Powder metallurgy or PM has been around for hundreds of years but only became recognized for its superiority in producing high quality parts within the last hundred years or so. Its early historical development is very closely related to sintering of ceramics. There is evidence that iron powders were fused into hard objects as early as 1200 B.C. In these early operations, iron was extracted by hand using a metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.
There are several process involved among them are producing metal powders (crushing, atomization, milling, etc.), mixing or blending of said powders, pressing/compacting powders(die press, extrusion method, etc.) and lastly sintering which is the process of heating the compacting metal powders to just below their melting points. After sintering, a variety of secondary processes and value-added operations may be performed to improve the final product.
The reason for the success of powder metallurgy is due to its many advantages when compared to other manufacturing process such as:
Fine surface finish and high dimensional accuracy is achieved. Hence it eliminates additional machining operations.
Provides materials which may be heat treated for increased strength or increased wear resistance
This process makes use of all the raw materials unlike press forming and casting. Hence there is no scarp. Eliminates of minimizes scrap losses by typically using more than 97% of the starting raw material in the finished part.
Porous parts can be produced which is not possible with other methods.
Combination of metals and non metals parts can be manufactured.
Even unskilled workers can handle powder metallurgy method.
Mass production using PM to produce small parts is very efficient and economical. It is much faster and cost effective since it minimizes scrap.
Carbide tools can only be produced using PM.
Now there are various improvements made to PM. Not only is the conventional PM process, referred to as press-and-sinter, used but there are many new techniques such as metal injection molding (MIM) process, hot isostatic pressing (HIP), and powder forging (PF).Components are also produced today from particle materials other than metal powders like cermets, intermetallic compounds, metal matrix composites, nanostructure materials and high-speed steels using PM processing techniques. But PM is not without its disadvantages a few of which have been identified below:
There is huge difficulty in storing and handling of powders as it would usually degrade with time and poses a significant fire risk.
Long parts are rather difficult to manufacture and it is extremely difficult to produce high purity powder.
PM parts have high porosity which may be a limitation in certain cases, has poor plastic properties and possess poor ductility.
For small scale production PM is not economical as it has high tooling cost (punches, dies, rolls, etc.) and the raw materials are very expensive.
The current trend in powder manufacturing and powder blending/mixing, emphasizing on the use of principles of sustainable design and development to design the process to meet the requirement.
Powder Manufacturing:
Any fusible material can be atomized. Several techniques have been developed which permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. Powders may be prepared by comminution, grinding, chemical reactions, or electrolytic deposition. Several of the melting and mechanical procedures are clearly adaptable to operations in space or on the Moon.
Powders of the elements Ti, V, Th, Nb, Ta, Ca, and U have been produced by high-temperature reduction of the corresponding nitrides and carbides. Fe, Ni, U, and Be submicrometre powders are obtained by reducing metallic oxalates and formats. Exceedingly fine particles also have been prepared by directing a stream of molten metal through a high-temperature plasma jet or flame, simultaneously atomizing and comminuting the material. On Earth various chemical- and flame-associated powdering processes are adopted in part to prevent serious degradation of particle surfaces by atmospheric oxygen.
Atomization
Atomization is accomplished by forcing a molten metal stream through an orifice at moderate pressures. A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. On Earth, air and powder streams are segregated using gravity or cyclonic separation. Most atomized powders are annealed, which helps reduce the oxide and carbon content. The water atomized particles are smaller, cleaner, and nonporous and have a greater breadth of size, which allows better compacting
Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance index used is the Reynolds number R = fvd/n, where f = fluid density, v = velocity of the exit stream, d = diameter of the opening, and n = absolute viscosity. At low R the liquid jet oscillates, but at higher velocities the stream becomes turbulent and breaks into droplets. Pumping energy is applied to droplet formation with very low efficiency (on the order of 1%) and control over the size distribution of the metal particles produced is rather poor. Other techniques such as nozzle vibration, nozzle asymmetry, multiple impinging streams, or molten-metal injection into ambient gas are all available to increase atomization efficiency, produce finer grains, and to narrow the particle size distribution. Unfortunately, it is difficult to eject metals through orifices smaller than a few millimeters in diameter, which in practice limits the minimum size of powder grains to approximately 10 μm. Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and re-melting a significant fraction of the grain boundary.
Centrifugal Disintegration
Centrifugal disintegration of molten particles offers one way around these problems. Extensive experience is available with iron, steel, and aluminum. Metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle. Opposite the spindle tip is an electrode from which an arc is established which heats the metal rod. As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls. A circulating gas sweeps particles from the chamber. Similar techniques could be employed in space or on the Moon. The chamber wall could be rotated to force new powders into remote collection vessels, and the electrode could be replaced by a solar mirror focused at the end of the rod.
An alternative approach capable of producing a very narrow distribution of grain sizes but with low throughput consists of a rapidly spinning bowl heated to well above the melting point of the material to be powdered. Liquid metal, introduced onto the surface of the basin near the center at flow rates adjusted to permit a thin metal film to skim evenly up the walls and over the edge, breaks into droplets, each approximately the thickness of the film.
Powder Blending/Mixing:
Rotary batch mixer yields high-quality blends for powder metallurgy
GKN Sinter Metals Inc. is the world's largest manufacturer of parts made by powder metallurgy, a molding process that produces complex shapes with a range of properties. GKN Sinter Metals, Auburn Hills, MI, has about 40 sites worldwide, producing components like gears, bearings, and pulleys for automotive engines and transmissions, lawn and garden products, home appliances, and power tools.
A critical part of GKN's operations is blending metal powders. At the company's Emporium, PA, plant, GKN relies primarily on a 205 cu ft (5.8 cu m) Rotary Batch Mixer from Munson Machinery Co. Inc., Utica, NY, to process millions of pounds of powder every year. The mixer handles truckload-size batches of just over 45,000 lb (20,412 kg), and provides the mixing efficiency necessary to formulate high-performance grades.To handle the weight, the mixer was configured with dual drives. The mixer can effectively process batch volumes as low as 5% of rated capacity, thus eliminating the need for multiple machines for smaller-batch processing.
Munson Rotary Batch Blenders utilize a lifter and baffle design that creates a four-way mixing action. Material is gravity-fed through a stationary intake port into the rotary mixing chamber, where it tumbles and turns in a low-intensity process that achieves complete particle distribution with no product degradation. Process times vary by user, but the mixing action of a Munson blender is efficient enough that homogeneous blends can be achieved in less than three minutes. For difficult-to-process materials that require shear, an optional Pin Intensifier can be installed near the discharge gate to break up agglomerates and assure homogenization.
Mixing operations at GKN begin with delivery of powdered metal in 5,000 lb (2,268 kg) cardboard bulk packs. The main ingredient is iron, which comprises 96% to 97% of blends. Other metals used in the most common formulations are copper (about 2%), which strengthens the iron, graphite (0.8% to 0.9%), to harden the blend, and an Acrowax lubricant (0.75%) that reduces friction of the iron particles during molding and acts as a mold release. Some blends use nickel instead of copper in the same proportion.
GKN develops blends from 58 recipes that are stored in a computer. Recipes list how much of each material goes into a blend. The process begins with manual weighing of bulk packs on a 25,000 lb (1,135 kg) capacity scale that's accurate to within 5 lb (2.26 kg) The bulk pack is then hoisted by fork-lift, inverted, and emptied into a cone-style hopper. The iron powder moves through a screening system and into a holding tank where other metals and additives are metered in, based on the formulation being produced. It takes 45 minutes to load the nine 5000 lb (2,260 kg) bulk packs of base metal and another 20 minutes to load the additives. The total batch weight of base material and additives is around 46,681 lb (21,174 kg).
Once the iron powder and additives are loaded, the material is gravity-fed through a slide gate into the mixing chamber. GKN's blending cycles last for one hour because of the time it takes to load the iron powder and additives. Process time could be much shorter, but maintaining the one-hour blending cycle assures batch-to-batch consistency. During processing the blender operates at a slow 9 rpm. The blend is discharged into the same bulk packs used for loading.
Rapid, gentle blending maximizes strength of powder metal parts
Allegheny Blending Technologies (ABT) specializes in custom blending of ferrous and non-ferrous powder metals that are sold to molders of powder metal parts. These blends of iron powder and additives (copper, nickel, graphite, manganese sulfide and dry lubricants) play important roles in products and equipment.
The blending process begins when iron powder arrives at ABT in loads consisting of 2.5 ton palletized cardboard boxes. Forklifts load the boxes into a powder dumper, which tilts and dumps the contents into a steel hopper positioned above a fine-mesh screener.
Additives arrive in either small bulk containers or paper sacks and are poured into individual hoppers that rest on coarse mesh screens. The company sift everything prior to blending. They ensure contaminants such as wood slivers from the pallets or bits of cardboard or plastic do not enter the mix, as foreign material could compromise the finished powdered metal part.
After screening, three floor scales weigh the powders before they are loaded into the blender. The iron is weighed on a 5 ton scale in 2.5 ton loads. The 500 lb (227 kg) scale weighs the nickel and copper additives. A 200 lb (91 kg) scale provides the greatest accuracy needed to weigh graphite and dry lubricants.
The iron powder is gravity-fed one box-load at a time from the steel hopper into the blender. Before the last 2.5 ton load is added, the hopper containing the additives is raised into position by a forklift for gravity discharging into the blender.
The rotary mixer used consists of a horizontal, rotating drum with a stationary inlet at one end and a stationary outlet with a discharge gate at the other. A self-adjusting face seal at the inlet allows dust-free operation. Internal baffles (mixing flights) and lifters create a four-way mixing action that tumbles, turns, cuts and folds material throughout the filling, mixing and discharging phases, achieving 100% batch uniformity and preventing the separation of ingredients of varying particle sizes.
The mixer can achieve 100% batch uniformity in three minutes, but ABT mixes 22.5 ton loads for about 15 minutes depending on the properties required for specific powder metal mixes. The rotary batch mixer saves time and ensures a quality product. A double cone blender for this size load would take more than 50 minutes to blend because the material tends to roll around rather than mix together. Plus, the long blend time builds more heat within the mix. When the mix gets hot, then you know you're rounding the particles, reducing the mixture's green strength.
A powdered metal part that has not undergone sintering is a green part. Green strength is something measured for the customers. The company found that green parts from material blended in the rotary mixer have a higher green strength than parts molded of material from double cone blenders.
Uniformity of powdered metal blends is crucial to the performance of the finished part. Sampling ensures homogeneity, but it is done primarily on new blends to obtain blending times. Once a blend's mix and mixing time have been mastered, there is no need to sample. But, if a customer has an order for a specific feature in a blend, such as apparent density, sample is a must to ensure that specification is met.
Bringing powdered metal blending in-house, rotary-style
When processing powdered metal compounds for products such as gyro rotors, flywheels, counterbalances, boring bars and grinding quills, the proportion of powdered metal ingredients must be precise, and the distribution of those ingredients 100 percent uniform.
Raw powder ingredients are weighed in floor hoppers equipped with load cells, and moved pneumatically into surge hoppers positioned above each blender. When an operator initiates a blending cycle, a surge hopper valve opens to discharge accumulated batch ingredients into the rotating blender.
Baffles in the rotating mixing chamber continually lift and direct material toward the discharge spout that is equipped with a head assembly designed specifically for Mi-Tech. While the powder will discharge from the blender on its own, a way of controlling how it is discharged into the intermediate bulk containers is needed. Once filled with blended powder, the bulk containers are moved using tow motors.
The company uses about 20 of these intermediate bulk containers for temporary storage of blended powders while the properties of the blends are verified, a process that can take several days to a week.
Thorough blending is crucial. There are industry standards for the various composite heavy metals. The powder has to be within specifications for tungsten, nickel, iron and so on, for each of the different materials. The mixing must be uniform. Otherwise the pressed parts would not have a consistent chemical analysis, which will affect the final part's properties as well as the sintering processes. To ensure the chemistry is right, test will be conducted after blending, taking samples to verify composition and to produce test parts for analysis of mechanical properties.
During the blending cycle the material is in motion 100 percent of the time, and because how homogenously the blenders mix the powders is known, and that static blends will not separate during storage, separation of blended ingredients is not a concern.
Once properties are verified, each 5500-lb (2500-kg) batch is typically metered from bulk containers into smaller barrels for transportation to various compaction processes. It is not efficient to constantly change over powders.
Phe blended powders are compressed into thousands of different shapes. Compacted powder not yet sintered is termed "green," and is subject to breakage if improperly handled. Broken pieces are loaded into a Pin Mill, also from Munson, reducing them into powders which are screened and blended with virgin powders up to certain percentages, depending on the blend being produced. By using green powders, a big buildup of powder that cannot be used can be prevented. Following compaction, green parts are sintered, and then machined to produce the final part.
