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This article takes an in depth look at Powder Metallurgy.
You will learn more about topics such as:
What is Powder Metallurgy?
The types of Powder Metallurgy processes
Parts and products made using Powder Metallurgy
Metals used in Powder Metallurgy
And much more…
Chapter One - What is Powder Metallurgy?
Powder metallurgy is a manufacturing process that produces precision and highly accurate parts by pressing powdered metals and alloys into a rigid die under extreme pressure. With the development and implementation of technological advances, powder metallurgy has become the essential process for the production of bushings, bearings, gears, and an assortment of structural parts.
The key to the accuracy and success of powder metallurgy is the sintering process that heats parts to bond the powder particles . The temperature in sintering is slightly below the melting point of the primary metal such that the bonds of the powdered particles are bound together.
Chapter Two - The Powder Metallurgy Process
The process of powder metallurgy is an ancient, unique method for forming shapes and designs from ferrous and non-ferrous metals. Powder metallurgy has been used for thousands of years as a way to produce household items and tools. It began as a method for mass producing products and parts in the middle of the first industrial revolution.
Until the early part of the 20th Century, the process was used sporadically but was not considered to be a viable production method. With the development of electricity and technological advances, powder metallurgy has found a place as a highly efficient and productive method for producing parts with high tolerances and minimal waste.
The four basic steps to the powder metallurgy process are powder preparation, mixing and blending, compacting, and sintering. These steps have been used over the centuries to produce a variety of products.
As with any manufacturing process, powder metallurgy has variations to accommodate the requirements of individual parts. The different methods and techniques have grown from the development of technological advances and engineering specifications. Four of the variations are conventional, injection molding, isostatic pressing, and metal additive manufacturing, which is the newest advancement.
Four Basic Steps of Powder Metallurgy
Properties of products produced using powder metallurgy are dependent on the characteristics and properties of the powder. One of the processes used to produce powder for powder metallurgy is melt atomization. In this process, liquid metal is broken into tiny droplets that cool and solidify into minute particles.
Though atomization is the most common method for producing powder, other processes include chemical reduction, electrolytic deposition, grinding, and thermal decomposition. Regardless of which process is used, all metals and alloys can be converted into a powder.
Prior to mixing and blending the powder, it is evaluated and tested for its appropriateness for the powder metallurgy process. The factors that are considered are flow rate, density, compressibility, and strength.
Mixing and Blending
In the mixing and blending process, powders are combined with other powders, binders, and lubricants to ensure the final part has the necessary characteristics. Blending and mixing can be completed wet or dry depending on the type of powder metallurgy process and the requirements of the part.
The four most common blending and mixing techniques are rotating drum, rotating double cone, screw mixer on the interior of a drum, and blade mixer on the interior of a drum. The image below shows a rotating double cone and rotating drum with three examples of blended powder below.
Compacting involves pressing and compressing the powder mixture into the desired shape or die. When done properly, compacting reduces potential voids and significantly increases the density of the product. The compressed and pressured form is referred to as a green compact, an indication that the part was formed by compacting.
Compacting pressure is between 80 MPa and 1600 MPa. Each type of metal powder requires a different amount of compacting pressure depending on its properties.
In soft powder compacting, the pressure is between 100 MPa and 350 MPa. For more resilient and harder metals, such as steel and iron, the pressure is between 400 MPa and 700 MPa.
Though the green compact has been stressed and pressed at extreme pressure, it is not strong enough to be used. In order to produce a permanent bond between the metal particles, the green compact is sintered or heated at high temperature. In essence, sintering produces the final usable product or part. The atmosphere of the sintering is controlled such that it has a sufficient amount of carbon to produce a neutral or carburizing environment, which determines the properties of the sintered materials.
Sintering is a heat treatment wherein large numbers of parts, in compacted form, are subjected to temperatures that are sufficient to cause the loose particles to unite and bond, forming a solid piece. The required temperature fluctuates in accordance with the type of metal but is always slightly lower than the metal’s melting temperature.
Compacting presses the particles of the green compact to form a shape. Regardless of the pressure applied during compacting, there are still minute porous spaces in the green compact. During sintering, the material is heated at high temperatures to close the porous spaces and strengthen the part.
Powder Metallurgy Processes
Conventional powder metallurgy follows each of the steps of basic powder metallurgy where the powder and alloy are mixed, compacted, and sintered. It is much like the ancient method of powder metallurgy with the added benefit of modern technology.
The unique benefit of injection molding is its ability to produce complex shapes in large quantities. The powders for injection molding are specially formulated with a binder, such as wax or a thermoplastic. The mixture, referred to as feedstock, is fed into the mold cavity of a normal injection molding machine. When the green compact is discharged from the mold, the binder is extracted by thermal processing or solvent. Any remaining binder dissipates during sintering.
The injection molding powder metallurgy process is very similar to plastic injection molding and high pressure die casting and produces the same intricate and complicated shapes of those processes. The difficulty with powder metallurgy injection molding is the amount of secondary finishing that is necessary. Its benefit is its ability to produce good dimensional tolerances in unlimited shapes and with unique geometric features.
Isostatic pressing is an equal pressure process wherein the same amount of pressure is applied to the entire surface of the workpiece. By applying pressure equally from all directions, every angle, curve, line, and depth of the part has the same density and microstructure. Isostatic pressing can be performed either cold or hot depending on the requirements of the piece being produced.
Cold isostatic pressing produces parts where pressing dies are not an option or the parts to be produced are very large and complex. A wide variety of materials can be used for isostatic processing. The amount of required pressure begins at a little under 5000 psi and goes as high as 100,000 psi. The powder is molded in elastomeric molds that may be wet or dry.
Hot isostatic pressing combines high temperatures with isostatic pressure. The combination of the two factors eliminates porosity, increases density, improves mechanical properties, and makes finished parts workable. Hot isostatic powder metallurgy reduces microshrinkage.
Metal additive manufacturing is a huge step forward in production processes and the next method for producing items we need. Sometimes referred to as three dimensional printing, it is a process that has been used for years to make prototypes for proposed components. Additive manufacturing builds parts layer by layer from a digital representation.
The process of metal additive manufacturing encompasses several technological advancements, is highly efficient, and does not produce any waste. It begins with a powder bed that is 20 µm (micrometers) to 100 µm thick. One or more lasers are applied to the bed to melt away the various layers. The process of melting continues layer by layer until the part is completely formed.
Chapter Three - Parts and Products Made Using Powder Metallurgy
After its return in the middle of the first industrial revolution, the use of powder metallurgy has steadily grown to become an essential part of the production of a wide variety of parts and products. As technology developed and electricity became more available, manufacturers explored processes that could produce high tolerance parts with little waste. In their search, they rediscovered powder metallurgy and its ability to produce parts accurately and economically.
The use of powder metallurgy is found in nearly every manufacturing company due to the fact that it is a technique that fits the demands of any application or production process. As producers search for more efficient and eco-friendly methods, many have chosen powder metallurgy since it provides a high degree of process control and limited waste.
In every part of life, from the bathroom shower to the gears and tools used to drill for oil, there are items made using powder metallurgy. Below are a few examples of the many thousands of parts, components, tools, instruments, and accessories that are made using this highly flexible and valuable process.
The use of powder metallurgy for the manufacture of automobile components is directly related to innovations in the development of new materials and pressing technology. The wide variety of materials available can be pressed, formed, and shaped with higher densities and strength.
The advantages of powder metallurgy for auto parts production include its ability to produce complex configurations using cost effective production methods. Being able to manufacture complex components without waste or scrap removes the need for finishing or machining.
Iron based and stainless steel powders are the most used in production of automobile components. Some of the components that are manufactured include:
Main bearing caps
Variable valve timing
Catalytic reduction system
Fuel system components
Jet Engine Turbine Disc
Turbines for aerospace are required to withstand extreme temperatures and varying pressures. The turbine disc is the core to which engine fans, compressors, and turbine blades are attached. Turbine discs are made from high temperature alloys that meet the grades and standards required. Turbine discs are machined and finished to achieve the highest level of performance.
Atomized powders and hot isostatic powder metallurgy are used to produce turbine discs. The process is complicated and difficult since it is necessary to avoid the introduction of unwanted particles or impurities.
Prosthetic Twin Connecting Plate
A twin connecting plate protects against accidental reaching into the joint and serves as a power transfer and connection between a prosthetic knee and the stretching device when the knee bends.
Surgeons require precision tools that provide excellent performance. Blades have to be honed to the finest point and ground to meet exacting specifications. Graspers, designed to assist in endoscopic operations, are a few millimeters in length with a complex design and must be stressed before being shipped. A part of graspers is a knot pusher that has the same dimensions and requirements as a grasper.
Gears are one of the most commonly produced parts when it comes to powder metallurgy. What producers have found is that powder metallurgy offers exceptional accuracy, which leads to longer gear life. Gears can be made with blind corners, eliminating the need for undercut relief.
When gears are produced using powder metallurgy, they easily combine with other components such as cams, ratchets, and other gears. Any form of gear can be produced using powder metallurgy including helical and involute gear forms.
Oil and Gas Industry
Hard metal and diamond cutting tools are a necessary part of the exploration for gas and oil. To meet the need, powder metallurgy produces tools made of austenitic and stainless steels. Valves and manifolds are made to meet precise dimensions and requirements using materials capable of enduring harsh, hazardous, and stressful conditions.
A market that is rapidly growing for powder metallurgy manufacturers is hand tools for DIY projects and lawn care. The precision and accuracy of powder metallurgy produces tools that last five times longer than those made using other processes. Tools made using powder metallurgy have wear resistance, exceptional toughness, and increased hardness retention . It is very likely that, in the near future, all hand tools will be made using powder metallurgy.
Chapter Four - Types of Metals Used in Powder Metallurgy
There are few limitations to the types of metals that can be used in the powder metallurgy process. Though there is a wide selection available, certain metals are used repeatedly due to their properties and characteristics. There are certain factors producers consider when making their metal selection.
The key factors in the selection process are corrosion resistance, hardness, tensile strength, impact toughness, and fatigue strength. Every metal meets some or all of these qualities. The requirements of the part to be produced determine the type of metal chosen.
Stainless steel is the first choice for the production of many components due to its many positive qualities, with corrosion and rust resistance being the main two. The series of stainless steels used for powder metallurgy production are 300 and 400. The versatility and adaptability of stainless steel make it an ideal choice for a wide range of applications.
Stainless steel series 316L has superior corrosion resistance, toughness, ductility, and resistance to acids. It is used for aerospace components, automotive parts, medical instruments, and parts for ship construction.
Copper powder and its alloys are corrosion and rust resistant and widely used in applications where there is a great deal of moisture. Copper alloys can be either pre-alloyed powders or elemental mixes. Bronze, a copper alloy made up of copper and tin, is used to produce self lubricating bearings.
The properties of nickel make it an ideal metal for use in the manufacture of parts for gas turbines and rocket engines since it is corrosion resistant and can withstand high temperatures. Monel, an alloy of copper and nickel, is exceptionally hard and resistant to the effects of saltwater.
Nickel is a silvery white metal that is malleable, hard, and ductile. It is a good electricity and heat conductor that dissolves slowly in dilute acids.
The majority of aluminum components are made using aluminum alloys since raw aluminum is soft, extremely pliable, and light. Regardless of combination with other metals, aluminum is lightweight and easy to shape. It is used in nearly every industry with the aerospace and automobile industries using it the most.
Iron powder is gray black crystalline powder with a density of 7.694 and melting point of 1837° C. For powder metallurgy, iron powder is produced using high pressure water atomization. It is sintered at 1121° C. Since iron is a soft metal, it is normally mixed with carbon to form steel. Of the various types of metals used for powder metallurgy, iron is one of the most common. Iron powder is mainly used to produce automobile parts such as shafts, rocker arms, and rotors for oil pumps.
The quality of iron powder depends on how it is produced. The common methods are atomization, reduction, and electrolysis with electrolysis producing the purest form.
Of the variety of metals available for the powder metallurgy process, titanium is the most expensive. It has a silvery appearance and is valued for its exceptional strength and corrosion resistance. Titanium powder is produced much like the other metals using a variety of processes. When titanium is processed into a powder, it takes on a gray or black color but retains the properties it has when in solid form.
Titanium powder is used to produce parts for aircraft, missiles, and chemical processing. The cost of titanium is due to the elaborate methods required to produce it, though less expensive methods are being perfected.
Soft Magnetic Composite (SMC)
Soft magnetic composites are created from iron that has each of the particles of iron coated with an insulation material prior to being compacted. The coated iron is pressed into solid form using a die, which is heat treated to cure the bond. The process makes it possible for a non magnetic material to become a magnetic material by adding electrical current. The created component has high resistivity and little eddy current loss.
Reinforced polyester material with carbon or glass fibers that are an inch or more long are the raw materials for the manufacture of SMCs. The long strands are spread in a resin bath made of vinyl ester, polyester, or epoxy. Parts are formed using the compression molding process, which makes it possible for SMCs to be formed into a wide range of parts with varying thicknesses, lengths, and intricate details. The molding process involves the compression of the mixture using a hydraulic press.
The powder metal manufacturing used to make SMCs is a way for producing low cost replacements for stacked laminations. Complex geometries with direct magnetic flux allow designers to reduce the size, weight, and waste from product design. The application of the resin material prior to compression of the part eliminates the need for lamination.
Chapter Five - Equipment Used for Powder Metallurgy
Each step of the powder metallurgy process requires special equipment that varies according to the process being used. Powder metallurgy is a method for producing structural parts and bearings using a variety of techniques and equipment. From blending and mixing the powder to sintering and cooling the finished part, specific specialized equipment is necessary to achieve positive results.
Every method of powder metallurgy begins with the production of the powder, which is completed to fit the needs of the part and its final use. The production of the powder determines the characteristics and properties of the part.
Water Atomization Powder Manufacturing Equipment
Atomization is one of the methods used to produce powder for powder metallurgy. An atomization machine uses high velocity water flow to atomize molten metal into particles and is capable of producing 10 kg to 10,000 kg of powder in each batch.
Compacting presses for powder metallurgy have the same construction and configurations as presses used for shaping metal and include mechanical, hydraulic, anvil, rotary, isostatic, and transfer molding presses. Powder compacting presses are able to compress a wide assortment of materials into the shapes, sizes, and densities any part may require.
The operation of a compression press happens in three stages. The first stage is the sliding stage where the powder material moves and fills pores. In the sliding stage, compression happens rapidly as pressure is applied. In the second stage, the pressure exceeds the pressure of the sliding stage, but little change occurs in the green compact. In the third and final stage, the pressure continually increases far beyond that of the second stage, the powder particles deform, and the density of the workpiece increases.
Though the part is fully formed after compacting, it is not stable enough for use as a component and needs to be heated to become a solid form. This is done by a sintering machine. The term sintering refers to a process that heats and forms a solid mass without melting or liquefying the mass. The heat applied during sintering is precision controlled such that it is just below the melting point of the powdered metal.
In the powder metallurgy process, sintering is the essential procedure that adds mechanical strength, density, and translucency to the workpiece.
An addition to the powder metallurgy process is laser sintering; it is a part of the additive manufacturing process. Laser sintering, also known as selective laser sintering, is a part of three dimensional technology and is used to sinter powder into a solid structure, much like traditional sintering.
Laser sintering forms the final part using a focused laser that coalesces the substrate of the powder into a solid mass. The forming process of the laser is guided by a computer aided design (CAD) file containing a model of the final part. The directions of the CAD file precisely aim the laser at points in space to bind material to form the final structure.
Chapter Six - Benefits of Using the Powder Metallurgy Process
To many, the steps of the powder metallurgy process may seem like a waste of time considering there are other methods less time consuming. Though there are negative factors regarding powder metallurgy, its many benefits have made it one of the most popular production methods.
A major factor in metal product production is the creation of homogenous and uniform parts with exceptional tolerances at high efficiency and low cost. It is for these reasons that powder metallurgy has become a vital part of the manufacturing of automobiles and aircraft.
The rapid growth of powder metallurgy is due to the many technological advances that have streamlined the process and made it so efficient. The many innovations have taken it from a curious production method to a low cost means of producing precision intricate parts.
The powder metallurgy process is an eco-friendly production method. An astounding 97% of the material used to produce parts through powder metallurgy becomes part of the final product. Powder metallurgy has very little waste and no scrap. Every piece of powder that enters the process is included in the finished component. In addition to being environmentally sound, the lack of waste is a significant cost saving.
Unlike other processes, powder metallurgy can blend and mix different metals and nonmetals into a product. Unusual and unique combinations can make up a single part with the use of a binding material. Complicated metallurgical procedures can be avoided as powder metallurgy can easily combine different materials and compact them into a single shape.
Powder metallurgy is capable of producing any shape, from complex and intricate designs to simple gears. The continuing growth of its technology is opening the door to new and diverse applications.
Near Net Shape
Parts and products produced using powder metallurgy do not require any secondary processing. Each part is near net shape, meaning they will not require any finishing. Additionally, powder metallurgy parts have exceptionally high dimensional accuracy.
The availability of raw materials is a constant problem for production operations since a lack of resources can slow or halt production. Raw materials for powder metallurgy are easily accessible and inexpensive. Powdered metals are very common and ordinary materials that are readily available from several producers. As powder metallurgy has grown, more and more suppliers have developed fabrication processes to meet the demand.
In mass production, it is essential that every part, from the first to the last, matches the dimensional requirements of its design. In certain processes, there is degradation that occurs during production that creates imperfections and differences in the individual parts. The uniformity and repeatability of the powder metallurgy process avoids such errors and ensures that every part has precise dimensions.
It is important that parts installed in cars, planes, and machinery have a long life span to avoid costly repairs and replacement. Parts produced using powder metallurgy have exceptional wear resistance and friction coefficient. These factors guarantee that when a powder metallurgy part is installed, it will last.
A unique feature of powder metallurgy is its ability to combine different metals to fabricate and produce magnetic components as well as control the magnetic properties. The compaction of powder material is the same process that is used to produce magnets, which makes it an easy property to add to powder metallurgy parts.
When a substance is homogeneous, its composition is the same regardless of where in the substance you pull the sample from. This quality is an indication that the properties of a part are consistent throughout its structure. Chemical homogeneity is essential for a component to have uniformity, strength, and durability. All powder metallurgy products have this characteristic; it is the reason they are so long lasting.
Powder metallurgy is a manufacturing process that produces precision and highly accurate parts by pressing powdered metals and alloys into a rigid die under extreme pressure.
The key to the accuracy and success of powder metallurgy is the sintering process that heats parts to bond the powder particles.
The process of powder metallurgy is an ancient, unique method for forming shapes and designs from ferrous and non-ferrous metals.
After its return in the middle of the first industrial revolution, the use of powder metallurgy has steadily grown to become an essential part of the production of a wide variety of parts and products.
There are few limitations to the types of metals that can be used in the powder metallurgy process.
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