|
5983 |
|
Wednesday, May 22, 2019, Saal Brüssel 2:40 PM Diffusion brazing and bonding |
|
Reaction-assisted bonding of Ti6Al4V alloy with Ti/Ni reactive nanostructured multilayers and interdiffusion behavior simulation employing molecular dynamics modelling
|
|
Hong Li* / Beijing University of Technology, P.R. China Linpai Yang / Beijing University of Technolog/College of Materials Science and Engineering, China Ying Ma/ Beijing University of Technolog/Institute of Laser Engineering, China Junli Yuan/ Beijing University of Technolog/College of Materials Science and Engineering, China Erika Hodúlóva/ Slovak University of Technology/Faculty of Materials Science and Technology, Slovakia Anming Hu/ Beijing University of Technolog/Institute of Laser Engineering, China |
|
Reaction-assisted bonded interfaces with reactive nano-multilayer film (RMLF) show a high potential for microsystem application due to the low joining temperatures, low bonding pressure and the resulting high mechanical strength of bonded joints. In this paper, Ti/Ni nano-multilayer film were prepared and diffusion bonding of Ti6Al4V alloy were performed with prepared RMLF as interlayer. The effect of Ti/Ni ratios, bonding pressure and bonding temperature on the joint strength was discussed. Molecular dynamics simulation was also applied to reveal the interdiffusion of Ti and Ni at elevated temperatures. Firstly, a Ti/Ni nano-multilayer film composed of alternating Ti and Ni layers with different stoichiometries was deposited onto the surface of polished Ti6Al4V alloy by magnetron sputtering. The nanoscale effect of nano-multilayers successfully reduces the melting point of the film, and the liquid phase can be formed at a lower temperature of 650°C according to DSC results. Then under 5-15 MPa, diffusion bonding processes were performed at 800°C for 60 min. Furthermore, diffusion bonding processes were carried out at three bonding temperatures (800°C, 850°C, 900°C) for 60 minutes under 5 MPa pressure for Ti/Ni nanomultilayer with an atomic ratio of 1:1. The microstructures of the interface and the mechanical performances were analyzed. Firstly, reliable joints can be obtained successfully with all three multilayer systems with different Ti/ Ni ratios (Ti/Ni=1:1, 1:2 and 1:3). A higher pressure and higher Ni content in the multilayers contributed to a higher shear strength. The highest shear strength of 160 MPa was achieved for the joint under 15 MPa using Ti/Ni multilayers with the stoichiometry ratio of Ti/Ni of 1:3 while the hardness of the joint was 6.9 GPa. The intermediate phases appeared to be combinations of hard intermetallics such as TiNi, TiNi3 and Ti2Ni frequently occurred in the Ti-Ni binary system. With regards to the influence of bonding temperature, when the temperature is 800°C, the joint has a low shear strength. However, when the temperature increases to 875°C, the joint strength exceeds 1000MPa for Ti/Ni multilayers with the stoichiometry ratio of Ti/Ni of 1:1 and 1:2. Ni atoms in the film rapidly diffuse into the base metals, which accelerates the gradual composition homogenization of the joint with decreased porosity. At bonding temperature of 900°C, the TLP process produces a strong, interface-free joint with no remnant reacted Ti/Ni nanofoil interlayer. Furthermore, aiming to evaluate the inter-diffusions of Ti and Ni atom, the profiles of mean square displacements (MSDs) of Ti and Ni atoms at different temperatures were first obtained with modified embedded atomic method. The diffusion coefficients and diffusion activation energy of Ti and Ni atoms were calculated by the Arrhenius equation and Einstein equation from MSD profiles. The influence of the Ti/Ni atomic ratio, heating temperature and vacancy on the interdiffusion behavior of Ti and Ni atoms at the interface are studied systematically. The variation of viscosity with temperatures for Ti/Ni RMLF with different atomic ratios were also simulated. The analysis indicates that temperature significantly affects the diffusion behavior of atoms, as the temperature increases, the diffusion coefficient increases gradually. Considering the comprehensive effect obtained by MD simulation, when the atomic ratio of Ti/Ni is close to 1.33, it has a small viscosity thus atoms diffusion can be accelerated, which is beneficial to the self-propagating exothermic reaction. The results could provide a theoretical basis for the modulation structure design and property improvement of Ti/Ni RMLF system. |