Arvind Kumar

Micromanufacturing: A review—part II

Micromanufacturing processes are expanding in their length and breadth as long as the related research and development (R&D) activities and applications are concerned. Products are getting miniaturized and their performance efficiency is getting enhanced by the addition of micro/nanofeatures and devices. In the set of these two articles (Part I and Part II), an attempt has been made to review the latest R&D activities of the selected micromanufacturing processes. This article (Part I) deals with a review of the literature related to attrition (subtractive, or machining and finishing) processes (both types—traditional and advanced) including microturning, micromilling, microdrilling, abrasive jet micromachining, laser beam micromachining, electrochemical micromachining, magnetorheological finishing, abrasive flow finishing, magnetic abrasive finishing, ion beam micromachining and so on. Apart from the subtractive processes, an overview of the X-ray lithography has also been presented. An attempt has been made to report some applications to help the readers to evolve more new applications of these processes. At the end of different sections/subsections, some research areas have been identified, which would hopefully fill the gaps between the theoretical analysis, experimental work and applications.

A numerical simulation of columnar solidification: influence of inertia on channel segregation

We investigate the role of the inertia of the flow through the dendritic mushy zone in the numerical prediction of channel segregations during columnar solidification. The contribution of inertia is included in the momentum transport equation through the quadratic Forchheimer correction term. The study reveals a significant influence of the Forchheimer term in the vicinity of the liquidus front, i.e. at high liquid fractions. The natural convective flow field in this region is modified due to the additional inertial drag. This strongly influences the convective transport of solute and thereby incurs a modification of the dynamics of the advancement of the mushy zone. The most notable consequence is a significant decrease in the predicted channel segregation.

Effects of carbon fillers on the tribological and mechanical properties of an epoxy-based polymer (SU-8)

SU-8, an epoxy-based negative photoresist polymer, is highly suitable for making micro-electro-mechanical systems (MEMS) structures. Despite fabrication advantages, its bulk mechanical and tribological properties are the main limitations for application as MEMS material. Carbon filler materials such as graphene, graphite and multi-walled carbon nanotube (MWCNT) are added to SU-8 for tribological and mechanical property enhancements. SU-8/(5 wt%) graphite composite has performed better for the steady-state coefficient of friction at all loads including for the speed effect. SU-8/(5 wt%) MWCNT has shown excellent wear resistance. At 10 wt% graphite content, SU-8/graphite is superior in tribological performance to other composites tested.

Substrate Melting and Re-solidification During Impact of High-Melting Point Droplet Material

In this paper, impact, spreading, and solidification of molten droplet on a dissimilar substrate along with substrate melting and its re-solidification are investigated numerically. Volume of fluid surface tracking method coupled with the solidification model within a one-domain continuum formulation is used to model the transient flow during the droplet impact, its subsequent spreading, and solidification. Evolution of melting front in the substrate is modeled by solving the governing equations for solidification in the substrate, too. Simulations are performed for the impact of a heated droplet on a substrate. The model predicts substrate melting, which can give better insight of bonding between the coating material and substrate, where droplet and substrate are of different materials. It is observed that melting in the substrate in the present case starts soon after the impact of the heated droplet. The depth and the width of the melting front in the substrate increase with the time and after reaching a maximum they start to decrease because of start of re-solidification from the melted edge. In the central part of the splat droplet solidifies, while the substrate remains melted which can enhance the coating strength and its bonding with the substrate.

Numerical Modeling of Impact and Solidification of a Molten Alloy Droplet on a Substrate

Most of the studies reported for droplet impact and spreading on a substrate in a thermal spray coating process assume that droplet material solidifies as a pure substance, i.e., phase change occurs at a fixed temperature. The alloy-type behavior of the droplet impact where it solidifies within liquidus and solidus temperature is not well reported. The role of formation of mushy zone and species composition variation during the coating layer formation while using a multi-constituent alloy material is not established. This work investigates the spreading and solidification characteristics of an alloy droplet impacting on a substrate. Two-dimensional axisymmetric model has been used to simulate the transient flow and alloy solidification dynamics during the droplet impingement process. Volume of fluid (VOF) surface tracking method coupled with the alloy solidification model within a one-domain continuum formulation is developed to describe the transport phenomena during the droplet impact, spreading, and solidification of an alloy droplet on a flat substrate. Using the model, the characteristics of alloy solidification in coating formation are highlighted.

Thermal Transport Phenomena in Multi-layer Deposition Using Arc Welding Process

The repair of steel plate using welding can be optimized by multi-layer deposition. In this numerical study, semi-automatic arc welding is used to deposit single-track multi-layers of mild steel on a same material mild steel plate. Three-dimensional transient numerical simulations of the transport phenomena involved in the melt pool are performed. The model considers heat transfer, convective and radiative losses, phase change, re-melting, solidification, and buoyancy and Marangoni convection driven fluid flow in the melt pool. The model predicts the temperature and velocity fields, and the evolution of melt pool shape and size.

Thermal Stress Analysis in Selective Laser Melting of Ti6Al4V Powder Layer

Selective laser melting (SLM) is a direct fabrication process that uses moving heat source for obtaining a 3D object from a digital design data. In the process, a steep temperature gradient develops due to localized heating causing differential expansion and contraction at different locations. Thus, stresses are generated, which remain in the material as residual stresses and affect the functionality of the part. This study presents an analysis of stresses developed in the SLM process. A thermo-mechanical model is developed for the prediction of residual stresses during processing of Ti-6Al-4V powder layer. The thermal model, considering Marangoni and natural convection in the melt pool, is used to deduce the temperature field, which is coupled with the mechanical model. Using the temperature field, an analysis is performed for the stresses developed due to solidification of the melt pool formed on the Ti-6Al-4V powder layer. The nature of the stresses during heating and cooling stages is studied. Stresses formed during melting in the vicinity of the melt pool region are compressive. During cooling, the stresses change their nature to tensile in the solidified melt pool region. These tensile stresses remain locked in the component as residual stresses even after cooling down period.

Modelling of Heat Transfer in Powder Bed Based Additive Manufacturing Process Using Lattice Boltzmann Method

One of the most promising additive manufacturing techniques is selective laser melting process. It is a complex process, which involves physical phenomena, such as absorption of the laser beam in the powder bed, melting and re-solidification, diffusive and radiative heat transport in the powder, diffusive and convective heat transport in the melt pool, gravity effects, etc. In this study, a two-dimensional lattice Boltzmann model is formulated to investigate melting of a uniformly packed powder bed under the irradiation of laser beam during the selective laser melting process. In the model, phase change of individual powder particle is considered mesoscopically. The results give an insight into the details of heat transfer and melting in the powder bed and formation of the mushy zone. These mesoscopic results can be useful to set parameters of the powder bed in additive manufacturing processes. The model developed can be applied to any powder bed based additive manufacturing process.

Finite Element Analysis of Melt Pool Characteristics in Selective Laser Spot Melting on a Powder Layer

In this work, volume contraction of powder layer and convective flow in the melt pool during laser spot melting of Ti–6Al–4V powder layer are investigated using a transient two-dimensional finite element model. An algorithm, coupled with the finite element model, accounting for volume contraction due to melting of porous powder to a denser liquid is proposed, which is thereafter used to understand the role of natural and Marangoni convection on the melt pool behaviour. Results for the melt pool characteristics, such as melt pool geometry, melt pool fluid flow dynamics and thermal behaviour are presented.

A Study of Solidification During Ice Slurry Generation in an Inclined Rectangular Cavity

The solidification process involved in ice slurry generation is accompanied by multiphase convection of liquid and solid phases. In this article, ice slurry generation in an inclined rectangular cavity has been investigated numerically and experimentally. The ice slurry is generated by freezing a hypereutectic aqueous ammonium chloride (H2O + NH4Cl) solution from one side of the cavity. In the numerical study, a validated macroscopic model, that consider solidification, multiphase convective flow, interfacial drag and particle sedimentation, is used to analyse the transport phenomena during ice slurry generation. The model predicts flow field, temperature, species and solid fraction distributions. In the experimental study, particle image velocimetry has been used for in situ study of the flow and the solidified and mushy zone thickness during solidification. The thermal history in the cavity at selected strategic locations is recorded by T-type thermocouples. The validation of the model and experimental and numerical results of evolution of solid fraction, temperature profile, multiphase velocity field and mass of ice slurry produced have been discussed.