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Knowledge of powder metallurgy (PM) process and sintering(2)

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-08-29 08:45:00
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4.1.2 Powder mixing

Powder mixing is to fully mix various metal powders and additives (such as lubricants, etc.) based on calculation of product ingredients and specific particle size distribution. This process is completed by powder mixing machine.

The addition of additives is mainly to improve forming characteristics of mixed powder. Addition of lubricants (such as zinc stearate, 0.25-1% mass ratio) can improve fluidity and increase compressibility of powder mixture. Before pressing and sintering, lubricant is removed by means of heating (e.g., zinc stearate in hot air at 375 -- 425oC).

The characteristics of mixed powder are usually expressed by uniformity degree. The higher the mixing degree is, the more uniform the mixing is. More conducive to products performance. However, powder mixing process should be careful, too intense mixing will cause deformation hardening, mutual wear of particles, peeling etc., so it must be in accordance with forming technical requirements and specifications.

4.1.3 Press forming

Powder forming is main and basic process. Its process includes weighing powder, loading powder, pressing, pressure holding and de-molding, etc.

There are many pressing methods, such as steel die pressing, fluid static pressing, three-way pressing, powder forging, extrusion, vibration pressing, high energy rate forming, etc. Commonly used are:

 (1) steel die pressing

Refers to normal temperature, mechanical press or hydraulic press, with a certain specific pressure (pressure often in 150-160MPa) forming the loose powder inside steel mold into a green part method. This forming technique is most widely used.

(2) Isostatic pressure of fluid

It is a method of forming a powder material by applying pressure from all directions simultaneously with a high-pressure fluid (liquid or gas).

(3) three-way suppression

It integrates characteristics of unidirectional steel die pressing and isostatic pressing. Density and strength obtained by this method are higher than those obtained by other forming methods. But it is suitable for parts with regular shape, such as cylinder, square, rectangle, sleeve, etc.

In addition, extrusion and rolling can be used to produce extruded or rolled products directly from powder state, such as bars, bars, sheets, components, etc. Different heating and processing can be selected according to different material and performance requirements. At present, this field of production is developing rapidly.

4.1.4 Pressing and sintering

Sintering is one of key processes of powder pressing technology. Only by proper sintering can obtained required mechanical and physical properties. In sintering process, atomic diffusion between particles occurs through high temperature heating, so the contact surfaces of green part particles are combined and become solid blocks. Sintering process is carried out in a special sintering furnace. Main technical factors are sintering temperature, holding time and atmosphere in furnace.

Due to composition and formulation of powder metallurgy products, sintering process is solid phase sintering or liquid phase sintering (such as special products cemented carbide and cerment). So-called solid phase sintering refers to powder remains solid at high temperature.

Sintering temperature of liquid phase exceeds melting point of some component powders, and coexistence of solid and liquid phase occurs at high temperature. Sintered body will be more compact and solid, which further guarantees the quality of sintered body. In fact, liquid phase is not allowed to flow completely freely at sintering temperature. For example, sintering temperature of tungsten-cobalt carbide varies from 1, 380cc to 1490cC with content of cobalt. Proper control of sintering temperature has a very important influence on properties of products. Generally, higher sintering temperature can facilitate diffusion of atoms between particles, so as to increase hardness and strength of sintered body. Sintering insulation time also affects quality of products. As insulation time is related to equipment situation and furnace loading quantity, insulation time of small parts is short while that of large parts is long. When liquid phase sintering occurs, if relative amount of liquid phase is large, lower limit sintering temperature is often adopted to prolong insulation time to prevent liquid phase exude from surface during sintering.

In order to understand and control sintering quality, list some problems that may occur in sintering process for helping engineers identify causes:

1) Warping is a common problem, which can be detected before parts are used. Warping increases rejection rates. Warping is generally caused by failure to support green part during sintering, or uneven density distribution (fluctuation) in green part. Former can be adjusted in  green part’s direction in furnace or used supports(generally made of refractory material) to correct, so that sintering deformation does not occur. Latter can only be solved in structural design and in previous process which causes uneven density distribution.

2) Over-sintering is another common problem. It may cause warping, swell or excessive grain growth in green part. The first two terms are generally easy to be found directly, and growth of grains is too large and can only be observed under a microscope. These problems are mainly caused by too high sintering temperature or too long insulation time.

Keywords in the article: powder metallurgy (PM)
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