Defect formation mechanism of laser additive manufacturing joint for large-thickness Ti6Al4V titanium alloy with Y2O3 nanoparticles

Abstract

Laser additive manufacturing is a method, through which parts are joined by laser melting deposition (LMD) layer by layer as opposed to the conventional welding processes. It presents superior advantages in joining large-size structural parts in aerospace due to its flexibility in technology and equipment. Owing to the large size of the welded joint and the complex welding process, the defect is easily produced in joining large structural parts. Given its detrimental influence on mechanical properties, defects are a major problem for the connection of large thickness components. This study aims to explore the formation mechanism of defects in joining 80 mm thick Ti6Al4V titanium alloy using laser additive manufacturing. The microstructure and element distribution detection of the welded joint is analyzed to verify the defect formation mechanism. It is indicated that defects during laser melting deposition (LMD) of large thickness Ti6Al4V titanium alloy can be classified into two types, porosity and lack of fusion (LOF). It is revealed that the irregular-shaped pores with a size of 1–5 μm can be observed in the joint area without nanoparticles due to larger dendrite spacing. Additionally, nanoparticles can reduce the porosity, but excessive content of nanoparticles will lead to metallurgical porosity, which decreases the performance of the weld joint. The LOF defects are caused by insufficient energy, which mostly appears between the layers or near the fusion line and has sharp edges.