Investigation of the Laser Powder Bed Fusion Process of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si Alloy

Laser powder bed fusion(LPBF)is an advanced manufacturing technology;however,inappropriate LPBF process parameters may cause printing defects in materials.In the present work,the LPBF process of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy was investigated by a two-step optimization approach.Subsequently,heat transfer and liquid flow behaviors during LPBF were simulated by a well-tested phenomenological model,and the defect formation mechanisms in the as-fabricated alloy were discussed.The optimized process parameters for LPBF were detected as laser power changed from 195 W to 210 W,with scanning speed of 1250 mm/s.The LPBF process was divided into a laser irradiation stage,a spreading flow stage,and a solidification stage.The morphologies and defects of deposited tracks were affected by liquid flow behavior caused by rapid cooling rates.The findings of this research can provide valuable support for printing defect-free metal components.

Ti-27Mo-1.5Nb无磁不锈定膨胀合金研究

研究了一种Ti-27Mo-1.5Nb新型无磁不锈定膨胀合金,该合金的膨胀系数为7.70×10-6/℃,磁导率为1.0003,同时研究了Ti-27Mo-1.5Nb合金的微屈服强度及其它机械性能和物理性能,该合金还是良好的耐腐蚀材料,试验表明,Ti-27Mo-1.5Nb合金是诸如静电陀螺等电真空领域中与高纯Al2O3匹配的良好封接材料。

Haemocompatibility of Ti-3Zr-2Sn-3Mo-25Nb biomedical alloy with surface heparinization using electrostatic self assembly technology

The haemocompatibility of Ti-3Zr-2Sn-3Mo-25Nb biomedical alloy was studied after surface heparinization. A layer of sol-gel TiO2 films was applied on the alloy samples followed by active treatment in the bio-functionalized solution for introducing the OH- and groups, and then the heparin was immobilized on the active TiO2 films through the electrostatic self assembly technology. It is shown that the heparinized films are mainly composed of anatase and rutile with smooth and dense surface. In vitro blood compatibility was evaluated by haemolysis test, clotting time and platelet adhesion behavior tests. The results show that the haemocompatibility of the alloy could be significantly improved by surface heparinization.

Preparation and osteoinduction of active micro-arc oxidation films on Ti-3Zr-2Sn-3Mo-25Nb alloy

A layer of porous film containing Ca and P was prepared by the micro-arc oxidation method on the surface of a novel near β biomedical Ti-3Zr-2Sn-3Mo-25Nb alloy, and then NH2- active group was introduced to the films by activation treatment. The phase composition, surface micro-topography and elemental characteristics of the micro-arc oxidation films were investigated with XRD, SEM, EDS and XPS, and the osteoinduction of the micro-arc oxidation films was tested using the simulated body fluid immersion, the in-vitro osteoblast cultivation test and animal experiment. The results show that the oxide layer is a kind of porous ceramic intermixture and contains Ca and P. The films in the simulated body fluid can induce apatite formation, resulting in excellent bioactivity. The cell test discovers that osteoblasts can grow well on the surface of micro-arc oxidation films. And the Ti-3Zr-2Sn-3Mo-25Nb biomedical alloy coated with active porous calcium-phosphate films shows better osteoinduction in vivo.

Hot deformation behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy with acicular microstructure

The characteristics of hot deformation of an α＋β titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si with acicular microstructure were studied using isothermal hot compressive tests in a strain rate range of 0.01-10 s^-1 at 860-1 100 ℃. The true stress-tree strain curves of alloy hot-compressed in the α＋β region exhibit a peak stress followed by continuous flow softening; whereas in the β region, the flow stress attains a steady-state regime. At a strain rate of 10 s^-1 and in a wide temperature range, the alloy exhibits plastic flow instability. According to the kinetic rate equation, the apparent activation energies are estimated to be about 633 kJ/mol in the α＋β region and 281 kJ/mol in the β region, respectively. The processing maps show a domain of the globularization process of a colony structure and α dynamic recrystallization in the temperature range of 860-960 ℃ with a peak efficiency of about 60%, and a domain of β dynamic recrystallization in the β region with a peak efficiency of 80%.

Quantitative analysis on microstructure evolution of Ti-6Al-2Zr-2Sn-2Mo-1.5Cr-2Nb alloy during isothermal compression

Isothermal compression tests of Ti-6Al-2Zr-2Sn-2Mo-1.5Cr-2Nb alloy were conducted at a Gleeble-1500 simulator in deformation temperature range of 1103–1243K, strain rate range of 0.01–5.00 s-1and height reduction range of 50 %–70 %. The effects of processing parameters on morphology, grain size and contents of a and b phases were discussed based on the quantitative microstructure examination, and the detailed explanation was shown. The results show that b transformed matrix will obviously grow up at higher deformation temperature or lower strain rate because of low grain growth activation energies. The content of a phase will decrease at higher deformation temperature or higher strain rate due to the phase transformation. Some elongated a or b grains exist at higher strain rate, implying that the dominant softening mechanism is dynamic recovery. The effect of height reduction on b transformed matrix is negligible, but the height reduction has some effects on the morphology of primary a phase.

Grain growth behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy during isothermal β heat treatments

Isothermal β heat treatments of Ti-6.5 Al-3.5 Mo-1.5 Zr-0.3 Si alloy were performed at the temperature of1040-1240 ℃ to examine the influence of heating conditions on grain growth of the alloy. The results show that the grain size increases with heating temperature and holding time increasing. Rapid β grain growth of the alloy takes place at the temperature of over 1140 ℃. The grain growth kinetics for the alloy follows the classical isothermal grain growth law.The growth time exponent(n) of 0.5651 and activation energy(Q) of 129.6 kJ mol-1 are determined. Finally, in order to determine the grain size under different heating conditions,the grain growth model of the alloy was established.

Microstructure evolution and mechanical properties of heat treated LCB titanium alloy

The influence of ageing time on microstructure and mechanical properties of low-cost beta （LCB） titanium alloy with a chemical composition of Ti-6.6Mo-4.5Fe-1.5Al was investigated. The correlation between microstructure and fatigue crack initiation and growth was also studied. Increasing ageing time tended to increase the volume fraction of the secondary α-precipitates, β-grain size and partial spheroidization of primary α-phase. The maximum tensile strength （1565 MPa） and fatigue limit （750 MPa） were obtained for the samples aged at 500 °C for 0.5 h, while the minimum ones of 1515 MPa and 625 MPa, respectively, were reported for the samples aged at 500 °C for 4 h. The samples aged at 500 °C for 4 h showed a transgranular fracture mode. However, the samples aged at 500 °C for 0.5 h revealed a mixture fracture mode of transgranular and intergranular. The formed cracks on the outer surface of the fatigue samples were found to propagate through the β-grains connecting the primary α-particles existing at the β-grain boundaries.

Influences of residual stress and micro-deformation on microstructures and mechanical properties for Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy produced by laser powder bed fusion

Laser powder bed fusion(LPBF)yields unique advantages during the fabrication of titanium alloys.In the present work,Ti-6.5 Al-3.5 Mo-1.5 Zr-0.3 Si alloy specimens with excellent mechanical performances were fabricated by LPBF.The as-built specimens displayed relatively high strength and ductility under modest volume energy densities(VEDs),whereas they manifested high strength with low ductility under high VEDs.To investigate the key reason of this phenomenon,the specimens were designed with two VEDs ranges of 60 J/mm^(3) and 85 J/mm^(3).Special attention was paid to the influences of residual stress and micro-deformation on microstructures and mechanical properties for the first time.The results indicated that the residual stresses and relative density of the 60 J/mm^(3) range specimens were higher than that of the 85 J/mm3 range specimens.Dislocation multiplication and dislocation movement promoted by the residual stress were hindered by the initialα’phase grain boundary(prior-α’GB),leading to the formation ofα’metastable structures.The mean tensile strength and elongation of the 60 J/mm^(3) range specimens were 1248.1 MPa and 12.3%,respectively,whereas the corresponding values for the 85 J/mm^(3) range specimens were 1405.3 MPa,5.0%,respectively.During deformation,the strength and ductility of the specimens were first improved by lamellar structures generated from prior-α’phases,and then effectively enhanced by the interaction between the{10–12}twins and dislocations.However,pores significantly reduced the ductility;hence,high VED specimens with large twins and numerous large pores increased the strength and reduce the ductility.

A method for prediction of unstable deformation in hot forging process by simulation

A method is proposed for prediction of the unstable deformation in hot forging process using both the determined thermomechnical parameter windows of the unstable deformation zones and finite element simulation. Taking Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy as the testing material, the thermomechnical parameter windows of the unstable deformation zones for the Ti-alloy are integrated into a commercial finite element simulation software platform. The distribution and variation of the unstable deformation zones of the alloy in hot compression process are simulated and predicted using the tailor-made finite element codes in the finite element platform. The simulation results tally with the physical experiments and the proposed method for simulation and prediction of the unstable deformation is thus verified and its efficiency is validated.