Manufacturing Technology II Exercise 2 Powder Metallurgy€¦ · Manufacturing Technology II...

Lehrstuhl für Technologieder Fertigungsverfahren

Laboratoriumfür Werkzeugmaschinenund Betriebslehre

Manufacturing Technology II

Exercise 2

Powder Metallurgy

WerkzeugmaschinenlaborLehrstuhl für

Technologie der FertigungsverfahrenProf. Dr. - Ing. F. Klocke

RWTH - AachenSteinbachstraße 53

52065 Aachen

Inhaltsverzeichnis

Fertigungstechnik II - Übung 2 2

Table of Contents1 Introduction .................................................................................................... 3

2 Technology and design related parameters................................................... 42.1 Potential and limitations of powder metallurgy............................................... 42.2 Sintering-oriented design ............................................................................... 6

3 Economic parameters .................................................................................... 9

4 Film: Steps in the operation and exemplar applications............................... 12

5 Tasks ........................................................................................................... 135.1 Tool design................................................................................................... 135.2 Manufacturing a sintered connecting rod ..................................................... 145.3 Manufacturing a sintered bronze shell ......................................................... 15

Einleitung


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 Sinteringtechnology 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.

Technologische und konstruktive Randbedingungen


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



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. 2.1.1. 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 kgisostatic compressionup to 600 kg

limited by the designof the presses

minimum mass 0,02 g limited by exact volumetric feed

dimensional accuracy without sizing IT 9-15with sizing IT 6-10in some cases IT 4

depends on material,density and strength

can be removed frommould after pressing

undercuts,drilled holes cross-wise tothe pressing direction

finishing-machiningrequired

Fig. 2.1.1: Geometric parameters for powder metallurgy



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 nothigher than 2,5*D, otherwise breakageof the punch or over-pressing

no small cross sections, otherwiseunequal density distribution

faces instead of sharp edges to reducethe risk of punch breakage

Fig. 2.2.1: Design guidelines for sintered pre-formed parts [1/4] (Source:

Association of Powder Metallurgists (König/Klocke Vol.4, P.56, Fig. 2-52)



avoiding tangential transitions to reducethe risk of breakage at the punches

avoiding acute angles and rounding-offs,in order to minimise the risk of breakageat the punches

avoiding circular profiles transverse to the compaction direction, otherwisethe punches become to pointed


punc

h

die

die

punc

h

punc

h

die

die

punc

h



dimensioning ofholes and fixed links:diameters resp. widths not smaller thanone 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 becomeshard to have a complete compaction of the teeth






bigger distance between the bottomof the tooth space and the internal bore,risk for punches

breaking througs if possible rounded,otherwise the tools become expensive

diameter tolerances not smaller thanIT 7, height tolerances not smaller thanIT 12.




Wirtschaftliche Randbedingungen


3 Economic parameters

The outcome of cost comparisons with rival processes, depends largely on the

characteristics of the parts, Fig. 3.1. 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.

piece number alloy geometry accuracy machining

after sintering

decr

ease

<<

pro

duct

ion

cost

s >>

in

crea

se

indirect coststool costssinteringcompactingpowder

>7,2

density

<6,8

small

high

MoCoCrNi

Fe

IT 6

IT 9

sump-tuous

little

Fig. 2.2.1: 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.

2.2.2.



step 3:2nd rough-machining

step 2:1st rough-machining

step 4:internal toothing

step 5:fine turning of planefaces

step 6:broaching the outsideprofileweight: 286 g

detent„forging and cutting“

step 1:forged blankweight: 590 g

detent„powder metallurgical manufacturing“

final partweight: 191 g

several turningoperations

sintered blank

powderweight: 327 g

compactingsinteringsizing

Fig. 2.2.2: Comparison of processes used to manufacture synchronous parts -

conventional forging and cutting versus sintering (Source: Krebsöge) König/Klocke

Vol.4, P.59, Fig. 2-56“

The shorter manufacturing sequence can be a decisive advantage of powder

metallurgy, as the exemplar application “Manufacture of a synchronous part”

shows, Fig. 2.2.2 and Fig. 2.2.3. 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 elements of the part during the pressing operation, since this

eliminates the need to produce these elements in a finish cutting operation.

source: Metallwerk Unterfranken, ZF

1 - 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

alternative manufacturing sequences

2 - sintering and cutting• compacting the green compact• sintering• sizing• turning the front side• turning the back side

1 - external toothing2 - oil pocket3 - internal toothing

1

2

3

detent

1

2

0

25

50

75

100

rel.

prod

uctio

n co

sts

Fig. 2.2.3: Shorter manufacturing sequence as a result of high-precision sintering

Film: Verfahrensschritte und Anwendungsbeispiele


4 Film: Steps in the operation and exemplar applications

Notes:

Übungsaufgaben


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. 5.1.1: Part

Übungsaufgaben


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.

compacting pressure

sint

ered

den

sity

Fig. 5.2.1: Sintered density over compacting pressure

Übungsaufgaben


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/mm2 .

• The material consists of 91 percentage volume of copper and 9 percentage

volume of tin (ρcu = 8.9 kg/dm3, ρSn = 7.2 kg/dm3).

• The powder density is ρpulver = 5.5 kg/dm3.h

= 10

0 m

mh p

di = 50 mm

da = 79,5 mm 140

150

160

170

180

190

6 6,5 7 7,5 8sintered density ρ [kg/dm3]

tens

ile s

treng

th R

mN

/ mm

²

200

[

]

Drawing of the component diagram I

Fig. 5.3.1: Part and diagram showing tensile strength over sintered density

Übungsaufgaben


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 hp before pressing.

Übungsaufgaben


c) How high can the tensile strength Rm of a part be when the available press has

a maximum pressing force of F = 1.2 MN?

8,0

7,0

6,0

5,00 200 400 600

sint

ered

den

sity

ρ [

kg/

dm3 ]

compacting pressure p [ N/mm2 ]

Fig. 5.3.2: Sintered density over compacting pressure

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