The AIM "Quality of HPDC parts" Workgroup are working for a proposal of classification

 

This paper represents the second part of the article published in September. In the first part of the paper the main frame of the classification of defects of High Pressure Die Cast (HPDC) part proposed by the WG “Quality of HPDC parts” of the Technical Committee “HPDC” of the Italian Association of Metallurgy was presented. The classification is a three-level, hybrid-type one. In the first level defects are grouped on the basis of their position (surface/internal and geometry defects) according to the typical control operations during which they can be detected and to the effects of defects on the functionality of the parts. The second level of the classification groups defect in classes according to their general metallurgical origin. Since the same metallurgical origin can be attributed both to surface and internal defects, corresponding names were adopted for level II classes. The specific defects are identified in the third level of the classification, better specifying their origin. In the present paper an introduction to the general classes of defects defined in level II as well a presentation of single defects identified in level III is proposed focusing on the above these features and without the aim of completing defect descriptions.

 

These class of defects are discontinuities formed when gas is in solution into molten metal or when, for different reasons, this latter contains gas bubbles.

The gas-related internal and surface defects will now be presented, stressing their common metallurgical origin.

Internal gas-related defects are spherical or round-shaped cavities (often known as porosity for their small size) characterized by their smooth surface.

Four different gas-related defects are proposed in level III (Fig. 7).

- Hydrogen porosity. When gas (typically hydrogen) is dissolved into molten metal, discontinuities originated durino solidification due to the abrupt reduction of hydrogen solubility in the solid phase/s. In fact, its rejection from the solid metal causes its concentration and bubble formation during solidification of the surrounding metal. Cavities are rather small and homogeneously distributed.

- Air entrapment porosity. It forms when air bubbles remain trapped in the liquid metal. This is the most frequent gas-related defect 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 the die cavity.

- Vapour entrapment porosity. Residual humidity on the die surface becomes vapour when it comes into contact to molten metal and then becomes trapped into it.

- Die-lubricant entrapment porosity. It forms when ‘uncontrolled’ lubricants combustion occurs as die-lubricant directly come into contact with molten metal and its combustion products become trapped into it.

The morphology and spatial distribution of defects as well as their brightness, colour and/or the presence of oxides on their surfaces can suggest the gas causing their formation.

Gas-related surface defects are commonly known with the term blisters.

These consist in small surface areas that blown up when the internal pressure of sub-surface gas related porosity plastically deformed the thin metallic surface layer that covers it (Fig. 6). The metal deformation occurs easily at relatively high temperatures, at which the metal yields under relatively low stress. This could happen both as the cast is ejected from the die or during following heat treating. Similarly during heat treating of coated cast parts, gases can flow out of the metal and the coating can blow up. This latter phenomenon, sometimes referred as outgassing, in the present classification is considered to give rise to blisters. Thus, blisters represent typical examples of defects of metallurgical origin that can be revealed or emphasized by following operations during the manufacturing process of the cast part.

 

The filling-related defects are caused by anomalous liquid metal flow. During filling of the die cavity, liquid or partially solidified metal veins at different temperatures and sometimes covered by oxide films can incorrectly come into contact (in some cases even with completely solidified metal) causing metallurgical inhomogeneities. The degree of imperfect metallic continuity can vary according to specific situations and it is particularly low when oxide films separate the solidifying flows. For these reasons, filling-related defects can be revealed when even limited mechanical stress is applied, such as for example during machining operations. Depending on their final location, these defects can result as surface or internal defects, but substantially remains of the same type. For this reason the same terms were adopted for level III: flow defects, laminations, cold shots (Fig. 8).

- Flow defect. This defect forms when a relatively cold metal portion, at least partially solidified and in some cases covered by an oxide film meets another warmer metal vein that can flow around it. When the flows meet close to the surface they give rise to a wrinkled surface or to linear depressions due to the deformation of the cooler and more viscous flow. In the present classification the term “flow defect” is adopted also for defects that are separately identified in other classifications (for example they could be referred in some cases as veins). In fact, the appearance of the defect can vary depending on the conditions of the metal flows at the moment of their confluence and on their location within the die cavity.

- Lamination. This defect, often considered only as a surface defect, consists in a thin surface metallic layer having a separation surface form the bulk metal almost parallel to the component surface and with imperfect adhesion to the inner metal. A lamination forms when a relatively warm metal vein flows between a cooler one and the die, also as a result of irregular movements or deformations of the die.

- Cold shot. It forms when a small portino of molten metal accidentally comes into contact to the die and rapidly cools. Its rapid solidification leads to a finer microstructure with respect to that of

the surrounding region, from which it could also be separated by a thin oxide layer. In any case the presence of a cold shot means the existence of an at least partial microstructural discontinuità with respect to other regions of the cast.

In some cases the metal flowing into the die can cause detachment of some of these rapidly solidified regions and can drive them inside the cavity, without completing their melting. Thus, cold shots can also be internal defects.

 

Shrinkage defects are metal discontinuities that form as a result of the volume contraction during solidification in regions where local metal filling is insufficient or even absent. This occurs in regions which are locally the last to solidify (hot spots). Such regions are often well inside the cast part, but in some cases they are so close to the die surface to give rise to surface defects.

When shrinkage defects are of internal type, they can be either in the form of interdendritic porosity, layer porosity or blowholes, depending on their size and location. In the case of surface defects, they are termed sinks. Some representative examples of these classes of defects are shown in Fig. 9.

Interdendritic porosity. It forms when liquid metal can not adequately flow inside interdendritic regions. The resulting small-size discontinuities are interconnected and can affect pressure tightness.

- Blowhole. It is relatively large shrinkage cavity, formed at hot spots. Its dimensions are related to the volume of the unsuitably filled region. Blow holes are characterized by rough and spongy surfaces for the presence of emergine dendrites as a consequence of their interrupted growth. Blowholes can also be surrounded by interdendritic porosity. Blowholes and interdendritic porosità can be considered as the extremes of a continuous range of defect sizes and distributions, that depends on a combination of factors including the alloy on one side and geometry/process parameters on the other. In many situations the identification of the particular shrinkage defect can be ambiguous.

- Layer porosity. In this case shrinkage defects are aligned along the neutral thermal axis/surface of a cast components. When the component thickness is far smaller than other dimensions these surfaces can locally be approximated as planes. Solidification fronts converge along these surfaces and liquid metal can not efficiently flow within the dendrites of the mushy zone, forming series of shrinkage discontinuities which can appear aligned on proper metallographic sections.

- Sink. It is a surface defect that occurs when, during the cast solidification, an hot spot localizes close to the metal/die interface. This could be for example the case of component with relatively wide plane surfaces or with internal corners. Thin metal layer (skin layer) that rapidly solidified when metal came in contact to the cooler die is not able to sustain stresses arising from the contraction of the sub-surface solidifying region and it plastically deforms (sinks) toward the interior of the cavity (see Fig. 9).

 

These defects consist in cracks formed during solidification or cooling to room temperature when tension stresses arising for the material contraction resulting from solidification or cooling exceed UTS at the local metal temperature. In the two cases defects are termed as hot tears and cracks, respectively. These terms in level III are the same for internal and surface defects since in general a single defect can be accounted both as surface and as internal defect.

- Crack. This defect forms at relatively low temperature (far from the solidification range) where the greater thermal contraction of the cast with respect to the die is prevented by a particolar part/die geometry. Tension stresses generated into the cast component can locally reach relatively high values, causing cracking. Cracks originated from ejection or handling operations can not be included in this type of defects.

- Hot tear. This defect arises when local UTS is exceeded in the solid portions of solidifying metal (often the last solidifying regions). Both cracks and hot tears often occur in regions of stress localization, either due to macroscopic geometrical reasons or to the presence of previously formed microstructural defects (as, for example, gas-related or interdendritic porosity).

 

The surface or the bulk material of HPDC parts can reveal the presence of phases different form the desired ones for each region: i.e. for the typical HPDC aluminium alloys a thin layer of greyish aluminium oxide on the surface, and a microstructure made of dendrites of the Al-rich a phase surrounded (in interdendritic regions) by eutectic structure.

Most of the undesired phases are of surface type, while in some cases they can be both internal or surface defects.

Most phases are undesired mainly for their higher hardness, stiffness, brittleness and for the fact that they create microstrctural discontinuities, resulting as crack nucleation sites or becoming part of the preferred crack path (either when the crack has a metallurgical origin or as it occurs after die-casting operations, such as for example during service). Particularly undesired phases, often in the form of small internal/surface particles, are non metallic inclusions (including oxides, dross). Also intermetallics or metallic phases can be some times identified as undesired phases. Lastly, metallic inclusions, i.e. metallic portions left in the die cavity by previous casting operations, and became trapped into the molten metal and represent metallurgical discontinuities.

Undesired surface phases can be revealed at visual inspections by unexpected colours. Among these defects are: contaminations and deposits. They refer to layers of non-metallic substances of variable thickness, adhesion and local distribution, deposited durino die-casting operations. More specifically, contaminations are signs of an interaction of the metal with the environment or with particular substances to which it locally comes in to contact. Deposits are related to substances transferred on the cast surface before its ejection from the die. One typical example of deposit is the excess of die-lubricant transferred from the die to the cast part.

 

This level II class of defects exists only for surface defects and it includes a set of unsuitable  urface morphologies caused by the interaction of the metal with the steel die. Most of these defects are a direct consequence of geometry modifications of the die surface and they are termed according to the degradation phenomenon occurred to the die: erosion, soldering, thermal fatigue

marks, corrosion of the die, in cavità build up (Fig. 10).

- Thermal-fatigue marks. These marks are a set of small net-textured reliefs, sometimes referred also as crocodile skin. They form from corresponding thermal fatigue cracks on the die surface formed and progressively extended during its service in particularly critical regions.

- Soldering. These defect consist in a localized lack of material on the cast component as a consequence of the formation of relief zones made of adherent intermetallic phases (Al-Si-Fe). Since aluminium alloy progressively layered over these intermetallics and sometimes partially locally detaches, the geometry of the defect on the cast parts can slightly vary. Soldering often occurs in region of the die exposed to liquid metal at relatively high temperature and flow rates.

- Erosion. This defect is caused by a progressive removal of the material from the steel die by erosive wear, which reflects in regions of excess material in the cast parts. The die erosion is generally more relevant in regions of the die where the liquid metal flows at particularly high flow rate and temperature. Erosion can also occur by cavitation (implosion of gas bubbles) at the

die surface.

- Corrosion of the die. This defect consists in surface roughness of the cast product resulting from a corresponding die surface area attacked by the environment (corrosion phenomena).

- In cavity build-up. Die lubricant or calcium carbonate can remain on the die and progressively accumulate (buildup) causing poor finishing of HPDC products.

- Ejection mark. This is a localized region of the cast part plastically deformed in the ejection direction. Such defects are often due to undercuts as a result of modifications of the geometry of the die or of the cast part (for example by one of the previously described erosion/soldering phenomena). The term “ejection marks” does not refer to the marks left on the components in the locations of the ejector pins and due to their incorrect geometry/position, which are considered as geometry defects.

 

The paper gives an introduction to one of the current actions undertaken by the WG “Quality of HPDC products” within the TC “HPDC” of the Italian Association of Metallurgy, concerning the classification and terminology of defects of HPDC products.

An initial survey of literature and industrially adopted classifications of defects in parts cast in metallic dies, revealed that the geometry-based and origin-based approaches are often mixed creating a wide range of hybrid classifications.

The proposed classification of HPDC products, discussed within the WG with the contribution of several foundries, is a multi-level, hybrid-type classification.

In the first level defects are grouped accordino to their position (surface/internal and geometry defects) following the typical inspection operations during which these defects can be detected and considering the effect of defects on the functionality of the components. The second level of the classification groups the defects into classes according to their general metallurgical origin. In this level analogies between the origin of internal/surface defects are highlighted.

The proposed classification does not define cause/defect correlations, but suggests starting points to the identification of specific causes. In order to better specify these features, the general classes of defects defined in level II were presented in the present paper. The specific defects taken into account for this classification are those identified in level III, where metallurgical origin is often more precisely specified by the adopted terminology. The action of the WG concerning defect classification of HPDC parts is still in progress with the definition of the multi-language terminology equivalence and with the compilation of the official document includine the proposed classification and terminology translations. For further information, suggestions and contributions, contact the authors of the present paper writing to the Italian Association of Metallurgy at info.aim@aimnet.it refering to “HPDC defect classification”.

 

The authors thank the members of the WG “Quality of HPDC parts” within the Technical Committee “High Pressure Die Casting” of the Italian Association of Metallurgy for their contribution to the definition of the presented classification and terminology of defects.