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.

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

Microstructure and wear resistance enhancement of cast steel rolls by laser surface alloying NiCr–Cr3C2

Although most of the near non-equilibrium microstructures of alloys produced by laser powder bed fusion (LPBF) additive manufacturing (AM) are being reported at a rapid rate, the accountable thermokinetics of the entire process have rarely been studied. In order to exploit the versatility of this AM process for the desired properties of built material, it is crucial to understand the thermokinetics associated with the process. In light of this, a three-dimensional thermokinetic model based on the finite element method was developed to correlate with the microstructure evolved in additively manufactured Ti6Al4V alloy. The computational model yielded the thermal patterns experienced at given location while building a single layer through multiple laser scans and a whole part through multiple layers above it. X-ray analysis of the resultant microstructure confirmed the presence of acicular martensitic (α′) phase of (002) texture within the build-plane. Computationally predicted magnitude of the thermal gradients within the additively manufactured Ti6Al4V alloy in different directions (X, Y, and Z) facilitated the understanding about the evolution of grain morphology and orientation of acicular martensite in prior β grains. The scanning electron microscopy observations of the alloy revealed the distinct morphology of phase precipitated within the martensitic phase, whose existence was, in turn, understood through predicted thermal history.

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Computational Assessment of Thermokinetics and Associated Microstructural Evolution in Laser Powder Bed Fusion Manufacturing of Ti6Al4V Alloy.

This study investigates the effects of high-energy electron-beam (1.4 MeV) irradiation on surface hardening and microstructural modification in a gray cast iron currently used for a diesel engine cylinder block. The gray cast-iron samples were irradiated in air using an electron accelerator. Afterward, their microstructure, hardness, and wear properties were examined. The original microstructure, which contained graphite flakes in a pearlitic matrix, was changed to martensite, ledeburite, and retained austenite, along with complete or partial dissolution of the graphite. This microstructural modification occurred only when the surface was irradiated with an input-energy density over 1.1 kJ/cm2, and it greatly improved the surface hardness and wear resistance. In order to investigate the complex microstructures, thermal analysis and simulation testing were also carried out. The results indicated that the irradiated surface was heated to the austenite-temperature region and then quenched to room temperature, which was enough to obtain surface hardening through martensitic transformation. The thermal analysis results matched well with the microstructures of the thermally simulated samples.

Surface hardening of a gray cast iron used for a diesel engine cylinder block using high-energy electron beam irradiation

Self-lubricating wear-resistant NiCr–Cr3C2–MoS2–CeO2 layers were fabricated on 38CrMoAl extruder screws by laser processing. The effect of scanning speeds on microstructure, phases, microhardness, and wear behavior was investigated. The obtained results indicate that the laser-processed layers had fine and nonuniform microstructures with undissolved MoS2 particles distributed on the matrix. With an increase of the laser-scanning speeds, the microstructures changed from hypoeutectic to hypereutectic, volume fraction of martensite increased, microhardness increased, and thickness and friction coefficients of the layers decreased. Wear resistance of the optimized layer was increased by 29.76 times compared with that of the substrate. The undissolved MoS2 was separated from the matrix on loading. In addition to the grain-refining and solution-strengthening effects, oxide films formed on the surface of the layers shielded them and enhanced their wear resistance. The crack or fracture behavior of the laser-processed layers on loading was determined by its toughness, which also had an important effect on the wear behavior of the processed layers.

Effect of scanning speeds on microstructure and wear behavior of laser-processed NiCr–Cr3C2–MoS2–CeO2 on 38CrMoAl steel

Pearlitic grey cast iron was surface melted using a 500 W CW CO2 laser at travel speeds 0.5–100 cm s−1. Detailed structural analysis of the laser modified layer was performed by optical and scanning electron microscopy (SEM), and microhardness depth profiles were measured. Temperature distribution was calculated from a three dimensional moving point source model, taking only the heat transfer into account. From the structural details observed in the austenitized zone some conclusions on the mechanism and kinetics of the pearlite austenite transformation at high heating rates were drawn. The melted zone consisted of primary austenite and ledeburite. At lower scanning speeds the structure was dendritic, at higher scanning speeds transition to dendritic-cellular structure was observed. From the secondary dendrite arm spacings the cooling rate during solidification was estimated as a function of the depth. Some discrepancies were found between our measurements and the literature data as well as predictions by the simple model neglecting convection in the melt.

J. Čermák

Papers

Mechanisms of phase transformations during laser treatment of grey cast iron