Process Optimization of Wire Based Laser Metal Deposition of Titanium

Abstract

Titanium alloys are used instead of steel and nickel-based alloys to lower the weight of turbines whenever it is applicable. Due to the high manufacturing costs of titanium, near-net-shape processes like laser metal deposition (LMD) processes are an approach to improve the production of new turbomachinery components. Additionally, these processes are also suitable for repair. LMD uses wire or powder as additional material. When highly reactive materials like titanium grade 5 (Ti6Al4V) are processed, wire-based laser metal deposition (LMD-W) processes are superior to powder-based processes due to the smaller reactive surface. Nowadays, three main challenges exist when titanium grade 5 (Ti6Al4V) is processed by additive manufacturing (AM): First of all the high affinity to oxygen combined with the increased brittleness of the material in case of a contamination with already low amounts of oxygen has to be faced. Secondly, the material is prone to distortion induced by thermal stress during the manufacturing process. Finally, the material has a complex bimodal microstructure, which has to be adjusted properly to generate optimal strength. The following publication will present how these technical challenges are faced. A local shielding gas concept demonstrates how flooding of the process chamber was avoided. The distortion was lowered by minimizing the heat input. Therefore, the laser spot was adapted. Its size was reduced to physical minimum nearly matching the size of the wire. To avoid process aborts, the proper feeding of the wire was improved. With this optimized process, it was possible to generate several specimens for metallurgical analysis. Finally, treatments to modify the alpha-martensitic-structure into a bimodal structure were performed. Summarizing the results show that the LMD-W process was improved to overcome the main challenges. Thereby the process has become suitable for manufacturing turbomachinery components made from titanium grade 5.