Innovative laser additive manufacturing technique was employed to successfully repair a titanium impeller, addressing challenges related to material integrity and complex geometry. The utilization of a positive-pressure chamber ensured proper gas shielding, resulting in multiple shiny silver layers. The added layers exhibited optimal hardness, matching that of the base metal since the laser additive manufacturing created a deposited layer with minimal Heat Affected Zone (HAZ) and a distinctive Widmanstätten microstructure in the fusion zone. This project showcases advancements in precision and efficiency for titanium impeller repairing.
Research Highlights
- The hardness achieved through laser additive manufacturing of Ti6Al4V matched that of the base metal, providing same characteristics as base metal for the intended application.
- Laser additive manufacturing produced layers with a minimal Heat Affected Zone (HAZ) and showcased a Widmanstätten microstructure in the fusion zone of the titanium impeller.
Challenges
Implication of Laser Additive Manufacturing for repairing titanium Impellers encounters challenges akin to those faced in the laser materials processing of titanium alloys, primarily centered around the critical issue of inadequate shielding during the process. Titanium’s vulnerability to oxidation, particularly beyond 425 degrees Celsius, emphasizes the need for sustained and sufficient gas shielding even after the melt pool has solidified. This challenge is exacerbated by the complex geometry of the impeller blades, which introduces some challenges in ensuring uniform added layers on each damaged blade. The project grapples with the imperative need to develop a shielding mechanism that effectively safeguards the titanium impellers throughout the additive manufacturing process, addressing both the material’s sensitivity to oxidation and the intricate design intricacies of the impeller blades.
Our Solution
The damaged area in a worn-out impeller machined to reach the functional base metal, and laser additive manufacturing was used for building up on those machined parts to meet the original geometry of the part. To apply additive manufacturing for the repair of a titanium impeller, several key steps were undertaken. First, recognizing titanium’s susceptibility to oxidation above 425 degrees Celsius, a trailing shielding mechanism was devised, illustrated in Figure 2. The additional layers created through this shielding approach appeared in pale yellow, deemed acceptable clads according to ASME standards (as indicated by their prescribed colors), but not sufficient for this application.
Nevertheless, the brittleness and hardness of materials is higher than the base metal when a pale-yellow clad surface is observed, prompting the development of an alternative shielding mechanism—a dedicated positive-pressure chamber. Utilizing this chamber, the laser additive manufacturing process was deployed for repairing the impeller while addressing the challenges associated with material brittleness and hardness.
Following the completion of the process, multiple shiny silver clads were produced, showcasing a microstructure detailed in Figure 3-A. This microstructure reveals the presence of α islands within the β background, commonly referred to as Widmanstätten in the literature. Figure 3-B presents a macrograph of one of the added layers. Remarkably, the Heat Affected Zone (HAZ) exhibited a minimal thickness, approximately 200 µm, owing to both the low heat input inherent of the process and the low heat conductivity of titanium grade 5 (7.1 W/m.K).
The hardness profile of the repaired impeller is depicted in Figure 4. Notably, the added layer exhibits higher hardness compared to the base metal, despite both being composed of the same Ti6Al4V material. It is essential to highlight that the hardness of the Heat Affected Zone (HAZ) is lower than both the added layer and base metal. This deviation is attributed to the tempering effect induced by the heat flow, leading to an increase in grain size in the affected area. The observed 200 µm thickness of the HAZ, as evident in the micrograph photos, is corroborated by the information provided in this hardness profile.
Outcomes
- Laser additive manufacturing process was used for repairing a complex geometry titanium impeller.
- Using a positive-pressure chamber, multiple shiny silver layers of titanium were deposited, ensuring effective gas shielding throughout the process.