Patent Publication Number: US-2005129562-A1

Title: Method for the manufacturing of sintered metal parts

Description:
FIELD OF THE INVENTION  
      The invention relates to a method for the manufacturing of sintered metal parts. Specifically the invention concerns a method for the manufacturing of sintered metal parts having a densified surface.  
     BACKGROUND OF THE INVENTION  
      One area of future growth in the utilization of powder metal parts is in the automotive industry. Of special interest within this field is the use of powder metal parts in more demanding applications, such as power transmission applications, for example, gear wheels. Problems with gear wheels formed by the powder metal process are that powder metal gear wheels have reduced bending fatigue strength in the tooth root region of the gear wheel, and low contact fatigue strength on the tooth flank compared with gears machined from bar stock or forgings. These problems may be reduced or even eliminated by plastic deformation of the surface of the tooth root and flank region through a process commonly known as surface densification. Products having a densified surface are described in e.g. the U.S. Pat. No. 5,711,187 and 6,171,546.  
      According to U.S. Pat. No. 5,711,187 a gear wheel is formed from a pressed and sintered powder metal blank, the surface of which is hardened by rolling. It is taught that the sintering is performed at high sintering temperatures up to 1350° C., i.e. at high sintering temperatures. No specific example concerning the exact temperatures used for the sintering are disclosed but normally the term “high sintering temperatures” means that the sintering is performed at about 1250° C. In addition to the high energy consumption the high sintering temperatures will negatively affect dimension tolerances of the sintered parts, which may lead to tolerance problems of the rolled part.  
      Also the U.S. Pat. No. 6,171,546 discloses a method for obtaining a densified surface. According to this patent the surface densification is obtained by rolling or, preferably, by shot peening of a green body of an iron-based powder. From this patent it can be concluded that the most interesting results are obtained if a pre-sintering step is performed before the final densification and sintering operations. According to this patent the sintering can be performed at 1120° C., i.e. at conventional sintering temperatures, but as two sintering steps are recommended the energy consumption will be quite considerable.  
      A simple and cost effective method requiring minimal dimensional change between the green and sintered compact for the preparation of gear wheels and similar products would thus be attractive and the main object for the present invention is to provide such a method. Another aspect of the invention is that lower energy consumption and accordingly lower energy costs may be achieved.  
     SUMMARY OF THE INVENTION  
      In brief it has now been found that by using a method involving the steps of compacting an iron or iron-based powder by high velocity compacting technique to a density above 7.2 g/cm 3 , low temperature sintering the obtained parts at and subsequently subjecting the parts to a surface densifying process such products having a densified surface can be produced.  
      Powder Types  
      Suitable metal powders which can be used as starting materials for the compaction process are powders prepared from metals such as iron. Alloying elements such as carbon, chromium, manganese, molybdenum, copper, nickel, phosphorous, sulphur etc can be added as particles, prealloyed or diffusion alloyed in order to modify the properties of the final sintering product. The iron-based powders can be selected from the group consisting of substantially pure iron powders, pre-alloyed iron-based particles, diffusion alloyed iron-based iron particles and mixture of iron particles or iron-based particles and alloying elements. Most preferable powders are prealloyed iron-based powders due to their high hardenability.  
      Compaction and Sintering  
      According to the present invention high velocity compaction, HVC, is used in order to obtain the products having the desired high density and narrow dimensional tolerances. An example of an equipment for HVC-compaction is the computer controlled percussion machine disclosed in U.S. Pat. No. 6,207,757 which is referred to above and which is hereby incorporated by reference. Particularly, the impact ram of such a percussion machine may be used for impacting the upper punch of a die including the powder in a cavity having a shape corresponding to the desired shape of the final compacted component. When supplemented with a system for holding a die, e.g. a conventionally used die, and a unit for powder filling (which may also be of conventional type) this percussion machine permits an industrially useful method for production of high-density compacts. Preferably a ram speed above 2 m/s is used in order to reach densities above 7.2 g/cm 3 .  
      The sintering according to the present invention is performed as low temperature sintering, i.e. below 1200° C., preferably below 1160° C. and most preferably between 1120° C. and 1160° C. Any conventional sintering furnace may be used and the sintering times may vary between about 15 and 60 minutes. The atmosphere of the sintering furnace may be an endogas atmosphere, a mixture between hydrogen and nitrogen or in vaccuum.  
      Important features of the inventive method in order to reach the advantages mentioned above are thus that the density of the compacted part is at least about 7.2 g/cm 3  and that the sintering can be performed at low temperatures.  
      Previously known methods of achieving high density of the sintered part are high temperature sintering or double pressing and double sintering.  
      The possibility of utilizing low sintering temperatures will reduce the energy consumption compared with that required for high temperature sintering. Additionally, the dimensional scatter of the part after low temperature sintering, e.g. within the temperature range of 1120° C. to 1160° C., is significantly smaller compared with the dimensional scatter after high temperature sintering. Narrow dimensional tolerances on the sintered blank are essential in order to reach a high quality of the surface densified part.  
      In comparison with methods involving double pressing and double sintering the method according to the present invention has the advantage that one pressing step and one sintering step are eliminated.  
      Additionally, by using this HVC technique for achieving a high green density it is possible to improve not only the mechanical properties of the final sintered part but also to increase the surface densifying depth.  
      Surface Densification  
      The surface densification may be performed by radial or axial rolling, shoot peening, sizing etc. A preferred method is radial rolling as this method provides short cycle times in combination with great densification depth. By the method of the invention a preferred densification depth of up to 1.5° mm, 2 mm and 3 mm or even higher can be obtained on cylindrical parts. For more complex parts such as gears, the achieved densification death is at least 0.3 mm, preferably at least 0.5 mm. The powder metal parts will obtain better mechanical properties with increasing densifying depth. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram showing the relationship between sintered density and the surface densifying depth.  
       FIG. 2   a  is a photomicrograph of a conventional compacted and surface densified sample.  
       FIG. 2   b  is a photomicrograph of a high velocity compacted and surface densified sample.  
       FIG. 3  is a diagram showing the standard deviation for the dimensional change between green and sintered components. 
    
    
      The invention is further illustrated by the following non-limiting examples.  
     EXAMPLE 1  
      Cylinders were compacted from a powder metallurgical composition consisting of the pre-alloyed powder Astaloy Mo with a graphite addition of 0.3% and amide wax lubricant. For obtaining green densities above 7.2 g/cm 3  high velocity compaction were used and for densities up to 7.2 g/cm 3  conventional uniaxial compaction were used. The cylinders were sintered at 1120° C., 30 minutes in an atmosphere of 90% nitrogen and 10% hydrogen. Surface densification were performed by radial rolling and the diameter reduction during rolling was 0.3 mm. Densification depths were evaluated using image analysis, and is defined as the shortest distance from the surface to the point where the density has decreased to 98% of theoretical density.  
      The relationship between sintered density of the blank and the acquired densification depth of the densified part can be seen in  FIG. 1 .  
     EXAMPLE 2  
      In  FIG. 2  photomicrographs of compacted, sintered and surface densified cylinders are shown. The cylinder in  FIG. 2   a  was conventionally compacted to a density of 7.0 g/cm 3  ard the achieved densified depth is 1 mm, while the cylinder shown in  FIG. 2   b  was high velocity compacted to a density of 7.5 g/cm 3  and the achieved densified depth is 2 mm.  
     EXAMPLE 3  
      High velocity compacted cylinders, sintered at 1120° C. for 30 minutes in an atmosphere of 90/10 N 2 /H 2  were surface densified by rolling in a two-roll burnishing machine. The diameter reduction during rolling was 0.3 mm. Polished cross sections of the rolled cylinders were investigated in light optical microscope and the densification depths were evaluated using image analysis. The acquired densification depths of the rolled cylinders are shown in table 1.  
                       TABLE 1                               Densification       Variant   Density   depth       #   (g/cm 3 )   (mm)                  A   7.49   2.5       B   7.48   2.5       C   7.42   2.3       D   7.42   2.0                  
 
     EXAMPLE 4  
      Gears with the gear data shown in table 2 were compacted and sintered using two different manufacturing routes: 
      1) High velocity compaction to a green density of 7.2, sintering at 1120° C. for 30 minutes    

      2) Uniaxial compaction to a green density of 7.1, sintering at 1300° C. for 120 minutes  
                           TABLE 2                                      No of teeth   18           Module (m n )   1.5875 mm           Pitch diameter (d)   28.575 mm                      
 
      The gears manufactured by route  1  reached a sintered density of 7.27 and the gears manufactured by route  2  reached a sintered density of 7.36.  
      The dimensional change from green to sintered state was measured on 20 gears from each manufacturing route. The standard deviation in dimensional change on four different dimensions are presented in  FIG. 3 .