I. Hemmati

Electron Microscopy Characterization of Ni-Cr-B-Si-C Laser Deposited Coatings

During laser deposition of Ni-Cr-B-Si-C alloys with high amounts of Cr and B, various microstructures and phases can be generated from the same chemical composition that results in heterogeneous properties in the clad layer. In this study, the microstructure and phase constitution of a high-alloy Ni-Cr-B-Si-C coating deposited by laser cladding were analyzed by a combination of several microscopy characterization techniques including scanning electron microscopy in secondary and backscatter imaging modes, energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The combination of EDS and EBSD allowed unequivocal identification of micron-sized precipitates as polycrystalline orthorhombic CrB, single crystal tetragonal Cr5B3, and single crystal hexagonal Cr7C3. In addition, TEM characterization showed various equilibrium and metastable Ni-B, Ni-Si, and Ni-Si-B eutectic products in the alloy matrix. The findings of this study can be used to explain the phase formation reactions and to tune the microstructure of Ni-Cr-B-Si-C coatings to obtain the desired properties.

Microstructure and Phase Formation in a Rapidly Solidified Laser-Deposited Ni-Cr-B-Si-C Hardfacing Alloy

In this study, microstructural evolutions and phase selection phenomena during laser deposition of a hardfacing Ni-Cr-B-Si-C alloy at different processing conditions are experimentally investigated. The results show that even minor variations in the thermal conditions during solidification can modify the type and morphology of the phases. Higher undercoolings obtained at faster cooling rates suppressed the primary borides and encouraged floret-shape mixtures of Ni and Cr5B3via a metastable reaction. Variations in the boride phases are discussed in terms of nucleation- and growth-controlled phase selection mechanisms. These selection processes also influenced the nature and proportion of the Ni-B-Si eutectics by changing the amount of the boron available for the final eutectic reactions. The results of this work emphasize the importance of controlling the cooling rate during deposition of these industrially important alloys using laser beam or other rapid solidification techniques.

Phase formation and properties of vanadium-modified Ni-Cr-B-Si-C laser-deposited coatings

A Ni–Cr–B-Si–C alloy powder was modified by addition of 2 and 5 wt% of vanadium to tackle the high cracking sensitivity of the original composition during laser deposition. The effects of vanadium on microstructure and phases were investigated by Scanning Electron Microscopy, Energy Dispersive Spectroscopy, and Transmission Electron Microscopy (TEM) and the changes in the hardness and cracking tendency of the deposits were evaluated. In comparison to the original composition, V-modified alloys produced deposits with lower hardness and moderately reduced cracking tendencies. Addition of vanadium transformed the nature and the morphology of the boride precipitates and added VC particles to the microstructure but did not induce a significant microstructural refinement. TEM characterizations confirmed that borides phases in the modified deposits consisted of alternating layers of CrB and (Cr1−xVx)B but the VC existed as independent particles which were formed on the boride precipitates. The final phase constitution of the modified alloys was dramatically influenced by the complete solid solubility between CrB and VB and the lack of solubility between Cr7C3 and VC. Addition of vanadium did not provide the phases which could act as nucleation sites to refine the microstructure of the deposits because VB had a tendency to dissolve in CrB and VC was formed at low temperatures on the boride phases. The outcomes of this study can be used to evaluate the effects of adding early transition metals such as vanadium on the microstructure and phase formations of the Ni–Cr–B-Si–C alloys.

Evolution of microstructure and properties in laser cladding of a Ni-Cr-B-Si hardfacing alloy

Ni-Cr-B-Si coatings are used in many industrial applications in order to improve wear and/or corrosion properties. These coatings have traditionally been deposited by thermal spray techniques but the laser cladding process is also being increasingly employed to produce Ni-Cr-B-Si coatings with superior functional properties. In this research, the microstructural evolutions and the phase formations in the laser cladding of a Ni-Cr-B-Si alloy and their effects on the hardness of the clad layer are discussed. Results of this study show various types of boride and carbide phases can form in each track of the clad layer. These changes demonstrate the potential for tuning the properties of these coating by controlling the processing conditions.

Compositional modification of Ni-base alloys for laser-deposition technologies

Cracking of the clad layers is a major issue in laser cladding of hard wear-resistant coatings such as Ni-Cr-B-Si-C alloys. One way to solve this problem is to increase the toughness of these alloys by compositional and microstructural modifications. The focus of this chapter is on defining a procedure to select alloying elements for microstructural refinement of Ni-Cr-B-Si-C alloys and assessing the effectiveness of such a refinement as a toughening mechanism to solve the cracking problem of laser deposited coatings. It is shown that the addition of Nb in specific quantities induces a significant microstructural refinement while preserving the original level of hardness. However, cracking susceptibility of the microstructurally refined deposits was not decreased because the continuous network of hard eutectics still provided an easy route for crack growth. Consequently, an effective toughening mechanism for Ni-Cr-B-Si-C alloy deposits should include not only refinement of the hard precipitates, but also modification of the eutectic structure.

The influence of processing speed on the properties of laser surface deposits

In this paper the influence of high cooling rate which is typical for laser surface engineering techniques, on the microstructure and properties of iron based laser deposits is studied experimentally. Four different Fe-based materials were deposited using laser cladding with powder injection at speeds that vary over a few orders of magnitudes. The microstructure of the coatings, type and quantity of their constituent phases were characterized using Scanning Electron Microscopy, Energy Dispersive Spectroscopy and Electron Backscatter Diffraction. The hardness was measured using Vickers indentation. With increasing cooling rate, the characteristic microstructure size decreases and the inter-dendritic phases became more refined for all coatings. However, the hardness behaves differently depending on the main contributing strengthening mechanism. The effect of cooling rates on the characteristic microstructure size, phase constitution, martensitic transformation and the interplay between the various strengthening mechanisms in producing the overall hardness of the iron based deposits will be discussed and explained.

Correlation between cladding speed, microstructure and hardness of laser-deposited steel coatings: The role of strengthening mechanism

In this study, three wear/corrosion-protective Fe-base alloys with different strengthening mechanisms including AISI 316L (boundary strengthening), Hoganas 3533 (boundary strengthening and carbides) and Carpenter MicroMelt 23 (martensite and carbides) were deposited using laser cladding with powder injection at speeds of 1, 10 a nd 100 m m/s. Microstructure of the coatings and the type and quantity of their constituent phases were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy and Electron Backscatter Diffraction. Hardness of the coatings was measured using Vickers indentation. Microstructural characteristic size, i.e. dendrite arm spacing or cell width of the coatings were measured on SEM images and the cooling rates during solidification were estimated using an analytical model. At higher cooling rates, the characteristic size decreased and the interdendritic phases became refined for all coatings. Nonetheless, hardness of MicroMelt 23 coatings remained unchanged and hardness of AISI 316L and Hoganas 3533 only slightly increased (10-20 percent). The role of cooling rates on the characteristic size, phase constitution, martensitic transformation and the interplay between various strengthening mechanisms in producing the overall hardness of the deposits are discussed.In this study, three wear/corrosion-protective Fe-base alloys with different strengthening mechanisms including AISI 316L (boundary strengthening), Hoganas 3533 (boundary strengthening and carbides) and Carpenter MicroMelt 23 (martensite and carbides) were deposited using laser cladding with powder injection at speeds of 1, 10 a nd 100 m m/s. Microstructure of the coatings and the type and quantity of their constituent phases were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy and Electron Backscatter Diffraction. Hardness of the coatings was measured using Vickers indentation. Microstructural characteristic size, i.e. dendrite arm spacing or cell width of the coatings were measured on SEM images and the cooling rates during solidification were estimated using an analytical model. At higher cooling rates, the characteristic size decreased and the interdendritic phases became refined for all coatings. Nonetheless, hardness of MicroMelt 23 coatings remained unchanged ...

Thickness and waviness of surface coatings formed by overlap : Modelling and experiment

Several surface engineering techniques are known that form a hard facing coating on an inexpensive substrate by a successive overlap of individual cladding tracks. Typical examples include laser cladding and laser additive manufacturing. Realistic predicting the final thickness and waviness of the coating as a function of geometry of single cladding track and their overlap are lacking in literature. In this contribution a recursive model for the calculation of the coating profile is presented. A few basic shapes of single tracks are presumed and on the basis of physical assumptions a recursive formula is deduced to construct a shape of the whole coating profile. Calculations of such profiles for different shapes of tracks and different overlaps show a dependence of the coating thickness and its waviness on these parameters. The model is tested experimentally for a laser cladding process, in which the laser track is formed by a continuous deposition of small metallic particles into the meltpool formed by a high power laser beam continuously moving over the substrate.

Characterization of protective layers produced by laser treatment

Metal matrix composite coatings prepared by laser melt injection (LMI) and thick metallic coatings (Iron, Nickel and Cobalt based) prepared by laser cladding were studied with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), orientation imaging microscopy (OIM) based on electron back scattered diffraction (EBSD) and by a combination of focused ion beam (FIB) and digital image correlation (DIC) of SEM images. Detailed information concerning the grain size and grain orientation together with texture characterization inside the coating will be presented. A possibility to determine phase characterization and orientation relationships between different phases by EBSD will be also demonstrated. Finally, an evaluation of internal stress on micro level inside the coatings and heat affected zones by a combination of focused ion beam and digital image correlation methods will be reported. Relationships between these microstructural characteristics and properties of the coatings will be shown and discussed in depth.Metal matrix composite coatings prepared by laser melt injection (LMI) and thick metallic coatings (Iron, Nickel and Cobalt based) prepared by laser cladding were studied with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), orientation imaging microscopy (OIM) based on electron back scattered diffraction (EBSD) and by a combination of focused ion beam (FIB) and digital image correlation (DIC) of SEM images. Detailed information concerning the grain size and grain orientation together with texture characterization inside the coating will be presented. A possibility to determine phase characterization and orientation relationships between different phases by EBSD will be also demonstrated. Finally, an evaluation of internal stress on micro level inside the coatings and heat affected zones by a combination of focused ion beam and digital image correlation methods will be reported. Relationships between these microstructural characteristics and properties of the coatings will be show...

WIT Transactions on Engineering Sciences

The main advantage of ultrashort laser pulses in manufacturing technology is their very high removal rate of material and high quality of microstructures with the smallest dimensions at 1 μm level. The accuracy is mainly due to almost absence of thermal diffusion into bulk material. In this paper we report the investigation on polycrystalline Cu sample surface treated by 6.7 ps laser pulses with 1030 nm laser light wavelength. Scanning electron microscopy micrographs reveal the presence of jet-like structures with spherical drop-like endings, solidified spheres and many bubble bursts at even lower fluence than the threshold value for the ablation is. Within the molten material the jet-like features are due to an explosion of bubbles originated in solid-liquid-vapor transitions. In the case of below-threshold irradiation the same objects can be seen along surface scratches, dot contaminations etc., which indicate an increase of the laser light absorption on these inhomogeneitiesThe main advantage of ultrashort laser pulses in manufacturing technology is their very high removal rate of material and high quality of microstructures with the smallest dimensions at 1 μm level. The accuracy is mainly due to an almost absence of thermal diffusion into bulk material. In this paper we report the investigation on polycrystalline Cu sample surface treated by 6.7 ps laser pulses with 1030 nm laser light wavelength. Scanning electron microscopy micrographs reveal the presence of jet-like structures with spherical drop-like endings, solidified spheres and many bubble bursts at even lower fluence than the threshold value for the ablation is. Within the molten material the jet-like features are due to an explosion of bubbles originated in solid-liquid-vapor transitions. In the case of below-threshold irradiation, the same objects can be seen along surface scratches, dot contaminations etc., which indicate an increase of the laser light absorption on these inhomogeneities.