Radovan Kovacevic | Southern Methodist University (original) (raw)
Papers by Radovan Kovacevic
The quality of laser welded-brazed (LWB) joints were strongly dependent on the main process param... more The quality of laser welded-brazed (LWB) joints were strongly dependent on the main process parameters, therefore the effect of laser power (3.2–4 kW), welding speed (60–80 mm/s) and wire feed rate (70–90 mm/s) on mechanical strength and surface roughness were investigated in this study. The comprehensive optimization process by means of response surface methodology (RSM) and desirability function was used for multi-criteria optimization. The experiments were planned based on Box– Behnken design implementing linear and quadratic polynomial equations for predicting the desired output properties. Finally, validation experiments were conducted on an optimized process condition which exhibited good agreement between the predicted and experimental results. AlSi3Mn1 was selected as the filler material for joining aluminum alloy 6022 and hot-dip galvanized steel in coach peel configuration. The high scanning speed could control the thickness of IMC as thin as 5 µm. The thermal simulations ...
Aluminum alloys are being increasingly applied in the automotive industry as a means to reduce ma... more Aluminum alloys are being increasingly applied in the automotive industry as a means to reduce mass. Their application to the vehicle structure is typically via a combination of either mechanical or fusion joining with adhesive bonding. Correspondingly, there has been a large effort in improving the adhesive bonding characteristics by changing the surface properties using different surface treatment techniques. One such method is the atmospheric arc discharge process which develops a specific surface roughness which can be leveraged to improve adhesive bonding. In this paper the effect of a textured surface by arc discharge on the failure mode and strength of adhesively bonded aluminum alloy sheets is investigated. A single-lap joint configuration is used for simulation and experimental analysis. A two-dimensional (2D) finite element method (FEM) involving the morphology of treated surfaces and using interfacial elements based on a cohesive zone model (CZM) are used to predict the j...
Magnesium is the lightest structural metal; therefore it has been used in a variety of industries... more Magnesium is the lightest structural metal; therefore it has been used in a variety of industries such as automotive, aerospace, electronics and defense. Among different joining processes, laser welding is advantageous due to its low heat input, high depth-to-width weld ratio and good mechanical properties. In this study, a fiber laser is used to weld AZ31B magnesium alloy in a lap joint configuration. Pores were formed in the weld bead that are caused by a thick oxide layer existing on the surface of as-received AZ31B samples. Process parameters including laser power and welding speed are studied to determine their effects on pore formation and maximum lap-shear load of the weld bead. Hardness and tensile tests are carried out to reveal the mechanical properties of the weld bead. A regression analysis is performed that establishes a mathematical relation between the process parameters and the weld maximum lap-shear load. Three regression models are compared in order to determine th...
It is a trend to use high-strength steels in the automobile industry because of their good formab... more It is a trend to use high-strength steels in the automobile industry because of their good formability, weldability, and high strength–volume ratio. In order to achieve quality control, it is necessary to monitor the welding process online. In this paper, acoustic signals generated during the laser welding process of high-strength steel DP980 were recorded and analysed. A microphone was used to acquire the acoustic signals. A spectral subtraction method was used to reduce the noise in the acoustic signals, and a Welch–Bartlett power spectrum density estimation method was used to analyse the frequency characteristics of the acoustic signals. The results indicate that good welds with full penetration (FP) could be clearly distinguished from bad welds with partial penetration (PP). An algorithm based on the different sound pressures between FP and PP was developed to identify the penetration state in the time domain. Another algorithm based on the different frequency characteristics fr...
Rapid Prototyping (RP) is a totally automatic generative manufacturing technique based on a “divi... more Rapid Prototyping (RP) is a totally automatic generative manufacturing technique based on a “divide-and-conquer” strategy called ‘slicing’. Simple slicing used on 2.5-axis kinematics of the existing RP machines is responsible for the staircase error. Although thinner slices will have less error, the slice thickness has practical limits. Visible Slicing overcomes these limitations. A few visible slices exactly represent the object. Each visible slice can be realized using a 3axis kinematics machine from two opposite directions. Visible slicing is implemented on Segmented Object Manufacturing (SOM) machine under development. SOM can produce soft large prototypes faster and cheaper with accuracy comparable to that of CNC machining.
It is well known that the residual stress of the friction-stir-welded joint affects the fatigue s... more It is well known that the residual stress of the friction-stir-welded joint affects the fatigue strength and life, and the corrosion performance. In this paper, a three-dimensional finite element model is established to study the thermal history and stress evolution in the butt welding of aluminium alloy 6061-T6, which will comprehensively provide insight into the friction-stir-welding process. The mechanical action of the shoulder and the thermomechanical effect of the tool (including shoulder and probe) are included in the comprehensive model. The orthogonal stress evolution of the weld is given in this paper. The relationship between the calculated residual stresses of the weld and the process parameters such as the tool traverse and rotational speeds, and the fixture release are investigated and presented.
In-process cleaning of oxides facilitates the high-quality welding and cladding of aluminum alloy... more In-process cleaning of oxides facilitates the high-quality welding and cladding of aluminum alloys when GTAW arc is used for melting aluminum in an open atmosphere. However, in order to understand the mechanism of cathodic cleaning, direct observation is needed. In this work, in order to visualize the physical processes underlying cathodic cleaning, a machine-vision system is developed, and the interaction of the variable-polarity arc with the aluminum surface is captured in real time by a high-speed camera. Surface studies are also performed to assist with the understanding of the oxide cleaning process. Real-time images and surface topography suggest that the cathode spots are responsible for removing oxides from the cathode surface during direct current electrode positive (DCEP) polarity. The cathodically cleaned zone expands over time. However, after the diameter of the cleaned zone reaches a specific value, the rate of its expansion decreases and stops. Unlike cathode spots of ...
International Congress on Applications of Lasers & Electro-Optics, 2012
Advances in Manufacturing
In this investigation, hybrid laser/arc welding (HLAW) was employed to join 8-mm-thick high-stren... more In this investigation, hybrid laser/arc welding (HLAW) was employed to join 8-mm-thick high-strength quenched and tempered steel (HSQTS) plates in the butt- and T-joint configurations. The influences of welding parameters, such as laser power, welding speed, stand-off distance (SD) between the arc of gas metal arc welding, and the laser heat source on the weld quality and mechanical properties of joints, were studied to obtain non-porous and crack-free fully-penetrated welds. The weld microstructure, cross-section, and mechanical properties were evaluated by an optical microscope, and microhardness and tensile tests. In addition, a finite element model was developed to investigate the thermal history and molten pool geometry of the HLAW process to join the HSQTS. The numerical study demonstrated that the SD had a paramount role in good synergy between the heat sources and the stability of the keyhole. For the butt-joint configuration, the results showed that, at a higher welding speed (35 mm/s) and optimum SD between the arc and laser, a fully-penetrated sound weld could be achieved. A non-porous weld in the T-joint configuration was obtained at a lower welding speed (10 mm/s). Microstructural evaluations indicated that the formation of residual austenite and the continuous network of martensitic structure along the grain boundary through the heat affected zone were the primary reasons of the softening behavior of this area. This was confirmed by the sharp hardness reduction and failure behavior of the tensile coupons in this area.
Journal of Manufacturing Science and Engineering
Residual stress (RS) is a major processing issue for selective laser melting (SLM) of metal alloy... more Residual stress (RS) is a major processing issue for selective laser melting (SLM) of metal alloys. Postprocessing by way of heat treatment or hot isostatic pressing is usually required for acceptable mechanical properties. In this work, laser shock peening (LSP) treatment on both SLM and cast aluminum A357 alloys are compared with regard to the development of beneficial near-surface compressive RS. Experiments are conducted using high energy nanosecond pulsed laser, together with a fast photodetector connected to a high-resolution oscilloscope and high-speed camera to identify detailed temporal and spatial laser pulse profiles to improve numerical predictions. Constitutive modeling for SLM A357 alloy is performed using finite element simulation and data obtained from X-ray diffraction (XRD) measurements. Since XRD-RS measurements are accompanied with significant machine-reported error, an effective method is introduced to quantify the material constitutive model uncertainty in term...
The International Journal of Advanced Manufacturing Technology
Robotized laser powder–directed energy deposition is a non-linear process, and the dynamic respon... more Robotized laser powder–directed energy deposition is a non-linear process, and the dynamic response of the system varies layer by layer. An adaptable PI controller with layer-dependent control gains was developed to ensure a constant melt pool width through the entire build. The laser power was selected as the control output variable, and the melt pool width was chosen as the control input variable. The performance of the controller was evaluated through deposition of thin wall samples. The results showed that the controller, by adjusting the laser power in real time, could successfully maintain the melt pool width and produce a more uniform and finer microstructure as compared to the sample with a constant laser power.
The International Journal of Advanced Manufacturing Technology
Laser wire-feed metal additive manufacturing (LWMAM) is a promising additive manufacturing techno... more Laser wire-feed metal additive manufacturing (LWMAM) is a promising additive manufacturing technology that is well suited to build from various materials near net shape mid- to large-size complex components with a high deposition rate and high material usage. It could find applications in diverse industrial sectors such as aerospace, automotive, oil and gas, and rapid tooling. To further gain the acceptance of this technology, the development of process planning software is identified as one of the main challenges. LWMAM has several specific features that complicate the process planning compared to other similar additive manufacturing processes. LWMAM is still in its infancy. In this study, key features involved in the LWMAM process were first identified by building a prismatic block. The corresponding strategies were proposed and tested. Then, a MATLAB-based process planning software was developed that consisted of modules for volume slicing, contour filling, track trimming/elongating, stair-step effect compensation, and post-processing. Free-form contours and a propeller were successfully built by using the developed software. The as-built shapes were measured and compared to the CAD models.
Lasers in Manufacturing and Materials Processing
Wear is a restricting factor for steel in hostile environments but can be abated by laser claddin... more Wear is a restricting factor for steel in hostile environments but can be abated by laser cladding. Laser cladding of steel poses a promising solution to wear resistance. The high power direct diode laser is employed for the laser cladding of thicknesses of about 1.6 mm. Recently, the light rare earth metals have become important additions to smart materials, particularly Lanthanum oxide (La2O3) and Cerium Oxide (CeO2), to improve microhardness and wear resistance. Layers of Ni-WC with 1% and 2% La2O3 and CeO2 as alloying elements were cladded onto A36 steel substrate. The wear resistance and microstructure were studied with XRD and SEM. The hardness to modulus of elasticity ratio (H/E) was used to analyze the wear. It was found that the addition of 1% La2O3 or 1% CeO2 to Nickel-based alloy (40% Ni-60% WC) improves the wear resistance.
The International Journal of Advanced Manufacturing Technology
The corrosion is a major limiting factor for steel in the harsh environments and can be controlle... more The corrosion is a major limiting factor for steel in the harsh environments and can be controlled by depositing protective passive coatings. Nickel-based alloy cladding of steel offers a solution by enabling better wear and corrosion resistance, and high bonding strength. The laser surface cladding of steel was conducted using a high-power direct diode laser. Layers of tungsten carbide in Ni (40% Ni-60% WC) combined with 1 and 2% lanthanum oxide (La2O3) and cerium oxide (CeO2) were deposited on ASTM A36 steel substrate. The X-ray diffraction and scanning electron microscopy were employed to study the morphology, microstructure, and phase evaluation of the cladded layer. Further, anodic polarization on the laser-cladded coupons in 3.5 wt% NaCl shows significant enhancement of the corrosion resistance. The addition of La2O3 and CeO2 improves the corrosion resistance and the hardness of the clad.
The International Journal of Advanced Manufacturing Technology
Heating and cooling cycles that occur during the fusion joining process generate a significant am... more Heating and cooling cycles that occur during the fusion joining process generate a significant amount of thermally caused residual stress that negatively influence the integrity and dimensional accuracy of the welded structures. The level of residual stress around the welding region was as large as the yield stress of the weld and was believed to have created an unwanted permanent deformation. In this paper, a three-dimensional thermo-metallurgical-mechanical finite element method (FEM), using the computational modeling software SYSWELD, was introduced to numerically study the residual stress fields and the distortion of thick welded plates of high-strength quenched and tempered steel (HSQTS) that were caused by hybrid laser/arc welding process in butt-joint configuration. The precision of thermal analysis results was verified prior to conducting mechanical analysis. The verified cooling curves obtained from the thermal simulation were acquired to predict the microstructure of the welding region. Employing the X-ray diffraction method (XRD), the field of residual stress was measured in order to validate the accuracy of the mechanical analysis. The results showed the numerically predicted temperature contours to be in a good agreement with the weld cross section as well as the temperature histories recorded by thermocouples. The microstructural evolution of the welding region revealed a relatively good consistency with the predicted phases based on the continuous cooling transformation (CCT) diagram and the numerically simulated cooling curves. It was shown that experimental and numerical results of the residual stress field and distortion matched well with a large distortion theory.
The International Journal of Advanced Manufacturing Technology
A robotized laser/wire directed energy deposition (RLW-DED) process has been employed, specifical... more A robotized laser/wire directed energy deposition (RLW-DED) process has been employed, specifically in applications where higher deposition rates or larger buildup envelopes are needed. However, this process might have limitations in printing certain complex shape parts. Fabricating parts with overhang sections, depending on the geometry, might cause a collision between the laser head and the buildup. Part segmentation and joining the elements back together has been presented to overcome those limitations. In this study, the welding of additively manufactured parts by RLW-DED has been proposed. Autogenous laser welding, performed at the same setup used for RLW-DED, was utilized to join the thin-walled 316LSi DED parts. Mechanical and microstructural testing were then performed on the welded samples. The results showed that the mechanical properties of welded DED parts are comparable with those of DED parts. Furthermore, a component of complex shape was fabricated to show the capability of the developed process. Therefore, the welding of RLW-DED parts can expand the application of 3D-printed parts in industry.
The International Journal of Advanced Manufacturing Technology
During the laser cladding process, thermal stresses are induced because of the high-energy input,... more During the laser cladding process, thermal stresses are induced because of the high-energy input, high temperature gradient, fast cooling rate, and inconsistency of the clad-substrate material. The induced thermal stresses not only increase the crack tendency, but also influence the mechanical performance of the deposited layer. In this study, a three dimensional (3D) uncoupled thermo-mechanical finite element (FE) model was established to simulate the stress evolution of laser cladding of cobalt-based coatings on mild steel A36. The temperature field was simulated first and then used as transient thermal loading to simulate the stress evolution. Stress distributions for three cases: single track on a flat substrate, double-track on a flat substrate, and double-track on a cylindrical substrate, were investigated in detail. To check the accuracy of the simulation results, validation experiments were carried out using an 8-kW high-power direct diode laser. The thermocouples were used to monitor the temperature cycles at several marked points. The cross-sections of single and double tracks on a flat substrate obtained experimentally were compared with the simulation results. The residual stress on the clad was experimentally determined by an X-ray diffraction machine. The experimentally obtained data showed a significant consistency with the prediction results.
Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science
Friction stir welding (FSW) is a solid-state jointing technology, in which the butted plates are ... more Friction stir welding (FSW) is a solid-state jointing technology, in which the butted plates are heated, plasticized and jointed locally by the plunged probe and shoulder moving along the joint line. The residual stresses due to the thermomechanical performance of the material and the constraint of the welded plates by the fixture are one of main concerns for this process. A prediction of the clamping force applied on the plates during FSW is expected to be helpful in controlling the residual stresses and weld quality. Furthermore, the prediction of the force history in FSW will be beneficial to understand the mechanics of the process and to provide valid models for controlling the process, especially in the case of robotic FSW. In this paper, a three-dimensional model based on a finite element method is proposed to study the thermal history and stress distribution in the weld and, subsequently, to compute mechanical forces in the longitudinal, lateral and vertical directions. The p...
Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture
This paper presents a new deposition process for directly building cylindrical parts of the 5356 ... more This paper presents a new deposition process for directly building cylindrical parts of the 5356 aluminium alloy with variable polarity gas tungsten arc welding (VPGTAW). The relationship between the geometric sizes of the deposited layer and the welding parameters is investigated. A machine vision sensor is used to monitor and control the arc length that is a key welding parameter in the achievement of a uniform deposition. By optimizing the depositing speed and the thickness of the depositing layer, there is no need for a cooling system to cool the part. Three-dimensional parts with different wall thicknesses and different shapes are successfully obtained. The surfaces of the deposited parts are smooth and uniform.
The quality of laser welded-brazed (LWB) joints were strongly dependent on the main process param... more The quality of laser welded-brazed (LWB) joints were strongly dependent on the main process parameters, therefore the effect of laser power (3.2–4 kW), welding speed (60–80 mm/s) and wire feed rate (70–90 mm/s) on mechanical strength and surface roughness were investigated in this study. The comprehensive optimization process by means of response surface methodology (RSM) and desirability function was used for multi-criteria optimization. The experiments were planned based on Box– Behnken design implementing linear and quadratic polynomial equations for predicting the desired output properties. Finally, validation experiments were conducted on an optimized process condition which exhibited good agreement between the predicted and experimental results. AlSi3Mn1 was selected as the filler material for joining aluminum alloy 6022 and hot-dip galvanized steel in coach peel configuration. The high scanning speed could control the thickness of IMC as thin as 5 µm. The thermal simulations ...
Aluminum alloys are being increasingly applied in the automotive industry as a means to reduce ma... more Aluminum alloys are being increasingly applied in the automotive industry as a means to reduce mass. Their application to the vehicle structure is typically via a combination of either mechanical or fusion joining with adhesive bonding. Correspondingly, there has been a large effort in improving the adhesive bonding characteristics by changing the surface properties using different surface treatment techniques. One such method is the atmospheric arc discharge process which develops a specific surface roughness which can be leveraged to improve adhesive bonding. In this paper the effect of a textured surface by arc discharge on the failure mode and strength of adhesively bonded aluminum alloy sheets is investigated. A single-lap joint configuration is used for simulation and experimental analysis. A two-dimensional (2D) finite element method (FEM) involving the morphology of treated surfaces and using interfacial elements based on a cohesive zone model (CZM) are used to predict the j...
Magnesium is the lightest structural metal; therefore it has been used in a variety of industries... more Magnesium is the lightest structural metal; therefore it has been used in a variety of industries such as automotive, aerospace, electronics and defense. Among different joining processes, laser welding is advantageous due to its low heat input, high depth-to-width weld ratio and good mechanical properties. In this study, a fiber laser is used to weld AZ31B magnesium alloy in a lap joint configuration. Pores were formed in the weld bead that are caused by a thick oxide layer existing on the surface of as-received AZ31B samples. Process parameters including laser power and welding speed are studied to determine their effects on pore formation and maximum lap-shear load of the weld bead. Hardness and tensile tests are carried out to reveal the mechanical properties of the weld bead. A regression analysis is performed that establishes a mathematical relation between the process parameters and the weld maximum lap-shear load. Three regression models are compared in order to determine th...
It is a trend to use high-strength steels in the automobile industry because of their good formab... more It is a trend to use high-strength steels in the automobile industry because of their good formability, weldability, and high strength–volume ratio. In order to achieve quality control, it is necessary to monitor the welding process online. In this paper, acoustic signals generated during the laser welding process of high-strength steel DP980 were recorded and analysed. A microphone was used to acquire the acoustic signals. A spectral subtraction method was used to reduce the noise in the acoustic signals, and a Welch–Bartlett power spectrum density estimation method was used to analyse the frequency characteristics of the acoustic signals. The results indicate that good welds with full penetration (FP) could be clearly distinguished from bad welds with partial penetration (PP). An algorithm based on the different sound pressures between FP and PP was developed to identify the penetration state in the time domain. Another algorithm based on the different frequency characteristics fr...
Rapid Prototyping (RP) is a totally automatic generative manufacturing technique based on a “divi... more Rapid Prototyping (RP) is a totally automatic generative manufacturing technique based on a “divide-and-conquer” strategy called ‘slicing’. Simple slicing used on 2.5-axis kinematics of the existing RP machines is responsible for the staircase error. Although thinner slices will have less error, the slice thickness has practical limits. Visible Slicing overcomes these limitations. A few visible slices exactly represent the object. Each visible slice can be realized using a 3axis kinematics machine from two opposite directions. Visible slicing is implemented on Segmented Object Manufacturing (SOM) machine under development. SOM can produce soft large prototypes faster and cheaper with accuracy comparable to that of CNC machining.
It is well known that the residual stress of the friction-stir-welded joint affects the fatigue s... more It is well known that the residual stress of the friction-stir-welded joint affects the fatigue strength and life, and the corrosion performance. In this paper, a three-dimensional finite element model is established to study the thermal history and stress evolution in the butt welding of aluminium alloy 6061-T6, which will comprehensively provide insight into the friction-stir-welding process. The mechanical action of the shoulder and the thermomechanical effect of the tool (including shoulder and probe) are included in the comprehensive model. The orthogonal stress evolution of the weld is given in this paper. The relationship between the calculated residual stresses of the weld and the process parameters such as the tool traverse and rotational speeds, and the fixture release are investigated and presented.
In-process cleaning of oxides facilitates the high-quality welding and cladding of aluminum alloy... more In-process cleaning of oxides facilitates the high-quality welding and cladding of aluminum alloys when GTAW arc is used for melting aluminum in an open atmosphere. However, in order to understand the mechanism of cathodic cleaning, direct observation is needed. In this work, in order to visualize the physical processes underlying cathodic cleaning, a machine-vision system is developed, and the interaction of the variable-polarity arc with the aluminum surface is captured in real time by a high-speed camera. Surface studies are also performed to assist with the understanding of the oxide cleaning process. Real-time images and surface topography suggest that the cathode spots are responsible for removing oxides from the cathode surface during direct current electrode positive (DCEP) polarity. The cathodically cleaned zone expands over time. However, after the diameter of the cleaned zone reaches a specific value, the rate of its expansion decreases and stops. Unlike cathode spots of ...
International Congress on Applications of Lasers & Electro-Optics, 2012
Advances in Manufacturing
In this investigation, hybrid laser/arc welding (HLAW) was employed to join 8-mm-thick high-stren... more In this investigation, hybrid laser/arc welding (HLAW) was employed to join 8-mm-thick high-strength quenched and tempered steel (HSQTS) plates in the butt- and T-joint configurations. The influences of welding parameters, such as laser power, welding speed, stand-off distance (SD) between the arc of gas metal arc welding, and the laser heat source on the weld quality and mechanical properties of joints, were studied to obtain non-porous and crack-free fully-penetrated welds. The weld microstructure, cross-section, and mechanical properties were evaluated by an optical microscope, and microhardness and tensile tests. In addition, a finite element model was developed to investigate the thermal history and molten pool geometry of the HLAW process to join the HSQTS. The numerical study demonstrated that the SD had a paramount role in good synergy between the heat sources and the stability of the keyhole. For the butt-joint configuration, the results showed that, at a higher welding speed (35 mm/s) and optimum SD between the arc and laser, a fully-penetrated sound weld could be achieved. A non-porous weld in the T-joint configuration was obtained at a lower welding speed (10 mm/s). Microstructural evaluations indicated that the formation of residual austenite and the continuous network of martensitic structure along the grain boundary through the heat affected zone were the primary reasons of the softening behavior of this area. This was confirmed by the sharp hardness reduction and failure behavior of the tensile coupons in this area.
Journal of Manufacturing Science and Engineering
Residual stress (RS) is a major processing issue for selective laser melting (SLM) of metal alloy... more Residual stress (RS) is a major processing issue for selective laser melting (SLM) of metal alloys. Postprocessing by way of heat treatment or hot isostatic pressing is usually required for acceptable mechanical properties. In this work, laser shock peening (LSP) treatment on both SLM and cast aluminum A357 alloys are compared with regard to the development of beneficial near-surface compressive RS. Experiments are conducted using high energy nanosecond pulsed laser, together with a fast photodetector connected to a high-resolution oscilloscope and high-speed camera to identify detailed temporal and spatial laser pulse profiles to improve numerical predictions. Constitutive modeling for SLM A357 alloy is performed using finite element simulation and data obtained from X-ray diffraction (XRD) measurements. Since XRD-RS measurements are accompanied with significant machine-reported error, an effective method is introduced to quantify the material constitutive model uncertainty in term...
The International Journal of Advanced Manufacturing Technology
Robotized laser powder–directed energy deposition is a non-linear process, and the dynamic respon... more Robotized laser powder–directed energy deposition is a non-linear process, and the dynamic response of the system varies layer by layer. An adaptable PI controller with layer-dependent control gains was developed to ensure a constant melt pool width through the entire build. The laser power was selected as the control output variable, and the melt pool width was chosen as the control input variable. The performance of the controller was evaluated through deposition of thin wall samples. The results showed that the controller, by adjusting the laser power in real time, could successfully maintain the melt pool width and produce a more uniform and finer microstructure as compared to the sample with a constant laser power.
The International Journal of Advanced Manufacturing Technology
Laser wire-feed metal additive manufacturing (LWMAM) is a promising additive manufacturing techno... more Laser wire-feed metal additive manufacturing (LWMAM) is a promising additive manufacturing technology that is well suited to build from various materials near net shape mid- to large-size complex components with a high deposition rate and high material usage. It could find applications in diverse industrial sectors such as aerospace, automotive, oil and gas, and rapid tooling. To further gain the acceptance of this technology, the development of process planning software is identified as one of the main challenges. LWMAM has several specific features that complicate the process planning compared to other similar additive manufacturing processes. LWMAM is still in its infancy. In this study, key features involved in the LWMAM process were first identified by building a prismatic block. The corresponding strategies were proposed and tested. Then, a MATLAB-based process planning software was developed that consisted of modules for volume slicing, contour filling, track trimming/elongating, stair-step effect compensation, and post-processing. Free-form contours and a propeller were successfully built by using the developed software. The as-built shapes were measured and compared to the CAD models.
Lasers in Manufacturing and Materials Processing
Wear is a restricting factor for steel in hostile environments but can be abated by laser claddin... more Wear is a restricting factor for steel in hostile environments but can be abated by laser cladding. Laser cladding of steel poses a promising solution to wear resistance. The high power direct diode laser is employed for the laser cladding of thicknesses of about 1.6 mm. Recently, the light rare earth metals have become important additions to smart materials, particularly Lanthanum oxide (La2O3) and Cerium Oxide (CeO2), to improve microhardness and wear resistance. Layers of Ni-WC with 1% and 2% La2O3 and CeO2 as alloying elements were cladded onto A36 steel substrate. The wear resistance and microstructure were studied with XRD and SEM. The hardness to modulus of elasticity ratio (H/E) was used to analyze the wear. It was found that the addition of 1% La2O3 or 1% CeO2 to Nickel-based alloy (40% Ni-60% WC) improves the wear resistance.
The International Journal of Advanced Manufacturing Technology
The corrosion is a major limiting factor for steel in the harsh environments and can be controlle... more The corrosion is a major limiting factor for steel in the harsh environments and can be controlled by depositing protective passive coatings. Nickel-based alloy cladding of steel offers a solution by enabling better wear and corrosion resistance, and high bonding strength. The laser surface cladding of steel was conducted using a high-power direct diode laser. Layers of tungsten carbide in Ni (40% Ni-60% WC) combined with 1 and 2% lanthanum oxide (La2O3) and cerium oxide (CeO2) were deposited on ASTM A36 steel substrate. The X-ray diffraction and scanning electron microscopy were employed to study the morphology, microstructure, and phase evaluation of the cladded layer. Further, anodic polarization on the laser-cladded coupons in 3.5 wt% NaCl shows significant enhancement of the corrosion resistance. The addition of La2O3 and CeO2 improves the corrosion resistance and the hardness of the clad.
The International Journal of Advanced Manufacturing Technology
Heating and cooling cycles that occur during the fusion joining process generate a significant am... more Heating and cooling cycles that occur during the fusion joining process generate a significant amount of thermally caused residual stress that negatively influence the integrity and dimensional accuracy of the welded structures. The level of residual stress around the welding region was as large as the yield stress of the weld and was believed to have created an unwanted permanent deformation. In this paper, a three-dimensional thermo-metallurgical-mechanical finite element method (FEM), using the computational modeling software SYSWELD, was introduced to numerically study the residual stress fields and the distortion of thick welded plates of high-strength quenched and tempered steel (HSQTS) that were caused by hybrid laser/arc welding process in butt-joint configuration. The precision of thermal analysis results was verified prior to conducting mechanical analysis. The verified cooling curves obtained from the thermal simulation were acquired to predict the microstructure of the welding region. Employing the X-ray diffraction method (XRD), the field of residual stress was measured in order to validate the accuracy of the mechanical analysis. The results showed the numerically predicted temperature contours to be in a good agreement with the weld cross section as well as the temperature histories recorded by thermocouples. The microstructural evolution of the welding region revealed a relatively good consistency with the predicted phases based on the continuous cooling transformation (CCT) diagram and the numerically simulated cooling curves. It was shown that experimental and numerical results of the residual stress field and distortion matched well with a large distortion theory.
The International Journal of Advanced Manufacturing Technology
A robotized laser/wire directed energy deposition (RLW-DED) process has been employed, specifical... more A robotized laser/wire directed energy deposition (RLW-DED) process has been employed, specifically in applications where higher deposition rates or larger buildup envelopes are needed. However, this process might have limitations in printing certain complex shape parts. Fabricating parts with overhang sections, depending on the geometry, might cause a collision between the laser head and the buildup. Part segmentation and joining the elements back together has been presented to overcome those limitations. In this study, the welding of additively manufactured parts by RLW-DED has been proposed. Autogenous laser welding, performed at the same setup used for RLW-DED, was utilized to join the thin-walled 316LSi DED parts. Mechanical and microstructural testing were then performed on the welded samples. The results showed that the mechanical properties of welded DED parts are comparable with those of DED parts. Furthermore, a component of complex shape was fabricated to show the capability of the developed process. Therefore, the welding of RLW-DED parts can expand the application of 3D-printed parts in industry.
The International Journal of Advanced Manufacturing Technology
During the laser cladding process, thermal stresses are induced because of the high-energy input,... more During the laser cladding process, thermal stresses are induced because of the high-energy input, high temperature gradient, fast cooling rate, and inconsistency of the clad-substrate material. The induced thermal stresses not only increase the crack tendency, but also influence the mechanical performance of the deposited layer. In this study, a three dimensional (3D) uncoupled thermo-mechanical finite element (FE) model was established to simulate the stress evolution of laser cladding of cobalt-based coatings on mild steel A36. The temperature field was simulated first and then used as transient thermal loading to simulate the stress evolution. Stress distributions for three cases: single track on a flat substrate, double-track on a flat substrate, and double-track on a cylindrical substrate, were investigated in detail. To check the accuracy of the simulation results, validation experiments were carried out using an 8-kW high-power direct diode laser. The thermocouples were used to monitor the temperature cycles at several marked points. The cross-sections of single and double tracks on a flat substrate obtained experimentally were compared with the simulation results. The residual stress on the clad was experimentally determined by an X-ray diffraction machine. The experimentally obtained data showed a significant consistency with the prediction results.
Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science
Friction stir welding (FSW) is a solid-state jointing technology, in which the butted plates are ... more Friction stir welding (FSW) is a solid-state jointing technology, in which the butted plates are heated, plasticized and jointed locally by the plunged probe and shoulder moving along the joint line. The residual stresses due to the thermomechanical performance of the material and the constraint of the welded plates by the fixture are one of main concerns for this process. A prediction of the clamping force applied on the plates during FSW is expected to be helpful in controlling the residual stresses and weld quality. Furthermore, the prediction of the force history in FSW will be beneficial to understand the mechanics of the process and to provide valid models for controlling the process, especially in the case of robotic FSW. In this paper, a three-dimensional model based on a finite element method is proposed to study the thermal history and stress distribution in the weld and, subsequently, to compute mechanical forces in the longitudinal, lateral and vertical directions. The p...
Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture
This paper presents a new deposition process for directly building cylindrical parts of the 5356 ... more This paper presents a new deposition process for directly building cylindrical parts of the 5356 aluminium alloy with variable polarity gas tungsten arc welding (VPGTAW). The relationship between the geometric sizes of the deposited layer and the welding parameters is investigated. A machine vision sensor is used to monitor and control the arc length that is a key welding parameter in the achievement of a uniform deposition. By optimizing the depositing speed and the thickness of the depositing layer, there is no need for a cooling system to cool the part. Three-dimensional parts with different wall thicknesses and different shapes are successfully obtained. The surfaces of the deposited parts are smooth and uniform.