Monitoring and Adaptive Control of Laser Processes

This paper presents a hybrid control system that is able to improve dimensional accuracy of geometrically complex parts manufactured by direct metal deposition process. The melt pool height is monitored by three high-speed charged couple device cameras in a triangulation setup. The melt pool temperature is monitored by a dual-color pyrometer. A two-input single-output hybrid control system including a master height controller and a slave temperature controller is used to control both height growth and melt pool temperature at each deposition layer. The height controller is a rule-based controller and the temperature controller uses a generalized predictive control algorithm with input constraints. When the melt pool height is above a prescribed layer thickness, the master height controller blocks control actions from the temperature controller and decreases laser power to avoid over-building. When the melt pool height is below the prescribed layer thickness, the temperature controller bypasses the height controller and dynamically adjusts laser power to control the melt pool temperature. This hybrid controller is able to achieve stable layer growth by avoiding both over-building and under-building through heat input control. A complex 3-D turbine blade with improved geometrical accuracy is demonstrated using the hybrid control system.

Control of melt pool temperature and deposition height during direct metal deposition process

Purpose – The purpose of this paper is to review the state of the art of laser powder deposition (LPD), a solid freeform fabrication technique capable of fabricating fully dense functional items from a wide range of common engineering materials, such as aluminum alloys, steels, titanium alloys, nickel superalloys and refractory materials.Design/methodology/approach – The main R&D efforts and the major issues related to LPD are revisited.Findings – During recent years, a worldwide series of R&D efforts have been undertaken to develop and explore the capabilities of LPD and to tap into the possible cost and time savings and many potential applications that this technology offers.Originality/value – These R&D efforts have produced a wealth of knowledge, the main points of which are highlighted herein.

Laser powder deposition

Laser beam drilling (LBD) is one of non contact type unconventional machining process that are employed in machining of stiff and high-strength materials, high strength temperature resistance materials such as; metal alloys, ceramics, composites and superalloys. Most of these materials are difficult-to-machine by using conventional machining methods. Also, the complex and precise holes may not be obtained by using the conventional machining processes which may be obtained by using unconventional machining processes. The laser beam drilling in one of the most important unconventional machining process that may be used for the machining of these materials with satisfactorily. In this paper, the attention is focused on the experimental and theoretical investigations on the pulsed Nd:YAG laser drilling of different categories of materials such as ferrous materials, non-ferrous materials, superalloys, composites and Ceramics. Moreover, the review has been emphasized by the use of pulsed Nd:YAG laser drilling of different materials in order to enhance productivity of this process without adverse effects on the drilled holes quality characteristics. Finally, the review is concluded with the possible scope in the area of pulsed Nd:YAG laser drilling. This review work may be very useful to the subsequent researchers in order to give an insight in the area of pulsed Nd:YAG laser drilling of different materials and research gaps available in this area.

Pulsed Nd:YAG laser beam drilling: A review

Research on molten pool temperature in the process of laser rapid forming

Optical focus control system for laser welding and direct casting

Any reliable automated production system must include process control and monitoring techniques. Two laser processing techniques potentially lending themselves to automation are percussion drilling and cutting. For drilling we investigate the performance of a modification of a nonintrusive optical focus control system we previously developed for laser welding, which exploits the chromatic aberrations of the processing optics to determine focal error. We further developed this focus control system for closed-loop control of laser cutting. We show that an extension of the technique can detect deterioration in cut quality, and we describe practical trials carried out on different materials using both oxygen and nitrogen assist gas. We base our techniques on monitoring the light generated by the process, captured nonintrusively by the effector optics and processed remotely from the workpiece. We describe the relationship between the temporal and the chromatic modulation of the detected light and process quality and show how the information can be used as the basis of a process control system.

Applications of optical sensing for laser cutting and drilling.

A nonintrusive optical sensor system is applied to real-time process control of the recently developed laser direct-casting process, in which a stream of metal powder is introduced into the beam of a high-power (500-W) cw laser to fabricate complex three-dimensional structures. The sensor system allows two critical parameters, temperature and build height, associated with this process to be monitored and controlled continuously. We achieved a height-sensing resolution of ±0.25 mm and temperature control with a resolution of ±10 °C at a typical working temperature of 1500 °C with an evident improvement in process quality, especially for complex workpieces comprising relatively high, thin walls at which the conductive heat transfer varies substantially as the process proceeds.

Optical sensor to monitor and control temperature and build height of the laser direct-casting process

We describe a real-time control system for high power CO2laser processing based on a sensor that detects the light generated by the process co-axially with the laser beam. This system provides control of the laser focal position relative to the workpiece in both laser welding and laser direct casting. In addition, it is used to measure and control the temperature of the melt pool in the laser direct casting process.We describe a real-time control system for high power CO2laser processing based on a sensor that detects the light generated by the process co-axially with the laser beam. This system provides control of the laser focal position relative to the workpiece in both laser welding and laser direct casting. In addition, it is used to measure and control the temperature of the melt pool in the laser direct casting process.

M D T Fox

Papers

Optical focus control system for laser welding and direct casting

Applications of optical sensing for laser cutting and drilling.

Optical sensor to monitor and control temperature and build height of the laser direct-casting process

Real-time process control in CO2 laser welding and direct casting: Focus and temperature

Real-time optical monitoring of gas shield condition during Nd:YAG laser welding