Zoran Bergant

Influence of laser deposition technique on surface integrity of 12 Ni maraging tool steel

The aim of this investigation is to study the material properties of two coating deposition techniques on 12 Ni maraging tool steel specimens. The first technique is the Nd:YAG laser cladding with coaxial powder injection and the second is the hybrid process of flame spraying with subsequent Nd:YAG laser remelting of sprayed coating. The subsequent precipitation annealing is necessary to obtain the material with similar properties as base material. The surface integrity was studied based on macro- and microstructural examination, chemical analysis and the microhardness and the residual stress distribution. All specimens were precipitation-hardened on required hardness. The through-depth residual stresses were measured and compared. The research shows that the material quality of laser-cladded layers is better than that of layers that were sprayed and laser remelted.

The optimisation of powder flame-spraying parameters using a Taguchi method

The paper reports on an experimental investigation conducted on the influences exerted by individual parameters of the flame spraying of a NiCrBSi coating on the obtained adhesive strength of the coating. An adjusted experimental setup was used to spray coatings of a uniform thickness on specimens of mild steel for a tensile adhesive test in accordance with the standard ASTM C633-01. The four process parameters optimised were the surface condition of a substrate, the preheating temperature of the substrate, the spraying distance and the type of oxy-acetylene flame. A Taguchi fractional factorial design was used to evaluate the effect of each parameter on the properties of the final coating. The individual relative influences of the factors were calculated using Analysis of Variance (ANOVA). A mean-value analysis was used to determine the parameter influences that enable the optimum performance of flame spraying. The quality of the coatings was confirmed by additional confirmation experiments made with optimal parameter settings.

Heat Treatment Effects of Laser Cladded 12 Ni Maraging Tool Steel with Ni-Co-Mo Alloys*

Abstract The aim of the research is to study the effect of a two-stage heat treatment process of two iron based Ni-Co-Mo alloys on microstructure, micro-hardness and residual stress distribution. The laser cladding with multi-jet powder injection was used to deposit different alloys on the 12 % nickel maraging tool steel specimens 1.2799 (SIST EN 10027-2) to study the effect on the materials properties. In Ni-Co-Mo-1 austenite alloy, the precipitation hardening had no effect on hardness even after a prolonged overaging for 16 hours at 720 C. The second alloy Ni-Co-Mo-2 has a chemical composition similar to the maraging steel substrate. After 3 hours of precipitation hardening at 550 C, the alloy hardness increased to the level of base material. The residual stresses were measured using the hole drilling method. The residual stress profiles into the depth show predominantly tensile residual stresses in the Ni-Co-Mo-1 deposit and compressive residual stresses in the Ni-Co-Mo-2 deposit. The results indicate the importance of the chemical composition on microstructure, hardness and residual stresses prior and after heat treatment as well as of the quality of the deposited material on the maraging steel.

Modelling of remelted and heat affected zone during laser alloying of C45 steel with nickel–based powder

The aim of the research is to develop a simplified numerical model to predict the formation of the melt pool and the heat affected zone in single track laser alloying of C45 steel with NiCrBSi powder. The developed finite element model is based on the temperature field calculation using Fourier law. The unknown coefficients such as surface absorption coefficient, volumetric efficiency and beam distribution coefficients are set according to cross–section geometry data, obtained from laser alloying experiment. The Nd:YAG solid state laser was used to study the influence of power and scan feed rate on remelted cross–section area and microstructure. The calculated height and depth of melt pool and heat affected zone are in fairly good agreement with the experimental data. The presented numerical model requires further refinement in order to take into account the complex physical phenomena during laser melting and alloying.

The Influence of Chemical Composition on Residual Stresses in NiCoMo Alloy Deposits on 12 Ni Maraging Steel

The in-plane residual stresses in laser cladded specimens, made of 12-nickel precipitation hardening maraging hot-working tool steel 1.2799 (SIST EN 10027-2) are analyzed using the hole drilling method. The CO2 laser was used to deposit the alloy NiCoMo-1 with significantly higher content of nickel and cobalt with austenitic microstructure at room temperature. The Nd:YAG laser was used to deposit the maraging alloy designated NiCoMo-2, with similar chemical composition as the base material. The comparison of residual stress field showed the sign and the magnitude of residual stresses depends on the chemical composition of the clad being deposited. The high tensile residual stresses were found in NiCoMo-1 layers and favorable compressive residual stresses were found in NiCoMo-2 layers. The metallurgical aspects of residual stress generation are discussed.

Effects of manufacturing technology on static, multi-frequency dynamic mechanical analysis and fracture energy of cross-ply and quasi-isotropic carbon/epoxy laminates

The magnitude of improvement of mechanical properties of laminates using expensive technologies is important to justify the investment in new equipment. The objective of this research is to evaluate the mechanical properties of a cross-ply and quasi-isotropic symmetrical plain weave carbon-epoxy laminate produced with a vacuum bagging method and an autoclave processing method for a given set of epoxy/carbon fabric types. Autoclave-processed laminates exhibit higher static strengths, higher moduli in tension, compression and bending but lower Charpy impact toughness. New findings about materials properties were deduced from dynamic multi-frequency tests between 1 Hz and 50 Hz where it was found that the activation energy was 1.8 times higher in autoclave-processed specimens. Autoclave laminates had, on an average, 1.7-times lower damping ratio in the glassy plateau region and a 3-times lower peak damping ratio in the glass transition region than wet lay-up specimens.

Simulation of Laser Alloying Process

The aim of the paper is to develop a simplified numerical model to predict the formation of the melt pool and the heat affected zone in single track laser alloying of C45 steel with NiCrBSi powder. The developed finite element model is based on the temperature field calculation using Fourier equations. The unknown coefficients such as surface absorption coefficient, volumetric efficiency and beam distribution coefficients are set according to cross-section geometry data, obtained from laser alloying experiment. The Nd:YAG solid state laser with multi-jet nozzle laser head with shielding gas argon was used to conduct experimental runs. The full factorial experimental design was used to evaluate the influence of power and scan feed rate on remelted cross-section area and microstructure. The calculated height and depth of melt pool and heat affected zone are in fairly good agreement with the experimental data. The presented numerical model require further refinement in order to take into account the complex physical phenomena during laser melting and alloying.

Determination of Residual Stresses After Laser Remelting of Flame Sprayed Fe-Ni-Co-Mo Coating

In this paper, the residual stresses on flat specimens, made from 12-nickel precipitation hardening maraging hot-working tool steel coated with Fe-Ni-Co-Mo and subsuquent laser remelted using Nd:YAG laser, were determined using the hole drilling method. In this newly proposed multi-step process for hot working tool steel reparation, the residual stresses are generated during each step by different physical mechanisms. The flame spraying is a highly productive technique to replace worn out material but with limited bonding to the substrate material. Subsequent laser remelting improves coatings bonding with substrate, coatings density and deoxidation to improve wear resistance, erosion and corrosion resistance of the flame sprayed coatings. After laser remelting, the tensile stresses in coatings slightly reduces or shifts into compressive which in turns improves the crack initiation resistance of the repaired surface of the thermomechanicaly loaded die casting tool.

Surface Repair of Tool Made of 12 Ni Maraging Steel by Laser Cladding of NiCoMo Powder

Surface repair experiments with Nd:YAG coaxial laser cladding of NiCoMo maraging powder were made on specimens from maraging steel (EN 10027-2, mat. no. 1.2799). The influences of different modes of laser-beam guidance with various powder mass flows and with different degrees of overlapping of individual traces on the dilution and the repair area were determined. The micro and macroscopic analyses of microsections of fusion zone (FZ), heat affected zone (HAZ) and through-depth microhardness were analysed after cladding and after subsequent solution and precipitation annealing. The microchemical (EDS) analysis was performed at various depths. The residual stresses in the clad face and in the clad toe were determined and compared, using the hole-drilling method.

Optimization of the Flame-Spraying Process and Improvement of Properties of a NiCrBSi Coating by Heat Treatment

The paper describes a study of adhesion strength of wear and corrosion resistant NiCrBSi coatings produced under different flame-spraying conditions on mild steel with 0.08 % carbon. Flame-sprayed coatings often show weak bonding strength to the substrate concerned; therefore, usability of a machine component treated in this way is limited only in case of lower mechanical loads on flame-spray coated parts, and at the same time it is subjected to wear and chemical influences. In order to measure adhesive strength of coatings, the standardized tensile adhesive test (TAT) was used. A Taguchi method was used to perform experiments and analyse results to find the most favourable combination of spraying parameters, i.e. surface roughness, preheat temperature, spraying distance, and type of oxy-acetylene flame, to provide a high-quality adhesive strenght with the substrate. Subsequent heat treatment of the NiCrBSi coating showed significant adhesion strength increase and improvement of microstructural properties.