This is the first in a series of blog posts on sintering by Dr. Jerry LaSalle, an expert in XPS surface analysis and physical and powder metallurgy. Dr. LaSalle’s areas of concentration include stainless steel passivation qualification, metallurgical failure analysis, additive manufacturing, and powder metallurgy consultation.
Ensuring Uniformity of Properties in Sintered Powder Metal Components
Powder metallurgy (PM) covers a wide range of processes in which unique materials are made from metal powders. PM processes, such as Press and Sinter and Metal Injection Molding (MIM), are used to produce high volumes (>100,000) of net and near-net shape parts, such as watch bezels, cell phone parts, gears, air bag components, printer parts, and parts for arthroscopic surgery tools for the automotive, consumer, electronic and medical markets.
One of the key processing steps in the manufacture of PM parts is sintering. Sintering is a process in which high temperatures (>1,000 °C) and special atmospheres are used to compress or densify the part in a manner similar to firing ceramic ware. Recent techniques in the field of additive manufacturing can also require sintering for densification, as it creates new alloys and larger-sized parts with special sintering needs.
Because significant shrinkage can occur during sintering (in some materials as much as 17%) and chemical interaction with the sintering atmosphere at these high temperatures can cause metallurgical properties to gradually develop, precise control of sintering parameters is required. Using the correct atmosphere composition, flow rates and temperature distribution is extremely important to make components which meet tight dimensions and mechanical properties in six sigma production goals that minimize variability by reducing defects.
The key metrics for successful sintering are chemical, microstructural and dimensional repeatability. The goal is to have six sigma yields with properties that compare to cast- or wrought-processed materials, and to maintain both “within-lot” and “lot-to-lot” repeatability in production runs exceeding 10K.
Example: The Challenge in Sintering the Stainless Steel Alloy 17-4PH
The alloy 17-4PH is a versatile “workhorse” of precipitation-hardening martensitic stainless steel that combines high strength and hardness to give it an excellent balance of corrosion resistance and high strength. It achieves its martensitic atomic arrangement through a very fine balance of Cr, Ni, Cu, and C, with particular sensitivity to carbon. Proper control of the carbon chemistry results in an essentially pure martensitic alloy having sought-after commercial properties. If the carbon is in excess of its upper limit (only 0.07wt%), the result will contain zones of austenite instead of martensite. Austenite has completely different properties than martensite. It has approximately 20% of the strength of martensite and does not have the capability to be further precipitation-hardened. Given the relatively low limit of carbon required to “poison” the martensite formation, the importance of uniform chemistry control during sintering becomes clear.
Figure 1. Metallographic cross section of sintered 17-4PH with poor hydrogen control.
During the sintering process, hydrogen acts as a reducing agent and prevents the steel from oxidizing. It also removes any carbon that is present around the powder during powder molding. Figure 1 shows a metallographic cross section of sintered 17-4PH, which had poor hydrogen atmosphere control during sintering. The micrograph shows an outside ring which is martensitic and a core with larger austenitic grains, indicating how inhomogeneous carbon distribution reveals itself in two different phases. An interior zone of austenite, resulting from inadequate carbon removal by the hydrogen of the sintering atmosphere, will have approximately 1/5 the target strength of that of controlled hydrogen, as well as other compromised properties. These results emphasize that the right analytical techniques are key to confirming proper sintering. This part will have to be scrapped, reducing yield and increasing cost.
Uniformity of properties in sintered parts can be compromised by inadequate carbon removal from the center of the part during sintering. Inadequate carbon removal may be the result of:
- Inadequate time for hydrogen to react at the part interior
- Too rapid a rise in temperature
- Inadequate flow of clean hydrogen
- Uneven distribution of hydrogen throughout the sintering furnace
- Uneven temperature distribution throughout the sintering furnace
The Production Furnace
It is important to define a process capable of achieving ideal debinding, carbon control, and microstructure for a given part thickness. However, it is also critical to ensure that a production furnace is qualified to produce uniform parts. All furnaces have different run signatures. This can be compared to the different flight “feel” of a small airplane vs. a jumbo jet. Successful sintering, then, involves first defining the ideal sintering parameters for a given part or alloy on a laboratory scale, and then working to scale the process to the production sintering furnace.
Figure 2 shows the inside of a medium-sized production furnace. In order to ensure six sigma production goals, the furnace must be qualified to be repeatable and uniform throughout the entire sintering furnace work zone, and during every subsequent sintering cycle. The failure of a few parts in a “dead zone” of a furnace, for example, can cause an entire lot of 10,000 parts to fail quality acceptance requirements.
Figure 2. Inside of medium production furnace.
The example of 17-4PH stainless steel sintered in hydrogen is just one example of the complex relationship between sintering parameters and final properties. Other alloy systems and their required sintering atmospheres include carbon steels sintered in nitrogen, titanium sintered in argon, nickel-based superalloys sintered in hi-vacuum, and tungsten heavy alloys sintered in moist hydrogen. The feedback between sintering process parameters and microstructure provides a loop, which defines a production cycle that can then be monitored to meet six sigma production yields. This is vitally important in the mature powder metal production processes such as MIM or press and sinter, and is most important in new additive manufacturing processes where sintering profiles need to be extended to new alloys and larger component sizes.