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Phase transition, microstructural evolution and mechanical properties of Ti–6Al–4V and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si joints brazed with Ti–Zr–Ni–Cu filler metal

Yang Zhenwen, Chen Yuhan, Niu Shiyu, Wang Ying, Han Yajing, Cai Xiaoqiang, Wang Dongpo

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 3

353--367

EN

Ti–6Al–4V (TC4) and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) joints were achieved via Ti–37.5Zr–15Ni–10Cu (wt%) filler metal when the brazing temperature was in the range from 950 °C (below β-transus) to 1040 °C (above β-transus) for 10–60 min. The role of brazing parameters in the microstructure evolution as well as mechanical properties of both base alloys and brazed joints was studied. The research analysis suggested that the typical interfacial microstructure was divided into five characteristic zones including reaction phases of α-Ti, β-Ti and (Ti, Zr)2CuNi. With the holding time prolonged, the sectionalized structure transformed into lamellar Widmanstätten structure in the brazing seam at the temperature of 950 °C, and the optimized shear strength reached 616 MPa at 950 °C for 60 min. Nevertheless, when increasing the brazing temperature to 1000 °C, the joint shear strength tended to be stable as the holding time exceeded 20 min due to the elemental homogenization, and the shear strength reached 627 MPa for the holding time of 20 min. Tensile test results showed that the mechanical properties of both TC4 and TC11 alloys were dramatically degraded at the heat treatment temperature of 1000 °C owning to the drastic grain coarsening and phase transition. Additionally, the plastic strain of TC4/TC11 joint brazed at 1000 °C for 20 min was 1.66%, while that of joint brazed at 950 °C for 60 min reached 2.01%. The variation in mechanical properties of base alloys as well as brazed joints under different thermal conditions revealed that the optimized temperature for brazing of titanium alloys was lower than β-transus with a long time.

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Relief of residual stress in Al2O3/Nb joints brazed with Ag-Cu-Ti/Cu/Ag-Cu-Ti composite interlayer

Zhao Yating, Wang Ying, Yang Zhenwen, Wang Dongpo

Archives of Civil and Mechanical Engineering

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2019

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Vol. 19, no. 1

1--10

EN

Ag-Cu-Ti/Cu/Ag-Cu-Ti composite interlayer was successfully designed to braze Al2O3 ceramic and Nb. The effect of the addition of Cu interlayer with various thicknesses on the microstructure, residual stress and mechanical properties of the brazed joints was investigated by finite element modeling (FEM) computation combined with experimental verification. The results showed that the layered Ag-Cu-Ag solid solution structure formed in the Al2O3/Nb brazed joints when the composite interlayer was used. Moreover, the thickness of TiO + Ti3Cu3O reaction layers adjacent to the Al2O3 ceramic substrate did not change obviously regardless of the Cu foil thickness. The maximum residual stress in the whole brazed joint always appeared in the Al2O3 ceramic substrate nearby the interlayer, but it was reduced from 384 MPa to 119 MPa when a 150 μm thick Cu foil was added. The variation of calculated residual stresses as a function of Cu foil thickness, which was verified by X-ray measurement, exhibited a consistent with Al2O3 ceramic strain energy. Thus, the calculation of Al2O3 ceramic strain energy could be a good criterion to evaluate the joint shear strength because the fracture occurred in the Al2O3 ceramic. The reduction of detrimental residual stress was primarily attributed to the increased plastic strain energy of Cu interlayer. The FEM and experiment results indicated that the ability of plastic deformation of the interlayer played a key role in determining the residual stress in the brazed joint, providing a method for improving the bonding properties of ceramic and metal.

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Microstructure and mechanical properties of Ti2AlNb alloy and C/C composite joints brazed with Ag–Cu–Zn and Ag–Cu–Zn/Cu/Ag–Cu–Ti filler metals

Hao Zhitao, Wang Dongpo, Yang Zhenwen, Wang Ying

Archives of Civil and Mechanical Engineering

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2019

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Vol. 19, no. 4

1083--1094

EN

C/C composite was successfully joined to Ti2AlNb alloy with inactive filler metal, Ag–Cu–Zn filler. In addition, a designed composite interlayer, Ag–Cu–Zn/Cu/Ag–Cu–Ti, was used to improve the properties of brazed joints. AlCu2Ti compound blocks were formed in brazing seam and the double reaction layers adjacent to C/C composite were AlCu2Ti and TiC when Ag–Cu–Zn filler was used to join C/C composite and Ti2AlNb. With the increase of brazing temperature and time, the amount of AlCu2Ti compound blocks increased and tended to accumulate together nearby the C/C composite, the double reaction layers thickened as well. In order to avoid the serious accumulation of brittle AlCu2Ti compound nearby C/C com-posite, Cu foil was added between the Ag–Cu–Zn filler and Ti2AlNb. Soft Cu but not brittle AlCu2Ti existed nearby the C/C composite with the addition of Cu foil, which is beneficial for the relief of residual stress near the C/C composite, and the shear strength of joint improved obviously. With the barrier of Cu foil, the thickness of reaction layers adjacent to C/C composite could be controlled easily by the addition of Ag–Cu–Ti foil. Correspondingly, the shear strength of this joint reached at 28 MPa, which was close to that of C/C itself.

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Effect of ultrasonic impact treatment on the microstructure and mechanical properties of diffusion-bonded TC11 alloy joints

Yang Zhenwen, Liu Qi, Wang Jiahui, Ma Zongqing, Wang Ying, Wang Dongpo

Archives of Civil and Mechanical Engineering

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2019

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Vol. 19, no. 4

1431--1441

EN

To derive the higher mechanical properties of diffusion-bonded joints at a low bonding temperature, ultrasonic impact treatment (UIT) was used to increase the dislocations and refine the microstructure of faying surfaces prior to diffusion bonding. The results show that a deformation layer with a thickness of 30 mm formed near the impacted TC11 alloy surface, with many dislocation tangles, substructures, and dislocation cells in this layer. Diffusion bonding experiments were performed for both original samples and UIT samples. For the original TC11 alloy, the average shear strength of the diffusion-bonded joints increased with an increase in the bonding temperature, and the joint strength was 550 MPa for joints bonded at 820 8C for 30 min under a pressure of 10 MPa. However, the average shear strength of the TC11 alloy joints with UIT reached 560 MPa for the joint that was diffusion-bonded at 780 8C, whereas it was only 370 MPa for the joint without UIT. Therefore, UIT of the alloy surface resulted in a joint with a high strength at a relatively low bonding temperature, which may be beneficial to inhibit the grain growth caused by the high temperature bonding process.