Increasing Laser Energy Density Research Articles

Experimental Studies on Laser Additive Manufacturing of Inconel-625 Structures Using Powder Bed Fusion at 100 µm Layer Thickness

This paper reports process development and material characterization studies of Inconel-625 (IN625) using laser powder bed fusion (LPBF)-based additive manufacturing at higher layer thickness (100 µm). Conventionally, layer thickness up to 50 µm is used in LPBF due to process instability issues at higher layer thickness. However, successful development of LPBF with higher layer thickness will yield higher build rate. Therefore, systematic parametric investigations are carried out by varying laser power (P), scan speed (v) and hatch spacing (h) from 150 to 450 W, 0.02 to 0.08 m/s and 0.150 to 0.350 mm, respectively, with 100 µm layer thickness. The obtained results are compiled as a function of combined parameter—laser energy density (LED). Samples with relative area density > 99% are achieved for LED of 150, 240 and 360 J/mm3. Geometrical studies show that the deviation from nominal length and range of height of the sample decreases and increases with an increase in LED, respectively. X-ray diffraction shows the presence of face-centered cubic γ-phase at all the conditions with fine crystallites. The microstructure is a mix of cellular and dendritic with the primary arm width increasing with LED. Micro-hardness studies show that the hardness decreases slightly with an increase in LED, while automated ball indentation tests indicate the increase in energy storage capability with increase in LED. The micro-hardness, yield strength and ultimate tensile strength of LPBF built IN625 structure at 100 µm are found to be higher than that of the conventional and laser directed energy deposited IN625 structures and similar to that of the LPBF built IN625 structures at lower layer thickness. The study provides insight into LPBF of IN625 at 100 µm layer thickness and paves way for fabricating components at higher layer thickness with favorable mechanical properties.

Improving the Forming Quality of Laser Dynamic Flexible Micropunching by Laser Pre-Shocking.

Laser pre-shocking (LPS) was introduced into the laser dynamic flexible micropunching process to refine the grain size of a workpiece to improve the forming quality of punched parts. T2 copper foils with five different grain sizes and seven different laser power densities with and without LPS were used for the experiment. The results showed that the grains are refined and the average surface roughness decreases after LPS. For copper foils annealed at 650 °C, the value of decreases from 0.430 to 0.363 µm. The increase in laser energy density and grain size leads to the deterioration of the fracture surface. LPS can improve the quality of the fracture surface. Compared with punched holes without LPS, the dimensional accuracy and shape accuracy of punched holes can be improved by LPS. When grain size is close to the thickness of the copper foil, the forming quality of the punched parts becomes uncertain, owing to the difference in the orientation of the initial grains. The instability of laser dynamic flexible micropunching can be reduced by LPS. Especially, the improvement of forming quality of the punched part brought by LPS is significant for the copper foils with coarse grains.

Analytical modelling and experimental study of the cladding characteristics of a laser powder-fed additive manufacturing process

To avoid the effects of the overlap phenomenon among powder particles, the concept of an effective particle number was introduced, and multi-parameter analytical models for laser powder-fed additive manufacturing (LPF-AM) cladding geometric characteristics were established based on the laws of energy and mass conservation. Experimental research on the LPF-AM of a FeCr alloy was carried out, and the theoretical results are discussed and compared with the experimental data from the perspective of the laser energy density and powder density. The results showed that the cladding area was proportional to both the laser energy density and powder density. The mixing area and dilution rate decreased gradually with increasing powder density, while they increased first and then decreased with increasing laser energy density. The analytical model calculation values fit well with the experimental measurements. The microstructure of the cladding layer and substrate showed good metallurgical bonding, and the micro-hardness variation from the mixing to the cladding zone ranged from 423 to 627 HV.

Surface modification of SiOx film anodes by laser annealing and improvement of cyclability for lithium-ion batteries

SiOx (x = 0.66, 0.76 and 1.04) film anodes with the different oxygen content were prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) through adjusting SiH4/N2O (R) gas flow ration. The introduction of oxygen can alleviate volume expansion in the discharge/charge process at the expense of cyclic specific capacity. To obtain both higher capacity and cyclically stable SiOx film anodes, KrF excimer laser system operated at λ = 248 nm can induce to form nanoparticles on the surface of SiO0.76 film, which is related to the formation of solid electrolyte interphase (SEI) area during charge/discharge process. The SEI area becomes larger and mechanically robust with increase of laser energy density, which suppresses structural degeneration for giving lithium-ion battery longer cycle life. At the laser energy density of 266 mJ/cm2, SiO0.76 annealed film anode delivers capacity of 1731 mAh/g and capacity retention of 94.8% after 300 cycles, which are much higher than that of as-deposited anode. Increasing laser energy density to 433 mJ/cm2, SiO0.76 film anode shows higher capacity retention of 97.4% after 300 cycles.

Influence of Laser Energy Density on Interfacial Diffusion Bonding and Surface Density of Chromium Coating by Multi-Arc Ion Plating on Zirconium Alloy

The chromium coating prepared by multi-arc ion plating on the surface of zirconium alloy is easy to fall off under extremely harsh conditions due to the defects of larger particles, pores and weak adhesion between the coating and the zirconium alloy substrate. Here we apply a new composite process for the fabrication of Cr coating by laser melting multi-arc ion plating technology. The numerical model of temperature field laser melting treatment was established, and based on the analysis of the above temperature field of laser melting treatment, the laser with an energy density of 9 × 104 J/cm2–18 × 104 J/cm2 was selected as the heat source. Laser energy density has a great influence on the diffusion behavior of Cr at the coating-substrate interface. When the energy density of laser is 9 × 104 J/cm2, the laser heat source cannot provide enough energy to melt the surface of the coating, however, the Cr element diffuses slightly at the membrane base interface, forming a Cr diffusion zone of about 4 μm. When the energy density of the laser increases to 13.5 × 104 J/cm2, the Zr element of the substrate diffused to the whole Cr coating, and the original Cr coating disappeared, forming a Zr-Cr diffusion and fusion zone of 30 μm. As the laser preparation process of the coating is a mixed sintering process of liquid phase sintering and solid phase sintering, the coating surface forms a dense zone and a loose zone respectively, resulting in the increase of porosity and particle rate. With the laser energy density increasing gradually, the Zr-Cr fusion zone is formed, and the coating surface particles were gradually refined and the pores were significantly reduced. With the further increase of laser energy density of 18 × 104 J/cm2, the Zr-Cr fusion zone was extended to 60 μm, and the porosity and particle rate were reduced to 0.3% and 0.4% respectively. In addition, the original stratification cracking of the film base at the fracture of the tensile specimen disappears, and the protrusion of the interface is formed. Coatings with dimple fractures were found, which indicates that the coating exhibited ductility and interfacial metallurgical bonding.

Atomic-level understanding of crystallization in the selective laser melting of Fe50Ni50 amorphous alloy

Selective laser melting (SLM), an emerging technology, has enabled the fabrication of large Fe-based amorphous alloys (FAAs) with complex geometry, unlocking the potential for the wider implementation. However, the almost unavoidable crystallization during SLM greatly degrades the performance of SLM fabricated FAAs. This study aims to understand the crystallization mechanism by investigating the atomic crystallization and cluster evolution in Fe50Ni50 FAA using molecular dynamics simulation under various laser powers and scanning speeds. It is observed that crystallization occurs easily at low scanning speed due to the increased fractions of body-centered cubic (BCC) bond pairs 1661 and 1441, with corresponding Voronoi polyhedrons <0,6,0,8> and <0,5,2,6>. The ease of crystallization is interpreted by increasing laser energy density, determined by elevating laser power and lowering scanning speed. Additionally, the suppression of crystallization is explained by the increasing average temperature variation rate, while effectively shortening the relaxation time for atomic rearrangement, inhibiting the increase of BCC crystal phases. This research is conducive to comprehend relevant atomic-level information on crystallization mechanism and microstructure evolution and provides an alternative way to inhibit crystallization during the manufacture FAAs with SLM.

Microstructures and unique low thermal expansion of Invar 36 alloy fabricated by selective laser melting

Invar 36 alloy, which presented extremely low coefficient of thermal expansion (CTE), was fabricated by selective laser melting with island scanning strategy. The microstructures and CTE were systematically characterized. The as-SLMed Invar 36 is mainly composed of fcc γ phase and sparse bcc α precipitates, which are consistent with the phases of the wrought one. The grains grow along the maximum temperature gradient directions, as the microstructure on the side surface is dominated by large columnar grains. The cross-section shows a large number of small columnar grains in the melt pool. The low laser energy density results in the lack-of fusion pores and corresponding high level porosity up to 15%. When the laser energy density is 99.2 J/mm3 and 198.4 J/mm3, excellent quality is obtained, as the porosity is only within 0.2–0.4%. Due to the porosity and nickel element evaporation, the CTE is 1.72–1.96 × 10–6 °C−1 that is lower than that of wrought one. Besides, lower than 99.2 J/mm3, the decreasing laser energy density leads to the reduction of CTE, since the pores provide extra space for internal thermal expansion. Higher than 99.2 J/mm3, the metal vaporization rate and vapor pressure are enhanced with the increasing laser energy density, and then the loss of nickel element leads to the reduction of CTE.

Selective laser melting of TiB2-Ti composite with high content of ceramic phase

An increasing need in customized ceramics and ceramic-metal composites has driven the development of powders feedstock and procedures for utilization of additive manufacturing for production of mechanically reliable composites. However, processing of materials with a high fraction of ceramic particles is still in its infancy. Herein we report on 3D printing of TiB2-TiB-Ti composites from TiB2-Ti powder mixture of high ceramic content (50 wt%TiB2) by an optimized process of selective laser melting. In-situ synthesized from the mixture of commercially pure Ti and TiB2 powders, the composites possess up to 20.4 GPa hardness despite of a relatively high porosity of around 8%. Improvement in hardness is mainly due to hardening effect of both TiB and TiB2 and correlated with an increase in fraction of needle-shaped TiB phase with an increase in laser energy density (LED). Depending on process parameters, an amount of the ceramic phases (needle-shaped TiB and coarse elongated TiB2) can be customized. The laser energy density significantly affects the development of microstructure and size of the ceramic grains as well as the formation of solidification cracks. This study demonstrates the capacity of AM through SLM to produce the composites of high percentage of ceramic phase.

Molecular dynamics simulation of coalescence kinetics and neck growth in laser additive manufacturing of aluminum alloy nanoparticles

Laser additive manufacturing emerged as an advanced manufacturing process to fabricate components in a layered fashion by fusing the powder particles. This process is multifaceted and pivotal to understand the underlying physics of the coalescence of powder particles during the process, which impacts the structural and mechanical properties of the build component. In this study, a classical molecular dynamics (MD) model is developed for the coalescence of pre-alloyed aluminum alloy (AlSi10Mg) particles during the laser additive manufacturing process. The model is employed to investigate the neck growth and coalescence kinetics of different pairs of particle size with changing the laser energy density from 7 to 17J/mm2. The simulation results reveal that the unevenly sized particles undergo complete coalescence as compared with even-sized particles, and the neck growth rate of AlSi10Mg particles increases with an increase in laser energy density. Based on the present investigation, it is established that the coalescence kinetics of the AlSi10Mg nanoparticles are governed by the surface and volume diffusion and the surface energy reduction during the joining of particles. This analysis will act as a guideline to design process parameters and quality control for the printing of new components.

Microstructure and mechanical properties of additive manufactured W-Ni-Fe-Co composite produced by selective laser melting

Tungsten composites face severe challenges in machining complex structures due to tungsten's high melting temperature. To explore solutions that enable fabrication of complex W composite parts by additive manufacturing, W-6Ni-2Fe-2Co (W90), W-12Ni-4Fe-4Co (W80) and W-18Ni-6Fe-6Co (W70) composites were consolidated by selective laser melting (SLM). The effects of laser process parameters and chemical compositions on densification, microstructures, phases, and tensile properties were investigated. With the increase of laser energy density, the density of the composite increases. Near full density with an absence of cracks and pores was achieved in the SLM-processed W70 composites. The typical microstructure consisted of un melted polyhedral W particles and the surrounding W-Ni-Fe-Co matrix with W dendrites. Alternating layered fine dendrite and coarse dendrite zones were visible in side views of the composites. The tensile properties of the W70 composite had a pronounced improvement with the increase of laser energy density. A maximum ultimate tensile strength of 1198 MPa was obtained in the SLM-processed W70 composite with elongation of 9.5%. The SLM-processed W-Ni-Fe-Co composites pave the way for new refractory metal alloys and complex shaped parts fabrication by additive manufacturing.

Influence of energy density on macro/micro structures and mechanical properties of as-deposited Inconel 718 parts fabricated by laser engineered net shaping

In the present study, the influence of laser energy density (LED) on geometrical characteristics, mechanical properties and microstructure of Inconel 718 parts fabricated by laser engineered net shaping (LENS) were investigated. Thin walls and block specimens were designed and fabricated under a total of 25 different energy density combinations. The material properties, including part dimensional shrinkage, surface roughness, surface hardness, and microstructures, including, porosity, geometrical dilution, average grain size and dendrite arm spacing (DAS), were evaluated. The results of this investigations indicate that with the increase of LED, the part shrinkage and surface roughness present a down and up trend, whereas the porosity, geometrical dilution, grain size and DAS increase with increasing of LED and there is no statistically significant effect of LED on the surface micro hardness. Taking account of the material characterization results, the energy density window for LENS process of Inconel 718 is suggested between 98.21 J/mm2 to 107.14 J/mm2. The outputs of this study could provide an insightful view of the effects of energy input on mechanical and microstructural properties of as-deposited Inconel 718 alloy by LENS process.

Influence of processing parameters on the microstructure and tensile property of 85 W-15Ni produced by laser direct deposition

The plate-like shape 85 W-15Ni parts were produced by laser direct deposition technology with different processing parameters (laser power and scanning speed). The influence of processing parameters and their corresponding laser energy density on the microstructural characterization, phase composition and tensile property of 85 W-15Ni samples was investigated. The results show that the relative density of samples increased with the laser energy density and the densification trend started to slow as the laser energy density reached 380–400 J/mm3, though the highest density value was obtained with laser energy of 425 J/mm3. With the increase of laser energy density, more disorder and fine W dendrites existed at the bonding region between deposition layers and more WW grain boundaries formed at the central region of the layer. The 85 W-15Ni samples produced with different processing parameters consisted of W and γ-Ni phase. To improve the tensile property, it is necessary to increase the laser energy density to obtain denser structure and reduce the residual pores or gaps. However, the excessive laser energy density resulted in the formation of more WW grain boundaries that were detrimental to the tensile property. The best tensile properties were obtained at the laser energy density of 395 J/mm3.

Effect of laser energy density on the microstructure, mechanical properties, and deformation of Inconel 718 samples fabricated by selective laser melting

Inconel 718 samples were fabricated by selective laser melting (SLM). The effects of line laser energy densities in the range 0.1–0.3 J/mm on the microstructure, mechanical properties, and deformation of these samples were investigated. The fewest pores and best mechanical properties were realized at 0.2 J/mm with a yield strength of 775 MPa, ultimate tensile strength of 1055 MPa, and elongation of 29.5%. Fine dimples were uniformly distributed on the fracture surface under 0.2 J/mm. An Electron Backscatter Diffraction (EBSD) test revealed a mixture of fine columnar and equiaxed grains that promote strength while maintaining high plasticity. The prisms cut from these samples bent outwards, indicating great residual stresses in the samples. Their deformation increased with increasing laser energy density. Therefore, stress release treatment is necessary for SLM parts.

Optimizing Collector-Emitter Saturation Voltage at 3000 V Insulated Gate Bipolar Transistors Using Laser Thermal Annealing

In this work, two thermal annealing processes (furnace and laser annealing) are adopted to obtain 3000 V breakdown voltage in insulated gate bipolar transistors (IGBTs) for propulsion control system of electric vehicles. In furnace annealing process (450 °C for 30 min), the parameter of Vce(sat) is 5.5–6.5 V once measuring at Ic = 2.0 A with EDS(electrical die sorting) analyser. However, annealing process was performed using a laser equipment to lower the Vce(sat). After back side grinding and back side implant, the sample wafer undergoes laser annealing process. From various annealing conditions it was found that the Vce(sat) was 1.6–1.7 V when measuring at collector current (Ic) of 2.0 A after annealing at pulse with of 1100 nm, 3-overlap and 3.5 J/cm2. From these results, it is found that the Vce(sat) is lower as increasing laser energy density and overlapping and the well-distributed Vce(sat) values over the entire chips appear as widening the laser pulse.

Selective laser melting of WC reinforced maraging steel 300: Microstructure characterization and tribological performance

Owing to the limited wear resistance, Maraging steels has limited applications in the harsh environments, leading to a shortened service-life or even failure. In this study, WC particles reinforced maraging steel 300 (MS300) composites were produced by selective laser melting (SLM) to enhance the tribological performance. The microstructure and phase analysis of SLM-produced composites were investigated by SEM-EDS. Dry sliding wear test and microhardness were used to investigate the mechanical properties of WC/MS300 composites. As laser energy density increases, the microhardness drops rapidly and the maximum value of 562.2 HV2 was measured at the linear energy density of 110 J/m. Dry sliding tests show that the wear rate was decreased by >1500 times through the WC reinforcement compared with pure MS300 sample, which emphasizes the superior wear-resistance properties of MS300/WC composite. Besides, the best wear-resistance performance at the improved linear energy density of 125 J/m. The study on wear mechanism shows that formation of carbide layer between WC and MS300 matrix significantly prevent the plowing effect. The present work provides an effectively method to improve the tribological performance MS300 through the reinforcement of WC particle by selective laser melting.

Laser surface modification of Mg-Zn-Gd alloy: microstructural, wettability and in vitro degradation aspects

Mg-Zn-Gd have great potential for biomedical applications owing to excellent bioactivity and non-toxicity properties. In the present study, laser surface melting (LSM) was carried out on newly developed Mg-1Zn -2Gd (wt%) alloy. Effects of laser energy on microstructural evolution, corrosion properties, surface energy, and hardness have been investigated. The surface modified sample processed at different energy densities showed fine grain structure in the melt zone compared to the untreated substrate. Grain refinement in the laser melted region improved the hardness by 60%. The surface roughness was found to be increased with increasing laser energy density. At higher energy density, removal of materials from the surface is enhanced, resulting in deeper grooves and higher surface roughness. The wettability studies indicated that the variations in surface geometry, grain size and surface roughness of LSM samples strongly influence the surface energy and hydrophilicity. Improved wetting of LSM sample was achieved owing to grain refinement and low surface roughness. The corrosion resistance determined by immersion and electrochemical methods of laser melted sample in Hank’s balanced salt solution improved considerably due to grain refinement, meltpool depth and uniform distribution of secondary phases.

Pulsed laser deposited NixMg1−xO film characterization and band gap investigation

In this letter, transparent NixMg1−xO film with good surface quality were obtained by advancing pulsed laser deposition method. The surface morphology and optical properties were investigated by changing the substrate temperature and laser energy density. It was found that the surface quality of NixMg1−xO can be improved greatly through the rising of substrate temperature or increasing laser energy density. In this case, the optimal deposition parameter was at 500 °C–5 J cm−2. The annealing process was proved having remarkable impact on the surface topography. The optical band gaps which range from 3.88 eV to 4.16 eV were obtained through UV–vis absorption spectrum, as the deposition temperature increasing, the blue shift of absorption edge happened. The annealing was found to broaden the band gaps of samples deposited at lower energy density but have no effect on the band gaps of samples deposited at higher energy density. The abnormal width of band gaps were found, we speculate that the element uniformity of film plays an important role in band gap engineering.

Synthesis, microstructure, and mechanical behaviour of a unique porous PHBV scaffold manufactured using selective laser sintering.

Selective Laser Sintering (SLS) is a promising technique for manufacturing bio-polymer scaffolds used in bone tissue engineering applications. Conventional scaffolds made using SLS have complex engineered architectures to introduce adequate porosity and pore interconnectivity. This study presents an alternative approach to manufacture scaffolds via SLS without using pre-designed architectures. In this work, a SLS process was developed for fabricating interconnected porous biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) scaffolds with large surface areas and relative porosities of up to 80%. These characteristics provide great potential to enhance cell attachment inside the scaffolds. The scaffold microstructure was dependent on the laser energy density (LED) during the SLS process. An increase in LED led to scaffolds with higher relative densities, stronger inter-layer connections, and a reduced quantity of residual powder trapped inside the pores. An increase in relative density from 20.3% to 41.1% resulted in a higher maximum compressive modulus and strength of 36.4 MPa and 6.7 MPa, respectively.

Effect of laser energy density on microstructures and mechanical properties of selective laser melted Ti-6Al-4V alloy

Abstract Ti-6Al-4V samples were fabricated by selective laser melting (SLM) with a series of different laser energy inputs. The microstructure presented a feature of fine acicular martensite due to the high cooling rate of the SLM process. All the relative density, micro-hardness and tensile properties initially increased and then decreased with increasing laser energy density. The fracture morphology exhibited a mixed mode of complex ductile and brittle fracture. All the results indicated that the laser energy density has a crucial effect on the microstructures and mechanical properties of the SLMed Ti-6Al-4V alloy. The correlations between the laser energy density and the microstructures and mechanical properties of the SLMed Ti-6Al-4V alloy were studied.

Selective laser melting of an equiatomic CoCrFeMnNi high-entropy alloy: Processability, non-equilibrium microstructure and mechanical property

The selective laser melting (SLM) of an equiatomic CoCrFeMnNi high-entropy alloy (HEA) powder was studied, with emphasis on its non-equilibrium microstructural evolution and mechanical properties. The printed sample density increases gradually with increasing laser energy density and then decreases at an over-high laser energy density. The microstructure of SLM printed HEA exhibits a large number dislocation pile-ups and lattice distortion. Of particular intrigue are the existence of nanotwins and tetragonal σ phase in SLM printed HEA; such nanotwins and σ phase have never been observed in either cast or deformed HEA with true strain below 3.7%. The grain refinement by rapid solidification and σ phase synergistically improve the mechanical properties, as compared with conventionally solidified HEA. Hot isostatic pressing (HIP) was performed to improve the densification, yielding the increase of tensile strength from 601 MPa of SLM printed sample to 649 MPa after HIP.