From Powder to Solid: The Material Evolution of Ti-6Al-4V during Laser Metal Deposition

Abstract

The additive manufacturing of titanium parts by laser metal deposition (LMD) offers a promising alternative to conventional machining of aviation parts. The technology enables the production of near net shape parts with higher deposition rates than powder bed-based processes. Ti-6Al-4V powder is fed directly into a high-power laser beam in order to form a deposition track on the underlying material. For three dimensional parts several tracks are stacked on top of each other. In this paper the material evolution from powder to a solid wall during LMD is investigated. Powder properties as well as the microstructure in deposited structures are thoroughly described and analyzed. The gained knowledge provides a deeper process comprehension and is an important step towards high-quality additive manufacturing of Ti-6Al-4V. At first, the influence of powder particle size on the LMD 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. As a result various solid phase transformations occur during the deposition of consecutive tracks. In addition the thermal boundary conditions change with increasing wall height and a heterogeneous microstructure is observed. It consists of 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.