Selective Laser Melting: obtaining the best fatigue performance

Selective Laser Melting: obtaining the best fatigue performance

In this article, I will talk about Additive Manufacturing and in this case Selective Laser Melting (SLM). In particular, the theme will focus on the role played by vacuum heat treatments in producing highly reliable components, i.e. characterized by a better quality of the material and an excellent fatigue resistance.

We will conclude by exposing the interesting experimental results obtained in laboratory, thanks to a fruitful collaboration with the University of Parma.

AM technology

Starting from a digital file and printing metal objects in 3 dimensions is no longer a science fiction novel technology, but a reality possible with the Additive Manufacturing.

Starting from a 3D design, it is possible to make metal components with remarkable mechanical strength that can be used in various sectors, from medical to aerospace. Thanks to Selective Laser Melting (SLM), the creation takes place using a powder bed and a high power laser source, all controlled in real time by a computer.

The role of the vacuum furnace in additive manufacturing

One of the tools to ensure that with this technology functional components can be realized for the concrete application are the vacuum furnaces; indeed, after the additive manufacturing process, heat treatments or surface mechanical finish treatments are fundamental.

Vacuum furnaces are therefore essential for post-process treatments and are suitable for processing a variety of materials, such as stainless steels, titanium superalloys, cobalt-chromium alloys and nickel superalloys.

Why heat treating under vacuum?

The products made by 3D printing are near-net-shape, in the sense that selective melting (SLM) yields a lower production rate than other conventional technologies, making it possible to obtain a more detailed component with more complex forms, with less material, which otherwise would be almost impossible to obtain.

The passage inside a vacuum furnace therefore appears to be of considerable importance, as it is characterized by the absence of oxygen and special diffusion pumps which, through precise temperature control, determine the desired vacuum conditions. The lack of oxygen is fundamental for medical and aerospace applications, for which oxide contamination must absolutely be avoided.

The vacuum heat treatments are different depending on the considered additive technologies.
Specifically, the heat treatments of debiding and sintering are indicated for the technologies of Fused Filament Fabrication (FFF) and of Binder Jetting, while the treatments of stress relieving and/or solubilization are carried out for the Selective Laser Melting (SLM) technology.

What makes a vacuum furnace ideal for treating near-net-shape products?
Continue reading and find out which are the 3 critical points to keep under control in Selective Laser Melting.

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Additive manufacturing: laboratory results

In order to make the research on heat treatments more effective, we have set up a special laboratory in which we have installed an all‑metal vacuum furnace H3 and with which various types of tests have been carried out (hardening, stress relieving, annealing, sintering, tempering, etc.). Moreover, in the same laboratory a machine for the deposition of PVD coatings with rotating cathodes has been positioned for a better surface finish of the samples made using additive technologies.

The results obtained from the analysis carried out in the TAV laboratory were aimed at:

  1. obtain a know-how in vacuum treatments and in the production of the relevant furnaces to support companies, universities and research centers;
  2. take part in partnerships and research projects for the development of these and other emerging technologies;
  3. prepare and publish scientific information texts.

Collaboration with the University of Parma

One of the most recent activities developed by TAV VACUUM FURNACES is a collaboration with the University of Parma on the topic of optimal fatigue resistance of specimens made with SLM technology. Fatigue is one of the most significant and complex parameters of industrial components; in the aeronautical field, in fact, fatigue is counted as the cause of a high percentage of breakages and it is for this reason that the focus of the study has focused on it in particular.

The objectives of the collaboration were as follows:

  • determining the behavior of fatigue resistance at high rpm on samples made with AM technology;
  • evaluate the effects of vacuum heat treatment on the fatigue strength of SLM components;
  • consider the impact of anisotropy of SLM components on fatigue resistance.

Fatigue performance of the components

That of Additive Manufacturing SLM is a process that presents three fundamental critical issues, which concern:

  • the quality of the material in terms of metallographic microstructures;
  • surface roughness;
  • the process residues present on the final component.

To obtain the experimental results that I will show you shortly, specimens were used in alpha/beta titanium superalloy (Ti-6Al-4V). The choice of this material was not accidental; it is a particularly light alloy, but resistant to corrosion and high temperatures. These characteristics make it the most suitable material for automotive, aerospace and biomedical engineering applications.

The research was carried out through the use of standard samples with different mechanical finishes and at a later stage, these were subjected to rotary bending tests for measuring fatigue resistance.

Looking at the graph below, you can see that as the mechanical surface finish increases, the fatigue resistance is also increased.

The two curves of the same color indicate that, with the same surface finish, the two standard samples were increased in two different orthogonal directions with respect to the plane on which the piece was built; the specimens therefore assumed a different behavior depending on the direction of growth and this demonstrates the relevance of the intrinsic anisotropic characteristic of the SLM technology.

Fatigue performance 

In the second graph, you can see the effect of heat treatment on some samples, indicated with red color curves. The trend of the curves shows that heat-treated specimens exhibit better behavior in terms of fatigue performance, compared to untreated ones. 

Fatigue performance

Conclusions

Additive Manufacturing it is undeniably the emerging technology in the field of engineering. The realization of components destined for strategic sectors requires processes and machinery that make a finished product and it is able to withstand the stresses deriving from the application in the field.

We have shown in the laboratory that vacuum furnaces represent a highly functional tool for performing post-heat treatment processes at Selective Laser Melting: the passage inside a vacuum furnace determines improvements in the microstructure of the material in terms of detensioning, isotropic behavior and surface roughness.

If you are interested in learning more, I invite you to read the article by Prof. Nicoletto of the Department of Industrial Engineering of the University of Parma: Metal additive manufacturing and vacuum heat treatments.

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Metal additive manufacturing and vacuum heat treatments

Discover how metal additive manufacturing makes it possible to build parts in real time and understand potential of subsequent vacuum heat treatments.

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