Upload
buihanh
View
227
Download
3
Embed Size (px)
Citation preview
Lehrstuhl für Technologieder Fertigungsverfahren
Laboratoriumfür Werkzeugmaschinenund Betriebslehre
Manufacturing Technology II
Exercise 1
Casting
WerkzeugmaschinenlaborLehrstuhl für
Technologie der FertigungsverfahrenProf. Dr. - Ing. F. Klocke
RWTH - AachenSteinbachstraße 53
52065 Aachen
Inhaltsverzeichnis
Fertigungstechnik II - Übung 1 2
Table of Contents1 Introduction .................................................................................................... 3
2 Requirement-oriented design of cast parts .................................................... 32.1 Casting faults ................................................................................................. 32.2 Shape and casting oriented design................................................................ 82.3 Load-oriented design ................................................................................... 102.4 Machining-oriented design ........................................................................... 11
3 Presenting and defining casting processes.................................................. 13
4 Exercises...................................................................................................... 144.1 Requirement oriented design of cast parts .................................................. 144.2 Selecting a casting process ......................................................................... 15
Einleitung
Fertigungstechnik II - Übung 1 3
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 information 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 relating 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
contraction, which is described by the term shrinkage. The volume contraction
over temperature, as recorded in the case of pure metals and eutectic alloys, is
shown in qualitative terms in Fig. 2.1.1. The shrinkage can be classified as liquid
shrinkage, solidification 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
shrinkage 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 appropriate feed technology.
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 4
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
different thickness. Whereas the thin rods have already solidified and can
therefore sustain only elastic deformation, the middle spar will continue to
contract, and will therefore 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
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 5
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 result 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
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 6
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 impurities 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
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 7
become 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
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 8
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
thickenings in cast parts. It is an important basic rule for the design of cast parts,
that material 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
processes 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
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 9
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)
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 10
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 principle 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)
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 11
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 subsequently
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 easily
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 milling 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)
Konstruktion von Gußbauteilen
Fertigungstechnik II - Übung 1 12
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)
Gießverfahren
Fertigungstechnik II - Übung 1 13
3 Presenting and defining casting processes
Notes:
Übungsaufgaben
Fertigungstechnik II - Übung 1 14
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
modified 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
Übungsaufgaben
Fertigungstechnik II - Übung 1 15
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 baseMaterial: GGMass: 1.5 tQuantity: 1
Turbine casingNodular cast ironMass: 15 tQuantity: 3
Extra car headlightAluminium alloyMass: 0.4 kgQuantity: 200,000
Cylinder linerLamellar cast ironMass: 1 tQuantity: 20
Turbine wheelCast steelMass: 1 kgQuantity: 50,000