F. Quintero

Laser cladding of bioactive glass coatings

Laser cladding by powder injection has been used to produce bioactive glass coatings on titanium alloy (Ti6Al4V) substrates. Bioactive glass compositions alternative to 45S5 Bioglass were demonstrated to exhibit a gradual wetting angle-temperature evolution and therefore a more homogeneous deposition of the coating over the substrate was achieved. Among the different compositions studied, the S520 bioactive glass showed smoother wetting angle-temperature behavior and was successfully used as precursor material to produce bioactive coatings. Coatings processed using a Nd:YAG laser presented calcium silicate crystallization at the surface, with a uniform composition along the coating cross-section, and no significant dilution of the titanium alloy was observed. These coatings maintain similar bioactivity to that of the precursor material as demonstrated by immersion in simulated body fluid.

CO2 laser cutting of slate

Slate is a natural stone which has the characteristic that shows a well-developed defoliation plane, allowing to easily split it in plates parallel to that plane which are particularly used as tiles for roof building. At present, the manufacturing of slate is mostly manual, being noisy, powdery and unsafe for the worker. Thus, there is a need to introduce new processing methods in order to improve both the working conditions and the quality of the products made of slate. Following the previous work focused on the drilling and cutting of slate tiles using a Nd : YAG laser, we present in this paper the results of the work carried out to explore the possibilities to cut slate plates by using a CO2 laser. A 1.5 kW CO2 laser was used to perform different experiments in which, the influence of some processing parameters (average power, assist gas pressure) on the geometry and quality of the cut was studied. The results obtained show that the CO2 laser is a feasible tool for a successful cutting of slate. r 2001 Elsevier Science Ltd. All rights reserved.

Study of melt flow dynamics and influence on quality for CO2 laser fusion cutting

The understanding of melt flow dynamics during fusion laser cutting is still a topic of great importance because this determines the quality characteristics of the processed workpiece. Despite the complexity of the experimental study of the physical processes involved in this technique, fusion laser cutting can be visualized during the processing of glass because this material absorbs the laser radiation provided by a CO2 laser but shows transparency to visible radiation. Then, we present in this work the results of the study of the melt flow dynamics during laser cutting of glass. Under different experimental conditions, the dynamics of the cutting front and its complete geometry (front wall inclination), and the evolution of the melt along the cut edge were analysed using a high-speed video camera to study the process. A phenomenon concerning the plasma plume formed during the process was observed, which has not been previously reported in the literature. This can displace the normal shock wave (MSD) commonly formed in the inlet kerf and can affect the assist gas flow into the kerf.On the other hand, the analysis of the recorded images allowed the determination of not only the amount of molten material along the cut edge but also the direction and velocity of the melt. Relevant processing parameters affecting the flow of molten material were assessed. These results were used as a basis to explain the different processes involved in the generation of dross, a typical imperfection appearing in laser cutting.

Three-dimensional bioactive glass implants fabricated by rapid prototyping based on CO2 laser cladding

Three-dimensional bioactive glass implants were produced by rapid prototyping based on laser cladding without using moulds. CO(2) laser radiation was employed to melt 45S5 and S520 bioactive glass particles and to deposit the material layer by layer following a desired geometry. Controlled thermal input and cooling rate by fine tuning of the processing parameters allowed the production of crack-free fully dense implants. Microstructural characterization revealed chemical composition stability, but crystallization during processing was extensive when 45S5 bioactive glass was used. Improved results were obtained using the S520 bioactive glass, which showed limited surface crystallization due to an expanded sintering window (the difference between the glass transition temperature and crystallization onset temperature). Ion release from the S520 implants in Tris buffer was similar to that of amorphous 45S5 bioactive glass prepared by casting in graphite moulds. Laser processed S520 scaffolds were not cytotoxic in vitro when osteoblast-like MC3T3-E1 cells were cultured with the dissolution products of the glasses; and the MC3T3-E1 cells attached and spread well when cultured on the surface of the materials.

Theoretical analysis of material removal mechanisms in pulsed laser fusion cutting of ceramics

It is well known that the efficiency of material removal mechanisms has a crucial influence on the performance and quality of the laser cutting process. However, they are very difficult to study since the physical processes and parameters which govern them are quite complicated to observe and measure experimentally. For this reason, the development of theoretical models to analyse the material removal mechanisms is very important for understanding the characteristics and influence of these processes.In this paper, a theoretical model of the pulsed laser fusion cutting of ceramics is presented. The material removal mechanisms from the cutting front are modelled under the assumption that the ceramic material may be, simultaneously, melted and evaporated by the laser radiation. Therefore, three ejection mechanisms are investigated together: ejection of molten material by the assist gas, evaporation of the liquid and ejection of molten material due to the recoil pressure generated by the evaporation from the cutting front.The temporal evolution of the material removal mechanisms and the thickness of the molten layer are solved for several laser pulse modes. Theoretical results are compared with experimental observations to validate the conclusions regarding the influence of frequency and pulse length on the cutting process.

Comparative study of the influence of the gas injection system on the Nd:yttrium-aluminum-garnet laser cutting of advanced oxide ceramics

Cutting of advanced oxide ceramics is still a difficult task. In this work, the possibility to effectively cut them using a Nd:YAG laser guided by an optical fiber is demonstrated. The key points are the aerodynamic interactions of the assist gas jet in the fusion laser cutting of ceramics. A comprehensive study of the influence of these aerodynamic interactions on the laser cutting of advanced oxide ceramics has been carried out. The characteristics of the heat affected zone (HAZ) were studied related to the efficiency of the assist gas to eject the molten material. It has been demonstrated that the HAZ can be avoided with a suitable design of the gas injection system combined with an appropriate selection of the values of the processing parameters. With the aim of improving the efficiency of the assist gas injection system, a new cutting head with an off-axis supersonic nozzle was developed. Furthermore, a comparison between the utilization of a conventional coaxial conical nozzle to inject the assist gas and the new system is presented. The results obtained give clear proof that the use of the new gas injection system leads to a great improvement on the cut quality by means of a more efficient removing of the molten material out of the cutting front. This result is of special interest in the laser fusion cutting of thick ceramic plates at high processing rates.

Laser cutting of 2024-T3 aeronautic aluminum alloy

Since the beginning of the aeronautic industry, aluminum alloys have played a crucial role in its development. Nowadays, different aluminum alloy families are the base material of many pieces of aerospace vehicles. In this work, a novel approach to process aluminum alloys is explored. The authors efforts are aimed to cut 2024-T3 plates by a CO2 laser. They used a novel laser cutting head assisted by a gas jet working in supersonic regime in order to accomplish this objective. This supersonic nozzle and the intrinsic geometry of the cutting head allow carrying out the processing of these alloys more efficiently than conventional cutting heads. The microstructural characterization, grain morphology, kerf dimensions, and surface finish of the cuts have been analyzed. The cut edges are free of dross and cracks and the heat affected zone is negligible. These successful results confirm laser cutting processing assisted by a supersonic assisting gas jet as a promising technique in the aerospace field.

On the conditions to produce micro- and nanofibres by laser spinning

Laser spinning is a new technique which has recently been demonstrated to produce ultralong amorphous ceramic nanofibres with controllable chemical compositions. A laser is employed to melt a small volume of the precursor material at high temperatures, while a supersonic gas jet is then used to rapidly elongate the molten material. The melt forms glass fibres as a result of its viscous elongation and rapid cooling by the convective heat transfer produced by the gas jet.This work analyses the relation between the operating conditions of laser spinning and the physical process that leads to the formation of nanofibres, with the purpose of controlling and designing the optimum conditions for the process. Two decoupled mathematical models were developed to study, on the one hand, the influence of the initial temperature and volume of the molten material on its elongation process, whereas the second model is solved to analyse the physics of the fusion front as a function of the process parameters and relate it to the process of producing the fibres. These models were verified and complemented by experimental tests studied by analyses of the products and direct observation of the fusion front using a high speed camera.

Toward smart implant synthesis: bonding bioceramics of different resorbability to match bone growth rates.

Craniofacial reconstructive surgery requires a bioactive bone implant capable to provide a gradual resorbability and to adjust to the kinetics of new bone formation during healing. Biomaterials made of calcium phosphate or bioactive glasses are currently available, mainly as bone defect fillers, but it is still required a versatile processing technique to fabricate composition-gradient bioceramics for application as controlled resorption implants. Here it is reported the application of rapid prototyping based on laser cladding to produce three-dimensional bioceramic implants comprising of a calcium phosphate inner core, with moderate in vitro degradation at physiological pH, surrounded by a bioactive glass outer layer of higher degradability. Each component of the implant is validated in terms of chemical and physical properties, and absence of toxicity. Pre–osteoblastic cell adhesion and proliferation assays reveal the adherence and growth of new bone cells on the material. This technique affords implants with gradual-resorbability for restoration of low-load-bearing bone.

Cutting of ceramic plates by optical fiber guided Nd:YAG laser

Advanced ceramics have excellent resistance to heat, wear, and corrosion and are used in many industrial applications were these properties are required. However, the high hardness and brittleness of ceramics make them difficult and expensive to machine by conventional manufacturing methods. In this article we present the results of the work carried out to analyze qualitatively the influence of some parameters such as the cutting speed and the assist gas pressure on the laser cutting of mullite-alumina plates. The experiments were performed with a 500 W pulsed Nd:YAG laser coupled to an optical fiber and a commercial cutting head. An inert gas (Ar) was used as assist gas, which was injected to the workpiece through a coaxial conical nozzle. Mullite-alumina plates having a thickness of 4 mm were cut at a maximum cutting speed of 5 mm/s. The results obtained demonstrate that the pulsed Nd:YAG laser connected to an optical fiber is a feasible and flexible tool for a successful cutting of components made of m...