10h at 800 ℃. With the temperature increasing to 900 ℃ and 1000 ℃ for 1h duration, the DCSs tended to partially disassemble, and the chemical segregations were removed. The chemical segregations at cell boundaries of the as-built 718 promoted the formation of γ', γ″ and δ phases during aging, leading to a spatially heterogeneous distribution of γ' and γ″ phases between the cell boundaries and interiors during short exposure time or at lower temperatures. Meanwhile, the coarsening of γ' and γ″ phases and the phase transformation of γ″ to δ at 700-800 ℃ were facilitated in the as-built 718 compared to the recrystallized ones. A unique precipitation distribution was observed at 700 ℃ that γ' and γ″ phases precipitated both at the cell boundaries and within cell interiors, while only γ' formed in the vicinity of cell boundaries. Time-temperature-transformation curves for additively manufactured and recrystallized 718 were constructed based on the microstructure analyses. This study indicates that conventional heat treatment procedures may be suboptimized for additively manufactured 718 in high temperature applications.

Micromechanical characteristics of titanium alloy (Ti-6Al-4V) made by laser powder bed fusion using an in-situ SEM micropillar compression technique

While titanium alloy (Ti-6Al-4V) made by laser powder bed fusion (L–PBF) exhibits complex deformation behaviors, its important micromechanical properties in relation to loading directions are not fully understood. This research aims to investigate the micromechanical behaviors of printed L–PBF Ti-6Al-4V alloys under vertical (i.e., the loading direction perpendicular to printed layers) and horizontal (i.e., the loading direction parallel to printed layers) compressions using in-situ scanning electron microscopy (SEM) micropillar techniques. Ti-6Al-4V alloys were L-PBF-printed using a 45° rotate scanning strategy with vertical and horizontal build directions. The microstructures of the two alloys were analyzed using the SEM with energy-dispersive X-ray spectroscopy (EDS). The titanium alloy micropillars were produced using focused ion beam (FIB) milling in the SEM. In-situ SEM micropillar compressions were conducted using a flat diamond indenter. Vertical alloy had smaller cross-patterned finer α′ martensite than horizontal one. While both vertical and horizontal micropillars showed elastic-plastic behaviors, the former had significantly higher yield, fracture, and compression strength values, as well as resilience and toughness, than the latter, leading to the formation of favorable shear bands. Both micropillars exhibited ductile fractures but had distinct failure mechanisms. The ductile fracture in the vertical micropillars was due to strain hardening, large plastic deformation, and shear band formation, while the ductile fracture in the horizontal ones was attributed to compression-induced bending and plastic buckling. The micromechanical characteristics of L–PBF Ti-6Al-4V materials provides an important insight into the small-scale deformation and failure mechanisms of the alloys influenced by loading directions.

In-situ synthesis of Yttria-based precipitates and their effects on Fe12Cr6Al in laser powder bed fusion

To advance the suitability of FeCrAl alloys for nuclear applications, radiation damage resistance, creep resistance, corrosion resistance, and mechanical properties enhancement are essential. A promising approach is the introduction of a high density of nitride and oxide nanoprecipitates within the alloy matrix. This study aimed to explore the impact of in-situ formed nitride and oxide precipitates on the properties of Fe12Cr6Al alloy by using a nitrogen-rich atmosphere and incorporating nano-sized Y2O3 (0.45 wt. %) during laser powder bed fusion (LPBF). As a result of adding Y2O3 nanoparticles to the Fe12Cr6Al alloy, the grain size decreased from 70 μm to 40 μm, and the Al-O, Al-N, and Al-N-O precipitates were replaced by Y-O, Y-Al-O, and Y-N precipitates. Additionally, the nitrogen contents increased from 220 ppm to 470 ppm. The sample with Y2O3 exhibited higher mechanical properties, with yield strength, tensile strength, and hardness (HV) increasing by 80 MPa, 112 MPa, and 23 HV, respectively. This study deviates from previous research on Y2O3-containing FeCrAl alloys by investigating the impact of in-situ synthesized Y-N nitrides on Fe12Cr6Al. The objective is to elucidate the combined effects of these precipitates on the properties of Fe12Cr6Al alloy in comparison to literature-reported oxide dispersion-strengthened (ODS) FeCrAl alloys, which were fabricated in an argon (Ar) atmosphere.

Grain boundary migration hysteresis during recrystallization of IN738LC alloy fabricated via laser powder bed fusion

The metastable microstructure formed during laser powder bed fusion (LPBF) significantly influences recrystallization behavior, which is crucial for flexible control of microstructure. To investigate these effects, various IN738LC alloys were fabricated by adjusting the volume energy density (VED) and then subjected to the same hot isostatic pressing (HIP) treatment. The microstructures before and after heat treatment were systematically analyzed using multi-scales characterization methods. The results indicate that the recrystallization fraction in HIP-treated samples decreased from 75% to 24% as the VED increased from 53 J/mm3 to 125 J/mm3. This reduction is primarily attributed to the increased grain boundary migration hysteresis caused by the higher density and larger diameter of MC-type carbide particles (i.e., MC particles). Subsequently, these HIP-treated samples were further processed using standard heat treatment (SHT) to evaluate their tensile properties at 23 °C and 900 °C. At 23 °C, the sample with the lowest recrystallization fraction (VED = 125 J/mm3) exhibited the best combination of strength and ductility, with an ultimate tensile strength (UTS) of 1422 MPa, yield strength (YS) of 932 MPa, and elongation of 21%. At 900 °C, the sample with a medium recrystallization fraction (VED = 75 J/mm3) demonstrated the best combination of strength and ductility, with an UTS of 626 MPa, YS of 420 MPa, and elongation of 15.5%.

Advancing laser powder bed fusion with non-spherical powder: Powder-process-structure-property relationships through experimental and analytical studies of fatigue performance

This study investigates the multifaceted interdependencies among powder characteristics (i.e., non-spherical morphology and particle size ranging 50-120 or 75-175µm), laser powder bed fusion (L-PBF) process condition (i.e., contouring), post-process treatments (i.e., hot isostatic pressing (HIP) and mechanical grinding) on the pore, microstructure, surface finish, and fatigue behavior of additively manufactured Ti-6Al-4V samples. Microstructure analysis shows a phase transformation α′ → α+β microstructure after HIP treatment (at 899±14 °C for 2h under the applied pressure of 1034±34bar) of the as-built Ti-6Al-4V parts. The findings from pore analysis using micro-computed tomography (μ-CT) show an increase in sub-surface pores when relatively smaller powders are L-PBF processed including contouring. Surface optical profilometry reveals a decrease in surface roughness when fine powder is L-PBF including contouring. Pore analysis conducted through μ-CT reveals that the presence of lack-of-fusion pores within the L-PBF processed coarse powder is more pronounced when compared to the fine powder. Furthermore, HIP treatment does not eliminate these pores. The fracture failure in as-printed parts occurs at the surface, while the combination of HIP and mechanical grinding alters crack initiation to subsurface pore defects. Fractography reveals that HIP and as-built samples followed the facet formation and pseudo-brittle fracture mechanisms, respectively. Fatigue life assessments, supported by statistical analysis, indicate that mechanical grinding and HIP significantly enhanced fatigue resistance, approaching the benchmarks set by wrought Ti-6Al-4V alloy. A fatigue prediction model which considers the surface roughness as a micro-notch has been used.

Mechanical Performance of Laser Powder Bed Fused Ti-6Al-4V: The Influence of Filter Condition and Part Location

Deteriorated filter condition in laser powder bed fusion (L-PBF) systems may negatively impact shield gas flow, causing inadequate spatter particle/plume removal, leading to laser beam attenuation and reduction in melt pool depth, and potentially causing more frequent formation of volumetric defects. This work investigated the effects of filter condition and part location on the micro-/defect-structure and mechanical behavior, including tensile and fatigue, of Ti-6Al-4V parts fabricated by L-PBF. Interestingly, within the manufacturer recommended service intervals, no specific effect of filter condition could be observed on the micro-/defect-structure or the mechanical behavior of the fabricated parts. However, the parts’ defect-structures were affected by their location, with ones located near the center of the build plate having less porosity than the ones located away. Although these defects did not affect the tensile properties, they frequently observed to initiate fatigue cracks; the critical defects sizes were often in the range of a few tens of micrometers. Therefore, their sensitivity to location resulted in the location dependence of the fatigue behavior.

Altered microstructure, phase transformation behaviors and shape memory response of CuAlMn shape memory alloys fabricated by selective laser melting

In this paper, nearly defect-free CuAlMn SMAs were successfully fabricated by selected laser melting (SLM). The result of microstructure show that only one type of thermal-induced martensite was detected in the prepared specimens, and the process parameters play a vital role in phase transformation behaviors. The alloy shows excellent ultimate tensile strength of 820 MPa and elongation of 11.5 %. Furthermore, the high shape memory response, i.e. shape recovery rate of 91.4 %–99.6 %, shape memory strain of 2.59 %–5.46 %, was presented under compressive strain not exceeding 8 %.

Effects of defects on the high-temperature performance of selective laser melting K418 superalloys: An in-situ 3D X-ray analysis

Defects are inevitable in selective laser melting process, significantly impacting the mechanical properties of materials and reducing their service life. In this study, the effects of various defects and their distribution on the high-temperature mechanical performance of the selective laser melted K418 superalloys were investigated via an in-situ 3D X-ray analysis and finite element method. The results showed that the selective laser melting process can significantly enhance the strength of the K418 sample, while degrading the fracture elongation. The sphericity and location of defects are the two key parameters influencing the mechanical performance. The defects with low sphericity at the sub-surface resulted in elevated local stress and strain, accounting for the significant degradation in fracture elongation. Locally increased stress and accumulated strain around lack of fusion defects at the sub-surface contribute to the initiation and propagation of crack. This study provides inspiration for understanding the correlation between the defects and mechanical properties.

Fabrication and tribological properties of bionic surface texture self-lubricating 60NiTi alloy via selective laser melting and infiltration

Due to its low elastic modulus, low density, non-magnetic and good lubricity, 60NiTi alloy is considered a novel fourth-generation aerospace bearing material. In this paper, 60NiTi biomimetic surface textured self-lubricating composites were fabricated by selective laser melting (SLM) and high-temperature fusion infiltration (HTFI). The effects of different slit widths (NTD), edge lengths (NTA), and soft metal Sn-Pb-Ag (SPA) infiltration on its tribological properties were studied. The results show that the NTA-SPA structural samples with medium texture density (ρ=16%) promoted the diffusion of the lubricant phase to the friction surface, resulting in a complete lubricant film. The COF stabilized at 0.2, and the wear rate decreased to 0.56×10-3 mm3/Nm, providing a more stable self-lubrication effect compared to the NTD-SPA sample.

Integration of sensors during laser powder bed fusion

AbstractLaser powder bed fusion (PBF‐LB/M) is an additive manufacturing (AM) process to produce high‐strength, functionally integrated, lightweight metal components which cannot be built conventionally. This article addresses the expansion of functionality by integrating sensors within closed cavities. For this reason, the building process was stopped just before the cavities were closed. After positioning the sensors, the building process was continued and a solid connection to the AM component was established.

Microstructural characterisation on reused Ti6Al4V powder in Direct Metal Laser sintering Additive manufacturing

One of the biggest advantages in metal additive manufacturing using laser powder bed fusion is low powder usage compared to traditional manufacturing methods. The process can be made more efficient by enabling metal powder to be reused. Recycling promotes environmental sustainability by minimizing material wastage and decreasing the demand for raw materials, making the metal additive manufacturing process more resource efficient.In this study, Ti6Al4V powder was recycled and reused up to 50 times. In order to understand the effects of reusing powder, the study included a detailed examination of both the powder and the processing method. The microstructure and characteristics of the recycled powder were examined in extensive detail.

On-line inspection of lattice structures and metamaterials via in-situ imaging in Additive Manufacturing

As advanced production capabilities are moving towards novel types of geometries as well as higher customization demands, a new and more efficient approach for process and part qualification is becoming an urgent need in industry. The layerwise nature of additive manufacturing (AM) potentially allows anticipating qualification tasks in-line and in-process, aiming at reducing the time and costs devoted to post-process inspections, enabling at the same time an early detection of defects since their onset stage. Such opportunity is particularly attractive in the presence of highly complex shapes like lattice structures or metamaterials, which have been increasingly investigated for industrial adoption in various sectors, aiming to achieve enhanced mechanical properties and innovative functionalities. This paper presents a novel methodology to inspect the geometry of lattice structures while the part is being built. The method is specifically designed to tackle the natural variability affecting layerwise images gathered in laser powder bed fusion. To this aim, it combines the segmentation of in-situ powder bed images of solidified layers with a data modelling approach to synthesize the 3-D shape of each unit cell into a 1-D profile representation. Such low-dimensional representation is suitable to quickly detect undesired distortions that may have a detrimental impact on final quality and performance. By using post-process X-ray computed tomography as ground truth reference, this study shows the effectiveness of the proposed approach for in-line inspection, opening a novel and cost-efficient way to address complex shape qualification for lattice structures in AM.

Solidification cracking susceptibility of alloy 718 during additive manufacturing and evaluating method

Although hot cracks frequently occur during metal additive manufacturing (AM), there are very few fundamental studies on the cracking behaviour and susceptibility. In this study, a horizontal tensile-type hot cracking test was investigated for evaluating the solidification cracking susceptibility of alloy 718 quantitatively during AM, particularly in laser powder bed fusion. The factors influencing the cracking susceptibility were also examined. The solidification cracking was reproduced by the melting of the AMed specimen with a tensile load under conditions of heat source simulating the AM process. The critical initial tensile stress for solidification crack initiation could apply as an evaluation index for cracking susceptibility. The critical initial stress decreased with increasing the laser power. A similar tendency was observed during the melting of the powders set on tiny groove for simulating AM process. The critical strain rate for solidification cracking (CST) was derived by a thermal elastic–plastic analysis using the critical initial stress measured experimentally. The CST at the laser power of 1125 W was higher than that of 1000 W for no crack condition. Therefore, high CST corresponding to penetration shape with high laser power induce to deteriorate the solidification cracking susceptibility.

Optimizing laser parameters and exploring building direction dependence of corrosion behavior in NiTi alloys fabricated by laser powder bed fusion

The microstructure and materials performance of Laser Powder Bed Fusion-fabricated (LPBFed) NiTi shape memory alloys (SMAs) varies depending on the laser parameters and building directions. This study initially optimized the laser processing parameters (laser power and scanning speed) for LPBFed NiTi SMAs to enhance their printing quality and mechanical properties. We determined that with fixed hatch spacing of 80 μm and layer thickness of 30 μm, the laser power of 105 W, and scanning speed of 600 mm/s produced the best results. Subsequently, the corrosion behavior of LPBFed NiTi SMAs built in three different building directions (0°, 45°, 90°) was investigated in 0.9 wt.% NaCl solution at 37 °C. The electrochemical data revealed that the corrosion current density (Icorr) decreased with increasing building direction angle (Icorr-90° <Icorr-45° <Icorr-0°). The corrosion resistance followed the order of 0° sample ˂ 45° sample ˂ 90° sample. These findings were attributed to the difference in the grain size and boundary density. Higher boundary densities enhanced the electron activity capacity and the element diffusion rates, facilitating the rapid formation of a protective film and thus improving the corrosion resistance. This new insight into the anisotropy of corrosion behavior relative to building directions can inform the design of NiTi-SMA products and improve their reliability in tissue engineering applications.

Physical and chemical surface modification by laser polishing of CuZn42 parts produced by laser powder bed fusion

The widespread use of Cu-Zn alloys containing lead (Pb) in plumbing applications poses significant health risks due to potential Pb leaching into drinking water. In response to international legislation aimed at reducing or eliminating Pb in metal alloys, there is an increasing demand for environmentally friendly brass. In this experimental work, authors show that during the Laser Beam Powder Bed Fusion (PBF-LB) process of the CuZn42 (CW510L) alloy, small particles only a few microns large mixed with large particles that are hundreds of microns in size, are spattered from the material. Large particles show average Zn/Cu ratio around 0.22, while for small particles it increases up to 3, against a nominal value of 0.72 for the CW510L alloy. The fallout of such particles on the produced part enriches surface of Zn, thus altering the surface chemical composition with an increase in Zn concentration beyond the acceptance limit of 43 at.%. To restore the standard chemical composition of the surface, a treatment based on the ablation of the surface material by a laser beam was proposed. Results clearly show that, after the laser treatment, the chemical composition of the surface is completely restored, and the standard properties recovered.

Additive Manufacturing of Fe-Mn-Al-C Lightweight Steel by Laser Powder Bed Fusion: The Role of Laser Scanning Speed on Forming Quality, Microstructure and Properties

Additive Manufacturing of Fe-Mn-Al-C Lightweight Steel by Laser Powder Bed Fusion: The Role of Laser Scanning Speed on Forming Quality, Microstructure and Properties

On the work-hardening behaviour of the additively manufactured Al-Si-Mg alloys: composite-like versus networked microstructure

Al-Si-Mg alloys are widely used for manufacturing components via laser powder bed fusion in various industrial applications where ductility and the capacity to accommodate the strain hardening are key criteria. The ductility of the laser powder bed-fused Al alloys has become a crucial property due to their fine cellular microstructure. Post-processing heat treatments improve ductility, but resultant microstructural changes affect the work-hardening behaviour, deformability, and uniform elongation values. This study aims to investigate the work-hardening capability, uniform elongation and deformability of AlSi7Mg and AlSi10Mg samples after different post-processing heat treatments by using tensile tests, optical and scanning electron microscopies. At aging temperatures below 200°C, the fully cellular structure of eutectic Si governs both the work-hardening behaviour and the strengthening mechanisms, despite the precipitation phenomenon. When direct-aging temperatures exceed 200°C, the coarsening of the Si-eutectic network modifies work-hardening behavior (Stages 1-3), accentuating the effects induced by the precipitates. Artificial aging highlights the role of precipitates in controlling both work-hardening properties and deformability.

Laser powder bed fusion of high-strength crack-free Al7075 alloy with the in-situ formation of TiB2/Al3Ti-reinforced phases and nucleation agents

The existence of solidification cracks caused by columnar grains in precipitation-hardened aluminium alloys limit the applicability of Al7075 components manufactured via laser powder bed fusion (LPBF) additive manufacturing. A novel approach was developed to co-incorporate submicron-sized B and micron-grade Ti6Al4V to eliminate hot cracks and to effectively transform coarse columnar grains into fine equiaxed grains, thus improving the mechanical performance of LPBF-fabricated modified Al7075 material. The grain refinement was mainly attributable to the heterogeneous nucleation promoted by the combination of in-situ-formed L12-Al3Ti and TiB2 nano-sized phases. After an optimised T6 heat treatment, excellent comprehensive mechanical properties were achieved, with a tensile strength of 460 MPa and an elongation of 13 %. This research provides an efficient and cost-effective path for addressing crack-sensitive metallic materials used for LPBF additive manufacturing processes.

Effect of cyclic loading on microstructure and plastic deformation in heat-treated Co–28Cr–6Mo alloy fabricated via laser powder bed fusion: an in situ synchrotron X-ray diffraction study.

We present a systematic microstructure-oriented plasticity investigation of an additively manufactured cobalt-based alloy aiming to relate microstructural changes to the fatigue resistance. A load-controlled cyclic test in the low-cycle fatigue regime was utilized to study mechanical response and microstructural changes in the alloy after reverse phase transformation heat treatment. Deformation-induced FCC → HCP phase transformation was followed using in situ synchrotron X-ray diffraction combined with ex situ electron backscatter diffraction and scanning electron microscopy. Scanning transmission electron microscopy provided experimental evidence of the presence of precipitates in the solution annealed sample. It was found, that a stepwise increase in plastic deformation is attributed to nucleation and growth of different martensitic crystallographic variants. These insights into plasticity and microstructural changes suggest that the reverse phase transformation heat treatment is an effective approach for improving the fatigue resistance of low stacking fault energy alloys, that are susceptible to deformation-induced martensitic transformation.

Effect of powder properties, process parameters, and recoating speed on powder layer properties measured by in-situ laser profilometry and part properties in laser powder bed fusion

In laser-based powder bed fusion of metals (PBF-LB/M), the powder layer is the link between the powder properties and the resulting part quality. Powder layer quality is a key metric related to powder spreadability and ultimately part quality, yet it is still unclear how it can be quantified. This is due to the difficulty of studying powder layer properties during the process. This study investigates the influence of powder properties, process parameters, and recoating speed on the surface roughness of the powder layer and the part, as well as on the effective thickness of the powder layer and solidified layer, and the resulting relative part density. Utilizing in-situ laser profilometry, high-resolution topographical data of the powder layer and the part surface were acquired, with minimal interference to the PBF-LB/M process. Six AlSi10Mg powders with varying particle size distribution, morphology, and flowability were processed using a wide range of recoating speeds and scan speeds to create powder layers with a wide range of properties. The results reveal a strong correlation between energy input and the effective powder layer thickness where lower scan speed results in an increased effective powder layer thickness due to material losses. Additionally, faster recoating decreases the powder layer density, which is moderated by the median particle size where the effect is strongest for fine powders. The surface roughness of the powder layer and top part surface are influenced by the recoating speed, energy input, and particle size, and they are strongly linked to each other. This highlights the importance of considering realistic substrate surface roughnesses in both powder spreading experiments and simulations. Finally, layer properties affect the process stability, resulting in small differences in relative part density.