Additive Manufacturing: Microstructure and Material Properties of Titanium-based Alloys: Laser Powder Bed Fusion - Session I
Program Organizers: Ulf Ackelid, Freemelt AB; Andrzej Wojcieszynski, ATI Powder Metals; Ola Harrysson, North Carolina State University; Sudarsanam Babu, University of Tennessee, Knoxville

Tuesday 2:00 PM
October 1, 2019
Room: B116
Location: Oregon Convention Center

Session Chair: Ola Harrysson, North Carolina State University

2:00 PM  
Toolpath analysis of void formation in PBFAM Ti-6Al-4V: Brett Diehl1; Abdalla Nassar1; Anil Chaudhary2; Brandon Baucher2; 1Pennsylvania State University; 2Applied Optimization
    Powder bed fusion additive manufacturing allows for the precise production of arbitrary geometries and is being rapidly adopted for aerospace, defense, and biomedical applications. However, near-surface lack of fusion and porosity remain primary concerns given their role in the fatigue life and mechanical strength of components. This work examines the role of the toolpath on flaw formation at contour-hatch interfaces. Ti–6Al–4V samples with various geometries and scanning strategies are produced using optimized (default) processing parameters. The size, distribution, and morphology of defects are investigated with high-resolution X-ray computed tomography (XCT). The distance the laser travels between neighboring hatch points along the contour was found to be a strong factor in void formation, particularly in geometries with smaller hatches.

2:20 PM  
Laser Powder Bed Fusion of Hydride-Dehydride Ti-6Al-4V Powder: Amir Mostafaei1; Ziheng Wu1; Nihal Sivakumar1; Anthony Rollett1; 1Carnegie Mellon University
    Feedstock powders that can be made without melting may be more economical for additive manufacturing but often exhibit non-spherical morphology. Hydride-Dehydride (HDH) Ti-6Al-4V powder, e.g., is used to manufacture parts via the laser powder bed fusion (LPBF) process. The effect of processing parameters such as power P and velocity V on the density, microstructure, surface roughness, and mechanical strength are studied. A P-V processing window for the LPBF of HDH Ti-6Al-4V powder is developed and defect formation mechanisms such as keyholing and lack of fusion pores are discussed. To study the laser-powder interaction, a high-speed synchrotron X-ray imaging technique is used to better understand conditions for (1) keyhole porosity, (2) threshold from conduction mode to keyhole based on laser power density and (3) the sequence of vaporization, depression of the liquid surface, instability, and then deep keyhole formation. The aim is to demonstrate the feasibility of LPBF with non-spherical powders.

2:40 PM  
Modeling Rapid Solidification Microstructures in Laser-melted TiNb Alloys: Joel Berry1; Aurelien Perron1; Jean-Luc Fattebert2; Joseph Mckeown1; Manyalibo Matthews1; 1Lawrence Livermore National Laboratory; 2Oak Ridge National Laboratory
    We simulate crystal growth kinetics and microstructure formation in TiNb alloys under conditions relevant to laser-based additive manufacturing using a phase field model for rapid solidification. The ultimate aim is to enable spatial control of microstructure and material properties by establishing quantitative relations between laser beam characteristics and resultant metal microstructure. Solid phase growth mode (planar, cellular, dendritic), compositional segregation, and characteristic microstructural length scales are mapped as functions of external process parameters and compared with measurements of the same quantities in single-track laser melting experiments. Transfer of our results to laser powder-bed fusion AM with spatial microstructural control will be discussed. This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, and supported by the Laboratory Directed Research and Development Program under project tracking code 18-SI-003.

3:00 PM  Cancelled
Multiple Strategies for Microstructural Control during Selective Laser Melting of a Beta Titanium Alloy: Chunlei Qiu1; 1Beihang University
    A titanium alloy was selectively laser melted under a pulsed laser mode. It is shown that with a powder layer thickness of 30 um, the as-fabricated samples show almost 100% density when a laser power of 200-400 W is used. Doubling the layer thickness increases porosity. Increasing laser exposure time, however, is beneficial for reduction in porosity. At the layer thickness of 30 um, samples are dominated by columnar grains. With increased laser power and exposure time, grain size increases continuously and texture becomes increasingly pronounced. Doubling layer thickness gives rise to considerable equiaxed grains, significantly randomising the grain structure. The sample with the finest columnar grains show high strengths and the best elongations (>13%). Samples with increased grain sizes show no ductility and failed in a brittle mode. The sample with least texture fails in an intergranular fracture mode and shows good tensile strengths but low elongation (1%).

3:20 PM  
Comparing Spherical and Irregular Powders in the Selective Laser Melting of Ti-53%Nb Alloy: Jhoan Sebastian Guzman Hernandez1; Rafael Nobre1; Enzo Nunes1; Daniel Bayerlein2; Railson Falcćo2; Edwin Leva2; Joćo Ferreira Neto3; Henrique Oliveira4; Victor Chastinet4; Fernando Gomes Landgraf1; 1University of Sao Paulo; 2Institute for Technological Research; 3Brazilian Metallurgy and Mining Company; 4National Service for Industrial Training
    Literature reports that irregular shaped powders have low flowability and low apparent density, factors that hinder their use in Selective Laser Melting, although its cost is potentially lower. This approach shows that it is possible to achieve porosity levels lower than 1% with irregular shaped powder produced by Hydrogenation-Dehydrogenation (HDH). Samples were produced with constant scanning speed (1000mm/s) and laser power levels of 200 and 300W. Due to the lower apparent density of the irregular powder, its layer thickness was twice larger than the 30µm thickness of the spherical powder procedure. Both power levels produced low porosity with spherical powders, but the irregular powder generated 13% porosity with 200W. The use of low interstitial content plasma atomized spherical powder resulted in lower hardness when compared to the higher interstitial content of the HDH irregular powder.