Abstract:
This invention relates to a method and apparatus for atomizing a liquid stream of metal or metal alloy. This invention relates to producing powders as well as to spray deposition process. During atomizing, a backpressure is created below the exit of the liquid delivery nozzle by the impingement of the atomization gas jets around the atomization zone. And this may block further atomization. The present invention provides a method of atomizing and an atomizing apparatus to control the backpressure. During atomizing, the intensities and directions of the atomization gas jets affects the atomization characteristics. The present invention provides a method of atomizing and an atomizing apparatus to control both the intensities and directions of the atomization gas jets.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for atomizing a liquid stream of metal or metal alloy. This invention relates to producing powders as well as to spray deposition process. 
     BACKGROUND OF THE INVENTION 
     For both powder production and spray deposition process, there are traditionally two kinds of atomization devices for atomizing a liquid stream of metal or metal alloys coming out of the liquid delivery nozzle into a spray of droplets. One is the &#34;Free Fall&#34; type of design, in which the stream of metal or metal alloy is atomized at a certain distance away from the exit of the liquid delivery nozzle. The other design is the &#34;Confined&#34; type of design, in which the stream of metal or metal alloy is atomized at the exit of the liquid delivery nozzle. The Confined type of atomization device gives more efficient and uniform transfer of energy from atomization gas to the stream of metal or metal alloy, due to the shorter distance between the atomization gas and the stream of metal or metal alloy and prefilming of the molten metal or metal alloy over the end of the liquid delivery nozzle. However, since the impingement point of the atomization gas is close to the exit of the liquid delivery nozzle, the molten metal or metal alloy is easier to freeze-up inside the liquid delivery nozzle, which blocks further atomization. The Free-Fall type atomization device doesn&#39;t have the freeze-up problem; however, the atomization efficiency is reduced compared to the Confined type of atomization device, resulting in coarser atomized powder and coarser microstructures due to a lower cooling rate. 
     During atomizing, a backpressure is created by the impingement of the atomization gas jets around the atomization zone below the exit of the liquid deliver nozzle. The backpressure has two effects. One effect is generating backsplash during atomization, in which molten metal or metal alloy is backsplashed upwards away from the atomization zone. The backsplashed molten metal or metal alloy may either deposit back onto the atomization device and block further atomization, or become coarse and irregular shaped powders, which may not be desired. Another effect is influencing the atomization rate, or the flow rate of the metal or metal alloy stream coming out of the liquid delivery nozzle. In the extreme, a complete blockage of the metal or metal alloy stream from coming out of the liquid delivery nozzle is likely to happen due to the backpressure. The present invention provides a method of atomizing and an atomizing apparatus to control the backpressure. 
     During atomizing, the intensities and directions of the atomization gas jets affect the atomization characteristics, such as atomization efficiency, atomization rate, the cooling rate of atomized droplets, trajectories and velocities of atomized droplets, shapes and sizes of atomized droplets, the spatial flux distribution of atomized droplets, etc. The intensities of the atomization gas jets are manipulated through controlling the pressure and/or flow rate of the atomization gas. However, the directions of the atomization gas jets are fixed by the design of the atomization device. In U.S. Pat. No.4,779,802, and U.S. Pat. No. 4,905,899, the atomization device is scanned to control the directions of the atomization gas jets. The present invention provides a method of atomizing and an atomizing apparatus to control both the intensities and directions of the atomization gas jets. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is to control the created backpressure, which, in turn, controls the backsplash and the atomization rate, or the flow rate of the metal or metal alloy stream coming out of the liquid delivery nozzle. Another aspect of the present invention is to control the atomization characteristics by controlling the intensities and directions of the atomization gas jets, which, in turn, controls the droplet characteristics, such as the variations of size, shape, temperature, heat content and microstructure of droplets, etc., and/or powder characteristics, such as powder size distribution, the powder shape distribution, the microstructure variations of powders, etc., and/or spray-deposit characteristics, such as the morphology, macrostructures and microstructures of the deposit, etc. 
     DESCRIPTION OF THE INVENTION 
     According to one aspect of the present invention there is provided a method of atomizing a liquid stream of metal or metal alloy consisting of the steps of: 
     teeming a stream of molten metal or metal alloy into an atomization device, 
     atomizing the stream with atomization gas to form droplets of metal or metal alloy, 
     and directing controlling fluid at atomization gas jets or at an atomization zone to control the backpressure and, if desired, the intensities and directions of the atomization gas jets. Preferably the atomization gas issues from first jets, and the controlling fluid issues from second jets directed at the atomization gas jets or at the atomization zone. The intensity, flow rate and pressure of the secondary jets are preset to control or are in-situ adjusted to in-situ control the backpressure and/or the intensities and directions of the atomization gas jets. The method may be for the production of powder to control the powder characteristics. Alternatively, the method may be for the production of spray deposits to control the deposit characteristics. Alternatively, the secondary jets may be so arranged, through which solid particles or whiskers of the same or different composition (either metallic or non-metallic) of the metal to be atomized are introduced into the controlling fluid which acts as a transport vehicle for the particles or whiskers to be co-deposited with the atomized droplets to form spray-deposited composite materials. Alternatively, the particles or whiskers are introduced from above the secondary jets, which also gives a mixture of the particles or whiskers with the spray to form spray-deposited composite materials. Suitably, the controlling fluid is an inert gas, such as Argon, Helium and Nitrogen, or Air. Alternatively, the controlling fluid may be cryogenic liquified gas which changes to a gaseous phase upon heating by the metal or metal alloy stream. The atomization gas is suitably an inert gas, such as Argon, Helium and Nitrogen, or air. The selection of gases is made in accordance with the compatibility with the liquid metal or metal alloy to be atomized. 
     According to another aspect of the invention there is provided an atomizing apparatus consisting of an atomization device for receiving a stream of molten metal or metal alloy to be atomized, means for directing atomization gas at the liquid stream to atomize the stream, and means for directing controlling fluid at atomization gas jets or at an atomization zone to control the backpressure and/or the atomization characteristics. In the preferred arrangement, the means for directing the atomization gas consists of primary jets and the means for directing the controlling fluid consists of secondary jets directed at the atomization gas jets or at the atomization zone. The intensity, flow rate and pressure of the secondary jets are preset to control or are in-situ adjusted to in-situ control the backpressure and/or the intensities and directions of the atomization gas jets. Suitably, the controlling fluid is an inert gas, such as Argon, Helium and Nitrogen, or air. Alternatively, the controlling fluid may be cryogenic liquified gas which changes to a gaseous phase upon heating by the metal or metal alloy stream. The atomization gas is suitably an inert gas, such as Argon, Helium and Nitrogen, or air. The selection of gases is made in accordance with the compatibility with the liquid metal or metal alloy to be atomized. 
     Alternatively, the apparatus may be used to produce spray deposits on a suitable collector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying in which: 
     FIG. 1 is a schematic sectional side elevation of a gas atomizing apparatus according to the invention; 
     FIG. 2 is a schematic side elevation of apparatus for producing powders including the atomizing apparatus according to the invention together with an alternative base arrangement; 
     FIG. 3 is a Process Map of Pu vs. PI for R=15 mm for water atomization 
     FIG. 4 is a Process Map of Pu vs. p 1  for R=20 mm for water atomization 
     FIG. 5 is a Process Map of Pu vs. p 1  for R=25 mm for water atomization 
     FIG. 6 shows the atomization phenomena for region A in the Process Map of Pu vs. p 1   
     FIG. 7 shows the atomization phenomena for region B in the Process Map of Pu vs. p 1   
     FIG. 8 shows the atomization phenomena for region C in the Process Map of Pu vs. p 1   
     FIG. 9 shows the atomization phenomena for region D in the Process Map of Pu vs. p 1   
     FIG. 10 shows the distributions of the powder sizes for each set of process parameters with the application of controlling fluid technique; 
     (a) A035 sample 
     (b) A036 sample 
     (c) A037 sample 
     (d) A038 sample 
     (e) A039 sample 
     (f) A040 sample 
     (g) A042 sample 
     (h) A043 sample 
     (i) A044 sample 
     FIG. 11 shows the mass distribution of powders produced with the application of controlling fluid technique; 
     FIG. 12 shows the variations of the intensities and directions of the atomization gas jets as the pressure of the controlling fluid varies; 
     (a) Pu=0.04 MPa 
     (b) Pu=0.06 MPa 
     (c) Pu=0.08 MPa 
     (d) Pu=0.10 MPa 
     (e) Pu=0.14 MPa 
     (f) Pu=0.20 MPa 
     (g) Pu=0.40 MPa 
    
    
     Table 1 lists the process parameters used for the production of powders with the application of controlling fluid technique; 
     Table 2 lists the first and second peak values of the distribution of powder sizes produced with the application of controlling fluid technique. 
     Table 3 lists the process parameters used to spray deposit Pb-50% Sn alloy preforms employing controlling fluid technique. 
     Reference Number Of Elements In The Drawings 
     1 . . . crucible or tundish 
     2 . . . liquid metal or metal alloy 
     3 . . . liquid delivery nozzle 
     4 . . . liquid metal or metal alloy stream 
     5 . . . primary gas atomization device 
     6 . . . primary atomization gas jets 
     7 . . . a spray of atomized droplets 
     8 . . . a secondary controlling fluid jets device 
     9 . . . controlling fluid jets 
     10 . . . atomization zone 
     11 . . . crucible/tundish metal dispensing system 
     12 . . . with liquid metal 
     13 . . . the gas atomization device 
     14 . . . the secondary controlling fluid jets device . . . 
     15 . . . inlet pipe 
     16 . . . separate inlet pipe 
     17 . . . a spray chamber 
     18 . . . a powder collection vessel 
     19 . . . a gas exhaust pipe 
     20 . . . a current to pneumatic pressure(P/I) converter 
     21 . . . controlling fluid control valve 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, an atomizing apparatus for gas atomizing liquid metal or alloy is shown consisting of a refractory or refractory lined crucible or tundish (1) for containing liquid metal or metal alloy (2). The crucible (1) has a liquid delivery nozzle (3) to provide a liquid metal or metal alloy stream (4) of a desired diameter. The liquid metal or metal alloy stream (4) teems into a central opening in a primary gas atomization device (5) which causes a number of atomization gas jets (6) to be directed at the liquid metal or metal alloy stream (4) so as to atomize the stream into a spray of atomized droplets (7). The primary atomization gas jets (6) preferably spray Nitrogen, Argon or Helium, but air may also be used. The atomizing assembly also consists of a secondary controlling fluid jets device (8), disposed upstream of the primary atomization gas jets (6), containing a number of controlling fluid jets (9) which apply Nitrogen, Argon, Helium, air, or cryogenic liquified gas jets to the atomization gas jets (6) or to the atomization zone (10). The pressure and flow rate of the controlling fluid applied at the secondary controlling fluid jets device (8) is controlled to manipulate the backpressure and the atomization characteristics. The controlling can be made in-situ during atomizing. 
     FIG. 2 shows the apparatus of FIG. 1 as applied to a powder production apparatus. In this figure, the crucible/tundish metal dispensing system (11) containing liquid metal (12), the gas atomization device (13) and the controlling fluid jets device (14) are positioned on a spray chamber (17). Atomization gas is supplied to the gas atomization device (13) via an inlet pipe (15), and controlling fluid is supplied to the controlling fluid jets device (14) via a separate inlet pipe (16). At the base of the spray chamber is a powder collection vessel (18), the chamber additionally containing a gas exhaust pipe (19). The flow rate of the controlling fluid applied at the secondary controlling fluid jets device (14) is controlled by activating the controlling fluid control valve (21) via a current to pneumatic pressure(P/I) converter (20). The controlling can be made in-situ during atomizing. 
     During atomizing, the backpressure is controlled by the controlling fluid jets device, which controls the extent of the backsplash and the atomization rate, or the flow rate of the metal or metal alloy stream coming out of the liquid delivery nozzle. In addition, the intensities and directions of the atomization gas jets are controlled by the controlling fluid jets device, which controls the atomization characteristics. Consequently, the droplet characteristics, such as the variations of size, shape, temperature, heat content and microstructure of droplets, etc., and powder characteristics, such as powder size distribution, the powder shape distribution, the microstructure variations of powders, etc., are controlled. The pressure and/or flow rate of the controlling fluid are in-situ adjustable during atomizing to in-situ control the backpressure and/or the intensities and directions of the atomization gas jets. 
     EXAMPLE OF THE USE OF NITROGEN GAS AS THE CONTROLLING FLUID IN THE ATOMIZATION OF WATER 
     The example below illustrates the principles of selecting the process parameters by illustrating the conditions used for the atomization of water employing the controlling fluid technique. Pu is the nitrogen gas pressure used for the controlling fluid jets device, p 1  is the nitrogen gas pressure used for the gas atomization device, and R is the vertical distance between the controlling fluid jets device and gas atomization device. 
     The principles of selection of R is discussed below for this example. When R&gt;25 mm, the controlling fluid jets device was too far from the gas atomization device, so that when the controlling fluid became large enough to surpress the backpressure, the water was atomized by the controlling fluid also, which rendered the controlling fluid jets device meaningless. When R&lt;5 mm. As a result, the R needed to be limited between 5 mm and 25 mm in this example. 
     The principles of selection of Pu and p 1  is discussed below for this example. FIGS. 3, 4, and 5 show the Process Maps of Pu vs. P 1  for R=15, 20, and 25 mm, respectively. In the figures, each map is divided into Regions A, B, C, and D. The effects of the controlling fluid jets device on the atomization characteristics of water for each Region are shown schematically in FIGS. 6 to 9, separately. In Region A, the controlling fluid jets are not able to surpress the backpressure completely. In Regions B and C, the backpressure is surpressed by the controlling fluid jets device; however, the water stream between the controlling fluid jets device and gas atomization device in Region C is more turbulent than that in Region B. Region D is the transition region between Region A and Regions B or C. In summary, Regions B and C are the regions suitable for water atomization in this example. 
     EXAMPLE OF THE USE OF NITROGEN GAS AS THE CONTROLLING FLUID IN THE PRODUCTION OF Pb-Sn POWDERS 
     The example below illustrates the conditions used for the production of Pb-50 wt % Sn powders. Table 1 lists the process parameters used for the production of powders. Pu is the nitrogen gas pressure used for the controlling fluid jets device, p 1  is the nitrogen gas pressure used for the gas atomization device, and R is the vertical distance between the controlling fluid jets device and gas atomization device. 
     Table 2 lists the first and second peak values of the distribution of powder sizes. For the condition of Pu=0, P 1  =0.30 MPa and R=20 mm, the backsplash created due to the backpressure was so severed that nearly no atomization took place, which resulted in no powder being produced. However, when the controlling fluid jets device was switched on and Pu was set to be 0.20 MPa, the backpressure was so controlled that backsplash was eliminated and the powder was produced as illustrated by the A038 production. Using controlling fluid to control the backpressure is demonstrated. 
     FIG. 10 shows the distributions of the powder sizes for each set of process parameters. It is shown that the first and second peak values of the distribution of powder sizes are controllable by varying the pressure and position of the controlling fluid jets. FIG. 11 shows the mass distribution of powders are controllable by varying the pressure and position of the controlling fluid jets. Using controlling fluid to control the atomization characteristics is demonstrated. 
     FIG. 12 shows the variations of the intensities and directions of the atomization gas jets as Pu varies. It is shown that the intensity of the atomization gas jets for Pu=0.14 MPa is relatively small compared to that for Pu=0.40 MPa, which gives a more scattered spray for the former. In addition, the direction of the atomization gas jets for Pu=0.14 MPa is also different from that for Pu=0.40 MPa, and the former has a larger included angle for the spray cone. Using controlling fluid to control the intensities and directions of the atomization gas jets is demonstrated. 
     A further application of the use of controlling fluid is in the production of spray deposits. In the production of spray deposits, liquid metal or metal alloy is atomized into a spray of droplets, which consists of a mixture of fully liquid, semi-solid/semi-liquid and solid particles. The resulting spray of metal droplets is directed onto an appropriate collector, where a preform is continuously deposited by these droplets. The process is essentially a rapid solidification technique with an integrated gas-atomizing/spray depositing operation. Deposits with different morphologies, such as tubes, billets, flat products, coated articles, etc., can be produced by manipulating the movement and shape of the collector, and by, in many situations, moving the spray itself. Such products can either be used directly or can be further processed normally by hot or cold working with or without the collector. 
     During atomizing, the backpressure is controlled by the controlling fluid jets device, which controls the extent of the backsplash and the atomization rate, or the flow rate of the metal or metal alloy stream coming out of the liquid delivery nozzle. In addition, the intensities and directions of the atomization gas jets are controlled by the controlling fluid jets device, which controls the atomization characteristics. Consequently, the droplet characteristics, such as the variations of size, shape, temperature, heat content and microstructure of droplets, etc., and spray-deposit characteristics, such as the morphology, macrostructures and microstructures of the deposit, etc., are controlled. The pressure and/or flow rate of the controlling fluid are in-situ adjustable during atomizing to in-situ control the backpressure and/or the intensities and directions of the atomization gas jets. Alternatively, the secondary controlling fluid jets may be so arranged, through which solid particles or whiskers of the same or different composition (either metallic or non-metallic) of the metal to be atomized are introduced into the controlling fluid which acts as a transport vehicle for the particles or whiskers to be co-deposited with the atomized droplets to form spray-deposited composite materials. Alternatively, the particles or whiskers are introduced from above the controlling fluid jets, which also gives a mixture of the particles or whiskers with the spray to form spray-deposited composite materials. 
     EXAMPLE OF THE USE OF NITROGEN GAS AS THE CONTROLLING FLUID IN THE PRODUCTION OF SPRAY-DEPOSITED PB-50% SN ALLOY PREFORMS 
     The example below illustrates the conditions used for the production of Pb-50% Sn spray-deposited preforms. Table 3 lists the atomization process parameters used to produce Pb-50% Sn powder employing the controlling fluid technique. In Example A, only atomization gas was used in the conventional manner of production of spray-deposited preforms. However, since the backsplash created due to the backpressure was so severe that nearly no atomization took place, which resulted in no preform being produced. In Example B, controlling fluid of Nitrogen was introduced by the controlling fluid jets device above the main atomization gas jets. Otherwise, the atomizing was carried out under identical conditions to Example A. The backpressure was so controlled by the controlling fluid jets device that backsplash was eliminated and a spray-deposited preform was produced. Using controlling fluid to control the backpressure in the spray deposition process was demonstrated. 
     
                       TABLE 1______________________________________        P.sub.I     Pu      RExperimental No.        (MPa)       (MPa)   (mm)______________________________________A035         0.40        0.20    25A036         0.30        0.30    25A037         0.20        0.20    15A038         0.30        0.20    20A039         0.20        0.30    20A040         0.40        0.40    20A042         0.30        0.40    15A043         0.40        0.30    15A044         0.20        0.40    25______________________________________ 
    
     
                       TABLE 2______________________________________                              second peak/Experimental No.        first peak                  second peak first peak______________________________________A035         177-250 μm                  53-74 μm 0.36A036         250-420 μm                  53-74 μm 0.24A037         250-420 μm                  88-105 μm                              0.31A038         250-420 μm                  53-74 μm 0.18A039         250-420 μm                  53-74 μm 0.17A040         250-420 μm                  53-74 μm 0.17A042         177-250 μm                  53-74 μm 0.34A043         177-250 μm                  53-74 μm 0.75A044         250-420 μm                  53-74 μm 0.29______________________________________ 
    
     
                       TABLE 3______________________________________Process Parameter           Symbol   Example A Example B______________________________________Metal Dispensing           T.sub.spray                    266       266Temperature(° C.)Metal Flow Rate(Kg/sec)           J.sub.melt                    0.18      0.18Atomization gas pressure(MPa)           P.sub.I  0.30      0.30Controlling fluid pressure(MPa)           Pu       0.00      0.20Vertical distance between the           R        20        20controlling fluid jets device andgas atomization device(mm)Spray Height(mm)           Z        600       600Results                  Process   Process                    Failed    Succeeded______________________________________