C. Navas

Tribological properties of laser clad Stellite 6 coatings on steel substrates

Abstract The laser cladding of diesel engine exhaust valves is an emerging technology that provides excellent results in terms of the wear and corrosion resistance of the repaired materials. This work focuses on the deposition by laser cladding of Stellite 6 alloy coatings on two different base materials: an AISI 1045 carbon steel and an AISI 304 stainless steel. Following a process optimisation criterion, several of the fundamental laser treatment parameters were varied in order to obtain defect-free coatings with good adherence and minimal dilution. The metallographic structure of the clad material, irrespective of the substrate used, consists of a typical dendritic microstructure in a solid solution of Co. The mechanical strength of the layers was analysed by measuring HV0·1 microhardness and sliding wear behaviour. Laser coatings achieved hardness greater than 500 HV0·1, and presented excellent dry sliding wear behaviour, with a dimensional wear coefficient k one and a half orders of magnitude lower than that of the substrates. A combined oxidation and abrasion mechanism dominated the wear of the Stellite 6 layers, while in the case of AISI 1045 steel substrate, the wear was caused mainly by a surface oxidation mechanism. For the AISI 304 steel, the wear resulted from a combined adhesion, abrasion and plastic deformation mechanism.

Direct laser cladding of stellite coatings and selection of process parameters

The direct laser cladding technique is applied for preparation of Co-based stellite SF6 protective coatings on the chromium steel base of a composition designed for the steam turbine blades. The cw 1.5 kW CO2 laser stand equipped with the multi-stream nozzle characterized by a powder feeding coaxial with the laser beam is used. It is observed that the increase of the base preheating temperature and the average temperature of the coating reduces the cracking susceptibility of the laser clads. This is accompanied by the decrease of the corrosion and wear resistances.

Experimental investigations of the laser cladding of protective coatings on preheated base material

The laser cladding technique was applied to obtain Co-based stellite SF6 coatings on the chromium steel base. The coatings were prepared by means of a direct cladding of metal powder using a 1.2 kW cw CO2 laser stand with a controlled preheating of the substrate material. Results of the metallographic tests revealed a fine-grained, dendritic microstructure and proper metallic bonding between substrate and coating. A nearly constant concentration of mian elements at different areas of the coating cross-section indicated on homogeneous chemical composition of the laser-cladded SF6 alloy samples. A significant decrease of the micro-crack number with increasing temperature of the base preheating was observed. This was accompanied by a drop of the wear and corrosion resistance.