Yes, in most cases. Traditionally we are trying to balance the density of a part so that they are similar throughout the part, however, sections of a part have the potential of being designed with lower or higher density.
At lower densities, part porosity is more skeletal in nature, and would enable liquids, such as oil, to pass through the part. However, as part density increases, those “pathways” are reduced or eliminated.
Best Practices for Welding Sintered Metals
Since powdered metal components are porous, the absorption of contaminants into PM parts poses a difficulty as well as a flame hazard for welding. Preventing contaminants from being absorbed into the part is critical in weld applications.
Simple Test to Identify the Presence of Porosity Contaminants in PM Parts
Steam treatment is a thermal process that creates a thin controlled oxide layer on the surface of an iron based metal component. Steam treatment can provide a component with increased corrosion resistance, hardness, density and magnetic properties. It can also be used to seal the porosity and improve its wear characteristic.
Steam treatment is a batch process with minimal inputs and has been proven to be a cost effective solution for many applications. Components transferred to steam treat must be kept clean and dry as it is necessary to avoid contaminants or residue on or in the structure prior to processing because it will impact of how well the oxide layer forms on the surface.
As in most thermal treatments, time, temperature and atmosphere are controlled to provide the optimal conditions for the expected finish. The desired properties of the component will dictate what time and process parameters are used for a given part. During a typical steam treatment process, parts are placed in a steam treat unit and heated to approximately 1000° F. Once the component is at temperature, steam is introduced and the water vapor reacts with the iron to form the oxide layer (magnetite - Fe3O4). After a designated period of time the component is removed from the unit and allowed to cool. The oxide appears on the component surface as a blue/black finish.
Components are copper infiltrated for a number of reasons. Some basic desired results are improvements to tensile strength, hardness, impact properties, and ductility. Copper-infiltrated components will also have a higher density.
Here's how copper infiltration works:
The base structure of the component has a known density, which is used to determine the amount of open porosity. A measured amount of copper is selected matching the amount of porosity to be filled. The copper fills the porosity during the sintering process (at temperatures above the melt temperature of copper) simply by placing the copper against the component prior to sintering. The >2000°F sintering temperature allows for the molten copper to flow into the component porosity through capillary action. Sintering is completed on a carrier (e.g. ceramic plate) so the copper stays on the component. Once the part is cooled, the copper is solidified within the structure.
Top Photo (right): Parts assembled with copper slugs ready for sintering. (Photo by Atlas Pressed Metals)
Bottom Photo (right): Microstructure of a part showing how copper infiltrates open porosity. (Photo by Dr. Craig Stringer - Atlas Pressed Metals)
Powdered metal tooling can render these types of testing a costly undertaking but there are alternative options. Unfortunately, it is not feasible to manufacture production components from any sort of temporary tooling. However, prototypes can be made from a PM blank, also known as a slug or puck, which is manufactured so that material characteristics, such as density and chemistry, closely mirror that of the desired production component. The component geometry can be machined by traditional methods, like milling, wiring or turning, from the PM slug. In some instances, after the machining operation is completed, and depending on how aggressively the slug is machined, the component may be re-sintered to ensure that residual stresses are relieved. In some instances, after the machining operation is completed, and depending on how aggressively the slug is machined, the component may be re-sintered to ensure that residual stresses are relieved. Heat treated or harder materials may require a pre-sintered slug so that the material is soft enough to machine, followed by a second, full sinter to reach the material's optimal physical properties. If the prototype component is to have a finish, any residual machining coolant must be removed prior to resin impregnation and/or the application of the finish.