Laser Glazing Research Articles

Transformations in laser track microstructures for a quasicrystal-reinforced Al-Cu-Fe-Cr alloy

Aluminum-quasicrystal composites are attractive candidate alloys for additive manufacturing, but the cost of evaluating such systems can be prohibitive. Recently, surface laser glazing studies have been used to show that desirable composite microstructures can be formed in an Al85Cu6Fe3Cr6 alloy under a wide range of laser processing parameters. Here the thermal stability of these laser track microstructures has been evaluated by performing in situ scanning transmission electron microscopy heating experiments on specimens produced by focused ion beam milling from the center of the tracks. The initial track microstructures exhibited a uniform phase mixture with 70 % by volume of I-phase quasicrystalline dispersoids in a FCC Al matrix with a film of Al2Cu at the interface. Preliminary ramped heating experiments were used to identify the temperatures at which transformations occurred and then isothermal experiments on fresh samples were used to monitor the progress of these processes. For temperatures of up to 450 °C the only significant changes were a redistribution and coarsening of the Al2Cu phase with a gradual increase in the Fe content of the phase. At higher temperatures, the I-phase decomposed to form a mixture of crystalline ω-Al7Cu2Fe and μ-Al4Cr phases. The I-phase decomposition proceeded by ejection of Cu and Fe with the remaining Cr-rich regions transforming to the Al4Cr approximant phase. The implications of these phenomena for use of the alloy in additive manufacturing are discussed.

Rapid identification of highly glass-forming alloys in Cu–Zr binary system by laser glazing

The high-efficient experimental identification of novel metallic glasses with high glass-forming ability (GFA) is crucial for breaking through the traditional trial-and-error approach, which is time-consuming and costly. A rapid detection method for highly glass-forming alloys using laser glazing was introduced and validated in a classic binary Cu–Zr system with well-known GFA across a wide compositional range. The vitrification degree induced by laser glazing was assessed for each selected alloy and shows a strong correlation with GFA determined by traditional rapid solidification methods. The combination of experimental kinetic analysis and theoretical finite element simulation revealed possible thermodynamic mechanisms of glass formation and crystallization in the heat-affected zone (HAZ) formed by laser glazing. The results offer insights into not only the rapid identification method of highly glass-forming alloys using laser glazing but also the mechanism of glass formation under fast heating and cooling processes, providing guidance on the construction of bulk metallic glasses through laser additive manufacturing.

Improvement of CMAS resistance of laser glazed and nano-modified YSZ thermal barrier coatings

The present investigation used atmospheric plasma spray (APS) as a method for the production of thermal barrier coatings composed of Yttria-stabilized zirconia (YSZ). A CO2 laser was used to remove surface cracks from the YSZ coatings' surface. Several parameter variations were applied to generate laser-glazed tracks on the coating surface, resulting in an impure surface microstructure with excellent surface properties. Scanning electron microscopy was utilized to analyse the tracks' microstructure. The optimal laser parameters were determined to be 77.7 W laser power, 215 mm laser distance, and, 120 mm s−1 scanning speed based on microstructural characteristics. Once the laser parameters had been optimized, a whole coating surface was laser glazed. In order to ascertain the impact of laser glazing, stability analysis, fracture toughness, and hardness evaluations were conducted on both unglazed and laser-glazed coatings. The growth of a network of cracks was seen on the surface. The surface roughness exhibited a reduction of about tenfold in comparison to the unglazed YSZ. The presence of the monoclinic ZrO2 phase within the coating structure was successfully eradicated. The laser glazing process resulted in enhancements to both the fracture toughness and surface hardness of the coating. Despite the fact that the gaps and imperfections on the coating surfaces were repaired during laser glazing, some cracked structures nonetheless appeared. Nano YSZ was deposited on laser-glazed surfaces by the electrophoretic method in order to eliminate cracks from laser-glazed surfaces. The experimental results obtained from conducting calcium-magnesium-alumina-silicate (CMAS) and hot corrosion tests on laser-glazed coatings indicate that the use of nano YSZ filler leads to a significant reduction in salt penetration during both glassy melt and hot corrosion processes.

Laser surface modification to improve the resistance of CMAS + molten salt coupling corrosion to thermal barrier coatings

In marine environment, thermal barrier coatings (TBCs) face coupling corrosion of calcium magnesium aluminum silicate (CMAS) and molten salt leading to premature failure. In this study, microstructure evolution of TBCs resulting from CMAS + NaVO3 (CN) coupling corrosion is investigated, and a corrosion protection method based on laser glazing is proposed. TBC degradation by the CN coupling corrosion is mainly due to the high penetration ability of the melt. Laser glazed TBCs have improved resistance to the coupling corrosion, however, the vertical cracks in the glazed layer provide paths for melt penetration. TBCs with double laser-glazed layer are designed, with vertical cracks in the glazed layer being bifurcated and staggered, which could largely suppress CN melt infiltration, and also exhibits excellent phase and microstructure stability in the coupling corrosion condition. This type of laser modified TBC is confirmed to be attractive against the coupling corrosion.

Hot corrosion behavior of CYSZ thermal barrier coating with optimized laser surface modification

The current study aims to investigate the influence of laser surface modification on the hot corrosion, fracture toughness, and hardness resistance of atmospheric plasma sprayed Ceria-Yttria Stabilized Zirconia (CYSZ) thermal barrier coating. Throughout the laser surface modification process, a variety of laser processing parameters are used to obtain the finest surface properties and optimum laser parameters. In this study, the whole coating surface is modified using the optimal parameters. The results show that 52.9 W laser power at a 220 mm laser distance and a 60 mm/s scanning speed were found as the optimum laser glazing parameters. A hot corrosion test is performed in a box furnace for 20 h at 1050 °C. Scanning electron microscopy (SEM) images show denser surfaces in laser-glazed coatings when compared to air-spraying coatings. A dense microstructure combined with the top coat is created in the coating cross-section, and a smooth microstructure with a coaxial fracture network is produced on the surface. The laser-glazed layer has superior mechanical characteristics to the as-sprayed coating, including hardness and fracture toughness. This layer also restricts the damaging monoclinic-tetragonal phase change for CYSZ TBC, blocking the infiltration of molten salt. The hot corrosion behavior of TBCs is improved by the inclusion of a dense laser-glazed layer.

Thermal stability of laser track microstructures in an Al-Cr-Co-Mn-Zr I-phase alloy

Laser glazing of solid metal surfaces is an efficient and cost-effective way to evaluate the potential of an alloy system for use in metal additive manufacturing (MAM), but there are concerns about the stability of the resulting microstructures. Here it is shown that in situ transmission electron microscopy heating experiments on specimens cut from laser glazing tracks using focused ion beam methods can provide a useful insight into the transformation phenomena that occur upon reheating. Examples of such experiments are presented for laser tracks in a powder-processed Al-Cr-Mn-Co-Zr alloy, which contains a complex distribution of icosahedral quasicrystal (I-phase) dispersoids in a polycrystalline FCC Al matrix. Initial ramped heating experiments were performed to identify suitable conditions for isothermal studies of transformation pathways. Tracks with the lowest laser energy input exhibited supersaturated solid solution microstructures. These were stable during ramped heating to around 375 °C. For isothermal exposures at 400 °C there was precipitation of Al4(Cr,Mn) both at grain boundaries and within the FCC Al matrix phase. In tracks with higher laser energy input, equiaxed I-phase dispersoids were formed in the matrix. These microstructures were stable during ramped heating to around 400 °C. In isothermal experiments at 425 °C, there was precipitation of Al4(Cr,Mn), followed by the onset of I-phase decomposition. The partially decomposed dispersoids exhibited an I-phase core surrounded by a shell of Al45(Cr,Mn)7. In isothermal experiments at 450 °C, the dispersoids decomposed completely to form Al4(Cr,Mn), indicating that Al45(Cr,Mn)7 is a metastable intermediate transformation product. These observations provide a useful guide to the transformations that might occur during MAM processing of this alloy system.

Tailorable shielded compliance of thermal barrier coatings through laser texturing and microstructural modification: microfeature design and validation

Thermal barrier coatings (TBCs) are widely used for protecting components from an aggressive environment and excessive temperatures. However, the top coat can lack thermal compliance and environmental resistance, promoting premature failure. This study provides an approach to design, produce, and evaluate the novel shielded-compliant TBCs to address these challenges. Top coat laser grooving can lower thermal mismatch levels and hinder crack propagation, whereas laser glazing can enhance strength and anti-corrosion performance. Material analysis for the glazed layer revealed large columnar grains alongside a stress-free structure compared to the as-sprayed coating. The digital image correlation (DIC) in-situ heating test showed vertical cracks after glazing already to be effective stress relief features, with the controlled grooves showing the most prominent compliance, as suggested by finite element analysis (FEA). Such a concept may be applied to any coating where increased thermal compliance or environmental protection is needed, making it a highly versatile tool.

Laser glazing of a quasicrystal-reinforced Al-Cu-Fe-Cr alloy: Implications for use in additive manufacturing

Quasicrystal-reinforced aluminum metal matrix composites exhibit combinations of properties that are attractive for metal additive manufacturing. It is challenging to evaluate such systems due to difficulties in modeling the solidification and microstructural evolution. Here, laser glazing in a powder bed fusion instrument was used as an efficient method to evaluate an Al85Cu6Fe3Cr6 alloy, and the resulting microstructures were characterized using electron microscopy. The arc-melted alloy contained only crystalline phases, but pre-glazing of the surface gave a refined microstructure comprising I-phase quasicrystals, Al2Cu and FCC Al. Individual laser tracks produced in this layer under different conditions exhibited an extremely fine and uniform phase mixture with over 70% by volume of I-phase in a FCC Al matrix with a thin film of Al2Cu at the interface, and no cracking or macrosegregation. The reasons for the formation of I-phase, rather than the D-phase that has been reported previously in this alloy, are discussed.

High-temperature wear behavior of HVOF sprayed, laser glazed, and laser cladded Stellite 6 coatings on stainless steel substrate

In this study, the Stellite 6 coating was applied on 316 stainless steel substrates through the high-velocity oxygen fuel method (ST-HVOF). Then, the laser-glazed sample (ST-Glazing) was deposited on the as-sprayed Stellite 6 coating using optimal conditions. Afterward, the Stellite 6 powder was optimized on the stainless-steel substrate by laser cladding process (ST-Clad) using 300 W and 5 mm/s of laser power and beam scanning speed, respectively. The microstructural characterization and phase analysis of ST-HVOF, ST-Clad, and ST-Glazing coatings were performed by field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) techniques. The microscopic observation results revealed that laser glazing significantly decreased the porosity of ST-HVOF (from about 2.3% to about 0.3%) and formed a dense coating with a uniform microstructure and strong adhesion to the substrate. Also, ST-Clad coating showed a porosity of 0.2% and a compact and uniform microstructure with a high-strength bonding to the substrate. Tribological evaluation of the coatings indicated a 67% and 58% increase in wear resistance of ST-Clad and ST-Glazing coatings compared to ST-HVOF coating, respectively. The high-temperature wear results suggested the following mechanisms for the studied coatings: plastic deformation mechanisms, wear adhesion, brittle fracture, and delamination.

Use of laser glazing to evaluate an Al-Cr-Co-Mn-Zr alloy containing icosahedral quasicrystalline dispersoids as a candidate material for additive manufacturing

Evaluating novel alloy systems for metal additive manufacturing (MAM) is challenging especially when metastable phases are involved. Here, laser glazing was used as a cost-effective method to evaluate the potential of a powder-processed Al-Cr-Mn-Co-Zr alloy for MAM. The alloy powder exhibited a nano-composite microstructure consisting of an FCC Al matrix plus icosahedral quasicrystal (I-phase) dispersoids, and this microstructure was retained in the as-consolidated alloy. Single laser tracks were produced on the surface of a polished coupon of the consolidated alloy at a range of laser powers and scan speeds in a commercial powder bed fusion (PBF) system. The resultant microstructures and mechanical properties of the laser tracks were investigated by electron microscopy studies and hardness measurements. The laser tracks showed no solidification cracking, and the only morphological defects were occasional pores. All of the phases in the substrate alloy were melted fully in the laser tracks, and four different types of microstructures (solid solution, equiaxed dispersoids, mixed and radial growths) were observed in the solidified tracks. The microstructures obtained depended upon the input energy densities due to the combinations of laser processing parameters. The laser tracks with the solid solution and equiaxed dispersoid microstructures gave the highest hardness values of 154–170 HV, which is comparable to that of the substrate alloy. These data indicate that there is considerable promise for the application of such I-phase alloys in MAM.

On the importance of interface stability in cellular automata models: Planar and dendritic solidification in laser melted YSZ

Laser-processing technologies are often applied to enhance the properties of ceramics, such as laser glazing of Yttria Stabilised Zirconia (YSZ). However, very limited attention was paid to the solidification phenomena and mechanisms of YSZ. In this paper, two coexisting solidification behaviours of laser-melted YSZ have been identified, namely grain bending (crystals with curved grain boundary geometry under the surface) and grain surface sealing (in-plane surficial crystals cover vertical columnar grains). Although these phenomena have been reported, this is the first time the two phenomena coexist in the same material. A new cellular automata (CA) approach has been proposed to explain the formation mechanisms of these two phenomena. This new CA method consists of the separation of growth modes into dendritic and planar growth, whose critical transition value is calculated based on the supercooling theory. Besides, the proposed model for planar growth is far less computationally expensive than the widely used decentred octahedron algorithm. A good agreement with the EBSD data of longitudinal cross-sections and the top surface has been observed which proves that the proposed method can become a more realistic and efficient way to predict the grain microstructure in laser processing, allowing to capture dendritic and planar growth simultaneously.

Finite element analysis on temperature field and stress distribution of thermal barrier coatings by laser modification and CMAS corrosion

Laser glazing to produce a modified layer on the surface of thermal barrier coatings (TBCs) is a promising method to alleviate calcium-magnesium-alumina-silicate (CMAS) attacks to coatings. In this study, finite element analysis is carried out to investigate the temperature field and stress distribution of TBCs after the laser glazing process and the modified TBCs after CMAS corrosion. Results revealed that along the direction of laser scanning, the principal stress was in a tensile state, which increased along the direction of the laser spot movement; while perpendicular to the laser scanning direction, the maximum principal stress appeared at the interface between the glazed layer and the unmodified coating, where could be a potential danger zone for crack initiation and coating spallation. For the modified TBCs with CMAS attack, in regions that are located in the range of 0–6 mm along the radial direction, the radial stress and maximum principal stress were high (∼1094 MPa), and the coating edge had complex shear stress state; as a result, these areas were easy to crack.

High strength Al–Li alloy development for laser powder bed fusion

With laser powder bed fusion (LPBF) of aluminum alloys growing in recent years, the research focus is progressively shifting from processing attempts on mature cast and wrought aluminum alloys to developing new aluminum alloy systems. This paper reports a novel Al–Li alloy specifically developed for the LPBF technology. The methodology of laser glazing on cast parts was applied to approximate solidification conditions of LPBF. Five variants of Al–Cu–Li ingots with different Sc and Zr contents were developed, and the optimized chemical composition was obtained and converted to Al–Cu–Li–Sc–Zr powder. This newly developed Al–Cu–Li–Sc–Zr alloy was successfully processed by LPBF with a high building rate and a low Li loss rate. The novel alloy shows excellent tensile properties including yield strength of 482 ± 1 MPa, ultimate tensile strength of 539 ± 1 MPa, and elongation of 8.8 ± 0.7%.

Microstructure design of the laser glazed layer on thermal barrier coatings and its effect on the CMAS corrosion

Calcium-magnesium-alumina-silicate (CMAS) corrosion is a great challenge for the long lifetime of thermal barrier coatings (TBCs). In this study, the surface microstructure of TBCs is tailored by laser glazing to improve the CMAS resistance, and the modified coatings are designed to have single and double laser-glazed layers. In the double laser-glazed coating, the vertical cracks in the glazed layer which penetrate directly for the case of the single laser-glazed coating become bifurcated and staggered. Under molten CMAS conditions, the coating with double laser-glazed layers highly resists to melt penetration, and the glazed layer exhibits excellent phase stability and structural integrity.

Thermal Shock Resistance and Thermal Insulation Capability of Laser-Glazed Functionally Graded Lanthanum Magnesium Hexaluminate/Yttria-Stabilised Zirconia Thermal Barrier Coating.

In this work, functionally graded lanthanum magnesium hexaluminate (LaMgAl11O19)/yttria-stabilised zirconia (YSZ) thermal barrier coating (FG-TBC), in as-sprayed and laser-glazed conditions, were investigated for their thermal shock resistance and thermal insulation properties. Results were compared with those of a dual-layered coating of LaMgAl11O19 and YSZ (DC-TBC). Thermal shock tests at 1100 °C revealed that the as-sprayed FG-TBC had improved thermal stability, i.e., higher cycle lifetime than the as-sprayed DC-TBC due to its gradient architecture, which minimised stress concentration across its thickness. In contrast, DC-TBC spalled at the interface due to the difference in the coefficient of thermal expansion between the LaMgAl11O19 and YSZ layers. Laser glazing improved cycle lifetimes of both the types of coatings. Microstructural changes, mainly the formation of segmentation cracks in the laser-glazed surfaces, provided strain tolerance during thermal cycles. Infrared rapid heating of the coatings up to 1000 °C showed that the laser-glazed FG-TBC had better thermal insulation capability, as interlamellar pores entrapped gas and constrained heat transfer across its thickness. From the investigation, it is inferred that (i) FG-TBC has better thermal shock resistance and thermal insulation capability than DC-TBC and (ii) laser glazing can significantly enhance the overall thermal performance of the coatings. Laser-glazed FG-TBC provides the best heat management, and has good potential for applications that require effective heat management, such as in gas turbines.

Microstructural characterization of modified plasma spray LZ/YSZ thermal barrier coating by laser glazing

One of the high potential material, lanthanum zirconate (LZ) is deposited on superalloy 625 due to its excellent phase stability, low thermal conductivity as well as the high melting point. However, microstructural defects were detected due to lamellar structure behaviour exhibited from the plasma-sprayed coating techniques. This paper investigated the effect of surface modification on LZ thermal barrier coating (TBC) by using laser glazing method. The double ceramic layer (DCL) TBCs are composed of NiCoCrAlYTa bond coating formed via the HVOF process while 8YSZ ceramic top coat and La2Zr2O7 as the third layer were produced by utilizing the APS technique. Nd:YAG pulsed laser has been used to modify the surface layer of plasma-sprayed LZ coatings. The microstructural morphologies of both as-sprayed and laser glazed samples were investigated using Scanning Electron Microscopy (SEM). The phases of the coatings were analysed with X-ray Diffractometry (XRD). The results showed that the laser glazing process reduced surface roughness attributed to the sealing of porosities and other structural defects of the coating. Finally, the impacts of laser energy input and laser distance on the characterization of the glazed layer are discussed.

Microstructural study and hot corrosion behavior of bimodal thermal barrier coatings under laser heat treatment

In this study, nanostructured YSZ powders were deposited on the Hastalloy X Superalloy substrate coated with a metallic bond coat by plasma spraying to produce a nanostructured thermal barrier coating with bimodal microstructure. After that, the coated samples were heat-treated using a Nd:YAG laser. Then, the microstructures of the conventional and nanostructured TBCs before and after the laser glazing process were examined using a scanning electron microscope (SEM). The coating phases were studied by X-ray diffractometry (XRD). The high-temperature corrosion behavior of the nanostructured plasma sprayed coating in the presence of Vanadium pentoxide and Sodium sulfate molten salt was compared with that of the conventional coatings before and after laser treatment at 1050 °C. The hot corrosion results showed that the coatings had a similar degradation mechanism based on which the corrosive molten salt reacted with the stabilizer of YSZ, producing hot corrosion products such as YVO4. It led to an unwanted phase transformation from tetragonal (t) to monoclinic (m) Zirconia and the final degradation of the TBC system. However, reducing molten salt penetration, decreasing surface roughness and, reduction of the specific surface area are three important mechanisms that improved hot corrosion resistance, finally extending the lifetime of the glazed samples. The results also showed that the nanostructured TBC had higher hot corrosion resistance in comparison with other samples.

Novel Al-X alloys with improved hardness

In this study, our goal is to design solid solution strengthened aluminum alloys for manufacturing technologies that involve high cooling rates. This investigation starts with an analysis of solid solution strengthening using first principles calculations to determine elastic property changes and local lattice distortions from the introduction of different elements into a host aluminum lattice. These results, coupled with both equilibrium and non-equilibrium solubility data, leads to the selection of cerium and cobalt as the primary candidate alloying elements. Alloys of AlCe and AlCo at concentrations of 0.5, 1.0, and 3.0 at. % are then synthesized and subjected to laser glazing to produce non-equilibrium microstructures. The microstructure and solid solution characteristics are determined using a combination of scanning electron microscopy and transmission electron microscopy. Furthermore, nanoindentation is used to measure the hardness showing that both candidate systems harden significantly after glazing. In addition, Al-1.0Co at. % achieves a hardness comparable to Al6061-T6. These results conclusively show that cerium and cobalt are promising elements in the next generation aluminum alloys which make use of non-equilibrium processing conditions such as additive manufacturing.

Microstructure modification of Y2O3 stabilized ZrO2 thermal barrier coatings by laser glazing and the effects on the hot corrosion resistance

Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) are prone to hot corrosion by molten salts. In this study, the microstructure of atmospheric plasma spraying YSZ TBCs is modified by laser glazing in order to improve the corrosion resistance. By optimizing the laser parameters, a ∼18 µm smooth glazed layer with some vertical cracks was produced on the coating surfaces. The as-sprayed and modified coatings were both exposed to hot corrosion tests at 700 and 1000 °C for 4 h in V2O5 molten salt, and the results revealed that the modified one had improved corrosion resistance. After hot corrosion, the glazed layer kept structural integrity, with little evidence of dissolution. However, the vertical cracks in the glazed layer acted as the paths for molten salt penetration, accelerating the corrosion of the non-modified coating. Further optimization of the glazed layer is needed in the future work.

Laser glazing of cold sprayed coatings for the mitigation of stress corrosion cracking in light water reactor (LWR) applications

The effect of laser power, traverse speed, and cold spray coating thickness were examined with the goal of mitigating and repairing stress corrosion cracking (SCC) in light water reactor (LWR) environment. For this purpose, SCC-susceptible Alloy 600 substrate material was coated with SCC-resistant Alloy 690 via the cold spray technique. The cold spray coated substrate was then laser-glazed using various laser parameters. Single pass and multiple pass laser glazed regions were created to determine the effects. For this purpose, the area of the fusion zone, depth of the fusion zone, and elemental composition of the cross section were examined by optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy. It was shown that chromium dilution from the laser glazed region into the substrate is not a significant factor in laser glazed cold spray coated samples. Separate laser glazed samples were subjected to ASTM 633C to determine coating adhesion. Finally, by using interrupted SCC crack growth rate tests, it was shown that low traverse speeds and high laser powers produce deep fusion zones within the cold sprayed coating and substrate that can be used to seal pre-existing cracks, thus stopping SCC growth.