J. M. Gallardo

Structure and properties of attrition-milled aluminium powder

Aluminium powder has been attrition milled in the presence of 1.5 wt% of a wax. The aim was to achieve a mechanically alloyed powder amenable to powder metallurgy processing. Changes in particle size and form, microstructure, hardness and other properties of powders with milling time has been studied. Under the experimental conditions employed, a process time of 10 h was selected for the mechanical alloying of Al powder. The powder milled in this way shows a Vickers microhardness (127 HV) more than six times higher than the starting powder (20 HV), a coarser particle size (mean particle size is doubled) and a better flowability.

Failure of gas turbine blades

The first-stage blades of a gas turbine had suffered a severe deterioration after around 10 500 h service. The expected service life was 40 000 h. Failure analysis (visual observations, studies by optical microscopy, scanning electron microscopy (SEM), SEM back-scattered electron (SEM-BSE), EDX, X-ray diffraction (XRD) and dimensional metrology) has been carried out. Blades, manufactured in the nickel superalloy CMSX-4, lost the protective coatings from their tips due to wear. Unprotected surfaces suffered high-temperature hot corrosion (Type-I corrosion). It is concluded that failure was mainly caused by an uneven clearance (out-of-line) between rotor and lining.

Games with fuzzy authorization structure

A cooperative game consists of a set of players and a characteristic function which determines the maximal gain or minimal cost that every subset of players can achieve when they decide to cooperate, regardless of the actions that the other players take. It is often assumed that the players are free to participate in any coalition, but in some situations there are dependency relationships among the players that restrict their capacity to cooperate within some coalitions. Those relationships must be taken into account if we want to distribute the profits fairly. In this respect, several models have been proposed in literature. In all of them dependency relationships are considered to be complete, in the sense that either a player is allowed to fully cooperate within a coalition or they cannot cooperate at all. Nevertheless, in some situations it is possible to consider another option: that a player has a degree of freedom to cooperate within a coalition. A model for those situations is presented.

Games with fuzzy permission structure: A conjunctive approach

Abstract A cooperative game consists of a set of players and a characteristic function which determines the maximal gain or minimal cost that every subset of players can achieve when they decide to cooperate, regardless of the actions that the other players take. A permission structure over the set of players describes a hierarchical organization where there are players who need permission from certain other players before they are allowed to cooperate with others. Various assumptions can be made about how a permission structure affects the cooperation possibilities. In the conjunctive approach it is assumed that each player needs permission from all his superiors. This paper deals with fuzzy permission structures in the conjunctive approach. In this model, players could depend partially on other players, that is, they may have certain degree of autonomy. First, we define a value for games with fuzzy permission structure that only takes into account the direct relations among players and provide a characterization for this value. Finally, we study a value for games with fuzzy permission structure which takes account of the indirect relations among players.

Influence of PCA Content on Mechanical Properties of Sintered MA Aluminium

Aluminium powder has been mechanically milled using different amounts of process control agent (PCA). Mechanically alloyed aluminium powder (MA Al) was prepared by attrition milling in the presence of 1.5 and 3wt.% of an EBS wax. Milling was carried out in vacuum during 10 h. Milled powders were consolidated by a press-and-sintering method. This consolidation method is not usually employed with MA Al powders. The amount of dispersed carbides formed in the Al powder increases with the percentage of PCA. These carbides restrain Al grain growth during sintering, resulting in consolidated compacts with a grain size of about 550 nm. Thus, these PM materials can be considered ultrafine grained materials. Due to grain refinement and dispersion strengthening, the tensile strength of MA Al specimens is increased remarkably.

Production of Al–Al3Ti powders by mechanical alloying and annealing

Abstract Mixed powders of Al and Ti (10 wt-%) have been mechanically alloyed in an attritor mill under vacuum or nitrogen atmosphere. Pure aluminium powders have also been prepared, in the same conditions, for comparison. After milling for 10 h, a metastable solution of Ti in an Al matrix is obtained, with ∼9 wt-%Ti dissolved in the matrix. The evolution of these powders during milling is reported. Their thermal stability has been studied using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), identifying the observed changes by X-ray diffraction (XRD). Annealing of these powders at different temperatures, up to a maximum of 625°C, produces the precipitation of new phases, such as Al4C3 and different structures of Al3Ti, as well as grain growth. The appearance of these second phases, and their influence on powder microhardness, has been characterised as a function of the selected heat treatment temperature.

Consolidation of mechanically alloyed aluminium by double cold-pressing and sintering

Abstract The present work describes a new processing route of mechanically alloyed aluminium powder. The attrition milled and degassed powder was processed by a fairly conventional PM method consisting of a double cycle of cold pressing and vacuum sintering. A systematic series of experiments were conducted to find optimum values of compaction pressures and sintering temperatures. The final consolidated compacts have relative densities and mechanical properties (hardness -at room temperature and at high temperature- modulus of rupture, ultimate tensile strength and ductility) comparable to those obtained using well-established hot-working processes.

Electrical conductivity of metal powder aggregates and sintered compacts

New equations for computing the electrical conductivity of powder aggregates and sintered compacts are proposed. In both cases, the effective or apparent conductivity is a function of the bulk material conductivity, the porosity of the sample and the tap porosity of the starting powder. Additional parameters are required for powder aggregates, such as the conductivity of the oxide covering the particles, the thickness of the oxide layers and the ease of descaling them. The new equations are valid from zero porosity to the tap porosity. Links between the equations and the percolation conduction theory are stated. Measurements of electrical resistance on sintered compacts and powder aggregates subjected to different pressures were performed. The proposed equations have been validated with these data. The electrical conductivity of both sintered compacts and powder aggregates of aluminium, bronze, iron and nickel was determined and compared to the equation predictions, resulting in notably good agreement.

Fuzzy restrictions and an application to cooperative games with restricted cooperation

Abstract The concept of restriction, which is an extension of that of interior operator, was introduced to model limited cooperation in cooperative game theory. In this paper, a fuzzy version of restrictions is presented. We show that these new operators, called fuzzy restrictions, can be characterized by the transitivity of the fuzzy dependence relations that they induce. As an application, we introduce cooperative games with fuzzy restriction, which are used to model cooperative situations in which each player in a coalition has a level of cooperation within the coalition. A value for these games is defined and characterized.

Green and sintered properties of consolidated mixtures of mechanically alloyed and elemental Al powders

Mechanically alloyed aluminium, MA Al, powder is difficult to consolidate. Consolidation often involves complex processing that includes a hot extrusion stage. An alternative consolidation method consisting of a press-and-sinter process has been developed at the University of Seville. Nevertheless, sintered MA Al compacts have a low ductility. In the present work, to improve the ductility of consolidated compacts, hard MA Al powder was blended prior to the consolidation processing with different amounts (10, 20, 30 wt%) of soft unmilled Al powder. The bimodal microstructure (hard/soft) of the final compacts makes it possible to balance strength and ductility values.