An Experimental Study on Bending Process of AISI 304 Steel Sheets by using Diode Laser Forming

Introduction

Laser forming is a non-contact method of producing bending and spatial forming, by which thin sheets are deformed due to a high thermal gradient inducted on a workpiece by means of high power laser beam. Main advantages of such technology over conventional sheet metal forming processes lie in absence of tooling, die and lubrication, high flexibility, possibility of automation and easiness of stirrup equipment (Vollertsen et al., 2004). The mechanism of laser forming is based on the shrinkage stresses arising during the cooling of a sheet previously irradiated with a laser source (Geiger & Vollertsen, 1993). The shrinkage in the sheet is minimum when compared to other conventional processes due to the very small volume of the heated material. In fact, laser bending has its roots in the bending by line heating using an oxy-fuel flame. Unlike oxy-fuel flame, laser beam is highly controllable in size and power. Because of this advantage, laser forming process is able to provide excellent reproducibility. The absence of a physical tooling system and the high gradients of temperature generated by laser beam complicates the setup of the process; therefore, in order to achieve the desired products in terms of geometrical shape, it is of crucial importance to assess the influence of main process parameters. Laser bending can be also used to deal with materials which are either extremely difficult or impossible to bend mechanically, such as titanium alloys (Walczyk & Vittal, 2000), brittle materials (Shen et al., 2009) and metal foams (Quadrini et al., 2010). Laser forming method was used for the precise adjustment of a rod (Qi & Namba, 2011) and to produce plastic deformation of sheet metal through shock waves induced by high energy pulsed laser (Jiang et al., 2013). An external mechanical force was added to a laser beam irradiation, in order to gain a 90-degree bending angle in aluminium alloys (Roohia et al., 2012). The thermal stress produced by heating with a CO₂ laser beam was employed to make a flat sheet from a sheet metal of protruded distortion (Ueda et al., 2011).

Laser forming, similarly to other laser material processing applications, such as laser heat treatment, involves many non-linear physical phenomena that include temperature distribution, stress field and microstructure variation, all of which are significantly inter-related. When compared with the input laser beam energy, the heat generated by the strain energy in the bending process is negligible (Hsieh & Lin, 2005). The rising of bending angle can be ascribed to three different mechanisms, which are dependent on workpiece material thermomechanical and geometrical properties and process parameters. In particular, it can be considered: Temperature Gradient Mechanism TGM, Buckling Mechanism BM, Upsetting Mechanism UM (Shen & Vollertsen, 2009). The TGM is usually used for analyzing the angular deformation of thick plates. In this case, a plate can only bend toward the laser beam. Due to the temperature gradient produced in the direction of thickness, the heated area on the top of the plate tends to expand while the surrounding material restricts this expansion. Therefore a compressive stress is produced on the top and a tensile stress at the bottom, causing plastic strain and deformation of the plate. The BM is usually used for analyzing very thin sheet metals. This mechanism occurs when the temperature gradient in thickness direction is small and the spot of laser beam is relatively large compared to the thickness (Jamil et al., 2011). In BM, the heated area expands in the longitudinal direction and is constrained by the cooler surrounding material. Upon cooling, in-plane shrinkage occurs and results in buckling of the plate if the magnitude of in-plane shrinkage is large enough. The sheet can be formed for either concave or convex shapes, therefore increasing the instability in the final shape. The UM is usually used for forming 3-D shapes of parts from plain sheets. In this case, a local change of the 2-D heating pattern affects the 3-D global curvature of the parts (Shi et al., 2012). This mechanism works well when the whole thickness of the parts are heated but no buckling occurs (Zhang & Michaleris, 2004).