Whether a component is produced through binder-jet additive manufacturing, metal injection molding or conventional press and sintering, lubricant removal continues to be one of the most common issues in sintering.
As with all technologies, new forming techniques have resulted in the development of new lubricants. The result is the need for even more understanding and process development in lubricant removal. A review of the “old rules of thumb,” current solutions and where lubricant removal technology is headed will help to lay out a roadmap for dealing with this issue today and into the future.
Background
There are two common steps to the manufacturing process of powder-metal (PM) components: shaping the powder into the desired component geometry and sintering the component to develop the desired material properties. In each step, lubricant added to the powder is a key consideration.
During the forming step (e.g., compaction), the lubrication is provided when it melts and moves to the surfaces of the die. Lubricant is necessary for the ejection of the compact, preventing cracking of the compact and improving tool life. Unfortunately, the lubricant that was so necessary in the shaping step must be removed from the PM compact before the particles may sinter together.
The lubricant coating each particle now acts as a barrier between the particles and may hinder the sintering process. Hence, the lubricant must be removed from the compact in the initial stages of the sintering process. Failure to adequately remove the lubricant may result in a number of problems within the part and the sintering furnace.
The most common type of lubricant used in the conventional PM process is ethylene bis-stearamide (EBS), also known as Acrawax-C from Lonza Group. This material is reported to have a melting point of 140°C (284°F) and a boiling point of 260°C (500°F). One may conclude that the lubricant should melt and boil out of the compact early in the sintering process, but this is not the case.
EBS was observed[2,3] as a function of temperature. EBS does not boil at 260°C (500°F). In fact, it remains as a liquid to approximately 540°C (1000°F) and then totally dissociates to carbon and hydrogen. The solid carbon, or soot, formed is the root cause of lubricant removal issues in the PM process (Fig. 2, online). Varying in degrees of severity, soot can be found on the parts, in the parts and in the furnace (Fig. 3).
Another important observation noted that once EBS melts, carbon begins to be present in the liquid as a solid until the entire solution becomes solid carbon. This was explained in a mechanism first proposed in lectures by Levanduski, et.al.[1] of Abbott Furnace Company.
Like most hydrocarbon chains, as EBS is heated, it begins to break down into smaller hydrocarbon chains. It will eventually break down to become the smallest hydrocarbon, methane. Thermodynamically, methane is no longer stable once it reaches approximately 550°C (1020°F), and it will dissociate into solid carbon and hydrogen.[2,3]
Powell, et.al.[2,3] also note that the density of the PM compact plays a major role in the lubricant removal process. As compaction technology improves to allow the production of compacts with a greater and greater as-compacted density, the time needed for EBS to come out of the compact becomes longer (Fig. 4). This forces us to reconsider the long-standing rule of thumb indicating 20 minutes was adequate for the removal of EBS.
Here we see that the rule of thumb would work well for lower-density compacts that were common in the past. However, this is no longer a valid rule. The time for the lubricant to come out of the compact, hence the time for the compact to be 140-540°C (284-1000°F), must increase with the desire to press to higher green densities.
There is another important step in this mechanism.[1] As EBS breaks down, it cannot be divided into an even number of methane molecules. The result is carbon being left behind. This explains the presence of the carbon in the liquid of the melting study.[2,3] This also points to another key aspect of the lubricant removal process. The lubricant comes out of the compact as a liquid and then boils to form a vapor. However, even though all of the liquid lubricant may be removed, there is still solid carbon that will be left behind as EBS dissociates.
Lubricant Removal Aids and Equipment
Over the years, a number of devices and processing tools have been developed to aid in the removal of lubricant and address the presence of carbon. The first sintering furnaces were single-box designs where the compact was preheated, and the lubricant removal took place in the front of the heated box. It was then determined that two heated boxes on the furnace helped with the lubricant removal and allowed for the 20-minute rule of thumb in the first heated box to be maintained. As the density of the compacts increased and the sintering loads increased, however, this was not enough to address all of the lubricant removal issues.
The bubbler was the first device that was added to furnaces to help in the removal of lubricants and address sooting (Fig. 5). The concept was to bubble nitrogen gas through water. The nitrogen would pick up moisture and carry it into the preheat section of the furnace. The injection location was typically placed two-thirds of the way into the preheat section (Fig. 6, online). This coincided with the location where most products would reach approximately 650°C (1200°F) because it could be seen in the furnace that this was the location where the soot began to form and collect.
To further the capability of the bubbler, the water was then heated to accelerate the formation of water vapor and was carried into the furnace. The degree of control of the bubble is limited because one only has the water temperature and nitrogen flow rate as adjustment variables. However, bubblers perform well for small to medium parts and production rates. They are capable of producing up to approximately 5 pounds/hour of water vapor.
Unfortunately, the variability in the performance of the bubbler increases at the higher flow rates because the large amount of nitrogen can often cause the water bath to become unstable and liquid water to be picked up by the gas. This liquid form of the water then causes surges in the actual amount of moisture being introduced into the furnace.
To address the control issues of the bubbler, a device was developed where water is dropped into a heater to form steam. The steam is then carried by a flow of nitrogen into the same injection location of the furnace as the bubbler. These are called FAST (furnace and atmosphere service technology) systems (Fig. 8).
Because the control variable is now the amount of water introduced to the heater, the degree of control is much better. The limiting factor is the heater size, which affects the amount of water vapor that can be produced. FAST systems are typically limited to approximately 2.5 pounds/hour of water vapor. The systems work well for low production rates and small parts with less lubricant.
For production rates and large parts that require more water vapor to react with the lubricant, systems were developed that used a gas burner to produce the atmosphere. The air-to-fuel ratio of the burner is adjusted to produce the moisture-containing atmosphere used to help in the lubricant removal process. A common example of this technology is a quality delube process (QDP, Fig. 9).
The burner gas is produced in a chamber above the product as it enters. The gas is then introduced to the product at the same location of the preheating step of the process as the other methods, approximately 650°C (1200°F).
Limited only by the size of the selected burner, the amount of water vapor that this type of system can produce is substantially larger than other systems. Due to the very large amount of moisture produced even when the burner is turned down to low fire, this system does not work well for small parts and light production loads. These loads do not carry enough carbon-producing lubricant to react with the large amount of produced moisture. The result is an oxidation of the parts that is evident by frosting and decarburization.
The system is controlled by a thermocouple located in the upper combustion chamber. Since the burner produces heat while generating the atmosphere, the amount of moisture is directly influenced by the temperature of the chamber. This is a source of variability.
Another significant drawback to this type of system is its maintenance. Burner performance will change over time. This requires a person knowledgeable in burner adjustment to routinely maintain the system.
Recognizing that the time to remove the lubricant from the compact continues to increase, the preheat sections of the furnace have continued to become longer, and technology has been developed to increase the time that the compact is in the optimal lubricant removal temperature range. One of these, referred to as Zone 0, is a simple addition of an insulated neck to the preheat section of the furnace (Fig. 11, online). The warm gases exiting the furnace pass over the incoming components, preheat them and help to start lubricant melting sooner.
A further addition to this simple approach is to add the ability to inject heated air into Zone 0. The air reacts with the excess combustible of the furnace atmosphere to produce heat. This addition to the system is called lubricant burner technology (LBT).
Although Zone 0 and LBT both help to decrease the time for lubricant removal, neither technology provides a means to deal with the carbon produced during the hydrocarbon-chain breakdown.
The Vulcan
Lack of control is the underlying drawback to all of the technologies developed to date. The temperature, time, sintering atmosphere composition and moisture to aid in reacting carbon are often directly connected or limited in the degree of their control.
A recent development focuses on the need for better and independent process control. This system is called the Vulcan (Fig. 12). It is a direct replacement for existing preheat box technologies.
Because the optimal temperature range for lubricant removal is between 140-540°C (284-1000°F), radiation heating, as is used in conventional furnaces, is not effective. The Vulcan uses convective heating with variable-speed fans to provide independent control of the heating rate and temperature profile of the compact.
The time in the optimal temperature range is controlled to 540°C (1000°F) or less by the length of the system and the heating in each zone. This time is adjustable to match the incoming density of the compact, leaving the rule of thumb behind.
The moisture is also independent of all the other variables. With the ability to produce from 0-12 pounds/hour of steam, the flow can be adjusted to provide as much or as little moisture needed to react with the carbon during the breakdown of EBS. This gives the same system the ability to process small parts and loads as well as large parts and heavy loads.
Combining the understanding of lubricant removal with the knowledge of atmosphere control and heat has produced a system that has been shown to function at an optimal level for a wide range of production levels and part sizes. Weight-loss studies have shown 100% lubricant removal for Interlube E and EBS without oxidizing or decarburizing the compact. 
For more information: Contact Stephen L. Feldbauer, Ph.D., director, Research and Development, Abbott Furnace Company, 1068 Trout Run Road, PO Box 967, St. Marys, PA 15857; tel: 814-781-6355; e-mail: sfeldbauer@abbottfurnace.com; web: http://www.abbottfurnace.com.
References:
- Edward Levanduski and Stephen L. Feldbauer, Ph.D., “Observations in Lubricant Removal,” Presentation, Special Interest Session, PowderMet 2010, Fort Lauderdale, FL., June 2010
- Robert Powell, Craig Stringer, Ph.D. and Stephen L. Feldbauer, Ph.D., “Lubricant Transport within a Powder Metal Compact during Pre-Sinter,” PowderMet 2013, Chicago, IL, June 2013
- Robert Powell, Craig Stringer, Ph.D. and Stephen L. Feldbauer, Ph.D., “Ongoing Investigation of Lubricant Transport within a Powder Metal Compact during Pre-Sinter,” PowderMet 2014, San Diego, CA, May 2014
