1、95FTM13 . Powder Metallurgy Gears - Expanding Opportunities by: W. Brian James, Hoeganaes and Howard Sanderow, Management mixing, compacting, and sintering. Variations to these basic steps such as infiltration, double prescingldouble sintering, and powder forging, may be used to achieve higher densi
2、ty parts. A sizing operation may be used to qualify critical part dimensions. Alternatively, a machining step may be added for the same purpose or to achieve a geometric feature not possible during rigid die compaction. PIM parts may be through hardened or surface hardened as required by the intende
3、d application. Why PIM for Gear Manufacture? The P/M process is particularly well-suited to the production of gears and spur, bevel, and helical gears may be made by powder metallurgy processing. The pressing cycle for a single level PIM part is presented in Figure 2 1. A charge of mixed powder is d
4、elivered to the die cavity by a feed shoe and the upper and lower punches are used to compact the powder. After the upper punch is withdrawn, the pressed compact is ejected by the lower punch and the feed shoe slides the part away from the die cavity. The cycle repeats as the feed shoe continues for
5、ward and refills the die cavity with another charge of powder. P/M compacting tooling for a wo-level cluster gear is illustrated in Figure 3 1. True involute gear forms are possible. Special features Opiionai ope rai ion Figure 1 : The Powder Metallurgy (PIM ) Process 1 1 COPYRIGHT American Gear Man
6、ufacturers Association, Inc.Licensed by Information Handling Servicessuch as keyways, drive lugs, splines, and cam contours may be incorporated during the compaction process. Lightening holes may be added to reduce part mass and P/M gears can be made with blind corners eliminating the undercut relie
7、f that is needed with cut gears - see Figure 4 2. PIM tooling provides consistent tooth form accuracy and surface finish over long production runs. Generally, because of their porosity, good surface finish, and consistent tooth form accuracy PIM gears reduce noise levels. Additional information on t
8、he types of gears suitable for powder metallurgy and the advantages of PIM gears is presented in subsequent sections of this paper. MATERIAL - PROCESS SELECTION Alloying Methods in Ferrous Powder Metallurgy The mechanical properties of PIM materials are directly related to their microstructure and t
9、he size, distribution, and morphology of the porosity they contain. Alloying additions are made to develop specific material performance characteristics. However, the manner in which the alloys are constituted has a significant effect on the porosity and microstructure of the final sintered product.
10、 ferrous P/M materials may be classified according to -I II II - 1. Cycle Start 2. Charging (Filling) Die 3. Compaction Begins with Powder 4. Compaction Completed 5. Ejection of Part 6. Recharging Die Figure 2 : PressingCycle for a Single Level PIM Part 1 ,- Upper Punch- e,: - PIM Cluster Gear (Dual
11、 Level) Die Lower Outer - Punch Figure 3 : Compacting Tooling for a Twolevel PIM Cluster Gear 1 the manner in which the alloy has been constituted : Admixed - The alloying additions are made to the . - iron powder base in the form of elemental or ferroalloy powders. This is the least expensive and m
12、ost commonly used alloying method. Since the iron powder base is unalloyed when the mix is pressed, admixed materials retain most of the compressibility of the iron base. The degree of alloying is limited by the diffusivtty of the alloying elements in iron at the sintering temperature, and the resul
13、ting microstructures are chemically heterogeneous. This type of material is also subject to powder segregation and dusting during handling and pressing. Partially Alloyed - The alloying additions are diffusion bonded to the base iron particles such that the compressibility of the base iron is essent
14、ially retained. These materials are often referred to as diffusion alloyed. lhe powders are highly compressible, and yield heterogeneous sintered microstructures consisting of lightly alloyed particle cores with a continuous network of more highly alloyed interparticle bonds. Figure 4 : PIM Gear Tee
15、th with Blind Corners can be Pressed 2 Prealloyed - The alloying elements, except for carbon, are added to the melt before atomization. This results in homogeneous microstructures and uniform hardness even on a microhardness level. Solution hardening of the powder particles by the alloy additions ge
16、nerally decreases the compressibility of the powders compared with admixed and partially alloyed materials. However, prealloyed powders that use molybdenum as their principal alloy addition have been developed with cornpressibilities approaching those of iron powders 3,4. Prealloyed powders have bet
17、ter hardenability than admixed or partially alloyed materials. The alloy additions must be in solution in order to contribute to hardenability. Hybrid - With the advent of highly compressible prealloyed powders, materials have been developed based on additions to these powders. For example, material
18、s in widespread use are based on an 2 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servicesaddition of 2 w/o (weight percent) nickel and 0.4 - 0.6 w/o graphite to a prealloyed powder with 0.85 wlo molybdenum as the principal alloy addition 5. Chrome-mangane
19、se steels have been introduced based on additions of high carbon ferrochromium and ferromanganese powders to a prealloyed steel (0.85 w/o Mo) 6-8. Recently introduced partially alloyed or diffusion alloyed powders have been based on a prealloyed base powder (1.5 w/o Mo) rather than a plain iron powd
20、er base 9. The Mechanical Properties of Ferrous P/M Materiais The mechanical properties of P/M materials are directly related to: Density Composition Microstructure P/M material designations, plus guaranteed and typical mechanical properties can be found in MPIF Standard 35 10 and ASTM B 783 11. The
21、se standards contain information on tensile, compressive, impact, and fatigue performance. Youngs modulus is listed also and is a linear function of density - Figure 5 10. Poissons ratio for P/M materials may be taken as 0.27 f 0.02. It takes considerable time for standards to be developed. Therefor
22、e, currently, many PIM materials are not covered in the standards. Designers and fabricators should consult material suppliers during their selection process to ensure that they are aware of new developments. In the unnotched condition, the fatigue strength of sintered steels is lower than that of w
23、rought materials, and, in some cases, to nodular cast iron. Parts however, usually contain geometric discontinuities that act as stress raisers and an application- oriented comparison between materials needs to be made in the notched condition. For P/M materials with densities above 7.1 g/cm% and no
24、tched specimens with notch factors above 2, the endurance limit data are largely comparable to those for wrought and cast materials - Figure 6 12. The tensile and fatigue properties of a ferrous P/M material depend on density and microstructure. The effect of process variables is seen for prealloyed
25、 materials in Figure 7a and for partially alloyed materials in Figure 7b 13. Single compaction processing (1PIS) is compared with double pressing/double sintering (2P2S). The prealloyed materials (FL-4605) were tested in the quench-hardened and tempered condition (tempered at 450 “F for 1 hour), whe
26、reas the partially alloyed materials (FD-48XX) were tested in the as- sintered condition. The effect of increasing the sintering temperature is shown for both types of material. Low Alloy Steels 6.4 6.6 6.8 7 7.2 7.4 Density (glcm) Figure 5 : Youngs Modulus as a Function of Density for Prealloyed Lo
27、w-Alloy PIM Materials The macroindentation hardness of P/M materials is really a composite hardness; material plus pores. The measured macrohardness is termed the apparent hardness of the material. Microindentation hardness measurements may be used to measure the matrix or particle hardness of the m
28、aterial. Since sliding and surface contact take place on a microscopic level (surface asperities), the microhardness of a P/M material influences the scoring resistance more than the macrohardness 14. A Systems Approach to Engineered PIM Materiais (The Interaction Between Material and Process Variab
29、les) A systems approach is required to optimize each stage in the production of a P/M part. There are a number of steps involved : Part Design Material Selection Mixing Compacting Sintering Post Sintering Operations While many parts are developed using a process of trial and error, modern design tec
30、hniques for load determination and stress measurement, computer aided design, and service load simulation, permit a systematic design procedure. The following steps are appropriate 15: Load assessment - This is often a difficult step. Measurement on similar parts may sometimes help. The anticipated
31、loading influences the choice of material and the dimensions of the part. Geometric shape of the part - The shape and dimensions of the part are often constrained by the available design space. The part geometry will be dictated by the function to be fulfilled, the loading, and the selected material
32、 and process, but should be adapted where possible to improve the ease of part manufacture. COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesFatigue Endurance Limit (10 psi) -i; 2 0.6 . Y -i Figure 6 : Influence of Notches on Relative Fatigue Endurance
33、Under Constant Amplitude Loading 12 Stress analysis (critical areas and maximum local stresses) - All parts contain features that act as stress raisers. The critical areas with the highest local stresses need to be determined by the design engineer. If they exceed allowable values they are potential
34、 failure sites. Allowable local stresses - These are a function of the selected material and its microstructure along with the local stress distribution. The latter depends on the part geometry and the loading mode. The ailowable local stresses can be derived from endurable stresses established by t
35、esting samples or parts. Statistical techniques should be used and appropriate safety margins applied. . Comparison between maximum and allowable local stresses - A design may be regarded as safe if the calculated or experimentally determined maximum local stresses do not exceed the allowable values
36、. Verification - It is necessary to test prototypes under service loading conditions to ensure the part design is safe. This can be carried out under controlled service conditions or by simulating service loading in the laboratory. Early consultation between the part designer, the part fabricator, t
37、he tool designer, the tool maker, the raw material supplier, and the end-user can help to ensure an optimal design and make sure all parties are aware of the critical and non-functional sections of the part. P/M alloys are engineered materials and, in order to achieve the best combination of strengt
38、h and toughness in P/M steels, it is necessary to optimize the properties of both the inter-particle and the intra-particle regions. The way the alloy is constituted will determine the hardenability of the material and will influence the nature of the porosity and the microstructure of the finished
39、part significantly. Reduced levels of porosity have been shown to improve the dynamic properties of PIM materials. 2425F + 2425F . . 50 2P2S 2050F (a) FL-4605 (prealloyed) Quench-Hardened Tempered 450F 20 10 50 100 150 200 Ultimate Tensile Strength (10 3psi) Fatigue Endurance Limit (10 psi) 70 I 4 I
40、 8 I I I 4 I I I I I 1 i 1 60 50 40 30 20 IO FWSXX (partially alloyed) As-Sintered 0.6 w/o gr. 2425F 2P2S 0.4 w/o gr. 2050F 2P2S - 0.4 wio gr. 1PlS 2050F 0.6 w/ogr. 0.4 w/o gr. 2425F 2300F 1PlS (gr. =graphite) tCII1II1,I 50 100 150 200 Ultimate Tensile Strength (10 3psi) Figure 7 : Rotating Bending
41、Fatigue Data for PIM Materials; (a) Partially Alloyed Materials (Distaloy 4800A). (b) Prealloyed Materials (Anconteel 4600V) 13 Two recent developments in ferrous powder metallurgy have been of particular significance from both a technical and commercial perspective. Binder treated premixes have bee
42、n found to reduce dusting and segregation, improve flow and die filling characteristics, and lead to greater consistency in parts making 16-17, The introduction of water atomized prealloyed powders with compressibilities comparable to those of iron powders has added considerable flexibility to the m
43、anner in which ferrous alloys are constituted 3.4.5. These new prealloyed powders, which use molybdenum as their principal alloy addition, may be combined with other alloy additions in a binder treated premix and used to develop an appropriate microstructure for applications which demand good static
44、 and dynamic mechanical properties. Binder Treated Premixes While admixed powders are the most susceptible to dusting and segregation, prealioyed and partially alloyed powders are also prone to dusting and segregation of lubricant and admixed graphite. This may happen during initial discharge of the
45、 mix, during transportation, while powder is being transferred to the feed hopper of the compaction press, or when the powder flows into the die cavity - see Figure 8 16. 4 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesFigure 8 : Dusting and Segregat
46、ion During the PIM Process 16 Respirable Dust (mglm 3, 3, i W Conventional Premix B ANCORBOND Premix 2.5 2 1.5 I 0.5 O Background Operator inside Press Figure 9 : Respirable Dust Reduction through use of Binder Treated Premix 5 Compositional variations which result from these demixing phenomena caus
47、e inconsistencies in the green and sintered properties of PIM parts. During part manufacturing, the improved powder flow and die filling characteristics result in more consistent part sectional densities and improved control of part mass. Improved die fill uniformity results in more consistency thro
48、ughout the entire part manufacturing process. Improved retention of alloy additions provides an economic advantage in terms of reduced amounts of green scrap, enhanced alloy efficiency, and a more pleasant and easier to maintain work environment. Plant cleanliness is dramatically improved and the am
49、ount of respirable dust in the immediate vicinity of the compaction press is reduced by an order of magnitude - see Figure 9 5. Binder treated premixes are produced using a proprietary mixing process that utilizes patented binders 16.17. Single Compaction Processing to High Densities The systems approach to engineered P/M materials has been enhanced recently by the introduction of new material/process technology 18-20. The new warm Compaction technology permits part densities ranging from 7.3 to 7.5 g/cm3, currently only accomplished by double pressing/double sinterin
