Fabricating With
Porous Metals

Many of our customers have used MKI's experience in the sintered porous metal field for the fabrication of their most difficult and challenging parts and assemblies. Our experience in all aspects of fabrication technology allows us to economically modify our Dynapore® materials to our customer's exact specifications. Feel free to inquire how MKI might be able to help you with your custom needs. Below you will find helpful information on how to effectively modify and work with Dynapore® materials.

In general, Dynapore® diffusion-bonded porous metals may be fabricated using standard sheet metal fabrication techniques. Using proper fabrication considerations, Dynapore® materials can be sheared, punched, rolled, formed and welded as necessary to fabricate OEM components. With proper processing, performance characteristics of Dynapore® normally remain virtually unchanged after fabricating.

A good rule of thumb in the modification of Dynapore® material is to employ the same techniques used to modify solid sheet metal of the same alloy in the same or thinner gauge. However, there are some limitations to the modification of Dynapore® material. These limitations are process and Dynapore® material specific. In some cases the porosity of our materials makes it necessary to employ specific techniques or settings that would otherwise be appropriate for much thinner gauge materials. For example, in welding porous metals with low density and high void volume, it is always advisable to use a minimum amount of heat to avoid overheating, burning or excessive melting. These same considerations normally apply to welding thin-gauge materials.

Other tips for general good practice include the following suggestions:

• Protect material surfaces during handling to avoid scratches or damage.
• Keep materials clean and dry.
• Avoid contact with plain steel or rusty surfaces.
• Wet processes should always be followed by thorough aqueous rinse using clean filtered or deionized water, preferably followed by oven drying at 300°F.
• Package carefully with protective material against porous surfaces.
  

Cutting techniques:

Some common forms of cutting Dynapore® include:

• Shearing
• Punching
• Laser cutting
• Wire EDM
• Waterjet cutting

When shearing or punching Dynapore® materials, the die clearances should be set to the same or less than the clearances which would be used to cut solid metal of the same gauge and alloy. Large die or blade clearance will ultimately yield parts with large edge burrs. The shearing or punching of Dynapore® materials will usually coin or densify the material around the sheared or punched edges. The degree of coining is dependent on the density or porosity of the material.

Laser cutting can be a good process for the cutting of shapes from Dynapore® sheet material but requires the proper setting of laser parameters in order to yield parts with relatively burr-free edge conditions. Inadequate laser parameters or use of standard (solid material) settings will yield parts with a mild to heavy edge burr depending on the thickness and density of the material. Dynapore® materials up to .125" thick can be cut by laser. Standard laser cutting tolerances are ±.010".

Waterjet cutting is a very effective process for the cutting of thin to heavy materials up to 2" thick with almost no edge burr after cutting. Waterjet cutting is typically not as accurate as laser cutting and also has a much larger kerf (jet cutting width). In the waterjet cutting process, the part is in contact with water from the cutting nozzle and the water bath of the machine. Steel or iron particles are usually present in the water which in turn form rust spots on the material after drying. Materials that are waterjet cut usually need a post-processing step for the removal of the iron particles. Standard waterjet cutting tolerances are ±.015".

Wire EDM cutting is the ideal method for cutting Dynapore® material but is also very expensive and therefore is restricted to the most demanding and critical applications. Some of the advantages of wire EDM cutting include:

• Cutting tolerances of up to ±.0005"
• Perfectly square edge cuts with no edge burr
• Perfect edge conditions with no smearing or closing of pores
• No heat affect zone
• No machining stresses imparted on the material

Wire EDM cut parts also need some post-processing for the removal of a brass film that attaches itself to the surface of the part during cutting. The brass film develops on the part from the burning of the brass cutting wire during machining.

Forming techniques:

Dynapore® can be formed using the following processes:

• Roll-forming
• Press brake forming
• Hydroforming

Conical and cylindrical elements of larger diameters may be roll-formed, while smaller diameter components may require step-forming on a press brake using hard tooling. In general, it is good practice to protect porous surfaces with slip sheets such as 24 gauge or thinner gauge aluminum or stainless steel. Forming into soft polyurethane is also recommended. Spinning, deep drawing and other methods which may smear or distort the material surfaces are not recommended, although some very deep draws have been accomplished using hydroforming.

Welding Techniques:

Most common welding technologies can be used for the welding of Dynapore® including:

• Manual or automatic TIG-welding (GTAW) or pulsed arc TIG (GTAW-P)
• Electron beam welding (EBW)
• Laser welding (LBW)
• Submerged arc welding (SAW)

Some key factors to successfully welding Dynapore® materials include proper weld joint design, proper welding parameters, and proper use of heat sinks. The more porous Dynapore® may present a real challenge for welding due to the high void volume and the possibility of melting holes in the filter mesh. Water-cooled copper chill bars are the best heat sinks, but in many cases aluminum or other materials may suffice. Use minimal heat and power, and avoid striking an arc directly on the porous surface. In the case where two pieces of material are butt welded together, it is advisable to complete the seam weld in one operation without stopping. Ample amounts of inert shielding gas should be employed, and may often be diffused or sparged through the porous material during welding.

Cleaning and post-treatments:

In many of the above processes, potential contamination may be introduced. For example, when fabricating 300 Series stainless steels, one must avoid contact with plain steel or iron ions which may lead to rust. Most shears, punch presses and press brakes have steel beds, blades and dies which may introduce free iron particles into the porous media. In instances where parts are laser cut, there are cases where the sparking from the laser crossing the steel support grid attaches itself to the media and introduces a possible area for rust. Waterjet cutting may also introduce iron particles into the porous media via any free iron present in the machine water bath. Many potential problems are best avoided with foresight and careful handling, but it is always advisable to finish parts with an appropriate cleaning treatment.

In the case of stainless steel parts, the best post-treatment is passivation per QQ-P-35C for the removal of free iron. This treatment should be followed by a thorough aqueous rinse, immersion in isopropanol, and a drying cycle such as vacuum bake-out. Other appropriate methods include steam cleaning, hot water power washing, ultrasonic cleaning and chemical cleaning. In some cases where potential dimensional distortion is not an issue, hydrogen annealing is an excellent cleaning and finishing method. Bright dipping or electropolishing may be useful for certain alloys and materials.

Rinsing and drying are very important if any chemical processes are used. Porous metals contain a high internal volume that acts as a sponge and retains liquids. Due to capillary action, it is often difficult to completely rinse and dry a porous material throughout its thickness. It is therefore useful to employ extra rinsing and longer drying times than would otherwise seem sufficient. After passivation, electropolishing or bright dipping, remember to rinse parts thoroughly!