Porous Sintered Metal Technology
After the material is selected, the powder metal processing method is critical for determining the final mechanical properties and porous characteristics of the porous component. Selection of the fabrication method is dependent on the powder characteristics and the type of porosity required by the application. The preparation of the powders, the use of additives, the compaction methods and the sintering conditions must be carefully controlled to produce uniform and repeatable porous characteristics.
Powder Preparation begins with the separation of the powder particles into the desired size distribution. In order to optimize the reproducibility and uniformity of the porosity, narrow powder particle size distributions are achieved by precision blending and screening of the powder particles. Once the desired particle size distribution is obtained, the powder must be properly blended prior to use in order to avoid segregation and to maximize uniformity. The apparent and tap densities of the powder cuts are normally measured to meet tight specifications since these characteristics will control further processing steps such as powder flow and die filling.
Additives such as lubricants, pore formers or binders are precisely mixed with the powder if required by the processing method. The powder cleanliness and chemical analysis must also be carefully monitored to maintain the chemistry requirements of the material and to avoid contamination with other materials being processed.
Compaction Methods and sintering are normally required to produce porous materials from stainless steel, nickel based alloys and nickel materials in order to achieve the best combination of mechanical strength, metallurgical properties and porosity. Lower density, higher permeability parts from these alloys can also result from using a gravity sintering process without compaction of the powder when the application does not require higher strength levels of compacted and sintered parts. Die compaction, isostatic pressing, and roll compaction increase the green strength of the part by cold welding of the particle to particle contact areas due to plastic deformation. As compaction force is increased, the density and green strength of the porous part increases which results in finer porosity and lower permeability than methods which do not use compaction prior to sintering. However, when compared to compacted structural P/M parts, the lower density and green strength limit the shape complexity of a porous component. For porous materials, uniform density is critical to the function of the parts since the largest porosity and greatest permeability will be in the lowest density region. Density variations of more than +/- 2.5% within the same component will cause significant difference in performance.
Die Compaction and sintering are the most common methods of improving mechanical properties for porous parts with length to diameter ratios of less than 5:1. Discs, cups, bushings and other shapes can be readily compacted by filling the powder into dies and using hydraulic or mechanical P/M presses. Typically, porous materials are manufactured to achieve the lowest density part in order to maximize porosity and permeability. Minimum green handling strength can be obtained by using lower compaction forces in the range of 700 - 2100 kg/cm² (5 - 15 tons/in²). By comparison, higher density structural P/M components require compaction pressures of 4200 - 8400 kg/cm² (30 - 60 tons/in²). Porous parts with 20 -50% green densities usually have minimal green strength which require careful handling. Higher green density parts in the 50 -80% range have sufficient green strength to allow With proper sintering, die compaction can hold dimensional tolerances within +/- 1%.
Die compaction with lower tooling forces can be accomplished with die wall lubrication or by adding less than 0.5% lubricant to the powder. In the case of coarser water atomized stainless steel powders, only 0.1 - 0.3% lubricant addition to the powder is generally used. Since smaller particle sizes of this same material are generally more rounded and have better particle packing (smaller pores), slightly higher pressing forces and lubricant additions are required for adequate green strength. Waxes and stearates are commonly added to the powder to reduce die wall friction and tool wear. However, since increasing the lubricant percentage also reduces green strength, minimizing lubricant additions is often a better alternative than obtaining maximum tool life service.
Isostatic Compaction is a common method of compaction for producing components such as tubes which have a length to diameter ratio greater than 3:1. Isostatic compaction can produce more uniform density porous components when compared to die compaction. The wet bag Cold Isostatic Pressing (CIP) process uses a hydraulic fluid to apply pressure to the tooling which is sealed after powder filling. As the hydraulic fluid is gradually pressurized, the soft polyurethane outer liner compresses the powder against the metal core rod to obtain sufficient green strength. This ´outside-in═ pressing process results in a green part with smooth inner surface and a rougher outer surface.
Sintering of porous metal is a critical balance between maximizing material properties and maximizing the open porosity and permeability. However, since permeability and material properties such as strength and ductility are generally inversely related, the desired balance of these characteristics normally occurs in a very small processing window. Sintering requires the proper compromise of temperature, time at temperature and atmosphere to arrive at the desired porosity characteristics. Porous components which are not adequately sintered exhibit poor mechanical properties due to lower density and to insufficient interparticle neck growth. Porous components which are exposed to excessive sintering conditions will result in lower permeability and higher densities than desired.
The sintering cycle can be subdivided into three main processes; preheating, sintering and cooling. Each process of the sintering cycle has unique atmosphere requirements which provide the optimum properties of the porous metal product just as with structural P/M components. The pre-heat and cooling portions of the sintering cycle must be closely controlled to achieve the proper metallurgical properties. Controlling the preheat conditions ensures adequate burn off of additives and lubricants as well as minimizing the distortion of the parts. The cooling conditions must be designed to provide maximum corrosion resistance and to avoid oxidation.
Sintering Temperature must be selected by considering the material, the powder shape and the powder particle size distribution. Sintering is normally accomplished at 70 - 90% of the material melting temperature. Finer powder particles require a lower sintering temperature since the surface energy driving force to initiate bond growth is much higher than for a coarser particle. Sintering at too high a temperature will also cause the formation of very large pores and non uniform porosity just prior to melting. Controlling the furnace temperature within +/- 1% of the optimum sintering temperature will achieve the best porosity uniformity and reproducible properties.
Sintering Time must be monitored to allow for a minimum exposure time at the desired sintering temperature. Sintering for at least 30 - 60 minutes at the maximum sintering temperature is recommended for most materials for sufficient bond formation and growth. Inadequate sintering time can lead to large variations in part shrinkage and final density causing porosity and permeability variations.
Sintering Atmosphere selection is critical for determining the metallurgical properties of the porous metal product. Since porous materials have much higher surface area than a similar size structural part, the atmosphere has more contact with surfaces throughout the part rather than just near the surface. Porous parts also contain relatively large amounts of trapped air in the pores which must be removed by purging or good atmosphere circulation in the furnace.
Mechanical strength and corrosion resistance properties are highly dependent on the interaction with the sintering atmosphere. Porous materials are commonly sintered in reducing atmospheres such as nitrogen-hydrogen mixtures, 100% hydrogen and dissociated ammonia or in vacuum. Reactive materials such as titanium require good vacuum sintering with a high purity, inert backfill gas. Nitrogen containing atmospheres can have a nitriding effect on some materials such as porous stainless steels. However, reduced corrosion resistance may result unless the cooling rate is carefully controlled to minimize the formation of chromium nitrides. A hydrogen atmosphere with a low dew point or vacuum sintering with a hydrogen backfill during cooling will produce the best combination corrosion resistance and mechanical properties for porous stainless steels. The cooling process cycle usually requires a reducing or oxygen free, inert gas atmosphere for best metallurgical properties. By preventing heavy oxidation or nitriding during cooling, a protective passive surface layer will form and good mechanical properties will result. Carbon containing atmospheres are not normally utilized in processing porous materials since higher carbon levels are usually detrimental to corrosion resistance.
Gravity Sintering is normally used to produce porous bronze components. Diffusion or liquid phase bonds form between the particles when loose (non-compacted) powders are heated to a temperature near the solidus temperature. Localized melting between the powder particle surfaces form necks during sintering and the shrinkage controls the characteristics of the interconnected porosity. As an example, spherical porous bronze powders are poured and vibrated into a graphite mold without compaction.
The bronze or tin coated copper powder is then sintered at 800 - 1000 °C (1472 - 1832 °F) for 20 minutes in a reducing atmosphere to form a liquid phase between the particles . Upon cooling, a strong metallurgical bond is formed at the contact points between the powder particles. Discs, cups, bushings or other simple shapes can be produced using this method. Typically, parts have a 1o draft angle to facilitate part release from the mold and have dimensional tolerances of +/- 3%.