D D2.1 Database on Defects - Stacast · PDF filedie is first in contact with a molten alloy at more than 700°C and, after 30-40 seconds, with a ... defect can be found both in HPDC

StaCast - New Quality and Design Standards for Aluminium Alloys Cast Products FP7-NMP-2012-CSA-6 - PROJECT N. 319188

www.stacast-project.org

StaCast: New Quality and Design Standards for Aluminium Alloys Cast Products – Deliverable D2.1: Database on Defetcs Authors: F. Bonollo, G. Timelli, E. Fiorese, E. Gariboldi, P. Parona, L. Arnberg

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COORDINATION AND SUPPORT ACTION

(SUPPORTING)

Call: FP7-NMP-2012-CSA-6 Support for standardisation needs

Project full title: New Quality and Design Standards for Aluminium Alloys Cast Products

Project acronym: StaCast

Co-ordinator name Franco BONOLLO Co-ordinator organisation name University of Padova – DTG

Co-ordinator email and fax [email protected] +39 0444 998889 Co-ordinator phone + 39 0444 998743

DELIVERABLE D2.1

Database on Defects

AUTHORS: F. BONOLLO, G. TIMELLI , E. FIORESE (UNIVERSITY OF PADOVA – DTG) E. GARIBOLDI, P. PARONA (ITALIAN ASSOCIATION OF METALLURGY – AIM) L. ARNBERG (UNIVERSITY OF TRONDHEIM – NTNU)

Date: March 29th, 2013




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Database on Defects

1. Introduction Defects are intrinsically generated by casting processes, due to several reasons. The final properties and in-service behaviour of castings are always related to microstructural features and to defects: both microstructure and defects are the results of process stages, alloys properties and dies & tools design. As an example, it should be considered that in HPDC filling stage extreme conditions are established: complexity of components leads to complex dies, and the high production rates required (up to 120 shots/h) lead to very high filling velocities for the molten alloy (up to 40 m/s) with strong generation of turbulence in the flow. Solidification takes place in few seconds, and the die is first in contact with a molten alloy at more than 700°C and, after 30-40 seconds, with a sprayed lubricant at room temperature. For these reasons HPDC (as well as other Aluminium alloys casting processes, such as permanent mold casting process) can be considered a “defect generating process”. Not only an average 5-10% scrap is typically produced, but the type, size and severity of defects are varying. From these considerations, it seems that the potential of high-pressure die-casting and permanent mold casting processes will be completely exploited only when the quality level will be perfectly optimised. In this regard, the analysis of defects allows the foundry to monitor the products quality respect to a quality standard. Further, the analysis of defects provide to the foundry useful correlations between defects type/distribution and their origin, so that it could be possible to define process modification for improving the quality. One of the current targets of the StaCast project is the compilation and dissemination of common tools to allow foundries to define a proper, comparable, quality standard. The first tool to be proposed is a common basis of language, i.e. a terminology and classification of defects in order to help die-casting foundries to face with increased confidence and ability the defect-related issues, to assure quality and reliability of their products.

2. The path towards a new Standard on Defect Classification 2.1. Previous classifications of defects There are three main approaches for defects classification of cast components proposed in literature or currently adopted by foundries:

- Cocks approach based on defects geometry/location - Campbell approach based on defects metallurgical origin/causes - NADCA approach based on defects morphology.

The first approach was proposed by Cocks and discerns between surface and internal defects (Table 1). The surface defects are visible by naked eye and impact both on the product aesthetics and functionality. On the other hand, the internal defects influence only the in-service properties of the component. As shown in Table 2, the second approach, proposed by Campbell, classifies defects on the basis of their metallurgical origin/causes (casting geometry, cast alloy, die characteristics, die lubrication, process parameters, etc.). The main advantage offered by this classification is the opportunity to design and adopt strategies to improve products quality. The main disadvantage of this approach is that the origin/causes of the defect must be defined concurrently to defect identification and that a single defect can be due to several concurring factors. Further, this approach is less suitable for




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direct application in foundries with respect to geometry/position-based approaches, where specific inspections can be proposed to reveal different defect groups. The third approach was proposed by NADCA (North America Die Casting Association) and is based on defects morphology. NADCA suggested seven defects categories, that are indicated by a letter (Table 3). Each category is divided into groups, that are divided into subgroups.

Category Class Type 1-General 2-Splash and shotting 3-Vortex

1-Cold shut

4-Lamination 1-Blisters 2-Sinks 2-Smooth irregularities 3-Lakes 1-Drag marks

1-Surface defects

3-Rough surface 2-Solder 1-Hydrogen

1-Gas porosity 2-Oxidizing gases 1-Gross 2-Intergranular

Coc

ks c

lass

ifica

tion

2-Internal defects 2-Shrink porosity

3-Cracks - hot tears

Table 1. Classification of casting defects according to Cocks.

Gas in solution (hydrogen) Gas entrapment during filling (air) Gas porosity Binder breakdown (core gases) Macro-porosity

Shrinkage porosity Micro-porosity (interdendritic) C

ampb

ell

clas

sific

atio

n

Hot tearing, cracks

Table 2. Classification of casting defects according to Campbell.

Category A-Metallic projection B-Cavities C-Discontinuities D-Defective surface E-Incomplete casting F-Incorrect dimension or shape

NA

DC

A

clas

sific

atio

n

G-Inclusion or structural anomalies

Table 3. Classification of casting defects according to NADCA.




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2.2. Proposal of new classification of defects Recently, the AIM (Italian Association of Metallurgy), after a 2-years survey carried out involving about 50 Al-alloys foundries, suggested a new classification approach, based on a 3-levels defects individuation: I) morphology/location of defects (internal, external, geometrical); II) metallurgical origin of defects (e.g. gaseous porosity, solidification shrinkage, etc.); III) specific type of defects (the same metallurgical phenomenon may generate various defects). The level I is based on morphology/location of defects, with reference to the investigation techniques suitable for their detection (visual inspections and controls involving the bulk material): there are internal and external (or surface) defects. Sub-surface defects (i.e. so close to the surface that they can affect external aspect detectable by conventional surface investigation techniques), are considered surface defects. Finally, the geometrical defects refer to the casting shape in terms of dimensions and tolerances. The level II is mainly focussed on the metallurgical origin of defects. Defects are grouped into several classes according to their general metallurgical origin:

- defects related to the presence of gas (gas-related defects); - defects related to material volume contraction during metal solidification (shrinkage

defects); - defects related to thermal contraction prevented by previously solidified metal or by the die

(thermal contraction defects); - defects related to incorrect filling of the die-cavity (filling defects); - defects related to metal/mould interaction; - defects related to the presence of unsuitable phases (undesired phases), originated by the

interaction of the metal with external environment during melting, casting, filling or extraction/ejection from the mould.

As previously observed, the knowledge of metallurgical origin could supply starting points for corrective actions (including process parameters). The level III is used to identify the specific types of defects. Usually, the term adopted to describe a particular type of defect allows a better definition of the metallurgical origin of the defect itself, which was preliminarily identified in the previous level. Such approach has been widely described in a report published [E. Gariboldi, F. Bonollo, P. Parona, Handbook of defects in HPDC, AIM, Milano], and certainly constitutes the basis for the first relevant StaCast Objective, i.e. the definition of a New Standard on defects classification. The present classification of defects is of hybrid type and multi-level, as schematically shown in Tables 4, 5 and 6. The proposal refers to metallurgically-based defects of HPDC and permanent mold casting products. Defects directly related to handling, finishing, machining operations following ejection from the die are excluded from the classification, even if they could be possible causes for product rejection. In this way, the range of defect types is not excessively wide.




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1st Level 2nd Level 3rd Level A1.1 Macro-shrinkage

A1.2 Interdendritic shrinkage A1 Shrinkage defects

A1.3 Layer porosity

A2.1 Air entrapment porosity

A2.2 Hydrogen porosity

A2.3 Vapour entrapment porosity A2 Gas-related defects

A2.4 Lubricant entrapment porosity

A3.1 Joint

A3.2 Lamination A3 Filling-related defects

A3.3 Cold shot

A4.1 Inclusion A4 Undesired phases

A4.2 Undesired structure

A5.1 Crack

A Internal defects

A5 Thermal contraction defects A5.2 Hot tear

Table 4. Classification of internal defects.

1st Level 2nd Level 3rd Level B1 Shrinkage defects B1.1 Sink

B2 Gas-related defects B2.1 Blister

B3.1 Joint and Vortex

B3.2 Lamination B3 Filling-related defects

B3.3 Cold shot

B4.1 Surface deposit B4 Undesired phases

B4.2 Contamination or inclusion

B5.1 Crack B5

Thermal contraction defects B5.2 Hot tear

B6.1 Erosion

B6.2 Soldering

B6.3 Thermal fatigue

B6.4 Ejection mark

B Surface defects

B6 Metal-die interaction defects

B6.5 Corrosion of the die

Table 5. Classification of surface defects.

1st Level 2nd Level 3rd Level C1 Lack of material C1.1 Incomplete casting

C2 Excess of material C2.1 Flash

C Geometrical defects C3 Out of tolerance C3.1 Deformed part

Table 6. Classification of geometrical defects.




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3. Database on Defetcs

INTERNAL DEFECTS

A Internal defects I A1 Shrinkage defects II A1.1 Macro-shrinkage III

A Internal defects I A1 Shrinkage defects II A1.2 Interdendritic shrinkage III

Definition A macro-shrinkage is a relatively large shrinkage cavity, formed inside an hot spot and due to the volume contraction during solidification. Morphology A macro-shrinkage is characterized by rough and spongy surfaces for the presence of emerging dendrites as a consequence of their interrupted growth. A macro-shrinkage can reach several mm in diameter (> 0.5 mm). It can be detected by means of radiographic, ultrasonic inspections and metallographic tests. Metallurgical origin The formation of a macro-shrinkage depends on mould-filling conditions, alloy physical properties (e.g. solidification range), geometry of the casting and process parameters. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 1. Macrograph of a macro-shrinkage [14].

Definition An interdendritic porosity consists of several cavities located in the interdendritic regions, and forms when the liquid afflux in these regions is inadequate to counterbalance the shrinkage of the metal during solidification. Morphology The interdendritic porosity is characterized by a net of long and narrow three-dimensional branches. The size of an interdendritic shrinkage varies between 10-150 µm. This defect can be detected by means of radiographic, ultrasonic inspections and metallographic tests. Metallurgical origin The formation of the interdendritic porosity is favored by a wide mushy zone, as could be the case of wide solidification range and low temperature gradients (e.g. in the last solidified regions of a thick casting). The interdendritic porosity could be a preferred path for gas, thus be deleterious for pressure tightness. This defect could also be a preferred path for crack propagation. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 2. Metal flow around dendrites [14].

References: 2, 3, 4, 9, 10, 11, 12, 13, 14.

References: 2, 3, 4, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 80.




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A Internal defects I A1 Shrinkage defects II A1.3 Layer porosity III

A Internal defects I A2 Gas-related defects II A2.1 Air entrapment porosity III

Definition A layer porosity consists of a set of shrinkage defects aligned typically along the neutral thermal axis/surface of the casting in its thin regions (where the component thickness is far smaller than the two other dimensions and the thermal gradient is lower than all adjacent points). Morphology A layer porosity is made up of a set of small shrinkage cavities laying on a surface, typically the neutral thermal one. The size of a layer porosity varies between 10-100 µm. This defect can be detected by means of radiographic inspections and metallographic tests. Metallurgical origin The layer porosity forms when the solidification fronts converge towards two surfaces and the last solidifying liquid metal cannot flow within the dendrites of the mushy zone. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 3. The formation of a layer porosity.

Definition The air entrapment porosity consists of small cavities due to air bubbles trapped inside the liquid metal. Morphology Air entrapment porosities appear as spherical or ellipsoidal cavities characterized by relatively smooth surfaces on which a thin oxide layer (due to the high-temperature interaction between air and the liquid metal) could be found. The final distribution of cavities within the casting depends on the path of the metal. The size of an air entrapment porosity is 10-2000 µm. This defect can be detected by means of radiographic, ultrasonic inspections and metallographic tests. Metallurgical origin Air entrapment porosity is the most frequent defect found in HPDC products. Air bubbles can form in turbulent liquid metal vein either when it is in the shot sleeve, in filling channels or inside die cavity.

Fig. 4. Entrapped air porosity in the casting [75].

References: 3, 4, 13, 15.

References: 2, 3, 4, 5, 14, 21, 22, 24, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 77, 78.




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A Internal defects I A2 Gas-related defects II A2.2 Hydrogen porosity III

A Internal defects I A2 Gas-related defects II A2.3 Vapour entrapment porosity III

Definition The hydrogen porosity consists of cavities due to the presence of hydrogen in the melt. Morphology Hydrogen porosity consists of approximately spherical cavities characterized by smooth and no-oxidized surface. Such cavities have a rather small size (0.05-0.5 mm of diameter) and are distributed almost homogeneously within the casting. This defect can be detected by means of radiographic, ultrasonic inspections and metallographic tests. Metallurgical origin Humidity can cause the presence of monoatomic hydrogen within the liquid metal at high temperature. Due to the abrupt reduction of hydrogen solubility in the solid phase, the solidifying region rejects hydrogen and this element concentrates in the liquid near the liquid/solid interface, where it combines into molecular form. In HPDC castings, hydrogen porosity is far less frequent than air entrapment porosity. This type of defect can be frequently found in permanent mold casting process.

Fig. 5. Micrograph of a hydrogen porosity [18].

Definition The vapour entrapment porosity consists of cavities caused by the residual humidity of the die. Humidity becomes vapour when it comes into contact with the molten metal. Morphology The vapour entrapment porosity is generally in the form of approximately spherical cavities with smooth surfaces. Due to the generally localized presence of humidity on the die, cavities are typically concentrated, even if in regions of the casting that could be far from the area where they originated. The size of a vapour entrapment porosity is 0.5-3 mm. This defect can be detected by means of radiographic, ultrasonic inspections and metallographic tests. Metallurgical origin The presence of humidity on the die surface could result from an excess of the water-based lubricant. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 6. Schematics of vapour entrapment porosity.

References: 3, 4, 14, 15, 18, 37, 38, 39, 40, 41, 42, 76.




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Definition The lubricant entrapment porosity forms when the gases - resulting from the decomposition of the lubricant - remain trapped into liquid metal in form of bubbles. Morphology The lubricant entrapment porosity is characterized by small, approximately spherical cavities with smooth surfaces. The surface of the cavities appears darker respect to the surface of gas-related defects due to the presence of combustion products on it. The size of a lubricant entrapment porosity varies between 0.5-3 mm. This defect can be detected by means of radiographic, ultrasonic inspections and metallographic tests. Metallurgical origin The lubricant entrapment porosity is caused by an excessive quantity of lubricant on the die surface and/or piston/sleeve coming into contact with molten metal. This type of defect can be found both in HPDC and permanent mold casting processes.

A Internal defects I A2 Gas-related defects II A2.4 Lubricant entrapment porosity III

A Internal defects I A3 Filling-related defects II A3.1 Joint III

Fig. 7. Schematics of lubricant entrapment porosity.

Definition A joint is a discontinuity of metallurgical type (either microstructural and/or metallic) rather than of geometrical-type (such as, for example, a cavity or a crack). Morphology The appearance of this defect depends on the conditions of the metal flows at the moment of their confluence and on their location within the die-cavity. This defect can be detected only by means of metallographic tests. Metallurgical origin The joint forms when a relatively cold liquid metal flow - at least partially solidified and in some cases covered by an oxide film - meets another warmer metal vein that can flow around it. The joint usually brakes along the previous interface of flows when relatively low tension stresses normal to it or shear stresses parallel to it are applied. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 8. Micrograph of a joint.

References: 2, 4, 14, 24, 26, 28, 29, 30, 31, 33, 34, 35, 37, 43, 44, 45, 46, 47, 48, 49.




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A Internal defects I A3 Filling-related defects II A3.2 Lamination III

A Internal defects I A3 Filling-related defects II A3.3 Cold shot III

Definition A lamination is a typical surface defect (see B3.2), but in some cases can be also considered as internal defect. The lamination is a sort of thin metallic layer (skin) with different microstructure in comparison to the material laying around and partly separated by a thin oxide film. Morphology This defect consists of a thin metallic layer having a separation surface from the bulk metal almost parallel to the component surface, with imperfect adhesion to the inner metal and with finer microstructure. This defect presents variable size and can be detected by means of ultrasonic inspections and metallographic tests. Metallurgical origin The lamination forms when the metal foil comes into contact with the die surface and cools down with a higher rate than the surrounding regions. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 9. Schematics of a lamination.

Definition Even if a cold shot always originates at the surface of the die, it can be found as internal defect when it is trailed by a liquid metal flow. The cold shot is a small amount of metal characterised by microstructural features much finer than the surrounding regions and separated by a thin oxide layer. Morphology The cold shot seems as a small amount of metal with a spherical or ellipsoidal shape. The size of a cold shot varies between 0.01-0.2 mm. This defect can be detected only by means of metallographic tests. Metallurgical origin The cold shot forms when the alloy flows turbulently with a front characterized by a considerable presence of drops (spray effect) and comes into contact with the surface of the die. In such conditions, the molten metal can approach the solidus temperature and rapidly solidifies. This type of defect is typical of HPDC process, because in this case the spray effect is more frequent.

Fig. 10. Micrograph of a cold shot [75].

References: 2, 4, 14, 24, 28, 29, 30, 31, 33, 34, 35, 37, 43, 44, 45, 46, 47, 49.

References: 2, 4, 14, 24, 28, 29, 30, 31, 33, 34, 35, 37, 43, 44, 45, 46, 47, 49, 75.




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A Internal defects I A4 Undesired phases II A4.1 Inclusion III

A Internal defects I A4 Undesired phases II A4.2 Undesired structure III

Definition Inclusions are typically non-metallic phases and include oxides and dross. Morphology The inclusion can be in the form of a particle or of a thin film. The size of an inclusion is major than 0.1 mm. This defect can be detected by means of radiographic, ultrasonic inspections and metallographic tests. Metallurgical origin In Al-alloys the most frequent type of inclusion is the aluminium oxide, i.e. alumina (Al2O3). It easily forms when the liquid metal comes into contact with air. Inclusions can also be other non-metallic phases, such as small portions of refractories (often silicon carbide) or dross. Because of their high hardness, inclusions can cause machining problems. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 11. Image of aluminium oxide [75].

Definition These are areas of different microstructure which are undesired mainly for their high hardness, stiffness, brittleness and because they create microstructural discontinuities. Morphology The morphology of undesired structures cannot be uniquely described and their size depends on the cell size. For example, the SDAS could be outside the acceptable limit for a specific region of the casting. This defect can be detected by means of metallographic tests. Metallurgical origin Undesired structures can include portions of previously produced castings (for example flash), accidentally left within the die-cavity and then embedded in the successive casting. Undesired structures represent microstructural discontinuities and could act as crack nucleation and propagation sites during cooling, finishing operations or in-service behaviour. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 12. Micrograph of a region with large dendrites [53].

References: 2, 4, 14, 24, 28, 34, 35, 37, 42,43, 44, 45, 46, 47, 50, 51, 52, 52, 76.

References: 2, 4, 52, 53, 54.




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A Internal defects I A5 Thermal contraction defects II A5.1 Crack III

A Internal defects I A5 Thermal contraction defects II A5.2 Hot tear III

Definition A crack is a geometrical discontinuity characterized by one dimension far smaller than the two others. Morphology A crack is a geometrical discontinuity characterized by one dimension far smaller than the two others. A narrow void volume lays within the two faced fracture surfaces which define crack. The length of a crack varies between 10 µm to several mm. This defect can be detected by means of ultrasonic inspections and metallographic tests. Metallurgical origin In HPDC products, such defect forms at relatively low temperature (far from the solidification range) when the greater thermal contraction of the casting with respect to the die is prevented by the die itself. Cracks can often occur in regions of stress localization, either due to macroscopic geometrical reasons or to the presence of microstructural defects. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 13. Schematics of a crack formation.

Definition A hot tear is a brittle crack formed in liquid portions of the mushy zone in the final stages of solidification. Morphology The surface of a hot tear typically displays a dendritic morphology and can be heavily oxidized since formed at high temperature. The length of an hot tear varies between 10 µm to several mm. This defect can be detected by means of metallographic tests. Metallurgical origin The hot tear usually forms in such alloys characterized by a wide solidification temperature range and in hot spot areas at stresses far below the tensile stress at the temperature. The hot tear can also be due of stress concentration for geometrical or microstructural reasons. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 14. Micrograph of a hot crack [4].

References: 2, 4.

References: 2, 3, 4, 14, 55, 56, 57.




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SURFACE DEFECTS

B Surface defects I B1 Shrinkage defects II B1.1 Sink III

B Surface defects I B2 Gas-related defects II B2.1 Blister III

Definition A sink is a surface depression related to the presence of a sub-surface shrinkage porosity. Morphology A sink looks like a surface depression toward the interior of the casting. It extends for several mm. This defect can be detected by means of visual, liquid penetrant inspections and metallographic tests. Metallurgical origin A sink occurs when, during the casting solidification, a hot spot localizes close to the metal/die interface. The skin layer - formed as a consequence of the contact with the die - is not able to sustain stresses arising from the contraction of the sub-surface solidifying region and plastically deforms. The sink is typically found in components with relatively wide plane surfaces or with sharp cross section changes. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 15. External surface of a sink [14].

Definition The blister is a porosity defect due to gases entrapment, with the only difference, in comparison with internal defect, that gases are entrapped within a sub-surface region. Morphology A blister is a small amount of material that blown up (with respect to the surrounding surface) in correspondence of a sub-surface gas porosity. A blister extends from 100 µm to several mm. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin A blister consists of small surface area that blown up when the internal pressure of sub-surface gas-related porosity is high enough to plastically deform the thin metallic layer that covers it. The metal deformation occurs easily at relatively high temperatures, when castings are ejected from the die or during following heat treatments. This type of defect can be found more frequently in HPDC process, because of turbulent die-filling and consequent air entrapment.

Fig. 16. Example of blisters [74].

References: 2, 14.

References: 2, 14, 58.




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B Surface defects I B3 Filling-related defects II B3.1 Joint and vortex III

B Surface defects I B3 Filling-related defects II B3.2 Lamination III

Definition The joints are surface wrinkles, slight depressions or simply alterations of castings visual features along the line corresponding to the interface between converging flows. A particular joint defect is the vortex, which forms on the surface when only one flow rolls itself up and generates a particular spiral distribution of oxide films and microstructures. Morphology A joint presents wrinkled surfaces or linear depressions due to the deformation of the cooler and more viscous flow. The surface of the casting can also be unaltered, but the presence of different microstructures of different flows is visible. The vortex has a characteristic spiral-shaped appearance on the surface of the casting. The joint and the vortex can be detected by means of visual inspections and metallographic tests. Metallurgical origin A joint forms when a relatively cold metal flow - at least partially solidified and in some cases covered by an oxide film - meets another warmer metal vein that can flow around it. As explained in the case of the corresponding internal defect (see A3.1), the metallic discontinuity can cause material detachment along it when even relatively low stress arises. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 17. Macrograph showing a vortex [74].

Definition A lamination is a thin surface metallic layer having a separation surface from the bulk metal almost parallel to the component surface and with imperfect adhesion to the inner metal. Morphology The lamination seems as a skin with different microstructure and partly separated from the bulk material by an oxide film. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin A lamination forms when a relatively warm vein at low viscosity flows between the steel-die and another cooler and partially solidified flow. Laminations could also form as a result of deformations of the die related to sudden pressure changes. The resulting metallic discontinuity can cause the partial or complete skin detachment along the interface, when even relatively low stresses arise or are externally applied. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 18. Surface lamination.

References: 2, 3, 4, 14.

References: 2, 4, 14, 24, 28, 29, 30, 31, 33, 34, 35, 37, 43, 44, 45, 46, 47, 49.




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B Surface defects I B3 Filling-related defects II B3.3 Cold shot III

B Surface defects I B4 Undesired phases II B4.1 Surface deposit III

Definition A cold shot is a small amount of metal characterised by microstructural features much finer than the surrounding regions and separated by a thin oxide layer. Morphology The cold shot looks like a small amount of metal and presents a spherical or ellipsoidal shape. The size of a cold shot varies between 0.01-0.2 mm. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin A cold shot forms when a small portion of liquid metal comes into contact with the surface of the die and rapidly solidifies (see A3.3). This condition occurs when the alloy flows turbulently and with a front characterized by a considerable presence of drops (spray effect). This type of defect can be found frequently in HPDC process, because in this case the spray effect is more frequent.

Fig. 19. Macrograph of a surface cold shot [14].

Definition A surface deposit can be a layer of various chemical composition, thickness, distribution and adhesion, which, for various reasons, deposited on the surface of the casting (without chemical interaction) during the process. Morphology A deposit appears as a surface region covered by particles of different chemical composition respect to the casting. A surface deposit usually extends for several mm. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin A lubricant excess, which can be transferred from the die to the casting, can cause a surface deposit formation. This type of defect can be found both in HPDC and permanent mold casting processes. Fig. 20. Macrograph of a deposit.

References: 2, 4, 14, 24, 28, 29, 30, 31, 33, 34, 35, 37, 43, 44, 45, 46, 47, 49.




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B Surface defects I B4 Undesired phases II B4.2 Contaminant or inclusion III

B Surface defects I B5 Thermal contraction defects II B5.1 Crack III

Definition A contaminant can be a layer of various chemical composition, thickness, distribution and adhesion, which, for various reasons, deposited on the surface of the casting during the HPDC process or later, but in some way related to the process. For inclusion definition refer to the corresponding internal defect (A4.1). Morphology When a contaminant defect occurs, the surface of the casting appears locally coloured differently from the other portions (or from the usual color related to the presence of a thin oxide layer). The size of a contaminant or an inclusion is major than 0.1 mm. These defects can be detected by means of visual inspections and metallographic tests. Metallurgical origin Contaminants are the result of interaction between metal and substances locally came into contact with it. Even corrosion of the casting can be included into this type of defect. This defect can be found both in HPDC and permanent mold casting processes.

Fig. 21. Macrograph of a contaminant.

Definition A crack is a defect that can widely extend within the casting, from surface to surface. For this reason, cracks have been included both within internal and surface defects. Refer to the corresponding internal defect (A5.1) for defect definition, causes and morphological features. Morphology A crack is a geometrical discontinuity characterized by one dimension far smaller than the two others. A narrow void volume lays within the two faced fracture surfaces which define crack. The length of a crack extends from 10 µm to several mm. This defect can be detected by means of visual, liquid penetrant, magnetic particle inspections and metallographic tests. Metallurgical origin A crack generally originates on a surface or in a sub-surface position, but it can propagate into internal regions of the casting until reaching the other surface/s. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 22. Image of a crack.

References: 2, 4.




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B Surface defects I B5 Thermal contraction defects II B5.2 Hot tear III

B Surface defects I B6 Metal/die interaction defects II B6.1 Erosion III

Definition A hot tear is a defect that can widely extend within the casting, from surface to surface. For this reason, hot tears have been included both within internal and surface defects. Refer to the corresponding internal defect (A5.2) for defect definition, causes and morphological features. Morphology The surface of a hot tear typically displays a dendritic morphology and can be heavily oxidized since formed at high temperature. The length of a hot tear extends from 10 µm to several mm. This defect can be detected by means of visual, liquid penetrant, magnetic particle inspections and metallographic tests. Metallurgical origin The hot tear usually forms in such alloys characterized by a wide solidification temperature range and in hot spot areas at stresses far below the tensile stress at the temperature. The hot tear can also be due of stress concentration for geometrical or microstructural reasons. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 23. Macrograph of a hot crack [57].

Definition Erosion is a defect that reproduces, in negative, a defect of the die caused by erosive phenomena. Morphology Erosion consists of a material excess on the casting caused by the steel removal from the die by erosive wear. The thickness of the erosion defect is 1-200 µm. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin The impact of the turbulent flow at high speed and high temperature on the die-cavity can lead to the progressive wear erosion. In addition to the above mentioned factors, erosion is related to the inclination angle of the metal flow with respect to the die surface and to the presence of particles or bubbles inside the liquid metal (cavitation). This type of defect can be found more frequently in HPDC process, due to high pressure and speed of the metal during the die-cavity filling.

Fig. 24. Image of a casting defect caused by die-erosion [75].

References: 2, 3, 4, 14, 55, 56, 57.

References: 2, 14, 55, 59, 60, 61, 75.




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B Surface defects I B6 Metal/die interaction defects II B6.2 Soldering III

B Surface defects I B6 Metal/die interaction defects II B6.3 Thermal fatigue III

Definition As for other metal/die interaction defects, the metallurgical origin of the die damage (soldering) reflects in the name of the casting defect. Morphology Soldering causes surface roughness or localized lack of material on the casting. The thickness of the soldering defect is 1-200 µm. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin The common metallurgical origin of soldering is the formation of intermetallic phases on the die surface and following adhesion of the aluminium alloy on them. Soldering often promptly occurs in regions of the die exposed to liquid metal at relatively high temperature and flow rates. Soldering can also easily take place in the zones of the die where thermal fatigue or erosion phenomena previously occurred. This type of defect can be found more frequently in HPDC process, due to high pressure and speed of the metal during the die-cavity filling.

Fig. 25. Example of soldering effects on the die [14].

Definition Thermal fatigue is the name commonly given to narrow relieves related to corresponding damage of the die. Morphology The defect consists of a set of narrow relieves on the surface of the casting, sometimes referred also as crocodile skin. The thickness of the thermal fatigue defect is 1-200 µm. This defect can be detected by means of visual, liquid penetrant, magnetic particle inspections and metallographic tests. Metallurgical origin As the time of service of the die increases, small cracks can form on its edges or surface, due to the repetition of stress-strain cycles (induced by the rapid heating and cooling stages). The liquid metal filters into cracks and gives rise to the relieves on the surface of the casting. This kind of defect appears on the casting independently from the specific process or relative parameters. This type of defect can be found more frequently in HPDC process, because of the high production rate and consequent severe damage of the die.

Fig. 26. A casting defect caused by the presence of cracks on the die [74].

References: 2, 14, 55, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71.

References: 2, 14, 55, 59, 60, 61, 62, 66, 67, 72, 74, 79.




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B Surface defects I B6 Metal/die interaction defects II B6.4 Ejection mark III

B Surface defects I B6 Metal/die interaction defects II B6.5 Corrosion of the die III

Definition The ejection mark is a defect related to the presence of an undercut in the die, that could be a result of modifications of the die-geometry (for example due to one of the previously described erosion/soldering phenomena). Morphology The ejection mark appears as a plastic deformation of the casting, that extends along the direction of the ejection from the die. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin When the ejection occurs at relatively high temperature, the presence of even a small undercut on the die causes the deformation of the casting around the undercut in the ejection direction. This type of defect can be found more frequently in HPDC process, because of the high production rate and consequent severe damage of the die.

Fig. 27. Image of an ejection mark.

Definition This defect consists of surface roughness of the casting due to damage of the die surface, attacked by the environment (corrosion). Morphology The surface of the product is characterized by high roughness, more evident in correspondence of the severely corroded areas of the die cavity. The thickness of the defect caused by the corrosion of the die is 1-200 µm. This defect can be detected by means of visual inspections and metallographic tests. Metallurgical origin The corrosion of the die is caused by the interaction with the environment and can be homogeneous or localized. An extremely severe corrosion of the die, either homogeneous or localized, can thus lead the casting to be out of tolerance or to have localized excess of material. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 28. Image of a casting obtained with a corroded die.




20

GEOMETRY DEFECTS

C Geometry defects I C1 Lack of material II C1.1 Incomplete casting III

C Geometry defects I C2 Excess of material II C2.1 Flash III

Definition An incomplete casting presents a local lack of material with respect to the geometry of the die cavity. Morphology The lack of material can be in order of several mm or cm. This defect can be detected by means of visual inspections. Metallurgical origin Due to an excessively high viscosity, a front portion of the metal flow can stop before the die cavity has been completely filled. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 29. Example of incomplete casting [31].

Definition A flash is an excess of material due to metal infiltration inside a thin gap between the die parts. Morphology The flash is a thin layer of material in excess, whose geometry roughly reproduces that of the die separation surface. The excess of material can be in order of several mm or cm. This defect can be detected by means of visual inspections. Metallurgical origin The flash is due to an insufficient clamping force of the machine, which cannot counterbalance the alloy pressure. Flashes are originated also by a different die thermal expansion. This type of defect can be found more frequently in HPDC process, because of the high pressure of the metal.

Fig. 30. Example of flash [74].




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C Geometry defects I C3 Out of tolerance II C3.1 Deformed part III

Definition A deformed part presents a geometrical non-conformity to its design geometry, even in absence of local excess or lack of material. Morphology The deformation of the casting is major than the established tolerances. This defect can be detected by means of visual inspections. Metallurgical origin The formation of the defect is related to the thermal contraction during cooling, that causes local stress inside the casting. The defect is more pronounced in castings ejected by the die at high temperature and presenting drastic thickness changes. This type of defect can be found both in HPDC and permanent mold casting processes.

Fig. 31. Schematics of out of tolerances castings.




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4. Dictionary of defects of HPDC and permanent mold casting processes DEFECT

CODE ITALIAN ENGLISH FRANCAIS DEUTSCH ESPAÑOL

A1.1 Macro-porosità Macro-shrinkage Souffloures Makroporosität Macroporosidad

A1.2 Porosità

interdendritica Interdendritic

shrinkage Microretassures Interdendritische Porosität

Porosidad interdendritica

A1.3 Porosità planare Layer porosity Microretassures

centralés des pieces Mittellinienporosität Porosidad planearea

A2.1 Porosità da aria

intrappolata Air entrapment

porosity Porositè d’air

enprisoné Luftporosität

Porosidad de aire atrapado

A2.2 Porosità da idrogeno

Hydrogen porosity Porositè de gaz Dissous dans le

metal Hydrogenporosität

Porosidad de hidrógeno

A2.3 Porosità da umidità

residua

Vapour entrapment

porosity Porositè

Dampfporosität/ /Dampf Feuchtigkeits-

einschluss

Porosidad de humedad restante

A2.4 Porosità da lubrificante

Lubricant entrapment

porosity

Porositè de gaz de poteyage

Porosität durch Verbrennungs-

produkteinschluss

Porosidad de lubricante

A3.1 Giunzione Joint Reprise Verbindung/ kaltflieβstelle

Unión

A3.2 Sfogliatura Lamination Friass Schülpe Foliación A3.3 Goccia fredda Cold shot Goutte froide kalter Tropfen Gota fría A4.1 Inclusione Inclusion Inclusion Einschluβ Inclusión

A4.2 Struttura non

desiderata Undesired structure

Structure ne pas demandée

Nicht gewunschte Struktur Estructura no

deseada A5.1 Cricca Crack Fissure Riβ Grieta A5.2 Cricca a caldo Hot tear Fissure à chaud Warmriβ Grieta en caliente B1.1 Ricalo Sink Retassure Schwindung Hundimiento B2.1 Blister Blister Cloque Blase Burbuja B3.1 Giunzione e vortice Joint and vortex Reprise Kaltstell und Wirbel Unión y remolino B3.2 Sfogliatura Lamination Eclatement Abblätterung/Schülpe Foliación B3.3 Goccia fredda Cold shot Goutte froide kalter Tropfen Gota fría B4.1 Depositi Surface deposits Dépot Schlackenhalde Depósitos

B4.2 Contaminazione o

inclusione Contaminant or inclusion

Trace de lubrifiant, trace de poteyage ou

inclusion

Verunreinigung/ Beschmutzung oder

Einschluss

Contaminación o inclusión

B5.1 Cricca Crack Fissure Riβ Grieta B5.2 Cricca a caldo Hot tear Fissure à chaud Warmriβ Grieta en caliente B6.1 Erosione Erosion Erosion Erosion Erosión B6.2 Metallizzazione Soldering Etamage Metallisierung Metalización

B6.3 Crettature da fatica

termica Thermal fatigue Craquelure Durch thermische Ermüdung

Grieta por fatiga térmica

B6.4 Segno di espulsione Ejection mark Arrachement Auswerfermarke Marca de expulsión

B6.5 Corrosione dello

stampo Corrosion of the

die Corrosion de moule

Druckgussform korrosion/

Korrosion der Druckgussform

Corrosión del molde

C1.1 Getto incompleto Uncomplete

casting Malvenue Unvollstandiges Gussteil Pieza incompleta

C2.1 Bava Flash Bavure Guβgrat Rebaba C3.1 Getto deformato Deformed part Deformation Deformiertes Guβteil Pieza deformada




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24

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Documents

D D2.1 Database on Defects - Stacast · PDF filedie is first in contact with a molten alloy at more than 700°C and, after 30-40 seconds, with a ... defect can be found both in HPDC