Research on Temperature and Molten Pool Monitoring in Laser Additive Manufacturing Process

In recent years, 3D printing, which is highly concerned by various countries, can realize the direct near-net forming of complex parts by means of laser layer-by-layer melting forming according to the computer three-dimensional model without any molds and workpieces. The technology has unique advantages such as short manufacturing cycle, high material utilization rate and good process flexibility. It has an important impact on the manufacturing industry and has extensive and important applications in industrial production, aerospace , medical and medical fields. However, many technical challenges have hindered the widespread application of laser additive manufacturing technology and the enormous potential it has. One of the biggest obstacles is the quality inspection of the final product, especially in areas where the quality of the product is extremely demanding, such as aviation. In the aerospace and medical medical fields, it is therefore necessary to monitor and control the laser additive manufacturing process. By monitoring the manufacturing process to reduce the occurrence of defects, improve the dimensional accuracy and mechanical properties of the product, and ultimately achieve the purpose of improving product quality.

At present, many researchers at home and abroad are studying the laser process monitoring in full swing. They have developed a number of systems that can monitor the laser additive manufacturing process. These systems only focus on the on-line detection of the physical parameters of the molten pool. Defects in the component are detected and reduced by feedback control. The laser monitoring process is mainly divided into two parts, one is data acquisition and the other is data processing. The data acquisition mainly has two parts, the shape of the molten pool and the temperature of the molten pool. The shape of the molten pool is generally obtained by a CCD camera or an infrared camera. The temperature of the molten pool is generally measured by a photodiode or a pyrometer. Data processing means that the measured data is processed and transmitted to the controller, and the controller updates the operating parameters of the system to effectively control the operation process of the system, thereby improving the quality of the product. It is worth noting that there are many control methods used by the controller, such as traditional PID control, fuzzy control, and artificial intelligence control such as neural network control. The most mature one is traditional PID control. The current research hotspot is Various artificial intelligence control methods.

The working process and results of a specific control system are described below. The experimental process is a laser metal deposition experiment, which aims to improve the shape accuracy of the product through control. Figure 1 is a laser metal deposition experimental process diagram. It can be found through experiments that the dimensional accuracy of the product has a great relationship with the thermal radiation signal during the manufacturing process. When the thermal radiation signal remains unchanged, the size of the molten pool is basically unchanged. The stability of the size of the molten pool will increase the dimensional accuracy of the product shape. 2 is a laser metal deposition monitoring process diagram, which uses an adaptive PID control method, the measured thermal radiation signal is input to an adaptive PID controller, the controller outputs a control signal, and the control signal acts on the laser emitter to adjust The laser power keeps the intensity of the thermal radiation signal substantially constant.

The outer dimensions of the molten pool and the heat radiation temperature are shown in Fig. 3 and Fig. 4. Comparing the two figures, it can be seen that the heat radiation signal of the process using the control system is relatively stable, basically maintaining the set value 2, corresponding to the melting. The size of the pool is also stable, the maximum and minimum difference between the molten pool is 0.1 mm; the heat radiation signal of the process without the control system is increased, the corresponding molten pool size is increased, and the maximum and minimum difference of the molten pool is 1.27 mm.

The macroscopic topography of the product is shown in Figure 5 and Figure 6. Comparing the two figures, it is obvious that the surface of the product using the control system is smoother, the height and width are more uniform, and the width change is reduced from 63.6% to 12.5%, it can be seen that this adaptive control system achieves the purpose of improving the shape accuracy of the product.
Of course, this is only one of the applications of the laser monitoring process. For different objectives, such as reducing product defects, improving product fatigue strength and various mechanical properties, the researchers have proposed various control systems and control methods.

Figure 5 Macroscopic appearance of the product without the control system Figure 6 Macroscopic appearance of the product using the control system

The current research indicates that many systems have not been applied in actual industrial production processes due to the complexity of integration of control systems and production processes, the limitations of measurement tools and sensors, and the difficulty of real-time control. The research is still in the development stage. It is believed that in the near future, with the continuous deepening of research, laser additive manufacturing monitoring technology will be more mature development and practical application.