Manufacturing Technology II. Exercise 1. Casting

Transcription

1 Lehrstuhl für Technologie der Fertigungsverfahren Laboratorium für Werkzeugmaschinen und Betriebslehre Manufacturing Technology II Exercise 1 Casting Werkzeugmaschinenlabor Lehrstuhl für Technologie der Fertigungsverfahren Prof. Dr. - Ing. F. Klocke RWTH - Aachen Steinbachstraße Aachen

2 Inhaltsverzeichnis Table of Contents 1 Introduction Requirement-oriented design of cast parts Casting faults Shape and casting oriented design Load-oriented design Machining-oriented design Presenting and defining casting processes Exercises Requirement oriented design of cast parts Selecting a casting process Fertigungstechnik II - Übung 1 2

3 Einleitung 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 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 ). The formation of shrinkage cavities in cast parts can be avoided by using appropriate feed technology. Fertigungstechnik II - Übung 1 3

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 ). Solid shrinkage is combated by providing an allowance (shrinkage allowance) in the mould. immediate before 1 filled casting mould 2 solidification 3 partially solidified cast part feeder liquid shrinkage liquid solidificaton shrinkage solid specific volume shrinkage behaviour of pure metals and eutectic alloys temperature 1 4 cooled down cast part solid shrinkage shrinkage cavity Fig : 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 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 Fertigungstechnik II - Übung 1 4

5 using a mould which is already convex, which will ensure that the lattice which is required, will be achieved after cooling. Fig 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. compression tension compression Fig : 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 Fertigungstechnik II - Übung 1 5

6 contrast to tension cracks, heat cracks are inter-crystalline. Heat cracks can be repaired by using good feed technology. length: material: mm GG-25 To meet the requirend tolerances acc. to DIN 1685 einzuhalten, the sand moulding was produced with a concave deformation of about 20 mm hergestellt. Due to that, the cast part is plane. Fig : 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 Fertigungstechnik II - Übung 1 6

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 measures shrinkage shrinkage feed technology cavities dimension smaller than specified distortion shrinkage cooling rates of cross sections with different thicknesses allowance of shrinkage 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. avoidance of material accumulation) segregation segregation of the melt during solidification smelting reduction measures inclusions gas bubbles oxide formation in the melt impurities in the melt solubility of the melt in the gas dimishes as the temperature falls smelting reduction measures allow melt to cool slowly; implement smelting reduction measures Fig : Typical casting faults and their causes Fertigungstechnik II - Übung 1 7

8 2.2 Shape and casting oriented design The risk that casting faults will occur, can be reduced by ensuring casting-oriented design. Fig 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. bad w better w good w x < w w w w bad good bad good shrinkage cavity risk of cracking bad good bad good shrinkage cavity risk of cracking risk of cracking shrinkage cavity Fig : 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 Fertigungstechnik II - Übung 1 8

9 essential to ensure that the part can be removed from the mould. Undercuts and through holes present particular problems in this respect, Fig Throughholes 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 puller core puller core puller core puller Fig : Principle of removability from the mould (Source: ZGV) Fertigungstechnik II - Übung 1 9

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 This principle is particularly important where fin design is concerned. Highly stressed cast parts if possible loading with pressure and not with tension! tension Zug p p p pressure Druck F F tension Zug F 1 a F F Druck compression tension Zug F 2 b compression Druck Fig : Load-oriented design of cast parts (Source: ZGV) (König/Klocke Vol. 4, P. 25, Fig. 2-19) Fertigungstechnik II - Übung 1 10

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 It is important to make provision for clamping. Parts can be fastened easily when there are clamping lugs (c.f. Fig ). Run-out space should be provided for the machining tools. This design principle is illustrated by the example of a clamping surfaces in Fig 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. machining-oriented design of clamping-surfaces machining-oriented design of working-surfaces bad good Fig : Machining-oriented design of cast parts (Source: ZGV) Fertigungstechnik II - Übung 1 11

12 A: finished part B: bad C: good Fig : Machining-oriented design of cast parts (Source: ZGV) (König/Klocke Vol. 4, P. 26, Fig. 2-20) Fertigungstechnik II - Übung 1 12

13 Gießverfahren 3 Presenting and defining casting processes Notes: Fertigungstechnik II - Übung 1 13

14 Übungsaufgaben 4 Exercises 4.1 Requirement oriented design of cast parts The drawing in Fig 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. P ü F Fig : Gas pressure tank Fertigungstechnik II - Übung 1 14

15 Übungsaufgaben 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 Fertigungstechnik II - Übung 1 15