World trends in metal 3D printers

World trends in metal 3D printers

Third column: OPM is working on the front line of research to mitigate “tensile residual stress, the enemy of fatigue strength!”

Tensile residual strength! This is the biggest enemy and difficulty for shaped objects fabricated with metal 3D printers.

Different companies are trying to improve this issue using various approaches.
Looking up “residual strength” on Wikipedia, we find the following.

The following is a quotation from the Japanese Wikipedia article on “Residual Stress”:

“Residual stress is the stress present in an object even after an external force has been removed. Due to Hooke's law, the strain corresponding to residual stress is called residual strain. Residual stress can be distributed in various ways, but because it satisfies the equilibrium conditions of the object, the positive and negative residual stresses are balanced throughout the object. There are times when the occurrence of residual stress is desirable, and times when it is not. Generally speaking, compressive residual stress improves strength, while tensile residual stress degrades strength. For example, laser peening adds beneficial compressive residual stress to metal parts such as turbine engine fan blades. It is also applied to the tempered glass used in smartphone displays, where it realizes glass that is large, thin, and resistant to cracking and scratches. However, the occurrence of unintended residual stress may cause premature failure of structures.” (End of quote)

The point is: When stress is used in mechanical parts, compressive stress is good, while tensile stress is the “root of many evils.”

Figure 1 shows what the compressive stress distribution of a shaped part looks like.

Fig. 1: Diagram of residual stress distribution in a shaped object (*conceptual illustration)

Fig. 1: Diagram of residual stress distribution in a shaped object (*conceptual illustration)

The diagram at left in Fig. 1 shows the situation with a shaped object on top of a plate. Here, the positive and negative residual stresses are balanced in the object as a whole, as described in the Wikipedia article.
However, if tensile stress preponderates, elastic deformation exceeding the elastic modulus of the plate part will arise, and warping will occur.
Now, if the plate is removed, as shown at right in Fig. 1, I think you can see what will happen. It will result in a block (part) with only tensile residual stress.
In the resulting state the overall sum of stresses, positive and negative, does not equal zero, so the object is extremely unbalanced.
When making a mold with a metal 3D printer, our company recommends the hybrid type which reuses plates.

To resolve this issue of tensile residual stress, solution heat treatment (i.e., “annealing”) is performed while in the condition with the shaped object on the plate, and this relaxes tensile stress. With commonly used maraging steel, however, the solution heat treatment temperature is 816°C, and if one looks at a shaped object after treatment, it is in a softened state in terms of dimensional precision, etc. After that, as aging treatment, heat treatment is performed again near 480°C to increase hardness, and in the heat history after the end of the metal 3D printer process, heating is performed two times.
Then the article is cut from the plate, secondary machining is performed, and the part is used.
You may be wondering what happens in this case, but researchers in the US and Europe have devised approaches for the laser irradiation pattern itself, and are accelerating research to reduce residual stress in shaped objects. Naturally, our company too is developing new irradiation patterns.
However, reduction can be achieved, but it doesn’t reach the point where it changes stress into compressive residual stress.
Also, we want to avoid performing solution heat treatment as far as possible, so at our company, and we have arrived at a point where it looks like we will be able to change inner surface residual stress of shaped objects simply by using an aging process immediately after shaping. We are still only halfway along, but we are diligently working on development.

See Table 1.

Table 1: Residual stress of a shaped object. No treatment vs. incorporating a process developed by OPM

Table 1: Residual stress of a shaped object. No treatment vs. incorporating a process developed by OPM

With the conventional process, tensile residual stress is 165MPa immediately after shaping, and 169MPa immediately after aging, but if our development process is incorporated, this changes to compressive residual stress of 134MPa immediately after shaping, and 154Mpa immediately after aging.

As the starting point of this development, we began to thinking urgently (even neglecting eating and sleeping) due to an emergency situation, namely: frequent occurrence of problems with stress corrosion cracking from the inside of water pipes of diecast molds, which experience punishing amounts of heat 3–4 times that of plastic molds. If this problem could not be avoided, it would cause steam explosions during mass production.

We’ve come to a point where we may be able to realize a concrete result.
We are convinced of the potential for this development to have an impact on the expansion of the metal 3D printer industry.