Consolidation of Compacts

Consolidation of metal powders generally entails compacting, followed by sintering. However, metal powders sometimes are sintered without compacting, which is called "loose powder sintering." This process is suitable for certain specialized P/M applications such as bronze filters and porous electrodes, as well as for model experiments to study the mechanism of sintering. Additionally, all powder injection molded structures begin sintering from this same condition once the binder is removed.

The simplest type of sintering is for compacts of a single metal powder, or compacts of homogeneous solid-solution alloy powders (alpha brass powder or an austenitic stainless steel powder). However, powder metallurgy alloys also can be produced by blending two or more constituent powders in proportions required to achieve the desired overall alloy composition. Constituent powders of the mixture may be composed of pure elements (elemental powders) or alloys containing two or more elements (master alloy powders). If a homogeneous alloy is to be produced from a mixture of powders, sintering must be controlled to achieve the required degree of homogenization.

Another alloying method is the use of prealloyed powders in which each powder particle has the same composition as the final part produced by compacting and sintering. Such prealloyed powders typically are produced by atomization of a molten alloy by impingement of a high-pressure gas or liquid stream. The fluid disperses the liquid alloy into small droplets to solidify into powder particles.

Homogenization of elemental powders occurs through interdiffusion of the chemical elements among the constituent powders of different composition. Often, it is a solid-state diffusion process. Complete homogenization during sintering is achieved during the sintering of mixtures of iron and graphite powder, where graphitic carbon goes into solid solution in gamma iron (austenite) at the sintering temperature.

Homogenization circumvents some of the disadvantages of prealloyed powder processing. Hardness and strength of prealloyed powder particles result in powder compacts with low densities and low green strengths when pressed at room temperature. Mixture of powder with the major component of the alloy present as an elemental powder (usually soft and ductile) typically exhibit high densities and high green strengths in the as-compacted condition. Furthermore, adjustments to alloy composition can be made readily by varying the proportions of constituent powders in the mixture, thus eliminating the need for additional powders of different composition. Mixtures composed of elemental powders normally can be obtained with high purity. Prealloyed powders, conversely, may contain elements other than the main components of the alloy because of pickup of impurities during melting prior to atomization.

The disadvantages of homogenization processing center around the need to remove compositional heterogeneities by thermal processing (often in conjunction with mechanical working) subsequent to compaction. Homogenization often involves more extensive elevated-temperature exposure than normal sintering; this added processing expense must be minimized by limiting processing conditions to levels that provide the required homogeneity. Also, in many cases, one element diffuses much faster as compared with the other, resulting in swelling and dimensional growth during sintering.

Activated Sintering. Sometimes, compacts pressed from a mixture of a high-melting-point metal with a small addition of a lower melting point metal are sintered, such as compacts of tungsten powder to which small amounts of nickel powder have been added. During sintering at temperatures below the melting points of both metals, the compacts densify at much lower temperatures than those of the high-melting-point metal. This is one type of activated sintering (Ref 2), which is discussed later in this article and elsewhere in this Volume.

Liquid-Phase Sintering. Compacts of mixtures of metal powders also may be sintered at temperatures that form a certain amount of liquid phase. This results in a much faster process, which is very popular in industry, because of the decreased sintering times. The process is widely used in sintering iron, copper, tungsten, cobalt, and cemented carbide compositions. The quantity of the liquid phase must be small enough to be held by capillary force within the skeleton of the remaining solid phase so that the compacts retain their shape. In this process, the liquid phase may be present through the duration of the sintering cycle. This is the case in the sintering of compacts from a mixture of tungsten, nickel, and iron powders (the so-called "heavy alloys"), and in the sintering of cemented carbides (combinations of tungsten and other refractory metal carbides with cobalt). Because the solution of small particles of the solid phase in the liquid and its reprecipitation on the large particles during sintering plays an important role, this method often is referred to as the solution and reprecipitation process of liquid-phase sintering.

In transient liquid-phase sintering, however, a liquid is formed when the compact is heated to the sintering temperature. The liquid is transformed into a solid by interdiffusion while the compact is at the sintering temperature, such as in the sintering of compacts from mixtures of copper and tin powders, in which an alpha bronze solid solution is formed.

Pressure-Assisted Sintering. The simplest form of pressure-assisted sintering is that which occurs by uniaxial hot pressing. A refractory mold (die) is inserted into a furnace, and both the powder and the die are heated while the punches are driven by an external pressure source. Hot isostatic pressing is another widely used process. This process is also called gas pressure bonding, because a high-pressure gas assists densification. In hot isostatic pressing, the powder is enclosed in a thin-walled container that is flexible but pressure-tight at sintering temperatures. A pressurized gas is uniformly applied to the container in a high-pressure chamber. The chamber contains a furnace to heat both the material and the gas. The hot consolidation of metal powders, where sintering and densification are combined by applying pressure to the powder at elevated temperature, is not discussed in detail in this article. Hot consolidation processes are discussed elsewhere in this Volume.

0 0

Post a comment