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What is Porous Metal?
For over a quarter century, SSI Technologies has made a variety of Engineered Porosity powder metal parts. Engineered Porosity takes advantage of the inherent porosity found in conventional powder metal processing and takes it to an extreme. By varying the interconnected 'void space' from 20% to 70% of the total volume and by controlling the size of these voids, our Engineered Porosity process provides components for a variety of applications.
Benefits of Porous Metal
Engineered Porosity offers a variety of benefits derived from the unique characteristics provided by this process. Well-informed designers utilize the wide potential of porous parts to develop creative solutions for challenging situations. More specifically, engineers are able to design components with:
- Precise Permeability Permeability, the measure of how much gas or fluid will pass through a part in a given time under a known pressure, can be maintained over a porosity range of 15 to 20%.
- Unique Flow Characteristics Engineered Porosity components can be designed to allow the flow of vapors while restricting or preventing the flow of liquids. This provides an excellent benefit for designers working with propane or liquid refrigerants, as well as applications requiring protection from water.
- Controlled Flow Rates Engineered Porosity components offer a significant advantage when compared with single hole, small orifice flow controllers. Providing precise flow control that ranges from micro-leaks to high volumes, the multitude of porous pathways minimizes potential blockage.
- Superior Corrosion Resistance The corrosion resistance of Engineered Porosity components is similar to that of the base metal. Frequently 316 stainless steel and alloys such as Hastelloy and Monel have been used for demanding applications.
- Fine-Tuned Filtration The mean pore size, defined by our standard system based on theoretical mean pore size, ranges from 3 microns to 250 microns. This allows effective filtration down to 1 micron.
- Superior Strength Engineered Porosity uses high temperature, controlled atmosphere sintering to achieve strong metallurgical bonds. This provides components with outstanding rigidity, shock resistance, and the ability to be 'cleaned'.
Porous Metal Process
While conventional PM processing sinters parts to 82%-88% of the theoretical density, Engineered Porosity materials are carefully processed to achieve densities from 30%-80% of the theoretical density. This process provides an interconnected network of pores bounded by high strength, metallurgically bonded stainless steel.
Engineered Porosity components, molded from a controlled mixture of metal powder and sacrificial 'pore-formers', are sintered in closely calibrated, high-temperature, hydrogen atmosphere furnaces. Controlling the chemistry and size of the 'pore-formers' as well as the size of the metal powders provides an almost infinite variety of porous networks. This diversity explains the variety of creative applications for this process.
High purity metal powders and sacrificial 'pore-formers' are selected to provide unique physical properties tailored to the customer's requirements. The powders are blended with lubricants which aid powder flow, green density, and green strength. Mixing metal powders with sacrificial 'pore-formers' allows opportunities to engineer a variety of material properties unique to Engineered Porosity components.
Powder metal mixes flow from the feedshoe into the die cavity where multiple punches compress it to half its original volume. Intense compacting forces first realign, then deform the metal particles, creating localized mechanical bonds, forming a 'green' part. Green parts are ejected and mechanically conveyed to minimize handling damage.
The initial sintering stage removes binders, lubricants, and 'pore-formers' and cleans the surface of the metal particles, ensuring excellent mechanical properties in the finished component. Time at temperature provides the energy that transforms the mechanical bonds into metallurgical bonds. High Temperature Sintering is essential to creating cohesive components especially when the percentage pore volume is high.
One benefit of PM is its net shape capability—often providing a finished component after the sintering operation. Sintered filters or other porous components can also be quickly pressed into an assembly. Oil impregnation utilizes PM's porous pathways as a built in lubrication reservoir. Other surface treatments or lubricants are also available to achieve a customer's specific requirements. Porous materials are difficult to machine without adversely affecting the porous pathway. The required flow dynamics should be carefully considered if features must be machined.