Powder-metallurgy (PM) research and development has been ongoing at the University of Missouri-Rolla for a good part of its 134-year history. Facilities and equipment are being continually upgraded and added to aid in improving powder metallurgy parts manufacturing, PM alloy development and improvement of PM part properties. The work at the UMR MS&E Dept. is targeted at powder characterization, pressing, hot pressing, powder injection molding, thermal analysis, ceramics processing, heat treatment, mechanical testing, intelligent control, machining and microstructural characterization.
Recently, Fluidtherm Technology Pvt. Ltd., Chennai, India, (www.fluidtherm. com) donated to the UMR Materials Science and Engineering Dept. a custom compact continuous sintering furnace (Fig. 1) designed and built specifically for research in powder metallurgy. The new equipment addition will provide a means to further advance PM technology, adding to the author's 20+ years of involvement in various areas of powder metallurgy including mechanical alloying, hard materials, aluminum metal-matrix composites, wear and corrosion resistant materials by PM and CVD and PVD of coatings from powder. Other areas investigated include the use of mechanical alloying as a means to create unique powders and compositions, including stainless steels, shape memory alloys, intermetallic alloys, hard materials and metal-matrix composites, as well as the development of different milling techniques to develop faster, cleaner and more economical methods of powder production.
New equipment for PM research
Fluidtherm's continuous sintering furnace is equipped with molybdenum disilicide heaters, high alumina refractories and a ceramic muffle, and is capable of operating at temperatures to 1400 C (2550 F) in air. In addition to air, any combination of nitrogen, hydrogen, argon and doping hydrocarbon gas can be used. All of these gases can be fed from cylinders into a mixing and feed panel.
The furnace incorporates a variable speed pusher mechanism at the charging end and a twin track cooling system at the discharge end. Slow cooling under protective atmosphere is performed in one track and pressure gas quenching-a new system developed by Fluidtherm for which a patent application has been filed-in the second track. This system has a high cooling rate (approaching 12 C/s) and can be operated at several levels of cooling intensity.
Process control instruments are provided with PC communication for data acquisition and recording. Many industrial cycles can be simulated by controlling pusher speed, preheat or burn-off temperature, hot zone temperature and atmosphere, and cool down zone conditions. The burn-off zone is also designed to perform second stage debinding for metal injection molded parts.
Preliminary plans for research using the Fluidtherm furnace include sinter hardening, causes of dimensional variation, toughness and fatigue improvement, metal injection molding, and metal-matrix composites.
The effects of processing parameters on the final properties of sinter hardened PM steels are being studied and process maps have been developed for transverse rupture strength, hardness, carbon level and dimensions of the steels investigated thus far. Figure 2 shows a representative process map. The new furnace will be used in this work to extend these studies into the area of the variability of mechanical properties. It also will be used to investigate the response of these steels to a pressure quench in place of being fan cooled.
Various metal-matrix composite materials based on aluminum, titanium, nickel and intermetallic matrices have been mostly fabricated using PM processes. The materials are intended for use in applications requiring high-temperature resistance, wear and corrosion resistance and high stiffness. In addition, surface composites of aluminum are being developed by friction stir processing hard powders into the surface layer.
The effects of metal injection molding processing parameters on mechanical properties, particularly impact toughness are being studied. Examination of parts made at several different companies shows that there can be a large difference in toughness and other properties that are sensitive to changes in the microstructure. There is also a greater dependence in materials that respond strongly to heat treatment, such as 17-4PH stainless steel. The average values of mechanical properties not only are dependent on processing conditions, but also the spread in values for one set of conditions can be considerable. These variations with processing can be reduced by choosing processing parameter regimes where these effects are greatly reduced.
An additional area of concern for metal injection molding companies is a lack of understanding of what factors control shrinkage as the size and geometry of parts are changed. The ability to predict the relationship between the size of the mold and the final part dimensions can pay large dividends in reducing tool rework time and expense. Studies show that the effect of molding parameters on part shrinkage can be modeled using Design of Experiments. The results predict regions of reduced distortion and lower variability of final dimensions. Figure 3 shows predicted shrinkage from the mold for a small metal injection molded part. One company has used some of these results to solve a problem with the dimensions of a customer's part. The new Fluidtherm furnace offers the capability to continue these studies on the effect of sintering parameters.
The variability of mechanical properties of press and sintered PM parts has been studied on a few large data sets. These data sets have mechanical property measurements on large numbers of samples (>60) produced in a single batch using the same processing conditions. For these data sets, the commonly used Gaussian distribution is compared with the more appropriate Weibull distribution (Fig. 4). Results show that the Gaussian distribution inaccurately models the actual data particularly at 3-sigma and higher. The Weibull distribution not only models the data better, but incorporates a threshold parameter, which can be used to determine the minimum expected value of a property. In addition, analysis of the data shows that most of the time there is more than one distribution in the data, indicating that there are multiple failure mechanisms.
Friction stir processing is being used to modify microstructure and composition of alloys including those produced by PM. Full density and chemical homogeneity can be achieved locally in the stirred region. The research is aimed at locally modifying a component to economically achieve higher performance.
In addition to aiding the projects described above, research plans using the Fluidtherm furnace include sintering studies, pressure gas quenching of PM steels, toughness and fatigue improvement in PM parts, and production of metal matrix composites. A research consortium involving metal injection molding also is planned, which also will play a major role in education.