Investigation Of Laser Clad Ticralsi System Multi-principal Element Alloys On Titanium Alloy

Titanium alloys play an important role in the field of aerospace, aviation, ships and weapons etc. because of their characteristics of low density, high specific strength, good fatigue and creep resistance, and excellent corrosion resistance. Nevertheless, their applications at high service temperature and in wear-intensive situations, have been limited by the poor tribological properties, insufficient oxidation resistance and risk of combustion. Surface modification is an attractive method to solve the aforementioned problems, while combining with the required bulk mechanical properties. Laser surface technology, characterized by an extremely high heating/cooling rate, allows the precise adaptation of surface properties. Multi-principal element alloy, containing several main elements at near equiatomic concentrations, instead of a single major element for the traditional alloy, is a new concept in alloy design. By appropriate selection of the alloying elements, these alloys can have their properties tailored to the desired performance, such as high hardness, good thermal stability, excellent wear, oxidation and corrosion resistance. Due to these properties, multi-principal element alloys are also considered attractive coating materials for enhancing surface behaviour.The purpose of the present study is to develop multi-principal element alloy coatings that improve simultaneously the oxidation resistance, tribological properties and burn-resistance of titanium alloys. The TiCrAlSi-V and TiCrAlSi-Ni multi-principal element alloys are deposited by laser cladding on Ti-6Al-4V substrate. The processing parameters, microstructure, thermal stability, tribological properties, oxidation behavior and burn-resistance are investigated. The CALPHAD method is employed to assist the analysis of phase constitution and burn-resistance. The major work completed and the results obtained are as follows:(1) TiCrAlSi-V and TiCrAlSi-Ni multi-principal element alloy coatings with few cracks and pores are obtained on Ti-6A1-4V substrates by using the optimized processing parameters. (2) SEM, XRD and EDS analysis show that, the as-clad TiCrAlSi-V coating is composed of (Ti,V)5Si3and a BCC solid solution. After annealing at800℃for24hours under vacuum, a new phase Al8(V,Cr)5precipites. The as-clad TiCrAlSi-Ni coating is composed of (Ti,Cr)5Si3and NiAl. After annealing at800℃for24hours under vacuum, there is no apparent change in phase composition. The results demonstrate that not all multi-principal element alloys form simple FCC or BCC solid solution phases, which are emphasized by many researchers.(3) A thermodynamic database for TiCrAlSi-V and TiCrAlSi-Ni multi-principal element alloys is established. Based on the database, the temperature-dependent equilibrium phase diagram for the coating materials are calculated by using the CALPHAD method. And the results are validated by experiment. The experimental phase composition for TiCrAlSi-V alloy agrees well with the thermodynamic calculations, while there is a small inconsistency for TiCrAlSi-Ni alloy.(4) After vacuum annealing, there is a small increase of hardness for the laser clad TiCrAlSi-V coating, which is due to the formation of Al8(V,Cr)5. There is no apparent difference of hardness for the TiCrAlSi-Ni coating before and after the heat treatment. Both coatings are much harder than the Ti-6A1-4V substrates.(5) The dry sliding wear tests showed the wear resistance of Ti-6Al-4V is significantly improved after laser clad TiCrAlSi-V and TiCrAlSi-Ni multi-principal element alloy coatings. At different frequencies, the specific wear rates of the coatings are much lower than that of the substrates. Especially for the TiCrAlSi-Ni coatings, they reduce the specific wear rates by two orders of magnitude. The friction coefficients of the TiCrAlSi-V coatings are much lower than that of the Ti-6A1-4V substrates. The Ti-6Al-4V substrates suffer adhesive wear and severe abrasive wear. The worn surface morphologies of the TiCrAlSi-V and TiCrAlSi-Ni multi-principal element alloy coatings indicate that, the test occurred in the mild wear regime, the main material loss mechanism includes oxidation, slight adhesive material transfer and fragmentation and spalling of the adhered material, as well as oxides.(6) The oxidation tests of Ti-6A1-4V alloys and the multi-principal element alloy coatings show that the laser clad TiCrAlSi-V and TiCrAlSi-Ni multi-principal element alloy coatings can effectively improve the oxidation resistance of Ti-6A1-4V alloys at800℃in air. The oxide scale formed on TiCrAlSi-V consists of SiO2, Cr2O3, TiO2, Al2O3and a small amount of V2O5, while the oxide scale on TiCrAlSi-Ni consists of Cr2O3, Al2O3, a small amount of NiO and SiO2. Both scales are continuous and dense, without any cracks and pores.(7) The burn resistance is investigated by calculating adiabatic temperature. The adiabatic temperatures of the TiCrAlSi-V and TiCrAlSi-Ni multi-principal element alloys are lower than Ti-6Al-4V alloy, indicating good burn resistance of the coatings. A primitive experimental evaluation on the burn resistance is conducted by laser ablation. The result agrees with the theoretical analysis.