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Showing papers on "Residual stress published in 2015"


Journal ArticleDOI
TL;DR: In this article, the Inconel-718 alloys were manufactured by selective laser melting (SLM) with 2×2mm 2, 3×3mm 2, 5×5mm 2 and 7×7mm 2 island scanning strategies.
Abstract: Inconel-718 has received an extensive using in mold industry. The selective laser melting (SLM) is providing an ideal means for manufacturing mold insert with complex geometrical features and internal architecture. During the manufacturing of high quality mold inserts with conformal cooling channel, the parameters play a vital role in the SLM process. In the study, the Inconel-718 alloys were manufactured by SLM with 2×2 mm 2 , 3×3 mm 2 , 5×5 mm 2 , and 7×7 mm 2 island scanning strategies. The microstructure, mechanical property, and residual stress were investigated by optical microscope, tensile test and Vickers micro-indentation, respectively. It can be found that the relative density increased with enlarging the island size; the results on the microstructure indicated that the cracks and more pores were detected in the 22-specimen; whilst the microstructures of all specimens were composed of fine dendritic grains, cellular, and columnar structures; the tensile testing suggested that the ultimate tensile strength and yield strength of all samples was similar; while the outcome of the residual stress showed that the value of residual stress was ranked in the following sequence: 22-specimen

367 citations


Journal ArticleDOI
TL;DR: In this paper, the surface convection generated by the deposition process is measured and implemented in the thermo-mechanical model and three depositions with different geometries and dwell times are used to validate the model using in situ measurements of the temperature and deflection as well as post-process measurements of residual stress.
Abstract: A thermo-mechanical model of directed energy deposition additive manufacturing of Ti–6Al–4V is developed using measurements of the surface convection generated by gasses flowing during the deposition. In directed energy deposition, material is injected into a melt pool that is traversed to fill in a cross-section of a part, building it layer-by-layer. This creates large thermal gradients that generate plastic deformation and residual stresses. Finite element analysis (FEA) is often used to study these phenomena using simple assumptions of the surface convection. This work proposes that a detailed knowledge of the surface heat transfer is required to produce more accurate FEA results. The surface convection generated by the deposition process is measured and implemented in the thermo-mechanical model. Three depositions with different geometries and dwell times are used to validate the model using in situ measurements of the temperature and deflection as well as post-process measurements of the residual stress. An additional model is developed using the assumption of free convection on all surfaces. The results show that a measurement-based convection model is required to produce accurate simulation results.

359 citations


Journal ArticleDOI
TL;DR: In this article, the fracture toughness and fatigue crack growth rate properties of selective laser melted (SLM) specimens produced from grade 5 Ti6Al4V powder metal have been investigated and three specimen orientations relative to the build direction as well as two different post-build heat treatments were considered.
Abstract: The fracture toughness (K 1c ) and fatigue crack growth rate (FCGR) properties of selective laser melted (SLM) specimens produced from grade 5 Ti6Al4V powder metal has been investigated. Three specimen orientations relative to the build direction as well as two different post-build heat treatments were considered. Specimens and test procedures were designed in accordance with ASTM E399 and ASTM E647 standard. The results show that there is a strong influence of post-build processing (heat treated versus ‘as built’) as well as specimen orientation on the dynamic behaviour of SLM produced Ti6Al4V. The greatest improvement in properties after heat treatment was demonstrated when the fracture plane is perpendicular to the SLM build direction. This behaviour is attributed to the higher anticipated influence of tensile residual stress for this orientation. The transformation of the initial rapidly solidified microstructure during heat treatment has a smaller beneficial effect on improving mechanical properties.

263 citations


Journal ArticleDOI
TL;DR: In this article, an X-ray diffraction technique and numerical simulation were used for investigating the residual stress in SLM samples fabricated from stainless steel 316L and Ti6Al4V alloy.
Abstract: Selective laser melting (SLM) has great potential in additive manufacturing because it enables the production of full-density complex parts with the desired inner structure and surface morphology. High temperature gradients as a result of the locally concentrated energy input lead to residual stresses, crack formation and part deformation during processing or after separation from the supports and the substrate. In this study, an X-ray diffraction technique and numerical simulation were used for investigating the residual stress in SLM samples fabricated from stainless steel 316L and Ti6Al4V alloy. Conclusions regarding directions and values of stresses in SLM objects are given.

259 citations


Journal ArticleDOI
TL;DR: In this article, the residual stress profiles were mapped using neutron diffraction in two simple prism builds of Inconel 718: one fabricated with electron beam melting (EBM) and the other with direct laser metal sintering.
Abstract: Residual stress profiles were mapped using neutron diffraction in two simple prism builds of Inconel 718: one fabricated with electron beam melting (EBM) and the other with direct laser metal sintering. Spatially indexed stress-free cubes were obtained by electrical discharge machining (EDM) equivalent prisms of similar shape. The (311) interplanar spacings from the EDM sectioned sample were compared to the interplanar spacings calculated to fulfill stress and moment balance. We have shown that applying stress and moment balance is a necessary supplement to the measurements for the stress-free cubes with respect to accurate stress calculations in additively manufactured components. In addition, our work has shown that residual stresses in electron beam melted parts are much smaller than that of direct laser metal sintered parts most likely due to the powder preheating step in the EBM process.

219 citations


Journal ArticleDOI
01 Oct 2015
TL;DR: In this paper, a finite element model is developed for predicting the thermo-mechanical response of Ti-6Al-4V during electron beam deposition, and both in situ distortion and post-process residual stress measurements suggest that stress relaxation occurs during the deposition of a single-bead wide, 16-layer-high wall built for model validation.
Abstract: In this work, a finite element model is developed for predicting the thermo-mechanical response of Ti-6Al-4V during electron beam deposition. A three-dimensional thermo-elasto-plastic analysis is performed to model distortion and residual stress in the workpiece and experimental in situ temperature, and distortion measurements are performed during the deposition of a single-bead-wide, 16-layer-high wall built for model validation. Post-process blind hole–drilling residual stress measurements are also performed. Both the in situ distortion and post-process residual stress measurements suggest that stress relaxation occurs during the deposition of Ti-6Al-4V. A method of accounting for such stress relaxation in thermo-elasto-plastic simulations is proposed where both stress and plastic strain are reset to 0, when the temperature exceeds a prescribed stress relaxation temperature. Inverse simulation is used to determine the values of the absorption efficiency and the emissivity of electron beam–deposited, wir...

213 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present an experimental investigation on the material properties and residual stress distributions of cold-formed high strength steel hollow sections, which are composed of 9 square hollow sections and 6 circular hollow sections.

142 citations


Journal ArticleDOI
TL;DR: In this article, a computational modeling approach to simulate residual stress formation during the electron beam melting (EBM) process within the additive manufacturing (AM) technologies for Inconel 718 is presented.
Abstract: A computational modeling approach to simulate residual stress formation during the electron beam melting (EBM) process within the additive manufacturing (AM) technologies for Inconel 718 is presented in this paper. The EBM process has demonstrated a high potential to fabricate components with complex geometries, but the resulting components are influenced by the thermal cycles observed during the manufacturing process. When processing nickel based superalloys, very high temperatures (approx. 1000 °C) are observed in the powder bed, base plate, and build. These high temperatures, when combined with substrate adherence, can result in warping of the base plate and affect the final component by causing defects. It is important to have an understanding of the thermo-mechanical response of the entire system, that is, its mechanical behavior towards thermal loading occurring during the EBM process prior to manufacturing a component. Therefore, computational models to predict the response of the system during the EBM process will aid in eliminating the undesired process conditions, a priori, in order to fabricate the optimum component. Such a comprehensive computational modeling approach is demonstrated to analyze warping of the base plate, stress and plastic strain accumulation within the material, and thermal cycles in the system during different stages of the EBM process.

141 citations


Journal ArticleDOI
TL;DR: In this paper, a thermal analysis of 316L stainless steel sheets during pulsed Nd-YAG laser-beam welding was carried out using three different heat sources employing SYSWELD.

137 citations


Journal ArticleDOI
TL;DR: In this article, the Anchorless Selective Laser Melting (ASLM) methodology is detailed and investigations into processing of a low temperature eutectic Al-Si binary casting alloy is explored.
Abstract: Rapid melt pool formation and solidification during the metal powder bed process Selective Laser Melting (SLM) generates large thermal gradients that can in turn lead to increased residual stress formation within a component. Metal anchors or supports are required to be built in-situ and forcibly hold SLM structures in place and minimise geometric distortion/warpage as a result of this thermal residual stress. Anchors are often costly, difficult and time consuming to remove and limit the geometric freedom of this Additive Manufacturing (AM) process. A novel method known as Anchorless Selective Laser Melting (ASLM) maintains processed material within a stress relieved state throughout the duration of a build. As a result metal components formed using ASLM do not require support structures or anchors. ASLM locally melts two or more powdered materials that alloy under the action of the laser and can form into various combinations of eutectic/hypo/hyper eutectic alloys with a new lower solidification temperature. This new alloy is maintained in a semi-solid or stress reduced state throughout the build with the assistance of elevated powder bed pre-heating. In this paper the ASLM methodology is detailed and investigations into processing of a low temperature eutectic Al-Si binary casting alloy is explored. Two types of Al powders were compared; pre-alloyed AlSi12 and elemental mix Al + 12 wt% Si. The study established an understanding of the laser in-situ alloying process and confirmed successful alloy formation within the process. Differential thermal analysis, microscopy and X-Ray diffraction were used to further understand the nature of alloying within the process. Residual stress reduction was observed within ASLM processed elemental Al + Si12 and geometries produced without the requirement for anchors.

133 citations


Journal ArticleDOI
TL;DR: In this paper, a CPM 9V steel powder has been deposited on H13 tool steel plate for repairing the die surface damage using a CW CO2 laser in conjunction with powder injection system.

Journal ArticleDOI
TL;DR: In this article, a numerical model of the multi-layer titanium-titanium nitride (Ti-TiN) multilayer has been formulated with multi-physics FEM, to find the optimal thickness of individual layers in a multi layer that can decrease interfacial axial and in-plane shear stress.

Journal ArticleDOI
TL;DR: In this article, the authors explore a promising new technique that potentially overcomes these issues based on nanoindentation testing of micro-pillars produced by focused ion beam milling of the films.
Abstract: The fracture toughness of thin ceramic films is an important material property that plays a role in determining the in-service mechanical performance and adhesion of this important class of engineering materials. Unfortunately, measurement of thin film fracture toughness is affected by influences from the substrate and the large residual stresses that can exist in the films. In this paper, we explore a promising new technique that potentially overcomes these issues based on nanoindentation testing of micro-pillars produced by focused ion beam milling of the films. By making the pillar diameter approximately equal to its length, the residual stress in the upper portion of the pillar is almost fully relaxed, and when indented with a sharp Berkovich indenter, the pillars fracture by splitting at reproducible loads that are readily quantified by a sudden displacement excursion in the load displacement behaviour. Cohesive finite element simulations are used for analysis and development of a simple relationship...

Journal ArticleDOI
TL;DR: In this article, a laser-aided direct metal deposition (DMD) was used to form a AISI 4340 steel coating on an aISI 4140 steel substrate and the defect density and microstructural property of the DMD coating were analyzed.

Journal ArticleDOI
TL;DR: In this article, the residual stress distribution induced by laser shock processing in a 2050-T8 aeronautical aluminium alloy with both X-ray diffraction measurements and 3D finite element simulation is investigated.

Journal ArticleDOI
TL;DR: In this paper, the authors evaluated fracture toughness and fatigue crack growth characteristics in selective laser-melted titanium 6Al-4V materials as a follow-on to a previous study on high cycle fatigue.
Abstract: Experimental investigation was conducted to evaluate the fracture toughness and fatigue crack growth characteristics in selective laser-melted titanium 6Al-4 V materials as a follow-on to a previous study on high cycle fatigue. For both the fracture toughness and crack growth evaluation, the compact tension specimen geometry was used. It was found that the fracture toughness was lower than what would be expected from wrought or cast product forms in the same alloy. This was attributed to the rapidly cooled, martensitic microstructure, developed in the parts. At low stress ratios, the crack growth rates were faster than in wrought titanium but became comparable at higher ratios. The fracture toughness appears to be higher when the crack is oriented perpendicular to the build layers. The difference in the average threshold and critical stress intensity values for the crack growth results for the three orientations was within the scatter of the data, so there was essentially no difference. The same was true for the empirically derived Paris Law constants. Residual stresses were likely to have overshadowed any variation in crack growth because of microstructural directionalities associated with build orientation.

Journal ArticleDOI
TL;DR: In this paper, a laser-engineered net shaping (LENS) based additive manufacturing technique was used for one step manufacturing of bimetallic structures of stainless steel and Ti6Al4V (Ti64) alloy.
Abstract: Laser Engineered Net Shaping (LENS™) is a commercially available additive manufacturing technique that was used for one step manufacturing of bimetallic structures of stainless steel and Ti6Al4V (Ti64) alloy. Two approaches were adopted for manufacturing these bimetallic structures. In the first approach, direct deposition of Ti64 on SS410 substrate and compositionally graded bimetallic structures were attempted without any intermediate bond layer. In the second approach, an intermediate NiCr bond layer (of thickness ∼750 μm) was deposited to minimize thermal and residual stresses for these bimetallic structures. Direct deposition of Ti64 was successful only for a couple of layers before the structures were delaminated. Compositionally graded bonding was unsuccessful with the formation of brittle intermetallics and related residual stresses causing delamination. Using an intermediate NiCr layer, bimetallic structures were successfully fabricated. Our work is focused on LENS™ based processing approach and related microstructural evolution towards bimetallic structures.

Journal ArticleDOI
TL;DR: In this article, an approach of optimizing the profile and magnitude of residual stress is proposed by analyzing the effects of depth of cut on the redistribution of residual stresses, and the results demonstrate that the distortion of thin-walled part is significantly affected by the residual stress generated after the material is cut away.

Journal ArticleDOI
TL;DR: In this article, a three dimensional finite element (FE) model was developed to simulate residual stress induced for laser cladding of AISI 4340 steel powder onto a similar substrate material.

Journal ArticleDOI
TL;DR: In this paper, the correlation between machining-induced residual stresses and distortion in a thin-walled work pieces is investigated, and the effect of work pieces thickness on distortion is investigated.
Abstract: One of the main problems in the machining of thin-walled parts made of high-strength aluminum alloys is distortion and dimensional instability after machining, which leads to an increase in scraped parts and the cost of production. In general, distortion and dimensional instability in machined parts made of aluminum alloy is a function of the residual stresses. In this article, the correlation between machining-induced residual stresses and distortion in a thin-walled work pieces is investigated. Several experiments are carried out under different machining conditions using two carbide and polycrystalline diamond (PCD) tools on a thin-walled cylinder made of AL7075-T6 alloy. Rates of variations in geometrical tolerances and distortion for all work pieces are measured. To study the effect of mechanical and thermal loads on the residual stresses and distortion, the machining force and temperature of cutting area are measured. Finally, the correlation between the residual stress and distortion is studied by measurement of residual stress on some work pieces. Similarly, in order to investigate the effect of work pieces thickness on distortion, several tests are carried out on three work pieces with different thicknesses. The results indicate that the force and temperature have direct effect on the residual stresses and distortion in the thin-walled parts.

Journal ArticleDOI
TL;DR: In this paper, the effect of severe plastic deformation induced by multiple sliding impacts by the specimen surface produced at ultrasonic impact treatment (UIT) on the stress-controlled fatigue response of the powder metallurgy Ti-6Al-4V alloy is studied.
Abstract: The effect of severe plastic deformation induced by multiple sliding impacts by the specimen surface produced at ultrasonic impact treatment (UIT) on the stress-controlled fatigue response of the powder metallurgy Ti–6Al–4V alloy is studied in this paper. Specimens of Ti–6Al–4V alloy were produced from Ti hydride precursor powders via the cost-effective blended elemental powder metallurgy technique. Structure investigations were performed by XRD, TEM, LM and SEM techniques. After UIT, fatigue strength was increased by about 60% on the base of 107 cycles, and lifetime was prolonged by two orders of magnitude at applied stress amplitudes of 300–400 MPa. The UIT process leads to approx. four times decrease in the surface roughness parameters. Increased by 65 and 20% microhardeness magnitudes are respectively registered on the top surface and on the depth of 100 μm. The UIT induced compressive stresses achieve about two thirds of the alloy yield stress. The hardness increase is shown to be coupled with the increased dislocation density, essentially refined of α+β microstructure and with randomization in α-grains orientations. Observations of cross-sections of the UIT processed specimens revealed the pores free near-surface layer of approx. 200 μm thick, which is formed thanks to micro-pore closure process promoted by high shear strains produced at the UIT induced sliding impacts. Analysis of fracture surfaces revealed subsurface cracks initiations and numerous fracture steps indicating on the cracks branching and deflection in the surface layers of the UIT processed specimens instead of superficial crack initiation and the grain boundary cleavages mainly observed in the pristine samples. Experimentally registered magnitudes of fatigue limit were successfully predicted by accounting for the effective stress intensity factor range ΔKth and observed by TEM microstructural units responsible for fatigue fracture (α-phase colonies, α-grains or micro-pores) and compared with literature data on PM Ti–6Al–4V alloy. Enhanced fatigue strength and prolonged lifetime of PM Ti–6Al–4V alloy after the UIT process are concluded to be associated with (i) minimized surface roughness; (ii) compressive residual stresses; (iii) UFG and nano-scale α+β microstructure; and (iv) micro-pore healing.

Journal ArticleDOI
TL;DR: In this article, a recast layer formed in Inconel alloy 718 (IN718) as a result of laser shock peening without an ablative layer was characterized and compared with surface condition of a sample peened with a protective overlay.

Journal ArticleDOI
TL;DR: In this article, the non-uniform steep residual stress profile arising from FOD of laser peened aerofoil leading edges varies as a function of fatigue crack growth under high cycle fatigue and mixed high and low cycle fatigue conditions.

Journal ArticleDOI
TL;DR: In this paper, the influence of tool nose radius on the residual stress distribution developed in Inconel 718 by finish turning is investigated, and the results show that an increase in tool nose diameter results in; increased radial cutting forces, increased microstructural deformation depth, higher near surface tensile stresses (up to 1550 MPa with a worn tool), and deeper tensile and compressive residual stress distributions.

Journal ArticleDOI
TL;DR: In this article, the effect of a significant parameter of LSP, the advancing direction of the laser scanning pattern, on the induced residual stress fields and the fatigue life of stainless steel 316L samples using experiments and 3D finite element analysis was investigated.

Journal ArticleDOI
TL;DR: In this article, a three-dimensional thermo-viscoelastic model to simulate the residual stress in composite laminates during curing was proposed using the differential constitutive law.

Journal ArticleDOI
TL;DR: In this paper, the effect of welding heat input and postweld natural aging on residual stress, microstructure, and precipitation distribution in different zones of dissimilar friction stir welding of 8mm thick plates of AA6082-T6 and AA7075 -T6 was investigated.

Journal ArticleDOI
TL;DR: In this article, the tendency of Co-based WC+NiCr composite layers to cracking during the laser cladding process was investigated using neutron diffraction and elucidate the correlation between the residual stress and the cracking in three types of cylindrical specimens.

Journal ArticleDOI
TL;DR: In this article, the effect of blank initial residual stress on component deformation was investigated by finite element simulation and experiment, and it was shown that the initial residual stresses in the blank was the main factor of deformation for three-frame monolithic beam, and the coupling action of the residual stress and machining-induced residual stresses aggravated the deformation.
Abstract: The principal influence factors on the monolithic component deformation were investigated by finite element simulation simulation and experiment. Initial residual stress of the blank, machining-induced residual stress, and coupling action of these two effect factors were considered. To study the effect of blank initial residual stress on component deformation, chemical milling was used to remove the machining-induced residual stress on the machined surface of the components. The research results show that the initial residual stress in the blank was the main factor of deformation for three-frame monolithic beam, and the coupling action of the initial residual stress and machining-induced residual stresses aggravated the deformation. The deformation caused by machining residual stress accounted for about 10 % of the total deformation of the component, and the deformation caused by the blank initial residual stress accounted for 90 % of the total deformation of the component. The finite element simulation results were compared with experimental results and found to be in good agreement.

Journal ArticleDOI
TL;DR: In this article, a detailed Raman line mapping near the edges of a monolayer graphene flake that is simply supported onto an epoxy-based photoresist (SU8)/poly(methyl methacrylate) matrix at steps as small as 100 nm is presented.
Abstract: The stress transfer mechanism from a polymer substrate to a nanoinclusion, such as a graphene flake, is of extreme interest for the production of effective nanocomposites. Previous work conducted mainly at the micron scale has shown that the intrinsic mechanism of stress transfer is shear at the interface. However, since the interfacial shear takes its maximum value at the very edge of the nanoinclusion it is of extreme interest to assess the effect of edge integrity upon axial stress transfer at the submicron scale. Here, we conduct a detailed Raman line mapping near the edges of a monolayer graphene flake that is simply supported onto an epoxy-based photoresist (SU8)/poly(methyl methacrylate) matrix at steps as small as 100 nm. We show for the first time that the distribution of axial strain (stress) along the flake deviates somewhat from the classical shear-lag prediction for a region of ∼2 μm from the edge. This behavior is mainly attributed to the presence of residual stresses, unintentional doping, ...