A. Molinari

Università degli Studi di Trento, Trient, Trentino-Alto Adige, Italy

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Publications (178)201.91 Total impact

  • Powder Technology 07/2015; 278:323. DOI:10.1016/j.powtec.2015.03.042 · 2.27 Impact Factor
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    ABSTRACT: Mechanical milling is a suitable technique to enhance various properties of copper by the severe plastic deformation involved during the process. Contamination from milling media is one of the major problems of mechanical alloying. In this study, the behaviour of copper powder during mechanical milling was investigated in order to minimize iron and chromium contaminations. Hence, three different parameters have been studied to highlight the high influence of ball milling parameters on the final products. The parameters included the type of process control agent (none, toluene or stearic acid), the ball-to-powder weight ratio (33:1 or 10:1) and finally, a change in the milling cycle (interrupted or continuous) in an interactive procedure according to the experimental results. As a result, the best morphology and contamination level combination was observed in powder milled with stearic acid; it was 10:1 for the ball to powder ratio when using a continuous milling cycle. Once the best milling conditions were determined, the resulting samples were exposed to spark plasma sintering (SPS). The sintering parameters were selected based on a previous thermal gravimetrical measurement of the milled powders to avoid residual porosity. The final density of all of the samples is very good, 99%, confirming an effective densification process and sintering activated by severely strain-hardened and nanostructured particles.
    Powder Technology 05/2015; 275. DOI:10.1016/j.powtec.2015.01.063 · 2.27 Impact Factor
  • C. Menapace, G. Cipolloni, A. Molinari
    Materials Science Forum 12/2014; 802:483-488. DOI:10.4028/www.scientific.net/MSF.802.483
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    ABSTRACT: The dry sliding wear behavior of two sintered and carburized steels with different Ni amounts has been investigated. The microstructure of the two steels comprises martensite, bainite, and the Ni-rich austenite. Under the sliding conditions investigated, wear is either oxidative or adhesive. In both cases, the lower amount of the soft Ni-rich austenite results in a better wear resistance. A design procedure for parts subject to dry sliding wear applications is proposed, based on the maximum acceptable wear depth, in order to evaluate the practical significance of the differences between the two materials.
    Journal of Materials Engineering and Performance 10/2014; 23(10):3630-3639. DOI:10.1007/s11665-014-1135-0 · 0.98 Impact Factor
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    ABSTRACT: A conservative approach to predict the contact fatigue behavior of a Powder Metallurgy steel, in the sinter-hardened condition and after shot peening, was proposed. It is based on the assumption that the contact fatigue crack nucleation is anticipated by the local plastic deformation of the material, which occurs when the maximum local stress exceeds the yield strength of the matrix. A model to predict contact fatigue crack nucleation has been proposed and validated by experiments. A local approach is used, based on the characteristics of the largest pores, which promote fatigue crack nucleation, and on the microhardness of the microstructural constituents where they are localized. Shot peening improves the contact fatigue resistance of the sinterhardened steel by 30%.
    Materials Science and Engineering A 09/2014; 614:81–87. DOI:10.1016/j.msea.2014.07.009 · 2.41 Impact Factor
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    ABSTRACT: In the conventional press and sinter process, dimensional change on sintering determines the precision of the final parts, providing that a good dimensional precision of green parts is ensured. Anisotropic dimensional change on sintering may be detrimental to the precision of Powder Metallurgy (PM) parts, and it should be considered in the design step. The effect of material and geometry on the anisotropic dimensional change is studied in this work. Four different iron alloys and five different geometries were considered. Dimensions were measured both on green and on sintered parts and the anisotropy of dimensional change was evaluated and correlated to the material and geometry. The effect of neglecting anisotropy in the design step was investigated, in terms of dimensional tolerances, which can be obtained with different process capabilities. A model to describe the effect of material and geometry on the anisotropic dimensional change is also being developed.
    International Journal of Precision Engineering and Manufacturing 09/2014; 15(9):1865-1873. DOI:10.1007/s12541-014-0540-5 · 1.50 Impact Factor
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    ABSTRACT: A systematic characterisation of the porosity in the bulk and surface regions of a sintered Cr-Mo low alloy steel was carried out using image analysis. Fractional porosity in the different regions varies, mainly due to the scatter of the maximum pore size. A higher porosity is found in the bulk region and lower porosity in the regions that contact the die surface during compaction. The maximum pore size is larger in the bulk region than in the surface layers. The large pores are more irregular. With increasing green density, both the fractional porosity and maximum pore size decrease. The fraction of load bearing section in the bulk and surface regions was calculated from fractional porosity and the shape factor of the pores and compared in the different regions. The load bearing section fraction and the maximum pore size were used to predict tensile and fatigue resistance for different densities. The data from the characterisation of the bulk images can predict tensile strength. For fatigue, where the crack nucleates in the surface regions, the use of bulk data underestimates the fatigue resistance.
    Materials Characterization 08/2014; 94. DOI:10.1016/j.matchar.2014.05.005 · 1.93 Impact Factor
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    ABSTRACT: In this work it has been investigated the hot deformation of a 2024 Al alloy obtained by spark plasma sintering of cryomilled powder. This behaviour was compared with the one of a sample produced with atomized powder. Different hot deformation responses were observed since their microstructures are different. In the milled sample grains are nanometric and ultrafine and contain some precipitates. A much higher concentration was observed in the atomized specimen, since this powder is a supersaturated solid solution and SPS causes the whole of the precipitation sequence, resulting in the extensive intragranular precipitation of incoherent particles. The different microstructures lead to a different hot compression behaviour. Milled alloy has a higher flow stress and shows a peculiar stress–strain curve characterized by a sharp peak of the flow stress followed by a rapid decrease. This work softening was correlated to the phenomenon of the dynamic precipitation occurring only in the milled specimen.
    04/2014; 3(2). DOI:10.1166/mat.2014.1166
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    ABSTRACT: Dilatometry experiments have been carried out to investigate the shrinkage kinetics on cold isostatic pressed iron specimens in the 550-730 degrees C temperature range, showing that dimensional contraction is much higher than that predictable on the basis of the shrinkage kinetics models, which neglect the effect of the prior cold compaction. The greater shrinkage is due to an enhanced diffusivity which may be attributed to the large density of structural defects accumulated in the powder particles during compaction (structural activity). A time depending effective lattice diffusion coefficient was determined, with an Arrhenius type dependence on temperature.
    Powder Metallurgy 02/2014; 57(1):61-69. DOI:10.1179/1743290113Y.0000000068 · 0.60 Impact Factor
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    International Journal of Powder Metallurgy 01/2014; 50(3):41-48. · 0.32 Impact Factor
  • Powder Metallurgy 12/2013; 56(5):420-426. DOI:10.1179/1743290113Y.0000000065 · 0.60 Impact Factor
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    Saliou Diouf, Anna Fedrizzi, Alberto Molinari
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    ABSTRACT: The effect of the surface overheating occurring during spark plasma sintering was investigated by means of a fractographic and microstructural analysis of specimens produced with large copper particles. The overheated layer is quite thick and its microstructure is clearly different from that of the bulk of the particles, comprising coarse columnar grains oriented along the radial direction. Such a microstructure indicates that the temperature at the surface of the particles may effectively overcome the melting point of the material, leading to fast melting and re-solidification.
    Materials Letters 11/2013; 111:17-19. DOI:10.1016/j.matlet.2013.08.056 · 2.27 Impact Factor
  • Enzo Castellan, Gloria Ischia, Alberto Molinari, Rishi Raj
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    ABSTRACT: We apply an in situ approach, whereby a polymer is incorporated into copper and evolves within the metal into the ceramic phase, to create a dispersion of hard particles in a metal. All constituents for the ceramic phase are contained within the organic polymer. The temperature for this polymer to ceramic conversion lies in the 1073 K to 1273 K (800 °C to 1000 °C) range. The process produces a nanoscale dispersion of the ceramic, which leads to high microhardness that remains unaltered at temperatures up to 1223 K (950 °C) (0.9T M). Apparently, the introduction of the ceramic phase leads to the retention of copper crystallite size of a few hundred nm, despite exposure to heat treatments at these very high temperatures. We call these materials polymer-derived metal-matrix composites.
    Metallurgical and Materials Transactions A 10/2013; 44(10). DOI:10.1007/s11661-013-1835-3 · 1.73 Impact Factor
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    ABSTRACT: Ti–6Al–2Sn–4Zr–6Mo is a a + b titanium alloy that combines the long-term strength at elevated temperature with the hardenability by heat-treatments.In this work the effect of conventional heat treatments on the microstructure is investigated,in relation with the hardness properties ofthe material, by Optical Microscopy (OM),Scanning Electron Microscopy (SEM),Energy Dispersion X-ray Spectroscopy (EDXS) and X-ray diffraction (XRD).According to the Rietveld method the quantitative phase analysis by XRD has been carried out, showing the evolution of the phases with the different heat treatments. Solution- treated samples exhibit a microstructure of a-phase crystals embedded in a matrix of retained- b phase and/or a00 martensite, depending onsolution treatment temperature;the a-phase amount depends on the solution-treatment temperature.The presence ofthe a00 martensite does not lead toa significant hardening ofthe material.The aging treatment induces the transformation ofthe retained- b phase and the martensite in a very fine a + b structure that hardens the alloy. The hardness is related to the amount of the fine(a + b)-phase and a linear dependence on the b-phase amount formed during the solution treatment can be identified.
    Journal of Alloys and Compounds 08/2013; 567:134-140. DOI:10.1016/j.jallcom.2013.03.046 · 2.73 Impact Factor
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    ABSTRACT: This work proposes a hypothesis for the interpretation of shrinkage anisotropy during sintering of an Fe–Cu–C alloy based on the effect of the structural modifications of the powder, due to the prior compaction, on the mass transport phenomena. Dislocations are introduced by cold compaction in the contact regions between particles, with different densities along the compaction direction and the transversal one. Therefore, the mass transport by volume diffusion is strongly activated in both directions, and a prevailing effect in the compaction direction is shown. The volume diffusion coefficients derived from the kinetic model correspond to the dislocation pipe diffusion mechanism.
    Powder Metallurgy 07/2013; 56(3):189-195. DOI:10.1179/1743290112Y.0000000043 · 0.60 Impact Factor
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    ABSTRACT: This work improves the systematic approach proposed in previous papers, aimed at defining design criteria specific for powder metallurgy (PM) parts subject to wear in application. Dry rolling–sliding tests have been performed on PM steels obtained under different process conditions, and the results have been used as a knowledge base in the proposed design procedure to evaluate the effect of the material variables (porosity and microhardness) and geometrical parameters (contact length) on wear resistance. The failure criterion considered here to ensure the functionality provides to compare the wear thickness with a value deriving from the dimensional tolerances. If the failure criterion is not matched, then a change in the material variables and/or geometrical parameters is proposed. The occurrence of plastic deformation and/or brittle contact at the surface region is also considered. Examples of application show that the most significant improvement is obtained by modifying both material variables and geometrical parameters.
    Powder Metallurgy 04/2013; 56(2):124-134. DOI:10.1179/1743290112Y.0000000032 · 0.60 Impact Factor
  • • C. Menapace, N. Vicente, A. Molinari
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    ABSTRACT: Powder preform forging is a technology that comprises the preparation of near net shape preforms through powder metallurgy and a subsequent hot forging in order to obtain the desired final shape. In this work, two Ti-6Al-4V powder preforms were sintered through spark plasma sintering (SPS) and then hot compressed in a horizontal dilatometer. Varying the temperature of the process, two full density preforms having different microstructures were produced: sintering at 950 degrees C, a plate-like alpha was obtained, whereas sintering at 1050 degrees C, an acicular alpha was obtained. The behaviour of the preforms under hot forging has been studied through hot compression tests carried out in a quenching and deformation dilatometer in a range of temperature and strain rates typically used in hot forging this alloy (850-1050 degrees C, 0.01-1 s(-1)). Hot workability has been evaluated by measuring the stresses required for deformation and by analysing both the stress-strain curves recorded during testing and the microstructures after deformation. The main microstructural phenomena occurring during hot compression were individuated. The best conditions for the hot forging operation of SPS preform are temperatures above beta transus, where the materials are deformed in a regime of dynamic recrystallisation, at every strain rate.
    Powder Metallurgy 02/2013; 56(2). DOI:10.1179/1743290112Y.0000000003 · 0.60 Impact Factor
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    ABSTRACT: The interface of bimetallic CoCrMo/cp2-Ti discs produced by either co-spark plasma sintering (SPS) or SPS bonding at a nominal temperature of 1273 K is made of a bonding layer and a diffusion layer. The bonding layer is made of three sublayers with different Co, Cr, Mo and Ti contents. The diffusion layer is made of two layers with different microstructure: beta titanium close to interface and lamellar alpha-beta over the whole diffusion distance of Co, Cr and Mo. The measured diffusion profiles correspond to a slightly higher (40 K) temperature than the measured one into the die wall during sintering. Some residual porosity is observed in the cobalt alloy due to the effect of titanium, which sinters at a lower temperature, reducing the efficiency of the particular heating mechanism of SPS.
    Powder Metallurgy 12/2012; 56(2). DOI:10.1179/1743290112Y.0000000040 · 0.60 Impact Factor
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    ABSTRACT: A PM approach is investigated in the context of extremely fine low temperature bainitic steels with a reasonable combination of ultimate tensile strength and uniform elongation. The strategy consists of three steps. First, ball milling is used for mechanical alloying and to create a large density of interfaces prior to sintering. In the second step, spark plasma sintering is used as a rapid consolidation process to retain a very fine and homogeneous structure after sintering. The isothermal heat treatment which leads to bainite can then be done at a low austenitisation temperature (860°C) for a few minutes, leading to a small initial austenite grain size. A fine grain size promotes the rapid transformation of the austenite into bainite. The characteristics of a nanostructured Fe–Ni–C bainitic steel obtained after mechanical alloying and nanostructuring of the powder, spark plasma sintering and final heat treatment are reported.
    Powder Metallurgy 09/2012; 55(4):256-259. DOI:10.1179/0032589912Z.00000000079 · 0.60 Impact Factor
  • S. Diouf, C. Menapace, A. Molinari
    Powder Metallurgy 07/2012; 55(3):228-234. DOI:10.1179/1743290111Y.0000000019 · 0.60 Impact Factor

Publication Stats

1k Citations
201.91 Total Impact Points

Institutions

  • 1990–2015
    • Università degli Studi di Trento
      • • Department of Industrial Engineering
      • • Departmental Area of Materials Engineering and Industrial Technologies
      Trient, Trentino-Alto Adige, Italy
    • University of Padova
      Padua, Veneto, Italy
  • 2011
    • University of Cambridge
      • Department of Materials Science and Metallurgy
      Cambridge, England, United Kingdom
  • 2010
    • Museo delle Scienze, Trento, Italy
      Trient, Trentino-Alto Adige, Italy
  • 2007–2008
    • Trent University
      Питерборо, Ontario, Canada
  • 2004
    • Università degli studi di Cassino e del Lazio Meridionale
      • Department of Mechanics, Structures, Environment and Territory (DIMSAT)
      Cassino, Latium, Italy
  • 1999–2003
    • Nottingham Trent University
      Nottigham, England, United Kingdom
  • 1990–1993
    • University of Florence
      • Dipartimento di Ingegneria Industriale
      Florens, Tuscany, Italy
  • 1985
    • Università degli Studi di Genova
      Genova, Liguria, Italy