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Papers by Niroj Maharjan

Research paper thumbnail of Post-Processing of Cold Sprayed Ti-6Al-4V Coatings by Mechanical Peening

Metals

Cold spray is an emerging additive manufacturing technology used in the aerospace industry to rep... more Cold spray is an emerging additive manufacturing technology used in the aerospace industry to repair damaged components made of expensive metal alloys. The cold sprayed layer is prone to surface integrity issues such as high porosity and inadequate bonding at the substrate-coating interface, which may cause premature failure of the repaired component. This study explored the use of mechanical peening as a post-processing method to improve the surface integrity of the cold sprayed component by modifying mechanical properties near the surface. Two mechanical peening processes, deep cold rolling (DCR) and controlled hammer peening (CHP), were utilized to improve cold sprayed Ti-6Al-4V coating on the Ti-6Al-4V substrate. Experimental results indicate that DCR and CHP increase the strength of the bond between the coating and substrate due to introduction of compressive residual stresses. In addition, porosity is also reduced by as much as 71%. The improvement is attributed to both the co...

Research paper thumbnail of Effects of post-processing route on fatigue performance of laser powder bed fusion Inconel 718

Research paper thumbnail of Fatigue performance of laser shock peened Ti6Al4V and Al6061‐T6 alloys

Fatigue & Fracture of Engineering Materials & Structures

Research paper thumbnail of Laser peening of 420 martensitic stainless steel using ultrashort laser pulses

Research paper thumbnail of Robotic hammer peening-induced martensite in austenitic steels: Spatial distributions of plastic deformation and phase transformation

Research paper thumbnail of Direct laser hardening of AISI 1020 steel under controlled gas atmosphere

Surface and Coatings Technology

Research paper thumbnail of Underwater laser hardening of bearing steels

Journal of Manufacturing Processes

Research paper thumbnail of Influence of operating parameters on morphology of laser hardened surfaces

High-Power Laser Materials Processing: Applications, Diagnostics, and Systems VII

Research paper thumbnail of Decarburization during laser surface processing of steel

Research paper thumbnail of Corrosion behavior of laser hardened 50CrMo4 (AISI 4150) steel: A depth-wise analysis

Research paper thumbnail of Surface Ablation of 52100 Bearing Steel Using Femtosecond Laser Irradiation

Surface Review and Letters

Laser ablation is a rapid-material removal technique with potential application in remanufacturin... more Laser ablation is a rapid-material removal technique with potential application in remanufacturing of bearings to clean damaged surfaces. This study reports the ablation morphology and change in surface composition of 52100 bearing steel irradiated with a femtosecond laser. A smooth ablated surface was achieved by operating the laser at fluence just above the ablation threshold, but higher fluences resulted in roughened surfaces with recast formation. Such coarse morphology is believed to result from higher energy deposition and surface melting. Minimal oxidation occurred at lower fluences. Periodic ripple patterns formed in the ablated area at low fluence; their morphology and possible formation mechanism is discussed.

Research paper thumbnail of Comparative study of laser surface hardening of 50CrMo4 steel using continuous-wave laser and pulsed lasers with ms, ns, ps and fs pulse duration

Surface and Coatings Technology

Research paper thumbnail of Laser surface hardening of AISI 1055 steel in water-submerged condition

Surface Review and Letters

Underwater laser hardening might produce better surface mechanical properties than conventional l... more Underwater laser hardening might produce better surface mechanical properties than conventional laser hardening in air due to additional cooling effect by water. However, it has not been studied in detail. This study investigates the effect of water layer on laser surface hardening of AISI 1055 steel. It is found that laser surface hardening is feasible with water layer up to 3[Formula: see text]mm above the steel surface. A higher surface hardness is achieved during underwater processing. This is attributed to fast cooling by water which facilitates complete martensitic transformation. Nevertheless, the hardened area is smaller than that in conventional laser hardening in air due to attenuation of laser energy. Above 3[Formula: see text]mm, the laser beam is severely attenuated due to formation of vapor plume. Furthermore, it is found that surface oxidation cannot be prevented completely even during underwater treatment, and the water movement results in random distribution of meta...

Research paper thumbnail of An Arrhenius equation-based model to predict the residual stress relief of post weld heat treatment of Ti-6Al-4V plate

Journal of Manufacturing Processes

Research paper thumbnail of Ablation morphology and ablation threshold of Ti-6Al-4V alloy during femtosecond laser processing

Research paper thumbnail of Design of Reversible Pump Turbine for its prospective application in Nepal

Most hydropower plants in Nepal are runoff type, which cannot supply the designed amount of energ... more Most hydropower plants in Nepal are runoff type, which cannot supply the designed amount of energy during dry season and peak demand period resulting in energy crisis. Therefore, pumped storage plants can be an ideal solution to meet the current energy needs of the country. Most of these storage plants make use of a single unit acting both as turbine as well as pump; hence aptly called Reversible Pump Turbine (RPT). This paper explains the new application concept for use of such RPTs as auxiliary unit to supplement the main power unit in hydropower plants of Nepal. It also summarizes the design process of such runner with reference to the parameters available at Chilime region in central part of Nepal. The design is performed in two steps. The first step is an analytical design, which gives an initial geometry of the RPT runner. The process resulted in a runner of larger size than a normal Francis runner for same parameters since it has to work as pump as well. The next step is an optimization procedure involving CFD analysis under which the simulation of the RPT in turbine mode yielded an efficiency of 88.71%. The detail laboratory experiments on a model RPT will be performed later to validate the results from CFD and determine the characteristics of the designed runner at different mode and operating conditions.

Research paper thumbnail of Laser surface hardening of bearing steels

dai and all other friends in Singapore and back home for being there for me both at good as well ... more dai and all other friends in Singapore and back home for being there for me both at good as well as hard times. In addition, I would also like to acknowledge NTU and Advanced Remanufacturing and Technology Center (ARTC) for supporting my PhD work and A*STAR SINGA for providing me the scholarship. Last but not least, I express my heartfelt thanks to my parents, sister and family for their ceaseless blessing, love and support.

Research paper thumbnail of Design of Reversible Pump Turbine for its prospective application in Nepal

Most hydropower plants in Nepal are run-off type, which cannot supply the designed amount of ener... more Most hydropower plants in Nepal are run-off type, which cannot supply the designed amount of energy during dry season and peak demand period resulting in energy crisis. Therefore, pumped storage plants can be an ideal solution to meet the current energy needs of the country. Most of these storage plants make use of a single unit acting both as turbine as well as pump; hence aptly called Reversible Pump Turbine (RPT). This paper explains the new application concept for use of such RPTs as auxiliary unit to supplement the main power unit in hydropower plants of Nepal. It also summarizes the design process of such runner with reference to the parameters available at Chilime region in central part of Nepal. The design is performed in two steps. The first step is an analytical design, which gives an initial geometry of the RPT runner. The process resulted in a runner of larger size than a normal Francis runner for same parameters since it has to work as pump as well. The next step is an optimization procedure involving CFD analysis under which the simulation of the RPT in turbine mode yielded an efficiency of 88.71%. The detail laboratory experiments on a model RPT will be performed later to validate the results from CFD and determine the characteristics of the designed runner at different mode and operating conditions.

Research paper thumbnail of Design and Development of a Reversible Pump Turbine Test Rig

Conference Presentations by Niroj Maharjan

Research paper thumbnail of Pumped storage concept and its potential application in Nepalese hydropower context–A case study of Chilime Hydropower Plant, Rasuwa, Nepal

Due to fluctuation in energy production from hydropower plants, the existing plants in Nepal cann... more Due to fluctuation in energy production from hydropower plants, the existing plants in Nepal cannot meet the current demand of energy resulting in the power cut during peak hours. Therefore, there is a dire need for an energy storage unit that can meet the surplus demand of energy during peak hours. A pumped storage plants can be used to store electrical energy during periods of low demand and consume the energy during peak energy demand periods. Such plants generally make use of Reversible Pump Turbines (RPTs). The following paper discusses the potential application of pumped storage system and RPT in context of existing and new hydropower plants in Nepal by studying the case of Chilime Hydropower Plant in Rasuwa, Nepal. The study revealed that the proper application of this technology could certainly aid in minimizing the energy crisis currently faced by Nepal.

Research paper thumbnail of Post-Processing of Cold Sprayed Ti-6Al-4V Coatings by Mechanical Peening

Metals

Cold spray is an emerging additive manufacturing technology used in the aerospace industry to rep... more Cold spray is an emerging additive manufacturing technology used in the aerospace industry to repair damaged components made of expensive metal alloys. The cold sprayed layer is prone to surface integrity issues such as high porosity and inadequate bonding at the substrate-coating interface, which may cause premature failure of the repaired component. This study explored the use of mechanical peening as a post-processing method to improve the surface integrity of the cold sprayed component by modifying mechanical properties near the surface. Two mechanical peening processes, deep cold rolling (DCR) and controlled hammer peening (CHP), were utilized to improve cold sprayed Ti-6Al-4V coating on the Ti-6Al-4V substrate. Experimental results indicate that DCR and CHP increase the strength of the bond between the coating and substrate due to introduction of compressive residual stresses. In addition, porosity is also reduced by as much as 71%. The improvement is attributed to both the co...

Research paper thumbnail of Effects of post-processing route on fatigue performance of laser powder bed fusion Inconel 718

Research paper thumbnail of Fatigue performance of laser shock peened Ti6Al4V and Al6061‐T6 alloys

Fatigue & Fracture of Engineering Materials & Structures

Research paper thumbnail of Laser peening of 420 martensitic stainless steel using ultrashort laser pulses

Research paper thumbnail of Robotic hammer peening-induced martensite in austenitic steels: Spatial distributions of plastic deformation and phase transformation

Research paper thumbnail of Direct laser hardening of AISI 1020 steel under controlled gas atmosphere

Surface and Coatings Technology

Research paper thumbnail of Underwater laser hardening of bearing steels

Journal of Manufacturing Processes

Research paper thumbnail of Influence of operating parameters on morphology of laser hardened surfaces

High-Power Laser Materials Processing: Applications, Diagnostics, and Systems VII

Research paper thumbnail of Decarburization during laser surface processing of steel

Research paper thumbnail of Corrosion behavior of laser hardened 50CrMo4 (AISI 4150) steel: A depth-wise analysis

Research paper thumbnail of Surface Ablation of 52100 Bearing Steel Using Femtosecond Laser Irradiation

Surface Review and Letters

Laser ablation is a rapid-material removal technique with potential application in remanufacturin... more Laser ablation is a rapid-material removal technique with potential application in remanufacturing of bearings to clean damaged surfaces. This study reports the ablation morphology and change in surface composition of 52100 bearing steel irradiated with a femtosecond laser. A smooth ablated surface was achieved by operating the laser at fluence just above the ablation threshold, but higher fluences resulted in roughened surfaces with recast formation. Such coarse morphology is believed to result from higher energy deposition and surface melting. Minimal oxidation occurred at lower fluences. Periodic ripple patterns formed in the ablated area at low fluence; their morphology and possible formation mechanism is discussed.

Research paper thumbnail of Comparative study of laser surface hardening of 50CrMo4 steel using continuous-wave laser and pulsed lasers with ms, ns, ps and fs pulse duration

Surface and Coatings Technology

Research paper thumbnail of Laser surface hardening of AISI 1055 steel in water-submerged condition

Surface Review and Letters

Underwater laser hardening might produce better surface mechanical properties than conventional l... more Underwater laser hardening might produce better surface mechanical properties than conventional laser hardening in air due to additional cooling effect by water. However, it has not been studied in detail. This study investigates the effect of water layer on laser surface hardening of AISI 1055 steel. It is found that laser surface hardening is feasible with water layer up to 3[Formula: see text]mm above the steel surface. A higher surface hardness is achieved during underwater processing. This is attributed to fast cooling by water which facilitates complete martensitic transformation. Nevertheless, the hardened area is smaller than that in conventional laser hardening in air due to attenuation of laser energy. Above 3[Formula: see text]mm, the laser beam is severely attenuated due to formation of vapor plume. Furthermore, it is found that surface oxidation cannot be prevented completely even during underwater treatment, and the water movement results in random distribution of meta...

Research paper thumbnail of An Arrhenius equation-based model to predict the residual stress relief of post weld heat treatment of Ti-6Al-4V plate

Journal of Manufacturing Processes

Research paper thumbnail of Ablation morphology and ablation threshold of Ti-6Al-4V alloy during femtosecond laser processing

Research paper thumbnail of Design of Reversible Pump Turbine for its prospective application in Nepal

Most hydropower plants in Nepal are runoff type, which cannot supply the designed amount of energ... more Most hydropower plants in Nepal are runoff type, which cannot supply the designed amount of energy during dry season and peak demand period resulting in energy crisis. Therefore, pumped storage plants can be an ideal solution to meet the current energy needs of the country. Most of these storage plants make use of a single unit acting both as turbine as well as pump; hence aptly called Reversible Pump Turbine (RPT). This paper explains the new application concept for use of such RPTs as auxiliary unit to supplement the main power unit in hydropower plants of Nepal. It also summarizes the design process of such runner with reference to the parameters available at Chilime region in central part of Nepal. The design is performed in two steps. The first step is an analytical design, which gives an initial geometry of the RPT runner. The process resulted in a runner of larger size than a normal Francis runner for same parameters since it has to work as pump as well. The next step is an optimization procedure involving CFD analysis under which the simulation of the RPT in turbine mode yielded an efficiency of 88.71%. The detail laboratory experiments on a model RPT will be performed later to validate the results from CFD and determine the characteristics of the designed runner at different mode and operating conditions.

Research paper thumbnail of Laser surface hardening of bearing steels

dai and all other friends in Singapore and back home for being there for me both at good as well ... more dai and all other friends in Singapore and back home for being there for me both at good as well as hard times. In addition, I would also like to acknowledge NTU and Advanced Remanufacturing and Technology Center (ARTC) for supporting my PhD work and A*STAR SINGA for providing me the scholarship. Last but not least, I express my heartfelt thanks to my parents, sister and family for their ceaseless blessing, love and support.

Research paper thumbnail of Design of Reversible Pump Turbine for its prospective application in Nepal

Most hydropower plants in Nepal are run-off type, which cannot supply the designed amount of ener... more Most hydropower plants in Nepal are run-off type, which cannot supply the designed amount of energy during dry season and peak demand period resulting in energy crisis. Therefore, pumped storage plants can be an ideal solution to meet the current energy needs of the country. Most of these storage plants make use of a single unit acting both as turbine as well as pump; hence aptly called Reversible Pump Turbine (RPT). This paper explains the new application concept for use of such RPTs as auxiliary unit to supplement the main power unit in hydropower plants of Nepal. It also summarizes the design process of such runner with reference to the parameters available at Chilime region in central part of Nepal. The design is performed in two steps. The first step is an analytical design, which gives an initial geometry of the RPT runner. The process resulted in a runner of larger size than a normal Francis runner for same parameters since it has to work as pump as well. The next step is an optimization procedure involving CFD analysis under which the simulation of the RPT in turbine mode yielded an efficiency of 88.71%. The detail laboratory experiments on a model RPT will be performed later to validate the results from CFD and determine the characteristics of the designed runner at different mode and operating conditions.

Research paper thumbnail of Design and Development of a Reversible Pump Turbine Test Rig

Research paper thumbnail of Pumped storage concept and its potential application in Nepalese hydropower context–A case study of Chilime Hydropower Plant, Rasuwa, Nepal

Due to fluctuation in energy production from hydropower plants, the existing plants in Nepal cann... more Due to fluctuation in energy production from hydropower plants, the existing plants in Nepal cannot meet the current demand of energy resulting in the power cut during peak hours. Therefore, there is a dire need for an energy storage unit that can meet the surplus demand of energy during peak hours. A pumped storage plants can be used to store electrical energy during periods of low demand and consume the energy during peak energy demand periods. Such plants generally make use of Reversible Pump Turbines (RPTs). The following paper discusses the potential application of pumped storage system and RPT in context of existing and new hydropower plants in Nepal by studying the case of Chilime Hydropower Plant in Rasuwa, Nepal. The study revealed that the proper application of this technology could certainly aid in minimizing the energy crisis currently faced by Nepal.