Virtual
Laser powder bed fusion of cost-effective hydride-dehydride Ti-6Al-4V powders
Amir Mostafaei, Illinois Institute of Technology
USA
Hydride-dehydride (HDH) Ti-6Al-4V with non-spherical powder morphology is not widely used in laser powder bed fusion (L-PBF). The effect of non-spherical particles with size distribution of 50-120 μm on powder flowability, packing density, and dynamics of laser-powder interaction is studied and parts with a density of 99.8 % is achieved. Microstructural observations and phase analyses show formation of columnar β grains with acicular α/α’ phases in as-built condition. The roughness of the as-fabricated samples is significant with an average roughness of Ra = 15.71±3.96 μm and a root mean square roughness of Rrms = 108.4±24.9 μm, however, both values are reduced to Ra = 0.19±0.04 μm and Rrms = 4.9±0.6 µm after mechanical grinding. Fatigue specimens are tested under fully-reversed tension-compression conditions of R = -1. The as-built samples failed from the surface with crack initiation mainly at micro-notches, whereas after mechanically grinding, crack initiation changed to subsurface defects such as pores. Minimizing surface roughness by mechanically grinding eliminates surface micro-notches which improves fatigue strength in the high cycle fatigue region. Fatigue notch factor calculations showed that the effect of surface roughness was significantly lower when HDH powder is used compared to standard spherical powder. X-ray diffraction analysis revealed an in-plane compressive stress, micro-strain and grain refinement on the surface of the mechanically ground samples. Fractography observations (macroscale) revealed a fully brittle fracture in the first stage of crack growth with a transition to a dominantly ductile fracture in in the third stage of crack growth. On the other hand, at the micro scale, even the brittle fracture regions showed evidence of ductile fracture within the α′ martensite laths. Results indicate that the cost-effective, non-spherical HDH powders used in this study are a viable raw material for use in the L-PBF process.
Powder Metallurgy of Titanium-Nickel Alloys: an Overview
Efrain Carreño-Morelli, University of Applied Sciences and Arts
Switzerland
Near stoichiometric titanium-Nickel alloys can exhibit shape memory behavior or superelastic properties depending on composition, which strongly influence the austenite-martensire transformation temperatures. These alloys are currently shaped into wires, tubes or tapes by costly processing routes which include difficult machining from starting from cast ingots. Alternative powder metallurgy routes have been explored, but the cost of prealloyed powders remains high, in the order of a semi-precious metal. In this work, the main research achievements on press and sintering, metal injection molding, tape casting and additive manufacturing of TiNi alloys will be summarized. The present status and the perspectives of PM TiNi applications will be discussed with focus on performance, powder availability and buy-to-fly ratio.
Formation of Amorphous Ti-Sn Alloy Powder by Mechanical Alloying of Intermetallic Powder Mixtures and Mixtures of Tin or Titanium with Intermetallics
P-Y Lee, National Taiwan Ocean University
Taiwan
In this study mechanical alloying was used to synthesize TixSn1−x alloys from mixtures of Ti3Sn and Ti6Sn5 intermetallic compound powders, and also from mixtures of Ti6Sn5 intermetallic compound powders and pure elemental Sn powders. The mechanically alloyed powders were amorphous in the range 0.40 ⩽ x ⩽ 0.85. This is the global first report on the successful preparation of Ti-Sn amorphous alloy powders. It is found that the morphological development during mechanical alloying of these powders is different from mechanical alloying using only pure ductile crystalline elemental powders. The thermal stability has been investigated. A high crystallization temperature of 580℃ was detected for amorphous Ti3Sn alloy powders.
Mechanical Behavior of Heterogeneous Ti Metal-Metal Composites
Y. Liu, Central South University
China
Ti-based composites with refractory metallic particles, such as Mo, W and Ta were prepared through powder metallurgy. These composites were then underwent thermal mechanical treatment to introduce refined heterogeneous structures, which may include Ti-rich, refractory metal-rich and partially-diffusional regions. Interestingly, all these Ti-base metal-metal composites have much higher strength than their conventional homogenous alloys, and also possess satisfactory ductility. The mechanical behavior was investigated through a modified heterogeneous deformation (HDI) induced stress model, rather than widely-used backstress model, and the underlying deformation mechanism in the aspect of microstructures was revealed as well. This talk is going to present a new kind of PM Ti-based composites, and new concept for designing high-performance materials.
Cost Down and Lean Design of High-Performance CP-Ti Via Additive Manufacturing
G. Chen, University of Science and Technology Beijing
China
Currently, additive manufacturing of high-performance Ti alloys suffers the high-cost powder feedstock and heavy dependence on rare-metal alloying. This works demonstrates an innovative and facile pathway towards low-cost HDH Ti powder feedstock for AM by gas-solid fluidization, which lowers its cost over 50 % as compared with the atomized powders. Besides, compositional tailoring during fluidization can be achieved via simultaneous reaction with ubiquitous interstitial elements for HDH CP-Ti powders. The as-fabricated CP-Ti by SLM using fluidized powders exhibited superior tensile properties (UTS=1155±6 MPa, YS=1083±29 MPa, EL=13±1.6 %) at room temperature, surprisingly comparable with Ti-6Al-4V. This can be attributed to the strengthening-and-toughening effect on CP-Ti induced by interstitial solid solution and nanoscale-interface microstructure. Contrary to the conventional metallurgical mechanism for Ti, this work presents SLMed CP-Ti with excellent synergy of strength and ductility while using HDH Ti powders, which surmounts the cost-performance dilemma and realizes lean design for high-performance Ti materials.