Lasers at Work – More Efficent Drilling and Welding

Except for computer controls, not since Henry Ford’s invention of the assembly line has a technology had a more significant effect on manufacturing. Since their invention a little more than 60 years ago, lasers have improved the speed and efficiency of even processes that have already benefited from automation. Two examples are drilling and welding, both of which are used extensively in high-volume industries such as automotive and aerospace.

Drilling holes cleaner and faster
The ability of lasers to drill small cylindrical holes, typically 0.3 to 1 mm in diameter, at angles from 15 to 90 degrees to a metal surface at rates between 0.3 and 3 holes per second has enabled the production of more efficient turbine engines such as those used for aircraft propulsion and for power generation. Increasing the number of cooling holes used in such engines has led to new designs that improve fuel efficiency, reduce noise, and lower NOx and CO emissions.

Laser drilling of cylindrical holes generally occurs through melting and vaporization (also referred to as “ablation”) of the work piece material through absorption of energy from a pulsed laser beam. The energy required to remove material by melting, called melt expulsion, is about 25% of that needed to vaporize the same volume, so a process that removes material by melting is often favored. Melt expulsion also results in a material removal rate of ten to hundreds of micrometers per pulse, one or two orders of magnitude higher than that typically achieved by vaporization. In order to achieve melt expulsion, however, some vaporization must occur.

Melt expulsion arises as a result of the rapid build-up of gas pressure, or recoil force, within a cavity created by evaporation. For melt expulsion to occur, a molten layer must form and the pressure gradients acting on the surface due to vaporization must be sufficiently large to overcome surface tension forces and expel the molten material from the hole. The temperature at which the recoil and surface tension forces are equal is the critical temperature for liquid expulsion. For instance, liquid expulsion from titanium can take place when the temperature at the center of the hole exceeds 3780ﹾK (3507ﹾC).

High speed welding
Laser beam welding (LBW) is a technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications, such as in the automotive industry, where the position of the laser beam is often controlled by a robot arm.

Laser beam welding has high power density, on the order of 1 MW/cm2, resulting in small heat-affected zones and high heating and cooling rates. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point. Penetration is maximized when the focal point is slightly below the surface of the work piece.

LBW is a versatile process, capable of welding carbon steels, stainless steel, aluminum and titanium. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the work piece.

Since depth of penetration is critical, especially when welding thicker work pieces, the power of the Laser Welding must be carefully controlled and it must be precisely directed. Internal optical components of the laser such as lenses, optical windows and mirrors supplied by Esco Custom Optics, are manufactured to extremely tight tolerances to assure assure optimum weld quality.