Composition Detection And Microstructure Analysis Of Laser Additive Manufacturing Of Titanium Alloys

Titanium alloy has been found various applications in aerospace, ocean andmedical due to its high specific strength, enhanced corrosion resistance, goodmechanical property and excellent biocompatibility. This thesis describes the fiberlaser additive manufacturing system with co-axial powder delivery system developedin the Intelligent Laser Manufacturing Center at Hunan University. We emphasize onthe Ti6Al4V microstructure evolution with different processing parameters and thedetection and analysis of laser induced plasma during Ti-Al binary alloying processusing this fiber laser additive manufacturing system.We design and develop a co-axial powder delivery system including powderdelivering, powder mixing, powder spraying, and the associated gas control andelectrical sensing and control systems. By integrating the co-axial powder deliverysystem with a high power IPG laser and a6-axis ABB robot together with aspectroscopic monitoring system, we develop the fiber laser additive manufacturingsystem. Design of experiment (DOE) is conducted for laser additive manufacturing ofTi6Al4V, from which the optimized manufacturing parameters are concluded.The laser induced plasma during laser additive manufacturing of Ti-Al binaryalloys are monitored using a spectrometer. Spectral line intensity ratio between twoTi-I atomic lines and two Al-I atomic lines are used to form four calibration curves forbinary Ti-Al alloying process with different composition. Linear fitting of thecalibration curve is performed and the best linear correlation coeff icient of fitting is0.9883with the largest error8.9%. Plasma temperature is estimated using Boltzmannplot of five Ti-I atomic lines in390to410nm and electron density is estimated usingStark broadening effect by Voigt fitting of Ti-II ionized line at348.48nm. Bothplasma temperature and electron density with different Ti-Al compositions are fittedusing quadratic fitting. Results show that with the increase of the Ti concentration,both the plasma temperature and the electron density increase, with gradually slowerincrease of plasma temperature and gradually faster increase of electron density.Optical microscopy(OM) and scanning electron microscopy (SEM) are used toanalyze the microstructure evolution of Ti6Al4V alloys with different manufacturi ngparameters, including laser power, scanning speed, powder feed rate and shielding gasflow rate. The results show that1) epitaxial growth of coarse columnar grains along the build direction is obtained at the bottom of the deposit and equiaxed-likemorphology is obtain at the top of the deposit. A transition from columnar grain toequiaxed grain exists around the top of the deposit;2) the microstructure evolves frompure titanium microstructure, lathlike structure, basketweave structure, finemartensite structure and equiaxed structure from the substrate to the top of the deposit.With the increase of laser power, the deposit width increases and the deposit heightfirst increases and then decreases;3) the equiaxed grain increases with coarsermicrostructure inside the grain with the increase of laser power. With the increase ofthe scanning speed, the microstructure inside the grain becomes thinner and longer.With the increase of the powder flow rate, deposit height increases and themicrostructure becomes finer. However, the change of the shielding gas has littleeffect on the deposit microstructure. We also find that mounting method inducesmartinsite transformation of the microstructure due to the extra stress applied duringpolishing. On the other hand, electrolytic polishing method introduces no phasetransformation and reveals the real microstructure of the sample.