Thick Co-based coating on cast iron by side laser cladding: Analysis of processing conditions and coating properties

Abstract

The objective of this work was to create Co-based coatings (compositionally close to Stellite 6) on compacted graphite and gray cast iron substrates with a high power laser (2 kW continuous Nd:YAG) cladding process. The relationships between the relevant laser cladding parameters (i.e. laser beam scanning speed, laser power and powder feeding rate) and the main geometrical characteristics of a single laser track (height, width, dilution, etc.) were examined. A gradual variation of a single processing parameter was used for an appropriate experimental analysis and statistical correlations study between main processing parameters and geometrical characteristics of an individual laser track. These relations lead to the design of a laser cladding processing map that can be used as a guideline for the selection and further tuning of proper processing parameters for laser cladding of extensive layer. The coatings with thickness from 1.0 to 3.3 mm were created on flat substrates without cracks and other major defects. The microstructural features of these coatings were studied using optical microscopy, scanning electron microscopy (Philips XL30 FEG), EDS (EDAX) and XRD. Mechanical properties were determined using microhardness measurement, scratch test (CSM Revetest) analysis at room temperature and using the tribotesting (CSM HT Tribometer) at room and elevated (up to 525°C) temperatures.

Metallic laser clad coatings: on the processing-microstructure-property relationships

“…The laser beam with wavelength of 1.06 µm was focused 16 mm above the substrate surface forming the circular beam spot size of 3 mm in diameter with a Gaussian distribution of energy density inside the beam on the treated surface. Side powder cladding setup [6] was used in which the powder is delivered by carrier gas (Argon) from a side against the cladding hill using the commercial powder nozzle (IWS Dresden) mounted under angle 35º to the surface normal. Four different laser beam scanning speeds S = 1, 2.5, 7.5 and 15 mm/s were used to vary the solidification rate.…”

“…It was found, that the laser cladding process is technologically a quite robust procedure. Laser power P, laser beam scanning speed S and powder feeding rate F (or combinations thereof) are the main technological parameters that control the laser cladding with the powder blowing technique [4][5][6]. Rapid solidification in the surface layer is the characteristic feature for the laser cladding process and in some conditions the coatings formed by cooling rates higher than 10 3 K/s show very fine, non-equilibrium or even amorphous microstructures.…”

“…These coatings are extensively used for hardfacing valves, shafts and many other industrial components [3]. The wear resistance of this type alloys and their laser clad coatings is attributed to the high hardness of chromium rich M 7 C 3 carbides [12] and to the formation of an oxidation protective layer at high temperatures [6]. The microstructure of rapidly cooled Stellite type alloys with carbon content lower than ~2 wt% is simply characterized as hypoeutectic structure containing face-centered cubic (fcc) Co rich primary dendrites (α-cobalt based solid solution) surrounded by an interdendritic eutectic with W, Cr and Co carbides.…”

Thick tool steel coatings with laser cladding

“…Laser cladding with a side blowing powder [3] was used in our experimental setup. Nd:YAG Rofin Sinar laser with the output power of 1.8 kW working in continuous regime was used as a heat source scanning the surface of the substrate materials with scanning speeds between 5 and 10 mm/s.…”

“…Laser cladding using the powder blowing technique [2,3] comprises fusion of an alloy powder layer to a substrate with minimum melting of the substrate. Melting starts at the surface and the particles being heated and melted when passing the laser beam are trapped in the melt pool.…”