Composite layer formation on Ti–6Al–4V surfaces by laser treatment using prepiaced SiC powder

Abstract

Composite layers produced by laser surface melting of Ti–6Al–4 V prep laced with either 5 or 10 vol.-% of 6 μm SiCp particles, using a 2·8 kW laser beam with 1, 1.5, or 2.5 mm radius rB and 112–560 MJ m−2 energy density E under a helium or nitrogen environment, were investigated in terms of microstructure, surface condition, and ceramic layer thickness. The tracks formed in a nitrogen atmosphere resulted indissolution of SiCp particles in the melt zone, but those produced in a helium environment had agglomerations of partially dissolved ceramic particles at the edges, and also in the melt pool of the track glazed at E = 112 MJ m−2. The MMC layer produced had a thickness ranging from 0·45 to 1·2 mm with increasing energy density; it was generally thicker with 10 vol.-% SiCp tracks, and those glazed under a nitrogen environment. The tracks produced in the helium environment were mostly free from surface cracking, but those in the nitrogen environment had cracking of varying intensity. The melt structure consisted of threadlike particles and dendrites for helium and nitrogen environments respectively. The 10 vol.-% SiCp tracks, glazed under a helium environment, gave threaded microstructures for E = 560 MJ m−2, and globular particles incorporating long dendrites for low energy density tracks. Pores were found only in the ceramic agglomerated areas and in those tracks glazed in a helium environment at 112 MJ m−2 energy density. The melt viscosity is assumed to be responsible for pore formation. Low energy density melting gave tracks with smooth surfaces in a helium environment, and distorted surfaces in a nitrogen environment; this distortion was found to diminish at high energy density.