Powder Metallurgy
Powder metallurgy (PM) is a highly effective method to create superior components by blending, pressing, and heat-treating metal powders. The most common elements in the PM process are copper, iron, aluminum, steel, tin, brass, and nickel. The technique reduces porosity to an acceptable degree by creating strong particle bindings. Also, it imparts durability and flexibility to the material. Powdered alloy parts find use in several industries and excel in tools that need precision design.
It is easy to alter the shape and size of materials to make special parts with powder metallurgy. Other processes like melting or forging cannot make such unique materials on a mass scale. Moreover, the PM process reduces the need for metal removal processes in the final product. Powder processes yield a wide range of products compared to fusing materials from alloys.
History
Humankind has been making materials, jewelry, and tools from metal powders for a long time. Mass manufacturing of PM-products started in the late-nineteenth century. But the powder metal business emerged as an industry only in the last century. Only in the last 30 years has it emerged as a technique to produce high-quality parts.
The PM Process
The PM process underwent a substantial change with the dawn of the industrial age, sophisticated tools, and technology. Furnaces easily maintain temperatures over 1100 °C today. Massive industrial presses can create forces exceeding several tons. Computers control process parameters to precision.
The PM procedure for making components or pieces usually follows a three-step refined process:
- Powder mixing
- Die compaction
- Sintering
- Powder mixing
Elements usually undergo crushing, grinding, and chemical reactions to take powder forms. Nitrides and carbides of titanium, vanadium, calcium, and uranium yield powders under high-temperature reduction. Iron and nickel powders come from reducing their metallic oxalates and formates. Other chemical and flame processes are also regular contributors to making metal powders.
Alloy powders undergo various operations to blend uniformly. The mixing occurs in fixed proportions to achieve optimum results. Ferrous powders form the largest volume of powders used in the industry.
- Powder compaction
Powder compaction mostly occurs under high pressure in a die using a vertical hydraulic or mechanical press. Pressure in the range of 80 psi to 1000 psi is typical. The compaction yields shapes when ejected from the die that are ready for use with minor alteration.
The compaction cycle involves:
- Filling the die cavity with metal powders.
- Compacting the powder in the die using punches.
- Ejecting shapes from the die.
The ejected green parts are cost-efficient and strong enough for transporting to the next phase. Compaction produces complex geometric shapes with areas typically between 0.1 square inches to 20 square inches.
Long-term reliability demands strong binding between alloy particles. The sintering process imparts much-needed strength for secondary operations.
- Sintering
The sintering process improves the bonding between the particles. Here, the furnace temperature is mostly below the melting point of the core component of the powder. Parts generally shrink in the process while their densities increase. But sintering can also expand sizes, conductivity, hardness and help meet other specifications.
Once on the conveyor, the parts move slowly towards the furnace chambers. The components go through a three-stage process. The first is a slow preheating stage to remove unwanted lubricants from the compaction process. Next, the green parts undergo heating in the 1400 °C to 2400 °C range in a controlled atmosphere in the furnace. The added gases keep oxidation at bay. Lastly, the parts pass through a cooling chamber.
Sintering imparts the desired properties to the parts. The entire cycle takes up to 1.5 hours, depending upon materials, desired shapes, and properties. Once done, the parts are ready for the machining and secondary operations stage. Metal injection molding, hot isostatic pressing, forging, and additive manufacturing are also other methods of the PM process.
Secondary operations
The PM process reduces additive or subtractive operations. But many components need unique shapes and designs. Secondary operations satisfy these requirements through:
- Repressing
- Sizing
- Coining
- Annealing
- Fastening and joining
- Heat and steam treatment
- Deburring, plating, and coating
- Machining
- Impregnation with oil, plastic, and resin
The secondary operations not only alter the size and shape but also improve the material’s properties. They increase product life and optimize its functions.
Advantages of Powder Metallurgy
Powder processes are flexible and can ignore solid-liquid phase changes. They do not need to melt the entire body of startup materials. So, the end products have controllable characteristics compared to casting and forging techniques.
The PM process holds several advantages over other metal-forming processes like:
- First-rate surface finish
- Scarcely any machining or scrap loss
- Easy for shaping and secondary operations
- Intricate designs made to precision that are usually impractical with other processes
- Excellent repeatability for creating components within a minimum tolerance range
- Bulk production and cost-effective
- High tensile strength gives long term durability to components
- Sustainable and green manufacturing
Generally, the products have lesser chemical and thermal constraints during manufacturing. As a result, the process is more environment-friendly than others.
Hazards do exist
Powder metallurgy has certain hazards. Due to their finely divided form, inhalation or ingestion of metal powders can be toxic. Threats to life and property are possible due to the use of special materials and processes. Aluminum, iron, and titanium powders need special handling. PM processes also create the danger of dust explosions.
Applications are universal
Components in critical applications like medical and healthcare devices use powder metallurgy. Others such as automotive, appliances, and firearms use a high percentage of PM components. The density, shape-making, and magnetic performance of PM materials make them the top choice across industries.
What lies ahead for Powder Metallurgy?
Powder Metallurgy is one of the best options to mass produce high-quality components. The roadmap ahead of it seeks advances in:
Material Density
Increased material density helps improve the structural and magnetic performance of the product. At the same time, a minor porous nature allows lubrication.
Melting points
High-quality materials from tungsten and molybdenum alloys have high melting points. So, the majority of PM component suppliers should have the technology to sinter at very high temperatures.
Magnetic composites
Soft magnetic composites can form highly complex shapes. The advent of 3D shapes helps in reducing the size of components.