1,720,963 research outputs found
Aging Behaviour of a 12.2Cr-10Ni-1Mo-1Ti-0.6Al Precipitation-Hardening Stainless Steel Manufactured via Laser Powder Bed Fusion
Full text linkThe combination of precipitation-hardening stainless steels (PH-SS) and laser powder bed fusion (LPBF) enables the manufacturing of tools for plastic injection moulding with optimised geometry and conformal cooling channels, with potential benefits in terms of productivity, part quality, and tool duration. Moreover, the suitability of LPBF-manufactured PH-SS in the as-built (AB) condition to be age-hardened through a direct aging (DA) treatment enables a great heat treatment simplification with respect to the traditional solution annealing and aging treatment (SA). However, plastic injection moulding tools experience severe thermal cycles during their service, which can lead to over-aging of PH-SS and thus shorten tool life. Therefore, proper thermal stability is required to ensure adequate tool life and reliability. The aim of the present work is to investigate the aging and over-aging behaviour of a commercially available PH-SS (AMPO M789) manufactured by LPBF in the AB condition and after a solution-annealing treatment in order to evaluate the effect of the heat treatment condition on the microstructure and the aging and over-aging response, aiming at assessing its feasibility for plastic injection moulding applications. The AB microstructure features melt pool borders, oriented martensite grains, and a cellular solidification sub-structure, and was retained during aging and over-aging. On the other hand, a homogeneous and isotropic martensite structure was present after solution annealing and quenching, with no melt pool borders, cellular structure, or oriented grains. The results indicate no significant difference between AB and solution-annealed and quenched specimens in terms of aging and over-aging behaviour and peak hardness (in the range 580–600 HV), despite the considerably different microstructures. Over-aging was attributed to both the coarsening of strengthening precipitates and martensite-to-austenite reversion (up to ~11 vol.%) upon prolonged exposure to high temperature. Based on the results, guidelines to aid the selection of the most suitable heat treatment procedure are proposed
Effect of heat treatment on microstructure and mechanical properties of a Fe-12.2Cr-10Ni-1Mo-1Ti-0.6Al precipitation hardening-stainless steel manufactured via laser powder bed fusion
No full textPrecipitation hardening-stainless steels (PH-SS) can be easily manufactured by laser powder bed fusion (LPBF) and hardened to high strength, enabling the production of molds and dies featuring conformal cooling channels with no cracks, high strength, and corrosion resistance. Previous works showed that LPBF-manufactured PH-SS can be strengthened through a one-step direct aging (DA) treatment, instead of the traditional solution annealing and aging (SA), with cost, time, and energy consumption reduction. However, significant differences remain between the microstructures of DA- and SA-treated parts, potentially affecting the resulting mechanical behavior. The present work investigates the microstructure and the mechanical behavior of a PH-SS manufactured by LPBF, AMPO M789, in the as-built (AB) condition and after SA and DA heat treatments. Compared to the AB state, both heat treatments led to a great increment of hardness (from ~ 320 HV to 570–585 HV), yield strength (152–164%) and tensile strength (76–84%) together with a reduction of ductility (31–46%) and impact toughness (93–95%). However, DA led to significantly higher ductility (+ 29%) and toughness (+ 25%) than SA despite a strength reduction of only 5% and a similar hardness. The better strength–ductility balance of DA was attributed to a higher austenite content (~ 8% vs. ~ 3%) and to the oriented microstructure resulting from LPBF, retained by DA and recovered by SA, which limited the cleavage fracture mode thus promoting a ductile behavior. Results indicated the feasibility of DA to replace SA for LPBF-manufactured PH-SS in industrial applications
Tensile behaviour of a hot work tool steel manufactured via Laser Powder Bed Fusion: effect of an innovative high pressure heat treatment
Full text linkThe combination of outstanding mechanical strength of tool steels and design freedom ensured by Additive Manufacturing (AM) processes is of a great interest for automotive applications. However, AM techniques produce peculiar defects, which affect the resulting mechanical behavior. For this reason, safety critical AM components are often subjected to hot isostatic pressing (HIP) to heal process defects. At the same time, tool steels require a proper quenching and tempering (QT) heat treatment to achieve high hardness and strength.
In the present work, the effect of an innovative high pressure heat treatment (HPHT) on the tensile properties of a hot work tool steel produced via Laser Powder Bed Fusion (LPBF) was investigated. LPBF samples were subjected to two post-process heat treatments: a conventional quenching and tempering heat treatment performed in vacuum (CHT), and an innovative HPHT combining HIP e QT in a single step, to heal LPBF defects and obtain high mechanical properties. HPHT featured the same quenching and tempering cycle of CHT but it was performed under high pressure in a HIP furnace and with longer austenitizing time to promote defect closure. Tensile tests indicated no significant effect on proof and tensile strength for HPHT compared to CHT, but a significant reduction of elongation after fracture. Fractographic analyses and fracture mechanics calculations indicated that both CHT and HPHT specimens failed via crack propagation from large LPBF defects when the stress intensity factor reached the material fracture toughness. Fractographic analyses indicated an incomplete defect closure during HPHT due to the presence of an oxide film on the inner surface of defects, thus justifying the same failure mechanism and strength of CHT samples. Instead, it was proposed that the reduced elongation could arise from coarsened grains due to the longer austenitizing
Room- and High-Temperature Fatigue Strength of the T5 and Rapid T6 Heat-Treated AlSi10Mg Alloy Produced by Laser-Based Powder Bed Fusion
Full text linkThe AlSi10Mg alloy produced by laser-based powder bed fusion (L-PBF) is widely used to produce high-value-added structural parts subjected to cyclic mechanical loads at high temperatures. The paper aims to widen the knowledge of the room- and high-temperature (200 °C) fatigue behavior of the L-PBF AlSi10Mg alloy by analyzing the fully reversed rotating bending test results on mechanically polished specimens. Two heat-treated conditions are analyzed: T5 (direct artificial aging: 4 h at 160 °C) and novel T6R (rapid solution: 10 min at 510 °C, artificial aging: 6 h at 160 °C). The study highlights that (i) the T6R alloy is characterized by higher fatigue strength at room (108 MPa) and high temperatures (92 MPa) than the T5 alloy (92 and 78 MPa, respectively); (ii) thermal exposure at 200 °C up to 17 h does not introduce macroscopical microstructural variation; (iii) fracture surfaces of the room- and high-temperature-tested specimens show comparable crack initiation, mostly from sub-superficial gas and keyhole pores, and failure propagation mechanisms. In conclusion, the L-PBF AlSi10Mg alloy offers good cyclic mechanical performances under various operating conditions, especially for the T6R alloy, and could be considered for structural components operating at temperatures up to 200 °C
Direct artificial aging of the PBF-LB AlSi10Mg alloy designed to enhance the trade-off between strength and residual stress relief
Full text linkThe AlSi10Mg alloy is one of the most studied alloys processed by the Powder Bed Fusion-Laser Beam (PBF-LB). Many already published works focus on post-process heat treatments to reduce residual stress or improve me- chanical strength. Instead, the present study aims to identify direct artificial aging (AA) heat treatment able to optimize both aspects, thus enhancing the trade-off between strengthening and residual stress relief for the PBF- LB AlSi10Mg alloy produced using a no-heated platform. Higher temperatures (190–240 ◦C) than those typically used in AA heat treatment were selected based on thermal analysis to benefit both residual stress relief and precipitation of reinforcing phases from the supersaturated solid solution of the metastable as-built alloy. The effects of AA heat treatment on mechanical properties (i.e. hardness) and residual stress were evaluated by plotting aging curves and by XRD and Raman analyses and demonstrated that different trade-offs between strengthening and stress relief can be achieved by tuning heat treatment parameters (temperature and time). In particular, the exposure at the lowest temperature (190 ◦C) induced a partial decrease in residual stress and a slight increase in hardness. By increasing heat treatment temperature and soaking time, the relief was more significant, whilst the decrease in hardness was rather limited. The results are supported by the microstructural changes observed on the samples due to the different heat treatment conditions applied and show the feasibility of designing an AA heat treatment for the PBF-LB AlSi10Mg alloy capable of satisfying the mechanical response required by the final application
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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