Mario Zadra

SPS, binderless WC powders, and the problem of sub carbide

Binderless tungsten carbide powders can be sintered to full density using spark plasma sintering through a combination of high heating rates and pressure. But although the pathway looks promising, there can be problems…

Tribological properties of surface engineered hot work tool steel for aluminium extrusion dies

Abstract A tribological test aimed at the simulation of aluminium hot extrusion was performed. A 6082-Al disc, induction heated up to a maximum surface temperature of 350°C, is allowed to rotate against a conformal hot work steel block, simulating the extrusion die. The test simulates the wear mechanism observed under in service conditions. After an initial period, the hot plasticised Al progressively adheres to the steel counterpart thus allowing direct Al–Al contact. The high shear stresses given by the strong adhesion produce the nucleation of deep cracks which strongly deteriorate the surface of the steel by delamination. Hot work tool steel (AISI H11) is employed as extrusion dies. Salt bath nitriding (Tenifer) is usually carried out to improve wear resistance. The possible application of PVD (CrN, TiAlN) and CVD (TiC + TiN) hard coatings, exhibiting lower compatibility versus Al, has been evaluated in the present work. The occurrence of two distinct damage regimes was detected. Regime 1 shows minor excursions of the friction coefficient μ and is followed by regime 2 which is characterised by fully plastic Al–Al contact, with large fluctuations of μ. A time to transition tT has been identified as indicative of the ability of a surface layer in delaying the occurrence of regime 2. tT is correlated to the chemical compatibility of the surface layer (compound layer, PVD or CVD coating) versus Al. Furthermore, tT is related to the mechanical stability of the surface layer. A generalised and severe form of wear is displayed by nitrided steel while a localised and less severe form of wear is displayed by duplex treated, nitriding + PVD steel. No wear traces were observed by CVD TiC + TiN coated steel, providing the best performance among the surface treatment investigated.

Development of a Hybrid Tool Steel Produced by Spark Plasma Sintering

The production of a new Powder Metallurgy (PM) tool steel obtained by Spark Plasma Sintering (SPS) powder blends of AISI H13 and AISI M2 is considered. The objective is to obtain a product, the properties of which could be modulated on the basis of the specific application considered. Sintering powders of four blends were obtained (20% H13–80% M2, 40% H13–60% M2, 60% H13–40% M2, and 80% H13–20% M2). Density, hardness, fracture toughness, and wear resistance were evaluated. Increasing the fraction of M2 improves hardness and wear resistance. Fracture toughness is correspondingly lowered. The toughness of H13 rich blends was negatively affected by the high oxygen content in the base powder.

Microstructure and Mechanical Properties of Nanostructured Aluminum Consolidated by SPS

Nanostructured aluminum powders were obtained by means of planetary ball milling with methanol as the Process Control Agent (PCA). The behavior, during milling, was considered measuring the microhardness and grain size at different milling times. Bulk near-full density samples were sintered using the Spark Plasma Sintering technology with different schedules: temperature of 500°C and 550°C, pressure of 30 MPa and 60 MPa and different modes of applying the pressure were changed in order to understand the behavior during sintering. The samples sintered at 500°C showed a density of about 2.4-2.61 g/cm3 while for that sintered at 550°C it was 2.65-2.67 g/cm3 depending on the applied pressure. All the samples retained their nanostructure with an increase of the grain size from about 46 up to 70-90 nm. Using X-ray diffraction and metallography the formation of Al4C3 carbides was detected for samples sintered at highest temperatures.

Spark Plasma Co-Sintering of Mechanically Milled Tool Steel and High Speed Steel Powders

Hot work tool steel (AISI H13) and high speed steel (AISI M3:2) powders were successfully co-sintered to produce hybrid tool steels that have properties and microstructures that can be modulated for specific applications. To promote co-sintering, which is made difficult by the various densification kinetics of the two steels, the particle sizes and structures were refined by mechanical milling (MM). Near full density samples (>99.5%) showing very fine and homogeneous microstructure were obtained using spark plasma sintering (SPS). The density of the blends (20, 40, 60, 80 wt % H13) was in agreement with the linear rule of mixtures. Their hardness showed a positive deviation, which could be ascribed to the strengthening effect of the secondary particles altering the stress distribution during indentation. A toughening of the M3:2-rich blends could be explained in view of the crack deviation and crack arrest exerted by the H13 particles.

Design of Layered Metal‐Ceramic FGMs Produced by Spark Plasma Sintering

The production of layered Nickel‐Alumina FGMs by means of Spark Plasma Sintering is studied by investigating the effect of sintering under temperature gradient on final properties, i.e. density, hardness and fracture toughness. Results demonstrate that alumina powder can be fully densified at 1400 °C, whilst a temperature of 1050 °C is sufficient to consolidate nickel powder. The temperature gradient, obtained with the proper design of the die configuration, is useful for reduction of thermal stresses, but does not permit the full consolidation of the intermediate layers.

Spark Plasma Sintering of Titanium Alloys for Biomedical Applications

A dense Ti6Al4V with a porous cp2-Ti surface layer was produced by Spark Plasma Sintering, using a Calcium Phosphate powder as space holder. The duplex porosity structure resulting from the space holder (large pores) and the uncompleted densification of the titanium powder (micrometric pores) has a very positive effect on both cell in-growth and cell proliferation.The porous structure decreases the fatigue resistance significantly, even if less than what reported in literature, likely due to the formation of a globular microstructure in the interface region of the substrate, where fatigue crack nucleation occurs. The porous surfaced specimens succeeded the adhesion by pull-off test, the resistance to static and cyclic shear stresses and the Taber abrasion test.