Changes of stainless steel powder

Objectives
There is high market potential for the production of metal parts using Additive Manufacturing (AM) technologies: In many applications, stainless steel (1.4404) with a good corrosion resistance is widely used. For example, in the field of passenger services or goods traffic with trains, or the automotive industry there are many applications, which can be produced cost-efficient by AM, e.g. brackets, hydraulic components. For serial production, deep knowledge on the robustness of part properties against variation of powder characteristics is required.

1.4404 is the most commonly used stainless steel, because it has a well-balanced property profile and different heat treatments allow to tune microstructure and properties for different requirements and applications. It has a very high wear resistance, with a very good ductility. This material offers a large range of applications because of the high corrosion resistance.

Motivation and aim
There are three key factors influencing the part quality of AM parts - the AM system including hard -& software, the process with numerous process parameters and the powder material itself. For series production, full control of all three factors is essential. During the use phase of powder, effects like out washing of fine fractions, pick up of oxygen as well as nitrogen change powder characteristics. In addition, there is a possibility of powder decomposition due to the powder handling process in the SLM machine. Therefore, the powder quality permanently changes during the manufacturing process.

Another point is the lot to lot variation of the powder quality inside the specified ranges. In Figure 1 for example the particle shape and the particle size distribution of atomized 1.4404 powder from one supplier are demonstrated. The specified particle size range was 20 – 60 μm. The identified average of the particle size is about 36.2 μm. As Figure 1 illustrates, the powder contains some small as well as big particles. Within the specified limits for particle size and particle chemistry there is a lot of freedom for lot to lot variations, the powder suppliers can go up to the range limits.

For conventional processing there is a broad data base giving the correlation with material properties. But for AM the experience concerning these correlations is limited. Scope of this project is to investigate the influence of relevant changes of powder characteristics on the AM process and the material as well as part properties.

Reference
[1] A. Riemer: Einfluss von Werkstoff, Prozessführung und Wärmebehandlung auf das
bruchmechanische Verhalten von Laserstrahlschmelzbauteilen. Forschungsberichte
des Direct Manufacturing Research Centers, Shaker Verlag, Paderborn,
2015.