Effect of addition of micron-sized lanthanum oxide particles on morphologies, microstructures and properties of the wire laser additively manufactured Ti-6Al-4V alloy

The bead geometry, microstructure and mechanical properties of wire laser additively manufactured (WLAM) Ti-6Al-4V samples reinforced by different contents of micron-sized lanthanum oxide (La2O3) particles were explored. The images attained through high-speed charge coupled device (HCCD) and metallographs showed that the addition of La2O3 changed the morphologies of the weld bead of Ti-6Al-4V alloy to lead to the significant reduction of the width of the deposited weld bead and remarkable growth of the weld penetration. The observation result attained through the scanning electron microscope (SEM) revealed that relative to the condition without the addition of micron-sized La2O3 particles, nano-sized La2O3 particles were uniformly distributed in the matrix after adding micron-sized La2O3 particles; beta grains and alpha phases were both greatly refined. Compared with the primitive La2O3 powders, the morphology of La2O3 in the matrix approximately presented a spherical shape. It indicated that La2O3 particles in the molten pool partly or completely decomposed during WLAM. The dispersedly distributed nano-sized La2O3 particles induced fine grain strengthening and dispersion strengthening. Therefore, the tensile strength and Vickers hardness of the deposit of the Ti-6Al-4V alloy rose with increasing addition amount of La2O3. However, the decomposition of La2O3 particles resulted in the formation of interstitial atoms (such as interstitial O atoms), thus leading to the decrease of the elongation of deposits. Without the addition of La2O3 particles, the deposit of the Ti-6Al-4V alloy fractured in the large-size lamellar alpha during the tensile test, in which the fracture showed micro-morphology of ductile fracture. After the addition of La2O3 particles, the deposit of Ti-6Al-4V alloy fractured at the grain boundary alpha phase during the tensile test and both cleavage and ductile morphologies were found at the fracture.