From powder to solid: The material evolution of Ti-6Al-4V during laser metal deposition

Enthalten in
Key engineering materials 
Start Page
135
End Page
147
Citation
Key Engineering Materials (770 KEM): 135-147 (2018)
Contribution to Conference
4th International Conference on Powder Metallurgy of Titanium, PMTi 2017 
Publisher DOI
10.4028/www.scientific.net/KEM.770.135
Scopus ID
2-s2.0-85047862290
Laser metal deposition (LMD) offers an alternative to conventional machining of titanium aviation parts, because near net shape parts can be produced with high deposition rates. In this paper the material evolution of Ti-6Al-4V during LMD is investigated. Powder properties as well as the microstructure in deposited structures are described and analyzed. At first, the influence of powder particle size on the process is quantified by creating two powder fractions with different sieving procedures. The used material is recycled Ti-6Al-4V powder from a powder bed-based AM process with particle sizes up to 150 µm. The powder is characterized according to current standards; apparent density, tap density and the flowability are determined as well as the particle size distribution. Additionally, the particle morphology is analyzed using electron beam microscopy. In order to link the powder properties to the LMD process and to identify impact factors to the feeding behavior the mass flow of both powder fractions is measured. Secondly, walls are manufactured with the characterized powder and the resulting microstructure is analyzed. Because of the layer-wise deposition and the resultant periodic heat input each layer experiences several thermal cycles. The thermal boundary conditions change with increasing wall height. The occurring solid phase transformation result in a heterogeneous microstructure with non-equilibrium phases (martensitic or massive α) and α+β lamellae. Based on an existing numerical model the thermal history of each layer is estimated and an explanation is presented for the complex sequence of solid phase transformations in each area of the structure.