The production of powder metal components can be summarized in three steps: powder preparation, compaction and sintering.
Metal powder is first formed and blended to a desired chemical composition. During this step, a lubricant, such as wax, is added to help in the ejection of compacts from the molding press. The metal powder mixture is then put into a punch and die tool set of a molding press, and compacted to form a “green” compact.
Although each step is important to the overall process, the sintering of the powder metal compact is one of the most important. It is during this step that the lubricant is removed from the green compact and the metal powder particles are bonded together to form the metallurgical properties of the final powder metal part.
Powder Metal Sintering
“Sintering” is a thermal process by which adjacent metal particles are chemically bonded to enhance the final properties of the powder metal compact.
Figure 1. Powder Metal Compact Before and After Sintering
Prior to the metal particle bonding, the metal particles must first come into contact with each other. The lubricant that was added to aid in compaction, now acts as a barrier preventing the particles from touching. The first step in sintering is to begin heating the compact and remove the lubricant. While the compact is heating, the lubricant begins to melt and eventually vaporize as it travels from the inside of the compact to the surface. Since most lubricants are hydrocarbons, they can be reacted by elevating the amount of moisture in the furnace atmosphere.
CxHy + (x)H2O → (x)CO + (1/2y + x )H2
The water reacts with the carbon of the lubricant to form Carbon Monoxide and Hydrogen that can be flushed out of the front of the furnace by the furnace atmosphere.
Oxide is present on the surfaces of all metal surfaces. This oxide layer is another barrier to the contact between the metal particles and their subsequent bonding. The oxide layer must be reduced to allow the particles to come in contact. The reduction is the result of the reaction between the furnace atmosphere and the oxygen in the oxide layer. A typical sintering furnace atmosphere will contain Hydrogen and/or Carbon Monoxide to react with the Oxygen on the surface of the metal particles.
Figure 2. Oxide Reduction Schematic
The reducing potential of the furnace atmosphere is typically measured using the dew point. The dew point is the temperature of the atmosphere at which the current amount of moisture would saturate the atmosphere. An atmosphere temperature that is less than the dew point temperature will result in water vapor being pushed out of the atmosphere. As the dew point temperature increases, the reducing potential of the atmosphere decreases.
The equilibrium dew point is a function of the chemistry of the powder. Elements that form a stronger bond with Oxygen, such as Chromium, require a larger driving force for the reduction of the oxide. At a given temperature, the equilibrium dew point will be lower for those oxide formations.
Figure 3. Equilibrium Dew Point
Once the particles come in contact, bonding will begin. The initial stage of the bonding occurs as small “necks” form between the metal particles.
Figure 4. Initial Stage of Bonding
Photograph provided by T. Murphy, Hoeganaes Corp.
Through diffusion and other mass transport mechanisms, material from the particles is carried to the necks, allowing them to grow as the particle bonding enters the intermediate stage. The intermediate stage of bonding is characterized by the pores beginning to round.
As the mass transport continues, the pores will become even more rounded and some will appear to be isolated away from the grain boundaries of the particles. This is referred to as the final stage of bonding.
The final step of the metal sintering process is to cool the bonded compact to a temperature at which it can be handled. This cooling is performed in an atmosphere that is no longer required to chemically react with the compact. The atmosphere in this stage of the process aids in the transport of the heat away from the compact and minimizes the re-oxidation of the compact during cooling.