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Abstract Selective laser melting, due to the high energy density input and the small interaction time (106 K/s), can result in an ultrafine microstructure and excellent mechanical properties. However, due to the nonuniform nature of the temperature distribution and the transition from liquid to solid, high residual stresses exist. High residual stresses in the parts can increase the risk of material distortion and cause many problems, such as dimensional inaccuracy or cracks. This work systematically investigated the influence of annealing on the microstructure and mechanical properties of SLM-processed AlSi7Mg alloy parts. The residual stresses relaxed significantly after annealing. The Vickers hardness and the tensile stress greatly reduced, while the elongation increased. The fracture mode of the as-fabricated sample was ductile and brittle mixed fracture, whereas the fracture morphology of the annealed sample presented many more dimples, and the elongation also increased, which indicated the ductile mode. [ABSTRACT FROM AUTHOR]

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. The use of high temperature pre-heating during an SLM build can assist in reducing thermal gradients, enable a more controlled cooling with the possibility to control/tailor as-built mechanical properties. In this work a high temperature SLM powder bed capable of pre-heating to 800 °C is used during processing of Ti6Al4V feedstock. The effect of powder bed temperature on residual stress formation, microstructure and mechanical properties was investigated. It was found that increasing the bed temperature to 570 °C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of α ′ martensitic microstructure into an equilibrium α+β microstructure. At 570 °C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively. [ABSTRACT FROM AUTHOR]

Abstract: Space effect of applied stresses on cracking was investigated by examining the influence of the stress gradient around an embedded particle on the cracking and fracture of the glass. On the other hand, the effect of the stress time was investigated by studying (1) the delayed cracking and fracture after unloading and (2) the effect of loading rate on the cracking and fracture. It is discovered that particles smaller than 0.2mm in diameter cause low risk of spontaneous breakage to tempered glass. Fracture strength but also the breakage risk increase with residual stresses on the glass surface. The results obtained from ball-on-ring tests indicate that the strength of glass would increase greatly with the loading rate, which confirmed the time dependence of the fracture in glass. [Copyright &y& Elsevier]

Abstract: The tensile behavior of a two-dimensional woven carbon/silicon carbide composite has been characterized both by monotonic and loading/unloading tests. The composite shows markedly non-linear behavior with evident hysteresis loops, appreciable residual strains and continuously decreasing moduli. Both the loop width and residual strain have a continuous acceleration with applied stress, implying the composite failure precedes matrix cracking saturation. The shape of hysteresis loops is characterized by an initial parabolic region followed by a linear region. The preexisting matrix microcracks in as-processed specimens and multiple microcracks accompanied by extensive fiber pull-out in the failed specimens were observed, indicating large thermal residual stresses and relatively weak interfacial bonding exist in the present composite, respectively. Then the interfacial sliding stress and residual stress were quantitatively derived from the hysteresis analysis, and further used to predict matrix cracking stress and stress–strain behavior. Excellent agreement between predictions and experimental results was obtained. [Copyright &y& Elsevier]

Abstract: Deep induction hardening has been performed on two batches of smooth cylindrical specimens with a hardening depth respectively around 2mm and 3mm. The distributions of axial and circumferential residual stresses are analysed for the two specimen batches by X-ray diffraction technique. The radial normal stress field is estimated through the use of the well known Moore and Evans correction. Finally, the experimental residual stresses are compared with those obtained from a multiphysic finite element modelling of the whole induction treatment process, including electromagnetic, thermal, metallurgical and mechanical phenomena. The simulated residual stress field is in good agreement with X-ray analysis especially at depths lower than one-tenth the specimen diameter. At deeper depths, a correction of the experimental X-ray analysis has been done to obtain realistic values. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to investigate the effect of uniaxial tension on the residual stress of cold-drawn pearlitic steel wire through X-ray diffraction in combination with layer removal technique. It is found that an extra 0.9% plastic strain is introduced to the steel wire due to the uniaxial tension using a universal electronic tensile machine. The XRD results indicate that uniaxial tension after drawing has dominate influence on the decrease of surface tensile residual stress and the corresponding stress gradient in the drawn wires. [Copyright &y& Elsevier]

Abstract: Case-hardened gear wheels, e.g., in vehicle transmissions are cyclically most stressed components. Their lifetime is determined by the interaction of loading, tooth flank load-capacity and tooth foot load-capacity. The latter is influenced by a multitude of parameters like mean loading or residual stress state appearing during operation. To evaluate the tooth-foot load-capacity, the knowledge of the mean stress and residual stress sensitivity of the fatigue strength of the material conditions at the sites critical for failure is necessary. The examinations on hand concerning means stress sensitivity and residual stress sensitivity usually deal with homogeneous materials. Systematic analyses for case-hardened conditions are missing. In this work case-hardened notched specimens with different residual stress states are investigated under cyclic bending. The results are analyzed using a local concept and an additional fracture mechanical approach. With the local concept it was possible to determine the crack initiation site which was partly at the surface and partly below the surface. The fracture mechanical approach was necessary to understand the crack stop behavior and to determine the site critical for failure. An uniform description of the lifetime behavior of unpeened and shot peened specimen could be achieved using a modified Haigh diagram which takes into account the local stress state at the site critical for failure. [Copyright &y& Elsevier]

Abstract: The application of shot-peening to improve performance and reduce scatter in the high cycle fatigue regime is common in aerospace production practice. Frequently, aerospace components are re-shot-peened during refit to “restore” compressive stresses that may have redistributed in service. Since shot-peening is an impact process that can cause various surface artifacts, it is not clear what effect this re-shot-peening process has on fatigue properties. In the current study, the effects of shot-peening and re-shot-peening on the profile of surface residual stress and the four-point bend fatigue behavior of Ti–6Al–4V(wt.%) alloy were investigated at room temperature and 150°C. A step-test method was used to determine the fatigue strength of the shot-peened and re-shot-peened specimens in different conditions. Shot-peening improved the fatigue strength of the alloy from about 65% σ y to 71% σ y (σ y is the yield strength of this alloy at room temperature). The fatigue limits of the shot-peened Ti–6Al–4V alloy at room temperature and 150°C were found to be almost identical. Residual stresses did not seem to redistribute significantly under the fatigue loading at room temperature and 150°C. Both shot-peening and re-shot-peening significantly enhanced the fatigue strength over that of the un-shot-peened alloy. No negative effects of re-shot-peening were observed. [Copyright &y& Elsevier]

Abstract: Ultrasonic cold forging technology (UCFT) utilizing ultrasonic vibration energy is a method to induce severe plastic deformation to a material surface, therefore, the structure of the material surface becomes a nano-crystal structure from the surface to a certain depth. It improves the mechanical properties; hardness, compressive residual stress, wear and fatigue characteristics. Applying UCFT to a rolling process in the steel industry is introduced in this study. First, the UCFT specimens of a tool steel (SKD-61/equivalent H13) are prepared and tested to verify the effects of the UCFT in a variety of mechanical properties, the UCFT is applied to the trimming knives in a cold rolling process. It has been determined that UCFT improves the mechanical properties effectively and becomes a practical method to improve productivity and reliability by about two times compared with the conventionally treated tooling in the trimming process in a cold rolling line. [Copyright &y& Elsevier]

Abstract: The effects of fatigue and fretting fatigue on the distribution of residual stresses in shot and laser shock peened Ti–6Al–4V samples have been investigated. Residual elastic strains have been determined using high-energy synchrotron X-ray diffraction. Laser shock peening introduces a considerable compressive residual stress, the compressive zone extending 1.5mm below the surface. The effects of fatigue loading have been investigated using a notched three-point bend geometry. The residual stress field was found to be largely insensitive to fatigue cycling, at least for the applied stress range studied. For fretting fatigue, while the residual stresses at depth were little affected, within 0.5mm of the surface significant stress relaxation was observed; the extent of relaxation being greatest in the direction parallel to the fretting direction. The states of residual stress have been quantified using the concept of eigenstrain, which quantifies the retained plastic misfit resulting from peening. Finite element modeling has been used to determine the eigenstrain profiles causing the measured elastic strain profiles, and the changes to these eigenstrain profiles due to fretting. Our results suggest laser shock peening confers much greater fretting fatigue resistance than traditional shot peening alone due to the much deeper compressive zone. [Copyright &y& Elsevier]

Abstract: Thermal residual stress analysis and failure simulation of particle-reinforced composites are performed based on thermomechanics and the finite element method (FEM) to investigate the effects of particles on strength and toughness. It is discovered that the residual stresses in matrix are controlled mainly by adjacent particles via stress superposition, and an expression for the residual stress is derived. The size of the residual stress zone in the matrix is found to increase with the particle size. The FEM failure simulation in uniaxial tension reveals two features: (i) the residual stress caused by the second phase particles in ceramic matrix would lead to strength degradation due to the weakest link theory; (ii) the second phase particles in the composite may enhance the fracture toughness because of the crack arrest and crack-deflection caused by residual compressive stress. These features have been confirmed by experimental results of TiN/Si3N4 composite. [Copyright &y& Elsevier]

Abstract: A unique approach introducing sialon polytypoids as a functionally graded material (FGM) bonding has been used to join silicon nitride and alumina. The various multilayered FGM samples ranging from 3 to 20 layers were sintered to fabricate a crack-free joining of heterogeneous ceramics. To calculate thermal stresses for the various multilayered FGM samples, the finite element analysis program (FEAP) was used. These analyses results matched experimental results and showed why some samples had large residual stresses that resulted in fracture. Moreover, the electron probe X-ray microanalysis (EPMA) from a crack-free FGM sample had a smooth concentration profile, which verifies the interface diffusion during sintering at each graded layer and confirms a successful joining. [Copyright &y& Elsevier]

Abstract: TiPdNi thin films with less than 2μm thickness were produced using ion beam assisted deposition (IBAD) on heated and unheated substrates. Films deposited on unheated substrates were found to be amorphous, and subsequently annealed to induce crystallization. Residual stresses in the films were evaluated using the Stoney equation after deposition, and after annealing. Films deposited using IBAD on unheated substrates were found to have slight compressive stress (−22.4MPa) while films deposited on heated substrates had a moderate tensile stress (176.2MPa). Annealed films experienced extensive tensile stress (598.3MPa), resulting in film failure. [Copyright &y& Elsevier]

Abstract: Commercially pure titanium was sandblasted with SiO2 particles of 200–300μm in diameter. It was found that the sandblasted samples exhibited an increase in fatigue strength by 11% over that of the untreated samples. The peak subsurface compressive residual stress produced by sandblasting was measured by XRD to be around 480MPa. Three distinct regions were observed in the sandblasted samples, namely the severely deformed surface layer, the region deformed mainly by twinning, and the substrate. After recovery treatment below 300°C, the surface layer of the sandblasted samples was transformed into a nano-crystalline structure, and its corrosion resistance was significantly improved. [Copyright &y& Elsevier]

Abstract: A novel fracture mechanics technique has been employed for the determination of crack growth rate and threshold stress intensity factor (K Iscc) for stress corrosion cracking (SCC) using small circumferential notch tensile (CNT) specimens. The technique was applied successfully for testing SCC susceptibility of spheroidal graphite (SG) cast iron in 5M NaOH at 100 and 120°C. Crack growth rate of SG cast iron in 5M NaOH solution at 100 and 120°C has been determined at different stress intensity factors (K I), and the K Iscc have been determined to be 11.2 and 9MPam1/2, respectively. The surfaces of fractured specimens have been examined by scanning electron microscopy (SEM) in order to establish intergranular propagation of stress corrosion cracks. CNT testing is a simple, relatively fast and cost-advantageous approach for generating crack growth rate and K Iscc data. [Copyright &y& Elsevier]

Abstract: The present paper addresses the accuracy of the reconstruction of depth profiles from X-ray diffraction lattice-strain analysis, combined with successive sublayer removal. In order to test the accuracy of reconstruction irrespective of experimental inaccuracies, X-ray diffraction was simulated for model stress-depth profiles and/or composition-depth profiles, reflecting those obtainable with surface engineering of materials, i.e. shot peening, carburizing of austenite and low temperature nitriding of stainless steel. Two principally different methods for the reconstruction of the actual stress and composition profiles were compared: [(I)] assuming that the lattice parameter determined at a specific depth, for a specific value for ψ is a weighted average over the actual lattice spacing profile for this ψ-direction; [(II)] assuming that the stress/strain determined at a specific depth is a weighted average over the actual stress/strain depth profile. On the basis of the results it is concluded that method (I) virtually avoids the occurrence of ghost stresses (stress artefacts) upon data evaluation. Substantial ghost stresses may occur upon data analysis using method (II) for strongly compositionally graded materials. [Copyright &y& Elsevier]

Abstract: There is a growing demand in hard turning industry to produce favorable surface integrity (surface finish, microstructure, microhardness, and residual stress) for improving component performance. Suitable process parameters will produce certain favorable surface integrity or at least avoid detrimental phase transformations to component performance. It is not clear if surface integrity is controllable using a set of selected process parameters. This research is to study the feasibility of obtaining four distinct types of surface integrity, which may have potential dramatic effects on fatigue life of hard-machined components. This study identifies surface integrity first and follows with the necessary conditions possible to create it in hard turning. Favorable surface integrity for optimal fatigue life can be produced using small feeds and sharp cutting tools. From the process point of view, tool wear is the dominant factor to promote white layer formation and yields a large variance of surface roughness, which may significantly deteriorate component life. A white layer could be more than 30% harder and a dark layer about 60% softer than the bulk material. [Copyright &y& Elsevier]

Abstract: Mechanical properties of NiO films developed at approximately 800°C in air for 26h on a 99.9% pure polycrystalline Ni substrates have been investigated by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The oxide layers, 8μm thick, exhibit a duplex structure with a slight {111} texture. The average stresses in the oxide films have been deduced by X-ray diffraction to be 330±35MPa in compression. Detailed analyses of the oxide films performed by atomic force microscopy show surface undulations on the nanometer scale located in some of the grains. These well-defined periodic structures are due to high growth stress heterogeneity from grain to grain in the oxide layer. From a linear analytical model, it is deduced that growth stresses as large as 4GPa can be developed locally during the high temperature oxidation process as a result of the strong in-plane anisotropy. [Copyright &y& Elsevier]