Engineering research paper

NEW RESEARCH DIRECTIONS
IN POWDER METALLURGY

R.L. ORBAN

TECHNICAL UNIVERSITY OF CLUJ-NAPOCA
103-105 MUNCII BLV.,400641 CLUJ-NAPOCA, ROMANIA

INTRODUCTION

Over the last years Powder Metallurgy (PM) has known an impressive development. This is eloquently highlighted by the upgraded classification of its present applications.The key factors of this development are its high potential in advanced material producing (some of them impossible to be produced by other technological methods), its known advantages in structural parts production , as well as the less influence of the PM companies’ activity than of the traditional, especially metallurgical, ones on the ecological system. As PM applications cover very diversified domains of the modern industry, their development continuously stimulate the research efforts for PM progress. The most important directions of these intensive researches will be analysed in this paper.

SINTERED PART TECHNOLOGY IMPROVING

Sintered parts have applications both as structural parts and as parts realized from special materials. For both, improving the processing technologies to overcome the known PM limitations concerning high mechanical properties, complex shape and large parts realization are well-defined research targets.

Researches for mechanical properties improving

The largest PM application from the tonnage point of view is to the structural parts, especially for the automotive industry, production [1]. They are commonly made of lowalloyed with C, Cu, Ni, Mo and, recently, also with Cr, Mn, Si steels. There are several research directions for their mechanical properties improving

Increasing powder compressibility/homogeneity, dust and hazard reduction

In the sintered structural partsIn the sintered structural parts production, water atomized iron powders are going to replace the sponge ones. Therefore, numerous researches are focussed on the atomization technology improvement for a better particle size distribution and a higher cleanliness obtaining. Beside those concerning the atomisation nozzle geometry and processing parameters of standard units optimisation, high-pressure water atomization (up to 150 MPa) is investigated to increase fine particle fraction, enhancing particle size distribution. Researches are too performed to introduce vacuum induction melting, degassing and pouring furnaces to replace the common ones. production, water atomized iron powders are going to replace the sponge ones.

Improving the die filling and lubrication processes
A high filling density and uniformity over the cross section of the die cavity is essential for good quality parts obtaining. It strongly depends on the powder flowing rate and, for a given powder, on the wall thickness, type of compacting lubricant, filling shoe movement in respect to wall direction. Numerous studies, some based on the process modelling, are carried out for its enhancing. Improving the filling system design, contour filling, using of agitating feed-shoes or vibratory feeders, powder fluidization/granulation with an organic binder

Warm compaction

Warm compaction is based on the notable decreasing of yield strength of the ferrite steels at heating (Fig. 4). Consequently, deformability of particles and implicitly, compressibility of such powders notable increases even at a moderate heating (up to 1500C), allowing a higher densification at the same applied pressure. A lubricant resistant at these temperatures, able to assure a good lubrication, without to decrease the powder
flowing capacity, and also a heating system of both powder and tooling is necessary, instead. Several companies developed such lubricants and heating systems

High velocity compaction

 Although compaction by explosion (shock compaction) has been used for several years, it was not extended due to its difficulties and low productivity. The dynamic compaction idea was recently reconsidered; new technological variants, less dangerous and of a high productivity, are being
developed.

Activated sintering

Sintering activation, especially by a transient liquid phase creation, is too in attention. It is possible by addition of small amounts of activators” - forming with the base powder an eutectic of a melting point below the sintering temperature. At ferrous materials, the most used activator is phosphorus . Some companies are even producing Fe-P powders or premixes . Boron, forming with Fe an eutectic at a lower content than P, caught too recently a high interest

Powder Injection Molding
Powder Injection Molding (PIM) combines the polymer injection technology with powder metallurgy. By injection of a powder-binder mixture (feedstock) in a die cavity, theoretically any shape can be realized. Investigation are presently done to obtain fine spherical powders of an increased flowing capacity, from a higher diversity of materials, to improve binder properties / feedstock rheological behavior for a better die
filling, to improve debinding / sintering / mechanical properties, dimensional control

Spray forming

In Spray Forming (SF, Fig. 13), the stream of droplets formed in a gas atomisation unit is accelerated by the gas jet to the rotating cavity of an open mold (substrate). The adjustable rotation of the mold allows a uniform compaction of the atomized particles.

Improving metal matrix properties

Sinter-hardening and particulate reinforcing are the most known ways of mechanical properties of steel matrix improving. To increase the sinter-hardening efficiency, modular furnaces with a convective cooling zone – by sintering gas re-circulation, were recently realized

SPECIAL/ADVANCED MATERIALS PRODUCTION

PM has numerous applications to special/advanced materials production - as sintered parts/coatings.

Sintering by infiltration of loose powders

Sintering by Infiltration of Loose Powders (SILP) is a relatively new technology that combines sintering with casting. It consists in a mold cavity filling with a selected mixture of powders, followed by its infiltration with an appropriate molten alloy

Mechanical Alloying

Mechanical Alloying (MA) is too a relatively new technology that enables a large variety of advanced materials obtaining. It consists of material milling in a high energy mill, process producing repeated powder particle deformation, work hardening, fracturing, clean surfaces forming, local heating, solid state welding,
re-fracturing and so on, under the impact energy transferred to them from collisions of the chaotic moving balls

CONCLUSIONS

The above presented developments and future trends in PM demonstrate the large aria of research directions that are still opened for its further development