Powder metallurgy is a metal fabricating process in which metal powder is compacted into some desired shape and sintered together. Sintering (in powder metallurgy) refers to the metallurgical bonding that takes place between individual powder particles as a result of pressure and elevated temperature (the sintering temperature is well below the melting temperature of the alloy). Conventional powder metallurgy processes usually involve four steps: powder production, powder blending, compaction, and sintering.
1. Powder Production - Metal powders may be made by many different methods. Atomization of molten metal is probably the most common. Metal of the desired composition is melted in a crucible (often in a vacuum induction furnace) that sits on top of the cooling tower. The molten metal is fed into the cooling tower through a nozzle at the top. Molten metal is atomized into spherical globules as it passes through the nozzle. These globules will solidify as they fall to the bottom. Many alloys require that the melting and atomization be done in a vacuum or an inert gas atmosphere to prevent an oxide layer from forming on the powder particles. A counterflow of inert gas is sometimes employed within the cooling tower to help break up the stream of molten metal coming out of the nozzle and to help control the cooling rate. The solidified powder is collected on the bottom of the tower. Other ways of manufacturing powder include the mechanical processing (such as crushing, grinding, etc.), chemical precipitation or decomposition, and electrolytic deposition.
Depending on the material and the powder production technique, powder particles may be spherical, acicular (shaped like a needle), in the shape of flakes, angular, or irregular in shape. The morphology (size and shape) of the powder plays an important role in how well the particles compact and what the ultimate density of the part will be. After the powder has been produced, it will be screened into the desired size ranges.
2. Blending - The next step in the manufacture of a powder metallurgy (P/M) part is to blend the powder with any necessary die lubricants, alloying additions, etc. Heats of powder are generally rather small and, as a consequence, several heats may be necessary to fill a large order. To insure uniformity of the product, it is often desirable to manufacture all the product from the same powder material. This can be accomplished by taking as many different heats of powder as required by the size of the order and mixing them into a homogeneous blend. Each heat will be divided up and distributed equally into
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of powder that can be efficiently blended together at one time. Each lot, or sub-blend as it is often called, thus has the same proportion of each heat of powder. Together the sub-blends constitute the master blend. Traceability and qualification testing (mechanical properties) are on a master blend basis.
3. Compaction - The powder can be compacted in a die or by extrusion, rolling, or numerous specialized methods. Compaction serves three basic purposes:
A. It gets the powder into the desired shape and roughly the desired size.
B. It increases density.
C. It imparts enough strength to the work piece for subsequent handling.
As an example, we'll examine compaction in a closed die. There is a substantial amount of empty space in the volume occupied by the powder because the apparent density of the powder is only 20-40% of the theoretical density of the metal. This, of course, means that we will have to add significantly more powder to the die than what is required to fill the actual die cavity. A punch (either mechanically or
hydraulically driven) is used to compress the powder into the die. The end of the punch may be shaped. The powder particles are squashed together and become mechanically interlocked. The density of the powder increases as a result of the compaction to typically 75-90% of the theoretical density. The work piece, or green compact as it is now referred to, is ejected from the die. It will undergo a slight increase in volume because of the elastic recovery of the powder particles. It is held together strictly by the interlocking of the powder particles. This gives the green compact sufficient strength to be handled, but drop it and in most cases it will crack or shatter.
There are many variations in how powder is compacted. Vibratory compaction utilizes a compacting press that has a mechanism for vibrating the die while powder is being compacted. This jostles the particles around producing more efficient packing thus increasing the apparent density. Isostatic pressing uses a flexible rubber or sheet metal mold instead of a die. The mold is evacuated, filled with powder, and then sealed. It is placed in a chamber which is then pressurized with a fluid (such as water, oil, or gas). The pressurized fluid causes the mold to squeeze the powder particles together thus compacting them. Because the work piece is subject to the same pressure over its entire surface area, isostatically compacted parts are much more
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uniform than mechanically compacted parts. Complex shapes and parts with high length-to-diameter ratios are often more easily compacted by isostatic pressing than by other methods.
4. Sintering - This is the step where a true metallurgical bond is formed in between the powder particles. The green compact is heated up to an elevated temperature (but well below the melting point) where the combination of internal pressure and heat causes bonding to take place. Note that there is no melting or fusing of the powder particles. Sintering is a complex process. There are many processes besides bonding that occur during sintering. These included densification, shrinkage, changes in the size and shape of pores, and even alloying where different metal powders have been blended together.
The final density and strength of the sintered part are dependent on many factors including the density of the green compact, sintering time and temperature, and the composition of the powder. The density of the sintered part is typically 5-20% above that of the green compact. All conventional P/M parts will contain some pores. Although the volume of pores is reduced during sintering, it is impractical to completely eliminate them because of the excessive time and temperature it would take.