Search

Laser surface texturing uses a pulsed laser that is scanned on the surface, wherein each pulse creates a micro-crater through material ablation. A variety of textures can be generated depending on the laser parameters and the overlap of the laser spots. This work presents a computational model that can predict the topography of a textured surface produced using a nanosecond pulsed laser. The model involves a multi-physics approach that considers laser ablation with plasma effects and the melt pool's fluid dynamics to obtain the crater profile for a single pulse. The 3D surface profile created from the multi-physics model is mathematically superimposed to mimic the spatial overlapping of multiple pulses. The model predicts surface topography when a laser is scanned along a linear track with successive overlapping tracks. The experiments have confirmed that the proposed model has an accuracy greater than 90% in predicting surface roughness (Sa), as well as volume parameters such as core void volume (Vvc) and valley void volume (Vvv). It was observed that the variation of these surface characteristics is highly non-linear with the process parameters. Furthermore, the model is used to design engineered surfaces to modify friction coefficient, adhesion, and leakage probability. It is demonstrated that the surface parameters for functional requirements can be modified significantly just by varying the overlap of the laser spots in different directions. The proposed model can be used to create textured surfaces for various applications through an appropriate choice of laser parameters and scanning parameters. [ABSTRACT FROM AUTHOR]

Laser assisted mechanical micromachining is a process that utilizes highly localized thermal softening of the material by continuous wave laser irradiation applied simultaneously and directly in front of a miniature cutting tool in order to produce micron scale three-dimensional features in difficult-to-machine materials. The hybrid process is characterized by lower cutting forces and deflections, fewer tool failures, and potentially higher material removal rates. The desktop-sized machine used to implement this process has a finite stiffness and deflects under the influence of the cutting forces. The deflections can be of the same order of magnitude as the depth of cut in some cases, thereby having a negative effect on the dimensional accuracy of the micromachined feature. As a result, selection of the laser and cutting parameters that yield the desired reduction in cutting forces and deflection, and consequently an improvement in dimensional accuracy, requires a reliable cutting force model. This paper describes a cutting force model for the laser-assisted microgrooving process. The model accounts for the effect of elastic deflection of the machine X-Y stages on the forces and accuracy of the micromachined feature. The model combines an existing slip-line field based force model with a finite element based thermal model of laser heating and a constitutive material flow stress model to account for thermal softening. Experiments are carried out on H-13 steel (42 HRC (hardness measured on the Rockwell 'C'scale)) to validate the force model. The effects of process parameters, such as laser power and cutting speed, on the forces are also analyzed. The model captures the effect of thermal softening and indicates a 66% reduction in the shear flow stress at 35 W laser power. The cutting force and depth of cut prediction errors are less than 20% and 10%, respectively, for most of the cases examined. DOI: 10.1115/1.3040076]

Laser directed energy deposition (DED) is one of the most promising additive manufacturing processes for restoring high-value components. The damaged components can have complex free-form shapes, which necessitate depositions with an inclined nozzle, where the gravity can adversely affect the powder flow dynamics and the powder catchment efficiency (PCE). PCE is defined as the fraction of the total mass flowrate entering the melt pool, and a low PCE can render the process inviable. In this paper, the effect of nozzle inclination on the powder flow dynamics and resulting PCEs have been studied. It was found that the powder flow dynamics is altered significantly in an inclined nozzle and results in an asymmetric and skewed powder jet. The PCE deteriorates rapidly with an increase in the nozzle inclination due to the progressive defocusing and falls below 20% at 75 deg. A discrete phase model has been developed to understand the powder flow dynamics at different inclinations and process conditions. The mass flow distribution asymmetries on the focal plane at various nozzle inclinations have been analyzed via the model. The model can predict PCEs at different nozzle inclinations with reasonable accuracy ranging from 5.4% at 0-deg inclination to 29.2% at 45-deg inclination. The carrier gas flow, particle size, and laser diameter affect the PCE significantly and can be used to counter the enhanced powder loss at large nozzle inclinations. Process maps have been developed to identify the favorable, acceptable, and low PCE regions to select optimal DED parameters. [ABSTRACT FROM AUTHOR]

Direct laser metal deposition (DLMD) is a promising additive manufacturing technique which has a huge potential in remanufacturing and restoration of high-value dies/molds and aerospace components. The residual stresses developed in the material deposited via DLMD affect the structural integrity of the restored components. The service life of the restored component will be compromised if tensile residual stresses are present in the deposited layer. The residual stresses originate due to differential thermal expansion/contraction and martensitic transformation-driven volumetric dilation and transformation-induced plasticity. The influence of martensitic transformation and processing conditions on the residual stresses of DLMD-processed components needs to be understood and modeled for sustainable repair. Hence, a finite element model has been developed to capture the coupled effect of thermomechanics and martensitic transformation on the evolution of residual stresses in DLMD. In this study, the individual and coupled effects of strains due to volume dilation and transformation-induced plasticity on residual stress evolution have been analyzed for the deposition of crucible particle metallurgy (CPM) 9 V on H-13 tool steel. The finite element model has been experimentally validated using X-ray and neutron diffractions. The inclusion of both transformation strains in the residual stress decreases the prediction errors of peak tensile residual stress from ~48% to ~15%. The fully coupled thermomechanical and metallurgical model has been used to obtain a critical linear mass density (m2/v) corresponding to the onset of a fully compressive longitudinal residual stress state in the deposited layer to ensure sustainable repair. [ABSTRACT FROM AUTHOR]

Nanopolishing processes are used in medical, industrial, telecommunication, optics, and military fields. Hydrodynamic polishing (HDP) is one of the prominent nanopolishing methods in creating nanopolished surfaces on hard and profiled surfaces, where rigid tool-based methods like diamond turning, grinding, and honing have many limitations. This work is focused on modeling of hydrodynamic polishing method. In this method, a film of abrasive suspension is formed between the work-piece surface and a rotating soft tool, which helps in nanopolishing. The past experimental research gives an insight into the process but the process has not been explicitly modeled. Consequently, besides experimental characterization, a numerical/mathematical model of hydrodynamic polishing process is important. This paper presents a model of the HDP process which takes into account the polishing process variables, such as, contact load, spindle speed, tool and work-piece material properties/geometry, and abrasive suspension properties. The response of the model is the pressure distribution and the abrasive film thickness in the polishing zone. To model the elastohydrodynamic process encountered in HDP, the pressure and the film thickness profiles of lubricated isothermal point contacts have been evaluated using the multilevel multi-integration (MLMI) scheme coded in C programming language. Finally, load, tool stiffness, speed, and particle concentration in the suspension have been implicitly correlated to the surface roughness (SR) to evolve a semi-empirical model for surface roughness as a function of mean film thickness and mean pressure. Empirical models for mean film thickness and mean pressure have also been developed as a function of process variables. These models have been developed from a Taguchi L27 orthogonal array wherein the mean pressure/film thickness values have been determined from the model and the average surface roughness values have been measured experimentally. It has been observed that the load does not affect the surface roughness significantly and mean pressure does not change with the change in abrasive size and spindle speed. Abrasive particle concentration has been found to be the most important parameter and it affects the surface roughness significantly. [ABSTRACT FROM AUTHOR]