Metal Powder Consolidation Processes
Powder metallurgy (PM) includes a number of consolidation processes using fine metal powders or particulate materials as the raw material. These processes require consolidation and provide varied benefits to the finished part depending on the consolidation process.
The two principal metal-forming consolidation process categories are press-and-sinter (also known as conventional powder metallurgy) and near-fully dense processes.
Conventional powder metallurgy, also known as press-and-sinter, offers many advantages over the other consolidation methods. It offers the lowest manufacturing cost, including modest tooling costs. It also produces the closest tolerances in the finished parts. Since it is a net-shape processing technology, it yields parts requiring little or no secondary machining operations. Lastly, it makes available to designers and fabricators a wide variety of material systems from which to choose.
Parts produced through the press-and-sinter process are subject to certain limitations as well. Tooling and the maximum press tonnage capabilities impose size and shape constraints on parts that can be fabricated. Annual production quantities dictate how quickly the costs of tool set-ups and maintenance can be amortized. Finally, the presence of residual porosity in the parts will cause certain physical and mechanical properties to be lower than those of the wrought material.
In this process, custom-blended metal powders are fed into a die, compacted into the desired shape, ejected from the die, and then sintered (solid-state diffused) at a temperature below the melting point of the base material in a controlled atmosphere furnace.
The compaction step requires the part to be removable from the die in the vertical direction with no cross movements of the tool members. The sintering step creates metallurgical bonds between the powder particles, imparting the necessary mechanical and physical properties to the part.
Typical Press-and-Sinter Products—gears, sprockets, cams, ratchets, levers, clutch plates, pressure plates, housings, pole pieces, bearings, bushings
Typical Markets Using Press-and-Sinter Parts—automotive, appliances, power tools, hydraulics, lawn and garden, agriculture, off-road equipment, motors, firearms, recreational equipment, hardware, business equipment
Metal Injection Molding
The advantages of the metal injection molding process lie in its capability to produce mechanical properties nearly equivalent to wrought materials, while being a net-shape process technology with good dimensional tolerance control. Metal injection molded parts offer a nearly unlimited shape and geometric-feature capability, with high production rates through the use of multi-cavity tooling.
The process utilizes fine metal powders (typically less than 20 micrometers) which are custom formulated with a binder (various thermoplastics, waxes, and other materials) into a feedstock which is granulated and then fed into multiple cavities of a conventional injection molding machine. After the “green” (un-sintered) component is removed, most of the binder is extracted by thermal or solvent processing and the rest is removed as the component is sintered (solid-state diffused) in a controlled atmosphere furnace.
The MIM process is very similar to plastic injection molding and high-pressure die casting, and it can produce much the same shapes and configuration features. However, it is limited to relatively small, highly complex parts that otherwise would require extensive finish machining or assembly operations if made by any other metal forming process.
Metal injection molding’s main limitation is in regards to size. Check with a MIM manufacturer to see if MIM is the right process for your part.
Typical Markets Using Metal Injection Molded Parts—computer peripherals, medical and dental devices, automotive, firearms, electronic packaging, consumer products
You’ll find a great deal more information about the MIM process at mimaweb.org, a dedicated site maintained by the Metal Injection Molding Association.
Isostatic pressing is generally used to produce large powder metallurgy parts to near-net shapes of varied complexity.
Unlike conventional PM (aka press-and-sinter), in which the powder is compacted through direct contact with tooling, isostatic pressing confines the metal powder within a flexible membrane or hermetic container which acts as a pressure barrier between the powder and the pressurizing medium, liquid or gas, that surrounds it. The use of this pressurizing system ensures a uniform compaction pressure throughout the powder mass and a homogeneous density distribution in the final product.
For Cold Isostatic Pressing (CIP), the container is typically a rubber or elastomeric material; the pressurizing medium is a liquid such as water or oil. Free of die frictional forces, the powder compact reaches a higher and more uniform density than would be obtained using conventional cold die compaction at the same pressure. In cold isostatic press processing, the part must be sintered (solid-state diffused) after removal from the mold.
For Hot Isostatic Pressing (HIP), the hermetic container for the powder is made of metal or glass and the pressurizing medium is a gas (inert argon or helium). At the elevated temperatures the process employs, the hermetic container deforms plastically to compact the powder within it. The combination of heat and pressure during the process eliminates the need for a supplemental sintering step. Removal of the HIP container after processing is an additional requirement not found in other powder metallurgy processes.
The advantages of the isostatic pressing process lie in its capability to produce parts of much larger sizes than is possible with other PM processes, with a virtually unlimited capability for complex shapes and geometric features. What’s more, it is applicable to difficult-to-compact and expensive materials such as superalloys, titanium, tool steels, stainless steel, and beryllium, with material utilization that is highly efficient. And, using the HIP process, parts can be produced that offer fully dense materials with isotropic mechanical properties equal or superior to those of cast and wrought materials.
Metal Additive Manufacturing
Metal additive manufacturing (MAM), or metal 3D printing, has the potential to profoundly change the production, time-to-market, and simplicity of components and assemblies. Unlike conventional or subtractive manufacturing processes, such as drilling, which creates a part by removing material, additive manufacturing builds a part using a layer-by-layer process directly from a digital model, without the use of molds or dies that add time, waste material, and expense to the manufacturing process. Additive manufacturing has been used as a design and prototyping tool for decades, but the focus of additive manufacturing is now shifting to the direct production of components, such as medical implants, aircraft engine parts, and jewelry.
Additive manufacturing is not a single type of technology or process. However, all additive manufacturing systems employ a common layer-by-layer approach, but they use a wide variety of technologies, materials, and processes.
Additive Manufacturing technologies utilizing metal powders:
- Laser Sintering (LM/SLS/SLFS)*
- Electron Beam Melting (EBM)*
- Selective Inkjet Binding (SIB)*
- Laser Powder Forming (LPF)
- Fused Deposition Modeling (FDM)/Extrusion
* Powder bed process
Powder forging begins with custom-blended metal powders being fed into a die, then being compacted into a “green” shape (meaning that the part has not yet been heat treated). This compact, called a “preform,” is different from the shape the final part will acquire after being forged. Again as in the conventional PM process, the green compact is sintered (solid-state diffused) at a temperature below the melting point of the base metal in a controlled atmosphere furnace, creating metallurgical bonds between the powder particles and imparting mechanical strength to the preform.
The heated preform is withdrawn from the furnace, coated with a high-temperature lubricant, and transferred to a forging press where it is close-die forged (hot worked). Forging causes plastic flow, thus reshaping the preform to its final configuration and densifying it, reducing its porosity to nearly zero.
Powder forging produces parts that possess mechanical properties equal to wrought materials. Since they’re made using a net-shape technology, powder forged parts require only minor secondary machining and offer greater dimensional precision and less flash than conventional precision forgings.
Parts fabricated through the powder forging process are subject to certain limitations. Tooling and the maximum press tonnage capabilities impose size and shape constraints on parts, just as in impression die hot forging. Annual production quantities in excess of 25,000 pieces are typically required to amortize the development costs of tool set-ups and maintenance. Finally, material systems are somewhat limited (all commercial PF products are steel).
Typical Powder Forged Products—connecting rods, cams, bearing races, transmission components
Typical Markets Using Powder Forged Parts—automotive, truck, off-road equipment, power tools
Unlike other forms of powder metallurgy, spray forming is not used to fabricate individual net-shape components. Rather, the process is used to produce semi-finished mill products in the form of billets, tubes, and sheet/plate.
The process consists of sequential stages of liquid metal atomization and droplet consolidation to produce a near-net-shape product. The as-sprayed material is close to full density with a fine, equiaxed grain structure, with mechanical properties that meet or exceed those of ingot-processed alloys.
Spray forming is known for its high rate of metal deposition, typically in the range of 0.5–5.0 lb./s. Commercial processing includes alloy steels, stainless steels, tool steels, superalloys, aluminum, and copper-base alloys.
Near Fully Dense Processes
The powder metallurgy process has expanded into the production of near-fully dense materials and parts, with residual porosity often being less than 1%. These processes use different compacting methods and may involve greater emphasis on high-alloy materials and enhanced sintering techniques.
These processes include: