A Working Laser Cutting Head: the emergence of laser cutting techniques has significantly improved our ability to create and modify materials (Courtesy of TradeMachines)

What Is Laser Cutting

Cutting is an intuitive application of laser devices. When converging a beam of photons onto a small spot, the resulting high energy density can easily vaporize materials. Thus, by moving the beam along a designed path, the workpiece can be separated into individual parts. Laser cutting techniques have now evolved into multiple forms, including vaporization cutting, fusion cutting, reactive fusion cutting, thermal stress cracking, scribing, and cold cutting. Each one of them involves different operating modes (pulsed or CW), energy level, and utilization or not of an assist gas. The choice of the optimal plan depends on material physical properties, light sources and other factors. Today, laser cutting has become the most common laser manufacturing technology, well-established in numerous industrial fields. The market share of laser cutting was 23% of the worldwide industrial laser applications in 2007, confirming it as one of the most firmly established laser techniques.

Applications Of Laser Cutting

Laser cutting is never built as an isolated technique, rather, it has to be incorporated with other production approaches so as to maximize manufacturing versatility and efficiency. Perhaps the most prominent example lies in the modern production lines of automobiles.

Laser cutting combined with hydroforming has become an overwhelming technological trend in manufacturing of automobile components. Hydroforming is a process in which tubes and other hollow members are produced by forcing a fluid into a preform. The first application obtaining noticeable interest was the rear axle suspension used in the 1994 BMW 5 series. Today, automotive industry adopts hydroforming to produce vehicle frames, engine cradles, roof pillars and suspensions particularly in trucks and SUVs.

Hydroforming assisted by laser cutting is able to produce automobile parts with increased plate size, thickness and accuracy. Compared with traditional cutting methods such as plasma cutting, this technique boasts not only improved efficiency and productivity, but also clean and accurate cut edges in both square and bevel configurations. In addition, distortions in components made by laser cutting are significantly low. Hence, parts fabricated this way usually demand minimal secondary processing. This is the major reason that all circular holes down to 3mm in diameter are now laser cut, rather than drilled or punched. The most significant rewards of high accuracy in laser cutting are gained downstream. Designs are carried out with minimal deviation, components fit together more closely, while seamless joints benefit the final automobiles with superior structural robustness.

An animated process of hydroforming (Courtesy of Wikipedia)

The first car constructed with prefabricated laser-cut metal parts was Ford Capri II. Today there is a great interest in the use of robot-mounted fiber optic beam delivery from a Nd:YAG laser for cutting features in hydroformed parts.

Comparison with Plasma Arc Cutting

Laser cutting challenges many conventional techniques including plasma arc and waterjet cutting, which utilize thermal and mechanical processes.

By accelerating a jet of hot plasma, plasma arc achieves cutting mainly on electrically conductive materials, and is particularly effective for metals of high thermal conductivity. Today, most plasma equipment with air or oxygen is employed for two-dimensional cutting of sheet materials. Plasma arc cutting is able to process materials at moderate speed. The edge quality can be improved by operations under water, and the productivity can be increased if using multiple torches held by gantries.

Nonetheless, compared with plasma arc technique, laser cutting exhibits superiority in several areas. First, it boasts a higher cutting speed on thick materials, especially when a square edge of high quality with a narrow kerf width is required. Additionally, unlike  plasma arc cutting which works mostly on metals, laser cutting also works on non-metal materials. Most importantly, laser cutting technique offers a higher accuracy and a smaller heat affected zone (HAZ). The non-contact mode of operation along with relatively low running costs makes laser cutting a preferred choice for many applications. In contrast, plasma arc cutting requires frequent replacements of the nozzle.

Comparison with Mechanical Cutting Techniques

Mechanical cutting – a traditional technique – also suffers from a series of problems. Cutting using dies often results in inconsistent size due to the existence of material elasticity. Knives cannot cope well with sharp profiles requiring rapid changes in direction. Compared to these methods, water jet cutting is now a popular mechanical approach adopted by many factories in the world.

A flat aluminum alloy sheet being cut by a laser (Courtesy of Petri Metsola)

Waterjet cutting uses pressurized water of up to 400MPa to cut materials. A high pressure pump forces the water through a small diameter sapphire or diamond nozzle (0.1-0.8mm) at speeds of about 800m/s. Abrasive particles when mixed with water can enable cutting of metals, composites and other hard materials. Silica, silicon nitride, garnet or alumina with diameters of 0.1-0.5mm are usual choices as abrasive particles. Water jet cutting is actually an erosion process and generates no HAZ. These properties are essential in some applications, especially in the aerospace industry. Waterjet cutting works great on inhomogeneous materials such as marble and concrete that could fracture on heating, and composites that delaminate when heated.

Compared to laser cutting, waterjet cutting shows better performance on reflective materials such as copper and aluminum, as well as heat-sensitive materials such as high alloy steels and titanium alloys. However, it suffers from a slow cutting speed, and hence a relatively low throughput. Most importantly, when cutting materials up to 100 mm thick, waterjet cutting exhibits problems with the edge profile. Due to the high energy of the jet, the top of the cut edge is smooth, but becomes rougher and striated lower in the workpiece as abrasive particles are scattered. In contrast, laser cutting features high accuracy of cut edges. It is more suitable for three-dimensional out-of-position cutting, and normally provides the best combination of quality and productivity.

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