Lino Costa

Laser powder deposition

Purpose – The purpose of this paper is to review the state of the art of laser powder deposition (LPD), a solid freeform fabrication technique capable of fabricating fully dense functional items from a wide range of common engineering materials, such as aluminum alloys, steels, titanium alloys, nickel superalloys and refractory materials.Design/methodology/approach – The main R&D efforts and the major issues related to LPD are revisited.Findings – During recent years, a worldwide series of R&D efforts have been undertaken to develop and explore the capabilities of LPD and to tap into the possible cost and time savings and many potential applications that this technology offers.Originality/value – These R&D efforts have produced a wealth of knowledge, the main points of which are highlighted herein.

Simulation of Phase Transformations in Steel Parts Produced by Laser Powder Deposition

Multilayer laser powder deposition is being used for the rapid manufacturing of fully dense near net shape components in a wide variety of materials. In this process parts are built by overlapping consecutive layers of a laser melted material. As a result of this overlapping, the material in each layer will undergo successive thermal cycles as new layers are deposited. Despite their short duration, these thermal cycles can activate solid-state transformations that lead to progressive modification of the microstructure and properties of the material. Since the thermal history of the material in the deposited part will differ from point to point, the part will present a complex and heterogeneous microstructure, and properties that differ from point to point. Given that the microstructure and property distribution in steel parts produced by laser powder deposition can only be predicted by modelling, a three-dimensional thermo-kinetic finite element model of laser powder deposition of tool steels was developed. In the present work this model was applied to the study of the influence of substrate size on the microstructure and properties of a six-layer wall of AISI 420 tool steel. The results show that the temperature field depends significantly on the size of the substrate, leading to distinct microstructures and properties in the final part. Introduction Laser powder deposition (LPD) [1-4] is a very promising technique for the rapid manufacture of fully dense steel components. Although the advantages of this flexible manufacturing process are widely recognised [1-5], there are still some factors restraining its wide acceptance by industry. Controlling and tailoring the material properties of the final part is one such factor. These properties are significantly affected by the solid-state transformations that may occur during deposition, induced by the consecutive thermal cycles created by successive layer overlapping. The lack of detailed understanding of these transformations and of their influence on the final properties of the deposited part may lead to irreproducible results. In order to ensure that the final part possesses an appropriate microstructure, one must know the effect of the processing parameters and part build-up strategy on the thermal cycle and microstructural changes. This knowledge cannot be obtained by trial and error, due to the large number of processing parameters that must be considered. Also, the results of such an approach might not be directly applicable to all cases because the specific geometry of individual parts strongly affects the thermal field in the part and its time evolution. A more satisfactory approach relies on computer aided engineering techniques, such as finite element analysis [6-9]. This approach requires a model describing the time evolution of the thermal field in the part during the deposition process, as well as a detailed knowledge of the solid-state phase transformations that occur in laser processed steels. The latter information is available in the work of several authors [10-15] on laser processed tool steels. In particular, Colaço et. al [11] showed that, due to their fast solidification and cooling rates, laser processed tool steels may contain

Solution-cast high-aspect-ratio polymer structures from direct-write templates.

This letter presents a novel strategy for template synthesis of polymer structures with laser machined substrates. User-designed patterns of submicrometer holes with aspect ratios >10:1 and depths >10 μm were produced by focusing 160 fs, 5.2 μJ laser pulses on the surface of fused silica with a high numerical aperture microscope objective. Some holes were enlarged by chemical etching. Polymer solutions were cast into the templates to create high-aspect-ratio polymer structures using replication. Engineered polymer structures prepared by this unique method are useful for a number of applications such as high surface area electrodes and biological substrates.

A Simple Correlation Between the Geometry of Laser Cladding Tracks and the Process Parameters

Laser cladding is a complex process and full featured cladding models require large computing times. However, from a practical standpoint, it is useful to establish simple correlation between the laser cladding parameters and the characteristics of the clad track, that can be used to select the processing parameters while avoiding the expensive and time consuming experiments. In this paper we present simple relationships that correlate the height and width of tracks obtained by the blown powder technique with the scanning speed, powder feed rate and catchement efficiency of the powder, for a given spot diameter and laser power. These relationships can be used to optimize the values of the parameters in order to obtain tracks with a shape of the cross section that minimizes defects due to overlapping or excessive dilution.

Cell interaction study method using novel 3D silica nanoneedle gradient arrays

Understanding cellular interactions with culture substrate features is important to advance cell biology and regenerative medicine. When surface topographical features are considerably larger in vertical dimension and are spaced at least one cell dimension apart, the features act as 3D physical barriers that can guide cell adhesion, thereby altering cell behavior. In the present study, we investigated competitive interactions of cells with neighboring cells and matrix using a novel nanoneedle gradient array. A gradient array of nanoholes was patterned at the surface of fused silica by single-pulse femtosecond laser machining. A negative replica of the pattern was extracted by nanoimprinting with a thin film of polymer. Silica was deposited on top of the polymer replica to form silica nanoneedles. NIH 3T3 fibroblasts were cultured on silica nanoneedles and their behavior was studied and compared with those cultured on a flat silica surface. The presence of silica nanoneedles was found to enhance the adhesion of fibroblasts while maintaining cell viability. The anisotropy in the arrangement of silica nanoneedles was found to affect the morphology and spreading of fibroblasts. Additionally, variations in nanoneedle spacing regulated cell-matrix and cell-cell interactions, effectively preventing cell aggregation in areas of tightly-packed nanoneedles. This proof-of-concept study provides a reproducible means for controlling competitive cell adhesion events and offers a novel system whose properties can be manipulated to intimately control cell behavior.

A Simplified Semi-Empirical Method to Select the Processing Parameters for Laser Clad Coatings

A semi-empirical method for selecting the processing parameter s of laser cladding is proposed. This phenomenological approach uses simple mathematical formulae, derived from a statistical analysis of measured data, to relate the laser cladding parameters with the geometric features of the clad track. Given the prescribed clad height and avai lable laser beam power, the proposed method allows to calculate values of the scanning speed and powder feed rate which are required to obtain low dilution, pore free coatings, fusion bonded to the substra te. To illustrate the application of this method, variable powder feed rate laser cladding e xperiments were carried out with Stellite 6 powder on mild steel substrates. In this technique t he laser beam power and radius and the processing speed are kept constant, while the powder feed rate is varied along a single track length according to a specified linear function. The expressions derive d from the model allowed to plot the experimental data in a coherent manner, revealing the combine d role of the different processing parameters.

On-Chip Open Microfluidic Devices for Chemotaxis Studies

Microfluidic devices can provide unique control over both the chemoattractant gradient and the migration environment of the cells. Our work incorporates laser-machined micro and nanofluidic channels into bulk fused silica and cover slip-sized silica wafers. We have designed “open” chemotaxis devices that produce passive chemoattractant gradients without an external micropipette system. Since the migration area is unobstructed, cells can be easily loaded and strategically placed into the devices with a standard micropipette. The reusable monolithic glass devices have integral ports that can generate multiple gradients in a single experiment. We also used cover slip microfluidics for chemotaxis assays. Passive gradients elicited from these cover slips could be readily adapted for high throughput chemotaxis assays.We have also demonstrated for the first time that cells can be recruited into cover slip ports eliciting passive chemoattractant gradients. This proves, in principle, that intravital cover slip configurations could deliver controlled amounts of drugs, chemicals, or pathogens as well as recruit cells for proteomic or histological analysis in living animals while under microscopic observation. Intravital cover slip fluidics will create a new paradigm for in vivo observation of biological processes.

A Microfluidic-Enabled Mechanical Microcompressor for the Immobilization of Live Single- and Multi-Cellular Specimens

A microcompressor is a precision mechanical device that flattens and immobilizes living cells and small organisms for optical microscopy, allowing enhanced visualization of sub-cellular structures and organelles. We have developed an easily fabricated device, which can be equipped with microfluidics, permitting the addition of media or chemicals during observation. This device can be used on both upright and inverted microscopes. The apparatus permits micrometer precision flattening for nondestructive immobilization of specimens as small as a bacterium, while also accommodating larger specimens, such as Caenorhabditis elegans, for long-term observations. The compressor mount is removable and allows easy specimen addition and recovery for later observation. Several customized specimen beds can be incorporated into the base. To demonstrate the capabilities of the device, we have imaged numerous cellular events in several protozoan species, in yeast cells, and in Drosophila melanogaster embryos. We have been able to document previously unreported events, and also perform photobleaching experiments, in conjugating Tetrahymena thermophila.

Patterned polymer matrix promotes stemness and cell-cell interaction of adult stem cells

BackgroundThe interaction of stem cells with their culture substrates is critical in controlling their fate and function. Declining stemness of adult-derived human mesenchymal stem cells (hMSCs) during in vitro expansion on tissue culture polystyrene (TCPS) severely limits their therapeutic efficacy prior to cell transplantation into damaged tissues. Thus, various formats of natural and synthetic materials have been manipulated in attempts to reproduce in vivo matrix environments in which hMSCs reside.ResultsWe developed a series of patterned polymer matrices for cell culture by hot-pressing poly(ε-caprolactone) (PCL) films in femtosecond laser-ablated nanopore molds, forming nanofibers on flat PCL substrates. hMSCs cultured on these PCL fiber matrices significantly increased expression of critical self-renewal factors, Nanog and OCT4A, as well as markers of cell-cell interaction PECAM and ITGA2. The results suggest the patterned polymer fiber matrix is a promising model to maintain the stemness of adult hMSCs.ConclusionThis approach meets the need for scalable, highly repeatable, and tuneable models that mimic extracellular matrix features that signal for maintenance of hMSC stemness.

Diffusion‐limited layer growth in spherical geometry: A numerical approach

In this article we present a detailed numerical analysis of the problem of diffusion‐controlled growth of a solid intermediate phase from two saturated solutions (solid solute and solid or liquid solvent). Spherical geometry is considered and the quasi‐steady‐state approximation used in earlier analytical models is eliminated. Comparison between our results and those from Larson’s model allows us to conclude that the use of a numerical scheme may lead to a significant gain in accuracy except when a solute‐rich sphere is converting into a dilute phase.