Patent Publication Number: US-6221125-B1

Title: Water-atomized spherical metal powders and method for producing the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part of U.S. Ser. No. 08/263,766, filed Jun. 22, 1994, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to spherical metal powders suitable for use in metal injection molding, and to a method for producing such powders. 
     2. Description of the Prior Art 
     Of the powdered raw materials used for metal injection molding, powdered alloys are produced by water-atomization or gas atomization. 
     When a melt of metal or alloy is formed into particles by water in the process of water atomization, the melt is cooled very rapidly so that the particles formed have a non-spherical shape. Their tap density ratio is therefore only 40-46%, and they cannot be used as a raw material for injection molding unless a large amount of an organic binder is blended with them. However, when the blending proportion of an organic binder is high, a long debinding time is needed and problems such as blistering or distortion arise with considerable frequency during the debinding process. 
     On the other hand, in the case of a gas-atomized powder, the particles produced are perfect spheres so that their tap density ratio is of the order of 60% and a sufficient injection moldability can be ensured, even with a low blending proportion of an organic binder. However, in this case, there is no interaction at all between particles after the debinding process. Thus, the strength of the debound body is reduced. Further, the gas-atomized powder is also more costly to produce than the water-atomized powder. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention to simultaneously overcome the disadvantages associated with a high blending proportion of organic binder required for the injection molding of non-spherical water-atomized particles having a tap density ratio as low as 40-46% and the problems caused due to the poor strength of molded debound bodies of gas-atomized particles. More specifically, an object of the present invention is to modify the non-spherical particles of a water-atomized powder into spherical particles so that the particle shape of the water-atomized powder becomes close to the particle shape of the gas-atomized powder and the blending proportion of organic binder can therefore be reduced compared to the proportion normally required for a conventional water-atomized powder in the injection molding process. A further object of the present invention is to provide debound bodies having a higher strength than in the case of the gas-atomized powder. As a result, there is provided a water-atomized spherical metal powder which ameliorates all the problems encountered in both types of atomized powders, i.e., water-atomized powder and gas-atomized powder. 
     Accordingly, this invention provides a water-atomized pherical metal powder having a spherical particle shape, an average particle size of 25 μm or less and a tap density ratio of 50-60%. 
     The present invention further provides a method for producing the above-mentioned water-atomized spherical metal powder wherein metal particles of non-spherical shape produced from molten metal by water-atomization are formed into particles of spherical shape by means of a high speed gas stream which causes high speed collisions to occur between the particles and between the particles and a collision target. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a scanning electron micrograph of a water-atomized powder which has been subjected to the high-speed gas treatment using a collision target according to the present invention. 
     FIG. 2 is a scanning electron micrograph of the water-atomized powder before the high-speed gas treatment. 
     FIG. 3 is a scanning electron micrograph of a gas-atomized powder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The metal used in this invention may be any metal normally used to produce powders for injection molding such as stainless steel AISI 316, Permendur, high speed steel M2 or the like. The high speed gas stream used for causing collisions to occur between metal particles and between metal particles and a collision target may be any gas flowing at a speed of 200 m/sec or more, such as air, or an inert gas, such as Ar, N 2 , etc. Due to collisions between the particles and collisions between the particles and a collision target by the high speed gas stream, the particles obtained by water-atomization undergo strong impacts and the surface projections on the particles are smoothed so that the particles overall become more spherical. However, the average particle size of the spherical metal powder thus obtained should not exceed 25 μm, otherwise the powder is no longer suitable for injection molding. 
     EXAMPLES 
     100 kg of powder prepared from a melt of each steel, shown in Table 1, by water-atomization was introduced in a jet mill (Nippon Pneumatic MFG., CO., Ltd., Type I-10), and treated by flowing an air stream at a flowing rate of 600 m/sec for 60 min. The average particle size of the treated powder was 10 pm. Table 1 shows the tap densities before and after the treatment for each water-atomized powder. 
     Table 1 shows the tap densities of the water-atomized powders before the high-speed gas treatment and the water-atomized powders after the high-speed gas treatment with or without using a collision target in the mill. The collision target was disposed in the direction of the flow of the high-speed gas and in the vicinity of the opening of a nozzle from which the atomized particles were expelled and brought to collide against the collision target by the high-speed gas. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Tap density 
                 Tap density after treatment 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 before 
                 without 
                 with 
               
               
                   
                   
                 treatment 
                 collision target 
                 collision target 
               
               
                   
                 True 
                 g/cm 3   
                 g/cm 3   
                 g/cm 3   
               
               
                   
                 density 
                 (Tap density 
                 (Tap density 
                 (Tap density 
               
               
                 Sample 
                 g/cm 3   
                 ratio) 
                 ratio) 
                 ratio) 
               
               
                   
               
               
                 Stainless 
                 8.03 
                 3.6 (44.8%) 
                 3.8 (47.3%) 
                 4.3 (53.5%) 
               
               
                 steel 
               
               
                 AISI 316 
               
               
                 Permendur 
                 8.30 
                 3.5 (42.2%) 
                 3.7 (44.6%) 
                 4.5 (54.2%) 
               
               
                 High speed 
                 8.18 
                 3.3 (40.3%) 
                 3.5 (42.8%) 
                 4.1 (50.1%) 
               
               
                 steel M2 
               
               
                   
               
            
           
         
       
     
     As is apparent from Table 1, the samples treated using a collision target showed higher tap density ratios than those treated without using the collision target, i.e., 6.2%, 9.6% and 7.3% higher tap density ratios for stainless steel (AISI 316), Permendur and high speed steel (M2), respectively, and the use of a collision target is effective in achieving an improved tap density. 
     Among the above sample powders, three kinds of powders of stainless steel AISI 316 (i.e. the untreated powder, the powder treated without using the collision target and the powder treated using the collision target) were examined for injection-moldability as follows. 91% by weight of each powder was blended with an organic binder consisting of 4.5% by weight of polyethylene, 3.7% by weight of paraffin wax and 0.8% by weight of stearic acid. 6 g of each of the resultant powder mixtures was charged into a flow tester made by Shimazu Seisakusho and subjected to an injection moldability test (flow test) at 170° C. under a load of 10 kgf, using a die of 1.0 mm in diameter and 1.0 mm in length. The injection pressure was 20 kgf. The test results are shown in Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Tap density 
                   
               
               
                   
                   
                 g/cm 3   
                 Injection 
               
               
                   
                   
                 (Tap density 
                 moldability 
               
               
                   
                 Stainless steel AISI 316 
                 ratio) 
                 (ml/sec) 
               
               
                   
                   
               
             
            
               
                   
                 Before treatment 
                 3.6 (44.8%) 
                 5.8 × 10 −2   
               
               
                   
                 After treatment without 
                 3.8 (47.3%) 
                 1.1 × 10 −1   
               
               
                   
                 collision target 
               
               
                   
                 After treatment with 
                 4.3 (53.5%) 
                 2.3 × 10 0   
               
               
                   
                 collision target 
               
               
                   
                   
               
            
           
         
       
     
     As is apparent from Table 2, the injection-moldability of the sample treated without using the collision target was 1.9 times that of the untreated sample, while the injection-moldability of the sample treated using the collision target was 39.7 times that of the untreated sample. That is, when water-atomized powder is treated by a high-speed gas using a collision target, the injection-moldability of the resultant treated powder is improved to approximately 40 times that before treating. Further, it is understood that the use of a collision target results in a substantial improvement in the injection-moldability to a level about 21 times the injection-moldability without using a collision target. 
     Further, the stainless steel AISI 316 powder obtained by the high-speed gas treatment using the collision target according to the present invention was injection-molded and debound. The thus obtained debound body was examined for defects (blistering and cracking) in order to compare with the water-atomized powder before the high-speed gas treatment and a conventional gas-atomized powder of the same stainless steel. The results are shown in Table 3. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                   
                 Organic 
                   
                   
                 Percent 
                   
               
               
                   
                 binder 
                   
                   
                 defective of 
               
               
                 Tap 
                 blending 
                   
                 Debinding 
                 debound 
               
               
                 density 
                 proportion 
                 Injection 
                 time 
                 body 
                 Cause of defect 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 (g/cm 3 ) 
                 (wt %) 
                 moldability 
                 (hr) 
                 (%) 
                 Blistering 
                 Cracking 
               
               
                   
               
            
           
           
               
            
               
                 Powder of the present invention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 4.3 
                 8 
                 good 
                 24 
                  0 
                 0 
                 0 
               
            
           
           
               
            
               
                 Comparative powders 
               
               
                 Water-atomized powder before treatment 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 3.6 
                  8 
                 unacceptable 
                 — 
                 — 
                 — 
                 — 
               
               
                 3.6 
                 10 
                 good 
                 36 
                 32 
                 32  
                 0 
               
            
           
           
               
            
               
                 Gas-atomized powder 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 4.8 
                 8 
                 qood 
                 24 
                 27 
                 0 
                 27  
               
               
                   
               
            
           
         
       
     
     In the case of the water-atomized powder before the treatment, when the blending proportion of an organic binder was increased to 10% by weight from 8% by weight, the injection-moldability was improved but the debound bodies showed a percent defective as high as 32%. Whereas the inventive powder showed a superior injection-moldability equal to that of the gas-atomized powder. Regarding the percent defective, the inventive powder showed a percent defective of 0%, whereas the gas-atomized powder showed a very high percent defective of 27% as compared with the inventive powder. 
     FIG. 2 is a scanning electron micrograph (×1000) showing the particle shape the AISI 316 steel water-atomized powder before the high-speed gas treatment. It can be seen from FIG. 2 that the untreated water-atomized powder is composed of non-spherical particles. 
     FIG. 1 is a scanning electron micrograph (×1000) of the same powder after the high-speed gas treatment using a collision target and shows that projections on the particles have been smoothed and the particles have become more spherical. 
     FIG. 3 is a scanning electron micrograph (×1000) of a gas-atomized powder consisting of perfectly spherical particles. 
     This invention produces a water-atomized metal powder wherein there are few particles of non-spherical shape, the particles being effectively spherical so that the blending proportion of organic binder used in injection molding can be reduced compared to the proportion used in a conventional water-atomized powder and the injection-moldability and tap density are improved while the strength of the resulting debound body is ensured. Moreover, the metal powder of the invention can be produced by a simple method. The strength of the debound body is fully maintained due to the fact that the particles of this powder are not as perfectly spherical as in the case of a gas-atomized powder, and still retain some degree of irregularity.