Relationship between powder characteristics and part properties in laser beam melting of Ti–6Al–4V, and implications on quality

The technology of laser beam melting (LBM) has lately become a true option for series production of functional end products. Metallic materials such as steel, aluminum, and titanium alloys may be processed to fully dense parts with mechanical properties meeting or even exceeding their conventionally manufactured counterparts. However, the use of LBM for industrial applications also requires reliable quality assurance methods along the entire process chain. Besides monitoring and controlling the LBM process itself, this implicates a thorough understanding of the relationship between the powder feedstock characteristics and the properties of the manufactured parts. This paper provides an in-depth analysis of how the characteristics of different Ti–6Al–4V powder feedstocks influence the usual quality criteria of parts. Typical characteristics of virgin powders from three different manufacturing routes, namely gas atomization, plasma atomization, and induction plasma spheroidization, are studied. Besides the investigation of particle morphology, particle size distribution, and chemical composition, powder flowability is determined using different measurement methods. Applying methods such as Hall flowmeter, angle of repose, Hausner ratio, avalanche angle, powder rheology, and shear tests, the powders are tested under static and dynamic stress states. From the different feedstocks, specimens are manufactured and the part properties such as density and static strength are determined. Correlation between powder characteristics and the resulting part properties is investigated. Depending on the different powder analyzation methods, recommendations for quality assurance are derived. It is shown that quality assurance of Ti–6Al–4V powder for LBM is a complex task. Powder material quality for this process cannot be ensured using a single measurement parameter or method as it is a result of the interactions between various properties. Such being the case, the determination of powder flowability leads to ambiguous results, irrespective of the selected measurement method. Presumably, the application of powder with good flowability according to a specific test will regularly result in parts with high density and acceptable mechanical properties. However, powder with comparably low flowability might also be applicable for the manufacturing of parts that meet the required quality criteria. As a consequence, solely employing a flowability test for powder quality assurance would possibly reject a number of powder batches that would turn out to be suitable for the LBM process.