Theoretical and experimental analyses of titanium sheet metal bending by nd:YAG laser

Laser Bending is a new non-contact method of forming sheet-metal components which does not require any special tools. This process is highly accurate and can be easily automated. This would save a great deal of time and cost which is essential for tooling design and fabrication in conventional processes. Laser bending also provides flexibility in process planning with various processes such as welding, cutting, cladding and surface treatment that can be carried out at a single Laser workstation. <br><br>In Laser Bending, the metal sheet is locally heated along a path by a laser beam. This forms a steep temperature gradient across the thickness i.e. between top and bottom surfaces. The top surface expands because of higher temperature, forcing bottom surface to contract. This bends the sheet away from the laser. As soon as the laser source is switched off, the surface starts to cool rapidly and the temperature gradient is reversed due to heat conduction. This results in bending the sheet towards the laser beam and thus giving it the final shape. Bending angle can be controlled by varying a) Laser Power, b) Scan Velocity, c) Beam Diameter, d) Sheet Thickness and e) Number of scans repetitions. <br><br>Laser bending has already been applied for micro-bending and adjusting curvatures of bent parts used in Computer hard disk drives. Other applications include bending thick plates for Ship-building, repairing welding distortions, straightening sheet-metal parts and sculpture making. Also Laser Bending can be a suitable process for bending high strength materials such as Titanium and its alloys, commonly used in Aerospace Industry and Medical Implants Fabrication.<br><br>Titanium is usually formed by special hot forming processes such as Hot Brake Forming and Superplastic Forming, which require special die designing and are only suitable for larger components. Laser Bending can be a better alternative to these processes because of its good controllability, less set-up time, ability to form complex shapes and repeatability. In this thesis, the feasibility of a 500W Nd:YAG Laser to bend Commercially Pure Grade-2 Titanium is examined. A theoretical model has been developed to predict the bending angle as a function of process parameters. The model is then verified with experiments on the Nd:YAG laser. Due to high temperatures involved in Laser bending and oxide film (α-case) formation, material properties change drastically and that reflects on reducing the bending angle per scan in multi-scan system. To reduce oxidation Inert gas shielding has been used. Different nozzle positions and inert gas combinations (Argon and Helium) have been used to enhance the bend quality and bend angle. Effects of these design changes on width of Heat Affected Zone (HAZ), thickness of sheet at HAZ, material hardness and final bending angle are discussed. As Grade-2 Titanium is a highly reflective material, different coatings such as graphite and thermal paint have been used to improve absorption of the laser beam. The effects of these coatings on bending angle and bend quality are covered.