The Effect of Direct Laser Deposition Process Parameters on Microstructure and Mechanical Properties of Ti-6Al-2Sn-4Zr-6Mo

Blown powder Direct Laser Deposition (DLD) is a type of Additive Manufacturing (AM) that is of interest to the aerospace industry as a method of performing high-integrity repairs of critical components. The properties of the deposited material are largely influenced by process parameters such as beam power, velocity, hatch spacing, beam radius and powder feed rate. It is critical for a high-quality repair, that the effect of these process parameters on the solidification microstructure and hence the mechanical properties are fully understood.
The work presented here focuses on quantifying the effect of process parameters on DLD of the α+β titanium alloy Ti-6Al-2Sn-4Zr-6Mo (Ti-6246). This alloy demonstrates high strength and good corrosion resistance and is a suitable replacement for Ti-6Al-4V in aerospace applications. This is due to its ability to perform at higher temperatures which is important as gas turbine engines push towards higher efficiencies and hence elevated operating temperatures.
A Design of Experiment (DoE) was used to map a potential process window that would be suitable for Ti-6246 DLD repair of compressor bladed disks (Blisks). The aim was to identify combinations of process parameters that resulted in a fully-dense defect-free build that produced repeatable mechanical properties comparable to the parent Ti-6246 blisk material.
Ten deposits were built with five different parameter sets using an RPM 557 laser deposition machine. Tensile specimens were machined from the build for uniaxial tensile testing. Small sections of each build were also retained for microstructural analysis, with the aim to correlate process parameters with the size of the resultant α+β lamellar microstructure. The α-lath width was found to generally increase with decreasing line energy density (beam power divided by velocity), although the effects of additional process parameters such as powder feed rate is also important and the influence of this is also explored.
The results from this work were used to determine response surfaces relating process inputs such as energy density to process outputs such as 0.2% yield stress. These were then used to provide recommendations for future work with the aim of optimizing the DLD process window for Ti-6246 as a suitable repair method.
The experimental work was supported by the development of a thermal model. This helped to inform how process parameters influenced the laser deposition conditions. The thermal model was calibrated against a thin-wall aerofoil-type build and reasonable agreement was found between predicted and measured melt depths for a range of process parameters. The thermal model also can help to provide predictions about the how further optimisation of the process window may affect mechanical properties.
Some of the key findings and outcomes of this work are:
• Development of an automated process to measure the size of Ti-6246 α+β lamellar microstructure produced by DLD. This automated process was validated using manual measurement techniques and was found to be a robust and trustworthy method that significantly decreases the time to gather microstructural data.
• Size of the α-laths were generally found to be <1µm, apart from a dendritic zone at the top of each of the builds which has remained fine due to lack of coarsening from repeated thermal cycles.
• Definition of a process window for the DLD of Ti-6246 which can produce dense builds with minimal defects (as revealed by both SEM and XCT analysis).
• Testing of Ti-6246 DLD builds showed mechanical properties (tensile strength, 0.2% yield stress and elongation) comparable to parent forged material and within requirements set by Rolls-Royce for repair purposes.
• Linear regression and response surface analysis showed that laser beam velocity (v) had the most effect on mechanical properties, particularly the 0.2% yield stress. Hatch spacing had little to no quantifiable effect on the mechanical properties.
• Recommendations for process optimisation and productivity gains include increasing the hatch spacing and/or beam velocity to increase productivity.
• Development of a Gaussian-based thermal model used to define a new parameter – melt pool saturation level (MPSL), this being the ratio between melting capacity of the laser and the actual amount of material being melted during the DLD process.
• The MPSL was used to calculate an upper limit to the PFR and DLD process inputs were used to define a lower limit or “aspirational” PFR. Hence, the model developed in this work is useful in an industrial setting as it can reduce the number of test deposits needed to down-select the best process parameters and therefore define a suitable process window.