Machinery's Handbook, 31st Edition
Electro-Thermal Processes 1347 effect results in a higher-quality cut than can be obtained with oxygen. Nitrogen-assisted cut edges suffer no discoloration and are oxide-free, so they are ready for subsequent welding or painting. Since the assist gas is inert, there is no addition of energy to the process, and achievable cutting speeds are lower than with oxygen. The high viscosity of molten steel (compared to that of iron oxide) can lead to dross adhesion at the bottom of the cut. This can be addressed by using very high assist gas pressures, resulting in high consumption of pure nitrogen and increased operating costs. Air-assisted cutting is often used with aluminum, but the air must meet high cleanliness and pressure requirements. The presence of reactive oxygen in air can enhance cutting speed, while still producing a clean cut edge in aluminum; however, this technique can require high investment costs for a setup that delivers clean air at elevated pressure and high flow rates. Instead, laser cutting with shop air, which is both inexpensive and com - monly available in industrial settings, can be used to cut relatively thin steel (both carbon and stainless steel) far more economically than using pure oxygen or nitrogen. Assist gas flow, pressure, purity, and delivery (standoff distance and jet shape) all affect cut characteristics, necessitating careful selection and monitoring of gas pressure, nozzle size, feed rate, laser power, and material thickness. For nitrogen-assisted cutting, high gas volumes and pressures are needed to successfully cut narrow kerfs, and selection of the gas nozzle size will directly affect volume capability. In many cases, a larger nozzle diameter is needed to cut a narrower kerf. For oxygen-assisted cutting, gas pressure plays a key role in determining cut quality: low pressure can result in incomplete cuts, while high pressure can lead to uncontrolled burning and a rough cut edge. Laser cutting equipment manufacturers provide a variety of nozzle types designed to provide assist gas flow characteristics tailored for optimal performance. Performance requirements and user preferences determine system priorities, such as cut edge quality (roughness, angle), cutting speed, gas consumption, and so on. Fig. 1 shows a laser cutting head with a commonly used double-nozzle design, which can accommodate two assist gas streams: an inner, primary assist gas stream and an outer, shield gas stream. These two streams could be fed by different gases, or by different supply pressures of the same assist gas to produce the desired outcome.
Laser Beam
Focusing Lens
Shield (Outer) Assist Gas Outer Nozzle
Primary (Inner) Assist Gas
Inner Nozzle
Workpiece Fig. 1. Laser Cutting Nozzle with Two Assist Gas Pathways
Laser Cutting Heat Effects: Laser cutting and machining normally exposes workpieces to high thermal energy for a brief period. This leads to formation of a narrow heat-affected zone (HAZ) and minor heat effects in the part. Plastics and composites generally are much more vulnerable to heat effects than metals; some materials, such as PVC, will give off hazardous fumes, and others may discolor, warp, or char. The thickness of the HAZ for a laser cut in thin material is small, often under 0.005 in. (0.13 mm), and is based on beam focus, assist gas, speed, and part material and thickness. The HAZ will be thicker around a cut in thicker material than in a thinner piece. Pulsing the laser can help to reduce HAZ thickness and other heat effects. Ultra-short pulse laser systems, some- times called femtosecond lasers, are available that create no HAZ in applicable materials. Efficiency loss during laser cutting occurs primarily due to heat conduction in the workpiece. In thermally conductive materials, a portion of the absorbed laser energy is
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online