Influence of the Total Porosity on the Properties of Sintered Materials—A Review

Porosity is a characteristic present in most sintered materials, full densification only being achieved in special cases. For some sintered materials, porosity is indeed a desired characteristic, serving for the intended application of the material. In any case, the porosity present in materials can have a strong effect on some of their properties, both structural and functional. In this paper, some of the expressions proposed to describe the influence of the total porosity on the effective properties of sintered materials are examined. Moreover, a universal expression (with two fitting parameters) valid to satisfactorily represent all the analysed behaviours is proposed. One of these parameters can be assimilated to the tap porosity of the powders used to manufacture the material. The properties examined were elastic moduli, ultimate strength, thermal and electrical conductivities, magnetic characteristics, and other properties directly related to these ones. The study is valid for sintered materials, both metallic and ceramic, with a homogeneous and non-texturised microstructure.

What We Should Consider for Full Densification when Sintering

To fully densify a powder compact, we should avoid two things: (i) entrapment of insoluble gases within pores and (ii) entrapment of isolated pores within grains. This paper describes general directions for promoting full densification in view of the above two points. Emphasis is placed on ways to potentially prevent pore entrapment in terms of grain growth control. Currently available techniques that can enhance densification while suppressing grain growth are briefly described, and their major mechanisms are discussed.

Densification, Microstructure and Properties of 90W-7Ni-3Fe Fabricated by Selective Laser Melting

The preparation of refractory tungsten and tungsten alloys has always been challenging due to their inherent properties. Selective laser melting (SLM) offers a choice for preparing tungsten and tungsten alloys. In this work, 90W-7Ni-3Fe samples were prepared by selective laser melting and investigated. Different process parameter combinations were designed according to the Taguchi method, and volumetric energy density (VED) was defined. Subsequently, the effects of process parameters on densification, phase composition, microstructure, tensile properties, and microhardness were investigated. Nearly a full densification sample (≥99%) was obtained under optimized process parameters, and the value of VED was no less than 300 J/mm3. Laser power had a dominant influence on densification behavior compared with other parameters. The main phases of 90W-7Ni-3Fe are W and γ-(Ni-Fe), dissolved with partial W. In addition, 90W-7Ni-3Fe showed a high tensile strength (UTS = 1121 MPa) with poor elongation (<1%). A high average microhardness (>400 HV0.3) was obtained, but the microhardness presented a fluctuation along building direction due to the inhomogeneous microstructure.

A Review on the Preparation of Magnesium-Based Alloys Prepared by Powder Metallurgy and the Evolution of Microstructure and Mechanical Properties

This paper reviews the manufacturing of magnesium-based alloys prepared by powder metallurgy (PM) technique and analysis of the effects of PM parameter on the developed microstructure, texture and mechanical properties. Powder metallurgy (PM) technique has been considered to produce magnesium product with consideration of less complex, finer grain and improved mechanical properties. Selection of PM route especially sintering to full densification determines a good diffusion path of alloy for interparticle bonding. This paper discusses the preparation and process parameter of each process in powder metallurgy routes, and the evolved microstructure including the crystallography texture and mechanical properties of the magnesium-based alloy product.

Microwave sintering of tungsten heavy alloys

To understand microwave sintering of heavy alloys with high tungsten content, 93W-4.9Ni-2.1Fe alloy was sintered using a 6 kW, 2.45 GHz microwave sintering furnace at 1783 K (1510˚ºC) and 1793 K (1520˚ºC). The alloy sintered at 1793 K (1520˚ºC) achieved full densification and had improved microstructural features, superior mechanical properties compared to 99.4% densification and relatively inferior properties obtained in the alloy sintered at 1783 K (1510˚ºC). This study also includes a comparison between microwave sintered and conventionally sintered 93W-4.9Ni-2.1Fe alloy (sintered at 1793K (1520ºC)). Contrary to the full densification and superior mechanical properties obtained in microwave sintering, conventional sintering at 1793K (1520ºC) resulted in only 99.6% densification and substantially inferior properties. Analyses of microstructure and fracture surface revealed that key microstructural parameters such as tungsten grain size, tungsten-tungsten contiguity, matrix volume fraction and also the fracture mode were significantly different between the alloys processed by the two routes. Possible reasons behind dissimilar densification, significantly different microstructures and mechanical properties obtained between these two modes of sintering, are also discussed in this study.

An investigation into reinforced and functionally graded lattice structures

Lattice structures are regarded as excellent candidates for use in lightweight energy-absorbing applications, such as crash protection. In this paper we investigate the crushing behaviour, mechanical properties and energy absorption of lattices made by an additive manufacturing process. Two types of lattice were examined: body-centred-cubic (BCC) and a reinforced variant called BCC z. The lattices were subject to compressive loads in two orthogonal directions, allowing an assessment of their mechanical anisotropy to be made. We also examined functionally graded versions of these lattices, which featured a density gradient along one direction. The graded structures exhibited distinct crushing behaviour, with a sequential collapse of cellular layers preceding full densification. For the BCC z lattice, the graded structures were able to absorb around 114% more energy per unit volume than their non-graded counterparts before full densification, 1371 ± 9 kJ/m3 versus 640 ± 10 kJ/m3. This highlights the strong potential for functionally graded lattices to be used in energy-absorbing applications. Finally, we determined several of the Gibson–Ashby coefficients relating the mechanical properties of lattice structures to their density; these are crucial in establishing the constitutive models required for effective lattice design. These results improve the current understanding of additively manufactured lattices and will enable the design of sophisticated, functional, lightweight components in the future.