Primary shaping - Powder Metallurgy

Transcription

1 Chair of Manufacturing Technology Primary shaping - Powder Metallurgy Manufacturing Technology II Exercise 2 Laboratory for Machine Tools and Production Engineering Chair of Manufacturing Technology Prof. Dr.-Ing. Dr.-Ing. E.h. F. Klocke

2 Table of Contents Table of Contents Introduction Technology and design related parameters Potential and limitations of powder metallurgy Sintering-oriented design Economic parameters Film: Steps in the operation and exemplar applications Tasks Tool design Manufacturing a sintered connecting rod Manufacturing a sintered bronze shell...15 Manufacturing Technology II - Exercise 2 2

3 1 Introduction Powder metallurgy encompasses the manufacture of metallic powder and the production of parts made from this powder by forming and sintering. The term Sintering is used to describe the heat treatment of powder or of a compact at temperatures below the melting point of the base material. The term Sintering technology encompasses all of the steps involved in the operation to manufacture a sintered part, with the exception of powder production. There are certain materials which can only be produced in a powder metallurgical operation, e.g. hard metals or alloys of metals with widely differing melting points. The technological potential and limitations of powder metallurgy will be explained in the first part of the exercise. The limitations of powder metallurgy stem partly from geometric boundary condition. The guidelines for sintering-oriented design are presented as a further focus of this exercise. Powder metallurgy competes with casting processes, solid forming and cutting techniques in a number of applications. The decision in favour of powder metallurgy, is therefore based not only on technological criteria. In fact, the economic parameters are frequently the decisive factor. The steps in the powder metallurgy will be shown in a short film along with typical sintered parts within the framework of this exercise. To conclude, the basic principles of sintering oriented design of pressing tools and sintered parts will be examined in detail. Manufacturing Technology II - Exercise 2 3

4 2 Technology and design related parameters 2.1 Potential and limitations of powder metallurgy Powder metallurgy offers possibilities in terms of material composition which are either completely impossible when casting techniques are used or which can be achieved only with considerable work and at great expense. Suitability for metallic and non-metallic materials combinations are also possible (Example: Cermets) Manufacture of defined material combinations Time-consuming and expensive via the casting process Example: High-purity metals, super-alloys Examples: Tungsten, molybdenum, tantalum, niobium Manufacture of refractory metals Cannot be produced using casting technology Manufacture of metals with a wide variance in melting points Manufacture by sintering with a liquid phase or via subsequent infiltration Example: tungsten-copper or molybdenum-silver switching contacts Manufacture of materials with hard materials in a ductile matrix Examples: Hard metals, stellites, high-speed metals Regulating a controlled pore space Disadvantage: 100% density impossible to achieve using sintering technology Infiltrating with plastic or metal which has a low melting point: Impermeable to oil and water Filling the pore space with oil: Lubricating sliding bearings Examples of highly porous parts: Filters, chokes, flame arresters Manufacturing Technology II - Exercise 2 4

5 The structure of parts produced using a powder-metallurgical process, has a high level of homogeneity and isotropy. The directed fibre orientation seen in forged or rolled parts, does not exist. There are process limitations in terms of the geometric design of sintered parts, due to the pressing operation which is required, Fig The maximum part size is limited by the efficiency and stability of the pressing tools under load. However even very large parts can be produced using special processes such as isostatic compression. In principle, even complex forms such as gear tooth forming, curves and shaped holes can be produced to meet exacting tolerances. However it is vital to ensure that the parts can be removed from the moulds after the pressing operation. In addition to the absolute process limitations, guidelines for sintering-oriented design have developed on the basis of the process sequence. These are explained in the next chapter. Criterion Limitations Note maximum mass coaxial pressing 3-5 kg isostatic compression up to 600 kg limited by the design of the presses minimum mass 0,02 g limited by exact volumetric feed dimensional accuracy without sizing IT 9-15 with sizing IT 6-10 in some cases IT 4 depends on material, density and strength can be removed from mould after pressing undercuts, drilled holes cross-wise to the pressing direction finishing-machining required Fig : Geometric parameters for powder metallurgy Manufacturing Technology II - Exercise 2 5

6 2.2 Sintering-oriented design The geometric design of sintered parts is subject to the restrictions imposed by the pressing operation, (c.f. table above). It is important to take account of the design data listed below in order to avoid punch failure, uneven distribution of density due to excess pressure or damage to the green compact in the form of cracking or flaking: Height/diameter 2,5 (= slenderness ratio of the part to be pressed) Avoid sharp edges, tangential junctions, sharp angles and pointed moulding plugs Narrow cross sections and bridges to be at least 2 mm thick Pressing tools to be as straightforward as possible; i.e. through-holes only in the round section, no finely interlocked knurling, no modulus < 0.5 in the case of gear wheels UNFAVOURABLE FAVOURABLE height H of the part to be pressed not higher than 2,5*D, otherwise breakage of the punch or over-pressing no small cross sections, otherwise unequal density distribution faces instead of sharp edges to reduce the risk of punch breakage Fig : Design guidelines for sintered pre-formed parts [1/4] (Source: Association of Powder Metallurgists (König/Klocke Vol.4, P.56, Fig. 2-52) Manufacturing Technology II - Exercise 2 6

7 UNFAVOURABLE FAVOURABLE avoiding tangential transitions to reduce the risk of breakage at the punches avoiding circular profiles transverse to the compaction direction, otherwise the punches become to pointed die avoiding acute angles and rounding-offs, in order to minimise the risk of breakage at the punches punch punch die die punch punch die Fig : Design guidelines for sintered pre-formed parts [2/4] (Source: Association of Powder Metallurgists (König/Klocke Vol.4, P.56, Fig. 2-52) UNFAVOURABLE FAVOURABLE dimensioning ofholes and fixed links: diameters resp. widths not smaller than one third of the component height. s and d > 2 mm. no fine toothed straight knurlings, due to difficult production of the tool crossed knurlings impossible at a modul smaller than 0,5 it becomes hard to have a complete compaction of the teeth Fig : Design guidelines for sintered pre-formed parts [3/4] (Source: Association of Powder Metallurgists (König/Klocke Vol.4, P.56, Fig. 2-52) Manufacturing Technology II - Exercise 2 7

8 UNFAVOURABLE FAVOURABLE bigger distance between the bottom of the tooth space and the internal bore, risk for punches breaking througs if possible rounded, otherwise the tools become expensive diameter tolerances not smaller than IT 7, height tolerances not smaller than IT 12. Fig : Design guidelines for sintered pre-formed parts [4/4] (Source: Association of Powder Metallurgists (König/Klocke Vol.4, P.56, Fig. 2-52) Manufacturing Technology II - Exercise 2 8

9 3 Economic parameters The outcome of cost comparisons with rival processes, depends largely on the characteristics of the parts, Fig The cost comparison increasingly favours sintering, the more exacting the requirements in terms of material characteristics, the closer the tolerances relating to complex shapes and the larger the quantity concerned. In principle, however, the investment cost for tools and equipment is high. Powder metallurgy frequently becomes the most favourable option from an economic point of view only when substantial quantities are involved. The automotive industry is therefore a typical area of application for powder metallurgy. decrease << production costs >> increase piece number alloy geometry accuracy small high Mo Co Cr Ni Fe indirect costs tool costs sintering compacting powder IT 6 IT 9 density >7,2 <6,8 machining after sintering sumptuous little Fig : Influence exerted by part characteristics on the manufacturing cost of sintered parts (König/Klocke Vol.4, P.61, Fig. 2-57) One of the special characteristics of powder metallurgy is the 100% use of material. This goes some way towards balancing out the disadvantages in terms of the higher cost of the powder in comparison with that of molten metal, Fig Manufacturing Technology II - Exercise 2 9

10 detent forging and cutting step 1: forged blank weight: 590 g step 2: 1 st rough-machining step 3: 2 nd rough-machining step 6: broaching the outside profile weight: 286 g step 5: fine turning of plane faces step 4: internal toothing detent powder metallurgical manufacturing several turning operations powder weight: 327 g final part weight: 191 g compacting sintering sizing sintered blank Fig : Comparison of processes used to manufacture synchronous parts - conventional forging and cutting versus sintering (Source: Krebsöge) König/Klocke Vol.4, P.59, Fig The shorter manufacturing sequence can be a decisive advantage of powder metallurgy, as the exemplar application Manufacture of a synchronous part shows, Fig and Fig The powder metallurgical process eliminates the need for numerous operations required when forged compacts are used. There is additional potential for rationalisation when it is possible to take account of certain geometrical Manufacturing Technology II - Exercise 2 10

11 elements of the part during the pressing operation, since this eliminates the need to produce these elements in a finish cutting operation. detent alternative manufacturing sequences 1 - external toothing 2 - oil pocket 3 - internal toothing source: Metallwerk Unterfranken, ZF forging and cutting cutting to length forging and punching burr removing annealing turning the front side turning the back side broaching the internal toothing plain turning broaching the external toothing milling the grooves milling the oil pockets 2 - sintering and cutting compacting the green compact sintering sizing turning the front side turning the back side rel. production costs Fig : Shorter manufacturing sequence as a result of high-precision sintering Manufacturing Technology II - Exercise 2 11

12 4 Film: Steps in the operation and exemplar applications Notes: Manufacturing Technology II - Exercise 2 12

13 5 Tasks 5.1 Tool design The workpiece shown below, is to be manufactured in a sintering operation. Sketch the tool required, in filling and pressing position. Fig : Part Manufacturing Technology II - Exercise 2 13

14 5.2 Manufacturing a sintered connecting rod A forged connecting rod is to be replaced by a sintered connecting rod with the same dimensions. The sintered connecting rod weights 576 g and is 24 g lighter than the forged one. a) Calculate the porosity P and name the special operation required in order to manufacture sintered parts with this level of porosity. b) In terms of machine-related factors, the density of the green compact achieved in the pressing operation is influenced by the level of compacting pressure. Please show the dependence of pressing density on compacting pressure in this case in comparison with the level of dependence when the workpiece is non-porous. sintered density compacting pressure Fig : Sintered density over compacting pressure Manufacturing Technology II - Exercise 2 14

15 h = 100 mm Primary shaping - Powder Metallurgy 5.3 Manufacturing a sintered bronze shell The bronze part shown, is to be produced in a powder metallurgical process. The following parameters must be taken into account: The tensile strength of the part should be at least 160 N/mm 2. The material consists of 91 percentage volume of copper and 9 percentage volume of tin (ρ cu = 8.9 kg/dm 3, ρ Sn = 7.2 kg/dm 3 ). The powder density is ρ pulver = 5.5 kg/dm d i = 50 mm h p [ ] tensile strength R m N/mm² d a = 79,5 mm ,5 7 7,5 8 sintered density ρ [kg/dm 3 ] Drawing of the component diagram I Fig : Part and diagram showing tensile strength over sintered density Manufacturing Technology II - Exercise 2 15

16 a) Calculate the porosity P, of the workpiece to be produced, taking account of the part characteristics which are required. b) Determine the height of the layer of powder h p before pressing. Manufacturing Technology II - Exercise 2 16

17 c) How high can the tensile strength R m of a part be when the available press has a maximum pressing force of F = 1.2 MN? 8,0 sintered density ρ [ kg/dm 3 ] 7,0 6,0 5, compacting pressure p [ N/mm 2 ] Fig : Sintered density over compacting pressure Manufacturing Technology II - Exercise 2 17