Patent ID: 12226833

DESCRIPTION OF THE PREFERRED EMBODIMENT

The main components of the atomizing device shown in the drawing are a melt chamber1, a powder chamber2(also called an atomizing chamber), an induction coil3arranged in the melt chamber1, and a nozzle plate4arranged between the two chambers1,2, in which an atomizer nozzle5serves to interconnect these two chambers1,2. The nozzle plate4is flat on the outlet side16and oriented perpendicularly to the flow direction of a melt stream8.

In the melt chamber1, which is under an argon pressure p1, the material to be atomized is partially introduced into the conical induction coil3with three windings in the form of a cylindrical rod7provided with a 45° tip6, as is basically known, for example, from DE 41 02 101 A1. The conicity of the induction coil3corresponds to the conicity of the tip6of the rod7to be atomized. The tip6and in particular the surface of the tip6is inductively heated by medium-frequency current flowing through the induction coil3until a molten phase is formed at the surface. This melt stream8runs down the conical surface and drips off the tip6in the form of a continuous pouring stream. The mass flow of the pouring stream forming the melt stream8can be varied over a wide range between 0.4 kg/min and 2.5 kg/min via the electrical power inductively coupled in. A melt stream between 0.8 and 1.5 kg/min is considered particularly suitable for atomization. During atomization, the rod7rotates slowly about its axis of symmetry S and moves continuously downward. The diameter D of rod7, which can be between 30 and 200 mm, and the set lowering speed determine the respective melt rate. Rod diameters D between 80 and 150 mm have proved to be particularly favorable from a process engineering point of view.

A linear suspension9, shown only schematically in the drawing, provides the height adjustability H of the induction coil3, by means of which the free fall height of the pouring stream up to the nozzle and thus, as mentioned above, the viscosity of the melt as it enters the nozzle can be varied. Distances between the atomizer nozzle5and the induction coil3of 3 to 100 mm have proven to be technically useful. At smaller coil distances, there is a risk of voltage flashover from the coil to the nozzle; at larger distances, there is a risk of splitting of the pouring stream before it enters the nozzle opening. Horizontal coil windings have also proven to be particularly advantageous, since they prevent the casting flow from being deflected by electromagnetic forces when it leaves the coil magnetic field, in contrast to rising coil windings.

The rotationally symmetrical atomizer nozzle5is located with its center in the axis of symmetry S of rod7and coil3having the distance H below the lowermost winding in the induction coil3. It is arranged in a separate nozzle insert11, which is detachably seated in the nozzle plate4, and is indirectly cooled by pressing with the pressure p1onto the water-cooled nozzle plate4. The melt stream8is radially enveloped by the gas flowing from the melt chamber1into the powder chamber2, constricted and accelerated through the circular opening of the atomizer nozzle5to at most the speed of sound at the nozzle outlet. The driving force for this is the positive pressure difference between the gas pressure in the melt chamber p1and the gas pressure p2in the powder chamber2. This pressure difference is at least 0.2 bar, at the highest 25 bar. Technically particularly advantageous pressure differences are in the range between 2 bar and 10 bar.

Even at high pressure differences p1−p2, the atomizing gas V in the atomizer nozzle5is accelerated at most to the speed of sound due to the exclusively convergent nozzle shape, since in the supersonic range a convergent nozzle profile acts as a diffuser and slows the gas down again. The higher the pressure difference p1−p2, the sooner the acoustic velocity limit is reached in the nozzle profile. As a consequence, the gas flow is not laminar, since the gas pressure immediately at the nozzle outlet is a function of the pressure difference and significantly higher than the ambient pressure p2in the powder chamber.

The atomizing gas causes pressure and shear stresses in the jet-shaped melt stream8, constricting and accelerating the latter. The melt velocity in the melt jet decreases radially from the outside to the inside. After leaving the atomizer nozzle5, the compressive and shear stresses are instantly relieved by the rupture of the melt jet filament12into individual droplets which solidify in the atomizing chamber to form spherical powder particles. Surprisingly, this does not require a laminar gas flow or gas velocities greater than the speed of sound. On the contrary, atomization exclusively in the sub-sonic range improves the sphericity of the powder particles and reduces the gas porosity compared to the known LAVAL atomization. This is achieved by an exclusively convergent nozzle profile, in which the nozzle flanks13are circular arc-shaped in cross-section in the form of a pitch circle having a radius R of 2 to 15 mm, preferably of 5 mm, and a height h of the atomizer nozzle5, which is smaller than the convergence circle radius R. A tangent T at the nozzle outlet has an angle W of <90° with respect to the nozzle outlet side. In the specific embodiment example, the height h is 4.5 mm with a radius of convergence R of 5 mm. The diameter of the nozzle d can vary from 2 to 20 mm. In the embodiment example, the nozzle diameter d is 10 mm With these parameters, a d50−value of 50 μm is achieved in a Ti alloy powder at a pressure of p1=4.5 bar and p2=930 mbar.

Furthermore, the nozzle insert11is made of a material specific to the species to be atomized, for example TiAl or titanium. Its diameter E can be between 20 and 200 mm, preferably 140 mm.

The rod7, for example, can be a so-called EIGA electrode having a diameter D of up to 150 mm. In the embodiment example shown, a diameter D of 115 mm has been selected.

For the induction coil3in the form of an internally cooled, conical coil made of copper with a pitch of 45°, the internal diameter I of the uppermost winding14can be up to 170 mm, specifically for example 130 mm, and the vertical distance G of the uppermost, middle and lowermost coil windings14,15,10can have a dimension of 3 to 20 mm, preferably 8 mm. The diameter F of the coil tube may be 10 to 30 mm, preferably 16 mm A rectangular cross-section is also possible.

The distance H between the underside of the induction coil3and the nozzle5is 10 mm.