Showing papers on "Necking published in 2022"

Laser aided additive manufacturing of spatially heterostructured steels

Abstract: Current heterostructured materials demonstrate the high potential to circumvent strength-ductility tradeoff but face challenges with unconfigurable hetero-zone distribution and mechanical anisotropy. This study explored laser aided additive manufacturing (LAAM) of non-laminar spatially heterostructured materials (SHM) with configurable architectures to combine superior properties from the constitutive AISI 420 stainless steel and C300 maraging steel to enhance the overall properties. It is proven that the hatch spacing (h) has a significant effect on the microstructural evolutions and mechanical properties of the SHMs as it affects the layer thickness and inter-dilution regions. The mechanical properties of the SHMs were evaluated at multi-scales. The sample with h of 1.5 mm possesses a high tensile strength of about 1.6 GPa along with a reasonable break elongation of 8.1%, showing a good strength-ductility combination. Micropillar compression tests were conducted to measure localised mechanical properties, which is critical to understand the strengthening mechanism. Additionally, the SHM substantiates a much higher strength than many lamellar and linear functionally graded materials reported in the literature. This can be explained by the rule-of-mixture effect and hetero-deformation induced strengthening (HDIS). Furthermore, in-situ deformation observation reveals multiple deformation bands in SHM, which delays necking and contributed to ductility in tandem with the transformation induced plasticity (TRIP) effect. The findings highlight a novel approach for the development of SHM with a tunable performance by using LAAM of multiple materials following the configurable architectures.

Laser aided additive manufacturing of spatially heterostructured steels

Abstract: Current heterostructured materials demonstrate the high potential to circumvent strength-ductility tradeoff but face challenges with unconfigurable hetero-zone distribution and mechanical anisotropy. This study explored laser aided additive manufacturing (LAAM) of non-laminar spatially heterostructured materials (SHM) with configurable architectures to combine superior properties from the constitutive AISI 420 stainless steel and C300 maraging steel to enhance the overall properties. It is proven that the hatch spacing (h) has a significant effect on the microstructural evolutions and mechanical properties of the SHMs as it affects the layer thickness and inter-dilution regions. The mechanical properties of the SHMs were evaluated at multi-scales. The sample with h of 1.5 mm possesses a high tensile strength of about 1.6 GPa along with a reasonable break elongation of 8.1%, showing a good strength-ductility combination. Micropillar compression tests were conducted to measure localised mechanical properties, which is critical to understand the strengthening mechanism. Additionally, the SHM substantiates a much higher strength than many lamellar and linear functionally graded materials reported in the literature. This can be explained by the rule-of-mixture effect and hetero-deformation induced strengthening (HDIS). Furthermore, in-situ deformation observation reveals multiple deformation bands in SHM, which delays necking and contributed to ductility in tandem with the transformation induced plasticity (TRIP) effect. The findings highlight a novel approach for the development of SHM with a tunable performance by using LAAM of multiple materials following the configurable architectures.

Toward qualification of additively manufactured metal parts: Tensile and fatigue properties of selective laser melted Inconel 718 evaluated using miniature specimens

Abstract: Combined with the topology optimization, additive manufacturing can be used to fabricate metal parts with complex shapes. However, due to the geometrical variations and microstructure heterogeneities of the additively manufactured metal parts, new standards with the use of miniature specimens are required for the evalutation of the spatial distribution of mechanical properties throughout the parts. Here, we conduct a systematic investigation on tensile and fatigue properties of selective laser melted Inconel 718 specimens with different thicknesses ranging from 0.1 mm to 1 mm. A “microstructure unit” that can well reflect the microstructure characteristic of selective laser melted materials is defined. The results reveal that premature necking with a dramatic drop in uniform elongation occurs if the ratio (t/d) of specimen thickness (t) to the "microstructure unit" size (d) is less than one. Premature necking is mainly attributed to the transition of strain localization behavior. We also propose a probabilistic statistical model for fatigue limit prediction based on the available fatigue data. It is recommended that the criterion of t/d ≥ 4 should be satisfied to ensure that the yield strength, the uniform elongation, and the fatigue limit determined by the miniature specimens are comparable with those determined by standard specimens. The findings may provdie a guide to the establishment of miniature specimen-based standards toward the qualification of additively manufactured metal parts.

Hot Deformation and Constitutive Modeling of TC21 Titanium Alloy

Abstract: Titanium alloys are extensively employed in the fabrication of various aviation structural parts, of which the most crucial processing step is hot working. In order to study the high-temperature deformation behavior of the TC21 titanium alloy, high-temperature tensile tests were performed. The results reveal that the flow stress of the material gradually decreases with an increased strain rate, and the stress increases rapidly with an increase in strain during the deformation of the alloy. Following this, flow stress gradually decreases. Flow stress decreases sharply, and the sample fractures when the appearance of necking and microvoids is observed. The Arrhenius and Radial basis function (RBF) neural network constitutive models are established in order to accurately describe the high-temperature deformation behavior of the material. In the modified Arrhenius model, strain rate indexes are expressed as a function of deformation temperature and strain rates; furthermore, the high prediction ability of the model was obtained. For the Radial basis function, the network parameters were obtained using the trial-and-error method. The established models could better forecast the flow stress of materials, and highly accurate results are obtained using the radial basis function model. The relationships between the stress index and the deformation activation energy with strain indicate that the primary deformation mechanism involves grain boundary slip and viscous slip of dislocations. The process of dynamic recrystallization primarily promotes the softening of the material.

Investigations on material properties and residual stresses in cold-formed high strength steel irregular hexagonal hollow sections

Abstract: This paper presents an experimental investigation on the material properties variation and residual stress distribution within the cold-formed high strength steel (HSS) irregular hexagonal hollow sections (IHexHS) with two different fabrication methods. The test specimens were manufactured through press-braking and gas metal arc welding (GMAW). Tensile coupons tests were conducted on specimens fabricated from the critical locations within cold-formed HSS irregular hexagonal hollow sections, namely the flat portions, corner portions of either half or quarter sections. New material models to predict the material properties for the tensile coupons with both rounded responses and yield plateau followed by significant strain hardening were proposed. In conjunction with conventional tensile coupon testing, non-contact digital image correlation (DIC) measurement through which strain fields along the gauge length before and after the occurrence of diffuse necking was carried out to obtain the accurate strain field after necking. Moreover, the residual stresses measurements for HSS IHexHS were also performed, membrane and bending residual stresses distributions on the investigated sections were measured in longitudinal directions with 59 strips cut by wire-cutting method and more than 708 strain readings obtained. Results of the residual stress distributions and magnitudes are presented and discussed. Based on the measurement results, predictive models for residual stress distribution were developed and can be subsequently applied for predicting structural behaviour of the cold-formed HSS IHexHS.

Overcoming the strength-ductility trade-off in refractory medium-entropy alloys via controlled B2 ordering

Abstract: Herein, we showed that controlled additions of Al, which provided a certain degree of B2 ordering, resulted in a ∼ 37%-enhancement of both yield strength and uniform elongation in Alx(NbTiZr)100-x (x = 0; 2.5; 5; 7.5 at.%) refractory medium-entropy alloys. The improvement of properties stemmed from the solid solution and short-range order strengthening, as well as from the alteration in a character of dislocation glide. The B2 ordering caused the formation of multiple dislocation bands and the activation of cross-slip, which improved the macroscopic stability of plastic flow and extended the strengthening stage, thereby postponing necking. GRAPHICAL ABSTRACT IMPACT STATEMENT Controlled Al-induced B2 ordering helps overcome the strength-ductility dilemma in refractory medium-entropy alloys due to solid solution and short-range strengthening coupled with the dislocation motion changing.

Characterization of the deformation behaviors under uniaxial stress for bicontinuous nanoporous amorphous alloys

Abstract: In this paper, the deformation behaviors of Cu50Zr50 bicontinuous nanoporous amorphous alloys (BNAMs) under uniaxial tension/compression are explored by molecular dynamics simulations. Scaling laws between mechanical properties and relative density are investigated. The results demonstrate that the bending deformation of the ligament is the main elastic deformation mechanism under tension. Necking and subsequent fracture of ligaments are the primary failure mechanism under tension. Under tensile loading, shear bands emerge near the plastic hinges for the BNAMs with large porosities. The typical compressive behaviors of porous structure are observed in the BNAMs with large porosities. However, for small porosity, no distinguished plateau and densification are captured under compression. The tension-compression asymmetry of modulus increases with increasing porosity, whereas the BNAMs can be seen as tension-compression symmetry of yield strength. The modulus and yield strength are negatively correlated with temperature, but a positive relationship between the tensile ductility and temperature is shown. This work will help to provide a useful understanding of the mechanical behaviors of the BNAMs.

Mechanical properties and deformation mechanisms of martensitic Ti6Al4V alloy processed by laser powder bed fusion and water quenching

Abstract: Martensitic Ti6Al4V samples were fabricated by laser powder bed fusion additive manufacturing, water quenching of wrought and the additively manufactured Ti6Al4V samples respectively, their microstructural characteristics, tensile properties and deformation behaviours have been investigated in this study. These samples showed differences in grain orientations, size of the prior β grains, thickness of the martensite (α′) laths, the lattice strain and presence of the β phase between the α′ laths. A relationship between the yield strength and the thickness of α′ laths and lattice strain has been developed. The significant difference in the ductility in terms of the total strain at fracture among the martensitic Ti6Al4V samples processed by the different routes has been observed and investigated. The additively manufactured Ti6Al4V showed the highest post-uniform plastic deformation (deformation after necking) with rapid reduction of stress, its failure mode was dominated by the inter-α′ quasi-cleavage and intra-α′ ductile fractures. The water quenched-wrought Ti6Al4V displayed the negligible post-uniform plastic deformation, the inter-prior β cleavage fracture was its dominant failure mode. The water quenched-additively manufactured Ti6Al4V produced the highest yield strength and medium post-uniform plastic deformation, during which the reduction of stress was much slower, its failure mode was dominated by the inter-α′ quasi-cleavage fracture. Plastic deformation was mainly taken by the sliding between α′ laths, however, grain reorientation and slip in α′ with the different scales in the martensitic Ti6Al4V processed by the different routes were also observed with the evidence of the changes in the relative intensities and shift of 2θ angles for the diffraction peaks in X-ray diffraction (XRD) spectra in the fractured samples compared to the as-processed samples.

Insights into the dynamics of non-Newtonian droplet formation in a T-junction microchannel

Abstract: Non-Newtonian shear-thinning droplet formation mechanism in a T-junction microchannel is experimentally investigated using aqueous solutions of xanthan gum as the dispersed phase and mineral oil as the continuous phase. Influences of both phase flow rates and polymer concentration on flow regime transition are explored. It is observed that the initial vertical expansion stage is present only for the Newtonian and lower shear-thinning systems. Droplet evolution rate shows the influence of continuous phase flow rate and shear-thinning properties on the dynamics of necking stages, viz. squeezing, transition, pinch-off, and filament thinning. Analysis of Ohnesorge number (Oh) reveals that inertial force dominates in the squeezing stage, whereas viscous and interfacial force control in the filament thinning stage. Longer and stable filament generation is detected as a discerning feature for non-Newtonian systems that appears more prominent with increasing dispersed phase shear-thinning properties. The results also indicate an inverse relation of droplet length with continuous phase flow rate and xanthan gum concentration, while the droplet formation frequency and its polydispersity vary directly with those parameters.

The grain size effect on strain hardening and necking instability revisited from the dislocation density evolution approach

Abstract: Revisiting the classical topic of the strain hardening behaviour from a perspective of the analytical model based on the evolution of dislocation density with strain, we extended the capacity of the single internal variable model towards predicting necking instability and re-examined the grain size dependence of the flow stress. An excellent agreement is observed between the model predictions of the necking strain and stress and the results of tensile testing of nickel polycrystals with grain sizes varied from sub-micrometres to hundred micrometres. The pivotal significance of the dynamic recovery in the occurrence of necking has been analysed and emphasised in the context of the discussion on the effect of grain size on flow stress. The most interesting corollary of the analysis made is that not only the simple modelling of the dislocation density evolution, which can be traced back from the very early stage of plastic flow in materials with different grain sizes, can be used for realistic approximation of the stress-strain behaviour during homogeneous deformation under constant plastic strain rate, but also for predicting the onset of necking instability with high confidence.

The grain size effect on strain hardening and necking instability revisited from the dislocation density evolution approach

Abstract: Revisiting the classical topic of the strain hardening behaviour from a perspective of the analytical model based on the evolution of dislocation density with strain, we extended the capacity of the single internal variable model towards predicting necking instability and re-examined the grain size dependence of the flow stress. An excellent agreement is observed between the model predictions of the necking strain and stress and the results of tensile testing of nickel polycrystals with grain sizes varied from sub-micrometres to hundred micrometres. The pivotal significance of the dynamic recovery in the occurrence of necking has been analysed and emphasised in the context of the discussion on the effect of grain size on flow stress. The most interesting corollary of the analysis made is that not only the simple modelling of the dislocation density evolution, which can be traced back from the very early stage of plastic flow in materials with different grain sizes, can be used for realistic approximation of the stress-strain behaviour during homogeneous deformation under constant plastic strain rate, but also for predicting the onset of necking instability with high confidence.

Effect of Welding Speed on Microstructure Evolution and Mechanical Properties of Friction Stir Welded 2198 Al-Cu-Li Alloy Joints

Abstract: In the present study, 2198 Al-Cu-Li alloys were successfully friction stir welded by using various welding speed ranges of 90~180 mm/min with an invariable rotation speed of 950 r/min. The effect of welding speed on microstructure evolution and mechanical properties of the joints was investigated. The results show that, with the welding speed decreasing, the size of the nugget zone (NZ) first increases and then decreases due to different welding temperatures. At a welding speed of 150 mm/min, the size of the NZ in all joints is the biggest and the “S” curve disappears. The equiaxed grains are finer, attributed to a higher degree of dynamic recrystallization, and a larger number of fine reprecipitated phase (δ’, β’ phases) particles are dispersively distributed in the NZ. Correspondingly, the joints have the highest tensile properties, and the tensile strength, yield strength and elongation are, respectively, 406 MPa, 289 MPa and 7.2%. However, compared to the base material, the tensile properties of all joints are reduced because a greater amount of δ’ and β’ phases particles are dissolved in the NZ. Only the joints produced at 150 mm/min are fractured in the TMAZ with detected deep dimples and tearing ridges, and a significant necking phenomenon is observed, which indicates a complete ductile fracture mode.

An improved modelling framework for strength and work hardening of precipitate strengthened Al–Mg–Si alloys

Abstract: A modelling framework for the strength and work hardening contributions in age hardened Al–Mg–Si alloys is presented. The modelling work is based on transmission electron microscopy (TEM) measurements of the needle-shaped precipitate lengths and cross sections, and on stress-strain curves up to fracture, obtained by tensile tests applying a necking correction. For strains beyond the uniform limit, further work hardening is found to be strongly suppressed by the presence of the precipitates. At strains up to about 10–15%, new models are proposed for the description of the work hardening at different aging conditions. A detailed way of accounting for the contribution from generation of dislocation loops around non-sheared precipitates is formulated, based on the precipitate size and shape distribution. However, this mechanism overestimates the work hardening, and as an alternative, a new mechanism is suggested, assuming that the precipitates act as obstacles for moving dislocations and contribute to restrict their average slip length, leading to increased storage rate of the dislocations. The model captures measured stress-strain curves at different aging conditions well.