Introduction to the Yangtze River Delta G60 Laser Alliance It is reported that this article introduces liquid chromatography in detail from the aspects of process simulation, monitoring and parameter optimization. At the same time, as high-entropy alloys, amorphous alloys and sin

Yangtze River Delta G60 Laser Alliance Introduction

It is reported that this article introduces liquid chromatography in detail from the aspects of process simulation, monitoring and parameter optimization. At the same time, as high-entropy alloy , amorphous alloy and single crystal alloys gradually show their advantages over traditional metal materials in liquid crystal materials, this article provides a comprehensive review of liquid crystal material systems. This article is the first part.

Abstract

In industries such as aerospace, petrochemical, and automotive, many parts of different machines are in high-temperature and high-pressure environments and are prone to wear and corrosion. Therefore, the wear resistance and stability at high temperatures need to be further improved. Laser cladding technology has the advantages of low dilution rate, small heat-affected zone, and good metallurgical bonding between the coating and the substrate. It is currently widely used in the repair and functional coating of mechanical parts. This article introduces liquid chromatography in detail from the aspects of process simulation, monitoring and parameter optimization. At the same time, as high-entropy alloys, amorphous alloys and single-crystal alloys gradually show their advantages over traditional metal materials in liquid crystal materials, this article provides a comprehensive review of liquid crystal material systems. Additionally, the use of liquid crystals in functional coatings and repair of mechanical parts is outlined. The existing problems and development trends of liquid crystal display technology are discussed.

1 Introduction

titanium alloy, magnesium alloy and other alloys have excellent properties, such as high specific strength, good toughness and low density. At the same time, due to its abundant reserves on the earth, it is widely used in aerospace, automobile industry and other fields. However, with the development of industry, these materials will be increasingly used in high temperature, high pressure and wear environments. The shortcomings of poor wear resistance and poor high temperature stability limit its application. In order to solve these problems, many surface strengthening technologies have been used to improve the wear resistance and corrosion resistance of these alloy surfaces, such as plasma spray , physical vapor deposition (PVD), chemical vapor deposition (CVD), Surfacing, carburizing, nitriding, etc.

Due to its high energy density, good coherence and good directionality, laser has been widely used for surface treatment of materials. laser surface treatment technology includes laser surface alloy, laser shot peening, laser cladding (LC), laser remelting, etc. It is worth mentioning that LC is a new surface strengthening and repair technology. Under laser irradiation, the cladding powder quickly melts and solidifies on the substrate surface. Due to the large temperature gradient, it will form a fine-grained and tough coating on the surface of the substrate. Compared with other surface strengthening technologies, it has the following advantages: (1) The coating can form a good metallurgical bond with the substrate, and the dilution rate and heat-affected zone are small; (2) Due to the large temperature gradient, fine microstructures can be formed ; (3) LC has the advantages of environmental protection, simplicity, flexibility and material saving. This article reviews the development status, existing problems and future development trends of liquid crystals from three aspects: liquid crystals, coating material systems and liquid crystal applications.

Structure of Ni60A+20 wt%WC composite coating prepared by LIHRC on A3 steel (a) E=20 J/mm2, ψ=55.1 g/dm2, (b) E=18.4 J/mm2, ψ=55.1 g/dm2, (c) E=20 J/mm2, ψ=61.7 g/dm2.

2 Laser cladding process

LC is a multidisciplinary technology that integrates laser technology, computer-aided manufacturing technology and control technology. LC is a complex physical, chemical and metallurgical process. This section introduces the development status of the LC process from the aspects of principles, simulation, monitoring and parameter optimization.

2.1. Process principle

LC uses a high-power laser as a heat source to form a cladding layer on the processed substrate. can be divided into four types according to the powder feeding method: coaxial powder feeding system, pre-placed powder feeding system, off-axis powder feeding system and wire feeding system. The most commonly used liquid chromatography methods are coaxial powder systems and prepositioned powder systems. Figure 1 is a schematic diagram of the coaxial powder system and the pre-placed powder system. When the powder is ejected from the powder feeding nozzle by the carrier gas, the laser beam irradiates the substrate to form a liquid molten pool.After interacting with the laser, the powder enters the liquid molten pool and forms a cladding layer when the powder feeding nozzle moves synchronously with the laser beam. Unlike coaxial powder systems, in pre-placed powder systems the cladding material is pre-placed on the substrate. Then, the pre-placed powder is melted by laser beam scanning, and the molten pool is rapidly cooled to form a cladding layer. LC samples can usually be divided into four parts: cladding zone (CZ), interface zone (IZ), heat-affected zone (HAZ) and substrate (SUB). Generally speaking, the pre-replacement powder system is simple to operate and has good cladding quality, but the penetration depth is difficult to control and the dilution is large. The coaxial powder system has higher laser utilization, but has higher quality requirements for cladding equipment.

Figure 1 Schematic diagram of the coaxial powder system and the pre-placed powder system.

2.2. Process simulation analysis

LC is the interaction process between laser, cladding material and substrate. Therefore, by establishing LC process simulation, the temperature, stress and flow field of molten pool under different process conditions can be better analyzed. . In practice, simulation analysis of the LC process plays an important role in improving the macromorphology, microstructure and performance of the cladding layer. Many scholars have simulated the powder deposition process, temperature field, stress field and microstructure of the cladding layer based on fluid mechanics and physical phase field processes.

In liquid crystals, the interaction of the powder with the laser, substrate and nozzle affects the distribution of the powder . The flow characteristics of the powder affect its utilization efficiency and the macromorphology of the cladding layer. The hydrodynamic properties of a powder are not only related to its particle size, shape and external air pressure, but also to the type of powder nozzle, as shown in Figure 2. In the interaction between powder and laser, the energy of the laser is absorbed, reflected and scattered by the powder, thereby increasing the temperature distribution of the flowing powder. The temperature distribution of the powder has a great relationship with the laser power and the distance between the nozzle and the laser focus. Therefore, appropriate laser power and distance between the nozzle and laser focus should be selected. Therefore, the energy of the powder distribution is all contained in the laser radiation area, and a uniform temperature distribution is obtained. The powder distribution near the molten pool has a great relationship with the matrix. Under the action of protective gas, the powder impacts the substrate and rebounds or disperses, thus affecting the distribution of the upper powder flow. Therefore, when conducting simulation analysis of the powder deposition process, the role of the matrix should be fully considered.

Figure 2 Calculation of nozzle and powder jet parameters.

The distribution of temperature field and flow field directly affects the macromorphology, microstructure and other physical and chemical properties of the cladding layer. The numerical simulation of temperature field and flow field is crucial for the design of process parameters in the LC process. Khamidullin et al. established a two-dimensional LC model and simulated the macromorphology, crystallization process, temperature field and velocity field of the cladding layer. Figure 3(c) is the simulation result of the macro morphology, velocity and temperature field of the two-dimensional cladding layer. It can be found that the simulation better reflects the actual macro and micro morphology of the cladding layer (Fig. 3(a)). Three flow types (Fig. 3(b)) can be clearly reflected. However, by comparing the macromorphology, temperature field, and stress field of the two- and three-dimensional cladding layers under low-speed powder feeding only, the model has good predictability. Therefore, this model should be further optimized. The finite element model of the LC process comprehensively considers fluid flow, heat transfer, surface tension and free surface motion, and has good prediction capabilities for heat input.

Figure 3 After the laser is turned on for 0.8 seconds, under the influence of the 0.5 kW Gaussian laser beam, the simulated microbead shape and the metal flow structure inside the microbeads are simulated.

During the LC process, thermal stress and residual stress will be generated. Therefore, the simulation analysis of the stress field provides a theoretical basis for effectively reducing defects such as cracks in the cladding layer. Ghorashi et al. considered the nonlinear kinematic hardening characteristics in multi-trajectory and introduced the cyclic plasticity theory into the LC Inconel 718 simulation model, which not only reduced the residual stress prediction error by about 50%, but also analyzed the cladding process The surface is loose. Zhang et al. analyzed the influence of induced thermal stress on the residual stress of the cladding layer by establishing the temperature field and stress field of single-track and multi-track cobalt-based coatings.However, the effect of induction preheating on single rails was only analyzed, so multi-rails should be further analyzed. In fact, liquid crystal is a process of multi-field interaction. Therefore, a comprehensive simulation model should be established to obtain the future microstructure and residual stress distribution under a fully coupled thermal-metallurgical-mechanical finite element model.

2.3. Process monitoring

LC, as an effective surface strengthening and repair technology, has been increasingly widely used, but sometimes there are problems with poor cladding layer quality and poor repeatability. However, the development of computer and sensing technology can help us better monitor the temperature field, the morphology of the molten pool, and the interaction process between the powder and laser, all of which are related to the internal microstructure, defects and geometry of the cladding layer. Accuracy is closely related. LC is a complex physical-chemical metallurgical process that can be better understood through temperature signals, image signals, and spectral signals.

image signals or spectral signals can be used to monitor the flow and distribution of powder, and then improve powder utilization efficiency by optimizing nozzle parameters. Gulyaev et al. used the optical diagnostic system Yuna (mainly composed of CMOS digital camera and spectrometer ) to study the effect of laser on the appearance, speed and temperature of powder flow under different gas flow rates. The monitoring results are shown in Figure 4. It can be seen that under the action of the laser, the powder flow expands from the original air flow conveying direction to a sector shape of 35°-40°. When the gas flow rate Gtr increases from 5 slpm to 15 slpm, both the average temperature of the powder flow and the average velocity in the direction of the laser beam increase. When the gas flow rate Gtr continues to increase to 20 slpm, the average speed and average temperature of the powder flow in the direction of the laser beam decrease, so there is a suitable gas flow rate to maximize the impact of the laser on the powder. At the same time, the flow rate of the molten pool under different process parameters was analyzed. However, under certain process parameters, it is difficult to obtain videos of sufficient quality due to large differences in brightness in the area of ​​interest. Therefore, illumination lasers using the bandpass filter may be considered in the future.

Figure 4 Trajectory, temperature and velocity distribution of powder particle flow. The

temperature sensor can monitor the fixed point temperature, the temperature distribution of the molten pool and the temperature distribution around the molten pool. The thermal history of is directly related to the growth of microstructure in the cladding layer. Gopinath et al. used an infrared pyrometer to monitor the thermal history of the molten pool and study the molten pool lifetime, cooling rate, microstructure and wettability of the cladding layer. Thermal history curves of the melt pool on the in-situ synthesized inconel718/TiC composite coating, obtained from an infrared pyrometer, allowed the location of the curing rack to be identified, allowing online identification of excessive dilution rates. The change in the slope of the solidification framework of TiC particles in the molten pool is an effective indicator for online evaluation of the state of TiC particles under different process parameters. Figure 5 is a typical thermal cycle recorded at 1200 W laser power and 200 mm/min scanning speed. This thermal cycle determines the formation of different phases and the mechanical properties of the coating/component. At the same time, the lifetime of the molten pool and the cooling rate for good wettability between WC and metal matrix can be determined by the thermal history.

Figure 5 Typical thermal cycle recorded during LC of Inconel 718+TiC at 1200 W laser power and 200 mm/min scan speed.

The physical and chemical changes of liquid crystal are extremely complex, so it is not enough to rely solely on the above three monitoring signals for adaptive control. More advanced sensors and monitoring equipment are needed to directly monitor gaps, thermal stress, dilution rates and other indicators.

2.4. Process parameter optimization

In the LC process, the dilution, aspect ratio, microstructure and mechanical properties of the cladding layer are closely related to the laser power, scanning rate, powder feeding rate, scanning method, defocus amount and other process parameters. Related. In order to obtain a cladding layer with fine structure, uniform composition and good mechanical properties, many scholars have analyzed the process parameters from different angles.

The influence of the cladding speed v and the distance L between the lens nozzle and the surface cladding on the size of the cladding track.

The relationship between the height H and width B values ​​and the cladding conditions can be observed in the above figure.Increasing the distance between the nozzle and the deposition surface by a factor of 1.4 reduces the width of the track by a factor of 1.1/1.2 and its height by a factor of 1.7/2.6. This is because as the nozzle/workpiece distance decreases, the laser beam experiences some defocusing and surface heating increases; however, it is cooler. This explains the reduction in monorail size. On the other hand, the speed of the laser spot is increased by 3 times, the width of the track is reduced by 1.15 times/1.3 times, and the height of the track is reduced by 2 times/2.9 times. This variation can be explained by the distribution of the deposited material volume over a longer length.

Proper laser power will reduce cracks, voids, and produce cladding layers of good quality and performance. High laser power causes cracking and deformation of the cladding layer, and when the laser power is too small, the powder will not be completely melted, resulting in local pilling and voids. Song et al. analyzed the effect of laser power on the macromorphology and microstructure of the coating. The results are shown in Figure 6. It can be found that as the laser power increases, the height, width and penetration of the cladding layer will increase. Most cracks originate from the heat-affected zone and extend in a direction perpendicular to the joint surface to the cladding surface. As the laser power increases, columnar dendrites, a small amount of equiaxed crystals, uniform columnar dendrites and grain growth appear at the bottom of the cladding layer. This is because as the power increases, the cooling rate gradually decreases, and the grain size is negatively correlated with it. As the laser power is reduced, the microstructures become finer. In addition to laser power, scanning speed also plays an important role in the formation of cladding layers.

Figure 6 Cross-section of K403 superalloy coating with different laser powers.

The process parameters that affect the surface morphology and internal microstructure of the cladding layer are usually not single, they often interact and influence each other. Therefore, it is very important to obtain the best combination of process parameters through various optimization algorithms and empirical formulas. Laser power, scanning speed and powder feeding speed are selected as the process parameters to be optimized, and the cladding height and dilution rate are the response targets for optimization. The optimal combination of process parameters that achieves maximum melting width, minimum melting height and appropriate dilution rate was found. The optimized parameter combination was verified through experiments, and the gray correlation value increased by 0.1533282. Wu et al. studied porosity and cracks in LC-NiCrBSi alloy coatings, and the results showed that linear energy density can be used to determine the threshold for eliminating macroporosity.

used the response surface method to obtain the process parameters such as laser power, scanning speed and powder feeding rate with minimum porosity. can effectively eliminate cracks by placing an insulating layer under the substrate that is preheated to 300°C. However, under optimal process parameters, a small number of pores still exist in the cladding layer. Therefore, it is expected that pore defects can be further reduced by optimizing LC equipment. Establishing an empirical formula between process parameters and cladding layer melting height, penetration depth and dilution rate can greatly reduce the number of optimization experiments and significantly improve cladding quality and efficiency.

Bax et al. proposed a system evaluation method based on the LC process parameter diagram of Inconel 718 single cladding. Not only was the semi-empirical relationship between laser power, scanning speed, powder feeding rate and cladding layer width, height, and area obtained, but also a process parameter diagram between process parameters and powder utilization rate was established. However, it is only applicable to single-track, so the research on multi-track should be further strengthened. Through single-track optimization experiments of LC amorphous Fe-Cr-B alloy, Reddy et al. established a model between powder deposition efficiency, dilution, porosity and process parameters, and verified it through experiments.

In short, there are many process parameters that affect the macromorphology, microstructure and performance of the cladding layer, and each process parameter also affects each other. Therefore, in practical applications, various process parameters should be comprehensively considered according to the requirements of the cladding layer.

2.5. Field-assisted laser cladding

can reduce the internal structural defects of the cladding to a certain extent by optimizing the process parameters, but sometimes holes, element segregation and structural unevenness still exist. In order to significantly reduce the impact of these defects on the microstructure and produce coatings with good properties.In recent years, many scholars have combined LC with other technologies to form induction heating laser cladding (LIHC), ultrasonic-assisted laser cladding, electromagnetic-assisted laser cladding and other technologies. The schematic diagram of the device structure is shown in Figure 7(a)(d)(h).

Figure 7 Schematic diagram of field-assisted LC and its beneficial effects.

Due to the large temperature gradient of liquid crystal, the coating is prone to defects such as cracks. research shows that induction preheating can reduce temperature gradients. The sensitivity of high-temperature induction preheating (temperature range) to heat-affected zone cracks was studied by Bidron et al. As shown in Figure 7(c), there are no cracks in the heat-affected zone on the 2 mm thick substrate, which can be attributed to the induction preheating temperature affecting the microstructure of the heat-affected zone, thereby changing the signs of cracks. In addition, the induction preheating temperature also has an important impact on the maximum deposition rate and laser energy efficiency. When the laser power and scanning speed remain unchanged, as the induction preheating temperature increases, the maximum deposition rate and laser energy efficiency increase, but the growth rate gradually decreases. Therefore, the induction preheating temperature should be controlled within an appropriate range.

Ultrasonic vibration, as an external physical field, has an important influence on the growth and solidification of microstructure and element distribution in the molten pool. Li et al. analyzed the microstructure, elemental composition and properties of LC-MMC coating assisted by ultrasonic vibration. As the ultrasonic power increases, the WC particles in the cladding layer seem to gather uniformly at the bottom and then reach the bottom, as shown in Figure 7(e). Therefore, under appropriate ultrasonic power, the ultrasonic cavitation effect and ultrasonic acoustic flow effect can overcome the shortcomings of uneven distribution of WC particles under gravity. The influence of ultrasonic vibration on the molten pool causes dendrite fracture and grain refinement, and promotes the decomposition of WC particles. As shown in Figure 7(f)(g) respectively. In ultrasonic vibration-assisted laser cladding, the mechanism of ultrasonic action on microstructure, pores and other defect growth needs further study.

The electromagnetic field mainly interacts with the electrons in the material, affecting the chemical reaction process, thereby affecting the microstructure and element distribution. is shown in Figure 7(j). Zhai et al. analyzed the dilution of the cladding layer under different electromagnetic fields and found that a stable magnetic field can significantly reduce the coating dilution rate, but the electromagnetic field has little effect on it. Analysis of the constituent phases of the coating revealed little variation in the phases in the different layers. This is because the electromagnetic field has little effect on the thermal conditions in the melt pool. As shown in Figure 7(i)E-H, when the ampere force in the same direction as the gravity is applied, the equivalent gravity acceleration increases. Therefore, the resultant buoyancy force acting on the pores increases accordingly, and the flow rate of the pores in the melt pool increases. Eventually, both porosity and pore size decrease. As shown in Figure 7(i)A–C, when an upward Ampere force is applied, it will be more difficult for the pores in the melt pool to overflow. However, it only changes the magnitude and direction of the electric field, so the microstructure of the cladding layer under changes in the direction of the magnetic field should be studied.

Source: Recent research and development status of laser cladding: A review, Optics & Laser Technology, doi.org/10.1016/j.optlastec.2021.106915

Reference: Composition optimization of low modulus and high-strength TiNb-based alloys for biomedical applications

Original work by Chen Changjun of the Yangtze River Delta Laser Alliance!