Questions About Powder Metallurgy

1. What is powder metallurgy?

Powder metallurgy is a technology that manufactures metal powders and uses metal powders (sometimes with a small amount of non-metallic powder) as raw materials to manufacture materials or products through mixing, forming and sintering. It includes two parts, namely:

(1) Manufacturing metal powders (also including alloy powders, hereinafter collectively referred to as "metal powders").

(2) Using metal powders (sometimes with a small amount of non-metallic powder) as raw materials, through mixing, forming and sintering, to manufacture materials (called "powder metallurgy materials") or products (called "powder metallurgy products").

2. What are the most prominent advantages of powder metallurgy?

There are two most prominent advantages of powder metallurgy:

(1) It can manufacture materials and products that cannot be manufactured or are difficult to manufacture using other processes, such as porous, sweating, shock-absorbing, sound-insulating materials and products, refractory metal materials and products such as tungsten, molybdenum, titanium, and metal-plastic, bimetallic and other composite materials and products.

(2) It can directly produce products that meet or are close to the finished product size requirements, thereby reducing or eliminating mechanical processing. Its material utilization rate can be as high as 95% or more. It can also replace copper with iron in some products, achieving "material saving and energy saving".

3. What is "iron-based"? What is iron-based powder metallurgy?

Iron-based refers to the composition of the material with iron as the matrix. Iron-based powder metallurgy refers to the general term for the process of manufacturing powder metallurgy materials and products (iron-based mechanical parts, anti-friction materials, friction materials, and other iron-based powder metallurgy materials) with iron as the main component by sintering (also including powder forging).

4. What are the main types of powder manufacturing methods used in powder metallurgy?

Powder manufacturing methods are mainly divided into two categories: physical and chemical methods and mechanical crushing methods. The former includes reduction method, electrolysis method and carbonyl method, etc.; the latter includes grinding method and atomization method.

5. What is the use of reduction method to make metal powder?

This method is a method of using a reducing agent to remove oxygen from metal oxides to obtain metal powder.

6. What is a reducing agent?

A reducing agent refers to a substance that can take away oxygen from an oxide. The reducing agent used to prepare metal powder refers to a substance that can remove oxygen from a metal oxide. As far as metal oxides are concerned, any substance whose affinity with oxygen is greater than the affinity of the metal with oxygen is called a reducing agent for the metal oxide.

7. What is the purpose of powder reduction annealing?

The purposes of powder reduction annealing are mainly the following three aspects:

(1) Remove the oxide film on the surface of metal powder particles;

(2) Remove foreign matter such as gas and moisture adsorbed on the surface of particles;

(3) Eliminate the work hardening of particles.

8. What are the general items for powder property determination used in powder metallurgy?

There are generally three items for powder property determination used in powder metallurgy: chemical composition, physical properties and process properties.

9. What are the main items of powder physical properties used in powder metallurgy?

The physical properties of powders used in powder metallurgy mainly include the following three items:

(1) Powder particle shape;

(2) Powder particle size and particle size composition;

(3) Powder specific surface area.

10. What are the main process properties of powders used in powder metallurgy?

The process properties of powders used in powder metallurgy mainly include the following five items:

(1) Bulk density;

(2) Tap density:

(3) Flowability;

(4) Compressibility;

(5) Formability.

11. What determines the shape of powder particles used in powder metallurgy? What are the main types?

Due to different powder preparation methods, the particle shapes are also different. Generally, there are irregular, flake, polyhedral, dendritic, granular, spherical, drop-shaped, fibrous...

12. What is the particle size of powder? What method is usually used to determine it?

Powder particle size refers to the size of powder particles. It is usually determined by screening.

13. What is the particle size composition of powder?

The particle size composition of powder is also called particle size distribution. It refers to the weight percentage of each level of powder in the powder.

14. What is the particle size range of powder?

The particle size range of powder refers to the particle size of powder particles that vary between two specified particle sizes. If the particle size range of a powder is -80+150 mesh, it means that the particle size of these powders is equal to or less than 80 mesh, but greater than 150 mesh. In other words, these powders pass through the 80 mesh sieve, but not the 150 mesh sieve.

15. What is the sieving of powder?

The sieving of powder refers to the method of sieving the powder particle size.

16. What is the sieve analysis of powder?

The sieve analysis of powder refers to the method of sieving the powder sample with a set of standard sieves to find the weight percentage of each level of powder to express the particle size distribution of the powder.

17. What is the mesh number of the sieve?

The mesh number of the sieve (such as the Taylor standard sieve) refers to the number of all meshes on a length of 1 inch.

18. What is the specific surface area of ​​powder?

The specific surface area of ​​powder refers to the sum of the surface areas of all particles of 1 gram of powder (c㎡ or ㎡), also known as the gram specific surface area.

19. What is the loose density of powder?

The loose density of powder refers to the mass of powder per unit volume measured by allowing the powder to flow freely into a standard container (measuring cup) under limited conditions and then scraping it flat, expressed in g/cm3.

20. What is the tap density of powder?

When the powder is freely left in a standard container, an arch bridge is formed due to friction between particles. If the powder is vibrated under limited conditions to collapse the arch bridge, the mass of powder per unit volume measured is called the tap density of powder.

21. What is the density of pressed green sheets?

The density of pressed green sheets is the average value of the actual mass per unit volume of pressed green sheets, expressed in g/cm3.

22. What are the relative density of pressed green sheets and the relative density of sintered parts?

The ratio of the density of the green compact to the density of a dense material of the same composition is called the relative density of the green compact; the ratio of the density of the sintered part to the density of a dense material of the same composition is called the relative density of the sintered part.

23. What is the fluidity of powder?

The fluidity of powder is a qualitative term that describes the properties of powder flowing through a defined hole. It is usually expressed by the time (s) taken for 50 grams of powder to pass through a flow funnel with an outlet aperture of 2.54mm and a cone angle of 60 degrees.

24. What is the compressibility of powder?

The compressibility of powder refers to the degree to which the powder can be compressed under a specified unit pressure (such as 392MPa, i.e. 4TF/cm²). This property is usually expressed by the green compact density. The compressibility of powder reflects the irreversible deformation ability of the powder when subjected to pressure, and is an important parameter in the physical properties of the powder.

25. What is the formability of powder?

The formability of powder refers to the ability of the powder to maintain a certain shape after forming. . It can be measured by drum test and expressed by the compressive strength or flexural strength of the pressed green body.

26. What is powder forming?

Powder forming is a metal processing process that produces solid material parts of predetermined shape and size by mixing metal or non-metal powder with a binder and then pressing or injecting it using specific equipment (such as a powder press or powder injection molding machine). This technology is widely used in a variety of industries such as aerospace, electronics, automobiles, and medical care, especially metal powder injection molding (MIM) technology, which can produce complex and high-precision parts, such as small three-dimensional structures. The main advantages of powder forming technology include the ability to mass-produce complex and high-precision parts, the ability to manufacture composite materials, and the high utilization rate of raw materials and high efficiency of mass production.

27. What are the special powder forming methods?

There are five main special powder forming methods:

(1) Isostatic pressing;

(2) Continuous forming;

(3) Pressureless forming;

(4) High-energy forming;

(5) Injection molding.

28. What is the purpose of powder mixing?

By mixing powder, powder components with different properties can form a uniform mixture, which is conducive to pressing and sintering, ensuring that the material of the product is uniform and the performance is stable.

29. What is the effect of the length of the powder mixing time on the powder?

The powder mixing time must be determined according to the specific requirements for the powder and the equipment conditions. If the time is too short, the mixing will be uneven; if the time is too long, many unfavorable factors will occur, such as work hardening of metal powders such as iron and copper, and changes in particle shape and particle size distribution.

30. What are the uses of the main parts in the pressing die?

The uses of the main parts in the pressing die are: female die, the outer surface of the forming blank; upper punch, the upper end face of the forming blank; lower punch, the lower end face of the forming blank; core rod, the inner surface of the forming blank; pressing sleeve, the outer surface and end face of the forming blank.

31. What are the material selection principles for manufacturing powder metallurgy molds?

The main parts of the powder metallurgy mold should be manufactured according to their specific use conditions, and the wear resistance, processability, cost and other factors of the material should be comprehensively considered and reasonably selected. Its hardness must reach HRC55 or above. Carbon tool steel, alloy tool steel and cemented carbide can meet the requirements of hardness and strength.

32. What materials are the parts of the pressing die made of?

The female mold and core rod can be made of carbon tool steel (T10A, T12A, etc.), alloy tool steel (GCr15, Cr12, 9CrSi, Cr12Mo, Cr12W, Cr12MoV, CrWMn, CrW5), high-speed steel (W18Cr4V, W9Cr4V, W12Cr4V4Mo), cemented carbide (steel-bonded cemented carbide, YG15, YG8); the punch can be made of carbon tool steel (T8A, T10A), alloy tool steel (GCr15, Cr12, 9CrSi, Cr12Mo, CrWMn, CrW5); the pressing sleeve can be made of alloy tool steel (GCr15, 9CrSi, Cr12, Cr12Mo, CrWMn, CrW5).

33. How to select mold materials in a targeted manner?

For pressing parts with large batches, materials with good wear resistance, such as high-speed steel and cemented carbide, must be used; for pressing parts with small batches, cheap materials such as carbon tool steel can be used. For pressing parts with complex shapes, materials such as alloy tool steel that are easy to process and have small heat treatment deformation should be used; for pressing soft metal powders such as copper and lead, carbon tool steel or alloy tool steel should be used; for pressing hard metal powder materials such as tungsten and molybdenum, as well as cemented carbide and friction materials, cemented carbide materials must be used. For pressing high-density pressing parts, materials with good wear resistance should be used; for high-precision pressing dies, wear-resistant materials should be used, and cemented carbide should be used as much as possible.

34. What is the heat treatment hardness of the main parts of the powder metallurgy pressing die?

The hardness of the main parts of the powder metallurgy pressing die is: the female die requires HRC60-63 for steel parts, HRC64-72 for steel-bonded carbide parts, and HRA88-90 for carbide parts; the core rod requires HRC60-63, the hardness of the slender core rod can be appropriately reduced, and the local hardness of the connection of the mobile core rod is HRC35-40; the punch requires HRC56-60; the pressing sleeve requires HRC53-57. The protective sleeve is not heat treated; or it is tempered and the hardness is HRC28-32.

35. What are single-layer compacts and double-layer compacts?

 Single-layer compacts refer to compacts made of powders with the same composition; double-layer compacts are compacts with two layers of powders with different compositions; multi-layer compacts are compacts made of more than two layers of powders with different compositions.

36. What are common compact defects and their causes?

Common compact defects and their causes are as follows:

(1) Uneven density.

The causes include: inconsistent compression ratios of different parts; low mold finish, increased friction resistance; insufficient lubrication; unreasonable part size (too large aspect ratio, too large length-to-wall thickness ratio...); incorrect pressing method.

(2) Cracks.

The causes include: uneven density; poor powder formability, large elastic aftereffect of pressed green sheets; incorrect demolding method; poor mold rigidity; mold has reverse taper.

(3) Edge and corner loss.

The causes include: poor powder formability; low density of pressed green sheets.

(4) Surface scratches.

The causes include: scratches on the mold surface; mold nodules.

37. What is powder metallurgy sintering?

Sintering is one of the main processes in powder metallurgy. Generally, the powder or pressed green sheets are heated to a temperature of 2/3 to 4/5 of the melting point of its main component, so that the particles undergo physical and chemical processes such as bonding to form the required material or product.

38. What is sintering protective atmosphere? What is its function?

Sintering protective atmosphere refers to the atmosphere used to prevent oxidation of powder products during sintering.

Protective atmosphere can prevent oxidation of sintered parts, reduce oxides on the surface of powder particles, remove gases and moisture adsorbed on the surface of powder particles, and ensure that iron-based powder products are neither decarburized nor carburized.

39. What are the commonly used sintering protective atmospheres?

Sintering protective atmospheres are generally reducing or neutral gases, such as hydrogen, decomposed ammonia, carbon monoxide, nitrogen and vacuum.

40. What are the uses of the porous characteristics of powder metallurgy materials?

Porosity is one of the important characteristics of powder metallurgy materials. Using this characteristic, we can:

(1) Make sweating materials. That is, impregnate low-melting-point substances in the pores of ordinary powder metallurgy materials. When working at high temperature, the impregnated substances melt and seep out, causing the material to "sweat" to dissipate heat. In this way, ordinary materials can replace expensive heat-resistant alloys, and further increase the use temperature of heat-resistant parts.

(2) Make filter materials. Used to filter gas, filtrate and filter poison, etc.

(3) Impregnation of antifriction agents to manufacture oil-containing and oil-free lubricated bearings; impregnation of spices to manufacture scented handicrafts, etc.

(4) In some cases, iron is used to replace non-ferrous metals such as copper and lead.

(5) Manufacture of vibration reduction, noise reduction, insulation and other materials.

(6) Increase the specific surface area of ​​the material and use it as a carrier of the substance (such as carrying catalysts, etc.).