Mixing of Fine Powders in Sound Assisted Fluidized Beds for Metallurgical Applications

Particle mixing is an essential and very important unit operation in industries dealing with powders and bulk solids. A number of published reviews and monographs on this subject appear in the literature and the various mechanisms of mixing of solid particles are discussed. Different types of mixers, such as tumbling mixers, convective mixers, high-shear mixers, including media mills and hammer mills, are also described in detail. However, this literature deals primarily with large, non-cohesive, spherical particles (i.e. mean particle sizes greater than 30µm), i.e. when the interparticle forces are small compared to the particle weight, and their role in mixing can be neglected. They rarely include micron or submicron-sized particle systems. Indeed, currently available mixers are not effective in disintegrating agglomerates made of highly-cohesive particles smaller than 10 µmin size; or they require either very intense shear or high velocity impacts, which indeed act as particle size reduction devices rather than as conventional mixers.

However, mixing of fine and ultra-fine particles, among them micro and nano-particles, has received increasing interest recently due to potential new applications such as in the manufacture of cosmetics, foods, plastics, detergents, catalysts, energetic, biomaterials, metallic components, and in microelectro-mechanical systems (MEMS). Among these different fields of applications, one of the most promising is represented by the metallurgical sector. In particular, in the Powder Metallurgy (PM) materials and/or components are made from metal powders, avoiding, or greatly reducing, the need to use metal removal processes (i.e. thereby drastically reducing yield losses in manufacture and often resulting in lower costs). Besides that, PM is also used to make unique materials impossible to get from melting or forming in other ways. Some important PM products are metal foams and tungsten carbide-cobalt (WC-Co) hard metals. In both cases the manufacturing process involves a mixing process between precursors in the form of fine powders and the quality of the final product strongly depends on the efficiency of this mixing step.

More specifically, metal foams are an interesting class of materials with very low specific weight and unusual physical, mechanical and acoustic properties due to their porous structure. For example, in the automotive industry, the use of metal foams reduces the weight of the vehicles, increases sound dampening, and energy absorption in case of a car-crash. In the classic PM manufacturing technique the first step consists of homogeneously mixing the metal powders (typically aluminium alloy) and the blowing agent (typically TiH₂). This mixture is compacted to produce a dense “foamable precursor material” through some compaction technique (e.g. hot isostatic compression, rod extrusion, powder rolling) in order to make sure that the blowing agent is well embedded within the metal matrix and to obtain a semi-finished product without any residual porosity. In the final step, the solid precursor is placed in a furnace and heated up to induce the decomposition and release of the blowing agent. However, this manufacturing process suffers from a very remarkable limitation, i.e. the PM precursor can only be foamed within simple molds, thus limiting the geometrical complexity of the components which can be produced using metal foams. A possible solution to this issue is the cold gas dynamic spray (CGDS) technology. The general concept is to spray the powder mixture, made of aluminium alloy powder, as metal matrix, and TiH₂ particles, as foaming agent, on a metallic substrate and then carry out the foaming process in order to obtain the final foamed component. The advantage of this technology is that the shape of the precursor can be ruled by using a complex shaped substrate or by imposing a complex trajectory at the spraying gun. However, a key factor of this process is to obtain a homogenous and deep mixing between the metal powders and the blowing agent particles, which are both micronic powders.

Likewise, WC-Co hard metals are widely used for a variety of machining, cutting, drilling, and other applications. The manufacturing process consists of five main steps: i) production of powders; ii) mixing; iii) pressing; iv) pre-sintering; v) sintering. However, each of these steps present some critical issues that ending up in reducing the efficiency of the production process and/or the quality of the final product. In particular, one of the major problem concerns the mixing of the powders, which are micronic. This step, generally performed by wet milling in ball mills, has a duration of 10 – 60 h to which 10 - 20 more hours must be added to dry the powders, thus strongly slowing down the entire production process. Therefore, finding an alternative and more efficient mixing technique would improve the efficiency of the entire production process.

Summing up, one of the main issues of both metal foams and hard metals manufacturing process is the powder mixing step. Indeed, in both cases the materials to be mixed are fine cohesive powders. As dry fine particles readily form large loose agglomerates (due to the strong interparticles forces such as van der Waals, electrostatic and moisture induced surface tension forces), mixing the individual constituents at the microscale (or even a submicron scale) is difficult; on the other hand, mixing only at a coarser (i.e., agglomerate) scale results in low-quality composites. Ideally, a mixing process between two or more powders should intimately mix all the species so that any small sample taken from the mixture would contain the same proportion of all the constituents, i.e. the individual materials have to be homogeneously dispersed. Clearly, the macroscopic mixing process must be designed to ensure that the shear flow is generated at the length scales at which mixing is desired, since if the external stresses generated are not large enough to break up the aggregates that have been created due to cohesive forces, mixing will not occur at scales smaller than the size of the aggregates. Therefore, the process of mixing must involve: i) a break-up of the agglomerated particles; ii) the dispersion of individual or small clusters of particles of one phase into the other.

In this framework, gas fluidization is one of the most effective available techniques in ensuring continuous powder handling and good mixing, chiefly because of the large gas–solid contact area. Even so, on the basis of their primary particle size and material density, fine and ultra-fine powders fall under the Geldart group C (<30 mm) classification, which means that fluidization is expected to be particularly difficult (i.e. characterized by plug formation, channeling and agglomeration) because of cohesive forces existing between particles and becoming more and more prominent as the particle size decreases. Because of the above-mentioned interparticle forces, cohesive fine particles are always found to be in the form of large-sized porous aggregates, rather than as individual particles, when packed together in a gaseous medium. Therefore, gas fluidization of these powders actually occurs in the form of particle aggregates, and their properties (size/density) highly affect the fluidization nature (i.e. primary particle size and density cannot be taken as representative parameters for predicting their fluidization behaviour), which, in turns, affects the quality of the mixing. In particular, the application of the acoustic fields induces a continuous break-up and reaggregation mechanism of the fluidizing aggregates, thus leading to the formation of hybrid aggregates, i.e. aggregates made of different powders. In other words, the mixing between the different powders actually occurs not only at a “global scale”, i.e. between aggregates made by only one powder, but also at a “local scale”, i.e. inside aggregates.

This work is focused on the application of sound assisted fluidization to mix metal powders for the production of: i) aluminium alloy metal foams; ii) WC-Co hard metals. In particular, a preliminary experimental campaign was carried out to characterize the powders from a fluid-dynamic point of view (i.e. by performing fluidization tests under both ordinary and sound-assisted fluidization conditions, 130-150dB/20-150Hz) and to optimize the process parameters (sound intensity/frequency and fluidization velocity) to perform the mixing process. Then, the mixing quality was assessed through SEM/EDS analysis of samples of the fluidized materials, taken from the bed at different times by means of an ad-hoc non-destructive sampling procedure, in order to determine the shape and the chemical composition of the aggregates.

Then, the mixtures obtained in the sound assisted fluidized bed: i) were sprayed and heated up to produce the metal foams; ii) were compacted and sintered to produce WC-Co hard metals. In both cases, the final manufactured products were further analysed to verify their structural quality. In particular, cross sections of the manufactured metal foams were observed by means of a SEM microscope to highlight the internal metallurgical phenomena as well as the distribution and morphology of foam cells. Likewise, the sintered WC-Co specimens were subjected to metallographic and microstructural examination (SEM analysis, magnetic properties analysis, relative density and Vickers hardness measures). The results obtained showed that sound assisted fluidization is a promising technology for the mixing of fine cohesive powders in the metallurgical sectors.