Sintering in powder metallurgy

That was the question asked in a recent survey run among a sample of design engineers working at diverse firms in the manufacturing industry.

The result: no one was able to give a 100% correct definition of sintering and how the sintering mechanism works. Moreover, we have noticed that 3D printers have been added to create further confusion on the subject in recent years.
 
Therefore, we have written this article to inform you, clearly and concisely, about what sintering is.


 

 



Definition

Let's start with a definition taken from a trustworthy Italian dictionary:
 
"Sintering is a process in which particles of a solid material in powder form, subjected to heating, approach and weld together, resulting in a compact piece of material. It is a widely used process in powder metallurgy [...] Typically, sintering is carried out at a temperature slightly below the melting temperature of the material." - Treccani
 
Sintering is employed to give strength and a solid structure to a given material, the goal is to shape the metal powders into a material for the final user. Sintering takes place in a unique, long hoven with diverse temperature zones.
 


Sintering process in powder metallurgy: 4 stages explained simply
 

We have provided a quick definition of what "sintering" is in metal powder metallurgy and the goals related to it. Now let’s dig deeper into the process in a simple and non-technical, yet thorough manner.

This process consists off the following four phases:

 
Let's analyze each phase.



The choice of alloy
 

The first stage of the sintering process depends on the alloy we have selected, and it consists of the mixing of these chosen components in powder form.  
This process depends on the characteristics we want to achieve for the product commissioned. 
 
The blends can be either standard or special:
 

 
But what is required to make these particular blends?
 

 
Even if a thorough understanding and planning of the product is required, most of the time empirical tests are necessary to obtain the right characteristics.
But this is not such a big deal.
 
Why?

Let’s go through the process together:
 

 
The second stage of the sintering process is called “the pressing phase”, and this is the moment in which the piece takes its shape.
 
The key elements of the pressing phase are:
 

 
Double-acting hydraulic presses act to deliver the “green” piece with the geometric characteristics required.

Pressing tonnages are gauged to fit the size of the parts, while the pressure is applied from both above and below.
But, first of all, a mold must be made. Mold employed in sintering processes are simpler and cheaper than the ones used in other manufacturing processes such as: M.I.M. (that we will define later on), plastic injection, molding, zamak or aluminium die casting and others. 
 
The part of the mold that shapes the outer of your part is called “the die”. As an example: if your part has a pentagon-shaped outer outline, the die will be a block of steel that is round on the outside and has a pentagon-shaped through cavity. An upper and lower punch connected to the upper and lower cylinders of the press complete the die. 
The result of this stage will be a block with a pentagonal shape. Even if it happens rarely, the height of the piece can be varied by working on the strokes of the punches.
 
A key advantage of sintering in powder metallurgy is the possibility to create very complex external shapes, and possibly internal ones as well. This is due to the fact that both the matrix that reflects the external shape, and the possible mobile punch placed inside to make a shaped hole, are made by wire EDM with centesimal tolerances.



 

 

To learn more, download the e-book, "How Metrology Can Help the Sintering Process."

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The sintering phase

What happens after the pressing phase?
 
After the pressing phase we can have three different situations depending on the size of the batches:

 
However,  we are still talking about compressed powder (fragile and not yet sintered), this means that a physical operator is needed even in medium sized batches. In this case there is an increase in costs.
 
Returning to the production stage, the block has reached the box. Here the positioning depends on the shape, weight and other factors.

The standard options are three:

 
In the case of our pentagonal block, given its imprecise shape the first option is the conceivable one.
 
Once done the real sintering phase takes place with the moving of all the cassettes containing the parts through the oven. The pieces pass inside the furnace with a controlled atmosphere and go through different temperatures. The time also differs depending on the characteristics of the part.
 
Even if there is a cooling phase, the parts are still very hot when exiting the hoven. Therefore, remaining for a few hours before the next phase. 
 
If tolerances are very tight a calibration step is required:
 

 
Calibration can also be carried out on manual presses, as a result this step can become more expensive than the part itself.
Now the production process is over.


 

 

 

After the 3 steps described in the sections above the sintered pieces lend themselves to various finishing processes:
 

 
In a nutshell, sintering in powder metallurgy is a heat treatment that, through 4 main steps, shapes metal powders with the aim of increasing its intrinsic strength and structural integrity, creating and providing the end-use parts.
 



Powder Metallurgy vs Metal Injection Molding vs Selective laser sintering 

 
Sintering in powder metallurgy we have described above, is an old and very different process compared to the selective laser sintering.



Selective laser sintering advantages and disadvantages

 
The main differences are these two:
 

Sintering in powder metallurgy is a process that is more accurate and cheap than the 3D printer, yet we will all need one in the future.
 

Let’s now summarize in a table the advantages and disadvantages of Selective Laser Sintering compared to Powder Metallurgy.


 

 


 

Metal Injection Molding vs Powder Metallurgy


Apart from the ambiguity defined in the above section, we will now define the main differences between sintering in powder metallurgy and metal injection molding (also known as M.I.M.).
 

The process is divided into 3 steps:

 
Let’s now summarize in a table the advantages and disadvantages of Metal Injection Molding compared to Powder Metallurgy.

 

 



Sintering advantages and disadvantages
 

We have gone through a comparison with the 2 main alternatives to sintering in powder metallurgy as a manufacturing process.
Now, we will highlight the advantages and limitations of this process.


PROS

 

CONS

 

We hope this article has been useful for you, we follow our customers in a step-by-step consulting approach and that’s the reason why we think that knowing their everyday work, together with the advantages and issues they face it’s the element that makes the difference.


To lear more about this topic, download our e-book on sintering.