# Interferometry of Gas-Phase Flows during Selective Laser Melting  
[PERSON]  
1 Laboratory of Innovative Additive Technologies, Moscow State University of Technology \"STANKIN\", 3a, Vadkovsky Lane, 127055 Moscow, Russia; [EMAIL_ADDRESS] (A.E.S. ); [EMAIL_ADDRESS] (R.S.K. ); [EMAIL_ADDRESS] (A.D.K. ); [EMAIL_ADDRESS] (A.V.G. )1  
[PERSON]  
1 Laboratory of Innovative Additive Technologies, Moscow State University of Technology \"STANKIN\", 3a, Vadkovsky Lane, 127055 Moscow, Russia; [EMAIL_ADDRESS] (A.E.S. ); [EMAIL_ADDRESS] (R.S.K. ); [EMAIL_ADDRESS] (A.D.K. ); [EMAIL_ADDRESS] (A.V.G. )1  
[PERSON]  
1 Laboratory of Innovative Additive Technologies, Moscow State University of Technology \"STANKIN\", 3a, Vadkovsky Lane, 127055 Moscow, Russia; [EMAIL_ADDRESS] (A.E.S. ); [EMAIL_ADDRESS] (R.S.K. ); [EMAIL_ADDRESS] (A.D.K. ); [EMAIL_ADDRESS] (A.V.G. )1  
[PERSON]  
1 Laboratory of Innovative Additive Technologies, Moscow State University of Technology \"STANKIN\", 3a, Vadkovsky Lane, 127055 Moscow, Russia; [EMAIL_ADDRESS] (A.E.S. ); [EMAIL_ADDRESS] (R.S.K. ); [EMAIL_ADDRESS] (A.D.K. ); [EMAIL_ADDRESS] (A.V.G. )1  
Footnote 1: email: {www.mdpi.com/journals/applsci/pdt  
Increasing the productivity of the SLM process is one of the main objectives [3]. The productivity largely depends on the scanning speed of a laser beam when growing products. Along with power and diameter of the laser beam, scanning speed is a parameter that determines the manufacturing of high-quality objects without pores, cracks and other defects of micro and macrostructures [4]. The balance between productivity and stable quality is sometimes reduced to an integral indicator characterizing the amount of energy transferred to the powder material per unit area (1):  
\[E_{D}=P/(DV), \tag{1}\]  
where \(P\) is laser power, \(D\) is the laser beam diameter and \(V\) is the laser scanning speed [4]. Increasing the scanning speed without changing the laser power leads to low-energy instability of the fused material, resulting in the formation of individual droplets. Such an effect arises as the result of capillary instability of molten material. A gradual increase in power to compensate this effect leads to an increase in the energy density in the laser spot. Over the past few years, the phenomenon of intense evaporation during SLM with a high energy density has been discovered in various scientific groups [5; 6; 7]. Evaporation not only leads to loss of mass and energy in the laser spot but also causes a jet flow in the gas phase, which entrains the powder particles from the adjacent areas of the powder layer; thus, playing a key role in mass transfer [8; 9]. This phenomenon was first explained by the evaporation of the material under laser irradiation and the entrainment of the ambient gas by the vapor jet due to Bernoulli effect [8]. This is the formation of the so-called denuded zone along the edges of the single track of molten material, which for a long time was an unresolved theoretical problem [10]. Besides, it was shown that the vapor recoil pressure significantly affects the melt pool and can lead to the formation of a keyhole. This effect determines the stability threshold of the melt pool and the entire SLM process at high energy densities, limiting the productivity [8; 9]. It is also noted in some works that the resulting vapor-gas jet affects the absorption of laser radiation, reducing its useful power in the SLM [10; 11; 12; 13]. Recent experimental works visualized the evaporation jet and particles it lifted and made it possible to estimate a typical jet velocity of several meters per second [5; 14; 15; 16; 17]. It experimentally confirms the importance of the gas-phase flow transporting the powder particles during SLM. Until now, theoretical concepts of the SLM mechanism have been based on the assumption of either direct interaction of laser radiation with powder particles, leading to their melting, or the absorption of a powder layer by a moving melt pool.