Numerical Simulation in the Melt Pool Evolution of Laser Powder Bed Fusion Process for Ti6Al4V.

In order to track the free interface of the melt pool and understand the evolution of the melt pool, the flow of fluid, and the interface behavior of gas and liquid, a physical model is developed by using the VOF method in this paper. Its characteristics are a combined heat source model, including a parabolic rotation and a cylindrical distribution, and a powder bed stochastic distributed model with powder particle size. The unit interface between the metallic and gas phase in the laser-powder interaction zone can only be loaded by the heat source. Only the first and second laser scanning tracks are simulated to reduce the calculation time. The simulation results show that process parameters such as laser power and scanning speed have significant effects on the fluid flow and surface morphology in the melt pool, which are in good agreement with the experimental results. Compared with the first track, the second track has larger melt pool geometry, higher melt temperature, and faster fluid flow. The melt flows intensely at the initial position due to the high flow rate in the limited melt space. Because there is enough space for the metal flow, the second track can obtain smooth surface morphology more easily compared to the first track. The melt pool temperature at the laser beam center fluctuates during the laser scanning process. This depends on the effects of the interaction between heat conduction or heat accumulation or the interaction between heat accumulation and violent fluid flow. The temperature distribution and fluid flow in the melt pool benefit the analysis and understanding of the evolution mechanism of the melt pool geometry and surface topography and further allow regulation of the L-PBF process of Ti6Al4V.

Material-structure-performance integrated laser-metal additive manufacturing.

Laser-metal additive manufacturing capabilities have advanced from single-material printing to multimaterial/multifunctional design and manufacturing. Material-structure-performance integrated additive manufacturing (MSPI-AM) represents a path toward the integral manufacturing of end-use components with innovative structures and multimaterial layouts to meet the increasing demand from industries such as aviation, aerospace, automobile manufacturing, and energy production. We highlight two methodological ideas for MSPI-AM-"the right materials printed in the right positions" and "unique structures printed for unique functions"-to realize major improvements in performance and function. We establish how cross-scale mechanisms to coordinate nano/microscale material development, mesoscale process monitoring, and macroscale structure and performance control can be used proactively to achieve high performance with multifunctionality. MSPI-AM exemplifies the revolution of design and manufacturing strategies for AM and its technological enhancement and sustainable development.

Carbon Nanotubes Enabled Laser 3D Printing of High-Performance Titanium with Highly Concentrated Reinforcement.

Zero- to two-dimensional nanomaterials have been incorporated into metal-matrices to improve the strength of metals, but challengingly, high-volume-fraction nanomaterials are difficult to disperse uniformly in metal matrices, severely degrading the ductility of conventionally processed metals. Here, a considerably dense uniform dispersion of in situ formed nanoscale lamellar TiC reinforcement (16.1 wt %) in Ti matrix is achieved through laser-tailored 3D printing and complete reaction of Ti powder with a small amount (1.0 wt %) of carbon nanotubes (CNTs). An enhanced tensile strength of 912 MPa and an outstanding fracture elongation of 16% are simultaneously achieved for laser-printed components, showing a maximum 350% improvement in "product of strength and elongation" compared with conventional Ti. In situ nanoscale TiC reinforcement favors the formation of ultrafine equiaxed Ti grains and metallurgically coherent interface with minimal lattice misfit between TiC lamellae and Ti matrix. Our approach hopefully provides a feasible way to broaden structural applications of CNTs in load-bearing Ti-based engineering components via laser-tailored reorganization with Ti.

Diode-pumped cw Alexandrite laser with temporally stable 6.5 W in TEM00 operation with prospect of power scaling.

We present the design of a longitudinally diode-pumped Alexandrite laser in continuous-wave operation and resulting performance data. A laser power of 6.5 W in fundamental mode operation was measured, which is, to the best of our knowledge, the highest laser power in fundamental mode operation yet reported. The laser crystal was pumped by two diode modules emitting at 637 nm. The pump radiation was polarization-combined and spatially symmetrized. The laser operates at an output power of 6.5 W with an optical-to-optical efficiency of 26%, temporally stable output with stability of 8% on ms timescale, a beam quality of M2 = 1.1 in both spatial directions and emission of an output wavelength of 752 nm. Measurements of the thermal dioptric power at pumping intensities up to 9.5 kW/cm2 support the appropriate approach of the design. Based on our results, we estimate the potential and show our concept for future scaling of the output power.

Development of a solvent-free polylactide/calcium carbonate composite for selective laser sintering of bone tissue engineering scaffolds.

Since large bone defects cannot be healed by the body itself, continuous effort is put into the development of 3D scaffolds for bone tissue engineering. One method to fabricate such scaffolds is selective laser sintering (SLS). However, there is a lack of solvent-free prepared microparticles suitable for SLS. Hence, the aim of this study was to develop a solvent-free polylactide/calcium carbonate composite powder with tailored material properties for SLS. Four composite powders with a composition of approximately 75 wt% polylactide (PLLA as well as PDLLA) and 25 wt% calcium carbonate (calcite) were prepared by a milling process based on GMP standards. Four different grades of polylactide were chosen to cover a broad inherent viscosity range of 1.0-3.6 dl/g. The composite material with the lowest inherent viscosity (1.0 dl/g) showed the best processability by SLS. This was caused by the small polymer particle diameter (50 µm) and the small zero-shear melt viscosity (400 Pa·s), which led to fast sintering. The SLS process parameters were developed to achieve low micro-porosity (approx. 2%) and low polymer degradation (no measurable decrease of the inherent viscosity). A biaxial bending strength of up to 75 MPa was achieved. Cell culture assays indicated good viability of MG-63 osteoblast-like cells on the SLS specimens. Finally, the manufacture of 3D scaffolds with interconnected pore structure was demonstrated. After proving the biocompatibility of the material, the developed scaffolds could have great potential to be used as patient-specific bone replacement implants.

Additive manufacturing of biodegradable metals: Current research status and future perspectives.

The combination of biodegradable metals and additive manufacturing (AM) leads to a revolutionary change of metal implants in many aspects including materials, design, manufacturing, and clinical applications. The AM of nondegradable metals such as titanium and CoCr alloys has proven to be a tremendous success in clinical applications. The AM of biodegradable metals including magnesium (Mg), iron (Fe), and zinc (Zn) is still in its infancy, although much progress has been made in the research field. Element loss and porosity are common processing problems for AM of biodegradable metals like Zn and Mg, which are mainly caused by evaporation during melting under a high-energy beam. The resulting formation quality and properties are closely related to material, design, and processing, making AM of biodegradable metals a typical interdisciplinary subject involving biomaterials, mechanical engineering, and medicine. This work reviews the state of research and future perspective on AM of biodegradable metals from extensive viewpoints such as material, processing, formation quality, design, microstructure, and properties. Effects of powder properties and processing parameters on formation quality are characterized in detail. The microstructure and metallurgical defects encountered in the AM parts are described. Mechanical and biodegradable properties of AM samples are introduced. Design principles and potential applications of biodegradable metal implants produced by AM are discussed. Finally, current research status is summarized together with some proposed future perspectives for advancing knowledge about AM of biodegradable metals. STATEMENT OF SIGNIFICANCE: Rapid development of research and applications on biodegradable metals and additive manufacturing (AM) has been made in recent years. Customized geometric shapes of medical metals with porous structure can be realized accurately and efficiently by laser powder bed fusion (L-PBF), which is beneficial to achieve reliable stress conduction and balanced properties. This review introduces the development history and current status of AM of biodegradable metals and then critically surveys L-PBF of Mg-, Fe-, and Zn-based metals from multiple viewpoints including materials, processing, formation quality, structural design, microstructure, and mechanical and biological properties. The present findings are summarized together with some proposed future challenges for advancing AM of biodegradable metals into real clinical applications.

Diode-pumped Q-switched Alexandrite laser in single longitudinal mode operation with Watt-level output power.

We present significantly improved performance data of a diode-pumped Q-switched Alexandrite laser in single longitudinal mode operation developed as a beam source for resonance lidar systems. The average output power of the laser-operating at the potassium resonance at 770 nm with a linewidth below 10 MHz-could be increased by a factor of five to the Watt-level by means of an optimized resonator design and pump scheme. The pulse energy is 1.7 mJ with a beam quality of M2≤1.1 in both spatial directions at a repetition rate of 500 Hz.

Influence of the material properties of a poly(D,L-lactide)/ß-tricalcium phosphate composite on the processability by selective laser sintering.

Complex 3D scaffolds with interconnected pores are a promising tool for bone regeneration. Such 3D scaffolds can be manufactured by selective laser sintering (SLS) from biodegradable composite powders. However, the mechanical strength of these scaffolds is often too low for medical application. We propose that the mechanical strength of laser-sintered scaffolds can be improved through composite powders with tailored properties (e.g., suitable powder particle size and melt viscosity for SLS). To prove this, two batches of a poly(D,L-lactide) (PDLLA)/ß-tricalcium phosphate (ß-TCP) composite powder with 50 wt% PDLLA and 50 wt% ß-TCP were synthesized. The two batches differed in polymer particle size, filler particle size, and polymer molecular weight. Both batches were processed with identical SLS process parameters to study the extent to which the material properties influence how well a PDLLA/ß-TCP (50/50) composite can be processed with SLS. In the SLS process, batch 2 showed improved melting behavior due to its smaller polymer particle size (approx. 35 µm vs. 50 µm) and its lower zero-shear melt viscosity (5800 Pa∙s vs. 17,900 Pa∙s). The better melting behavior of batch 2 led to SLS test specimens with lower porosity compared to batch 1. In consequence, the batch 2 specimens exhibited a larger biaxial bending strength (62 MPa) than the batch 1 specimens did (23 MPa). We conclude that a tailored composite powder with optimized polymer particle size, filler particle size, and polymer molecular weight can increase the achievable mechanical strength of laser-sintered scaffolds.

Glass-ceramic coating material for the CO2 laser based sintering of thin films as caries and erosion protection.

OBJECTIVES: The established method of fissure-sealing using polymeric coating materials exhibits limitations on the long-term. Here, we present a novel technique with the potential to protect susceptible teeth against caries and erosion. We hypothesized that a tailored glass-ceramic material could be sprayed onto enamel-like substrates to create superior adhesion properties after sintering by a CO2 laser beam. METHODS: A powdered dental glass-ceramic material from the system SiO2-Na2O-K2O-CaO-Al2O3-MgO was adjusted with individual properties suitable for a spray coating process. The material was characterized using X-ray fluorescence analysis (XRF), heating microscopy, dilatometry, scanning electron microscopy (SEM), grain size analysis, biaxial flexural strength measurements, fourier transform infrared spectroscopy (FTIR), and gas pycnometry. Three different groups of samples (each n=10) where prepared: Group A, powder pressed glass-ceramic coating material; Group B, sintered hydroxyapatite specimens; and Group C, enamel specimens (prepared from bovine teeth). Group B and C where spray coated with glass-ceramic powder. All specimens were heat treated using a CO2 laser beam process. Cross-sections of the laser-sintered specimens were analyzed using laser scanning microscopy (LSM), energy dispersive X-ray analysis (EDX), and SEM. RESULTS: The developed glass-ceramic material (grain size d50=13.1mm, coefficient of thermal expansion (CTE)=13.310-6/K) could be spray coated on all tested substrates (mean thickness=160µm). FTIR analysis confirmed an absorption of the laser energy up to 95%. The powdered glass-ceramic material was successfully densely sintered in all sample groups. The coating interface investigation by SEM and EDX proved atomic diffusion and adhesion of the glass-ceramic material to hydroxyapatite and to dental enamel. SIGNIFICANCE: A glass-ceramic material with suitable absorption properties was successfully sprayed and laser-sintered in thin films on hydroxyapatite as well as on bovine enamel. The presented novel technique of tooth coating with a dental glass-ceramic using a CO2-laser holds a great potential as a possible method to protect susceptible teeth against caries and erosion.

Influence of additive multilayer feature on thermodynamics, stress and microstructure development during laser 3D printing of aluminum-based material.

A transient three dimensional model for describing the temperature behavior, thermo-capillary convection, microstructure evolution and the resultant mechanical properties during selective laser melting of AlN/AlSi10Mg composite is proposed. The powder-solid transformation, temperature dependent physical properties and the preservation of the heat are taken into account. The effect of the additive manufacturing multilayer feature on the molten pool dynamics, cooling rate, crystal size, microstructure morphology, micro-hardness and types of the residual stress has been investigated. It shows that the operating temperature and the thermo-capillary convection obtained within the molten pool generally increases as the processing multilayers are successively added, while the thermal effect depth is negatively reduced. The preferential direction of the heat diffusion generally changes from a downward pattern, then to the slightly strengthened horizontal direction and finally to a typically horizontal one for various deposited layers being processed. Therefore, the microstructure of the solidified part along the building direction (Region I to Region V) undergoes an interesting transformation: directional columnar cellular microstructure, crosswise-extended cellular microstructure, refined cellular microstructure, fragmentation microstructure and the coarse cellular microstructure. The tensile stress and the compressive stress are comprehensively obtained within the finally solidified layers, significantly influencing the micro-hardness.

Transverse pump-probe microscopy of moving breakdown, filamentation and self-organized absorption in alkali aluminosilicate glass using ultrashort pulse laser.

We present time and space resolved transverse pump-probe measurements of the free electron and defect generation induced by nonlinear absorption of ultra short pulsed laser radiation in unhardened Corning Gorilla glass. The applied setup exhibits a 100 fs probe pulse duration and an independent pump pulse duration up to 5 ps. Hence, our work comprises the absorption of ultra short pulsed laser radiation at a wavelength of 800 nm and pulse energies from 10 µJ to 50 µJ up to a delay of 6 ns. Our investigations reveal different absorption regimes like filamentation and moving breakdown as well as the formation of permanent modifications. Finally, the deposition of multiple pulses in the incubation regime is examined, observing a self-organizing absorption effect.

Development and characterization of a coronary polylactic acid stent prototype generated by selective laser melting.

In-stent restenosis is still an important issue and stent thrombosis is an unresolved risk after coronary intervention. Biodegradable stents would provide initial scaffolding of the stenosed segment and disappear subsequently. The additive manufacturing technology Selective Laser Melting (SLM) enables rapid, parallel, and raw material saving generation of complex 3- dimensional structures with extensive geometric freedom and is currently in use in orthopedic or dental applications. Here, SLM process parameters were adapted for poly-L-lactid acid (PLLA) and PLLA-co-poly-ε-caprolactone (PCL) powders to generate degradable coronary stent prototypes. Biocompatibility of both polymers was evidenced by assessment of cell morphology and of metabolic and adhesive activity at direct and indirect contact with human coronary artery smooth muscle cells, umbilical vein endothelial cells, and endothelial progenitor cells. γ-sterilization was demonstrated to guarantee safety of SLM-processed parts. From PLLA and PCL, stent prototypes were successfully generated and post-processing by spray- and dip-coating proved to thoroughly smoothen stent surfaces. In conclusion, for the first time, biodegradable polymers and the SLM technique were combined for the manufacturing of customized biodegradable coronary artery stent prototypes. SLM is advocated for the development of biodegradable coronary PLLA and PCL stents, potentially optimized for future bifurcation applications.

Precise ablation of dental hard tissues with ultra-short pulsed lasers. Preliminary exploratory investigation on adequate laser parameters.

This study aimed to evaluate the possibility of introducing ultra-short pulsed lasers (USPL) in restorative dentistry by maintaining the well-known benefits of lasers for caries removal, but also overcoming disadvantages, such as thermal damage of irradiated substrate. USPL ablation of dental hard tissues was investigated in two phases. Phase 1--different wavelengths (355, 532, 1,045, and 1,064 nm), pulse durations (picoseconds and femtoseconds) and irradiation parameters (scanning speed, output power, and pulse repetition rate) were assessed for enamel and dentin. Ablation rate was determined, and the temperature increase measured in real time. Phase 2--the most favorable laser parameters were evaluated to correlate temperature increase to ablation rate and ablation efficiency. The influence of cooling methods (air, air-water spray) on ablation process was further analyzed. All parameters tested provided precise and selective tissue ablation. For all lasers, faster scanning speeds resulted in better interaction and reduced temperature increase. The most adequate results were observed for the 1064-nm ps-laser and the 1045-nm fs-laser. Forced cooling caused moderate changes in temperature increase, but reduced ablation, being considered unnecessary during irradiation with USPL. For dentin, the correlation between temperature increase and ablation efficiency was satisfactory for both pulse durations, while for enamel, the best correlation was observed for fs-laser, independently of the power used. USPL may be suitable for cavity preparation in dentin and enamel, since effective ablation and low temperature increase were observed. If adequate laser parameters are selected, this technique seems to be promising for promoting the laser-assisted, minimally invasive approach.

Single-pass high-harmonic generation at 20.8 MHz repetition rate.

We report on single-pass high-harmonic generation (HHG) with amplified driving laser pulses at a repetition rate of 20.8 MHz. An Yb:YAG Innoslab amplifier system provides 35 fs pulses with 20 W average power at 1030 nm after external pulse compression. Following tight focusing into a xenon gas jet, we observe the generation of high-harmonic radiation of up to the seventeenth order. Our results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.

Manufacturing of individual biodegradable bone substitute implants using selective laser melting technique.

The additive manufacturing technique selective laser melting (SLM) has been successfully proved to be suitable for applications in implant manufacturing. SLM is well known for metal parts and offers direct manufacturing of three-dimensional (3D) parts with high bulk density on the base of individual 3D data, including computer tomography models of anatomical structures. Furthermore, an interconnecting porous structure with defined and reproducible pore size can be integrated during the design of the 3D virtual model of the implant. The objective of this study was to develop the SLM processes for a biodegradable composite material made of ß-tricalcium phosphate (ß-TCP) and poly(D, L)-lactide (PDLLA). The development of a powder composite material (ß-TCP/PDLLA) suitable for the SLM process was successfully performed. The microstructure of the manufactured samples exhibit a homogeneous arrangement of ceramic and polymer. The four-point bending strength was up to 23 MPa. The X-ray diffraction (XRD) analysis of the samples confirmed ß-TCP as the only present crystalline phase and the gel permeations chromatography (GPC) analysis documented a degradation of the polymer caused by the laser process less than conventional manufacturing processes. We conclude that SLM presents a new possibility to manufacture individual biodegradable implants made of ß-TCP/PDLLA.

Determination of temporal and spatial concentration gradients in hydrogel beads using multiphoton microscopy techniques.

Multiphoton microscopy is a promising technique to detect spatially and temporally resolved concentration gradients of chemical compounds, e.g., reactants in hydrogel-encapsulated biocatalysts. In contrast to current techniques, the improved spatial and temporal resolution of this method in data acquisition and its ability to measure hydrogel beads facilitates the identification of various kinetic phenomena. To our knowledge, multiphoton microscopy is used here for the first time to examine diffusion, mass transfer, and reaction in immobilized hydrogel systems. In a first step, the phenomena of diffusion and diffusion-coupled mass transfer through the phase interface are investigated in the bead center. Finally, the complete system--consisting of diffusion, mass transfer, and enzymatic reaction--is observed by measuring concentration gradients along the bead radius with temporal and spatial resolution. This metrology enables a subsequent mechanistic model identification, which in turn leads to an enhanced knowledge of reaction kinetics and supports the design of biotechnological processes. This task was only possible due to excellent spatial (25 microm) and temporal (5 s) resolution and the accuracy (+/-1%) achieved by using a multiphoton microscopy setup.

Visualising the procedures in the influence of water on the ablation of dental hard tissue with erbium:yttrium-aluminium-garnet and erbium, chromium:yttrium-scandium-gallium-garnet laser pulses.

The exact mechanism of the ablation of tooth hard tissue with most common wavelengths, which are 2,940 nm and 2,780 nm, is not yet clear. There are several different theories, but none of them has yet been established. Concepts and methods of looking at these mechanisms have been based on heat formation and transformation, and mathematical calculations evaluating the outcome of ablation, such as looking at the shape of cuts. This study provides a new concept, which is the monitoring of the direct interactions between laser light, water and enamel, with a high-speed camera. For this purpose, both the above-mentioned wavelengths were examined. Bovine anterior teeth were prepared as thin slices. Each imaged slice had a thickness close to that of the beam diameter so that the ablation effect could be shown in two dimensional pictures. The single images were extracted from the video-clips and then were animated. The following steps, explaining the ablation procedures during each pulse, were seen and reported: (1) low-output energy intensity in the first pulses that did not lead to an ablative effect; (2) bubble formation with higher output energy density; (3) the tooth surface during the pulse was covered with the plume of vapour (comparable with a cloud), and the margins of ablation on the tooth were not clear; (4) when the vapour bubble (cloud) was collapsing, an additional ablative process at the surface could be seen.

Ablative targeting of fatty-tissue using a high-powered diode laser.

BACKGROUND AND OBJECTIVE: Concerning current clinical practice, laser-assisted lipoplasty is still secondary to other procedures. In order to evaluate effects of thermal interaction with fatty-tissue, a near infrared diode laser was examined under reproducible conditions. METHODS: Based on optical spectroscopy of fatty-tissue, a high-powered diode laser (lambda = 940 nm) was used to irradiate n = 59 fat samples of fresh corpses in non-contact mode. Thermal effects were histologically evaluated by computer based metric measurements. Calculated values included ablation rate (AR) and the ratio of cavity diameter to diameter of collateral damage (CCD(ratio)). Pearson's correlation and analysis of covariance (ANCOVA) were used for statistical evaluation. P values of less than 0.05 were considered to indicate statistical significance. RESULTS: Regarding the conditions examined, irradiances from 250 to 400 W/cm(2) revealed both increased ablation capacities and decreased collateral damages. An average irradiance of 370+/-0 W/cm(2) shows an average CCD(ratio) of 2:1 and an average AR of 9.98+/-7.65 mm(3)/second. CONCLUSION: Near infrared high-powered diode laser energy proved to be eligible for tissue protective ablation of fat in vitro. Further studies are necessary to improve efficiency and safety of this procedure.