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Powder Metallurgy (Sintering)

2022/10/31
[Archival Edition] A thorough explanation of powder metallurgy from a professional perspective!
METAL COLUMN

What is powder metallurgy?

Powder metallurgy can be broadly divided into two types: press compression molding (PM) and metal powder injection molding (MIM). Here, we will explain press compression molding (PM). In press compression molding, metal powder is filled into a die, press compression molded, and then sintered in a high-temperature furnace. When the molded body is heated at high temperatures, the particles bond together and become an alloy. Various types of quenching and plating are available as surface treatments.

What kind of dies are used in powder metallurgy?

"Generally" the mold structure for powder metallurgy mainly consists of an upper punch, a lower punch, a die, and a core. The material used for the mold is mainly die steel such as SKD51 or SKD11. In addition, by using a multi-stage punch, it is possible to handle more complex shapes. Multi-stage punches are also used in large press machines.

Powder metallurgy manufacturing process

Powder metallurgy manufacturing process
1. Blending and mixing
② Molding
③ Sintering
④ Post-processing

1. Blending and mixing
Depending on the properties of the desired product, various metal powders are mixed in predetermined ratios and then homogenized in a mixer.

② Molding
The mixed metal powder is poured into a mold and compression molded at room temperature using an automatic molding machine.

③ Sintering
The compressed compact is placed in a sintering furnace and heated in gas below its melting point to sinter it.

④ Post-processing
The aim is precision and strength. The post-processing mainly involves sizing (re-pressing), machining, and heat treatment before the product is completed.

Powder metallurgy manufacturing process: mixing

Two or more types of metal powders are mixed together as raw materials. They are then completely dried using a spray dryer or similar to produce metal powder. The main raw materials used for metal powder are iron, copper, stainless steel, titanium, tungsten, and aluminum. The most commonly used materials are combinations of carbon, copper, nickel, phosphorus, molybdenum, and manganese. Care must be taken to avoid segregation during the mixing process. (Segregation refers to a state in which powders are separated and not mixed evenly.) Generally, premixed powders mixed by the raw material manufacturer are sometimes used.

Powder metallurgy manufacturing process: molding

Metal powder is placed in a mold and compressed into a shape using a press.
Because we use extremely high-precision molds, we are able to mass-produce the same shapes with high precision. Setting up the molds takes about 45 minutes to 2 hours, depending on the molding tonnage and shape.

Powder metallurgy manufacturing process: sintering

The molded product made by compressing metal powder is sintered in a sintering furnace. Each grain of metal powder bonds together to create a hard, durable product. The sintering temperature and time vary depending on the material. Generally, copper-based powder alloys are sintered at 750 degrees, iron-based powder alloys at 1130 degrees, and stainless steel alloys at 1300 degrees in a dedicated vacuum sintering furnace.

Powder metallurgy manufacturing process: Post-process

If there is a demand for improved precision or strength, we perform machining such as sizing (repressure and straightening), drilling and tapping, polishing such as flat grinding, heat treatment such as steam treatment, carburizing and quenching, high-frequency quenching, surface treatment such as plating, etc. We also handle assembly such as pressing in ball bearings and crimping flange plates.

Powder types and production methods

The main metal powders used as materials are pure iron powder, stainless steel powder, high speed steel powder, alloy steel powder, and copper powder. There are many other types, and the production methods are also different. Furthermore, even metal powders of the same element can have different properties depending on the production method.
We mainly carry out oxide reduction processes using atomization, melt spinning, rotating electrode, pulverization, and other chemical processes.

Atomization method

The atomization method is a method of melting metals such as iron, copper, and aluminum, then solidifying them by spraying them with high-pressure gas or water to create metal powder. The metal melted in a melting furnace is poured into a small crucible. The liquid metal that flows out of the hole in the bottom of the crucible is sprayed with high-pressure gas or water, causing it to scatter and solidify, and is produced in a powder form. This is called atomized powder.

Oxide reduction method

The oxide reduction method is a method for producing powders of iron, copper, etc. by reducing fine oxide powders with gases such as hydrogen, carbon monoxide, and ammonia decomposition gas. By adjusting the reduction temperature and time, the particle size of the produced powder can be easily adjusted, making it possible to obtain metal powders with good moldability and sinterability.

Electrolysis

The electrolysis method is a method of producing metal powder by electrolysis, depositing powder on the negative electrode. When electrolysis is performed, metal dissolution and oxygen generation usually occur on the positive electrode, while metal deposition and hydrogen generation occur on the negative electrode. Using this mechanism, metal powder is produced, dehydrated, dried, and separated by particle size for use. This is called electrolytic powder.

Grinding method

Crushing → Mainly using a stamp mill, the raw powder is flattened.
Classification → Using an air classifier such as a cyclone, the particle size distribution is adjusted according to the product.
Glazing → The powder is used for surface treatment to give the coating or printed surface gloss and leafing properties (the property where metal powder floats to the surface, creating a mirror effect).
Mixing → If paste processing is required depending on the product type, the powder is mixed with a processing agent.

Uniform droplet spraying method

In the uniform droplet atomization method, molten metal is dripped evenly from a heat-resistant container in an inert gas atmosphere.
Using a special dripping technique, the dripped metal solidifies into a spherical metal ball.

Heat Treatment Method

Heat treatment: Powders produced by various manufacturing methods are used as raw materials and are heat-treated to improve their properties or to alloy them to obtain new alloy powders or partially alloyed powders.
Crushing → The powder solidified by heat treatment is crushed.
Setsubun → The product is sieved into several stages according to particle size in order to adjust the particle size distribution according to the intended use.
Mix until the powder is uniform.
Standards → Check whether the product complies with the standards.

Powder types and production methods

Dissolution Process

The melting process is an essential step for preparing the desired alloy, removing impurities, and atomizing it, but since it can become contaminated with undesirable elements due to reactions between the alloy and the crucible or atmosphere, various melting processes are used according to the type of alloy, as described below. The heat source for melting is usually electricity, and induction heating or arc melting with carbon electrodes is used, but electron beam heating and plasma arc heating are also used.

Air Dissolution

Air melting is a melting method that uses a crucible, and when it is necessary to prevent the molten metal from oxidizing, flux, inert gas, or conductive slag is used. The energy used for melting is usually induction heating or arc melting with carbon electrodes. Although it is not necessarily suitable for alloys that contain a large amount of elements that are easily oxidized, it is widely used when necessary because it is economical and productive. Vacuum melting Vacuum melting is used for alloys that are not suitable for air melting, as the crucible is placed in a vacuum container and melted in a vacuum state. In addition to regular induction heating, electron beams and plasma arc heating, which will be described later, may also be used as a heating method. After melting, the pressure is returned to atmospheric pressure using an inert gas or the like, and then gas atomization is performed.

Water-cooled copper crucible melting (floating melting)

Crucibles are usually made of ceramic or carbon, but this method is used when certain alloy elements react with the crucible and normal molten metal cannot be obtained.
The molten metal is suspended in an inert gas without contact using the principle of electromagnetic levitation, and nominally in a copper crucible, but by using copper, which has good thermal conductivity, and cooling it with water, the molten alloy itself solidifies and forms a layer even if it comes into contact with the crucible surface. This is a method to obtain clean molten metal by constantly cooling it and forming a so-called second crucible, preventing copper from penetrating into the alloy. This melting method is also used when casting titanium alloys.

Plasma Melting

This is a crucible-less melting method used to melt the tip of wire or rod material or powder particles with an ultra-high temperature plasma arc, and then atomize the molten metal using centrifugal force, etc. The extremely high temperature of the plasma arc has the advantage of being able to instantly melt high-melting point metals, intermetallic compounds, carbides, etc. There is also a method in which the plasma jet itself melts and atomizes at the same time.

The benefits of powder metallurgy

Mass production using molds allows for significant cost reduction compared to cutting products.
◎Near net shape can be achieved with high-precision molds, reducing post-processing
◎The weight of parts can be reduced
◎The porous properties can be utilized to impregnate the product with oil.
◎It is possible to mold metals with "high melting points" that are difficult to cast.
◎Homogeneous metal molding is possible even for alloys that are difficult to cast.

Disadvantages and drawbacks of powder metallurgy

×Not suitable for producing parts with long overall length
×Dimensional accuracy may decrease due to expansion and contraction during sintering.
× A certain amount of mold investment is required, so it is not suitable for producing parts with small total lots.
×Compared to steel, it is not as dense, so it is not suitable for areas where high strength is required.

Strength of powder metallurgy

Powder metallurgy materials (each type symbol) are clearly defined by JIS standards. Mechanical properties are expressed by radial crushing strength for oil-impregnated bearing materials, and tensile strength for machine part materials. Hardness is not specified, but for SMF4030 it is around HRC28. If you want to increase the hardness, heat treatment (carburizing and quenching) is performed in a later process.

The difference between powder metallurgy and sintering

"Sintering" refers to the phenomenon in which metal powder is heated and solidifies into a dense object called a sintered body, but sintering is generally understood to mean powder metallurgy. Sintering also often refers to press molding (PM). MIM is also included in powder metallurgy, but it is called MIM.

Difference between powder metallurgy and MIM

Powder metallurgy is abbreviated as PM, which is an abbreviation of Powder Metallurgy. It is called powder metallurgy. MIM is abbreviated as MIM, which is an abbreviation of Metal Injection Molding. It is called MIM (=Mim). Powder metallurgy is made into a compact by "compression molding press", and becomes a sintered body through the sintering process. In contrast, MIM is made into a compact by "injection molding", and becomes a sintered body through the debinding and sintering processes.

What we can make with powder metallurgy

Auto Parts
Motorcycle Parts
Motor parts
OA equipment parts
Amusement related parts
Housing equipment (toilet) parts

Auto Parts

We manufacture bearings for power window devices and moving gears for power reclining seats.

Motorcycle Parts

We manufacture exterior parts for the key insertion area.

Motor parts

We manufacture drive system parts such as gears, pulleys, sprockets, and gear heads that are attached to the end of motor shafts.

Office equipment parts

We have a strong track record in manufacturing copper-based oil-retaining bearings for the paper transport parts of copy machines and fax machines, as well as various mechanical structural parts (sintered) used in drive parts, and can handle small lot orders of many different items. We also have experience in manufacturing sliding parts used in the hinge parts of computers.

Amusement related parts

We manufacture mechanical parts for pachinko balls and medal counters.

What can be made with powder metallurgy: Parts for housing facilities (toilets)

We manufacture mechanical parts for toilet seat lid opening and closing devices.

If you have any powder metallurgy problems, come to Kiyota!

We will promptly introduce you to the most suitable domestic and foreign manufacturers based on your budget, product size, product specifications, and the difficulty of processing. We will also propose costs based on materials and shapes. We will also provide one-stop quotations for other manufacturing methods (cutting, MIM, etc.). At the same time, we will assist those in charge of material procurement with sourcing.  

FAQ


Q: What is powder metallurgy?

Answer: Powder metallurgy is a process in which metal powder is put into a mold, pressed and compressed into a compact, and then placed in a high-temperature furnace to sinter it. The compact is then heated at high temperatures, causing the particles to sinter together and bond to form an alloy.

Q: What are the benefits of ordering from Kiyota?

A: We will promptly introduce you to the best domestic and overseas manufacturers based on your budget, product size, product specifications, and processing difficulty. We will also suggest ways to reduce costs by selecting the right material and shape.