American Machinist

Slab laser finds a home. (includes related article)


BECAUSE OF ITS SUPERIOR beam quality the YAG-slab laser Offers significant advantages over the conventional, or rod-type, as a materials-processing tool. Although General Electric's aircraft-engine plant in Evendale, Ohio, may be the only current user, others are apt to follow as the benefits become more widely recognized.

The slab laser, developed by GE, dates back to 1972 and US Patent 3.633.126, Multiple Internal Reflection Face Pumped Laser, granted the firm's J.P. Chernoch and WS. Martin. After successful designs for military applications, spawning ground for many of today's industrial lasers, a prototype system from GE R&D (Schenectady, NY) was installed at the engine plant in late 86.

That use of a slab, or rectangular cross-section, lasing crystal was a major advance in YAG-laser technology was quickly apparent, according to Dr Todd J. Rockstroh, project engineer at the plants's MTL Laser Center. Nickel-superalloy samples 2-in. thick were percussion-drilled in less than 2 min, and 1/2-in. stock was cut at 7 ipm, he reported at the ICALEO Conference of the Laser Institute of America (Orlando, Fla) in Boston last November. His was one of several presentations on slab lasers made at this conference.

The GE face-pumped laser, as the slab laser is also called, has an average continuous power of 50-500 W, a pulse duration of 0.6-2.5 msec, a repetition rate of 0-70 Hz, and a pulse energy of 70 J/pulse in a 1.5-msec pulse. There are two at the plant, one residing in a 3 x 4 x 9 ft enclosure at the top of a materials-processing center Fig 1) built by S.E. Huffman, which has six axes of motion (plus two for a trepanning mechanism) and a work envelope of 6 x 9 x 4 ft. The optical-path length varies from 9-19.5 ft.

Beam quality, expressed in mm-mrad, is the product of beam size and beam divergence. The lower the value, the better the quality and the greater the power density. As shown in Fig 2, the beam quality of the slab laser is at least twice that of the best rod type in the 100- to 250-W average power range, and far better still than rod types capable of 100-400 W Moreover, unlike the latter beam quality improves slightly with increasing power-to "7 mm-mrad (worst axis) at 500 W and [to] over 1-kW," according to Rockstroh. The other drawings in the group refer to drilling and cutting performance during initial trials at the plant.

These initial trials revealed, he noted, that the laser could process materials [nickel superalloys] at twice the speed of rod-laser systems" with similar metallurgical quality, or be applied so as to only marginally improve processing rates but double quality. The drilling data pertains to percussion-drilled holes having 0.0 I 8-in. entrance diameter, 0.013-in. exit diameter, thus .0025-in. end-to-end taper, through 2-in.-thick samples, one of which appears in Fig 1.

The laser is "the preferred choice for integration into large machine tools with `flying' optics and delivery via fiberoptics," Rockstroh said. Output power is scalable with crystal size, and the slab is the natural' multipass amplifier geometry [for] improving output power and gain while minimizing beam distortion." In multipass configurations, hole geometries typical of aircraft-engine components have been made with little taper and average recast (melted metal resolidified on the hole wall) of only 0. …

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