E.S. Caballero

Nanocrystalline Al Composites from Powder Milled under Ammonia Gas Flow

The production of high hardness and thermally stable nanocrystalline aluminium composites is described. Al powder was milled at room temperature in an ammonia flow for a period of less than 5 h. NH3 dissociation during milling provokes the absorption, at a high rate, of nitrogen into aluminium, hardening it by forming a solid solution. Controlled amounts of AlN and Al5O6N are formed during the subsequent sintering of milled powders for consolidation. The pinning action of these abundant dispersoids highly restrains aluminium grain growth during heating. The mean size of the Al grains remains below 45 nm and even after the milled powder is sintered at 650°C for 1 h.

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.

Production of Aluminium Based MMCs by Short-Time Ammonia Gas Flow Milling

In order to produce metal matrix composites (MMCs), aluminium powder was milled for a total time of 5 hours. Aluminium nitride was the ceramic reinforcement chosen to improve the mechanical behaviour of the aluminium matrix. In order to form it in situ, an ammonia gas flow was incorporated during a certain period of the milling process. Two different conditions of NH3 flow during milling were studied: short time (5 min) and long time (3 h). In both cases, milling started with a 2 h period of mechanical alloy in vacuum (5 Pa). Then, NH3 was incorporated during the stipulated time (5 min or 3 h), after which the milling process continued under vacuum to complete 5 hours. The powders were cold pressed and vacuum sintered to produce compacts. The results showed that compacts with better mechanical properties are obtained when short duration ammonia gas flow is used. The use of short flows provides good control of the amount of ceramic second phases formed. This allows the produced compacts to reach ultimate tensile strength higher than 400 MPa.

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.

On the sintering conditions of CP titanium powder

The effect of sintering conditions on compacts made from commercially pure titanium (CP Ti) powder has been studied. Compacts were cold pressed to ensure interconnected porosity, and then, sintered with different temperature-time cycles. A detailed study has been carried out to determine the influence of the sintering conditions on mechanical properties. Properties result very sensitive to the sintering cycle, especially to the speed of the heating stage. A difference of 30 minutes in the duration of the heating ramp makes the ultimate tensile strength to change in almost 200 MPa.

Use of short-time NH3 gas flow during Al milling

Samples from mechanically alloyed aluminium powder were prepared by a simple press and sintering method in order to study the influence of a flow of ammonia gas during a short time of the milling process. All milling experiences were carried out at room temperature for a total of 10 hours. Millings were carried out in vacuum or under confined ammonia. This last type allows to incorporate nitrogen-rich second phases, mainly aluminium nitride (Al3CON) and oxynitride (Al5O6N), after powder sintering. To control the amounts of the second phases, a new milling type, using ammonia gas flow during 5 minutes followed by vacuum milling, was carried out. Testing of sintered samples shows that milling using ammonia, both confined and in flow, substantially improves mechanical properties. Furthermore, the use of a very short time gas flow allows obtaining compacts with similar tensile strength (485 MPa) to those obtained after milling in confined ammonia for 10 h.

Influence of composition on pitting corrosion of (MA)-Al reinforced with carbides and nitrides

Immersion tests from 2 to 96 h in 3.5 wt. % NaCl solution were used to study the corrosion behaviour of mechanically alloyed aluminium in-situ reinforced with carbides and nitrides. Samples were polished before the test, and then, image analysis techniques were used to characterize the initial porosity. After the immersion tests, samples were again analysed to obtain the pitting characteristics. The main conclusion was that the decrease of porosity in the specimens, as well as the formation of nitrides, increases the corrosion resistance.