E.J. Herrera

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.

Sintering of Al/AlN composite powder obtained by gas–solid reaction milling

Abstract Attrition milling in ammonia gas was used to obtain aluminum composite powder by partial nitriding of Al powder. The composite powder is very hard, thermally stable and shows excellent sinterability. The reason for the high sinterability is not clear at this stage of the research. Samples consolidated by cold pressing and vacuum sintering achieve a hardness of 167 HB and a bending strength of 920 MPa.

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.

Thermal and electrical conductivities of sintered powder compacts

Abstract A simple new equation for calculating thermal and electrical conductivities of powder sintered compacts is proposed. In this equation, the effective conductivity of the sintered compact is a function of the solid material conductivity, the degree of compact porosity and the packing porosity of the starting powder. The latter parameter determines to a great extent the compact pore structure, since it depends on powder particle size, shape and distribution. The new equation is physically applicable from zero porosity to the packing porosity. The conductivity equation has been evaluated using sintered compacts of iron, nickel, titanium and aluminium. The compacts were prepared by cold pressing and sintering. Results are in very good agreement with theoretical predictions.

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.

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.

Wear of aluminium-base materials processed by mechanical milling in air or ammonia

Abstract The tribological behaviour of mechanically alloyed (MA) aluminium-base materials has been studied as a function of processing methods. In particular, a new sintered MA aluminium material, milled under an ammonia atmosphere (MA Al-NH 3 ) and consolidated by a single cycle of cold compaction and sintering, is compared with conventional MA aluminium (MA Al-air), milled in confined air and consolidated by a double cycle of compaction and sintering. The wear behaviour of the aluminium-base materials has been studied in a pin-on-disk tribometer, sliding against AISI 52100 steel pins. Dry wear tests have been carried out under variable load and temperature, to show that milling conditions, specifically milling atmosphere, have a strong influence both on mechanical properties and wear resistance. The higher wear resistance of MA Al-NH 3 with respect to MA Al-air increases with the severity of the contact conditions. Wear mechanisms are discussed from SEM observation of wear tracks, wear debris morphology and transfer tribolayers.

Electrical conductivity of sintered powder compacts

Abstract Some equations for calculating the electrical conductivity of porous materials are reviewed and their applicability to sintered powder compacts is discussed. A previous equation proposed by the authors, in which the effective conductivity of a sintered compact is composed of a function of the fully dense material conductivity, the compact porosity degree and tap porosity of the starting powder, is reviewed. The primary aim of the present work is to establish a connection between such equation and the percolation conduction theory.

Permanent elimination of the yield-point phenomenon in AISI 430 stainless steel by skin-pass rolling

The different yielding behaviour of skin-pass and skin-pass plus annealed samples of AISI 430 stainless steel and its dependence on substructure has been studied. Skin-pass specimens show no yield elongation in tensile testing and do not strain age at room temperature. According to TEM observations, this seems to be due to submicroscopic precipitation of low-carbon carbonitrides — most probably Cr2 (C,N) — preferably on grain boundaries. The locking of interstitials (N, C) by these precipitates could explain the absence of discontinuous yielding. On the other hand, annealing at 700‡C of skin-pass samples dissolves the carbonitride precipitates, interstitial solutes are able to segregate on dislocations, and the pinned dislocations give rise to yield-point phenomena.

Some case histories of wear-related failures of metallic materials

Abstract Several case histories of wear-related failures of metallic parts are presented. In addition to failure analysis, corrective measures are outlined. The materials studied were austenitic manganese steels, tool steels, cast iron and a nickel alloy. Wear-induced failures occurred by mechanisms of abrasion, cavitation and erosion-corrosion. Failures were due to improper choice of material, process embrittlement, the use of inappropriate alloys, faulty heat treatment, poor design, defective materials, lack of environmental control or a combination of some of the above causes.