Control Of Microstructure And Properties Of Wire And Arc Additive Manufactured Titanium Alloy With An Heterogeneous In-situ Enhanced Structure

Titanium alloy is an important structural material in the aerospace industry.Additive manufacturing technology is a new way of rapid and low-cost manufacturing of titanium alloy integral structural parts,which promotes the development and production of modern aircraft and other major equipment.The major problem of additive manufactured titanium alloy is the large temperature gradient and periodic high-temperature thermal cycle,which inevitably leads to the formation of large-size columnar crystals.Promoting the in-situ synthesis of reinforcing phase through microalloying is a simple and effective method to regulate the microstructure of titanium alloy,but the improvement of alloy strength is accompanied by the sacrifice of plasticity.Therefore,the synergistic improvement of strength and plasticity has become a key problem.To solve this problem,this paper proposes an in-situ synthesis of heterogeneous structure reinforced titanium alloy in the process of wire and arc additive manufacturing,and realizes the synergistic improvement of strength and plasticity of titanium alloy by adjusting the distribution structure of reinforcing phase.Firstly,a cold metal transition(CMT)arc was used to fabricated Ti-6Al-4V titanium alloy with trace boron.Due to the macro segregation of boron during solidification,the number of Ti B whiskers synthesized in situ gradually increased from the bottom to the top of a single deposition layer,forming a gradient cyclic reinforced structure in the whole component.The strength of titanium alloy with gradient cyclic reinforced structure increased by 17% to 1089 MPa,and the elongation remained at 8%without significant reduction.After adding up to 0.15 wt.% boron,the Ti B whiskers synthesized in situ still maintained the distribution structure of the cyclic gradient.With the further addition of boron,the strength of titanium alloy increased and the plasticity decreased.The strength of Ti-6Al-4V-0.15 B alloy reached 1178 MPa and the plasticity decreased to 5.2%.In order to make the additive manufacturing components have better forming accuracy and more boron addition,tungsten inert gas shielding(TIG)arc combined with interlayer adding boron powder was used to prepare laminated alternating structure reinforced titanium alloy,and the addition amount of boron was up to 0.45 wt.%.The laminated alternating structure reinforced titanium alloy was composed of pure titanium alloy layer and Ti B whisker reinforced layer alternately.The interface between layers was clear and the combination was intact.The strength of titanium alloy was increased by more than 16%,and the elongation in vertical and horizontal directions were 7% and4%,respectively.Then,the feasibility of adjusting the thickness of the laminated structure by adjusting the wire feeding speed was explored.Finally,the titanium alloy with uniform distribution of Ti B whiskers was fabricated using a TIG arc.The reinforcement effect in vertical and horizontal directions was only 10% and 7%,and the elongation was reduced to 5% and 3%.Compared with two kinds of heterogeneous reinforced titanium alloys,it was proved that the heterogeneous structure has a better strengthening effect.Based on various strengthening theories,the strengthening mechanism of the heterogeneous structure was analyzed.It is found that fine grain strengthening and load transfer strengthening were the main strengthening mechanism,and the heterogeneous distribution of Ti B whiskers effectively maintained the plasticity of the alloy.Based on the research results,further design ideas of cross-scale heterogeneous structures were prospected.