Papers by Author: Erica Anna Squeo

Abstract: Stainless steel sheet metals were laser bent by means of a high power diode laser at different values of power and scan velocity. The laser power ranged from 100 to 300 W (with an increment of 50 W); two scan speeds were used, 4 and 8 mm/s, and the number of passes was 2, 4 or 6. In the experimentation, the values of bending angle, microstructure and residual stresses of the laser bended sheet metals were analyzed with regard to the input variables. In particular, residual stresses were evaluated by means of X-ray analysis in terms of first and second order stress. Measurements were performed on the convex surface of the sample in the laser beam action zone. The bending process was numerically simulated by means of a thermo-mechanical finite element model, implemented to predict the sheet metal bending angle as a function of the laser power and scan velocity. The residual stress distribution was extracted from the numerical simulations and its agreement with the experimental observations was discussed. As a general conclusion, the effect of multiple scans is hardly simulated by thermo-mechanical models which do not take into account the material annealing during forming.

Abstract: In this study, an easy and innovative technique for laser welded butt joint characterization is proposed. AISI 304 sheets, 1.25 mm in thickness, were welded in the butt configuration by means of a high power diode laser (HPDL). Different combinations of the process parameters were considered. For each combination, during the process different thermal cycles were induced in the material generating heat affected zones, which were subsequently tested by the double indentation test method. This test was performed on the sheets by means of two co-axial flat indenters, 1 mm in diameter, made of tungsten carbide (WC); the upper indenter penetrates at a constant rate (0.1 mm/min) into the material, whereas the lower indenter acts as a support. On each side of the seam, 10 indentations were performed at fixed positions with different distance from the centre of the seam, to obtain a load map correlated to the mechanical properties. By comparing the maps of the different laser welded joints, a clear effect of the laser scan speed and the power was found. Furthermore, a wake field effect is recognizable, in fact the loads are symmetric as regards the seam, and there is a clear trend in the direction of the laser path. These results confirm the effectiveness of this method, which is also suitable for on-line application because a very small indentation is left on the sheet.

Abstract: A process innovation is proposed by the authors to weld aluminum alloy tubes by means of a high power diode laser. In order to make the temperature profile uniform along the entire welding line, multiple passes of the laser source along the welding path can be performed at very high scan speeds. In the current study, this effect is achieved by focusing the standstill laser spot on the external surface of the aluminum alloy tubes which were put in rotation at high speed. The tubes were clamped together by using a threaded rod passing inside the tubes. Experimental tests were performed by changing the aluminum alloy heat treatment (6060 T1 and 6060 T5), the length of the samples to weld (80 and 100 mm), and the laser power (ranging from 800 to 950 W). The outer diameter of the tube was 10 mm, the thickness was 1 mm, and the rotational speed was kept constant at 1000 rpm. The welding time was monitored during the tests and ranged from 30 to 100 s in dependence on the material and process parameters. Specimens were extracted from the joints to perform tensile tests so as to measure the tensile strength. In the best condition, a high tensile strength was obtained (about 140 MPa).

Abstract: An easy and innovative technique for metal sheet characterization is described. A double indentation is performed on sheets by means of two co-axial small diameter flat indenters made of WC. A very small indentation is left on the sheet, so as to consider this technique a non destructive one, particularly suitable for on-line application. The proposed method was tested on sheets of aluminum alloy (6082 T6) with several thicknesses (nominally 0.6, 0.8, 1 and 1.5 mm). Double indentations were performed changing indenter diameter (1 and 2 mm) and testing rate (from 0.05 to 1 mm/min). In order to make a comparison with indentation tests, flat specimens were cut from the same sheets and standard tensile tests were performed. A very good correlation was found between indentation and tensile test results, showing the effectiveness of the proposed method. A suitable data normalization is necessary to correctly compare indentation and tensile data. The best results were obtained using the smaller diameter indenter. The testing rate seems to be not relevant in the experimented range, suggesting that a fast procedure can be defined on purpose for on-line application.