Promoting the columnar to equiaxed transition and grain refinement of titanium alloys during additive manufacturing

Powder metallurgy (PM) of titanium is a potentially cost-effective alternative to conventional wrought titanium. This article examines both traditional and emerging technologies, including the prod...

https://www.tandfonline.com/doi/pdf/10.1080/09506608.2017.1366003

Powder metallurgy of titanium – past, present, and future

Controlling the microstructure and properties of wire arc additive manufactured Ti-6Al-4V with trace boron additions

Room temperature tensile ductility is an important property of titanium (Ti) and titanium alloys for structural applications This article reviews the dependency of tensile ductility on oxygen for α-Ti, (α+β)-Ti and β-Ti alloys fabricated via traditional ingot metallurgy (IM), powder metallurgy (PM) and additive manufacturing (AM) or three-dimensional printing methods and recent advances in understanding the effect of oxygen on ductility Seven mechanisms have been discussed based on case studies of individual titanium materials reported in literature The dependency of ductility on oxygen is determined by both the composition and microstructure of the titanium alloy For Ti–6Al–4V (wt-%), as sintered Ti–6Al–4V shows a critical oxygen level of about 0·33 wt-% while additively manufactured Ti–6Al–4V exhibits different critical levels ranging from about 0·22% to well above 0·4% depending on microstructure Rare earth (RE) elements are effective scavengers of oxygen in titanium materials even just wi

Review of effect of oxygen on room temperature ductility of titanium and titanium alloys

A transmission electron microscopy and three-dimensional atom probe study of the oxygen-induced fine microstructural features in as-sintered Ti–6Al–4V and their impacts on ductility

A comprehensive study has been made of the sintering and microstructural evolution of Ti–10V–2Fe–3Al compacted from titanium and master alloy powder blends. The densification of Ti–10V–2Fe–3Al at ≤1300 °C occurs by solid-state sintering with apparent activation energy of 163 ± 13 kJ/mol, which falls into the reported activation energy range for the self-diffusion of titanium. The sintered density depends primarily on the titanium powder size while the sintered microstructure and mechanical properties depend mainly on the master alloy type or diffusion of vanadium. The real challenge for the fabrication of Ti–10V–2Fe–3Al by sintering is not densification, which is trivial with relatively fine titanium powder. Rather, it is the realisation of a desired microstructure through master alloy selection or design of the sintering pathway, which determines the distribution of the principal alloying element V, a slow diffuser in β-Ti. This distinguishes it from the sintering of lean alloys, where the focus has predominantly been on densification. The use of 10V–2Fe–3Al master alloy produced more uniform microstructures and therefore much better ductility than the use of 85V–15Al at similar densities.

Sintering of Ti-10V-2Fe-3Al and mechanical properties

An addition of ≤0.5 wt% lanthanum boride (LaB 6 ) to powder metallurgy commercially pure Ti (CP-Ti), Ti–6Al–4V and Ti–10V–2Fe–3Al (all in wt%) resulted in improved sintered density, substantial microstructural refinement, and noticeably increased tensile elongation. The addition of LaB 6 led to scavenging of both oxygen (O) and chlorine (Cl) from the titanium powder during sintering, evidenced by the formation of La 2 O 3 and LaCl x O y . The pinning effect of La 2 O 3 , LaCl x O y and TiB inhibited prior-β grain growth and resulted in subsequent smaller α-laths. The formation of nearly equiaxed α-Ti phase is partially attributed to the nucleation effect of α-Ti on TiB. The improved sintered density was caused by B from LaB 6 rather than La, while excessive formation of La 2 O 3 and TiB with an addition of >0.5 wt% LaB 6 resulted in a noticeable decrease in sintered density. The improved tensile elongation with an addition of ≤0.5 wt% LaB 6 was mainly attributed to the scavenging of oxygen by LaB 6 , partially assisted by the improved sintered density. However, an addition of >0.5 wt% LaB 6 led to the formation of large La 2 O 3 aggregates and more brittle TiB whiskers and therefore decreased tensile elongation. Balanced scavenging of O is thus important. The optimal addition of LaB 6 was 0.5 wt% but this may change depending on the powder size of the LaB 6 to be used.

The effect of lanthanum boride on the sintering, sintered microstructure and mechanical properties of titanium and titanium alloys

Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti–Ni alloys because small additions of Si lower the solidus of Ti–Ni alloys appreciably (>200 °C by 1 wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti–3Ni alloy at 1300 °C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti5Si3 form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti5Si3 phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si.

The effect of Si additions on the sintering and sintered microstructure and mechanical properties of Ti-3Ni alloy

A small addition (≤0.5 wt%) of cerium silicide (CeSi 2 ) to powder metallurgy (PM) commercially pure Ti (CP-Ti), Ti–6Al–4V and Ti–10V–2Fe–3Al (all in wt%) results in substantial microstructural refinement and noticeably improved ductility with marginally improved sintered density. CeSi 2 is unstable and decomposes between 1423 K and 1473 K. The Si goes into solid solution in β-Ti and is responsible for the improved sintered density while the Ce scavenges both oxygen (O) and chlorine (Cl) from the Ti powder and therefore improves tensile ductility. The resulting CeO 2 and CeCl x O y particles generally exist along or close to the prior-β grain boundaries. The substantial microstructural refinement in terms of both the prior-β grain size and the subsequent α-Ti lath size is attributed to the grain boundary pinning effect of the CeO 2 particles. The optimum concentration of CeSi 2 is approximately 0.5 wt%, beyond which both the sintered density and tensile elongation drop with increasing addition of CeSi 2 . CeSi 2 can be a practical form of Ce addition to PM Ti alloys for impurity scavenging, microstructural refinement and tensile ductility improvement.

Impurity scavenging, microstructural refinement and mechanical properties of powder metallurgy titanium and titanium alloys by a small addition of cerium silicide

Titanium aluminide based alloys are difficult to consolidate by pressureless sintering. A systematic study has been made of the effect of small additions of copper on the sintering densification, microstructure and mechanical properties of gamma Ti–48Al–2Cr–2Nb (in at% throughout). The density of the Ti–48Al–2Cr–2Nb alloy sintered at 1375 °C for 120 min increased consistently from 74% theoretical density (TD) without Cu to >98%TD with an addition of up to 2 at%Cu. The enabling effect of a small addition of Cu is due to the formation of a wetting liquid, supported by dilatometry, differential scanning calorimetry (DSC) and Thermal-Calc analyses. Rapid sintering shrinkage occurred around 1251 °C and 1370 °C corresponding to liquid formation. Transmission electron microscopy (TEM) identified the formation of a Cu- and Cr-enriched hexagonal close-packed Ti(Al, Cr, Cu, Nb) phase with a=0.520 A and c=0.803 A. Its presence increased with increasing Cu content. The compressive, tensile and flexure properties of the as-sintered Ti–48Al–2Cr–2Nb–2Cu alloy were assessed. The tensile properties are similar or superior to those of the Ti–48Al–2Cr–2Nb alloy fabricated by metal injection moulding and sintering while the compression strength is about twice that of the as-cast Ti–48Al–2Cr–2Nb.

S.D. Luo

Papers

Sintering of Ti-10V-2Fe-3Al and mechanical properties

The effect of lanthanum boride on the sintering, sintered microstructure and mechanical properties of titanium and titanium alloys

The effect of Si additions on the sintering and sintered microstructure and mechanical properties of Ti-3Ni alloy

Impurity scavenging, microstructural refinement and mechanical properties of powder metallurgy titanium and titanium alloys by a small addition of cerium silicide

The sintering densification, microstructure and mechanical properties of gamma Ti–48Al–2Cr–2Nb alloy with a small addition of copper