Progress in Numerical Simulation of the Laser Cladding Process

Laser Cladding is one of the developing manufacturing techniques used for diverse applications such as coating, repairing and prototyping. Complex processing phenomena and the formation and growth of thin clad of few micrometers to millimeters range in most cases are yet to be fully understood. However, in recent past, several numerical models have been reported to get some understanding of physical, dynamic and metallurgical phenomena of this process. This article reviews the progress of numerical simulations spanning over three distinct stages of the process to model powder flow dynamics, melt pool and clad properties. For each stage, the governing equations, the effect of process variables and experimental validation techniques have been discussed. Specifically, we have outlined some of the underlying assumptions in the current numerical models which can act as pointers for further improvement of the existing numerical models. Authors recommend that numerical simulation results have to be complemented with experimental results to achieve better clad properties

Laser Cladding for use in Extreme Tribological Interfaces

Coatings are common in engineering applications for protecting the surface of components, either from exposure to environmental conditions or from contact with other components. Laser cladding is a coating technique which allows for thicker coatings of various alloys that enable high load bearing interfaces to operate at a wider range of loads or for longer, for example by increasing durability. This is of great benefit to the railways industry as well as other heavy industries, such as the steel industry. Laser clad coatings have been used extensively in other industries such as oil and gas for increasing the durability of drilling components; in mining and earth moving equipment, for increasing the durability of the components that come in contact with hard soil and rocks. Both are extreme interfaces. In this study, new interfaces and extreme conditions for new industries are investigated, by highlighting the laser clad coating advantages, when used under extreme conditions. The extreme test conditions have not been investigated in published literature, especially with the use of laser clad coatings. This project evaluated the performance of laser cladding coatings on railway components such as the wheel and rail. Other interfaces found in machinery in the steel industry were considered, specifically in the rolling of steel. A variety of interfaces were evaluated by modelling and testing, such as rolling-sliding, high pressure water jet erosion and impact. Three clad materials were identified as suitable for the chosen interfaces, martensitic stainless steel (MSS), Stellite 6 (Co-Cr) and a two-layer clad of Inconel 625 with Technolase. The clad parameters were fixed, resulting in constant material grades, allowing the coatings used in different interfaces to be comparable. The materials choice was based on published research on similar interfaces. Tests were performed on existing test rigs for rolling-sliding and bending tests. The impact test was performed on a rig modified specifically for this study, while a bespoke rig was built for the erosion test. Metallographic techniques were used for all materials, to prepare the samples for characterisation using optical and electron microscopy, as well as nanoindentation and microhardness. Pre- and post-test material analysis was performed. The use of computer modelling was considered mainly for the generation of test parameters, while the results from testing were compared to existing data. Key findings highlight that the use of the selected clad materials under the chosen extreme interfaces can have a positive effect on the durability of the coating, mainly by increasing the wear resistance properties of the coating. Furthermore, the two-layer clad coating showed promising results in stopping crack propagation to the substrate. The test results can be used in predictive tools by researchers in academia, as well as in industry, as a way of introducing laser cladding applications to interfaces of engineering products. Furthermore, the performance of the chosen materials indicates that this study may be used as the basis for selecting similar clad coatings for pilot trials or large scale testing

Experimental Investigation of Laser Cladding Bead Morphology and Process Parameter Relationship for Additive Manufacturing Process Characterization

During the past two decades, basic and applied research has led to an in-depth understanding of the cladding process as well as to a variety of potential applications. Industry had been reluctant to adopt this technology mainly due to high investment costs, and the unpredictable and nonlinear behavior of the process. However, the repair and refurbishment sector of production engineering is flourishing. Most engineering applications require high strength and corrosion resistant materials for long term reliability and performance of the components; consequently, laser cladding (LC) has been explored as a viable solution for an additive manufacturing (AM) approach. Laser cladding is one of the material AM processes used to produce a metallurgically well-bonded deposition layer and now it has been integrated into the industrial manufacturing lines to create a quality surface. To obtain a desired-quality resulting part, a deep understanding of the process mechanisms is required since laser cladding is a multiple-parameter-dependent process. Developing a bead shape to process parameter model is challenging due to the nonlinear and dynamic nature of the LC environment. This introduces unique predictive modeling challenges for both single bead and overlapping bead configurations. A set of cladding experiments have been performed for single and multiple bead scenarios, and the effects of the transient conditions on the bead geometry for these scenarios have been investigated. It is found that the lead-in and lead-out conditions differ, corner geometry influences the bead height, and when changing the input power levels, the geometry values oscillate differently than the input pulses. The dynamic, time varying heating and solidification, for multiple layer scenarios, leads to challenging process planning and real time control strategies. Models are developed for single and overlapping beads using the analysis of variance (ANOVA) and Generalized reduced gradient (GRG) approach along with regression analysis to determine the process trends and the best modeling approaches. Since laser cladding (LC) process has potential to make 3D components; determination of the fill volume for the ‘near net shape’ and the appropriate fill rate is the primary challenge. Although the additive approach reduces many issues related to process planning, there are still issues related to accuracy, surface finish, and build time that require improvement

Real-Time Closed-Loop Control of Microstructure and Geometry in Laser Materials Processing

Laser Materials Processing (LMP) is currently one of the fastest growing technologies of the 21st century. Different categories of this technology such as Laser Additive Manufacturing (LAM) and Laser Heat Treatment (LHT) have now paved the way for more versatile methods of manufacturing that were not possible through conventional manufacturing methods. The localized laser heat source provides advantages such as minimal dilution, minimal distortion, small heat affected zones, and improved localized geometry and quality. However, these advantages come at a price, which is the number of inputs, outputs and process parameters involved that make the LMP a complex process for mainstream manufacturing. Current industrial LMP platforms require an extensive amount of manual tuning and process knowledge in order to achieve high quality production. Nonetheless, because of process sensitivity and lack of automation in LMP machines, the material and mechanical properties of LMP-manufactured products are highly inconsistent. Therefore, to take advantage of the technology’s benefits and to establish LMP into the mainstream manufacturing technology, it is highly essential to develop a fully automated closed-loop LMP process that can intelligently control important output characteristics in real-time. In this research, an automated real-time closed-loop process will be studied and developed to simultaneously control two of the most important LMP output properties: (1) microstructure and (2) geometry. A multi-objective thermal-geometry monitoring and control module is developed to enable closed-loop control of microstructure and geometry properties of the LMP process. Geometry features such as clad height of the LAM process are directly monitored through a CCD camera. Geometry control is achieved by direct control of absolute geometrical values in real-time. An infrared thermal image acquisition system is integrated with the CCD-based imaging system to monitor real-time thermal dynamics. Thermal dynamics of the process such as the cooling rate, melt pool temperature, and heating rate are recorded directly in real-time through a specific set of thermal image analyses algorithms. Microstructure control is defined as control of consistency and stability of a desired set of microstructures for specific materials correlated with a set of perceived thermal dynamics and thermal signatures offline. Therefore, by directly controlling the desired set of correlated thermal dynamics in real-time, a consistent controlled microstructure is guaranteed during the process. A complete closed-loop control process is developed by integrating the monitoring system, LMP system and a multi-input-multi-output controller system. LHT and LAM experiments are conducted with thermal monitoring to understand and predict microstructue, hardness and geometry characteristics in real-time. Microstructure features such as martensitic formation and phase transformations are correlated with real-time thermal cooling/heating rates and melt pool temperatures to develop a microstructure prediction method. Important geometry properties such as hardened depth are also correlated with the thermal dynamics to identify a suitable feedback signal for closed-loop control of the depth, which cannot be monitored by a CCD camera. Thermal patterns are identified for online control of the hardness during single-track and multi-track LHT and LAM processes. Furthermore, an accurate and computationally efficient thermal dynamics model is developed and validated for the LHT and LAM processes for real-time estimation of the thermal dynamics of the process with limited information of the thermal boundaries. The dynamic model is integrated into a state observer feedback control system to provide model-based closed-loop control of the thermal dynamics. The intelligent closed-loop process is evaluated for different case studies of single-track and multi-track laser heat treatment and laser additive manufacturing. The real-time control of microstructure and hardness is achieved in the LHT process through a closed-loop control of the peak temperature. State observer feedback control of the peak temperature is also evaluated for the LHT process. Single-input-single-output control of the clad height and cooling rate are also incorporated for individual real-time control of the microstructure and geometry. Finally, an integrated microstructure and geometry control of the LAM process is constructed and tested for single-track and multi-track LAM depositions, to provide consistent material properties with controlled clad height. As a result of the closed-loop multi-input-multi-output control, the consistency and quality of the LMP manufacturing processes have increased significantly. The controller is capable of eliminating the effect of process and environmental disturbances such as irregular workpiece geometries or undesired heat accumulations. As a result, the developed closed-loop system significantly reduces the extensive amount of time and effort required for manual tuning of LMP setups, and automatically adjusts the process inputs to achieve the desired material and geometry properties. In addition, it also provides an essential tool for obtaining in-process knowledge of the LMP manufacturing process