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.
Undoubtedly, a better understanding and the further development of approaches for damage tolerant component design of AM parts are among the most significant challenges currently facing the use of ...these new technologies.
This article presents a thorough overview of the discussion at an international workshop on the topic. It aims to provide a review of the parameters affecting the damage tolerance of parts produced by additive manufacturing (shortly, AM parts) with special emphasis on the process parameters intrinsic to the AM technologies, the resulting defects and the residual stresses. Based on these aspects, basic concepts are reviewed and critically discussed specifically for AM materials:-Criteria for damage tolerant component design;-Criteria for the determination of fatigue and fracture properties;-Strategies for the determination of the fatigue life in dependence of different manufacturing conditions;-Methods for the quantitative characterization of microstructure and defects;-Methods for the determination of residual stresses;-Effect of the defects and the residual stresses on the fatigue life and behaviour.We see that many of the classic concepts need to be expanded in order to fit with the particular microstructure (grain size and shape, crystal texture) and defect distribution (spatial arrangement, size, shape, amount) present in AM (in particular laser powder bed fusion). For instance, 3D characterization of defects becomes essential, since the defect shapes in AM are diverse and impact the fatigue life in a different way than in the case of conventionally produced components. Such new concepts have immediate consequence on the way one should tackle the determination of the fatigue life of AM parts; for instance, since a classification of defects and a quantification of the tolerable shapes and sizes is still missing, a new strategy must be defined, whereby theoretical calculations (e.g. finite element modeling) allow determining the maximum tolerable defect size, and non-destructive testing (NDT) techniques are required to detect whether such defects are indeed present in the component. Such examples show how component design, damage and failure criteria, and characterization (and/or NDT) become for AM parts fully interlinked. We conclude that the homogenization of these fields represents the current challenge for the engineer and the materials scientist.
In recent years, several experimental approaches have been adopted to study and understand the mechanism and improve the ferroelectricity of fluorite-type hafnia-based ferroelectric materials. In ...this regard, significant efforts have been made to elucidate the role of top electrode and bottom electrode (TE and BE) materials in defining the ferroelectricity in such systems, especially in terms of induced mechanical tensile stress by these materials during the process of annealing. However, the effect of the electrode material was not investigated both at TE and BE, and despite numerous efforts, there is still a lack of accurate and systematic understanding. In this report, we have carried out a systematic investigation on the effect of TE and BE materials having different coefficient of thermal expansion (CTE), by changing the electrode material one at a time, both at the top and bottom. The influence of the TE was confirmed using TE/Hf 0.5 Zr 0.5 O 2 (HZO)/TiN structure in which the BE was fixed as TiN, and the influence of the BE was confirmed using TiN/HZO/BE structure by fixing TiN as the TE. As revealed by polarization versus electric field and residual stress analysis, smaller CTE of the electrode was found to result in higher tensile stress in the HZO films during the annealing process, facilitating the formation of higher ferroelectric o-phase and thereby resulting in greater ferroelectricity. Although the influence of TE and BE on the ferroelectric property of HZO films was found to show similar trends according to the CTE value of the electrodes, the influence of TE on the ferroelectric property of the HZO capacitors is found to be mainly due to the variation in the induced mechanical tensile stress; pulse switching measurement and X-ray photoelectron spectrometer (XPS) analysis suggest that in case of BE, both the induced mechanical tensile stress and the interfacial dead layer were found to play a significant part. As a result, BE was found to have a greater influence on ferroelectricity of the HZO capacitors when compared with that of TE. The highest remnant polarization of 48.2 and <inline-formula> <tex-math notation="LaTeX">58.7~\mu \text{C} </tex-math></inline-formula>/cm 2 was obtained for W with the lowest of CTE of <inline-formula> <tex-math notation="LaTeX">4.5\times 10^{-6}/^{\circ }\text{C} </tex-math></inline-formula> in both the configurations. The results obtained in this article are expected to provide a new way out to optimize the interface quality and ferroelectricity in HZO-based capacitors.
•Probabilistic study of the shape of welding residual stresses at the weld toe through the thickness direction in both cases.•When welding introduces compressive residual stresses at the ...surface.•When welding introduces tensile residual stresses at the surface.•Contribution of welding residual stresses to the fatigue life (failure) of the structures.•Prediction of the welding residual stresses shape by knowing only the magnitude of the surface residual stresses.
Welding is a joining process that is associated with heating cycles which leads to considerable change in local material microstructure and the formation of high welding Residual stresses (RS) in the welded joint. Residual stresses can have a detrimental effect on the fatigue strength of welded joints. In this paper, previously published data from measurements of residual stresses in various types of welded joints are compiled. In total, more than 100 test results are studied covering steels with yield strengths between 307 MPa and 1050 MPa in different welded details (butt joints, longitudinal and transverse attachments, cruciform joints, as well as K-joints) with varying thicknesses.
The collected data is used to study the distribution of welding residual stresses (regardless of the welding parameters) at weld toe and through the thickness of the welded plate. Probabilistic analysis is then used to arrive at a model that represents the value and distribution of residual stresses in welded joints. This model is used to predict and explain the scatter in fatigue test data from recent fatigue testing of welded samples.
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•Fatigue tests on ‘as-built’ AlSi10Mg samples manufactured by SLM.•Moderate correlation between defects at the fracture origin and roughness parameters.•Fatigue strength model based ...on Kitagawa diagram including the effect of residual stresses.
This study investigated the effects of the ‘as-built’ condition on the fatigue properties of an AlSi10Mg aluminium alloy manufactured by Selective Laser Melting (SLM) in different orientations. The results showed a moderate correlation between defects/surface features at the fracture origin and the roughness measurements. On the other hand, a fatigue strength analysis based on the size of features at fracture origin and residual stress just below the surface proved to be successful. Although considering the synergistic effect of different variables increased the scatter in fatigue life data points, a fatigue strength model including short crack effect and residual stresses made it possible to estimate the EIFS close to the real critical features. Results for life prediction models based on EIFS showed that 87% of the estimated number of cycles to failure are within a factor of 0.5–2 with respect to the experimental number of cycles to failure.
•Shot-peening process, compared with machined specimens, increases by 21.8% fatigue limit.•Quasi-static tests show more stress relaxation with compressive loads.•High stress magnitudes produce ...relaxation along the complete fatigue life.•Surface stresses relaxation model for SP is obtained.•FWHM is a good indicator for defining the relaxation process.
Shot-peening treatment was applied to a quenched and tempered DIN 34CrNiMo6 steel to improve its high-cycle R: −1 axial fatigue strength. Compared with the machined condition, the increase in the fatigue limit was 21.8%. S-N curves for shot-peened and the as machined condition were presented and compared with those obtained in previous research for rotating bending fatigue, including curves for mirror-polished specimens.
The applied shot-peening treatment in this work (Isp: 8A and 200% coverage) for quenched and tempered (Q + T) DIN 34CrNiMo6 steel introduced a compressive residual stress field and an increase in surface roughness, as well as minor variations in microstructure, hardness and the FWHM (full width of the diffraction peak at half maximum intensity) parameter.
The introduced compressive residual stress field tended to reduce when an external stress is applied. This was due to the onset of plastic strain. In this paper, two types of quasi-static tests were conducted by applying an axial stress with six different magnitudes and in the two directions (compressive or tensile). This was in order to assess their influence on the relaxation of surface residual stresses. Due to the introduced compressive residual stresses, if the applied stress was compressive, the onset of plastic deformations was achieved with a lower stress magnitude.
In addition, surface residual stress relaxation under cyclic applied stress was evaluated at four different stress magnitudes. Due to the cyclic-softening behaviour of this Q + T steel, its cyclic mechanical properties must be considered to assess the onset of plastic strains. With the experimental data, a logarithmic model to predict the evolution of surface residual stresses with the number of cycles for different applied stress magnitudes was presented.
In general, the as-received steel components before welding have usually suffered from numerous manufacturing processes. There is no doubt that initial residual stresses (IRS) remain in the steels ...with different magnitudes and even signs if no any stress-relieving process is applied before welding. Nevertheless, the relationship between IRS and welding residual stresses (WRS) is still not unclear yet, especially in ultra-high strength steels (UHSS). The current work aims to elucidate the influence of IRS on WRS in ultra-high strength steel S960. The X-ray diffraction (XRD) was applied to measure both IRS and WRS here. The experimental results exhibit that IRS in the weld area are completely eliminated because of the metallurgical and mechanical melting effect, which have no influence on WRS. In the heated zone out of the weld area, initial longitudinal residual stress (σRSL) can significantly affect welding σRSLonly if it is in tension. In the cold area, WRS are the combination of IRS and welding-induced residual stresses (WIRS). Furthermore, IRS play the dominant role here in general. No matter how much and what sign IRS are in the cold area, IRS here almost have no impact on WRS in the weld area.
Metal additive manufacturing (AM) refers to any process of making 3D metal parts layer-upon-layer via the interaction between a heating source and feeding material from a digital design model. The ...rapid heating and cooling attributes inherent to such an AM process result in heterogeneous microstructures and the accumulation of internal stresses. Post-processing heat treatment is often needed to modify the microstructure and/or alleviate residual stresses to achieve properties comparable or superior to those of the conventionally manufactured (CM) counterparts. However, the optimal heat treatment conditions remain to be defined for the majority of AM alloys and are becoming another topical issue of AM research due to its industrial importance. Existing heat treatment standards for CM metals and alloys are not specifically designed for AM parts and may differ in many cases depending on the initial microstructures and desired properties for specific applications. The purpose of this paper is to critically review current knowledge and discuss the influence of post-AM heat treatment on microstructure, mechanical properties, and corrosion behavior of the major categories of AM metals including steel, Ni-based superalloys, Al alloys, Ti alloys, and high entropy alloys. This review clarifies significant differences between heat treating AM metals and their CM counterparts. The major sources of differences include microstructural heterogeneity, internal defects, and residual stresses. Understanding the influence of such differences will benefit industry by achieving AM metals with consistent and superior balanced performance compared to as-built AM and CM metals.
The origin of rigidity in disordered materials is an outstanding open problem in statistical physics. Previously, a class of 2D cellular models has been shown to undergo a rigidity transition ...controlled by a mechanical parameter that specifies cell shapes. Here, we generalize this model to 3D and find a rigidity transition that is similarly controlled by the preferred surface area S0: the model is solid-like below a dimensionless surface area of s 0 S 0 V ¯ 2 3 5.413 with V ¯ being the average cell volume, and fluid-like above this value. We demonstrate that, unlike jamming in soft spheres, residual stresses are necessary to create rigidity. These stresses occur precisely when cells are unable to obtain their desired geometry, and we conjecture that there is a well-defined minimal surface area possible for disordered cellular structures. We show that the behavior of this minimal surface induces a linear scaling of the shear modulus with the control parameter at the transition point, which is different from the scaling observed in particulate matter. The existence of such a minimal surface may be relevant for biological tissues and foams, and helps explain why cell shapes are a good structural order parameter for rigidity transitions in biological tissues.
Shot peening is one of the most effective surface strengthening treatment technologies in which compressive residual stresses are induced beneath the specimen surface. Effects of various factors on ...the distribution of residual stress profile induced by shot peening have been investigated by many researchers. However, initial residual stresses are one of the important factors which affect the shot peening residual stress.
This study is aimed to present comprehensive numerical and experimental study on the effect of initial residual stresses on the shot peened specimen. Initial residual stresses were induced using a four-point bending rig and grinding. Incremental center hole drilling (ICHD) technique was employed to measure residual stresses on bent, ground, shot peened, bent plus shot peened and ground plus shot peened specimens. Numerical analyses of these processes were performed to provide quantitative comparison of different combinations of residual stresses. The comparison with experimental results helped to have a better understanding on how shot peening residual stresses were redistributed. Furthermore, the surface hardness was measured for all specimens.
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•Redistribution of residual stresses by shot peening has been studied experimentally and numerically.•Finite element analyses of random shot impacts provided were combined with bending and grinding simulations.•Interesting results were obtained on how the shot peening stresses redistribute tensile or compressive initial stresses.•Residual stresses for all cases were experimentally measured.
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