By: Angel Ravelo, Date: 02/12/2018
Summary
Powder metallurgy is a sustainable, modern manufacturing method for rapidly forming unique and complex parts. Atomized metals are pressed into a die and go through a sintering process to make a finished part. This method is popular for its ability to avoid the metal removal processes found as results of other manufacturing modes.
Background
Though an early form of PM was used by the ancient Incas to make jewelry, industrial manufacturing of parts using PM did not begin until the 19th century. These early operations extracted iron powder by hand. Recent technological advances have made this process much more popular in the last few decades.
Application
PM is frequently used to make unique parts that are essential to many industries for a variety of applications; some examples include tungsten carbide (WC), metal gears, bearings and diamond tools.
Powder manufacturing can be very highly automated and is able to avoid the hassle of metal removal found in other processes, leading to reduced yield losses and cutting costs for the manufacturer. Powder manufacturing also allows the manufacturer to control the density of the finished product; if a part can be allowed some porosity it can save on material drastically. Various additives can be mixed into the powder to alter some of the properties to suit many needs. PM is also able to make much more complex parts than would be possible with conventional forms of manufacturing, such as forging techniques.
PM is a fairly high resolution method that can be used to make high precision finished parts with typical tolerances in the range of ± 0.005".
Powder manufacturing is not very viable for low production volume as the tooling and equipment required to manufacture parts is very expensive. Running a PM operation could also be hazardous as workers are exposed to atomized metal in the air around them.
Process Details
As seen in the graphic above, there are basically 3 or 4 main steps in the process to get from solid metals to a finished product.
- The base metal(s) and any additives are atomized and mixed together to create a very fine powder consistency of micron-sized metal particulates. Iron is typically used as a base metal and varying amounts of carbon, nickel, copper and/or a lubricating wax are mixed in to tune the finished product's mechanical properties. The atomization process can be done in several different ways but the most common involves forcing molten metal through a small opening as it is blasted by water (or gas) jets at high pressures.
- Metal powder is then mechanically pressed into a die by a punch tool with large pressure (on the order of 150MPa to 700 MPa) and ejected from the die. This creates a part that matches the shape of the desired product, but is only loosely held together due to the compaction process. The part can usually be handled at this stage, but will break apart with very little force. The pressure applied by the press directly influences the density of the final product; higher pressures will decrease the porosity of the part. The manufacturer therefore has selective control of how much material they must use depending on the intended application of the part.
- The sintering process heats the metal powder to a temperature that approaches the melting temperature in order to densify the material without allowing it to liquefy. This process removes most of the pores within the material and makes a more solid part overall. Preferential heating of conductive metals is performed using a high electrical current to create a nonuniform cross section as desired, though a more conventional furnace is used for electrically insulating material. The heating process is carried out in a precisely controlled, endothermic gas environment at a standard pressure of 1 atm. The gases prevent oxidation of the metal as it is heated and protect material properties that would otherwise degrade in an uncontrolled environment.
- The last two (optional) steps before the completion of a final product include secondary machining and finishing. These processes basically remove excess material and provide the desired surface finish. This step mostly improves the aesthetics of the part but also serves to ensure the finished product is of suitable precision and quality for their intended application.
One advantage of the powder manufacturing process is that most steps are automated and thus does not require skilled workers to operate the various steps in the process.
Costs
Typically, equipment and tooling costs are the most expensive part of running a powder manufacturing line throughout a part's production lifetime. In order to be viable, powder manufacturing is generally reserved for very high production volume parts that can allow little to no variation in geometry, density, etc. While the atomization process to create the stock powder necessary is generally more expensive than ingots of metals used in other manufacturing methods, the total amount of material saved in long term PM processes offsets this initial cost and is more profitable in the end. Powder metallurgy is most viable when material costs are at most around 20% of the total cost to produce a part. Downtime between production jobs should be minimized in order to recuperate as much overhead and equipment costs as possible.