Defect-free laser powder bed fusion of Ti–48Al–2Cr–2Nb with a high temperature inductive preheating system

Abstract

In the industrial panorama, laser powder bed fusion (LPBF) systems enable the near net shaping of metal powders into complex geometries with unique design features. This makes the technology appealing for many industrial applications, which require high performance materials combined with lightweight design, lattice structures and organic forms. However, many of the alloys that would be ideal for the realisation of these functional components are classified as difficult to weld due to their cracking sensitivity. γ-TiAl alloys are currently processed via electron beam melting (EBM) to produce components for energy generation applications. The EBM process provides crack-free processing thanks to the preheating stages between layers, but lacks geometrical precision. The use of LPBF could provide the means for higher precision, and therefore an easier post-processing stage. However, industrial LPBF systems employ resistive heating elements underneath the base plate which do not commonly reach the high temperatures required for the processing of γ-TiAl alloys. Thus, elevated temperature preheating of the build part and control over the cooling rate after the deposition process is concluded are amongst the features which require further investigations. In this work, the design and implementation of a novel inductive high temperature LPBF system to process Ti-48Al-2Cr-2Nb is presented. Specimens were built with preheating at 800°C and the cooling rate at the end of the build was controlled at 5°C min −1 . Crack formation was suppressed and apparent density in excess of 99% was achieved.

Additive manufacturing of TiAl-based alloys

“…A conceptual design of a new pre-heating system (Fig. 3) for the LPBF technology was recently experimented with by Caprio et al [27]. With the novel design, the authors were able to maintain the preheating temperature of the base plate and the previously Fig.…”

“…The keyhole effect phenomenon occurs because of excessive energy absorption into the powder bed [46]. Despite the presence of the pores in the TiAl samples the current approach of maintaining a high preheating temperature throughout the manufacturing process as proposed by Caprio [27], have demonstrated that the goal of manufacturing the crack-free TiAl components via LPBF is becoming a reality. The next step is how to produce the crack-free samples without pores.…”

“…The successfully using of the EBM technology to manufacture the TiAl-based alloys is due to the preheating system ( Fig. 2A) employed during the EBM manufacturing process [27]. Due to the preheating system, the powder bed remains at high temperature in the order of 1000°C [22] causing the thermal gradient and residual stress to be lowered resulting in the manufacturing of crack-free TiAlbased alloys.…”

“…However, EBM manufacturing technology lacks geometrical precision due to the melt pool dimension [22,26,28]. The lack of geometrical precision requires larger machining allowances and the post-processing stage becomes very complicated due to the complex geometry of the product [27]. Machining the products to the final precise dimensions would result in wasting the material.…”

“…2. The layout design of AM machines; (A) EBM with a pre-scanning heating system (B) LPBF with a pre-heating system at the based plate [27].…”