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Mechanical losses consume about 40% of the total energy developed by a typical automotive engine. The piston–cylinder system accounts for 50% of the engine frictional losses, of which 70% to 80% is...

In the present study, laser periodic structuring of titanium-based alloy (Ti-6Al-4V) has been carried out using Ti:Sapphire laser with the wavelength of 800 nm and pulse duration of 3 ps and 100 fs with varying peak fluence and scan speed. After laser irradiation, the topography of the surface has been recorded using 3D surface profilometer. Laser surface processing leads to the formation of a periodically patterned surface with the average ablation depth of 37.25 µm-42.13 and average surface roughness of 0.991-1.862 µm as compared to 0.169 µm average roughness of as-received Ti-6Al-4V. In the microstructure, there is presence of fine periodic ripples with an average ripple width of 0.48 µm to 0.54 µm when processed with 3 ps laser and the average ripple width of 0.17 μm in addition to the presence of very fine pits, deposited particle, and oxide dispersed surface when processed with 100 fs laser. The microhardness of the surface is improved (395 VHN-373 VHN ) as compared to 282 VHN of as-received Ti-6Al-4V. There is a significant improvement in the corrosion resistance in terms of a decrease in corrosion rate (0.0037 to 0.0008 mm/year) in laser surface processed sample as compared to as-received Ti-6Al-4V (0.0932 mm/year) in Hank’s solution and also increase in pitting corrosion resistance in terms of increase in critical potential for pit formation (Epit) under a few employed parameters with both 3 ps laser (at a laser fluence of 0.063 J/cm2 and a scan speed of 20 mm/sec) and 100 fs laser (at a laser fluence of 0.63 J/cm2 and at a scan speed of 60 mm/sec).

The present study investigates susceptibility of Inconel 617 to liquation cracking in laser-based welding techniques such as autogenous laser welding (ALW) and laser-MIG hybrid welding (LHW) with v...

The present work is directed towards understanding the role of filler addition in laser-MIG hybrid welding (LHW) process as compared with that in autogenous laser welding (ALW) process during welding of 10-mm-thick maraging steel plates. Addition of filler wire is beneficial in improving the edge bridging between the parts to be welded and also in improving the fusion zone microstructure. Single-pass LHW of the 10-mm-thick maraging steel plates was performed using a combination of 3.5-kW CO2 laser and synergic pulse MIG welding power source at a welding speed of 1 m/min. MIG filler wire with reduced solutes such as Mo and Ti contents was used for the LHW process. Simultaneously, double-sided single-pass ALW of similar plates was performed. The role of filler wire addition on the microstructure and mechanical properties of the LHW fusion zone (FZ) was studied and compared with that of ALW FZ. The usage of filler wire resulted in less solutes in the LHW FZ after welding, and hence, the volume fraction of “reverted austenite (RA)” formed during aging was minimized when compared with ALW FZ. During transverse weld tensile testing, the ALW welds yielded higher tensile properties when compared with the LHW weld due to their fine grain structure and high number density of precipitates. Whereas the KIc fracture toughness of the ALW FZ was low with the KIc value of 49.5 MPa√m due to the presence of the high amount of RA as compared with LHW which exhibited the KIc value of 77.5 MPa√m. The study vividly brings out the advantages of LHW process in improving the microstructure of the fusion zone due to the addition of filler wire.

The effect of zinc coating on the weld brazability of dissimilar Al–steel joint made by the cold metal transfer (CMT) process using the passive infrared (IR) thermography sensing technique was investigated using a numerical model. The numerical model was developed to understand the heat transfer phenomena during the CMT process. In the developed model, a thin thermally resistive layer (TTRL) was introduced at the interface of the two joining metals in order to manipulate the absence of zinc deposition on the steel sheet. The model could clearly identify the lack of weld deposition. The results were compared with experimental observations. Two cases are considered in this paper. The first study identifies the sensitivity of the IR thermography technique and the second one reveals the resolution in defect detection. The developed 3D model can be used as a tool to identify the defects caused during welding and can provide insights into the online monitoring of cold metal transfer joining process.

Cold metal transfer welding, a low-heat input process, was employed to weld-braze 6061-T6 aluminium alloy to galvanized interstitial free steel using Al–Si-based (4043 and 4047) filler wires in lap fillet configuration. The effects of current pulsing and filler composition on interface morphology and joint strength were investigated. Pulsing, as well as Si content in the filler, affected the interfacial intermetallic compound layer morphology and joint strength. The detailed micro-area X-ray diffraction studies of seam/steel interface revealed the presence of binary (Al–Fe type) and ternary (Al–Fe–Si type) intermetallic phases for joints made with 4043 and 4047 filler, respectively. Lap shear tests proved that joints made with 4047 filler having ternary intermetallic phases were stronger and fractured in the braze seam. However, joints made with 4043 filler having binary intermetallic phases recorded interfacial failure at lower loads than joints made with 4047 filler. Therefore, the type and morphology of the interfacial intermetallic compounds were found to influence the joint performance.

Comparitive analysis of the corrosion resistance of H32 5052 Al alloy and galvanized mild steel lap joints fabricated by a) Metal Inert Gas (MIG) Welding-brazing and b) Cold metal transfer (CMT) was done by keeping wire feed rate and welding speed constant. The weld interfaces of 5052 Al and mild steel were characterized using Field emission scanning electron microscopy, X-ray diffraction and nano-indentation measurements. Weight loss tests were done as per ASTM G 67-04 on the lap joints. Results indicated formation of Al-Fe-Si layer at the weld bead-mild steel interface in both the techniques. The layer was found to be Aluminium rich in MIG –braze sample whereas it was iron rich in the CMT sample. The layer thickness was found to be higher in the toe region than the head region in both samples. The Al-Fe-Si layer preferentially dissolved during the weight loss test of the welds. The thickness of Al-Fe-Si layer for MIG-braze welds was greater resulting in higher intergranular corrosion.

Laser brazing of aluminum 6082 to galvanized steel was carried out with different laser power followed by mechanical and microstructural characterization. Hardness, tensile and low cycle fatigue tests were conducted for mechanical property evaluation, while microstructural analyses were consisted of optical microscopy, SEM and XRD studies. Microstructures revealed the presence of different eutectics at the inter-dendritic region, whereas, ternary Al-Fe-Si and binary iron-rich intermetallics were at the steel interface. Asymmetrical hardness profile throughout the braze joint with lowest value in the brazed region was noted. The sample, brazed with 4 kW laser power, showed best tensile strength, and failed through the base material. Low cycle fatigue tests showed cyclic stabilization at lower strain amplitudes. Fatigue failure occurred in the brazed region and exhibited fatigue striations, flow lines and intergranular cracking.

Partial replacement of steel by Aluminium (Al) alloys is a promising approach adopted by automotive sector to improve fuel efficiency without compromising on the strength. However, this results in the generation of dissimilar welds of Al alloys and steel. Understanding the corrosion behavior of Al alloys–steel welds as a function of welding parameters is critical in the successful application of such alloys. The present study reports the intergranular corrosion behavior of T6 heat-treated AA6061 Al alloy-galvanized mild steel lap joints, welded by metal inert gas welding–brazing technique as a function of different welding parameters, viz. weld speed (4-6 m/min) and wire feed rate (0.8-0.9 m/min), following ASTM G 67-04. Weld characterization was performed by field emission scanning electron microscopy (FESEM), x-ray diffraction and nano-hardness measurement. Results indicate heavy dissolution and metal loss at the interface. A high volume fraction of Al-Fe intermetallics was precipitated at the weld interfaces, resulting in high hardness and higher localized corrosion. The thickness of Al-Fe intermetallic layer increased with increasing wire feed rate and lower weld speed which enhanced the severity of galvanic and intergranular corrosion.

Laser, a high intensity, precise, flexible and clean heat source is used as a manufacturing tool in several industrial sectors. With the advent of more robust, energy efficient, cheaper and low footprint lasers such as fiber lasers, laser based manufacturing is increasing rapidly. Laser based processes can be classified into macro processes (cutting, welding, cladding, alloying, drilling, brazing) and micro processes (micro texturing, scribing, micro drilling etc.) depending on the scale of effects induced during the process. The Centre for Laser Processing of Materials (CLPM) at ARCI, established about 15 years ago has been carrying out R&D on several processes for application in industry. Some of the recently developed industrial applications such as laser micro texturing of automotive components, laser deposition based repair of pressure die casting dies, laser brazing of aluminium alloys to steel, laser hardening of bearing steels and laser metal additive manufacturing are presented here.

Laser micromachining is an advanced machining process in which machining is achieved by focusing a laser beam to melt and vaporize the material. The primary aim of this work is to fabricate a control grid for an electron gun using laser micromachining. Initially, line scribing and 2D profiling experiments are performed on a 130-μm molybdenum plate to compare the surface quality and material removal rate of nanosecond and femtosecond lasers. The effects of laser processing parameters such as average power, repetition rate, and the feed rate on the width, depth, material removal rate, and cut quality of both the nanosecond and femtosecond lasers are studied. During micromachining using the nanosecond laser, melting and recasting of the metal around the machined sites are observed, resulting in the formation of heat-affected zone. During machining using the femtosecond laser, ultrafast laser pulses are used, which result in the absence of heat-affected zone. The surface roughness obtained using the femtosecond laser for creating a 2D profile is 0.187 μm, while using the nanosecond laser, the roughness value obtained is 1.89 μm. The femtosecond laser is used to successfully machine the 3D profile of the control grid, adopting the optimized parameters obtained from the line scribing and 2D profiling experiments. The average width of the grid line was measured as 149.89 μm which is very close to the required dimension of 150 μm.

Online (passive) thermographic inspection of overlap joints of aluminium and zinc coated steel sheets made by cold metal transfer weld brazing process was explored. Different experimental trials were conducted for demonstrating the feasibility of thermographic inspection to detect the porosities, improper weld bead and to differentiate the pre weld temperature. The whole process was monitored using infrared cameras in different wavelength region. Image analysis algorithms were developed to reconstruct the thermal images that contain the signatures of the weld defects and to extract the pre weld temperature and its evolution with distance from the centre of the weld torch. Post-weld radiography lends strong support to the observations.

Maraging steel 250 grade plates of 10 mm thickness were welded in single pass using a laser-arc hybrid welding (LHW) setup comprising 3.5 kW CO2 laser and synergic pulse metal inert gas (MIG) welding power source at a welding speed of 1 m/min. The influence of single-pass welding on the bead characteristics, microstructure, and mechanical properties was investigated. The size and volume fraction of reverted austenite was effectively reduced in the fusion zone. Moreover, the width of the heat-affected zones (HAZ) was reduced and the microhardness results did not show significant softening in the HAZ after post weld aging. Tensile testing of the welds in transverse direction showed 97.3% weld efficiency. The fusion zone exhibited KIc fracture toughness of 77.4 MPa√m which was affected by the distribution pattern of reverted austenite. The study vividly brings out the process advantages of LHW for accomplishing thick section welds of maraging steel in single pass with narrow groove and lesser filler wire con...

This work presents a detailed microstructure-property correlation of high power diode laser nitrided titanium based on its dendrite and martensite microstructures, structural phase and microhardness. For the laser nitrided titanium, cooling rate was estimated for the first time using empirical power law relationship by considering the martensite width and dendrite arm spacings and the results were validated by analytical thermal model. A processing window was formulated for laser gas nitriding of titanium by relating the experimental parameters such as laser processing parameters, nitrogen gas pressure and titanium nitride phases. The novelty of this work is demonstration of laser nitriding of titanium in laminar flow nitrogen gas environment using a simple acrylic container setup that avoids design and fabrication of a complex gas nozzle delivery system. In this setup, a thin transparent acrylic sheet was used as optical window instead of expensive quartz glass, to transmit high power diode laser beam at 980 nm wavelength to irradiate the substrate work piece in a closed container. This simple laser nitriding technique has created characteristic golden colored TiN surface with high surface hardness. Dendrite and martensite microstructures of the laser nitrided titanium were analyzed by optical and FESEM microscopy. EDAX analysis revealed the presence of nitrogen in laser nitrided titanium and X-ray diffraction confirmed the TiN phases. The microhardness of laser nitrided titanium is six times higher than its substrate.

Three integrated ANN-GA, ANN-SA and ANN-Quasi Newton methodology has been developed and implemented according to the following way to determine optimised input parameter setting for maximum welding strength during laser-MIG hybrid welding of aluminium alloy plates. Finally, significance of optimised parameters has been determined by ANOVA. Variation of welding strength with individual process parameters have been tested through main effect plots and interaction plots. Three soft computing based integrated models such as, ANN-GA, ANN-SA and ANN-Quasi Newton have been developed.Those models predicted and optimised welding strength during hybrid CO2 laser-MIG welding process.Best ANN architecture (3-11-1 network) predicts welding strength with mean absolute percentage errors less than 2%.ANN-GA shows best optimisation performance with only 0.09% experimental validation error.Welding speed shows maximum influence on welding strength and an increase in welding speed decreases welding strength. In this paper, artificial neural networks (ANNs), genetic algorithm (GA), simulated annealing (SA) and Quasi Newton line search techniques have been combined to develop three integrated soft computing based models such as ANN-GA, ANN-SA and ANN-Quasi Newton for prediction modelling and optimisation of welding strength for hybrid CO2 laser-MIG welded joints of aluminium alloy. Experimental dataset employed for the purpose has been generated through full factorial experimental design. Laser power, welding speeds and wires feed rate are considered as controllable input parameters. These soft computing models employ a trained ANN for calculation of objective function value and thereby eliminate the need of closed form objective function. Among 11 tested networks, the ANN with best prediction performance produces maximum percentage error of only 3.21%. During optimisation ANN-GA is found to show best performance with absolute percentage error of only 0.09% during experimental validation. Low value of percentage error indicates efficacy of models. Welding speed has been found as most influencing factor for welding strength.

Strain localization and premature splitting in the vicinity of outer heat affected zone is a major concern in the formability performance of laser welded similar material dual phase (DP) steels during stretch forming. Hence, the present work aims at evaluation of limiting drawing ratio (LDR) of tailor welded blanks (TWBs) fabricated using two dissimilar material combinations of DP and interstitial free (IFHS) steels using laboratory scale deep drawing test. Finite element models of deep drawing of parent materials and TWBs were developed incorporating anisotropy of sheet metals and non-homogenous properties of the weld zone. The numerical simulations were performed incorporating theoretical forming limit diagram (FLD) to predict LDR, thinning development and failure location. The LDR of both the TWBs was lower compared to that of parent metals due to non-uniform plastic deformation with localized thinning failure at cup bottom in the weaker material side. The failure propagated along a direction parallel and close to weld line as observed in experimental results. The global forming performance of different TWBs was characterized and compared in terms of formability ratio from the deep drawing, limiting dome height and Erichsen cupping test data. It was found that the extent of decrease in formability due to increase in strength ratio was not so severe in deep drawing process unlike stretch forming process. It was also proposed through rigorous finite element studies that LDR of TWBs could be improved with restricted weld movement by shifting the initial weld line position, and the same was validated through deep drawing experiments.

Tailor-Welded Blanks (TWBs) are obtained by welding together sheets of same or different metals of different thicknesses to produce a single blank prior to subjecting it to a forming process. Designers nevertheless need to overcome the difficulty faced in the forming of TWBs due to material property changes in the two parent metal sheets which is leading to weld line shift. The present work is aimed at studying the formability behavior of TWBs of two different steels viz. Interstitial free steels and Dual phase steels with respect to laser welding and Tungsten inert Gas welding. The formability of the TWBs and the base materials has been studied for differential strengths and analyzed with the help of LS Dyna Non-linear Finite element software. From the results, it was found that the weld shift can be altered by varying the strength parameters of the parent materials.

In this work, joining of aluminium alloy AA6061-T6 to Interstitial Free steel using pulsed gas metal arc welding process has been attempted. The effect of different surface conditions of steel (viz, galvanized, galvanealed and uncoated) and gap between the sheets on braze joint formation have been investigated. Galvanized steel surface showed good bead width, joint formation and lap shear strength compared to the other two combinations. Interface gap has not affected the wetting behaviour significantly but presence of a gap of 300 μm or so helped in escape of zinc vapour during the process there by avoiding formation of any crevice or macroporosity in the joint. Features and properties of the joint are characterized by metallography, fractography, energy dispersive spectroscopy (EDS) and lap shear tests. Load carrying capacity of Al-Galvanized steel was highest (222 N/mm of seam length) compared to other combinations, aided by better wetting due to presence of Zn on the surface and minimum porosity due to interfacial gap provided during brazing.

In the present study, laser cladding of AISI H13 tool steel powder has been undertaken on hardened and tempered AISI H13 tool steel components using a 6 kW fiber coupled diode laser both in continuous wave and pulsed mode with coaxial powder feeder to explore the possibility of reclamation/refurbishment of large AISI H13 tool steel components. Laser parameters in terms of laser power, processing speed and powder feed rate have been optimized to achieve acceptable clad-substrate integrity with optimum deposition height and width without any surface cracks and porosity. A detailed study of the effect of post-cladding heat treatment (both by laser assisted surface heating and conventional tempering at 550 °C) on the microstructure, phase aggregate, crystallite size, micro-strain and residual stress was carried out. Subsequently, the microhardness of the clad zone was evaluated. Pulsed laser cladding (heat input controlled) with 50 Hz frequency and 50% duty cycle with 10 ms ON-time developed a microstructure comprising martensite, retained austenite and mixed carbides. The maximum improvement in hardness (considered as a convenient index of strengthening) by laser cladding reached up to 650 VHN, which is about 45% higher than the hardness of hardened and tempered substrates. Post-cladding tempering at 550 °C for 2 h led to the development of uniform microstructure without fine carbide precipitated along grain boundaries or inter-dendritic regions taking the hardness to 600–650 VHN level. Laser reheating (tempering) of the clad zone with a 1000 W laser power showed a more refined microstructure with maximum hardness up to 680–700 VHN. Residual stress, developed on the surface after laser cladding, was compressive in nature and measured higher in magnitude after pulsed laser cladding. Magnitude of this residual compressive stress marginally decreased after post-cladding tempering either by conventional or laser assisted route. Thus, the novelty of the present study lies in the continuous or pulsed mode laser cladding assisted reclamation of AISI H13 hot working dies with similar composition powder, followed up with conventional or laser assisted tempering, leading to the development of about 45% higher hardness (than as hardened and tempered conditions) and retention of residual compressive stress on the surface after cladding.

Laser and laser hybrid welding are among the processes chosen for joining Indian reduced activation ferritic martensitic (INRAFM) steel that will be used for fabrication of Indian test blanket modules (TBMs) to be tested in ITER. In the present study, a 3.5 kW slab CO2 laser in autogenous mode and hybridized mode with MIG were successfully employed to make butt joints in plates 6 mm and 12 mm thick respectively. Initially, bead-on-plate (BOP) experiments were conducted to study the effect of various laser and laser hybrid welding parameters such as laser power, welding speed, MIG torch angle, wire feed rate, composition of torch gas, gas shielding arrangement, stand off, distance between arc and laser, and focal position of the laser with respect to the surface. The resultant weld beads were evaluated for weld penetration, width of the weld, and width of the HAZ. Based on BOP welding studies, parameters were chosen for carrying out butt welding experiments using square butt for 6 mm plates and Y-g...

In the present study, laser welding was carried out with CO2 laser on a 2 mm thick aluminum alloy (6061-T6) sheet to study laser weldability of this heat-treatable aluminum alloy. Screening experiments were performed initially to enumerate the power density requirement for proper coupling of the laser with the material. Bead on plate experiments were conducted to recognize situation under which stable keyhole and welds with minimum defects (porosity and cracks) can be produced. Recognized process window (3 kW power, 4 m/min welding speed, and 18 L/min Helium flow rate) obtained from initial trial was used in butt welding experiment and the resulted weld was characterized by macroscopic and microscopic observation, microhardness study, and tensile test. The microstructure reveals porosity and crack-free welds and hardness data confirmed presence of heat-affected zone (HAZ) and huge drop in overall hardness after welding. After post-weld treatment (aging treatment), the hardness was increased marginally, bu...

Surface hardening of low carbon hot rolled C–Mn steel has been successfully performed by high power diode laser with an achievable case depth of about 300 μm. The laser treated samples have been characterised using optical microscope, scanning electron microscope, transmission electron microscope, and X-ray diffraction techniques. Higher hardness level is achieved in laser surface hardened zone (≈300 HV) than in the base alloy region (≈140 HV). The variation in hardness as a function of distance across the laser tracks is observed during multi-track laser hardening. Laser hardened steel sheets show enhanced mechanical strength (YS: 383–443 MPa, UTS: 476–506 MPa) with the lowering of percentage total elongation (23–28%) compared to the base alloy (YS: 351 MPa, UTS: 450 MPa and total elongation is 32%). Strain hardening exponent (‘n’) has been evaluated from true tensile stress–strain diagram and it shows a similar nature for both base alloy and laser treated steels. The microstructure in the base alloy region consists of a mixture of ferrite and pearlite, whereas predominantly lath martensite is present in the laser hardened surface layer. The improvement of mechanical strength is discussed in terms of the formation of this hardened layer on the surface.

The present study concerns laser surface hardening (LSH) and melting (LSM) of AISI H13 tool steel using a high power continuous wave diode laser. Depth of surface hardened or melted layer increases with increase in incident laser energy density. Surface melting occurs at a higher laser energy density (>75 J/mm 2 ) and leads to the formation of inhomogeneous microstructure comprising non-uniform distribution of retained austenite, carbides (along inter-dendritic boundary) and martensite with their respective volume fractions varying with depth. Application of intermediate laser energy density (50–75 J/mm 2 ) yields a hardened layer with dispersion of ultrafine mixed carbides (M 23 C 6 , M 7 C 3 , MC or M 2 C). Laser treatment with a very low laser energy density ( 2 ) leads to formation of an over-tempered microstructure consisting of low carbon martensite and coarse carbide precipitates. Micro-tensile studies with specially machined samples from the surface melted zone following LSM with a laser energy density of 100 J/mm 2 records a high yield strength of 1310 MPa along with poor ductility, marked by brittle failure. On the other hand, a similar sample from laser surface hardened zone treated with a laser energy density of 62.5 J/mm 2 yielded even higher yield strength of 1460 MPa with a maximum elongation of 3.6%. Though both LSH and LSM produced higher yield strength compared to hardened and tempered AISI H13 tool steel, LSH yielded a combination of higher elongation (3.6%), than that after LSM (0.97%), with high yield strength and hence was considered a better option.

An investigation of laser dressing of metal-bonded diamond blades is reported. The influence of laser irradiation on the diamond particles and bond material was reported, and the bonding material w...

Laser drilling of precise holes in thermal barrier coated Ni based superalloys has been studied. The interplay between various hole geometrical features such as hole shape, taper, barrelling, undercut, etc. and laser parameters such as pulse energy, pulse width and pulse repetition rate have been examined. The hole diameters are seen to follow a linear dependence on the incoming laser power densities for pulse width up to 2·0 ms. However, such a linear dependence was not observed for a pulse width of 3·0 ms. It was found that high pulse energy and short pulse width (high power density) gave crack free recast layer, whereas low pulse energy and longer pulse width (low power density) gave microcracks in the heat affected layer of superalloy. The significant barrelling observed in IN718 material at low power density values is due to multiple reflection of the incident beam from the cavity in combination with plasma formation at the evaporation front and trapping of the incident radiation causing exce...

Ti–6Al–4V was laser surface nitrided using a rectangular beam intended for large area coverage at minimal processing time. The laser nitriding performed at different energy densities yielded the formation of ~120 µm thick TiN layer. Metallurgical characterization was performed using Optical Microscope (OM), Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) analysis. A hardness improvement up to 760 HV 0.2 was achieved compared to the base material hardness of 315 HV 0.2 . The tribological characterization of laser nitrided Ti-alloy exhibited superior performance, indicating the possibility of using it as a light weight disk brake rotor than the heavier gray cast iron.

Laser surface hardening, when applied to complex geometries, poses a challenge in the terms of obtaining uniform hardness throughout the hardened area because of variable heat sink effects. In this work, an iterative numerical approach was used to estimate the required modulation in laser power to achieve a uniform surface temperature throughout the process zone. Firstly, a transient thermal model for the laser–material interaction was developed using the finite element method for a rectangular spot of 8 × 5 mm. The temperature-dependent material properties were used to bring in nonlinear effects in the analysis so as to predict the hardened zone dimensions more precisely. The numerical model was validated by carrying out laser hardening experiments using a 6-kW diode laser. The validated numerical model was used with an iterative technique aided by conditional looping to achieve a uniform surface temperature during the laser hardening of the complex geometry with a variable heat sink. The developed iterative approach can be effectively used on any geometry with a variable heat sink to obtain a constant surface temperature throughout the process zone.

304B4 Borated austenitic stainless steel is widely used in the nuclear industry due to high neutron absorption efficiency. In the present investigation, autogenous bead-on-plate (BoP) laser welding studies were carried out on 3 mm thick 304B4 grade stainless steel using a 3.5 kW slab CO 2 laser. Influence of variables such as laser power, welding speed, shielding gas and laser beam mode on microstructure and mechanical properties were studied. Dye penetrant testing, macrostructural analysis, bead geometry measurements, microhardness survey, and microstructural analysis in both as-weld and post-weld heat treated conditions were carried out. The macrostructural and bead geometry analyses of the welds have shown that the welds were free from cracks in the fusion zone (FZ) and also in the heat affected zone (HAZ) for all the welding parameters studied. The Gaussian mode has given a very narrow weld width compared to donut mode. During welding use of helium and nitrogen has reduced the width of the FZ and HAZ. The as-weld micro hardness was more than double the base metal, and the peak hardness was shifted from the centre to the fusion boundaries with the increase in heat input. The PWHT has reduced the hardness of both the FZ and HAZ. In summary, usable laser welding parameters for welding 3 mm thick 304B4 grade stainless steel have been identified.

Laser welding investigations were carried out on dissimilar Ti–SS combination. The study is aimed to improve the weld strength and ductility by minimizing harmful intermetallics and taking advantage of high cooling rates in laser welding. Results of continuous wave 3.5 kW CO2 laser welding of totally dissimilar combination of Titanium and stainless steel (304) have been discussed. Bead on plate welding experiments were conducted to identify the laser welding parameters using depth of penetration as criteria. The welding of dissimilar combination has been attempted both autogenously and with interlayers such as Vanadium (V) and Tantalum (Ta) in the form of laser cladding as well as strip. Autogenous welds were carried out by varying the laser power, welding speed and position of the laser beam with respect to the joint centre. The resultant welds are characterized by macrostructure analysis, SEM/EDAX and XRD and as welded tensile test in UTM. The autogenous welds have exhibited extensive cracking even when welded at high speeds or by manipulating the beam position with respect to the joint. Similarly Vandaium as interlayer could not achieve crack free joint. A joint with 40 MPa strength could be made with Ta as interlayer. Results and analysis of these variants of laser welded joints are reported and discussed.

Well-aligned nitrogen-doped carbon nanotube (N-CNTs) film was fabricated on silicon substrate by thermal chemical vapor deposition process with varying the growth temperature. The effect of growth temperature on morphology, microstructure and crystallinity for the growth of N-CNTs was studied. At all growth temperatures, the bamboo-like morphology of graphene layers with compartments in CNTs were observed in transmission electron microscope micrographs. The doping level and the type of nitrogen-related moieties were determined by X-ray photoelectron spectroscopy analysis. The compartment distance decreases with increase in nitrogen doping level in hexagonal graphite network. The increase in nitrogen doping level in N-CNTs will lead to decrease in crystallinity and in-plane crystallite size. Field emission study of nitrogen-doped carbon nanotubes grown at optimum parameters showed that they are good emitters with a turn-on and threshold field of 0.3 and 1.6 V/μm, respectively. The maximum current density was observed to be 18.8 mA/cm2 at the electric field of 2.1 V/μm. It is considered that the enhanced field emission performance of doped nanotube is due to the presence of lone pairs of electrons on nitrogen atom that supplies more electrons to the conduction band.

The development of nanostructured, hard, and wear-resistant composite coating of Al2O3, TiB2, and TiN on low carbon steel (AISI 1025) substrate by combined self-propagating high-temperature synthesis (SHS) and laser surface alloying (LSA) has already been communicated in the author’s previous publications. The coefficient of friction of the coating (with WC-Co as counterbody) is also found to have reduced substantially with the presence of free hexagonal boron nitride (hBN) in it. The occurrence of free hBN has been made possible by putting excess hBN in the precursor mixture which is supposed to undergo combined SHS and LSA. This has also been reported in one of the previous publications. This reduction in coefficient of friction is achieved at a cost of a marginal reduction in the microhardness as well as the specific wear rate as the free hBN in the coating matrix causes some softening. Laser post-treatment causes distinct enhancement in the microhardness and wear resistance of the coating while augmenting the favorable effect of free hBN on the coefficient of friction.

Modified 9Cr–1Mo steel plates of 6 mm thickness have been laser welded using CO2 laser. The effects of beam intensity and overall heat input (168-1500J/mm) on the bead characteristics, microstructure and mechanical properties of the welds have been investigated by varying the laser welding parameters such as laser beam mode, power and welding speed. The microhardness survey carried out on the welds after post-weld heat treatment did not reveal any soft zones in the intercritical heat affected zone for welds made with a heat input of up to 420 J mm−1. The tensile strengths of the welds were comparable to that of the base material. Charpy impact tests on subsize specimens revealed that the welds have good toughness. δ-Ferrite was observed in the fusion zone of the welds made at heat input of 700 J mm−1 and above, the content of which increased with the increased heat input.

Hard and wear resistant Al 2 O 3 –TiB 2 –TiN composite coatings have been developed on low carbon steel (AISI 1025) substrate by following two different routes involving laser surface treatment. In the first (termed ‘ in-situ ’ process), reinforcing phases TiB 2 and TiN, as well as the matrix Al 2 O 3 of the composite are synthesized in-situ by laser-triggered self-propagating high temperature synthesis (SHS) from a mixture of Al, TiO 2 and h-BN and coated onto the substrate surface by laser surface alloying (LSA). In the second (termed ‘ ex-situ ’ process), the constituents Al 2 O 3 , TiB 2 and TiN of the coating are provided directly as a pre-placed precursor powder mix and laser surface alloyed onto the substrate. Of these two laser assisted manufacturing procedures, it is of interest to determine the one that is more appropriate for the development of a hard, wear resistant coating. In the present work, investigation of the comparative merits and demerits of Al 2 O 3 –TiB 2 –TiN coatings produced by in-situ and ex-situ processes is attempted through analysis of microstructure and evaluation of mechanical and tribological properties.

Laser induced keyhole (KH) and weld pool formation during laser beam welding (LBW) can alter mechanical properties of weld seams. In this regard, deep penetration high power continuous wave (CW) CO 2 laser induced KH was generated in mild steel (MS) in both ambient and under water medium. Under water, KH was deeper and narrower as compared with KH formed in ambient condition. Contact mechanical studies of both the types of weld zones were carried out by measuring hardness and frictional properties. A significant rise in hardness was observed in fusion zone (FZ) regardless of the medium used. The KH and weld pool mediated LBW fusion zone showed lower coefficient of friction. Scratch size was also found to decrease in FZ due to increased hardness. The mechanical properties of FZ such as hardness, coefficient of friction and scratch size were correlated with microstructure composed of fine grained structure in the weld zones.

A nanostructured, hard and wear resistant composite coating of Al2O3, TiB2 and TiN isformed on low carbon steel (AISI 1025) substrate by combined self-propagating high-temperaturesynthesis (SHS) and laser surface alloying (LSA). A precursor powder mixture of Al, TiO2 and hBNpreplaced on the substrate surface undergoes self-propagating high-temperature synthesis (SHS) atthe high temperatures induced by the incidence of laser. The products of SHS are subsequently laseralloyed onto the substrate to form the coating. The high power laser beam is again made to pass overthe coating with a view to remelt the coating top layer. This laser post-treatment causes distinct enhancements in microhardness and tribological performance by the modification of themicrostructure of top portion of the coating.

This study concerns laser surface hardening (LSH) of austempered SAE 52100 steel to enhance hardness and wear resistance. Following LSH with selected laser parameters, surface microstructure and mechanical properties were evaluated by optical/scanning electron microscopy, X-ray diffraction, hardness measurement and wear studies, and thermal profile modeling. The results suggest that LSH could develop residual compressive stress and high hardness and wear resistance in this steel without affecting the bainitic core.

An experimental Al–Cu–Li–Mg–Ag–Zr type alloy in the form of 13.7 mm thick plates was studied for its fusion characteristics using gas metal arc welding (GMAW) and pulsed gas metal arc welding (P-GMAW). High copper 2319 filler of 1.6 mm diameter was used. The burn-off characteristics of 2319 filler wire in GMAW and P-GMAW were experimentally determined, including the relation between pulse current and pulse duration for the desired one-drop detachment per pulse (ODPP) condition and feasible range of pulse parameters. The effect of welding parameters on bead geometry and shape relationships was investigated through beadon-plate experiments in the welding current range above the spray transition current. Reasonably good weld beads were obtained in P-GMAW at currents as low as 194 A and welding speeds of 45 cm min–1. P-GMAW yielded significantly higher weld penetration compared to GMAW.

Laser surface texturing process involves creation of microfeatures, e.g., tiny dimples, usually distributed in a certain pattern, covering only a fraction of the surface of the material that is being treated. The process offers several advantages for tribological applications, including improved load capacity, wear resistance, lubrication lifetime, and reduced friction coefficient. In the present study, the surface modification of gray cast iron, using millisecond (λ = 1,064 nm), nanosecond (λ = 1,064 nm) and femtosecond (λ = 800 nm) pulse duration laser irradiation, is adopted to establish a particular geometrical pattern with dimple features and dimensions, to improve wear and friction behavior. The effect of various laser processing parameters, including laser pulse energy, pulse duration and processing speed, on the performance characteristics of the laser-treated samples is investigated. The microtextured surfaces were produced on gray cast iron using different millisecond (0.5 ms), nanosecond (40 ns) and femtosecond (120 fs) laser source with the dimple depth between 3 and 15 μm. The coefficient of friction for the untextured surface was ~0.55, millisecond laser textured ~0.31, nanosecond laser textured ~0.02 and femtosecond laser ~0.01, under normal force of 50 N and sliding speed of 63 mm/s. Surface texturing of the gray cast iron surface using femtosecond pulse duration resulted in significant improvement in wear resistance in comparison to the untextured as well as millisecond and nanosecond laser-textured surface. © 2014 Springer-Verlag Berlin Heidelberg.

Substitutional nitrogen doped aligned carbon nanotubes (NACNTs) with uniform height and high packing density of arrays was synthesized by using the liquid injection CVD process. Transmission electron micrographs indicated that the NACNTs are having nano-bell morphology of graphene layers with series of internal compartments. Field emission study of NACNTs showed that they are good emitters with low turn-on and threshold field. The maximum current density was observed to be 18.8 mA/cm2 at electric field of 1.89 V/μm.

Few-walled carbon nanotubes (CNTs) were synthesized by the arc discharge process. The study pertains to analyze the effect of buffer gas pressure (Helium) and density of graphite anodes on morphology and yield of CNTs. The synthesis of CNTs was carried out at different buffer pressures of Helium ranging from 80 Torr to 600 Torr using different anode densities specifically 1.6 and 1.8 gm/cc. FE-SEM and TGA studies indicated that the highest percentage of MWCNTs was formed around 450 Torr pressure as compared to other pressures. It was observed that the variation of gas pressure affects the current drawn capacity and also the evaporation rate of the graphite electrode. Micro-structure of CNTs synthesized at 450 Torr consists of 8 concentric graphene layers. Improved crystallinity with better thermal stability was observed in case of high density graphite anode.

In this paper, hybrid laser-arc welding process has been studied based on the relative position of the laser and the arc (i.e. laser-leading and arc-leading arrangement) and, the effects of welding parameters, such as the laser power, arc current, arc voltage and the welding speed on the weld bead were investigated. The study indicates that the welding direction has a significant effect on the weld bead and weld pool behaviour. The result shows that laserleading configuration shows better bead characteristics when compared to arc-leading configuration. This is because in the laser-leading case molten metal flow is inward, while in the arc-leading case the metal flow is outward leading to variation in solidification front resulting in lack of synergic effects of both processes.

Laser as a high intensity heat source can be effectively used for joining of materials by fusion welding and brazing in autogenous or in hybrid modes. In autogenous mode, welding is done in conduction , deep penetration , and keyhole mode. However, due to inherently high energy density available from a laser source, autogenous keyhole welding is the most popular laser welding mode. But, it has certain limitations like need for extremely good joint fit-up, formation of very hard welds in steel , keyhole instability, loss of alloying elements, etc. To overcome these limitations, innovative variants such as laser-arc hybrid welding , induction-assisted welding , dual beam welding , etc., have been developed. Using laser heat, brazing can be performed by melting a filler to fill the joints, without melting the base materials. Accomplishing laser-based joining as mentioned above requires appropriate choice of laser source, beam delivery system, processing head with appropriate optics and accessories. Basic principles of various laser-based joining processes, laser system technology, process parameters, metallurgical effects on different base materials, joint performance, and applications are explained in this chapter.

This chapter discusses the microstructure and slurry erosion characteristics of laser surface alloyed coatings that are used to mitigate the erosion issues of 13Cr–4Ni martensitic stainless steel. The chapter first describes the present scenario of surface modification methods, such as conventional welding, surface hardening/nitriding, external coatings, etc., which are being utilized to minimize the silt erosion problems of components in hydropower plants. The chapter also explains newer coating methods, such as laser surface alloying. It then describes the usefulness towards erosion mitigation in hydropower plant applications of laser surface alloying of 13Cr–4Ni steels with commercial coating powders such as Colmonoy 88 and Stellite 6. Erosion performances of coatings and substrates are evaluated using a slurry jet erosion test-rig with various parameters, using commercial silica sand and river sand. An evaluation of the effect of boron carbide addition on the erosion performances of laser surface alloyed coatings is given. An attempt has also been made to correlate the quantitative results (erosion performances) with qualitative results (erosion mechanisms) by extensive studies of eroded samples using scanning electron microscopy. By the end of the chapter, a correlation between measured and predicted erosion rates has been established in power-law formulation.

The present work describes an investigation of the effect of 3 different parameters of laser surface alloying—i.e., laser scanning speed (LSS), nozzle stand-off distance (NSOD) and laser beam scan-off distance (LBSOD) on coating height, depth and width. Nickel-based Colmonoy 88 alloy powder has been deposited on 13Cr-4Ni steel by single-step process of laser surface alloying. Laser power and powder feed rate were maintained at 3kW and 25 g/min, respectively. L8 orthogonal array has been designed to study these 3 parameters at 2 levels each. The results of single pass with extent of dilution, surface hardness and microstructures produced by different conditions are presented and discussed. For a specified NSOD and LBSOD, there was a decrease in coating height and depth with increase in LSS. Coating height and depth were not affected much by increase in NSOD. From the present investigation, optimized parameters were identified for enhanced hardness, minimum dilution and desired coating height and coating depth.