Spatter prevention during the laser drilling of selected aerospace materials

The aerospace industry has been employing laser drilling to produce high quality closely spaced holes in turbine engine components for effusion cooling. However, laser drilled holes are inherently associated with the deposition of spatter as the ejected material resolidifies and adheres around the periphery of the holes. Work has been conducted to investigate the effectiveness of an anti-spatter composite coating (ASCC) for the drilling of closely spaced array (2 mm hole pitch) holes in three aerospace materials (IN 100, Nimonic PK 33 and 263 alloys) with an optically delivered 400 W Nd:YAG laser. The ASCC comprised of a mixture of ceramic filler particles embedded in a silicone elastomer matrix was applied on the surfaces of the alloy sheets before the process of laser percussion drilling. The work shows that the ASCC effectively prevents the deposition of spatter such that laser drilled through-holes are produced whilst maintaining the as-received surface characteristic for the various assist gases (O 2, air, N 2 and Ar) tested. It was observed that wetting contact angles for the ASCC and one of alloys (Nimonic 263) were 118° and 46°, respectively. Such a difference was identified to be primarily due to the low surface tension of the silicone elastomer matrix and as a result, the ejected molten material would not wet the surface of the ASCC. The spatter prevention mechanism is based on the provision of a physical constriction (i.e. ASCC hole) at the drilling site (i.e. around the to-be drilled alloy substrate hole). Due to this constriction and the sufficient thermal resistance of the ASCC, the free space on the surfaces of the alloy substrates is eliminated such that the spatter is continually ejected out of the drilling site by the high-pressure forces from the assist gas jet and the recoil pressure.