Partha Saha

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.

Surface Integrity of C-40 Steel Processed with WC-Cu Powder Metallurgy Green Compact Tools in EDM

Electric discharge machining (EDM) is an electrothermal process where recast layer on the machined surface and heat-affected (HAZ) zone just below the machined surface are common phenomena. Thus, the assessment of surface integrity in EDM is a very important task. In this study, an attempt was made to modify the surface integrity of C-40 steel in EDM. WC-Cu powder metallurgy (P/M) green compact tools were used so that the tool material could be transferred to the work surface and thereby the surface characteristics could be altered. WC was used as tool material because of its ability to produce a hard layer over the work surface. A detailed experimental study was performed. The effect of various process parameters on the surface characteristics were presented with the support of analyses such as material transfer rate (MTR), tool wear rate (TWR), surface roughness, optical microscopy, scanning electron microscopy (SEM), energy dispersive x-ray (EDX), x-ray diffraction (XRD), microhardness testing, etc. It was established that the surface integrity of the workpiece changes with the desired deposition of a hard composite layer of WC and copper over the work surface.

Surface modification of C40 steel using WC-Cu P/M green compact electrodes in EDM

This study deals with surface modification of C-40 grade steel using P/M (powder metallurgy) green compacts of WC?Cu mixture as the EDM tool during a die sinking operation in hydrocarbon dielectric fluid. An L8 Taguchi array is used to investigate the process by varying tool parameters like composition and compaction pressure and EDM operating parameters like polarity, peak-current and pulse-duration. An optimal condition, for getting a uniform layer over the surface, is achieved using OEC (overall evaluation criteria). At the optimum condition, a layer thickness of 48.9μm and an increase in hardness up to 1632Hv are achieved.

Investigations Into the Influence of Weld Zone on Formability of Fiber Laser-Welded Advanced High Strength Steel

In this study, two different dual phase steel grades DP980 and DP600, and IFHS steel sheets were laser welded by a 2-kW fiber laser. The weld quality of these three different LWBs was assessed with the help of microstructure, micro-hardness and transverse tensile tests. Tensile testing of longitudinal and miniature samples was performed to evaluate the mechanical properties of the weld zone. Formability of parent materials and LWBs were assessed in bi-axial stretch forming condition by Erichsen cupping test. To validate the weld zone properties, 3-D finite element models of Erichsen cupping test of LWBs was developed, and the failures in the deformed cups were predicted using two theoretical forming limit diagrams. It was observed that hardness of the fusion zone and HAZ in laser welded DP600 and IFHS steels was more compared to the respective parent metal. However, 29% reduction in hardness was observed at the outer HAZ of DP980 steel weldments due to tempering of martensite. Reduction of formability was observed for all the LWBs with two distinct failure patterns, and the maximum reduction in formability was observed in the case of DP980 LWBs. The presence of the soft zone is detrimental in forming of welded DP steels.

Multi-objective optimization in wire-electro-discharge machining of TiC reinforced composite through Neuro-Genetic technique

This paper proposed a Neuro-Genetic technique to optimize the multi-response of wire electro-discharge machining (WEDM) process. The technique was developed through hybridization of a radial basis function network (RBFN) and non-dominated sorting genetic algorithm (NSGA-II). The machining was done on 5vol% titanium carbide (TiC) reinforced austenitic manganese steel metal matrix composite (MMC). The proposed Neuro-Genetic technique was found to be potential in finding several optimal input machining conditions which can satisfy wide requirements of a process engineer and help in efficient utilization of WEDM in industry.

Synthesis of calcium hydrogen phosphate and hydroxyapatite coating on SS316 substrate through pulsed electrodeposition.

The orthopaedic implants for human body are generally made of different biomaterials like stainless steels or Ti based alloys. However, it has been found that from surface properties point of view, none of these materials is attractive for fast tissue or cell growth on the surface of implant. This is one of the most important criteria to assure quick bonding between implant and body tissues vis-à-vis minimum recovery time for the patient. Keeping in view of the above facts, this work involves the pulsed electro-deposition coating of biocompatible hydroxyapatite and its group compounds from a diluted bath of calcium and phosphate salt at various current densities over the biomaterial sheet of SS316. SEM study confirms different morphologies of the coatings at different current densities. Characterization techniques like X-ray diffraction, SEM with EDX and FTIR have been used to confirm the phase and percentage quantity of hydroxyapatite compound in the depositions. This coating can serve as a medium for faster tissue growth over the metallic implants.

PREDICTION OF FORMABILITY OF BI-AXIAL PRE-STRAINED DUAL PHASE STEEL SHEETS USING STRESS BASED FORMING LIMIT DIAGRAM

Dual phase (DP) steel is of great interest for automotive part manufacturers due to its excellent combinations of strength and formability. Complex components involving three-dimensional stampings are usually fabricated through multistage sheet forming operations. The ability of a sheet metal to be deformed into a specific desired shape by distributing strain over arbitrary tool surface depends on complex interaction of material, process and design variables. The strain-based forming limit diagram (e-FLD) is often used as a measure of formability in the press shop due to convenience of measuring the limiting strain. However, it was reported by previous researchers that the e-FLD of sheet metal shifts after pre-strain due to the initial forming operations. Hence, this work proposes a mathematical framework for constructing σ-FLD of different pre-strained sheets incorporating Barlat-89 yield criterion with different hardening laws. The formability of biaxially pre-strained DP600 was evaluated experimentally in two stages. The forming behaviour of pre-strained material was predicted by finite element model using the σ-FLD, and the predicted results matched very closely with the experimental data. It was also observed that the σ-FLD was robust and underwent insignificant changes due to the change in the pre-strain path.

Fabrication of cylindrical micro tools by micro electrochemical form turning operation

Cylindrical micro tools of tungsten material have various applications in micro devices and systems because of its high rigidity and toughness. This article deals with the fabrication of cylindrical micro tools from tungsten rod by micro electrochemical form turning operation using KOH solution as electrolyte. The experiments were conducted with an indigenously developed setup with the variation of different experimental parameters to get good cylindrical shape of the tool. Cylindrical micro tools of diameter less than 50 µm and length of up to 1 mm have been fabricated through this process from tungsten rods of initial diameter of 1.5 mm within the time duration of 15 min. This process produces cylindrical micro tools from a much larger initial diameter of tungsten rod and with low lead time, as compared to the reported literatures. The variation of the cylindrical length of the micro tool according to the variation of parameters such as pulse on time, applied voltage and concentration of electrolyte has been analyzed, and the optimum parameter settings for getting the better shape and surface finish of the cylindrical electrode are electrode gap distance of 100 µm, frequency of the pulse power supply of 500 kHz, voltage of 18 V, electrolyte concentration of 3 M and pulse on time of 1400 ns.

Back propagation neural network based modeling of multi-responses of an electrical discharge machining process

This work attempts to model the electrical discharge machining (EDM) process using artificial neural network (ANN) with back propagation as the learning algorithm. The three most important parameters, namely, material removal rate (MRR), tool wear and surface roughness have been considered as a measure of the process performance. A large number of experiments have been carried out over a wide range of machining conditions to study the effect of input parameters on the machining performance. The experimental data is used for the training and verification of the model. Testing results demonstrate that the model is suitable for predicting the response parameters accurately.

Prediction of formability of laser-welded dual-phase steel by finite element analysis:

Dual-phase steels are being applied in the automobile sectors for their higher strength with reasonable work hardening exponent. However, applications of these steels in tailor welded blanks for automobile part manufacturing involve laser welding and subsequent sheet forming. The thermal cycle during laser welding changes the local properties of the weld zone, which affects the formability. Hence, the present study was targeted to understand the effect of laser welding in formability of dual-phase steel. Laser welding of 1.2-mm-thick dual-phase steel with tensile strength of 600 MPa (DP600) and 980 MPa (DP980) was performed using 2-kW fibre laser set-up to fabricate two different laser welded blanks. The laser power and scan speed during welding were selected as 1.8 kW and 1000 mm/min, respectively. Weld quality was accessed using microhardness, metallography and transverse tensile tests. Formability of the laser welded blanks was evaluated in terms of cup height by Erichsen cupping test. It was observed that formability of welded blanks reduced compared to parent metals. The soft zone was observed in the heat-affected zone of DP980 welded specimens, and hence, reduction in formability was more. Finite element simulation of Erichsen cupping test was performed using LS-DYNA explicit finite element code.