Research On Key Technique Of Laser Deposition Repair Titanium Alloy

Laser deposition repai（rLDR）is a kind of novel remanufacturing technology based onthe development of laser cladding and rapid prototyping technology, which uses laser heatto combine the cladding layers and substrate with metallurgical bonding, and restore itsdimensions and mechanical properties. It has an extensive application prospects since threedimensional near-net-shape dense metal components can be directly remanufactured. Inthis paper, the transient temperature field, microstructural characterization, control methodwith magnetic field and as well as mechanical properties of laser deposition repair titaniumalloy have been analyzed and discussed combining methods of FEM numerical simulation,experimental study and theoretical analysis. On the basis of building laser deposition repairsystem, the research on key technique of repair typical damage of titanium alloy structurewas conducted, and the following contents have been systematically studied such as laserdeposition process, the microstructure and internal quality control and mechanicalproperties. It will have an important theoretical value and practical significance not onlyfor rich solidification theory but also for promoting the development and the technology inindustry.In view of active chemical properties of titanium alloy, it has a strong affinity foroxygen, nitrogen and carbon in air, so the experimental system is consist of six modulessuch as laser, dynamic inert gas protection system, motion control system, powder feedingsystem, magnetic stirring auxiliary system and the control software. Then the keycomponents powder nozzle and the electromagnetic stirring device are designed.Three-dimensional finite element model of powder-delivery transient temperaturefields in LDR has been built up and relatively comprehensive numerical simulation hasbeen carried out with parametric programming methods, and the laser process parameterson the influence of the temperature field in LDR process and the variation rule of thedistribution of temperature gradient have been analyzed. Simulation results indicated thatthe temperature gradient were extremely high，reached105~106℃/m, also it can be find that temperature fields of the deposition layers and the substrate changed periodically anddynamically and points in different positions presented different thermal cyclescharacteristics and change trends of temperature gradient, which can provide a theoreticalbasis for study of microstructure and defects formation.The macrostructural and microstructural characterization of laser deposition repairTA15titanium alloy were systematically studied. The process experiment of laser meltingand laser deposition were carried out and the optimal process parameters were obtained,which are used to laser repair typical titanium alloy component damage and the functionalgradient sample. The forming mechanisms of titanium alloy microstructure were deeplyanalyzed according to the temperature field simulation results and approximation rapidsolidification theory. The microstructure of laser deposition repair TA15titanium alloy hasthe typical characteristics of epitaxial growth and it is mainly consisted of β columnarcrystal with0.5mm~1.5mm width and1mm~5mm height, which is coarse and throughoutmany deposition layers. The repaired component experienced a continuous microstructuraltransition from substrate duplex microstructure with equiaxed α and lamellar α/β in therepaired substrate to the epitaxial coarser columnar β with basket weave α/β throughheat-affected zone. In the heat affected zone the microstructure changed from bimodalstructure to the basket weave structure gradually, and the repaired zone is characterized bya fine α/β lamellar microstructure with various random orientations, and the width of αphase is about0.4~0.5μm, which should be related to the rapid solidification and phasetransformation during the process. The Al, Zr, Mo and V elements were uniformlydistributed without fluctuation and segregation from the substrate to the repaired zone forlaser rapid solidification characteristics, and there is a dense metallurgical bond betweenthe repaired zone and the substrate with little defects such as porosity and ill bonding. Themicrostructure of laser deposition repair functional gradient sample was investigated, Theresults showed that the in-situ TiC reinforced phase distributes on deposition layers evenlyin the form of big dendrite, large graininess and little graininess structure along thegradient direction.In order to control the laser deposition repair microstructure, electromagnetic stirringis introduced to the LDR system. The effects of magnetic field on the macrostructures,microstructures and micro-hardness of the laser repaired zone are studied by changing itsintensity or rotating speed. The results show that with more strong intensity or higherrotating speed of magnetic field α lamellar structure length is reduced and its thickness is about0.4~0.5μm in certain parameters. And the micro-hardness of repaired zone isimproved with fine α phase.Distribution of LDR parts micro-hardness shows an increasing trend from substrate,heat affected zone and repaired zone. Tensile test at room temperature indicates that tensilestrength and plasticity of LDR parts is approaching to the forging TA15alloy, andannealing has no effect on repaired sample tensile strength. Charpy impact test of groovedamage sample shows that its has lower impact toughness than substrate, but its hightensile strength indicates substrate and repaired zone have good bonding strength. The laserdeposition repair functionally graded sample has good wear resist property at room.Theincrease of micro-hardness and good wear resistant property is due to the fine grain sizeand dispersion strengthening.