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 terms of a joint probability mass function. Conventionally, most constitutive behavior research for LSP involves deterministic material modeling. Predicted RS using deterministic approaches fail to reflect real-world variations in the materials, laser treatment, or RS measurements. A discretized Bayesian inference is used to quantify the rate-dependent plasticity material model parameters as a joint probability function. RS are then characterized as random fields, which provides far greater insight into the practical ability to attain desired residual stresses. Moreover, for identical LSP treatments, it is determined that the material models are significantly different for the SLM and the conventional cast A357 aluminum alloys, resulting in much lower magnitude of compressive RS in the SLM alloy.

Injection molding of plastic optical lenses prevails over many other techniques in both efficiency and cost; however, polymer shrinkage during cooling, high level of uneven residual stresses, and refractive index variations have limited its potential use for high precision lens fabrication. In this research, we adopted a newly developed strong graphene network to both plano and convex fused silica mold surfaces and proposed a novel injection molding with graphene-coated fused silica molds. This advanced injection molding process was implemented in the molding of polymer-based plano-concave lenses resulting in reduced polymer shrinkage. In addition, internal residual stresses and refractive index variations were also analyzed and discussed in detail. Meanwhile, as a comparison of conventional injection mold material, aluminum mold inserts with the same shape and size were also diamond machined and then employed to mold the same plano-concave lenses. Finally, a simulation model using moldex3d was utilized to interpret stress distributions of both graphene and aluminum molds and then validated by experiments. The comparison between graphene-coated mold and aluminum mold reveals that the novel injection molding with carbide-bonded graphene-coated fused silica mold inserts is capable of molding high-quality optical lenses with much less shrinkage and residual stresses with a more uniform refractive index distribution.

Prediction of residual stresses induced by any additive layer manufacturing process greatly helps in preventing thermal cracking and distortion formed in the substrate and deposition material. This paper presents the development of a model for the prediction of residual stresses using three-dimensional finite element simulation (3D-FES) and their experimental validation in a single-track and double-track deposition of Ti-6Al-4V powder on AISI 4130 substrate by the microplasma transferred arc (µ-PTA) powder deposition process. It involved 3D-FES of the temperature distribution and thermal cycles that were validated experimentally using three K-type thermocouples mounted along the deposition direction. Temperature distribution, thermal cycles, and residual stresses are predicted in terms of the µ-PTA process parameters and temperature-dependent properties of substrate and deposition materials. Influence of a number of deposition tracks on the residual stresses is also studied. Results reveal that (i) tensile residual stress is higher at the bonding between the deposition and substrate and attains a minimum value at the midpoint of a deposition track; (ii) maximum tensile residual stress occurs in the substrate material at its interface with deposition track. This primarily causes distortion and thermal cracks; (iii) maximum compressive residual stress occurs approximately at mid-height of the substrate material; and (iv) deposition of a subsequent track relieves tensile residual stress induced by the previously deposited track.

Additive manufacturing (or three-dimensional (3D) printing) is constantly growing as an innovative process for the production of complex-shape components. Among the seven recognized 3D printing technologies, fused deposition modeling (FDM) covers a very important role, not only for producing representative 3D models, but, mainly due to the development of innovative material like Peek and Ultem, also for realizing structurally functional components. However, being FDM a production process involving high thermal gradients, non-negligible deformations and residual stresses may affect the 3D printed component. In this work we focus on meso/macroscopic simulations of the FDM process using abaqus software. After describing in detail the methodological process, we investigate the impact of several parameters and modeling choices (e.g., mesh size, material model, time-step size) on simulation outcomes and we validate the obtained results with experimental measurements.

Thermal history and residual stresses in dissimilar friction stir welding (FSW) of AA2024 and AZ31 were studied under different tool offsets using a coupled Eulerian–Lagrangian (CEL) finite element model and a mechanical model. Welding experiments and residual stresses' measurements were conducted to validate the models. Comparisons between the experimental and numerical results indicated good agreement. The maximum temperature in the welded zone was predicted to be slightly lower than 400 °C, regardless of offset, and that its location shifted with tool offset from the advancing side (AS) to the retreating side (RS). Longitudinal residual stresses changed from tensile under the tool shoulder to compressive beyond this region and it appeared to be the dominant stress component. The transverse stresses were tensile and of lower magnitude. Both the longitudinal and transverse residual stresses have their maximum values within the weld zone near the end of the weld length. For both peak temperatures and residual stresses, higher values were obtained at the AS with no tool offset and 1 mm offset to the AS, and at the RS with 1 mm offset to the RS. Lower residual stresses and better weld quality were obtained with tool offset to the aluminum side.

Residual stress, characteristic of surface integrity, is a great issue in cutting process for its significant effects on fatigue life and dimension stability of the machined parts. From a practical viewpoint, residual stress is generated in a dynamic tool-part engagement process, instead of a process with nominal cutting loads. This is the challenge that we have to handle, so as to achieve better predictive methods than the previously recorded approaches in literatures which ignore the dynamic effects on residual stress. This paper presents an analytical method for the prediction of residual stress in dynamic orthogonal cutting. A mechanistic model of the dynamic orthogonal cutting is provided, considering the indentation effect of the cutting edge during the wave-on-wave cutting process. Following the calculation of plastic strains by incremental analysis in mechanical loading, analytical solution of the residual stress due to distributed plastic strains in half-plane is obtained based on inclusion theory. Without relaxation procedures, the two-dimensional (2D) distribution of residual stress in dynamic cutting process is predicted for the first time. A delicately designed dynamic orthogonal cutting experiment is realized through numerical control (NC) lathe. The periodic residual stress distribution is predicted using the proposed approach, which is then validated against the X-ray diffraction measurements.

Finite element modeling (FEM) of machining-induced residual stresses (RS) takes place over two consecutive steps: a cutting step and a relaxation step. In the latter, the workpiece is left to cool down after deactivating all external loads. The current work focuses on the relaxation step, and how different strain components, material plasticity, and workpiece edge deflections affect the final state of different RS components. First, a two-dimensional arbitrary-Lagrangian–Eulerian (ALE) plane strain thermomechanical explicit model was used to simulate dry orthogonal cutting. After that, the relaxation process was modeled using three approaches: (1) the classical approach, (2) a new approach that is first presented here, and (3) a modified approach that was developed earlier by the current author. In the classical approach, the same exact machined workpiece is relaxed, considering all stress/strain components and material plasticity. On the other hand, the new approach uses a pure elastic one-dimensional thermal relaxation model, in the cutting direction, and assumes that the workpiece edges normal to the cutting direction remain so. The differences between the RS predicted by the new and classical approaches reflected the combined effects of the examined parameters. The role of each parameter was isolated using three different versions of the modified approach. The current findings confirmed that for orthogonal cutting, the stress relaxation process can be considered as a one-dimensional pure elastic thermal relaxation process. Also, the workpiece edges normal to the cutting direction deflect during relaxation, contributing to the final state of RS.

The use of magnesium (Mg) alloy has been continuously on the rise with numerous expanded application in transportation/aerospace industries due to their lightweight and other areas, such as biodegradable medical implants. It was shown recently that machining can be used to improve the functional performance of Mg-based products/components, such as corrosion resistance, through engineered surface integrity. In this paper, the behavior of AZ31B Mg alloy in cryogenic machining was discussed firstly. The surface integrity can be significantly improved by introducing the ultrafine grained (UFG) layer due to the severe plastic deformation (SPD) effect during cryogenic machining. The mechanisms of microstructure evolution and plastic deformation were analyzed based on the experimental findings in literature. A physics-based constitutive model involving material plasticity and grain refinement is developed based on both slip and twinning mechanisms and successfully implemented in a finite-element (FE) analysis with multiple cutting passes to predict the microstructure evolution by nanocrystalline grain refinement and other improvement of the surface integrity in the cryogenic machining of AZ31B Mg alloy. With a more quantitative assessment, the FE model results are further discussed for grain refinement, changes in microhardness, residual stresses, and slip/twinning mechanism with the apparent SPD taking place due to rapid cryogenic cooling.

Thermal shrinkage of the added material can distort the manufactured part and generate residual stresses. Experiments are carried out on growing the beams of rectangular cross section. The beams bend with formation of a concave top surface. The distortion is characterized by the curvature radius. The curvature radius significantly increases with the beam height, however, its variation with the layer thickness is within the experimental uncertainty. The proposed mathematical model assumes sequential addition of thermally expanded elastic layers. It explains the experiments and indicates the existence of finite limits for the stress and the deformation fields and the curvature radius at small layer thickness. The proposed model can be applied to predict residual stresses and deformations arising in complicated parts.

Hard turning is becoming increasingly considered by industry as a potential substitute for grinding. However, it is greatly hurdled by surface integrity problems such as tensile residual stress and white layer, which are generally found to have negative effects on the stress corrosion, wear resistance, and fatigue life of the machined parts. This paper investigates white layer formation and morphology in hard turning process using various process parameters, taking into account the effects of heat treatment which results in microstructure and hardness differences on bulk materials. Samples undergone three typical heat treatment processes are prepared and then machined using different cutting speeds and radial feed rates. Optical microscope, scanning electron microscope (SEM), and X-ray diffraction (XRD) are employed to analyze the microstructures of white layer and bulk materials after varies heat treatments and cutting processes. Through the studies, we find the existence of a cutting speed threshold, below which no white layer forms for both the low and medium-temperature tempering. The threshold value increases; however, the white layer thickness decreases under the same cutting conditions, for the low and medium-temperature tempering, respectively. Also, we find that the white layer thickness and the scattering of it along the cutting direction on the surface increases with cutting speed and radial feed rate. White layer with wavy morphology can be found in samples after quenching at high cutting speed. We first discover that the pitch of the white layer with wavy morphology is similar to the displacement of tool at the time a segment of the serrated chips forms. Also, the surface residual stresses of the samples are measured. Relationship between white layer and residual stresses is presented. Based on the relationship we reveal that high temperature is more dominant than volume expansion for white layer formation.

Gas nitriding is a common surface treatment practice to improve the wear resistance of AISI H13 hot extrusion die cavities. However, due to the presence of complex and sharp features of die cavities, it has been observed that nonuniform nitride layer develops in these regions. Moreover, the formation of compound layer in the surface vicinity of nitrided surfaces leads to the development of transformation-induced stresses. The present work presents the application of the phase field method in predicting the evolution of the nitride layers and associated residual stresses during the gas nitriding of AISI H13 tool steels. Nitriding process is modeled and simulated in line with experimental setup, which uses automated two-stage controlled nitriding process. Some representative samples having commonly used geometric features are manufactured and nitrided for validation purpose. Both experimental and numerical results are found in close agreement in terms of nitrogen concentration and corresponding microhardness profiles. The results show that high stresses are induced at the surface due to formation of the nitride layers, and these stresses are found to be higher at the sharp corners. In view of the current results, some process and design strategies are suggested for improved and more effective nitriding treatment of hot extrusion dies used in the industry.

In this paper, residual stresses due to indentation and rolling of a rigid cylinder on a finite plate at a very high rolling load with a relative peak pressure of 22 are examined by two-dimensional plane strain finite element analyses using abaqus for the first time. In the finite element analyses, the roller is modeled as rigid and has frictionless contact with the finite plate. The geometry of the finite plate and its boundary conditions are assigned to correspond to those of fillet rolling of crankshafts with the constraint in the rolling direction. Finite element analyses with different meshes for single indentation on an elastic flat plate under plane strain conditions are first carried out, and the results are benchmarked with those of the elastic Hertzian solutions to establish the requirement of the finite element meshes for acceptable numerical results. The results show that the accuracy of computational results is limited by the discretization of the finite element analysis by a plot of the contact width as a function of the load. For accurate peak pressure, a total of at least eight linear elements are needed. Finite element analyses with different meshes for single indentation on an elastic–plastic flat plate under plane strain conditions are then carried out. The plate material is modeled as an elastic–plastic power-law strain hardening material with a nonlinear kinematic hardening rule for loading and unloading. The computational results are compared to establish the requirement of the finite element meshes for acceptable numerical results within 4 mm distance to the rolling surface for the crankshaft fatigue analyses. The computational results for rolling at the relative peak pressure of 22 show that the symmetric Hertzian or modified Hertzian pressure distribution should not be used to represent the contact pressure distribution for rolling simulation, while the computational results for rolling at the relative peak pressure of 5 show that the symmetric Hertzian or modified Hertzian pressure distribution may be used to represent the contact pressure distribution for rolling simulation. The computational results for the rolling case also show a significantly higher longitudinal compressive residual stress and a lower out-of-plane compressive residual stress along the contact surface when compared to those for the single indentation case. The results suggest that the effects of rolling must be accounted for when two-dimensional finite element analyses of crankshaft sections are used to investigate the residual stresses due to fillet rolling of the crankshafts under the prescribed roller loads. Due to the boundary conditions of the finite plate, the compressive residual stresses are larger when compared to those when the boundary conditions of the finite plate are fully relaxed.

Laser cladding (LC) is a material deposition technique, in which a laser beam is used to deposit one or several layers of a certain clad material onto a substrate to improve its wear or corrosion resistance. It can also be used for structural repair. Consequently, it is of interest to characterize the residual stresses and the microstructure along with the clad geometry as a function of process parameters. A 100 W fiber laser and focusing optics capable of producing very small spot sizes (∼10 μm) have been integrated with a micromachining center. This paper focuses on providing a comprehensive metallurgical and mechanical characterization of microscale LC of preplaced powdered mixture of cobalt and titanium on IS 2062 (ASTM A36) substrate. Parametric studies were conducted by varying the scanning velocity, laser power, and spot size to produce clad layers well bonded to the substrate. The results show that the width and height of the cladding increases up to 28% and 36%, respectively, due to the variation in the laser parameters. An increase of up to 85% in the microhardness is observed in the cladded layer with presence of Ti–Co intermetallic compounds at the interface, highlighting the application of the process in improving subsurface properties of existing components. The residual stresses obtained in the cladded layer are compressive in nature, indicating the potential application of this technique for repair of structures. In addition, a finite element model has been developed for predicting the clad geometry using a moving Gaussian heat source. Molten region is determined from the thermal model and Tanner's law has been used to account for spreading of the molten layer to accurately predict the clad geometry. The model predicts clad geometry with reasonable prediction errors less than 10% for most cases with stronger dependence on scan velocities in comparison to laser power.

A finite element (FE) approach is developed to investigate the laser spot welding (LSW) of flat-plate solar absorbers and the stress and distortion fields that develop after fabrication and during operation. Numerical calculations at two different levels are carried out. At a microscopic scale, the details of a spot weld are analyzed. At a macroscopic level, a global approach is used to simulate the joining of the pipeline to the absorber plate and the “restoration” (flattening) process of the absorber. The simulated welding-induced distortion is compared with experimental measurements. The thermomechanical behavior of a solar absorber under working conditions is also studied and operational stresses and the critical locations for structural failure are reported.

Cracks and delamination, resulting from residual stresses, are a barrier in the world of additive manufacturing and selective laser melting (SLM) that prohibits the use of many metals in this field. By preheating the baseplate, thermal gradients are lowered and stresses can be reduced. In this work, some initial tests were performed with M2 high speed steel (HSS). The influence of preheating on density and mechanical and physical properties is investigated. The paper shows many promising results for the production of SLM parts in materials that are very sensitive to crack formation and delamination. When using a preheating of 200 °C, crack-free M2 HSS parts were produced with a relative density of 99.8%.

Aero-engine manufacturers are continuously striving to improve component performance and reliability while seeking to increase the efficiency of manufacturing to reduce costs. Efficiency gains by using higher rates of material removal, however, can be counter-productive if they give rise to surface anomalies that distort the material microstructure and reduce the resistance of the material to fatigue crack nucleation. This paper investigates the effect of hole making processes and parameters on surface integrity and the initiation of cracks from low-cycle fatigue (LCF). It reports the dependence of elevated temperature (600 °C) low-cycle fatigue performance of nickel alloy RR1000 from surfaces produced from hole making and subsequent surface conditioning. As-machined surfaces include a reference “damage-free” surface, and two distorted microstructures: (i) a white layer, produced to a depth of 5 and 10 μm and (ii) a distorted gamma prime (γ') structure, produced to a depth of 10 and 15 μm. The effect of shot peening damage-free and 10 μm deep white layer surfaces was also evaluated. It was found that the presence of white layer significantly reduced fatigue performance compared with that shown by the damage-free surface, regardless of whether the white layer was subsequently shot peened or not. In contrast, surfaces showing distorted γ' structures produced much less debit in fatigue life and only from a depth of 15 μm. These results have been rationalized from an examination of fracture surfaces and from measurement of residual stresses before and after fatigue testing. This research is of particular importance as it is among the few reports that quantify the effect of different levels of work piece surface integrity on the fatigue life of a nickel-based superalloy that has been developed for critical rotating components in aero-engine applications.

This paper presents the state of surface integrity produced on hardened-high strength 41Cr4 steel after hard machining and finish ball burnishing. Surfaces machined by sequential machining processes were characterized using 2D and 3D surface roughness parameters. Moreover, detailed functionality of the generated surfaces was performed using a set of 3D functional roughness parameters. Among the characteristics of the surface layer, its microstructure, the distribution of microhardness and the residual stresses were determined. This investigation confirms that ball burnishing allows producing surfaces with lower surface roughness and better service properties than those generated by cubic boron nitride (CBN) finish hard turning operations.

This research evaluates the fatigue properties of Ti-6Al-4V specimens and components produced by Electron Beam additive manufacturing. It was found that the fatigue performance of specimens produced by additive manufacturing is significantly lower than that of wrought material due to defects such as porosity and surface roughness. However, evaluation of an actual component subjected to design fatigue loads did not result in premature failure as anticipated by specimen testing. Metallography, residual stress, static strength and elongation, fracture toughness, crack growth, and the effect of post processing operations such as machining and peening on fatigue performance were also evaluated.

Residual stresses and refractive index of molded glass lenses are important quality indicators of their optical performance. In this research, the control of residual stresses and refractive index variation of molded glass lenses were experimentally investigated by postmolding annealing. Residual stresses were quantitatively measured using a circular polariscope. Refractive index was reconstructed and calculated by an optical setup based on Mach–Zehnder interferometer. In addition, geometry of the aspherical surface of lens was also evaluated before and after annealing. The comparison between the measured results before and after annealing showed that residual stresses and refractive index variation were well controlled and the shape of the aspherical surface was largely retained. This comprehensive experimental study demonstrated a suggestion to improve quality of the compression molded glass lens by postmolding annealing for high-precision optical applications.

Laser shock peening (LSP) is a surface treatment technique similar to conventional shot peening. The laser induced plasma causes plastic deformations and compressive residual stresses that are useful for developing improved properties in the fields of resistance to fatigue, wear or stress corrosion cracking. The actual distribution of residual stresses is extremely important while designing for improved fatigue life using laser shock peening, as fatigue cracks would initiate from the weakest point in the structure. In this paper, the variations in distribution of residual stresses due to laser shock peening are studied with a focus on two materials, annealed 1053 and hardened 52100 AISI steels. A 3D finite element model was developed to study the actual distributions of the residual stresses due to laser shock peening. The effect of hardness on the distribution of the residual stresses and the presence of tensile residual stresses in the surrounding regions of the impact is analyzed. Much larger variations in the residual stress distributions were observed in case of the 1053 steel as compared to hardened 52100 steel. A comprehensive analysis of the simulation results was performed in order to address and explain this behavior. It was observed that the extent of overlap would also affect the variations in the residual stress distributions. The tensile residual stresses present in the areas surrounding the shocked region were also analyzed based upon the extent of overlap and the hardness of the material. It was observed that the ratio of peak tensile to compressive residual stresses developed in 1053 steel was much higher as compared to that in the hardened 52100 steel.