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Spring 2023 Contents: Issue No. 58
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Beam manipulation techniques in laser welding including trepanning head, wobble head, and galvo head
In this nugget, we review different mechanisms for beam manipulation that greatly expand capabilities of laser welding systems.
Download an article on Beam Manipulation for Laser Welding
(Below is text of the article without figures; if you would like to download pdf copy with figures and tables, please click on the link above)
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Laser welding is one of the high energy density processes that deliver energy in a non-contact manner; the other is electron beam welding, often known as e-beam. While e-beam has its limitations (need to work in a vacuum, and parts to be welded need to be electrically conductive), one of its main benefits is the ability of the electron gun to move the weld spot by changing the electromagnetic fields. Such movements could be actuated at seemingly infinite speed, leading to a very high linear speeds not feasible by any mechanical x-y tables. On the other hand, laser systems were limited to keeping the beam fixed at given location, and the part was moved on an x-y table or a rotary. Due to inertia of an x-y table, linear speeds were limited, and sudden changes in direction even more so. If you have been following developments in the laser welding world, you probably have heard of fiber lasers and disk lasers which have significantly improved beam quality compared to conventional YAG lasers in terms of lower beam-parameter-product which leads to smaller spot size for a given focal length resulting in very high energy density. High energy density allows welds to be operated in the keyhole mode thus producing deep and narrow welds. However, one of the constraints of the keyhole mode is the requirement for high linear speeds; a limitation with x-y tables. Laser and ancillary manufacturers have solved this problem by designing systems than manipulate the beam by moving light weight mirrors and prisms in the focus head to produce linear speeds of the order of 1000 mm/sec, much faster than a typical x-y table at 100 mm/sec. There are three beam manipulations mechanisms that are commercially available. The oldest one being the trepanning head which manipulates the beam by passing the collimated beam through a set of counter-rotating prisms. Prism offset and rotation speeds can be adjusted to produce profiles such as circles, triangles, and rectangles, amongst a few others. Based on beam quality and focal length, circles of the order of 50 mm diameter can be made at a rotation speed of 50 Hz; see Figure 1(a). A more recent invention is the wobble head that also has moving optical elements in the head and is able to manipulate the beam in circles, figure-of-eights, and linear rastering. The wobble head is able to rotate the beam at frequencies up to 1 kHz but at a smaller radius of 5 mm compared to the trepanning head. Weld made with the wobble head, as shown in a weld section in Figure 1(b) below, can produce a wide shallow weld with a lower power laser which otherwise (without wobble) would have required a much higher power laser. (Download pdf version to see photos) Figure 1. Photo (a) on the left shows top view of weld made with trepanning head on aluminum; photo (b) on the right shows a wide weld made with small spot size and a wobble head. Both trepanning head and wobble head are able to move the beam around but in a limited x-y range and pattern, so usually these heads are paired with a conventional x-y motion system to obtain a full range of motion, but again limited by x-y system inertia. Such limitation is overcome by galvo heads where the collimated beam is bounced off two galvo mirrors and focused through the large diameter lens such that the scan field is typically of the order of 6”x6” (150 mm x 150 mm) although larger lens diameters with fields up to 1000 mm x 1000 mm are available. With such a large working zone, need for x-y motion may not be necessary depending on the application. Galvos also have almost limitless ability to program shapes and profiles, and thus can reproduce profiles of trepanning and wobble heads, in addition to many others. Galvo heads are frequently used to make linear welds at speeds of up to 1200 mm/sec. (Note that maximum speed possible with galvo heads can be much higher and are used for marking applications.) Galvo heads can also be used to move the beam in a spiral pattern to produce a single large spot, and can be used to make multiple individual spot welds over a large area. Figure 2(a) shows a large spot weld on copper made with spiral pattern starting from the center. Figure 2(b) shows a transverse section of linear CW weld made at speeds of the order of 1000 mm/sec. The only downside to galvos is that as the work field and focal lengths increases, the input beam has to be a smaller diameter to get a reasonable spot size on the work piece. (Download pdf version to see photos) Figure 2. Photo (a) on the left shows a large spot weld made with a spiral pattern of beam movement; photo (b) on the right shows a transverse section of linear weld; both made with a galvo head. With the advent of the fiber laser and disk lasers systems, laser welding has significantly increased the opportunities compared to conventional YAG lasers. However, to truly capitalize on their capabilities, the user has to couple good beam quality with suitable beam manipulation which will allow the engineer to vastly expand the reach and scope of the laser system. (Note: Equipment specifications discussed in this newsletter are based on presently available information; please contact vendors for updated capabilities and specs)_________________________________________________