Issue 53

P. Ferro et alii, Frattura ed Integrità Strutturale, 53 (2020) 252-284; DOI: 10.3221/IGF-ESIS.53.21

of the melt pool and L as its length, using results from numerical simulation is it possible to predict the melt pool stability conditions [80] bypassing a fluid-dynamic analysis. Both numerical simulation and experiments showed that lack of fusion defects form during the first laser beam tracks (Fig. 22). This is because a stationary condition between the heat transmitted by the laser beam and the heat absorbed by the layer is not yet reached at the beginning of the process. Powder has a low thermal conductivity and thus the fusion zone is low. As the mass of consolidated material increases, the heat conduction becomes more efficient and the fusion zone increases reaching a stationary dimension. This suggests that a greater heat input would be necessary at the beginning of the process, during the transitory phase, in order to avoid defects.

Figure 22: Lack of fusions defects (identified by blue arrows) formed during laser scanning, comparison between experimental and numerical results obtained by Bruna-Rosso et al. [93]. The volume shrinkage induced by the transformation from powder to consolidate material was simulated by different authors [96-97]. The layer is divided into two sub-layers according to the powder porosity. As the powder reached the melting temperature, the upper sub-layer is removed by using for example the ‘birth and death’ numerical technique. In [84] the volume shrinkage is simulated by using the moving-mesh method. When the temperature at the top surface reaches the melting point and dT/dt > 0, the top surface moves downward with a speed v s that depends on the initial powder porosity. Numerical models that take into account this phenomenon were found more realistic compared to previous ones above all in the prediction of the thermal gradient in vertical direction and molten pool depth [84]. Schwalbach et al. [98] showed in their work how the amount of latent heat be negligible compared to the sensible heat. Using the Stefan number, defined by the ratio of sensible to latent heat:

E sens H sl

 1

T 0 T m 

St 

C p (T)dT

(38)

H sl

where H sl is the mass specific enthalpy of the solid-liquid transformation and C p is the mass specific heat capacity, it was observed that for Ti-6Al-4V alloy, the value of St = 3.81 means that sensible heat is roughly four times the latent heat of transformation.

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