In Laser Cladding, a differentiation must be made between cladding by brazing and cladding by welding regarding process parameters and the resulting material properties. Results of investigations of bronze cladding on steel parts produced by Laser Deposition Brazing will be presented. This means that a strong metallurgical bond is realized by diffusion processes by Laser Deposition Brazing, but the steel base material is not molten. The coatings were characterized by hardness distribution measurements from the bronze cladding to the steel substrate, by measuring the size of the heat-affected zone and by porosity measurements. This combination of a steel substrate and a local bronze coating is used industrially in many tribological applications, such as plain bearings or hydraulic pumps etc. The bronze offers excellent tribological properties. In some cases, the bronze is used as a complete solid part. However, applying the bronze locally to a steel base body instead of using a complete solid bronze component, offers the advantage of the higher modulus of elasticity of the steel, which provides greater stability of shape with regard to possible elastic deformations as these coated parts are exposed to high mechanical loads, it is essential that a high coating quality is achieved by laser cladding and that the properties are extensively and purposefully characterized. The production technology, the characterization and the industrial applications of such bronze coated steel parts are presented and explained in this contribution.

Laser cladding shows an increasing interest to apply high-quality tailored surface coatings, as well as 3D-deposits for repair and additive manufacturing of metallic parts. The industry is requesting powerful technologies that maintain the quality advantages of the laser technology, but also make the process more productive, as well as time and cost efficient. At Fraunhofer IWS a Laserline fiber-coupled diode laser of 20 kW power has been employed for over a decade to develop competitive coating solutions. The deposition rates achieved with this technology are comparable to those of common PTA technique, while at the same time bringing significant advantages in terms of reduced heat affected zone, distortion and savings in material resources.

The applications of Wire Arc Spraying (WAS) include large-area corrosion protection coatings e.g. the protection of off-shore wind power plants. While WAS is cost efficient and well-known, the inherent instabilities can lead to coating defects and subsequent vulnerabilities in the corrosion protection coating. The occurrence of these process-related fluctuations cannot be predicted by deterministic models. However, these fluctuations can be monitored in situ, analyzed and finally minimized. A sensor unit is set up on the free jet of a WAS process using ZnAl15 wire. Voltage, amperage, noise and wire feed rate are measured in situ at a sampling rate of 80 MHz. Following a design of experiments approach, 64 different parameter settings are run and measured. For that purpose, voltage, atomizing gas and wire feed rate of the free gas jet have been varied. A generalized linear model (GLM) is trained on the dataset. A Fast Fourier Transformation (FFT) in conjunction with smoothing filters is conducted. Adopting the GLM enabled the calculation of parameters that minimize process fluctuations. Plots in the form of response surfaces depict the influence of the varied parameters on the process stability. A signal analysis using FFT revealed major periodic changes of the voltage in the range of 0.5-1 kHz next to process control-related frequencies at 20 kHz. The mounting and structuring of the data as well as the calculation of key figures is fully automated. Due to the high degree of automation, large quantities of data can be processed. In the future, a simplified version of the adopted sensor unit may be adopted to optimize parameters in an autonomous way. This can ensure not only the minimization of process fluctuations for any chosen feed wire, but also indicate irregularities in the process. The high-resolution recording and automated analysis of the data allows the determination of optimized parameters as well as major underlying frequencies.

Hybrid plasma spraying combines deposition of coatings from coarse powders and liquids (suspensions or solutions) so that the benefits of both routes may be combined. In this study, failure evolution of early-stage thermal barrier coatings (TBCs) with hybrid YSZ-YSZ and YSZ-Al 2 O 3 top-coats deposited by hybrid water/argon-stabilized plasma torch was evaluated. In-situ bending experiment was carried out in SEM to assess potential influence of the secondary miniature phase addition on the coating failure during mechanical loading. Adapted high-resolution open-source strain-mapping code GCPU_Optical_flow was used to track evolution of the local coating failure. For the tested coatings, addition of miniature phase did not weaken the hybrid coating microstructure as the crack propagation was practically insensitive to the presence of the secondary phase and dissimilar splat boundaries. Main micromechanisms of the top-coat failure were thus splats cracking, loss of cohesion (splat debonding), and mutual splat sliding.

The Ti-6Al-4V alloy is widely used in aerospace applications for its excellent mechanical properties, however, it presents low wear resistance. It is often coated with a cermet using high-velocity oxy-fuel (HVOF) spraying to improve its wear performance. The Cr3C2-NiCr cermet becomes particularly interesting since it is non-carcinogenic, compared to traditional cermet coatings containing tungsten-cobalt compounds. While the improvement in wear resistance of Ti-6Al-4V with this coating has been demonstrated, its impact on the fatigue performance of the alloy remains to be studied. This is precisely the aim of this study, which focuses on the fatigue life of a Cr3C2-25NiCr-coated Ti-6Al-4V alloy. Among the various influencing factors, surface preparation represents a significant source of crack initiation, particularly in the case of sandblasted surfaces. Indeed, the inclusion of fragmented alumina particles can produce stress concentration zones. Thus, laser texturing, which is a method involving the creation of anchoring points through controlled ablation, can be considered today as a less harmful surface preparation technique. The results obtained from cyclic tensile fatigue tests with a stress ratio of 0.1 for these two surface preparation methods are presented in this paper.

Revealed as a process for surface functionalization and repair, cold spray is currently used as a reliable additive manufacturing process thanks to its ability to fabricate dense solid-state deposits with high deposition efficiency. However, cold-sprayed deposits generally present limited mechanical and structural properties due to manufacturing defects such as microporosities and weak interfacial particle bonding. As solutions, post-processing methods such as heat treatment or hot isostatic pressing are proposed to reduce manufacturing defects and optimize final deposit properties. This paper investigates the heat treatment effect on structural and mechanical features of cold sprayed 3D Aluminium part by comparing deposits properties evolution with the additive growth in the as sprayed and heat-treated states. Thus, a study is carried out to identify the right heat treatment conditions for optimizing deposits properties.

Amorphous alloys have attracted extensive attention due to their unique atomic arrangement and excellent properties. However, the application in practical engineering is seriously limited due to the size, crystallization and other problems. Laser additive manufacturing technology has the characteristics of high heating, cooling rate and point by point melting deposition, which provides a new idea for the preparation of amorphous alloys. Zr 50 Ti 5 Cu 27 Ni 10 Al 8 amorphous alloy was prepared on the surface of pure zirconium substrate by selective laser melting technology. The composition and structure of the samples were characterized. The results show that the samples are mainly composed of amorphous phase, and the crystallization mainly occurs in the superimposed zone of heat affected zone. With the decrease of laser power, the area of crystallization zone and the number of crystallization particles decrease. However, if the laser power is too low, there will be non-fusion defects and cracks, which will seriously affect the forming quality and amorphous rate of amorphous alloy.

During the impact and solidification of thermal spray droplets on a substrate, the density increases when the droplet solidifies. Depending on the material, the changes in density could be significant. For example, aluminum oxide's density changes by 66%, while the changes are 12% and 19% for nickel and copper, respectively. For zirconia, this change is 24%. The effect of such densification on the dynamic of the droplet impact and the formation of porosity could be dramatic. In this study, the effect of shrinkage of a molten droplet during solidification on droplet impact is numerically investigated for several materials. Results for the impact of molten alumina, nickel, copper, and zirconia droplets on both smooth and rough surfaces are presented. The results of variable density cases are compared with those assuming constant density. The effect of thermal shrinkage is particularly vital in the interaction of two impacting droplets. The shrinkage promotes the formation of additional pores.

High amorphous phase formation tendency, a desirable microstructure and phase composition and silicon evaporation are the challenges of spraying Yb 2 Si 2 O 7 environmental barrier coatings (EBCs). This research addresses these issues by depositing as-sprayed high crystalline Yb 2 Si 2 O 7 using atmospheric plasma spray (APS) without any auxiliary heat-treating during spraying, vacuum chamber, or subsequent furnace heat treatment, leading to considerable cost, time, and energy savings. Yb 2 Si 2 O 7 powder was sprayed on SiC substrates with three different plasma powers of (90, 72 and 53 kW) and exceptional high crystallinity levels of up to ~91% and deposition efficiency of up to 85% were achieved. The silicon mass evaporation during spraying was controlled with a short stand-ff distance of 50 mm, and an optimum fraction of Yb 2 SiO 5 secondary phases (<20 wt.%) was evenly distributed in the final deposits. The desirable microstructure, including a dense structure with uniform distribution of small porosities, was observed. The undesirable vertical crack formation and any interconnected discontinuities were prevented. Reducing the plasma power from 90 kW to 53 kW, while conducive for mitigating the silicon mass loss, was detrimental for microstructure by increasing the fraction of porosities and partially melted or unmelted fragments. The gradual decrease of the coating temperature after deposition alleviated microcracking but has an insignificant effect on the crystallinity level. Coatings annealed close to their operating temperature at 1300 °C for 24 hours demonstrated sintering and a crack healing effect, closing the tiny microcracks through the thickness. An improved coating composition was detected after annealing by the transformation of Yb 2 SiO 5 to Yb 2 Si 2 O 7 (up to ~10 wt.%).

As a critical technology, thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades, however, the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats, three new bond coat alloys have been designed and developed, and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys, such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase, absence of mixed / spinel oxides, and TGO self repair ability, which cannot be achieved by the existed MCrAlY alloys.

Because of their high specific strength, carbon fiber reinforced plastics (CFRPs) are widely used in the aerospace industry. Metallization of CFRP by cold spraying as a surface modification method can improve the low thermal resistance and electrical conductivity of CFRP without the need for high heat input. Herein, we cold spray a Sn coating on cured CFRP substrates and examine the Sn/epoxy interface. The results suggest that the Sn coatings are successfully obtained at a gas temperature of 473 K and indicate no severe damage to the CFRP substrates. The stress and plastic strain distributions at the cross-section of the Sn/CFRP interface when a Sn particle is impacted onto the CFRP substrate are obtained using the finite element method.

The product quality of selective laser melting (SLM) is closely related to the alloy powder characteristics, including the size distribution and the oxygen content. In this work, the 316L stainless steel powder was prepared by a vacuum atomization furnace and sieved into a normal-sized distribution range from 15 to 53 μm with a median diameter of 37.4 μm, and a fine-sized distribution range from 10 to 38 μm with a median diameter of 18.9 μm. Then they were mixed with each other in different proportions. The results show that, under the condition of the same SLM parameters, the SLM part, with adding a large amount of fine-sized powder, has a lower density and strength, as well as more holes and spheroidized particles, compared with the SLM part with adding a small amount of finer-sized powder. Furthermore, the 316L stainless steel powder with a high oxygen content was prepared by a non-vacuum atomization furnace. Although the 316L stainless steel powder with a high oxygen content can be evenly spread in the SLM process, the surface layer of the powder is easy to form an oxide film during the cooling and solidification of powder inside the molten pool. Under the action of thermal stress, the small crack forms and expands along the oxide film, eventually leading to large cracks inside the melt channel.

When testing the thermal cycling resistance of thermal barrier coatings, the surface temperature of the materials must be controlled so that test results can be used for coating life prediction. In this study, the temperature at the surface of plasma-sprayed TBCs was controlled during thermal shock testing using feedback from a double-color IR thermometer and high-rate cooling. The results are presented and discussed, highlighting the capability of the recently designed thermal shock test.

In this investigation, thermally sprayed cylindrical specimens are machined by turning with different cutting speeds. To ensure that process-induced shearing loads do not cause delamination, a fine helical dovetail structure is cut into the substrate before it is coated with FeCrNi alloy by air plasma spraying. Dovetail structures with different geometries were produced and their effectiveness is compared. The finish-machined surfaces of the FeCrNi coatings were examined and characterized with respect to feed marks, cracks, open pores, pull outs, and residual stresses. It is shown that surface roughness and the number of pull outs decrease with increasing cutting speed while residual stresses remain relatively unchanged, except for the orientation of the first principal stress.

This study investigates the effect of thermal cycling on cold-spray chromium coatings deposited on steel substrates. First, equilibrium stress states are determined for different coating thicknesses. Next, the potential for crack initiation and growth is simulated based on periodic heating and cooling cycles. The corresponding crack driving forces are characterized using interface stresses and energy release rate as a function of the thermal cycles. The effects of coating thickness, embedded microcracks, and initial residual stress on the driving forces are investigated systematically to demonstrate the risk of coating fracture and delamination.

In recent studies, crack formation was observed in oxidized areas of wire-arc sprayed Zn-Al coatings. As corrosion tests show, these cracks allow electrolyte to penetrate the coating, reducing effective service lifetime. Wire-arc sprayed coatings usually exhibit tensile residual stresses with the potential to cause such cracking. To determine the extent of that potential, the stress state of Zn-Al coatings was measured and correlated with corrosion test results. Residual stress was obtained using the sin2ψ method based on XRD analysis and the results are combined with those of previous studies, forming a hypothesis for the root cause of crack formation in wire-arc sprayed Zn-Al coatings, its effects, and its control.

This study investigates the effect of suspension plasma spraying (SPS) parameters on inner diameter coatings produced from yttria suspensions, one in water and one in ethanol. Thermal spray trials were conducted at different spray distances, transverse speeds, and spray angles, with and without a water shroud. The coatings obtained were then examined in order to assess the influence of each parameter and the effect of water cooling on substrate temperature, porosity, vertical cracking, nodule formation, surface roughness, and deposition rate. Key findings and correlations are presented in the paper along with recommended practices and potential improvement pathways.

This study assesses the applicability of high-speed laser cladding for producing iron-based alloy coatings, in particular, CrNi duplex steel, FeCrV, and FeCrNiB. Process parameters are optimized for 150 µm thick claddings on mild steel using different laser power levels, surface speeds, and preheating temperatures. Claddings are also produced on cylindrical substrates of different diameters to investigate dependency on component geometry. Duplex steel was found to be highly processable by high-speed laser cladding. In contrast, crack-free FeCrV claddings can only be produced on small diameter surfaces, and only with preheating, while FeCrNiB could not be applied at all without cracking.

Nondestructive Evaluation and Testing (NDE&T) techniques have been played vital roles in property characterization, process development and quality control of various thermal spray coatings. Besides conventional NDE&T lab methods such as eddy current test (ECT) for thickness measurement and fluorescent penetrant inspection (FPI) for cracking detection, some latest NDE techniques have been developed, demonstrated and applied to evaluate and characterize thermal sprayed coatings recently. The improved and innovative NDE methods provide more capable and accurate measurement to inspect on surface morphology, 2D and 3D coating porosity, oxide content, interface debonding, as well as other types of coating features, defects or specific properties. In this work, some non-contact NDE techniques and their applications were investigated and discussed based on several case studies of thermal sprayed coatings. Laser confocal microscopy had been used for characterizing surface morphologies and roughness profiles of HVOF WC-based coatings with 2D and 3D mapping methods. In particular, thermal wave imaging and ultrasonic micro imaging methods were used to detect the suspicious existence of lateral coating separation within or at the MCrAlY coating-substrate interfaces. Laser dimension sensoring method exhibited the extended capability of in-situ coating thickening measurements on turbine blade and vane. The latest non-contact NDE techniques demonstrated their unique and strong capability for in-situ and ex-situ coating characterization, process and quality control and coating failure analysis.

Functionally graded thermal barrier coatings (FG-TBCs) with a gradual composition change from the MCrAlY bond coat (BC) layer to the yttria stabilized zirconia (YSZ) TBC layer is one way to decrease the residual stresses at the BC/TBC interface during a thermal cycling process. At high temperatures, the oxidation resistance of TBCs is also important which would affect the cyclic life of the coatings. In this paper, two YSZ materials were used to make FG-TBCs and conventional duplex TBCs by applying air-plasma spray (APS) technique. The coated samples were thermal cycled with 24-hours heating at 1100 °C followed by air cooling. The results showed that the FG-TBCs performed earlier failure than the duplex ones. Microstructure study indicated that the poor oxidation resistance of the graded mixed layer was responsible for the coating failure. In addition, the influence of spraying parameters on the coating life was discussed.