Laser brazing of TiAl intermetallic compound using precious brazing filler metals

The applicability of laser brazing technique to bonding of TiAl intermetallic compound was investigated. Five kinds of filler metals such as gold, sliver, palladium and titanium alloys were employed for brazing. Diode laser brazing of TiAl intermetallic compound was carried out at laser power 300–450 W, travelling velocity 3.0–5.0 mm/s and wire feeding speed 20.0 mm/s with shield gas (Ar) at flow rate 15 L/min. According to the preliminary investigation of the filler metal selection, the gold-silver-copper filler metal (BAu-12) was selected as the suitable brazing filler metal for TiAl intermetallic compound. The filler metal did not completely penetrate and infiltrate the joint gap at lower heat input conditions, and the centreline cracking as well as serious erosion occurred in the braze metal at higher heat input conditions, while sound joints could be obtained by optimising processing parameters. The centreline cracking in the braze metal would be caused by the formation of brittle compounds attributed to the contamination (erosion) of the base metal into the filler metal. The theoretical approaches to the erosion and wetting/flowing phenomena during laser brazing process were made by the computer simulation. We customised the flow modelling software (FLOW-3D) to enable us to analyse the metal flow problem during laser brazing by coupled with the erosion behaviour. The simulations of the filler metal BAu-12 showed that it wetted/spread the base metals and infiltrated the joint gap with 0.5 mm when the laser power was increased. However, it did not completely infiltrate the joint gap when the brazing clearance was 0.3 mm. The amount of base metal erosion concurrently increased with an increase in the laser power at any brazing clearances. The computed wetting/flowing and erosion profiles in laser braze joints were fairly consistent with the experimental ones. The joint strength of TiAl intermetallic compound with the filler metal BAu-12 at laser power of 380 W attained to approx. 350 MPa being higher than 80 % of the base metal strength at any brazing clearances between 0.3 and 0.5 mm.