Laser aided manufacturing of composite materials
Cement-based materials are prevalent materials in almost all construction fields, such as buildings, roads, bridges, dams and nuclear power plants. The cement-based materials can be deteriorated under some unfavorable circumstances, such as high chloride environments, poorly controlled concrete casting and unexpected overloading. For this reason, the demand for the removal of deteriorated parts is steadily increasing. Conventional cutting methods for cement-based materials are typically used diamond saw cutting, wire cutting, and water-jetting. However, these methods are time consuming and uncertain removal due to relatively low precision. In addition, undesirable vibration could cause micro-cracks in the sound concrete structure, which could lead to the removal of essential cement-based materials sections. Furthermore, there are additional drawbacks, including noise and dust generation during the cement-based materials removing process. On the other hand, Laser-Aided Manufacturing (LAM) has many advantages, such as no tool wear due to no physical contact, low Heat-Affected Zone (HAZ), high precision, high energy density, low noise level, flexible processing, high productivity and high processing speed. Based on these advantages, laser scabbling has been studied to remove the surface material of concrete using a high-power laser beam. However, this process is only limited to the removal of surface layers. To remove more volume of concrete, a higher laser beam energy is required. Recently, since a high-power laser beam of 10 kW is commercially available and can be focused on very small spots up to 10 um by manipulating the optics, laser cutting can be applied to remove the large volumes of cement-based materials. In addition, laser ablation techniques are considered as the alternative technology to remove the contaminated concrete of about 76.2 mm thickness in nuclear power plants. By decontaminating the concrete surface using the laser ablation technique, the generation of radioactive concrete waste can be significantly reduced.
Bio-degradable polymer material
The polymer materials, which is having the high productivity can be easily modeled by heat and pressure. In addition, it has the advantages of its low prices, so it is used in various industry field. Since the polymer are not easily decomposed, the developments of eco-friendly materials are actively carried out due to environmental pollution of wastes, soil degradation, and leakage of endocrine disruptors. Poly Lactic Acid (PLA), the most widely used biodegradable polymer material, is produced by chemical method for fermenting the sugar and starch obtained from plants. PLA has the advantages, such as good clarity, high strength, and low toxicity. Based on these benefits, PLA is used as food and film packaging, disposable products, and medical materials. Conventional joining methods for PLA include the adhesives, Friction Stir Welding (FSW), and Ultrasonic Welding (USW). However, these methods have the disadvantages, such as high noise level, limitation for three-dimensional shapes, requiring high technical skills of operator, and significant damage to substrate. Especially, FSW forms the hole in the end of the weld part due to probe coupled in tool. In comparison to traditional joining methods, the laser processing has advantages, such as non-contact method, high precision, minimization of crack. Furthermore, since the laser processing has low Heat-Affected-Zone (HAZ), it can minimize the heat deformation of material. Therefore, this study applies pulsed fiber laser to PLA, and observes the effect of laser parameter on the change of microstructure and chemical properties of weld part.
Electrode cutting and structuring for Secondary battery
Nowadays the Li-ion batteries (LIBs), which are firstly commercialized in 1991, are a popular alternative energy source due to environmental pollution and fossil fuel depletion. The LIBs have many advantages compare to other secondary batteries such as high-power density, high energy density, long lifespan, and low self-discharge rate. Thus, that is used to many applications such as cordless products, IT devices, and electric vehicles. Meanwhile, LIBs, which have been developed for 30 years, have various issues in performance and manufacturing processes. We are proposing a solution to various issues of conventional LIBs through laser processing. The conventional separation process of electrodes mainly includes die cutting and knife cutting. These methods result in defects of electrodes, such as burr and bending edge due to wear of tools as time goes on. The defects cause internal short circuits and explosion due to locally concentrated current. We are researching on cutting electrodes by laser to reduce defects caused by wear on tools and to provide a high quality cutting edge. The LIBs have problems related to thickness and density of active material layer. For example, the thicker and denser active material layer has increasing energy density but decreasing power density. Therefore, between energy density and power density have a trade-off relationship. To solve this problem, we tried to make 3D electrodes by laser structuring technology. The Li-ion diffusion rate of 3D electrodes is improved due to the formed groove by laser. Consequently, the reduction of power density can be minimized although the active material layer is thick and dense. Currently, commercial LIBs include graphite anode, and the capacity of conventional LIBs has almost reached its limit. Therefore, an anode with a capacity higher than graphite anode is needed to obtain a higher capacity battery. Lithium metal is expected to be a material to solve this problem. However, Li dendrite is formed on the li metal surface during battery operation. The formed li dendrite dramatically reduces battery performance. We plan to control Li dendrite growth by inducing Li+ into grooves, which is formed by laser (lighting rod effect).
Dissimilar welding for Secondary battery manufacturing
The manufacturing of lithium-ion battery is raising the interest of manufacturers due to the massive demand of this battery in the power supply and energy storage system industry. However, the desire of the batteries with better performance and capacity is crucial. Therefore, researchers and manufacturers are putting hands on the table to improve the manufacturing process as well as experiment with the potential materials for the battery. Above all, joining in the battery should be considered as the most important factor because it affects not only mechanical properties but also the working duration of the battery. The existed joining technologies in the battery have shown significant drawbacks and limitations. This is due to the different materials involved as well as the characteristics of the joining technology itself. Fortunately, researchers have found the most suitable substitute method using laser beam. Laser beam welding features with various advantages and characteristics which help manufacturers to solve the problems of conventional joining methods. Therefore, the research of laser welding for the joints in battery is continuously carried out. The research that we are conducting includes laser welding for the battery case and tab in cylindrical battery cells using cost-effective laser technology. The weld is conducted in two configurations with and without the use of metal tube as the application of the metal has been proven to provide positive effect on the weld. Another research is carried out on the laser welding of dissimilar Cu-Al. This is a typical weld in battery as the materials of Al and Cu are the most common materials used in battery. In this study, we aim to reduce the weld defects such as porosities, voids, and spatters. With these studies, we hope that our result would contribute to the development of battery technology as well as the application of laser technology in industries.
레이저 어블레이션은 재료의 레이저 가공에서 중요한 역할을 한다. 레이저 어블레이션 가공의 적용 분야는 복잡한 표면 패턴을 제작하거나 고정밀가공에서 미세구조를 변경하는데 사용된다. 이때 레이저 절제는 파장, 전력 및 펄스 폭과 같은 레이저 매개변수의 영향을 받는다. 또한, 레이저와 재료 간의 상호 작용 특성은 레이저 매개 변수로 제어할 수 있다. 레이저 어블 레이션은 원자 사이의 결합을 끊을 수 있는 충분한 에너지가 재료 표면에 흡수될 때 발생한다. 레이저와 재료의 상호작용 중에 특정 임계 값 이상의 에너지가 조사되면 표면에서 절제가 발생한다. 그러나, 레이저 에너지가 임계 값 이하로 조사되면 크레이터가 거의 생성되지 않는다. 또한, 물리적, 화학적 반응으로 인한 표면변화가 발생하지 않는다. 레이저 매개 변수에서 절제 임계 값은 최소 레이저 에너지로 정의된다. 다양한 변수에 따라 레이저 가공에 큰 영향을 미치기 때문에 중요한 역할을 한다. 레이저 어블레이션 임계 값은 레이저 매개변수에 따라 증가하거나 감소하며 그에 따른 레이저 어블레이션 결과도 다르게 나타난다. 레이저 어블레이션 임계 값을 예측하기 위해서는 크게 조사되는 펄스 수를 조절해야 되며, 각각의 펄스 수에 따른 크레이터 사이즈를 측정하여 임계 값을 예측할 수 있다.
Advanced design for manufacturing processes
Development of self De/Anti-icing surface
The aviation and railway industries are experienced with attached ice on metal surface in winter season. For example, freezing airfoil of airplane causes delay and the attached ice on railways causes derailment and slow operation. Also, the attached ice and water contacted on metal surface reduce metal’ residual life. Currently, these problems are solved by de-icing and anti-icing. De-icing is the process of removal of snow and attached ice by mechanical and chemical mechanisms. Anti-icing technology is delaying the form of ice and converting hydrophilic surface to hydrophobic surface. Hydrophilic surface is well wetted surface and its contact angle is lower than . In contrast, hydrophobic surface is hardly wetted surface and its contact angle is higher than . Recently, de-icing and anti-icing technologies use a chemical compound to cover surface with anti-icing fluid, which creates protective layer. However, using a compound is non-ecofriendly and can be affected by weather. For example, rain and snow may reduce its hydrophobicity of metal surface. To solve this, Laser Surface Treatment (LST) is proposed. The market size of LST exceeds to 180 million and GDP is reported at about 27%. Laser surface treatment has many advantages. (e.g. prevention of corrosion, self-cleaning and hydrophobic surface) Therefore, this research objective is the fabrication for hydrophobic surface of metal using LST. Hierarchical structures are induced by laser surface treatment, and surface free energy on the irradiated surface is reduced. This change is the main reason why hydrophilic surfaces are converted to hydrophobic surfaces. To establish this transformation, the effect of surface morphology change induced by laser irradiation is investigated. In addition, the effect of surface free energy on wettability is proved.