Harris L. Marcus

Elastic constants versus melting temperature in metals

Examen detaille des relations empiriques existant entre les constantes elastiques et la temperature de fusion de metaux afin de determiner le domaine dans lequel ils peuvent etre utilises. Importance des valeurs de limite de solubilite des alliages

Selective laser sintering of alumina with polymer binders

The selective laser sintering (SLS) process is used to prepare test bars from Al2O3/polymer binder powders. Finds that binder‐coated A12O3 particles formed bars that were approximately twice as strong as could be formed from mixtures of alumina and polymer binder at the same binder level and processing conditions. In mixed systems, bar strengths increased nearly in proportion to increases in polymer binder content over the 20‐40 per cent volume binder range. Parts made in any particular laser scanning mode showed optimum values for strength and density as the laser energy density was systematically increased from 2‐8cal/cm2. Suggests that optima result from the counteracting influences of energy density on binder fusion and thermal degradation. The optimum energy density is mode or geometry sensitive and shifts to lower values as the laser scanning vector is reduced. Concludes that this behaviour is probably the result of the lower heat losses. Equivalently better utilization of laser energy is associated with the shorter scan vectors. Some of the SLS fabricated bars were infiltrated with colloidal alumina, fired to remove the binder, and sintered at 1,600°C to achieve alumina bars with 50 per cent relative densities, interconnected porosity, and strengths between 2 and 8MPa.

Post‐processing of selective laser sintered metal parts

Gives a brief overview of post‐processing of selective laser sintered (SLS) metal parts to improve structural integrity and/or to induce a material transformation. Presents results which show the effect of post‐processing liquid phase sintering temperature and time on material properties. Describes the hot isostatic pressing process, and discusses its application to SLS metal parts. Results gained from using this process show that it is suitable for achieving almost full‐density parts.

Residual stress measurements on Al-graphite composites using X-ray diffraction

Abstract During the fabrication of an aluminum matrix-graphite fiber composite, a residual stress state is introduced at the matrix-fiber interface because of the different thermal expansion coefficients of aluminum and graphite. These stresses can be different along different directions because of the anisotropy of the graphite fiber. In this study an X-ray diffraction technique was used to measure these residual stresses. It was found that large stresses exist along the fiber direction. Lower stresses were observed in the transverse direction. Some samples were quenched in liquid nitrogen and the residual stress was measured at room temperature. Approximately 30% reduction relative to the residual stress of non-quenched samples was observed. The results can be related to interface structures and transverse properties of the composites.

Selective Laser Sintering of Alumina Using Aluminum Binder

Abstract Selective Laser Sintering (SLS) process has been used to make shapes from Al203 using Al as binder. SLS is a rapid manufacturing process that uses data from Solid Modeling systems to guide a laser beam and rapidly form 3-D shapes from powder without any part specific tooling. The aluminum melts under the laser and bonds the alumina particles. Some of the aluminum reacts with the ambient (air) to form alumina. The residual aluminum is oxidized in a subsequent he at-treatment step. The effect of parameters in the SLS step and heat-treating step on the mechanical properties and density of the part is examined. Linear expansion of the parts with oxidation heat-treatment is also examined.

NANOPHASE MATERIALS IN SOLID FREEFORM FABRICATION

Solid freeform fabrication (SFF) is a manufacturing technology that produces parts directly from computer-aided design databases. Examples of the SFF approach are selective laser sintering (SLS) and selective laser reactive sintering (SLRS), both of which have the potential to directly produce structurally sound metallic or ceramic parts. The development of suitable materials systems that can optimize the SLS or SLRS processes are critical to this technology. For instance, nanocomposites, in which the constituents are mixed on a nanometer scale, have the potential to provide important advantages in the SLS and SLRS processes. One strategy is to design and develop nanocomposites in which one nanosize component has a lower melting point than the other nanosize component, either of which can serve as the matrix phase. The nanoscale dispersion of the low-melting component can aid the sintering process during SLS or SLRS. In this article, the philosophical basis for SLS and SLRS of nanocomposites is discussed. Conceptual design of nanocomposite systems and the SLS/SLRS results of a few exploratory systems are presented.

The behavior of temper embrittlement in a secondary-hardening 4.2wt.%Mo-0.4wt.%C-0.06wt.%P steel

Abstract Grain boundary embrittlement associated with phosphorus grain boundary segregation in a 4.2wt.%Mo-0.4wt.%C-0.06wt.%P steel was studied by Auger electron spectroscopy, scanning electron microscopy and hardness measurements. It was found that this steel exhibits a secondary-hardening behavior during tempering and high grain boundary embrittlement susceptibility accompanied by secondary hardening. From the information on the variations in hardness, the phosphorus grain boundary concentration and the percentage of intergranular fracture as functions of tempering temperature, it was shown that grain boundary embrittlement is enhanced by the combined effects of the increase in phosphorus grain boundary segregation and the increase in hardness during tempering in the secondary-hardening temperature range. Also, it was found that phosphorus grain boundary segregation and grain boundary embrittlement are strongly affected by the amount of free molybdenum in the matrix.

High-energy, high-rate consolidation of tungsten and tungsten-based composite powders

Abstract Tungsten and tungsten-based heavy alloys are well known for their superior mechanical properties at elevated temperatures. However, unalloyed tungsten is difficult to consolidate owing to its very high melting temperature (3683 K). The additions of small amounts of low-melting elements such as iron, nickel, cobalt and copper, facilitate the powder processing of dense heavy alloys at moderate temperatures. Energetic high-current pulses have been used recently for powder consolidation. In this paper, the use of a homopolar generator as a power source to consolidate selected tungsten and tungsten-based alloys is examined. Various materials were consolidated including unalloyed tungsten. WNb, WNi, and tungsten heavy alloys with boron carbide. The effect of process parameters such as pressure and specific energy input on the consolidation of different alloy systems is described in terms of microstructure and property relationships.

High Tc dual phase Ag-YBa2Cu3O7-x composites prepared by selective laser sintering and infiltration

Bulk porous samples of YBa2Cu3O7−x were made from powders by selective laser sintering, a near-net-shape forming technology requiring no part-specific tooling. The porous parts were densified by infiltrating silver into the pores, resulting in a dense, dual-phase superconducting composite. The laser-processing parameters were varied to obtain the optimum microstructure. The laser-sintered parts required oxygen annealing after infiltration to restore the orthorhombic, superconducting structure. X-ray diffraction (XRD) and Tc measurements indicated that some impurity phases were present in samples processed under aggressive laser conditions.

Ceramic Composites Fabricated by Selective Laser Sintering

Abstract Selective laser sintering (SLS) uses a computer controlled laser beam to create objects directly from the CAD data without part-specific tooling. In this paper, the effect of material and processing parameters on the microstructure and mechanical properties of an alumina/aluminum phosphate composite fabricated by SLS is described. The precursor material is a blend of alumina and ammonium phosphate powder. This paper deals primarily with the effect of alumina particle size and material composition on the strength of the composite. A constitutive equation is also proposed to relate the mechanical properties of the composite to its microstructural characteristics.