The green strength of powder metal part is only as strong as chalk.
So, how do you improve the strength?
The sintering process in powder metallurgy (PM) significantly boosts the strength, density, and structural integrity of the parts. Sintering heats the metal powder below its melting point, rather than melting it.
Let’s go into depth:
Did you know this manufacturing method dates back to ancient Egypt? People used metal powder to make tools and even precious jewelry! Since the 19th century, with the demand of complex parts increasing, powder metallurgy has developed rapidly. Today, powder metallurgy has become an advanced production process for making high-precision parts.
This manufacturing process begins by mixing metal powders into a uniform powder mixture, then pressing powdered metal under intense pressure into molds. Finally, sintering these shaped parts in a controlled atmosphere to effectively fuse the particles together.
Sintering process unfolds in three key stages: preheating, main sintering, and cooling. Each stage is crucial for achieving the beneficial properties such as tensile strength, mechanical strength.
Pre-sintering is the initial stage where the powdered metal component is gradually heated to 500-900°C.
Why is this necessary?
Removing lubricants and binders from the compacted powder particles is a primary goal. These lubricants and binders, while aiding in compaction, must be removed to prevent bonding and densification between powders
Furthermore, pre-sintering reduces oxides on the surface of the metal powders. These oxides must be reduced before the powder particles can be sintered and densified to produce high-performance workpieces.
Lubricants, such as zinc stearate, produce carbon monoxide when heated and decomposed. Carbon monoxide exceeding 18 vol% is easy to react to produce carbon or carbon dioxide, which will form carbon deposits on the surface of the workpiece. This will make the surface of the workpiece look like bubbles or bulges. You can control the sintering atmosphere flow rate to be faster to solve this problem.
This is where the magic happens.
In this stage, temperatures escalate to about 70-90% of the metal’s melting point. Take, for example copper-based parts, which are sintered at roughly 820°C, iron-based parts at around 1120°C, and stainless steel parts nearing 1250°C.
There are three stages of high-temperature sintering.
Initial Stage: The compacted powder particles start to form face contacts (necks), and the material begins to densify slightly. During this stage, most of the pores in the parts are interconnected.
Intermediate Stage: As time passes, the necks gradually become larger, and the grains begin to grow. This leads to a decrease in porosity and a significant increase in density.
Final Stage: At this point, most of the pores are isolated. The gas in the pores slows down further densification. Additionally, during the final stage, the grain growth rate increases significantly.
The figure below shows the relative density change of the workpiece during high temperature sintering process.
Therefore, after high-temperature sintering, the mechanical properties of green compact are significantly improved, such as hardness, strength and wear resistance.
The cooling stage follows the main sintering process. It solidifies and stabilizes the parts. Gradually cooling from high to room temperature preserves the part’s structural integrity established during sintering.
Cooling rates are critical in shaping the material’s microstructure. Slower rates are recommended because they refine the microstructure and enhance mechanical properties like toughness and strength.
Effective cooling helps relieve stresses that develop during the sintering process. If not properly managed, these stresses could lead to warping or cracking as the parts cool.
Moreover, adjusting the furnace atmosphere can improve surface properties or control oxidation levels.
For instance, creating a protective oxide layer during cooling significantly enhances the corrosion resistance of certain metals. For instance, creating a protective oxide layer during cooling boosts the metal’s resistance to corrosion.
Sintering process specifically enhances the following mechanical properties of parts:
Sintering significantly enhances both the hardness and strength of materials, making them capable of handling substantial loads and stresses.
The process increases density and strengthens particle bonds, boosting wear resistance—vital for parts that endure constant friction.
Enhanced properties from sintering enable materials to resist repeated stress, promoting long-term durability and reliability.
Through controlled sintering, materials become more flexible and less likely to crack under stress, which is crucial for sustaining structural integrity.
Since sintering occurs below the melting point of the material used, it conserves more energy than traditional melting processes.
Sintering also betters the magnetic properties of materials compared to casting or machining, making it ideal for applications needing efficient magnetic responses.
Different advanced sintering furnaces serve various needs within the advanced powder metallurgy process, such as accurate sintering control for magnetic applications. Below we detail the primary types of furnaces used:
Pusher Furnaces:
These furnaces are specifically designed to handle high-volume, consistent production of powder metallurgy parts. Pusher furnaces transport parts through multiple temperature zones to optimize the sintering process.
These furnaces are used for sintering a wide range of metal parts, particularly when uniform heating and controlled atmosphere are critical.
Compared to pusher furnaces, belt furnaces offer a continuous, smooth transport mechanism. This feature reduces handling and minimizes potential damage to parts.
Vacuum furnaces perform sintering processes under a high vacuum to prevent oxidation and contamination from atmospheric gases.
These furnaces are ideal for sintering advanced materials that require ultra-clean environments to maintain material integrity and properties.
The following is a comparison of the main sintering furnaces.
Sintering Furnaces types | Capacity | Cost Implication | Sintering Time |
Pusher Furnaces | About 3 tons per cycle | Initial investment is high and subsequent production costs are low. The cost is approximately US$0.41 to 0.55/KG. | Fast(about 3.5h per cycle) |
Conveyor belt furnace | About 2 tons per cycle | Initial investment is high and subsequent production costs are low. The cost is approximately US$0.41 to 0.55/KG. | Fast(about 4.5h per cycle) |
Vacuum Furnaces | About 0.2 tons per cycle | The initial investment and subsequent production costs are high. The cost of is approximately US$2.07 to 4.14/KG. | Low(about 20h per cycle) |
The sintering atmosphere affects the mechanical properties, appearance, and production cost of the workpiece. Its main functions are as follows:
Often used for its inert properties, nitrogen prevents oxidation during the sintering process, maintaining the purity of powdered metal. It’s particularly common in sintering stainless steel and high-speed steels to avoid unwanted chemical reactions.
Hydrogen has the characteristics of fast diffusion speed in metals and strong ability to reduce oxides. It is ideal for stainless steel, tungsten and molybdenum.
Argon is relatively inert and will not react with highly active elements such as titanium and chromium.
A vacuum atmosphere is used to remove air from the sintering process environment, minimizing oxidation and contamination. It’s ideal for sintering advanced materials and components requiring high precision and purity.
Gasbarre, headquartered in PA, United States, was founded in 1973, to design and manufacture equipment for the powder metallurgy industry.
Abbott Furnace Company, based in PA, United States, manufactures industrial furnaces. They mainly produce sintering furnaces, annealing furnaces, tempering furnaces, and special furnace products.
Centorr Vacuum Industries, based in Nashua, New Hampshire, USA, manufactures high-temperature vacuum and controlled atmosphere furnaces.
Challenge in sintering process
From our extensive experience in the powder metallurgy industry, controlling the product’s shrinkage ratio presents a significant challenge in the sintering process. So we typically create basic molds to test the expansion and shrinkage ratios in new sintering furnaces, significantly reducing the risk of dimensional deviations. However, controlling dimensional changes in every new product type remains a complex task.
Do you have any effective solutions to this challenge, or are there aspects of powder metallurgy sintering that still confuse you?
Leave a comment below and share your thoughts.
I’m here to hear from you!
Smaller batch furnaces can manage anything from a few kilograms to several hundred kilograms of material each cycle, making them perfect for specialized or customized production.
In contrast, larger industrial-scale continuous furnaces, like conveyor belt furnaces, are capable of processing several tons of material in a single cycle, making them well-suited for mass production.
Sintering furnaces differ greatly in length depending on their design and intended use.
Batch furnaces, which are generally utilized for small-scale production, usually measure between 10 and 30 meters.
In contrast, continuous furnaces, aimed at high-volume production, often extend beyond 30 meters to accommodate a steady flow of parts through multiple heating zones.
Belt speeds in sintering furnaces usually vary from 100 to 130 mm/min.
Adjusting this speed is critical for controlling sintering times accurately, ensuring that parts are exposed to the necessary temperatures for the proper duration.
This precision is essential for achieving optimal densification and the desired mechanical properties of the parts.
Usually, sintering leads to a rougher surface on the finished parts.
To get a smoother surface, you often need to do additional processes after sintering, like machining or grinding.
These steps help make the parts look better and function more effectively.
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