PSI - Issue 8

418 P. Conti et al. / Procedia Structural Integrity 8 (2018) 410–421 Author name / Structural Integrity Procedia 00 (2017) 000–000 Fig. 11 synthetizes the results of the DOE analysis with regards to maximum temperature in the entire process. The bar diagram shows the relative influence of the physical parameters and their cross effects. It is evident that the influence of the Young modulus is null, as expected; the heat capacity has not a very significant influence but the parameter k, thermal conductivity, has a relevant importance as it represent the main cooling mechanism. We can argue that a reliable FE model requires a precise knowledge of the thermal conductivity, more experimental studies must investigate the evolution of the thermal conductivity in the powders at different void content and temperature. Fig. 12 synthetizes the results of the DOE analysis with regards to maximum stress along the diagonal in the entire process. Here all the parameters have some significant influence but, of course, the most important factor is Young modulus. Again, the thermal conductivity is important but a combined effect of specific heat capacity and conductivity is relevant too; this last result can suggest that the important parameter could be a combination of the two formers and therefore thermal diffusivity, �/��� , could be a more meaningful parameter for the optimization of the SLM process.

A = Thermal conductivity B = Specific heat capacity C = Young modulus

A = Thermal conductivity B = Specific heat capacity C = Young modulus

Fig. 11 – Effects on maximum temperature

Fig. 12 – Effect on maximum stress

3.2. Optimization of the technological parameters The scope of this analysis is the comparison of different sets of three technological parameters (laser power, overlap and beam speed). For every parameter three levels were considered (Tab III). Tab. III – Parameters levels Low level Mean level High level Laser power [W] 100 120 150 Scan speed [m/sec] 25 50 100 Overlap [%] 0 25 50 A set of ten FE analyses were carried on. Two different aspects can be considered in order to compare the performance of the parameter set: the maximum temperature and percentage of melted elements. Tab. IV summarizes the results. On the basis of the first 8 test of Tab. IV it appears that the most significant parameter is the speed of the laser beam. This result is highlighted in fig.13 - based on DOE analysis of the first 8 tests - where the influence of the different parameters is displayed. The parameter sets of the first 8 tests are in accordance with an L8 orthogonal test array (R.K. Roy, 2001). Tests no. 9 to no. 10, which do not belong to an orthogonal array, confirmed the results. The last column of Tab. IV shows that in many tests the quantity of elements melted was too low, only the tests with low speed yield acceptable performances Two results are displayed in fig.14 and 15. Fig 14 refers to test no. 6: only 70% of the elements are melted because the heat does not penetrate inside the layer. Fig. 15, corresponding to test no. 9, shows a comparison between the melted elements and the initial meshed model; here 97% of the elements are melted.