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VDG spec-P201Volume Deficits of Non-Ferrous Metal Castings.pdf

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VDG SPEC P201VOLUME DEFICITS OF NON FERROUS METAL CASTINGS
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Page 1, VDG Specification P 201 revised and updated 2002-10-16 VEREIN DEUTSCHER GIESSEREIFACHLEUTE Volume Deficits of Non-Ferrous Metal Castings P 201 May 2002 VDG SPECIFICATION Specification by the Special Technical Committees Die Casting and Light Metal Casting of the German Foundrymen’s Association (VDG) Contents Page 1 Scope 2 1.1 General 2 1.2 Alloys 2 1.3 Casting processes 2 2 Explanatory notes 2 2.1 Porosity 2 2.1.1 Shrinkage cavities 2 2.1.2 Gas porosity 3 2.2 Other flaws 3 3 Possible effects of porosity 3 3.1 Static strength 3 3.2 Dynamic strength 4 3.3 General leak tightness and mating surfaces 4 3.4 Surface and heat treatment 4 4 Evaluation of casting porosity 4 5 Description of porosity 5 5.1 Pore classes 5 5.1.1 Pore class designation 5 5.1.2 Reference surfaces for the pore classes 6 5.1.3 Porosity class determination 7 5.1.4 Examples for pore class designations 7 5.2 Test options 8 5.2.1 Radiography test with image intensifier 8 5.2.2 Ultrasonic test 8 5.2.3 Leak tightness test 8 5.2.4 Visual test 9 5.2.5 Density test 9 5.2.6 Metallographic test and section test 9 6 Drawing notes 9 6.1 Collective notes 9 6.2 Notes for defined areas 9 7 References, standards, guidelines 10 Annex 1 Definition of further flaws 12 Annex 1.1 Cold flow marks 12 Annex 1.2 Build-up (stick marks) 12 Annex 1.3 Draw marks 12 Annex 1.4 Burrs 12 Annex 1.5 Hot cracks 12 Annex 1.6 Further characteristics of cast components 12 Annex 2 Microstructure examples of pore classes 0, 4 and 8 13 Annex 3 Examples of different porosity in general and in dependence on the reference surfaces (informative) 14 To be obtained from VDG-Informationszentrum Giesserei, Postfach 10 51 44, D-40042 Düsseldorf, Telefon (02 11) 68 71-254 To be quoted only with permission of the German Foundrymen’s Association The English translation is believed to be accurate. In case of discrepancies the German version shall govern. Klass.Nr: A6001 QUELLE: NOLIS (Norm vor Anwendung auf Aktualit?t prüfen!/Check standard for current issue prior to usage) Page 2, VDG Specification P 201 1 Scope 1.1 General This VDG specification applies to non-ferrous metal castings. It aims at a description of casting quality requirements as well as their uniform presentation in technical documentation. The scope of this specification is restricted to inner and outer volume deficits (porosity). Other flaws such as sink marks, cold flow marks, build-up, draw marks, burrs and hot cracks are not subject of this specification. Porosity can be minimised if designer and caster collaborate. 1.2 Alloys The scope of this specification is restricted to aluminium, magnesium and zinc base alloy castings acc. to the relevant standards (DIN EN 1706 “Aluminium“, DIN EN 1753 “Magnesium“, DIN EN 12844 “Zinc“). 1.3 Casting processes This VDG specification is restricted to die casting including related special casting processes, such as squeeze casting and thixocasting, as well as to sand casting and ingot casting procedures. 2 Explanatory notes 2.1 Porosity 2.1.1 Shrinkage cavities Shrinkage cavities result from the thermophysical properties of the cast materials during solidification. It is impossible to manufacture castings free of shrinkage cavities. Metallic materials in liquid and solid aggregate state have different densities and therefore different specific volumes. The transition from liquid to solid state causes a change in volume (Figure 1). For commonly used non- ferrous metal alloys, their volume decreases during cooling (volume deficits, solidification contraction). Solid contraction Fluid contraction Solidification contraction Temperature, °C Specific volume, cm 3 /g T MT M = Melting point Figure 1 Specific volume of aluminium depending on temperature As generally outer shell and sprue of a part solidify early, a volume deficit occurs in the casting. This causes cavities in the casting. By means of a suitable design of the casting, e.g. avoidance of differences in the wall thickness, and optimum design of the casting system, this volume deficit can be minimised. Shrinkage cavities are hollow and have a more or less rugged shape. In case the outer shell cannot withstand the stresses which occur during shrinkage, this may result in shrinkage cracks or hot cracks. If casting and mould are designed in an appropriate way, these cracks and cavities can be cured by refeeding the affected areas. Alloys with a wide solidification interval are especially sensitive to hot cracks. If cavities and pores can be observed with the emmetropic eye, this is called macroporosity, if not, it is called microporosity. 2.1.2 Gas porosity Gas pores can have: – Thermodynamic causes – Fluidic causes 2.1.2.1 Thermodynamically caused gas porosity In liquid metals elementary gases generally are better soluble than in solid metals. Therefore gases are exuded during solidification, which leads to pore formation. Page 3, VDG SpecificationP 201 The pores accumulate in the area of remaining solidification. In alloys with a distinct solidification interval, gas exudation preferably takes place between the dendrite arms. As a result, gas pores caused by exudation generally do not have a round contour. The contour rounding depends on the gas content of the melt. 2.1.2.2 Gas porosity caused by fluidics During mould filling, gases are included as a result of metal flow. These gases are either gases from the ambient air or gases that result from the thermal contact reaction between the melt and the moulding material or the material of the casting mould and/or accessory casting materials such as release agents, die lubricants etc. Gas pores caused by fluidics generally are round as a result of transitional stresses between gas and melt. Note: In general, both types, shrinkage cavities and gas pores occur in combination. 2.2 Other flaws In addition to porosity other flaws may occur which can influence the casting quality, such as cold flow marks, built-up, draw marks, burrs and hot cracks. These flaws are mentioned in this specification and defined in Annex 1; however they are not subject of this specification. As far as possible, reference is made to other guidelines and standards that refer to these flaws and their test methods. 3 Possible effects of porosity According to type and properties of the component as well as to load case, pores in castings can affect strength, leak tightness under pressure, surface characteristics and/or appearance of the component. In the case of technical grade components special attention shall be paid to the effects of porosity on component strength. The same porosity can have different effects on statically and dynamically stressed components. In both cases points of force application, stress intensity and areas of stress concentration should be given in order to ensure suitable selection of pore classes. 3.1 Static strength When a force is applied to a component, this causes stress in the cross section under load. This stress is proportional to the quotient of force and cross sectional area. If the cross section is minimised (weakened) because of pores, the stress increases. As soon as the resulting stress exceeds the elastic limit of the material, permanent deformation occurs which can lead to fracture. In addition to this increase in stress caused by the cross- sectional minimisation, a notch effect arises, depending on pore geometry. The most critical aspect in the case of static stress is the portion of pores related to the cross sectional area and therefore the weakening of the cross section . Under bending stress and torsional stress, the position of porosity in relation to the neutral fibre is to be observed. Especially in the case of shrinkage pores, porosity is located in the excess material area and therefore near the neutral fibre. As a result, strength reduction in the total cross section is proportional to the surface portion of porosity in good approximation. 3.2 Dynamic strength Concerning the dynamic strength of a component, besides the material the notch effect is of great importance. Geometric contours, inhomogeneity caused by oxide films, inclusions, microstructure components, intermetallic compounds etc., and casting defects can lead to notches having a more serious effect than pores. Depending on their shape, their position relative to the casting surface and their arrangement, pores have different notch effects. The notch effect – Increases with the area occupied by porosity and the pore diameter – Decreases with better roundness and greater distance of the pores from the casting surface Page 4, VDG Specification P 201 Examinations concerning the fatigue strength of aluminium cast alloys have revealed a decrease of the fatigue strength by approx. 15 to 20 % [1 to 11], when the porosity is increased from pore class 0 to pore class 8 (see figure in Annex 2 [12]). 3.3 General leak tightness and mating surfaces When gas pores, shrinkage cavities and hot cracks are directly connected with the casting skin (open pores, sink marks) or when they are cut during machining, this may lead to leakage of the components or mating surfaces depending on pore distribution. In connection with the requirement for leak tightness under pressure, especially hot cracks and interconnected cavities are to be considered as critical. Depending on their shape and size, pores can cause damage and/or impairment of seals. 3.4 Surface and heat treatment If the components have been coated, electroplated or heat treated, surface porosity can cause points of discontinuity and/or surface blisters. During heat treatment (annealing, thermal drying of paints, etc.) of castings, the elevated temperature causes a strength reduction of the material. As a result, the internal pressure causes deformation and/or blistering, especially of gas-filled pores. This applies in particular to die cast components (see 2.1.2.2). Even in die casting, blistering can be prevented to a great extent by applying forced ventilation to the die. 4 Evaluation of casting porosity The degree of porosity depends on the material, the manufacturing process, the process-compatible design of the component, its function and the permissible degree of porosity. In general, it can be noted that an enhancement of the porosity requirement leads to increased efforts for production and testing, and therefore to increased costs. Two types of porosity have to be distinguished: macroporosity and microporosity: Macroporosity includes all pores, the size and shape of which can be specified with the emmetropic human eye or an auxiliary means with the same resolution (such as X-ray method). This applies to pores with a minimum extension of 0.5 mm. Microporosity includes all pores, the shape and size of which cannot be reliably evaluated with the naked eye. This includes pores up to a maximum diameter of 0.5 mm. The minimum pore size to be determined depends on the resolution of the auxiliary means used. Porosity requirements shall be guided by the component requirements (static strength, fatigue strength, leak tightness under pressure and function of machined surfaces, appearance of unfinished casting surfaces). Evaluation criteria and measures shall be defined by manufacturer and customer by the time of ordering. Area-specific criteria should be explicitly defined on one component (see 5.1). Publications [12 to 19] can help defining evaluation criteria and measures. 5 Description of porosity 5.1 Pore classes In order to describe all porosity requirements of a component, different pore classes can be defined for sub-areas of the component. Experience shows that it is difficult to manufacture complex components in only one pore class. 5.1.1 Pore class designation The pore class designation consists of the following parameters: Load case: The load case parameter can be designated as follows: S for components under static stress mainly D for components under dynamic stress mainly F for components with special requirements on functional surfaces Page 5, VDG SpecificationP 201 G for components without further specified requirements Porosity: The porosity parameter for the surfaces agreed upon states the maximum permissible porosity in percent for the load cases G, S and D, and the maximum permissible number of defined pores related to a reference surface for the load case F. The reference surface is always square, triangular (isosceles) or round, with its shape depending on the respective component geometry (see Figure 2 and 5.1.2). Diameter: The specification of the diameter parameter is optional. It specifies the maximum permissible comparison diameter of single pores. Optionally, manufacturer and customer can agree on the mean length or on the mean pore diameter or on the equivalent diameter. (The comparison diameter is the smallest diameter possible including all pores; the equivalent diameter is the diameter of a circle of the same area.) Additional parameters: The specification of the additional parameters Z 1to Z n(from German “Zusatz“) is optional. They can be used separately or in multiple combinations and assume the following values: An Distance between adjacent pores. This parameter specifies the minimum edge distance between two adjacent pores. The minimum edge distance is the diameter of the smaller one of two adjacent pores multiplied by the factor n in mm. (A = distance; from German “Abstand“) The value for n shall be agreed upon by manufacturer and customer. M Centre of the component wall. This parameter can only be used in connection with the diameter parameter. Localised porosity is only permissible in the centre (M; from German “Mitte“) of the component wall. Localised porosity is an accumulation of single pores. Necessary prerequisites for the presence of localised porosity are: – The localised porosity diameter is greater than the maximum permissible size of single pores. – The distance between adjacent pores is smaller than the diameter of the smallest of these pores. C Excess material. This parameter can only be used in connection with the diameter parameter. Localised porosity is only permissible in excess material and joint areas (heat centres = C). R Component wall centre area. The parameter R is only permissible for pore classes D10 to D30. (e.g. D10: mainly dynamically stressed with a maximum permissible porosity of 10 %) The specified porosity class only applies to the centre area (R) of the component wall (inner third of the relevant wall thickness). In the two outer thirds porosity class D4 shall be adhered to. Pn Pore size. This parameter can only be used in connection with the dia
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