A. Roy Choudhury

Direct selective laser sintering of iron-graphite powder mixture

In the present work, laser sintering of a powder mixture of iron and graphite (99.22 and 0.78%, respectively) was carried out using a pulsed Nd-YAG laser. The paper reports experimental details with evolution of the microstructures and evaluation of some of the physical and mechanical properties of the resultant sintered material. The obtained results indicate suitability of the laser sintered material for some special applications. # 2003 Elsevier Science B.V. All rights reserved.

Adaptive slicing with cubic patch approximation

In adaptive slicing, the number of layers is drastically reduced by using sloping layer walls. For both vertical (2.5D slices) and sloping (ruled slices) outer walls, the strategies for determining slice height generally consider a number of vertical sections along the contour of a slice. Surface deviation error is calculated at these sections and slice height subsequently determined. Instead, a method is proposed which calculates error at every part of the surface. This method approximates the outer wall between two successive contours by a series of taut cubic spline patches. It is proposed that the deviation between such a patch and the actual surface is a better and more exhaustive estimate of surface error. Results show that the predicted number of slices is slightly higher than that predicted by existing methods for sloping layer walls.

A study on laser drilling of thin steel sheet in air and underwater

In laser drilling of a thin stainless steel sheet in air with Nd:YAG laser pulses of 0.5–1 ms durations it was observed that the 0.5 ms duration laser pulse was more effective in drilling a through-hole than the relatively longer laser pulses with proportionately more energy. Further, laser drilling could be readily done when the sheet was placed at the focal point of the lens and below it but not above the focal point. On the other hand, the underwater laser drilling could be done when the sheet was placed above the focal point. An attempt has been made to explain these experimental observations considering various processes involved in laser drilling in air and underwater. While the recoil pressure of the vapor and plasma played an important role in laser drilling in air; the radial gradient of recoil pressure of evaporation, the Marangoni force induced by the surface tension gradient in melt pool and the cavitation effect of bubble collapse were believed to be responsible for the material removal in un...

Volume deviation in direct slicing

Purpose – To calculate the volume deviation between a CAD model and built‐up part in 5‐axis laminated object manufacturing employing direct slicing with first‐order approximation.Design/methodology/approach – It is proposed here that the deviation between the CAD model and the built‐up part, which is normally calculated as a linear dimension in specific 2D sections of the CAD model, be treated as a volume (as it actually is), for higher accuracy in subsequent calculations. An algorithm has been developed and implemented for identification and calculation of volume deviation, considering all possibilities.Findings – It has been conclusively shown that volume deviation consideration results in improved feature recognition and less approximation.Research limitations/implications – Increase in complexity of the CAD model leads to a considerable increase in the volume deviation computation time. Future research in this area would focus on optimization and calculation of the slice heights based on volume deviat...

Application of curved layer manufacturing for preservation of randomly located minute critical surface features in rapid prototyping

Purpose – Layered manufacturing with curved layers is a recently proposed rapid prototyping (RP) strategy for the manufacture of curved, thin and shell-type parts and the repair of worn surfaces, etc. The present investigation indicates another possible application area. In case of flat-layered RP of computer-aided design models having randomly located, small-dimensioned but critical surface features, adaptive slicing is resorted to. Large number of thin slices have to be employed to preserve the critical features. In contrast, a considerably lower number of curved thin slices would be required to preserve such surface features in case of RP with curved layers. Design/methodology/approach – The method of preservation of critical features by RP with curved layers is formulated and demonstrated for two clusters of critical features on the surface of a part. A minimum number of such curved layers is identified by application of genetic algorithms (GAs) in case of a simple example. GA evolves the shape of the...

Rapid Manufacture in Light Metals Processing

As several of the free form fabrication processes progress with continuous process and material improvements, the feasibility of Rapid Manufacturing becomes more and more of a reality. Defined as the use of a Computer Aided Design (CAD) based automated additive manufacturing process to construct parts that are used directly as finished products and components, some of the rapid manufacturing processes are already competing with traditional processes such as injection moulding and progress is being made in applying the new technologies to the processing of metals, envisioning additive manufacture of high strength parts of unlimited complexity. While there have been quite a few successful attempts in the rapid production of complex medical implants using titanium alloys, 3D printing of sand moulds opens up yet another rapid manufacturing front, allowing for the rapid casting of aluminium and magnesium alloys. The effectiveness of such processes is yet to be researched in terms of process and product characteristics and the overall economy. This paper attempts to review some of the promising rapid manufacturing technologies for light metals processing and presents results of experimental investigations conducted to evaluate the effectiveness of the rapid casting process currently researched at the Rapid Product Development Centre of AUT University.

Hardfacing of AISI304 steel: fabrication of oxide-boride-nitride ceramic matrix composite layer by laser-assisted high temperature chemical reaction

Composition, structure and properties of the products of self-propagating high-temperature synthesis (SHS) are characterised by some distinctive features. High heating rate, fast cooling after rapid completion of the reactions and steep temperature gradients make SHS very effective in producing in situ composites with ceramic reinforcements. In the present work, hardfacing of AISI304 substrates has been done by fabricating a hard ternary ceramic matrix composite layer of Al2O3–TiB2–TiN by laser surface treatment at different scan speeds. The formation of the surface layer is due to laser-triggered SHS followed by laser melting. A mixture of Al, TiO2 and hBN has been used as a precursor for the SHS reaction. The study of the microstructure of the as-fabricated composite layer reveals the co-existence of TiB2 and TiN phases in the nanometric size range in Al2O3 matrix. The presence of all the phases has been confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The average grain sizes were calculated for the reinforcing phases and found to be 36 and 66 nm for TiB2 and TiN, respectively, for the ceramic layer fabricated with a scan speed of 10 mm s−1, whereas 21 and 53 nm have been observed for TiB2 and TiN, respectively, for the ceramic layer fabricated with the scan speed of 5 mm s−1. The understanding of the chemical synthesis in the SHS reaction mentioned here and the process of development of the reinforced composite in the fabrication of the hardfaced layer over steel surface will be immensely helpful in the discernment of the mechanical properties and, thus, finding the target area for the usage of this product. The virtues of the process and formation of the hard composite are reflected well in the microhardness achieved in the fabricated layers, as it is significantly higher than that of the substrate (AISI304 steel). In addition, indentation with a Berkovich tip in a nano-indentation set-up helped in further evaluation of the composite’s hardness and elastic modulus. The property spectrum of the composite, as reported here, indicates its suitability in various wear-intensive applications.