Remote Laser Welding with High Power Fiber Lasers

Paul Kah, Jinhong Lu, Jukka Martikainen, Raimo Suoranta

Laboratory of Welding Technology and Laser Processing, Department of Mechanical Engineering, Lappeenranta University of Technology, Lappeenranta, Finland.

DOI: 10.4236/eng.2013.59083                

Abstract

The introduction of the high power fiber laser with brilliant beam quality has enabled the rapid development of remote laser welding (RLW). This paper presents a theoretical review of remote laser welding. As a promising technology, RLW offers increased flexibility, high operational speed, and reduced cycle time to process a wide range of workpieces. This study presents the typical characteristics of RLW with high power fiber lasers. It also investigates the influence of process parameters such as laser power, welding speed, shielding gas supply, beam inclination and focal position on the weld seam quality.

Keywords

Remote Laser Welding; Scanner; Fiber Laser; Process Parameters

1. Introduction

Remote laser welding (RLW), a non-contact robotic laser welding technique, has been developed and implemented to improve the productivity and flexibility of conventional laser welding [1]. The first experimental analysis of RLW was reported by John Macken in 1996 [2]. The remote welding process is characterized by a long focal length (up to 1600 mm), a high-power and brilliant-quality laser source, and beam deflection by the scanner [3-5]. Compared to conventional laser welding, the remote welding technology offers increased flexibility, higher working speed, and reduced cycle time [3,6].

Thus far, RLW has been studied and implemented with a variety of high-power laser sources. In [1], the RLW systems are implemented with CO₂ and Nd: YAG lasers. In [7], the RLW experiments are carried out with Nd: YAG and disk lasers. RLW applications with highpower fiber lasers have been introduced in [6,8]. With a wavelength of 1080 nm, these high-power fiber lasers enable beam delivery through the fiber, enhancing spatial flexibility and accurate focusing. By contrast, it is impossible to deliver the beam of CO₂ lasers via an optical fiber due to its rather long wavelength of 10.6 µm [7]. The suitable wavelength of high-power fiber lasers, combined with excellent beam quality, makes them a promising alternative to conventional CO₂ and Nd: YAG lasers in RLW applications.

The operating principle of RLW is normally based on using a scanner to deflect and position the laser beam onto the surface of the workpiece travelling at high speed [9] and, at present, 2D-scanners are the most widely adopted scanners in remote welding applications. The 2D-scanner unit is a galvanometer system, in which two lightweight mirrors are used and rotated by motors. The system can handle a laser power of up to 5 kW and is more economical than 3D-scanners [10].

Remote laser welding the technology is not yet very widespread, although it clearly has potential in the automotive industry, such as in the seating, body in white and interior parts. RLW has replaced resistance spot welding with its increased laser usability rate and reduced process times in car body construction [6].

However, RLW still faces many challenges; in predicting process behavior, in ensuring acceptable and reliable weld quality, and in dealing with issues such as shielding gas supply, clamping and coated sheet metals [11,12]. Compared to conventional laser welding, a greater number of process parameters have to be taken into account in remote welding applications. These parameters can be primarily divided into the areas of beam quality, processing parameters, and material properties [6].

2. Remote Laser Welding

2.1. Principle

Remote laser welding is not a new technology, but is based on the principle where a scanner deflects and positions the focused laser beam over the workpiece from a distance of typically 1000 - 1600 mm [13]. The first experiment in laser keyhole welding with 1600 mm focal length, by John Macken in 1996, is acknowledged as forming the cornerstone of remote laser welding technology [11]. The scanner enables the translation of the laser beam into large working areas of 1m × 1 m or over 2 m³ 3D working volume with a welding speed of up to 30 m/min [14].

Remote laser welding can be principally implemented with two modes; the scanner-integrated system and the robot-based system [15]. The scanner-integrated system utilizes a scanning unit (usually a 2D scanner) for positioning and focusing of the laser beam, as shown in Figure 1(a) [11]. RLW with the robot-based system is accomplished with a long focal length laser optic and a 6- axis robot, in which the robot serves for positioning of the laser beam on the surface of the workpiece, as illustrated in Figure 1(b) [11].

Compared to robot-based RLW, the scanner-integrated RLW system offers shorter processing times and higher accuracy in many applications. However, the laser beam quality requirements of scanner-integrated RLW are much higher than for robot-based RLW systems. Table 1 presents the typical performance of scanner-integrated and robot-based RLW systems versus conventional laser welding.

2.2. Requirements

The typical requirements for RLW to make sound welds can be divided into three categories: the scanner for delivering and positioning the beam adequately, a highpower laser with sufficient quality for the long focal length of the system, and proper control of process parameters [16].

2.2.1. Scanner

The scanner is used to guide and rapidly position the beam on the surface of the workpiece along the desired weld path [16]. The lightweight and highly dynamic scan head enables extremely fast movement of the laser beam between welds, which means that positioning requires much less time than in the conventional laser welding process [17].

Figure 2 shows the typical elements of a scan head [9]. A scanner unit consists of a group of mirrors and lenses [9,18]. In remote welding, the laser beam first passes through the lenses. The smaller lens moves along the optical axis in order to change the focal position. Afterwards, the laser beam is deflected and guided successively by the mirrors X and Y. Finally, the laser beam is focused on the workpiece precisely along the desired weld seams [9]. Figure 3 illustrates a classic 2D-scanner system including a seam tracking sensor, a line projector,

Table 1. Comparison of remote and conventional laser welding processes [4,13,15].

(a)(b)

Figure 1. (a) Scanner-integrated RLW system and (b) Robot-based RLW system without scanner optics [11].

Figure 2. Typical elements of a scan head [9].

Figure 3. Main components of the 2D-scanner system [19].

and a high dynamic 2D-scanner unit [19].

2.2.2. High-Power Fiber Laser

High-power fiber lasers have recently been developed with attractive characteristics for materials processing applications [20]. High-power fiber lasers with brilliant beam quality can produce ultra-high peak power density of several MW/mm², which is important for high speed RLW with long focal length [21].

Based on a number of studies, high-power fiber lasers have multiple advantages, such as [22,23]:

●      High electric efficiency.

●      Excellent beam quality.

●      Relatively low operational costs due to the long lifetime.

●      High flexibility in production because of the beam delivery with fibers.

●      High absorption coefficient for thin sheets of most metals.

●      Compact design and mobility.

Figure 4 shows an RLW cell with a high-power fiber laser anchored by a robot and equipped with welding head fixtures. It has been shown that remote fiber laser welding allows an increase in weld processing speeds, a reduction in consumables, such as weld wire and weld guns, less tooling and part fixturing, and a decrease in on-going maintenance costs typically associated with conventional welding processes [24].

Conflicts of Interest

The authors declare no conflicts of interest.

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