Jyoti Mazumder

A two-dimensional transient model for convection in laser melted pool

A two-dimensional transient model for convective heat transfer and surface tension driven fluid flow is developed. The model describes the transient behavior of the heat transfer process of a stationary band source. Semi-quantitative understanding of scanning is obtained by a coordinate transformation. The non-dimensional forms of the equations are derived and four dimensionless parameters are identified, namely, Peclet number (Pe), Prandtl number (Pr), surface tension number(S), and dimensionless melting temperature(@#@ Tm*@#@). Their governing characteristics and their effects on pool shape, cooling rate, velocity field, and solute redistribution are discussed. A numerical solution is obtained and presented. Quantitative effects of Prandtl number and surface tension number on surface velocity, surface temperature, pool shape, and cooling rate are presented graphically.

Additive Manufacturing: Current State, Future Potential, Gaps and Needs, and Recommendations

Additive manufacturing (AM), the process of joining materials to make objects from three-dimensional (3D) model data, usually layer by layer, is distinctly a different form and has many advantages over traditional manufacturing processes. Commonly known as “3D printing,” AM provides a cost-effective and time-efficient way to produce low-volume, customized products with complicated geometries and advanced material properties and functionality. As a result of the 2013 National Science Foundation (NSF) Workshop on Frontiers of Additive Manufacturing Research and Education, this paper summarizes AMs current state, future potential, gaps and needs, as well as recommendations for technology and research, university–industry collaboration and technology transfer, and education and training.

Transport phenomena during direct metal deposition

The evolution of temperature and velocity fields during direct metal deposition with coaxial powder injection was simulated using a self-consistent three-dimensional model based on the solution of the equations of mass, momentum, energy conservation, and solute transport in the liquid pool. The basic physical phenomena, including heat transfer, phase changes, mass addition, fluid flow, and interactions between the laser beam and the coaxial powder flow, were considered in the model. The level-set method was implemented to track the evolution of the liquid/gas interface. The temperature and velocity fields, liquid/gas interface, and energy distribution at liquid/gas interface at different times were simulated. For verification purposes, the cladding depth and height were compared with experimental results.

Pulsed laser deposition of hydroxyapatite thin films on Ti–6Al–4V: Effect of heat treatment on structure and properties

Hydroxyapatite (HA) is an attractive biomaterial that has been widely used as a coating for dental and orthopedic metal implants. In this work, HA coatings were deposited on Ti-6Al-4V substrates by laser ablation of HA targets with a KrF excimer laser. Deposition was performed at ambient temperature under different working pressures that varied from 10(-4) to 10(-1) torr of oxygen. The as-deposited films were amorphous. They were annealed at 290-310 degrees C in ambient air in order to restore the crystalline structure of HA. The coatings morphology, composition and structure were investigated by scanning electron microscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction techniques. Mechanical and adhesive properties were examined using nanoindentation and scratch tests, respectively. The stability of the HA coatings was tested under simulated physiological conditions. This study reveals that the combination of pulsed laser deposition and post-deposition annealing at 300 degrees C have the potential to produce pure, adherent, crystalline HA coatings, which show no dissolution in a simulated body fluid.

LASER CLADDING OF ZIRCONIUM ON MAGNESIUM FOR IMPROVED CORROSION PROPERTIES

Laser cladding of Mg-2 wt% Zr, and Mg-5wt% Zr powder mixture onto magnesium was carried out. The microstructure of the laser clad was studied. From the microstructural study, the epitaxial regrowth of the clad region on the underlying substrate was observed. Martensite plates of different size were observed in transmission electron microscope for Mg-2wt% Zr and Mg-5wt% Zr laser clad. The corrosion properties of the laser clad were evaluated in sea-water (3.5% NaCl). The position of the laser claddings in the galvanic series of metals in sea-water, the anodic polarization characteristics of the laser claddings and the protective nature and the stability of the passivating film formed have been determined. The formation of pitting on the surface of the laser clad subjected to corrosion has been reported. The corrosion properties of the laser claddings have been compared with that of the commercially used magnesium alloy AZ91 B.

Control of the wetting properties of an AISI 316L stainless steel surface by femtosecond laser-induced surface modification

A simple and effective method without vacuum to control the wetting properties of AISI 316L stainless steel using femtosecond laser pulses at high repetition rate has been developed. Both hydrophilic and hydrophobic surfaces were formed by creating micro-conical structures on the surface with femtosecond laser irradiation in air. The scan speed was found to be an effective parameter in controlling micro-cone morphology, size and number densities and contact angles during surface wettability experiments. It was found during surface wettability experiments that the contact angle of water varied from 0? (superhydrophilic) to 113? on laser micro-cone textured surfaces depending on processing conditions. Additionally, a superhydrophobic AISI 316L stainless steel surface was created (contact angle ?150?) with a functionalized silane coating on already hydrophobic surface geometry.

Laser cladding of Mg-Al alloys

Among rapid solidification processing methods, laser cladding is a unique and promising technique which can be used to increase the corrosion resistance of materials. This paper describes the improvement of the laser-cladding process for magnesium-based alloys and an investigation of the effect of the laser-cladding technique upon the microstructure and the corrosion resistance of magnesium-based alloys. The cladding apparatus and techniques have been adapted for magnesium cladding to overcome the oxidation and high vapour pressurerelated problems. Laser-clad Mg27Al73, Mg53Al47, and Mg72Al28 have been obtained. Eutectic phases were observed in the two magnesium-rich alloys and in the interface of the aluminiumrich alloy. Polycrystalline structure was formed in Mg27Al73. Laser-clad Mg27Al73 was found to be superior to laser-clad Mg-5 wt% Zr, Mg-2 wt% Zr, cast AZ91 B and cast magnesium in corrosion properties.

Temperature and composition profile during double-track laser cladding of H13 tool steel

Multi-track laser cladding is now applied commercially in a range of industries such as automotive, mining and aerospace due to its diversified potential for material processing. The knowledge of temperature, velocity and composition distribution history is essential for a better understanding of the process and subsequent microstructure evolution and properties. Numerical simulation not only helps to understand the complex physical phenomena and underlying principles involved in this process, but it can also be used in the process prediction and system control. The double-track coaxial laser cladding with H13 tool steel powder injection is simulated using a comprehensive three-dimensional model, based on the mass, momentum, energy conservation and solute transport equation. Some important physical phenomena, such as heat transfer, phase changes, mass addition and fluid flow, are taken into account in the calculation. The physical properties for a mixture of solid and liquid phase are defined by treating it as a continuum media. The velocity of the laser beam during the transition between two tracks is considered. The evolution of temperature and composition of different monitoring locations is simulated.

Feedback Control of Melt Pool Temperature During Laser Cladding Process

Laser cladding is a multiple-parameter-dependent process, and a feedback control is critical for the process stabilization. This paper presents a generalized predictive control strategy with input constraints to stabilize the melt pool temperature during a high-power diode laser cladding process. A dual-color pyrometer was used to monitor the melt pool temperature. A state-space dynamic model relating the laser drive signal (laser power) to the melt pool temperature was identified experimentally using the subspace method. A generalized predictive controller with input constraints was implemented in real time using the state-space model. The closed-loop process was able to track the melt pool temperature to a reference temperature profile. Laser cladding of H13 tool steel on a substrate with uneven surface showed that the closed-loop process was able to compensate for an under-fill with 3-mm depth after 40-layer depositions.

Composition control in laser surface alloying

Laser surface alloying, a process of growing interest for local surface modification, relies upon a suitable composition and microstructure for satisfactory on-the-job performance. This paper reports the results of an initial systematic study of laser surface alloying nickel onto AISI 1020 steel substrates using a statistical experimental design technique. The objective was to relate processing conditions to dimensions, solute content, and microstructural refinement of the laser alloyed zones. Solute content was of principal concern as it is the single most important factor affecting the properties of laser surface alloys. The effects of varying the laser power, beam diameter, and speed on the width, depth, nickel content, and fluctuations in nickel content are reported. Interactions between process parameters are discussed, the reproducibility assessed, contour plots for solute content drawn. Dimensionless plots are developed that relate average solute content and microstructural refinement to process parameters. Previously published data for alloying chromium into 1018 steels are shown to contain similar trends. It is felt that such an approach would facilitate selection of processing conditions to obtain reproducibly the compositions and microstructures necessary for gainful utilization of laser surface alloys.