Secondary Operations: Machining, Welding & Thermal Processing
Secondary operations are normally required to enhance metallurgical properties and to allow easier use in the intended application. Secondary Operations are widely used to enhance the part shape, the porosity and the surface characteristics of a porous part. The part shape complexity can be enhanced by sizing, machining and forming. Secondary sizing processes can improve the roundness, flatness and other dimensional tolerances which cannot be held during sintering. Sizing operations will increase the density of the part and reduce the surface porosity that contacts the tooling.
Secondary machining operations are used to produce features such as threads or tighter dimensional tolerances which cannot be readily controlled during compaction. Conventional machining of porous materials will close off the majority of the surface porosity due to metal smearing. Machining can be utilized to blind off the porosity and fluid flow in selected areas depending on the application. Controlled chemical etching of a machined surface can restore the surface porosity and still retain the tighter dimensional tolerances produced by machining. Other machining methods such as electrical discharge machining (EDM) are also used to avoid closing off the surface porosity.
Joining methods such as brazing, welding, epoxy gluing, sinter bonding and mechanical interlocking are used to create porous metal assemblies. Welding of porous materials to other porous materials or to solid materials becomes much more complex than welding solid materials. The addition of filler metal, the reduction of weld heat input and the use of special welding gases are required to overcome welding problems associated with higher porosity and gases trapped in the pores. Weld joint design is extremely important to allow the proper heat distribution and minimize the weld heat input to prevent leaks and large holes that can form during cooling.
Sinter bonding is a thermal processing method used to form a metallurgical diffusion bond between two parts such as between a porous part and assembly hardware. The diffusion bonds formed are significantly lower than welded or brazed joints since the sinter bonds are only formed at the local contact points. However, the sinter bonds are significantly stronger than just a press fit alone and can ensure that the porous component will not be easily removed.
Thermal processing can also be utilized for stress relief, removal of oxidation, burn off of organic contaminants introduced by secondary operations, and for passivation of the metal surfaces. Heat treatment in a reducing atmosphere can also restore a uniform oxide layer upon cooling which forms a passive surface on the porous metal in order to increase corrosion resistance.