Issue 53

P. Ferro et alii, Frattura ed Integrità Strutturale, 53 (2020) 252-284; DOI: 10.3221/IGF-ESIS.53.21 254 underlying solidified powder or incomplete particles melting [5]. Finally, swelling is a defect similar to the humping phenomenon in welding and it occurs due to surface tension effects related to the melt pool geometry [8]. It is well known that sound parts can be obtained by process parameters optimization. A formidable challenge is understanding the complex interactions between the processing parameters and the metallurgical and mechanical properties of additively manufactured components [9,10]. Process parameters are numerous and collect building direction, layer thickness, platform temperature, spot diameter, source power, scanning strategy, hatch distance, point distance (for non-continuous laser source), scan speed, exposure time. Fig. 2 summarizes some geometrical parameters in spot-to-spot fabrication process that need to be accurately calibrated. For example, although good overlapping of two consecutive tracks is necessary to ensure a good relative density of printed parts, exaggerated overlapping will reduce the printing speed and may be accompanied with negative phenomena like balling effect. On the other hand, the material to be processed is characterized by physical, thermal, metallurgical and mechanical properties that influence the choice of process parameters, as well. Finally, the geometry of the part plays an important role in the fabrication process. Every change in the geometry will change the way that the AM machine performs its fabrication routine affecting the properties of the resulting solid [11,12]. One way to study the interaction between final part properties and processing parameters is to carry out different ‘trial and error’ experimental tests. Figure 2: Definition of some scanning parameters (a). Spot-to-spot fabrication process, where ‘s’ is the point distance, ‘  ’ is the laser beam spot, and h is the hatch distance (b). This strategy is time intensive, requires extensive raw materials and is not always successful in identifying optimal processing parameters for specific applications. For instance, when a new component (or material) is planned to be produced (or shaped) by additive manufacturing, manufactures print small specimens with varying process parameters until a defects-free specimen is obtained. Results of the tests are often summarized by using the volumetric energy density parameter defined by Eq. (1) [14]: Ed  P v s hd (1) where P is the heat source power, vs is the scan speed, h is the hatch distance and d is the layer thickness. In an alternative formulation d can stand for the beam size. For chosen values of d and h, the volumetric energy density allows summarizing several combinations of power-scan speed values or power-exposure time (with v s = s/exposure time) values in a single ‘master diagram’ as shown in Fig. 3. Even if the master diagram of Fig. 3 can be used as a guideline in process design, limits in its use are quite evident. It is strongly material and process dependent; so that if the material or a process parameter, different from power or scanning speed, is changed, the master diagram must be change also. Further, as observed by Prashanth et al. [15] and Bertoli et al. [16], the applicability of Eq. (1) is still at stake, even though it has been widely used in literature for optimizing the SLM parameters [17,14]. Eq. (1) may not properly represent the effective energy transferred to melt the powder bed, therefore it needs to be improved involving the material properties and the laser-powder interactions. Another drawback of the above described experimental procedure is that, moving from small samples to real components production, a further improvement of process parameters could be necessary because of the geometric effects that are not taken into account in the parametric study.