Research Status of Powder Metallurgy Densification Technology

Powder metallurgy is a technology that uses metal powder or a mixture of metal powder and non-metal powder as raw materials, and consolidates the powder through various processes to obtain metal materials, tool steels, composite materials and various products. Powder metallurgy products have been widely used in aerospace, mold manufacturing, biomedicine, national defense and military industries.

Powder metallurgy consists of important processes such as powder preparation, pressing, high-temperature sintering and post-processing. Among them, pressing and high-temperature sintering are the densification processes of powder metallurgy.

1.1 Pressing
Powder pressing is the process of realizing the change from loose filling to tight filling of particles. The transmission and distribution of force change, resulting in uneven density and strength distribution of each part of the green sheet, thus generating a series of complex phenomena during the pressing process.
In actual production, the appropriate pressing process is usually selected according to the shape characteristics of the green sheet to improve the density and uniformity of the green sheet. In recent years, scholars from various countries have studied a variety of new pressing technologies, such as rapid omnidirectional pressing, warm pressing technology, high-speed pressing, etc.

(1) Warm pressing technology
Warm pressing technology uses a special warm pressing mixed powder. Under a pressing pressure not exceeding 690MPa and a sintering temperature of 1393K, the density can reach 7.2~7.5g/cm3 through one pressing and sintering.
Warm pressing is a simple process for preparing high-density, high-strength powder metallurgy parts, and the cost is relatively low. Only a small amount of adjustment to conventional equipment is required to obtain metallurgical parts with a density close to 7.5g/cm3. The warm pressing process is considered to be an important technological breakthrough in the preparation of powder metallurgy parts since the 1990s due to its high density, high performance and low cost.

(2) Powder high-speed pressing technology
Powder high-speed pressing technology is a powder pressing technology that drives a heavy hammer by hydraulic or gravity to impact the powder at a speed of 2~30m/s, thereby prompting the powder to complete the densification process within 20ms.

Powder high-speed pressing technology has the characteristics of high green compact density, relatively uniform green compact density distribution, high forming efficiency, low production cost, high green compact geometric accuracy, long mold service life, and excellent green compact comprehensive performance.

1.2 High-temperature sintering
Sintering is the most important process in the powder metallurgy production process and has always been the focus of research. High-temperature sintering is a heat treatment process that heats the powder block to a temperature lower than the melting point of the main component to make it bond into a material with higher strength. It is an effective means to increase the density of the powder green compact. The following introduces two commonly used metal powder sintering processes.
(1) Hot isostatic pressing process
Hot isostatic pressing (HIP) process is an important means of densification of high-performance powder metallurgy products. The general process flow of HIP is: fill the package with pressed powder, place it in the furnace of the hot isostatic press, and apply high temperature and high pressure at the same time. After the multiple coupling densification of the powder particles, a fully dense component is obtained.

When HIP is used for the molding or post-processing of metals and metal-based composite materials, it can effectively remove defects inside castings, reduce cracks inside parts, and improve product performance. The main metal materials involved are nickel-based high-temperature alloys, cobalt-based high-temperature alloys, titanium alloys, aluminum alloys, steel, copper alloys, etc.

(2) Spark plasma sintering
Spark plasma sintering technology (SPS) is a sintering technology that uses the upper and lower punches of a graphite mold and the energized electrodes to simultaneously apply a DC pulse current and a pressing pressure to the powder body, and achieves rapid consolidation and forming of the powder body through plasma activation and thermoplastic deformation. It is also called plasma activated sintering and plasma assisted sintering.

Spark plasma sintering working principle diagram

As shown in the figure above, SPS densification can be regarded as the superposition of discharge between powder particles, Joule heat generated by energization, and pressure under the action of pulse current. In addition to the two sintering factors of heating and pressurizing, surface discharge between powder particles can promote local high temperature and local melting of the surface; high-temperature plasma sputtering and discharge shock can effectively remove impurities (such as surface oxides, etc.) and gas adsorption on the surface of powder particles. These factors can promote the sintering process.

Spark plasma sintering has high heating efficiency and fast densification rate, which can greatly improve the sintering efficiency of powder.

Factors affecting the densification of metal powders
The densification process of powders under high temperature and high pressure is very complicated. According to finite element simulation research experiments, the factors affecting the densification of metal powders are mainly pressing method, friction resistance, and aspect ratio of the pressed green sheet.

(1) Pressing method: The pressing methods are mainly unidirectional pressing and bidirectional pressing. In order to reduce the pressure drop during the pressing process of metal powders and obtain a green sheet with more uniform density, the powder can be pressed by bidirectional pressing.

(2) Friction resistance: Friction resistance is a key factor affecting the density of the green sheet and its distribution uniformity. Studying the influence of different friction conditions on the powder compression densification process is conducive to improving the pressing process of powder metallurgy products. When the friction conditions are improved, the stress gradient of the green compact decreases, the density is more uniform, and the pressing force is also reduced.

(3) Green compact height-to-diameter ratio: The green compact height-to-diameter ratio is a shape factor that must be referred to in the powder metallurgy product compression molding process. Under the same compression process and friction conditions, green compacts with a small height-to-diameter ratio can obtain a more uniform density distribution.
Therefore, in the actual production of powder metallurgy products, overly slender parts should be avoided. A bidirectional pressing process can be adopted, and lubrication conditions can be improved to improve the density uniformity of the product.

Powder metallurgy technology has been widely used in the preparation of high-performance metals and alloys of various compositions. The density and distribution of powder metallurgy materials are key factors affecting the mechanical properties and dimensional accuracy of powder metallurgy products. Uneven densification will lead to uneven densification and shrinkage deformation of various parts of the powder body, resulting in scattered mechanical properties of various parts of the powder component. Therefore, the development of high-density and high-performance powder metallurgy products has become the development direction and research focus of powder metallurgy technology.