کنترل ارتفاع لیزر سیم فلزی رسوب مبتنی بر کنترل یادگیری تکرار ی و 3D اسکن

Laser Metal-wire Deposition is an additive manufacturing technique for solid freeform fabrication of fully dense metal structures. The technique is based on robotized laser welding and wire filler material, and the structures are built up layer by layer. The deposition process is, however, sensitive to disturbances and thus requires continuous monitoring and adjustments. In this work a 3D scanning system is developed and integrated with the robot control system for automatic in-process control of the deposition. The goal is to ensure stable deposition, by means of choosing a correct offset of the robot in the vertical direction, and obtaining a flat surface, for each deposited layer. The deviations in the layer height are compensated by controlling the wire feed rate on next deposition layer, based on the 3D scanned data, by means of iterative learning control. The system is tested through deposition of bosses, which is expected to be a typical application for this technique in the manufacture of jet engine components. The results show that iterative learning control including 3D scanning is a suitable method for automatic deposition of such structures. This paper presents the equipment, the control strategy and demonstrates the proposed approach with practical experiments.

Production of complex metal structures, by means of traditional manufacturing, often requires expensive precision castings or oversized forgings that need extensive machining. For large and complex structures that are manufactured in small quantities using high-cost materials, e.g. jet engine components, traditional methods lead to significant production costs, due to high scrap rates and long lead-times. Rapid manufacturing techniques based on additive layer manufacturing have therefore gained an increased attention due to their ability to fabricate fully dense metal shapes without the need of dies or extensive machining. If rapid manufacturing can be included as a supporting technique to traditional manufacturing methods, the production costs and lead-times can be decreased. The flexibility of rapid manufacturing can also allow for late design changes or repair of worn out parts. Moreover, the technique enables the use of other materials and new designs, utilizing, e.g. sheet metals, which can help to reduce the total weight of the final component. Since their first introduction three decades ago, additive manufacturing techniques for metal have been developed and commercialized in the industry under names such as Direct Metal Deposition [1], Laser Engineered Net Shaping [2], Shaped Metal Deposition (SMD) [3], Selective Laser Melting [4], and Electron Beam Melting [5]. Apart from the SMD system, which utilizes metal wire, the parts are built from powderized feedstock either in a powder-feed process [1] and [2] or a powder-bed process [4] and [5]. The heat source used for melting the additive material is usually a high power laser, an electron beam or a tungsten inert gas (TIG) welding source. Traditionally, the powder based processes have been developed towards the manufacture of small and complex geometries with less focus on the deposition speed. For large structures with moderate complexity, such as flanges or bosses, it is more rewarding to use wire based techniques since these will give a better surface finish, could lead to better material quality, and also higher deposition rates [6] and [7]. The use of wire will also give better process efficiency and cleaner working environment since all wire that is fed into the melt pool is utilized, in comparison to a substantial amount of scattered powders that are not melted when currently available powder based techniques are used. Basic process characteristics of wire-based additive manufacturing and material properties of deposited beads and 3D parts have been studied by many authors [3], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] and [20]. Combination of wire and powder feeding has also been reported [21], [22] and [23]. A conclusion can be drawn that a wire-based deposition process is sensitive to wire position and orientation relative to the melt pool and the deposition direction. Careful tuning of the wire feed rate, the heat input, and the travel speed are also important in order to obtain defect-free beads. Moreover, in [24] it is further argued that in order to achieve good process stability for multi layered deposition, continuous process monitoring and control of, e.g. the wire feed rate, is also necessary. In [25] a camera-based monitoring system is developed for closed loop control of straight-bead deposition and in [18] a temperature monitoring system is investigated for a laser metal-wire deposition process. However, apart from the mentioned references this subject has not yet been investigated to any great extent in the literature. By contrast, there are several publications on this subject for powder-based systems, e.g. monitoring and process control using cameras [26], [27] and [28], closed-loop height control using photo diodes [29], [30], [31] and [32], powder flow control based on motion system speed profile [33], temperature measurements using pyrometers [34], [35] and [36]. However, transferring these results to wire-based deposition systems is not a straightforward task since the two processes are dissimilar in many ways. A robotized laser metal-wire deposition system has been developed at University West, Sweden, in close cooperation with Swedish industry. For this process a camera-based monitoring system and closed loop control of bead width and height has been developed and demonstrated for straight bead deposition [25]. The work presented in this paper is a continuation of that work. The main contribution is a generalized height controller based on iterative learning [37] that can cope with arbitrary deposition patterns. It uses height information from preceding layers, obtained by means of 3D scanning, and prevents the deterministic disturbances (which the controller learns during the deposition), and compensates for non-deterministic temporary disturbances. The result is a stable deposition process with small errors and low control activity. The material considered in this paper is Ti–6Al–4V deposited on plates of same material.

A laser metal wire deposition cell has been built up at University West with a monitoring system, featuring cameras and a 3D scanner. The monitoring system enables on-line visual feedback, both of the process and the deposited layer's topological profile. Use of a scanner to produce 3D images of each layer gives a good insight into the process-induced disturbances and thus enables control of the process. This information can also be a valuable input for future modeling and simulation, or for development of Off-Line Programming tools. The structure of the measurement system allows for synchronized storage of all measured data into a database, which simplifies the extraction and the analysis of the experimental data. Using the developed monitoring system an in-process height controller has been developed and demonstrated. The control algorithm is based on iterative learning. The control signal adjusts the wire feed rate based on 3D scanned data of the deposited part. The results show that the developed ILC is able to learn the traits of a specific part on-line without any a priori knowledge of the deterministic disturbances. This is important since deposition of several adjacent beads within a layer generate height variations which depend on the chosen parameters in a complex way, and are therefore hard to predict. The controller is thereby able to compensate for local changes and maintain a smooth flat surface throughout the deposition. The experimental work is conducted using Ti–6Al–4V, but the results can be generalized to other materials as well. Furthermore, the controller is useful for arbitrary components and deposition patterns, while the scanner is mainly intended for small-size bosses.