Laser Additive Manufacturing of Titanium Aluminides for Turbomachinery Applications

Abstract

The defect-free processing of TiAl alloy TNM™-B1 by means of Laser Powder Bed Fusion (LPBF) is demonstrated by manufacturing of an automobile turbocharger wheel. Similar precision cast material was used as reference. TNM™-B1 was manufactured crack free with a density > 99.5% using elevated process temperatures above the brittle-to-ductile transient temperature (BDTT). The preheating temperature was provided by an induction preheating system. To minimize oxygen pick up during the LPBF process, the process atmosphere was actively cleaned using a gas-purification system. Produced test samples were analyzed in as-built and heat-treated condition regarding density, micro structure and phases by means of a Light Optical Microscope (LOM) and Scanning Electron Microscopy (SEM). Micro hardness was measured according to Vickers. Oxidation measurements were performed by means of carrier-gas hot extraction. Mechanical properties were determined using room temperature tensile tests. The final automobile turbocharger wheel was analyzed for defects using a Micro-Computer Tomography scanner (MCT). Besides bulk test samples, thin-walled specimens can be manufactured with sufficient density. Depending on the process parameters, an oxygen content < 1000 ppm could be reached. The as-built microstructure consists of lamellar (α2+γ) colonies and nearly globular γ as well as β/β0 at the grain boundaries. High cooling rates in the magnitude of 105 to 106 K/s provide small grain sizes of 1–7 μm. Hardness measurements reveal an increased hardness (515-560HV0.3) compared to cast material (390HV0.3). Samples for tensile tests show tensile strength around 840 MPa and a total elongation of 1.1% for LPBF-manufactured and hot isostatic pressed (HIP) samples. The CT analysis of the turbocharger wheel confirms that complex geometries made of TiAl can be additively manufactured free of cracks.

Influence of Combined Heat Treatment and Hot Isostatic Pressure (HT-HIP) on Titanium Aluminide Processed by L-PBF

“…The last two cycles were conducted under high temperature and pressure to generate compressive stresses that help to close micropores and cracks generated during the L-PBF. The heat treatment and isostatic pressure conditions were determined according to previous studies [ 11 , 37 ].…”

Optimization potentials of laser powder bed fusion: A conceptual approach

“…Areal and point EDS inspections were conducted on the virgin powder and samples, yielding favorable results. However, production in the automotive industry does not only require turbine blades, but other components around the mixture charging of the exhaust side are also under high-temperature stress and require flow-optimized structures [116]. This type of application is also suitable for self-fluxing Ni-based alloy coatings as they are widely used under conditions of wear, corrosion, and high temperatures [117].…”

Laser based manufacturing of titanium aluminides

“…The maximum build height tested is 60 mm, the maximum width 10 mm. In [20] is demonstrated the successful build of a complete automobile turbocharger wheel out of TNMTM-B1 with LPBF. In the present study, process parameters were developed to produce equally dense and crack free parts out of GE4822 and 4522XDTM.…”

“…On selected samples, pressure strength tests were performed at 700°C results of which are presented in Figure 5. [8,20]. It is assumed, that further decrease in oxygen during the process chain could be favorable to increase the mechanical properties, which therefore is a remaining challenge for LAM not only of TiAl but several intermetallic materials.…”

“…Current research includes EBM of e.g. GE4822 [9,19], Laser powder bed fusion (LPBF) of TNMTM [8,20], and Direct energy deposition (DED) of GE4822 and TNMTM [7,10,21]. Potential in Laser additive manufacturing is provided by the possibility to evade the necessity for a costly vacuum atmosphere and build on already 3-dimensional structures.…”