Fátima Ternero

On the compressibility of metal powders

ABSTRACT A new equation relating the porosity of green compacts and the applied external pressure during the cold die compaction of metal powders is proposed. All of the parameters in the model have a clear physical meaning. These parameters are those related to the plastic behaviour of the material, as well as to the ‘structural resistance’ of the powder mass. Also the friction between the powders and die walls is considered, as a kind of constraint that diminishes the local pressure borne by the fully dense material. The model includes, as a key parameter, the tap porosity of the powders (an extremely useful property that contains the morphometric information of the powder). The proposed model has been experimentally checked with the compressibility curves obtained with five metal powders of different types. The agreement between the model and experimental data is reasonable over the tested pressure range.

In Situ Synthesis of Al-Based MMCs Reinforced with AlN by Mechanical Alloying under NH3 Gas

Aluminum matrix composites (AMCs) reinforced by aluminum nitride were prepared by mechanical alloying followed by a simple press and sintering method. Milling began under vacuum and after a period of between 1 and 4 h, NH3 gas flow (1 cm3/s) was incorporated until the total milling time of 5 h was reached. Results show that in addition to the strain hardening taking place during mechanical alloying, NH3 plays an additional role in powder hardening. Thereby, the properties of the sintered compacts are strongly influenced by the amount of N incorporated into the powders during milling and the subsequent formation of AlN during the consolidation process. The obtained AMC reaches tensile strengths as high as 459 MPa and hardness much higher than that of the as-received aluminum compact.

Amorphous Phase Formation and Heat Treating Evolution in Mechanically Alloyed Al-Ti Powders

This paper focuses on the microstructural characterization of Al25Ti75, Al37Ti63, Al50Ti50, Al63Ti37 and Al75Ti25 powders mixtures prepared by mechanical alloying (MA). The high-energy ball-milling, up to 75 h, of aluminium and titanium powders leads to a nanocrystalline or an amorphous structure. It is showed that a stable amorphous Al–Ti phase with uniform elemental distribution forms after 50 h of milling in Al50Ti50 alloy. Heat treatment of the different alloys leads to the crystallization of AlTi3, AlTi, Al2Ti and Al3Ti intermetallic compounds. A comprehensive study by laser granulometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) was carried out on the structure, surface morphology and thermal behaviour of the MA Al-Ti mixtures, both of milled and heat treated powders.

Medium-Frequency Electrical Resistance Sintering of Highly Oxidized Iron Powder

Highly oxidized iron powders were consolidated by means of the medium-frequency electrical resistance sintering technique (MF-ERS). In order to activate the powders and to disperse the oxides coating the particles, prior to the consolidation process, powders were milled in a high-energy mill for 7 minutes. Structural and mechanical characterisations of electrically consolidated compacts were carried out in order to study the effect of two main processing parameters (current intensity and heating time). The compact properties resulted to be very sensitive to these parameters, especially to the current intensity. A change from 5 kA to 10 kA in the current intensity makes the porosity to fall from 30% to 8%. Moreover, using a higher current intensity (10 kA) increases the mechanical properties of the final compacts: micro-hardness change in almost 50 HV, up to 104 HV 1, and compression resistance by around 500 MPa, up to 569 MPa.

Medium-Frequency Electrical Resistance Sintering and Electrical Discharge Consolidation of Metallic Powders

Compacts of iron powders were prepared by medium-frequency electrical resistance sintering (MF-ERS) and electrical discharge consolidation (EDC). Structural and mechanical characterization was carried out in order to study the effect of the main processing parameters (current intensity and sintering time in MF-ERS and voltage and capacity in EDC). The compact properties resulted to be quite sensitive to the consolidation method and parameters. Porosities around 8% and microhardness of about 120 HV were reached. It is concluded that the MF-ERS process can be a best option for the consolidation of cemented carbide composites with composition WC-6wt.%Co. MF-ERS compacts of this composite show a very low porosity and reasonable uniform microstructure, preserving the original ultrafine grain size and an adequate hardness with a very quick processing cycle of the order of one second.