SLS Powder Analysis with the Anton Paar Powder Cell

Introduction

The aim of this Application Report is to showcase a method for characterizing metal powders by selective laser sintering (SLS) with the new Anton Paar Powder Cell.

Two batches of a cobalt chromium nickel iron alloy (both described as micro melt CCM-MC) were analyzed. In the aviation industry, such alloys are usually used for turbine blades, although other applications are possible. Such powders are usually made by atomization, whereby a stream of molten metal is broken into droplets by the introduction of either an air or water jet, and then rapidly cooled in a water (or oil) curtain[1]. The two batches of micromelt CCM-MC examined in this report are unlikely to differ in chemical makeup or particle shape; however, visual inspection of the powders showed a slight difference in cohesion. Thus, the powders are assigned as cohesive (Powder A) and cohesionless (Powder B). The powders were tested for magnetic moments, and both were found to be paramagnetic.

A preliminary grain-size-distribution analysis of the two powders was carried out. This is not strictly necessary, but it does show how minuscule changes in grain-size distribution can have greater effects on applications than the size data would suggest. Powder flow is more sensitive than most competing methods, and can thus show such effects.

The grain-size-distribution analysis was carried out with a dry-dispersive cell by using static light scattering. The main difference appears to be a minuscule addition of fines in the more cohesive sample. Nonetheless, the correlation between particle size and flow is not simple. Up to a threshold, fine addition does indeed affect cohesion,[2],[3],[4] but the effect depends on many factors, including chemical composition, static electricity and moisture[5]. To gain an accurate picture of the flow behavior, analysis of particle size alone is not sufficient.

The Anton Paar Powder Flow Cell was used to subject the powders to an array of measurement procedures. The Powder Flow Cell allows diverse measurements to be made on metal and other powders, even by untrained personnel.

A method to simulate the process of raking a blade over the metal powder was also devised, a process that is similar to what is encountered in the preparation of an SLS layer.

[1] F. Thummler and W. Thomma, "The Sintering Process," Metallurgical Reviews 115, June (1967)

[2] Powders and Bulk Solids Behavior, Characterization, Storage and Flow by Dietmar Schulze, Springer, Hamburg, Germany, (2008)

[3] Mohammadi, M. S. & Harnby, N. Bulk density modeling as a means of typifying the microstructure and flow characteristics of cohesive powders. Powder Technol. 92, 1–8 (1997).

[4] Santomaso, a, Lazzaro, P. & Canu, P. Powder flowability and density ratios: the impact of granules packing. Chem. Eng. Sci. 58, 2857–2874 (2003).

[5] Abdullah, E. C. & Geldart, D. The use of bulk density measurements as flowability indicators. Powder Technol. 102 (1999).

 

 

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