Primary Shaping - Casting Differences in the cross-sections of a part cause distortion. Distortion...

Chair of Manufacturing Technology

Primary Shaping - Casting

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


Manufacturing Technology II - Exercise 1 2

Table of Contents

Table of Contents ......................................................................................................2

1 Introduction .........................................................................................................3

2 Requirement-oriented design of cast parts ......................................................3

2.1 Casting faults ................................................................................................3 2.2 Shape and casting oriented design...............................................................8 2.3 Load-oriented design ..................................................................................10 2.4 Machining-oriented design..........................................................................11

3 Presenting and defining casting processes ...................................................13

4 Exercises ...........................................................................................................14

4.1 Requirement oriented design of cast parts .................................................14 4.2 Selecting a casting process ........................................................................15



1 Introduction

“Casting” as a forming method provides a means of producing complex parts in one forming operation. However, the high level of design freedom is limited by process-specific characteristics. Principles and guidelines in relation to requirement oriented design of cast parts, will therefore be one of the focuses of this exercise. The lecture, in which the various casting processes were presented, is supplemented here by in-formation relating to the selection of the most suitable casting process for the task in hand, which will backed up by examples. The exercise will conclude with tasks relat-ing to casting-oriented design and to the selection of a suitable casting process.

2 Requirement-oriented design of cast parts

2.1 Casting faults

Cooling a cast workpiece from melting to room temperature causes volume contrac-tion, which is described by the term shrinkage. The volume contraction over tempera-ture, as recorded in the case of pure metals and eutectic alloys, is shown in qualita-tive terms in Fig. 2.1.1. The shrinkage can be classified as liquid shrinkage, solidifica-tion shrinkage and solid shrinkage.

The cooling rate is inversely proportional to the volume of the cast part, i.e. thinner sections solidify more rapidly than thick ones.

These two characteristics are at the root of typical casting faults, which are described in the following:

Shrinkage cavities:

The inner area of a cast cross section normally solidifies last. Shrink holes, or shrink-age cavities form to balance out the volume deficit caused by shrinkage (c.f. Fig. 2.1.1). The formation of shrinkage cavities in cast parts can be avoided by using ap-propriate feed technology.



Dimensions smaller than specified:

Dimensions smaller than specified, are the result of shrinkage. Liquid shrinkage can be balanced out by adding to the melt via the feed attachment (c.f. Fig. 2.1.1). Solid shrinkage is combated by providing an allowance (shrinkage allowance) in the mould.

43

21

1 2 3

4

shrinkage behaviour of puremetals and eutectic alloys

spec

ific

volu

me

filled casting mould

feeder

cast part

immediate beforesolidification

liquidshrinkage

solid

liquid

partially solidified

solidificatonshrinkage

cooled downcast part

shrinkage cavity

solid shrinkagetemperature

Fig. 2.1.1: Volume contraction when pure metals (and eutectic alloys) cool down from their molten state

Distortion:

Differences in the cross-sections of a part cause distortion. Distortion is illustrated in Fig. 2.1.2 on the basis of the example of a closed lattice with cross sections of differ-ent thickness. Whereas the thin rods have already solidified and can therefore sus-tain only elastic deformation, the middle spar will continue to contract, and will there-fore be subjected to tensile stresses whilst compressive strain will occur in the rods. In addition to this, the two connecting struts will form a concave arch. This can be remedied by balancing out the cross sections or by using a mould which is already convex, which will ensure that the lattice which is required, will be achieved after cooling.



Fig. 2.1.3 shows a practical example of an arched form deviation. In the manufacture of the front section of a 13 m long machine base for a grinding machine, the form was produced with a bow of 20 mm. After cooling, the cast part was straight as a re-sult of distortion.

tens

ion

com

pres

sion

com

pres

sion

Fig. 2.1.2: Distortion due to different cooling in sections of varying thickness (source: ZGV) (König/Klocke Vol. 4, P. 23, Fig. 2-16)

Tension cracks:

Residual stresses occur as a result of extreme changes in the cross section when the cast structure solidifies. The offset yield stress can even be exceeded due to the stresses and tension cracks begin to form. The risk that tension cracks will appear, can be reduced by avoiding material build-up and sharp-edged transitional areas, which can cause high levels of notch stress.

Heat cracks:

Heat cracks develop when small residues of liquid phase remain in a cast part which has largely solidified. Solidification shrinkage causes heat cracks. The risk that heat cracks will develop, is particularly high when the volume contraction is hampered, by the more rapid solidification of thin sections, for example. In contrast to tension cracks, heat cracks are inter-crystalline. Heat cracks can be repaired by using good feed technology.



length: 13.270 mmmaterial: GG-25

To meet the requirend tolerancesacc. to DIN 1685 einzuhalten,the sand moulding was producedwith a concave deformation ofabout 20 mm hergestellt. Due tothat, the cast part is plane.

Fig. 2.1.3: One-part front section of a grinding machine base, cast in a concave mould in order to compensate for residual stresses (Source: Krupp) (König/Klocke Vol. 4, P. 24, Fig. 2-17)

Segregation:

Segregation is the term used to describe localised concentrations of one alloying element or of impurities. Segregation can be suppressed by the implementation of smelting reduction measures such as killed casting.

Inclusions:

Metal melts are susceptible to oxide formation. There are also non-metallic inclusions in metal melts due to impurities. When the material solidifies, the oxides and impuri-ties are enclosed in the structure. Smelting reduction measures can suppress the formation of oxides in some cases.

Gas bubbles:

The gas solubility of metal melts diminishes as the temperature falls. Considerable amounts of gas are released, particularly in the transitional stage from a liquid to a solid state. If the gas bubbles cannot rise freely to the surface of the melt, they be-come enclosed in the cast part. Technological and smelting reduction measures such as slow cooling of the melt, can prevent gas bubbles from forming.



casting faults cause avoidance measuresshrinkagecavities

shrinkage feed technology

dimensionsmaller thanspecified

shrinkage allowance of shrinkage

distortion cooling rates of cross sectionswith different thicknesses

casting-oriented design (e.g. same cross sections)

heat cracks shrinkage feed technology, casting-oriented design(e.g. avoidance of material accumulation)

stress cracks residual stresses casting-oriented design (e.g. avoidanceof material accumulation)

segregation segregation of the meltduring solidification

smelting reduction measures

inclusions oxide formation in the meltimpurities in the melt

smelting reduction measures

gas bubbles solubility of the melt in the gasdimishes as the temperature falls

allow melt to cool slowly; implementsmelting reduction measures

Fig. 2.1.4: Typical casting faults and their causes



2.2 Shape and casting oriented design

The risk that casting faults will occur, can be reduced by ensuring casting-oriented design. Fig. 2.2.1 shows guidelines for the design of junction points and wall thicken-ings in cast parts. It is an important basic rule for the design of cast parts, that mate-rial accumulation should be avoided. Differences in wall thickness cannot always be avoided, for functional reasons. Gradual transitions, e.g. via radii, are more efficient than sharp-edged transitional areas.

w ww

w

w

wx

< wgoodbetterbad

bad good bad good

shrinkagecavityrisk ofcracking

risk

of

crac

king

bad good bad good

risk ofcracking

shrinkagecavity

shrinkagecavity

Fig. 2.2.1: Design guidelines for junction points and wall thickening of cast parts (Source: ZGV) (König/Klocke Vol. 4, P. 25, Fig. 2-18)

After casting, the cast part must be removed from the mould. In the case of proc-esses involving lost moulds and permanent models (e.g. hand moulding, shell mould casting) and processes involving lost moulds and lost models (e.g. precision casting, full mould casting), the casting mould is destroyed after casting. This is not possible in the case of processes which use permanent moulds (e.g. chilled casting, die-casting). When these processes are used, it is therefore essential to ensure that the



part can be removed from the mould. Undercuts and through holes present particular problems in this respect, Fig. 2.2.2. Through-holes can be produced in die-casting operation using movable permanent cores, for example. It is vital to ensure at the design stage, that the cores can be pulled out of the cast part, without causing any damage to the part.

core pullercore puller

core pullercore puller

Fig. 2.2.2: Principle of removability from the mould (Source: ZGV)



2.3 Load-oriented design

Knowledge of the level and direction of all forms of stress and strain arising in the course of the operation, is an important prerequisite for the load-oriented design of cast parts. Care should be taken to ensure that cast parts which are exposed to high levels of load, are subjected to pressure but not to tensile force, Fig. 2.3.1. This prin-ciple is particularly important where fin design is concerned.

DruckZug p

p p

F1

F2

Druck

Zug

Zug

Drucka

b

F

F

tension pressure

tension

compression

compression

tension

F

F

Highly stressed cast partsif possible loading with pressureand not with tension!

Fig. 2.3.1: Load-oriented design of cast parts (Source: ZGV) (König/Klocke Vol. 4, P. 25, Fig. 2-19)



2.4 Machining-oriented design

The majority of cast parts require a metal-cutting finishing operation before they are fit for industrial use. There are some ground rules which must be observed:

■ It is vital to take account of the machining technology which will subse-quently be used. The surfaces which will be machined, must be designed so as to be production-environment friendly. For example, a drilling axis which is normal to the surface of the tool, prevents the drill from running off centre, Fig. 2.4.1.

■ It is important to make provision for clamping. Parts can be fastened eas-ily when there are clamping lugs (c.f. Fig. 2.4.1.1).

■ Run-out space should be provided for the machining tools. This design principle is illustrated by the example of a clamping surfaces in Fig. 2.4.2. The machining allowance in Model B, must be worked off in a time-consuming operation in the corner area. The provision of a tool run-out area (Model C), permits the corner to be produced relatively easily in mill-ing and planing operations.

■ Residual stresses which cause part distortion can develop as a result of a metal cutting operation.

bad

machining-oriented designof clamping-surfaces

machining-oriented design of working-surfaces

good

Fig. 2.4.1: Machining-oriented design of cast parts (Source: ZGV)



A: finished part B: bad C: good

Fig. 2.4.2: Machining-oriented design of cast parts (Source: ZGV) (König/Klocke Vol. 4, P. 26, Fig. 2-20)



3 Presenting and defining casting processes

Notes:



4 Exercises

4.1 Requirement oriented design of cast parts

The drawing in Fig. 4.1.1 shows a gas pressure tank, which is to be produced in a casting process. However, the drawing has a number of faults which must be modi-fied before a model is produced.

a) First mark and label the points where there are faults. b) Then modify the drawing, eliminating these faults.

Pü

F

Fig. 4.1.1: Gas pressure tank



4.2 Selecting a casting process

The following workpieces are to be manufactured in a casting process.

State one process which is suitable for each part and give reasons for your choice.

Part Process Reason

Machine tool base

Material: GG

Mass: 1.5 t

Quantity: 1

Turbine casing

Nodular cast iron

Mass: 15 t

Quantity: 3

Extra car headlight

Aluminium alloy

Mass: 0.4 kg

Quantity: 200,000

Cylinder liner

Lamellar cast iron

Mass: 1 t

Quantity: 20

Turbine wheel



Cast steel

Mass: 1 kg

Quantity: 50,000

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Primary Shaping - Casting Differences in the cross-sections of a part cause distortion. Distortion...

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