Showing papers on "Hardening (metallurgy) published in 2019"

Perspective on hetero-deformation induced (HDI) hardening and back stress

Abstract: Heterostructured materials have been reported as a new class of materials with superior mechanical properties, which was attributed to the development of back stress. There are numerous reports on ...

The hardening soil model: Formulation and verification

Abstract: A new constitutive model is introduced which is formulated in the framework of classical theory of plasticity. In the model the total strains are calculated using a stress-dependent stiffness, different for both virgin loading and un-/reloading. The plastic strains are calculated by introducing a multi-surface yield criterion. Hardening is assumed to be isotropic depending on both the plastic shear and volumetric strain. For the frictional hardening a non-associated and for the cap hardening an associated flow rule is assumed. First the model is written in its rate form. Therefor the essential equations for the stiffness modules, the yield-, failure- and plastic potential surfaces are given. In the next part some remarks are given on the models incremental implementation in the Plaxis computer code. The parameters used in the model are summarized, their physical interpretation and determination are explained in detail. The model is calibrated for a loose sand for which a lot of experimental data is available. With the so calibrated model undrained shear tests and pressuremeter tests are back-calculated. The paper ends with some remarks on the limitations of the model and an outlook on further developments.

Heat treatment of Inconel 718 produced by selective laser melting: Microstructure and mechanical properties

Abstract: Inconel 718 (IN718) samples have been produced by selective laser melting (SLM). The effects of solution temperature, time and cooling rate as well as aging hardening on the microstructure and mechanical properties of SLMed IN718 have been studied. It is found that the as-fabricated IN718 is characterized with fine cellular dendrites with Laves phase precipitating in the subgrain boundaries, which is profoundly different from cast and wrought materials and needs different heat treatment schedules. The relationship between the minimum solution time and solution temperature is established and it provides a basis for the selection of solution treatment parameters. In addition, decreasing the cooling rate of solution treatment will contribute to the precipitation of strengthening phases. The precipitation temperatures of γ′ and γ″ are about the same for SLMed and wrought IN718, but the former has a faster aging response. The tensile properties of SLMed IN718 can be tuned in a large range by properly varying the microstructure. The highest elongation of 39.1% can be obtained after solution treatment (water quenching) without aging treatment and the highest yield and tensile strength (1374/1545 MPa) can be obtained after the direct aging treatment. The match of strength and ductility is able to be tailored by controlling the amount of strengthening phases, which can be realized by adjusting the cooling rate of solution treatment and aging time.

Contribution of Mg2Si precipitates to the strength of direct metal laser sintered AlSi10Mg

Abstract: Microstructure of deformed DMLS-AlSi10Mg under quasi-static loading was studied using TEM to elaborate the strengthening mechanisms. In addition to Orowan (due to presence of Si precipitates), Hall-Petch (due to eutectic Si walls), and dislocation hardening (due to pre-existing entangled dislocations) mechanisms, Mg2Si precipitates (colonies) contributed to the strength of alloy by impeding dislocation motion. The level of this contribution was evaluated as ~13 MPa by comparing the modeled and measured yield strength.

Influence of post treatment on microstructure, porosity and mechanical properties of additive manufactured H13 tool steel

Abstract: Additive manufacturing (AM) is an attractive manufacturing technology in tooling applications. It provides unique opportunities to manufacture tools with complex shapes, containing inner channels for conformal cooling. In this investigation, H13, a widely used tool steel, was manufactured using a laser powder bed fusion method. Microstructure, tensile mechanical properties, hardness, and porosity of the AM H13 after stress relieve (SR), standard hardening and tempering (SR + HT), and hot isostatic pressing (SR + HIP + HT) were investigated. It was found that the microstructure of directly solidified colonies of prior austenite, which is typical for AM, disappeared after austenitizing at the hardening heat treatment. In specimens SR + HT and SR + HIP + HT, a microstructure similar to the conventional but finer was observed. Electron microscopy showed that SR and SR + HT specimens contained lack of fusion, and spherical gas porosity, which resulted in remarkable scatter in the observed elongation to break values. Application of HIP resulted in the highest strength values, higher than those observed for conventional H13 heat treated in the same way. The conclusion is that HIP promotes reduction of porosity and lack of fusion defects and can be efficiently used to improve the mechanical properties of AM H13 tool steel.

Effect of natural ageing or pre-ageing on the evolution of precipitate structure and strength during age hardening of Al–Mg–Si alloy AA 6016

Abstract: In this study we demonstrate the complementarity of transmission electron microscopy (TEM) and atom probe tomography (APT) in studying the early stages of phase decomposition in the age-hardening Al–Mg–Si alloy AA 6016. Samples are subjected to natural ageing at ambient temperature or artificial pre-ageing at elevated temperature in order to produce different types of atomic clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their number density, size and compositions, whereas TEM is applied to analyse and quantify number density, sizes and types of the hardening precipitates during artificial ageing. Finally, the particle statistics derived by APT and/or TEM were utilized to predict the mechanical properties of the various samples and conditions analysed.

Microstructure and Properties of Novel Heat Resistant Al–Ce–Cu Alloy for Additive Manufacturing

Abstract: The microstructure and properties of the novel heat resistant Al–3Ce–7Cu alloy produced by selective laser melting were investigated. Fine Al11Ce3 and Al6.5CeCu6.5 eutectic phases were found in the microstructure. Annealing at temperatures in the 250–400 °C range leads to a decrease in the hardness. Hardness has larger values after annealing at 350 and 400 °C than at 250 °C due to the precipitation of nanosized particles. The low hardness after quenching and aging at 190 °C is caused by quench stress relief and the absence of aging hardening because of poor solid solution. The as-printed yield strength, ultimate tensile strength and elongation are 274 MPa, 456 MPa and 4.4%, respectively. High mechanical properties of the Al–3Ce–7Cu alloy were demonstrated by high temperature tension and compression tests.

CD-MPM: continuum damage material point methods for dynamic fracture animation

Abstract: We present two new approaches for animating dynamic fracture involving large elastoplastic deformation. In contrast to traditional mesh-based techniques, where sharp discontinuity is introduced to split the continuum at crack surfaces, our methods are based on Continuum Damage Mechanics (CDM) with a variational energy-based formulation for crack evolution. Our first approach formulates the resulting dynamic material damage evolution with a Ginzburg-Landau type phase-field equation and discretizes it with the Material Point Method (MPM), resulting in a coupled momentum/damage solver rooted in phase field fracture: PFF-MPM. Although our PFF-MPM approach achieves convincing fracture with or without plasticity, we also introduce a return mapping algorithm that can be analytically solved for a wide range of general non-associated plasticity models, achieving more than two times speedup over traditional iterative approaches. To demonstrate the efficacy of the algorithm, we also develop a Non-Associated Cam-Clay (NACC) plasticity model with a novel fracture-friendly hardening scheme. Our NACC plasticity paired with traditional MPM composes a second approach to dynamic fracture, as it produces a breadth of organic, brittle material fracture effects on its own. Though NACC and PFF can be combined, we focus on exploring their material effects separately. Both methods can be easily integrated into any existing MPM solver, enabling the simulation of various fracturing materials with extremely high visual fidelity while requiring little additional computational overhead.

Characterization of SLM-fabricated Inconel 718 after solid solution and precipitation hardening heat treatments

Abstract: The microstructure and hardness of solid solution heat treated (ST) and precipitation hardened Inconel 718 parts fabricated with selective laser melting are investigated. The temperature range for the ST is between 970 and 1250 °C, while the two-step precipitation hardening was done at 760 and 650 °C, each for 10 h. The result demonstrates the effects of homogenization and consequently the effects of aging on the microstructure and hardness of the samples studied. Complete recrystallization occurred for the specimens ST at and higher than 1180 °C. The grain structures of ST specimens qualitatively appear identical with those specimens ST and aged, implying that aging does not induce noticeable changes in the grain structures. Precipitation hardening generates uniformly distributed good yield of ellipsoidal γ″ precipitates with average size of minor and major axis of 11–17 nm and 48–81 nm, respectively. In addition, smaller quantities of γ′ precipitates with an average size of 24 nm are observed for the aged specimens. Increasing the hold time of ST for a particular temperature leads to coarsening of γ″ precipitates, which have a negative impact on the hardness of the material. After aging, the hardness of the specimens is increased by 32–43% relative to that of the as-printed specimen. The increments in hardness for the specimens ST at and lower than 1100 °C (and aged) are the result of the combined effects of hardening precipitates and strain associated with the lattice defects, such as dislocation networks and subgrain boundaries that remain undissolved. The microstructures of the specimens ST at higher temperatures (e.g., 1250 °C) have attained minimal lattice defects due to completed recrystallization. Hence, the increment in hardness for these specimens after aging is mainly due to the hardening precipitates. Needle-shaped δ phase is also precipitated along/near grain boundaries during solid solution heat treatment at 970 °C. Formation of δ phase can consume a lot of Nb, which otherwise be used for the precipitation of hardening phases.

High power diode laser surface hardening of AISI 4130; statistical modelling and optimization

Abstract: Laser surface hardening of AISI 4130 carbon steel was conducted with a high power diode laser using Response Surface Methodology. Scanning speed, laser power and focal plane position were considered as the input process variables and cross sectional geometry of the hardened area, average micro-hardness and the ferrite phase percentage were considered as process responses. The effect of parameters on the responses variations were investigated using analysis of variance. Microstructure evaluation of the laser hardened zone was performed using optical and field emission scanning electron microscopy. Results indicated that by increasing the laser power and decreasing the scanning speed and focal plane position, higher surface hardness with more penetration in depth, higher average micro-hardness and minimum ferrite percentage will be achieved. Finally, the process was optimized by desire ability approach based on the applied statistical analyses. Minimum value of percentage of the ferrite and maximum value of the other responses are considered as optimization criteria. The recommended optimized results were validated using the experimental tests. The results show that the hardness of the diode laser hardening process is 3 times of the hardness of the base metal. Laser-overlapping scanning is performed in the optimum setting and effect of overlapping percentage is investigated.

Rheological Property Criteria for Buildable 3D Printing Concrete.

Abstract: Fresh concrete used in 3D printing should ensure adequate yield stress, otherwise the printed concrete layer may suffer intolerable deformation or collapse during the printing process. In response to this issue, an analytical study was carried out to derive the initial yield stress and hardening coefficient of fresh concrete suitable for 3D printing. The maximum shear stress distribution of fresh concrete was calculated using a stress transformation equation derived from the equilibrium condition of forces. In addition, the elapsed time experienced by fresh concrete during the printing processes was estimated and was then substituted into the elapsed time-yield stress function to calculate the yield stress distribution. Based on these results, an algorithm capable of deriving both the initial yield stress and the hardening coefficient required for printing fresh concrete up to the target height was proposed and computational fluid dynamics (CFD) analyses were performed to verify the accuracy of the proposed model.

Microstructural characterisation and in-situ straining of additive-manufactured X3NiCoMoTi 18-9-5 maraging steel

Abstract: Additive manufacturing (AM) is an advanced technology used for the manufacture of products that have intricate shapes and complex inner geometries. Various metal powders can be used for AM; however, the resulting microstructures will differ profoundly from those obtained via the casting, heat treatment, or thermomechanical processing of metals with the same chemical composition. This is because of the rapid heating and cooling rates used during three-dimensional (3D) printing. Further complications arise from the repeated heating and cooling of some regions, which is owed to the step-by-step formation of the solidified layers. A powder consisting of 1.2709 (X3NiCoMoTi 18-9-5) low-carbon maraging steel was used in an AM experiment. Given the high residual stresses that exist within printed metals, a post-processing heat treatment is desirable to limit the risk of cracking. In this study, solution annealing and hardening treatments were applied to the printed samples to induce changes in their microstructures and mechanical properties. The mechanical properties and microstructures of the builds were characterised and compared to those of a bar of conventional steel with the same chemical composition. During tensile loading, the fracture that was initiated at the sites of metallurgical defects was observed in situ.

Strong and ductile beta Ti–18Zr–13Mo alloy with multimodal twinning

Abstract: Body-centred cubic (BCC) Ti–18Zr–13Mo (wt%) alloy displays excellent yield strength (≈800 MPa), stable hardening (rate > 1500 MPa) and uniform ductility >18%, resulting from multi-TWIP (multiple tw...