Cinzia Menapace

Interpretation of effect of matrix hardening on tensile and impact strength of sintered steels

Abstract Sintered steels in the density range 6.7-7 g cm- 3 and with a wide range of matrix hardnesses were produced with the main objective of understanding the interplay between porosity and matrix strength in determining their tensile and impact behaviour. The materials can be subdivided into two groups, characterised by different deformation and fracture mechanisms. The first group pertains to the as sintered materials (with a microhardness lower than 350HV0.1).They are characterised by a strain hardening stage before fracture, and both tensile strength and impact fracture load increase as density and matrix strength are increased. The second group pertain to the heat treated materials with a higher matrix hardness. These materials are characterised by a macroscopically brittle behaviour, since fracture takes place before general yielding. Because of the high matrix hardness, the peak stresses at the pore edges are not relieved by plastic deformation, and the pores behave as internal cracks. Fracture is then attained once the local applied stress intensity factor becomes equal to the matrix fracture toughness.

Working hypothesis for origin of anisotropic sintering shrinkage caused by prior uniaxial cold compaction

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.

Spark plasma sintering and hot compression behaviour of AZ91 Mg alloy

Abstract In the present investigation, the microstructural characterisation of the AZ91 Mg alloy produced by spark plasma sintering (SPS), as well as the evaluation of its hot compression behaviour, has been performed. Based on the differential scanning calorimetry analyses of the starting powders, three SPS cycles are investigated, using temperatures of 400 and 450°C, and at 450°C with previous solubilisation soaking at 420°C. Despite different microstructural and hardness characteristics, the three alloys display similar hot compression behaviour. At 200°C, the formation of an unstable crack, which propagates at 45° with respect to the loading axis, is observed after the occurrence of the peak load. At higher testing temperatures, after reaching the peak stress, the flow stress decreases slowly with increased strain of ∼0·51. Such behaviour corresponds with the observation of an accelerated cracking due to the propagation of decohesions at the interparticle regions. Ultimately, SPS allowed for attainment of high relative density; however, the sintering degree of the materials was quite low.

Surface self-densification in boron alloyed austenitic stainless steel and its effect on corrosion and impact resistance

Abstract A boron alloyed AISI 316L, sintered in pure hydrogen at 1250°C, was investigated to study the effect of the microstructure, with particular reference to the near full dense and boride free surface layers, on corrosion resistance and impact properties. The near full dense and homogeneous austenitic surface layer results from the flow of the liquid towards the bulk of the specimens during sintering. It significantly improves the corrosion resistance, while impact properties are strongly influenced by the bulk microstructure, where the boride network creates favourable conditions for both nucleation and propagation of crack.

Properties of AZ91 alloy produced by spark plasma sintering and extrusion

Abstract Mg alloys are characterised by several promising properties, including a good biocompatibility. In this work, a commercial AZ91 powder was used to produce cylindrical specimens by spark plasma sintering (SPS), and the specimens were further consolidated by hot extrusion. The SPS materials were found to be quite brittle because of the low bonding between the original powders. After hot extrusion, however, they displayed a ductile behaviour as revealed by room temperature tensile tests and hot compression tests. The metallographic investigation showed that extrusion induced a dynamic recrystallisation with grain refinement, but also an increase in the β phase content, which reduced the corrosion resistance of the materials.

Shrinkage kinetics during early stage sintering of cold isostatically compacted iron powder

Abstract Dilatometry experiments have been carried out to investigate the shrinkage kinetics on cold isostatic pressed iron specimens in the 550–730°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.

Selective laser melting of Ti6Al7Nb with hydroxyapatite addition

Purpose – The purpose of this paper was to obtain by means of selective laser melting and then characterize biocomposites of medical-grade Ti6Al7Nb with hydroxyapatite (2 and 5 vol.%) and without hydroxyapatite, as reference. Design/methodology/approach – Rectangular samples were manufactured with the same scanning strategy; the laser power was between 50 W and 200 W. Processed samples were analysed by means of optical microscopy, scanning electron microscopy and microhardness. Findings – The results showed that despite the very short processing times, hydroxyapatite decomposed and interacted with the base Ti6Al7Nb material. The decomposition degree was found to depend on the applied laser power. From the porosity and bulk microstructure point of view, the most appropriate materials for the purposed medical applications were Ti6Al7Nb with hydroxyapatite processed with a laser power of 50 W. Originality/value – The originality of the present work consists in the study of the behaviour and interaction of hy...

Study of microstructural transformations and dimensional variations during liquid phase sintering of 10% tin bronzes produced with different copper powders

Abstract The microstructural evolution during sintering of 10% bronzes obtained by mixing elemental powders was studied, in order to investigate the mechanisms responsible for microstructural homogenisation, consolidation of the green compacts, and dimensional variation. The effect of the copper powder was also studied using an electrolytic and an atomised commercial powder. As a consequence of the different surface area and surface chemistry, and of the presence of internal pores and microcavities in the atomised copper powder, the two mixes display significantly different dimensional behaviour but, under specific time and temperature conditions, they give rise to the same dimensional variation and the same microstructural characteristics.

Spark plasma sintering of cryomilled copper powder

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.

Nanostructured bainitic steel obtained by powder metallurgy approach: structure, transformation kinetics and mechanical properties

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.