A. Molinari

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

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Publications (260)233.16 Total impact

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    ABSTRACT: Press and sinter is a near net-shape technology, and its cost effectiveness is strictly related to the geometrical and dimensional precision of the component itself. In this work an analytical model to describe the dimensional variations due to the sintering process of iron components has been proposed and discussed. This model has been developed using experimental data coming from a well designed sampling. The sampling is a set of axisymmetric geometries, rings and cylinders, having different diameters and heights. The different features (diameters and heights) have then been compared with the dimensional variations to study the influence of the geometry on the dimensional variations. Every sample has been measured both in the green state and after the sintering process. The dimensional variations concerning the diameters and the heights have been evaluated. The measuring procedure has been implemented using a coordinate measuring machine. The sintering process has been carried at three different temperatures under the same operating conditions. The anisotropy of the dimensional variations has been studied and described within the proposed model, introducing the anisotropy parameter (K). Basically the parameter K identifies the difference between the dimensional variations occurring in an ideal isotropic volumetric change and the actual anisotropic volumetric variation. The model can describe the difference between the dimensional variations occurring on the compaction plane (diameters) and the dimensional variations occurring along the compaction axis (height). The effect of the geometry and the sintering temperature on the anisotropy of the dimensional variations has been evaluated.
    No preview · Article · Oct 2015 · La Metallurgia Italiana
  • Alberto Molinari · Elisa Torresani · Cinzia Menapace · Mats Larsson
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    ABSTRACT: The anisotropy of dimensional change on the sintering of iron was investigated by dilatometry. Dimensional changes are different along the longitudinal and transversal directions, and shrinkage is more pronounced parallel to the compaction direction. This phenomenon is particularly pronounced during the early stage of sintering, in the alpha field below the Curie temperature. The results were elaborated according to the kinetics model for shrinkage to calculate an effective diffusion coefficient along the two directions. Such an effective diffusion coefficient is higher parallel to the compaction direction than perpendicular to it, and both are larger than the diffusion coefficient calculated on the basis of the activation energy reported in the literature for pure iron. This discrepancy is attributed to the defectiveness introduced by cold compaction in the particle contact regions, which may enhance diffusivity owing to the dislocation pipe mechanism, which, in turn, is particularly intense below the Curie temperature. This interpretation may also justify anisotropy of shrinkage because the powder particles are inhomogeneously deformed by uniaxial cold compaction. The enhanced diffusion coefficient increases with time and shows a maximum at temperatures below the Curie point. This trend was discussed with reference to the self-activated sintering mechanism.
    No preview · Article · Oct 2015 · Journal of the American Ceramic Society

  • No preview · Article · Jul 2015 · Powder Technology
  • G. Cipolloni · M. Pellizzari · A. Molinari · M. Hebda · M. Zadra
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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.
    No preview · Article · May 2015 · Powder Technology
  • L. Emanuelli · M. Biesuz · S. Libardi · P. Marconi · A. Molinari
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    ABSTRACT: The influence of the mechanical properties of a diffusion bonded Ni-Cu-Mo sintered steel on the residua! stresses and the plastic deformation promoted by both steel and ceramic shot peening was investigated. While steel shots deform the surface more extensively than ceramic ones, leading to a thicker surface densification, ceramic shots are more effective in terms of maximum compressive residual stresses. The increase in the yield strength of the base material enhances residual stresses and reduces plastic deformation. The strain induced transformation of Ni-rich austenite in martensite causes a slight decrease of tensile elongation and of impact energy in specimens sintered at low temperature.
    No preview · Article · Mar 2015 · La Metallurgia Italiana
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    Simone Zanzarin · Sven Bengtsson · Alberto Molinari
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    ABSTRACT: High power diode laser with coaxial powder injection was used to deposit single tracks of cobalt alloys with tungsten carbides on to a carbon steel plate in order to study carbide dissolution in the metallic matrix. Two kinds of cobalt alloy powders (Stellite 12 and Stellite 21) having different content of tungsten and carbon were used. Processing parameters were changed to obtain different cladding conditions, and effect of substrate preheating was evaluated too. Carbides dissolution, measured with image analysis and EDXS analysis, increases with the increase in the laser power. A low carbon and the absence of tungsten in the matrix powder promote carbides dissolution. Substrate preheating also leads to a higher dissolution. Matrix microhardness of the Stellite 21 increases with the increase in carbides dissolution, while matrix microhardness of the Stellite 12 does not change. Hardness of the coatings is slightly influenced by the dissolution phenomena.
    Full-text · Article · Feb 2015
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    ABSTRACT: Sizing is studied in this work as a post-sintering operation aimed at improving the dimensional and geometrical precision of sintered parts. The required dimensional and geometrical characteristics are obtained by the plastic deformation due to sizing, which is related to the applied stress. In this work, the relationships between applied force, resulting deformation, attainable geometrical characteristics have been investigated. By means of the data recorded by a hydraulic press, force-displacement curves have been derived. The analysis of these curves allowed identifying the elastic deformation of part and tool, as well as the plastic deformation of the part. The plastic deformation has been correlated to the actual dimensional changes measured on the part, as well as to the change in the required geometrical characteristic (conicity). On the basis of these relationships, a design procedure to optimise the sizing strategy has been proposed.
    No preview · Article · Dec 2014 · Powder Metallurgy
  • C. Menapace · G. Cipolloni · A. Molinari

    No preview · Article · Dec 2014 · Materials Science Forum
  • Ibrahim Metinoz · Ilaria Cristofolini · Alberto Molinari
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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.
    No preview · Article · Oct 2014 · Journal of Materials Engineering and Performance
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    ABSTRACT: A conservative approach to predict the Rolling Contact Fatigue (RCF) behavior of two different sintered and heat treated steels with heterogeneous microstructure was proposed. It is based on the assumption that the RCF crack nucleation is anticipated by the local plastic deformation of the material, which occurs when the maximum local stress, calculated using equations (1), (2) and (3), exceeds the yield strength of the matrix, calculated using equation (5). Two steels were considered, having composition, density, fractional porosity and elastic constants reported in Table 1. The theoretical predictions were validated by contact fatigue experiments carried out with a test configuration (disk-on-disk) shown in Figure 2. The fraction of the load bearing section, which influences the maximum stress, has been calculated by equation (4); the shape factor of the pores was measured by Image analysis on metallographic images (Figure 2), and both the whole of the pore population, as well as the larger pores corresponding to 10% of the whole population were considered, obtaining the results reported in Table 2. Since the microstructure of the two steels is heterogeneous (Figure 3), not only the mean microhardness reported in Figure 4 was considered, but even the microhardness of the microstructural constituents where the large pores are located. This way, a mean approach and a localized approach were used to implement the theoretical model. Figure 5 shows the results of theoretical prediction and of the experimental validation in case of material A under a mean pressure of 600 MPa. The yield strength of the matrix (5a) and the maximum stress profile (5b) were calculated with the two approaches above described, and the difference between yield strength and maximum stress is plotted in fig. 5c. Only the localized approach predicts plastic deformation in the subsurface layers and, in turn, crack nucleation, as actually observed in the metallographic section of the tested specimen. Indeed, large pores are localized in the softer constituent. In case of material B at the same mean pressure (figure 6), the two approaches do not predict crack nucleation, which in fact does not occur. On increasing mean pressure up to 1 GPa (figure 7), the mean approach predicts crack nucleation, whilst the localized one does not predict it. The experimental verification does not show any crack. In this case, the large pores are localized in the harder constituent, and the mean approach underestimates the resistance to plastic deformation of the matrix subject to the enhanced stress. The theoretical model proposed works satisfactorily in predicting the contact fatigue behavior of the two materials (it also has been verified on other sintered steels), provided that the peculiar characteristics of the microstructure of these materials are taken into account, which means that the model has to applied with a local approach.
    No preview · Article · Oct 2014 · La Metallurgia Italiana
  • S. Zanzarin · S. Bengtsson · L. Maines · A. Molinari
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    ABSTRACT: High power diode laser with coaxial powder injection was used to deposit single tracks of austenitic stainless steel on to a carbon steel plate, and the influence of heat input and powder density on energetic efficiency of the process, as well as on some geometrical features of the clad and on dilution was investigated. The energetic efficiency, calculated as the energy used to form the clad and the Heat Affected Zone, tends to increase with the powder density, while it decreases on increasing heat input. The combination of a high powder density and a low heat input is expected to optimize the energetic efficiency. In these conditions, the chemical dilution of the clad is minimized whilst aspect ratio is rather low but still acceptable. Moreover, the energy spent for the powder and the substrate can be correlated to the iron contamination of the clad.
    No preview · Article · Sep 2014 · La Metallurgia Italiana
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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.
    No preview · Article · Sep 2014 · International Journal of Precision Engineering and Manufacturing
  • Ibrahim Metinoz · Ilaria Cristofolini · Alberto Molinari
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    ABSTRACT: The dry-sliding wear behavior of two high-density, sinter-hardened, powder metallurgy (PM) steels was found to depend on the formation and stability of a protective oxide layer on the contact surface. The presence of soft nickel-rich austenite in a nickel-alloyed steel prevents the formation of the layer and, therefore, the nickel-free steel shows superior wear resistance. A design procedure for parts subject to dry-sliding wear applications is proposed based on the maximum acceptable wear depth. The influence of sliding conditions on wear depth is calculated and the possibility of improving the performance of parts by varying the material characteristics and part geometry is discussed.
    No preview · Article · Sep 2014 · International Journal of Powder Metallurgy
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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%.
    No preview · Article · Sep 2014 · Materials Science and Engineering A
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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.
    No preview · Article · Aug 2014 · Materials Characterization
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    ABSTRACT: A superaustenitic stainless steel containing 20 w/o Cr, 19 w/o Ni, and 6.3 w/o Mo (balance iron) was sintered in a vacuum furnace varying the sintering atmosphere and temperature. Sintering was carried out with and without nitrogen backfilling at temperatures in the range 1,250 degrees C-1,350 degrees C. The two conditions were compared in order to assess the effect of microstructural changes induced by nitrogen pickup on density, microstructure, mechanical properties, and corrosion resistance. Sintering in nitrogen suppresses the formation of sigma phase, whereas sintering in vacuum leads to an enhanced densification due to the formation of delta ferrite at high temperature. A post-sinter solution anneal was required to eliminate the sigma phase in specimens sintered in vacuum. Mechanical properties are significantly affected by nitrogen pickup in specimens with a low level of porosity and hence a high fraction of the cross-sectional area carrying the load. Ductility is affected negatively by sigma phase, and a solution anneal is needed to recover tensile elongation. Corrosion behavior in sulphuric acid depends primarily on sintered density and porosity, and on sigma phase in NaCl solution.
    Full-text · Article · Jun 2014 · International Journal of Powder Metallurgy
  • K. Bendo Demetrio · C. Menapace · A. Molinari
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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.
    No preview · Article · Apr 2014
  • A. Molinari · E. Bisoffi · C. Menapace · J. Torralba
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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.
    No preview · Article · Feb 2014 · Powder Metallurgy
  • S. Diouf · C. Menapace · M. D’Incau · A. Molinari · G. Ischia
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    ABSTRACT: The densification and sintering behaviour of a cryomilled copper powder (grain size of 17±2 nm and dislocation density of 6.26±0. 04×1016 m-2) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400°C. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.
    No preview · Article · Dec 2013 · Powder Metallurgy
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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.
    Full-text · Article · Nov 2013 · Materials Letters

Publication Stats

2k Citations
233.16 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
  • 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
  • 2004
    • Università degli studi di Cassino e del Lazio Meridionale
      • Department of Mechanics, Structures, Environment and Territory (DIMSAT)
      Cassino, Latium, Italy
  • 1998
    • Nottingham Trent University
      Nottigham, England, United Kingdom